YAG Absorber Research Articles

Electron spin resonance study of an annealed Cr,Mg:YAG epitaxial saturable absorber

We have demonstrated the use of electron spin resonance (ESR) spectroscopy to observe the influence of high-temperature annealing in an oxidizing atmosphere on Cr3+ ion concentration changes in an epitaxial Cr,Mg:YAG saturable absorber. For this purpose, a set of samples of epitaxial Cr,Mg:YAG/YAG structures differing in both the concentration of Cr and Mg ions and the concentrations ratio of these ions have been synthesized using liquid phase epitaxy. The X-ray diffraction analysis and optical transmission spectra measurements were performed to characterize the obtained layers. The usefulness of the ESR technique for monitoring certain high-temperature induced changes in epitaxial Cr,Mg:YAG absorber has been verified.

Efficient continuous-wave and passively Q-switched lasers based on disordered Nd:Ca0.7La0.3Al12O19 crystals

A new neodymium-doped (Nd3+-doped) Ca0.7La0.3Al12O19 (Nd:CLA) disordered crystal was successfully grown by the Czochralski method. The performance of diode-pumped Nd:CLA (a-cut and c-cut) lasers was demonstrated for the first time. For the c-cut crystal continuous-wave (CW) operation, we obtained the highest output power of 2.08 W at the wavelength of 1053.2 nm, and the slope efficiency was 23% versus the incident pump power. By inserting a Cr4+:YAG absorber, we successfully achieved a passively Q-switched laser, and the shortest pulse width was 18.36 ns with a peak power of 364.6 W. Compared with a-cut crystal, c-cut crystal has superior laser characteristics.

High-peak-power subnanosecond pulse characteristics in a doubly Q-switched and mode-locking green laser with electro-optic modulator and Cr4+:YAG

By simultaneously employing an electro-optic (EO) modulator and Cr4+:YAG in a dual-loss-modulated Q-switched and mode-locking (QML) Nd:Lu0.15Y0.85VO4/KTP green laser, a subnanosecond single mode-locking (ML) green pulse with high peak power underneath a Q-switched envelope is obtained. Three Cr4+:YAG absorbers with different initial transmissions are used in the experiment. The pulse characteristics versus incident pump power under different initial transmissions and modulation frequencies are measured. The shortest pulse duration of 349 ps is obtained with a pulse energy as high as 371 µJ, corresponding to a peak power of 1.063 MW.

Efficient Continuous-Wave 1053-nm Nd:GYSGG Laser With Passively Q-Switched Dual-Wavelength Operation for Terahertz Generation

Research on an efficient continuous-wave Nd:GYSGG laser at 1053 nm with excellent stability is demonstrated. The maximum output power is 4.17 W, corresponding to the conversion efficiency of 33.9% and the slope efficiency of 42.92%. Using a Cr:YAG absorber, pulsed dual-wavelength operation at 1053 and 1058.4 nm is obtained, of which the maximum single pulse energy and peak power are 172.1 μJ and 26.1 kW, respectively, when the pulse width is 6.6 ns and the repetition rate is 4.3 kHz. A polarization property is observed, owing to the anisotropy of Cr:YAG. This stably Q-switched dual-wavelength laser is a good pump source for the generation of a terahertz wave at 1.53 THz.

Determination of parameters of Cr4+:YAG crystalline absorbers from saturation of absorption

A method for determining absorption cross sections of chromium ions in the ground and excited states in addition to their concentration in Cr4+:YAG crystalline absorbers was presented. The method was based on a study of the saturation of absorption, the measurement of the absorber transmittance as a function of laser-pulse energy, and the modeling of this function that gave the best fit of the experimental data. Absorption cross sections and concentrations of chromium ions were determined for seven absorber samples. The χ2-confidence intervals and correlation matrices for those parameters were calculated. The parameters of different samples were shown to vary over rather wide ranges. Hence, it was necessary to characterize beforehand a specific absorber sample in order to calculate correctly the parameters of lasers with Cr4+:YAG absorbers.

>100-kW linearly polarized pulse fiber amplifier seeded by a compact efficient passively Q-switched Nd:YVO4 Laser

We thoroughly develop compact high-peak-power Nd:YVO4/Cr4+:YAG passively Q-switched lasers (PQS) as the seed source of the fiber amplifier. We exploit a nearly hemispherical cavity to reach the second threshold criterion and systematically consider the thermal lensing effect and the mode-size matching in the overall optimization. Employing a Cr4+:YAG absorber with 70% initial transmission, we obtain a 50-kHz seed pulse train with the pulse duration of 4.8 ns and the pulse energy of 22 μJ at a pump power of 5.4 W. Injecting this seed laser into a polarization maintained Yb-doped fiber, the pulse energy and peak power at a pump power of 16 W are enhanced up to 178 μJ and 37 kW, respectively. We also use an absorber with 40% initial transmission to generate a 25 kHz pulse train with the pulse duration of 1.6 ns and the pulse energy of 36 μJ at a pump power of 5.4 W. With this seed laser, we find that the surface damage of the fiber limits the maximum pulse energy and peak power to be 192 μJ and 120 kW, respectively.

