Theoretical and experimental study on the self-Raman laser with Nd:YVO/sub 4/ crystal

The intracavity photon densities and the initial population inversion density were assumed to be Gaussian distributions in the rate equations of the laser diode end-pumped actively Q-switched intracavity Raman laser. These space-dependent rate equations were solved numerically. In the experiment, an efficient self-Raman laser was realized based on the multifunctional Nd:YVO/sub 4/ laser crystal with the acoustooptic Q-switch. The output, temporal, and spectral characteristics of the self-Raman laser were investigated experimentally in detail. The performance of the self-Raman laser was studied numerically, and the theoretical results showed the main trends of the intracavity Raman laser, which are in agreement with the experimental ones.     

主动调Q腔内和频拉曼激光器的数值模拟

推导了LD泵浦主动调Q腔内和频拉曼激光器的归一化速率方程组,由实际的实验参数确定了方程组中七个综合参量的合理取值范围,然后用数值模拟的方法得到各个综合参量的大小对于和频光的脉冲峰值功率、单脉冲能量和脉冲宽度的影响,分析发现归一化和频因子F和归一化拉曼增益系数M之间能够相互匹配,通过数值拟合得到F和M在不同归一化反转粒子数密度N下的匹配方程,由此得出F和M的匹配值呈线性关系,应用此线性方程可以指导实验中参数的优化,使激光器获得更大的和频转换效率。最后利用拟合的结论对报道的实验进行计算和分析并给出优化措施。     

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     

Theoretical characterization of Raman oscillation with intracavitypumping of fiber lasers

Theoretical results relating to the generation of continuous-wave (CW) output from fiber lasers that are internally pumped with light generated from the stimulated Raman effect are presented. This investigation establishes the important fiber and resonator parameters, such as the fiber length and glass composition, dopant concentration, and pump power required to realize this new form of fiber laser arrangement. Three examples are studied: the Ho³⁺-doped silica fiber laser that is pumped at a wavelength of 1.15 μm, the Er ³⁺-doped silica fiber laser which is pumped at 1.48 μm and, the Tm³⁺-doped silica fiber laser which Is pumped at 1.625 μm. These three examples cover first Stokes pumping, second Stokes pumping, and first Stokes pumping with direct dopant absorption of the pump light, respectively. The simulations involve the use of simple numerical models comprising the spatially dependent field propagation equations (under the slowly varying field approximation) and the rate equations for the population densities. It is established that intracavity Raman pumping of fiber lasers with first Stokes radiation is efficient when the losses at the pump, Stokes and laser wavelengths are kept low (<10 dB/km). It is also established that second Stokes pumping is, even with direct absorption of the pump light, theoretically quite efficient and, as a result, the Er³⁺-doped silica fiber laser which is pumped with second Stokes radiation at 1.48 μm may provide the best demonstration of intracavity Raman pumping     

LD泵浦被动调Q腔内和频拉曼激光器速率方程

运用非线性光学的和频理论推导出被动调Q腔内和频拉曼激光器速率方程理论中的和频项,在被动调Q拉曼激光器速率方程的基础上,推导被动调Q腔内和频拉曼激光器速率方程,再将方程归一化,得到八个综合参量。推导出和频脉冲峰值功率和单脉冲能量的归一化表达式。数值模拟观察各参量对和频脉冲峰值功率、单脉冲能量和脉冲宽度的影响。对各参量影响进行分析,发现和频因子与归一化拉曼增益系数需要相互匹配才能实现高效率的和频光输出。同时,比较了被动调Q腔内和频拉曼激光器与被动调Q腔内倍频拉曼激光器的数值模拟结果。     

斯托克斯光种子法固体相干反斯托克斯拉曼频移器的理论研究

以波动方程和受激拉曼散射(SRS)物质方程为基础,采用光种子法,建立了固体相干反斯托克斯拉曼频移器的归一化耦合波方程,研究了晶体中反斯托克斯光转换效率。在脉冲抽运条件下分析了归一化增益系数G、归一化相位失配系数ΔK以及一阶斯托克斯光种子的归一化光场振幅ψs0三个变量对固体相干反斯托克斯拉曼频移器的影响,并作出了一系列相应曲线,由所得曲线估算了各归一化变量的合理取值范围。分析结果表明,在ΔK=0时,通过增大sψ0来打破拉曼增益抑制的影响,其转换效率峰值可达到44%。而当sψ0较弱时,可选取合适的相位失配系数,反斯托克斯光转换效率可达40%。     

