Characterization of the train-average time-frequency parameters inherent in the low-power picosecond optical pulses generated by the actively mode-locked semiconductor laser with an external single-mode fiber cavity

The specific approach to characterizing the train-average parameters of low-power picosecond optical pulses with the frequency chirp, arranged in high-repetition-frequency trains, in both time and frequency domains is elaborated for the important case when the semiconductor laser is matched by an external single-mode fiber cavity and operates in the active mode-locking regime. This approach involves the joint Wigner time-frequency distributions, which can be created for those pulses due to exploitation of a novel interferometric technique. Practically, the InGaAsP/InP-heterolaser generating at the wavelength 1320 nm was used during the experiments carried out and an opportunity of reconstructing the corresponding joint Wigner time-frequency distributions was successfully demonstrated.     

Measurement-induced decoherence and Gaussian smoothing of the Wigner distribution function

We study the problem of measurement-induced decoherence using the phase-space approach employing the Gaussian-smoothed Wigner distribution function. Our investigation is based on the notion that measurement-induced decoherence is represented by the transition from the Wigner distribution to the Gaussian-smoothed Wigner distribution with the widths of the smoothing function identified as measurement errors. We also compare the smoothed Wigner distribution with the corresponding distribution resulting from the classical analysis. The distributions we computed are the phase-space distributions for simple one-dimensional dynamical systems such as a particle in a square-well potential and a particle moving under the influence of a step potential, and the time-frequency distributions for high-harmonic radiation emitted from an atom irradiated by short, intense laser pulses.     

Wavelength-tunable high-power picosecond pulses from a fiber-pumped diode-seeded high-gain parametric amplifier

We describe a high-gain optical parametric amplifier (OPA) and optical parametric generator (OPG) system pumped by a fiber chirped-pulse amplification source based on novel large-mode-area fiber components. 45-nJ OPG thresholds and 34% pump-signal conversion efficiencies are obtained for ~4-ps pulses. OPA gains in excess of 75 dB and pump-signal conversion efficiencies of ~39% (external) are demonstrated. Furthermore, we show that injection seeding of such high-gain parametric devices by use of a low-power continuous-wave beam results in high-power (>18-kW) picosecond pulses with a time-bandwidth produce of ~0.65, some ten times less than from a free-running OPG. Using such an approach, we obtain 2.7-ps 50-nJ pulses at 1.310 mum .     

Generation of 10-GHz duty-cycle tunable square optical pulse in an SOA-based mode-locked fiber laser

We demonstrate the generation of 10-GHz optical square pulses by injecting a picosecond pulse train into an SOA-based mode-locked fiber laser. The novel scheme exploits nonlinear effects and gain saturation phenomenon in the semiconductor optical amplifier (SOA). This technique uses gain-compression dynamics between the input pulses and the generated ones in gain-saturated SOA to form square pulses. The center wavelength of the generated optical square pulse can be tuned from 1530 to 1570 nm by adjusting the center wavelength of the optical band pass filter (OBPF) in the SOA-based mode-locked fiber ring laser. The duty cycle of the output pulse can be tuned from 12.7 to 88.4%, which strongly depends on the input power and intra-cavity power.     

一种基于非线性光纤环镜开关特性的超短光孤子产生方法

提出了一种利用非线性光纤环镜的开关特性将连续波同时转化为亮孤子和暗孤子的新方法 ,即让连续波和另一波长的调制脉冲串共同耦合入光纤环镜 ,交叉相位调制使得一部分连续波被环镜透射 ,其余部分被反射 ;再让透射波和反射波分别在反常色散光纤和正常色散光纤中传输 ,自相位调制和群速度色散之间的相互作用使得透射波和反射波分别演化为亮、暗孤子。数值计算表明 ,该方法不仅可产生脉宽比调制脉冲窄、重复频率比调制脉冲高的亮孤子和暗孤子 ,而且几乎可将全部的连续波能量转化为孤子能量。     

Ultrafast all-optical first- and higher-order differentiators based on interferometers

We demonstrate that a conventional two-arm interferometer can implement first-order temporal differentiation of ultrafast arbitrary optical waveforms. Straightforward extension of this technique to nth-order optical differentiation is also suggested. This approach is experimentally demonstrated by an efficient and accurate first- and second-order temporal differentiation of (sub-)picosecond Gaussian optical pulses.     

