A 526\,mJ Subnanosecond Pulsed Hybrid-Pumped Nd:YAG Laser

Jiang-Peng Shi(石江鹏)$^{1\hyperlink{s}{**}}$, Jian-Guo Xin(辛建国)$^{1}$, Jun Liu(刘军)$^{1}$, Jia-Bin Chen(陈家斌)$^{2}$, Sher Zaman$^{3}$

doi:10.1088/0256-307X/34/7/074208

⇦Chinese Physics Letters, 2017, Vol. 34, No. 7, Article code 074208⇨ A 526 mJ Subnanosecond Pulsed Hybrid-Pumped Nd:YAG Laser Jiang-Peng Shi(石江鹏)1**, Jian-Guo Xin(辛建国)1, Jun Liu(刘军)1, Jia-Bin Chen(陈家斌)2, Sher Zaman3 Affiliations 1School of Optoelectronics, Beijing Institute of Technology, Beijing 100081 2School of Automation, Beijing Institute of Technology, Beijing 100081 3Department of Physics, Karakoram International University, Gilgit 15100, Pakistan Received 8 May 2017 ^**Corresponding author. Email: winnerbit@126.com Citation Text: Shi J P, Xin J G, Liu J, Chen J B and Zaman S 2017 Chin. Phys. Lett. 34 074208 Abstract A hybrid-pumped Nd:YAG pulse laser with a double-pass two-rod configuration is presented. The focal length of offset lens is particularly studied to compensate for the thermal lens effect and depolarization. For input pulse energy of 141 μJ with pulse duration of 754 ps, the pulse laser system delivers 526 mJ pulse energy and 728 ps pulse width output at 10 Hz with pulse profile shape preservation. The energy stability of the laser pulse is less than 3%, and the beam quality factor $M^2$ is less than 2.26. DOI:10.1088/0256-307X/34/7/074208 PACS:42.60.By, 42.60.Gd, 42.60.Da © 2017 Chinese Physics Society Article Text Solid-state sub-nanosecond lasers with high pulse energy are of great interest for applications in various fields such as laser-induced periodic surface structures (LIPSSs),[1] material processing,[2] shock ignition,[3] nonlinear optics[4] and biomedicine.[5] Particularly in laser processing, lasers in hundreds of picoseconds with high energy of joule level have the merits of small scope of thermal effect, high precision and high speed. Up to now, there are many methods to achieve short pulse width, such as mode locking, simulated Brillouin scattering (SBS) pulse compression, Q-switching and regenerative amplification. Compared with these techniques, passively Q-switching offers a reliable method to achieve a short-pulse operation with the virtues of simplicity, low cost and high stability. Passively Q-switching can be accomplished by either continuous-wave pumping or pulsed pumping techniques.[6] Pulsed pumping can simultaneously reduce the thermal effect of the gain medium[7] and improve the instability of the pulse period.[8,9] The repetition rate of the laser could be tuned by changing the repetition rate of pump light. Generally, the pulse energy delivered from passively Q-switched microchip lasers is too low (typically up to hundreds of micro joules).[10] A master oscillator power amplifier (MOPA) can be used to boost the seed source to higher pulse energy. In 1994, Zayhowski et al.[11] first achieved a picosecond microchip laser system based on the passively Q-switching technique using Cr$^{4+}$:YAG as the saturable absorber. Pumped by the unfocused 1.2 W output of a fiber coupled diode, 11 μJ pulse energy was obtained with 337 ps duration at a pulse repetition rate of 6 kHz in a single frequency TEM$_{00}$ mode. To achieve the sub-nanosecond lasers with higher energy and power, a series of trials were made on passively Q-switched lasers based on MOPA configuration. In 1999, Druon et al.[4] demonstrated a very efficient multi-pass amplifier with a microchip laser as seed source. A maximal output of 510 mW after four passes was obtained with the pulse duration of 500 ps at the wavelength of 1064 nm. By the frequency conversion technique, this system has been optimized to deliver 45 kHz, 300 ps UV laser with an energy per pulse of close to 1 μJ. Since 2007, Liu et al.[12] have reported their works on amplifying the short pulse laser. In their studies, 1064 nm, 100 μJ, $\sim$1 ns seeding pulse was amplified up to 400 mJ with the divergence angle of 1 mrad. A few years later, Chuchumishev et al.[10] demonstrated a sub-nanosecond single-frequency MOPA laser system consisting of a multistage amplifier. Near-diffraction-limited pulses (240 μJ, 830 ps at 0.5 kHz) are amplified up to 13 mJ, whilst preserving pulse duration and beam quality. In this Letter, laser pulses in durations of a few hundreds of picoseconds with high energy output based on the hybrid pumped master oscillator power amplifier is presented. With this technique, a 141 μJ pulse energy and 754 ps pulse width input are amplified to 526 mJ pulse energy and 728 ps pulse width with pulse profile shape preservation, high beam quality, and high pulse energy stability. The microchip seed source is shown in Fig. 1. The pump laser of the seed in this experiment is a fiber coupled 808 nm laser diode with the core diameter of 400 μm and the numerical aperture (NA) of 0.22 from BWT Beijing. A grin-lens was used to collimate and focus the pump light. A ${\it \Phi}$5 mm $\times$ 5 mm piece of Nd$^{3+}$:YAG doped at 1.5 at.% was employed as the gain medium. The entrance face of this gain medium was coated for high transmission at 808 nm and high reflectivity at 1064 nm, while the second face was coated for high reflectivity at 808 nm and high transmission at 1064 nm. The saturable absorber was a ${\it \Phi}$5 mm $\times$ 2.85 mm piece of Cr$^{4+}$:YAG with an initial transmission of 25%. The Cr$^{4+}$:YAG was antireflection (AR) coated for high transmission at 1064 nm on the entrance face, and the second surface with a reflectivity of 60% at 1064 nm was used as the output coupler during the experiment.

