An optoelectronic device based on interferometry in a resonant microcavity has been proposed for use in measuring the direction of arrival of a circularly polarized beam of light from a beacon laser. In comparison with prior optoelectronic direction sensors, the proposed device would be simple, compact, and lightweight, in that it would contain no moving parts and no imaging optics and could be fabricated from semiconductor crystals. Moreover, it should be possible to integrate readout electronic circuits directly onto the device; these circuits would perform all needed intermediate signal processing to generate electrical output indicative of the angle of incidence of the laser beam on the device. In one potential application, the beacon would be located at one end of a free-space optical-communication link and would be used as a target for aiming a transmitting telescope located at the other end of the link. In another potential application, the beacon could be used similarly as a target for measuring angles precisely in land surveying.
The device, denoted an optical pointer on a chip (OPOC), would be divided into four quadrants, each of which would contain a wave plate in series with a blazed transmission-type diffraction grating, a Fabry-Perot etalon serving as a resonator, and a photodetector (see figure). Each wave plate would transform the circular polarization of the laser beam into the linear polarization optimum for the blaze of the grating in its quadrant. A complete explanation of the principle of operation would entail a great deal of mathematical derivation and, as such, would greatly exceed the space available for this article. In summary:
- The device would exploit the sharp dependence of the transmissivity of a Fabry-Perot resonator on the internal angle (b) of refraction or diffraction.
- The spatial period of the diffraction grating would be chosen so that normally incident light would be diffracted to a desired order inside the resonator at a desired angle b.
- The blazes in the opposite quadrants of the grating would oppose each other in such a manner as to select opposite diffraction orders (e.g., the +jth and the -jth) in opposite quadrants.
- The net result of the aforementioned features is that the currents generated in the photodetectors in opposite quadrants would respond antisymmetrically to a deviation of the laser beam from normal incidence. To a first-order approximation for an angle that is a small fraction of a radian, the angle of incidence in a plane containing the outer corners of quadrants A and D would be proportional to
where IA and ID are the photodetector currents for quadrants A and D, respectively. Similarly, the angle of incidence in the plane containing the corners of quadrants B and C would be proportional to
Several design parameters [e.g., the thickness of the Fabry-Perot etalon, b, the reflectivity (preferably close to 1) of the mirror layers on the etalon, the laser wavelength, the indices of refraction of the device materials] affect the factor of proportionality and other aspects of the angular response of the device. It has been estimated that resolution of the order of tens of nanoradians could be obtained by suitable choice of design parameters and execution of design by mass-production techniques now in use in the semiconductor industry.
This work was done by John Sandusky of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to Intellectual Property group, JPL, Mail Stop 202-233, 4800 Oak Grove Drive, Pasadena, CA 91109, (818) 354-2240. Refer to NPO-21117.