- Email: [email protected]
Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser
Optical Fiber Technology xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser X.D. Chen a,⇑, Q. Sun b, H. Li a, M.H. Zhao a a b
State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, Hefei, Anhui 230037, China Division of Metrology in Optics and Laser, National Institute of Metrology, Beijing 100013, China
a r t i c l e
i n f o
Article history: Received 7 March 2013 Revised 15 May 2013 Available online xxxx Keywords: Stimulated Raman scattering Hollow-core photonic crystal fiber Single-mode Fiber laser Q-switched
a b s t r a c t We report a compact all-fiber gas Raman light source based on hydrogen-filled HC-PCF gas cell with a SM Q-switched fiber laser as the Raman pump source. Theoretical model of the hydrogen SRS based on HC-PCF pumped with Q-switched fiber laser is also analyzed. Simulated results show that the Stokes pulse is much narrower than the pump pulse, and both the Stokes pulse duration and conversion efficiency are increased with the pump pulse energy of the same duration pump pulses. The experimental results are in agreement with the theoretical results in change trend. For the 120 ns pump pulses with the repetition rate of 5 kHz, the threshold pump energy and conversion efficiency at the threshold are 1.146 lJ and 2.69%. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The Raman light source based on the gas stimulated Raman scattering (SRS) effect is very useful in many applications, such as nonlinear optics, spectroscopy and trace gas detection [1–3], because of its different Raman frequency-shift for different gases. The gas Raman cell is the standard component of achieving the gas Raman light source. A long-standing difficulty in gas SRS based on the traditional gas Raman cell is very high threshold pump power, because the power density of the pump beam is difficult to be kept at a high level in the Raman interaction length due to the limit of the optical diffraction effect. To decrease the threshold of the gas SRS, hollow-core photonic crystal fibers (HC-PCFs) filled with high pressure gases have been used as the gas Raman cell. The threshold may be decreased by more than three orders in magnitude compared to that based on the traditional gas Raman cell [4]. Based on the gas-filled HC-PCF cell, the gas SRS of low threshold has been demonstrated by using Q-switched solid-state lasers or continuous wave (CW) high-power double-clad (DC) fiber lasers as the Raman pump sources [5–9]. Taking into account the fact that the all-fiber structure has the characteristics of high stability and much convenience, we have demonstrated an all-fiber gas Raman light source pumped with a
single-mode (SM) Q-switched fiber laser followed by a DC fiber amplifier [10]. However, this pump design becomes good expensive and less convenient for practical applications, and also results in high threshold pump energy required by the gas Raman light source because of the high splicing loss between the SM fiber and DC fiber. Due to the gas Raman cell is sealed by splicing HCPCF to SM fiber [11], the SM Q-switched fiber lasers are expected to be more suitable for producing the Raman Stokes wave. To date, there has been no report on gas Raman light source pumped by such SM Q-switched fiber laser. In this letter, we propose a compact all-fiber gas Raman light source based on hydrogen-filled HC-PCF gas cell with a SM Q-switched fiber laser as the Raman pump source. Theoretical results show that the Stokes pulse is much narrower than the pump pulse, and both the Stokes pulse duration and conversion efficiency are increased with the pump pulse energy of the same duration pump pulses. In the experiment, the proposed gas Raman light source generates the Stokes wave at 1135.7 nm with the pump wave at 1064.7 nm. For the 120 ns pump pulses with a repetition rate of 5 kHz, the threshold pump energy and conversion efficiency at the threshold are 1.146 lJ and 2.69%. The experimental results are in agreement with the theoretical results in change trend. We believe that the proposed gas Raman light source with the simple design, high stability and low threshold will have many important applications.
