End-pumped Laser Research Articles

Analysis of Thermal Effects within Cylindrical Shape Solid-State Laser Rod

In this work, a temperature-dependent analytical model was modified to predict the thermal effects of diode laser double–end-pumped cylindrical laser rod under Gaussian pump beam distribution. Heat load and temperature distribution were analyzed using the Kirchhoff integral transform method. Results show that a maximum temperature difference of approximately 69.61 K was obtained on each side face of the laser rod at a maximum power of 40 W (equally divided on each face). The total thermal focal length of approximately 34.64 mm was calculated under the Gaussian pumping profile. The finite element method code incorporated with well-verified software was used to numerically verify the obtained results, where the analytical and numerical results are highly matched. The results reveal that the total thermal focal length produced in a double–end-pumped geometry is two times less than that obtained from a single-end-pumped geometry.

Crack formation process in ceramic Nd:YAG

We estimated the crack formation process by analyzing the crack type and thermal stress of 2 at.% and 4 at.% ceramic Nd:YAGs damaged by pump power in an end-pumped ceramic laser. Lateral cracks, radial cracks, and median cracks occurred sequentially in the ceramic Nd:YAG when the tensile stress caused by the pump power increased over 160 MPa and decreased.

Digital laser for on-demand intracavity selective excitation of second harmonic higher-order modes.

In this article, we demonstrate selective excitation of second harmonic higher-order modes inside a diode end-pumped solid-state laser resonator that comprises of a nonlinear potassium titanyl phosphate (KTP) crystal and a digitally addressed holographic end-mirror in a form of a reflective phase-only spatial light modulator (SLM). The emitted second harmonic higher-order modes at 532 nm are generated by an intracavity nonlinear KTP crystal that is pumped by high-order fundamental modes operating at 1064 nm. The fundamental modes are digitally controlled by displaying a computer-generated hologram in the form of a grey-scale image to the SLM screen for on-demand high-order modes. The phase matching of the fundamental mode to the generated frequency-doubled mode is achieved by controlling the phase of the digital hologram to either achieve a high or quasi-degree of orbital angular momentum conservation. We show that we can intracavity generate frequency-doubled high-order Laguerre-Gaussian modes and Hermit-Gaussian modes that are either quasi or fully reproducible in the far-field. To the best of our knowledge, this is the first laser to generate frequency-doubled on-demand higher-order modes inside the cavity at the visible (green) wavelength of 532 nm.

Pushing the limit of pulse duration in Q-switched solid-state lasers with high gain

We demonstrate a 1064 nm, mJ-level, sub-nanosecond end-pumped Q-switched laser with weak feedback and high gain. We obtain the pulse duration of 449.7 ps, namely 1.2 times of the round-trip time, as well as the maximum energy of 0.26 mJ at the repetition rate of 1 kHz. The pulse energy is further scaled to 3.6 mJ by a double-passed amplifier, resulting in the peak power of 8.1 MW at the highest repetition rate of 2 kHz. Moreover, both the classical Q-switching theory and weak-feedback Q-switching theory are utilized to analyze the experimental results. The simulated results support that the Q-switched lasers with weak feedback and high gain, is effective for approaching single roundtrip time, which is the pulse duration limit. Smaller pulse duration is expected by virtue of this simple and compact configuration.

Pulse Characteristics of Cavityless Solid-State Laser * *Supported by the National Natural Science Foundation of China under Grant No. 62875100

We propose a theoretical model (cavityless pulsed solid-state-laser theory) to analyze the pulse characteristics of cavity-less solid-state lasers. A high gain Nd:YVO 4 end-pumped cavityless laser system is adopted to verify the theoretical model. It shows that the performance of output energy and pulse width achieved in cavityless configuration is better than that in resonator configuration when the small-signal gain reaches the saturated level. The simulation results calculated by our theoretical model agree very well with the experimental results. This agreement proves the validity of our theoretical model, which has great importance for theoretical analyses of high gain pulsed laser.

Experimental study on spherical aberration and beam quality factor of LD end-pumped laser

A method for judging the locations of dynamic stable zones boundaries and mode separations is presented by spherical aberration “transient variation” Experiments on spherical aberration in dynamic stable zones of an end-pumped laser are carried out. Based on the inversion of the wavefront information, the “beam quality deterioration zone” is investigated. The multiple relationships among the thermal focal length of the low-order modes are calculated. Based on the discussion of experimental results, the changing law of the sign of spherical aberration is studied. A theoretical model of the relation among the cavity configuration, spherical aberration and beam quality can be established in addition. The experimental results agree well with the theory.

