We demonstrate a high energy nanosecond (ns) slab Yb:YAG master oscillator power-amplifier (MOPA) laser system with high beam quality, operating at room temperature. The MOPA system consists of a Q-switched Yb:YAG rod oscillator and a four-pass Yb:YAG slab amplifier. The oscillator, utilizing a Yb:YAG rod with medium doping concentration (2.0 at.%), delivers a near-diffraction-limited beam with an energy of 2.4 mJ, a repetition rate of 100 Hz, and a pulse duration of 52 ns. The Yb:YAG slab has a 10:1 large aspect ratio and is side-pumped by five laser diode bars via a microlens array shaping system to homogenize and focus the pump beam. The seed pulse energy is amplified to 49 mJ, achieving a laser gain of 20 times, and the experimental result is in fair agreement with the numerical simulation. The average beam quality factor M-2 is measured to be 1.35. The tabletop area of the laser setup is 0.2 m(2), which indicates this scheme has the potential for an application that needs high compactness.

We demonstrated a gain-switched Ho:YAG monolithic laser pumped by an actively Q-switched Tm:YLF laser. Furthermore, we simulated and analyzed the effects of various factors on pulse duration of the gain-switched Ho: YAG laser. By employing the monolithic laser structure with the gain-switching method, the Ho:YAG laser cavity length was shortened to 5 mm as short as possible. The shortest pulse duration of 1.83 ns with the peak power of 0.48 MW was obtained at 1.5 kHz repetition rate, with an optical-to-optical conversion efficiency of 38.2 %. To the best of our knowledge, these are the shortest pulse duration and highest peak power reported for a 2.1 mu m source from a Ho:YAG gain-switched laser system.

We demonstrate what we believe to be for the first time a high efficiency, high brightness laser source by external cavity spectral beam combination (SBC) in the fast axis based on the 1000 mu m ultra-broad area laser diode (UBALD). Regarding the slow axis beam quality of UBALD, we propose a feedback model of the slow axis angle energy distribution in the oscillation process. By adding a structure with a filtering effect in the external cavity, both theory and experiment show that the beam quality of the slow axis can be effectively improved. With a 25-emitter UBALD stack, an SBC maximum CW output power reaches up to 730.6 W with an electro-optical efficiency of 63.7%. The SBC spectrum consists of 25 discrete peaks with a central wavelength of similar to 967 nm and an overall spectrum width of 27.2 nm. The beam quality M 2 values along the fast and slow axes are measured to be 5.2 and 22.8 at an output power of 623.4 W, corresponding to a brightness as high as 562.3 MW/(cm2 center dot sr). (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Abstract A compact High energy end-pumped Yb:YAG rod laser oscillator with nanosecond (ns) pulse was demonstrated. By combining the quasi three-level characteristics of Yb: YAG and the steps method simulation, the crystal parameters were optimized. By using a 2.0 at.% doped Yb:YAG rod with 10 mm length, a 1030 nm laser output energy of 18 mJ with 100 Hz, and ~ 30 ns was achieved through direct oscillator. The beam quality M2 factor was 1.13. The setup of the whole system had a compact footprint of 300 × 400 mm2. Hopefully, the end-pumped Yb:YAG with a moderate doped concentration provides an alternative solution for a compact seed source for high energy 100 Hz ns laser.

We propose and experimentally demonstrate a novel spectral beam combing (SBC) method based on a single slab laser cavity. By establishing the quasi-three-level oscillator model of Yb:YAG crystal, the output characteristics and gain bandwidth of Yb:YAG crystal are investigated theoretically on the basis of the above theoretical model. In a proof-of-principle experiment, the SBC of seven beamlets in a wide-emission-spectrum Yb:YAG slab is realized with an architecture of 7 beamlets which are generated by seven pairs of mini-mirrors and mini-lens, a transform lens, and a multi-layer dielectric grating. Here, a maximum output power of 241 W is achieved, with a 4.1 horizontal beam quality (HBQ) and the output spectrum range spans from $1029.0~\sim ~1031.5$ nm. To the best of our knowledge, this is the first SBC demonstration of solid-state laser based a single slab.