Femtosecond hybrid mode-locked semiconductor laser and amplifier dynamics

We describe the generation of femtosecond high power optical pulses using hybrid passive-active mode-locking techniques. Angle stripe geometry GaAs/AlGaAs semiconductor laser amplifiers are employed in an external cavity including prisms and a stagger-tuned quantum-well saturable absorber. An identical amplifier also serves as an optical power amplifier in a stretched pulse amplification and recompression sequence. After amplification and pulse compression this laser system produces 200 fs, 160 W peak power pulses. We discuss and extend our theory, and supporting phenomenological models, of picosecond and subpicosecond optical pulse amplification in semiconductor laser amplifiers which has been successful in calculating measured spectra and time-resolved dynamics in our amplifiers. We have refined the theory to include a phenomenological model of spectral hole-burning for finite intraband thermalization time. Our calculations are consistent with an intra-band time of approximately 60 fs. This theory of large signal subpicosecond pulse amplification will be an essential tool for understanding the mode-locking dynamics of semiconductor lasers and for analysis of high speed multiple wave-length optical signal processing and transmission devices and systems based on semiconductor laser amplifiers.     

Blue 489-nm picosecond pulses generated by intracavity frequency doubling in a passively mode-locked optically pumped semiconductor disk laser

We report the generation of blue 489-nm picosecond laser pulses by intracavity second-harmonic generation in a mode-locked optically pumped InGaAs vertical-external-cavity surface-emitting laser. Mode locking achieved by a semiconductor saturable absorber mirror generated 5.8-ps-long sech²-shaped pulses at an emission wavelength of 978 nm and a repetition rate of 1.88 GHz. Intracavity frequency doubling in a 5-mm-long lithium triborate crystal generated blue picosecond pulses with a spectral width of 0.15 nm and an average output power of up to 6 mW.     

利用较完善模型研究半导体光放大器对皮秒光脉冲的放大

提出了一个描述半导体光放大器（SLA）对皮秒脉冲的放大这一物理过程的较为完善的物理模型，并数值分析了光脉冲经SLA放大后的上升时间和下降时间.结果表明：随着SLA偏置电流的增大，上升时间将缩短而下降时间将延长；输入脉冲的大峰值功率将加速上升时间的缩短和下降时间的延长；增益压缩对脉宽为几个皮秒的输入脉冲的上升时间和下降时间有明显的影响，而对脉宽为几十皮秒的输入脉冲可近似认为没有影响；增益非对称和漂移强烈影响上升和下降时间. 关键词： 半导体光放大器 皮秒光脉冲 上升和下降时间     

Red picosecond pulses generated by frequency doubling a Raman amplified widely tunable 1.3 microm fiber ring laser

We generated 0.66 microm picosecond pulses by second-harmonic generation of the Raman amplified output of a 1.3 microm actively mode-locked fiber ring laser in a periodically poled potassium titanyl phosphate (PPKTP) waveguide. The ring laser produced 9 ps pulses at a 20 GHz repetition frequency, was tunable over 1284-1330 nm, and was based on a semiconductor optical amplifier and a Mach-Zehnder amplitude modulator. The Raman amplifier served both to amplify the ring laser and to compress the pulses as solitons. The spectral flexibility of the amplifiers and the modulator should enable similar configurations to be made at other wavelengths and facilitate efficient frequency doubling in waveguides to other visible wavelengths.     

Effect of pulse duration on plasmonic enhanced ultrafast laser-induced bubble generation in water

Bubbles generated in water by focusing femtosecond and picosecond laser pulses in the presence of 100 nm gold nanoparticles have been investigated in the fluence range usually used for efficient cell transfection (100–200 mJ/cm²). Since resulting bubbles are at the nanoscale, direct observation using optical microscopy is not possible. An optical in-situ method has been developed to monitor the time-resolved variation in the extinction cross-section of an irradiated nanoparticle solution sample. This method is used to measure the bubbles lifetime and deduce their average diameter. We show that bubbles generated with femtosecond pulses (40–500 fs) last two times longer and are larger in average than those generated with picosecond pulses (0.5–5 ps). Controlling those bubble properties is necessary for optimizing off-resonance plasmonic enhanced ultrafast laser cell transfection.     