⇑ Corresponding author. E-mail address: [email protected] (X.D. Chen). 1068-5200/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.yofte.2013.06.002
Please cite this article in press as: X.D. Chen et al., Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser, Opt. Fiber Technol. (2013), http://dx.doi.org/10.1016/j.yofte.2013.06.002
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X.D. Chen et al. / Optical Fiber Technology xxx (2013) xxx–xxx
2. Experimental setup The proposed all-fiber gas Raman light source system is shown schematically in Fig. 1. It consists of a Raman pump source and a gas Raman cell. The Raman pump source is a SM Q-switched Ytterbium-doped fiber laser (YDFL), which is made of a SM fiber pigtailed acousto-optic modulator (AOM), a wavelength division multiplexing (WDM) coupler, a 3.2-m long YDF with absorption 528 dB/m at 976 nm, and two fiber Bragg gratings (FBGs). A SM fiber pigtailed 975 nm laser diode (LD) with maximum output power of 280mW provides the pump power for YDFL. FBG1 and FBG2 used in YDFL have the same center wavelength of 1064.7 nm and bandwidth of 0.2 nm, but different reflectivity of 70% and 99.7%, respectively. The AOM has an insertion loss of 1.5 dB and the extinction radio more than 30 dB in 1064 ± 5 nm. The modulation frequency and rise time of the AOM can be set to any value between 5 and 80 kHz, and 10 and 200 ns, respectively. The gas Raman cell is a SM fiber pigtailed HC-PCF gas cell. A 30 m-long HC-PCF (Crystal Fiber A/S, HC-1060-02) with the mode field radius of 3.25 lm at 1060 nm and the transmission losses of 0.09 and 0.13 dB/m at 1060 and 1135 nm is hydrogenfilled to form the all-fiber high-pressure gas cell. The HC-PCF gas cell with SM fiber pigtails is very stable, which are made with the technique in Ref. [11], and the hydrogen pressure in the HCPCF gas cell is evaluated to be about 8–9 atm. The whole transmission loss of the HC-PCF gas cell, including the linear absorption and the additional insertion loss, is measured to be 9 dB at 1064 nm. The gas Raman light source output is split by a 1064/1135 nm WDM coupler (WDM2) with the isolation of 35 dB, and then analyzed with an optical spectrum analyzer (OSA, Agilent 86140B), and a fast photo-detector (UltraFast 20SM) followed by a 600 MHz oscilloscope (OSC, Agilent MSO 8064A), respectively. 3. Modeling and simulation In this section, we will analyze the hydrogen SRS based on HCPCF pumped with a Q-switched fiber laser theoretically. Because the output power of the SM Q-switched fiber laser is limited, which is difficult to meet the threshold of the high-order Stokes waves, only the first-order Stokes wave is discussed in this model. The following equations describe the nonlinear interactions between the pump pulses and Stokes pulses [12],
ns @P s @Ps g þ ¼ R P s P p as P s c @t @z Aeff np @P p @Pp g ks þ ¼ R Ps Pp ap Pp c @t @z Aeff kp
the effective mode area and Raman gain coefficient associated with the pump and Stokes wave. c is the light velocity. The pump wavelength kp is 1064 nm, and the first-order Stokes wavelength ks is based on a 587 cm1 frequency shift of the rotational Raman level of hydrogen, so the Stokes wavelength is 1135 nm. For HC-1060-02 HC-PCF, the mode field of the first-order Stokes wave equals to that of the pump wave approximately, so the effective mode area Aeff can be written as 2pr 2p [13], where rp is the mode field radius of HC-PCF at 1060 nm. The Raman gain coefficient gR is taken to be 1cm/GW at a pump wavelength of 1064 nm for hydrogen pressure inside the HC-PCF of about 8–9 atm [6], and the refractive indexes np and ns are about 1.0. Given the Gaussian Q-switched pump pulses with the pulse duration of 120 ns, the shapes of the Stokes pulses and pump pulses are simulated, respectively. Figs. 2 and 3 show the simulated results for different pump pulse energy. From these results, we can see that the Stokes pulses are much narrower than the pump pulses. The reason may be only the central part of the pump pulse, which is over the SRS threshold, can convert into the Stokes pulse. Moreover, the leading edge of the Stokes pulse is always steeper than the trailing edge. This is because that the Raman gain is not only determined by the instantaneous power of the pump pulse, but also depends on the accumulated energy as the pump pulse building up [12,14].When the pump pulse duration is 120 ns, the Stokes pulse duration is simulated to be 28.53 and 52.89 ns for the pump pulse energy of 1.14 and 1.33 lJ, respectively. Fig. 4 gives the simulated Stokes pulse duration and conversion efficiency as function of the pump pulse energy when the duration of the pump pulse is 120 ns. It can be clearly seen that both the Stokes pulse duration and conversion efficiency are increased with the pump pulse energy. As the pump pulse energy increases, the central part width of the pump pulse that is over the SRS threshold power may broaden, which also means that more part of the pump pulse can be converted into the Stokes pulse, resulting that both the Stokes pulse duration and conversion efficiency increase with the pump pulse energy. When the pump pulse energy are 1.14 and 1.33 lJ, the Stokes pulse energy are 0.084 and 0.269 lJ, corresponding to the conversion efficiencies are 7.36% and 20.33%, respectively.
