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High-peak power multi-wavelength picosecond pulses generated from a BaWO4 Raman-seeded optical parametric amplifier
Optics Communications 282 (2009) 2960–2963
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High-peak power multi-wavelength picosecond pulses generated from a BaWO4 Raman-seeded optical parametric amplifier Shifeng Du a, Yuxian Shi a, Dongxiang Zhang a, Qinan Li a, Baohua Feng a,*, Jing-Yuan Zhang b,*, Jing-Cun Zang c a
Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA c College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, China b
a r t i c l e
i n f o
Article history: Received 22 December 2008 Received in revised form 26 March 2009 Accepted 27 March 2009
PACS: 33.20.Fb 42.65.Yj 42.65.Dr 42.65.–k
a b s t r a c t We report the generation of high-peak power multi-wavelength picosecond laser pulses using optical parametric amplification (OPA) in BBO seeded with pulses generated in a 5-mm length BaWO4 crystal by stimulated Raman scattering of 18-ps laser pulses at 532 nm. The maximum output energy of the amplified first-Stokes component at 559.7 nm was about 1.76 mJ. The corresponding maximum peak power, pulse duration and spectral line width were measured to be 117.3 MW, 15 ps and 18.0 cm1, respectively. The multi-wavelength picosecond laser pulses were in the visible and near infrared ranges. Using this Raman-seeded OPA technique, the beam quality of the stimulated Raman scattering pulses can be improved. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Stimulated Raman scattering Optical parametric amplifier BaWO4 crystal Raman-seeded Picosecond laser pulses
1. Introduction Stimulated Raman scattering (SRS) of pico- and femto-second laser pulses, which can emit stimulated radiation with high-peak power, step-tunable wavelength and narrow spectral line width, has attracted much attention for their wide applications in medicine, biology and ultra-fast laser spectroscopy. In the past, SRS in gases [1], liquids [2] and solids Raman materials [3] has been often used for frequency shifting through the Stokes- and anti-Stokescomponents generated [4]. Generally speaking, SRS in solid-state crystals has a possibility of designing all-solid-state lasers emitting radiation in specified spectral regions [5,6]. Recently, BaWO4 crystal is known to have the characteristics of becoming a promising solidstate Raman crystal due to its advantages of thermal and mechanical properties, narrow line width of the vibrational modes, and especially high concentration of active Raman centers [5,7–14]. Much attention has been paid on Q-switched BaWO4 Raman laser * Corresponding authors. E-mail addresses: [email protected] (B. Feng), jyzhang@georgiasouthern. edu (J.-Y. Zhang). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.03.056
ˇ erny´ and co-workers reported a passively Q-switched output. C Raman laser on BaWO4 crystal with output energy of 1.46 mJ and pulse duration of 3.5 ns for the first-Stokes component at 1180 nm [9]. Chen et al. reported an actively Q-switched Nd:YAG/BaWO4 Raman laser with average output power and pulse duration of 1.56 W and 24 ns at the same wavelength [10]. Basiev et al. reported a 50-ns, 1.34-lm acoustic-optically Q-switched Nd:YAG/ BaWO4 Raman laser with output energy of 6 mJ for the first-Stokes [11]. The maximum peak power of the above-mentioned Raman laser pulses were at the level of kW. In comparison, the nanosecond pump pulse for SRS is suitable for realizing Raman laser out. However, when the pico- and femto-second laser pulses are used as the pump beam for SRS, the conversion efficiency for such short pump pulses would be severely decreased in comparison with by longer pulse excitation due to the transient behavior [15,16]. For the BaWO4 crystal, in the transient regime of SRS with 50-ps pump pulses duration, the Raman gain coefficient is decreased by about 2.5 times compared with that of the steady-state [8]. Therefore, it is a drawback for achieving high energy output of ultra-short pulses. In addition, because the SRS process stars from the quantum noise it is a difficult task to optimize the spatial beam quality and
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S. Du et al. / Optics Communications 282 (2009) 2960–2963
the stability of the output [17]. According to Refs. [8,18], the beam profile and the divergence of the SRS radiation were badly dependent on the intensity of the pump beam, which may represent a limitation in beam shaping of the Raman laser output. There are reports on Stimulated Raman amplification with picosecond excitation, and it was found that it was useful to apply seeding to reduce the pumping threshold and improve the parameter of the Stokes-shifted radiation in the transient regime [16,19–21]. Recently, Vodchits and co-workers have conducted experimental investigation on OPA used as an energy amplifier for amplifying multi-frequency SRS pulses generated in a gas mixture of compressed hydrogen and methane. The maximum output energy of the amplified SRS pulse was 72 lJ [22], and it was also found that the Raman-seeded OPA was more convenient than the multi-pass OPG/OPA systems [23] for generation of pulses with a narrow spectral line width. However, no study has been done in using the OPA system as an energy amplifier in amplifying SRS pulses generated in the solid-state Raman crystal so far. In this paper, we report the generation of high-peak power multi-wavelength picosecond pulses using a BBO-OPA seeded with pulses generated in a 5-mm long BaWO4 crystal by SRS pumped by 18-ps laser pulses at 532 nm. This system, operating at 10 Hz, produces the maximum output energy of as high as 1.76 mJ for the amplified first-Stokes component at 559.7 nm, which corresponds to a peak power as high as 117.3 MW. This system delivers output in the visible and near infrared ranges with narrow spectral line width and good beam quality. The use of Raman-seeded OPA system not only amplifies the energy of SRS components, but also extends the spectral range of the SRS lines to the middle infrared through signal-to-idler down conversion.
