John Donoghue

Efficient low-intensity optical phase conjugation based on coherent population trapping in sodium.

We have observed optical phase-conjugate gain (>50) in sodium vapor, using low-intensity pump lasers (1 W/cm2), with a response time of the order of 1 μs. Coherent population trapping is experimentally identified as the phase-conjugate mechanism. A theoretical model is presented that supports these observations by showing that coherent population trapping can write large-amplitude nonlinear-optical gratings at laser intensities well below those needed to saturate the optical transitions.

Superparallel holographic correlator for ultrafast database searches

We describe a superparallel holographic optical correlator that performs two-dimensional spatial and angular multiplexing simultaneously. The key step in this architecture is the use of a holographic multiplexer to split a query image into many copies before it applies them to the holographic database. A holographic demultiplexer, in conjunction with an aperture, is used to identify the location and the angle of the brightest correlation peak. This architecture uses only O(square root of N) detector elements to search through N unsorted images in a single query. We demonstrate the basic features of this architecture, using three spatial locations with eight angle-multiplexed images in each location.

Hybrid optoelectronic correlator architecture for shift-invariant target recognition

In this paper, we present theoretical details and the underlying architecture of a hybrid optoelectronic correlator (HOC) that correlates images using spatial light modulators (SLMs), detector arrays, and field programmable gate array (FPGA). The proposed architecture bypasses the need for nonlinear materials such as photorefractive polymer films by using detectors instead, and the phase information is yet conserved by the interference of plane waves with the images. However, the output of such an HOC has four terms: two convolution signals and two cross-correlation signals. By implementing a phase stabilization and scanning circuit, the convolution terms can be eliminated, so that the behavior of an HOC becomes essentially identical to that of a conventional holographic correlator (CHC). To achieve the ultimate speed of such a correlator, we also propose an integrated graphic processing unit, which would perform all the electrical processes in a parallel manner. The HOC architecture along with the phase stabilization technique would thus be as good as a CHC, capable of high-speed image recognition in a translation-invariant manner.

Adaptive filtering with organic photorefractive materials via four-wave mixing

In prior work, we exploited the nonlinearity inherent in four-wave mixing in organic photorefractive materials for adaptive filtering. In this paper, we extend our work further and demonstrate new applications which involve: dislocation, scratches and defect enhancement. With the availability of the organic photorefractive materials with large space-bandwidth product, it should open the possibility of using the adaptive filtering techniques in quality control systems.

Characterization of optical correlation via dynamic range compression using organic photorefractive materials

In prior work, we demonstrate optical correlation via dynamic range compression in two-beam coupling using thin-film organic materials. In this paper, we continue the effort; characterize the performance of this correlator for variety of input. We successfully demonstrated correlation results almost free of cross- correlation and noise for extremely complicated noisy image were the signal image consist of several targets and reference image superposed of many templates.

Performance comparison of photorefractive two-beam coupling correlator with optimal filter based correlators

The photorefractive joint transform correlator (JTC) combines two features. The first is embedded semi-adaptive optimality which weighs the correlation against clutter and noise in the input and the second is the intrinsic dynamic range compression nonlinearity which improves several metrics simultaneously without metric tradeoff. The performance of this two-beam coupling joint transform correlator scheme is evaluated against several other well-known correlation filters that have been developed during the last three decades. The result shows that the two-beam coupling joint transform scheme is a very robust correlator with respect to standard evaluation metrics for different sets of data.

Analytical analysis of adaptive defect detection in amplitude and phase structures using photorefractive four-wave mixing

In this work, brief theoretical modeling, analysis, and novel numerical verification of a photorefractive polymer based four wave mixing (FWM) setup for defect detection has been developed. The numerical simulation helps to validate our earlier experimental results to perform defect detection in periodic amplitude and phase objects using FWM. Specifically, we develop the theory behind the detection of isolated defects, and random defects in amplitude, and phase periodic patterns. In accordance with the developed theory, the results show that this technique successfully detects the slightest defects through band-pass intensity filtering and requires minimal additional post image processing contrast enhancement. This optical defect detection technique can be applied to the detection of production line defects, e.g., scratch enhancement, defect cluster enhancement, and periodic pattern dislocation enhancement. This technique is very useful in quality control systems, production line defect inspection, and computer vision.

Optical Phase Conjugation in the Double Raman System

The double Raman (or double A system) has been the subject of much recent interest for applications such as lasing without inversion and precision laser frequency translation. We now show that coherent population trapping (CPT) in the double Raman system can be used to dramatically improve the performance of nonlinear optical materials that are based on resonant systems.