Optimization of the pulse energy of GaAs saturable absorber Q-switched lasers

By taking into account the single-photon absorption (SPA) and two-photon absorption (TPA) processes of GaAs, the Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density, and the pumping and the spontaneous emission during the pulse formation, the new normalized rate equations of a GaAs saturable absorber Q-switched laser are solved. The key parameters of an optimally coupled GaAs saturable absorber Q-switched laser are determined, including the optimal normalized coupling parameter and the optimal normalized GaAs saturable absorber parameters, which can maximize the pulse energy, and a group of general curves are generated for the first time, which clearly show the dependence of the optimal key parameters on the parameters of the gain medium, the GaAs saturable absorber, and the resonator. In addition, the influence including the space variation, the pumping and the spontaneous emission is also shown. Sample calculations for a diode-pumped Nd³⁺:YVO₄ laser with a GaAs saturable absorber are presented to demonstrate the use of the curves and the relevant formulas.     

Optimization of pulse width of double passively Q-switched lasers with GaAs and Cr-doped saturable absorbers

Compared with a single passively Q-switched laser, a double passively Q-switched laser with a GaAs saturable absorber and a Cr⁴⁺-doped saturable absorber can produce more symmetric and shorter pulses with high pulse peak power. New normalized coupled rate equations for a double passively Q-switched laser with both a GaAs saturable absorber and a Cr⁴⁺-doped saturable absorber are solved numerically, where the single-photon absorption (SPA) and two-photon absorption (TPA) processes of GaAs are combined. The Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density are considered. The optimization of a double passively Q-switched laser to obtain the shortest pulse width is performed, and a group of general curves are generated for the first time. The curves clearly show the dependence of the optimal normalized parameters on the parameters of the gain medium, the GaAs saturable absorber, the Cr⁴⁺-doped saturable absorber and the spatial distributions of the intracavity photon density. Sample calculations for a diode-pumped Nd³⁺:YVO₄ laser with both a GaAs saturable absorber and a Cr⁴⁺:YAG saturable absorber are presented to demonstrate the use of the curves and the related formulas.     

Yb-doped passively mode-locked fiber laser with Bi_2Te_3-deposited

In this study we present an all-normal-dispersion Yb-doped fiber laser passively mode-locked with topological insulator(Bi2Te3) saturable absorber. The saturable absorber device is fabricated by depositing Bi2Te3 on a tapered fiber through using pulsed laser deposition(PLD) technology, which can give rise to less non-saturable losses than most of the solution processing methods. Owing to the long interaction length, Bi2Te3 is not exposed to high optical power, which allows the saturable absorber device to work in a high power regime. The modulation depth of this kind of saturable absorber is measured to be 10%. By combining the saturable absorber device with Yb-doped fiber laser, a mode-locked pulse operating at a repetition rate of 19.8 MHz is achieved. The 3-d B spectral width and pulse duration are measured to be 1.245 nm and317 ps, respectively.     

Chirped and chirp-free femtosecond pulses in passively mode-locked dye lasers

Femtosecond light pulse generation in a ring laser and its propagation properties are described. The basic principles and some theoretical aspects of the pulse generation process in dye lasers are summarized taking into account the influence of the amplifier, of the saturable absorber, of the glass path and of mirrors. Phase modulation arising from multilayer dielectric mirrors has been calculated and measured by determination of an equivalent intracavity glass path length. The main source of phase modulation (chirp) seems to be the saturable absorber.     

The effect of hard mode locking in a dye laser

The influence of the relaxation time of a saturable absorber on the dynamics of a quasi-continuous ring laser is investigated. It is shown that in a dye laser with a fast absorber, bistability arises; thus two multimode regimes coexist. The regime of shallow stochastic modulation is established spontaneously. Transition to the mode-locking regime accompanied by a sharp increase in the number of generated modes is feasible only under the action of an initiating pulse (hard mode locking). The analytical criterion for hard locking has been obtained. If the criterion is not satisfied, the only stable regime of a laser is mode locking. In this case the transient process is the longer the smaller the relaxation time of a saturable absorber.     

Optimization of GaAs semiconductor saturable absorber Q-switched lasers

The expressions of pulse characteristics such as output energy, peak power, and pulse width are obtained by solving the coupled rate equations describing the operation of GaAs semiconductor saturable absorber Q-switched lasers. The key parameters of an optimally coupled GaAs saturable absorber Q-switched laser are determined and several design curves are generated from these expressions for the first time. These key parameters include the optimal normalized coupling parameters and the optimal normalized saturable absorber parameters that maximize the output energy or maximize the peak power, and the corresponding normalized energy, normalized peak power, and normalized pulse width. Using the expressions and design curves, one can predict the pulse characteristics and perform the design of an optimally coupled GaAs saturable absorber Q-switched laser.     

