YAG Rod Research Articles

Errata: High-repetition-rate multiwavelength polarized air cooled solid-state laser source for light detection and ranging applications

Advances in laser technology and nonlinear optical techniques can be effectively utilized for light detection and ranging (LIDAR) applications in space and atmospheric sciences to achieve better flexibility and control of the available optical power. Using such devices, one can achieve highly accurate and resolved measurement of the distribution for atmospheric scattering layers. In the present investigation, a diode-double-end-pumped high-repetition-rate, multiwavelength Nd:YAG laser is designed, fabricated, and various laser beam parameters are characterized for LIDAR applications. Nonlinear optical techniques are employed to generate higher harmonics like 532, 355, and 266 nm for various spectral studies. The studies of laser crystal parameters of Nd:YVO4 and Nd:YAG are performed using a diode-double-end-pumped configuration to extract maximum output power in the fundamental mode. The fractional thermal loading and effective stimulated emission cross section for 4F3/24I11/2 transition of 1.1 at. % doped Nd:YVO4 slab and 0.7% doped Nd:YAG rod is determined and compared using a convex-plane resonator configuration stabilized by pump-power-induced thermal lensing effect. Lower fractional thermal loading, larger effective stimulated emission cross section, and naturally polarized output enable these lasers to be more suitable for highly accurate and resolved measurement of the distribution for atmospheric scattering layers.

High-repetition-rate multiwavelength polarized air cooled solid-state laser source for light detection and ranging applications

Advances in laser technology and nonlinear optical techniques can be effectively utilized for light detection and ranging (LIDAR) applications in space and atmospheric sciences to achieve better flexibility and control of the available optical power. Using such devices, one can achieve highly accurate and resolved measurement of the distribution for atmospheric scattering layers. In the present investigation, a diode-double-end-pumped high-repetition-rate, multiwavelength Nd:YAG laser is designed, fabricated, and various laser beam parameters are characterized for LIDAR applications. Nonlinear optical techniques are employed to generate higher harmonics like 532, 355, and 266 nm for various spectral studies. The studies of laser crystal parameters of Nd:YVO4 and Nd:YAG are performed using a diode-double-end-pumped configuration to extract maximum output power in the fundamental mode. The fractional thermal loading and effective stimulated emission cross section for 4F3/24I11/2 transition of 1.1 at. % doped Nd:YVO4 slab and 0.7% doped Nd:YAG rod is determined and compared using a convex-plane resonator configuration stabilized by pump-power-induced thermal lensing effect. Lower fractional thermal loading, larger effective stimulated emission cross section, and naturally polarized output enable these lasers to be more suitable for highly accurate and resolved measurement of the distribution for atmospheric scattering layers.

Birefringence-induced bifocusing for selection of radially or azimuthally polarized laser modes

We develop a round-trip matrix diagonalization method for quantitative description of selection of radially or azimuthally polarized beams by birefringence-induced bifocusing in a simple laser resonator. We employ different focusing between radially and tangentially polarized light in thermally stressed laser rods to obtain low-loss stable oscillation in a radially polarized Laguerre-Gaussian, LG(0,1)*, mode. We derive a free-space propagator for the radially and azimuthally polarized LG(0,1)* modes and explain basic principles of mode selection by use of a round-trip matrix diagonalization method. Within this method we calculate round-trip diffraction losses and intensity distributions for the lowest-loss transverse modes. We show that, for the considered laser configuration, the round-trip loss obtained for the radially polarized LG(0,1)* mode is significantly smaller than that of the azimuthally polarized mode. Our experimental results, obtained with a diode side-pumped Nd:YAG rod in a flat-convex resonator, confirm the theoretical predictions. We achieved a pure radially polarized LG(0,1)* beam with M(2)=2.5 and tens of watts of output power.

