- Email: [email protected]
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber
Optics Communications 347 (2015) 64–67
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber Qi Song a, Guoju Wang a, Bingyuan Zhang a,n, Qingli Zhang b, Wenjun Wang a, Minghong Wang a, Guihua Sun b, Yong Bo c, Qinjun Peng c a School of Physics Science and Information Engineering, Shandong key laboratory of optical communication science and technology, Liaocheng University, Liaocheng 252000, China b The Key Laboratory of Photonic Devices and Materials of Anhui, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2015 Received in revised form 17 February 2015 Accepted 2 March 2015 Available online 4 March 2015
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber is demonstrated. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. In addition, a simple way to fabricate few layers graphene oxide saturable absorbers has been reported. & 2015 Elsevier B.V. All rights reserved.
Keywords: Q-Switched Mode-locked Dual-wavelength Graphene oxide Nd:GYSGG
1. Introduction Multi-wavelength mode-locked lasers exhibit versatile applications in remote sensing, metrology, biomedical diagnostics, terahertz (THz) radiation generation, pump-probe measurement, component characterization and telecommunications [1–4]. Compared with single-wavelength laser generating its output at one center wavelength, multi-wavelength ultrafast lasers can simultaneously yield pulse trains at different center wavelengths. Some techniques have been proposed to generate multi-wavelength pulse, including dual gain media [5], stimulated Raman scattering [6] and crystals with the disordered structure, such as Nd:GGG [7], Nd:Sc0.2Y0.8SiO5 [8], Nd:CNGG, Nd:CLNGG, Nd:CTGG and Nd:CaYAlO4 [9–13]. Among them, Nd:GYSGG crystal with an inhomogeneous broadening of the emission spectra is beneficial to generating ultra-short pulses and multi-wavelength laser pulse [14–16]. The emission cross-sections and excited-state lifetime were 1.58 10 19 cm2 and 220 μs, respectively. There are some previous studies on continuous-wave of multi-wavelength [16–18] and passively mode-locking of single-wavelength operation [19] with the Nd:GYSGG crystal. To our knowledge, multi-wavelength mode-locking or Q-switched mode-locking (QML) operation with n
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.optcom.2015.03.002 0030-4018/& 2015 Elsevier B.V. All rights reserved.
this crystal has not been realized yet. Recently graphene and graphene oxide absorbers are believed to be promising saturable absorbers of gapless linear dispersion of Dirac electrons, which shows that they can be operated over a very broad range of wavelengths [20]. In this paper, the passively dual-wavelength Q-switched mode-locking operation of the disordered Nd:GYSGG crystal was obtained using the graphene oxide saturable absorber for the first time. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured.
2. Experimental setup 2.1. Preparation and characterization of GO saturable absorber GO was prepared from expanded acid washed graphite flakes by a modified Hummers method [21]. First, 2 g of graphite was added into 40 ml of H2SO4. Then, 1 g of NaNO3 was added. whereafter, the beaker was put into cold water bath to keep the temperature below 4 °C. 6 g of KMnO4 was added into the mixture, which was vigorously stirred. After addition of the oxidant, the beaker was heated and kept at 20 °C with continuous stirring. Afterward it can be left at room temperature overnight. Soon afterwards, the beaker was put into a water bath at a temperature of 32–38 °C and stirred. The mixture was then heated to 90 °C and
Q. Song et al. / Optics Communications 347 (2015) 64–67
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Fig. 3. Schematic setup of the diode-end-pumped Nd: GYSGG laser.
crystalline structure; the D-peak located at 1352 cm 1, which arises from the breathing modes of sp2 atoms in a ring [22]. 2.2. Setup of the diode-end-pumped Nd: GYSGG laser
Fig. 1. UV–vis–NIR transmissivity spectrum of GO saturable absorber.
kept these conditions for 20 min. 80 ml of alcohol and 5 ml of H2O2 were added in order to stop the reaction and dispersed graphene. HCl solution was used to remove the sulfate ions. To obtain uniform saturable absorber of high quality and suitable size in a quite simple way, 0.2 ml of the alcohol dispersed graphene oxide solution was added into 8 ml of alcohol. Then the graphene oxide solution was stirred by magnetic stirring apparatus for 5 min with the rotation rate of 500 rpm. Subsequently, quartz sheet with the diameter of 30 mm submerged into the prepared graphene oxide solution completely. The quartz substrate with graphene oxide was left at room temperature overnight to remove the alcohol thoroughly. The UV–vis–NIR spectrophotometer was used to characterize the transmissivity of the saturable absorber. Fig. 1 shows the UV–vis–NIR transmissivity spectrum of GO saturable absorber. From Fig. 1 we can see the GO saturable absorber can be employed as saturable absorber in a wide wavelength range from 500 nm to 1100 nm. The transmissivity are 91.1% at 1057 nm and 91% at 1061 nm respectively. The Raman spectrometer (RM 2000) was used to characterize the Raman spectrum of the saturable absorber. Fig. 2 shows the Raman spectrum of the deposited graphene oxide layer. Two Raman peaks can be identified in this spectrum: the G-peak at around 1600 cm 1, due to the bond stretching of sp2 atoms in the
Fig. 3 shows the schematic of the Nd:GYSGG laser cavity. The Q-switched mode-locking laser experimental setup was based on a standard Z-folded cavity with the length of about 1.5 m. The pump source used in the experiment was a fiber-coupled LD with the emission wavelength centered at 808 nm. The output beam of the LD was focused into the Nd:GYSGG crystal with a spot radius of about 0.1 mm using a focusing system. The Nd:GYSGG crystal with the dimensions of 3 3 5 mm3 was employed in the experiment. M was a flat mirror. M1 and M2 were concave mirrors with the radius of curvature of 400 mm, the flat mirror OC was an output couplers with the transmissions of 3% at 1.06 μm laser wavelength. The Nd:GYSGG crystal was mounted in a water-cooled copper block with the temperature of 22 °C. The GO deposited on the quartz sheet was employed as the saturable absorber. The laser spot radius on the GO saturable absorber was 0.12 mm. The laser pulse was recorded by a fast photo-detector (ET-3000) and an oscilloscope (Agilent DSO54832b).The fluorescence spectrum of Nd:GYSGG crystal excited by 808 nm laser is shown in Fig. 4 (measured by an Ocean Optics HR2000þ spectrum analyzer). The resolution of the spectrum analyzer is about 1 nm (FWHM). It is obviously seen in Fig. 4, there are two comparable peaks around 1060 nm due to the Stark splitting of the energy levels, indicating the possibility of multi-wavelength operation [23,24].
3. Experimental results In the experiment, continuous wave operation was obtained
Fig. 2. Raman spectrum of the GO formed on the quartz substrate.
Fig. 4. Fluorescence spectrum of Nd:GYSGG ranging from 950 nm to 1100 nm.
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Q. Song et al. / Optics Communications 347 (2015) 64–67
Fig. 5. The output power versus the incident pump power of the continuous wave (CW) and Q-switched mode-locking (QML).
with the optical conversion efficiency of 6.2% firstly. Then the GO saturable absorber was inserted into the laser cavity. When the pump power increases to 2.2 W, Q-switched operation can be obtained. When the pump power is 2.6 W, the Q-switched modelocked operation was obtained. In order to protect the crystal and GO saturable absorber, the maximum pump power is 5 W. The relationship of the output power versus pump power is shown in Fig. 5. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The slope efficiencies for CW and QML laser were 10% and 6.3%, respectively. Fig. 6(a) and (b) shows temporal shape of a Q-switched pulse envelope and pulse train of the QML Nd:GYSGG laser respectively. The mode-locked pulse inside the Q-switched envelope is quite stable with the repetition rate of 100 MHz. According to the expanded oscilloscope traces of the modelocked pulse within the Q-switched envelope shown in Fig. 6(b), the mode-locked pulse width can be estimated by the formula [25,26]:
Fig. 6. (a) Temporal shape of a Q-switched pulse envelope and (b) pulse train of the QML Nd:GYSGG laser.
2 2 2 2 treal = tmeasure − tprobe − toscilloscope ,
here, treal is the real time of the pulse, tmeasure is the measured rise time, tprobe is the rise time of the probe and toscilloscope is the rise time of the oscilloscope. In our experiment, the average rise time of the mode-locked pulses is 0.59 ns. The rise time of oscilloscope is 0.35 ns and the rise time of the probe is also about 0.175 ns. Assuming that the pulse width is approximately 1.25 times more than the rise time, the estimated mode-locked pulse width is about 441 ps. The output spectrum is shown in Fig. 7 (measured by an Ocean Optics HR2000 þspectrum analyzer). Two laser lines are found simultaneously at 1057.28 nm and 1060.23 nm, which is exactly consistent with the fluorescence spectrum shown in Fig. 4. The separation between wavelengths is quite stable, and the intensities for different wavelength lines were nearly equal. The relative powers of the two lasing wavelength does not stay constant. There is a slight jitter, which was attributed to the mode competition. Considering that the line separation for dual wavelength oscillation regime is optimal for difference frequency generation, this laser material might be employed to generate THz-wave at 0.78 THz with a suitable nonlinear crystal. 4. Conclusion In this paper, the performance of passively dual-wavelength
Fig. 7. Spectrum of Nd:GYSGG laser.
Q-switched mode-locked Nd:GYSGG laser was investigated using the GO as the saturable absorber. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm was obtained, which can be used to generate THz-wave at 0.78 THz. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. In addition, a simple way to fabricate few layers
Q. Song et al. / Optics Communications 347 (2015) 64–67
GO saturable absorbers has been reported. With further optimization, this graphene oxide saturable absorber can be widely used in mode-locked solid state lasers.
[11]
[12]
Acknowledgments [13]
This work is partially supported by the National Natural Science Foundation of China (Nos.:61275147, 11375081 and 51172236), the Natural Science Foundation of Shandong Province (Nos.: ZR2012AL11 and ZR2014FL030) and a grant from the Special Construction Project Fund for Shandong Province Taishan Mountain Scholar.
