A continuous-mode model of the mode locking in a pump-modulated laser

The active mode-locking process of the multimode laser with an external pump modulation is theoretically investigated in the frequency domain within the framework of the continuous-mode approximation. Intermode interaction and mode-coupling effects, including both AM and FM modulations, are naturally considered in a hierarchical equation of the mode components derived from the multimode Maxwell-Bloch equations. It is reduced to a continuous-mode equation that can be solved analytically in a stationary case, and used to discuss the spectral line shape and the phase dynamics of mode-components as a function of modulation amplitude and detuning of the modulation frequency. We predict a novel oscillation existing below the threshold of the ordinary complete mode-locking: The intensity of the total electric field yields a stable pulse train but its phase varies irregularly in time. This semi-locked state is characterized by a nonlinear chirping, an asymmetric spectrum, and drifting phases of the field mode-components.     

Theoretical analysis of picosecond pulse development of passively mode-locked Nd-glass lasers

A realistic numerical theory of the temporal and spectral development of picosecond light pulses in a passively mode-locked Nd-glass laser is presented. The effects of the inhomogeneous gain profile of the active medium are considered. The calculations include the pulse development in the prelaser, the linear laser and the mode-locking region. In the mode-locking region twophoton absorption and self-phase modulation in the active medium as well as the bleaching dynamics of the mode-locking dye are included. The influence of self-phase modulation on the temporal pulse shape in active media of finite spectral gain width is analysed. The theoretical results are compared with the experimental findings.This paper is dedicated to Professor W.Kaiser for his 60th birthday.     

Detuning effects of FM mode-locking in atmospheric CO2 lasers

We describe a study of the detuning of the intracavity FM modulation of the homogeneously broadened laser applied to CO₂ systems. The analysis is done by working in the frequency domain and by describing the field in terms of discrete amplitudes and phases. The coupled mode equations are solved for modulation frequencies near the axial mode-separation frequency. The asymmetry of the mode-locking behaviour due to the dispersive behaviour of the active medium is verified experimentally. We have measured both the pulse and the phase shift of the pulse with respect to the modulation signal as a function of detuning. The experimental results show clearly the asymmetry with detuning, in agreement with theory.     

Fourier domain mode locking pulse fiber laser

A novel pulse fiber laser is proposed based on the modulation of the lasing wavelength. Different from conventional amplitude or phase modulated mode-locking fiber lasers, the proposed fiber laser is with a so-called Fourier domain mode locking (FDML) operation (i.e., the cavity round-trip time of the optical pulse should match the period that the open time window of the tunable filter appears for the lasing wavelength or its harmonics). An FDML pulse fiber laser with a cavity round-trip frequency of 19.46 kHz is demonstrated and optical pulses with repetition rate of 19.46 kHz are achieved when the wavelength modulation frequency is 9.73 kHz.     

Mechanism of pulse shaping in femtosecond lasers based on the high-order soliton

The mechanism of pulse shaping in Kerr-lens mode-locking Ti : sapphire lasers is investigated. Considering the balanced interaction between self-phase modulation, introduced by Kerr effect in Ti : sapphire, and group-velocity dispersion provided by prism-pair, pulse-splitting reflected in the intensity autocorrelation trace and pulse-compression display that the pulses outputting from different positions in laser cavity have different durations. We have concluded that the solitonlike pulse shaping that results from the competition between intracavity self-phase modulation within the Ti : S and negative group-velocity dispersion play dominant role in pulse evolution in Kerr-lens mode-locking lasers.     

Generation of the numerator=2 rational harmonic mode-locked pulses in fiber ring lasers

In conventional rational harmonic mode-locking, optical pulse trains with the repetition rate of(pn + 1)fc are generated when the modulation frequency of the in-cavity modulator is set at fm=(n + 1/p)fc, where n and p are both integers, fc is the fundamental cavity frequency. In this paper, we report that rational harmonic mode locking phenomenon takes place in the fiber lasers when the modulation frequency is set at fm =(n + 2/p)fc. The pulse generations are experimentally demonstrated when the numerator of the rational corresponds to 2 in 5th and 7th order rational harmonic mode-locking.     

Mode-locked CW dye lasers

Mode-locking of CW dye lasers are described. Three different methods: active mode-locking by acoustic loss modulation, pumping with mode-locked pulses and passive mode-locking by a saturable absorber, all resulted in stable continuous trains of tunable mode-locked pulses of various duration. Best results were obtained using passive mode-locking by a saturable absorber. The pulse lengths for this case were measured by auto-correlation technique to be as short as 1.5 ps. Factors influencing the mode-locking and possible improvements are discussed.     

Saturable absorber mode-locking and laser pulse compression

The theoretical predictions of two alternative models of saturable absorber mode-locking in dye lasers are compared. The predicted mode-locking and pulse compression regimes are considered for both flashlamp-pumped and cw dye lasers, and some questions raised thereby are tested by numerical simulation of pulse propagation.     

有源耦合腔激光器锁模特性研究

本文在理论上首次发现有源耦合腔激光器的自启动锁模和时域调制特性,并对其进行了全面的分析和计算.对有源耦合腔激光器的自启动锁模机制和时域调制特性给予详尽的数值计算结果和清晰的物理图象解释,提出在有源耦合腔锁模激光器中可用连续泵浦源代替锁模泵浦源,而且有产生更短的光脉冲可能性.     