Study of thermal effects, considering birefringence, on phase distortion of beam in a side pumped Nd:YAG rod using BEM

The present study uses boundary element method (BEM) in linear approximation to solve heat transfer equation. Differential equations of displacement are numerically solved through BEM to obtain strain tensor in an Nd:YAG crystal diode transversely pumped in a three fold geometry. Variation of refractive index, due to the temperature distribution and induced birefringence, which arises from photo-elastic effect, is obtained in the volume of the rod. Then the phase shift imposed on emerging beam from the rod is calculated by solving the equation of path of ray along the laser rod. Using the obtained results, effective focal distance and astigmatism of the beam for different types of polarization are calculated.

Theoretical investigation of feasibility of Yb:YAG as laser material for nanosecond pulse emission with large energies in the Joule range

The long fluorescence lifetime of the upper laser level of Yb:YAG seems to make it an attractive material for the realization of Q-switched pulses with large pulse energy. The consequences of the spectroscopic parameter set of Yb:YAG for the feasibility of a large pulse energy laser, which emits pulses with nanosecond duration, are addressed on the basis of a rate equation model. The rate equations are analytically solved and applied to the optical side pumping of Yb:YAG rods. The thermal population of the lower laser level and the saturation of the laser material are discussed in specific.

Numerical treatment of the temperature distribution in end-pumped composite laser rods

Analytical and numerical studies of the thermal characteristics of conventional and composite laser rods in laser-diode endpumped geometry are reported in detail using Nd : YAG rods as an example. It has been shown that taking into account the temperature dependence of the heat conductivity, the change of the temperature in the active element can be signicantly higher compared to the data for the temperature-independent heat conductivity coefcient. It has been found that the dependence of heat conductivity on temperature causes the non-parabolic heat distribution even in the region inside the pump beam with the top hat intensity distribution. It has also been shown by direct numerical simulations that the undoped entrance section of the composite rod signicantly reduces the peak temperature rise inside the crystal.

104 W high stability green laser generation by using diode laser pumped intracavity frequency-doubling Q-switched composite ceramic Nd:YAG laser

By use of CW diode laser stacked arrays, side-pumping Q-switched composite ceramic Nd:YAG rod laser based on a type II KTP crystal intracavity frequency-doubled, a high power high stability green laser has been demonstrated. Average output power of 104 W is obtained at a repetition rate of 10.6 kHz with a diode-to-green optical conversion efficiency of 10.9%. For the average output power of about 100 W, the measured pulse width is 132 ns with power fluctuation of less than 0.2%. The experimental results show that the green laser system using this novel ceramic Nd:YAG offers better laser performance and output stability than the traditional single Nd:YAG crystal green laser system with the same operating conditions and experimental configuration.

Efficient generation of blue light by intracavity frequency doubling of a cw Nd:YAG laser with LBO

We report on the efficient room-temperature operation of 4F 3/2– 4I 9/2 transition in a diode pumped Nd:YAG laser operating at 946 nm. An output power of 5.1 W and a slope efficiency of 23.6% at 946 nm have been obtained. Different LBO crystals of length 3×3×10 mm 3, 3×3×15 mm 3, 3×3×18 mm 3 were selected as frequency doubling material for comparison. A maximum single-ended output power of 1.3 W at 473 nm was achieved by frequency doubling with an optical conversion efficiency of 5%. When the Nd:YAG rod was replaced by the one with high reflectivity coating at 473 nm on the pump side, the output power of blue light was almost twice that without high reflectivity coating for 473 nm at the same pump power level. Moreover, the theoretical optimum length of LBO crystal for intracavity frequency doubling was discussed, and the experimental results made a good agreement with it.

Performance studies of diode-side-pumped CW Nd:YAG laser in copper coated optical pump cavity

This paper reports the performance studies of Nd:YAG rods with different diameters and doping concentration in the CW diode-side-pumped geometry. We have used p-polarized diodes to pump the active medium and we studied the performance of different rods in terms of thermal lensing, its fluorescence profile, power output, and M 2 factor of the laser beam. We achieved a maximum of 378 W of output power for an input pump power of 757 W, which corresponds to optical conversion efficiency close to 50%. The slope efficiency of the system was 56.4%. These efficiencies are the highest reported to the best of our knowledge and we achieved with ∅ 4 mm × 100 mm Nd:YAG rod at 1.1-at% doping.

