Controlling waveform of light pulse with light in optical fibers

We present our experimental demonstration of controlling waveform of a signal pulse by using two beams of continuous wave (CW) generated by stimulated Brillouin scattering (SBS) in optical fibers with different frequencies. Waveform of the signal pulse is modulated in an “all-optical” way according to total input power. This method is only suitable for light pulse with nanosecond width. We provide a new method for controlling light with light.     

高非线性微结构光纤中基于受激布里渊散射的慢光延迟

全光连续可调的慢光技术在全光网络和光信息处理等领域具有重要的应用前景. 利用自行设计并拉制的高非线性微结构光纤, 实验研究了基于受激布里渊散射的可调谐慢光延迟. 采用单抽运光和单级延迟方案, 当抽运光功率为162.6 mW时, 在长度为120 m的高非线性微结构光纤中获得了最大76 ns的延迟量, 相当于0.76个脉冲宽度. 通过调节抽运光功率的大小, 可以实现对慢光延迟量的可调谐.该慢光延迟方案具有延迟量大、 全光可调谐及与现有光通信系统兼容等优势. 关键词： 慢光 微结构光纤 受激布里渊散射     

Low distortion, continuously tunable, positive and negative time delays by slow and fast light using stimulated Brillouin scattering

We demonstrate a single optical timing module that can provide both tunable delay and advancement with low distortion using a reconfigurable gain medium based on stimulated Brillouin scattering. Dual-stage intensity modulation method is used to achieve optimized gain profiles for both slow- and fast-light operation. Using 6.5 ns Gaussian pulses, we demonstrate a continuous temporal adjustment from a fractional advancement of 0.31 to a fractional delay of 0.82, giving more than a full pulse width of total tunability.     

Ultrasonic enhanced Brillouin light scattering in water

Brillouin light spectra in water was experimentally investigated by a scanning Fabry-Perot interferometer. The optical properties of the scattering media were modulated by a plane ultrasound wave. An enhancement up to 10 times of the Brillouin signal by modulation with the ultrasound wave has been seen clearly. The dependence of the Brillouin signal amplitude on the ultrasound frequency, amplitude and the position of the ultrasound field based to the light beam was investigated experimentally. Very strong dependence of the enhancement with experimental conditions was obtained.     

Slow and fast light: basic concepts and recent advancements based on nonlinear wave‐mixing processes

After a review of the basic concepts of slow and fast light, recent advancements based on nonlinear wave‐mixing processes are described. As a nonlinear medium, the authors focus on a liquid crystal light valve showing that it allows obtaining a large control of the group delay, with a maximum fractional delay of 1, and a deceleration of light pulses down to group velocities as small as 0.2 mm/s. A theoretical model accompanies the observations and accounts for them in the general framework of two‐wave mixing in the light valve. At the end, a high‐sensitivity interferometer is presented as an example of slow light applications.     

Numerical study on SBS slow light systems using a super-Gaussian filtered incoherent pump

We numerically investigate the performance improvement and its limitations using super-Gaussian filtered incoherent pumping in Stimulated Brillouin Scattering (SBS) slow light systems, especially for 2.5-Gb/s return-to-zero (RZ) OOK data streams. As compared to regular Gaussian filtering schemes, super-Gaussian filters can reduce the signal broadening without sacrificing the generated delay. On the other hand, although the figure-of-merits (FOM) in terms of the normalized delay and signal Q-factor products can be improved as well, additional sub-pulses may appear and subsequently the curve of FOM versus slow-light bandwidth exhibits a fluctuated behavior.     

基于SBS效应激光波长对光学材料损伤的影响

从耦合波理论出发,采用纵向受激布里渊散射(SBS)模型,通过时域有限差分法数值求解了瞬态SBS耦合波方程组,得到了泵浦光、Stokes光强度及声波产生应力随时间的分布;研究了在三种常见波长激光（基频、倍频和三倍频）作用下SBS效应的发展过程及其对光学材料损伤过程的影响。结果表明：种子光作用主要体现在SBS起振阶段,一旦进入稳态阶段,Stokes光强及应力发展的最大值、稳态值依赖于泵浦光的大小;在相同脉冲宽度下,激光波长越短,SBS发生越早,发展越快,更易造成光学元件的力学损伤;三倍频激光产生最大应力为基频光产生最大应力的100倍,且相比于倍频激光,产生最大应力的时刻要提前15 ns。     

矩形谱宽带光抽运的布里渊慢光中脉冲失真的分析

在小信号增益条件下推导出了高斯光脉冲经过矩形谱宽带光抽运的布里渊慢光系统后输出脉冲时域振幅包络的近似解析表达式,适用于抽运光频谱具有陡峭上升(下降)沿和平坦顶部的情况.定量分析和比较了光纤色散以及受激布里渊散射增益不均衡和增益所致色散效应对延迟信号光脉冲失真的影响.计算结果表明:当信号光脉冲的频谱处于布里渊有效增益谱内时,解析解与数值解符合,布里渊增益所致三阶色散效应是导致延迟信号光脉冲失真的主要物理因素,制约了窄脉冲延迟量的提高.     

