Diffraction-based femtosecond pulse shaping with a two-dimensional spatial light modulator

A new diffraction-based method is proposed and demonstrated for simultaneous shaping of both the phase and amplitude of femtosecond laser pulses by use of a phase-only two-dimensional spatial light modulator. The method suppresses certain types of temporal replica features ordinarily observed in femtosecond pulse shaping owing to imperfections in modulator devices and allows for multiplexed outputs suitable for use in various applications.     

Compact optical correlator based on one phase-only spatial light modulator

An optical correlator that utilizes one phase-only spatial light modulator (SLM) combined with a mirror is proposed and demonstrated. This system is compressed by displaying the input and filter pattern on different parts of the same SLM. The background noise is separated from the correlation signal by superimposing a high-frequency carrier with the filter pattern except in the zero-frequency regions, which will improve the signal-to-noise ratio in pattern recognition. Our architecture is compact and the cost is relatively low by utilizing only one SLM.     

Intense femtosecond pulse shaping using a fused-silica spatial light modulator

We present the design concept of a setup of a pulse shaper to be used for high-power femtosecond lasers. The pulse shaper is constructed from a high-damage threshold fused-silica spatial light modulator and a 4-f optical system based on the design concept to avoid optical damage. We have successfully demonstrated a pulse compression of 20 fs, 5 mJ pulses obtained from a 1 kHz repetition rate Ti:sapphire chirped pulse amplification system at an average power of 5 W.     

Femtosecond pulse shaping using a two-dimensional liquid-crystal spatial light modulator

We introduce a programmable, high-rate scanning femtosecond pulse shaper based on a two-dimensional liquid crystal on a silicon spatial light modulator (SLM). While horizontal resolution of 1920 addressable pixels provides superior fidelity for generating complex waveforms, scanning across the vertical dimension (1080 pixels) has been used to facilitate at least 3 orders of magnitude speed increase as compared with typical liquid-crystal SLM-based pulse shapers. An update rate in excess of 100 kHz is demonstrated.     

Hollow Gaussian beam generated by beam shaping with phase-only liquid crystal spatial light modulator

Based on the refractive beam shaping system, the transformation of a quasi-Gaussian beam into a dark hollow Gaussian beam by a phase-only liquid crystal spatial light modulator (LC-SLM) is proposed. According to the energy conservation and constant optical path principle, the phase distribution of the aspheric lens and the phase-only LC-SLM can modulate the wave-front properly to generate the hollow beam. The numerical simulation results indicate that, the dark hollow intensity distribution of the output shaped beam can be maintained well for a certain propagation distance during which the dark region will not decrease whereas the ideal hollow Gaussian beam will do. By designing the phase modulation profile, which loaded into the LC-SLM carefully, the experimental results indicate that the dark hollow intensity distribution of the output shaped beam can be maintained well even at a distance much more than 550 mm from the LC-SLM, which agree with the numerical simulation results.     

Femtosecond direct space-to-time pulse shaping in an integrated-optic configuration

We demonstrate femtosecond operation of an integrated-optic direct space-to-time pulse shaper for which there is a direct mapping (no Fourier transform) between the spatial position of the masking function and the temporal position in the output waveform. The apparatus is used to generate trains of more than 30 pulses as an ultrafast optical data packet over approximately an 80-ps temporal window.     

Synthesis of fractal light pulses by quasi-direct space-to-time pulse shaping

We demonstrated a simple diffractive method to map the self-similar structure shown in squared radial coordinate of any set of circularly symmetric fractal plates into self-similar light pulses in the corresponding temporal domain. The space-to-time mapping of the plates was carried out by means of a kinoform diffractive lens under femtosecond illumination. The spatio-temporal characteristics of the fractal pulses obtained in this way were measured by means of a spectral interferometry technique assisted by a fiber optics coupler (STARFISH). Our proposal allows synthesizing suited sequences of focused fractal femtosecond pulses potentially useful for several current applications, such as femtosecond material processing, atomic, and molecular control of chemical processes or generation of nonlinear effects.     

Controllable generation of partially coherent light pulses with direct space-to-time pulse shaper

We demonstrate the possibility of creating user-defined partially coherent light pulses by means of a slight modification of the direct space-to-time pulse shaper. Specifically, we generate a mutual coherence function that corresponds to the independent-elementary-pulse representation model. The theoretical limits in the parameter of global coherence and the efficiency of the system are studied. Our result opens the door to a new way of quantum control in laser-assisted chemical reactions, namely, control by partial coherence.     

利用纯相位型液晶空间光调制器实现空心光束

为了提高激光系统的整体效率,需要将出射光强整形为空心分布。采用纯相位型液晶空间光调制器整形近高斯分布光强到空心分布。基于能量守恒定律和等光程原理,分析了纯相位型液晶空间光调制器光束整形系统,得出了整形系统所需的相位分布。采用衍射光学方法,数值模拟了整形效果,讨论了入射光束束腰半径和强度分布等因素对整形效果的影响。利用纯相位型液晶空间光调制器实验实现了空心光束,实验测得的转换效率大于99%。     

Hyperresolving phase-only filters with an optically addressable liquid crystal spatial light modulator

Hyperresolving (sometimes called 'superresolving' or 'ultraresolving') phase-only filters can be generated using an optically addressable liquid crystal spatial light modulator. This approach avoids the problems of low efficiency, and coupling between amplitude and phase modulation, that arise when using conventional liquid crystal modulators. When addressed by a programmed light intensity distribution, it allows filters to be changed rapidly to modify the response of a system or permit the investigation of different filter designs. In this paper we present experimental hyperresolved images obtained using an optically addressable parallel-aligned nematic LCD with two zone Toraldo type phase-only filters. The images are compared with theoretical predictions.     