2. Experimental setup A schematic of our experimental setup is shown in Fig. 1. A mode-locked Nd:YAG laser (EKSPLA: PL2143B) delivering pulses at 532 nm and 355 nm with a repetition rate of 10 Hz was used as the pumps for Raman and OPA, respectively. The output at 532 nm with 18-ps duration and horizontal polarization was focused by lens L1 (U = 25.4 mm, f = 100 mm) into a 5-mm long BaWO4 crystal (BC) to generate multi-frequency SRS components after passed through a 6-mm diameter pinhole (P1). The SRS pulses were collected and collimated by lens L2 (U = 25.4 mm, f = 50 mm). Only the center part of the collimated Raman pulses beam was used (or chose) by a 5-mm diameter pinhole (P2) and then injected into a type-I phase-matched BBO crystal cut at hc = 28.5o, Uc = 0o, which was used as the energy amplifier for the
SRS pulses. A dichroic mirror M1, which was highly reflective at 532 nm, was used to remove the residual pump beam. The output at 355 nm with 15-ps duration and vertical polarization was used as the pump beam for the BBO-OPA. The diameter of the pump beam was 5 mm. The injected seed beam was collinear with the pump beam by the use of a dichroic mirror M4, which was highly reflective at 355 nm. The temporal overlap of the seed and pump beams for the OPA in the BBO crystal was precisely controlled by an optical delay line. Behind the BBO crystal, the residual pump beam at 355 nm was removed by another dichroic mirror M5. The output of the amplified signal and idler was separated by a prism. 3. Results and discussion By removing the lens L2 and putting a screen behind the BaWO4 crystal, we observed the beam profile and the divergence of the scattered radiation varied with the 532-nm pump beam intensity. At low pump energy, there was only one SRS component and the cone was filled and solid. With increased pump energy, the higher order Stokes and anti-Stokes components show up off-axis. Since the higher order SRS is generated by lower order SRS, some of the energy of the first-Stokes component in the solid cone was used and the dark ‘‘absorption” cones can be seen. In the meantime, the generated cones for higher order SRS components were off-axis because of phase-matching condition and become hollow. The directions of SRS radiation cones are determined by the phasematching condition of the pump beam (K 0 ), the first Stokes (K 1 ), the nth-order anti-stokes (K n ), and nth-stokes ðK n Þ : K 0 þ K n1 ¼ K 1 þ K n and K 0 þ K 1 ¼ K n1 þ K n [24,25]. Hence, these characteristics are the limitation for achieving stable Raman laser spatial beam profile and divergence. Furthermore, more and more SRS Raman components can be generated when the pump energy for SRS increases. With a pump energy of 0.81 mJ to illuminate the BaWO4 crystal, up to nine Stokes and anti-Stokes spectral components, including the forth- (k = 444.8 nm), third- (k = 464.0 nm), second(k = 484.5 nm), first-anti-Stokes (k = 507.2 nm), first- (k = 559.7 nm), second- (k = 590.2 nm), third- (k = 624.3 nm), forth- (k = 662.6 nm) and fifth-Stokes (k = 705.5 nm), were generated. Fig. 2 shows the energy relation between the pump beam for SRS and the generated SRS pulses without OPA amplified. The energy of the generated SRS pulses was measured by an energy probe (J3) connected to a dual channel energy meter (COHERENT model: EPM2000) behind the mirror M1. The maximum output energy of the generated SRS pulses of 46.2 lJ was achieved with pump beam energy of about 1 mJ, which corresponds to energy conversion efficiency of 4.62%. Reference [8] reported the energy conver-
Energy of SRS pulses(µJ)
50 40 30 20 10 0 0.2
0.4
0.6
0.8
1.0
Pump Energy for SRS (mJ) Fig. 1. Scheme of the experimental setup. L1-4, lenses; P1-2, pinholes; M1, dichroic mirror (highly reflective at 532 nm); M2, silver mirror; M3-5, dichroic mirrors (highly reflective at 355 nm); BC, BaWO4 crystal.
