- Email: [email protected]
NdYVO4 laser with EO and middle SESAM
Optics Communications 285 (2012) 5347–5350
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Stable sub nanosecond pulse generation from dual-loss-modulated QML YVO4/NdYVO4 laser with EO and middle SESAM Gang Zhang a,n, Shengzhi Zhao b,n, Guiqiu Li b, Kejian Yang b, Dechun Li b, Kang Cheng b, Yonggang Wang c a
Colllege of Science, China University of Petroleum (East China), Qingdao 266555, China School of Information Science and Engineering, Shandong University, Jinan 250100, China c Research Center for Applied Sciences, Academia Sinica, Taiwan, China b
a r t i c l e i n f o
abstract
Article history: Received 5 May 2012 Received in revised form 28 July 2012 Accepted 3 August 2012 Available online 23 August 2012
By using electro-optic (EO) modulator and middle semiconductor saturable absorber mirror (SESAM), a diode-pumped dual-loss-modulated Q-switched mode-locked (QML) YVO4/NdYVO4 laser at 1.06 mm is presented. A stable pulse train with shorter pulse duration and higher peak power is generated. The experimental results show both the pulse width of the Q-switched envelope and the number of the mode-locked pulse underneath the Q-switched envelope decrease with increasing pump power. When the pump power exceeds 8.76 W, there is only one mode-locked pulse lying underneath a Q-switched envelope. This kind of stable sub-nanosecond pulse has the repetition rate of 1 kHz. The evaluated mode-locked pulse width is 950 ps and the corresponding peak power is 398 kW. & 2012 Elsevier B.V. All rights reserved.
Keywords: Dual-loss-modulation Sub-nanosecond pulse Q-switched envelope Pulse stability
1. Introduction High-peak power and short-pulse lasers have attracted great attention in recent years because they have many practical applications, such as fiber-sensing, telecommunications, micro-machining, and so on. The pulse width is one of the most important parameters in short pulse lasers because it decides the dynamical process of the inter-reaction between laser and materials. Generally, the continuous wave mode-locked (CWML) lasers can generate the ultra short optical pulses with the pulse width between hundreds of fs and dozens of ps, and the repetition rate is in tens of MHz, which depends on the cavity length [1–3]. The pulse widths of the Q-switched lasers range from several ns to hundreds of ns, and the pulse repletion rate is in several Hz dozens of kHz [4–7]. The passively microchip lasers can operate in sub-nanosecond pulse regime, but the output parameters have the poor stability and the difficult controllability [8,9]. So the pulsed lasers with the pulse width of sub-nanoseconds are expected. Simultaneously QML lasers involve two dynamical processes of Q-switching and mode-locking, from which the mode-locked pulses with sub nanosecond duration can be generated [10–12]. Especially, the stable QML pulses with the high energy and the optional repetition rate can be obtained in the doubly QML lasers by the integrations of the active modulator and the passive saturable absorption, in which the repetition rate of the Q-switched envelope
is controlled by the active acousto-optic modulator while the modelocked pulses inside the Q-switched envelope depend on both the actively modulated loss and the saturable absorption [13–16]. However, there are many mode-locked pulses in the single Q-switched envelope and each one has different pulse energy. Moreover, two kinds of the repetition rates exit in this QML laser, i.e. the repetition rates of the Q-switched envelope and the modelocked pulse under the Q-switched envelope, resulting in the limitation of the application fields. If the number of mode-locked pulses lying underneath the Q-switched envelope is limited to one, the stable sub-nanosecond pulses with higher peak power can be generated, in which only the repetition rate of the Q-switched envelope can be exited. As far as we know, there is no the related report on this kind of stable sub-nanosecond pulse lasers. In this paper, by simultaneously employing EO modulator and SESAM, a diode-pumped doubly QML YVO4/NdYVO4 laser at 1.06 mm is realized for the first time. The experimental results show the pulse peak power increases and the pulse widths of Q-switched envelope decrease with increasing pump power. When the pump power exceeds 8.76 W and the repetition rate 1 kHz of EO, there is only one mode-locked pulse lying in a Q-switched envelope, in which the stable sub-nanosecond pulse with 1 kHz repetition rate is generated. The evaluated mode-locked pulse width is 950 ps and the corresponding peak power is 398 kW.
