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100-W, 100-h external green generation with Nd:YAG rod master-oscillator power-amplifier system
1 October 2000
Optics Communications 184 (2000) 231±236
www.elsevier.com/locate/optcom
100-W, 100-h external green generation with Nd:YAG rod master-oscillator power-ampli®er system Y. Hirano *, N. Pavel, S. Yamamoto, Y. Koyata, T. Tajime Mitsubishi Electric Corporation, Information Technology Research and Development Center, 5-1-1 Ofuna, Kamakura, Kanagawa 247-8501, Japan Received 20 June 2000; accepted 7 August 2000
Abstract A high repetition rate diode-pumped Nd:YAG rod master-oscillator power-ampli®er system was designed, fabricated and tested. The system provides a maximum average power of 222 W at a repetition rate of 2.5 kHz with a pulse width of 51 ns. With an external two-stage KTP crystal architecture, this system produced a 131 W green average power with a frequency conversion eciency as high as 65.2%. To examine the system reliability, the 100-h continuous operation was done at a green power level of 100 W. The decreasing rate of green power was observed to be 0.07% hÿ1 . Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Laser ampli®ers; Nd:YAG laser; Second harmonic generation
In recent years, diode-pumped solid-state lasers have signi®cantly enhanced the performances of solid-state lasers giving a higher eciency and a higher average output power at both fundamental and frequency-doubled wavelengths. Recently, Le Garrec et al. [1] demonstrated an average power of 106 W at 532 nm with a diode side-pumped Nd:YAG rod laser with an intracavity KTP crystal in a Z cavity. The laser that was acousto-optically (AO) Q-switched was able to operate for more than 1 h at an average green power of 100 W. The repetition rate was 27 kHz, and the pulse duration was about 200 ns. Honea et al. [2] reported 140 W of multimode 532-nm light from a diode-array
*
Corresponding author. Fax: +81-467-41-2519. E-mail addresses: [email protected], [email protected] (Y. Hirano).
hirano-
end-pumped AO Q-switched Nd:YAG oscillator that used a KTP crystal in a V cavity. The repetition rate and pulse duration were 30 kHz and 160 ns, respectively. Chang et al. [3] demonstrated the highest average green-output power of 315 W generated by a single diode-pumped solid-state laser using a KTP crystal. The same author achieved the 1000-h continuous operation at an average green-output power of 170 W with an LBO crystal [4]. A master-oscillator power-ampli®er (MOPA) system with a diode-pumped Nd:YAG slab geometry has been reported; this produces 900 W infrared power while operating at frequencies up to 100 Hz and 165 W of green power at 5 J pulseÿ1 , 33 Hz and a beam quality of 1.5 times the diffraction limit [5]. Extending the repetition rate from 100 Hz to 2.5 kHz while using a zigzag slab master oscillator, a phase conjugated mirror and a
0030-4018/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 0 ) 0 0 9 4 0 - 8
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Y. Hirano et al. / Optics Communications 184 (2000) 231±236
two-stage zigzag slab ampli®er, the performance of the previous system was extended to nearly 700 W IR power with a beam quality of 1.1 times the diraction limit [6]. Using an external KTP crystal, this system produced 175 W average green-output power with a frequency conversion eciency of 45%. A more than 1 h continuous operation at average green power level of 160 W has been performed. A MOPA system with a ¯ash-lamp pumped Nd:YAG rod geometry has been reported, which produces 200 W infrared power at a pump repetition rate of 100 Hz emitting 20 passively Q-switched pulses for each pump duration [7]. Using a two-rod birefringence compensation technique and phase conjugated mirror in doublepass ampli®er stage, a beam quality of M 2 < 3 was obtained. This system produced 103 W greenoutput power with an external KTP crystal. Recently, we have reported a 2.4 kHz repetition rate rod-type diode-pumped MOPA system that provides 108 W of average infrared power with a pulse width of 47 ns [8]. The beam was charac-
terized by an M 2 factor of 2.5. The performances of this scheme have been improved and the results are presented here. The new system provides a maximum average output power of 222 W. The repetition rate is 2.5 kHz (25% duty cycle), and the pulse width is 51 ns. With an external two-stage KTP crystal architecture, this system produced an average green power of 131 W with a frequency conversion eciency as high as 65.2%. A 100-h continuous operation for average green power of 100 W was responsible for long-time operation of the system. These results are believed to be the highest average green power and the longest time operation at green power level of 100 W reported for a rod-type diode-pumped MOPA system. The schematic diagram of the MOPA system that includes a master oscillator, a beam forming stage, the ampli®er, and the SHG stage is shown in Fig. 1. The master oscillator and the beam forming stage were described in detail elsewhere [8]. Brie¯y, the electro-optically Q-switched resonator consists of two diode-pumped Nd:YAG rods (4 mm in
Fig. 1. The MOPA architecture with an external doubler ± P: polarizer, QWP: quarter-wave plate, HWP: half-wave plate and DS: dichroic splitter.