Fig. 2. Dependence of the output of SRS on the pump beam energy for SRS at 532 nm.
S. Du et al. / Optics Communications 282 (2009) 2960–2963
(a) Intensity (a. u.)
sion efficiency of the first Raman component at 560 nm can be as high as 85% for direct Raman generation in a 50-mm length BaWO4 crystal by a 50-ps laser pulses at 532 nm with a double-pass configuration. Therefore, the low energy conversion efficiency in our experiment was possibly due to the short length BaWO4 crystal available. In the meantime, the attempt for achieving high Raman energy by using higher pump beam energy was unsuccessful because high pump beam intensity would cause optical breakdown in the BaWO4 Raman medium. It can be seen from Fig. 2 that with the 18-ps 532-nm laser pulses as the pump beam for SRS, the output energy of SRS pulses was rather limited. It is clear that in order to produce multi-wavelength and high-energy output with good beam quality, a highly efficient energy amplifier – OPA is necessary. In our experiment, the pump beam energy for SRS was kept a constant at 0.86 mJ, which was useful for achieving multi-line SRS generation and maintaining a stable spatial beam profile and divergence angle. By collecting and collimating the generated SRS pulses by lens L2 and choosing the center part of the SRS pulses beam by a on-axis 5-mm diameter pinhole (P2) for amplification in the 355-nm pumped BBO-OPA system, high-peak power laser pulses with good beam quality of OPA-amplified Raman were achieved. All SRS components can be amplified in this OPA system individually by rotating the angle of BBO crystal to achieve phasematching. The wavelength and energy for the seeded and amplified SRS components were measured behind the prism, as shown in Table 1. It includes the fourth-, third-, second-, first-anti-Stokes, first-, second-, third-, fourth- and fifth-Stokes, denoted as AS4, AS3, AS2, AS1, S1, S2, S3, S4 and S5, respectively, and the corresponding idler of the 355-nm pumped OPA at 1751, 1506, 1324, 1180, 970.7, 890.8, 823.0, 763.6 and 713.0 nm. The spectral range of the amplified step-tunable Raman laser sources was from 444.8 nm to 1751 nm, which indicates that the use of Raman seeded OPA system not only amplifies the energy of SRS components, but also extends the tuning range of the SRS lines to the middle infrared through signal-to-idler down conversion. This advantage is unique for the Raman-seeded OPA technique in extending the spectral tuning range of short laser pulses. It can be seen from Table 1 that, with pump beam energy of 5.83 mJ at 355 nm, the maximum output energy of the amplified first-Stokes component at 559.7 nm was about 1.76 mJ for the signal and 0.62 mJ for the idler. The corresponding energy conversion efficiency was 30.2% from the pump beam for the OPA to the amplified signal, and the total efficiency would be up to 40.8%, if the output energy of the idler was also included. This was higher than the similar OPG/OPA, in which the seeder was started from quantum noise [26]. In a direct Raman generator based on a 50-mm long BaWO4 crystal and pumped by a 50-ps laser pulses at 532 nm, the maximum energy of 0.23 mJ was achieved for the first Raman component at 560 nm with a double-pass configuration, which yielded
Seed Signal
3000 2000 1000 0 -100
-1
18.0 cm
-1
17.2 cm
-50
0
50
100 -1
Relative wavenumber (cm )
(b) Intensity (a. u.)