4. Experimental results and discussions
ð1Þ ð2Þ
Initially, the modulation frequency and rise time of the AOM used in the Q-switched YDFL are fixed at 5 kHz and 200 ns respectively. In this experiment, the polarization state of the Q-switched
In (1) and (2), Pp is the pump power, Ps is the first-order Stokes power. ki , ni, and ai are the wavelengths, refractive indexes, and transmission losses of the HC-PCF gas cell, respectively. The indexes p and s apply to the pump and Stokes wave. Aeff and gR are
Fig. 1. Configuration of the gas Raman light source. YDF: ytterbium-doped fiber; LD: laser diode; WDM: wavelength division-multiplexing coupler; AOM: acoustooptic modulator; FBG: fiber Bragg grating; HC-PCF: hollow-core photonic crystal fiber.
Fig. 2. Simulated output pulse shapes with the pump pulse energy of 1.14 lJ when the duration of the pump pulse is 120 ns.
Please cite this article in press as: X.D. Chen et al., Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser, Opt. Fiber Technol. (2013), http://dx.doi.org/10.1016/j.yofte.2013.06.002
X.D. Chen et al. / Optical Fiber Technology xxx (2013) xxx–xxx
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Fig. 3. Simulated output pulse shapes with the pump pulse energy of 1.33 lJ when the duration of the pump pulse is 120 ns.
YDFL is natural light. Fig. 5 gives the measured output spectrum for different pump powers of the SM Q-switched YDFL. As shown in the figure, besides the Q-switched pump pulses at 1064.7 nm, a new wave line at 1135.7 nm is emerged abruptly from the noise on the OSA when the YDFL pump power reaches 186mW. The frequency spacing between the new wave line and pump wavelength is 587 cm1, which right equals to the first-order Stokes frequencyshift of the rotational Raman level of hydrogen. According to the threshold measurements methods in Ref. [7], the threshold pump energy is about 1.146 lJ. Continuing to increase the YDFL pump power, the amplitude of the Stokes wave is also observed to be increased. In order to investigate the energy conversion behavior between the pump pulses and Stokes pulses, with the help of the fast photodetector and oscilloscope, the shapes of the Stokes pulses and pump pulses, are measured in the experiments, respectively. Figs. 6 and 7 show the measured results for different YDFL pump powers. From these figures one can clearly see that the Stokes pulses are much narrower than the pump pulses, and the leading edge of the Stokes pulse is steeper than trailing edge, which is consistent with the theoretical results. When the YDFL pump powers are 186 and 200mW, the pump pulse durations are measured to be about 120 ns, which are almost independent on the YDFL pump power. However, the durations of the Stokes pulses are measured to be increased from 10 to 20 ns. The shapes of the pump pulses in experiments deviate from gaussian distribution, which results that the durations of the Stokes pulses in our experiments are only in agreement with the theoretical results in change trend.
Fig. 4. Simulated first-order Stokes pulse duration and conversion efficiency as function of the pump pulse energy when the duration of the pump pulse is 120 ns.
