- Email: [email protected]
1 Hz, 1 ps, terawatt Nd: glass laser
1 November 1994
OPTICS COMMUNICATIONS EUEVIER
Optics Communications 112 ( 1994) 37-42
1 Hz, 1 ps, terawatt Nd: glass laser K.Yamakawa
L*, H. Sugio, H. Daido, M. Nakatsuka,
Y. Kate, S. Nakai
Institute ofLaser Engineering, Osaka University, 2-6 Yamada-oka, Osaka 565, Japan
Received 24 May 1994
Abstract Chirped-pulse amplification of picosecond pulses is demonstrated in a compact Nd:glass laser system. A Nd: phosphate glass slab laser is used to produce terawatt pulses as a power amplifier. The detailed characterization of the zigzag slab amplifier is described. The laser system produces > 1 J, 1.O ps pulses at a 1 Hz repetition rate.
1. Introduction
The interaction of short laser pulses ( < 1 ps) with matter has been becoming an attractive method of producing ultrashort X-ray pulses [ 11. These X-ray pulses could be used for various applications such as time resolved X-ray spectroscopy, X-ray microscopy, lithography and X-ray laser pumping [ 2 1. For many of these applications, high-energy X-ray pulses are desired. Recently, X-ray energy conversion effrcienties as high as 20% have been demonstrated in femtosecond laser-produced plasmas [ 3 1. The application of chirped-pulse amplification (CPA) has made it possible to produce high peak power, ultrashort laser pulses [ 4 1. This technique has been applied to a variety of solid-state materials, including Nd: glass [5,6], alexandrite [7], Ti:sapphire [8,9], and Cr : LiSrA1F6 [ lo]. The advantages of using Ti : sapphire or Cr : LiSrAIFs as the oscillator and amplifier include a large gain-bandwidth, making it possible to both produce and amplify pulses of less l
Corresponding author.
’ Present address: Advanced Science Research Center, Japan Atomic Energy Research Institute, Naka, Ibaraki 3 11-O1, Japan. Email: [email protected].
than 100 fs duration up to a few hundred milijoules, at a high repetition rate ( N 10 Hz). However they cannot match the glass laser systems in energy per pulse. Neodymium-doped glasses as an amplifier are superior over crystalline hosts owing to their large energy storage, low nonlinear refractive index, and the commercial availability of large volumes with high optical quality. Picosecond or subpicosecond lasers with the highest energy per pulse have only been based on Nd:glass [ 6,111. However, the limitation with flash lamp pumped Nd : glass amplifiers is a low repetition rate due to the poor thermal characteristics of laser glasses. The pulse repetition rate of the joule class Nd: glass laser systems is normally one shot every 10 minutes, which is not convenient for medical or biological applications. The lower thermal conductivity of glass could be overcome by the concept of slab geometry [ 12,131. Many laboratories have developed neodymium-doped solid-state slab lasers as high-average power sources. Recently, a flash lamp pumped Nd : glass slab amplifier system produced an average power exceeding 100 W [ 141. The moving slab Nd: glass laser has been used to produce 0.5 GW, 11 ps injected pulses from a cw mode-locked Nd:glass laser without CPA technique [ 15 1. However, there has been no report of a Nd : glass slab amplifier used
0030-4018/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SsDIOO30-4018112 (1994) 000-000
K. Yamaknwa et al. / Optics Communications 112 (1994) 3 7-42
38
Nd:YLF oscillator Single-mode fiber 7-mm amolifier 7-mm amplifier
Regenerative amplifier
I
Beam expander
Coupling
optics
Slab amplifier
+
1J 1P-s 1 Hz
Fig. 1. Schematic of the high-repetition-rate terawatt Nd: glass laser system. PC: Pockels cell; PL: thin film polarizer; WP: quarter wave plate; VSF: vacuum spatial filter.
for chirped pulse amplification at the terawatt level. In this paper we describe the performance of a compact Nd: glass laser system capable of producing 1.0 ps pulses with a peak power exceeding 1 TW. A flash lamp pumped Nd: phosphate glass slab amplifier is used to produce 1 J compressed pulses in double-pass configuration. Since this laser can be operated at a 1 Hz repetition rate, more than 1 W of average power is produced after compression.