[14]
[15]
[16]
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H. Jiang, Continuous-wave and passively Q-switched laser performance with a disordered Nd: CLNGG crystal, Opt. Express 17 (2009) 19015–19020. H.H. Yu, H.J. Zhang, Z.P. Wang, J.Y. Wang, Y.G. Yu, Z.B. Shi, X.Y. Zhang, M. H. Jiang, High-power dual-wavelength laser with disordered Nd: CNGG crystals, Opt. Lett. 34 (2009) 151–153. G.Q. Xie, D.Y. Tang, W.D. Tan, H. Luo, S.Y. Guo, H.H. Yu, H.J. Zhang, Diodepumped passively mode-locked Nd: CTGG disordered crystal laser, Appl. Phys. B 95 (2009) 691–695. D.Z. Li, X.D. Xu, J.Q. Meng, D.H. Zhou, C.T. Xia, F. Wu, J. Xu, Diode-pumped continuous wave and Q-switched operation of Nd: CaYAlO4 crystal, Opt. Express 18 (2010) 18649–18654. C.L. Sun, K. Zhong, J.Q. Yao, D.G. Xu, X.L. Cao, Q.L. Zhang, J.Q. Luo, D.L. Sun, S. T. Yin, Diode-pumped continuous-wave quasi-three-level Nd: GYSGG laser at 937 nm, Opt. Commun. 294 (2013) 229–232. C.L. Sun, K. Zhong, C.G. Zhang, J.Q. Yao, D.G. Xu, F. Zhang, S.T. Yin, Stimulated emission cross section of the 4F3/2-4I11/2 of Nd: GYSGG, Laser Phys. Lett. 9 (2012) 410. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, D.L. Sun, J.Q. Luo, S. T. Yin, Continuous-wave and passively Q-switched laser performance of a disordered Nd: GYSGG crystal, Opt. Commun. 284 (2011) 5734–5737. K. Zhong, J.Q. Yao, C.L. Sun, C.G. Zhang, Y.Y. Miao, R. Wang, D.G. Xu, F. Zhang, Q. L. Zhang, D.L. Sun, S.T. Yin, Efficient diode-end-pumped dual-wavelength Nd, Gd: YSGG laser, Opt. Lett. 36 (2011) 3813–3815. K. Zhong, C. Sun, J. Yao, D. Xu, X. Xie, X. Cao, S. Yin,Efficient, Continuous-wave 1053-nm Nd: GYSGG laser with passively Q-switched dual-wavelength operation for terahertz generation, IEEE J. Quantum Electron. 49 (2013) 375–379. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, S.T. Yin, Diode‐ pumped passively mode‐locked Nd: GYSGG laser, Laser Phys. Lett. 8 (2011) 787–790. J.L. Xu, X.L. Li, Y.Z. Wu, X.P. Hao, J.L. He, K.J. Yang, Graphene saturable absorber mirror for ultra-fast-pulse solid-state laser, Opt. Lett. 36 (2011) 1948–1950. W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, K.M. Abramski, Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser, Opt. Express 20 (2012) 19463–19473. P. Zhao, S. Ragam, Y.J. Ding, I.B. Zotova, Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser, Opt. Lett. 35 (2010) 3979–3981. E.B. Petersen, W. Shi, A.C. Pirson, N. Peyghambarian, A.T. Cooney, Efficient parametric terahertz generation in quasi-phase-matched GaP through cavity enhanced difference-frequency generation, Appl. Phys. Lett. 98 (2011) 121119. S. Zhao, J. Zhao, G. Li, K. Yang, Y. Sun, D. Li, J. An, J. Wang, M. Li, Doubly Q-switched laser with electric-optic modulator and GaAs saturable absorber, Laser Phys. Lett. 3 (2006) 471. Y. Zhang, S.Z. Zhao, D.C. Li, K.J. Yang, G.Q. Li, G. Zhang, K. Cheng, Diodepumped doubly Q-switched mode-locked YVO4/Nd:YVO4/KTP green laser with AO and GaAs saturable absorber, Opt. Mater. 33 (2011) 303–307.
Optics Communications 347 (2015) 64–67
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber Qi Song a, Guoju Wang a, Bingyuan Zhang a,n, Qingli Zhang b, Wenjun Wang a, Minghong Wang a, Guihua Sun b, Yong Bo c, Qinjun Peng c a School of Physics Science and Information Engineering, Shandong key laboratory of optical communication science and technology, Liaocheng University, Liaocheng 252000, China b The Key Laboratory of Photonic Devices and Materials of Anhui, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2015 Received in revised form 17 February 2015 Accepted 2 March 2015 Available online 4 March 2015
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber is demonstrated. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. In addition, a simple way to fabricate few layers graphene oxide saturable absorbers has been reported. & 2015 Elsevier B.V. All rights reserved.
Keywords: Q-Switched Mode-locked Dual-wavelength Graphene oxide Nd:GYSGG
1. Introduction Multi-wavelength mode-locked lasers exhibit versatile applications in remote sensing, metrology, biomedical diagnostics, terahertz (THz) radiation generation, pump-probe measurement, component characterization and telecommunications [1–4]. Compared with single-wavelength laser generating its output at one center wavelength, multi-wavelength ultrafast lasers can simultaneously yield pulse trains at different center wavelengths. Some techniques have been proposed to generate multi-wavelength pulse, including dual gain media [5], stimulated Raman scattering [6] and crystals with the disordered structure, such as Nd:GGG [7], Nd:Sc0.2Y0.8SiO5 [8], Nd:CNGG, Nd:CLNGG, Nd:CTGG and Nd:CaYAlO4 [9–13]. Among them, Nd:GYSGG crystal with an inhomogeneous broadening of the emission spectra is beneficial to generating ultra-short pulses and multi-wavelength laser pulse [14–16]. The emission cross-sections and excited-state lifetime were 1.58 10 19 cm2 and 220 μs, respectively. There are some previous studies on continuous-wave of multi-wavelength [16–18] and passively mode-locking of single-wavelength operation [19] with the Nd:GYSGG crystal. To our knowledge, multi-wavelength mode-locking or Q-switched mode-locking (QML) operation with n
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.optcom.2015.03.002 0030-4018/& 2015 Elsevier B.V. All rights reserved.
this crystal has not been realized yet. Recently graphene and graphene oxide absorbers are believed to be promising saturable absorbers of gapless linear dispersion of Dirac electrons, which shows that they can be operated over a very broad range of wavelengths [20]. In this paper, the passively dual-wavelength Q-switched mode-locking operation of the disordered Nd:GYSGG crystal was obtained using the graphene oxide saturable absorber for the first time. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured.
2. Experimental setup 2.1. Preparation and characterization of GO saturable absorber GO was prepared from expanded acid washed graphite flakes by a modified Hummers method [21]. First, 2 g of graphite was added into 40 ml of H2SO4. Then, 1 g of NaNO3 was added. whereafter, the beaker was put into cold water bath to keep the temperature below 4 °C. 6 g of KMnO4 was added into the mixture, which was vigorously stirred. After addition of the oxidant, the beaker was heated and kept at 20 °C with continuous stirring. Afterward it can be left at room temperature overnight. Soon afterwards, the beaker was put into a water bath at a temperature of 32–38 °C and stirred. The mixture was then heated to 90 °C and
Q. Song et al. / Optics Communications 347 (2015) 64–67
65
Fig. 3. Schematic setup of the diode-end-pumped Nd: GYSGG laser.
crystalline structure; the D-peak located at 1352 cm 1, which arises from the breathing modes of sp2 atoms in a ring [22]. 2.2. Setup of the diode-end-pumped Nd: GYSGG laser
Fig. 1. UV–vis–NIR transmissivity spectrum of GO saturable absorber.
kept these conditions for 20 min. 80 ml of alcohol and 5 ml of H2O2 were added in order to stop the reaction and dispersed graphene. HCl solution was used to remove the sulfate ions. To obtain uniform saturable absorber of high quality and suitable size in a quite simple way, 0.2 ml of the alcohol dispersed graphene oxide solution was added into 8 ml of alcohol. Then the graphene oxide solution was stirred by magnetic stirring apparatus for 5 min with the rotation rate of 500 rpm. Subsequently, quartz sheet with the diameter of 30 mm submerged into the prepared graphene oxide solution completely. The quartz substrate with graphene oxide was left at room temperature overnight to remove the alcohol thoroughly. The UV–vis–NIR spectrophotometer was used to characterize the transmissivity of the saturable absorber. Fig. 1 shows the UV–vis–NIR transmissivity spectrum of GO saturable absorber. From Fig. 1 we can see the GO saturable absorber can be employed as saturable absorber in a wide wavelength range from 500 nm to 1100 nm. The transmissivity are 91.1% at 1057 nm and 91% at 1061 nm respectively. The Raman spectrometer (RM 2000) was used to characterize the Raman spectrum of the saturable absorber. Fig. 2 shows the Raman spectrum of the deposited graphene oxide layer. Two Raman peaks can be identified in this spectrum: the G-peak at around 1600 cm 1, due to the bond stretching of sp2 atoms in the
Fig. 3 shows the schematic of the Nd:GYSGG laser cavity. The Q-switched mode-locking laser experimental setup was based on a standard Z-folded cavity with the length of about 1.5 m. The pump source used in the experiment was a fiber-coupled LD with the emission wavelength centered at 808 nm. The output beam of the LD was focused into the Nd:GYSGG crystal with a spot radius of about 0.1 mm using a focusing system. The Nd:GYSGG crystal with the dimensions of 3 3 5 mm3 was employed in the experiment. M was a flat mirror. M1 and M2 were concave mirrors with the radius of curvature of 400 mm, the flat mirror OC was an output couplers with the transmissions of 3% at 1.06 μm laser wavelength. The Nd:GYSGG crystal was mounted in a water-cooled copper block with the temperature of 22 °C. The GO deposited on the quartz sheet was employed as the saturable absorber. The laser spot radius on the GO saturable absorber was 0.12 mm. The laser pulse was recorded by a fast photo-detector (ET-3000) and an oscilloscope (Agilent DSO54832b).The fluorescence spectrum of Nd:GYSGG crystal excited by 808 nm laser is shown in Fig. 4 (measured by an Ocean Optics HR2000þ spectrum analyzer). The resolution of the spectrum analyzer is about 1 nm (FWHM). It is obviously seen in Fig. 4, there are two comparable peaks around 1060 nm due to the Stark splitting of the energy levels, indicating the possibility of multi-wavelength operation [23,24].
3. Experimental results In the experiment, continuous wave operation was obtained
Fig. 2. Raman spectrum of the GO formed on the quartz substrate.
Fig. 4. Fluorescence spectrum of Nd:GYSGG ranging from 950 nm to 1100 nm.
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Q. Song et al. / Optics Communications 347 (2015) 64–67
Fig. 5. The output power versus the incident pump power of the continuous wave (CW) and Q-switched mode-locking (QML).
with the optical conversion efficiency of 6.2% firstly. Then the GO saturable absorber was inserted into the laser cavity. When the pump power increases to 2.2 W, Q-switched operation can be obtained. When the pump power is 2.6 W, the Q-switched modelocked operation was obtained. In order to protect the crystal and GO saturable absorber, the maximum pump power is 5 W. The relationship of the output power versus pump power is shown in Fig. 5. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The slope efficiencies for CW and QML laser were 10% and 6.3%, respectively. Fig. 6(a) and (b) shows temporal shape of a Q-switched pulse envelope and pulse train of the QML Nd:GYSGG laser respectively. The mode-locked pulse inside the Q-switched envelope is quite stable with the repetition rate of 100 MHz. According to the expanded oscilloscope traces of the modelocked pulse within the Q-switched envelope shown in Fig. 6(b), the mode-locked pulse width can be estimated by the formula [25,26]:
Fig. 6. (a) Temporal shape of a Q-switched pulse envelope and (b) pulse train of the QML Nd:GYSGG laser.
