Mobile adaptive holographic laser hydrophone

A mobile adaptive holographic laser hydrophone based on a dynamic hologram formed in a photorefractive crystal is developed. The hydrophone sensitivity is to –163 dB (rel. 1V/μPa) or 8.6 · 10^?3 rad/Pa in the frequency range of 1–15 kHz. Field tests of the hydrophone were performed in water area of a sea bay. The results confirm the efficiency of the use of measuring systems based on adaptive holographic interferometers to solve problems of recording weak signals (acoustic, hydroacoustic, and others) under non-laboratory conditions.     

Design of new seismometer based on laser Doppler effect

In order to improve the resolution of seismic acquisition, a new seismic acquisition system based on tangential laser Doppler effect with an optimized differential optical configuration is proposed. The relative movement of the inertia object and the immobile frame is measured by laser Doppler effect, which can avoid the electromagnetic and thermometric interference, and the adoption of frequency-modulated (FM)transmission can improve the ability of anti-jamming. The frequency bandwidth is properly determined by analyzing the frequency of the Doppler signal. The velocity, displacement, acceleration, and frequency to be measured can be real-time acquired by frequency/velocity (F/V) converting the FM Doppler signal.A 100-dB dynamic range and the linear frequency range of 1.0 to 1000 Hz are realized.     

Self-starting Ti:sapphire holographic laser oscillator

We demonstrate, for the first time to our knowledge, operation of a holographic laser oscillator that uses laser-pumped Ti:sapphire (Ti(3+) : Al(2)O(3)) as the gain medium. The device is self-starting and self-adaptive by virtue of spontaneous gain-grating formation. We present experimental results of the system that include gain-switched pulses of 25-60-ns duration in a TEM(00) mode and as much as 11 mJ of output energy from a plane output coupler and 47 mJ from an intracavity polarizer port.     

结合Retinex和HVS的自适应全息水印算法

提出一种水印算法的新思路,可以自适应地确定水印嵌入的区域和强度。基于HVS的人眼掩蔽效应对图像亮度和纹理敏感度的量化指标进行计算,图像先后两次分块。依据评分选择最佳嵌入区域后对其进行单尺度Retinex分解,将其分解为用于评价亮度信息的照度分量和评价纹理信息的反射分量。根据评价结果,设置不同的水印嵌入强度分别嵌入图像小波域中频的各分块中。提取的水印通过数学形态学处理以提高其可辨识度。嵌入水印的不可见性得到最大化实现(PSNR=40.0),鲁棒性较佳,能抵御信号处理和几何攻击。能根据图像特征自适应分析水印的最佳嵌入位置,在保证隐蔽性的同时嵌入尽可能多的水印信息;水印的安全性也得以保障,是一种应用面广、适应性强的数字水印算法。     

用全息光学元件实现对眼睛的激光防护

反射式全息滤光片具有很好的滤光特性,可作为一种激光防护元件.本文对这种元件的波长选择性,角度选择性,所能承受的功率密度进行分析,并讨论了实际应用的可能性.     

Time-average holographic interferometry with a sinusoidally modulated laser diode

Laser diodes have the features of a single-mode operation and a frequency tunability. Holographic interferometry is described for investigating the vibrating amplitude and phase with a frequency-modulated laser diode. Sinusoidal wavelength change by varying laser injection current produces the sinusoidal phase modulation of relative phase difference between the reference and vibrating object paths. Holograms are made by time-average exposure of three-type recording modes with a laser diode. The phase of vibrating object is extracted from the measurements of Bessel-type fringe irradiances in three kinds of time-average holographic reconstruction. Experimental results of phase and amplitude for a vibrating cantilever object are shown.     

Direct laser writing defects in holographic lithography-created photonic lattices

As a well-established laser fabrication approach, holographic lithography, or multibeam interference patterning, is known for its capability to create long-range ordered large-volume photonic crystals (PhCs) rapidly. Its broad use is, however, hampered by difficulty in inducing artificially designed defects for device functions. We use pinpoint femtosecond laser ablation to remove and two-photon photopolymerization to add desired defective features to obtain photonic acceptors and photonic donors, respectively, in an otherwise complete PhC matrix produced by holographic lithography. The combined use of the two direct laser writing technologies would immediately make holographic lithography a promising industrial tool for PhC manufacture.     