A high efficiency high brightness continuous wave (cw) cryogenic a-cut Nd:YAlO 3 (Nd:YAP) laser is demonstrated with three wavelengths at 1064, 1072 and 1079 nm. By exploring the temperature dependence of the fluorescence and the absorption spectra from the a-cut Nd:YAP crystal, the feasibility of multi-wavelength laser operation is analyzed. At an optimal temperature of ~180 K, a maximum output power for dual-wavelength operation is reached 2.1 W at an incident pump power of 4.6 W, corresponding to a slope efficiency up to 50%. The average beam quality factor is measured to be 1.17, indicating a brightness of ~130MW/cm 2 ·sr. These results present, to the best of authors knowledge, the highest brightness and slop efficiency for any multi-wavelength Nd:YAP laser. Such brightness multi-wavelength laser source with high beam quality has important applications in scientific research, laser radar and astronomy domains, especially in obtaining terahertz (THz) sources.

A Ne-liquefaction system cooled by a G-M cryocooler was developed. The heat load and liquefied rate of the system were analyzed theoretically. The neon gas was liquefied over 340 min while the condenser chamber was cooled down to 26.7 K. The gas pressures of the heat exchanger inlet and condenser are directly proportional to its absolute temperature, which decreased with the reduction of cryostat system temperature. The pressure of the heat exchanger inlet decreased from 105 kPa to 100.4 kPa with the pressure of the condenser decreased from 101 kPa to 100.1 kPa. Neon gas with a purity of 99.999% was used in liquefaction and the mass flow is set to 0.173 g/s. The production rate of liquid neon is determined to be 0.52 L/h. The liquid neon was collected in a glass vacuum dewar with a transparent window, which allows to observe the liquid level and conduct the optical tests. The refractive index and transmittance of the 940 nm infrared in liquid neon are 1.103 and 99.85%, respectively. The parameters will provide a valuable database on infrared equipment with liquid neon as a cryogen.

Nanosecond (ns) pulsed laser with high average power and high pulse repetition rate above 50 kHz is a potential solution for laser cutting, laser welding, laser cleaning and many other industry processing scenarios. Although Nd:YAG is a widely used solid-state gain medium, it is difficult to obtain ns pulsed laser with repetition rate above 50 kHz due to its limited stimulated emission cross section and thermal distortion under high pump intensity. In this paper, a kilowatt-level 100 kHz high repetition rate ns Nd:YAG master oscillator power amplifier (MOPA) laser system is reported, and a general optimization method was used to obtain a 205 W seed laser with a high repetition rate of 100 kHz. After beam shaping elements, the seed laser was amplified to 1008 W by a two-rod Nd:YAG preamplifier and a two-rod Nd:YAG main amplifier. The pulse-to-pulse stability factor of the pulsed laser was 0.961 and the pulse width was measured as 142.8 ns. The beam parameter product in the horizontal axis and vertical axis were measured as BPPx = 2.81 mm∙mrad and BPPy = 2.78 mm∙mrad respectively. This is the first time to obtain a kilowatt-level average power ns pulsed laser with repetition rate above 50 kHz using Nd:YAG, and the compact MOPA system is also suitable for power scaling and other practical use.

A high-efficiency and high-power vertical-cavity surface-emitting laser (VCSEL) side-pumped rod Nd:YAG laser with temperature adaptability are demonstrated. The VCSEL side-pumped laser module is designed and optimized. Five VCSEL arrays are symmetrically located around the laser rod and a large size diffused reflection chamber is designed to ensure a uniform pump distribution. Furthermore, the absorbed pump power distribution of the rod is simulated to verify the uniformity of the pump absorption. Finally, a proof-of-principle experiment is performed in short linear cavity laser with single laser module. A continuous-wave output power of 658 W at 1064 nm is obtained, the corresponding optical-to-optical efficiency is 52.6%, and the power variations are ±0.7% over 400 s and ±3.1% over the temperature range from 16 °C to 26 °C. To the best of our knowledge, this is the highest output power and the highest optical-to-optical efficiency ever reported for VCSEL pumped solid-state lasers. By inserting a telescopic module into the cavity and optimizing the TEM 00 mode volume, the average beam quality is measured to be M 2 = 1.34 under an output power of 102 W. The experimental results reveal that such a high power rod laser module with temperature stability is appropriate for field applications.