2 2 2 2 treal = tmeasure − tprobe − toscilloscope ,
here, treal is the real time of the pulse, tmeasure is the measured rise time, tprobe is the rise time of the probe and toscilloscope is the rise time of the oscilloscope. In our experiment, the average rise time of the mode-locked pulses is 0.59 ns. The rise time of oscilloscope is 0.35 ns and the rise time of the probe is also about 0.175 ns. Assuming that the pulse width is approximately 1.25 times more than the rise time, the estimated mode-locked pulse width is about 441 ps. The output spectrum is shown in Fig. 7 (measured by an Ocean Optics HR2000 þspectrum analyzer). Two laser lines are found simultaneously at 1057.28 nm and 1060.23 nm, which is exactly consistent with the fluorescence spectrum shown in Fig. 4. The separation between wavelengths is quite stable, and the intensities for different wavelength lines were nearly equal. The relative powers of the two lasing wavelength does not stay constant. There is a slight jitter, which was attributed to the mode competition. Considering that the line separation for dual wavelength oscillation regime is optimal for difference frequency generation, this laser material might be employed to generate THz-wave at 0.78 THz with a suitable nonlinear crystal. 4. Conclusion In this paper, the performance of passively dual-wavelength
Fig. 7. Spectrum of Nd:GYSGG laser.
Q-switched mode-locked Nd:GYSGG laser was investigated using the GO as the saturable absorber. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm was obtained, which can be used to generate THz-wave at 0.78 THz. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. In addition, a simple way to fabricate few layers
Q. Song et al. / Optics Communications 347 (2015) 64–67
GO saturable absorbers has been reported. With further optimization, this graphene oxide saturable absorber can be widely used in mode-locked solid state lasers.
[11]
[12]
Acknowledgments [13]
This work is partially supported by the National Natural Science Foundation of China (Nos.:61275147, 11375081 and 51172236), the Natural Science Foundation of Shandong Province (Nos.: ZR2012AL11 and ZR2014FL030) and a grant from the Special Construction Project Fund for Shandong Province Taishan Mountain Scholar.
[14]
[15]
[16]
References [1] M. Mielke, G.A. Alphonse, P.J. Delfyett, 168 channels 6 GHz from a multi wavelength mode-locked semiconductor laser, IEEE Photon. Technol. Lett. 15 (2003) 501–503. [2] W.W. Zhang, J.Q. Sun, J. Wang, L. Liu, Multi wavelength mode-locked fiber-ring laser based on reflective semiconductor optical amplifiers, IEEE Photon. Technol. Lett. 19 (2007) 1418–1420. [3] J. Xia, Y.F. Lü, H.L. Liu, X.Y. Pu, Diode-pumped Pr3 þ : LiYF4 visible dual-wavelength laser, Opt. Commun. 334 (2015) 160–163. [4] C.L. Yang, L. Xia, Y.W. Wang, D.M. Liu, Stabilized 51-wavelength erbium-doped fiber ring laser based on high nonlinear fiber, Opt. Commun. 318 (2014) 171–174. [5] Y.J. Huang, Y.S. Tzeng, C.Y. Tang, S.Y. Chiang, H.C. Liang, Y.F. Chen,Efficient, high-power terahertz beating in a dual-wavelength synchronously modelocked laser with dual gain media, Opt. Lett. 39 (2014) 1477–1480. [6] H. Zhang, X. Chen, Q. Wang, P. Li, Dual-wavelength actively Q-switched diodeend-pumped ceramic Nd: YAG/BaWO4 Raman laser operating at 1240 and 1376 nm, Laser Phys. Lett. 11 (2014) 105806. [7] H.T. Huang, J.L. He, B.T. Zhang, J.F. Yang, J.L. Xu, C.H. Zuo, X.T. Tao, V3 þ : YAG as the saturable absorber for a diode-pumped quasi-three-level dual-wavelength Nd: GGG laser, Opt. Express 18 (2010) 3352–3357. [8] S.D. Liu, L.H. Zheng, J.L. He, J. Xu, X.D. Xu, L.B. Su, K.J. Yang, B.T. Zhang, R. H. Wang, X.M. Liu, Passively Q-switched Nd: Sc0.2Y0.8SiO5 dual-wavelength laser with the orthogonally polarized output, Opt. Express 20 (2012) 22448–22453. [9] G.Q. Xie, D.Y. Tang, W.D. Tan, H. Luo, H.J. Zhang, H.H. Yu, J.Y. Wang, Subpicosecond pulse generation from a Nd: CLNGG disordered crystal laser, Opt. Lett. 34 (2009) 103–105. [10] H.H. Yu, H.J. Zhang, Z.P. Wang, J.Y. Wang, Y.G. Yu, Z.B. Shi, X.Y. Zhang, M.
[17]
[18]
[19]
[20] [21] [22]
[23]
[24]
[25]
[26]
67
H. Jiang, Continuous-wave and passively Q-switched laser performance with a disordered Nd: CLNGG crystal, Opt. Express 17 (2009) 19015–19020. H.H. Yu, H.J. Zhang, Z.P. Wang, J.Y. Wang, Y.G. Yu, Z.B. Shi, X.Y. Zhang, M. H. Jiang, High-power dual-wavelength laser with disordered Nd: CNGG crystals, Opt. Lett. 34 (2009) 151–153. G.Q. Xie, D.Y. Tang, W.D. Tan, H. Luo, S.Y. Guo, H.H. Yu, H.J. Zhang, Diodepumped passively mode-locked Nd: CTGG disordered crystal laser, Appl. Phys. B 95 (2009) 691–695. D.Z. Li, X.D. Xu, J.Q. Meng, D.H. Zhou, C.T. Xia, F. Wu, J. Xu, Diode-pumped continuous wave and Q-switched operation of Nd: CaYAlO4 crystal, Opt. Express 18 (2010) 18649–18654. C.L. Sun, K. Zhong, J.Q. Yao, D.G. Xu, X.L. Cao, Q.L. Zhang, J.Q. Luo, D.L. Sun, S. T. Yin, Diode-pumped continuous-wave quasi-three-level Nd: GYSGG laser at 937 nm, Opt. Commun. 294 (2013) 229–232. C.L. Sun, K. Zhong, C.G. Zhang, J.Q. Yao, D.G. Xu, F. Zhang, S.T. Yin, Stimulated emission cross section of the 4F3/2-4I11/2 of Nd: GYSGG, Laser Phys. Lett. 9 (2012) 410. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, D.L. Sun, J.Q. Luo, S. T. Yin, Continuous-wave and passively Q-switched laser performance of a disordered Nd: GYSGG crystal, Opt. Commun. 284 (2011) 5734–5737. K. Zhong, J.Q. Yao, C.L. Sun, C.G. Zhang, Y.Y. Miao, R. Wang, D.G. Xu, F. Zhang, Q. L. Zhang, D.L. Sun, S.T. Yin, Efficient diode-end-pumped dual-wavelength Nd, Gd: YSGG laser, Opt. Lett. 36 (2011) 3813–3815. K. Zhong, C. Sun, J. Yao, D. Xu, X. Xie, X. Cao, S. Yin,Efficient, Continuous-wave 1053-nm Nd: GYSGG laser with passively Q-switched dual-wavelength operation for terahertz generation, IEEE J. Quantum Electron. 49 (2013) 375–379. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, S.T. Yin, Diode‐ pumped passively mode‐locked Nd: GYSGG laser, Laser Phys. Lett. 8 (2011) 787–790. J.L. Xu, X.L. Li, Y.Z. Wu, X.P. Hao, J.L. He, K.J. Yang, Graphene saturable absorber mirror for ultra-fast-pulse solid-state laser, Opt. Lett. 36 (2011) 1948–1950. W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, K.M. Abramski, Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser, Opt. Express 20 (2012) 19463–19473. P. Zhao, S. Ragam, Y.J. Ding, I.B. Zotova, Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser, Opt. Lett. 35 (2010) 3979–3981. E.B. Petersen, W. Shi, A.C. Pirson, N. Peyghambarian, A.T. Cooney, Efficient parametric terahertz generation in quasi-phase-matched GaP through cavity enhanced difference-frequency generation, Appl. Phys. Lett. 98 (2011) 121119. S. Zhao, J. Zhao, G. Li, K. Yang, Y. Sun, D. Li, J. An, J. Wang, M. Li, Doubly Q-switched laser with electric-optic modulator and GaAs saturable absorber, Laser Phys. Lett. 3 (2006) 471. Y. Zhang, S.Z. Zhao, D.C. Li, K.J. Yang, G.Q. Li, G. Zhang, K. Cheng, Diodepumped doubly Q-switched mode-locked YVO4/Nd:YVO4/KTP green laser with AO and GaAs saturable absorber, Opt. Mater. 33 (2011) 303–307.
Optics Communications 347 (2015) 64–67
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber Qi Song a, Guoju Wang a, Bingyuan Zhang a,n, Qingli Zhang b, Wenjun Wang a, Minghong Wang a, Guihua Sun b, Yong Bo c, Qinjun Peng c a School of Physics Science and Information Engineering, Shandong key laboratory of optical communication science and technology, Liaocheng University, Liaocheng 252000, China b The Key Laboratory of Photonic Devices and Materials of Anhui, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2015 Received in revised form 17 February 2015 Accepted 2 March 2015 Available online 4 March 2015
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber is demonstrated. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. In addition, a simple way to fabricate few layers graphene oxide saturable absorbers has been reported. & 2015 Elsevier B.V. All rights reserved.
Keywords: Q-Switched Mode-locked Dual-wavelength Graphene oxide Nd:GYSGG
1. Introduction Multi-wavelength mode-locked lasers exhibit versatile applications in remote sensing, metrology, biomedical diagnostics, terahertz (THz) radiation generation, pump-probe measurement, component characterization and telecommunications [1–4]. Compared with single-wavelength laser generating its output at one center wavelength, multi-wavelength ultrafast lasers can simultaneously yield pulse trains at different center wavelengths. Some techniques have been proposed to generate multi-wavelength pulse, including dual gain media [5], stimulated Raman scattering [6] and crystals with the disordered structure, such as Nd:GGG [7], Nd:Sc0.2Y0.8SiO5 [8], Nd:CNGG, Nd:CLNGG, Nd:CTGG and Nd:CaYAlO4 [9–13]. Among them, Nd:GYSGG crystal with an inhomogeneous broadening of the emission spectra is beneficial to generating ultra-short pulses and multi-wavelength laser pulse [14–16]. The emission cross-sections and excited-state lifetime were 1.58 10 19 cm2 and 220 μs, respectively. There are some previous studies on continuous-wave of multi-wavelength [16–18] and passively mode-locking of single-wavelength operation [19] with the Nd:GYSGG crystal. To our knowledge, multi-wavelength mode-locking or Q-switched mode-locking (QML) operation with n
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.optcom.2015.03.002 0030-4018/& 2015 Elsevier B.V. All rights reserved.
this crystal has not been realized yet. Recently graphene and graphene oxide absorbers are believed to be promising saturable absorbers of gapless linear dispersion of Dirac electrons, which shows that they can be operated over a very broad range of wavelengths [20]. In this paper, the passively dual-wavelength Q-switched mode-locking operation of the disordered Nd:GYSGG crystal was obtained using the graphene oxide saturable absorber for the first time. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured.