Diode-pumped passively Q-switched Nd:YVO4 laser with GaAs saturable absorber

The intracavity photon density is assumed to be of Gaussian spatial distributions and its longitudinal variation is also considered in the rate equations for a laser diode(LD)end-pumped passively Q-switched Nd:YVO4 laser with GaAs saturable absorber.These space-dependent rate equations are solved numerically.The dependences of pulse width,pulse repetition rate,single-pulse energy,and peak power on incident pump power are obtained.In the experiment,the LD end-pumped passively Q-switched Nd:YVO4 laser with GaAs saturable absorber is realized and the experimental results are consistent with the numerical solutions.     

Control of the pulse width of a laser-diode end-pumped passively Q-switched solid-state laser

We theoretically and experimentally study different techniques to control the pulse width of a laser-diode-pumped passively Q-switched solid-state laser. It is shown that varying the laser beam radius in the saturable absorber and the pump beam radius in the gain medium provide an efficient means to control the pulse width. The experiments performed on a laser-diode-pumped Nd:YVO₄ laser passively Q-switched by a Cr⁴⁺:YAG saturable absorber are consistent with the theoretical calculations obtained from the rate-equations model, in which the intracavity photon density is assumed to be Gaussian spatial distribution, and the longitudinal variation of the intracavity photon density and the pump beam spatial distribution are also considered.     

Cr^4＋：YAG在对撞脉冲锁模的Nd：YAG激光器中实现被动锁模的研究

在理论上提出了Ｃｒ＾４＋：ＹＡＧ作为可饱和吸收体在Ｎｄ：ＹＡＧ激光器中实现被动锁模的可能性，运用速率方程组导出在强激光脉冲作用下Ｃｒ＾４＋：ＹＡＧ激发态吸收的恢复时间和饱和光强。实验上在带有抗共振环结构的接近于介稳腔的脉冲式Ｎｄ：ＹＡＧ激光器中用Ｃｒ＾４＋：ＹＡＧ作为可饱和吸收体实现被动锁模运转，得到能量１３．５ｍＪ、平均脉宽１８０ｐｓ的锁模脉冲序列。     

Mathematical modeling of the intracavity Q-switched Nd-glass laser

The temporal characteristics of the Q-switched Nd-glass laser have been numerically investigated. A mathematical model describing the dynamic emission and different physical processes has been adapted. This model allows the investigation of the nonlinear saturable absorber effects on the mode characteristics of the Nd-glass laser, and studying the affects of the laser input parameters on the output laser pulse characteristics.Numerical solutions of a nonlinear rate equation system predict the generation of nanosecond pulses of Q-switched Nd-glass laser. The solutions estimate the laser density and the relative population inversion of the Nd-glass laser rod and saturable absorber for different emission regimes. The estimated results of the laser output pulse characteristics are in a good agreement with the other calculated and experimental results.     

Generation of wide-bandwidth pulse with graphene saturable absorber based on tapered fiber

Wide-bandwidth pulses were generated with a dispersion-managed erbium-doped passively mode-locked fiber laser based on a graphene saturable absorber. The graphene saturable absorber was composed of a tapered fiber deposited with graphene fabricated by liquid-phase exfoliation. The output pulse had a 3-dB bandwidth of 13.6 nm, which is the widest spectrum ever achieved with graphene-tapered-fiber saturable absorbers.     

脉冲LDA泵浦Nd∶YVO4/GaAs被动调Q锁模激光器

用脉冲激光二极管阵列（LDA）作为泵浦源、微柱透镜阵列和透镜导管作为耦合系统,以As＋注入GaAs可饱和吸收片作为被动调Q锁模元件,实现了Nd∶YVO4激光器调Q锁模运转.调Q运转阶段,激光器每泵浦脉宽内输出一个调Q脉冲,调Q脉宽7ns.调Q锁模运转阶段,初始透过率60％的GaAs晶片对调Q包络内的锁模脉冲的调制深度达到95％以上,锁模脉冲重复频率991 MHz.研究了加在LDA上的电压、方波脉冲的脉宽和重复频率对调Q锁模脉冲特性的影响,并对实验结果进行了讨论.     

碳纳米管锁模双包层光纤激光器的实验研究

本文采用双包层掺镱光纤作为增益介质,用单壁碳纳米管作为饱和吸收体,获得最高输出功率为336 mW的锁模脉冲激光.用飞秒激光诱导水击穿法直接在单模光纤上制备出D形区,通过在D形光纤上滴涂单壁碳纳米管溶液,成功制备出碳纳米管饱和吸收体,并对其饱和吸收特性进行测试,发现其调制深度为27％.利用该饱和吸收体作为锁模器件,制备出具有环形腔结构的锁模光纤激光器.当抽运功率为4W时,获得了脉宽为93.8 fs,中心波长为1083.8 nm,3 dB谱宽为8.6 nm,重复频率为5.59 MHz,平均功率为336 mW的飞秒脉冲激光输出.     