An integrated aberration-compensating module (IACM), consisting mainly of an adjustable slab-aberration compensator, a one-dimensional Shack-Hartmann wavefront sensor, and a data processor, which meet the urgent requirements of correcting the specific wavefront aberrations of a slab laser based on an off-axis stable-unstable resonator, is designed and experimentally demonstrated. Benefits include compactness, robustness, simplicity, automation, and cost-effectiveness. The particular wavefront aberrations of the 9 kW level quasi-continuous-wave Nd:YAG slab laser, which have characteristics of asymmetry, large amplitude and gradient, high spatial frequency, and low temporal frequency, were measured and theoretically analyzed. In the experiment, the wavefront aberrations of the slab laser were corrected by the IACM. At the average output power of 9 kW, the diffraction-limited factor β was improved from 20.3 times diffraction limit (DL) to 3.6 times DL. The peak-to-valley and root-mean-square values of aberrations were reduced from 9.6 to 0.85 µm and from 2.86 to 0.18 µm within five iterations of the IACM, respectively. Moreover, The IACM is capable of maintaining the compensating surface figure after power-off.

We present an investigation on the spectrum broadening in continuous-wave, sub-nanometer linewidth high power fiber amplifiers caused by the multiple four-wave mixing (FWM) process. The spectrum broadening employing two different types of narrow linewidth seeds, including the multi-longitudinal-mode seed and the broadened single frequency seed generated by high speed phase modulation, is studied. It is shown both theoretically and experimentally that the multi-longitudinal-mode seed experiences serious spectrum broadening induced by the FWM among various longitudinal modes, while the modulated seed can maintain the spectrum profile during the amplifying process even with some noise fluctuation. The different broadening results are mainly caused by the random phase distribution of the multiple waves. It is further explained by an exact solution of the degenerate FWM with three waves. The theoretical predictions on the spectrum and power dependence of the output laser linewidth are in quantitative agreement with the experimental results up to kilowatt.

We propose a possible way to achieve coherent beam combining (CBC) for the optical parametric process. We point out that the essential difficulty is to lock the phase of an individual wave in the three-wave-mixing process. The phase evolution law in an optical parametric amplifier for both cases of phase matching and quasi-phase matching is studied theoretically. The results indicate that, under the condition of strong pumping, a higher intensity injection signal can lock its phase to be its initial value, while for a weaker injection signal the phases of the signal and idler waves exhibit chaos-like evolution. Based on this understanding, we propose a ring configuration for an optical parametric oscillator (OPO) with the appropriate intensity seed injection. The numerical studies indicate this OPO configuration can achieve phase-locking and wavelength-locking simultaneously. This inspires a possible design to achieve CBC for OPOs.

We proposed and experimentally demonstrated an approach to achieving a narrow spectrum kilowatt-level fiber master oscillator power amplifier (MOPA). The MOPA is seeded by a narrow spectrum random laser with Yb-doped fiber gain and has the spectral broadening-free feature. The narrow spectrum property is sustained during power scaling due to the suppression of nonlinear process. The frequency and time domain of the random laser are well characterized. The proposed one-stage scheme eventually produces 1.105- and 1.073-kW output at 1064 and 1067 nm, and the 3-dB bandwidth are 0.40 and 0.47 nm, corresponding to 0.38- and 0.37-nm random laser seed, respectively. The beam quality of $\text {M}^{2}\approx 1.4$ is achieved at 1.105-kW power level, and the proposed MOPA has the potential of wavelength tunability by simply tuning the fiber Bragg grating of random laser seed.

The self-pulsing phenomenon in kilowatt level narrow-linewidth fiber amplifiers with white noise source (WNS) phase-modulation is observed experimentally. It possesses the obvious threshold of the pump power and prevents the narrow-linewidth fiber lasers from further power scaling. The experimental study shows that known explanations are not applicable here and indicates that occurrence of self-pulsing is closely related to Stimulated Brillouin Scattering (SBS) process. The theoretical discussion reveals that the spikes in the modulated spectrum are the critical factor that SBS threshold is lower than the theoretical estimation. The 1+1 dimensional SBS model analysis predicts that self-pulsing originates from forward second order Stokes pulses, which is in good qualitative agreement with the experimental data.