Fig. 1. Scheme of the passively Q-switched microchip laser.

Fig. 2. Schematic diagram of the double-pass pulse laser system. HW, half-wave plate; PBS, polarization beam splitter; FR, Faraday rotator; QR, 90$^{\circ}$ quartz rotator; CL, compensation lens; QW, quarter-wave plate; and HR, high-reflectivity mirror.

The schematic diagram of the double-pass laser pulse amplifier is shown in Fig. 2. A half-wave plate was used to adjust the polarization state of the seed laser. A telescope system composed of a biconcave lens and a plano convex lens was used to expand the diameter of seed laser to 8.5 mm. A polarizer, a half-wave plate, and a Faraday rotator composed an isolator to protect the seed laser from damage by reflected light of the amplifier. The amplifier was composed of two Nd:YAG rods (130 mm long, 9 mm in diameter, doped at 0.6 at.%), which were pumped by a xenon-lamp in a diffuse reflecting chamber. The cross section of the chamber was a circular track, and the material of the chamber reflector was Morgan ceramics. The gain medium and flashlamp were cooled by circulating cooling water at 20$^{\circ}\!$C. A homemade flashlamp power supply providing variable voltage in the range of 400–1500 V was used as the voltage source for the flashlamp. The energy-storage capacitor of the flash lamp power supply is 100 μF. A 90$^{\circ}$ quartz rotator between two identically pumped laser rods can achieve depolarization compensation. What is more, a biconcave lens coated for high transmission at 1064 nm with a proper focal length ($f=-1500$ mm) can compensate for the thermal lensing effect. The angle between the principal axis of the quarter-wave plate and the vibration direction of p-polarized light is 45$^{\circ}$. A totally reflecting mirror coated at 0$^{\circ}$ and a quarter-wave plate made the polarization direction of the input laser beam rotated from p-polarization to s-polarization and reflected back for double passing through two Nd:YAG rods. The second polarizer (PBS2) coupled the amplified laser (s-polarization light) out, and the depolarization light of second-pass was output from PBS1. Both the drive source of laser diode and flashlamp power supply were external voltage level triggered by connecting them to DG535 digital delay and pulse generator (Stanford Research System, Inc.). We can change the time delay between the input seed laser and the pumping pulse of xenon lamp by tuning time delay between the drive source of laser diode and flashlamp power supply to optimize the output energy. Total pump energy of approximately 91 J was employed by the two-stage Nd:YAG rod amplifier to achieve high gain. The xenon lamp pumping configuration caused a serious heat load on the amplifier. Therefore, the beam quality would become much worse after the seed laser passed the double-pass amplifier without compensation. In this work, a quartz rotator and a suitable negative lens were inserted between two Nd:YAG rods to eliminate the thermal lensing effect and the thermally induced birefringence effect. The mode-volume matching in two rods was achieved very well, thus the degradation of beam quality caused by the first Nd:YAG rod can be compensated for by the second one. The maximum depolarization loss of 25.4 mJ was measured. Figure 3 shows the simulation of beam radius in this configuration with negative lens compensation and without compensation using reZonator (an optical simulation software). Furthermore, this simulation was in good agreement with the experimental results.

Fig. 3. Beam radius in the double-pass amplifier with compensation lens (CL) and without CL.

Fig. 4. Intensity distribution of the seed beam: (a) 2D distribution of near field, and (b) 3D distribution of near field.

