End-pumped Laser Research Articles

The symmetry change of the transverse modes in ring-LD end-pumped Nd:YVO[formula omitted] laser with an étalon

A straightforward method for converting the symmetry of a diode end-pumped laser with an annular pump beam has been proposed. By introducing an étalon into the cavity, we observed a change in the spatial symmetry of the transverse mode profile from rectangular to cylindrical both in degenerate and non-degenerate ranges. A simulation of the transmittance distribution of the beam through the étalon after filtering is given. Our experimental results indicate that this method of altering the symmetry maintains relative stability during the tilting of the étalon.

The frequency degeneracy range of transverse mode in an off-axis end-pumped solid-state lasers

The frequency degeneracy range of transverse mode in an off-axis end-pumped solid-state lasers

Structure, spectroscopy and LD end-pumped laser performance of an Er,Pr:LuYSGG crystal operated at ∼2.8 μm

We demonstrate the structure, spectroscopy, and laser performance of an Er3+ and Pr3+ co-doped LuYSGG crystal grown by the Cz method.

Reduction of depolarization loss in a Tm-doped sesquioxide ceramic laser using a ring-shaped pump beam.

We report on a simple method for reduction of the depolarization loss in an end-pumped Tm:Y2O3 ceramic laser by using a near-field ring-shaped pump beam. Initially, we theoretically derive the expression of the depolarization loss in a bulk laser end-pumped with a near-field flat-top-hat or ring-shaped beam, where a significant reduction of depolarization loss in the latter case is presented. Experimental verification is thereafter carried out with a Tm:Y2O3 ceramic laser employing these two different pump configurations. It shows that the experimentally measured depolarization losses are close to the simulated values; the loss in the case of the annular-beam pump is almost 18 times lower than that with a quasi-top-hat beam at a same absorption pump power of 7.4 W. This work, as a proof-of-principle study, indicates that depolarization loss in the end-pumped bulk lasers can be significantly reduced simply by using a ring-shaped pump beam.

Improved phase-locking of laser arrays by pump shaping.

We introduce a method to enhance the phase-locking quality and duration of an end-pumped laser array by precisely shaping its pump beam to overlap with the array. Shaping the pump beam results in a significant improvement in lasing efficiency and reduces the pump power required to reach the lasing threshold compared to a typical uniform pumping configuration. Our approach involves shaping a highly incoherent laser beam by addressing smaller segments of the beam with higher local spatial coherence. We demonstrate a remarkable increase in the laser array output brightness by up to a factor of 10, accompanied by a substantial extension in the phase-locking duration.

Experimental generation of nondiffracting Mathieu beams using a thin-disk laser configuration

Nondiffracting modes were experimentally generated in an end-pumped thin-disk laser with a new kind of axicon-based stable laser resonator. The eigenvalue and eigenvector of the resonator were investigated based on the geometrical optics view. Ytterbium ions doped in the YAG crystal as an active medium were then utilized in this configuration. The experimental generation of the Mathieu–Gauss and Ince–Gauss modes was performed and also discussed during the thin-disk deformation.

Numerical simulation of thermal effects in laser diode end-pumped solid-state lasers utilizing finite element method

Based on the finite element numerical analysis method, the temperature field distribution in the crystal with time is simulated by using the PDE-tool in MATLAB. The critical factors affecting the crystal transient temperature distribution are simulated and analyzed. According to the simulation parameters and results, a simplified method for solving the transient heat conduction equation is proposed. The transient thermal focal length is solved to obtain its distribution expression and an approximate analytical solution. The results provide a theoretical basis for the thermal analysis process of suppressing or utilizing the transient thermal effect in high-energy solid-state lasers.

