High-power non-astigmatic TEM<Subscript>00</Subscript> and vortex mode generation in a compact bounce laser design

A novel design of a diode-side-pumped bounce amplifier laser is described that can produce a high-power TEM₀₀ mode and Laguerre–Gaussian (LG) vortex mode operation with high circularity and low astigmatism. The design utilises control of pump size and bounce angle such that the laser mode experiences substantially circular symmetric gain and spherical thermal lensing, in a highly compact fundamental mode resonator. In a simple plane–plane Nd:YVO₄ laser resonator with no other optical components and total length of just 13.4 cm, highly circularised TEM₀₀ operation is produced at wavelength 1064 nm, with 14.2 W of power and high beam quality M ²<1.05. Stable Q-switched TEM₀₀ operation is demonstrated up to 900 kHz and second harmonic power of 7.2 W at 532 nm is generated with conversion efficiency up to 64%. Minor modification of the design to exploit the spherical aberration of the thermal lens is shown to allow for generation of a doughnut-shaped LG vortex mode with average power 16.6 W, maintaining extremely low astigmatism. Q-switched operation of the vortex mode is accomplished up to 400 kHz. A detailed numerical thermal analysis of the amplifier design is performed and gives quantitative agreement with experimental results.     

Theoretical investigation on thermal lensing effects of Yb:KY(WO4)2 in diode-pumped lasers

Based on the features of laser diode end-pumped lasers, a thermal model of Yb:KY(WO₄)₂(Yb:KYW) with square cross-section was established. Considering the heat transfer on side faces, the anisotropic of thermal conductivity and the latest reports about thermo-optic coefficient, thermal expansion coefficient and thermal conductivity, temperature distribution, end-pumped face distortion and thermal lens focal length of Yb:KYW were more precisely obtained using finite difference method to solve Poisson equation in a rectangular Cartesian coordinate for the first time. At the pump power of 14 W, the highest temperature located at the center of end-pumped face was 243.8 °C, the highest distortion was 0.28 μm, and the thermal lens focal length was 5.4 cm along z-axis and −4.9 cm along x-axis. The results show that thermal lensing effects in the b-cut Yb:KYW were mainly determined by the anisotropic thermal expansion of Yb:KYW, and further present thermal lensing effects become weaker after considering the heat transfer. This work is significant for compensating the thermal lensing effect and improving the resonator stability of diode-pumped anisotropy crystal lasers.     

Laser performance and thermal lensing in high-power diode-pumped Yb:KGW with athermal orientation

A comparative, experimental study of the high-power diode-pumped laser performance and thermal lensing properties between standard b-cut Yb:KGW and Yb:KGW cut along an athermal direction is presented. The results show that thermal lens properties in both the b-cut and the athermal direction-cut crystals are determined by anisotropic thermal expansion in Yb:KGW. Thermal gradients due to the pump beam cause thermal lensing even in the athermal direction-cut geometry. The thermal lens is much weaker and less astigmatic in the athermal direction-cut crystal, for the same absorbed power. These properties allow generation of better-quality laser beams with the athermal direction-cut crystal as compared to the b-cut crystal. PACS 42.55.Xi; 42.60.Jf; 42.70.Hj     

Measurement of thermal lensing in a power amplifier of a terawatt Ti:sapphire laser

We have measured detailed thermal lensing in a power amplifier of a terawatt Ti:sapphire laser operating at 50 Hz. The thermal lensing in the amplifier was evaluated by measuring the optical path difference (OPD) using a Shack–Hartmann-type wavefront sensor. It was found that the radial dependence of the OPD was almost quadratic in the pumping region, despite inhomogeneous pumping. Therefore, a simple spherical lens or convex mirror effectively compensates for the thermal lens in our amplifier. We found that the thermal lens profile was temporally stable, and did not degrade the pointing stability of the amplified laser pulses. We also found that the time constant of the thermal distortion in our power amplifier was approximately 0.5 s. Received: 3 September 2001 / Revised version: 23 January 2002 / Published online: 14 March 2002     

Thermal eigenmode amplifiers for diffraction-limited amplification of ultrashort pulses

The design of high-repetition, high-average power, multipass amplifiers in which the pump power induced thermal lensing within the amplifier is used to create an equivalent lens waveguide for the production of diffraction-limited beams is analyzed.     

Numerical modeling of the thermal lensing effect in a grazing-incidence laser

The numerical modeling of thermal lensing effect is investigated in a grazing-incidence laser. The deformation of the bounce face is introduced into the modeling for the first time, and the Gaussian distribution of the pump light and the anisotropic heat conduction are considered. The results indicate that the proportion of the deformation on the bounce face to the thermal lensing effect is as high as 80% for small grazing-incident angle of 5°. The thermal lensing effect sensitively depends on the pump power, grazing-incident angle and the pump distribution in a grazing-incidence bounce geometry laser.     