Y. Hirano et al. / Optics Communications 184 (2000) 231±236
diameter, 60 mm in length, Nd doping of 0.8%) with a 90° rotator placed between them. This provides maximum average output power of 32 W (energy of 12.8 mJ, pulse repetition rate of 2.5 kHz, 25% duty cycle). The output beam is a Gaussian beam with a spot size of 1.4 mm. The pulse duration was 43 ns (FWHM de®nition). The beam forming stage [8] transforms this Gaussian beam into a beam of top-hat distribution, and adjusts its diameter to match the input aperture of the ampli®er. This beam of top-hat distribution is suitable for energy extraction from the ampli®er rods with uniform pump distribution. The ampli®er stage contains two Nd:YAG rods (4 mm in diameter, 110 mm in length, Nd doping of 0.8%). The diode laser consists of a 12-layered SDL-3244-MBL array, with dimensions of 20 and 10 mm for the fast and slow axes, respectively. Four-diode lasers are used for every head. The total diode output peak power was 2.88 kW with a maximum 720 W average power for every laser head. To obtain a uniform pump distribution in the rod for minimizing the in¯uence of thermal aberration for the propagating beam, a highly diused cross-axis delivery (HiDiCAD) pumping cavity [9], in which the diode laser slow axis and the rod axis are parallel to each other, has been devised for the ampli®er heads. This new design leads to a highly ecient con®nement of the pump light, a uniform pump-beam distribution in the rod under various pump-power levels, and a lower sensitivity to radius and Nd concentration of the rod and shifts of the diode wavelength. A relay optics maintains a top-hat beam pro®le from the ®rst to the second rod, maximizing the extraction energy from two rods in double-pass propagation, and eliminating the optics damage produced by Fresnel diraction and reducing the alignment sensitivity of the system. After ®rst-pass ampli®cation, the beam is re¯ected back through the ampli®er chain by the high re¯ectivity mirror of the telescope. The thermal birefringence and thermal bifocusing that occurred in the rods are compensated by two rods with 90° rotator scheme which averages the transmitted optical path lengths and wavefronts for radial and tangential polarization components. To adjust the focal length of the telescope, the
233
Fig. 2. Ampli®er single- and double-pass average power as a function of diode-laser average power for input signal energies of 4 and 10 mJ. Triangles indicate the depolarization losses.