Robust quantum dot exciton generation via adiabatic passage with frequency-swept optical pulses

The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We report on the robust population inversion in a single quantum dot using an optical technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser pulses whose frequency is swept through the resonance. This observation in photoluminescence experiments is made possible by introducing a novel optical detection scheme for the resonant electron hole pair (exciton) generation.     

Engineerable femtosecond pulse shaping by second-harmonic generation with Fourier synthetic quasi-phase-matching gratings

We describe a pulse-shaping technique that uses second-harmonic generation with Fourier synthetic quasi-phase-matching gratings. We demonstrate both amplitude and phase tailoring by generating a picosecond squarelike pulse as well as trains of femtosecond pulses with a terahertz-range repetition rate from either a compressed or a chirped pump pulse.     

超短光脉冲波形研究

本文讨论了光脉冲技术发展中的脉冲波形测量问题。介绍了一种通用的光脉冲波形测量新方法,该方法可以给出光脉冲的波形、脉宽以及光脉冲内含的频率啁啾信息。这种测量方法已应用于半导体激光器产生ps光脉冲的实验研究,其可行性得到了验证。     

Coherent interaction of picosecond pulses in a fluorescent medium and the timing of pulse trains for two-wavelength excite and probe measurements

Coherent interaction between two cw, picosecond pulse trains of the same wavelength absorbed by a fluorescent material and the effect on the emission has been studied. As the path difference between the two beams is varied, the infra-red emission from colour centres in KCl: Na or KCl: Li fluctuates in phase with the transmitted intensity of the weaker of the two beams. The effect may be applied to the accurate relative timing of cw pulse trains of different wavelengths for use in two-wavelength, excite and probe measurements. A small fraction of the pump pulse train is superimposed on the probe beam and passes along precisely the same optical path. Coherent interaction between the two pulse trains at the pump frequency causes modulation of the transmitted probe beam intensity when pump and probe pulses are accurately synchronised.     

Picosecond passively mode-locked GaSb-based semiconductor disk laser operating at 2 μm

We report on a passively mode-locked optically pumped GaSb-based semiconductor disk laser producing stable picosecond optical pulses at a 1.95 μm wavelength. The gain mirror was comprised of a 15 quantum well InGaSb/GaSb structure. A fast semiconductor saturable absorber mirror with three InGaSb/GaSb quantum wells was used to attain self-starting mode-locked operation at a fundamental repetition rate of 881.2 MHz. The laser produced pulses with 30 pJ energy and a duration of 1.1 ps within a factor of 2 of the Fourier limit.     

Passive synchronization of two independent laser oscillators with a Fabry-Perot modulator

By optical modulation of the reflectivity of an intracavity nonlinear Fabry-Perot semiconductor mirror, the pulse train from a passively mode-locked picosecond Nd:YVO(4) laser oscillator is synchronized to an independent femtosecond-mode-locked Ti:sapphire laser. We obtain stable synchronized pulse trains at central wavelengths of 1064 and 850 nm, and the Ti:sapphire laser is still independently tunable over a large wavelength range. The tolerable cavity-length difference between the two laser oscillators exceeds 20mu;m .     

All-optical programmable shaping of narrow-band nanosecond pulses with picosecond accuracy by use of adapted chirps and quadratic nonlinearities

We experimentally demonstrate pure optical pulse picosecond shaping of narrow-bandwidth nanosecond pulses. The method used is based on the manipulation in the spectral domain of strongly chirped femtosecond pulses and on the use of either frequency addition or frequency difference.     

Generation of 20-GHz picosecond pulse trains in the normal and anomalous dispersion regimes of optical fibers

In this paper, we numerically and experimentally study two methods to generate 20-GHz pulse trains at 1550 nm from a dual-frequency beat-signal. The first method is based on the multiple four-wave mixing temporal compression occurring in the anomalous dispersion regime of a standard optical fiber (SMF). In the second original method, the initial sinusoidal signal is first converted into a parabolic pulses train through nonlinear propagation in a normally dispersive fiber. A subsequent linear compression in an anomalous dispersive fiber leads to well-separated picosecond pulses.