Experimental determination of the thermal lens parameters in a broad area semiconductor laser amplifier

Due to the build-up of temperature gradients along their width, semiconductor laser diodes tend to be affected by thermal lensing effects. We propose a simple and easy-to-implement experiment in order to determine the thermal lens coefficient in a broad area semiconductor laser amplifier during operation. The results obtained are compared to simulations of the temperature distribution in the laser structure. In order to further validate our method, we compare the measured M ² value of a free running broad area laser diode with the calculated M ² of such a laser under the influence of a thermal lens as predicted by diffraction theory of first-order optical resonators. The experimental results are seen to be in good agreement with the calculations.     

Influences of pump beam distribution on thermal lensing spherical aberration in an LD end-pumped Nd:YAG laser

A numerical investigation is made on the thermal lensing and spherical aberration effect in an LD end-pumped Nd:YAG laser. Based on the finite element method (FEM), the laser rod temperature distribution is calculated and the focal length of the thermal lens is deduced, the influences of pump beam on the thermal lensing spherical aberration are mainly studied. The results show the thermal lens which focal length varied with the radial coordinate r is not an ideal lens. Given the heat dissipation boundary conditions, the radial dependent focal length will be shortened when the pump power raised or the waist radius of the pump beam reduced, meanwhile the radial differences of the focal length will decrease when the pump power increased. For a Super-Gaussian profile pump beam, the higher the exponent number, the more similar to flat top the pump beam is, and the less the radial differences of the focal length are.     

Sub-nanosecond single-frequency 10-kHz diode-pumped MOPA laser

A single-axial-mode, passively Q-switched (PQS) diode-pumped Nd:YAG laser, generating a diffraction-limited beam train of ≈40–60 μJ, ∼500-ps pulses with adjustable repetition rate in the range 1–10 kHz, was efficiently amplified by a single side-pumped Nd:YVO₄ bounce amplifier. After double-pass amplification, ≈1-MW pulse peak power with 577-ps duration and 545-μJ energy was achieved, still maintaining diffraction-limited beam performance. The average output power was 5.45 W at 10 kHz, corresponding to 13% extraction efficiency. The high brightness of this laser system seems ideal for nonlinear optics and some particular laser processing applications.     

Thermal effects investigation and cavity design in passively mode-locked Nd:YVO4 laser with a SESAM

A diode-pumped passively mode-locked Nd:YVO₄ laser with a five-mirror folded cavity is presented by using a semiconductor saturable absorber mirror (SESAM). The temperature distribution and thermal lensing in laser medium are numerically analyzed to design a special cavity which can keep the power density on SESAM under its damage threshold. Both the Q-switched and continuous-wave mode-locked operation are experimentally realized. The maximum average output power of 8.94 W with a 9.3 ps pulse width at a repetition rate of 111 MHz is obtained under a pump power of 24 W, correspondingly the optical slope efficiency is 39.2%.     

Experimental determination of effective stimulated emission cross-section in a diode pumped Nd:YVO4 micro-laser at 1064 nm with various doping concentrations

A practical method is described and used for determination of the effective stimulated emission cross-section (σ_e) in an operating diode pumped Nd:YVO₄ micro-laser at 1064 nm with various doping concentrations. In this method a micro-laser is formed by keeping a small piece of the sample in a plane–plane resonator under semi-monolithic configuration and a fiber coupled diode laser (808 nm) was used for pumping. The pump power induced thermal lensing effect was used to make the cavity stable. In thermally stabilized solid-state lasers the cavity parameters change dynamically with the pump power and hence the overlap integrals become a function of the absorbed pump power. In our method the overlap integrals were estimated by measuring the thermal lens focal length at the threshold. The value of σ_e of Nd:YVO₄ crystal with different doping concentrations obtained by this method were in well agreement with the reported values.     

气体激光器中的类透镜效应分析

本文讨论了类负透镜介质一般曲面镜光学谐振腔,并用来分析气体激光器中的热透镜效应。对平凹腔连续波二氧化碳激光器进行了模式分析。结果表明,热透镜效应对输出功率和频率特性影响相当大,对基模输出激光器的影响最为突出。最后,简单地讨论了离子气体激光器和某些其它激光器中的可能热透镜效应。     

Characterization of 1.06 μm optical vortex laser based on a side-pumped Nd:GdVO<Subscript>4</Subscript> bounce oscillator

We directly produced for the first time a high-power 1.06 μm vortex mode from a diode-pumped Nd:GdVO₄ bounce amplifier. A maximum output of 17.8 W was achieved for a pump power of 55 W. The corresponding optical efficiency from the diode to the output was above 30%.     