纯相位空间光调制器实现振幅调制的技术研究

纯相位空间光调制器能够方便地模拟各种相位型光学元件,但不能直接模拟振幅型元件。使用两种方法,即棋盘相格法和闪耀光栅法,把由这两种方法生成的计算全息图加载到纯相位空间光调制器上,通过搭建的光束变换系统进行实验验证。实验结果表明,棋盘相格法和闪耀光栅法都能很好地实现振幅调制,用闪耀光栅法生成的叉状振幅光栅,给拉盖尔-高斯光束的合束带来了一种新的方法。     

Femtosecond pulse shaping with space-to-time conversion based on planar optics

We propose a novel structure of planar optical configuration for implementation of the space-to-time conversion for femtosecond pulse shaping. The previous apparatuses of femtosecond pulse shaping are 4f Fourier-transforming type system that is usually large, expensive, difficult to align. The planar integration of free-space optical systems on solid substrates is an optical module with the attractive advantages of compact, reliable and robust. This apparatus is analyzed in details and the design of the particular lens for femtosecond pulse shaping based on planar optics is presented.     

Millimeter-wave arbitrary waveform generation with a direct space-to-time pulse shaper

By using tailored pulse sequences from a novel, 1.5-microm direct space-to-time pulse shaper driving a high-speed photodetector, we have achieved, for the first time to our knowledge, millimeter-wave arbitrary waveform generation at center frequencies approaching 50 GHz. By appropriately designing the driving optical pulse sequences, we demonstrate the ability to synthesize strongly phase- and frequency-modulated millimeter-wave electrical signals on a cycle-by-cycle basis.     

All optical phase-only filtering correlation with binarized inputs by a ferroelectric liquid-crystal spatial light modulator

All optical phase-only filtering correlator is constructed with an optically addressed ferroelectric liquid-crystal spatial light modulator (FSLM) for a binarized input object and a twisted-nematic liquid-crystal spatial-light modulator for a computer-generated phase-only filter. In order to improve the discrimination capability of a phase-only filtering correlator, a binarized amplitude with maximum contrast in a FSLM is used for an object input. The contrast of binarized intensities can be maximized by adjusting the rotation angles of a FSLM and polarizers. The threshold level of binarized input objects can be controlled by changing the power of a laser incident on a FLSM. Experimental results with a high discrimination capability are presented to support the numerical correlation performance.     

Tunable pulse repetition-rate multiplication using phase-only line-by-line pulse shaping

We demonstrate a method for all-optical, tunable pulse repetition-rate multiplication of a mode-locked laser based on spectral line-by-line control. In particular, two-to-five-times repetition-rate multiplication of a 9 GHz source is achieved with very high fidelity.     

Femtosecond optical packet generation by a direct space-to-time pulse shaper

We demonstrate femtosecond operation of a direct space-to-time pulse shaper in which there is direct mapping (no Fourier transform) between the spatial position of the masking function and the temporal position in the output waveform. We use this apparatus to generate trains of 20 pulses as an ultrafast optical data packet over an ~40-ps temporal window.     

Chirp control in the direct space-to-time pulse shaper

The dispersion properties of the direct space-to-time pulse shaper are investigated for the first time to our knowledge. We demonstrate that phase-front curvature of the input spatial profile leads to a chirp in the output temporal waveform, which one can compensate for by varying the separation between the pulse-shaping lens and slit. Furthermore, the output intensity profile remains invariant as the chirp is manipulated. These properties are fundamentally different than in the well-known Fourier-transform pulse shaper.     

基于纯位相液晶空间光调制器的可变焦透镜的实现

提出一种基于纯位相液晶空间光调制器(LC-SLM)实现变焦透镜的方法。根据液晶空间光调制器的位相调制原理,以相息图控制LC-SLM来调制入射光波,并衍射产生透镜波面,改变相息图的衍射距离,可实现变焦位相菲涅耳透镜的功能。分析衍射孔径、衍射距离、相息图位相阶数等参数对变焦位相菲涅耳透镜的影响,并以点光源相息图为例对该方法的可行性进行了实验验证。实验结果表明:通过改变计算衍射距离,提出的方法可得到焦距不同的透镜,且易于控制,为三维扫描显示的实现提供了可行性依据。     

Controlling the evolution of nondiffracting speckle by complex amplitude modulation on a phase-only spatial light modulator

In this work we investigate the structure of nondiffracting speckle fields, both experimentally and theoretically. We are able to produce very good agreement between the experimentally recorded and theoretically calculated fields by using complex amplitude modulation on a phase-only spatial light modulator to implement controlled ring-slit experiments for the generation of nondiffracting speckle fields. The structure of the nondiffracting speckle due to binary and continuous phase modulations for both a uniform and a normal distribution is investigated. We find that we are able to engineer whether the nondiffracting field will appear as speckle or a structured zero-order Bessel beam by adjusting the standard deviation in the distribution. Having the ability to control where in the spectrum, from fully-developed nondiffracting speckle to a symmetric zero-order Bessel beam, the nondiffracting field will exist can prove to be a useful resource in the non-destructive testing of materials.     

液晶空间光调制器光谱整形的理论研究

 将液晶空间光调制器用于超高斯种子脉冲光谱整形,重点分析了调制器参数对压缩脉冲信噪比的影响,以此为依据对其结构参数进行优化设计,以降低其影响。对整形、放大及压缩过程的理论模拟表明:应用液晶空间光调制器进行光谱整形时,能够有效克服钕玻璃啁啾脉冲放大系统的非线性效应,在放大器输出端保持原始光谱宽度,但液晶空间光调制器对光谱的调制作用使压缩脉冲信噪比降低。