averaged thermal lensing of two rods are compensated, and suitable propagation in second-pass ampli®cation can be obtained. The quarter-wave plate, QWP, and polarizer, P, serve as power coupling from the ampli®er. By a standard Findlay±Clay analysis [10], the coecient K, which relates the small-signal gain to medium length product g0 L to the diode-pump power, was estimated to be 0.00195 Wÿ1 . The average ampli®er power as a function of diode power is shown in Fig. 2. For an ampli®erinput signal of 10 mJ and a diode-laser power of 1260 W, the average maximum power of 148 W for single-pass ampli®cation was obtained. Under the same conditions, the MOPA system provided an average maximum power of 222 W with a depolarization loss of 6%. The laser beam M 2 factor that was measured by a Wavefront Meter Class2D CCD camera (Shack Hartmann detector) was 5:18 5:19 and the pulse duration was 51 ns. A type-II KTP crystal is very useful for green generation because of its high nonlinear coecient, small walk-o angle and large tolerance parameters. However, the gray-tracking behavior [11±13] of KTP prevents this crystal from operating at high peak and high average power levels. Boulanger et al. [14] suggested that this problem could be partially solved by reducing the green intensity inside the crystal. A two-stage KTP
234
Y. Hirano et al. / Optics Communications 184 (2000) 231±236
crystal architecture was applied to maximize the conversion eciency with long interaction length, and minimize the back conversion that occurred by phase mismatch. The noble two-crystal frequency conversion technique called ``quadrature conversion'', that does not suer from back conversion, has been reported previously [15]. In this technique, two crystals are arranged so that the green beam generated in the ®rst crystal passes through the second crystal with orthogonal polarization state for the generated green beam in the second crystal. Although a single output green beam can be obtained using this technique, the high peak and high average green output from the ®rst crystal is absorbed by the second crystal and, consequently, gray-tracking damage and temperature phase-mismatch problem are likely to be induced in the second crystal. Thus, we applied the two-stage KTP crystal arrangement in which the output green beam from the ®rst crystal is picked up between two crystals. Although the output green beam from the SHG stage is obtained in two separate linear-polarized beams in this arrangement, these beams can be combined, if required, with an external polarizer. The lengths of two KTP crystals were 15 mm. The crystal temperature was set to 70°C. A telescope placed in front of the ®rst KTP (Fig. 1) reduced the fundamental beam intensity to less than 40 MW cmÿ2 . The vertical polarization of the MOPA beam was rotated to 45° which is the angle required for type II doubling by the HWP2 half-wave plate, and the incident infrared power was varied using the HWP1 wave plate. The average green power as a function of average power of the MOPA is shown in Fig. 3. For an average input power of 201 W, ®rst KTP provides 119-W average green power with a net frequency conversion eciency of 59.2%. A dichroic beam splitter (DS) removed the green power after the ®rst KTP, and the transmitted infrared power was incident on the second KTP crystal. This results in a maximum green power of 12 W with a frequency conversion of 17.8%. The frequency conversion eciency of the KTP stage as a function of MOPA power was also shown in Fig. 3. A maximum green power of 131 W was obtained for a net conversion eciency of 65.2%.
Fig. 3. Average green power and conversion eciency as a function of MOPA average power.
Fig. 4. Frequency-conversion eciency as a function of beam power density for ®rst KTP. The phase mismatch (rad cmÿ1 ) is the parameter.
Fig. 4 shows the expected conversion eciency as a function of infrared beam intensity for the ®rst KTP. The continuous lines were obtained following the theory of Dmitrev et al. [16], considering however the top-hat distribution of the laser beam and triangular temporal shape of the pulses. A good agreement between the experimental data and the theoretical curve that corresponds to a phase mismatch of only 1.0 rad cmÿ1 was obtained. For the second KTP, a phase mismatch of 1.5 rad cmÿ1 was found, indicating again
Y. Hirano et al. / Optics Communications 184 (2000) 231±236
235
Fig. 5. Long time operation of the developed system.
