二极管泵浦准三能级固体激光器的新进展

对比了掺Nd3 、Yb3 、Ho3 、Tm3 等稀土离子的准三能级固体激光的特性,分析了它们在圆棒、板条、薄片、光纤等构型的固体激光器中的工作性能.着重总结了高平均功率掺Yb3 固体激光器的最新进展,最后简要介绍了本研究小组在高功率Yb:YAG/YAG复合板条激光器和光纤激光器两方面的最新研究结果.     

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     

Optimization of fiber-coupled laser-diode end-pumped lasers:influence of pump-beam quality

A general model has been developed for the optimization of fiber-coupled laser-diode pumped lasers by including the effect of pump-beam quality into the mode-overlap integrals. The numerical results of the mode-overlap integrals have been fitted to an analytical function of pump spot size, laser mode size and the parameter β, which is in terms of laser-diode beam quality and properties of the active medium. This analytical function allowed us to obtain explicit formulae for the threshold pump power, the slope efficiency, and the optimum pump spot size. With these formulae, the optimum mode size and the maximum output efficiency can be easily calculated for arbitrary values of β and input power. From the numerical results, we obtained an analytical expression to relate the required input power and pump-beam quality to the desired output efficiency and properties of an active medium. The present model provides a straightforward procedure to design the laser resonator and the optical coupling system for optimization. To illustrate the utility of the present model, a Nd:YAG laser pumped by fiber-coupled laser diodes is considered and optimized     

Passively Q-switched lasers

Passively Q-switched, diode pumped, all solid-state lasers are an elegant, yet simple, means to generate single-frequency laser pulses in the nanosecond and sub-nanosecond regions. The majority of the work on these systems has been in the 1-/spl mu/m region, but recent developments at eye-safe and mid-infrared wavelengths of both diode-pumped lasers and solid-state saturable absorbers will probably lead to practical, passively Q-switched lasers at longer wavelengths. However, longer wavelength material properties are expected to only allow the generation of longer duration pulses. In the 1-/spl mu/m region, near-term advances are anticipated in the development of higher average power systems operating at multikilohertz pulse rates, with pulse energies exceeding 1 mJ and corresponding improvements at second, third, fourth, and even fifth harmonic wavelengths.     

Modeling of end-pumped CW quasi-three-level lasers

A plane wave model has been developed to describe the dynamic behavior of end-pumped CW quasi-three-level lasers. We have derived algebraic equations and analytic expressions from a set of coupled rate equations. The analytic expression predicts that characteristics of a laser are affected not only by the Boltzmann occupation factor of the pump states, but also by that of the laser states. To illustrate the utility of the model, an end-pumped Yb:YAG laser oscillator was investigated and optimized. We have examined the threshold value of the pump beam, pump beam depletion in the gain medium, optimal crystal length, and laser output power for a given pump power. Finally, we have shown the pump and laser intensities and the population of each manifold as a function of crystal length     

LED end-pumped Nd:YAG lasers

Experimental results are reported on the room-temperature operation of Nd:YAG lasers end pumped with an LED. The radiation from a 10-percent-efficient 0.46-mm-diam domed LED was coupled to the end of a 0.46-mm-diam × 5.0-mm-long laser rod with a large hemispherical reflector. At 20°C, a multimode laser power of 0.25 mW was obtained at an LED current of 250 mA. By measuring the variation of threshold pump power with rod temperature and the laser power versus pump power just above threshold, both the laser cavity loss and the output mirror transmission were determined. A round-trip cavity loss as low as 0.022 percent was measured. A calculation of the fractional pump power absorbed in the rod for the LED spectrum gave 56 percent for a 5-mm-long rod while measurements showed that 30 to 35 percent of the LED power was being absorbed indicating an LED-to-laser rod coupling efficiency of 54 to 63 percent for this arrangement. With such efficient absorption of pump power and low cavity loss, end-pumped Nd:YAG lasers with high slope efficiencies above threshold are possible.     

Pump source requirements for end-pumped lasers

A simple model is developed to relate laser design and properties of a gain medium to the requirements on a pump source for end-pumped lasers or amplifiers. A novel technique for scaling the pump power or energy for end-pumped lasers is presented and analyzed in terms of this model. The results indicate that power or energy scaling of the pump source for end-pumped lasers is not limited by geometry; however, the results also show that some minimum pump energy or power is required     

Q-switched semiconductor diode lasers

Diode lasers with an intracavity electroabsorption modulator have been operated with full on/off modulation at rates of 3 GHz. In addition, modulation of the lasers has been shown up to a detector-limited frequency of 6 GHz. A new model of these devices, which includes amplified spontaneous emission and high gain is developed in this paper. A quasi-static gain approximation is introduced and the dynamics of the electron and photon population are modeled by three coupled nonlinear difference equations which can be numerically solved with very little computation time. The model predicts the possibility of a new mode ofQ-switched operation with the capacity for repetition rates of tens of gigahertz and binary pulse position modulation at rates of the order of 10 Gbits/s.     