2. Experiment setup n
Corresponding authors. E-mail addresses: [email protected] (G. Zhang), [email protected] (S. Zhao). 0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.08.009
The experimental arrangement is shown schematically in Fig. 1. The pump source is a commercial fiber-coupled laser-diode
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G. Zhang et al. / Optics Communications 285 (2012) 5347–5350
Fig. 1. Experimental setup of dual-loss-modulated QML YVO4/NdYVO4 laser.
(Coherent, FAP system), which works at the maximum absorption wavelength (808 nm) of the Nd3 þ ions. The laser crystal is an a-cut YVO4/Nd:YVO4 composite crystal for relieving the thermal lens effect. This crystal is fabricated by the thermal diffusion bonding technique with a dimensions of 4 4 (3þ8) mm3. The Nd:YVO4 in composite crystal has a Nd3 þ doped concentration of 0.5 at%. One end facet of the Nd: YVO4 crystal is antireflective (AR) coated at 1064 nm, while the other end facet of YVO4 is AR coated at 808 nm and 1064 nm as the pump end. The pump light is focused into the YVO4/Nd:YVO4 by two coupling lenses of 5 cm focal length. The EO modulator (BBO crystal, the repetition rate 1–10 kHz) with a polarizer and l/4 plate is used as active Qswitch while a middle semiconductor saturable absorber mirror (SESAM) is used as the passive one. Compared to other passive saturabe absorber, the middle SESAM has the advantages of simplicity, easy growth, low cost, without Bragg reflectivity layer and resonant structure. The saturation fluence of this SESAM (Fsat, S) is 90 mJ/cm2 and the modulation depth of SESAM (DR) is 6%. A simple four-mirror-Z-fold-cavity is used to simultaneously provide the proper spot sizes in the pump beam and a tightly focused spot size on SESAM. The different arm lengths (L1, L2, and L3) are set as 25, 65, and 10 cm, respectively. The whole length of the folded cavity is about 100 cm. The input mirror M1 is a flat mirror with high transmission (HT) coated at 808 nm and high reflection (HR) at 1064 nm while M2 and M3 are HR-coated at 1064 nm with radii of curvature of 500 and 150 mm, respectively. M4 was also a flat mirror with a 6.5% transmission coated at 1064 nm. By considering the thermal lens effect of the gain medium and using ABCD matrix method, the beam radii of the gain medium and SESAM can be calculated. The calculated results show that the beam radii depend on the pump power. When the pump power increases from 1.5 W to 10.25 W, the beam radius of the gain medium varies from 490 mm to 285 mm while the beam radius of SESAM varies from 80 mm to 99 mm. The pulse temporal profile and the repetition frequency are recorded by a digital oscilloscope (500 MHz bandwidth and 2.5 Gs/s sampling rate, Tektronix Inc., USA) and a photo detector (New Focus, model 1623) with a rising time of 1 ns. The output power was measured by the power meter (MAX 500AD, Coherent Inc., USA).
Firstly, the continuous-wave characteristics of the YVO4/ Nd:YVO4 laser are studied. The threshold pump power is 331 mW, and the average output power up to 2.2 W in a nearly diffraction-limited beam is obtained at the maximum pump power of 10.25 W, corresponding to an optical-optical conversion efficiency of 21%. Then the EO and SESAM are inserted into the cavity, the oscillation threshold is 1.5 W and the dual-lossmodulated laser operates in the regime of QML. The pulse output power almost increases lineally with pump power until 8.76 W. When the pump power exceeds 8.76 W, the average output power begins saturating. The highest output power of 378 mW is obtained at the pump power of 10.25 W. Fig. 2 gives the average output power versus pump power. Meanwhile we can know the laser output is elliptically polarized light by using analyzer. According to the repetition rate and the average output power, the pulse energy of single Q-switched envelope versus the incident pump power for the QML laser can be calculated, which is shown in Fig. 3. From Fig. 3, it can be seen that the pulse energy of the singly Q-switched envelope almost increases lineally with pump power until 8.76 W. When the pump power exceeds 8.76 W, the pulse energy begins saturating, which is similar o the average output power. The obtained maximum pulse energy is 378 mJ for the doubly QML laser at 10.25 W. The pulse width of the Q-switched envelope versus incident pump power and the typical variations of the mode-locked number are shown in Fig. 4. It can be seen that the pulse width of the Q-switched envelope always decreases with the increase of the incident pump power due to the gradually saturated SESAM, resulting in the decrease of the mode-locked number underneath the Q-switched envelope. When the pump power exceeds 8.76 W, the obtained pulse width of the Q-switched envelope is shorter than the cavity roundtrip transmit time 6.7 ns. So there is only
Fig. 2. Output powers versus incident pump power.