a good agreement between numerical simulations and experimental values. To examine the reliability of the operation of the MOPA system, this was operated continuously for 100 h with a starting green power of 106 W. Fig. 5 shows the MOPA infrared power, the total green power and the green power for each KTP crystal as a function of time. At the end of the experiment, the green power was reduced to 97.4 W. The frequency-conversion eciency varied from 67% at the beginning of the test to 62.3% at the end. The decreasing rate of the MOPA power was 0.01% hÿ1 as deduced from a linear ®t of the output power to time dependence. This decrease was attributed to (i) decrease of the diode-laser power in the oscillator that decreases the oscillator beam power, and (ii) decrease of the diode-laser power in the ampli®er stage that decreases the extraction energies from the rods. The green power delivered by the ®rst KTP crystal decreases with a slope of 0.11% hÿ1 . Based on the ®nal inspection of this KTP (Fig. 6), these decreases were attributed to the gray-tracing eect outside the beam area. No gray tracing eect in the beam area was observed. The decrease in the conversion ef®ciency of ®rst KTP increases the infrared power incident on the second KTP. This results in an increasing rate of 0.3% hÿ1 of the average green power delivered by the second KTP. Finally, the total decreasing rate of green power resulted in a rate of 0.07% hÿ1 .
Fig. 6. A photo of the ®rst KTP after 100 h of operation.
In conclusion, we developed a diode-pumped Nd:YAG rod MOPA system that delivered 222 W average power with a beam quality of M 2 5:2. The repetition rate was 2.5 kHz with a pulse width of 51 ns. By means of an external two-stage KTP crystal architecture, the system produced 131 W an average green power at a frequency-conversion eciency of 65.2%. The system was operated continuously for 100 h, starting with an initial green power of 106 W. The decreasing rate of green power was characterized as 0.07% hÿ1 . To our knowledge, these results are the the highest average green power and the longest time operation at 100-W green power ever reported for a rodtype diode-pumped MOPA system.
References [1] B.J. LeGarrec, G.J. Raze, P.Y. Thro, M. Gilbert, Opt. Lett. 21 (1996) 1990. [2] E.C. Honea, C.E. Ebbers, R.J. Beach, J.A. Speth, J.A. Skidmore, M.A. Emanuel, S.A. Payne, Opt. Lett. 23 (1998) 1203. [3] J.J. Chang, E.P. Dragon, I.L. Bass, in postdeadline papers of CLEO Õ98, May 3±8, San Francisco, 1998, paper CPD2. [4] J. Chang, E. Dragon, C. Ebbers, I. Bass, C. Cochran, in: W.R. Bosenberg, M.M. Fejer (Eds.), Advanced Solid-State Lasers, vol. 19, OSA Trends in Optics and Photonics Series, Washington, D.C. (1998), p. 300.
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[5] R.J. St. Pierre, D.W. Mordaunt, H. Injewan, J.G. Berg, R.C. Hilyard, M.E. Weber, M.G. Wickham, G.M. Harpole, R. Senn, IEEE J. Sel. Top. Quant. Electron. 3 (1997) 53. [6] R.J. St. Pierre, et al., IEEE J. Sel. Top. Quant. Electron. 3 (1997) 64. [7] S. Seidel, G. Mann, Proc. SPIE 2989 (1997) 204. [8] Y. Hirano, N. Pavel, S. Yamamoto, Y. Koyata, T. Tajime, Opt. Commun. 170 (1999) 275. [9] N. Pavel, Y. Hirano, S. Yamamoto, Y. Koyata, T. Tajime, Appl. Opt. 39 (2000) 986. [10] D. Findlay, R.A. Clay, Phys. Lett. 20 (1966) 277.
[11] F.C. Zumsteg, J.D. Bierlin, E.E. Gier, J. Appl. Phys. 47 (1976) 4980. [12] J.M. Layocono, D.N. Laiocono, T. McGee, M. Babb, J. Appl. Phys. Lett. 72 (1992) 2705. [13] J.P. Feve, B. Boulanger, G. Manier, H. Albrecht, Appl. Phys. Lett. 70 (1997) 277. [14] B. Boulanger, M.M. Fejer, R. Blachman, P.F. Bordui, Appl. Phys. Lett. 65 (1994) 2401. [15] D. Eimerl, IEEE J. Quant. Electron. QE-21 (1987) 1361. [16] V.G. Dmitriev, G.G. Gurdadyan, D.N. Nikogosyan, Handbook of Nonlinear Optical Crystals, Springer, Berlin, 1997.
