Undoped End Research Articles

20 W CW laser operation of Tm:YAP single crystal fiber

We report the development of a low-loss, thulium-doped yttrium aluminum perovskite (Tm:YAP) single-crystal fiber (SCF), fabricated using the Czochralski method. This SCF features undoped YAP end caps that are directly grown on both ends, aimed at improving thermal management and enhancing the guidance of pump light. This innovative design also removes differences in crystallographic orientation, exhibiting a propagation loss of 0.0112 cm−1 at 1.94 µm. When pumped by diode lasers at 788 nm, the SCF laser delivers continuous-wave output powers of 11.9 W and 20.6 W under single-end and double-end pumping configurations, respectively. The slope efficiencies achieved are 53.2 % and 40.7 %, relative to the incident pump power. To the best of our knowledge, this is the inaugural report of a Tm:YAP SCF laser.

Thermal and population lensing of Yb:YLF at cryogenic temperature

We present what is, to our knowledge, the first detailed study of thermal and population lensing in cryogenically cooled Yb:YLF. Using a rod-shaped a-cut Yb:YLF crystal, the thermal lens strength is measured as a function of absorbed pump power for both E//a and E//c polarizations for pump power levels up to 600 W. Our experiments revealed that the thermal lensing behavior in Yb:YLF is quite asymmetric. For the E//a polarization, the thermal lens is rather strong and always converging for both horizonal and vertical axes. For the E//c configuration, thermal lensing is rather weak, and its sign could be different in different axis, and furthermore, the sign of thermal lensing is observed to be dependent on the absorbed pump power level. Simulations and detailed experimental measurements show that the observed peculiar behavior is a result of competition between the photo-elastic effect and thermo-optic effect: as their relative strength varies at different pump power levels. We have also measured the contribution of surface bulging into thermal lensing and found it to be relatively small in our crystals with un-doped end caps. Lastly, the population lensing is measured in Yb:YLF for the first time, and it is found to be quite weak as well, and the polarizability difference parameter of Yb:YLF is determined as (3 ± 1.5) x 10−34 m3.

63 W wing-pumped Tm:YAG single-crystal fiber laser.

We present a high-power continuous-wave (CW) Tm:YAG single-crystal fiber (SCF) laser wing-pumped by laser diodes at 791 nm. A maximum output power of 63.3 W is achieved at ∼ 2.01 µm, corresponding to a slope efficiency of 34.2%. This is, to the best of our knowledge, the highest power obtained from the SCF laser in the 2 µm spectral range. In addition to the wing pumping scheme, the large surface-to-volume ratio of such fiber-geometry crystalline rod with diffusion-bonded undoped YAG end caps are benefited for the spatial uniform distribution of pump intensity and thermal load, and thus improving the power scalability.

Simulation and experimental results of a high-gain two-stage and double-pass off-axis Nd:YVO4 picosecond laser amplifier.

A high-gain two-stage double-pass off-axis Nd:YVO4 picosecond laser amplifier has been developed. The comprehensive influence of crystal doping concentration and pump beam quality on the small-signal gain of the Nd:YVO4 amplifier is theoretically analyzed with a model developed by considering energy transfer upconversion and pump light absorption saturation effect. The thermal effect of Nd:YVO4 crystal with different doping concentrations, undoped end cap lengths, and pump beam quality is investigated as well. Based on the theoretical analysis, a high-gain two-stage and double-pass off-axis Nd:YVO4 amplifier based on picosecond fiber seed source is realized by choosing long-composite low-doping Nd:YVO4 crystal and with high brightness laser diode pumping, delivering a gain of 28 dB and an output pulse energy of 67.5 µJ at a repetition rate of 200 kHz.

Thermal Lensing Reduction in Conventional and Composite Nd:YAG Laser Rod

A finite-element method (FEM) wasused to simulate numerically the effect of thermal lensing in YAG rods doped with Nd ion using LASCAD software. The temperature distribution and thermal lensing focal length of the composite laser rod (YAG/Nd:YAG) with one undoped end cap was considered and compared with conventional laser rod (Nd:YAG) by applying software. Results show that thermal lensing effects were reduced by a factor of 2 using (YAG/Nd:YAG) composite rod at pump power of 20W.

