Pierre Ambs

Optical Computing: A 60-Year Adventure

Optical computing is a very interesting 60-year old field of research. This paper gives a brief historical review of the life of optical computing from the early days until today. Optical computing ngenerated a lot of enthusiasm in the sixties with major breakthroughs opening a large number of nperspectives. The period between 1980 and 2000 could be called the golden age with numerous new ntechnologies and innovating optical processors designed and constructed for real applications. nToday the field of optical computing is not ready to die, it has evolved and its results benefit to new nresearch topics such as nanooptics, biophotonics, or communication systems.

Optimal multicriteria approach to the iterative Fourier transform algorithm

We propose a unified approach to the multicriteria design of diffractive optics. A multicriteria version of the direct binary search that allows the user to adjust the compromise between the diffraction efficiency and the signal-to-noise ratio already exists. This technique has proved to be extremely powerful but also very time consuming. We extend this multicriteria approach to the iterative Fourier transform algorithm, which helps to reduce the computation time dramatically, especially for multilevel domains. Simulations as well as experimental validations are provided.

High-speed degree-of-polarization imaging with a ferroelectric liquid-crystal modulator

Images describing polarimetric characteristics of objects are more and more used when studying complex scenes. In some cases, only the depolarization phenomenon, mostly due to scattering, proves of interest and estimating the degree of polarization (DOP) is sufficient. For a linearly polarized beam under normal incidence, when the materials in the scene exhibit neither birefringence nor dichroism, this comes down to an intensity measurement in two crossed directions. Nevertheless, the classical optical setup, consisting mainly of a rotating polarizer used as a polarization state analyzer, gives good-quality images, but cannot provide dynamic information. In this paper, we propose to use a ferroelectric liquid-crystal modulator in an imaging polarimeter. We demonstrate the use of this modulator as a polarization state analyzer for frame rates up to about 400 Hz. Provided a fast camera is used, we show that the polarimeter allows us to surpass the classical video frame rate. We propose a setup using a rapid CCD camera (up to 233 Hz). Images in DOP are of very high quality and hold their own against images obtained with a classical polarizer. With a faster camera, we were able to obtain images at up to 400 Hz.

Characterization and applications of a pure phase reflective liquid crystal spatial light modulator

We present the characterization of a pure phase reflective liquid crystal spatial light modulator. This modulator is electrically addressed and is based on nematic liquid crystal. Its performance in terms of frame rate, phase modulation versus gray level and wavefront distortion is experimentally evaluated. It is shown that after adding phase compensation and applying an appropriate Look-Up-Table, this spatial light modulator is an excellent candidate for applications such as dynamic diffractive optical elements, wavefront generation and dynamic zoom lens.

Target tracking correlator assisted by a snake-based optical segmentation method

Abstract To improve the efficiency of a target tracking correlator, we have developed a snake-based segmentation method of extracting the target from the reference window. This method uses the snake method in the spatial domain. The purpose is to darken the target background in order to avoid its disruptive influence on the tracking. Even if the contour does not fit the target perfectly, the reference window contains enough details for us to obtain a good correlation peak. With a new optimisation criterion, an optical image preprocessing and an iris diaphragm, the extraction is achieved with a negligible reaction time in comparison with the target tracking. A complete tracking correlator based on the joint transform architecture, including a holographic edge enhancement filter and a “snake”-based optical segmentation was constructed and tested successfully.

Filter implementation technique for multicriteria characterization of coding domains in the joint transform correlator

An improved method for implementing correlation filters in the joint transform correlator architecture is proposed. We derived the method from computer-generated holography techniques. It allows us to use any correlation filters, especially ones that provide an optimal trade-off between noise robustness, peak sharpness, and optical efficiency, with any spatial light modulator (SLM). This method also allows for an objective comparison of the performance of the coding domains of various SLMs.

Active contour segmentation by use of a multichannel incoherent optical correlator.

We describe an optoelectronic incoherent multichannel processor that is able to segment an object in a real image. The process is based on an active contour algorithm that has been transposed to optical signal processing to accelerate image processing. This implementation requires exact-valued correlations and thus opens attractive perspectives in terms of optical analog computation. Furthermore, this optical multichannel processor setup encourages incoherent processing with high-resolution images.

Implementation of optimal trade-off correlation filters with an optimized resolution technique

Several techniques can be used for the optical implementation of correlation filters onto spatial light modulators whose coding domain has a limited extent. Using the general framework of optimal trade-off filters, we compare them in the case of a 4f architecture and demonstrate that oversampling certain parts of the filters can improve performance.

Experimental implementation of a joint transform correlator using synthetic discriminant functions

Abstract A joint transform correlator (JTC) was implemented in the Optical Science Department at CERT. In order to ensure a rotational invariance, the reference image is replaced by a synthetic discriminant function (SDF) image displayed on an amplitude spatial light modulator (SLM). Numerical simulations are performed and then compared to experiments. We show that the pseudo-inverse SDP (PI-SDF) gives accurate experimental results. The training ability of the PI-SDF allows it to recognize the reference object unambiguously, even if the input view does not explicitly belong to the training set. Then, we propose to replace our amplitude device with a ternary phase and amplitude device, in order to display positive and negative images: this allows us to display any kind of SDF filters, e.g. OT-SDF or MINACE-SDF, which reduce the side lobes, with a good resistance to noise.

Alternative Model of a Subwavelength Diffractive Lens Proposed for PV Cells Applications

Recently, several approaches have been proposed to concentrate and to separate spectrum bands of sunlight using a planar design. Fresnel lenses are widely used in concentrating photovoltaic systems, to achieve the concentration of sunlight onto the surface of solar cells. A diffractive lens superimposed with a grating, in a single integrated optical device, has been proposed to obtain the sunlight spectrum splitting and beam concentration (SSBC) simultaneously. Here, we present an alternative approach to achieve SSBC functionality using only a diffractive lens. Because of the subwavelength structures used, a high efficiency for a broadband light spectrum is demonstrated through simulations. We propose a theoretical model for a subwavelength diffractive lens (SWDL), which has been elaborated with optimal parameters. The model proposed is suitable to be used for low concentration (1-10×) PV cell systems. The optical efficiency obtained is ~70% after separating apart of the sunlight spectrum (400-1100) nm into two bands (400-800) and (800-1100) nm. A new electromagnetic modeling tool based on a hybrid finite difference time domain angular spectrum method propagator has been used to analyze the performance of the proposed model. The proposed SWDL achieves the functionality of an SSBC over a small distance and can be integrated into a compact PV cells system.