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High efficiency diode-pumped slab oscillator and amplifier for space-based application
Optics & Laser Technology 43 (2011) 559–562
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
High efficiency diode-pumped slab oscillator and amplifier for space-based application Xiuhua Ma n, Jinzi Bi, Xia Hou, Weibiao Chen Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
a r t i c l e in fo
abstract
Article history: Received 15 November 2009 Received in revised form 24 June 2010 Accepted 29 July 2010 Available online 21 August 2010
The design and performance of conduction-cooled, laser diode-pumped oscillator and amplifier slab laser featuring high efficiency, high pulse energy and high beam quality for space-based application are reported. The oscillator was a diode-pumped Q-Switched Nd:YAG slab laser using unstable resonator, and the amplifiers were two zig-zag Nd:YAG slabs based on a side-pumped slab geometry. A near diffraction-limited output of 450 mJ in a 10 ns pulse at a repetition rate of 20 Hz was obtained, corresponding to an optical-to-optical conversion efficiency of over 20%. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Zig-zag slab Diode-pumped lasers Conduction-cooled
1. Introduction In recent years, high efficiency and high beam quality laser technology has made great progress. High efficiency and high beam quality have been achieved through the use of: (1) diode pumps that reduce the waste heat and decrease thermallyinduced aberrations; (2) a zig-zag slab geometry that compensates for aberrations in one dimension; (3) precise control of the temperature distribution in the slab; (4) an unstable resonator to provide a large volume single transverse mode; (5) masteroscillator-power-amplifier (MOPA) to achieve high pulse energy and narrow pulse width. For space-based lasers used for wind, atmosphere, aerosols and vegetation measurment, high pulse energy 300–500 mJ, short pulse duration 10 ns and high beam quality are required. These lasers can be achieved by diode-pumped master oscillator power amplifiers owning to their high efficiecy, robustness, reliability and compact packaging. High beam quality and short pulse duration are determined by the oscillator, pulse energy and power are determined by amplifier. For space-based lasers, conduction cooling is a more reliable method for heat removal compared with liquid cooling, as it has simplified construction, protects the total internal reflection (TIR) of the zig-zag slab laser median, reduces vibration and saves energy. Conduction-cooled Q-Switched zig-zag slab lasers recommended strongly to be used in spaceborne platforms, and several lasers have been used in space-based lidar systems, such as Clementine (a lunar altimeter) [1], Mars Orbiter Laser Altimeter (MOLA2) [2], Near Earth Asteroid Rendezvous (NEAR) [3], n
Corresponnding author. E-mail address: [email protected] (X. Ma).
0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.07.016
Geoscience Laser Altimeter System (GLAS) [4–6] and Mercury Laser Altimeter (MLA) [7]. For the potential application of a spaceborne lidar, there is great motivation to develop conductioncooled all solid-state zig-zag slab lasers. In this paper, a laser-diode-pumped master oscillator power amplifier architecture was chosen to achieve high efficiency, high pulse energy and high beam quality laser output. The design and experiment result of the laser will be discussed.
2. Design of the oscillator and amplifier The experiment setup of the oscillator and amplifier is shown in Fig. 1a. The oscillator consists of Nd:YAG zig-zag slab, polarizer, Q-switch, quarter wave plate (QW), high reflective mirror (HRM) and output coupler with variable reflectivity mirror (VRM). The oscillator, an Nd:YAG slab with uncoated Brewster angle ends, was pumped from one end and conduction-cooled with a 5 7 mm2 aperture, was 78.8 mm long and provided a 10-bounce zig-zag path through the medium. The laser head is shown in Fig.1b. To increase the absorption of the pump light and improve pump efficiency, the pumped surface was AR coated at 808 nm pump wavelength. A SiO2 layer with thickness of 2.5 mm and HR coatings were deposited on the cooling surface. Retroflection of the pump light increases the absorption efficiency and helps to make the spatial distribution of absorbed power uniform across the width of the slab. The other two sides of the slab were ground to reduce specular reflections that could lead to parasitic oscillations. The zig-zag slab was bonded to the t/w alloy featuring the same coefficient of thermal expansion by a thermally conductive adhesive for heat removing. A 60 bar laser diode array was used to pump from the bottom with the fast axis
