On the nature of optically controlled delay of a stokes pulse amplified by stimulated Brillouin scattering in optical fiber

In this work we show through an analytic solution of the set of two coupled dynamic equations for the slowly varying amplitudes of the Stokes optical field and material density in stimulated Brillouin scattering (SBS), its analysis and a set of experiments, that the SBS-induced delay of an output Stokes pulse which is amplified by the SBS interaction is due exclusively to the inertia of the acoustic wave excitation.     

Long Optical Delay Lines Enhanced by Ring Configuration in Optical Fibres

A long optically controlled delay line enhanced by ring configuration is demonstrated by using the group-velocity control of signal pulses based on stimulated Brillouin scattering. In experiment, two optical fibre ring cavities are used： one is used as the Brillouin laser, providing single-mode Stokes wave as probe wave; the other is used as the Brillouin amplifier, working as slow light medium. We achieve a maximum time delay of 215ns using the ring Brillouin amplifier, five times larger than the input probe pulse width of 40ns. In the meantime, a considerable pulse broadening is observed, which agrees well with the theoretical prediction based on linear theory.     

Generation of short light pulses under the stimulated Brillouin scattering in fibers with an optimal feedback

The generation of short light pulses (1 ns) in single mode fibers under pumping by wide laser pulses (of a microsecond duration) due to the backward stimulated Brillouin scattering (SBS) is numerically investigated. The influence of the acoustic diffraction is taken into account. The cases of acoustic waveguide and anti-waveguide fibers are considered. For an acoustic anti-waveguide fiber, a dependence of overlap integral S on the acoustic mode number n has a sharp peak in the region of n 100. Computer simulations have demonstrated the energy conversion of the pump wave into short pulses of the signal (Stokes) wave in the case of synchronous pumping. The optimal length of the fiber should be approximately equal to the half-length of the pump pulse. The bypass time of the Stokes pulse of the optimal circuit fiber and the feedback loop must be equal to the repetition period of the pump pulse. An importance of acoustic mode structure of the fiber for the process of forming pulse train in shown. We have found that the acoustic anti-waveguide fibers with a small core (a < 3 m) can be preferable for obtaining the stable train of compressed pulses.     

Modulation instability of the stimulated Brillouin scattering in fibers in a presence of acoustic diffraction

An influence of acoustic wave diffraction on the modulation instability of the backward stimulated Brillouin scattering in fibers is numerically investigated, when a small feedback for the Stokes wave is present. The cases of acoustic waveguide and anti-waveguide fibers are considered. It is shown that a presence of acoustic diffraction may expand the region of occurrence of modulation instability and may lead to 2–5 times increasing the peak amplitude of output Stokes pulses. In this case, a stable train of short powerful Stokes pulses of nanosecond duration is produced at the output.     

Stimulated Brillouin scattering generation in ns pulsed double-cladding fiber amplifier

This paper proposes a study both in theory and experiment on the phenomenon of stimulated Brillouin scattering (SBS) in a double-cladding fiber amplifier. The distortion characteristic of the 200-ns is observed through a stationary coupled-wave SBS model including the second-order Stokes wave for double-cladding fiber amplifier. The first-order Stokes wave is amplified during backward propagation to such an intense peak power that it can generate second-order Stokes wave. The stochastic aspect of pulse distortion induced by SBS is also experimentally demonstrated in the single frequency 200 ns pulse amplifier with 12-m Yb³⁺ doped double-cladding fiber. The backward SBS pulse is observed when the pulse peak power is up to 3.3 W, and the pulse width of SBS is narrower than that of the input signal. In the meantime, to overcome the difficulty of the pulse spectrum measurement, a novel method is put forward to measure the SBS frequency shift using the F–P interferometer with free spectral rang of 30 G, showing a good agreement with the theoretical estimation. SBS is the main factor that limits the output pulse peak power in the amplification of the single-frequency pulse.     

DCM中C_3H_6O受激拉曼散射荧光增强特性

本文报道了DCM增强C3H6O受激拉曼散射荧光增强特性研究结果。实验测量了在不同的DCM浓度、不同的泵浦激光强度以及不同的Stokes入射信号条件下Stokes波输出能量,发现当DCM浓度为2×10-4mol/L时,Stokes波最大增强因子达到5.7,Stokes强度随入射激光增强而增强,过强的Stokes输入信号则可能导致Stokes波增益饱和。     

水中受激布里渊散射微弱Stokes信号光的高增益放大

采用脉冲宽度为7.2 ns的种子光注入式倍频Nd:YAG脉冲激光器,以CS₂为放大介质,实验并理论研究了水中受激布里渊散射微弱Stokes光的信号增益随延迟时间、放大器池长、抽运光能量的变化规律. 结果表明,当抽运光脉冲相对信号光脉冲延迟进入放大器,且延迟时间为脉冲宽度的一半,抽运光能量略低于介质受激布里渊散射阈值,选择合适的放大器池长可获得最佳的信号增益. 适当选择抽运光能量,亦可实现微弱信号光的线性放大. 实验中采用独立双池放大系统,当水中Stokes信号光的能量为1 pJ时,信 关键词： 布里渊放大器 信号增益 延迟时间 抽运光能量     

