In order to validate the theoretical work and computer simulations, a simple laser speckle experiment was performed. The polarimetric phase retrieval algorithm successfully recovered a three bar object from a series of noisy autocorrelations formed from collected laser speckle images.
The laboratory experiment was conducted with available laboratory hardware. The laboratory experiment is not completely representative of the (proposed) large- scale system designed to recover remote satellite images; however, it does serve to reinforce the theory and development presented in this research. The choice of labo- ratory hardware was purely out of convenience and availability. Much improvement in experimental performance is available via hardware and experimental design. Even so, the chosen laboratory hardware does perform reasonably similar in function to the proposed sensor system.
A traditional charge-coupled device (CCD) camera and polarizing film was used to observe two channels of data from a coherently illuminated object. For this lab-
oratory experiment, the hardware configuration is simplified by using a continuous wave (CW) source vice a pulsed laser source. Sufficient light level is achieved by appropriate integration time at the camera.
The laboratory setup is depicted in Fig. 6.7. The test sensor consists of a Pho- tometrics Cascade 512B camera without a lens, with removable polarization analyzer placed in front of the camera aperture. The camera is an electron-multiplying charge- coupled device sensor. The camera array is 512 × 512 pixels with a 16µm pitch. A spatially coherent source at 630 nm was used to back illuminate a target set. A laser line filter at 630 nm was inserted at the camera aperture to minimize background light.
Figure 6.7: Diagram of Laboratory Setup
The target set consists of a glass plate completely opaque where no object exists and transparent where the object exists. The experimental object consisted of three identically sized bars. To emulate polarization effects, the side opposite illumination of two of the bars were covered with polarizing film aligned to the same axis. To emulate random surface roughness, a randomizing phase screen was placed between the source and the bar target consisting of highly fibrous, white paper. The paper phase screen was moved for each collection frame to emulate random phase perturbations and produce statistically independent laser speckle images.
The propagation path included a 0.5m lens at the target plane to emulate far- field (or Fraunhofer) propagation to the camera array plane. Laser power was not
a consideration as the source was placed near the target plane. Camera integration time was selected after several preliminary tests and then held constant throughout the experiment.
For extremely remote sensing such as imaging of space-borne objects, the over- all path length is very large compared to distance where atmosphere turbulence is encountered enabling us to consider the atmospheric turbulence as a single, uniformly distributed phase screen. In this research, a layered atmosphere and scintillation are not considered. Experimentally, a small path length is used to ensure only uniform atmosphere is encountered.
In an operational system with LADAR backscatter, the target geometry, sur- face roughness and propagation distances typically produce the desired laser speckle effects. For the laboratory experiment, the paper phase screen enabled experimen- tation using back illumination and simplified the overall experiment. See Ref. [44] for experimentation with a backscatter setup. The paper phase screen produced low light-levels where read noise dominates the detection process, though sufficient for the experiment. However, the paper did exhibit spatially dependent surface roughness for the spatial sampling size produced by the camera. The correlography technique assumes spatially independent surface roughness (see Sec. 1.2.1). To overcome the effects of spatially dependent surface roughness and low-light levels, each 512 × 512 laser speckle image was segmented into 16, 128 × 128 laser speckle images. Each 128 × 128 laser speckle image contains the statistical nature of the target and can be processed independently. Target spatial resolution is lost but SNR is improved by a factor of four and spatially independent surface roughness is gained due to coarser sampling in the target plane. In order to minimize reflections from lab equipment and background light, the propagation path was enclosed in a light baffle. An image of the back illuminated target taken with the test camera (with lens and without polarizer) of the bar target is shown in Fig. 6.8. The image depicts the bright center bar and the two side bars with reduced brightness due to the effects of the polarizing film.
Figure 6.8: Image of Back Illuminated Bar Target Set
Figure 6.9 depicts the recovered image using the polarimetric phase retrieval algorithm. The recovered image was produced after 28 iterations. The three bar target set is clearly depicted with the side bars reduced in intensity compared to the center bar. Figure 6.10 depicts the recovered image using the single-channel algorithm developed by Schulz and Snyder [38]. For visual comparison, this image was 28 iterations and using the same starting guess as the two-channel solution. For the single-channel, Schulz-Snyder algorithm and the same iteration number, three bars are clearly discernable; however, the bar shape is more rounded and less defined compared to the two-channel polarimetric algorithm.
Figure 6.9: Recovered Image From Experimental Data; Two-Channel Algorithm, 28 Iterations
Figure 6.10: Recovered Image From Experimental Data; Single-Channel Algorithm, 28 Iterations