2. Front end of the laser system A schematic of the laser system is shown in Fig. 1. A 1053 nm laser pulse of 100 ps duration from a Qswitched and mode-locked Nd: YLF oscillator was focused into a polarization preserving single-mode fiber of 1.O km length. Self-phase modulation (SPM ) and group velocity dispersion (GVD) in the fiber typical produces a nearly linear chirp with 230 ps duration and 4.4 nm bandwidth. Since pulses having a duration of N 1 ns are necessary to extract a joule en-
ergy from small-scale amplifiers, a pulse stretcher is required before amplification. The stretcher uses two 1800 lines/mm holographic gratings used in a nearLittrow configuration placed within the focal points of a telescope. The telescope is made of two ARcoated 1 m focal-length lenses, and each grating is 70 cm away from the lens. The time resolved spectrum of the stretched, chirped-pulse was observed with a 1 m spectrometer coupled to a 2 ps Hamamatsu streak camera. The result is shown in Fig. 2. Four passes through the system results in stretching the pulse to 880 ps having a bandwidth of 3.0 nm. This frequency chirp appears to be highly linear within the dynamic range of the measurement system. The bandwidth is adjusted to the transportation bandwidth of the regenerative amplifier which is limited by gain narrowing to 1.8 nm. After the stretcher and Faraday isolator, 1.5 nJ remains to seed the regenerative amplifier (Continuum RGA 24-6) operating at 6 Hz. it consists of two cavity mirrors, a pair of 4 mm x 65 mm glass amplitiers, two Pockels cells, and a quarter-wave plate. A
K. Yamakawa et al. /Optics Communications 112 (1994) 37-42
39
0
0.5
1.0
1.5
1053
1056
wavelength [nm] Fig. 2. Time resolved spectrum of the chirped pulse after the pulse stretcher.
thin film polarizer is used to inject and eject the laser pulse. A 90” rotator placed between the two heads are used to cancel the birefringent depolarization introduced by the amplifiers. Two different phosphate glasses (Hoya HAP-3 and HAP-4) are used in the amplifier heads which have gain peaks at 1052.5 nm for HAP-3 and 1054 nm for HAP-4, respectively to increase an effective bandwidth of the regenerative amplifier [ 161. During 34 round-trips the pulse experiences a net amplification of 1.7~ lo6 in the TEMoo mode with an energy as high as 2.5 mJ. Due to the high gain amplification a gain narrowing leads to a bandwidth reduction to 1.8 nm. This effective gain-bandwidth is larger than that exhibited by either glass type alone. The amplified pulse from the regenerative amplifier is passing through a pulse slicer to eliminate the pre- and post-pulses that reflect from the switchedout polarizer with each round trip inside the cavity. The selected pulse is then amplified by two 7 mm diameter amplifiers (Hoya HAP-4 ) to produce a pulse energy up to 100 mJ at a 1 Hz repetition rate. The first amplifier is designed to use a double-pass config-
uration. This amplifier produces pulse energies of 35 mJ, which are then further amplified by the next single-pass 7 mm amplifier to 100 mJ.