Optics Communications 184 (2000) 231±236
www.elsevier.com/locate/optcom
100-W, 100-h external green generation with Nd:YAG rod master-oscillator power-ampli®er system Y. Hirano *, N. Pavel, S. Yamamoto, Y. Koyata, T. Tajime Mitsubishi Electric Corporation, Information Technology Research and Development Center, 5-1-1 Ofuna, Kamakura, Kanagawa 247-8501, Japan Received 20 June 2000; accepted 7 August 2000
Abstract A high repetition rate diode-pumped Nd:YAG rod master-oscillator power-ampli®er system was designed, fabricated and tested. The system provides a maximum average power of 222 W at a repetition rate of 2.5 kHz with a pulse width of 51 ns. With an external two-stage KTP crystal architecture, this system produced a 131 W green average power with a frequency conversion eciency as high as 65.2%. To examine the system reliability, the 100-h continuous operation was done at a green power level of 100 W. The decreasing rate of green power was observed to be 0.07% hÿ1 . Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Laser ampli®ers; Nd:YAG laser; Second harmonic generation
In recent years, diode-pumped solid-state lasers have signi®cantly enhanced the performances of solid-state lasers giving a higher eciency and a higher average output power at both fundamental and frequency-doubled wavelengths. Recently, Le Garrec et al. [1] demonstrated an average power of 106 W at 532 nm with a diode side-pumped Nd:YAG rod laser with an intracavity KTP crystal in a Z cavity. The laser that was acousto-optically (AO) Q-switched was able to operate for more than 1 h at an average green power of 100 W. The repetition rate was 27 kHz, and the pulse duration was about 200 ns. Honea et al. [2] reported 140 W of multimode 532-nm light from a diode-array
*
Corresponding author. Fax: +81-467-41-2519. E-mail addresses: [email protected], [email protected] (Y. Hirano).
hirano-
end-pumped AO Q-switched Nd:YAG oscillator that used a KTP crystal in a V cavity. The repetition rate and pulse duration were 30 kHz and 160 ns, respectively. Chang et al. [3] demonstrated the highest average green-output power of 315 W generated by a single diode-pumped solid-state laser using a KTP crystal. The same author achieved the 1000-h continuous operation at an average green-output power of 170 W with an LBO crystal [4]. A master-oscillator power-ampli®er (MOPA) system with a diode-pumped Nd:YAG slab geometry has been reported; this produces 900 W infrared power while operating at frequencies up to 100 Hz and 165 W of green power at 5 J pulseÿ1 , 33 Hz and a beam quality of 1.5 times the diffraction limit [5]. Extending the repetition rate from 100 Hz to 2.5 kHz while using a zigzag slab master oscillator, a phase conjugated mirror and a
0030-4018/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 0 ) 0 0 9 4 0 - 8
232
Y. Hirano et al. / Optics Communications 184 (2000) 231±236
two-stage zigzag slab ampli®er, the performance of the previous system was extended to nearly 700 W IR power with a beam quality of 1.1 times the diraction limit [6]. Using an external KTP crystal, this system produced 175 W average green-output power with a frequency conversion eciency of 45%. A more than 1 h continuous operation at average green power level of 160 W has been performed. A MOPA system with a ¯ash-lamp pumped Nd:YAG rod geometry has been reported, which produces 200 W infrared power at a pump repetition rate of 100 Hz emitting 20 passively Q-switched pulses for each pump duration [7]. Using a two-rod birefringence compensation technique and phase conjugated mirror in doublepass ampli®er stage, a beam quality of M 2 < 3 was obtained. This system produced 103 W greenoutput power with an external KTP crystal. Recently, we have reported a 2.4 kHz repetition rate rod-type diode-pumped MOPA system that provides 108 W of average infrared power with a pulse width of 47 ns [8]. The beam was charac-
terized by an M 2 factor of 2.5. The performances of this scheme have been improved and the results are presented here. The new system provides a maximum average output power of 222 W. The repetition rate is 2.5 kHz (25% duty cycle), and the pulse width is 51 ns. With an external two-stage KTP crystal architecture, this system produced an average green power of 131 W with a frequency conversion eciency as high as 65.2%. A 100-h continuous operation for average green power of 100 W was responsible for long-time operation of the system. These results are believed to be the highest average green power and the longest time operation at green power level of 100 W reported for a rod-type diode-pumped MOPA system. The schematic diagram of the MOPA system that includes a master oscillator, a beam forming stage, the ampli®er, and the SHG stage is shown in Fig. 1. The master oscillator and the beam forming stage were described in detail elsewhere [8]. Brie¯y, the electro-optically Q-switched resonator consists of two diode-pumped Nd:YAG rods (4 mm in
Fig. 1. The MOPA architecture with an external doubler ± P: polarizer, QWP: quarter-wave plate, HWP: half-wave plate and DS: dichroic splitter.