利用激光全息干涉测量梁的微小位移

全息干涉测量利用二次曝光记录物体在不同载荷状态下的相对位移场.通过在干板上记录和比较不同状态产生的光波的干涉,可以得到在不同载荷时干涉条纹随物体位移的变化情况,实现对物体微小变形、微小位移量的测量.本文利用激光全息干涉技术测量了金属梁的微小位移量,计算得到金属梁的弹性模量和挠度.     

Signal quality improvement of holographic data storage using adaptive two-dimensional filter

Holographic data storage is being widely studied for the purpose of developing next-generation large optical memories. A prospective use of this type of memory is in building image archives in large-scale data centers. In particular, demand for energy conservation at data centers, and therefore for holographic data storage, is growing. In holographic data storage, interference between bits occurs owing to wave aberration in the optical system, shrinkage of the medium, and crosstalk noise from neighboring holograms during multiplex recording; as a result of the interference, the reproduced image deteriorates and the bit error rate (BER) increases. In this study, to reduce the BER in both off-axis-type recording and coaxial-type recording, a two-dimensional finite impulse response (FIR) filter is applied to a reproduced image that has been recorded by angle multiplex recording and shift multiplex recording. First, for the optimization of the FIR filter coefficients, the linear minimum mean square error (LMMSE) method is applied; this method optimizes the coefficients by reducing the BER. Furthermore, for evaluating the optimization performance of the LMMSE method, the optimization performance is compared with that of the real-coded genetic algorithm (RCGA), which has the capability to search a wide range of coefficients. The optimization by the LMMSE method has been found to be excellent for off-axis-type recording but not for coaxial-type recording. It is speculated that this is because of the brightness irregularity in the reproduced image, resulting from crosstalk. On the other hand, a marked reduction in the BER is observed using the RCGA, despite the brightness irregularity. In this study, the effectiveness of the LMMSE method for signals recorded by coaxial-type recording, in which large brightness irregularity is expected, is examined using automatic gain control (AGC). It is found that the application of AGC reduces the BER even in the case of coaxial-type recording.     

Use of holographic interferometer to investigate local deformations of adaptive mirrors

A method is described for using and monitoring local deformations of adaptive mirrors by means of a holographic interferometer. Interference patterns of the mirror surface are obtained and permit qualitative and quantitative analysis of the deformations.Translated from Voprosy Metrologicheskogo Obespecheniya Izmereniya Parametrov Tekhnologicheskikh Lazerov, pp. 46–48, 1984.     

Multiwatt continuous-wave adaptive laser resonator

We present, for what we believe is the first time, results of continuous-wave diode-pumping of a Nd:YVO (4) laser with an adaptive gain-grating resonator. The system is shown to produce more than a 7-W output in a TEM>(00) single longitudinal mode with a laser beam propagation parameter M(2) of <1.3 and <1.1 in the x and y axes, respectively. We demonstrate the self-adaptive abilities of the resonator by spatial correction of an intracavity aberrator for both injected and self-starting versions of the cavity.     

高功率激光系统中全息“热像”效应

对高功率激光系统中存在的一种类似于小尺度自聚焦的非线性过程“热像”效应进行了理论分析和模拟计算。当一束经过遮光物调制的激光光束通过非线性介质后,在随后的光路上将产生一个亮点,即“热像”。“热像”处的光强比背景光强大几倍,因此位于“热像”附近位置的光学元件很可能遭受损伤。从全息成像的角度对“热像”的形成过程进行了分析。结果表明：“热像”的位置处于以非线性介质为中心与遮光物大约对称的地方,“热像”的光强是原始光强的(1+B)2倍。计算机模拟与理论计算结果吻合较好,验证了上述结果。     

A tunable dye laser with a composite holographic wavelengths selector

Composite holographic diffraction gratings, each recorded with a different wavelength, are utilized to achieve simultaneous multiple wavelengths operation of a tunable dye laser. By properly selecting the recording wavelengths and adjusting the efficiency of the holographic gratings, the tuning range and output powers at the various wavelengths can be controlled.     