3. Performance of the Nd : glass slab power amplifier The pulse is then spatially shaped by a soft aperture and collimated to 10 mm diameter by a beam expander before being amplified in the slab power amplifier. This final amplifier consists of a Nd : phosphate slab glass (Hoya LHG-5,8 wt.% doping) used in double pass which brings the energy level of the pulse to over a joule. The slab amplifier that we used here was developed for a high average power laser oscillator [ 17 1. The Brewster-cut slab glass are 228 mm long, 65 mm wide, and 5 mm thick and designed for a zig-zag optical path of 20 internal reflections. The transmission wavefront distortion through the optical pass measured by an interferometer is less than A/6 (A=633 nm). The laser glass is pumped by eight Xe flash lamps which have an arc length of 177.8 mm and a bore diameter of 6 mm from both sides of
40
K. Yamakuwa et al. /Optics Communications 112 (1994) 37-42
the slab surfaces. Shied glasses were used to separate the slab cooling channel from the lamp cooling channel. The laser glass and Xe flash lamps are cooled with N, gas and pure water, respectively. Fig. 3 shows the distribution of stored energy density in the longitudinal cross section of the laser glass. This measurement was made with input flash lamp energies of 2.5 and 3.0 W, respectively. The stored energy density at both ends of the laser glass is N 35% lower than that at the center. A non-uniform gain distribution may be caused by the shield glass holders, which partially cuts the pump light from the flash lamps to the gain medium at both ends of the laser glass. Single- and double-pass gain measurements are made with this amplifier as a function of lamp energy as shown in Fig. 4. A single-pass gain of 6.5 is obtained at 3.0 kJ of pump energy. Double-pass gains of 42 are also observed at the same pump energy. All measurements are made at a factor of 5 below the saturation fluence of LHG-5 glass (4.5 J/cm2). The 10 mm diameter output beam from the beam expander is compressed in the x-direction and expanded in the y-direction by coupling optics based on cylindrical lenses of 3 mmx48 mm in the slab amplifier to cover the slab glass fully. Double passing in the slab amplifier results in an output as high as 2 J with input flash lamp energy of 2.0 kJ. The average fluence is estimated to be 1.4 J/cm2. Relay imaging a top-hat beam will further improve energy extraction in the B-integral limited amplifier chain, because the top-hat spatial profile allows a greater fill factor in
1
2
3
pump energy [kJ]
Fig. 4. Single- and double-pass gain versus the lamp energy in the slab amplifier.
the amplifier crystal without significant diffraction effects. This improvement should result in an amplified pulse at the 5-6 J level. Fig. 5 shows the measured spectra evolution during the amplification. These spectra were measured at the output of the regenerative amplifier, the singlepass rod amplifier and the slab power amplifier. Because the wavelength for maximum gain in the regenerative amplifier (1053.3 nm) was different from the central wavelength of the un-amplified chirped pulse (1053.0 nm), the spectrum of the amplified pulse was shifted by 0.3 nm toward a longer wavelength. At slab amplifier output, the bandwidth of the chirped pulse remained 1.64 nm centered at 1053.5 nm. This result indicates that no additional gain narrowing caused by the slab amplifier was observed. We could not also observe any distortion of the chirped spectrum indicating the absence of saturation effects on the amplified pulse.
4. Pulse compression and beam quality
x position
[cm]
Fig. 3. Measured stored energy density distribution of the slab amplifier along the slab x axis at pump energies of 2.5 and 3.0 kJ, respectively.
The amplified pulse is re-collimated by the same coupling optics to 10 mm diameter and then expanded by a 3.5 xvacuum spatial filter. Finally, the output beam double passed through a grating pulse compressor to remove the positive frequency chirp. The final pulse compression stage consists of a double pass through a pair of ruled gold-coated reflection gratings separated by 18.5 m. The gratings have a groove density of 1200 lines/mm, a blaze angle of
41
K. Yamakawa et al. /Optics Communications 112 (1994) 37-42
1056.3
1053.3 wavelength
1050.3
1056.5
1053.5 wavelength
[nm]
1050.5 [nm]
1056.5
1053.5 wavelength
1050.5 [nm]
Fig. 5. Spectra measured at the output of the regenerative amplifier, the single-pass rod amplifier and the slab power amplifier.
.z 2
-+
1 .O ps
[FWHY]
2 r
O_ I 0
1 20
I
time
I 40
I
1 60
[ps]
and measuring the spot size at the focus with a CCD camera. The 35 mm diameter beam is focused with a 1.5 m focal-length lens. The result indicates that the output beam divergence is 130 grad in the x-direction and 520 prad in the y-direction. The degradation in the y-direction is believed top be due to thermally induced effects caused by the nonuniform pumping and the thermal conduction through the glass edges. The beam divergence in the y-direction can be improved by using a cylindrical lens.
Fig. 6. Streak camera trace of the compressed pulse.
