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     

Dual-wavelength diode-pumped laser operation of N p-cut and N g-cut Tm:KLu(WO4)2 crystals

Simultaneous continuous-wave laser oscillation at two wavelengths has been observed and studied in a diode-pumped monoclinic N _p-cut Tm:KLu(WO₄)₂ for different transmission of the output coupler. The maximum output power reached 1.15 W with a slope efficiency of 20.4 % with respect to the absorbed power for polarization parallel to the N _m optical axis. In an analogous N _g-cut crystal, the dual-wavelength laser operation is accompanied by polarization switching with increasing pump power and the switching point depends on the output coupling. The thresholds are slightly higher, and the slope efficiency reached a maximum of 25.5 % for polarization parallel to N _m at low pump levels, but at high pump levels, the oscillating polarization is parallel to N _p, reaching maximum output power of 3.09 W. Simple modelling with rate equations taking into account reabsorption losses explains qualitatively the complex behavior observed in the continuous-wave laser experiments with this anisotropic biaxial laser crystal.     

Quasi-CW diode pumped Nd:KGW laser at low temperature

We have studied the laser performance of Nd: KGW and Nd: YAG rods by monitoring the 1.06 μm laser emission at 78.5 K and 300 K. Our observations indicate that the laser efficiency from Nd: KGW is much higher than from Nd: YAG at these two temperatures. The laser threshold in Nd: KGW is lower at 78.5 K than at 300 K but it is always lower than that in Nd:YAG.     

Laser performance and thermal lens of diode-pumped Nd,Y-codoped:SrF_2 single crystals

We demonstrate the laser performances of Nd, Y：SrF2 crystals with Nd＇3＋ concentrations of 0.15 and 0.43 at.%. The sample with 0.43 at.% Nd3＋ concentration yields a maximum output power of 1.023 W at 1056.9 nm with a slope efficiency of 53%. The focal length of the thermal lens is analyzed for the 0.15 at.% Nd3＋-doped crystal sample. An improved cavity is designed considering the thermal lens. The maximum output power is 464 mW at 1056.9 nm, with a slope efficiency of 36.1%. The wavelength is tuned within the range of 1049.74-1059.13 nm.     

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.     

闪光灯抽运的Nd∶KGW和Nd∶YAG激光器的性能比较

用重复频率 1Hz、脉冲宽度 12 0 μs的闪光灯侧面抽运 ,在相同的实验条件下 ,对比了Nd∶KGW晶体与Nd∶YAG晶体的激光性能。在自由运转时 ,分别获得 1 0 %和 0 44 %的斜率效率和 0 5 4J、0 .76J的外推阈值能量 ,获得的最大输出能量为 384mJ和 16 5mJ。调Q运转时 ,Nd∶KGW激光器与Nd∶YAG激光器的斜率效率分别是0 16 %和 0 0 7% ,阈值能量为 5 9J和 12 5J。     

Output characteristics of a flashlamp pumped Nd:YAG laser at 1444 nm

An investigation of the output characteristics of Nd:YAG lasers operating at a wavelength of 1444 nm using a Xe-flashlamp of 450 Torr, a Xe-flashlamp of 700 Torr, and a Kr-flashlamp of 700 Torr is reported. The highest electrical-to-optical conversion efficiency of 1.83% was attained using the Kr-flashlamp of 700 Torr with a repetition rate of 20 Hz, a pumping pulse width of 144 μs, and an input energy of 31.7 J. The investigation concluded that output energy depends on the temperature of cooling water. The results showed a 3% decrease in output energy at an input energy of 31.7 J as the temperature of cooling water was increased from 23 to 38 °C.     

High-efficiency flashlamp-pumped Nd:KGW laser

The laser performance of a Nd:KGW rod has been studied in a single flashlamp cavity in the free running as well as in the Q-switched mode of operation at input energies ranging from 1–25J. The results of Nd:KGW have been compared with Nd:YAG operated under identical experimental conditions. The laser extraction efficiency of the Nd:KGW rod was observed to be 2.5 times higher at a much lower threshold than that of the Nd:YAG rod. The intrinsic slope efficiency was determined to be 2.25% and 8.840% for Nd:YAG and Nd:KGW rods, respectively.     

Effect of Nd concentration on the Nd:KGW laser

The effect of Nd concentration on the laser performance at 1064 nm from Nd:KGW has been studied in a single flash-lamp cavity. The Nd concentrations used were 3, 5, and 6 at.% in 6 mm diameter and 80 mm long KGW rods. The 5 at % Nd-doped KGW rod was found to have the highest laser extraction efficiency as compared to the 3 at.% and 6 at.% rods.     

Temperature dependence of efficiency and thermal lensing of diode-laser-pumped Nd:YAG lasers

Efficiencies, lasing thresholds and thermal lensing of a Nd:YAG rod side-pumped with two 10 W diode-laser-arrays are measured for various rod temperatures between 80 K and 380 K. The threshold is reduced by a factor of 2.5–4 on cooling from room temperature to about 100 K, while the slope efficiency is increased by between 0% and 50% depending on the reflectivity of the outcoupling mirror. Extrapolation to a 240 W pumped laser gives an increase in optical efficiency of 25% to 30%. The thermal lensing measured with a beam deflection and an interferometric method is reduced by a factor of 10.     

