Saturable absorber design for efficient Q-switch laser operation

Q-switching properties of a laser with a saturable absorber (SA) are investigated. We propose an original method to optimize the transverse concentration profile of the SA inserted in the cavity. The resulting design considerations give the way to an optimal use of the SA over its whole transverse section. The optimization problem is formulated as a variational problem and then solved analytically using the calculus of variations. Replacing an uniform profile by an optimized profile, it is possible with the same number of molecules to increase the absorption at low intensity from a factor 1.9, while transmission at high intensity remains near to 100%. We further demonstrate that the optimization does not affect the transverse pattern properties of the laser emission (M² near 1).     

Steady-state mode-locking operation in a laser with a saturable absorber

It is shown that a simple correspondence exists between the form of radiation for a steady-state mode-locking operation and the well-known solution of the one-mode problem. The dependence of the amplitude and shape of the pulses on the amount of absorber is determined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 15, No. 2, pp. 221–226, February, 1972.     

Time-domain master equation for pulse evolution and laser mode-locking

Using the well-known analogy between the space and time domains we derive a temporal master equation (ME) operator which can be applied to any cavity containing dispersive and filtering elements, phase or amplitude modulators, and one nonlinear element. The cavity properties are described in terms of 2 × 2 `KIJL' matrices. We show that this ME correctly reproduces the cavity mode structure in the linear limit. Numerical simulation of an actively mode-locked Fabry–Perot laser with the nonlinear medium at an end mirror gives results in excellent agreement with those found using the more conventional Huygens' integral method. Using a simple perturbation approach based on the nonlinear Schrödinger equation (NLS) we also show that the field in this laser is soliton-like, and give analytic expressions for the soliton parameters.     

利用被动锁模技术实现超短脉冲激光输出实验

基于被动锁模技术,设计了结构紧凑的Z型谐振腔,采用半导体可饱和吸收镜作为被动锁模元件,利用激光二极管泵浦增益线宽较宽的Nd∶YVO4晶体,最终实现了连续锁模的脉冲输出.在输入功率为8.20W时,获得了输出功率为1.72W的1 064nm连续锁模脉冲激光输出,光-光转换效率达21%,锁模脉冲重复频率为110MHz.     

Saturable absorption dynamics of mode-locking dye DODCI

0 –S₁–S_m–S_n multilevel system. An S_m→S₁ absorption recovery time of τ^S1 _A=(600±100) fs is determined. The picosecond and femtosecond pulse excitation leads to S₀→S₁ ground-state absorption bleaching and S₁→S_m first excited-state absorption bleaching. The excited-state absorption cross-sections σ^S1 _ex(S₁→S_m) and σ^Sm _ex(S_m→S_n) are determined. Received: 3 June 1996     

Fundamental and harmonic mode-locking in erbium-doped fiber laser based on graphene saturable absorber

Graphene-based passively mode-locked erbium doped fiber laser is presented. Multilayer graphene was obtained by mechanical exfoliation of pure graphitic block and deposited on the fiber ferrule. The mode-locking performance was investigated under various laser pumping conditions. The laser could operate at fundamental repetition rate of 16.34 MHz with 844 fs pulse duration and 30 mW average output power. Also harmonic-mode locking of the laser is demonstrated. 294 and 800 MHz repetition rates were obtained (corresponding to the 18th and 49th harmonic of the fundamental repetition frequency, respectively) with nearly transform-limited pulses.     

On the possibility of mode-locking a dye laser by a fast saturable absorber

The problem of mode-locking a fully relaxing saturable (dye) laser medium by an instantaneously saturating (fast) absorber is theoretically considered when group velocity dispersion and Kerr-type nonlinearity are present. Off-resonant chirped solutions are found whose minimum duration is determined by the relative losses (saturable and nonsaturable) and by the saturation depth of the absorber. The addition of nonlinear elements can only broaden the stability range of the solution with respect to the group velocity dispersion.     

High-power passively Q-switched mode-locking Nd:GdVO4 laser with LT-InGaAs saturable absorber

The generation of passively Q-switched mode-locking (QML) pulse has been obtained from a diode-pumped Nd:GdVO₄ laser with a LT-InGaAs wafer as saturable absorber as well as output coupler. Under the incident pump power of 10 W, an average output power of QML was 1.8 W with a Q-switched repetition rate of 280 kHz. The pulse duration of Q-switched pulse is about 160 ns and mode-locked pulse within the Q-switched envelope had a repetition rate of 410 MHz. It is indicated that the present LT-InGaAs is a very promising device in the field of mode locking solid-state laser, and we are sure that it will be complete pure cw mode locking with single beam output easily after further optimizing in the parameter such as saturation fluence, modulation depth, recovery time and damage threshold in semiconductors.     

Erratum: “On two models of light pulse delay in Saturable absorber”

Optics and Spectroscopy -     

Mode-locked He-Ne laser pulse compression and expansion

By means of an electro-optic internal modulator it is possible to obtain from a laser He-Ne recurring pulses whose duration (about 500 psec) is the inverse of the Ne amplifying spectral bandwidth. The optical frequency of these pulses, passing through an external modulator is translated or linearly modulated. The duration of the linear frequency modulated pulses, whose spectrum is widely broadened, is then reduced by travels into a dispersive interferometric structure (optical dispersive delay line). A pulse compression ratio at least equal to 3 has been obtained.     