Dynamic effects of temperature-dependent aberration factors on the beam quality of LD end-pumped lasers

Considering the temperature dependence of the thermal conductivity of the gain medium, the aberration factor of the thermal lens becomes a function of temperature. The influence of the aberration factor on critical spot size is analyzed, the actual value of critical spot size is recalculated, and the range of beam quality deterioration is re-pointed. An asymmetric flat cavity experiment is built, and a symmetrical flat cavity is used as the supporting experiment. The widened beam quality degradation range and the dynamic operating point of the fundamental mode beam are pointed out. Experimental results match the simulation.

Exploring the influence of pump beam quality on designing millijoule diode-end-pumped passively Q-switched lasers.

To practically predict the design criteria of diode-end-pumped passively Q-switched (PQS) lasers with energy scaling to millijoule region, an analytical model with longitudinally spatial dependence is derived to investigate the influence of pump beam quality. In comparison with PQS theory that considers transverse spatial dependence only, it is found that the threshold pump power can be up to 5 times larger when the beam quality factor was 80. This result indicates the importance of considering pump beam quality when designing PQS lasers especially for operation at high pump power level. The theoretical results are verified by a series of PQS experiments. The influence of thermal lensing effect on cavity design is further discussed to obtain good laser quality. Finally, an end-pumped millijoule PQS laser is successfully realized based on the theoretical analysis and the resonator design.

Optimization of the output performance of optically pumped semiconductor disk lasers

In order to obtain higher output power and better beam quality in semiconductor disk lasers, the characteristics of the gain chip and the parameters of laser cavity are investigated theoretically and experimentally in this paper. A 1085 nm semiconductor disk laser with an output power of 2 W is demonstrated, and the M2 factor is about 1.1, which means the beam quality is good. Optimization based on wafer design, pumping method and thermal management is carried out, and the corresponding output performances are discussed. The temperatures of the active regions and the output powers of lasers with front-pumping and end-pumping schemes are compared. The results show that the front-pumped laser could tolerate bigger pump power and obtain higher output power owing to its better heat dissipation, while the end-pumped laser has smaller pump threshold and higher slope efficiency due to its better mode-matching between the pump spot and the laser spot. This investigation may provide a general guidance for better laser performance of a semiconductor disk laser.

Modified Frantz–Nodvik equation for CW end-pumped high-repetition-rate picosecond laser amplifier

The modified Frantz–Nodvik equation considering the pulse repetition rate (PRR) for continuous-wave end-pumped picosecond laser amplifier is theoretically developed for the first time by analogizing Q-switch theory and solving rate equations. Based on the modified Frantz–Nodvik equation, a simple finite-element slice model is established to simulate the output characteristics of the end-pumped high-repetition-rate picosecond amplifier. Moreover, the validity of the theory and model is well verified by the experimental results of a two-staged diode-end-pumped Nd : YVO4 amplifiers using a picosecond fiber laser with adjustable PRRs as the seed source. The agreement between the experimental and theoretical results illustrates that the as-developed pulse amplification theory and model are a powerful tool for designing and optimizing adjustable-high-repetition-rate solid-state picosecond laser amplifiers.

Analysis of Thermal Effect in Thin Disk Laser: A Review

Since its first invitation in 1993, solid-state diode end-pumped thin-disk laser attracts much attention due to its significant improvements in the solid-state laser system. It had many advantages; it has high optical and electrical efficiency, high produced peak power, a simple cooling system, and high beam quality more than the usual solid-state diode laser system. The thin-disk laser (TDL) permits an effective pulse mode, which permits a new type of very short-pulsed laser system that could be used in the industry. Due to the influence of the many factors such as effective cooling system, the type of pumping, and the used thin crystal, a one-dimensional heat flow through thin-disk laser could be achieved which permits excellent enhancement in the beam quality. Finally, in this work, a review is made to the works devoted to discussing the basic principle and the solutions used to analyze the thermal effect in the thin disk laser.