36.87”, and a ruled area of 102 mmx 128 mm (Milton Roy Co. ). The p-polarized laser beam is incident on the gratings at an angle of 20 degrees with respect to normal. The double-pass efficiency of the grating compressor is N 55%. (Hamamatsu A single-shot streak camera FESCASOO) with a temporal resolution of 600 fs is used to measure the compressed pulses. A typical streak camera trace of the 1.1 J compressed pulse is shown in Fig. 6. The temporal FWHM is 1.0 ps. The bandwidth of the compressed pulse is 1.64 nm, resulting in a time-bandwidth product of 0.443. The contrast (defined as the ratio of the peak to background intensity) of the compressed pulse has not yet been measured at this point, since the streak camera has a poor dynamic range of about 50 : 1. The contrast of the compressed pulse may be of the order of lo3 due to nonlinearity and negatively-chirped pulse components generated by SPM in the fiber [ 18,6]. Temporal windowing by fast Pockels cell switches can improve this ratio to > 10’ [ 6 1. We have determined the spatial beam quality of the compressed pulse by focusing the attenuated output
5. Repetitively operation of the laser system The repetitively operation of the laser system is tested at 1 Hz under 600 l/min Nz gas flow. The pulseto-pulse stability is typically less than + 5% under this condition. It takes 30 seconds to reach the steady state. The far-field beam quality is however decreased due to thermal lensing effect in the slab amplifier at this repetition rate. Near-diffraction-limited beam quality requires reduction of the repetition rate to l/6 Hz to ensure complete thermal recovery of the slab amplifier. The moving slab Nd : glass laser could be operated faster than 10 Hz and should be capable of producing an average power of > 10 W while maintaining the terawatt power level [ 19 1.
6. Conclusion We have developed a compact Nd : glass laser system capable of producing 1 TW pulses with 1.0 ps duration at a 1 Hz repetition rate. A high gain and efficient Nd : phosphate glass slab laser was charac-
42
K. Yamakawa et al. /Optics Communications 112 (1994) 37-42
terized for chirped pulse amplification as the power amplifier. We could not observe any distortion and gain narrowing of the chirped spectrum in the slab amplifier. More than 1 W of average power is produced after compression. This laser system is currently being used for high energy ultrashort X-ray pulse generation at a moderate repetition rate. Relay imaging the top-hat beam spatial profile should result in picosecond compressed pulses at the multiterawatt level.
Acknowledgements
We wish to thank T. Togawa for his help in the early stages of this project, and I. Ishida of Continuum K.K. for his technical support K. Yamakawa acknowledges the support of a fellowship of the Japan Society for the Promotion of Science. This work was partly supported by the Institute for Laser Technology and KANSAI Electric Power Co.
References [ 1 ] M.M. Mumane, H.C. Kapteyn and R.W. Falcone, Phys. Rev. Lett.62 (1989) 155. [2]H.C.Kapteyn,Appl.Optics31 (1992)4931. [3] M.M. Mumane et al., in: Short-Wavelength Coherent Radiation: Generation and Application, Vol. 11 ( 1991) 28 1. [4] D. Strickland and G. Mourou, Optics Comm. 56 (1985) 219.