Y. Hirano et al. / Optics Communications 184 (2000) 231±236
diameter, 60 mm in length, Nd doping of 0.8%) with a 90° rotator placed between them. This provides maximum average output power of 32 W (energy of 12.8 mJ, pulse repetition rate of 2.5 kHz, 25% duty cycle). The output beam is a Gaussian beam with a spot size of 1.4 mm. The pulse duration was 43 ns (FWHM de®nition). The beam forming stage [8] transforms this Gaussian beam into a beam of top-hat distribution, and adjusts its diameter to match the input aperture of the ampli®er. This beam of top-hat distribution is suitable for energy extraction from the ampli®er rods with uniform pump distribution. The ampli®er stage contains two Nd:YAG rods (4 mm in diameter, 110 mm in length, Nd doping of 0.8%). The diode laser consists of a 12-layered SDL-3244-MBL array, with dimensions of 20 and 10 mm for the fast and slow axes, respectively. Four-diode lasers are used for every head. The total diode output peak power was 2.88 kW with a maximum 720 W average power for every laser head. To obtain a uniform pump distribution in the rod for minimizing the in¯uence of thermal aberration for the propagating beam, a highly diused cross-axis delivery (HiDiCAD) pumping cavity [9], in which the diode laser slow axis and the rod axis are parallel to each other, has been devised for the ampli®er heads. This new design leads to a highly ecient con®nement of the pump light, a uniform pump-beam distribution in the rod under various pump-power levels, and a lower sensitivity to radius and Nd concentration of the rod and shifts of the diode wavelength. A relay optics maintains a top-hat beam pro®le from the ®rst to the second rod, maximizing the extraction energy from two rods in double-pass propagation, and eliminating the optics damage produced by Fresnel diraction and reducing the alignment sensitivity of the system. After ®rst-pass ampli®cation, the beam is re¯ected back through the ampli®er chain by the high re¯ectivity mirror of the telescope. The thermal birefringence and thermal bifocusing that occurred in the rods are compensated by two rods with 90° rotator scheme which averages the transmitted optical path lengths and wavefronts for radial and tangential polarization components. To adjust the focal length of the telescope, the
233
Fig. 2. Ampli®er single- and double-pass average power as a function of diode-laser average power for input signal energies of 4 and 10 mJ. Triangles indicate the depolarization losses.