连续腔内倍频拉曼激光器的归一化理论解析

腔内倍频拉曼激光器是获得黄光激光光源的重要途径,目前尚未有直接的表达式描述连续腔内倍频激光器的功率输出对抽运和激光器参数的赖关系。以速率方程为基础,对腔内倍频连续拉曼激光器的理论模型进行归一化处理,得到平面波近似下连续腔内倍频拉曼激光器的归一化速率方程组。对此方程组进行求解,得到描述激光器输出的表达式各变量以及参量与输出变量之间的归一化表达式,根据表达式获得了描述激光器运行的理论曲线。     

考虑腔内光强高斯分布时调Q激光器的速率方程及其解

在主动调Ｑ激光器速率方程中考虑腔内光强的高斯分布,数值求解该速率方程得至归一化的脉冲能量、峰值功率和脉冲宽度与泵浦条件之间的关系,并做图表示,同时与采用平面波近似时所得结果进行比较,并对相关问题进行讨论。     

Raman oscillation with intracavity second harmonic generation

We use a plane-wave analysis to examine a Raman oscillator containing an intracavity second harmonic interaction that frequency doubles the circulating first-order Stokes radiation. We find that there is an optimum ratio between the nonlinear coupling in the Raman medium and the nonlinear coupling in the frequency doubler. We also find that higher order Stokes radiation should be suppressed with the optimum choice of nonlinear coupling in the frequency doubler. We present numerical integration results that model the stimulated Raman scattering and second harmonic generation in three spatial dimensions. Quantum efficiencies as large as 48% are predicted from planewave theory and 43% are obtained from numerical integration of the equations containing transverse effects     

Raman oscillation with intracavity sum-frequency generation

We use a plane-wave analysis to examine a Raman oscillator containing an intracavity sum-frequency interaction that frequency sums the circulating first-order Stokes radiation with the pump radiation. We find that there is an optimum ratio between the nonlinear coupling in the Raman medium and the nonlinear coupling in the sum-frequency generator. We also find that higher order Stokes radiation should be suppressed with the optimum choice of nonlinear coupling in the sum-frequency interaction. Numerical integration of the equations containing transverse effects predicts a time-averaged power-conversion efficiency of 61.4% for conversion of 532- to 273.5-nm radiation using a CW mode-locked frequency-doubled Nd:YAG laser with Ba(NO₂) ₃ for the Raman material and CsLiB₆O₁₀ (CLBO) for the sum-frequency material     

Stokes-Anti-Stokes Iterative Resonator Method for Modeling Raman Lasers

We present a novel modeling method to describe the steady-state and transient regimes of a continuous-wave pumped Raman laser emitting both Stokes and anti-Stokes photons. Our so-called "Stokes-anti-Stokes iterative resonator method" evaluates for every half round-trip time the longitudinal distribution of the intracavity pump, Stokes and anti-Stokes fields propagating in forward and backward directions. Although this Stokes-anti-Stokes iterative resonator method is widely applicable, its most important asset resides in its ability to accurately model Raman lasers that feature cavity enhancement of the pump power and that emit both Stokes and anti-Stokes photons. Important here is that our modeling method correctly incorporates the longitudinal intracavity field distributions, the generation of anti-Stokes photons, and the interference effects between incident and intracavity pump fields, and that it describes not only the lasers' steady-state operation but also their transient characteristics. We demonstrate for both a hydrogen-based and a silicon-based Raman laser with pump cavity enhancement that the Stokes-anti-Stokes iterative resonator method performs better than the modeling methods presently used for these categories of Raman lasers. Finally, to demonstrate the potentialities of our modeling method, we numerically simulate, for the first time according to our knowledge, the anti-Stokes emission generated by a silicon-based Raman laser     

Optimization of doubly Q-switched lasers with both an acoustic-optic Modulator and a Cr/sup 4+/-doped saturable absorber

By considering the Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density as well as the influence of the acoustic-optic (AO) Q-switch, we provide the coupled rate equations for a doubly Q-switched laser with both an AO modulator and a Cr /sup 4+/ -doped saturable absorber (SA). These coupled rate equations are solved numerically. The key parameters of an optimally coupled doubly Q-switched laser are determined,and a group of general curves are generated for the first time. These key parameters include the optimal normalized coupling parameter, the optimal normalized SA parameters and the normalized parameters of the AO Q-switch, which can maximize the output energy. Meanwhile, the corresponding normalized energy, the normalized peak power and the normalized pulsewidth are given. The curves clearly show the dependence of the optimal key parameters on the parameters of the gain medium, the SA, the AO Q-switch,the resonator and the spatial distributions of the intracavity photon density. Sample calculations for a diode-pumped Nd/sup 3/+:YVO/sub 4/ laser with both an AO modulator and a Cr/sup 4+/ :YAG SA are presented to demonstrate the use of the curves and the related formulas.     