2962
3000
Seed Signal
2000
14.8 cm
1000
15.5 cm
0 -100
-1
-1
-50
0
50
100 -1
Relative wavenumber (cm ) Fig. 3. (a) Spectral profile of the 559.7-nm first-Stokes seed pulse (square) and amplified signal pulse (circle); and (b) spectral profile of the 590.2-nm secondStokes seed pulse (square) and amplified signal pulse (circle).
the maximum energy conversion efficiency of 85% [8]. In fact, the energy of OPA-amplified Raman components can be further increased by choosing a longer BaWO4 crystal. The output energy stability of the amplified 559.7-nm first-Stokes in our work was better than 1.8%. The divergence angle of the amplified 559.7-nm first-Stokes was measured to be 0.68o. The spectral line width of the generated laser pulses was measured by use of a 0.5-m spectrometer (SpectraPro-500i). The measured typical spectra of the 559.7-nm first-Stokes seed and amplified signal are shown in Fig. 3a. The full width at half maximum (FWHM) of the first-Stokes seed pulse was 17.2 cm1, while the corresponding spectral width for the amplified signal pulse was 18.0 cm1. They are about the same. As has been pointed out by reference [27] that for longer wavelengths where the acceptance angle of the BBO crystal becomes larger and the spectral line width of OPO/OPA without further spectral selection becomes broad. However, using the Raman-seeded OPA system, the spectral line width of the OPA amplified pulses is mainly determined by the
Table 1 Wavelength and output energy of SRS seed and parametrically amplified signal and idler. SRS components: AS4, AS3, AS2, AS1, S1, S2, S3, S4, and S5, respectively. The pump beam energy at 355 nm was 5.83 mJ. SRS components
Signal branch
Idler branch
Assignment
Energy (lJ)
Wavelength (nm)
Energy (mJ)
Wavelength (nm)
Energy (mJ)
AS4 AS3 AS2 AS1 S1 S2 S3 S4 S5
<0.01 0.02 009 029 2.03 1.35 0.75 0.11 <0.01
444.8 464.0 484.5 507.2 559.7 590.2 624.3 662.6 705.5
0.09 0.38 0.49 0.64 1.76 1.66 1.13 0.73 0.21
1751 1506 1324 1180 970.7 890.8 823.0 763.6 713.0
0.02 0.12 0.13 0.25 0.62 0.61 0.51 0.29 0.15
S. Du et al. / Optics Communications 282 (2009) 2960–2963
3.5
Data Gauss fit
Intensity (a.u.)
3.0 2.5 2.0
2963
from 444.8 nm to 1751 nm, which was significantly extended by means of signal-to-idler down-conversion. In contrast to the SRS amplification, where the high energy and thermal lensing effect remain to be problematic, the beam quality of the OPA amplified SRS pulses has been improved.
15 ps
1.5
Acknowledgements
1.0 0.5 0.0 0
10
20
30
40
50
60
Time (ps) Fig. 4. Autocorrelation traces of the amplified 559.7-nm first-Stokes.
spectral line width of the seed and the spectrum of the amplified pulse at long wavelength remained narrow. This was demonstrated for the 590.2-nm second-Stokes showing in Fig. 3b. The FWHM of the second-Stokes seed and amplified signal were 15.5 cm1 and 14.8 cm1, respectively, which were corresponding narrower than the above values for the first-Stokes. The spectral line width of the amplified 705.5-nm fifth-Stokes signal was also measured to be as narrow as 8.6 cm1. The pulse duration is an important parameter for a short pulses system. The typical intensity autocorrelation traces of the amplified 559.7-nm first-Stokes were measured, as shown in Fig. 4. It can be seen from Fig. 4 that the pulse duration of the amplified SRS pulse was 15 ps (FWHM) assuming a Gaussian-shaped temporal intensity profile. In our experiment, the pulse durations of the amplified second- and third-Stokes signals were also measured to be 14.8 ps and 13.3 ps, respectively, which were a little shorter than the value for the amplified first-Stokes signal. 4. Conclusion In summary, we have demonstrated efficient generation of high-peak power, multi-wavelength picosecond laser pulses by OPA in BBO seeded with SRS-components generated in a 5-mm length BaWO4 crystal, which was pumped with 18-ps laser pulses at 532 nm. With pump energy of 5.83 mJ at 355 nm for OPA, the maximum output energy of 1.76 mJ for the amplified 559.7-nm first-Stokes was achieved. The corresponding pulse duration, spectral line width and maximum peak power were 15 ps, 18.0 cm1 and 117.3 MW, respectively. The output energy fluctuation of the amplified 559.7-nm first-Stokes was better than 1.8%. The spectral range of the OPA-amplified Raman laser sources was step-tunable
This work is supported by the National Natural Science Foundation of China under Grant No. 60438020 and the National Basic Research Program of China (No. 2007CB613205). J.-Y. Zhang thanks the support of Georgia Southern University.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
High-peak power multi-wavelength picosecond pulses generated from a BaWO4 Raman-seeded optical parametric amplifier Shifeng Du a, Yuxian Shi a, Dongxiang Zhang a, Qinan Li a, Baohua Feng a,*, Jing-Yuan Zhang b,*, Jing-Cun Zang c a
Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA c College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, China b
a r t i c l e
i n f o
Article history: Received 22 December 2008 Received in revised form 26 March 2009 Accepted 27 March 2009
PACS: 33.20.Fb 42.65.Yj 42.65.Dr 42.65.–k
