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We present a high-average-power green laser based on second harmonic conversion of a laser diode-pumped master oscillator Nd:YAG power amplifier system. The power amplifier chain includes a stimulated Brillouin scattering (SBS) cell that was used a phase-conjugate mirror to double-pass scheme. That suppresses the thermal phase distortion and compresses the pulse duration. The fundamental beam output power was 670 W with a pulse width of 7.9 ns. A second harmonic power of 335 W with a 4.8-ns pulse width and 80-mJ pulse energy was produced using a LiB3O5 (LBO) crystal.
© 2016 Optical Society of America
1. Introduction
High-average-power green lasers are useful tools for many applications, including laser annealing and laser peening, and act as pump sources for Ti:sapphire laser and optical parametric amplification (OPA) systems. Kojima et al. reported a 412 W (8 kHz, 47 ns) green output for laser annealing [1]. Their system was constructed using a laser diode (LD)-pumped Nd:YAG master oscillator power amplifier (MOPA) system with multiple transverse modes. Powerlase Photonics Ltd. (Crawley, West Sussex, UK) reported a 290 W (10 kHz, 75 ns) green laser system based on a LD-pumped Q-switched Nd:YAG laser with polarizing multiple transverse modes [2]. However, the pulse durations achieved in these previous works are too long for use in OPAs. St. Pierre et al. demonstrated an active tracker laser (ATLAS) based on a zigzag slab Nd:YAG amplifier with a stimulated Brillouin scattering (SBS) phase-conjugate mirror (PCM) [3]. They obtained average output power of 175 W (2.5 kHz, 30 ns) and beam divergence of 1.5 times the diffraction limit. Kiriyama et al. produced an output power of 132 W (1 kHz, 30 ns) to pump a Ti:sapphire laser using a Nd:YAG slab amplifier with an SBS-PCM [4]. Riesbeck et al. reported a green laser with output power of 132 W (2.8 kHz, 160 ns) based on a Nd-doped yttrium-aluminum-perovskite (Nd:YAP) laser system with a fiber SBS-PCM [5,6]. These green laser systems, however, provided pulse durations of more than 30 ns for their applications. In contrast, a high-average-power laser with much shorter pulse length is desirable for use in optical parametric chirped pulse amplification (OPCPA) systems, which would be a powerful tool for generation of high harmonics in the extreme ultraviolet (EUV) region. An OPCPA system would require a high-peak-power and high-average-power visible laser to act as the pump source. For efficient pumping of OPCPA, the required pulse duration is much less than 10 ns. Negel et al. reported a picosecond green laser with output power of 820 W (300 kHz, 2.7 mJ, 7.7 ps) using a Yb:YAG thin disk [7]. The M2 value of the focusable component was better than 2, but the beam wavefront was not as good.
In this paper, we report a green laser system that produces an average power of 335 W. The pulse energy was 80 mJ (4.8 ns pulse width, 16.9 MW peak power) with 15 kHz operation in 28 ms bursts with a 10 Hz repetition rate (duty cycle of 28%). This system is a MOPA laser with frequency doubling. A continuous-wave (CW) fiber laser was used as the master oscillator with precise oscillation wavelength tuning. The pulse-sliced output with a 15 kHz repetition rate was boosted by amplifier chains that included fiber amplifiers and Nd:YAG rods pumped by LDs. A SBS-PCM was installed as a high-reflectivity-mirror in a double-pass configuration in the main amplifier. A cooled LiB3O5 (LBO) crystal was used for second harmonic generation (SHG).
In our laser system, a relay imaging system was introduced to maintain beam quality throughout the laser system. This relay imaging system confined the amplified spontaneous emission (ASE) light that co-propagated with the seed laser light. In the case where the pulsed laser’s pulse train is amplified by the CW-pumped amplifier, the pulse contrast of the input laser is generally an important issue. The CW-pumped amplifier generates a large amount of ASE, which then degrades the pulse contrast and the extraction efficiency. This ASE component increases the thermal effect in the frequency doubling crystal, and degrades the net harmonic conversion efficiency. To improve the pulse contrast, we inserted a Pockels cell into the fiber amplifier chain. The SBS-PCM, which improves the beam quality, also acted as a pulse shortening device and the CW ASE remover. As a result, the SBS-PCM was useful for improvement of the harmonic conversion efficiency.