3. Experimental results and discussions For the dual-loss-modulated QML laser with the active modulator and the passive saturable absorber, the repetition rate of the Q-switched envelope depends on the actively modulated rate. As the dual-loss-modulated QML laser with EO and saturable absorber in [16], more stable pulse train with shorter pulse width, higher pulse energy and peak power can be generated at the repetition rate of 1 kHz than those generated at higher repetition rate. So in the experiment, we set 1 kHz as the repetition rate of EO modulator.
Fig. 3. Pulse energy versus pump power.
G. Zhang et al. / Optics Communications 285 (2012) 5347–5350
one mode-locked pulse in the Q-switched envelope and the repetition rate of the mode-locked pulse is equal to the modulation rate of EO. The pulse width of the mode-locked pulse is even shorter than those obtained in Q-switched lasers [4–7]. A typical pulse train of oscilloscope trace is presented in Fig. 5. The pulseto-pulse amplitude fluctuation of the pulse train is less than 2%, which is more stable than common mode-locked lasers. The temporal shape of the single mode-locked pulse is shown in Fig. 6, in which it is evident there is only one mode-locked pulse. Meanwhile some noise can be seen in this figure. This is because the active modulator, which is used for shaping the mode-locked pulse in our experiment, cannot provide enough loss to clear away all the satellite pulses. It is worthwhile to apply the definition of critical intra-cavity pulse energy for distinguishing the QML and cw mode-locking operation of SESAM in our experiment. The critical intra cavity pulse energy (Ep,c) is defined as Ep,c ¼ (Fsat,L Aeff,L Fsat,S Aeff,S)1/2 [17], where Fsat,L is the saturation fluence of laser medium, given by Fsat,L ¼hn/ms, h is the Planck’s constant, n is the lasing frequency, s is the stimulated emission cross section, m¼2 for standing wave cavity. Aeff,S, Aeff,L are the effective mode areas on SESAM and in the laser medium. For Ep oEp,c, the laser belongs to the QML operation. With a set of parameters and s ¼25 10 19 cm2, the critical intra cavity pulse energy Ep,c E45.7 10 8 J. EP can be calculated by EP ¼PTR, where TR is the cavity roundtrip time, P¼1/2 Pout(1 þR)/(1 R), Pout is the output power, R is the reflectivity of the output mirror. The calculated value of EP is about 3.78 10 8 J. It is obvious that the Ep oEp,c, so the QML laser operates on our experimental condition. For this mode-locked pulse in Q-switched envelope, the pulse widths generally lie within the range of sub-seconds. According to the expanded oscilloscope traces of the mode-locked pulse, the mode-locked pulse width can be approximately estimated by the formula t2r ¼t2m t2p t2o, where tm is the measured average rise time which is about the time from about 10–90%, tp is the responding time of detection device, tr is the real rise time, to is the rise time of the oscilloscope [16,18]. Assuming that the pulse width is approximately 1.25 times more than the rise time, it can
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Acknowledgments
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This work is partially supported by the National Science Foundation of China (61078031) and the Natural Science Foundation of Shandong Province (ZR2011FM012).
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Incident pump power (W) Fig. 7. Average mode-locked pulse peak power versus the incident pump power.
be calculated that the mode-locked pulse width is about 950 ps. According to the above results, the average mode-locked pulse peak power for the dual-loss-modulated QML YVO4/NdYVO4 laser can be calculated, which is shown in Fig. 7. The obtained peak power in single mode-locked pulse laser is twice than that obtained in common QML laser and the maximum peak power of 398 kW is obtained at the pump power of 10.25 W.
