Relaxation Oscillations and Pulse Stability in Harmonically Mode-Locked Semiconductor Lasers

In this paper, we discuss pulse dynamics in harmonically mode-locked semiconductor lasers and present the conditions necessary for stability. In a laser mode-locked at the Nth harmonic, the pulse energy fluctuations have (N+1) different modes of relaxation oscillations. Different modes correspond to different patterns for the energy fluctuations in the N different pulses inside the laser cavity. In the higher order relaxation oscillation modes, the energy fluctuations are negatively correlated in different pulses inside the laser cavity, and these modes can cause instability. Gain saturation on time scales of the order of the pulse width (dynamic gain saturation) stabilizes pulse energy fluctuations with respect to relaxation oscillations. The precise limits on the stable operating regime depend on the gain dynamics at both slow and fast time scales. We also discuss harmonic mode-locking in the presence of a slow saturable absorber. Dynamic loss saturation in a saturable absorber can work against dynamic gain saturation and limit the stability range for harmonic mode-locking.     

用可变波长抗共振环实现Nd：YAP介质碰撞脉冲锁模

利用一种能适用于不同波长的各种激光晶体碰撞脉冲锁模特性研究的实用抗共振环，成功地实现了Ｎｄ：ＹＡＰ介质１．０７９μｍ波长的碰撞脉冲锁模。     

Mode-locked multisegment resonant-optical-waveguide diode laserarrays

The first mode-locked operation of resonant optical waveguide (ROW) semiconductor laser arrays is reported. The well-behaved emission patterns of such arrays allow coupling to external cavities with efficiencies comparable to those achieved by using single-element lasers. Single- and multisegment lasers are employed to achieve active, passive, and hybrid mode-locking. The use of an arrayed gain region is effective in increasing the saturation energies of gain and absorber segments, resulting in high pulse energies. Pulses are generated that have well-suppressed secondary pulsations, with pulsewidths as short as 5.6 ps and peak powers of over 3 W in a collimated beam with a single main lobe     

Simulation study on semiconductor laser mode locking usingnonlinear coupling in multimode-interference waveguide amplifiers

The feasibility of a passively mode-locked semiconductor laser with a multimode-interference (MMI) waveguide amplifier is numerically studied using a two-dimensional time-domain beam-propagation method. In an appropriately designed ring cavity, a pulse can be compressed from a few hundred picoseconds to several picoseconds, as a result of the interplay between linear coupling and gain saturation in the MMI waveguide amplifier. The asymptotically stable pulse peak position and pulsewidth imply the feasibility of passively mode-locking semiconductor lasers. The frequency chirping of the compressed pulse and the lateral distribution of the output beam are numerically analyzed in detail. Our simulations show that mode locking can be implemented within a fairly broad range of injection current and cavity alignment     

Calculation of the colliding pulse mode locking in CW dye ring lasers

A calculation of the influence of a transient absorber grating on the pulse parameters of a dye laser in the "colliding pulse mode-locking" (CPM) regime is given. It is shown that, compared to other passive methods, considerably shorter pulses with a more favorable stability region can be obtained. Within the stability range of single pulse operation the shortest pulses in the CPM are obtained by operating far above threshold where the saturation energy of the amplifier is much greater than that of the absorber. Deviation from an amplifier position that is symmetric with respect to the amplification of the counter-running pulses leads to different energies of the pulses and to a strong decrease of the stability range. Taking the dispersive properties of absorber and amplifier into consideration, equations for the time dependent phase of the laser pulse are established and solved by an analytical ansatz. The resulting pulse chirp is shown to be mainly dependent on the detuning of the pulse frequency from the center frequency of the saturable absorber. The estimated values for the pulse duration and the down-chirp for typical experimental conditions of a rhodamine 6G-DODCI CPM laser agree well with the measured ones.     

Tunable monolithic colliding pulse mode-locked quantum-well lasers

The tunabilities of both the wavelength and the pulse-width of monolithic mode-locked semiconductor lasers are demonstrated. Pulses shorter than 1.6 ps, tunable over 8.8 mu m, have been generated by a temperature-tuned monolithic colliding pulse mode-locked (CPM) quantum-well laser. For a fixed wavelength, the pulse-width is independently controlled from 1.2 ps to longer than 3 ps by external bandpass filters. Near transform-limited time-bandwidth products of 0.34 were maintained throughout the tuning processes.<>     

Theory for passive mode-locking in semiconductor laser structuresincluding the effects of self-phase modulation, dispersion, and pulsecollisions

