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Compact 120 TW Ti:sapphire laser system with a high gain final amplifier
ARTICLE IN PRESS
Optics and Lasers in Engineering 44 (2006) 130–137
Compact 120 TW Ti:sapphire laser system with a high gain final amplifier Xiaoyan Liang, Yuxin Leng, Lihuang Lin, Haihe Lu, Wenyao Wang, Yunhua Jiang, Bin Shuai, Hanlin Peng, Baozhen Zhao, Cheng Wang, Wenqi Zhang, Zhengquan Zhang, Ruxin Li, Zhizhan Xu State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 1 October 2004; received in revised form 2 February 2005; accepted 1 March 2005 Available online 13 June 2005
Abstract A 120 TW/36 fs laser system based on Ti:sapphire chirped-pulse amplification (CPA) has been successfully established in our lab. The final four pass Ti:sapphire amplifier pumped by an energetic single-shot Nd:YAG—Nd:glass laser was designed and optimized. With 24 J/8 ns pump energy at 532 nm, 300 mJ/220 ps chirped pulse was amplified to 5.98 J in this amplifier, and a total saturated gain of 20 was achieved. The focused intensity of compressed beam could reach to 1020 W/cm2 with the M 2 of 2.0. r 2005 Elsevier Ltd. All rights reserved. Keywords: CPA; Amplification; Gain; Pulse duration
1. Introduction The very high-energy laser system with ultrashort pulse duration has been widely used in high-field applications such as electron acceleration, self-focusing and Corresponding author. Tel.: +86 21 699 182 56; fax: +86 21 699 18800.
E-mail address: [email protected] (X. Liang). 0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.03.011
ARTICLE IN PRESS X. Liang et al. / Optics and Lasers in Engineering 44 (2006) 130–137
131
harmonic generation, ICF fast ignitor, astrophysics and quantum electrodynamics [1]. Since the mid-1980s, the technique of chirped-pulse amplification (CPA) [2] opened new avenues for the production of these kinds of lasers without optical damage to amplifiers and optical components. Recently, the development of CPA has become more attractive and matured with the development of large aperture ultra-broadband solid-state laser materials. The two gain media, Nd:glass and Ti:sapphire, are mainly employed in this amplification, and TW- and PW-class laser systems have been developed based on Nd:glass [1,3]. Compared with Nd:glass, Ti:sapphire has more attractive characteristics. Firstly, the gain bandwidth of Ti:sapphire is more than ten times than that of Nd:glass , which means that Ti:sapphire can support femtosecond output with at least 6.0 fs pulse duration, but Nd:glass can be worked only on subpicosecond. Secondly, though the saturation fluence (Es) is 1 J/cm2 for Ti:sapphire and 5 J/cm2 for Nd:glass, Ti:sapphire has much more small signal gain coefficient (g0 ) so that it requires lower length of amplifier medium and a tabletop high-energy laser system could be built up. To date, CPA laser system based on Ti:sapphire with peak power of 100 TW with femtosecond pulse duration has been demonstrated in 10 Hz [4,5] and single shot [6,7], and even a PW class CPA system has been built up in Japan [8]. Though Nd:glass cannot be used as an active medium for ultrashort pulse CPA laser system, Nd:glass amplifier technology is broadly used as the pump source of high-energy in Ti:sapphire CPA laser system because of its reliable running and high-energy throughput [6–8]. For the design of TW-class and PW-class CPA laser system, the final amplifier is the main stage to get the high-energy output; however, the parasitic amplification increases with the increased pump intensity and this makes the gain decrease quickly. In addition, the narrowing of pulse bandwidth has also to be considered to maintain the compression of the pulse. Therefore, the optimization of final amplifier with high pump intensity is very important. In this report, we present a 120 TW CPA laser system based on a high gain multi-pass final amplifier with a large aperture Ti:sapphire disk. This amplifier was pumped by a well-established high-energy Nd:YAG—Nd:glass laser, and the saturation gain of 20 was reached in it while the spatial beam quality is about two times of a diffraction-limited Gaussian beam. By our knowledge, this is the highest gain got from a high-intensity pumped final amplifier. After compression, the output energy achieved was 4.33 J with a pulse duration of 36 fs.
