Fast cylindrical microlenses are an ideal solution for correcting astigmatism and beam ellipticity in laser diodes. Many laser diode applications require a diffraction limited non- astigmatic circular beam. The traditional way of accomplishing this is by use of an anamorphic prism pair in combination with a cylindrical lens. Through proper design it is possible to encompass the effects of these three elements in one microlens. Such an element is known as a virtual point source (VPS) microlens. The VPS microlens reduces the divergence of the fast- axis of a laser diode to that of the slow axis while at the same time forcing the fast and slow axis to appear to emanate from one point. Since a microlens has dimensions similar to those of multimode fibers it allows a laser diode to maintain its compact size while producing a diffraction limited circular wavefront. The design and implementation of the VPS microlens has been reported. A typical microlens has a working distance of 30 micrometers and requires sub-micron positioning accuracy as well as mounting techniques that can maintain this precision lens alignment over time. We address the issues of microlens characteristics, alignment tolerances, and bonding techniques.
We have investigated the effect on a laser diode's frequency of optical feedback from a short external cavity made from a precision cylindrical microlens collimator. Since the microlens collimates the laser output radiation to a diffraction limited beam, reflected light from the planar surfaces of the microlens is reimaged at the output facet, providing very efficient optical feedback into the laser cavity. Feedback from the external cavity can be used to control the longitudinal mode characteristics and emission frequency of the laser by varying the microlens position. Based on the results of these experiments we are developing a simple, compact, and robust tunable laser diode system that utilizes an orthogonal pair of collimating microlenses to form a short external cavity for longitudinal mode and frequency control. The resulting system will generate a circular, collimated beam of single frequency, tunable laser radiation.
This paper provides an overview of the design and operation of Lockheed's portable optical correlator. The design issues for a compact correlator are given along with the performance characteristics of the Fourier transform lens. The liquid crystal spatial light modulators are described and the system level description of the optical correlator given.
Future high performance miniature optical correlators will require specially designed Fourier Lenses. This paper presents optical design considerations and results for several small, high performance Fourier Lenses. These lenses are designed to match up with various spatial light modulators (SLMS) to utilize the full resolution of the SLMS. The miniature designs are compared in size and performance to optics used in the current LMSC Subcompact Hybrid Optical Correlator. The results presented illustrate that the high level of performance of our current correlator can be retained and extended in a miniature correlator, and that the size can be dramatically reduced. The new designs are optimized for use with spatial light modulators having various numbers of pixels and pixel spacings.
Optical design considerations and performance results are presented for two compact hybrid optical correlators and a novel, subcompact correlator model; all operate with two metal-oxide spatial light modulators (SLMs) at HeNe and diode laser wavelengths. The 4F optical path of the former two correlators approximates 25 in., while that of the subcompact unit requires less than 13 in. for a 4F-equivalent path. The results obtained demonstrate superior performance relative to available devices.
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