560
X. Ma et al. / Optics & Laser Technology 43 (2011) 559–562
parallel to length of the slab. Each bar provided a typical pulse energy 12 mJ at 70 A pulsed operating current in a squaredshaped pulse of 200 ms duration. The output was collimated by a plano-convex cylindrical lens fabricated from ZF6 with refractive index 1.76. This made the spatial distribution of the pump power uniform along the width of the slab. In order to maintain a large beam waist in the cavity and high optical quality laser output from the oscillator, an unstable cavity with Gaussian Reflectivity Mirror (GRM) couple was used. The optimized cavity length was 40 cm, the radius of curvature of the variable-reflectivity output coupler and the rear mirror were 1.5 and 2 m, respectively. The magnification of unstable cavity was 1.30. The VRM output coupler had a peak reflectivity of 0.3 and a spot size (half-width at 1/e2 of the intensity field) of 2 mm. The Nd:YAG dual slab amplifiers used in the amplifier section were oriented at near normal incidence that simplified AR
coatings design. The slab amplifiers were 96 mm long, had 5 7 mm2 cross sections and were coated on the total internal reflection (TIR) surface. All of the waste heat was removed by a liquid-cooled panel through the heat pipe. The beam from the oscillator with a 4 4 mm2 square profile double passed through the amplifiers to improve their extraction efficiency. Separation of the input and output beams was accomplished through the standard polarization separation techniques. The linearly polarized oscillator beam was transmitted through a polarizer at near normal incidence and was output to the amplifiers. The beam was then reflected by the same polarizer after double passed a Quarter Waveplate (QW) in the amplifier chain. The amplifier performance was modeled using simple Frantz and Nodvic [8] approach. Values of 70 W/bar and 72 bars/amplifier were used in the models. The calculated results of amplifier extraction for single pass and double pass with the signal pulse energy 40, 60, 80, 100 mJ from the oscillator are shown in Fig. 2. The maximum predicted pulse energies of 360 and 460 mJ were extracted from the amplifier for one and double pass amplifier.
3. Experiment result of the oscillator and amplifier
Fig. 1. (a) Optical schematic of the oscillator and amplifier, (b) The laser head.
Fig. 3 shows the measured pulse energy as a function of the LD optical pump energy at a repetition rate of 20 Hz with pulse energies of 20, 30, 40, 50, 60 mJ input to the amplifier. The output pulse energy linearly increased with the pump energy. The maximum output pulse energies of 350 and 450 mJ were extracted from the amplifier for single and double pass amplification, which was well in accordance with the model prediction of Fig. 2. This corresponds to optical-to-optical conversion efficiencies of 14% and 20%, respectively. The pulse shape from the amplifier compared with the pulse shape from the oscillator is shown in Fig. 4. The output pulse duration was shortened from 11.3 ns (FWHM) from the oscillator to 10 ns (FWHM) owing to saturation effect. The peak power of the laser output was 45 MW. To evaluate the spatial properties for the laser beam, the beam quality measurement was made using a Spiricon M2-200 laser beam analyzer for the highest output pulse energy, and the measurement result is shown in Fig. 5. The zig-zag axis had an M2 of 2.3 and the non-zigzag axis had an M2 of 1.6. As the input beam quality has an M2 of 1.4 in both axes, this implies that the output beam quality from the dual amplifiers is near diffraction limited. Finally the beam polarization ratio is
0.5
0.4 Ein=40mJ Ein=60mJ Ein=80mJ Ein=100mJ
Output Energy/J
0.3
0.45 0.4 Output Energy/J
0.35
0.25 0.2 0.15
0.35 0.3 0.25 0.2
0.1
0.15
0.05
0.1
0
0
0.2
0.4
0.6 0.8 1 Pump energy/J
1.2
1.4
1.6
Ein=40mJ Ein=60mJ Ein=80mJ Ein=100mJ
0.05 0.2
0.4
0.6
0.8 1 1.2 Pump energy/J
1.4
1.6
Fig. 2. Typical modeled result of amplifier extraction for one pass and double pass with different pulse energy input to the amplifiers.
X. Ma et al. / Optics & Laser Technology 43 (2011) 559–562
0.5
0.35
0.45
Ein=0.02J
Ein=0.04J
0.4
Ein=0.04J
Ein= 0.05J
0.35
Ein=0.02J
0.3
Output Energy/J
Ein=0.03J
0.25 Output Energy/J
561
Ein=0.06J
0.2 0.15 0.1
Ein=0.03J Ein= 0.05J Ein=0.06J
0.3 0.25 0.2 0.15 0.1
0.05 0
0.05 0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Pump Energy/J
2
0
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Pump Energy/J
2
Fig. 3. Experiment result of amplifier energy extraction for the one and double pass with different pulse energy input to the amplifiers.