We present a theory for passive mode-locking in semiconductor laser structures using a semiconductor laser amplifier and absorber. The mode-locking system is described in terms of the different elements in the semiconductor laser structure. We derive mode-locking conditions and show how other mode-locking parameters, like pulse width and pulse energy, are determined by the mode-locking system. System parameters, like bandwidth, dispersion, and self-phase modulation are shown to play an important role in mode-locking conditions and results. We also discuss the effects of pulse collisions and positions of the mode-locking elements inside the cavity on mode-locking stability and show that these effects can be easily included in the presented model. Finally, we give a number of design rules and recommendations for fabricating passively mode-locked lasers     

Theory of intracavity frequency doubling in passively mode-lockedfemtosecond lasers

The authors construct a quantitative theoretical model of an intracavity frequency-doubled and passively mode-locked laser, supported by experimental observations with a colliding pulse mode-locked femtosecond dye laser. The major findings are that for second-harmonic conversion efficiencies consistent with continuing laser operation (<5%): (1) a stable mode-locking regime always exists, although it narrows somewhat with increasing conversion efficiency; and (2) the duration of the fundamental pulses can always be preserved, even in the femtosecond time domain, by readjusting saturable gain and saturable loss under the same conditions. Both the model and observations contrast with previous studies of actively mode-locked and synchronously mode-locked lasers containing intracavity frequency-doubling crystals     

Monolithic colliding-pulse mode-locked quantum-well lasers

Integration of the whole mode-locked laser onto a single piece of semiconductor offers a number of advantages, including total elimination of optical alignment processes, improved mechanical stability, and the generation of short optical pulses at much higher repetition frequencies. Semiconductor laser processing technologies were used to implement the colliding-pulse mode-locking (CPM) scheme, which is known to effectively shorten the pulses and increase stability, on a miniature monolithic semiconductor cavity. The principles of and recent progress in monolithic CPM quantum-well lasers are reviewed     

A physical model for mode-locking process of colliding pulsemode-locked multiquantum-well semiconductor laser

In this paper, we first present a physical model for mode-locking process of the colliding pulse mode-locked multiquantum-well laser diode (CPM-MQW-LD). The absorption of saturable absorber on pulse leading edge and the scattering of the transient grating generated in absorption and gain regions on pulse trailing edge are considered. As an example, the mode-locking process of a multiquantum-well structure laser and the effect of transient grating on pulse width and peak power are studied     

高功率掺镱全固态飞秒激光器

开展了激光二极管泵浦的高功率掺镱全固态飞秒激光器的研究。利用Yb:LYSO晶体实现了高功率高效率的半导体可饱和吸收镜锁模飞秒振荡器,分别在1 035、1 042 nm实现了3 W的稳定锁模运转,相应的脉冲宽度分别为351、287 fs,斜效率分别为88.2%和89.7%;通过将增益介质与克尔介质分开,利用大功率多模LD直接泵浦Yb:CYA晶体实现了高功率的克尔透镜锁模飞秒振荡器,脉冲宽度70 fs,平均输出功率2.52 W,重复频率50 MHz,获得了50 nJ的单脉冲能量且峰值功率达到0.71 MW。表明上述掺镱晶体在高功率二极管泵浦全固态激光器领域中具有非常优异的性能。     

10×30 GHz pulse train generation from semiconductor amplifierfiber ring laser

A multiwavelength, fiber ring laser source, is demonstrated. It generates 10 wavelength channels, simultaneously mode-locked and synchronized at 30 GHz, each producing 7-ps pulses. The mode-locking technique relies on the gain saturation of the semiconductor amplifier from an external optical pulse train to impose the simultaneous mode-locking of the 10 wavelengths     

Electron–Hole Plasma-Induced Spectral Blueshift of Optical-Data Injection Mode-Locked Semiconductor Optical Amplifier Fiber Laser

Under an optical nonreturn-to-zero (NRZ) data injection at 10 Gbit/s, the 10-GHz mode-locking and pulsed return-to-zero (RZ) clock extraction from a semiconductor optical amplifier (SOA) based fiber ring is investigated in this paper. The diagnoses on gain and intracavity-power-controlled anomalous blueshifted spectrum and subpicosecond timing jitter are demonstrated. By increasing the injecting power of the optical NRZ data from ${-}3$ to 8 dBm into the SOA bias at different currents, the mode locking is completed with a dc level greatly decreasing from 480 to 50 $mu$ W (only 1.5% of the mode-locked pulse power at 3 mW), corresponding to a pulse/dc amplitude contrast ratio up to 18 dB. Increasing the SOA bias current up to 350 mA significantly suppresses the timing jitter from 1.8 ps to 345 fs, and the extracted RZ clock pulse is shortened from 55 to 27 ps. The pulsewidth of the amplified SOAFL is compressed from 11 ps to 836 fs after dispersion compensation. At constant data injection level, the increasing SOA bias or gain oppositely redshifts the mode-locked SOA fiber ring laser (SOAFL) spectrum by 5 nm. The amplifier spontaneous emission of SOA at short wavelength region (${sim} {hbox {1520}}$ nm) is eliminated with increasing NRZ data power, whereas the mode-locking gain peak arises and blueshifts from 1558 to 1552 nm due to the band-filling effect. Such a blueshift in mode-locking spectrum becomes more significant in SOA at lower bias (or gain) condition. A theoretical model interprets the correlation between the nonlinear gain suppression-induced variation of electron–hole plasma in SOA and the blueshifted mode-locking SOAFL spectrum, which is occurred when the gain saturation condition for the SOA becomes more pronounced.      