Fig. 5. Measured output spectra for different YDFL pump powers of (a) 180, (b) 186 and (c) 200mW.
When the YDFL pump power are 186 and 200mW, corresponding to the pump pulse energy are 1.146 and 1.336 lJ, the average output powers of the Stokes pulses are measured to be 0.117 and 0.182 mW, corresponding to the pulse energy of 0.0234 and 0.0364 lJ. If the insertion loss of about 1.2 dB in the pump input port of the HC-PCF gas cell is deducted from the pump pulses,
Fig. 6. Measured output pulse shapes with the YDFL pump power of 186mW. (a) Pump pulse. (b) Stokes pulse.
Please cite this article in press as: X.D. Chen et al., Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser, Opt. Fiber Technol. (2013), http://dx.doi.org/10.1016/j.yofte.2013.06.002
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show that, the Stokes pulse is much narrower than the pump pulse, and both the Stokes pulse duration and conversion efficiency are increased with the pump pulse energy of the same duration pump pulses. The experimental results are in agreement with the theoretical results in change trend. For the 120 ns pump pulses with the repetition rate of 5 kHz, the threshold pump energy and conversion efficiency at the threshold are 1.146 lJ and 2.69%. The compact gas Raman light source has the simple design and excellent stability so that it can find important applications on nonlinear optics, spectroscopy, and trace gas detection, etc. References
Fig. 7. Measured output pulse shapes with the YDFL pump power of 200mW. (a) Pump pulse. (b) Stokes pulse.
the conversion efficiencies increase from 2.69% with the pump pulse energy of 1.146 lJ to 3.58% with the pump pulse energy of 1.336 lJ. It is worthwhile to point out that, during the course of making the HC-PCF gas cell, the splicing between HC-PCF and SM fiber may destroy the periodical structure of HC-PCF, so the splicing loss of the HC-PCF gas cell output port is very high [11]. The output Stokes pulse energy is attenuated badly, which results that there is a discrepancy between the theoretical computations and the experimental results. 5. Conclusions In this paper, we have proposed a compact all-fiber gas Raman light source based on hydrogen-filled HC-PCF gas cell with a SM Qswitched fiber laser as the Raman pump source. Theoretical results
[1] S.K. Freeman, Applications of Laser Raman Spectroscopy, Wiley Interscience, New York, 1974. [2] C. Guntermann, V.S. Gathen, H.F. Dobele, Raman shifting of Nd:YAG laser radiation in methane: an efficient method to generate 3-lm radiation for medical uses, Appl. Opt. 28 (1989) 135–138. [3] Z. Chu, U.N. Singh, T.D. Wilkerson, Multiple Stokes wavelength generation in H2, D2, and CH4 for lidar aerosol measurements, Appl. Opt. 30 (1991) 4350– 4357. [4] F. Benabid, J.C. Knight, G. Antonopoulos, P.St.J. Russell, Stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber, Science 298 (2002) 399–402. [5] F. Benabid, G. Bouwmans, J.C. Knight, P.St.J. Russell, Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen, Phys. Rev. Lett. 93 (2004) 123903-1–123903-4. [6] F. Benabid, G. Antonopoulos, J.C. Knight, P.St.J. Russell, Stokes amplification regimes in quasi-cw pumped hydrogen-filled hollow-core photonic crystal fiber, Phys. Rev. Lett. 95 (2005) 213903-1–213903-4. [7] F. Benabid, F. Couny, J.C. Knight, T.A. Birks, P.St.J. Russell, Compact, stable and efficient all-fibre gas cells using hollow-core photonic crystal fibres, Nature 434 (2005) 488–491. [8] F. Couny, F. Benabid, O. Carraz, Enhanced SRS in H2 filled hollow core photonic crystal fibre by use of fibre Bragg grating, J. Opt. A: Pure Appl. Opt. 9 (2007) 156–159. [9] F. Couny, F. Benabid, P.S. Light, Subwatt threshold cw Raman fiber-gas laser based on H2-filled hollow-core photonic crystal fiber, Phys. Rev. Lett. 99 (2007) 143903-1–143903-4. [10] X.D. Chen, Q.H. Mao, Q. Sun, J.S. Zhao, P. Li, S.J. Feng, An all-fiber gas Raman light source based on a hydrogen-filled hollow-core photonic crystal fiber pumped with a Q-switched fiber laser, Chin. Phys. Lett. 28 (2011) 074201-1– 074201-4. [11] Q. Sun, Q.H. Mao, E.M. Liu, R.Z. Rao, H. Ming, Hollow-core photonic crystal fiber high-pressure gas cell, Proc. SPIE 7004 (2008). 700455-1–4. [12] Y. Wang, Dynamics of stimulated Raman scattering in double-clad fiber pulse amplifiers, IEEE J. Quantum Electron. 41 (2005) 779–788. [13] W.P. Urquhart, P.J. Laybourn, Effective core area for stimulated Raman scattering in single-mode optical fibres, IEE Proc. 132 (1985) 201–204. [14] R.L. Carman, F. Schimizu, C.S. Wang, N. Bloembergen, Theory of Stokes pulse shapes in transient stimulated Raman scattering, Phys. Rev. A 2 (1970) 60–72.
