Christopher J. Solomon

Imaging a coherently illuminated object after double passage through a random screen

We show that the average intensity spectrum for an object illuminated and viewed through the same random screen contains diffraction-limited information on the Fourier modulus of the object amplitude. For physical clarity, the theory is first presented for a two-pinhole, Michelson aperture and then extended to include the nonredundant and full-aperture cases. Theoretical predictions are confirmed by detailed computer simulations. We propose and implement a method for the recovery of the Fourier modulus of the object.

Optimal wavefront reconstruction from a Shack-Hartmann sensor by use of a Bayesian algorithm

An optimal algorithm has been derived for the reconstruction of random wavefronts using information on local slopes obtained with Hartmann-type sensors. The approach is based on calculation of the Bayesian posterior mean of the distribution. The accuracy of the method has been investigated via derivation of the error covariance matrix and subsequent calculation of the residual mean square error for waves which have propagated through the turbulent atmosphere. The dependence of the accuracy on the spatial correlation scale of the wavefront, the number of sensor channels and their signal-to-noise ratio is also discussed.

Observation of the enhancement of coherence by backscattering through turbulence

Abstract We present results of an experiment in which a plane mirror is coherently illuminated and viewed through a turbulent cell. Using a two-pinhole aperture in the pupil plane for both illumination and viewing, we have obtained time-averaged images containing fringes. This gives qualitative confirmation of theoretical predictions claiming that the enhanced back-scattering effect may be used to image deterministic objects.

Double passage imaging through a random screen using a non-redundant aperture

Abstract When a deterministic object situated in the near or far-field is coherently illuminated and viewed through the same random screen, the time-averaged image contains diffraction-limited information. We demonstrate that the use of a non-redundant aperture considerably simplifies the retrieval of this information. The effects of the number of sub-apertures, finite sub-aperture size and finite exposure time on the imaging process are discussed. The optimal procedure for recovery of object spatial frequencies is shown to be sequential, using a two pinhole or Michelson-Fizeau aperture. Computer-simulated reconstructions of simple object distributions are presented.

Use of polarisation in double passage imaging through a random screen

Abstract It has recently been proposed that a deterministic object which is obscured by a random screen may be imaged by the double passage method. This essentially involves illuminating and viewing the object through a non-redundant aperture and time averaging over the statistics of the random screen, the time-averaged image thereby yielding the Fourier modulus of the object. We propose a simple method of obtaining this information without time averaging by manipulating the polarisation states of the illuminating and detected light.

USE OF POLARIZATION IN INTERFEROMETRY AFTER DOUBLE PASSAGE THROUGH TURBULENCE

We report what are to our knowledge the first experimental results of coherence enhancement that use polarization to separate coherent and incoherent paths.

Imaging a coherently illuminated object through a random screen by using a dilute aperture

We consider the properties of the average-energy spectrum of an object that is coherently illuminated and then viewed after it passes through a random screen. We show that use of a dilute aperture permits the retrieval of diffraction-limited information for a statistically varying object. The factors influencing the accuracy of this procedure are discussed. Results of a computer simulation that are consistent with our theoretical predictions are presented.

Use of double passage and spatial filtering in imaging through turbulence

Abstract It has been predicted theoretically that the combination of spatial filtering together with double-passage illumination and viewing of a deterministic object leads to an improvement in image quality. The results of experimental investigations of this effect are presented.

COHERENT IMAGING THROUGH A RANDOM SCREEN - AN INTERFEROMETRIC APPROACH

Recent work on imaging coherently illuminated objects through a time-varying random screen has demonstrated the possibility of recovering the Fourier modulus of the object spectrum. We suggest an approach which, although limited to restricted conditions, enables both the modulus and the phase of the object to be recovered from the time-averaged image spectrum.

Double-passage imaging through turbulence

Recent progress in the understanding of imaging when an object is both illuminated and viewed through the same random screen is reviewed, with particular emphasis on the use of non-redundant apertures.