Fig. 4. Temporal profile of the amplified Q-switched laser pulse compared with the pulse profile from the oscillator.
Fig. 5. 20 Hz, 450 mJ per pulse beam quality measurement.
562
X. Ma et al. / Optics & Laser Technology 43 (2011) 559–562
great than 99% through measurement of the ratio of the p and s components.
4. Conclusion In this paper, the development of a diode-pumped, high energy, high efficiency and high beam quality conduction-cooled Nd:YAG laser for potential space-based application was discussed. Greater than 450 mJ with 10 ns pulse duration and high beam quality laser output has been achieved at a repetition rate of 20 Hz. The laser can be scaled to 500 mJ–1 J at a repetition rate of over 100 Hz through use of a master oscillator power amplifier. This type of laser will be developed in the near future and also opens up new areas of interest in space-based platform and its applications.
References [1] Kushina Mark E, Russek Joe B. Clementine: diode-pumped laser qualification. SPIE 1995;2379:137. [2] Afzal RS. Mars observer laser altimeter: laser transmiter. Opt Appl 1994;33:3184. [3] Cole TD, EI-Dinary A. Laser radar instrument for the Near-Earth Asteroid Rendezvous (NEAR) mission. SPIE 1996;2748:122. [4] Dallas JL, Stephen MA. Demonstration and characterization of a multibillionshot, 2.5 mJ, 4 ns, Q-switched Nd:YAG laser. Opt Appl 1996;35:1427. [5] Afzal RS, Fan TY. Single-mode high peak power passively Q-switched diodepumped Nd:YAG laser. Opt Lett 1997;22:1314. [6] Afzal RS, Selkner MD. Simple high-efficiency TEM00 diode-laser pumped Q-switched laser. Opt Lett 1995;20:465. [7] Krebs Danny J, Yu Anthony W. Compact, passively Q-switched Nd:YAG laser for the MESSENGER mission to Mercury. Opt Appl 1715;2005:44. [8] Frantz LM, Nodvik JS. Theory of pulse propagation in a laser amplifier. Appl Phys 1963;34:2346.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
High efficiency diode-pumped slab oscillator and amplifier for space-based application Xiuhua Ma n, Jinzi Bi, Xia Hou, Weibiao Chen Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
a r t i c l e in fo
abstract
Article history: Received 15 November 2009 Received in revised form 24 June 2010 Accepted 29 July 2010 Available online 21 August 2010
The design and performance of conduction-cooled, laser diode-pumped oscillator and amplifier slab laser featuring high efficiency, high pulse energy and high beam quality for space-based application are reported. The oscillator was a diode-pumped Q-Switched Nd:YAG slab laser using unstable resonator, and the amplifiers were two zig-zag Nd:YAG slabs based on a side-pumped slab geometry. A near diffraction-limited output of 450 mJ in a 10 ns pulse at a repetition rate of 20 Hz was obtained, corresponding to an optical-to-optical conversion efficiency of over 20%. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Zig-zag slab Diode-pumped lasers Conduction-cooled
1. Introduction In recent years, high efficiency and high beam quality laser technology has made great progress. High efficiency and high beam quality have been achieved through the use of: (1) diode pumps that reduce the waste heat and decrease thermallyinduced aberrations; (2) a zig-zag slab geometry that compensates for aberrations in one dimension; (3) precise control of the temperature distribution in the slab; (4) an unstable resonator to provide a large volume single transverse mode; (5) masteroscillator-power-amplifier (MOPA) to achieve high pulse energy and narrow pulse width. For space-based lasers used for wind, atmosphere, aerosols and vegetation measurment, high pulse energy 300–500 mJ, short pulse duration 10 ns and high beam quality are required. These lasers can be achieved by diode-pumped master oscillator power amplifiers owning to their high efficiecy, robustness, reliability and compact packaging. High beam quality and short pulse duration are determined by the oscillator, pulse energy and power are determined by amplifier. For space-based lasers, conduction cooling is a more reliable method for heat removal compared with liquid cooling, as it has simplified construction, protects the total internal reflection (TIR) of the zig-zag slab laser median, reduces vibration and saves energy. Conduction-cooled Q-Switched zig-zag slab lasers recommended strongly to be used in spaceborne platforms, and several lasers have been used in space-based lidar systems, such as Clementine (a lunar altimeter) [1], Mars Orbiter Laser Altimeter (MOLA2) [2], Near Earth Asteroid Rendezvous (NEAR) [3], n
Corresponnding author. E-mail address: [email protected] (X. Ma).