averaged thermal lensing of two rods are compensated, and suitable propagation in second-pass ampli®cation can be obtained. The quarter-wave plate, QWP, and polarizer, P, serve as power coupling from the ampli®er. By a standard Findlay±Clay analysis [10], the coecient K, which relates the small-signal gain to medium length product g0 L to the diode-pump power, was estimated to be 0.00195 Wÿ1 . The average ampli®er power as a function of diode power is shown in Fig. 2. For an ampli®erinput signal of 10 mJ and a diode-laser power of 1260 W, the average maximum power of 148 W for single-pass ampli®cation was obtained. Under the same conditions, the MOPA system provided an average maximum power of 222 W with a depolarization loss of 6%. The laser beam M 2 factor that was measured by a Wavefront Meter Class2D CCD camera (Shack Hartmann detector) was 5:18 5:19 and the pulse duration was 51 ns. A type-II KTP crystal is very useful for green generation because of its high nonlinear coecient, small walk-o angle and large tolerance parameters. However, the gray-tracking behavior [11±13] of KTP prevents this crystal from operating at high peak and high average power levels. Boulanger et al. [14] suggested that this problem could be partially solved by reducing the green intensity inside the crystal. A two-stage KTP
234
Y. Hirano et al. / Optics Communications 184 (2000) 231±236
crystal architecture was applied to maximize the conversion eciency with long interaction length, and minimize the back conversion that occurred by phase mismatch. The noble two-crystal frequency conversion technique called ``quadrature conversion'', that does not suer from back conversion, has been reported previously [15]. In this technique, two crystals are arranged so that the green beam generated in the ®rst crystal passes through the second crystal with orthogonal polarization state for the generated green beam in the second crystal. Although a single output green beam can be obtained using this technique, the high peak and high average green output from the ®rst crystal is absorbed by the second crystal and, consequently, gray-tracking damage and temperature phase-mismatch problem are likely to be induced in the second crystal. Thus, we applied the two-stage KTP crystal arrangement in which the output green beam from the ®rst crystal is picked up between two crystals. Although the output green beam from the SHG stage is obtained in two separate linear-polarized beams in this arrangement, these beams can be combined, if required, with an external polarizer. The lengths of two KTP crystals were 15 mm. The crystal temperature was set to 70°C. A telescope placed in front of the ®rst KTP (Fig. 1) reduced the fundamental beam intensity to less than 40 MW cmÿ2 . The vertical polarization of the MOPA beam was rotated to 45° which is the angle required for type II doubling by the HWP2 half-wave plate, and the incident infrared power was varied using the HWP1 wave plate. The average green power as a function of average power of the MOPA is shown in Fig. 3. For an average input power of 201 W, ®rst KTP provides 119-W average green power with a net frequency conversion eciency of 59.2%. A dichroic beam splitter (DS) removed the green power after the ®rst KTP, and the transmitted infrared power was incident on the second KTP crystal. This results in a maximum green power of 12 W with a frequency conversion of 17.8%. The frequency conversion eciency of the KTP stage as a function of MOPA power was also shown in Fig. 3. A maximum green power of 131 W was obtained for a net conversion eciency of 65.2%.
Fig. 3. Average green power and conversion eciency as a function of MOPA average power.
Fig. 4. Frequency-conversion eciency as a function of beam power density for ®rst KTP. The phase mismatch (rad cmÿ1 ) is the parameter.
Fig. 4 shows the expected conversion eciency as a function of infrared beam intensity for the ®rst KTP. The continuous lines were obtained following the theory of Dmitrev et al. [16], considering however the top-hat distribution of the laser beam and triangular temporal shape of the pulses. A good agreement between the experimental data and the theoretical curve that corresponds to a phase mismatch of only 1.0 rad cmÿ1 was obtained. For the second KTP, a phase mismatch of 1.5 rad cmÿ1 was found, indicating again
Y. Hirano et al. / Optics Communications 184 (2000) 231±236
235
Fig. 5. Long time operation of the developed system.