a b s t r a c t We report the generation of high-peak power multi-wavelength picosecond laser pulses using optical parametric amplification (OPA) in BBO seeded with pulses generated in a 5-mm length BaWO4 crystal by stimulated Raman scattering of 18-ps laser pulses at 532 nm. The maximum output energy of the amplified first-Stokes component at 559.7 nm was about 1.76 mJ. The corresponding maximum peak power, pulse duration and spectral line width were measured to be 117.3 MW, 15 ps and 18.0 cm1, respectively. The multi-wavelength picosecond laser pulses were in the visible and near infrared ranges. Using this Raman-seeded OPA technique, the beam quality of the stimulated Raman scattering pulses can be improved. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Stimulated Raman scattering Optical parametric amplifier BaWO4 crystal Raman-seeded Picosecond laser pulses
1. Introduction Stimulated Raman scattering (SRS) of pico- and femto-second laser pulses, which can emit stimulated radiation with high-peak power, step-tunable wavelength and narrow spectral line width, has attracted much attention for their wide applications in medicine, biology and ultra-fast laser spectroscopy. In the past, SRS in gases [1], liquids [2] and solids Raman materials [3] has been often used for frequency shifting through the Stokes- and anti-Stokescomponents generated [4]. Generally speaking, SRS in solid-state crystals has a possibility of designing all-solid-state lasers emitting radiation in specified spectral regions [5,6]. Recently, BaWO4 crystal is known to have the characteristics of becoming a promising solidstate Raman crystal due to its advantages of thermal and mechanical properties, narrow line width of the vibrational modes, and especially high concentration of active Raman centers [5,7–14]. Much attention has been paid on Q-switched BaWO4 Raman laser * Corresponding authors. E-mail addresses: [email protected] (B. Feng), jyzhang@georgiasouthern. edu (J.-Y. Zhang). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.03.056
ˇ erny´ and co-workers reported a passively Q-switched output. C Raman laser on BaWO4 crystal with output energy of 1.46 mJ and pulse duration of 3.5 ns for the first-Stokes component at 1180 nm [9]. Chen et al. reported an actively Q-switched Nd:YAG/BaWO4 Raman laser with average output power and pulse duration of 1.56 W and 24 ns at the same wavelength [10]. Basiev et al. reported a 50-ns, 1.34-lm acoustic-optically Q-switched Nd:YAG/ BaWO4 Raman laser with output energy of 6 mJ for the first-Stokes [11]. The maximum peak power of the above-mentioned Raman laser pulses were at the level of kW. In comparison, the nanosecond pump pulse for SRS is suitable for realizing Raman laser out. However, when the pico- and femto-second laser pulses are used as the pump beam for SRS, the conversion efficiency for such short pump pulses would be severely decreased in comparison with by longer pulse excitation due to the transient behavior [15,16]. For the BaWO4 crystal, in the transient regime of SRS with 50-ps pump pulses duration, the Raman gain coefficient is decreased by about 2.5 times compared with that of the steady-state [8]. Therefore, it is a drawback for achieving high energy output of ultra-short pulses. In addition, because the SRS process stars from the quantum noise it is a difficult task to optimize the spatial beam quality and
2961
S. Du et al. / Optics Communications 282 (2009) 2960–2963
the stability of the output [17]. According to Refs. [8,18], the beam profile and the divergence of the SRS radiation were badly dependent on the intensity of the pump beam, which may represent a limitation in beam shaping of the Raman laser output. There are reports on Stimulated Raman amplification with picosecond excitation, and it was found that it was useful to apply seeding to reduce the pumping threshold and improve the parameter of the Stokes-shifted radiation in the transient regime [16,19–21]. Recently, Vodchits and co-workers have conducted experimental investigation on OPA used as an energy amplifier for amplifying multi-frequency SRS pulses generated in a gas mixture of compressed hydrogen and methane. The maximum output energy of the amplified SRS pulse was 72 lJ [22], and it was also found that the Raman-seeded OPA was more convenient than the multi-pass OPG/OPA systems [23] for generation of pulses with a narrow spectral line width. However, no study has been done in using the OPA system as an energy amplifier in amplifying SRS pulses generated in the solid-state Raman crystal so far. In this paper, we report the generation of high-peak power multi-wavelength picosecond pulses using a BBO-OPA seeded with pulses generated in a 5-mm long BaWO4 crystal by SRS pumped by 18-ps laser pulses at 532 nm. This system, operating at 10 Hz, produces the maximum output energy of as high as 1.76 mJ for the amplified first-Stokes component at 559.7 nm, which corresponds to a peak power as high as 117.3 MW. This system delivers output in the visible and near infrared ranges with narrow spectral line width and good beam quality. The use of Raman-seeded OPA system not only amplifies the energy of SRS components, but also extends the spectral range of the SRS lines to the middle infrared through signal-to-idler down conversion.