Effective Selection of the TEM00-Mode in a Powerful Nd:YVO4-Laser With the 808 nm Diode Pumping

The problems of an effective selection of the TEM00-mode and thermo-optical distortions in a Nd:YVO4 laser with the longitudinal diode pumping are considered. In an experiment with a 40-watt single-end pumping of the composite YVO4/Nd:YVO4 crystal, the single-mode output power of 24 W at an optical efficiency of ηopt = 57.1% (the slope efficiency of 63.3%) is obtained. Apparently, this is the greatest power for lasers based on the Nd-doped yttrium orthovanadate with the longitudinal pumping at λ = 808 nm from one end of the active crystal. Evaluations of the thermal effects show that when using the Nd:YVO4-crystal (the length 20 mm, the diameter 3 mm, and the doping of 0.27 atomic %) having two undoped ends and the double-end diode pumping with total power of 170 W, one can expect obtaining a 100-watt level of the output power in the TEM00-mode from a single active element.

Nd:GdVO4 ring laser pumped by laser diodes

The design and operation of a laser diode-pumped Nd:GdVO4 ring laser is described. A composite crystal (Nd:GdVO4/YVO4) with undoped ends is single-end pumped by a fiber-coupled laser diode (LD) at 808 nm. A four-mirror ring cavity is designed to keep the laser operating unidirectionally, which eliminates spatial hole burning in the standing-wave cavity. This laser can operate either as continuous wave (CW) or Q-switched. The single-frequency power obtained was 9.1 W at 1063 nm. Q-switched operation produced 0.23 mJ/pulse at 20 kHz in the fundamental laser.

1 319-nm single-frequency output from dif fusion-bonded monolithic nonplanar Nd:YAG ring resonator with undoped end

An efficient 1 319-nm Nd:YAG single-frequency laser is demonstrated in a diffusion-bonded nonplanar ring oscillator (NPRO) with an undoped end. The thermal model of diffusion-bonded NPRO is generated to analyze the temperature field and thermal focal length. A stable single-frequency output power of up to 1.55 W is obtained at 1 319 nm.

Efficient single-mode laser with longitudinal diode pumping

A single-mode laser with unidirectional longitudinal pumping by laser diodes with and a power of is studied. In the mode, the output laser power is with the optical efficiency (slope efficiency 63.3%), which, as far as we know, is the best result for lasers with longitudinal pumping at from one face of the active crystal. Estimates of thermal effects show that, using a crystal (length , diameter , dopant concentration 0.27 at%) with two undoped ends and bidirectional diode pumping with a total power of , one can obtain an output power of in the mode from one active element.

A Continuous-Wave Diode-Side-Pumped Tm:YAG Laser with Output 51 W

A compact diode-side-pumped Tm:YAG laser is presented, which can output 51 W of cw power at 2.02μm. The Tm:YAG rod is side pumped by nine diode arrays with the central wavelength of 783nm and the with bandwidth of about 2.5 nm at 25° C. To decrease the thermal effect on the both ends and dissipate the heat effectively, one composite Tm:YAG rod with the undoped YAG end caps and the screw threads on the side surface of the rod is used as the laser crystal. The maximum optical-to-optical conversion efficiency of the 2.02-μm laser output is 14.2%, with a slope efficiency of 26.8%.

Comparative study of LD-pumped Tm:YAG and Tim:LuAG lasers

Widely tunable performances of Tm:YAG and Tm:LuAG lasers near 2 μm are demonstrated. The emission spectra of Tm3+ in these two single crystals are reported and analyzed. The lifetimes of the 3 F 4 state in thulium-doped YAG and LuAG are 10.1 and 9.2 ms, respectively. In addition, it is proven that the Tm:YAG crystal with two bonded undoped YAG ends is promising for high-efficient operation.

室温下二极管泵浦的复合Tm:YAG激光器

The lasing characteristics of composite Tm:YAG rod with undoped ends at room temperature are reported. The maximum output power at 2.015 µm is 6.97 W and the slope efficiency is 41.45%. Focusing point and output coupler are changed to find the optimization condition. The relationship between operation temperature and output power is discussed. Comparison between Tm:YAG and composite Tm:YAG testifies the superiority of composite crystal.

Study on the composite Nd:YAG crystal by laser micro-Raman spectroscopy

The composite Nd:YAG crystal with undoped YAG end caps was fabricated by thermal bonding technique. Scanning electron microscopy (SEM) result shows that the width of the bonded interface was about 0.5 μm. The laser micro-Raman was used to study the bonded interface of the composite crystal. The results of Raman spectra and the bonded interface image indicate that the undoped YAG and Nd:YAG crystals have been well bonded together. Furthermore, it can be drawn that the structure and the composition of the bonded interface are the same as the undoped YAG crystal, and that the Nd3+ ions do not enter the bonded interface and the YAG side. This property is advantageous to decrease the thermal effect of the boned interface and to improve the laser stability.