When setting the peak power 30 W, the pulse duration 300 μs and the repetition rate 10 Hz on the pumping laser diode, the master oscillator provided 754 ps laser pulse with single pulse energy of 168 μJ at 1064 nm wavelength. The energy stability of the seed laser was less than 1%. The measured beam quality factors $M^2$ of the seed laser were respectively 1.52 and 1.57 in the two orthogonal directions. The 2D and 3D intensity distributions of the seed beam were given in Fig. 4, which were detected by a charge coupled device (WinCamD-UCD12, USA). It can be seen that the beam intensity of the seed laser was an ideal Gaussian distribution.

Fig. 5. Output energy as a function of the amplifier voltage.

Fig. 6. The energy stability of the amplified laser and the depolarization loss of output energy.

Figure 5 represents the experimental curve of output energy with the change of amplifier voltage, at 10 Hz repetition rate. When the input pulse energy was 141 μJ, the maximum output energy of 526 mJ was obtained in double-passing amplification with the amplifier voltage of 1350 V. Figure 6 shows the energy stability measurement of the amplified laser and the depolarization loss of output energy. The energy jitter of the output energy is less than 3%, and the depolarization loss of energy is less than 5%. Compared with the seed laser, the reasons for the energy stability degeneration are instability of flashlamp pumping energy and thermal effect distortion in the laser amplifier. In this experiment, the output pulse energy was detected by an LE-3B laser energy meter (Physcience Opto-Electronics Co. Ltd., Beijing, China). As shown in Fig. 7, the pulse width at full width half maximum (FWHM) of the seed laser is 754 ps, and the FWHM of the amplified output pulse laser with output energy of 526 mJ is 728 ps. It is found that the amplified pulse preserved nearly the same profile shape as the input pulse profile, which is almost identical to that of the seed laser without waveform distortion. The pulse signals were detected by a high-speed photo detector (DET02AFC Si biased Detector, Thorlabs, Ltd) with the typical rising time of 50 ps, and the signals were cascaded into a 1-GHz-bandwidth LeCroy Wavepro 7000 digital oscilloscope.

Fig. 7. Temporal pulse profiles of master oscillator seed output (a) and amplified laser output (b).

Fig. 8. Intensity distribution of amplified laser output: (a) 2D distribution of near field, (b) 3D distribution of near field, (c) 2D distribution of far field, and (d) 3D distribution of far field.

Figure 8 represents the beam characteristics of the output laser from polarizer PBS2 in both near-field and far-field, which were measured by a charge coupled device (WinCamD-UCD12, USA). It can be seen that the beam intensity of the amplified laser was nearly a Gaussian distribution. Figure 9 shows the squared beam radii measured at different distances under the output energy of 526 mJ after a lens with the focal length of 180 mm. The beam quality factors in the $X$ and $Y$ axes are $M^{2}=2.14$ and $M^{2}=2.26$, respectively. The reason for the beam quality degeneration is spatial distortion such as thermal distortion, non-uniform pumping and diffraction effect in the amplifier.

Fig. 9. Measured beam quality of the amplified laser: (a) the horizontal direction, and (b) the vertical direction.

In summary, a sub-nanosecond pulsed laser, which exhibits the advantage of the pulse temporal profile preservation, has been realized with high energy (maximum energy 526 mJ), pulse duration of 728 ps, repetition rate of 10 Hz, high beam quality, and high pulse energy stability. The compensation for the thermal effect is particularly studied to improve the beam quality. With this compensation method, the depolarization loss of energy is less than 5%. What is more, the energy jitter of this laser system is less than 3%. The beam qualities of $M^{2}=2.14$ in the $X$ axis and $M^{2}=2.26$ in the $Y$ axis in the orthogonal directions are obtained. References Subnanosecond-laser-induced periodic surface structures on prescratched silicon substrateFemtosecond, picosecond and nanosecond laser ablation of solidsGeneration of 360 ps laser pulse with 3 J energy by stimulated Brillouin scattering with a nonfocusing schemeHigh-repetition-rate 300-ps pulsed ultraviolet source with a passively Q-switched microchip laser and a multipass amplifierCompact sub-nanosecond wideband laser source for biological applicationsEfficient high-energy passively Q-switched Nd:YLF/Cr^4+:YAG UV laser at 351Â nm with pulsed pumping in a nearly hemispherical cavityEfficient operation of a room-temperature Nd:YAG 946-nm laser pumped with multiple diode arraysCost-effective low timing jitter passively Q-switched diode-pumped solid-state laser with composite pumping pulsesComparison of CW pumping and Quasi-CW Pumping for a Passively Q-switched Nd:YAG LaserSingle-frequency MOPA system with near-diffraction-limited beam qualityDiode-pumped passively Q-switched picosecond microchip lasersHigh-energy single longitudinal mode 1Â ns all-solid-state 266Â nm lasers

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