2. Laser system design Our 120 TW laser system is a typical CPA configuration based on Ti:sapphire crystal, which is upgraded by a added optimized Ti:sapphire amplifier to our previously presented 23 TW CPA system [9]. It consists of an oscillator, a pulse stretcher, a regenerative amplifier, three stages of multi-pass amplifiers and a pulse compressor in a vacuum chamber. The scheme set up is shown in Fig. 1.
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SML Ti:sapphire Oscllator FR
Ti:S
Compressor
OC
Verdi V-5
G
5W
Output Stretcher G
Reg.Amplifier
PC
FR
VC
G
Ti:S
Pre-Amplifier / 10Hz
Power Amplifier / 10Hz
Final Amplifier /single-shot
Ti:S Ti:S PC
Ti:S
BE BE
BE
Fig. 1. Scheme of 120 TW/36 fs Ti:sapphire CPA laser setup.
Seed pulses were derived from a Ti:sapphire self-mode locked oscillator pumped by a 5 W frequency-doubled cw diode-pumped Nd:YVO4 laser (Verdi, Coherent Inc.). The oscillator produces a 82 MHz 5 nJ train of pulses with 24 fs duration, and operates on 790 nm center wavelength with 42.3 nm FWHM bandwidth. According to Ref. [7], the extended pulse duration of 4200 ps is necessary for terawatt CPA system. After passing through a Faraday rotator, the pulse train from the oscillator was stretched by a typical all-reflective Offner-triplet stretcher [10]. The dispersion introduced by this stretcher could be fully compensated by a diffraction grating based compressor, which normally could reach the efficiency of more than 70%. Our stretcher consists of a single 1200-grooves/mm gold-coated ruled grating, one concave spherical mirror and one convex cylindrical mirror. In our design, the large size of optical elements and careful alignment could minimize the spectral clipping and spatial chirp in it. This is very helpful for getting the shorter pulse duration after the compression. The stretched pulses were then amplified in a regenerative amplifier, which was pumped by 50 mJ pulses of 532-nm radiation at 10 Hz. This regenerative amplifier provided a net gain of approximately 107 for 12 cavity round trips, which led to 2 mJ output with 220 ps pulse duration (FWHM). In order to minimize the prepulse, two good-quality crossed-glan polerizers and one pockels cell (PC) were used after the regenerative amplifier as a single-pulse selector. Then the beam was up-collimated through a Galilean telescope to nearly 6 mm and sent into a 4-pass pre-amplifier. In this amplifier, a 10 10 15 mm3 Ti:sapphire crystal was pumped from both sides by 400 mJ of energy at 532 nm, and 70–80 mJ of energy was got with good beam quality (M 2 1:1).