Please cite this article in press as: X.D. Chen et al., Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser, Opt. Fiber Technol. (2013), http://dx.doi.org/10.1016/j.yofte.2013.06.002
Contents lists available at SciVerse ScienceDirect
Optical Fiber Technology www.elsevier.com/locate/yofte
Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser X.D. Chen a,⇑, Q. Sun b, H. Li a, M.H. Zhao a a b
State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, Hefei, Anhui 230037, China Division of Metrology in Optics and Laser, National Institute of Metrology, Beijing 100013, China
a r t i c l e
i n f o
Article history: Received 7 March 2013 Revised 15 May 2013 Available online xxxx Keywords: Stimulated Raman scattering Hollow-core photonic crystal fiber Single-mode Fiber laser Q-switched
a b s t r a c t We report a compact all-fiber gas Raman light source based on hydrogen-filled HC-PCF gas cell with a SM Q-switched fiber laser as the Raman pump source. Theoretical model of the hydrogen SRS based on HC-PCF pumped with Q-switched fiber laser is also analyzed. Simulated results show that the Stokes pulse is much narrower than the pump pulse, and both the Stokes pulse duration and conversion efficiency are increased with the pump pulse energy of the same duration pump pulses. The experimental results are in agreement with the theoretical results in change trend. For the 120 ns pump pulses with the repetition rate of 5 kHz, the threshold pump energy and conversion efficiency at the threshold are 1.146 lJ and 2.69%. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The Raman light source based on the gas stimulated Raman scattering (SRS) effect is very useful in many applications, such as nonlinear optics, spectroscopy and trace gas detection [1–3], because of its different Raman frequency-shift for different gases. The gas Raman cell is the standard component of achieving the gas Raman light source. A long-standing difficulty in gas SRS based on the traditional gas Raman cell is very high threshold pump power, because the power density of the pump beam is difficult to be kept at a high level in the Raman interaction length due to the limit of the optical diffraction effect. To decrease the threshold of the gas SRS, hollow-core photonic crystal fibers (HC-PCFs) filled with high pressure gases have been used as the gas Raman cell. The threshold may be decreased by more than three orders in magnitude compared to that based on the traditional gas Raman cell [4]. Based on the gas-filled HC-PCF cell, the gas SRS of low threshold has been demonstrated by using Q-switched solid-state lasers or continuous wave (CW) high-power double-clad (DC) fiber lasers as the Raman pump sources [5–9]. Taking into account the fact that the all-fiber structure has the characteristics of high stability and much convenience, we have demonstrated an all-fiber gas Raman light source pumped with a
single-mode (SM) Q-switched fiber laser followed by a DC fiber amplifier [10]. However, this pump design becomes good expensive and less convenient for practical applications, and also results in high threshold pump energy required by the gas Raman light source because of the high splicing loss between the SM fiber and DC fiber. Due to the gas Raman cell is sealed by splicing HCPCF to SM fiber [11], the SM Q-switched fiber lasers are expected to be more suitable for producing the Raman Stokes wave. To date, there has been no report on gas Raman light source pumped by such SM Q-switched fiber laser. In this letter, we propose a compact all-fiber gas Raman light source based on hydrogen-filled HC-PCF gas cell with a SM Q-switched fiber laser as the Raman pump source. Theoretical results show that the Stokes pulse is much narrower than the pump pulse, and both the Stokes pulse duration and conversion efficiency are increased with the pump pulse energy of the same duration pump pulses. In the experiment, the proposed gas Raman light source generates the Stokes wave at 1135.7 nm with the pump wave at 1064.7 nm. For the 120 ns pump pulses with a repetition rate of 5 kHz, the threshold pump energy and conversion efficiency at the threshold are 1.146 lJ and 2.69%. The experimental results are in agreement with the theoretical results in change trend. We believe that the proposed gas Raman light source with the simple design, high stability and low threshold will have many important applications.