[5] F.G. Patterson, R. Gonzales and M.D. Perry, Optics Lett. 16 (1991) 1107. [ 61 K. Yamakawa, H. Shiraga, Y. Kato and C.P.J. Barty, Optics Lett. 16 (1991) 1593. [ 71 M. Pessot, J. Squier, G. Mourou and D. Hatter, Optics Lett. 14 (1989) 797. [S] J.D. Kmetec, J.J. Macklin and J.F. Young, Optics Lett. 16 (1991) 1001. [9] A. Sullivan, H. Hamster, H.C. Kapteyn, S. Gordon, W. White, H. Nathel, R.J. Blair and R.W. Falcone, Optics Lett. 16 (1991) 1406. [lo] T. Ditmire and M.D. Perry, Optics Lett. 18 ( 1993) 426. [ 111 C. Rouyer, E. Mazataud, I. Allais, A. Pierre, S. Seznec, C. Sauteret, G. Mourou and A. Migus, Optics Lett. 18 ( 1993) 214. [ 121 W.S. Martin and J.P. Chemoch, U. S. Patent 3,633,126, 1972. 1’3 G.F. Albrecht, J.M. Eggleston and R.A. Petr, IEEE J. Quantum Electron. QE-22 ( 1986) 2092. ]14 C.B. Dane, L.E. Zapata and L.A. Hackel, in: Conf. on Lasers and Electra-Optics, 1993 OSA Technical Digest Series, Vol. 11 (Optical Society of America, Washington, DC, 1993) paper CWI 1. [‘5 S. Basu and R.L. Byer, IEEE J. Quantum Electron. QE-26 (1990) 149. [ 161 M.D. Perry, F.G. Patterson and J. Weston, Optics Lett. 15 (1990) 381. [ 171 B.T. Kim, Y. Oishibashi, T. Kanabe, M. Nakatsuka and S. Nakai, IEEE Photo. Tech. Lett. 2 ( 1990) 604. [ 181 Y.-H. Chuang, D.D. Meyerhofer, S. Augst, H. Chen, J. Peatross and S. Uchida, J. Opt. Sot. Am. B 8 ( 1991) 1226. [ 191 H. Sekiguchi, H. Kawakami, T. Eguchi, J.R. Untemahrer, S. Amano and T. Mochizuki, in: Conf. on Lasers and ElectroOptics, 1992 OSA Technical Digest Series, Vol. 12 (Optical Society of America, Washington, DC., 1992) paper CTuE3.
OPTICS COMMUNICATIONS EUEVIER
Optics Communications 112 ( 1994) 37-42
1 Hz, 1 ps, terawatt Nd: glass laser K.Yamakawa
L*, H. Sugio, H. Daido, M. Nakatsuka,
Y. Kate, S. Nakai
Institute ofLaser Engineering, Osaka University, 2-6 Yamada-oka, Osaka 565, Japan
Received 24 May 1994
Abstract Chirped-pulse amplification of picosecond pulses is demonstrated in a compact Nd:glass laser system. A Nd: phosphate glass slab laser is used to produce terawatt pulses as a power amplifier. The detailed characterization of the zigzag slab amplifier is described. The laser system produces > 1 J, 1.O ps pulses at a 1 Hz repetition rate.
1. Introduction
The interaction of short laser pulses ( < 1 ps) with matter has been becoming an attractive method of producing ultrashort X-ray pulses [ 11. These X-ray pulses could be used for various applications such as time resolved X-ray spectroscopy, X-ray microscopy, lithography and X-ray laser pumping [ 2 1. For many of these applications, high-energy X-ray pulses are desired. Recently, X-ray energy conversion effrcienties as high as 20% have been demonstrated in femtosecond laser-produced plasmas [ 3 1. The application of chirped-pulse amplification (CPA) has made it possible to produce high peak power, ultrashort laser pulses [ 4 1. This technique has been applied to a variety of solid-state materials, including Nd: glass [5,6], alexandrite [7], Ti:sapphire [8,9], and Cr : LiSrA1F6 [ lo]. The advantages of using Ti : sapphire or Cr : LiSrAIFs as the oscillator and amplifier include a large gain-bandwidth, making it possible to both produce and amplify pulses of less l
Corresponding author.
’ Present address: Advanced Science Research Center, Japan Atomic Energy Research Institute, Naka, Ibaraki 3 11-O1, Japan. Email: [email protected].