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In the power amplifier, the beam size was expanded to 16 mm. The Ti:sapphire disk has a size of 25 25 20 mm3 and was pumped from both sides by two Nd:YAG lasers producing 2.0 J at 532 nm. With the increase of pump energy, the output energy and the beam quality were affected seriously by the effect of thermal lens in Ti:sapphire. Though the amplifier could routinely deliver the energy of 700 mJ, we make it work at the level of 500 mJ in order to prevent the effect of thermal lens. The Ti:sapphire disk used in the final amplifier has a size of 54 40 23 mm3 with antireflection coatings on both surfaces (R&D Center for Laser & Opto-Electronic Materials, SIOM, China). Its absorption at 532 nm is 1.3 cm1 and FOM value is 255. The chirped pulse from the stage of power amplifier was up-collimated to nearly 32 mm diameter through a Galilean telescope. By using a soft aperture, the central beam part of 24 mm diameter with better uniformity was sent to the final amplifier. The energy contained in this area was 300 mJ, so the fluence insert in the amplifier was in a lower level, 66.3 mJ/cm2. Under these conditions, the theoretical calculations were made based on the mode of FrantzNodvik [11] to simulate the gain and output energy for every pass of the final amplifier. qIðz; tÞ ¼ sðoÞNðz; tÞIðz; tÞ, qz
(1)
qNðz; tÞ 2 ¼ sðoÞNðz; tÞIðz; tÞ, qt _o
(2)
where sðoÞ is the emission cross section, Iðz; tÞ is the pulse intensity and Nðz; tÞ is the population inversion, 2* is a dimensionless population saturation factor. Because the exited-state lifetime is much longer than the laser pulse duration, for first pass, the material is treated as a four-level system. In next passes, it is treated as a three-level system. The population inversion of medium and the input intensity for the next pass p þ 1 are the same as the output of last pass [12]. ðpþ1Þ I in ðtÞ ¼ I ðpÞ out ðtÞ,
(3)
ðpþ1Þ ð0Þ ¼ N ðpÞ N tot tot ðtÞ,
(4)
Based on our calculations and pump energy, a four-pass scheme of amplifier could achieve the maximum output. The pump source was a Nd:YAG–Nd:glass laser system, as shown in Fig. 2. It consists of an Nd:YAG oscillator/amplifier, a Farady isolator, a spatial filter and two Nd:glass amplifiers. The 10-ns pulses from a Q-switched flash lamp pumped Nd:YAG oscillator/amplifier could generate 2 J energy per pulse with 1 Hz repetition rate. After up-collimated, it was sent to a spatial filter to improve the beam uniformity. Then the pulse was amplified through a two-pass 35 mm diameter Nd:glass amplifier and a single pass 40 mm diameter Nd:glass amplifier to deliver the output energy of 50 J at the wavelength of 1064 nm. IR output from the amplifiers was frequency doubled to 532 nm by the use of a 60 mm diameter 35 mm long KDP
ARTICLE IN PRESS 134
X. Liang et al. / Optics and Lasers in Engineering 44 (2006) 130–137 Nd:YAG oscillator
Nd:YAG amplifiers
φ 35 Nd:glass amplifier
λ/2
24J@532nm
λ/4
Faraday Isolator
Spatial Filter
φ 40 Nd:glass amplifier
KDP
Fig. 2. Scheme of Nd:YAG–Nd:glass oscillator/amplifier laser as the pump of final amplifier.
crystal. The type II phase matching was chosen for high SHG conversion, and the conversion efficiency of the IR to the harmonic reached to nearly 50%, which yields an output pulse energy of 24J per pulse at 532 nm. The laser can operate with 3 shots per hour. The frequency-doubled energy was beam-split and image relayed to 24 mm diameter at both the faces of the Ti:sapphire amplifier crystal, so the pump intensity in crystal reached 5.3 J/cm2. Since the pump intensity in final amplifier was in high level, in order to keep high gain the parasitic amplification has to be considered. From the previous report [7], we know that the threshold of parasitic lasing depends on the aspect ratio, which is defined as the ratio of the diameter of the pumped region to the crystal thickness. In our case, it is 1.0, and the threshold of parasitic amplification keep a small value. On the other hand, the design of our final amplifier layout also takes into consideration the timing of the pump arrival to the arrival of the chirped short pulse. Prior to solving the parasitic lasing problems, small amounts of timing jitter resulted in large gain variations in these final amplifiers. Our experiment confirms that, with the increase of time jitter, the parasitic amplification increases quickly. So in the operation of our system, it is optimized and kept less than 5 ns. By these optimizations the final 4-pass amplifier delivered the maximum energy of up to 5.98 J output with 24 J pump energy, which means that a total saturation gain of 20 was reached. The output characteristic via different pump energy was measured and compared with the theoretical calculation, as shown in Fig. 3. The experimental results agree well with our theoretical simulation. From the calculated curve we find that the amplifier was operated in the saturation region, and the output energy fluctuation was not so sensitive to that of pump laser. The narrowing and red-shifting of the spectrum often take place in successive steps of amplification because of finite gain bandwidth of the amplifier material. In our case, the spectrum from our oscillator and the final amplifier were also measured with a fiber spectrum meter (SD2000, Ocean Optic Inc.), as shown in Fig. 4. The FWHM bandwidth of the output pulse was narrowed from 42.3 to 37 nm with center wavelength red shifting from 790 to 794 nm.