Continuous-wave, 152 W diode-end-pumped Nd:YAG laser operating at 946 nm

A high-power continuous-wave (cw) Nd:YAG laser operating at 946 nm by utilizing a quasi-three-level transition is reported. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser and a simple plane-concave cavity. At an incident pump power of 40.2 W, a maximum cw output of 15.2 W at 946 nm is obtained, achieving a slope efficiency of 45%. To the best of our knowledge, this is the highest output at 946 nm ever generated by diode-pumped Nd:YAG lasers. In addition, at an incident pump power of 15.2 W, a 1.25 W blue output at 473 nm is achieved with a simple compact three-element cavity and a type-I lithium triborate (LiB(3)O(5)) crystal as a frequency doubler.

High-beam-quality, 5.1J, 108Hz diode-pumped Nd:YAG rod oscillator–amplifier laser system

A high efficiency, high beam quality diode-pumped Nd:YAG master oscillator power-amplifier (MOPA) laser with six amplifier stages is demonstrated. The oscillator with two-rod birefringence compensation was designed as a thermally determined near hemispherical resonator, which presents a pulse energy of 223 mJ with a beam quality value of M2 = 1.29 at a repetition rate of 108 Hz. The MOPA system delivers a pulse energy of 5.1 J with a pulse width of 230 μs, a M2 factor of 3.6 and an optical-to-optical efficiency of 38.5%. To the best of our knowledge, this is the highest pulse energy for a diode-pumped Nd:YAG rod laser operation with a high beam quality and a pulse width of hundreds of microseconds at a repetition rate of over 100 Hz.

Generation of 2.1 W Continuous Wave Blue Light by IntracavityDoubling of a Diode-End-Pumped Nd:YAG Laser in a 30 mm LBO

A folded four-mirror cavity with a composite Nd:YAG rod is optimized toobtain high efficient cw 473 nm blue output. The laser could operatestably in the region of the thermal-lens focal length from 20 mm to70 mm. LBO is used for intracavity frequency doubling of the 946 nmtransition of Nd:YAG and the optimum LBO length is investigated. Amaximum output power of 2.1 W in the blue spectral range at 473 nm isachieved with 30-mm-long LBO, corresponding to an optical conversionefficiency of 9.1%.

High-power and high-quality, green-beam generation by employing a thermally near-unstable resonator design

We have obtained green-beam quality of M2 = 6.2 at an average output power of 120 W by intracavity frequency doubling of a diode-side-pumped, Q-switched Nd:YAG rod laser with a repetition rate of 10 kHz and an optical-to-optical conversion efficiency of 15.2%. To achieve high-beam quality at high average power, the laser employs a thermally near-unstable resonator design with two-rod birefringence compensation in an L-shaped flat-flat cavity. The output power fluctuation of the green laser remains less than 0.9% in 4 h.

The overall phase shift and lens effect calculation using Gaussian boundary conditions and paraxial ray approximation for an end-pumped solid-state laser

In this work, the inhomogeneous equation of heat conduction was exactly solved by applying inhomogeneous boundary conditions for laser crystals of aspect ratio=1 (aspect ratio=radius of the laser rod/length of the laser rod). We have shown that the paraxial ray approximation leads the solution to be a function ofr2, that is, the approximation is equivalent to a situation in which a homogeneous pump source is used. The solution was then used to derive expressions for the overall phase shift, focal length of the thermal lens and the end effect induced curvature of the end face. The expressions were then applied to Nd:YAG laser medium. The result shows a meaningful correction of the order of 0.001 cm to the focal length of Nd:YAG rod for 3 W source power and beam waist of 100 μm.

Core-doped Ceramic Nd:YAG Laser

We report on a diode end-pumped composite core-doped ceramic Nd:YAG laser. The ceramic Nd:YAG rod consists of a centrally doped region of 1.5 mm in diameter in a 3 mm rod. An output power of 144 W was achieved with an absorbed pump power of 226 W corresponding to an opt.-opt. efficiency of 64% by longitudinal pumping with fiber-coupled laser diodes.