Analysis of laser thermal effect of funnel-shape beam generated by spherical lens pump system

The uniformity of temperature distribution of an end-pumped laser can be improved simply and effectively by introducing the correct amount of spherical aberration into the pump system. An iterative model of optical-thermal coupling is proposed to calculate the temperature distribution of the end-pumped laser amplifier. The results showed that, the thermal effect of the laser gain medium in the coupling system composed of spherical lenses decreased significantly compared with the coupling system composed of aspherical lenses. This is proved by a neodymium-doped yttrium orthovanadate (Nd:YVO4) amplification experimental system, which was end-pumped by a fiber-coupled laser diode (LD) with the optimized wavelength of 878 nm. A comparative study of the two pump systems showed that the decrease in the gain medium thermal wavefront peak-to-valley (PV), the thermal lens effect, and an improvement in beam quality are still observed in the pump amplification experiment with spherical aberration. At last, the relationship between the spherical aberration and the output brightness of the amplifier was established, which provides guidance for further optimization of the spherical aberration of coupling pump systems and improving the beam quality.

Beam-Shaping Optimization of the diode bar end-pumped laser

Beam-Shaping Optimization of the diode bar end-pumped laser

Thermal lens analysis in a diode-pumped 10 Hz 100 mJ Yb:YAG amplifier

We address the power scaling issue in end-pumped laser rod amplifiers by studying, experimentally and numerically, the magnitude of thermal lensing in a high-energy diode-pumped Yb:YAG crystal. The spatio-temporal temperature profile of the gain medium and the focal length of the induced thermal lens are determined numerically. The influence of the repetition rate and pumping power on the temperature distribution is analyzed. Experimental measurements covered repetition rates between 1 and 10 Hz and up to 4 kW pumping power.

Improvement of beam quality by pump homogenization of Innoslab laser amplifier

The effect of pump homogenization on the beam quality of the output laser is studied for a compact partially end-pumped hybrid-cavity slab laser amplifier, i.e. Innoslab laser amplifier. Using ZEMAX simulation software, two pump coupling modes, direct imaging and homogenization with waveguide were designed, and their effects on the uniformity of light intensity distribution in the slow axis (crystal width) direction of laser diode arrays were compared. It is found that the pump uniformity is significantly improved after the waveguide is homogenized and the influence of the laser diode array luminescence dead pixels on the pump imaging is weakened. Furthermore, the effect of pumping homogenization on the performance of Innoslab laser amplifier is verified by experiments: the uniformity of slow-axis pumping increases from 90.6% to 95.4% after homogenization with waveguide, and the quality of output laser beam M2 increases from 2.41 to 1.55 after three-pass amplification, and the self-oscillation in the cavity is effectively suppressed.

Sharp focusing of superpositions of Hermite–Gaussian modes locked in fractionally degenerate cavity

We investigate, experimentally and numerically, sharp focusing of superpositions of Hermite–Gaussian modes obtained at transverse mode locking in an end-pumped laser with a fractionally degenerate cavity. Being focused with a lens, these beams create quasi-waists that are shorter and narrower as compared to the waist of a zero-order Gaussian mode. Comparison with the case of Laguerre–Gaussian superpositions showed that both mode families demonstrate similar sharp-focusing behavior in terms of the longitudinal and transverse scales, but the superpositions of Hermite–Gaussian modes have substantially non-zero on-axis intensity between the quasi-waists.

A new approach to evaluate temperature distribution and stress fracture within solid state lasers

This work presents a new approach to determine the maximum allowable pump power that causes fractures in solid-state lasers. By considering temperature-dependent-thermal conductivity, heat equation was solved and an analytical thermal model of laser diode double end-pumped cylindrical laser rod was deduced. Taking into account two unequal pump radii focused on each rod faces, the temperature distribution and thermal stresses components were calculated at the two end faces of the rod under top-hat pump profile. The results showed that at the maximum total pump power of 200 W (80 W on the right side and 120 W on the left side) the rod is subject to fracture.

Asymmetric behavior of threshold pump power of Nd:YAG diode end-pumped laser in critical cavity configurations

The essential asymmetry in the dependences of threshold pump power on the cavity length of an Nd:YAG laser in degenerate (critical) cavity configurations under inhomogeneous diode end-pumping was investigated experimentally and theoretically. These dependences were numerically calculated using an iterative method based on the expansion of the field in Laguerre–Gaussian modes of a cold cavity. A complex gain profile, representing not only active medium gain but also phase distortions due to electronic and/or nonstationary thermal lenses, was introduced into the numerical calculations to obtain the asymmetric threshold pump power dependences on the cavity length in critical cavity configurations. In the case of the Nd:YLF crystal the threshold dependences, observed under the same experimental conditions, were symmetric and could be explained with pure real gain profile.

Research on 200 mJ semiconductor end-pumped MOPA Laser

Using the high-power laser diode array (LDA) end-pumped Nd:YAG laser bar method, combined with the design of the convex-concave unstable cavity, the average energy of the small beam divergence laser output was achieved about 83 mJ under 20 Hz operating conditions. Then taking the laser as the oscillation source, the same method of the LDA end-pumped Nd:YAG laser rod was used for energy amplification to form an LDA end-pumped master oscillator-power amplification (MOPA) laser, which finally obtained the pulsed laser output with a repetitive frequency of 20 Hz, an average energy of >200 mJ, a divergence angle < 2.1 mrad and an energy fluctuation < ±2.5%. The laser has a optical-to-optical conversion efficiency of about 14.6%, a volume of 175×91×49 mm3, a mass of < 1 kg, and a divergence angle of < 0.3 mrad after transmitted through an 8× transmission antenna.