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7
Experiment calculated
6
Output Energy (J)
5
4
3
2
1
0 5.0
7.5
10.0
12.5
15.0
17.5
20.0
22.5
25.0
27.5
Pump Energy (J) Fig. 3. Output characteristics of final amplifier. The curve is the calculated output energy versus the input pump energy and the squares are the measured value.
After the amplifier, the output beam was up-collimated and sent into the compressor, which generally consists of two grating and a roof mirror. The transmission of the compressor was 72.4%, yielding a compressed output pulse energy of 4.33 J with pulse duration of 36 fs, which implies a peak power for the pulse of 120 TW. The compressed pulse was measured with single-shot autocorrelator (SAA. Inc.), the recorded trace was shown in Fig. 5. The output laser fluence in final amplifier has reached 1.32 J/cm2, this is above the saturation fluence of Ti:sapphire. Therefore, the maximum energy extracted from the amplifier is limited by the saturation fluence of Ti:sapphire, the output is difficult to be improved any more in our current case. After expanding the diameter of beam size and improving the uniformity of the pump beam by using hexagonal matrix mirrors, we believe that our system could get more efficiency output. For the application of a CPA laser system, the approaching focused laser intensity is crucial. In our system, the spatial beam quality is often more crucial and was measured with Shack-Hartmann-type wavefront sensor in different positions before compressed. By analyzing of the measured results, the value of M 2 is 2.2. With an f/3 off-axis parabolic mirror, focused intensities should reach to 1020 W/cm2 with this beam quality.
3. Conclusion We have developed a high peak power Ti:sapphire laser system producing 120 TW short pulse with 36 fs pulse duration. This system could provide an ultrahigh
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oscillator (FWHM 42.3nm) amplifier (FWHM 37nm)
1.0
Intensity (a.u.)
0.8
0.6
0.4
0.2
0.0 720
740
760
780
800
820
840
860
880
wavelength (nm) Fig. 4. Measured spectrum of oscillator and after the final amplifier.
1.0
FWHM 36fs Intensity (a.u.)
0.8
0.6
0.4
0.2
0.0 -100
-50
0
50
100
Time (fs) Fig. 5. Measured autocorrelation trace of the compressed pulse.
intensity (1020 W/cm2) pump source for high-field physics interactions. By the optimization of the final 4 pass amplifier based on a large aperture Ti:sapphire disk, the parasitic amplification was effectively suppressed and the amplifier reached a total saturation gain of 20 with output of 5.98 J when the pump intensity reached 5.3 J/cm2. This is the highest gain got in a multi-pass final amplifier. After compression, the output energy of the laser was 4.33 J with 36 fs pulse duration. These results make us believe that a high saturation gain could be achieved in a high intensity pumped amplifier by the optimization of suppressed parasitic lasing. It
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means that the number of amplifier stages could be decreased using the higher gain single amplifier. It will then reduce the complexity and be more economic in building high power CPA laser system. References [1] Mourou GA, Party CPJ, Perry MD. Phys Today 1998;22. [2] Strichland D, Mourou G. Opt Commun 1985;56:219. [3] Perry MD, Pennington D, Stuart BC, Tietbohl G, Britten JA, Brown C, Harman S, Golick B, Kartz M, Miller J, Powell HT, Vergino M, Yanovsky V. Opt Lett 1999;24:160. [4] Yamakawa K, Aoyama M, Matsuoka S, Kase T, Akahane Y, Takuma H. Opt Lett 1998;23:1468. [5] Pittman M, Ferre S, Rousseau JP, Notebaert L, Chambaret JP, Cheriaux G. Appl Phy B 2002;74:529. [6] Bonlie JD, Patterson F, Price D, White B, Springer P. Appl Phy B 2000;70:S155. [7] Kalachnikov MP, Karpov V, Schonnagel H, Sandner W. Laser Phys 2002;12:368. [8] Aoyama M, Yamakawa K, Akahane Y, Ma J, Inoue N, Ueda H, Kiriyama H. Opt Lett 2003;28:1594. [9] Lin L, Xu Z, Li R, Wang W, Jiang Y, Yang X, Leng Y, Zhang Z, Wang Y, Zhang W. Chin J Laser 2004;31:134. [10] Cheriaux G, Rousseau P, Salin F, Chambaret JP, Walker B, Dimauro LF. Opt Lett 1996;24:414. [11] Siegman AE. Laser. Mill Valley, CA: Univ. Sci. Books; 1986. [12] Yamakawa K, Barty PJ. IEEE J Sel Topics Quantun Electron Ultrafast Opt. 2000;6:658.