2. Laser system
Figure 1 shows a schematic diagram of the laser system. This system consists of a fiber front-end, the Nd:YAG rod pre-amplifier chain with a small clear aperture, and the Nd:YAG main rod amplifier chain with the large aperture. Seed laser light was generated using a CW master oscillator (OSC) that was based on a polarization-maintaining Yb-doped single-mode fiber (PM-YDF). The oscillator cavity was composed of a pair of fiber Bragg gratings (FBGs) with reflectivities of 99% and 85%, thus ensuring a bandwidth of 0.15 nm (full width at half maximum: FWHM). To match the oscillation wavelength with the peak gain of the main amplifier, the temperatures of the FBGs are controlled over the 1064.1 to 1065.3 nm range using a Peltier device. The OSC delivered 40 mW of CW light. This CW output was amplitude-modulated to obtain a 15 kHz pulse train, for which the pulse duration was 15 ns (FWHM), using a lithium niobate amplitude modulator (LN-AM). The pulse train was amplified using single mode YDF (SM-YDF) and Yb-doped polarization-maintaining large mode-area-fibers (LMA-YDFs) pumped by a fiber-coupled 978 nm CW LD. The average output power obtained was 350 mW. A Pockels cell (PC) switch was inserted in front of the last LMA-YDF to reduce the pump-induced ASE noise and the CW noise component that originated from the leakage of the LN-AM. The position of the PC was decided to make a maximum pulse energy. Next, the laser light was spatially filtered using a 2 mm diameter hard aperture. The output power of this fiber front-end was 200 mW.
Fig. 1 Schematic diagram of the laser system. PM-YDF: polarization maintaining Yb-doped fiber; OSC: laser oscillator; SMF: single mode fiber; LN-AM: lithium niobate amplitude modulator; LMA-YDF: large-mode-area Yb-doped fiber; AMP: amplifier; LP: afocal lens pair; M: mirror; PBS: polarizing beam splitter cube; HWP: half wave plate; 90R: 90° rotator; FR: Faraday rotator; L: lens; HA: hard aperture; Rod: Nd:YAG rod amplifier; SBS-PCM: stimulated Brillouin scattering phase-conjugate mirror; MA: main amplifier.
The pre-amplifier consisted of two LD-pumped Nd:YAG multi-pass amplifiers stages. The amplifier heads were RB20-1C2 and RD40-1C2 made by Northrop Grumman. The rod diameters were 2 mm and 4 mm for the first (double-pass) and second (four-pass) stages, respectively. In both multi-pass stages, a 90° polarization rotator made from quartz was inserted between the two YAG rods to reduce the thermal birefringence loss. To compensate for the thermal lens effect of the YAG rods, the distance among two convex lens which set between the amplifier head was adjusted [8]. The focal length of the convex lens was 203 mm. The maximum output power of the pre-amplifier was approximately 15 W.