Optical fibre-grating pulse compression

Numerical simulation is used to consider non-linear pulse propagation in fibres and subsequent pulse compression in a dispersive delay line. It is shown that for small initial pulse powers the conventional non-linear Schrödinger equation (NSE) is quite accurate to describe the process of pulse propagation in fibres. In this case initially symmetrical pulses undergo squaring and spectral broadening in fibres, and frequency chirp is linearized over most of the pulse, while shapes of the pulse, spectrum and frequency chirp remain symmetrical at the output of the fibre. There is a certain optimum fibre lengthZ _opt which is determined by the initial pulse parameters and fibre characteristics for pulse compression in the dispersive delay line. When the fibre lengthZ>Z _opt the optical wave breaking effect distorts the linearity of the frequency chirp and thus deteriorates the quality of the compressed pulse. The region of NSE approximation accuracy is determined. It is demonstrated that at increase of the initial pulse power (initial pulse width makes no difference) the NSE approximation becomes inaccurate. So the pulse dynamics in the fibre were described by the modified NSE derived in the higher-order approximation of the method of slowly varying amplitudes from Maxwell's equations. In this case the shock wave appears at the trailing edge of the pulse, which accelerates the wave breaking process. This results in a decrease of the optimum fibre length and deterioration of compressed pulse parameters, compared with the NSE case. Spectral windowing of the extreme Stokes components of the pulse spectrum permits significant improvement in the quality of the compressed pulse. The main features of the compression of pulses with asymmetrical initial shape are also considered.     

Time-resolved coherent anti-Stokes Raman scattering with a femtosecond soliton output of a photonic-crystal fiber

We demonstrate time-resolved coherent anti-Stokes Raman scattering (CARS) by using a frequency-tunable femtosecond soliton output of a silica photonic-crystal fiber (PCF) as a Stokes field. This approach allows quantum beats originating from two close Raman modes to be resolved in the time-domain CARS response. The nonresonant CARS background is efficiently suppressed by introducing a delay time between the probe pulse and the pump-Stokes pulse dyad, suggesting a convenient fiber-optic format for the Stokes source in time-resolved CARS and allowing sensitivity improvement in PCF-based CARS spectroscopes and microscopes.     

DCM荧光增强C3H6O 受激拉曼散射（SRS）的增益特性研究

报道荧光染料DCM对丙酮SRS的高效放大, 研究了DCM中Stokes波的增益特性. 改变DCM乙醇溶 液浓度，变化进入DCM的Stokes光强度以及抽运激光功率，测量并分析了Stokes波增益与上 述条件的关系. 研究结果发现，当DCM浓度为 2×10^-4mol/L时，Stokes波 获得最大功 率增益达到57；抽运激光功率增加或进入DCM的Stokes信号增强时Stokes波输出增强，但 过强的Stokes 信号将导致增益饱和. 关键词： 受激拉曼散射 荧光增强Stokes波 丙酮 DCM     

Simultaneous intracavity optical parametric oscillation and stimulated Raman scattering pumped by a doubly passively Q-switched Nd:GGG laser

By using a doubly passively Q-switched Nd:Gd₃Ga₅O₁₂(Nd:GGG) laser with Cr⁴⁺:YAG and GaAs as saturable absorbers as pump laser, simultaneous intracavity optical parametric oscillation and stimulated Raman scattering based on a single X-cut KTiOPO₄ (KTP) crystal have been realized. Under an incident diode pump power of 10.5 W, the output powers at the signal wave near 1,569 nm and the first Stokes emission near 1,094 nm were 218 and 72 mW, corresponding to the optical-to-optical conversion efficiencies of 2.08 and 0.69 %, respectively. The measured shortest pulse duration at the signal wave near 1,569 nm was 580 ps, generating a pulse peak power of 43.7 kW, while the minimum pulse duration at the first Stokes emission near 1,094 nm was 1.61 ns. By adjusting the tilt angle of the KTP crystal, up to the third Stokes scattering was also obtained.     

基于时分复用技术的甚多束光脉冲产生系统

提出了一种基于时分复用技术的全光纤、全固化的用于惯性约束聚变驱动器的甚多路光脉冲产生系统.系统中采用单纵模振荡器输出连续激光信号,利用时分复用技术结合高速电光调制技术实现序列脉冲的产生和甚多束脉冲的独立整形.采用偏振无关的声光调制技术实现甚多束脉冲的独立输出.每个序列脉冲包含8个子脉冲,子脉冲间隔设置为120ns,对子脉冲独立整形和选单后将其传输放大至微焦耳量级输出.实验成功验证了采用时分复用技术完全可以实现序列脉冲输出,各子脉冲可以独立任意整形且最后的单束输出能量为1.275μJ.     