基于一种优化的梳状布里渊增益谱实现对任意周期信号的零展宽快慢光

在理论分析任意信号实现无畸变快慢光所需条件的基础上,根据周期信号的频谱特点,推导出一种优化的梳状布里渊增益谱的普适公式.利用该普适公式,可根据所需时间延迟或超前量,方便地得到周期信号每个离散频谱分量所对应的各离散布里渊增益谱的强度和增益谱峰值相对于频谱分量的偏移量,从而确定整个梳状布里渊增益谱的形状.通过对正弦信号、周期性高斯信号和周期性方波信号的数值计算,结果表明,利用该优化的梳状布里渊增益谱可实现这三种周期信号在零展宽下的快慢光,证实了所得普适公式的正确性.此外,通过调节各离散布里渊增益谱的强度和偏移量,时间延迟或超前量连续可调.     

Slow light in photonic crystals waveguide constructed with symmetrically perturbed structure

Symmetrically perturbed photonic crystal waveguide can be constructed by inserting perturbative dielectric rods into photonic crystal waveguide structure with whose rods’ radius distributed according to a certain proportion. Slow light properties in this new structure are studied by using the plane wave expansion method (PWM). In this paper, schemes of adjusting radius of perturbative dielectric rods and adjusting the dielectric constant of perturbative dielectric rods are proposed to optimize slow light properties. The result shows that the scheme for adjusting radius of perturbative rods can realize larger average slow light bandwidth and efficiently control the NDBP value of the waveguide, but it contributes little to obtain smaller group velocity. The scheme for adjusting dielectric constant of perturbative rods can realize smaller group velocity, but can only obtain smaller slow light bandwidth and cannot efficiently enlarge NDBP value of waveguide. Both optimization schemes proposed in this paper realize group velocity that is two magnitudes smaller than the vacuum speed of light meanwhile maintaining large NDBP and low GVD region. Our results provide important theoretical basis for the potential application offered by symmetrically perturbed photonic crystal in future optical networks.     

Slow light transmission in a photonic crystal power splitter with parallel outputs

In this paper, we investigate coupling of light to slow modes in a photonic crystal power splitter composed of a Y-junction and two 60° bends. First, a combination of two cascaded bends which is commonly used in integrated photonic crystal circuits is studied in slow light frequency regime. We propose a structure that its transmission spectrum covers the high group-index frequencies near the band edge. Also, by structural modifications, high transmission near to 95% is achieved in slow light bandwidth. Next, we study the complete structure of a photonic crystal power splitter with parallel outputs based on a Y-junction integrated with two 60° bends. Using modified bends and reducing sharpness of Y-junction, the efficiency of splitting increases in both high and low group-index frequency bands. The optimized structure has an average efficiency of 82% in slow mode regime. This structure can be used in photonic crystal based slow light devices, such as Mach-Zehnder interferometers.     

Slow light engineering in polyatomic photonic crystal waveguides based on square lattice

In this paper, the slow light properties of the polyatomic Photonic Crystal (PhC) which has multiple different air holes in each primitive cell are investigated. A slow light waveguide with “U-type” group index-frequency curve, which results in nearly constant group index over large bandwidth, is achieved using this new photonic crystal geometry based on the square lattice. Also, the radius and position of the innermost rows of small air holes have been modified to investigate the feasibility of controlling the dispersion relation by subtle structural modification. Numerical results demonstrate that decreasing the group velocity effectively and meanwhile maintaining a large Normalized Delay-Bandwidth Product (NDBP) can be achieved by only modifying the radius of the innermost rows of small air holes. Shifting the innermost rows of small air holes toward the waveguide core is highly beneficial to enlarge the slow light bandwidth, but it contributes nothing to the promotion of NDBP. Our results provide important theoretical basis for the potential application offered by the polyatomic photonic crystal in future optical networks.     

用SF6气体作SBS相位共轭镜的激光谐振腔

利用高压SF_6气体中的受激布里渊散射作相位共轭镜,将其用于Nd:YAG激光谐振腔,得到了能量在100mJ左右的脉冲激光输出.与利用液体介质的受激布里渊散射相位共轭腔的激光输出相比,其光斑的空间模式大大改善,基本达到TEM_(00)模,功率密度在150MW/cm~2输出能量的稳定性也有所提高.     

Pulse compression effect based on stimulated Brillouin scattering light storage in an optical fiber

The pulse compression effect in SBS light storage is numerically investigated. We demonstrate theoretically that the compressed width of retrieved data-pulses is not only related with temporal profile of data-pulse, but also spectrum and temple profile of control-pulse. The data-pulses with steep rising edge can be compressed after they are retrieved. The optimum compression effect takes place when a 2-ns-long exponential data-pulse interacts with a 1.5-ns-long chirped Gaussian control-pulse, where the data-pulse width is compressed to 1.73-ns-long and the readout efficiency is the biggest. A 2-ns-long rectangular data-pulse is also compressed to 1.73-ns-long when it encounters to a 1.5-ns-long rectangular control-pulse, but the readout efficiency isn’t maximum value. The results are significant for increasing storage capacity and variable bits all-optical buffering.     