4. Conclusion In conclusion, a dual-loss-modulated Q-switched mode-locked YVO4/NdYVO4 laser with EO modulator and SESAM is demonstrated. The QML laser characteristics have also been measured for different pump power. When the pump power exceeds 8.76 W and the repetition rate 1 kHz of EO, there is only one mode-locked pulse in a single Q-switched envelope. The evaluated modelocked pulse width is about 950 ps. The maximum peak power of 398 kW is obtained at the pump power of 10.25 W. The experimental results show that the dual-loss modulated QML laser with EO and SESAM is an efficient method for the generation of stable sub nanosecond pulses with high peak power.
[1] L. Krainer, R. Paschotta, M. Moser, U. Keller, Applied Physics Letters 77 (14) (2000) 2104. [2] Viktor E. Kisel, Sergey V. Kurilchik, Anatol S. Yasukevich, Sergey V. Grigoriev, Sofya A. Smirnova, Nikolay V. Kuleshov, Optics Letters 33 (19) (2008) 2194. [3] J. Kong, D.Y. Tang, S.P. Ng, B. Zhao, L.J. Qin, X.L. Meng, Applied Physics B 79 (2004) 203. [4] Haohai Yu, Huaijin Zhang, Zhengping Wang, Jiyang Wang, Zongshu Shao, Minhua Jiang, Optics Express 15 (2007) 3206. [5] T.T. Kajava, A.L. Gaeta, Optics Letters 21 (16) (1996) 1244. ¨ [6] Liu Junhai, Bernd Ozygus, Suhui Yang, Jurgen Erhard, Ute Seelig, Adalbert Ding, Horst Weber, Xianlin Meng, Li Zhu, Lianjie Qin, Chenlin Du, Xinguang Xu, Zongshu Shao, Journal of the Optical Society of America B 20 (4) (2003) 652. [7] Zhigang Li, Z. Xiong, N. Moore, G.C. Lim, W.L. Huang, D.X. Huang, Optics Communications 237 (2004) 411. [8] A.A. Demidovich, P.A. Apanasevich, L.E. Batay, A.S. Grabtchikov, A.N. Kuzmin, V.A. Lisinetskii, V.A. Orlovich, O.V. Kuzmin, V.L. Hait, W. Kiefer, Applied Physics B 76 (5) (2003) 509. [9] John J. Zayhowski, Optics Letters 22 (3) (1997) 169. [10] Y.F. Chen, S.W. Tsai, S.C. Wang, Optics Letters 25 (2000) 1442. [11] D.J. Kuizenga, D.W. Phillion, T. Lund, A.E. Siegman, Optics Communications 9 (1973) 221. [12] Ja-Hon Lin, Hou-Ren Chen, Hsin-Han Hsu, Ming-Dar Wei, Kuei-Huei Lin, Wen-Feng Hsieh, Optics Express 16 (2008) 16538. [13] Prasanta Kumar Datta, Sourabh Mukhopadhyay, Susanta Kumar Das, Luca Tartara, Antonio Agnesi, Vittorio Degiorgio, Optics Express 12 (17) (2004) 4041. [14] J.-H. Lin, K.-H. Lin, H.-H. Hsu, W.-F. Hsieh, Laser Physics Letters 5 (4) (2008) 276. [15] S. Zhao, G. Li, D. Li, K. Yang, Y. Li, M. Li, T. Li, K. Cheng, G. Zhang, H. Ge, Laser Physics Letters 7 (1) (2010) 29. [16] Tao Li, Shengzhi Zhao, Zhuang Zhuo, Kejian Yang, Guiqiu Li, Dechun Li, Optics Express 18 (2010) 10315. ¨ [17] C. Honninger, R. Paschotta, F. Morier-Genoud, M. Moser, U. Keller, Journal of the Optical Society of America B 16 (1999) 46. [18] D.Weller, Relating wideband DSO rise time to bandwidth: lose the 0.35, EDN December 12, 2002, 89–94.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Stable sub nanosecond pulse generation from dual-loss-modulated QML YVO4/NdYVO4 laser with EO and middle SESAM Gang Zhang a,n, Shengzhi Zhao b,n, Guiqiu Li b, Kejian Yang b, Dechun Li b, Kang Cheng b, Yonggang Wang c a
Colllege of Science, China University of Petroleum (East China), Qingdao 266555, China School of Information Science and Engineering, Shandong University, Jinan 250100, China c Research Center for Applied Sciences, Academia Sinica, Taiwan, China b
a r t i c l e i n f o
abstract
Article history: Received 5 May 2012 Received in revised form 28 July 2012 Accepted 3 August 2012 Available online 23 August 2012
By using electro-optic (EO) modulator and middle semiconductor saturable absorber mirror (SESAM), a diode-pumped dual-loss-modulated Q-switched mode-locked (QML) YVO4/NdYVO4 laser at 1.06 mm is presented. A stable pulse train with shorter pulse duration and higher peak power is generated. The experimental results show both the pulse width of the Q-switched envelope and the number of the mode-locked pulse underneath the Q-switched envelope decrease with increasing pump power. When the pump power exceeds 8.76 W, there is only one mode-locked pulse lying underneath a Q-switched envelope. This kind of stable sub-nanosecond pulse has the repetition rate of 1 kHz. The evaluated mode-locked pulse width is 950 ps and the corresponding peak power is 398 kW. & 2012 Elsevier B.V. All rights reserved.