五甲川红外锁模染料的研究

合成了 6个最大吸收波长在 1.0 6 μm附近的五甲川红外染料 (两个为不对称结构 )。结构经红外 ,H1核磁 ,元素分析表征 ,并测量了其 1,2 -二氯乙烷溶液的电子吸收光谱 ,范围在 10 52～ 10 95nm之间。它的 1,2 -二氯乙烷溶液对 1.0 6 μm波长的 Nd3 :YAG激光进行锁模 ,全部锁模成功 ,波形稳定。脉宽为 2 6 .8～ 35ps。     

Phase correlation between longitudinal modes in semiconductor self-pulsating DBR lasers

Phase correlation leading to self-pulsation (SP) in semiconductor distributed Bragg reflector (DBR) lasers is investigated experimentally and theoretically. Under proper biasing conditions, the laser oscillates with three main modes and we observe that each two-modes beating provides SP with identical spectral linewidth. Under the same operating conditions, the measured spectral linewidths of the beating modes are much larger than the linewidth of the self-pulsating signal. These results demonstrate the natural occurrence of passive mode-locking (PML) and phase correlation in semiconductor DBR lasers. A model based on multimode coupled-wave rate equations, including four-wave mixing (FWM), is developed to describe PML and SP in the gain region of the laser cavity. This model demonstrates that the existence of phase correlation between longitudinal modes is due to FWM.     

Chirp and stability of mode-locked semiconductor lasers

A numerical study of mode-locked semiconductor lasers is presented with special attention to the chirp characteristics and to dispersion-related criteria for stable pulse-train emission. The dependence of the pulse chirp upon the refractive-index change, both with carrier density and carrier temperature changes, is discussed. The experimental observation of blue-chirped pulses for passive mode-locking in contrast to red-chirped pulses for active mode-locking is found to be due to the different contributions of gain and absorber media to the refractive-index change. In addition, it is revealed that the boundary of the stable operation regime is critically influenced by the spectral characteristics of laser and external cavity. Design considerations toward the achievement of high pulse energy, narrow spectral bandwidth, and linear chirp are given     

A scheme of picosecond pulse shaping using gain saturationcharacteristics of semiconductor laser amplifiers

To obtain high power, well shaped picosecond pulses from gain-switched semiconductor lasers, the use of dynamic gain saturation characteristics of semiconductor laser amplifiers was investigated theoretically and experimentally. A configuration of a reflected-wave amplifier (RWA) with single-side external coupling is introduced for pulse shaping, which is found to be suitable for enhancing dynamic gain saturation. By a combination of a distributed feedback laser oscillator at 1.3 μm in wavelength and a reflected-wave amplifier of 400 μm cavity length with asymmetric facet reflectivities of 0.01% and 30%, single-mode optical pulses with almost no tailing, full width at half maximum of 15 ps, and peak power exceeding 50 mW were obtained without pulse broadening, despite the considerable tail structure of the incident pulse     

半导体可饱和吸收镜实现高频脉冲激光研究进展

介绍了作为固体激光器、半导体激光器和光纤激光器被动锁模吸收体的半导体可饱和吸收镜(SESAM)的基本原理和制作方法。详细阐述了利用半导体可饱和吸收镜对同体激光器和光泵垂直外腔面发射半导体激光器进行被动锁模，获得重复率为几吉赫到几百吉赫的超短脉冲激光的方法。     

Tm3+/ Ho3+离子掺杂中红外超快激光技术研究进展（特邀）

稀土离子Tm3+/ Ho3+ 掺杂中红外2 μm波段超快激光由于广泛的应用前景成为近十余年来激光领域的研究热点之一。文中首先综述了稀土离子Tm3+/Ho3+掺杂固体/光纤2 μm波段超快激光锁模技术进展,包括主动锁模技术以及饱和吸收、克尔透镜、非线性偏振旋转、非线性光环形镜、非线性多模干涉等被动锁模技术;其次,结合激光增益介质及色散管理技术回顾了Tm3+/ Ho3+掺杂固体和光纤锁模激光脉冲宽度压缩进展;再次,总结了Tm3+/ Ho3+大能量/高功率超快激光技术及进展;最后,对2 μm波段超快激光发展趋势进行了总结和展望。