Optics and Lasers in Engineering 44 (2006) 130–137
Compact 120 TW Ti:sapphire laser system with a high gain final amplifier Xiaoyan Liang, Yuxin Leng, Lihuang Lin, Haihe Lu, Wenyao Wang, Yunhua Jiang, Bin Shuai, Hanlin Peng, Baozhen Zhao, Cheng Wang, Wenqi Zhang, Zhengquan Zhang, Ruxin Li, Zhizhan Xu State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Received 1 October 2004; received in revised form 2 February 2005; accepted 1 March 2005 Available online 13 June 2005
Abstract A 120 TW/36 fs laser system based on Ti:sapphire chirped-pulse amplification (CPA) has been successfully established in our lab. The final four pass Ti:sapphire amplifier pumped by an energetic single-shot Nd:YAG—Nd:glass laser was designed and optimized. With 24 J/8 ns pump energy at 532 nm, 300 mJ/220 ps chirped pulse was amplified to 5.98 J in this amplifier, and a total saturated gain of 20 was achieved. The focused intensity of compressed beam could reach to 1020 W/cm2 with the M 2 of 2.0. r 2005 Elsevier Ltd. All rights reserved. Keywords: CPA; Amplification; Gain; Pulse duration
1. Introduction The very high-energy laser system with ultrashort pulse duration has been widely used in high-field applications such as electron acceleration, self-focusing and Corresponding author. Tel.: +86 21 699 182 56; fax: +86 21 699 18800.
E-mail address: [email protected] (X. Liang). 0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.03.011
ARTICLE IN PRESS X. Liang et al. / Optics and Lasers in Engineering 44 (2006) 130–137
131
harmonic generation, ICF fast ignitor, astrophysics and quantum electrodynamics [1]. Since the mid-1980s, the technique of chirped-pulse amplification (CPA) [2] opened new avenues for the production of these kinds of lasers without optical damage to amplifiers and optical components. Recently, the development of CPA has become more attractive and matured with the development of large aperture ultra-broadband solid-state laser materials. The two gain media, Nd:glass and Ti:sapphire, are mainly employed in this amplification, and TW- and PW-class laser systems have been developed based on Nd:glass [1,3]. Compared with Nd:glass, Ti:sapphire has more attractive characteristics. Firstly, the gain bandwidth of Ti:sapphire is more than ten times than that of Nd:glass , which means that Ti:sapphire can support femtosecond output with at least 6.0 fs pulse duration, but Nd:glass can be worked only on subpicosecond. Secondly, though the saturation fluence (Es) is 1 J/cm2 for Ti:sapphire and 5 J/cm2 for Nd:glass, Ti:sapphire has much more small signal gain coefficient (g0 ) so that it requires lower length of amplifier medium and a tabletop high-energy laser system could be built up. To date, CPA laser system based on Ti:sapphire with peak power of 100 TW with femtosecond pulse duration has been demonstrated in 10 Hz [4,5] and single shot [6,7], and even a PW class CPA system has been built up in Japan [8]. Though Nd:glass cannot be used as an active medium for ultrashort pulse CPA laser system, Nd:glass amplifier technology is broadly used as the pump source of high-energy in Ti:sapphire CPA laser system because of its reliable running and high-energy throughput [6–8]. For the design of TW-class and PW-class CPA laser system, the final amplifier is the main stage to get the high-energy output; however, the parasitic amplification increases with the