Simplified technique for subnanosecond pulse generation and injection mode-locking of a XeCl laser

A line narrowed XeCl oscillator/amplifier has been used to generate pulses of 100 ps duration by a combination of stimulated Brillouin scattering and optical breakdown. In addition, a Brillouin mirror has been used to control the amplified spontaneous emission in an injection mode-locked XeCl oscillator.     

NAB对撞增强相位共轭腔快锁模特性

本文分析了NAB[NdAl_3(BO_3)_4]自激活激光晶体脉冲被动锁模的优越性。研究了对撞增强自起动自泵浦近简并四波混频相位共轭腔的形成条件及其在锁模过程中的作用。首次演示了NAB对撞增增强快速压缩脉冲锁模过程。获得了平均7ps的超短脉冲的实验结果。     

InGaAs/GaAs saturable absorber for diode-pumped passively Q-switched mode-locking of Tm:YAP laser

A passively Q-switched mode-locking of diode-pumped TmYAP laser by use of InGaAs/GaAs as a saturable absorber is reported. The maximum Q-switched mode-locking output power was 480 mW near the wavelength range of 1.94 μm with pulse repetition frequency of 26.47 kHz. The beam quality was also measured to be 1.12 ± 0.02.     

Two-stage optical pulse compression for a mode-locking SOAFL driven by an asymmetric-duty-cycle square wave

We demonstrate ultra-short optical pulses by using a two-stage compression of pulses from a mode-locking SOAFL driven by the injection of a periodic square wave with an asymmetric duty cycle. Through cascading a linear compressor of 110-m dispersion compensation fiber and a soliton compressor of 12-m single-mode fiber, the optical pulses can be shortened to 55 fs with a timing jitter of 0.45 ps. In the ninth-order soliton compression, the spectral linewidth of the mode-locking SOAFL pulses can be broadened by the self-phase-modulation effect. Also, the compression performance includes a time-bandwidth product up to 0.437 and a pulse-compressing ratio of 54.     

Computational model for simultaneous Q switching and mode-locking in a solid state laser with a saturable absorber

A numerical model of rate equations for a four-level solid state laser with Cr⁴⁺:YAG saturable absorber including excited state absorption ESA is presented. The cavity is divided into a large number of disks and the model is solved for each disk and its local corresponding photon flux. The flux array is shifted for each recurrence simulating the movement of photons inside the cavity during the round trip. This simulator can describe the mode-locking phenomenon and can be used to simulate simultaneous mode locking and Q switching with a saturable absorber.     

Single picosecond UV pulse generation by mode-locking of an excimer laser-pumped dye laser

A single picosecond ultraviolet pulse has been generated based on mode-locking of a dye laser pumped by a long pulse XeCl laser to serve as the input source for a high-power ps KrF laser system. A short-pulse uv dye laser (BBQ) pumped by an additional XeCl laser was used to selectively amplify a single pulse from a mode-locked pulse train with the pulse separation of 3.2 ns. The amplified single pulse was frequency-doubled to 248 nm with the pulse duration of 20 ps.     

Passive harmonic mode-locking of Er-doped fiber laser using CVD-grown few-layer MoS_2 as a saturable absorber

Passive harmonic mode locking of an erbium-doped fiber laser based on few-layer molybdenum disulfide(MoS2)saturable absorber(SA) is demonstrated. The few-layer MoS2 is prepared by the chemical vapor deposition(CVD) method and then transferred onto the end face of a fiber connector to form a fiber-compatible Mo S2SA. The 20 th harmonic modelocked pulses at 216-MHz repetition rate are stably generated with a pulse duration of 1.42 ps and side-mode suppression ratio(SMSR) of 36.1dB. The results confirm that few-layer MoS2 can serve as an effective SA for mode-locked fiber lasers.     

Passive mode-locking of an Ar+-laser with rhodamine 6g as saturable absorber and double mode-locking of the pump- and dye laser system

The synchronization of the modes of the Ar⁺-pump laser by using a dye as saturable absorber is proposed. Besides, the passive mode-locking is extended to double mode-locking. In this case the dye simultaneously acts as a synchroneously pumped laser medium.     

高稳定性、高质量脉冲压缩飞秒光纤激光系统研究

报道了一款用于眼科手术的高稳定性、高质量脉冲压缩飞秒光纤主振荡功率放大系统. 振荡器采用混合腔方式增加了光纤双折射, 改善了非线性偏振旋转锁模在低重复频率下无法自启动的缺点, 提高了飞秒光纤激光器自启动锁模的稳定性. 研究了脉冲预啁啾与飞秒脉冲压缩质量的关系, 通过实验证明了光纤放大器的入射脉冲保持负的预啁啾状态可以得到最好的脉冲压缩结果. 实验最终实现了满足自启动锁模条件的重复频率19.4 MHz、平均功率1.2 W、脉冲宽度183 fs的激光脉冲, 目前这台激光器是国内首台具有自主知识产权并应用于动物角膜实验的飞秒光纤激光器.