The Nd:YAG main amplifier consisted of a double-pass amplifier (the first stage, called MA-1) and a single-pass amplifier (the second stage, called MA-2). Before injection into MA-1, the beam was expanded to a diameter of 6 mm, and filtered by a 5 mm diameter hard aperture. In MA-1, four YAG rods were used; each of these rods was a 145-mm-long composite ceramic crystal rod with a 7 mm diameter Nd-doped (0.6 at%) area surrounded by a 1.5-mm-thick undoped clad layer. Outside diameter of those is 10 mm. A side pumping geometry was used. Six LDs were arranged around the laser medium with 50 mm distance from the center of the rod. The LD modules pumped the central part (75 mm in length) of each rod, and these modules were operated in burst mode with a 28% duty cycle at 10 Hz to prevent thermal fracture of the Nd:YAG rods. The 90° rotators and the vacuum telescopes with convex lens (focal length is 150 mm) pairs (LP) were also inserted to compensate for thermal effects in the same way as they were used in the pre-amplifier chain. The length of the telescope can be changed by motorized stage with a bellows. The back mirror of MA-1 was replaced with a sealed glass cell (300 mm length) filled with Fluorinert liquid FC-75 (3MTM) which was processed membrane filtration [9]. The laser light was focused to the SBS-PCM by a 150 mm convex lens after expanded by factor of 2 with an afocal lens pair. The power input to MA-1 was controlled using a half-wave plate and a polarizing beam splitter cube while the pump power was fixed, so that the thermal lensing effect and the small signal gain were also fixed. The whole length of the MA-1 was about 5 m. The maximum average output power was 200 W at a total pumping power of 4000 W. The pulse width was shortened to 7.9 ns. The output beam diameter was also reduced to 3.5 mm because of the spatial filtering effect of the SBS-PCM. After MA-1, the beam diameter was then expanded by a factor of 1.5. In MA-2, four amplifier heads, that were similar to those in MA-1, were used. The diameter of the Nd-doped area of the ceramic YAG was 10 mm, and the thickness of the undoped ceramic area was 1.5 mm. The whole length of the MA-2 was 3 m. An average output power of 700 W was achieved at a total pumping power of 4000 W with the same duty cycle.
Finally, the amplified laser pulse was converted to the second harmonic using a 15-mm-long type-I LBO crystal. The LBO crystal was kept at a temperature of 23.0°C using two Peltier cooling devices with an endothermic capacity of 32.7 W. A Faraday rotator was inserted in front of the LBO crystal to avoid amplifier damage due to back-reflection from the LBO crystal surfaces. The depolarization loss of this Faraday rotator was approximately 3% at an input power of 700 W.
3. Results
3.1 Reflection properties of SBS-PCM at high repetition rates
Figure 2 shows the reflectivity of the SBS-PCM for 15 ns pulses as a function of input energy and the repetition rate parameter. The data acquired when using a flash-lamp pumped, Q-switched (15 ns), injection locked Nd:YAG laser operating at 10 Hz are also plotted for comparison. The reflectivity of the SBS-PCM was estimated based on the power transmitted through the SBS cell and the reflected power. The SBS-PCM reflectivity for the high-average-power laser was substantially lower than that for the 10 Hz input, and the maximum reflectivity observed was 44% for an input pulse energy of 45 mJ (corresponding to an average power of 127 W). The SBS threshold energy was approximately 5.5 mJ for the high repetition rate and 3 mJ for 10 Hz operation, respectively. This discrepancy is caused by differences in the beam quality and the ASE and CW noise levels. In the high-average-power laser case, the focused beam intensity is degraded by thermally-induced phase aberrations, which result in reduced SBS reflectivity. However, this phenomenon can be used as a transverse mode filtering method to improve the beam quality, because the SBS-PCM does not reflect the high spatial frequency components of the phase-distorted beam. Therefore, the SBS-PCM acts as a pulse cleaner to remove the ASE and the CW noise that originated in the fiber front-end and the pre-amplifier. Additionally, the energy in the leading edge at the pulse front is expended to enhance an acoustic phonon. As a result, a pulse shortening behavior that is dependent on the input energy is expected in the laser system shown in Fig. 1 [10].
Fig. 2 Reflectivity of SBS-PCM for 10 kHz (solid circles), 15 kHz (gray circles) and 30 kHz (white triangles) repetition rates. The data taken at the 10 Hz repetition rate (white circles) are also plotted for comparison.