2. Experimental setup 2.1. Preparation and characterization of GO saturable absorber GO was prepared from expanded acid washed graphite flakes by a modified Hummers method [21]. First, 2 g of graphite was added into 40 ml of H2SO4. Then, 1 g of NaNO3 was added. whereafter, the beaker was put into cold water bath to keep the temperature below 4 °C. 6 g of KMnO4 was added into the mixture, which was vigorously stirred. After addition of the oxidant, the beaker was heated and kept at 20 °C with continuous stirring. Afterward it can be left at room temperature overnight. Soon afterwards, the beaker was put into a water bath at a temperature of 32–38 °C and stirred. The mixture was then heated to 90 °C and
Q. Song et al. / Optics Communications 347 (2015) 64–67
65
Fig. 3. Schematic setup of the diode-end-pumped Nd: GYSGG laser.
crystalline structure; the D-peak located at 1352 cm 1, which arises from the breathing modes of sp2 atoms in a ring [22]. 2.2. Setup of the diode-end-pumped Nd: GYSGG laser
Fig. 1. UV–vis–NIR transmissivity spectrum of GO saturable absorber.
kept these conditions for 20 min. 80 ml of alcohol and 5 ml of H2O2 were added in order to stop the reaction and dispersed graphene. HCl solution was used to remove the sulfate ions. To obtain uniform saturable absorber of high quality and suitable size in a quite simple way, 0.2 ml of the alcohol dispersed graphene oxide solution was added into 8 ml of alcohol. Then the graphene oxide solution was stirred by magnetic stirring apparatus for 5 min with the rotation rate of 500 rpm. Subsequently, quartz sheet with the diameter of 30 mm submerged into the prepared graphene oxide solution completely. The quartz substrate with graphene oxide was left at room temperature overnight to remove the alcohol thoroughly. The UV–vis–NIR spectrophotometer was used to characterize the transmissivity of the saturable absorber. Fig. 1 shows the UV–vis–NIR transmissivity spectrum of GO saturable absorber. From Fig. 1 we can see the GO saturable absorber can be employed as saturable absorber in a wide wavelength range from 500 nm to 1100 nm. The transmissivity are 91.1% at 1057 nm and 91% at 1061 nm respectively. The Raman spectrometer (RM 2000) was used to characterize the Raman spectrum of the saturable absorber. Fig. 2 shows the Raman spectrum of the deposited graphene oxide layer. Two Raman peaks can be identified in this spectrum: the G-peak at around 1600 cm 1, due to the bond stretching of sp2 atoms in the
Fig. 3 shows the schematic of the Nd:GYSGG laser cavity. The Q-switched mode-locking laser experimental setup was based on a standard Z-folded cavity with the length of about 1.5 m. The pump source used in the experiment was a fiber-coupled LD with the emission wavelength centered at 808 nm. The output beam of the LD was focused into the Nd:GYSGG crystal with a spot radius of about 0.1 mm using a focusing system. The Nd:GYSGG crystal with the dimensions of 3 3 5 mm3 was employed in the experiment. M was a flat mirror. M1 and M2 were concave mirrors with the radius of curvature of 400 mm, the flat mirror OC was an output couplers with the transmissions of 3% at 1.06 μm laser wavelength. The Nd:GYSGG crystal was mounted in a water-cooled copper block with the temperature of 22 °C. The GO deposited on the quartz sheet was employed as the saturable absorber. The laser spot radius on the GO saturable absorber was 0.12 mm. The laser pulse was recorded by a fast photo-detector (ET-3000) and an oscilloscope (Agilent DSO54832b).The fluorescence spectrum of Nd:GYSGG crystal excited by 808 nm laser is shown in Fig. 4 (measured by an Ocean Optics HR2000þ spectrum analyzer). The resolution of the spectrum analyzer is about 1 nm (FWHM). It is obviously seen in Fig. 4, there are two comparable peaks around 1060 nm due to the Stark splitting of the energy levels, indicating the possibility of multi-wavelength operation [23,24].
3. Experimental results In the experiment, continuous wave operation was obtained
Fig. 2. Raman spectrum of the GO formed on the quartz substrate.
Fig. 4. Fluorescence spectrum of Nd:GYSGG ranging from 950 nm to 1100 nm.
66
Q. Song et al. / Optics Communications 347 (2015) 64–67
Fig. 5. The output power versus the incident pump power of the continuous wave (CW) and Q-switched mode-locking (QML).
with the optical conversion efficiency of 6.2% firstly. Then the GO saturable absorber was inserted into the laser cavity. When the pump power increases to 2.2 W, Q-switched operation can be obtained. When the pump power is 2.6 W, the Q-switched modelocked operation was obtained. In order to protect the crystal and GO saturable absorber, the maximum pump power is 5 W. The relationship of the output power versus pump power is shown in Fig. 5. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The slope efficiencies for CW and QML laser were 10% and 6.3%, respectively. Fig. 6(a) and (b) shows temporal shape of a Q-switched pulse envelope and pulse train of the QML Nd:GYSGG laser respectively. The mode-locked pulse inside the Q-switched envelope is quite stable with the repetition rate of 100 MHz. According to the expanded oscilloscope traces of the modelocked pulse within the Q-switched envelope shown in Fig. 6(b), the mode-locked pulse width can be estimated by the formula [25,26]:
Fig. 6. (a) Temporal shape of a Q-switched pulse envelope and (b) pulse train of the QML Nd:GYSGG laser.
2 2 2 2 treal = tmeasure − tprobe − toscilloscope ,
here, treal is the real time of the pulse, tmeasure is the measured rise time, tprobe is the rise time of the probe and toscilloscope is the rise time of the oscilloscope. In our experiment, the average rise time of the mode-locked pulses is 0.59 ns. The rise time of oscilloscope is 0.35 ns and the rise time of the probe is also about 0.175 ns. Assuming that the pulse width is approximately 1.25 times more than the rise time, the estimated mode-locked pulse width is about 441 ps. The output spectrum is shown in Fig. 7 (measured by an Ocean Optics HR2000 þspectrum analyzer). Two laser lines are found simultaneously at 1057.28 nm and 1060.23 nm, which is exactly consistent with the fluorescence spectrum shown in Fig. 4. The separation between wavelengths is quite stable, and the intensities for different wavelength lines were nearly equal. The relative powers of the two lasing wavelength does not stay constant. There is a slight jitter, which was attributed to the mode competition. Considering that the line separation for dual wavelength oscillation regime is optimal for difference frequency generation, this laser material might be employed to generate THz-wave at 0.78 THz with a suitable nonlinear crystal. 4. Conclusion In this paper, the performance of passively dual-wavelength
Fig. 7. Spectrum of Nd:GYSGG laser.
Q-switched mode-locked Nd:GYSGG laser was investigated using the GO as the saturable absorber. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm was obtained, which can be used to generate THz-wave at 0.78 THz. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. In addition, a simple way to fabricate few layers
Q. Song et al. / Optics Communications 347 (2015) 64–67
GO saturable absorbers has been reported. With further optimization, this graphene oxide saturable absorber can be widely used in mode-locked solid state lasers.
[11]
[12]
Acknowledgments [13]
This work is partially supported by the National Natural Science Foundation of China (Nos.:61275147, 11375081 and 51172236), the Natural Science Foundation of Shandong Province (Nos.: ZR2012AL11 and ZR2014FL030) and a grant from the Special Construction Project Fund for Shandong Province Taishan Mountain Scholar.
[14]
[15]
[16]
References [1] M. Mielke, G.A. Alphonse, P.J. Delfyett, 168 channels 6 GHz from a multi wavelength mode-locked semiconductor laser, IEEE Photon. Technol. Lett. 15 (2003) 501–503. [2] W.W. Zhang, J.Q. Sun, J. Wang, L. Liu, Multi wavelength mode-locked fiber-ring laser based on reflective semiconductor optical amplifiers, IEEE Photon. Technol. Lett. 19 (2007) 1418–1420. [3] J. Xia, Y.F. Lü, H.L. Liu, X.Y. Pu, Diode-pumped Pr3 þ : LiYF4 visible dual-wavelength laser, Opt. Commun. 334 (2015) 160–163. [4] C.L. Yang, L. Xia, Y.W. Wang, D.M. Liu, Stabilized 51-wavelength erbium-doped fiber ring laser based on high nonlinear fiber, Opt. Commun. 318 (2014) 171–174. [5] Y.J. Huang, Y.S. Tzeng, C.Y. Tang, S.Y. Chiang, H.C. Liang, Y.F. Chen,Efficient, high-power terahertz beating in a dual-wavelength synchronously modelocked laser with dual gain media, Opt. Lett. 39 (2014) 1477–1480. [6] H. Zhang, X. Chen, Q. Wang, P. Li, Dual-wavelength actively Q-switched diodeend-pumped ceramic Nd: YAG/BaWO4 Raman laser operating at 1240 and 1376 nm, Laser Phys. Lett. 11 (2014) 105806. [7] H.T. Huang, J.L. He, B.T. Zhang, J.F. Yang, J.L. Xu, C.H. Zuo, X.T. Tao, V3 þ : YAG as the saturable absorber for a diode-pumped quasi-three-level dual-wavelength Nd: GGG laser, Opt. Express 18 (2010) 3352–3357. [8] S.D. Liu, L.H. Zheng, J.L. He, J. Xu, X.D. Xu, L.B. Su, K.J. Yang, B.T. Zhang, R. H. Wang, X.M. Liu, Passively Q-switched Nd: Sc0.2Y0.8SiO5 dual-wavelength laser with the orthogonally polarized output, Opt. Express 20 (2012) 22448–22453. [9] G.Q. Xie, D.Y. Tang, W.D. Tan, H. Luo, H.J. Zhang, H.H. Yu, J.Y. Wang, Subpicosecond pulse generation from a Nd: CLNGG disordered crystal laser, Opt. Lett. 34 (2009) 103–105. [10] H.H. Yu, H.J. Zhang, Z.P. Wang, J.Y. Wang, Y.G. Yu, Z.B. Shi, X.Y. Zhang, M.