光纤中基于双布里渊增益线的受激布里渊散射快光

为了解决受激布里渊散射快光在高吸收区产生损耗的问题,通过分析普通单模光纤中双线泵浦产生的双布里渊增益线特性及在增益峰间实现脉冲的超光速传输理论,利用有限元法数值模拟了双布里渊增益线处受激布里渊散射引起的快光特性。结果表明,当频率分离因子大于0.596时,可以观察到双增益峰;当频率分离因子在1~5.25范围内时,两个泵浦波产生的双增益峰之间可以明显地产生快光;当频率分离因子为1.75时,在双布里渊增益线之间的最大时间提前可达25 ps。当频率分离因子为2.42时,三阶色散所对应的归一化色散长度为无穷大,三阶色散可以得到完全补偿;当频率分离因子大于2.464时,脉冲展宽因子趋近于1,可以实现无畸变传输,但时间提前量小于13.52 ps。本文的研究结论对于在布里渊增益区实现快光具有一定的理论意义,并对设计基于受激布里渊散射快光器件具有理论指导作用。     

Tunable slow and fast light with large bandwidth in the band-edge of gain spectrum of the wide band fiber optical parametric amplifier

We propose a novel method theoretically to generate the slow and fast light with large bandwidth and low gains, which is based on the parametric process in fiber. In our scheme, the wide band fiber optical parametric amplifier is employed and the whole signal bandwidth should be located at a certain frequency range of the band-edge of gain spectrum, and then signal waves will be delayed or advanced with low signal gains because of the peculiar feature of signal gain and phase shift. By changing the pump power, the delay time is continuously-tunable optically. The ultimate delay bandwidth and the delay bandwidth product are constrained by the shape of time delay spectrum. Our simulation verifies that 22.4 ps delay or advanced time for the bandwidth of 10 GHz with little distortion can be obtained at certain wavelengths in the optical communication waveband, and their gains are nearly zero. The tunable range is from 0 ps to 22.4 ps for the signal bandwidth of 10 GHz, and it is from 0 ps to 15.6 ps for the bandwidth of 15 GHz. This type of slow and fast light in wide band FOPA has the potential capability to produce the tunable slow and fast light for large bandwidth with low signal gains in future.     

利用受激布里渊散射能量反射率测量液体介质运动黏度方法的研究

本文提出了一种利用受激布里渊散射(SBS)能量反射率的变化测定不同温度时的液体介质运动黏度的方法.液体介质的运动黏度随温度而变化,这导致了介质增益系数和SBS能量反射率的变化,因此通过SBS能量反射率的变化可测定不同温度时的液体介质的运动黏度.在Countinuum Nd: YAG种子注入激光系统中测定了不同温度时的水的运动黏度并与运动黏度计测定的结果进行了比较.用本方法获得的不同温度时的水的运动黏度测量值与运动黏度计测量值很接近,相对误差小于5%. 关键词： 受激布里渊散射 SBS能量反射率 温度 运动黏度     

Slow light in photonic crystal with combinations of gradient dielectric constant

A line defect waveguide formed by a new kind of two-dimensional photonic crystal with combinations of gradient varied dielectric was designed and the properties of slow light will been studied by using the Plane Wave expansion Method (PWM). The result shows that the group velocity is slower when the corresponding combination of gradient dielectric constants are with higher dielectric constant and lower interval. In our new structure of photonic crystal, the corresponding group velocity can reach as low as 0.06c (c: light velocity in vacuum), two orders magnitude lower than light velocity. Meanwhile, the group velocity dispersion effect is relatively small.     

A study of pulse shape in electromagnetically induced transparency based slow light

Slow light of four different pulses are demonstrated and analyzed in cesium vapor based on electromagnetically induced transparency. Each pulse, generated with an identical temporal width, experiences very different slow light effects due to its temporal and spectral distributions simultaneously. Aiming at the applications such as timing, instead of communication, we obtained two optimized ones among the four different shaped pulses. Firstly, the single-exponential pulse is more appropriate to maximize the time delay than other three pulses; secondly, the cosine pulse shows advantage to minimize the distortion (broadening) of the slowed pulse. Finally, using optical filter model, we present a convenient simulation method in theory; moreover, the theoretical results show excellent agreement with experiments.     

Measurement of stimulated Brillouin scattering （SBS） threshold based on waveform variation of SBS optical limiting

This paper proposes a method for measuring the stimulated Brillouin scattering (SBS) threshold based on waveform variation of SBS optical limiting. The output waveforms for different pump power densities are numerically simulated, and validated in the Nd:YAG seed-injected laser system. The results indicate that SBS does not take place in the case of a low pump power density and thus the output power scales up linearly with pump power. Once the pump power density exceeds the SBS threshold, SBS takes place and thereby the energies are transferred from pump to Stokes. As a result, a small shoulder appears in the trailing edge of the output waveform, which provides another method to determine the SBS threshold.