increased pump intensity and this makes the gain decrease quickly. In addition, the narrowing of pulse bandwidth has also to be considered to maintain the compression of the pulse. Therefore, the optimization of final amplifier with high pump intensity is very important. In this report, we present a 120 TW CPA laser system based on a high gain multi-pass final amplifier with a large aperture Ti:sapphire disk. This amplifier was pumped by a well-established high-energy Nd:YAG—Nd:glass laser, and the saturation gain of 20 was reached in it while the spatial beam quality is about two times of a diffraction-limited Gaussian beam. By our knowledge, this is the highest gain got from a high-intensity pumped final amplifier. After compression, the output energy achieved was 4.33 J with a pulse duration of 36 fs.
2. Laser system design Our 120 TW laser system is a typical CPA configuration based on Ti:sapphire crystal, which is upgraded by a added optimized Ti:sapphire amplifier to our previously presented 23 TW CPA system [9]. It consists of an oscillator, a pulse stretcher, a regenerative amplifier, three stages of multi-pass amplifiers and a pulse compressor in a vacuum chamber. The scheme set up is shown in Fig. 1.
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132
SML Ti:sapphire Oscllator FR
Ti:S
Compressor
OC
Verdi V-5
G
5W
Output Stretcher G
Reg.Amplifier
PC
FR
VC
G
Ti:S
Pre-Amplifier / 10Hz
Power Amplifier / 10Hz
Final Amplifier /single-shot
Ti:S Ti:S PC
Ti:S
BE BE
BE
Fig. 1. Scheme of 120 TW/36 fs Ti:sapphire CPA laser setup.
Seed pulses were derived from a Ti:sapphire self-mode locked oscillator pumped by a 5 W frequency-doubled cw diode-pumped Nd:YVO4 laser (Verdi, Coherent Inc.). The oscillator produces a 82 MHz 5 nJ train of pulses with 24 fs duration, and operates on 790 nm center wavelength with 42.3 nm FWHM bandwidth. According to Ref. [7], the extended pulse duration of 4200 ps is necessary for terawatt CPA system. After passing through a Faraday rotator, the pulse train from the oscillator was stretched by a typical all-reflective Offner-triplet stretcher [10]. The dispersion introduced by this stretcher could be fully compensated by a diffraction grating based compressor, which normally could reach the efficiency of more than 70%. Our stretcher consists of a single 1200-grooves/mm gold-coated ruled grating, one concave spherical mirror and one convex cylindrical mirror. In our design, the large size of optical elements and careful alignment could minimize the spectral clipping and spatial chirp in it. This is very helpful for getting the shorter pulse duration after the compression. The stretched pulses were then amplified in a regenerative amplifier, which was pumped by 50 mJ pulses of 532-nm radiation at 10 Hz. This regenerative amplifier provided a net gain of approximately 107 for 12 cavity round trips, which led to 2 mJ output with 220 ps pulse duration (FWHM). In order to minimize the prepulse, two good-quality crossed-glan polerizers and one pockels cell (PC) were used after the regenerative amplifier as a single-pulse selector. Then the beam was up-collimated through a Galilean telescope to nearly 6 mm and sent into a 4-pass pre-amplifier. In this amplifier, a 10 10 15 mm3 Ti:sapphire crystal was pumped from both sides by 400 mJ of energy at 532 nm, and 70–80 mJ of energy was got with good beam quality (M 2 1:1).