An external cavity diode laser using a volume holographic grating

This study presents an external cavity diode laser (ECDL) system, utilizing a volume holographic grating (VHG) and a microfabricated silicon flexure as the VHG holder. The laser design is aimed for easy assembly, controllability, and better stability of the laser cavity. The laser frequency was stabilized to a D₂ transition of rubidium at 780.247 nm, with a mode-hop-free tuning range of 16 GHz and 9.6 GHz with and without feed-forward on the diode injection current. The measured linewidth was 850 kHz in 500 s, qualified for laser cooling experiments.     

Self-deflection of laser beams during holographic recording in photorefractive crystals

We have observed dynamic deflection of laser beams reflected from a crystal surface during recording of dynamic holograms in a photorefractive crystal. A theory describing the principal experimental facts is presented. The model takes into account the nonlinear interaction of space-charge waves. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 253–256 (25 August 1999)     

Self-adaptive Nd:YLF holographic laser with selectable wavelength operation

We demonstrate, for the first time to our knowledge, operation of a self-adaptive holographic laser with wavelength selection, which uses Nd:YLF as the gain medium. The holographic laser was self-starting and, by virtue of the temporal dynamics in writing the gain grating, passively Q-switched the cavity to produce pulses with 20–50 ns duration. A pulse energy of 184 mJ was obtained from the system with beams having a TEM₀₀ spatial profile. Single-longitudinal-mode operation was observed with near-transform-limited bandwidth. Received: 30 July 2002 / Revised version: 1 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: s.y.lam@ic.ac.uk     

Enhancing the sensitivity of an adaptive holographic interferometer using non-Bragg diffraction orders

The signal-to-noise ratio of the output of an adaptive holographic interferometer (AHI) based on a Bi(12)TiO(20) crystal is investigated. We show experimentally that the sensitivity of an AHI using the non-Bragg orders of diffraction in a thin photorefractive material is more than an order of magnitude greater than that of an AHI employing two-wave mixing in photorefractive volume holograms.     

MEMS-based adaptive optics scanning laser ophthalmoscopy

We have developed a compact, robust adaptive optics (AO) scanning laser ophthalmoscope using a microelectromechanical (MEMS) deformable mirror (DM). Facilitated with a Shack-Hartmann wavefront sensor, the MEMS-DM-based AO operates a closed-loop modal wave aberration correction for the human eye and reduces wave aberrations in most eyes to below 0.1 microm rms. Lateral resolution is enhanced, and images reveal a clear cone mosaic near the foveal center. The significant increase in throughput allows for a confocal pinhole whose diameter is less than the Airy disc of the collection lens, thereby fully exploiting the axial resolution capabilities of the system.     

Self-starting Nd:YAG holographic laser oscillator with a thermal grating

Self-starting operation of a self-conjugating loop oscillator is achieved, for the first time to our knowledge, by use of an optically induced thermal hologram in a nonsaturable absorbing medium. Self-Q-switched single-longitudinal-mode pulses of 12-ns duration are obtained in a near-diffraction-limited spatial mode and with an output energy of >250 mJ . Good correction of intracavity phase distortions is obtained, as well as good extraction efficiency in the laser mode volume. It is shown that one has to set an additional nonreciprocal phase-shift element in the loop to ensure resonant operation of the system.     

Converging shock on laser plasma: Density profiles by holographic interferometry

A double pulse ruby laser was used to obtained interference patterns from a laser produced plasma in helium. From patterns obtained at different times the electron density evolution was inferred by Abel inversion, with suitable approximations. There was a net evidence for electron density increase due to the action of a converging shock wave produced by reflection.