9A5 - Wide field active imaging

Abstract

In this paper we report the results of some experiments in active imaging-image processing in which the pictorial information is placed within the laser cavity. By the use of an active medium which provides high gain over a wide aperture, in conjunction with a highly degenerate conjugate resonator, we have been able to obtain diffraction limited resolution with an information content orders of magnitude greater than previously reported. The resonator used is the Flat-Field Conjugate Resonator (FFCR), which can support as many as 107modes of nearly equal Q when properly designed lenses are used. As active medium we used the pulsed hollow cathode Hg+laser. In the present FFCR, two plane mirrors are imaged one upon the other by means of two achromatic doublets (focal lengths about 200 mm) located between the ends of the discharge tube and the mirrors. Active imaging is accomplished by masking one of the mirrors and observing the pattern resulting on the other mirror. Masks used include pinholes, wire meshes, and photographic transparencies as well as patterns etched in thin metallic films that were used directly as mirrors. With the doublets working near f/10 , the resolution over a 15mm diameter field was greater than 100 lines/mm, which corresponds quite well to the observed resolution of the passive system. The resonator was thus capable of actively imaging photographic transparencies having more than 106bits. One characteristic of active imaging in this type of resonator is that all the modes have a large common active volume, and the suppression of some of the modes enhances the intensity of the others. Furthermore, the nonlinear behavior of a laser as an oscillator allows sharp discrimination between differing loss levels in the cavity. These effects have been observed, and their possible application to contrast enhancement and image dissection is discussed.