Keywords: Dual-loss-modulation Sub-nanosecond pulse Q-switched envelope Pulse stability
1. Introduction High-peak power and short-pulse lasers have attracted great attention in recent years because they have many practical applications, such as fiber-sensing, telecommunications, micro-machining, and so on. The pulse width is one of the most important parameters in short pulse lasers because it decides the dynamical process of the inter-reaction between laser and materials. Generally, the continuous wave mode-locked (CWML) lasers can generate the ultra short optical pulses with the pulse width between hundreds of fs and dozens of ps, and the repetition rate is in tens of MHz, which depends on the cavity length [1–3]. The pulse widths of the Q-switched lasers range from several ns to hundreds of ns, and the pulse repletion rate is in several Hz dozens of kHz [4–7]. The passively microchip lasers can operate in sub-nanosecond pulse regime, but the output parameters have the poor stability and the difficult controllability [8,9]. So the pulsed lasers with the pulse width of sub-nanoseconds are expected. Simultaneously QML lasers involve two dynamical processes of Q-switching and mode-locking, from which the mode-locked pulses with sub nanosecond duration can be generated [10–12]. Especially, the stable QML pulses with the high energy and the optional repetition rate can be obtained in the doubly QML lasers by the integrations of the active modulator and the passive saturable absorption, in which the repetition rate of the Q-switched envelope
is controlled by the active acousto-optic modulator while the modelocked pulses inside the Q-switched envelope depend on both the actively modulated loss and the saturable absorption [13–16]. However, there are many mode-locked pulses in the single Q-switched envelope and each one has different pulse energy. Moreover, two kinds of the repetition rates exit in this QML laser, i.e. the repetition rates of the Q-switched envelope and the modelocked pulse under the Q-switched envelope, resulting in the limitation of the application fields. If the number of mode-locked pulses lying underneath the Q-switched envelope is limited to one, the stable sub-nanosecond pulses with higher peak power can be generated, in which only the repetition rate of the Q-switched envelope can be exited. As far as we know, there is no the related report on this kind of stable sub-nanosecond pulse lasers. In this paper, by simultaneously employing EO modulator and SESAM, a diode-pumped doubly QML YVO4/NdYVO4 laser at 1.06 mm is realized for the first time. The experimental results show the pulse peak power increases and the pulse widths of Q-switched envelope decrease with increasing pump power. When the pump power exceeds 8.76 W and the repetition rate 1 kHz of EO, there is only one mode-locked pulse lying in a Q-switched envelope, in which the stable sub-nanosecond pulse with 1 kHz repetition rate is generated. The evaluated mode-locked pulse width is 950 ps and the corresponding peak power is 398 kW.
2. Experiment setup n
Corresponding authors. E-mail addresses: [email protected] (G. Zhang), [email protected] (S. Zhao). 0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.08.009
The experimental arrangement is shown schematically in Fig. 1. The pump source is a commercial fiber-coupled laser-diode
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G. Zhang et al. / Optics Communications 285 (2012) 5347–5350
Fig. 1. Experimental setup of dual-loss-modulated QML YVO4/NdYVO4 laser.