Nonlinear absorption and refraction effect on thermal lensing of solid-state laserrods

In this work, the effect of nonlinear absorption and refraction on thermal lensing of an end-pumped solid-state laser rod is investigated. At high input pump powers and on the focused region of thermal lensing, nonlinear responses of the active medium will be important. By considering the nonlinear absorption and refraction, the intensity distribution inside the rod is obtained. A convergence lens models the intensity dependency of the refractive index. By modeling of the laser rod with some contributed effective lenses, the stability condition of a typical laser resonator is investigated.     

Effective pulse recompression after nonlinear spectral broadening in picosecond Yb-doped fiber amplifier

We report the performance of a picosecond master-oscillator power amplifier (MOPA) system based on a diode-pumped solid-state (DPSS) seed laser and Yb-doped fiber amplifier. An average power of 28 W at ∼200 MHz repetition rate is achieved by using only one amplification stage. We found that positive nonlinear phase shift induced by nonlinear effect in the active fiber can be effectively compensated by a grating pair. A pulse duration of ∼1.6 ps is shown after recompression.     

Influence of neodymium concentration on the strength of thermal effects in continuous-wave diode-pumped Nd:YVO4 lasers at 1064 nm

The influence of neodymium ion concentration on the continuous-wave power performance of diode-pumped Nd:YVO₄ lasers was experimentally investigated by using three crystals of approximately the same total absorption but different Nd concentrations of 0.5, 1, and 3 at.%. In situ measurements indicate that strong thermal lensing occurs in the highly doped sample (3 at.% Nd) and precludes scaling of the output power to large values. Results also show that undesirable thermal effects are dramatically reduced with decreasing active-ion concentration. The best power performance was obtained with a 10-mm-long Nd:YVO₄ sample that had 0.5 at.% Nd. In this case, up to 6.5 W of continuous-wave output power was obtained at 1064 nm by using a 3% transmitting output coupler.     

Diode end pumped Nd:YAG laser at 946 nm with high pulse energy limited by thermal lensing

Diode lasers with peak powers in the kW range and pulse durations of micro- to milli-seconds have been available since several years. Pumping solid state lasers with such sources yield high output pulse energies in spiking or Q-switched operation. The output energy is limited by the thermal lens effects, which are measured and calculated. The time dependent heat conduction equation in the laser crystal is solved numerically to predict the overall temperature rise and thermal lensing. The thermally induced optical path difference is approximated by a quadratic distribution to obtain the focal length f of the thermal lens. The thermal lens coefficient K=1/(f⋅P _av), which depends only weakly on the heat transfer coefficient H of the laser crystal to the heat sink, decreases exponentially with increasing pump frequency until the steady state is reached. Experiments were done with a Nd:YAG crystal at different pump frequencies up to 100 Hz. The thermal lens coefficients obtained by the power maxima of asymmetric flat-flat resonators agree with our calculations.     

Control of thermal effects for high-intensity Ti:sapphire laser chains

We have experimentally studied thermal effects in highly pumped Ti:sapphire crystals in the 140-340 K temperature range. We demonstrate that a critical temperature exists, which depends on pumping and probing conditions, below which detrimental thermal effects completely disappear. In particular, we show that in a typical high-intensity Ti:sapphire amplifier, thermal lensing and wave-front distortions can be suppressed keeping the crystal temperature lower than ~230 K.     

Thermal-lens measurement of a quasi steady-state repetitively flashlamp-pumped Cr, Tm, Ho:YAG laser

A simple technique using cavity stability conditions is demonstrated to measure the effective dynamic thermal lensing of a flashlamp-pumped Cr, Tm, Ho:YAG laser operated in a low repetition-rate regime of 4–7 Hz. By measuring the flashlamp energy where a stable resonator becomes unstable, the effective focal length of the laser rod can be calculated. When no mode restricting aperture is used in the cavity, the thermal lens behaviour displays a significant deviation from the commonly assumed thin lens approximation.     

Transient thermal lensing measurement in a laser diode pumped NdxY1−xAl3(BO3)4 laser using a holographic shearing interferometer

A new technique for measuring a thermal lens, using a holographic shearing interferometer is presented. This technique was used to measure transient thermal lensing in a laser diode pumped NYAB laser. The measured thermal lensing power was proportional to the pumping laser diode intensity, with a gradient of 1.1 × 10⁻¹ m⁻¹ mm²/W. The transient response time of the thermal lens was 1.5 s, this value being consistent with the temporal decline of the second harmonic power.