2. Experimental setup A schematic of our experimental setup is shown in Fig. 1. A mode-locked Nd:YAG laser (EKSPLA: PL2143B) delivering pulses at 532 nm and 355 nm with a repetition rate of 10 Hz was used as the pumps for Raman and OPA, respectively. The output at 532 nm with 18-ps duration and horizontal polarization was focused by lens L1 (U = 25.4 mm, f = 100 mm) into a 5-mm long BaWO4 crystal (BC) to generate multi-frequency SRS components after passed through a 6-mm diameter pinhole (P1). The SRS pulses were collected and collimated by lens L2 (U = 25.4 mm, f = 50 mm). Only the center part of the collimated Raman pulses beam was used (or chose) by a 5-mm diameter pinhole (P2) and then injected into a type-I phase-matched BBO crystal cut at hc = 28.5o, Uc = 0o, which was used as the energy amplifier for the
SRS pulses. A dichroic mirror M1, which was highly reflective at 532 nm, was used to remove the residual pump beam. The output at 355 nm with 15-ps duration and vertical polarization was used as the pump beam for the BBO-OPA. The diameter of the pump beam was 5 mm. The injected seed beam was collinear with the pump beam by the use of a dichroic mirror M4, which was highly reflective at 355 nm. The temporal overlap of the seed and pump beams for the OPA in the BBO crystal was precisely controlled by an optical delay line. Behind the BBO crystal, the residual pump beam at 355 nm was removed by another dichroic mirror M5. The output of the amplified signal and idler was separated by a prism. 3. Results and discussion By removing the lens L2 and putting a screen behind the BaWO4 crystal, we observed the beam profile and the divergence of the scattered radiation varied with the 532-nm pump beam intensity. At low pump energy, there was only one SRS component and the cone was filled and solid. With increased pump energy, the higher order Stokes and anti-Stokes components show up off-axis. Since the higher order SRS is generated by lower order SRS, some of the energy of the first-Stokes component in the solid cone was used and the dark ‘‘absorption” cones can be seen. In the meantime, the generated cones for higher order SRS components were off-axis because of phase-matching condition and become hollow. The directions of SRS radiation cones are determined by the phasematching condition of the pump beam (K 0 ), the first Stokes (K 1 ), the nth-order anti-stokes (K n ), and nth-stokes ðK n Þ : K 0 þ K n1 ¼ K 1 þ K n and K 0 þ K 1 ¼ K n1 þ K n [24,25]. Hence, these characteristics are the limitation for achieving stable Raman laser spatial beam profile and divergence. Furthermore, more and more SRS Raman components can be generated when the pump energy for SRS increases. With a pump energy of 0.81 mJ to illuminate the BaWO4 crystal, up to nine Stokes and anti-Stokes spectral components, including the forth- (k = 444.8 nm), third- (k = 464.0 nm), second(k = 484.5 nm), first-anti-Stokes (k = 507.2 nm), first- (k = 559.7 nm), second- (k = 590.2 nm), third- (k = 624.3 nm), forth- (k = 662.6 nm) and fifth-Stokes (k = 705.5 nm), were generated. Fig. 2 shows the energy relation between the pump beam for SRS and the generated SRS pulses without OPA amplified. The energy of the generated SRS pulses was measured by an energy probe (J3) connected to a dual channel energy meter (COHERENT model: EPM2000) behind the mirror M1. The maximum output energy of the generated SRS pulses of 46.2 lJ was achieved with pump beam energy of about 1 mJ, which corresponds to energy conversion efficiency of 4.62%. Reference [8] reported the energy conver-
Energy of SRS pulses(µJ)
50 40 30 20 10 0 0.2
0.4
0.6
0.8
1.0
Pump Energy for SRS (mJ) Fig. 1. Scheme of the experimental setup. L1-4, lenses; P1-2, pinholes; M1, dichroic mirror (highly reflective at 532 nm); M2, silver mirror; M3-5, dichroic mirrors (highly reflective at 355 nm); BC, BaWO4 crystal.
