Engineering topics
Publications and source records attributed to Liu, Duncan T. H..
A new mathematical model for describing radiation-induced attenuation in optical fibers ispresented. Unlike the existing empirical power law, the new expression is dose- rate dependent andcan be used to predict low-dose-rate induced fiber loss occurring in space from the normally high-dose-rate results obtained in a ground-based laboratory. The new theory is in good agreement withthe experiment.
A real-time GaAs and liquid-crystal television- (LCTV-) based optical correlator is demonstrated. The speed of this correlator (video rate) is limited by the LCTV's; with faster spatial light modulators, the potential speed of a GaAs-based correlator may be 1000 frames/s or higher. Comparisons are made between VanderLugt and joint transform configurations and between degenerate and nondegenerate four-wave mixings. The edge-enhancement effect and the Bragg diffraction effect are also discussed.
A potential application of the photorefractive time-integrating correlator is the real-time radar jamming interference rejection system, using the adaptive filter method; a fast photorefractive crystal is needed for adapting a rapidly changing jamming signal. An effort is presently made to demonstrate and characterize a GaAs-based photorefractive time-integrating correlator, since GaAs crystals are 2-3 orders of magnitude faster than most other alternatives.
We demonstrate a new optical correlator in which the correlation peak intensity is increased when the matched input object is moving. The basic configuration of the correlator is the same as a VanderLugt optical correlator consisting of a photorefractive crystal. The principal of this new correlator is based on the dynamic grating erasure property of photorefractive materials. The detail of this principle is described.
The resolution of a target-tracking optical novelty filter is discussed in terms of the response time of the nonlinear medium, the speed of the target, and the resolution of the input device. Optical novelty filters using a faster nonlinear medium may have a higher output resolution. This is particularly true in the case of tracking high-speed targets. The potential of implementing high-resolution optical novelty filters using photorefractive GaAs is investigated experimentally.
The unique features of photorefractive compound semiconductors are presented. The advantages of this class of nonlinear optical materials for optical processing are illustrated with examples using GaAs and InP. The difference between GaAs and InP in the laser power density requirement is discussed.
Recent experimental results on matrix-vector multiplication and multiple four-wave mixing using GaAs are presented. Attention is given to a simple concept of using two overlapping holograms in GaAs to do two matrix-vector multiplication processes operating in parallel with a common input vector. This concept can be used to construct high-speed, high-capacity, reconfigurable interconnection and multiplexing modules, important for optical computing and neural-network applications.
Photorefractive semiconductors have the attractive features of fast response times and operation at near-infrared wavelengths. This has opened some new research opportunities in the field of photorefractive nonlinear optics which is significant for applications in real-time image processing and optical computing. This paper presents recent experimental demonstrations of several basic optical information processing techniques using photorefractive GaAs crystals. The results of these demonstrations illustrate that photorefractive compound semiconductors has a great potential as a new medium for light beam interaction based on the dynamic holographic principle.
A real-time optical correlator based on GaAs and liquid-crystal TV (LCTV) is demonstrated. The demonstrated system has a video-frame rate limited by the speed of the LCTVs; if faster spatial-light modulators are used, the potential frame rate of a GaAs-based correlator can be as fast as 1000 frames/sec under experimental conditions. Comparisons are made between VanderLugt and joint transform and between degenerate and nondegenerate four-wave mixing. The edge-enhancement effect and the Bragg diffraction effect are discussed.
Experimental results of two-beam coupling gain efficiency in an undoped, semiinsulating GaAs crystal are reported. The highest gain coefficient measured is about 4.5/cm under the condition of an applied electric field of 13 kV/cm and a grating periodicity of 20 microns. The experimental results and theoretical calculations are in reasonable agreement with each other.
This paper reports on the first observation of a photorefractive gain coefficient as high as 2.6/cm in the undoped liquid-encapsulated Czochralski-grown GaAs crystals at 1.06 microns under a dc electric field of 13 kV/cm without using the moving grating technique.The absorption coefficient of the crystals used is 1.3/cm, showing that a net gain has been achieved. This measured gain coefficient is close to the predicted theoretical value.