[17]
[18]
[19]
[20] [21] [22]
[23]
[24]
[25]
[26]
67
H. Jiang, Continuous-wave and passively Q-switched laser performance with a disordered Nd: CLNGG crystal, Opt. Express 17 (2009) 19015–19020. H.H. Yu, H.J. Zhang, Z.P. Wang, J.Y. Wang, Y.G. Yu, Z.B. Shi, X.Y. Zhang, M. H. Jiang, High-power dual-wavelength laser with disordered Nd: CNGG crystals, Opt. Lett. 34 (2009) 151–153. G.Q. Xie, D.Y. Tang, W.D. Tan, H. Luo, S.Y. Guo, H.H. Yu, H.J. Zhang, Diodepumped passively mode-locked Nd: CTGG disordered crystal laser, Appl. Phys. B 95 (2009) 691–695. D.Z. Li, X.D. Xu, J.Q. Meng, D.H. Zhou, C.T. Xia, F. Wu, J. Xu, Diode-pumped continuous wave and Q-switched operation of Nd: CaYAlO4 crystal, Opt. Express 18 (2010) 18649–18654. C.L. Sun, K. Zhong, J.Q. Yao, D.G. Xu, X.L. Cao, Q.L. Zhang, J.Q. Luo, D.L. Sun, S. T. Yin, Diode-pumped continuous-wave quasi-three-level Nd: GYSGG laser at 937 nm, Opt. Commun. 294 (2013) 229–232. C.L. Sun, K. Zhong, C.G. Zhang, J.Q. Yao, D.G. Xu, F. Zhang, S.T. Yin, Stimulated emission cross section of the 4F3/2-4I11/2 of Nd: GYSGG, Laser Phys. Lett. 9 (2012) 410. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, D.L. Sun, J.Q. Luo, S. T. Yin, Continuous-wave and passively Q-switched laser performance of a disordered Nd: GYSGG crystal, Opt. Commun. 284 (2011) 5734–5737. K. Zhong, J.Q. Yao, C.L. Sun, C.G. Zhang, Y.Y. Miao, R. Wang, D.G. Xu, F. Zhang, Q. L. Zhang, D.L. Sun, S.T. Yin, Efficient diode-end-pumped dual-wavelength Nd, Gd: YSGG laser, Opt. Lett. 36 (2011) 3813–3815. K. Zhong, C. Sun, J. Yao, D. Xu, X. Xie, X. Cao, S. Yin,Efficient, Continuous-wave 1053-nm Nd: GYSGG laser with passively Q-switched dual-wavelength operation for terahertz generation, IEEE J. Quantum Electron. 49 (2013) 375–379. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, S.T. Yin, Diode‐ pumped passively mode‐locked Nd: GYSGG laser, Laser Phys. Lett. 8 (2011) 787–790. J.L. Xu, X.L. Li, Y.Z. Wu, X.P. Hao, J.L. He, K.J. Yang, Graphene saturable absorber mirror for ultra-fast-pulse solid-state laser, Opt. Lett. 36 (2011) 1948–1950. W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, K.M. Abramski, Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser, Opt. Express 20 (2012) 19463–19473. P. Zhao, S. Ragam, Y.J. Ding, I.B. Zotova, Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser, Opt. Lett. 35 (2010) 3979–3981. E.B. Petersen, W. Shi, A.C. Pirson, N. Peyghambarian, A.T. Cooney, Efficient parametric terahertz generation in quasi-phase-matched GaP through cavity enhanced difference-frequency generation, Appl. Phys. Lett. 98 (2011) 121119. S. Zhao, J. Zhao, G. Li, K. Yang, Y. Sun, D. Li, J. An, J. Wang, M. Li, Doubly Q-switched laser with electric-optic modulator and GaAs saturable absorber, Laser Phys. Lett. 3 (2006) 471. Y. Zhang, S.Z. Zhao, D.C. Li, K.J. Yang, G.Q. Li, G. Zhang, K. Cheng, Diodepumped doubly Q-switched mode-locked YVO4/Nd:YVO4/KTP green laser with AO and GaAs saturable absorber, Opt. Mater. 33 (2011) 303–307.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber Qi Song a, Guoju Wang a, Bingyuan Zhang a,n, Qingli Zhang b, Wenjun Wang a, Minghong Wang a, Guihua Sun b, Yong Bo c, Qinjun Peng c a School of Physics Science and Information Engineering, Shandong key laboratory of optical communication science and technology, Liaocheng University, Liaocheng 252000, China b The Key Laboratory of Photonic Devices and Materials of Anhui, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2015 Received in revised form 17 February 2015 Accepted 2 March 2015 Available online 4 March 2015
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber is demonstrated. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. In addition, a simple way to fabricate few layers graphene oxide saturable absorbers has been reported. & 2015 Elsevier B.V. All rights reserved.
Keywords: Q-Switched Mode-locked Dual-wavelength Graphene oxide Nd:GYSGG
1. Introduction Multi-wavelength mode-locked lasers exhibit versatile applications in remote sensing, metrology, biomedical diagnostics, terahertz (THz) radiation generation, pump-probe measurement, component characterization and telecommunications [1–4]. Compared with single-wavelength laser generating its output at one center wavelength, multi-wavelength ultrafast lasers can simultaneously yield pulse trains at different center wavelengths. Some techniques have been proposed to generate multi-wavelength pulse, including dual gain media [5], stimulated Raman scattering [6] and crystals with the disordered structure, such as Nd:GGG [7], Nd:Sc0.2Y0.8SiO5 [8], Nd:CNGG, Nd:CLNGG, Nd:CTGG and Nd:CaYAlO4 [9–13]. Among them, Nd:GYSGG crystal with an inhomogeneous broadening of the emission spectra is beneficial to generating ultra-short pulses and multi-wavelength laser pulse [14–16]. The emission cross-sections and excited-state lifetime were 1.58 10 19 cm2 and 220 μs, respectively. There are some previous studies on continuous-wave of multi-wavelength [16–18] and passively mode-locking of single-wavelength operation [19] with the Nd:GYSGG crystal. To our knowledge, multi-wavelength mode-locking or Q-switched mode-locking (QML) operation with n
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.optcom.2015.03.002 0030-4018/& 2015 Elsevier B.V. All rights reserved.
this crystal has not been realized yet. Recently graphene and graphene oxide absorbers are believed to be promising saturable absorbers of gapless linear dispersion of Dirac electrons, which shows that they can be operated over a very broad range of wavelengths [20]. In this paper, the passively dual-wavelength Q-switched mode-locking operation of the disordered Nd:GYSGG crystal was obtained using the graphene oxide saturable absorber for the first time. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured.
2. Experimental setup 2.1. Preparation and characterization of GO saturable absorber GO was prepared from expanded acid washed graphite flakes by a modified Hummers method [21]. First, 2 g of graphite was added into 40 ml of H2SO4. Then, 1 g of NaNO3 was added. whereafter, the beaker was put into cold water bath to keep the temperature below 4 °C. 6 g of KMnO4 was added into the mixture, which was vigorously stirred. After addition of the oxidant, the beaker was heated and kept at 20 °C with continuous stirring. Afterward it can be left at room temperature overnight. Soon afterwards, the beaker was put into a water bath at a temperature of 32–38 °C and stirred. The mixture was then heated to 90 °C and
Q. Song et al. / Optics Communications 347 (2015) 64–67
65
Fig. 3. Schematic setup of the diode-end-pumped Nd: GYSGG laser.
crystalline structure; the D-peak located at 1352 cm 1, which arises from the breathing modes of sp2 atoms in a ring [22]. 2.2. Setup of the diode-end-pumped Nd: GYSGG laser
Fig. 1. UV–vis–NIR transmissivity spectrum of GO saturable absorber.
kept these conditions for 20 min. 80 ml of alcohol and 5 ml of H2O2 were added in order to stop the reaction and dispersed graphene. HCl solution was used to remove the sulfate ions. To obtain uniform saturable absorber of high quality and suitable size in a quite simple way, 0.2 ml of the alcohol dispersed graphene oxide solution was added into 8 ml of alcohol. Then the graphene oxide solution was stirred by magnetic stirring apparatus for 5 min with the rotation rate of 500 rpm. Subsequently, quartz sheet with the diameter of 30 mm submerged into the prepared graphene oxide solution completely. The quartz substrate with graphene oxide was left at room temperature overnight to remove the alcohol thoroughly. The UV–vis–NIR spectrophotometer was used to characterize the transmissivity of the saturable absorber. Fig. 1 shows the UV–vis–NIR transmissivity spectrum of GO saturable absorber. From Fig. 1 we can see the GO saturable absorber can be employed as saturable absorber in a wide wavelength range from 500 nm to 1100 nm. The transmissivity are 91.1% at 1057 nm and 91% at 1061 nm respectively. The Raman spectrometer (RM 2000) was used to characterize the Raman spectrum of the saturable absorber. Fig. 2 shows the Raman spectrum of the deposited graphene oxide layer. Two Raman peaks can be identified in this spectrum: the G-peak at around 1600 cm 1, due to the bond stretching of sp2 atoms in the
Fig. 3 shows the schematic of the Nd:GYSGG laser cavity. The Q-switched mode-locking laser experimental setup was based on a standard Z-folded cavity with the length of about 1.5 m. The pump source used in the experiment was a fiber-coupled LD with the emission wavelength centered at 808 nm. The output beam of the LD was focused into the Nd:GYSGG crystal with a spot radius of about 0.1 mm using a focusing system. The Nd:GYSGG crystal with the dimensions of 3 3 5 mm3 was employed in the experiment. M was a flat mirror. M1 and M2 were concave mirrors with the radius of curvature of 400 mm, the flat mirror OC was an output couplers with the transmissions of 3% at 1.06 μm laser wavelength. The Nd:GYSGG crystal was mounted in a water-cooled copper block with the temperature of 22 °C. The GO deposited on the quartz sheet was employed as the saturable absorber. The laser spot radius on the GO saturable absorber was 0.12 mm. The laser pulse was recorded by a fast photo-detector (ET-3000) and an oscilloscope (Agilent DSO54832b).The fluorescence spectrum of Nd:GYSGG crystal excited by 808 nm laser is shown in Fig. 4 (measured by an Ocean Optics HR2000þ spectrum analyzer). The resolution of the spectrum analyzer is about 1 nm (FWHM). It is obviously seen in Fig. 4, there are two comparable peaks around 1060 nm due to the Stark splitting of the energy levels, indicating the possibility of multi-wavelength operation [23,24].
3. Experimental results In the experiment, continuous wave operation was obtained
Fig. 2. Raman spectrum of the GO formed on the quartz substrate.
Fig. 4. Fluorescence spectrum of Nd:GYSGG ranging from 950 nm to 1100 nm.
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Fig. 5. The output power versus the incident pump power of the continuous wave (CW) and Q-switched mode-locking (QML).
with the optical conversion efficiency of 6.2% firstly. Then the GO saturable absorber was inserted into the laser cavity. When the pump power increases to 2.2 W, Q-switched operation can be obtained. When the pump power is 2.6 W, the Q-switched modelocked operation was obtained. In order to protect the crystal and GO saturable absorber, the maximum pump power is 5 W. The relationship of the output power versus pump power is shown in Fig. 5. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The slope efficiencies for CW and QML laser were 10% and 6.3%, respectively. Fig. 6(a) and (b) shows temporal shape of a Q-switched pulse envelope and pulse train of the QML Nd:GYSGG laser respectively. The mode-locked pulse inside the Q-switched envelope is quite stable with the repetition rate of 100 MHz. According to the expanded oscilloscope traces of the modelocked pulse within the Q-switched envelope shown in Fig. 6(b), the mode-locked pulse width can be estimated by the formula [25,26]:
Fig. 6. (a) Temporal shape of a Q-switched pulse envelope and (b) pulse train of the QML Nd:GYSGG laser.