than 100 fs duration up to a few hundred milijoules, at a high repetition rate ( N 10 Hz). However they cannot match the glass laser systems in energy per pulse. Neodymium-doped glasses as an amplifier are superior over crystalline hosts owing to their large energy storage, low nonlinear refractive index, and the commercial availability of large volumes with high optical quality. Picosecond or subpicosecond lasers with the highest energy per pulse have only been based on Nd:glass [ 6,111. However, the limitation with flash lamp pumped Nd : glass amplifiers is a low repetition rate due to the poor thermal characteristics of laser glasses. The pulse repetition rate of the joule class Nd: glass laser systems is normally one shot every 10 minutes, which is not convenient for medical or biological applications. The lower thermal conductivity of glass could be overcome by the concept of slab geometry [ 12,131. Many laboratories have developed neodymium-doped solid-state slab lasers as high-average power sources. Recently, a flash lamp pumped Nd : glass slab amplifier system produced an average power exceeding 100 W [ 141. The moving slab Nd: glass laser has been used to produce 0.5 GW, 11 ps injected pulses from a cw mode-locked Nd:glass laser without CPA technique [ 15 1. However, there has been no report of a Nd : glass slab amplifier used
0030-4018/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SsDIOO30-4018112 (1994) 000-000
K. Yamaknwa et al. / Optics Communications 112 (1994) 3 7-42
38
Nd:YLF oscillator Single-mode fiber 7-mm amolifier 7-mm amplifier
Regenerative amplifier
I
Beam expander
Coupling
optics
Slab amplifier
+
1J 1P-s 1 Hz
Fig. 1. Schematic of the high-repetition-rate terawatt Nd: glass laser system. PC: Pockels cell; PL: thin film polarizer; WP: quarter wave plate; VSF: vacuum spatial filter.
for chirped pulse amplification at the terawatt level. In this paper we describe the performance of a compact Nd: glass laser system capable of producing 1.0 ps pulses with a peak power exceeding 1 TW. A flash lamp pumped Nd: phosphate glass slab amplifier is used to produce 1 J compressed pulses in double-pass configuration. Since this laser can be operated at a 1 Hz repetition rate, more than 1 W of average power is produced after compression.
2. Front end of the laser system A schematic of the laser system is shown in Fig. 1. A 1053 nm laser pulse of 100 ps duration from a Qswitched and mode-locked Nd: YLF oscillator was focused into a polarization preserving single-mode fiber of 1.O km length. Self-phase modulation (SPM ) and group velocity dispersion (GVD) in the fiber typical produces a nearly linear chirp with 230 ps duration and 4.4 nm bandwidth. Since pulses having a duration of N 1 ns are necessary to extract a joule en-
ergy from small-scale amplifiers, a pulse stretcher is required before amplification. The stretcher uses two 1800 lines/mm holographic gratings used in a nearLittrow configuration placed within the focal points of a telescope. The telescope is made of two ARcoated 1 m focal-length lenses, and each grating is 70 cm away from the lens. The time resolved spectrum of the stretched, chirped-pulse was observed with a 1 m spectrometer coupled to a 2 ps Hamamatsu streak camera. The result is shown in Fig. 2. Four passes through the system results in stretching the pulse to 880 ps having a bandwidth of 3.0 nm. This frequency chirp appears to be highly linear within the dynamic range of the measurement system. The bandwidth is adjusted to the transportation bandwidth of the regenerative amplifier which is limited by gain narrowing to 1.8 nm. After the stretcher and Faraday isolator, 1.5 nJ remains to seed the regenerative amplifier (Continuum RGA 24-6) operating at 6 Hz. it consists of two cavity mirrors, a pair of 4 mm x 65 mm glass amplitiers, two Pockels cells, and a quarter-wave plate. A
K. Yamakawa et al. /Optics Communications 112 (1994) 37-42
39
0
0.5
1.0
1.5
1053
1056
wavelength [nm] Fig. 2. Time resolved spectrum of the chirped pulse after the pulse stretcher.
thin film polarizer is used to inject and eject the laser pulse. A 90” rotator placed between the two heads are used to cancel the birefringent depolarization introduced by the amplifiers. Two different phosphate glasses (Hoya HAP-3 and HAP-4) are used in the amplifier heads which have gain peaks at 1052.5 nm for HAP-3 and 1054 nm for HAP-4, respectively to increase an effective bandwidth of the regenerative amplifier [ 161. During 34 round-trips the pulse experiences a net amplification of 1.7~ lo6 in the TEMoo mode with an energy as high as 2.5 mJ. Due to the high gain amplification a gain narrowing leads to a bandwidth reduction to 1.8 nm. This effective gain-bandwidth is larger than that exhibited by either glass type alone. The amplified pulse from the regenerative amplifier is passing through a pulse slicer to eliminate the pre- and post-pulses that reflect from the switchedout polarizer with each round trip inside the cavity. The selected pulse is then amplified by two 7 mm diameter amplifiers (Hoya HAP-4 ) to produce a pulse energy up to 100 mJ at a 1 Hz repetition rate. The first amplifier is designed to use a double-pass config-
uration. This amplifier produces pulse energies of 35 mJ, which are then further amplified by the next single-pass 7 mm amplifier to 100 mJ.
