A passively Q-switched sub-nanosecond master oscillator power amplifier (MOPA) laser system at 1064 nm has been reported in this paper. The master oscillator was a passively Q-switched YAG/Nd:YAG/Cr4+:YAG microchip laser, yielding a pulse energy of 0.14 mJ and a pulse width of ∼490 ps at repetition rates of 500 Hz and 1 kHz. After passing a double-pass side-pumped Nd:YAG amplification system, the pulse energy reached 7.6 mJ and 1.7 mJ at 500 Hz and 1 kHz, respectively. The spatial beam deformation caused by the thermally induced birefringence was investigated numerically and experimentally.
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As one of the most important laser applications, lidar has advantages of high spatial and temporal resolution, compactness and strong interaction with the atmospheric constitutes. Because of the stability, power efficiency and the availability of photodetectors with high efficiency, Nd3+-doped solid-state lasers are widely used in long range lidar [1–4]. In recent years, the advancement of photon counting detectors and timers promotes the development of single photon lidar and finds a balance between long distance and high accuracy detection [5,6]. Lasers with sub-nanosecond pulse width are the key component of the single photon lidar to reduce the temporal ambiguity and increase the detection accuracy.
In recent years, several methods have been proposed to obtain laser pulses with short pulse width including laser diode (LD) modulation, mode-locking technique and Q-switching technique [6–11]. The LD modulation requires large bandwidth electrical waveform generators and high-speed intensity modulators. The repetition rate of the passively mode-locking lasers is not actively controlled. Both these approaches have a low output energy and need a multi-stage amplifier to increase the energy for applications. Q-switching technique is an effective approach to produce sub-nanosecond laser pulses with high pulse energy. Compared to actively Q-switching, passively Q-switching technique has advantages of compactness, simplicity and single frequency operation [12–16]. In 2008, a Nd:YAG/Cr4+:YAG microchip laser was presented, yielding a pulse energy of 0.97 mJ and a pulse width of 460 ps at a repetition rate of 100 Hz . In 2011, Bhandari et al. reported a linearly polarized passively Q-switched sub-nanosecond laser using  cut Cr4+:YAG. Laser pulses with a pulse energy of 3 mJ and a pulse width of 365 ps were achieved at a repetition rate of 100 Hz . Because the average power is limited by the heat generation in the gain media, several heat-dissipation structures have been developed to reduce the thermal effects [17–19]. In 2014, Ge et al. developed a composite YAG/Nd:YAG/YAG transparent ceramic, with heat removed from the two-faces of Nd:YAG . In 2017, Zheng et al. reported a tiny integrated laser with a sapphire/Nd:YAG based a distributed face cooling chip, achieving a high capability of thermal reduction . The master oscillator power amplifier (MOPA) configuration enables the output laser beam characteristics well defined at low energy by the oscillator, while the energy scaling is accomplished by the amplifier [20–22]. The severe thermal effects in the amplifier restrict the performance of MOPA system, but there are few investigations on the thermally induced birefringence in MOPA configurations [23–25].
In this paper, we demonstrated a passively Q-switched sub-nanosecond MOPA laser system at 1064 nm. The master oscillator was a YAG/Nd:YAG/Cr4+:YAG microchip laser. Laser pulses with a pulse width of ∼490 ps and a pulse energy of 0.14 mJ were obtained at repetition rates of 500 Hz and 1 kHz. Two LD side-pumped Nd:YAG modules were used to scale up the pulse energy. In order to explain the spatial beam deformation during the amplification process, the thermally induced birefringence in the LD side-pumped modules was investigated numerically and experimentally. The simulated beam profiles were in good agreement with experiment results. After amplification with a double-pass side-pumped amplification system, the pulse energy was amplified to 7.6 mJ and 1.7 mJ, respectively. The maximum peak power was calculated to be 15.5 MW at the repetition rate of 500 Hz. By using a focusing lens with a focal length of 15 mm, air breakdown was observed with an output pulse energy over ∼3 mJ at 500 Hz.
