DETAILS

Incoherent v. coherent

Optical computing using a coherent source is clearly the simplest to compute and understand, but it has many practical drawbacks. If one uses a transparency to create an input pattern, our coherency is affected by changes in thickness in the film. Also, any imperfections or dust on the optical components will create an unwanted interference pattern due to scattering effects. Incoherent processing, on the other hand, takes into account only intensity, not phase, so just about anything, from a television to a piece of paper can act as the input source. Also, imperfections in the system have little effect on incoherent light due to its redundant, multichannel nature [6]. On the other hand, incoherent computing can never record phase information, unlike coherent optical computing.

Filter fabrication

For coherent processing, both amplitude and phase information can be stored on a film, similar to the theory behind a hologram. The Mach-Zender interferometer can be used to overlay a reference signal and our desired impulse response onto a film which stores both phase and the amplitude.

With the Rayleigh Interferometer, the reference signal is focused through the aperture one focal length in front of lens L3. This is then collimated by lens L3, and thus overlayed with the desired impulse response onto the film. One expense of this setup is the elimination of spatial frequencies of the reference signal [4].

A better way of performing this filtering process is through the Vander Lugt filter [1]. This setup produces the crosscorrelation of the input (f) with our reference filter (h) at the output [1]. To do so, we must take our collimated light source and have a portion of it strike the reference mask, while the rest of it is directed to our photoresist film (see Figure 3). The light that passes through the reference mask is Fourier transformed by a lens L₂ and focused onto the same film [1]. This reference light will only appear when the spectral lines of the input and the reference match. The output plane will display the source at the center, but as outlined in Goodman, the convolution and crosscorrelation of the input and reference will appear off-axis [1].

Filter fabrication for simple incoherent processing can be made by either a film or a simple transparency. By placing a CCD camera at the Fourier transform plane of our system with the desired pattern as the source, the Fourier transform can easily be captured and printed onto a transparency with a laser printer.