Control of the pulse duration in one- and two-axis passively Q-switched solid-state lasers

We study theoretically and experimentally different methods to control the pulses emitted by solid-state lasers passively Q-switched by a saturable absorber. We explore one- and two-axis laser schemes allowing to control the pulse duration, which is ruled by the saturation powers of the transitions in the absorber and in the gain medium. In one-axis lasers, it is shown that the adjustment of the pump and laser beam sizes in the active medium and in the absorber provides an efficient means to control the pulse temporal shape and duration. Furthermore, a two-axis laser cavity supporting so-called forked-eigenstate operation permits to freely adjust the parts of the mode power which circulate in the gain medium and in the absorber. In this case, a lengthening of the pulse duration up to 500 ns is obtained with an increase of the average output power. The theoretical results obtained by using rate equations adapted to each cavity geometry are in close agreement with experiments performed on a diode-pumped Nd³⁺:YAG laser Q-switched by a Cr⁴⁺:YAG saturable absorber. The relevance of the different techniques to control the pulse durations in the framework of potential applications is discussed. Received 3 December 2001     

Optimization of passively Q-switched laser with V:YAG saturable absorber

By considering the Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density, the coupled rate equations for a diode-pumped passively Q-switched laser with V³⁺:YAG saturable absorber are given. These coupled rate equations are solved numerically and the key parameters of an optimally coupled passively Q-switched laser with V³⁺:YAG at 1342 nm are determined. These key parameters include the parameters of the gain medium, the saturable absorber and the resonator, which can maximize the pulse energy of singly Q-switched pulse. The optimal calculations for a diode-pumped passively Q-switched a-Nd:GdVO₄ laser with V³⁺:YAG saturable absorber are presented to demonstrate the numerical simulation applicable.     

Pump power and saturable absorber effect on a colliding pulse-modelocked dye laser emitting pulses with 47 fs

We present a quantitative experimental study of the effects of pump power and saturable absorber concentration on the performance of a colliding pulse modelocked dye laser. Pulses as short as 47 fs were generated without the use of any additional (intracavity or extracavity) pulse compression scheme. The laser has a well defined stability region in the pump power × saturable absorber concentration plane, and exhibits a strong bistable behavior in the limits of this stability region.     

Two-wavelength operation of a femtosecond ring dye laser

We report on two-wavelength operation of a CPM ring dye laser employing Rhodamine 6G as gain medium and a new styryl dye as saturable absorber. Two trains of femtosecond pulses at different wavelengths are simultaneously generated under proper alignment of the laser. The secondary pulse train is attributed to the laser action of the absorber dye. Auto- and cross-correlation measurements are performed to determine the temporal characteristics of the laser in the double mode-locking regime.     

Quantum-dot-based saturable absorber for femtosecond mode-locked operation of a solid-state laser

A quantum-dot-based saturable absorber has been demonstrated to initiate the generation of femtosecond pulses from a passively mode-locked solid-state laser. Control and tuning of the pulse duration from 58 ps to 158 fs was achieved. The 158 fs transform-limited pulses at 1280 nm are the shortest pulses that were produced from the Cr:forsterite laser passively mode locked by an InAs/InGaAs quantum-dot semiconductor saturable absorber mirror.     

Femtosecond pulse generation in a linear passive mode-locked dye laser

The femtosecond pulse generation in a c.w. pumped linear passive mode-locked rhodamine 6G-DODCI (3,3-diethyloxadicarbo cyanine iodide) dye laser is studied experimentally. Colliding pulse mode-locking (CPM) is achieved by placing the saturable absorber jet in the centre of the linear ring resonator. The laser performance is studied as a function of the saturable absorber concentration and of the absorber jet detuning from the central position (CPM position). Without a prism pair in the resonator pulse durations down to 140 fs were obtained. Detuning the absorber jet from the CPM position resulted in a trailing pulse tail ofcirca 900 fs duration. The dependence of the laser performance on the prism pair positioning is investigated experimentally and analysed theoretically. At the prism pair balanced position, stable pulses of about 50 fs duration were generated independent of the lateral detuning of the absorber jet out of the resonator centre. The dependence of the laser wavelength on the absorber concentration is compared with theoretical predictions. In an appendix the ray-tracing inside the linear resonator is simulated by an ABCD matrix calculation for Gaussian beams.     

Pulse compression in AO Q-switched diode-pumped Nd:GdVO4 laser with Cr4+:YAG saturable absorber

By simultaneously using both an acoustic-optic (AO) modulator and a Cr⁴⁺:YAG saturable absorber in the cavity, for the first time, a diode-pumped doubly Q-switched Nd:GdVO₄ laser has been realized. The pulse duration is obviously compressed in contrast to the actively acoustic-optic Q-switched laser. By considering the Gaussian transversal distribution of the intracavity photon density and the longitudinal distribution of the photon density along the cavity axis as well as the influence of turnoff time of the acoustic-optic (AO) Q-switch, we provide the coupled rate equations for a diode-pumped doubly Q-switched Nd:GdVO₄ laser with both an acoustic-optic (AO) modulator and a Cr⁴⁺:YAG saturable absorber. These coupled rate equations are solved numerically, and the dependence of pulse width, pulse energy and peak power on the incident pump power at different pulse repetition rates is obtained. The numerical solutions of equations agree well with the experimental results.This revised version was published online in August 2005 with a corrected cover date.