2. Experimental setup
Figure 1 shows the experimental setup of the sub-nanosecond MOPA laser system. A LD pumped passively Q-switched YAG/Nd:YAG/Cr4+:YAG microchip laser is utilized as the master oscillator. The pump source is a fiber coupled 808 nm LD with a core diameter of 400 µm. The pump laser beam is re-imaged into the composite crystal by a pair of aspherical lenses with a coupling ratio of 1:1.5. The composite crystal consists of a 1.5-mm-long undoped YAG crystal, a 1.1-at.%-doped 1.5-mm-long Nd:YAG crystal and a 1.5-mm-long Cr4+:YAG crystal with an initial transmission of 50%. The undoped YAG crystal is used to reduce the thermal effect in the gain medium. The Cr4+:YAG crystal is cut in the  direction to obtain polarized output [26–29]. One end of the composite rod is coated for high reflection (HR) at 1064 nm and high transmission (HT) at 808 nm while the other end is coated for partial transmission at 1064 nm. A Faraday isolator is utilized to prevent potential feedback-light-induced damage to the master oscillator.
Because the polarization direction of the laser is rotated by 45° after passing through the Faraday isolator, a half-wave plate was inserted as a compensator. After passing through a polarized beam splitter (PBS) and a plane mirror M1 (HR at 1064 nm at 45°), the laser pulses are introduced to LD side-pumped module I and LD side-pumped module II. In order to reduce the size of the MOPA laser system, the two LD side-pumped modules are placed serially. Besides, they are also specially designed, with the pumping uniformity and the thermal-lensing effect of the gain medium improved obviously . Each of the LD side-pumped modules has a Nd:YAG rod with a dimension of Φ5 mm × 80 mm and a doping concentration of 0.8 at.%. In order to prevent parasitic oscillation, the Nd:YAG rods are cut with 2° wedge on both facets. The maximum pump energy of each LD side-pumped Nd:YAG module is 500 mJ at 500 Hz, while it is 360 mJ at 1 kHz to avoid thermal damage under high average power. The plane mirror M2 is HR coated at 1064 nm. After passing through the quarter-wave plate twice the polarization direction of the laser beam is rotated by 90° and the laser beam is outputted from the PBS.
The temperature of the LD, the composite crystal and the two LD side-pumped Nd:YAG modules is maintained at 25°C with a water-cooling system. A digital pulse generator (DG535, Stanford Research Systems Inc.) is used to control the system timing. The pump pulse duration of the LD, LD side-pumped module I and LD side-pumped module II is set to be 230 µs. The pulse characters are measured by a fast photodiode (ET-3500, Electro-Optics Technology, Inc. rising time: 25 ps) and recorded by a digital oscilloscope (DPO7104, Tektronix Inc. bandwidth: 1 GHz).
3. Results and discussion
3.1 Output performance of the master oscillator
The experiments were carried out at repetition rates of 500 Hz and 1 kHz. The pulse energy and pulse width were ∼0.14 mJ and ∼490 ps both at 500 Hz and 1 kHz, respectively. The temporal pulse trains and the single pulse profiles of the master oscillator at 500 Hz and 1 kHz are shown in Fig. 2. To show the amplitude stability of the pulse trains, 100 pulses were captured in Fig. 2(a) and Fig. 2(b). The laser pulses showed good amplitude stability at both 500 Hz and 1 kHz. The corresponding coefficient of variations (CV, the ratio of the standard deviation to the mean) were calculated to be ∼3.6% and ∼5.4%, respectively. Because of serious thermal effects at the high repetition rate, the amplitude stability got worsen at 1 kHz, compared to that at 500 Hz. The pulse profiles are given in Fig. 2(c) and Fig. 2(d), which show a good symmetry.
The spatial beam distributions of the master oscillator were measured by a laser beam analyzer (LBA-712PC-D, Spiricon Inc.), as shown in Fig. 3. The beam profile had a good symmetry at 500 Hz and agreed with the Gaussian distribution. However, because of the high pump power at 1 kHz, the beam profile presented an approximative diamond distribution. The reason for this phenomenon can refer to the analysis of the thermally induced birefringence in the LD side-pumped modules in the next section.