⇑ Corresponding author. E-mail address: [email protected] (X.D. Chen). 1068-5200/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.yofte.2013.06.002
Please cite this article in press as: X.D. Chen et al., Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser, Opt. Fiber Technol. (2013), http://dx.doi.org/10.1016/j.yofte.2013.06.002
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X.D. Chen et al. / Optical Fiber Technology xxx (2013) xxx–xxx
2. Experimental setup The proposed all-fiber gas Raman light source system is shown schematically in Fig. 1. It consists of a Raman pump source and a gas Raman cell. The Raman pump source is a SM Q-switched Ytterbium-doped fiber laser (YDFL), which is made of a SM fiber pigtailed acousto-optic modulator (AOM), a wavelength division multiplexing (WDM) coupler, a 3.2-m long YDF with absorption 528 dB/m at 976 nm, and two fiber Bragg gratings (FBGs). A SM fiber pigtailed 975 nm laser diode (LD) with maximum output power of 280mW provides the pump power for YDFL. FBG1 and FBG2 used in YDFL have the same center wavelength of 1064.7 nm and bandwidth of 0.2 nm, but different reflectivity of 70% and 99.7%, respectively. The AOM has an insertion loss of 1.5 dB and the extinction radio more than 30 dB in 1064 ± 5 nm. The modulation frequency and rise time of the AOM can be set to any value between 5 and 80 kHz, and 10 and 200 ns, respectively. The gas Raman cell is a SM fiber pigtailed HC-PCF gas cell. A 30 m-long HC-PCF (Crystal Fiber A/S, HC-1060-02) with the mode field radius of 3.25 lm at 1060 nm and the transmission losses of 0.09 and 0.13 dB/m at 1060 and 1135 nm is hydrogenfilled to form the all-fiber high-pressure gas cell. The HC-PCF gas cell with SM fiber pigtails is very stable, which are made with the technique in Ref. [11], and the hydrogen pressure in the HCPCF gas cell is evaluated to be about 8–9 atm. The whole transmission loss of the HC-PCF gas cell, including the linear absorption and the additional insertion loss, is measured to be 9 dB at 1064 nm. The gas Raman light source output is split by a 1064/1135 nm WDM coupler (WDM2) with the isolation of 35 dB, and then analyzed with an optical spectrum analyzer (OSA, Agilent 86140B), and a fast photo-detector (UltraFast 20SM) followed by a 600 MHz oscilloscope (OSC, Agilent MSO 8064A), respectively. 3. Modeling and simulation In this section, we will analyze the hydrogen SRS based on HCPCF pumped with a Q-switched fiber laser theoretically. Because the output power of the SM Q-switched fiber laser is limited, which is difficult to meet the threshold of the high-order Stokes waves, only the first-order Stokes wave is discussed in this model. The following equations describe the nonlinear interactions between the pump pulses and Stokes pulses [12],
ns @P s @Ps g þ ¼ R P s P p as P s c @t @z Aeff np @P p @Pp g ks þ ¼ R Ps Pp ap Pp c @t @z Aeff kp