a good agreement between numerical simulations and experimental values. To examine the reliability of the operation of the MOPA system, this was operated continuously for 100 h with a starting green power of 106 W. Fig. 5 shows the MOPA infrared power, the total green power and the green power for each KTP crystal as a function of time. At the end of the experiment, the green power was reduced to 97.4 W. The frequency-conversion eciency varied from 67% at the beginning of the test to 62.3% at the end. The decreasing rate of the MOPA power was 0.01% hÿ1 as deduced from a linear ®t of the output power to time dependence. This decrease was attributed to (i) decrease of the diode-laser power in the oscillator that decreases the oscillator beam power, and (ii) decrease of the diode-laser power in the ampli®er stage that decreases the extraction energies from the rods. The green power delivered by the ®rst KTP crystal decreases with a slope of 0.11% hÿ1 . Based on the ®nal inspection of this KTP (Fig. 6), these decreases were attributed to the gray-tracing eect outside the beam area. No gray tracing eect in the beam area was observed. The decrease in the conversion ef®ciency of ®rst KTP increases the infrared power incident on the second KTP. This results in an increasing rate of 0.3% hÿ1 of the average green power delivered by the second KTP. Finally, the total decreasing rate of green power resulted in a rate of 0.07% hÿ1 .
Fig. 6. A photo of the ®rst KTP after 100 h of operation.
In conclusion, we developed a diode-pumped Nd:YAG rod MOPA system that delivered 222 W average power with a beam quality of M 2 5:2. The repetition rate was 2.5 kHz with a pulse width of 51 ns. By means of an external two-stage KTP crystal architecture, the system produced 131 W an average green power at a frequency-conversion eciency of 65.2%. The system was operated continuously for 100 h, starting with an initial green power of 106 W. The decreasing rate of green power was characterized as 0.07% hÿ1 . To our knowledge, these results are the the highest average green power and the longest time operation at 100-W green power ever reported for a rodtype diode-pumped MOPA system.
References [1] B.J. LeGarrec, G.J. Raze, P.Y. Thro, M. Gilbert, Opt. Lett. 21 (1996) 1990. [2] E.C. Honea, C.E. Ebbers, R.J. Beach, J.A. Speth, J.A. Skidmore, M.A. Emanuel, S.A. Payne, Opt. Lett. 23 (1998) 1203. [3] J.J. Chang, E.P. Dragon, I.L. Bass, in postdeadline papers of CLEO Õ98, May 3±8, San Francisco, 1998, paper CPD2. [4] J. Chang, E. Dragon, C. Ebbers, I. Bass, C. Cochran, in: W.R. Bosenberg, M.M. Fejer (Eds.), Advanced Solid-State Lasers, vol. 19, OSA Trends in Optics and Photonics Series, Washington, D.C. (1998), p. 300.
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[5] R.J. St. Pierre, D.W. Mordaunt, H. Injewan, J.G. Berg, R.C. Hilyard, M.E. Weber, M.G. Wickham, G.M. Harpole, R. Senn, IEEE J. Sel. Top. Quant. Electron. 3 (1997) 53. [6] R.J. St. Pierre, et al., IEEE J. Sel. Top. Quant. Electron. 3 (1997) 64. [7] S. Seidel, G. Mann, Proc. SPIE 2989 (1997) 204. [8] Y. Hirano, N. Pavel, S. Yamamoto, Y. Koyata, T. Tajime, Opt. Commun. 170 (1999) 275. [9] N. Pavel, Y. Hirano, S. Yamamoto, Y. Koyata, T. Tajime, Appl. Opt. 39 (2000) 986. [10] D. Findlay, R.A. Clay, Phys. Lett. 20 (1966) 277.
[11] F.C. Zumsteg, J.D. Bierlin, E.E. Gier, J. Appl. Phys. 47 (1976) 4980. [12] J.M. Layocono, D.N. Laiocono, T. McGee, M. Babb, J. Appl. Phys. Lett. 72 (1992) 2705. [13] J.P. Feve, B. Boulanger, G. Manier, H. Albrecht, Appl. Phys. Lett. 70 (1997) 277. [14] B. Boulanger, M.M. Fejer, R. Blachman, P.F. Bordui, Appl. Phys. Lett. 65 (1994) 2401. [15] D. Eimerl, IEEE J. Quant. Electron. QE-21 (1987) 1361. [16] V.G. Dmitriev, G.G. Gurdadyan, D.N. Nikogosyan, Handbook of Nonlinear Optical Crystals, Springer, Berlin, 1997.