0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.07.016
Geoscience Laser Altimeter System (GLAS) [4–6] and Mercury Laser Altimeter (MLA) [7]. For the potential application of a spaceborne lidar, there is great motivation to develop conductioncooled all solid-state zig-zag slab lasers. In this paper, a laser-diode-pumped master oscillator power amplifier architecture was chosen to achieve high efficiency, high pulse energy and high beam quality laser output. The design and experiment result of the laser will be discussed.
2. Design of the oscillator and amplifier The experiment setup of the oscillator and amplifier is shown in Fig. 1a. The oscillator consists of Nd:YAG zig-zag slab, polarizer, Q-switch, quarter wave plate (QW), high reflective mirror (HRM) and output coupler with variable reflectivity mirror (VRM). The oscillator, an Nd:YAG slab with uncoated Brewster angle ends, was pumped from one end and conduction-cooled with a 5 7 mm2 aperture, was 78.8 mm long and provided a 10-bounce zig-zag path through the medium. The laser head is shown in Fig.1b. To increase the absorption of the pump light and improve pump efficiency, the pumped surface was AR coated at 808 nm pump wavelength. A SiO2 layer with thickness of 2.5 mm and HR coatings were deposited on the cooling surface. Retroflection of the pump light increases the absorption efficiency and helps to make the spatial distribution of absorbed power uniform across the width of the slab. The other two sides of the slab were ground to reduce specular reflections that could lead to parasitic oscillations. The zig-zag slab was bonded to the t/w alloy featuring the same coefficient of thermal expansion by a thermally conductive adhesive for heat removing. A 60 bar laser diode array was used to pump from the bottom with the fast axis
560
X. Ma et al. / Optics & Laser Technology 43 (2011) 559–562
parallel to length of the slab. Each bar provided a typical pulse energy 12 mJ at 70 A pulsed operating current in a squaredshaped pulse of 200 ms duration. The output was collimated by a plano-convex cylindrical lens fabricated from ZF6 with refractive index 1.76. This made the spatial distribution of the pump power uniform along the width of the slab. In order to maintain a large beam waist in the cavity and high optical quality laser output from the oscillator, an unstable cavity with Gaussian Reflectivity Mirror (GRM) couple was used. The optimized cavity length was 40 cm, the radius of curvature of the variable-reflectivity output coupler and the rear mirror were 1.5 and 2 m, respectively. The magnification of unstable cavity was 1.30. The VRM output coupler had a peak reflectivity of 0.3 and a spot size (half-width at 1/e2 of the intensity field) of 2 mm. The Nd:YAG dual slab amplifiers used in the amplifier section were oriented at near normal incidence that simplified AR
coatings design. The slab amplifiers were 96 mm long, had 5 7 mm2 cross sections and were coated on the total internal reflection (TIR) surface. All of the waste heat was removed by a liquid-cooled panel through the heat pipe. The beam from the oscillator with a 4 4 mm2 square profile double passed through the amplifiers to improve their extraction efficiency. Separation of the input and output beams was accomplished through the standard polarization separation techniques. The linearly polarized oscillator beam was transmitted through a polarizer at near normal incidence and was output to the amplifiers. The beam was then reflected by the same polarizer after double passed a Quarter Waveplate (QW) in the amplifier chain. The amplifier performance was modeled using simple Frantz and Nodvic [8] approach. Values of 70 W/bar and 72 bars/amplifier were used in the models. The calculated results of amplifier extraction for single pass and double pass with the signal pulse energy 40, 60, 80, 100 mJ from the oscillator are shown in Fig. 2. The maximum predicted pulse energies of 360 and 460 mJ were extracted from the amplifier for one and double pass amplifier.
3. Experiment result of the oscillator and amplifier
Fig. 1. (a) Optical schematic of the oscillator and amplifier, (b) The laser head.