Ultra-short optical pulse shaping using semiconductor optical amplifier

Optical ultra-short pulse compression and amplification using semiconductor optical amplifier (SOA) is presented. Using pump-probe pulse configuration, we present an SOA model which includes the nonlinear effects such as, spectral hole burning (SHB), carrier heating (CH), two-photon absorption (TPA) and group velocity dispersion (GVD) taking into account gain spectrum effect. Then by adjusting time delay between the pump pulse and probe pulse we use the model for simultaneous compression and amplification of probe pulse. We also analyze the four wave mixing (FWM) signal during pulse compression process. It is shown that dispersive effect of GVD on output probe pulse becomes more important for larger cavity length and probe-pump pulses relative time delays.     

Zur Theorie der nichtstationären stimulierten Raman-Streuung

The theory of transient stimulated Raman scattering is developed taking into account the deplation of the pump pulse. Two coupled differential equations describing the conversion of a laser pulse in a Stokes wave are derived with the aid of a semiclassical nonstationary perturbation calculation without the phonon concept. The equations are solved both analytically by a successive approximation and by computer calculations. Estimations are given of the delay of the Stokes pulse, the gain and a characteristic length concerning the conversion.     

Soliton generation via continuous stokes acoustic self-scattering of hypersonic waves in a paramagnetic crystal

A new mechanism is proposed for continuous frequency down-conversion of acoustic waves propagating in a paramagnetic crystal at a low temperature in an applied magnetic field. A transverse hypersonic pulse generating a carrier-free longitudinal strain pulse via nonlinear effects is scattered by the generated pulse. This leads to a Stokes shift in the transverse hypersonic wave proportional to its intensity, and both pulses continue to propagate in the form of a mode-locked soliton. As the transverse-pulse frequency is Stokes shifted, its spectrum becomes narrower. This process can be effectively implemented only if the linear group velocity of the transverse hypersonic pulse equals the phase velocity of the longitudinal strain wave. These velocities are renormalized by spin-phonon coupling and can be made equal by adjusting the magnitude of the applied magnetic field. The transverse structure of the soliton depends on the sign of the group velocity dispersion of the transverse component. When the dispersion is positive, planar solitons can develop whose transverse component has a topological defect of dark vortex type and longitudinal component has a hole. In the opposite case, the formation of two-component acoustic “bullets” or vortices localized in all directions is possible.     

A new type of solution-concentration sensor using the effects of generation and detection of bulk acoustic waves with interdigital transducers

A new type of solution-concentration sensor using a layered planar structure consisting of piezoelectric substrate-liquid solution layer-piezoelectric substrate is presented. Two interdigital transducers (IDTs) are deposited, respectively, at the two parallel interfaces of the layered structure. Under given conditions, the first IDT deposited at one interface and driven by a tone burst can generate a high frequency bulk acoustic wave (BAW) pulse into the solution layer only. This BAW pulse propagates through the solution layer and impinges on the second IDT deposited at the other interface. The second IDT then produces an electrical pulse output, containing information of the solution concentration of the liquid layer via the carrier phase delay of the BAW pulse propagation. Based on the theoretical consideration, a high sensitivity solution-concentration sensor using BAW generation and detection with the IDTs is designed and fabricated. Its feasibility and performance are also experimentally examined.     

脉冲发电机电源宽频变化情况下锁相技术仿真

针对在基波频率变化情况下传统锁相环无法进行正确锁相的问题,提出了基于同步参考坐标系的追踪型三相软件锁相环。三相电压信号(V_a、V_b、V_c)经同步坐标变换后,得到两相静止坐标(V_α、V_β)信号,再经过旋转坐标变化后获得(V_d、V_q)信号;V_q信号经过低通滤波后,进行动态信号延迟,其延迟的时间由输出的频率信号获得;把延迟前后的信号做差,经过PID调节后输出为角频率,对角频率积分后获得稳定基波正序相位。为了验证该算法,基于matlab建立了仿真模型。仿真结果表明,该锁相环在电源基波频率线性变化情况下其相移时间小于0.2ms,在脉冲发电机电压基波频率非线性变化情况下需要经过一个周波也能获得精准地锁相,其运用于有源电力滤波器有效地抑制了脉冲发电机的输出谐波。     

An acoustic switch

The benefits derived from the development of acoustic transistors which act as switches or amplifiers have been reported in the literature. Here we propose a model of acoustic switch. We theoretically demonstrate that the device works: the input signal is totally restored at the output when the switch is on whereas the output signal nulls when the switch is off. The switch, on or off, depends on a secondary acoustic field capable to manipulate the main acoustic field. The model relies on the attenuation effect of many oscillating bubbles on the main travelling wave in the liquid, as well as on the capacity of the secondary acoustic wave to move the bubbles. This model evidences the concept of acoustic switch (transistor) with 100% efficiency.     

Controllable optical delay line using a Brillouin optical fiber ring laser

A controllable optical delay line using a Brillouin optical fiber ring laser is demonstrated and a large timedelay is obtained by cascading two optical fiber segments. In experiment, a single-mode Brillouin opticalfiber ring laser is used to provide Stokes wave as probe wave. We achieve a maximum tunable time delayof 61 ns using two cascading optical fiber segments, about 1.5 times of the input probe pulse width of 40ns. In the meantime, a considerable pulse broadening is observed, which agrees well with the theoreticalprediction based on linear theory.