椭圆高斯分布半导体激光器泵浦被动调Q激光器的优化

在考虑泵浦光束和初始反转粒子数椭圆高斯分布的条件下，确定了新的调Q耦合速率方程。在椭圆高斯分布近似下，通过数值分析的方法针对脉冲能量进行优化，首次得到了归一化的脉冲能量和反射镜反射率等关键参数与无量纲变量z的关系曲线。文中以半导体激光器贴近端面泵浦，Cr4+:YAG作为饱和吸收体的Nd:YVO4被动调Q激光器为例进行了脉冲能量优化计算，在小信号透过率为75%，反射率91.8%时，对应最大脉冲能量1.5 J，峰值功率81 W，脉冲宽度18.4 ns。选用小信号透过率为73.4%的饱和吸收体和反射率为87%的反射镜进行实验验证，得到的脉冲能量为1.25 J，峰值功率为76.4 W，脉冲宽度为16.3 ns。理论计算与实验结果基本相符。     

Q-switched unidirectional ring lasers

Traveling-wave operation of a ruby laser has been obtained by the use of a 5° Faraday rotator. Mode selection in a TW ruby laser with electroopticalQswitching is discussed.     

Injection-locked passively Q-switched lasers

The time-dependent behavior of an injection locked single-mode passivelyQ-switched laser may be described by three coupled first-order rate equations relating the laser gain, the absorber loss, and the optical power density. Injection locking results in a substantial increase in the mean output power and the output pulse energy and higher pulsation rate than when freely running. Stability analysis allows the determination of the maximum injected power that allows pulsations.     

Q-switched operation of quasi-three-level lasers

A theoretical model for longitudinally pumped quasi-three-level lasers under Q-switched, repetitively Q-switched, and cw operating conditions is developed. The model is applied using 8% Ho:YAG as the gain medium, and it is found that for typical conditions the effective energy storage time can be much less than the upper state lifetime, in agreement with experiment but not predicted by earlier theories. This effect is attributed to details of laser design and pumping conditions, as opposed to being an intrinsic property of the gain material     

LD纵向抽运固体激光器中的耦合系统

介绍了目前在LD纵向抽运的固体激光器中可以采用的各种耦合系统，分析了它们的优缺点及适用范围，重点研究了排球面柱透镜系统和透镜导管耦合系统。     

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 Cr⁴⁺ :YAG absorber     

Coherent combining of Q-switched fibre lasers

The power rising of an actively Q-switched erbium-doped fibre laser by using two coupled cavities with amplifying fibres in an interferometer configuration is reported. This study shows that the pulse peak power obtained is 1.7 higher than in a case of a unique laser. This concept brings some novel perspectives for scaling the output peak power of singlemode Q-switched fibre lasers.     

Dynamics of passively Q-switched microchip lasers

Self-pulsing in passively Q-switched microchip lasers is investigated in detail. The typical range of values of the parameters motivates a new analysis of the laser rate equations. We determine basic properties of the laser intensity oscillations such as threshold conditions, repetition rate, pulsewidth, peak power, and pulse energy. We show that these oscillations appear through a quasi-vertical Hopf bifurcation located slightly above the lasing threshold. Our bifurcation results are verified numerically by modeling a microchip laser experiment with Nd:YAG as the gain medium and YAG:Cr as saturable absorber. Our results agree with the experiment to within 10%     

Pulse characteristics of Q-switched fiber lasers

A theoretical investigation of the anticipated properties of optically end-pumpedQ-switched fiber lasers is reported. This analysis stresses the effect of the transverse structure of the interacting pump and signal (guided) waves, in particular of the fiberVnumber, on the device performance. In Nd : YAG single crystal fibers thresholds as low as a fraction of a milliwatt may be achieved; pulses with peak powers of hundreds of watts and subnanosecond widths may be generated under laser diode pumping conditions.     

Multipulsing behavior of electrooptically Q-switched lasers

The influence of the piezooptical effect on postlasing in electroopticallyQ-switched Nd:YAG lasers is discussed and a simple method for its elimination is presented.     

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 Nd³⁺:YVO₄ laser is presented to demonstrate the use of the curves and the related formulas