Fig. 3 shows the measured pulse energy as a function of the LD optical pump energy at a repetition rate of 20 Hz with pulse energies of 20, 30, 40, 50, 60 mJ input to the amplifier. The output pulse energy linearly increased with the pump energy. The maximum output pulse energies of 350 and 450 mJ were extracted from the amplifier for single and double pass amplification, which was well in accordance with the model prediction of Fig. 2. This corresponds to optical-to-optical conversion efficiencies of 14% and 20%, respectively. The pulse shape from the amplifier compared with the pulse shape from the oscillator is shown in Fig. 4. The output pulse duration was shortened from 11.3 ns (FWHM) from the oscillator to 10 ns (FWHM) owing to saturation effect. The peak power of the laser output was 45 MW. To evaluate the spatial properties for the laser beam, the beam quality measurement was made using a Spiricon M2-200 laser beam analyzer for the highest output pulse energy, and the measurement result is shown in Fig. 5. The zig-zag axis had an M2 of 2.3 and the non-zigzag axis had an M2 of 1.6. As the input beam quality has an M2 of 1.4 in both axes, this implies that the output beam quality from the dual amplifiers is near diffraction limited. Finally the beam polarization ratio is
0.5
0.4 Ein=40mJ Ein=60mJ Ein=80mJ Ein=100mJ
Output Energy/J
0.3
0.45 0.4 Output Energy/J
0.35
0.25 0.2 0.15
0.35 0.3 0.25 0.2
0.1
0.15
0.05
0.1
0
0
0.2
0.4
0.6 0.8 1 Pump energy/J
1.2
1.4
1.6
Ein=40mJ Ein=60mJ Ein=80mJ Ein=100mJ
0.05 0.2
0.4
0.6
0.8 1 1.2 Pump energy/J
1.4
1.6
Fig. 2. Typical modeled result of amplifier extraction for one pass and double pass with different pulse energy input to the amplifiers.
X. Ma et al. / Optics & Laser Technology 43 (2011) 559–562
0.5
0.35
0.45
Ein=0.02J
Ein=0.04J
0.4
Ein=0.04J
Ein= 0.05J
0.35
Ein=0.02J
0.3
Output Energy/J
Ein=0.03J
0.25 Output Energy/J
561
Ein=0.06J
0.2 0.15 0.1
Ein=0.03J Ein= 0.05J Ein=0.06J
0.3 0.25 0.2 0.15 0.1
0.05 0
0.05 0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Pump Energy/J
2
0
0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Pump Energy/J
2
Fig. 3. Experiment result of amplifier energy extraction for the one and double pass with different pulse energy input to the amplifiers.
Fig. 4. Temporal profile of the amplified Q-switched laser pulse compared with the pulse profile from the oscillator.
Fig. 5. 20 Hz, 450 mJ per pulse beam quality measurement.
562
X. Ma et al. / Optics & Laser Technology 43 (2011) 559–562
great than 99% through measurement of the ratio of the p and s components.
4. Conclusion In this paper, the development of a diode-pumped, high energy, high efficiency and high beam quality conduction-cooled Nd:YAG laser for potential space-based application was discussed. Greater than 450 mJ with 10 ns pulse duration and high beam quality laser output has been achieved at a repetition rate of 20 Hz. The laser can be scaled to 500 mJ–1 J at a repetition rate of over 100 Hz through use of a master oscillator power amplifier. This type of laser will be developed in the near future and also opens up new areas of interest in space-based platform and its applications.
References [1] Kushina Mark E, Russek Joe B. Clementine: diode-pumped laser qualification. SPIE 1995;2379:137. [2] Afzal RS. Mars observer laser altimeter: laser transmiter. Opt Appl 1994;33:3184. [3] Cole TD, EI-Dinary A. Laser radar instrument for the Near-Earth Asteroid Rendezvous (NEAR) mission. SPIE 1996;2748:122. [4] Dallas JL, Stephen MA. Demonstration and characterization of a multibillionshot, 2.5 mJ, 4 ns, Q-switched Nd:YAG laser. Opt Appl 1996;35:1427. [5] Afzal RS, Fan TY. Single-mode high peak power passively Q-switched diodepumped Nd:YAG laser. Opt Lett 1997;22:1314. [6] Afzal RS, Selkner MD. Simple high-efficiency TEM00 diode-laser pumped Q-switched laser. Opt Lett 1995;20:465. [7] Krebs Danny J, Yu Anthony W. Compact, passively Q-switched Nd:YAG laser for the MESSENGER mission to Mercury. Opt Appl 1715;2005:44. [8] Frantz LM, Nodvik JS. Theory of pulse propagation in a laser amplifier. Appl Phys 1963;34:2346.