Simulation and experimental investigation of beam distortions in end-pumped laser rod amplifiers

Power scaling of end-pumped laser rod amplifiers is challenging due to beam distortions at high powers. It is difficult to understand these distortions since several physical effects also contribute simultaneously to the problem. We present a detailed investigation of the beam distortions by numerical simulation and experiments in different gain regimes (passive regime with no gain, small signal gain regime, and saturated gain regime) to decouple effects caused by thermal lensing from aberrations induced by the transversal gain profile. The simulation and experiment are in good agreement and show the influence of the gain regime in the details and reveal a significant impact of the transversal gain profile on the output beam quality.

Dynamically tunable multi-lobe laser generation via multifocal curved beam

Beams with curved properties, represented by Airy beam, have already shown potential applications in various fields. Here we propose a simple method to achieve a multifocal curved beam (MCB). The scheme is based on the ability of microspheres to control the distribution of the light field. Combined with the caustic effect, the dynamic control of the beam curvature and the foci can be realized. The simulation results confirm the mechanism behind this phenomenon. Furthermore, MCB is applied experimentally into the end-pumped microchip laser. This work has verified the theory of MCB and achieved a dynamically tunable multi-lobe laser, which has a wide application prospect.

Dual-wavelength end-pumped Rb-Cs vapor lasers.

Diode-pumped alkali vapor lasers (DPALs) have been rapidly developed because of their excellent performances. However, there have been few reports about DPALs with multiple wavelengths until now. The effects of the output features on the waist size and position of both Rb and Cs pump beams were first theoretically investigated using a kinetic model for an end-pumped dual-wavelength Rb-Cs laser. Then, a continuous-wave (CW) laser was experimentally constructed. Finally, the hybrid CW-modulated output with two wavelengths was also successfully obtained for the first time in development of alkali lasers. The results might be helpful in applications for laser communication and ranging/radar in the future.

Industrial 1.5 kW single-stage end-pumped fiber laser

An industrial prototype of single-stage end-pumped fiber laser at 1080 nm has been demonstrated and developed. The corresponding optical-to-optical conversion efficiency is 73.3% with a pump power of 2066 W and output laser power of 1515 W. The total laser power fluctuation is less than 0.9% within one hour. The stimulated Raman scattering (SRS) is well suppressed. The cladding light stripper (CLS) is optimized and integrated with three regions of particle mixture and gel.

Investigation of multi-segmented Nd:YAG/NdYVO4 crystals and their laser performance end-pumped by a fiber coupled diode laser

A novel flat-flat resonator consisting of two crystals (Nd:YAG+Nd:YVO4) is established to demonstrate the feasibility of spectral line matching for output power scale-up in end-pumped lasers. The optimal choices for multi-segmented crystals are 40 mm length with 0.2% doping concentration(Nd3+) and 16 mm with 0.2% for the Nd:YAG and Nd:YVO4, respectively. A maximum continuous-wave output power of 79.2 W is reported at 1064 nm with Mx2 =4.82(My2 =5.48) with pumping power of 136 W in the multi-segmented crystals (Nd:YAG+Nd:YVO4). Compared to conventional composite crystals (Nd:YAG+Nd:YAG), the optical conversion efficiency of multi-segmented crystals (Nd:YAG+Nd:YVO4) from 808 nm to 1064nm is enhanced from 30% to 58.8%.

High-power and high optical conversion efficiency diode-end-pumped laser with multi-segmented Nd:YAG/Nd:YVO4**Project supported by the National Defense Pre-Research Foundation of China (Grant No. 9140A020105).

A novel flat–flat resonator consisting of two crystals (Nd:YAG + Nd:YVO4) is established for power scaling in a diode-end-pumped solid-state laser. We systematically compare laser characteristics between multi-segmented (Nd:YAG + Nd:YVO4) and conventional composite (Nd:YAG + Nd:YAG) crystals to demonstrate the feasibility of spectral line matching for output power scale-up in end-pumped lasers. A maximum continuous-wave output power of 79.2 W is reported at 1064 nm, with , , and a pumping power of 136 W in the multi-segmented crystals (Nd:YAG + Nd:YVO4). Compared to conventional composite crystals (Nd:YAG + Nd:YAG), the optical-optical conversion efficiency of multi-segmented crystals (Nd:YAG + Nd:YVO4) from 808 nm to 1064 nm is enhanced from 30% to 58.8%, while the laser output sensitivity as affected by the diode-laser temperature is reduced from 55% to 9%.