Industrial 6 kW high-stability single-stage all-fiber laser oscillator based on conventional large mode area ytterbium-doped fiber

We have demonstrated an industrial 6 kW single-stage end-pumped all-fiber laser oscillator based on a conventional large mode area ytterbium-doped fiber (YDF) with 30 μm core diameter and 600 μm inner-cladding diameter. As a result, the fiber oscillator achieved a maximum output power of 6.07 kW at a central wavelength of ∼1080 nm with a slope efficiency of ∼65.8%. The power of Raman Stoke light was 21.6 dB smaller than the signal light at the output power of 6.07 kW. For industrial application, the stimulated Raman scattering effect was further suppressed by employing a 50 μm-core delivery fiber, and no sign of dynamic transverse mode instability comes into view during the whole experiment. The power stability measurement at ∼6 kW operation was carried out for continuous 2 h, and the power fluctuation was within 0.31%. To the best of our knowledge, this is the first detailed demonstration of industrial 6 kW high-stability single-stage all-fiber laser oscillator based on conventional YDF. The results provide a significant guidance for the construction of more than 6 kW output power single-stage end-pumped all-fiber laser systems.

Thermal analysis of end pumped fiber lasers subjected to jacket fluid cooling

End pumped lasers are highly efficient lasers particularly in diode lasers using micro lenses. The common cooling method for end-pumped systems is using water jacket or copper tube surrounding the laser rod. In this paper, the temperature distribution within a water jacket and a fiber laser end pumped by a top hat beam is studied analytically. The temperature distribution is obtained by considering the radial heat convection with fully developed laminar flow neglecting the axial heat conduction. The effect of laser dimensions and the Brinkman number on the temperature distribution are presented. The results indicate that the temperature distribution is strongly dependent on the Brinkman number. The results are presented in dimensionless form so that they can be applied to any end-pumped laser rod and fluid types. The main output of this work is that it is better for cooling purposes to have low Br values.

Microchannel Heat Sink with High Thermal Conductivity Path for Diode Partially End-Pumped Slab Laser

The thermal effect of diode pumped solid-state laser (DPSSL) has always been a main problem limiting the further improvement of laser performance. In this paper, based on the special heat load distribution of the laser crystal of diode partially end-pumped slab (Innoslab) laser, a multi-stage manifold rectangular microchannel heat sink (MCHS) with high thermal conductivity path (HTCP) is designed. The multi-stage manifold provides the MCHS more fluid-solid heat transfer area near the heat gathering area, while the HTCP provides a path for heat transmission from pump end to the body of MCHS. According to simulation calculation, the highest temperature of laser crystal has been reduced by nearly 50? due to the addition of HTCP. Then the MCHS is fabricated by MEMS technology and the laser crystal is replaced by simulated heat source. Compared with the MCHS without the HTCP, the thermal resistance of the MCHS with HTCP is reduced by 6.4%, which proves the effectiveness of HTCP in heat dissipation of Innoslab laser crystal.

Numeric Multi-Dimensional (r, z, t) Analysis Method for Compact Yb³⁺:YAG End-Pumped, Passively Q-Switched Lasers

We have applied a new simplified combination of numerical methods for studying the time and three-dimensional space dependence of quasi-three-level Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :Yttrium Aluminum Garnet (YAG) end-pumped lasers passively Q-switched by a Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> :YAG saturable absorber. We base our 3-D model on iterative, efficient, time- and space-dependent numerical propagation of the optical field through the laser cavity. The complex-valued laser field is coupled to the Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> :YAG and Cr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> :YAG media via complex optical permittivities, which are subsequently altered by gain/loss intensity saturation. The calculation is simplified using the radial symmetry of the system, with the cavity round-trip time as the smallest increment for updating the permittivities. We also include the effects of field diffraction in an intra-cavity air gap. For specified CW spatial pump conditions, self-consistent repetitively pulsed solutions for the laser field in a flat-flat or flat-convex mirror cavity are found with no ad hoc laser mode size or shape assumptions; these solutions are not Gaussian modes. We concentrate on compact lasers with multi-Watt average output power, operating at modest pulse energy (~1.0 mJ), high repetition rate (~5 kHz) and short pulse duration (~1.5 ns). Typical room-temperature pump-to-laser slope power efficiencies exceeding 50% are predicted, depending on laser pump and cavity loss parameters. Model results agree well with recently published experimental data.