3. Performance of the Nd : glass slab power amplifier The pulse is then spatially shaped by a soft aperture and collimated to 10 mm diameter by a beam expander before being amplified in the slab power amplifier. This final amplifier consists of a Nd : phosphate slab glass (Hoya LHG-5,8 wt.% doping) used in double pass which brings the energy level of the pulse to over a joule. The slab amplifier that we used here was developed for a high average power laser oscillator [ 17 1. The Brewster-cut slab glass are 228 mm long, 65 mm wide, and 5 mm thick and designed for a zig-zag optical path of 20 internal reflections. The transmission wavefront distortion through the optical pass measured by an interferometer is less than A/6 (A=633 nm). The laser glass is pumped by eight Xe flash lamps which have an arc length of 177.8 mm and a bore diameter of 6 mm from both sides of
40
K. Yamakuwa et al. /Optics Communications 112 (1994) 37-42
the slab surfaces. Shied glasses were used to separate the slab cooling channel from the lamp cooling channel. The laser glass and Xe flash lamps are cooled with N, gas and pure water, respectively. Fig. 3 shows the distribution of stored energy density in the longitudinal cross section of the laser glass. This measurement was made with input flash lamp energies of 2.5 and 3.0 W, respectively. The stored energy density at both ends of the laser glass is N 35% lower than that at the center. A non-uniform gain distribution may be caused by the shield glass holders, which partially cuts the pump light from the flash lamps to the gain medium at both ends of the laser glass. Single- and double-pass gain measurements are made with this amplifier as a function of lamp energy as shown in Fig. 4. A single-pass gain of 6.5 is obtained at 3.0 kJ of pump energy. Double-pass gains of 42 are also observed at the same pump energy. All measurements are made at a factor of 5 below the saturation fluence of LHG-5 glass (4.5 J/cm2). The 10 mm diameter output beam from the beam expander is compressed in the x-direction and expanded in the y-direction by coupling optics based on cylindrical lenses of 3 mmx48 mm in the slab amplifier to cover the slab glass fully. Double passing in the slab amplifier results in an output as high as 2 J with input flash lamp energy of 2.0 kJ. The average fluence is estimated to be 1.4 J/cm2. Relay imaging a top-hat beam will further improve energy extraction in the B-integral limited amplifier chain, because the top-hat spatial profile allows a greater fill factor in
1
2
3
pump energy [kJ]
Fig. 4. Single- and double-pass gain versus the lamp energy in the slab amplifier.
the amplifier crystal without significant diffraction effects. This improvement should result in an amplified pulse at the 5-6 J level. Fig. 5 shows the measured spectra evolution during the amplification. These spectra were measured at the output of the regenerative amplifier, the singlepass rod amplifier and the slab power amplifier. Because the wavelength for maximum gain in the regenerative amplifier (1053.3 nm) was different from the central wavelength of the un-amplified chirped pulse (1053.0 nm), the spectrum of the amplified pulse was shifted by 0.3 nm toward a longer wavelength. At slab amplifier output, the bandwidth of the chirped pulse remained 1.64 nm centered at 1053.5 nm. This result indicates that no additional gain narrowing caused by the slab amplifier was observed. We could not also observe any distortion of the chirped spectrum indicating the absence of saturation effects on the amplified pulse.
4. Pulse compression and beam quality
x position
[cm]
Fig. 3. Measured stored energy density distribution of the slab amplifier along the slab x axis at pump energies of 2.5 and 3.0 kJ, respectively.