2 2 2 2 treal = tmeasure − tprobe − toscilloscope ,
here, treal is the real time of the pulse, tmeasure is the measured rise time, tprobe is the rise time of the probe and toscilloscope is the rise time of the oscilloscope. In our experiment, the average rise time of the mode-locked pulses is 0.59 ns. The rise time of oscilloscope is 0.35 ns and the rise time of the probe is also about 0.175 ns. Assuming that the pulse width is approximately 1.25 times more than the rise time, the estimated mode-locked pulse width is about 441 ps. The output spectrum is shown in Fig. 7 (measured by an Ocean Optics HR2000 þspectrum analyzer). Two laser lines are found simultaneously at 1057.28 nm and 1060.23 nm, which is exactly consistent with the fluorescence spectrum shown in Fig. 4. The separation between wavelengths is quite stable, and the intensities for different wavelength lines were nearly equal. The relative powers of the two lasing wavelength does not stay constant. There is a slight jitter, which was attributed to the mode competition. Considering that the line separation for dual wavelength oscillation regime is optimal for difference frequency generation, this laser material might be employed to generate THz-wave at 0.78 THz with a suitable nonlinear crystal. 4. Conclusion In this paper, the performance of passively dual-wavelength
Fig. 7. Spectrum of Nd:GYSGG laser.
Q-switched mode-locked Nd:GYSGG laser was investigated using the GO as the saturable absorber. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm was obtained, which can be used to generate THz-wave at 0.78 THz. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. In addition, a simple way to fabricate few layers
Q. Song et al. / Optics Communications 347 (2015) 64–67
GO saturable absorbers has been reported. With further optimization, this graphene oxide saturable absorber can be widely used in mode-locked solid state lasers.
[11]
[12]
Acknowledgments [13]
This work is partially supported by the National Natural Science Foundation of China (Nos.:61275147, 11375081 and 51172236), the Natural Science Foundation of Shandong Province (Nos.: ZR2012AL11 and ZR2014FL030) and a grant from the Special Construction Project Fund for Shandong Province Taishan Mountain Scholar.
[14]
[15]
[16]
References [1] M. Mielke, G.A. Alphonse, P.J. Delfyett, 168 channels 6 GHz from a multi wavelength mode-locked semiconductor laser, IEEE Photon. Technol. Lett. 15 (2003) 501–503. [2] W.W. Zhang, J.Q. Sun, J. Wang, L. Liu, Multi wavelength mode-locked fiber-ring laser based on reflective semiconductor optical amplifiers, IEEE Photon. Technol. Lett. 19 (2007) 1418–1420. [3] J. Xia, Y.F. Lü, H.L. Liu, X.Y. Pu, Diode-pumped Pr3 þ : LiYF4 visible dual-wavelength laser, Opt. Commun. 334 (2015) 160–163. [4] C.L. Yang, L. Xia, Y.W. Wang, D.M. Liu, Stabilized 51-wavelength erbium-doped fiber ring laser based on high nonlinear fiber, Opt. Commun. 318 (2014) 171–174. [5] Y.J. Huang, Y.S. Tzeng, C.Y. Tang, S.Y. Chiang, H.C. Liang, Y.F. Chen,Efficient, high-power terahertz beating in a dual-wavelength synchronously modelocked laser with dual gain media, Opt. Lett. 39 (2014) 1477–1480. [6] H. Zhang, X. Chen, Q. Wang, P. Li, Dual-wavelength actively Q-switched diodeend-pumped ceramic Nd: YAG/BaWO4 Raman laser operating at 1240 and 1376 nm, Laser Phys. Lett. 11 (2014) 105806. [7] H.T. Huang, J.L. He, B.T. Zhang, J.F. Yang, J.L. Xu, C.H. Zuo, X.T. Tao, V3 þ : YAG as the saturable absorber for a diode-pumped quasi-three-level dual-wavelength Nd: GGG laser, Opt. Express 18 (2010) 3352–3357. [8] S.D. Liu, L.H. Zheng, J.L. He, J. Xu, X.D. Xu, L.B. Su, K.J. Yang, B.T. Zhang, R. H. Wang, X.M. Liu, Passively Q-switched Nd: Sc0.2Y0.8SiO5 dual-wavelength laser with the orthogonally polarized output, Opt. Express 20 (2012) 22448–22453. [9] G.Q. Xie, D.Y. Tang, W.D. Tan, H. Luo, H.J. Zhang, H.H. Yu, J.Y. Wang, Subpicosecond pulse generation from a Nd: CLNGG disordered crystal laser, Opt. Lett. 34 (2009) 103–105. [10] H.H. Yu, H.J. Zhang, Z.P. Wang, J.Y. Wang, Y.G. Yu, Z.B. Shi, X.Y. Zhang, M.
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H. Jiang, Continuous-wave and passively Q-switched laser performance with a disordered Nd: CLNGG crystal, Opt. Express 17 (2009) 19015–19020. H.H. Yu, H.J. Zhang, Z.P. Wang, J.Y. Wang, Y.G. Yu, Z.B. Shi, X.Y. Zhang, M. H. Jiang, High-power dual-wavelength laser with disordered Nd: CNGG crystals, Opt. Lett. 34 (2009) 151–153. G.Q. Xie, D.Y. Tang, W.D. Tan, H. Luo, S.Y. Guo, H.H. Yu, H.J. Zhang, Diodepumped passively mode-locked Nd: CTGG disordered crystal laser, Appl. Phys. B 95 (2009) 691–695. D.Z. Li, X.D. Xu, J.Q. Meng, D.H. Zhou, C.T. Xia, F. Wu, J. Xu, Diode-pumped continuous wave and Q-switched operation of Nd: CaYAlO4 crystal, Opt. Express 18 (2010) 18649–18654. C.L. Sun, K. Zhong, J.Q. Yao, D.G. Xu, X.L. Cao, Q.L. Zhang, J.Q. Luo, D.L. Sun, S. T. Yin, Diode-pumped continuous-wave quasi-three-level Nd: GYSGG laser at 937 nm, Opt. Commun. 294 (2013) 229–232. C.L. Sun, K. Zhong, C.G. Zhang, J.Q. Yao, D.G. Xu, F. Zhang, S.T. Yin, Stimulated emission cross section of the 4F3/2-4I11/2 of Nd: GYSGG, Laser Phys. Lett. 9 (2012) 410. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, D.L. Sun, J.Q. Luo, S. T. Yin, Continuous-wave and passively Q-switched laser performance of a disordered Nd: GYSGG crystal, Opt. Commun. 284 (2011) 5734–5737. K. Zhong, J.Q. Yao, C.L. Sun, C.G. Zhang, Y.Y. Miao, R. Wang, D.G. Xu, F. Zhang, Q. L. Zhang, D.L. Sun, S.T. Yin, Efficient diode-end-pumped dual-wavelength Nd, Gd: YSGG laser, Opt. Lett. 36 (2011) 3813–3815. K. Zhong, C. Sun, J. Yao, D. Xu, X. Xie, X. Cao, S. Yin,Efficient, Continuous-wave 1053-nm Nd: GYSGG laser with passively Q-switched dual-wavelength operation for terahertz generation, IEEE J. Quantum Electron. 49 (2013) 375–379. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, S.T. Yin, Diode‐ pumped passively mode‐locked Nd: GYSGG laser, Laser Phys. Lett. 8 (2011) 787–790. J.L. Xu, X.L. Li, Y.Z. Wu, X.P. Hao, J.L. He, K.J. Yang, Graphene saturable absorber mirror for ultra-fast-pulse solid-state laser, Opt. Lett. 36 (2011) 1948–1950. W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, K.M. Abramski, Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser, Opt. Express 20 (2012) 19463–19473. P. Zhao, S. Ragam, Y.J. Ding, I.B. Zotova, Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser, Opt. Lett. 35 (2010) 3979–3981. E.B. Petersen, W. Shi, A.C. Pirson, N. Peyghambarian, A.T. Cooney, Efficient parametric terahertz generation in quasi-phase-matched GaP through cavity enhanced difference-frequency generation, Appl. Phys. Lett. 98 (2011) 121119. S. Zhao, J. Zhao, G. Li, K. Yang, Y. Sun, D. Li, J. An, J. Wang, M. Li, Doubly Q-switched laser with electric-optic modulator and GaAs saturable absorber, Laser Phys. Lett. 3 (2006) 471. Y. Zhang, S.Z. Zhao, D.C. Li, K.J. Yang, G.Q. Li, G. Zhang, K. Cheng, Diodepumped doubly Q-switched mode-locked YVO4/Nd:YVO4/KTP green laser with AO and GaAs saturable absorber, Opt. Mater. 33 (2011) 303–307.
Optics Communications 347 (2015) 64–67
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber Qi Song a, Guoju Wang a, Bingyuan Zhang a,n, Qingli Zhang b, Wenjun Wang a, Minghong Wang a, Guihua Sun b, Yong Bo c, Qinjun Peng c a School of Physics Science and Information Engineering, Shandong key laboratory of optical communication science and technology, Liaocheng University, Liaocheng 252000, China b The Key Laboratory of Photonic Devices and Materials of Anhui, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2015 Received in revised form 17 February 2015 Accepted 2 March 2015 Available online 4 March 2015
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber is demonstrated. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. In addition, a simple way to fabricate few layers graphene oxide saturable absorbers has been reported. & 2015 Elsevier B.V. All rights reserved.
Keywords: Q-Switched Mode-locked Dual-wavelength Graphene oxide Nd:GYSGG
1. Introduction Multi-wavelength mode-locked lasers exhibit versatile applications in remote sensing, metrology, biomedical diagnostics, terahertz (THz) radiation generation, pump-probe measurement, component characterization and telecommunications [1–4]. Compared with single-wavelength laser generating its output at one center wavelength, multi-wavelength ultrafast lasers can simultaneously yield pulse trains at different center wavelengths. Some techniques have been proposed to generate multi-wavelength pulse, including dual gain media [5], stimulated Raman scattering [6] and crystals with the disordered structure, such as Nd:GGG [7], Nd:Sc0.2Y0.8SiO5 [8], Nd:CNGG, Nd:CLNGG, Nd:CTGG and Nd:CaYAlO4 [9–13]. Among them, Nd:GYSGG crystal with an inhomogeneous broadening of the emission spectra is beneficial to generating ultra-short pulses and multi-wavelength laser pulse [14–16]. The emission cross-sections and excited-state lifetime were 1.58 10 19 cm2 and 220 μs, respectively. There are some previous studies on continuous-wave of multi-wavelength [16–18] and passively mode-locking of single-wavelength operation [19] with the Nd:GYSGG crystal. To our knowledge, multi-wavelength mode-locking or Q-switched mode-locking (QML) operation with n
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.optcom.2015.03.002 0030-4018/& 2015 Elsevier B.V. All rights reserved.
this crystal has not been realized yet. Recently graphene and graphene oxide absorbers are believed to be promising saturable absorbers of gapless linear dispersion of Dirac electrons, which shows that they can be operated over a very broad range of wavelengths [20]. In this paper, the passively dual-wavelength Q-switched mode-locking operation of the disordered Nd:GYSGG crystal was obtained using the graphene oxide saturable absorber for the first time. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured.