Fig. 2. Dependence of the output of SRS on the pump beam energy for SRS at 532 nm.
S. Du et al. / Optics Communications 282 (2009) 2960–2963
(a) Intensity (a. u.)
sion efficiency of the first Raman component at 560 nm can be as high as 85% for direct Raman generation in a 50-mm length BaWO4 crystal by a 50-ps laser pulses at 532 nm with a double-pass configuration. Therefore, the low energy conversion efficiency in our experiment was possibly due to the short length BaWO4 crystal available. In the meantime, the attempt for achieving high Raman energy by using higher pump beam energy was unsuccessful because high pump beam intensity would cause optical breakdown in the BaWO4 Raman medium. It can be seen from Fig. 2 that with the 18-ps 532-nm laser pulses as the pump beam for SRS, the output energy of SRS pulses was rather limited. It is clear that in order to produce multi-wavelength and high-energy output with good beam quality, a highly efficient energy amplifier – OPA is necessary. In our experiment, the pump beam energy for SRS was kept a constant at 0.86 mJ, which was useful for achieving multi-line SRS generation and maintaining a stable spatial beam profile and divergence angle. By collecting and collimating the generated SRS pulses by lens L2 and choosing the center part of the SRS pulses beam by a on-axis 5-mm diameter pinhole (P2) for amplification in the 355-nm pumped BBO-OPA system, high-peak power laser pulses with good beam quality of OPA-amplified Raman were achieved. All SRS components can be amplified in this OPA system individually by rotating the angle of BBO crystal to achieve phasematching. The wavelength and energy for the seeded and amplified SRS components were measured behind the prism, as shown in Table 1. It includes the fourth-, third-, second-, first-anti-Stokes, first-, second-, third-, fourth- and fifth-Stokes, denoted as AS4, AS3, AS2, AS1, S1, S2, S3, S4 and S5, respectively, and the corresponding idler of the 355-nm pumped OPA at 1751, 1506, 1324, 1180, 970.7, 890.8, 823.0, 763.6 and 713.0 nm. The spectral range of the amplified step-tunable Raman laser sources was from 444.8 nm to 1751 nm, which indicates that the use of Raman seeded OPA system not only amplifies the energy of SRS components, but also extends the tuning range of the SRS lines to the middle infrared through signal-to-idler down conversion. This advantage is unique for the Raman-seeded OPA technique in extending the spectral tuning range of short laser pulses. It can be seen from Table 1 that, with pump beam energy of 5.83 mJ at 355 nm, the maximum output energy of the amplified first-Stokes component at 559.7 nm was about 1.76 mJ for the signal and 0.62 mJ for the idler. The corresponding energy conversion efficiency was 30.2% from the pump beam for the OPA to the amplified signal, and the total efficiency would be up to 40.8%, if the output energy of the idler was also included. This was higher than the similar OPG/OPA, in which the seeder was started from quantum noise [26]. In a direct Raman generator based on a 50-mm long BaWO4 crystal and pumped by a 50-ps laser pulses at 532 nm, the maximum energy of 0.23 mJ was achieved for the first Raman component at 560 nm with a double-pass configuration, which yielded
Seed Signal
3000 2000 1000 0 -100
-1
18.0 cm
-1
17.2 cm
-50
0
50
100 -1
Relative wavenumber (cm )
(b) Intensity (a. u.)