The amplified pulse is re-collimated by the same coupling optics to 10 mm diameter and then expanded by a 3.5 xvacuum spatial filter. Finally, the output beam double passed through a grating pulse compressor to remove the positive frequency chirp. The final pulse compression stage consists of a double pass through a pair of ruled gold-coated reflection gratings separated by 18.5 m. The gratings have a groove density of 1200 lines/mm, a blaze angle of
41
K. Yamakawa et al. /Optics Communications 112 (1994) 37-42
1056.3
1053.3 wavelength
1050.3
1056.5
1053.5 wavelength
[nm]
1050.5 [nm]
1056.5
1053.5 wavelength
1050.5 [nm]
Fig. 5. Spectra measured at the output of the regenerative amplifier, the single-pass rod amplifier and the slab power amplifier.
.z 2
-+
1 .O ps
[FWHY]
2 r
O_ I 0
1 20
I
time
I 40
I
1 60
[ps]
and measuring the spot size at the focus with a CCD camera. The 35 mm diameter beam is focused with a 1.5 m focal-length lens. The result indicates that the output beam divergence is 130 grad in the x-direction and 520 prad in the y-direction. The degradation in the y-direction is believed top be due to thermally induced effects caused by the nonuniform pumping and the thermal conduction through the glass edges. The beam divergence in the y-direction can be improved by using a cylindrical lens.
Fig. 6. Streak camera trace of the compressed pulse.
36.87”, and a ruled area of 102 mmx 128 mm (Milton Roy Co. ). The p-polarized laser beam is incident on the gratings at an angle of 20 degrees with respect to normal. The double-pass efficiency of the grating compressor is N 55%. (Hamamatsu A single-shot streak camera FESCASOO) with a temporal resolution of 600 fs is used to measure the compressed pulses. A typical streak camera trace of the 1.1 J compressed pulse is shown in Fig. 6. The temporal FWHM is 1.0 ps. The bandwidth of the compressed pulse is 1.64 nm, resulting in a time-bandwidth product of 0.443. The contrast (defined as the ratio of the peak to background intensity) of the compressed pulse has not yet been measured at this point, since the streak camera has a poor dynamic range of about 50 : 1. The contrast of the compressed pulse may be of the order of lo3 due to nonlinearity and negatively-chirped pulse components generated by SPM in the fiber [ 18,6]. Temporal windowing by fast Pockels cell switches can improve this ratio to > 10’ [ 6 1. We have determined the spatial beam quality of the compressed pulse by focusing the attenuated output
5. Repetitively operation of the laser system The repetitively operation of the laser system is tested at 1 Hz under 600 l/min Nz gas flow. The pulseto-pulse stability is typically less than + 5% under this condition. It takes 30 seconds to reach the steady state. The far-field beam quality is however decreased due to thermal lensing effect in the slab amplifier at this repetition rate. Near-diffraction-limited beam quality requires reduction of the repetition rate to l/6 Hz to ensure complete thermal recovery of the slab amplifier. The moving slab Nd : glass laser could be operated faster than 10 Hz and should be capable of producing an average power of > 10 W while maintaining the terawatt power level [ 19 1.
6. Conclusion We have developed a compact Nd : glass laser system capable of producing 1 TW pulses with 1.0 ps duration at a 1 Hz repetition rate. A high gain and efficient Nd : phosphate glass slab laser was charac-
42
K. Yamakawa et al. /Optics Communications 112 (1994) 37-42
terized for chirped pulse amplification as the power amplifier. We could not observe any distortion and gain narrowing of the chirped spectrum in the slab amplifier. More than 1 W of average power is produced after compression. This laser system is currently being used for high energy ultrashort X-ray pulse generation at a moderate repetition rate. Relay imaging the top-hat beam spatial profile should result in picosecond compressed pulses at the multiterawatt level.
Acknowledgements
We wish to thank T. Togawa for his help in the early stages of this project, and I. Ishida of Continuum K.K. for his technical support K. Yamakawa acknowledges the support of a fellowship of the Japan Society for the Promotion of Science. This work was partly supported by the Institute for Laser Technology and KANSAI Electric Power Co.
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