2. Experimental setup 2.1. Preparation and characterization of GO saturable absorber GO was prepared from expanded acid washed graphite flakes by a modified Hummers method [21]. First, 2 g of graphite was added into 40 ml of H2SO4. Then, 1 g of NaNO3 was added. whereafter, the beaker was put into cold water bath to keep the temperature below 4 °C. 6 g of KMnO4 was added into the mixture, which was vigorously stirred. After addition of the oxidant, the beaker was heated and kept at 20 °C with continuous stirring. Afterward it can be left at room temperature overnight. Soon afterwards, the beaker was put into a water bath at a temperature of 32–38 °C and stirred. The mixture was then heated to 90 °C and
Q. Song et al. / Optics Communications 347 (2015) 64–67
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Fig. 3. Schematic setup of the diode-end-pumped Nd: GYSGG laser.
crystalline structure; the D-peak located at 1352 cm 1, which arises from the breathing modes of sp2 atoms in a ring [22]. 2.2. Setup of the diode-end-pumped Nd: GYSGG laser
Fig. 1. UV–vis–NIR transmissivity spectrum of GO saturable absorber.
kept these conditions for 20 min. 80 ml of alcohol and 5 ml of H2O2 were added in order to stop the reaction and dispersed graphene. HCl solution was used to remove the sulfate ions. To obtain uniform saturable absorber of high quality and suitable size in a quite simple way, 0.2 ml of the alcohol dispersed graphene oxide solution was added into 8 ml of alcohol. Then the graphene oxide solution was stirred by magnetic stirring apparatus for 5 min with the rotation rate of 500 rpm. Subsequently, quartz sheet with the diameter of 30 mm submerged into the prepared graphene oxide solution completely. The quartz substrate with graphene oxide was left at room temperature overnight to remove the alcohol thoroughly. The UV–vis–NIR spectrophotometer was used to characterize the transmissivity of the saturable absorber. Fig. 1 shows the UV–vis–NIR transmissivity spectrum of GO saturable absorber. From Fig. 1 we can see the GO saturable absorber can be employed as saturable absorber in a wide wavelength range from 500 nm to 1100 nm. The transmissivity are 91.1% at 1057 nm and 91% at 1061 nm respectively. The Raman spectrometer (RM 2000) was used to characterize the Raman spectrum of the saturable absorber. Fig. 2 shows the Raman spectrum of the deposited graphene oxide layer. Two Raman peaks can be identified in this spectrum: the G-peak at around 1600 cm 1, due to the bond stretching of sp2 atoms in the
Fig. 3 shows the schematic of the Nd:GYSGG laser cavity. The Q-switched mode-locking laser experimental setup was based on a standard Z-folded cavity with the length of about 1.5 m. The pump source used in the experiment was a fiber-coupled LD with the emission wavelength centered at 808 nm. The output beam of the LD was focused into the Nd:GYSGG crystal with a spot radius of about 0.1 mm using a focusing system. The Nd:GYSGG crystal with the dimensions of 3 3 5 mm3 was employed in the experiment. M was a flat mirror. M1 and M2 were concave mirrors with the radius of curvature of 400 mm, the flat mirror OC was an output couplers with the transmissions of 3% at 1.06 μm laser wavelength. The Nd:GYSGG crystal was mounted in a water-cooled copper block with the temperature of 22 °C. The GO deposited on the quartz sheet was employed as the saturable absorber. The laser spot radius on the GO saturable absorber was 0.12 mm. The laser pulse was recorded by a fast photo-detector (ET-3000) and an oscilloscope (Agilent DSO54832b).The fluorescence spectrum of Nd:GYSGG crystal excited by 808 nm laser is shown in Fig. 4 (measured by an Ocean Optics HR2000þ spectrum analyzer). The resolution of the spectrum analyzer is about 1 nm (FWHM). It is obviously seen in Fig. 4, there are two comparable peaks around 1060 nm due to the Stark splitting of the energy levels, indicating the possibility of multi-wavelength operation [23,24].
3. Experimental results In the experiment, continuous wave operation was obtained
Fig. 2. Raman spectrum of the GO formed on the quartz substrate.
Fig. 4. Fluorescence spectrum of Nd:GYSGG ranging from 950 nm to 1100 nm.
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Fig. 5. The output power versus the incident pump power of the continuous wave (CW) and Q-switched mode-locking (QML).
with the optical conversion efficiency of 6.2% firstly. Then the GO saturable absorber was inserted into the laser cavity. When the pump power increases to 2.2 W, Q-switched operation can be obtained. When the pump power is 2.6 W, the Q-switched modelocked operation was obtained. In order to protect the crystal and GO saturable absorber, the maximum pump power is 5 W. The relationship of the output power versus pump power is shown in Fig. 5. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The slope efficiencies for CW and QML laser were 10% and 6.3%, respectively. Fig. 6(a) and (b) shows temporal shape of a Q-switched pulse envelope and pulse train of the QML Nd:GYSGG laser respectively. The mode-locked pulse inside the Q-switched envelope is quite stable with the repetition rate of 100 MHz. According to the expanded oscilloscope traces of the modelocked pulse within the Q-switched envelope shown in Fig. 6(b), the mode-locked pulse width can be estimated by the formula [25,26]:
Fig. 6. (a) Temporal shape of a Q-switched pulse envelope and (b) pulse train of the QML Nd:GYSGG laser.
2 2 2 2 treal = tmeasure − tprobe − toscilloscope ,
here, treal is the real time of the pulse, tmeasure is the measured rise time, tprobe is the rise time of the probe and toscilloscope is the rise time of the oscilloscope. In our experiment, the average rise time of the mode-locked pulses is 0.59 ns. The rise time of oscilloscope is 0.35 ns and the rise time of the probe is also about 0.175 ns. Assuming that the pulse width is approximately 1.25 times more than the rise time, the estimated mode-locked pulse width is about 441 ps. The output spectrum is shown in Fig. 7 (measured by an Ocean Optics HR2000 þspectrum analyzer). Two laser lines are found simultaneously at 1057.28 nm and 1060.23 nm, which is exactly consistent with the fluorescence spectrum shown in Fig. 4. The separation between wavelengths is quite stable, and the intensities for different wavelength lines were nearly equal. The relative powers of the two lasing wavelength does not stay constant. There is a slight jitter, which was attributed to the mode competition. Considering that the line separation for dual wavelength oscillation regime is optimal for difference frequency generation, this laser material might be employed to generate THz-wave at 0.78 THz with a suitable nonlinear crystal. 4. Conclusion In this paper, the performance of passively dual-wavelength
Fig. 7. Spectrum of Nd:GYSGG laser.
Q-switched mode-locked Nd:GYSGG laser was investigated using the GO as the saturable absorber. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm was obtained, which can be used to generate THz-wave at 0.78 THz. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. In addition, a simple way to fabricate few layers
Q. Song et al. / Optics Communications 347 (2015) 64–67
GO saturable absorbers has been reported. With further optimization, this graphene oxide saturable absorber can be widely used in mode-locked solid state lasers.
[11]
[12]
Acknowledgments [13]
This work is partially supported by the National Natural Science Foundation of China (Nos.:61275147, 11375081 and 51172236), the Natural Science Foundation of Shandong Province (Nos.: ZR2012AL11 and ZR2014FL030) and a grant from the Special Construction Project Fund for Shandong Province Taishan Mountain Scholar.
[14]
[15]
[16]
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H. Jiang, Continuous-wave and passively Q-switched laser performance with a disordered Nd: CLNGG crystal, Opt. Express 17 (2009) 19015–19020. H.H. Yu, H.J. Zhang, Z.P. Wang, J.Y. Wang, Y.G. Yu, Z.B. Shi, X.Y. Zhang, M. H. Jiang, High-power dual-wavelength laser with disordered Nd: CNGG crystals, Opt. Lett. 34 (2009) 151–153. G.Q. Xie, D.Y. Tang, W.D. Tan, H. Luo, S.Y. Guo, H.H. Yu, H.J. Zhang, Diodepumped passively mode-locked Nd: CTGG disordered crystal laser, Appl. Phys. B 95 (2009) 691–695. D.Z. Li, X.D. Xu, J.Q. Meng, D.H. Zhou, C.T. Xia, F. Wu, J. Xu, Diode-pumped continuous wave and Q-switched operation of Nd: CaYAlO4 crystal, Opt. Express 18 (2010) 18649–18654. C.L. Sun, K. Zhong, J.Q. Yao, D.G. Xu, X.L. Cao, Q.L. Zhang, J.Q. Luo, D.L. Sun, S. T. Yin, Diode-pumped continuous-wave quasi-three-level Nd: GYSGG laser at 937 nm, Opt. Commun. 294 (2013) 229–232. C.L. Sun, K. Zhong, C.G. Zhang, J.Q. Yao, D.G. Xu, F. Zhang, S.T. Yin, Stimulated emission cross section of the 4F3/2-4I11/2 of Nd: GYSGG, Laser Phys. Lett. 9 (2012) 410. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, D.L. Sun, J.Q. Luo, S. T. Yin, Continuous-wave and passively Q-switched laser performance of a disordered Nd: GYSGG crystal, Opt. Commun. 284 (2011) 5734–5737. K. Zhong, J.Q. Yao, C.L. Sun, C.G. Zhang, Y.Y. Miao, R. Wang, D.G. Xu, F. Zhang, Q. L. Zhang, D.L. Sun, S.T. Yin, Efficient diode-end-pumped dual-wavelength Nd, Gd: YSGG laser, Opt. Lett. 36 (2011) 3813–3815. K. Zhong, C. Sun, J. Yao, D. Xu, X. Xie, X. Cao, S. Yin,Efficient, Continuous-wave 1053-nm Nd: GYSGG laser with passively Q-switched dual-wavelength operation for terahertz generation, IEEE J. Quantum Electron. 49 (2013) 375–379. B.Y. Zhang, J.L. Xu, G.J. Wang, J.L. He, W.J. Wang, Q.L. Zhang, S.T. Yin, Diode‐ pumped passively mode‐locked Nd: GYSGG laser, Laser Phys. Lett. 8 (2011) 787–790. J.L. Xu, X.L. Li, Y.Z. Wu, X.P. Hao, J.L. He, K.J. Yang, Graphene saturable absorber mirror for ultra-fast-pulse solid-state laser, Opt. Lett. 36 (2011) 1948–1950. W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. G. Sobon, J. Sotor, J. Jagiello, R. Kozinski, M. Zdrojek, M. Holdynski, P. Paletko, J. Boguslawski, L. Lipinska, K.M. Abramski, Graphene oxide vs. reduced graphene oxide as saturable absorbers for Er-doped passively mode-locked fiber laser, Opt. Express 20 (2012) 19463–19473. P. Zhao, S. Ragam, Y.J. Ding, I.B. Zotova, Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser, Opt. Lett. 35 (2010) 3979–3981. E.B. Petersen, W. Shi, A.C. Pirson, N. Peyghambarian, A.T. Cooney, Efficient parametric terahertz generation in quasi-phase-matched GaP through cavity enhanced difference-frequency generation, Appl. Phys. Lett. 98 (2011) 121119. S. Zhao, J. Zhao, G. Li, K. Yang, Y. Sun, D. Li, J. An, J. Wang, M. Li, Doubly Q-switched laser with electric-optic modulator and GaAs saturable absorber, Laser Phys. Lett. 3 (2006) 471. Y. Zhang, S.Z. Zhao, D.C. Li, K.J. Yang, G.Q. Li, G. Zhang, K. Cheng, Diodepumped doubly Q-switched mode-locked YVO4/Nd:YVO4/KTP green laser with AO and GaAs saturable absorber, Opt. Mater. 33 (2011) 303–307.