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In the power amplifier, the beam size was expanded to 16 mm. The Ti:sapphire disk has a size of 25 25 20 mm3 and was pumped from both sides by two Nd:YAG lasers producing 2.0 J at 532 nm. With the increase of pump energy, the output energy and the beam quality were affected seriously by the effect of thermal lens in Ti:sapphire. Though the amplifier could routinely deliver the energy of 700 mJ, we make it work at the level of 500 mJ in order to prevent the effect of thermal lens. The Ti:sapphire disk used in the final amplifier has a size of 54 40 23 mm3 with antireflection coatings on both surfaces (R&D Center for Laser & Opto-Electronic Materials, SIOM, China). Its absorption at 532 nm is 1.3 cm1 and FOM value is 255. The chirped pulse from the stage of power amplifier was up-collimated to nearly 32 mm diameter through a Galilean telescope. By using a soft aperture, the central beam part of 24 mm diameter with better uniformity was sent to the final amplifier. The energy contained in this area was 300 mJ, so the fluence insert in the amplifier was in a lower level, 66.3 mJ/cm2. Under these conditions, the theoretical calculations were made based on the mode of FrantzNodvik [11] to simulate the gain and output energy for every pass of the final amplifier. qIðz; tÞ ¼ sðoÞNðz; tÞIðz; tÞ, qz
(1)
qNðz; tÞ 2 ¼ sðoÞNðz; tÞIðz; tÞ, qt _o
(2)
where sðoÞ is the emission cross section, Iðz; tÞ is the pulse intensity and Nðz; tÞ is the population inversion, 2* is a dimensionless population saturation factor. Because the exited-state lifetime is much longer than the laser pulse duration, for first pass, the material is treated as a four-level system. In next passes, it is treated as a three-level system. The population inversion of medium and the input intensity for the next pass p þ 1 are the same as the output of last pass [12]. ðpþ1Þ I in ðtÞ ¼ I ðpÞ out ðtÞ,
(3)
ðpþ1Þ ð0Þ ¼ N ðpÞ N tot tot ðtÞ,
(4)
Based on our calculations and pump energy, a four-pass scheme of amplifier could achieve the maximum output. The pump source was a Nd:YAG–Nd:glass laser system, as shown in Fig. 2. It consists of an Nd:YAG oscillator/amplifier, a Farady isolator, a spatial filter and two Nd:glass amplifiers. The 10-ns pulses from a Q-switched flash lamp pumped Nd:YAG oscillator/amplifier could generate 2 J energy per pulse with 1 Hz repetition rate. After up-collimated, it was sent to a spatial filter to improve the beam uniformity. Then the pulse was amplified through a two-pass 35 mm diameter Nd:glass amplifier and a single pass 40 mm diameter Nd:glass amplifier to deliver the output energy of 50 J at the wavelength of 1064 nm. IR output from the amplifiers was frequency doubled to 532 nm by the use of a 60 mm diameter 35 mm long KDP
ARTICLE IN PRESS 134
X. Liang et al. / Optics and Lasers in Engineering 44 (2006) 130–137 Nd:YAG oscillator
Nd:YAG amplifiers
φ 35 Nd:glass amplifier
λ/2
24J@532nm
λ/4
Faraday Isolator
Spatial Filter
φ 40 Nd:glass amplifier
KDP
Fig. 2. Scheme of Nd:YAG–Nd:glass oscillator/amplifier laser as the pump of final amplifier.