Energy transfer in flashlamp pumped organic dye lasers

The pumping characteristics of dye lasers using multiple dyes are discussed. It turns out that varying the dye concentration may change considerably the portion of the pump light spectrum which is effective in creating population inversion. Thus the effect of an energy transfer dye depends strongly on the concentration of the lasing dye. Multiple dye systems are shown to have larger tuning ranges than single dye systems, and should exhibit improved tuned linewidth as well as better dye stability. Experimental results are presented for a common dye combination (Rhodamine 6G and Coumarin 6). Measurements were made both by directly mixing the auxiliary dye with the lasing dye, and also by using the auxiliary dye as a fluorescent spectral filter placed between the pump light source and the laser cell; the results are compared.     

Comparison of cw laser performance of Nd:KGW, Nd:YAG, Nd:BEL, and Nd:YVO4 under laser diode pumping

4 crystals under low-power laser diode end-pumping. Output power dependencies on the pump power and the pump wavelength of these diode-pumped solid state lasers were investigated. The high Nd³⁺ concentration of the Nd:KGW samples used in our measurements as well as up-conversion and exited-state absorption processes in Nd:KGW cause the reduced laser output power dependence on the pump wavelength which was experimentally observed. At pump levels up to 270 mW a slope efficiency of η_sl≈46% was reached for the Nd:KGW laser. Nd:KGW microchip laser operation with a slope efficiency of η_sl≈50% was demonstrated. Thermal lensing in Nd:KGW at pump powers up to 3 W was measured. Received: 4 August 1997     

Reduction of beam divergence of a thermal lensing insensitive Nd:YAG laser

In a previous paper we proposed an active resonator designed especially for Nd:YAG lasers. The beam generated by this resonator had a flat-top beam profile and a divergence that was insensitive to the thermal lensing effect. The beam generated by this resonator was, however, too divergent to be focused into a fibre. With the help of numerical simulations, we have found a way to reduce this divergence, and to render the far-field of the laser beam fine enough to be coupled into a fibre with 600 μm core and NA = 0.2.     

Laser dye DCM: CW,synchronously pumped,cavity pumped and single-frequency performance

Laser dye DCM exhibits a tuning range of 605 to 725 nm with a lasing efficiency as high as 34% when pumped by the 488 nm line of the argon ion laser, placing it among the most efficient and broadly tunable dyes known. Performance of the dye is characterized for four laser systems: 1) continuous wave, 2) synchronously pumped (SP), 3) cavity dumped synchrompously pumped (SPCD) and 4) single-frequency ring dye laser. Pulse peak powers were as high as 520 W and 2.8 kW for SP and SPCD systems respectively.     

Laser emission under flashlamp pumping of a new crystal: Nd-doped strontium-lanthanum-aluminate (Nd:ASL)

1-x Nd_yLa_x-yMg_xAl_12-xO₁₉) is a new material, tunable in the 1–1.1 μm band. We present the first results of pulsed laser emission of this crystal under flashlamp pumping. We obtain a slope efficiency of 3.3% with a maximum average power of 40 W at 40 Hz in free-lasing regime. Nd:Cr codoping of ASL is also tested, but with much lower efficiency. Received: 12 June 1998/Revised version: 30 September 1998     

Superradiant guided modes in flashlamp pumped capillary dye lasers

Superradiant self-guiding emission from capillary dye lasers is reported. Capillary cells with very small inner diameter (50–200 μm) and standard large bore (2–6 mm) flashlamps have been used. The heat released by the pumping pulse to the active medium (Rhodamine 6G in ethanol) produces a lens-like distribution of the refractive index, which in turn gives rise to low-loss guided propagation in the cell. Near-field and far-field patterns turn out to be almost uniform at low dye concentration and become markedly ring-shaped at high dye concentration, denoting the presence of ‘whispering gallery’ modes. A theoretical analysis of the mode propagation in lens-like active media is carried out, and the field intensity distribution of Gaussian modes are evaluated for some cases of interest. The mode selection mechanism which determines the excitation of the modes is discussed.     

Comparative laser study of Nd:KGW and Nd:YAG near 1.3 μm

A comparative study of Nd:KGW and Nd:YAG laser crystals pumped by flashlamp has been conducted near 1.3 μm with output energy up to 1 J and at a repetition rate up to 50 Hz. An average power of 23 W for KGW in free-running mode was achieved with a total efficiency better than 2.8 % for the Nd:KGW and 1.8 % for the Nd:YAG. Received: 9 December 1996 / Revised version: 10 February 1997     

Laser gain in femtosecond microstructured Nd:MgO:LiNbO3 crystals

Femtosecond laser writing has been used to create 1D and 2D diffracting microstructures in Nd³⁺:MgO co-doped LiNbO₃ crystals. The main characteristics of the diffracting device, such as its polarization dependence and diffraction efficiency have been investigated, showing that first order diffraction efficiencies in excess of 35% can be achieved. We have demonstrated that continuous wave laser oscillation from Nd³⁺ ions at 1.06 μm can be generated through the integrated diffraction gratings, thus leading to non-collinear intracavity laser propagation. The laser performance, in terms of laser thresholds and slope efficiencies, is reported and explained. The achievement of a controlled non-collinear propagation is an interesting feature in multi-frequency lasers, optical communication and optical switching. PACS 42.55.Rz; 42.70.Hj; 42.25.Fx