2962
3000
Seed Signal
2000
14.8 cm
1000
15.5 cm
0 -100
-1
-1
-50
0
50
100 -1
Relative wavenumber (cm ) Fig. 3. (a) Spectral profile of the 559.7-nm first-Stokes seed pulse (square) and amplified signal pulse (circle); and (b) spectral profile of the 590.2-nm secondStokes seed pulse (square) and amplified signal pulse (circle).
the maximum energy conversion efficiency of 85% [8]. In fact, the energy of OPA-amplified Raman components can be further increased by choosing a longer BaWO4 crystal. The output energy stability of the amplified 559.7-nm first-Stokes in our work was better than 1.8%. The divergence angle of the amplified 559.7-nm first-Stokes was measured to be 0.68o. The spectral line width of the generated laser pulses was measured by use of a 0.5-m spectrometer (SpectraPro-500i). The measured typical spectra of the 559.7-nm first-Stokes seed and amplified signal are shown in Fig. 3a. The full width at half maximum (FWHM) of the first-Stokes seed pulse was 17.2 cm1, while the corresponding spectral width for the amplified signal pulse was 18.0 cm1. They are about the same. As has been pointed out by reference [27] that for longer wavelengths where the acceptance angle of the BBO crystal becomes larger and the spectral line width of OPO/OPA without further spectral selection becomes broad. However, using the Raman-seeded OPA system, the spectral line width of the OPA amplified pulses is mainly determined by the
Table 1 Wavelength and output energy of SRS seed and parametrically amplified signal and idler. SRS components: AS4, AS3, AS2, AS1, S1, S2, S3, S4, and S5, respectively. The pump beam energy at 355 nm was 5.83 mJ. SRS components
Signal branch
Idler branch
Assignment
Energy (lJ)
Wavelength (nm)
Energy (mJ)
Wavelength (nm)
Energy (mJ)
AS4 AS3 AS2 AS1 S1 S2 S3 S4 S5
<0.01 0.02 009 029 2.03 1.35 0.75 0.11 <0.01
444.8 464.0 484.5 507.2 559.7 590.2 624.3 662.6 705.5
0.09 0.38 0.49 0.64 1.76 1.66 1.13 0.73 0.21
1751 1506 1324 1180 970.7 890.8 823.0 763.6 713.0
0.02 0.12 0.13 0.25 0.62 0.61 0.51 0.29 0.15
S. Du et al. / Optics Communications 282 (2009) 2960–2963
3.5
Data Gauss fit
Intensity (a.u.)
3.0 2.5 2.0
2963
from 444.8 nm to 1751 nm, which was significantly extended by means of signal-to-idler down-conversion. In contrast to the SRS amplification, where the high energy and thermal lensing effect remain to be problematic, the beam quality of the OPA amplified SRS pulses has been improved.
15 ps
1.5
Acknowledgements
1.0 0.5 0.0 0
10
20
30
40
50
60
Time (ps) Fig. 4. Autocorrelation traces of the amplified 559.7-nm first-Stokes.
spectral line width of the seed and the spectrum of the amplified pulse at long wavelength remained narrow. This was demonstrated for the 590.2-nm second-Stokes showing in Fig. 3b. The FWHM of the second-Stokes seed and amplified signal were 15.5 cm1 and 14.8 cm1, respectively, which were corresponding narrower than the above values for the first-Stokes. The spectral line width of the amplified 705.5-nm fifth-Stokes signal was also measured to be as narrow as 8.6 cm1. The pulse duration is an important parameter for a short pulses system. The typical intensity autocorrelation traces of the amplified 559.7-nm first-Stokes were measured, as shown in Fig. 4. It can be seen from Fig. 4 that the pulse duration of the amplified SRS pulse was 15 ps (FWHM) assuming a Gaussian-shaped temporal intensity profile. In our experiment, the pulse durations of the amplified second- and third-Stokes signals were also measured to be 14.8 ps and 13.3 ps, respectively, which were a little shorter than the value for the amplified first-Stokes signal. 4. Conclusion In summary, we have demonstrated efficient generation of high-peak power, multi-wavelength picosecond laser pulses by OPA in BBO seeded with SRS-components generated in a 5-mm length BaWO4 crystal, which was pumped with 18-ps laser pulses at 532 nm. With pump energy of 5.83 mJ at 355 nm for OPA, the maximum output energy of 1.76 mJ for the amplified 559.7-nm first-Stokes was achieved. The corresponding pulse duration, spectral line width and maximum peak power were 15 ps, 18.0 cm1 and 117.3 MW, respectively. The output energy fluctuation of the amplified 559.7-nm first-Stokes was better than 1.8%. The spectral range of the OPA-amplified Raman laser sources was step-tunable
This work is supported by the National Natural Science Foundation of China under Grant No. 60438020 and the National Basic Research Program of China (No. 2007CB613205). J.-Y. Zhang thanks the support of Georgia Southern University.
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