High-efficiency pulse compression with intracavity Raman amplifiers

High-efficiency pulse compression using intracavity Raman amplifiers has been computed. The energy of a pump laser stored in a lossless cavity is extracted at the Stokes frequency by means of Raman amplification of an input Stokes pulse. Calculations are made for both long and short duration input Stokes pulses for different lossless cavities. As an example, we use a hydrogen-argon mixture as the Raman medium and 1.5 J/cm²energy fluence stored in the lossless cavity at the ruby frequency. By comparing amplified Stokes pulses to a 30 ns pulse duration conventional ruby laser delivering the same energy fluence, pulse shortening factors larger than 20 are computed with quantum conversion efficiencies higher than 80 percent. These values compare favorably to backward Raman amplification. Moreover, this technique is proved to be able to provide a pulse compression rate larger than 14, even for a broad-band laser, which is impossible with backward Raman amplification. This technique could be used with any laser, even with absorbing laser media (excimer lasers) provided pump energy is stored in the lossless cavity by shifting of the laser frequency with any nonlinear process.     

A kilohertz repetition rate 1.9 μm H2 Ramanoscillator

Raman conversion of a high-repetition-rate Q-switched Nd:YAG laser using a gaseous H₂ Raman medium is reported. With a H₂ cell placed in a focusing intracavity Raman oscillator, 3 W of average power at 1.9 μm was obtained from a 15-W 1.06-μm laser operating at 2 kHz. Although the pump beam was multimode, the Stokes output was diffraction limited. At kilohertz repetition rates, conversion efficiencies were improved with a flowing gas cell which substantially reduced the thermal lensing effect in the Raman medium. A rate equation approach was used to model the intracavity conversion process     

An intracavity 16-μm Raman laser: a theoretical investigation

A theoretical model describing the dynamics of an intracavity 16-μm laser was developed. This laser consists of a TEA-CO₂ laser with an intracavity Raman cell. The Raman medium could be either hydrogen or deuterium, depending on the required wavelength. Stokes, anti-Stokes, and pump coupling were considered. Output energies as well as pulse shapes were calculated. Optimum output coupling was also determined. The model predicted that the intracavity Raman laser could be a viable alternative to a conventional multipass Raman cell for generating intense 16-μm pulses     

Electronic Stabilization of Continuous-Wave and Pulsed Lasers Based on Macroscopic Rate-Equation Modelling

We present a macroscopic laser rate-equation model based on measurable laser parameters, allowing easy system identification. A numerical simulation based on the model is used in the design and testing of electronic laser feedback systems for intensity noise suppression and Q-switched pulse stabilization. A novel pulse energy control scheme is also presented, including experimental results.     

Highly efficient Raman frequency converter with strontium tungstate crystal

A highly efficient extracavity Raman laser pumped by the infrared nanosecond laser pulses is presented utilizing the recently recommended Raman medium-strontium tungstate crystal (SrWO/sub 4/). The maximum conversion efficiency of the first and second Stokes pulses both reached about 50%, and the maximum total conversion efficiency of the first and second Stokes was obtained to be 70% in the experiment. The conversion efficiency dependence on the polarization and the temporal characteristics of the Stokes and pump pulses were also studied. A theoretical model for the solid-state extracavity Raman laser was established based on the radiation transfer equations describing stimulated Raman scattering processes, and solved numerically. This model can accurately predict the energy transfer dynamics observed in extracavity Raman lasers.     

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.     

1.54μm腔内喇曼激光器实验研究

喇曼激光器作为一种将基波频率转换成斯托克斯频率的高量子效率转换装置,实现激光频率的扩展,日益受到人们的重视。腔外喇曼频移过程,国内、外都有过充分的研究,而腔内喇曼频移激光器投入实际使用则只是近几年才发展起来。本文报道了脉冲Nd:YAG1.06μm基频经高压甲烷气体在封闭式激光腔内喇曼频移成1.54μm一阶斯托克斯脉冲输出的实验研究结果。     

激光二极管抽运主动调Q Nd:GdVO4自受激拉曼激光器

采用激光二极管（LD）抽运、主动调Q的方式，利用c向切割的Nd：GdVO4晶体的自受激拉曼散射（self-SRS）效应，实现了结构紧凑、高效的脉冲拉曼激光器。在输入功率为1．8W，主动调Q10kHz时，自受激拉曼激光器产生了稳定的1176nm的斯托克斯（Stokes）脉冲光，斯托克斯光的单脉冲能量为10μJ，脉冲宽度为19ns。此时，自受激拉曼散射的阈值仅为510mW，斯托克斯光的转换效率为5．6％。实验结果表明，有效的自受激拉曼变频可以通过一个c向切割的Nd：GdVO4晶体，采用主动调Q的方式来实现。