(Coherent, FAP system), which works at the maximum absorption wavelength (808 nm) of the Nd3 þ ions. The laser crystal is an a-cut YVO4/Nd:YVO4 composite crystal for relieving the thermal lens effect. This crystal is fabricated by the thermal diffusion bonding technique with a dimensions of 4 4 (3þ8) mm3. The Nd:YVO4 in composite crystal has a Nd3 þ doped concentration of 0.5 at%. One end facet of the Nd: YVO4 crystal is antireflective (AR) coated at 1064 nm, while the other end facet of YVO4 is AR coated at 808 nm and 1064 nm as the pump end. The pump light is focused into the YVO4/Nd:YVO4 by two coupling lenses of 5 cm focal length. The EO modulator (BBO crystal, the repetition rate 1–10 kHz) with a polarizer and l/4 plate is used as active Qswitch while a middle semiconductor saturable absorber mirror (SESAM) is used as the passive one. Compared to other passive saturabe absorber, the middle SESAM has the advantages of simplicity, easy growth, low cost, without Bragg reflectivity layer and resonant structure. The saturation fluence of this SESAM (Fsat, S) is 90 mJ/cm2 and the modulation depth of SESAM (DR) is 6%. A simple four-mirror-Z-fold-cavity is used to simultaneously provide the proper spot sizes in the pump beam and a tightly focused spot size on SESAM. The different arm lengths (L1, L2, and L3) are set as 25, 65, and 10 cm, respectively. The whole length of the folded cavity is about 100 cm. The input mirror M1 is a flat mirror with high transmission (HT) coated at 808 nm and high reflection (HR) at 1064 nm while M2 and M3 are HR-coated at 1064 nm with radii of curvature of 500 and 150 mm, respectively. M4 was also a flat mirror with a 6.5% transmission coated at 1064 nm. By considering the thermal lens effect of the gain medium and using ABCD matrix method, the beam radii of the gain medium and SESAM can be calculated. The calculated results show that the beam radii depend on the pump power. When the pump power increases from 1.5 W to 10.25 W, the beam radius of the gain medium varies from 490 mm to 285 mm while the beam radius of SESAM varies from 80 mm to 99 mm. The pulse temporal profile and the repetition frequency are recorded by a digital oscilloscope (500 MHz bandwidth and 2.5 Gs/s sampling rate, Tektronix Inc., USA) and a photo detector (New Focus, model 1623) with a rising time of 1 ns. The output power was measured by the power meter (MAX 500AD, Coherent Inc., USA).
Firstly, the continuous-wave characteristics of the YVO4/ Nd:YVO4 laser are studied. The threshold pump power is 331 mW, and the average output power up to 2.2 W in a nearly diffraction-limited beam is obtained at the maximum pump power of 10.25 W, corresponding to an optical-optical conversion efficiency of 21%. Then the EO and SESAM are inserted into the cavity, the oscillation threshold is 1.5 W and the dual-lossmodulated laser operates in the regime of QML. The pulse output power almost increases lineally with pump power until 8.76 W. When the pump power exceeds 8.76 W, the average output power begins saturating. The highest output power of 378 mW is obtained at the pump power of 10.25 W. Fig. 2 gives the average output power versus pump power. Meanwhile we can know the laser output is elliptically polarized light by using analyzer. According to the repetition rate and the average output power, the pulse energy of single Q-switched envelope versus the incident pump power for the QML laser can be calculated, which is shown in Fig. 3. From Fig. 3, it can be seen that the pulse energy of the singly Q-switched envelope almost increases lineally with pump power until 8.76 W. When the pump power exceeds 8.76 W, the pulse energy begins saturating, which is similar o the average output power. The obtained maximum pulse energy is 378 mJ for the doubly QML laser at 10.25 W. The pulse width of the Q-switched envelope versus incident pump power and the typical variations of the mode-locked number are shown in Fig. 4. It can be seen that the pulse width of the Q-switched envelope always decreases with the increase of the incident pump power due to the gradually saturated SESAM, resulting in the decrease of the mode-locked number underneath the Q-switched envelope. When the pump power exceeds 8.76 W, the obtained pulse width of the Q-switched envelope is shorter than the cavity roundtrip transmit time 6.7 ns. So there is only
Fig. 2. Output powers versus incident pump power.