A 3 W continuous-wave 589 nm yellow laser based on the intracavity sum frequency generation in a V-shaped cavity

This paper reports an all-solid-state yellow laser, which is based on two Nd：YAG rods connected in series and the intracavity sum frequency generation in a KTP crystal by use of a V-shaped folding configuration, and continuous-wave 589 nm light with the output power of 3 W is obtained. In order to improve the output power of the yellow light, two means are introduced. One means is optimizing the space overlapping of two fundamental beams by optimized designing of the oscillators, and the other is optimizing the intracavity power ratio of two fundamental beams by choosing proper pump powers of the two beams.

Correction of spherical and azimuthal aberrations in radially polarized beams from strongly pumped laser rods

Spherical aberration of heavily pumped Nd:YAG rods was corrected by use of spherical relay optics selected to add conjugate amounts of aberration. Wavefront measurements showed elimination of spherical aberration. Correction of spherical aberration allowed 250 W of power to be generated in a radially polarized, birefringence-free oscillator (60% more power than without correction). Scale-up of wavefront maintenance was demonstrated in a two-rod amplifier module (6.3 kW electrical pump power). Radial polarization and spherical aberration correction together eliminated the main aberrations in uniformly pumped rod-based lasers. Rotating adjacent pump chambers substantially reduced the multifold aberrations induced by nonuniformity of the azimuthal pumps. DeltaM2 in the radially polarized beam was 0.3 and 1.4 with and without aberration correction, respectively, in each two-pump chamber module. Analysis predicts further improvements when higher-order aberration correction is applied.

Thermal lens compensation by convex deformation of a flat mirror with variable annular force

An optical surface with convex parabolic shape over a large area has been created using a 12.7-mm-diameter annular pusher to deform a flat 25.4-mm-diameter mirror. The deformable mirror assembly has been modeled using finite element analysis software as well as analytical solutions. The measured parabolic surface deformation shows good agreement with those models. Mirror performance was studied using a Shack–Hartman wavefront sensor and the mirror has been applied to compensate thermal lensing in a Nd:YAG rod amplifier.

비대칭 구조로 이루어진 불안정 공진기형 Nd : YAG 레이저의 빔질 분석

제2조화파 발생용으로 사용되는 볼록평면거울로 구성된 비대칭 불안정 공진기형 Nd:YAG 레이저의 빔질 특성을 수치적으로 분석하였다. 분석은 <TEX>$M^2$</TEX>, 공진기 내에서의 빔허리 직경, 모드 부피의 계산을 통해서 이루어 졌다. 수치적 계산 결과는 레이저 헤드가 볼록레이저 거울 근방에 위치한 비대칭 공진기 구조가 될 때 고효율의 제2조화파를 발생할 수 있는 조건이잘 충족됨을 보였다. We numerically investigate the characteristic of single Nd:YAG rod laser with the convex-plane resonator configuration in order to get the best condition for high efficient intra-cavity second harmonic generation(SHG). The beam quality is analyzed with ABCD matrixes including the thermal lensing characteristic of Nd:YAG rod. The analysis is focused on stability, M2, beam waist and mode-volume of laser beam inside the resonator. The best conditions for SHG is obtained when a laser rod is set near the curved laser mirror.

One hundred and twenty one W green laser generation from a diode-side-pumped Nd:YAG laser by use of a dual-V-shaped configuration

To ensure high-average-power and long-lifetime second harmonic generation, a dual-V-shaped configuration was designed. In this thermally near-unstable resonator, large fundamental mode size in Nd:YAG rod and small one in LBO crystal could be generated, which improved both the beam quality and the power density. A green output power of 121 W with pulse width of 68 ns and repetition rate of 10 kHz was obtained. To the best of our knowledge, the dual-V-shaped configuration is the first time used for green laser with average output power of more than 100 W.