Figure 3 shows the output beam patterns of MA-1 when observed using a normal high-reflectivity mirror (HRM) and when using the SBS-PCM. In the HRM case, the near-field pattern (Fig. 3(a)) had a flat-top profile because of the image relay of the hard aperture. The far-field pattern (Fig. 3(b)), which was observed with a 286 mm focal length lens, seems to be of good quality and shows a spot size of 190 µm × 264 µm (1/e2), which is consistent with the diffraction-limited value of 165 μm. However, as shown in Fig. 3(c), many of the higher spatial frequency components exist around the first lobe. In the SBS-PCM case, the near-field pattern (Fig. 3(d)) showed an almost Gaussian distribution. The spot size of the far-field pattern (Fig. 3(e)) was 1.4 times of the diffraction limit (156 µm). The SBS-PCM therefore worked as a spatial filter and removed the higher spatial frequency components, as shown in Fig. 3(f).
Fig. 3 Near- and far-field patterns of MA-1 output beam with normal high-reflectivity mirror (upper photographs, (a)–(c)) and with SBS-PCM (lower photographs, (d)–(f)). (a) and (d): near-field patterns; (b) and (e): far-field patterns observed with a 286-mm focal length lens; (c) and (f): overexposed far-field patterns (by a factor of five) corresponding to (b) and (e), respectively.
3.2 Advantages of SBS-PCM for second harmonic conversion
To confirm the advantages of the SBS-PCM, we checked its SHG performance by placing a LBO crystal (type-I, 15-mm-thick) immediately after MA-1. The pulse energy of the input fundamental was varied by adjusting the rotation angle of the half-wave plate in front of MA-1. Figure 4(a) shows the variation in the second harmonic conversion efficiency with respect to the fundamental pulse energy. The pulse repetition rate was 10 kHz and the duty cycle was 28%. Solid circles indicate the second harmonic conversion efficiency when observed using the SBS-PCM, and the open circles represent the case where the SBS-PCM was replaced with a HRM. As shown in Fig. 4(b), the pulse width was shortened to 9 ns from 15 ns by the SBS-PCM. Thus, for comparison in Fig. 4(a), we adjusted the pulse width to the equivalent value in the HRM case of 9 ns, as shown in Fig. 4(c), using the LN-AM in the front-end. In fact, the pulse duration of the SBS-PCM is long as the input energy into the one increases, so that this situation is disadvantageous to the SBS-PCM. While, the cross section of the SBS-PCM is 1.6 times as small as the one of the HRM as shown in Fig. 3. Even considered these conditions, the difference of the conversion efficiency is remarkable. The SBS-PCM improved both the pulse contrast and the beam quality. Therefore, the second harmonic conversion efficiency was improved by a factor of 2 by introduction of the SBS-PCM.
Fig. 4 (a): Second harmonic conversion efficiency. Closed and open circles denote data measured when using the SBS-PCM and the high-reflectivity mirror, respectively. (b) and (c): Temporal profiles of fundamental light for a pulse energy of 35 mJ in the SBS-PCM and HRM cases, respectively.
3.3 Characterization on final output fundamental beam
Figure 5(a) shows a pulse train of the output laser from MA-2 at 10 kHz repetition rate. A sampling rate of the oscilloscope was 500M points/s. The pulse stability was not so good. A coefficient of variation (C.V.) for the peak value was about 9.2%. The burst pulse train is shown in Fig. 5(b). The sampling rate is 10M points/s. The pumping wavelength of the LDs were slightly swept during the pumping time. As a result, the laser gain was also swept, and the energy of the pulse front decreased. While, the C.V. of the averaged power during 5 minutes was about 1%. In case of the high repetition laser, an amount of a gain recovery depends on the input laser pulse energy. When the large input pulse incidents to the amplifiers, the gain for the next one will be not recovered and decrease. While, in case of the small input pulse, the gain for the next pulse will increase. As a result, the deviation of the pulse energy is emphasized. If the SBS-PCM is inserted the high repetition laser system, the pulse train becomes more unstable because the reflectivity of the SBS-PCM and the duration of the reflected pulse depend on the laser pulse energy. In our laser system, a part of the pulse train was lost when the repetition rate is more than 30 kHz. For the OPCPA, a timing jitter is very important things. In our previously report [11], the timing jitter due to the SBS process was hardly observed. To obtain the stable operation on the SBS-PCM laser system, the input laser pulse energy for the SBS-PCM always has to be over the threshold energy. The SBS-PCM makes the laser light passed through until the energy of the past light is over to the threshold. This phenomenon causes to vary a pulse peak timing. If the past energy surpasses the reflection threshold before the pulse peak passes through the SBS-PCM, the timing jitter of the pulse peak will be suppressed. It will be a good result that the laser pulse is slowly increased and quickly decreased.