3. Experimental results and discussions For the dual-loss-modulated QML laser with the active modulator and the passive saturable absorber, the repetition rate of the Q-switched envelope depends on the actively modulated rate. As the dual-loss-modulated QML laser with EO and saturable absorber in [16], more stable pulse train with shorter pulse width, higher pulse energy and peak power can be generated at the repetition rate of 1 kHz than those generated at higher repetition rate. So in the experiment, we set 1 kHz as the repetition rate of EO modulator.
Fig. 3. Pulse energy versus pump power.
G. Zhang et al. / Optics Communications 285 (2012) 5347–5350
one mode-locked pulse in the Q-switched envelope and the repetition rate of the mode-locked pulse is equal to the modulation rate of EO. The pulse width of the mode-locked pulse is even shorter than those obtained in Q-switched lasers [4–7]. A typical pulse train of oscilloscope trace is presented in Fig. 5. The pulseto-pulse amplitude fluctuation of the pulse train is less than 2%, which is more stable than common mode-locked lasers. The temporal shape of the single mode-locked pulse is shown in Fig. 6, in which it is evident there is only one mode-locked pulse. Meanwhile some noise can be seen in this figure. This is because the active modulator, which is used for shaping the mode-locked pulse in our experiment, cannot provide enough loss to clear away all the satellite pulses. It is worthwhile to apply the definition of critical intra-cavity pulse energy for distinguishing the QML and cw mode-locking operation of SESAM in our experiment. The critical intra cavity pulse energy (Ep,c) is defined as Ep,c ¼ (Fsat,L Aeff,L Fsat,S Aeff,S)1/2 [17], where Fsat,L is the saturation fluence of laser medium, given by Fsat,L ¼hn/ms, h is the Planck’s constant, n is the lasing frequency, s is the stimulated emission cross section, m¼2 for standing wave cavity. Aeff,S, Aeff,L are the effective mode areas on SESAM and in the laser medium. For Ep oEp,c, the laser belongs to the QML operation. With a set of parameters and s ¼25 10 19 cm2, the critical intra cavity pulse energy Ep,c E45.7 10 8 J. EP can be calculated by EP ¼PTR, where TR is the cavity roundtrip time, P¼1/2 Pout(1 þR)/(1 R), Pout is the output power, R is the reflectivity of the output mirror. The calculated value of EP is about 3.78 10 8 J. It is obvious that the Ep oEp,c, so the QML laser operates on our experimental condition. For this mode-locked pulse in Q-switched envelope, the pulse widths generally lie within the range of sub-seconds. According to the expanded oscilloscope traces of the mode-locked pulse, the mode-locked pulse width can be approximately estimated by the formula t2r ¼t2m t2p t2o, where tm is the measured average rise time which is about the time from about 10–90%, tp is the responding time of detection device, tr is the real rise time, to is the rise time of the oscilloscope [16,18]. Assuming that the pulse width is approximately 1.25 times more than the rise time, it can
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Acknowledgments
Pulse peak power (kW)
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multiple mode-locked pulses single mode-locked pulse
This work is partially supported by the National Science Foundation of China (61078031) and the Natural Science Foundation of Shandong Province (ZR2011FM012).
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Incident pump power (W) Fig. 7. Average mode-locked pulse peak power versus the incident pump power.
be calculated that the mode-locked pulse width is about 950 ps. According to the above results, the average mode-locked pulse peak power for the dual-loss-modulated QML YVO4/NdYVO4 laser can be calculated, which is shown in Fig. 7. The obtained peak power in single mode-locked pulse laser is twice than that obtained in common QML laser and the maximum peak power of 398 kW is obtained at the pump power of 10.25 W.
4. Conclusion In conclusion, a dual-loss-modulated Q-switched mode-locked YVO4/NdYVO4 laser with EO modulator and SESAM is demonstrated. The QML laser characteristics have also been measured for different pump power. When the pump power exceeds 8.76 W and the repetition rate 1 kHz of EO, there is only one mode-locked pulse in a single Q-switched envelope. The evaluated modelocked pulse width is about 950 ps. The maximum peak power of 398 kW is obtained at the pump power of 10.25 W. The experimental results show that the dual-loss modulated QML laser with EO and SESAM is an efficient method for the generation of stable sub nanosecond pulses with high peak power.
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