By using the travelling knife-edge method, the beam radius variations of the master oscillator were measured, as shown in Fig. 4. The beam quality factors in the two orthogonal directions were calculated to be Mx2 = 1.47 and My2 = 1.50 at 500 Hz. The beam quality factors were Mx2 = 1.66 and My2 = 1.69 at 1 kHz.
3.2 Analysis for the thermally induced birefringence in LD side-pumped Nd:YAG modules
The Nd:YAG crystal has a cubic lattice and exhibits optical isotropy in the unstressed case. Under high power pumping condition, the thermally induced birefringence becomes serious. Figure 5 shows the setup for investigating the thermally induced birefringence in the laser amplifier. Because of the thermally induced birefringence in the LD side-pumped modules, the polarization direction changes during the amplification process, and the extraordinary light is separated out by the PBS II. Two charged coupled devices (CCD) and the corresponding filters are used to capture the ordinary and extraordinary beam distributions.
The fluorescence intensity distributions across the section of the Nd:YAG rods in the LD side-pumped modules were displayed by a CMOS camera and are shown in Fig. 6. The fluorescence intensity distributions showed a good uniformity across the section of the gain mediums. The difference between the fluorescence intensity distributions was mainly caused by the emission spectrum difference of the diode bars. It can be assumed that the pump distribution was independent of the angular direction.
Because the Nd:YAG rods are cut in the  direction, the indicatrix changes from a sphere in the unstressed case to an ellipsoid in the stressed situation [30,31]. The indicatrix across the section of the Nd:YAG rod is a ellipse, with one axis (nr) along the radial direction and the other (nφ) along the angular direction. Then the polarization ratio of a single point in the amplified laser beam can be expressed as
As for the T(r), we have proposed a method to calculate the temperature difference across the section of the Nd:YAG rod based on the fluorescence intensity distribution in our previous work . Figure 7 shows the temperature difference distribution along the radial direction under a pump condition of 500 Hz, 500 mJ and 1 kHz, 360 mJ, respectively. The temperature differences of the two LD side-pumped modules were small. The maximum temperature differences were about 12.5 K and 18.7 K at 500 Hz, 500 mJ and 1 kHz, 360 mJ, respectively. Based on the simulated temperature differences, the corresponding phase differences along the radial direction were calculated, as shown in Fig. 8. The maximum phase differences were about 2.1 and 3.2 at 500 Hz, 500 mJ and 1 kHz, 360 mJ, respectively.
The captured fluorescence intensity distributions across the section of the Nd:YAG rods in the LD side-pumped modules showed a good uniformity and the amplification in this experiment could be classified as the small-signal amplification . Therefore, the assumption that the laser beam profiles kept Gaussian distribution during the amplification process was made in the simulation. The laser beam profiles with different polarization directions could be given, as shown in Fig. 9. With the pump energy increasing, the phase difference between the center and the boundary increased and the extraordinary light moved toward the center of the Nd:YAG rods. Because the maximum phase difference was about δmax = 3.2×2 = 6.4, bigger than 2π at 1 kHz and 720 mJ, four entire extraordinary beams were observed in Fig. 9(d1).
The measured beam profiles with different pump energy at 500 Hz and 1 kHz are shown in Fig. 10 and Fig. 11, respectively. It could be seen that there was good agreement between theoretical and experimental results. The extraordinary light moved toward the beam center, with the pump power increasing. Four entire extraordinary beams were also observed at 1 kHz and 720 mJ as shown in Fig. 11(d1), which was in coincidence with the simulation and verified the accuracy of the calculated phase difference. It can be seen that the laser beam profiles were nearly in Gaussian distribution with the increasing of pump energy. Besides, the thermally induced birefringence mainly affected the boundary of the Nd:YAG rods at low pump energy, and the diffraction phenomenon could be observed in Fig. 10(a1) and Fig. 11(a1).
