Three-dimensional object recognition by use of digital holography

We present a technique for performing three-dimensional (3D) pattern recognition by use of in-line digital holography. The complex amplitude distribution generated by a 3D object at an arbitrary plane located in the Fresnel diffraction region is recorded by phase-shifting interferometry. The digital hologram contains information about the 3D object's shape, location, and orientation. This information allows us to perform 3D pattern-recognition techniques with high discrimination and to measure 3D orientation changes. Experimental results are presented.     

Anisotropic diffraction of stratified volume holographic grating in photorefractive media

We have studied the anisotropic diffraction properties of the stratified volume holographic gratings recorded in photorefractive media using the anisotropic coupled wave theory. It is shown that the diffraction efficiency of such system exhibit the uniform periodic Bragg selectivity properties. In addition the dependence of the stratified volume holographic optical elements (SVHOEs) diffraction properties on the buffer-layer thickness, grating-layer thickness, number of modulation layers, and total thickness of system are discussed in detail.     

Anisotropic Bragg diffraction of finite-sized volume holographic grating in photorefractive crystals

Anisotropic diffraction of uniform plane wave by finite-sized volume holographic grating in photorefractive crystals is considered. It is found that the anisotropic diffraction can take place when some special conditions are satisfied. The diffracted image is obtained in experiment for the anisotropic Bragg diffraction in Fe:LiNbO₃ crystals. A coupled wave analysis is presented to study the properties of anisotropic diffraction. An analytical integral solution for the amplitudes of the diffracted beams is submitted. A trade off between high diffraction efficiency and the deterioration of reconstruction fidelity is analyzed. Numerical evaluations also show that the finite-sized anisotropic volume grating exhibits strong angular and wavelength selectivity. All the results are useful for the optimizing design of VHOE based on finite-sized volume grating structures.     

Analysis of photorefractive volume holographic lenses with different recording and read-out wavelengths

The photorefractive volume holographic lenses for converting a plane wave to a lateral-spread spherical wave with different recording and reconstructing wavelength has been studied. Using the coupled-wave theory, the half-analytical solutions for the amplitudes of the space harmonics of the field inside the transmission geometry are presented. The values of the off-Bragg parameter at the reconstructed process and the diffracted beam's amplitude distribution are analyzed. In addition, the dependences of diffraction properties on the focal length of recording spherical wave and on the geometrical dimensions of the grating are discussed. Furthermore, the focusing properties of this photorefractive holographic lens are analyzed.     

Three-dimensional object recognition from digital Fresnel hologram by wavelet matched filtering

We propose and demonstrate a digital holographic technique for 3D object recognition and classification. One Fresnel digital hologram of each of the 3D objects to be classified is recorded. The electronic holograms are processed digitally to retrieve 3D object information as 2D digital complex images. We use this method to classify four physical objects in a 3D scene into two classes. Results are presented from an experiment to demonstrate the proof of the concept.     

Theoretical study on lifetime of thermal-fixed volume holographic phase grating in photorefractive crystals

<正>We review the dark decay of the electronic holographic phase grating before thermal fixing,and deduce the general analytic expression of the lifetime of thermal-fixed ionic holograms in the photorefractive crystal,by means of analogizing.Because the ions are optically inactive,the lifetime of thermal-fixed ionic holograms is only closely relate to the ionic decay rate which is determined by the conductivity of ionic species at a given temperature.We theoretically analyze and numerically simulate the influences on the lifetime of ionic grating from the crucial factors in the experiment and application.The results reveal that low temperature,low ion-concentration,and large grating spacing are advantages for extending the life of the thermal-fixed volume holographic phase grating in photorefractive crystal.     

Demonstration of a spatial-spectral holographic LIDAR range-Doppler processor

We present a new approach to laser interferometric Doppler and ranging (LIDAR) processing using spatial-spectral holography (SSH). In this approach, broadband optical signals from a random noise or frequency-modulated laser are transmitted and reflected off remote targets. The return signals interfere spatially and spectrally with a local copy of the original transmit signal in an SSH medium, resulting in spectral gratings that have a spectral period inversely proportional to the LIDAR target's range and a position proportional to the target's Doppler (or velocity). These gratings are subsequently read out by a slowly chirped source onto a parallel detector array, and the velocity and range of the targets are inferred. We present the theoretical framework that describes the function of the LIDAR processor, as well as proof-of-concept experimental results.     

Depth-resolved holographic imaging through scattering media by use of a photorefractive polymer composite device in the near infrared

We report full-field depth-resolved holographic imaging through scattering media by use of a photorefractive polymer device in the near infrared. Real-time images through seven scattering mean free paths are acquired in a few seconds with a broadband superluminescent diode at 794 nm. A depth resolution of 15 microm and a transverse resolution of better than 42 microm are achieved. The photorefractive devices used are based on a poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone dimalenitrile charge-transport network doped with the electro-optic chromophore 1-(2'-ethylhexyloxy)-2,5-dimethyl-4-(4"-nitrophenylazo)benzene.     

Diffraction study of volume holographic gratings in dispersive photorefractive material for femtosecond pulse readout

Based on the modified Kogelnik diffraction efficiency equation, the diffraction intensity spectrum and the total diffraction efficiency of volume gratings in photorefractive media are studied systematically. Taking photorefractive InP:Fe crystal as an example, the effect of the grating parameters and the pulse width on the diffraction properties are presented, in particular under the influence of crystal material dispersion. Under the combined effects, the diffraction pulse profiles and the total diffraction efficiency are shown. Also, the diffraction properties with and without crystal material dispersion are compared. These studies indicate that the properties of the diffraction beams can be controlled by the holographic grating parameters; this property can be used for pulse shaping applications.     

Development of narrow-linewidth diode lasers by use of volume holographic transmission gratings

We present two diode laser setups that employ volume holographic transmission gratings to provide optical feedback. The advantage of this kind of grating is high diffraction efficiency and the possibility to place optical elements on both sides of the grating. This allows for advanced external cavities and adjustable feedback efficiency. The first setup is a diode laser in the Littman configuration with the transmission grating replacing the conventional reflection grating. The second setup improves the frequency selectivity by substituting the feedback mirror with a passive resonator. This grating-enhanced external cavity diode laser (GECDL) achieves excellent frequency stability. A prototype of the GECDL setup demonstrates an intrinsic linewidth of 7 kHz and an operation range that covers the full amplification profile of the laser diode.