The polarization-dependent transverse mode properties in a few-mode nonpolarization-maintaining (non-PM) fiber amplifier with PM fiber laser seeding are investigated. In addition, the effect of spectral broadening on degeneration of the degree of polarization (DOP) is studied. The experimental results show that a high DOP can be achieved if the output laser of the non-PM fiber amplifier has good beam quality and a narrow linewidth without any broadening. A kilowatt (kW) fiber laser system with PM fiber seed and a non-PM fiber amplifier is demonstrated with the DOP at around 90% by controlling the beam quality and spectrum broadening. With the active polarization control technique, the system can achieve a kWlinearly polarized output with 6.5 GHz linewidth. The M-2 is around 1.1, and the polarization extinction ratio is about 14.5 dB. Unfortunately, the random self-pulsing issue arose when the output power exceeded 1093 W. We believe that a higher power of linearly polarized laser output based on adaptive polarization control in a non-PM fiber amplifier will be obtained if the self-pulsing issue is solved. (C) 2017 Optical Society of America

We demonstrate a high-efficiency, high-average-power, CW master oscillator power amplifier based on a conduction-cooled, end-pumped Yb:YAG slab architecture at room temperature (RT). Firstly, the CW amplification property is theoretically analyzed based on the kinetics model for Yb:YAG. To realize high-efficiency laser amplification extraction for RT Yb:YAG, not only intense pump but also a high-power seed laser is of great importance. Experimentally, a composite Yb:YAG crystal slab with three doped and two un-doped segments symmetrically is employed as the gain medium, which is end-pumped by two high-power, 940-nm diode lasers. A high-power, narrow-spectral-width, 1030-nm fiber seed laser then double passes the composite slab to realize efficient power amplification. For 0.8-kW seed input, maximum output power of 3.54 kW is obtained at 6.7 kW of pump power, with the optical conversion efficiency of 41% and the highest slope efficiency of 59%. To the best of our knowledge, this is the highest power and efficiency reported for Yb:YAG lasing at RT except thin-disk lasers.

An active phase locking of a large-scale fiber array with thirty channels has been demonstrated experimentally. In the experiment, the first group of thirty phase controllers is used to compensate the phase noises between the elements and the second group of thirty phase modulators is used to impose additional phase disturbances to mimic the phase noises in the high power fiber amplifiers. A multi-level phase dithering algorithm using dual-level rectangular-wave phase modulation and time division multiplexing can achieve the same phase control as single/multi-frequency dithering technique, but without coherent demodulation circuit. The phase locking efficiency of 30 fiber channels is achieved about 98.68%, 97.82%, and 96.50% with no additional phase distortion, modulated phase distortion I (±1 rad), and phase distortion II (±2 rad), corresponding to the phase error of λ/54, λ/43, and λ/34 rms. The contrast of the coherent combined beam profile is about 89%. Experimental results reveal that the multi-level phase dithering technique has great potential in scaling to a large number of laser beams.

A direct-liquid-cooled side-pumped Nd:YAG multi-disk QCW laser resonator is presented, in which the oscillating laser propagates through multiple thin disks and cooling flow layers in Brewster angle. Twenty Nd:YAG thin disks side-pumped by LD arrays are directly cooled by flowing deuteroxide at the end surfaces. A laser output with the highest pulse energy of 17.04 J is obtained at the pulse width of 250 μs and repetition rate of 25 Hz, corresponding to an optical-optical efficiency of 34.1% and a slope efficiency of 44.5%. The maximum average output power of 7.48 kW is achieved at the repetition rate of 500 Hz. Due to thermal effects, the corresponding optical-optical efficiency decreases to 30%. Under the 12.5 kW pumping condition while not oscillating, the wavefront of a He-Ne probe passing through the gain module is as low as 0.256 μm (RMS) with the defocus and tetrafoil subtracted.

A low-threshold Raman effect in a kilowatt ytterbium-doped narrowband fiber amplifier system is reported. The Raman Stokes light at 1120 nm is achieved with the total output power of only ~400 W, indicating that the Raman threshold of this kilowatt codirectional pumped continuous wave fiber amplifier is much lower than the predicted value estimated by the classic formula. To figure out the mechanism of this phenomenon, simulations based on the general stimulated Raman scattering (SRS) model are analyzed indicating that the key factor is the coupling between four-wave mixing (FWM) and SRS. The simulation results are in good agreement with our experiments.