Optics Communications 347 (2015) 64–67
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber Qi Song a, Guoju Wang a, Bingyuan Zhang a,n, Qingli Zhang b, Wenjun Wang a, Minghong Wang a, Guihua Sun b, Yong Bo c, Qinjun Peng c a School of Physics Science and Information Engineering, Shandong key laboratory of optical communication science and technology, Liaocheng University, Liaocheng 252000, China b The Key Laboratory of Photonic Devices and Materials of Anhui, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 January 2015 Received in revised form 17 February 2015 Accepted 2 March 2015 Available online 4 March 2015
Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber is demonstrated. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. In addition, a simple way to fabricate few layers graphene oxide saturable absorbers has been reported. & 2015 Elsevier B.V. All rights reserved.
Keywords: Q-Switched Mode-locked Dual-wavelength Graphene oxide Nd:GYSGG
1. Introduction Multi-wavelength mode-locked lasers exhibit versatile applications in remote sensing, metrology, biomedical diagnostics, terahertz (THz) radiation generation, pump-probe measurement, component characterization and telecommunications [1–4]. Compared with single-wavelength laser generating its output at one center wavelength, multi-wavelength ultrafast lasers can simultaneously yield pulse trains at different center wavelengths. Some techniques have been proposed to generate multi-wavelength pulse, including dual gain media [5], stimulated Raman scattering [6] and crystals with the disordered structure, such as Nd:GGG [7], Nd:Sc0.2Y0.8SiO5 [8], Nd:CNGG, Nd:CLNGG, Nd:CTGG and Nd:CaYAlO4 [9–13]. Among them, Nd:GYSGG crystal with an inhomogeneous broadening of the emission spectra is beneficial to generating ultra-short pulses and multi-wavelength laser pulse [14–16]. The emission cross-sections and excited-state lifetime were 1.58 10 19 cm2 and 220 μs, respectively. There are some previous studies on continuous-wave of multi-wavelength [16–18] and passively mode-locking of single-wavelength operation [19] with the Nd:GYSGG crystal. To our knowledge, multi-wavelength mode-locking or Q-switched mode-locking (QML) operation with n
Corresponding author. E-mail address: [email protected] (B. Zhang).
http://dx.doi.org/10.1016/j.optcom.2015.03.002 0030-4018/& 2015 Elsevier B.V. All rights reserved.
this crystal has not been realized yet. Recently graphene and graphene oxide absorbers are believed to be promising saturable absorbers of gapless linear dispersion of Dirac electrons, which shows that they can be operated over a very broad range of wavelengths [20]. In this paper, the passively dual-wavelength Q-switched mode-locking operation of the disordered Nd:GYSGG crystal was obtained using the graphene oxide saturable absorber for the first time. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured.
2. Experimental setup 2.1. Preparation and characterization of GO saturable absorber GO was prepared from expanded acid washed graphite flakes by a modified Hummers method [21]. First, 2 g of graphite was added into 40 ml of H2SO4. Then, 1 g of NaNO3 was added. whereafter, the beaker was put into cold water bath to keep the temperature below 4 °C. 6 g of KMnO4 was added into the mixture, which was vigorously stirred. After addition of the oxidant, the beaker was heated and kept at 20 °C with continuous stirring. Afterward it can be left at room temperature overnight. Soon afterwards, the beaker was put into a water bath at a temperature of 32–38 °C and stirred. The mixture was then heated to 90 °C and
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Fig. 3. Schematic setup of the diode-end-pumped Nd: GYSGG laser.
crystalline structure; the D-peak located at 1352 cm 1, which arises from the breathing modes of sp2 atoms in a ring [22]. 2.2. Setup of the diode-end-pumped Nd: GYSGG laser
Fig. 1. UV–vis–NIR transmissivity spectrum of GO saturable absorber.
kept these conditions for 20 min. 80 ml of alcohol and 5 ml of H2O2 were added in order to stop the reaction and dispersed graphene. HCl solution was used to remove the sulfate ions. To obtain uniform saturable absorber of high quality and suitable size in a quite simple way, 0.2 ml of the alcohol dispersed graphene oxide solution was added into 8 ml of alcohol. Then the graphene oxide solution was stirred by magnetic stirring apparatus for 5 min with the rotation rate of 500 rpm. Subsequently, quartz sheet with the diameter of 30 mm submerged into the prepared graphene oxide solution completely. The quartz substrate with graphene oxide was left at room temperature overnight to remove the alcohol thoroughly. The UV–vis–NIR spectrophotometer was used to characterize the transmissivity of the saturable absorber. Fig. 1 shows the UV–vis–NIR transmissivity spectrum of GO saturable absorber. From Fig. 1 we can see the GO saturable absorber can be employed as saturable absorber in a wide wavelength range from 500 nm to 1100 nm. The transmissivity are 91.1% at 1057 nm and 91% at 1061 nm respectively. The Raman spectrometer (RM 2000) was used to characterize the Raman spectrum of the saturable absorber. Fig. 2 shows the Raman spectrum of the deposited graphene oxide layer. Two Raman peaks can be identified in this spectrum: the G-peak at around 1600 cm 1, due to the bond stretching of sp2 atoms in the
Fig. 3 shows the schematic of the Nd:GYSGG laser cavity. The Q-switched mode-locking laser experimental setup was based on a standard Z-folded cavity with the length of about 1.5 m. The pump source used in the experiment was a fiber-coupled LD with the emission wavelength centered at 808 nm. The output beam of the LD was focused into the Nd:GYSGG crystal with a spot radius of about 0.1 mm using a focusing system. The Nd:GYSGG crystal with the dimensions of 3 3 5 mm3 was employed in the experiment. M was a flat mirror. M1 and M2 were concave mirrors with the radius of curvature of 400 mm, the flat mirror OC was an output couplers with the transmissions of 3% at 1.06 μm laser wavelength. The Nd:GYSGG crystal was mounted in a water-cooled copper block with the temperature of 22 °C. The GO deposited on the quartz sheet was employed as the saturable absorber. The laser spot radius on the GO saturable absorber was 0.12 mm. The laser pulse was recorded by a fast photo-detector (ET-3000) and an oscilloscope (Agilent DSO54832b).The fluorescence spectrum of Nd:GYSGG crystal excited by 808 nm laser is shown in Fig. 4 (measured by an Ocean Optics HR2000þ spectrum analyzer). The resolution of the spectrum analyzer is about 1 nm (FWHM). It is obviously seen in Fig. 4, there are two comparable peaks around 1060 nm due to the Stark splitting of the energy levels, indicating the possibility of multi-wavelength operation [23,24].
3. Experimental results In the experiment, continuous wave operation was obtained
Fig. 2. Raman spectrum of the GO formed on the quartz substrate.
Fig. 4. Fluorescence spectrum of Nd:GYSGG ranging from 950 nm to 1100 nm.
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Fig. 5. The output power versus the incident pump power of the continuous wave (CW) and Q-switched mode-locking (QML).
with the optical conversion efficiency of 6.2% firstly. Then the GO saturable absorber was inserted into the laser cavity. When the pump power increases to 2.2 W, Q-switched operation can be obtained. When the pump power is 2.6 W, the Q-switched modelocked operation was obtained. In order to protect the crystal and GO saturable absorber, the maximum pump power is 5 W. The relationship of the output power versus pump power is shown in Fig. 5. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The slope efficiencies for CW and QML laser were 10% and 6.3%, respectively. Fig. 6(a) and (b) shows temporal shape of a Q-switched pulse envelope and pulse train of the QML Nd:GYSGG laser respectively. The mode-locked pulse inside the Q-switched envelope is quite stable with the repetition rate of 100 MHz. According to the expanded oscilloscope traces of the modelocked pulse within the Q-switched envelope shown in Fig. 6(b), the mode-locked pulse width can be estimated by the formula [25,26]:
Fig. 6. (a) Temporal shape of a Q-switched pulse envelope and (b) pulse train of the QML Nd:GYSGG laser.
2 2 2 2 treal = tmeasure − tprobe − toscilloscope ,
here, treal is the real time of the pulse, tmeasure is the measured rise time, tprobe is the rise time of the probe and toscilloscope is the rise time of the oscilloscope. In our experiment, the average rise time of the mode-locked pulses is 0.59 ns. The rise time of oscilloscope is 0.35 ns and the rise time of the probe is also about 0.175 ns. Assuming that the pulse width is approximately 1.25 times more than the rise time, the estimated mode-locked pulse width is about 441 ps. The output spectrum is shown in Fig. 7 (measured by an Ocean Optics HR2000 þspectrum analyzer). Two laser lines are found simultaneously at 1057.28 nm and 1060.23 nm, which is exactly consistent with the fluorescence spectrum shown in Fig. 4. The separation between wavelengths is quite stable, and the intensities for different wavelength lines were nearly equal. The relative powers of the two lasing wavelength does not stay constant. There is a slight jitter, which was attributed to the mode competition. Considering that the line separation for dual wavelength oscillation regime is optimal for difference frequency generation, this laser material might be employed to generate THz-wave at 0.78 THz with a suitable nonlinear crystal. 4. Conclusion In this paper, the performance of passively dual-wavelength
Fig. 7. Spectrum of Nd:GYSGG laser.
Q-switched mode-locked Nd:GYSGG laser was investigated using the GO as the saturable absorber. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm was obtained, which can be used to generate THz-wave at 0.78 THz. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. In addition, a simple way to fabricate few layers
Q. Song et al. / Optics Communications 347 (2015) 64–67
GO saturable absorbers has been reported. With further optimization, this graphene oxide saturable absorber can be widely used in mode-locked solid state lasers.
[11]
[12]
Acknowledgments [13]
This work is partially supported by the National Natural Science Foundation of China (Nos.:61275147, 11375081 and 51172236), the Natural Science Foundation of Shandong Province (Nos.: ZR2012AL11 and ZR2014FL030) and a grant from the Special Construction Project Fund for Shandong Province Taishan Mountain Scholar.
[14]
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