Numerical study of active mode locking in pulsed solid-state lasers

Transient mode locking of spiking solid-state lasers is studied with the help of two theoretical approaches. The first of them is an extension of the previous transient theory with single-mode rate equations to find the limited spike build-up time depending on the modulation index. The second is based on semiclassical multimode equations describing both spiking behaviour and formation of the picosecond pulse from the initial noise. The derivation of the equations as well as a comparison of results from both approaches are given. The obtained numerical results agree with experimental data. It is shown that the pulsewidth in a spiking laser can approach the small value predicted by the steady-state theory. This minimal pulsewidth can be reached at a smaller modulation index than predicted by the standard transient mode locking theory. The minimal modulation index for generation of good picosecond pulses free of noise is found to be 0.6 in our case. Losses connected with amplitude modulation have a negligible effect on spike build-up time and pulse duration.     

40 GHz Mode-Locked Semiconductor Lasers: Theory, Simulations and Experiment

We study both theoretically and experimentally typical operation regimes of 40 GHz monolithic mode-locked lasers. The underlying Traveling Wave Equation model reveals quantitative agreement for characteristics of the fundamental mode-locking as pulse width and repetition frequency tuning, as well as qualitative agreement with the experiments for other dynamic regimes. Especially the appearance of stable harmonic mode-locking at 80 GHz has been predicted theoretically and confirmed by measurements. Furthermore, we derive and apply a simplified Delay-Differential-Equation model which guides us to a qualitative analysis of bifurcations responsible for the appearance and the breakup of different mode-locking regimes. Higher harmonics of mode-locking are predicted by this model as well.     

The production and measurement of ultrashort pulses in a ruby layer

The two-photon flourescence from a whole pulse train is measured photoelectrically for a variety of laser conditions. A compound mode-locking solution containing DDI and DCI is found to reduce pulse lengths. An etalon which cancels gain-narrowing also has this effect. Intensity-dependent variations are found in the two-photon fluorescence profile which suggest a threshold for self-phase modulation.     

Effect of Spontaneous and Carrier Noise on Mode-Locked Hybrid Soliton Pulse Source

Noise characteristics of mode-locked hybrid soliton pulse source (HSPS) where fiber Bragg grating is used as external cavity is presented. Model is based on time-domain solution of coupled-mode equations including spontaneous noise. Relative intensity noise (RIN) is calculated by using numerical solution of these equations. It is shown that inclusion of carrier density noise in the rate equations is necessary for the accurate analysis of noise in mode-locked lasers. It is also found that transform-limited pulses are not generated over a wide tuning range around the fundamental mode-locking frequency with spontaneous and carrier density noise sources if noise level is high. Therefore mode-locking range where transform limited pulses are obtained reduces.     

Q-switched mode-locking femtosecond all-fiber laser based on Er3+/Yb3+ codoped phosphate fiber

A Q-switched mode-locking femtosecond all-fiber laser based on a 2 cm long homemade Er³⁺/Yb³⁺ codoped phosphate glass fiber has been reported. By using the nonlinear polarization evolution technique, a nearly 100% modulation depth of mode-locking pulse train is achieved. At a pump power of 410 mW, the energy of each Q-switched envelope, whose width is about 220 μs, is 10 μJ, while the duration of mode-locking pulse within the Q-switched envelope is 318-fs.     

Passive harmonic mode-locking in a fiber laser with nonlinear polarization rotation

On basis of numerical simulation and analytical treatment, we investigate the multiple pulse passive mode-locking of a fiber laser with nonlinear loss due to the nonlinear polarization rotation technique. Various additional mechanisms resulting in the ordering of ultrashort pulses inside the laser resonator and in the realization of harmonic passive mode-locking are analyzed. Among which are the active harmonic modulation of the intracavity loss or of the refractive index, the passive modulation of the index due to the inertial properties of intracavity elements, and the amplification modulation due to the depletion and the relaxation of the gain. The velocity of pulses relative to each other is determined, it results in attractive or repulsive pulse-pulse interaction, and allows to evaluate the conditions of ordering of the pulses inside the cavity.     

High Repetition Rate Optical Pulse Generation Using a Rational Harmonic Mode-Locking Technique in a Fiber Ring Laser

By applying a small frequency detuning to the RF modulation frequency, pulse trains with repetition rates of integer multiples of the RF modulation frequency have been obtained in an actively mode-locking fiber ring laser. Optical pulse trains with repetition rates of 20 GHz, 30 GHz and 40 GHz were generated respectively here. Furthermore, an interesting phenomenon of generation of pulse train with repetition rate half the modulation frequency was observed.     

C. 超短激光光源

本文对超短激光脉冲的产生、压缩技术的最新发展做了详细评述。     

Nd,Cr:YAG自调Q自锁模激光器的研究

实验采用光纤耦合激光二极管泵浦,得到了二极管泵浦Nd,CrYAG自调Q自锁模1.064 μm激光输出,其锁模调制深度达到了近100%,其输出功率最高可达1.9 W,平均输出功率稳定性优于1%.实验表明平均输出功率、重复频率和脉冲宽度随泵浦功率增加而上升.