crystal. The type II phase matching was chosen for high SHG conversion, and the conversion efficiency of the IR to the harmonic reached to nearly 50%, which yields an output pulse energy of 24J per pulse at 532 nm. The laser can operate with 3 shots per hour. The frequency-doubled energy was beam-split and image relayed to 24 mm diameter at both the faces of the Ti:sapphire amplifier crystal, so the pump intensity in crystal reached 5.3 J/cm2. Since the pump intensity in final amplifier was in high level, in order to keep high gain the parasitic amplification has to be considered. From the previous report [7], we know that the threshold of parasitic lasing depends on the aspect ratio, which is defined as the ratio of the diameter of the pumped region to the crystal thickness. In our case, it is 1.0, and the threshold of parasitic amplification keep a small value. On the other hand, the design of our final amplifier layout also takes into consideration the timing of the pump arrival to the arrival of the chirped short pulse. Prior to solving the parasitic lasing problems, small amounts of timing jitter resulted in large gain variations in these final amplifiers. Our experiment confirms that, with the increase of time jitter, the parasitic amplification increases quickly. So in the operation of our system, it is optimized and kept less than 5 ns. By these optimizations the final 4-pass amplifier delivered the maximum energy of up to 5.98 J output with 24 J pump energy, which means that a total saturation gain of 20 was reached. The output characteristic via different pump energy was measured and compared with the theoretical calculation, as shown in Fig. 3. The experimental results agree well with our theoretical simulation. From the calculated curve we find that the amplifier was operated in the saturation region, and the output energy fluctuation was not so sensitive to that of pump laser. The narrowing and red-shifting of the spectrum often take place in successive steps of amplification because of finite gain bandwidth of the amplifier material. In our case, the spectrum from our oscillator and the final amplifier were also measured with a fiber spectrum meter (SD2000, Ocean Optic Inc.), as shown in Fig. 4. The FWHM bandwidth of the output pulse was narrowed from 42.3 to 37 nm with center wavelength red shifting from 790 to 794 nm.
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7
Experiment calculated
6
Output Energy (J)
5
4
3
2
1
0 5.0
7.5
10.0
12.5
15.0
17.5
20.0
22.5
25.0
27.5
Pump Energy (J) Fig. 3. Output characteristics of final amplifier. The curve is the calculated output energy versus the input pump energy and the squares are the measured value.
After the amplifier, the output beam was up-collimated and sent into the compressor, which generally consists of two grating and a roof mirror. The transmission of the compressor was 72.4%, yielding a compressed output pulse energy of 4.33 J with pulse duration of 36 fs, which implies a peak power for the pulse of 120 TW. The compressed pulse was measured with single-shot autocorrelator (SAA. Inc.), the recorded trace was shown in Fig. 5. The output laser fluence in final amplifier has reached 1.32 J/cm2, this is above the saturation fluence of Ti:sapphire. Therefore, the maximum energy extracted from the amplifier is limited by the saturation fluence of Ti:sapphire, the output is difficult to be improved any more in our current case. After expanding the diameter of beam size and improving the uniformity of the pump beam by using hexagonal matrix mirrors, we believe that our system could get more efficiency output. For the application of a CPA laser system, the approaching focused laser intensity is crucial. In our system, the spatial beam quality is often more crucial and was measured with Shack-Hartmann-type wavefront sensor in different positions before compressed. By analyzing of the measured results, the value of M 2 is 2.2. With an f/3 off-axis parabolic mirror, focused intensities should reach to 1020 W/cm2 with this beam quality.
3. Conclusion We have developed a high peak power Ti:sapphire laser system producing 120 TW short pulse with 36 fs pulse duration. This system could provide an ultrahigh
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oscillator (FWHM 42.3nm) amplifier (FWHM 37nm)
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wavelength (nm) Fig. 4. Measured spectrum of oscillator and after the final amplifier.
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Time (fs) Fig. 5. Measured autocorrelation trace of the compressed pulse.
intensity (1020 W/cm2) pump source for high-field physics interactions. By the optimization of the final 4 pass amplifier based on a large aperture Ti:sapphire disk, the parasitic amplification was effectively suppressed and the amplifier reached a total saturation gain of 20 with output of 5.98 J when the pump intensity reached 5.3 J/cm2. This is the highest gain got in a multi-pass final amplifier. After compression, the output energy of the laser was 4.33 J with 36 fs pulse duration. These results make us believe that a high saturation gain could be achieved in a high intensity pumped amplifier by the optimization of suppressed parasitic lasing. It
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