Optical signal processing with spatial light modulators

Optical correlation is not the only part of optical information processing that benefits from the properties of spatial light modulators (SLM's). SLM's can display programmable images encoded in amplitude and phase, and enable many applications in image processing and dynamic holography. Starting from the measurement of the amplitude and phase modulation of SLM's, at the beginning for the needs of optical correlation systems, I have gradually switched to the study of original active lens systems and wavefront sensors.

Coding domains

From 1992, I have committed myself to the signal processing description of SLM's. I have especially discussed the use of liquid crystal televisions in optical information processing through the coding domains in amplitude and phase. I have discussed measurement methods for these screens [1, 5]. Indeed, the quality of the measurement of the coding domain has a direct influence on the performances that will be achieved by an optical system including SLM's. Together with Philippe Réfrégier, we have determined the transformation of the statistical characteristics of noise affecting an image that is caused by its display on a SLM [2].

Optical adaptive filtering of microwave signals

I participated in the analysis of optical systems for the adaptive filtering of microwave signals using a SLM [3,4]. My contribution to this work was limited to the analysis and modeling of the devices.

Active pupil

I proposed and demonstrated the original principle of an active pupil (a pupil plane is also called a Fourier plane in paraxial optical systems), in which a SLM, for instance a liquid crystal television, governs the transfer function of an optical system [5]. I used this principle for analog optical image processing operations, such as edge detection and high-pass filtering, and then the improvement of the resolution of two dimensional sensors by deconvolution of micro-scanned images using imaging system including an active pupil [7]. In the frame of the doctoral research of Dominique Delautre, who was interested in a new large field-of-view heterodyne detection technique, we used the active pupil system to simulate experimentally the effects of atmospheric turbulence and of various phase defects that degrade the heterodyne efficiency [6].


Fig. 1: Schematic of the principle of the active pupil system [5].

(a) (b) (c)

Fig. 2: From Ref. [5]; (a) Phase image displayed on the SLM (phase levels are represented by grey levels in the figure); (b) Image of a test pattern as observed in the focal plane of the imaging objective; (c) Same image but with the focus adjusted manually to show that the lens type image (b) has changed the global focal length with no perceptible image distortion. The focus can then be changed at will without any moving mechanical part.

Wavefront sensors

Based on a suggestion by Jean-Pierre Huignard, and with the help of two master degree level students, I developed a new wavefront sensor based on the principle of the Hartmann test. This sensor makes use of a SLM for the sequential sampling of the incident wavefront [8, 9] (Fig. 4). In comparison to other wavefront sensors such as Hartmann-Shack wavefront sensors, the dynamic range to sensitivity ratio is higher, but the acquisition speed is lower. The Hartmann wavefront scanner can be seen as a trade-off insisting more on precision than on operation rate. The device is patented.




Fig. 3: Example of a wavefront measured using the Hartmann wavefront scanner [8] ; (a) slopes measured along two directions; (b) wavefront reconstructed on the basis of Legendre polynomials.

References

  1. V. Laude, S. Mazé, P. Chavel, and Ph. Réfrégier, ``Amplitude and phase coding measurements of a liquid crystal television,'' Opt. Commun. 103, 33-38 (1993).
  2. Ph. Réfrégier and V. Laude, ``Spatial fluctuations of optical fields modulated with spatial light modulators and noisy input signals,'' J. Opt. Soc. Am. A 12, 1338-1345 (1995).
  3. O. Durand, D. Dolfi, V. Laude, J.-P. Huignard, and J. Chazelas, ``Optical architecture for adaptive filtering of microwave signals,'' Opt. Lett. 21, 803-805 (1996).
  4. D. Dolfi, J. Tabourel, O. Durand, V. Laude, J.-P. Huignard, and J. Chazelas, ``Optical architectures for programmable filtering and correlation of microwave signals,'' IEEE Trans. Microwave Theory Tech. MTT-45, 1467-1471 (1997).
  5. V. Laude, ``Twisted-nematic liquid crystal active lens,'' Opt. Commun. 153, 134-152 (1998).
  6. D. Delautre, S. Breugnot, and V. Laude, ``Measurement of the sensitivity of heterodyne detection to aberrations using a programmable liquid-crystal modulator,'' Opt. Commun. 160, 61-65 (1999).
  7. V. Laude and C. Dirson, ``Liquid-crystal active lens: application to image resolution enhancement,'' Opt. Commun. 163, 72-78 (1999).
  8. V. Laude, S. Olivier, C. Dirson, and J.-P. Huignard, ``Hartmann wavefront scanner,'' Opt. Lett. 24, 1796-1798 (1999).
  9. S. Olivier, V. Laude, and J.-P. Huignard, ``Liquid-crystal Hartmann wavefront scanner,'' Appl. Opt. 39, 3838-3846 (2000).