the effective mode area and Raman gain coefficient associated with the pump and Stokes wave. c is the light velocity. The pump wavelength kp is 1064 nm, and the first-order Stokes wavelength ks is based on a 587 cm1 frequency shift of the rotational Raman level of hydrogen, so the Stokes wavelength is 1135 nm. For HC-1060-02 HC-PCF, the mode field of the first-order Stokes wave equals to that of the pump wave approximately, so the effective mode area Aeff can be written as 2pr 2p [13], where rp is the mode field radius of HC-PCF at 1060 nm. The Raman gain coefficient gR is taken to be 1cm/GW at a pump wavelength of 1064 nm for hydrogen pressure inside the HC-PCF of about 8–9 atm [6], and the refractive indexes np and ns are about 1.0. Given the Gaussian Q-switched pump pulses with the pulse duration of 120 ns, the shapes of the Stokes pulses and pump pulses are simulated, respectively. Figs. 2 and 3 show the simulated results for different pump pulse energy. From these results, we can see that the Stokes pulses are much narrower than the pump pulses. The reason may be only the central part of the pump pulse, which is over the SRS threshold, can convert into the Stokes pulse. Moreover, the leading edge of the Stokes pulse is always steeper than the trailing edge. This is because that the Raman gain is not only determined by the instantaneous power of the pump pulse, but also depends on the accumulated energy as the pump pulse building up [12,14].When the pump pulse duration is 120 ns, the Stokes pulse duration is simulated to be 28.53 and 52.89 ns for the pump pulse energy of 1.14 and 1.33 lJ, respectively. Fig. 4 gives the simulated Stokes pulse duration and conversion efficiency as function of the pump pulse energy when the duration of the pump pulse is 120 ns. It can be clearly seen that both the Stokes pulse duration and conversion efficiency are increased with the pump pulse energy. As the pump pulse energy increases, the central part width of the pump pulse that is over the SRS threshold power may broaden, which also means that more part of the pump pulse can be converted into the Stokes pulse, resulting that both the Stokes pulse duration and conversion efficiency increase with the pump pulse energy. When the pump pulse energy are 1.14 and 1.33 lJ, the Stokes pulse energy are 0.084 and 0.269 lJ, corresponding to the conversion efficiencies are 7.36% and 20.33%, respectively.
4. Experimental results and discussions
ð1Þ ð2Þ
Initially, the modulation frequency and rise time of the AOM used in the Q-switched YDFL are fixed at 5 kHz and 200 ns respectively. In this experiment, the polarization state of the Q-switched
In (1) and (2), Pp is the pump power, Ps is the first-order Stokes power. ki , ni, and ai are the wavelengths, refractive indexes, and transmission losses of the HC-PCF gas cell, respectively. The indexes p and s apply to the pump and Stokes wave. Aeff and gR are
Fig. 1. Configuration of the gas Raman light source. YDF: ytterbium-doped fiber; LD: laser diode; WDM: wavelength division-multiplexing coupler; AOM: acoustooptic modulator; FBG: fiber Bragg grating; HC-PCF: hollow-core photonic crystal fiber.
Fig. 2. Simulated output pulse shapes with the pump pulse energy of 1.14 lJ when the duration of the pump pulse is 120 ns.
Please cite this article in press as: X.D. Chen et al., Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser, Opt. Fiber Technol. (2013), http://dx.doi.org/10.1016/j.yofte.2013.06.002
X.D. Chen et al. / Optical Fiber Technology xxx (2013) xxx–xxx
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Fig. 3. Simulated output pulse shapes with the pump pulse energy of 1.33 lJ when the duration of the pump pulse is 120 ns.