Modal reconstruction of transverse mode-locked laser beams

Transverse mode-locking in an end-pumped solid state laser by amplitude modulation with an acousto-optic modulator was investigated. Using the stochastic parallel gradient descent algorithm the modal power coefficients and the modal phases of the transverse mode-locked (TML) laser beam were reconstructed from the measured spatial and spatio-temporal intensity distributions, respectively. The distribution of the reconstructed modal power coefficients revealed that the average mode order of the transverse mode-locking process could be increased by a factor of about 8 compared to previous works, corresponding to an increase in the normalized oscillation amplitude by a factor of about 3. Furthermore, we found that besides a non-Poissonian modal power distribution, strong aberrations of the modal phases occurred in the experiment, resulting in a deformation of the oscillating spot. Additionally, we demonstrated the generation of up to four spots oscillating simultaneously on parallel traces by operating the TML laser on a higher mode order in the orthogonal direction to the transverse mode-locking process. TML lasers are of interest, e.g., for beam scanning purposes, as they have the potential to enable spot resolving rates in the multi-GHz regime.

Study on the dynamic control of spherical aberration on the stable zone of the first three low-order modes of LD end-pumped lasers

The stable zones of the first three low-order modes of the LD end-pumped laser dynamically controlled by spherical aberration are proposed. Considering the temperature dependence of thermal conductivity, an aberration factor representing spherical aberration is calculated. Based on the multiples of the thermal focal lengths of the fundamental mode, TEM01 and TEM10, their dynamic stable zones were confirmed. An asymmetrical LD end-pumped Nd:YVO4 laser wavefront information measurement experiment was performed. The primary spherical aberration, the beam quality factor and the output power were measured, and their changes with the pumping power were analyzed to determine the dynamic stable zone. The experimental results agree well with the theoretical simulations.

Temperature distribution and stress analysis of end pumped lasers under Gaussian pump profile

Modified modeling of thermal effects within a dual end-pumped cylindrical shape laser medium was presented. The temperature distribution and the maximum hoop stress were determined by solving the heat conductance equation in a cylindrical medium. Two different incident beams of 0.6 mm waist radii were suggested, namely, effective and the Gaussian beam radii, where the effective beam radius is smaller by a factor of $$\sqrt 2$$ than the Gaussian beam radius. Under these considerations, the radial temperature difference due to an effective beam radius was found to be 23% higher than that for a Gaussian beam radius. The calculations show that the pump power does not affect the location of the maximum hoop stress as the spot does. A discrepancy within 1% was obtained by comparing the obtained results with the experimental work in the literature.

Microchip Nd:YAG and Nd:YVO4 Lasers Pumped by VHG Wavelength-Stabilized Laser Diode

We adopted a single-mode, single-wavelength volume holographic grating (VHG) wavelength-stabilized wavelength laser diode (LD) as a pumping LD for an end-pumped microchip Nd:YAG and Nd:YVO4 lasers we developed during CW and pulse operations. Higher optical-optical and slope efficiencies during CW operation have been obtained than when using a VHG LD experimentally. Output laser power is insensitive to the temperature of the LD when using a wavelength-stabilized LD and can remain stable and almost constant until the temperature of LD increases up to 40°C. The improved optical-optical conversion efficiency of 58% for the Nd:YVO4 laser has been obtained and calculated the output laser power during CW operation and compared it with the experimental results. We found that the output laser power of the Nd:YVO4 laser using the VHG wavelength-stabilized LD was more than twice as high as that using an LD without VHG. When the ambient temperature increases, the difference in output laser power should be large. In the future, a low-cost end-pumped microchip laser that does not require a temperature control should be developed.