The output energy characteristic of the single-pass amplification is shown in Fig. 12. With a pump energy of 1000 mJ, the pulse energy of the ordinary light and the extraordinary light reached 1.04 mJ and 0.23 mJ respectively at 500 Hz, corresponding to an amplification factor of ∼9.1 and an overall polarization ratio of 81.9%. With a pump energy of 720 mJ, the pulse energy of the ordinary light and the extraordinary light reached 0.52 mJ and 0.17 mJ respectively at 1 kHz, corresponding to an amplification factor of ∼4.9 and an overall polarization ratio of 75.4%. The smaller overall polarization ratio of the laser beam after the single-pass amplification at 1 kHz was in accordance with the more serious thermally induced birefringence. Due to the low extraction efficiencies and high amplification factors, it could be concluded that the amplification process in this experiment belonged to the small-signal amplification, which confirmed our assumption.
3.3 Output performance of the double-pass amplification
The double-pass amplification experiment was carried out with the experimental setup shown in Fig. 1. The temporal pulse trains and single pulse profiles after amplification are shown in Fig. 13. The corresponding CVs were calculated to be ∼5.0% and ∼7.3% at 500 Hz and 1kHz, respectively. The laser pulses maintained a good amplitude stability after the amplification at repetition rates of 500 Hz and 1 kHz. Besides, the pulse profiles kept the pulse width of ∼490 ps after amplification.
The output energy characteristic of the double-pass amplification is shown in Fig. 14. Under the maximum pump energy of the two LD side-pumped Nd:YAG modules, the output energy of the sub-nanosecond MOPA laser system reached 7.6 mJ and 1.7 mJ at 500 Hz and 1 kHz, respectively. The corresponding peak power were 15.5 MW and 3.5 MW, respectively. The maximum amplification factors reached 54.3 and 12.1, respectively. Due to the restriction of high pump power, the output energy was lower at 1 kHz compared to that at 500 Hz.
The beam radius variations after the double-pass amplification were measured, as shown in Fig. 15. The beam quality factors were calculated to be Mx2 = 2.15 and My2 = 2.17 at 500 Hz, while they were Mx2 = 2.94 and My2 = 3.05 at 1 kHz. The beam quality deterioration in the amplification is supposed to be related with the spherical phase aberration caused by the thermal effects . The utilization of the two special-designed LD side-pumped modules with good pumping uniformity was useful to reduce the spherical phase aberration and relieve the deterioration of the beam quality after amplification.
Laser beam profiles after the double-pass amplification at different pump repetition rates and pump energy were measured, as shown in Fig. 16. The laser beam profiles were affected by the thermally induced birefringence obviously. As the maximum optical path difference between the ordinary and extraordinary light was more than 2λ under the double-pass amplification at 1 kHz, two zero points appeared in the laser beam profile along the 45° direction in each quadrant, as shown in Fig. 16(b4). The thermally induced birefringence mainly affected the border of the laser beam profiles. Besides the laser beam became more divergent due to the thermal lens effects. The returned extraordinary light was not able to be captured due to the restriction of the Faraday isolator’s aperture.
By using a focusing lens with a focal length of 15 mm, air breakdown was observed with the output energy of ∼3 mJ at 500 Hz. Figure 17 shows the captured air breakdown under the maximum output energy at 500 Hz. It showed the potential in the application of laser diagnostics and laser ignition.
In this paper, a high-pulse-energy sub-nanosecond MOPA laser system at 1064 nm was developed. The master oscillator was a YAG/Nd:YAG/Cr4+:YAG microchip laser, producing stable sub-nanosecond laser pulses with a pulse width of ∼490 ps. After amplification with two LD side-pumped Nd:YAG modules, the pulse energy reached 7.6 mJ and 1.7 mJ at 500 Hz and 1 kHz, respectively. The maximum peak power of 15.5 MW was achieved at the repetition rate of 500 Hz. By using a focusing lens, air breakdown was observed at 500 Hz. The thermally induced birefringence in the LD side-pumped Nd:YAG module was investigated to explain the spatial beam deformation. Future work will be conducted to compensate the thermally induced birefringence and improve the beam quality factors . The high-pulse-energy, sub-nanosecond MOPA laser system is a promising laser source for many industrial and scientific applications including lidar and laser ignition.
National Natural Science Foundation of China (61605032, 61705165, 61775167, 61975150); Natural Science Foundation of Tianjin City (18JCZDJC37900, 19JCZDJC38400).
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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