Figure 6 shows the output beam patterns of MA-2 when observed using the SBS-PCM on MA-1. The near file pattern (Fig. 6(a)) was a Gaussian like distribution, as same as Fig. 4(d). The far-field pattern (Fig. 6(b)), which was observed with a 286 mm convex lens, seems to be of good quality. The spot size of the far-field pattern was 2.0 times of the diffraction limit (105 µm). However, as shown in Fig. 6(c), higher order spatial components increased.
Fig. 6 Near- and far-field patterns of MA-2 output beam. (a): near-field patterns; (b): far-field patterns observed with a 286-mm focal length lens; (c): overexposed far-field patterns (by a factor of five) corresponding to (b).
3.4 Second harmonic generation by LBO
When selecting one of the candidate nonlinear crystals, KTiOPO4 (KTP), LBO and barium borate (BBO), for use as second harmonic converter materials for the maximum fundamental power of 700 W, we must consider many important issues, including each material’s nonlinear optical coefficient (d coefficient), acceptance angle, temperature characteristics, walk-off and laser-induced damage threshold. The KTP crystal has a high nonlinear optical coefficient. Its angular and thermal acceptances are also broad. However, the KTP damage threshold is the lowest of the three candidates, and more its nonlinear refractive index is the highest among these three crystals. The acceptance angle of BBO is small and its walk-off is large. The LBO crystal is highly suitable for high-average-power SHG because the LBO crystal has a high damage threshold, a large acceptance angle, small walk-off and a low nonlinear refractive index. We therefore selected the LBO crystal for high-average-power SHG.
Figure 7(a) shows the average output power and the conversion efficiency of the second harmonic light as a function of the fundamental power. The pulse repetition rate was 15 kHz, and the input pulse width was approximately 7.9 ns (Fig. 7(b)). An averaged second harmonic power of 335 W was achieved for a fundamental average power of 670 W with a duty cycle of 28%, and the corresponding conversion efficiency was 50%. The pulse energy of the second harmonic reached 80 mJ, with a pulse width of 4.8 ns (Fig. 7(c)).
Fig. 7 (a): Averaged second harmonic power and conversion efficiency as a function of fundamental beam power. (b) and (c): Temporal profiles of the fundamental and second harmonic pulses, respectively.
4. Conclusions
We have developed a high-average-power green laser system. A SBS-PCM was inserted into the high-repetition-rate laser system to improve both the beam quality and the pulse contrast. The reflectivity of the SBS-PCM for the high-average-power (127 W) laser beam was approximately 44%. There were no break-down in the SBS medium. The pulse width of the amplified laser was shortened by approximately half (~7.9 ns) and the improvement in the second harmonic conversion efficiency when using the SBS-PCM was clarified.
A pulse stability was not so good. The C.V. for the pulse to pulse stability was 9.2%. While, the C.V. for the averaged energy is about 1% for 5 minutes. The pulse stability will be improved by the pulse forming. To obtain the jitter free pulse, the laser pulse should be slowly increased and quickly decreased.
For a fundamental beam input power of 670 W at a repetition rate of 15 kHz, a green beam with 335 W output power and conversion efficiency of 50% was achieved, corresponding to pulse energy of 80 mJ with burst operation of a 28% duty cycle. The shortest pulse duration was 4.8 ns, and thus the peak power rose to more than 16.9 MW. This laser system will be useful for materials processing applications and ultra-short-pulse laser systems based on CPA and OPCPA.
Acknowledgment
Part of this work was performed under the auspices of the New Energy and Industrial Technology Development Organization, Japan (NEDO) under the contract subject “Advanced Laser and Processing Technology for Next-generation Materials Project”.
References and links
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