YDFL is natural light. Fig. 5 gives the measured output spectrum for different pump powers of the SM Q-switched YDFL. As shown in the figure, besides the Q-switched pump pulses at 1064.7 nm, a new wave line at 1135.7 nm is emerged abruptly from the noise on the OSA when the YDFL pump power reaches 186mW. The frequency spacing between the new wave line and pump wavelength is 587 cm1, which right equals to the first-order Stokes frequencyshift of the rotational Raman level of hydrogen. According to the threshold measurements methods in Ref. [7], the threshold pump energy is about 1.146 lJ. Continuing to increase the YDFL pump power, the amplitude of the Stokes wave is also observed to be increased. In order to investigate the energy conversion behavior between the pump pulses and Stokes pulses, with the help of the fast photodetector and oscilloscope, the shapes of the Stokes pulses and pump pulses, are measured in the experiments, respectively. Figs. 6 and 7 show the measured results for different YDFL pump powers. From these figures one can clearly see that the Stokes pulses are much narrower than the pump pulses, and the leading edge of the Stokes pulse is steeper than trailing edge, which is consistent with the theoretical results. When the YDFL pump powers are 186 and 200mW, the pump pulse durations are measured to be about 120 ns, which are almost independent on the YDFL pump power. However, the durations of the Stokes pulses are measured to be increased from 10 to 20 ns. The shapes of the pump pulses in experiments deviate from gaussian distribution, which results that the durations of the Stokes pulses in our experiments are only in agreement with the theoretical results in change trend.
Fig. 4. Simulated first-order Stokes pulse duration and conversion efficiency as function of the pump pulse energy when the duration of the pump pulse is 120 ns.
Fig. 5. Measured output spectra for different YDFL pump powers of (a) 180, (b) 186 and (c) 200mW.
When the YDFL pump power are 186 and 200mW, corresponding to the pump pulse energy are 1.146 and 1.336 lJ, the average output powers of the Stokes pulses are measured to be 0.117 and 0.182 mW, corresponding to the pulse energy of 0.0234 and 0.0364 lJ. If the insertion loss of about 1.2 dB in the pump input port of the HC-PCF gas cell is deducted from the pump pulses,
Fig. 6. Measured output pulse shapes with the YDFL pump power of 186mW. (a) Pump pulse. (b) Stokes pulse.
Please cite this article in press as: X.D. Chen et al., Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser, Opt. Fiber Technol. (2013), http://dx.doi.org/10.1016/j.yofte.2013.06.002
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show that, the Stokes pulse is much narrower than the pump pulse, and both the Stokes pulse duration and conversion efficiency are increased with the pump pulse energy of the same duration pump pulses. The experimental results are in agreement with the theoretical results in change trend. For the 120 ns pump pulses with the repetition rate of 5 kHz, the threshold pump energy and conversion efficiency at the threshold are 1.146 lJ and 2.69%. The compact gas Raman light source has the simple design and excellent stability so that it can find important applications on nonlinear optics, spectroscopy, and trace gas detection, etc. References
Fig. 7. Measured output pulse shapes with the YDFL pump power of 200mW. (a) Pump pulse. (b) Stokes pulse.
the conversion efficiencies increase from 2.69% with the pump pulse energy of 1.146 lJ to 3.58% with the pump pulse energy of 1.336 lJ. It is worthwhile to point out that, during the course of making the HC-PCF gas cell, the splicing between HC-PCF and SM fiber may destroy the periodical structure of HC-PCF, so the splicing loss of the HC-PCF gas cell output port is very high [11]. The output Stokes pulse energy is attenuated badly, which results that there is a discrepancy between the theoretical computations and the experimental results. 5. Conclusions In this paper, we have proposed a compact all-fiber gas Raman light source based on hydrogen-filled HC-PCF gas cell with a SM Qswitched fiber laser as the Raman pump source. Theoretical results
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Please cite this article in press as: X.D. Chen et al., Compact all-fiber gas Raman light source based on hydrogen-filled hollow-core photonic crystal fiber pumped with single-mode Q-switched fiber laser, Opt. Fiber Technol. (2013), http://dx.doi.org/10.1016/j.yofte.2013.06.002