Performance enhancement of sub-nanosecond diode-pumped passively Q-switched Yb:YAG microchip laser with diamond surface cooling

We experimentally confirm that diamond surface cooling can significantly enhance the output performance of a sub-nanosecond diode-end-pumped passively Q-switched Yb:YAG laser. It is found that the pulse energy obtained with diamond cooling is approximately 1.5 times greater than that obtained without diamond cooling, where a Cr(4+):YAG absorber with the initial transmission of 84% is employed. Furthermore, the standard deviation of the pulse amplitude peak-to-peak fluctuation is found to be approximately 3 times lower than that measured without diamond cooling. Under a pump power of 3.9 W, the passively Q-switched Yb:YAG laser can generate a pulse train of 3.3 kHz repetition rate with a pulse energy of 287 μJ and with a pulse width of 650 ps.

Modeling the pulse shape of Q-switched lasers to account for terminal-level relaxation

To account for the effect of lower-level relaxation, we have derived a characteristic equation for describing the laser pulse from the modified rate equations for Q-switched lasers. The pulse temporal profile is related to the ratio of the lower-level lifetime to the cavity lifetime and the number of times the population inversion density is above the threshold. By solving the coupled rate equations numerically, the effect of terminal-level lifetime on pulse temporal behaviour is analysed. The mode is applied to the case of a diode-pumped Nd:YAG laser that is passively Q-switched by a Cr4+:YAG absorber. Theoretical results show good agreement with the experiments.

Maximum value of the pulse energy of a passively Q-switched laser as a function of the pump power

The finite recovery time Ts of the bleached absorber is presented as one of the possible mechanisms accounting for the increase-maximum-decrease in pulse energy E with the pumping rate Wp in cw-pumped passively Q-switched solid-state lasers, by analytically evaluating the sign of the derivative partial differentialE/ partial differentialWP. The results show that, in the low pump regime (T>Ts, T is the interpulse period), the initial population density ni remains constant, the final population density nf decreases with Wp, and this results in a monotonic increase of E with Wp. In the high pump regime (T<Ts), ni decreases but nf remains the same with Wp; this results in a monotonic decrease of E with Wp. At the critical region (T approximately Ts), E reaches its maximum value. A cw-pumped Yb:YAG laser passively Q switched by a Cr4+:YAG absorber is demonstrated to confirm this model. The theoretical model is also applied to the analysis of three previously reported passive Q switching solid-state [Nd:GdVO4, Nd+:LaSc3(BO3)4 (Nd+:LSB), and Nd:YAG] lasers experiments.

수동 큐스위칭 Nd:YAG 레이저에서 포화흡수체 Cr4+:YAG의 초기 광흡수 효과와 출력 특성

To understand the characteristics of the passively Q-switched Nd:YAG laser output energy with :YAG saturable absorbers, the transmissions of :YAG and the inversion population densities of Nd:YAG at the onset of Q-switch were experimentally analysed. The measured transmissions at the onset of Q-switch were 0.700.02 and 0.620.02 for the 0.48 and 0.38 of initial transmission, respectively. It means that the initial transmission loss of :YAG absorber is reduced in a low Q-state due to the initial absorbing effect of :YAG. In pumping stage, :YAG has absorbing processes due to the fluorescence and amplified spontaneous emissions of the Nd:YAG even if there is no laser oscillation. The minimum population inversion densities for Qswitch were approximately 3.7 and 4.0 , respectively. At the beginning of Q-switch, the number density of ions in the ground state of :YAG was approximately 1.4 and the ratio of the ground to the excited state of absorbing ions was 0.44 both. The modified theoretical output energies with the initial absorbing effect were 18 and 18.5 mJ. The measured output energies were 171 and 181.5 mJ, respectively. The quantum extraction efficiencies of Q-switch were 0.32 both. The theoretical Q-switched output results with the initial absorbing effect of the saturable absorber are a good agreement with the experimental results.

Modeling pulse shape of Q-switched lasers

A general approach describing the pulse shape and temporal width of Q-switched lasers has been derived. Based on a normalized intermediate variable describing the ratio of population inversion density at the beginning of Q-switch to that at threshold, a generalized characteristic equation that governs passively and actively Q-switched lasers is derived. From the characteristic equation, the pulse symmetry property and pulse width can be described and easily calculated for any given operating parameters, and the waveform of the laser pulse can be reconstructed without solving the laser rate equations. An approach is also given for the case of intracavity frequency-doubled Q-switched lasers. Theoretical results show agreement with experiments for a diode pumped Nd:YAG laser system that is passively Q-switched by a Cr/sup 4+/:YAG absorber.