Source linewidth effects in temporal imaging of Gaussian Schell-model pulses

A transform-limited Gaussian pulse generated from an externally modulated stationary source is launched within a temporal imaging system composed of a second-order dispersion followed by a time lens and a subsequent quadratic dispersion. We consider the effect of the statistical properties of the emitted light for temporal imaging. In particular, it is shown that the design parameters that ensure a received signal with the minimum root-mean-square (rms) width achievable, which is called the temporal image of the incident pulse, are strongly dependent on the coherence properties of the input waveform. Finally, limitations on the temporal resolution of the setup are highlighted and a realistic numerical example is provided.     

Real-Time Fourier Transformer System Using Transmissive Fiber Gratings

The duality between the equations that describe the paraxial diffraction of light beams in free space and the temporal dispersion of narrowband pulses in dielectrics allows one to deduce a time-domain equivalent to the spatial Fraunhofer approximation. This equivalence is used to design real-time optical Fourier transformer systems, which are shown to be realizable by using linearly chirped intermodal couplers (transmissive fiber gratings) as the dispersive media. In particular, the design of an intermodal coupler to carry out Fourier transformations is presented and it is shown that this design exhibits very good correlation between theoretical and simulated results.     

Temporal filtering using time lenses for optical transmission systems

An optical signal processing scheme using time lenses in a 4-f configuration for optical communication systems is proposed. The first time-lens combined with a dispersive element such as an optical fiber produces the Fourier transform of the input signal and the second time lens combined with an optical fiber placed after the temporal filter produces the inverse Fourier transformation. Typically, in an optical signal processing scheme based on space/time-lens, the signal at the output is space/time-reversed because of the direct Fourier transformation after the spatial/temporal filter, which is undesirable for a practical optical communication system. Here, we propose a technique to implement both direct and inverse Fourier transformation using time lenses which has no spatial analogue. As a result, the bit sequence at the output is not time-reversed. Two applications of the proposed scheme, a demultiplexer for wavelength division multiplexing (WDM) systems and a higher-order dispersion compensator, have been discussed and numerically implemented.     

Real-Time Fourier Transformer System Using Transmissive Fiber Gratings

The duality between the equations that describe the paraxial diffraction of light beams in free space and the temporal dispersion of narrowband pulses in dielectrics allows one to deduce a time-domain equivalent to the spatial Fraunhofer approximation. This equivalence is used to design real-time optical Fourier transformer systems, which are shown to be realizable by using linearly chirped intermodal couplers (transmissive fiber gratings) as the dispersive media. In particular, the design of an intermodal coupler to carry out Fourier transformations is presented and it is shown that this design exhibits very good correlation between theoretical and simulated results.     

硫系玻璃在无热化长波红外广角镜头中的应用

设计了一种工作波段为8~12μm、有效焦距为5mm、F数为2、视场角为110°的无热化长波红外广角镜头.根据红外无热化光学设计的基本原理,用了常规红外材料硫化锌、硒化锌和硫系玻璃材料制备的六片镜片,通过合理地分配各个镜片的光焦度以及相互间空气间隔等参量,在全视场角为110°的范围内实现接近衍射极限的成像效果.为了更好地控制系统像差,设计利用了硫系玻璃易于精密模压制备非球面的优点,在两片硫系玻璃镜片上设计了3处非球面.设计结果显示:系统在-40℃~60℃的温度范围内均可实现品质良好的红外成像,光学调制函数全视场内均大于0.4,同时在110°视场角时畸变控制在5%以下;实现了在较大视场角条件下控制红外广角镜头的畸变以及系统无热化等设计要求.该系统体积紧凑、质量较轻,整体设计符合民用红外车载镜头的使用要求.     

利用LED的投影系统光源设计

设计了利用多颗LED(Light Emitting Diode,发光二极管)阵列组成的扩展面光源.经过合理的聚光设计使之符合某些投影设备对亮度要求不是很高,但结构紧凑、性能稳定、使用寿命长的要求.结合数学建模和软件模拟的方式设计了一种小巧的反光杯,利用反光杯把LED近180°的发散光束汇聚到60°左右;然后再用一一对应的透镜阵列汇聚为平行光;最后采用柯勒照明方式把较大的面光源阵列汇集到LCD投影屏幕上,从而达到了较高亮度且具有很高均匀性的目的.     

A time-compensated spectral filter for transform-limited tunable picosecond pulse generation from spectro-temporal-selection dye lasers

Diffraction and transform-limited picosecond tunable pulses are generated from Spectro-temporal-Selection (STS) dye lasers by using a new extra-cavity filter. This filter is based on a grazing-incident grating and arranged in the configuration of a folded dispersive delay line. Thus, it provides both high spectral selectivity and controllable temporal compensation for elimination of pulse broadening. Direct production of diffraction- and transform-limited picosecond dye laser (10 µJ, 50 ps) pulses spectrally adjustable between 398 and 702 nm is demonstrated in a compact device, with 8 ns pump pulses from a nanosecond nitrogen laser.     

Complete temporal pulse characterization based on phase reconstruction using optical ultrafast differentiation (PROUD)

A simple and general technique for recovering the phase profile of a given optical waveform from temporal intensity measurements is introduced and experimentally demonstrated. The proposed method involves the measurement of the temporal intensity profiles at the input and output of a linear optical time differentiator. The signal phase profile can be unambiguously recovered from these intensity measurements using a direct and noniterative algorithm. Given that ultrafast optical differentiators can be readily implemented in all-fiber or free-space platforms, the proposed technique could be applied over time waveforms with durations ranging from the subpicosecond to the nanosecond regime.     

Proposal for arbitrary-order temporal integration of ultrafast optical signals using a single uniform-period fiber Bragg grating

A simple and practical all-fiber design for implementing arbitrary-order temporal integration of ultrafast optical waveforms is proposed and numerically investigated. We demonstrate that an ultrafast photonics integrator of any desired integration order can be implemented using a uniform-period fiber Bragg grating (FBG) with a properly designed amplitude-only grating apodization profile. In particular, the grating coupling strength must vary according to the (N-1) power of the fiber distance for implementing an Nth-order photonics integrator (N=1,2,...). This approach requires the same level of practical difficulty for realizing any given integration order. The proposed integration devices operate over a limited time window, which is approximately fixed by the round-trip propagation time in the FBG. Ultrafast arbitrary-order all-optical integrators capable of accurate operation over nanosecond time windows can be implemented using readily feasible FBGs.     

基于Wollaston棱镜的图像复分快照式成像光谱仪设计

设计一种以Wollaston棱镜为分光元件的图像复分快照式成像光谱系统,主要包括前置望远物镜、准直镜、Wollaston棱镜组、成像镜和补偿滤光片。此类光学系统可以一次曝光获取同一目标景物在不同波长下的二维信息。光束经过Wollaston棱镜组分光,为了使不同波长对应景物不至于重叠,要求分束角度比较大,这样进入成像镜的光线入射角度相对较大,无疑增加了成像镜的设计难度。分析了基于Wollaston棱镜的图像复分快照式成像光谱仪的原理及特点,设计了一套完整的成像光谱系统。全系统结构复杂,光学系统的光阑必须匹配好。为了使得不仅单个镜头成像质量良好,而且镜组之间能够良好的衔接,将前置望远物镜设计为像方远心结构,准直镜设计为物方远心结构。全系统采用多重结构,使得16个谱段在56线对处的MTF值均接近衍射极限,点列图中RMS值基本都在艾里斑以内,系统成像质量良好。