Calculation of three-dimensional thermal-gradient-induced stress birefringence in slab lasers

The thermal-gradient-induced stress birefringence and depolarization losses were reported previously for a Yb:YAG slab in which the plane-strain approximation applied. In this paper we present analyses of these phenomena applying a full three-dimensional treatment of deposited pump power and cooling in a Yb:YAG slab. We treat both straight-through and zigzag propagation and the cases of the slab length along the crystalline [1 1 1] and [1 1 0] axes. We also describe the losses in a slab with undoped Brewster ends and pump edges.

Improvement of thermal effects in a diode-end-pumped, composite Tm:YAG rod with undoped ends

We present comparative studies of the laser performance of a diode-end-pumped, composite Tm:YAG rod with undoped ends. Efficient heat removal in a composite rod can reduce the peak of the rise in temperature in the 2-mm-long active segment by 19% compared with that in a noncomposite rod by theoretical calculation. The threshold pump power was reduced by 10%, and the focal power of the thermal lens was improved by 15%. These improvements permit high output performance and flexible design of laser cavities in diode-end-pumped Tm:YAG lasers.

Efficient 946-nm laser operation of a composite Nd:YAG rod with undoped ends

We present comparative studies on laser performance of diode end-pumped, composite Nd:YAG rods with undoped ends at 946 nm. Efficient heat removal in a composite rod can reduce the peak of rise in temperature in the active segment to 61% by theoretical calculation. The maximum, continuous-wave output power of 1.5 W, which is three times higher than a conventional noncomposite rod, was obtained at an absorbed pump power of 8 W because of the 55 deg reduction in temperature rise that was estimated experimentally. The performance and characteristics of composite rods in quasi-three-level laser operation are analyzed and discussed in detail, taking into account the reduction in reabsorption loss, which is strongly temperature dependent.

Cooling schemes for longitudinally diode laser-pumped Nd:YAG rods

Longitudinal diode laser pumping causes pronounced inhomogeneous heating of the laser material. The effect of the thermal load on the mechanical and optical properties is significantly influenced by the method used for cooling the pumped surface. Four methods for longitudinally pumped Nd:YAG rods were compared experimentally and with finite-element (FE) analysis. The pumped surface exposed to air or to flowing water, a sapphire plate pressed onto the pumped surface and composite rods, having an undoped pumped end, were investigated. The comparison includes temperature and stress distributions with the emphasis on thermal lensing. Careful validation of the FE code with experiments allowed to numerically comparing the cooling methods under identical conditions.

Reduction of thermal effects in a diode-end-pumped, composite Nd:YAG rod with a sapphire end

We present what is to our knowledge the first theoretical and experimental estimation of the thermal characteristics of a composite Nd:YAG rod with an Al2O3 (sapphire) end in diode-end-pumped geometry. The peak temperature rise in the active segment is calculated to be reduced to 66% and the focal length of the thermal lens was measured to be reduced by 20% compared with a noncomposite Nd:YAG rod. By using a composite rod we successfully demonstrated the improvement of high-power performance that is due to reduction of thermally induced birefringence, which has not been observed in a composite rod with an undoped YAG end.

Analytical and experimental studies on the characteristics of composite solid-state laser rods in diode-end-pumped geometry

In this paper, we report analytical and experimental studies on the characteristics of end-pumped composite laser rods with undoped end, using mainly Nd:YAG rods as an example. It is found that the peak temperature rise in a composite rod decreases to <70% of that in a noncomposite crystal. Thermal stress is dramatically reduced to <60% by employing the composite rod structure, We also demonstrate high-power operation of the diode-end-pumped composite Nd:YVO/sub 4/ rod and a maximum CW output power of 9.3 W was achieved, which is about 1.5 times higher than that in the noncomposite rod. This high-power performance of the composite rod is primarily attributed to the reduction of thermal stress inside the rod.

115-W Tm:YAG diode-pumped solid-state laser

A compact diode-pumped Tm:YAG laser capable of generating greater than 100 W of CW power at 2 /spl mu/m has been demonstrated. A scalable diode end-pumping architecture is used in which 805-nm radiation, coupled to the wing of the Tm/sup 3+3/H/sub 6/-/sup 3/H/sub 4/ absorption feature, is delivered to the end of the laser rod via a lens duct. To facilitate thermal management, undoped YAG end caps are diffusion bonded to the central doped portion of the laser rod. For 2% and 4% Tm-doped rods of the same length, the lower doping level results in higher power, indicating that cross relaxation is still efficient while offering lower thermal stress and reduced absorption at the laser wavelength. Output powers for various output coupler reflectivities are compared to the predictions of a quasi-three-level model. Thermal lensing, cavity stability, and stress-induced birefringence measurements are described. The beam quality was analyzed with the 2% Tm-doped rod and a flat output coupler, yielding M/sup 2/ values of 14-23.