Shengzhi Zhao

Optimization of Cr/sup 4+/-doped saturable-absorber Q-switched lasers

Cr/sup 4+/-doped saturable absorbers are characterized by long excited state lifetime and appreciable excited state absorption. In the paper, we first solve the three coupled rate equations describing the operation of Cr/sup 4+/-doped saturable-absorber passively Q-switched lasers to obtain the expressions of pulse characteristics such as output energy, peak power, and pulsewidth. We then determine the key parameters of an optimally coupled passively Q-switched laser as functions of two variables concerning the amplifying medium, saturable-absorber medium, and pump level, and generate several design curves. These key parameters include the optimal normalized coupling parameter and the optimal normalized saturable-absorber parameter which maximize the output energy (or maximize the peak power, or minimize the pulsewidth), and the corresponding normalized energy, normalized peak power, and normalized pulsewidth. The results are valid for not only Cr/sup 4+/-doped saturable-absorber Q-switched lasers but also any other lasers passively Q-switched by saturable absorbers with long excited state lifetime and appreciable excited state absorption. Using the expressions and design curves, with the aid of a simple hand calculator, one can predict the pulse characteristics and perform the design of an optimally coupled passively Q-switched laser.

Modeling of passively Q-switched lasers

The intracavity photon density and the initial population-inversion density in the diode-pumped passively Q-switched laser rate equations are assumed to be Gaussian spatial distributions. These space-dependent rate equations are solved numerically, and a group of general curves are generated. These curves clearly show the dependence of the pulse characteristics on the pump- to laser-mode size ratio and the parameters of the gain medium, the saturable absorber, and the resonator. They can also be used to estimate the pulse parameters of an arbitrary diode-pumped passively Q-switched laser and to determine the optimal pump- to laser-mode size ratio. In addition, these curves are not limited to diode-pumped lasers. Sample calculations for a diode-pumped Nd3+:YVO4 laser and a flash-lamp pumped Nd3+:YAG laser are presented to demonstrate the use of the curves and the related formulas.

Modeling of diode-pumped actively Q-switched lasers

The intracavity photon density and initial population inversion density in the diode-pumped actively Q-switched laser rate equations are assumed to be Gaussian distributions. These space-dependent rate equations are solved numerically and a group of general curves are generated. By using these curves and the related formulas, the pulse parameters of an arbitrary actively Q-switched laser can be easily estimated and an optimally coupled Q-switched laser can be designed. A sample calculation for a Q-switched Nd/sup 3+/:YVO/sub 4/ laser is presented to demonstrate the use of the curves and the related formulas.

A new model of laser-diode end-pumped actively Q-switched intracavity frequency doubling laser

The intracavity photon density and the initial population-inversion density are assumed to be Gaussian spatial distributions in the rate equations of a laser-diode end-pumped actively Q-switched intracavity-frequency-doubling laser. In addition, the influence of the pump rate, the thermal effect in the gain medium and the change of the photon density along the cavity axis have been taken into account. These coupled rate equations are solved numerically, and the dependences of pulse width, single-pulse energy and peak power on incident pump power are obtained for the generated-green-laser pulses. In the experiment, a laser-diode end-pumped actively Q-switched Nd:YVO/sub 4//KTP laser with acoustic-optic-modulator is realized and the experimental results agree with the numerical solutions.

Dual-loss-modulated Q-switched and mode-locked YVO 4 /Nd:YVO 4 /KTP green laser with EO and Cr 4+ :YAG saturable absorber

By simultaneously employing the electro-optic (EO) modulator and Cr(4+):YAG saturable absorber, a diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) YVO(4)/NdYVO(4)/KTP green laser is presented. In comparison with the singly passively QML green laser with Cr(4+):YAG, the dual-loss-modulated QML green laser with EO and Cr(4+):YAG can generate more stable pulse train with deeper modulation depth, shorter pulse width, greater pulse energy and higher peak power. For the dual-loss-modulated QML green laser, at a pump power of 18 W and a repetition rate of 1 kHz, the pulse width and the pulse energy of the Q-switch envelope as well as the peak power of QML green laser are 42.1 ns, 360 microJ and 382 kW, respectively, corresponding to the pulse width compression 62%, the pulse energy improvement 10 times and the QML peak power increase 40 times when compared with that of the singly passively QML green laser.

Diode-pumped passively Q-switched and mode-locked Nd:GdVO 4 laser at 1.34 µm with V:YAG saturable absorber

Using V:YAG as the saturable absorber, a diode-pumped passively Q-switched and mode-locked Nd:GdVO4 laser at 1.34µm is realized. Nearly 100% modulation depth of mode-locking has been achieved. The width of the mode-locked pulse is estimated to be less than 460ps with 125MHz repetition rate within an about 1µs-long Q-switched pulse envelope. A maximum output power of 220mW and Q-switched pulse energy of 10.5µJ is obtained. Using the hyperbolic secant function methods, a fluctuation rate equation model considering the Gaussian distribution of the intracavity photon density and the population inversion in the gain medium as well as the ground-state population intensity of the saturable absorber has been proposed to describe the mode-locking process of diode-pumped Nd:GdVO4/V3+:YAG laser. With the space-dependent rate equations solved numerically, the theoretical calculations reproduce the laser characteristics well.

Passively Q-switched self-frequency-doubling Nd3+:GdCa4O(BO3)3 laser with GaAs saturable absorber

Nd 3 + :GdCa 4 O(BO 3 ) 3 , i.e., Nd:GdCOB, is a new self-frequency-doubling laser crystal. By using a GaAs saturable absorber as a passive Q-switched, the Q-switched Nd:GdCOB laser at 0.53 μm is successfully realized. Meanwhile, the pulse width and the single pulse energy are measured and the numerical solutions of the coupling wave rate equations are in agreement with the experimental results.

Optimization of dye Q-switched lasers

Analytical expressions for all key parameters of the optimally coupled Q-switched laser are derived. These parameters include the optimal reflectivity of the output mirror, the optimal initial transmission of the dye, and the maximum attainable output energy and corresponding peak power and pulse width for a given pump level. Meanwhile, according to the optimization theory of dye Q-switched lasers, a NAB miniature dye Q-switched laser is studied experimentally and experiment shows a good agreement with the theory. >

Influence of thermal effect on KTP type-II phase-matching second-harmonic generation

Abstract By considering the influence of phase mismatching resulting from the thermal effect of KTiOPO 4 (KTP), the type-II phase-matching second-harmonic-wave (SHW) power is calculated under the small-signal and the near-field approximations. The influences of thermal effect on the SHW intensity profile and the nonlinear conversion efficiency are analyzed in detail. The calculated results show that the thermal effect can result in a more severe reduction in conversion efficiency at high fundamental-wave (FW) power or/and long KTP length for a fixed FW beam width in comparison with the case where only walking-off effect is considered. It also shows that it is the combined influences of thermal effect and walking-off effect of KTP that result in the distortion of harmonic intensity distribution.

Watt-level passively Q-switched Er:Lu₂O₃ laser at 2.84 μm using MoS₂.

Efficient diode-pumped passively Q-switched Er:Lu2O3 laser operation at 2.84 μm was realized. A few-layer MoS2 nanosheet film on a YAG substrate, was fabricated and employed as saturable absorber (SA) in a short plane-plane cavity. Under an absorbed diode laser pump power of 7.61 W, an average output power of 1.03 W was generated with a pulse duration of 335 ns and a repetition rate of 121 kHz, resulting in a pulse energy of 8.5 μJ.