Quadrant Photodetector Research Articles

Reflection micro-Fresnel lenses and their use in an integrated focus sensor

Blazed reflection micro-Fresnel lenses and their use in an integrated focus sensor are proposed. Theoretical analysis indicates that the optical characteristics of reflection Fresnel lenses can be improved compared with a conventional transmission micro-Fresnel lens. These reflection Fresnel lenses were fabricated using electron-beam lithography and exhibited the diffraction-limited focusing characteristics with 71% high efficiency. The focus sensor has a folded optical path and includes a beam splitter integrated with thin film components, such as a reflection elliptical Fresnel lens and a quadrant photodetector. The reflection elliptical Fresnel lens in the focus sensor exhibiting excellent astigmatic characteristics agreed with the theoretical results, and the focus error signal was detected. This sensor can be developed as the optical head of an optical disk system.

Stability analysis of direct-detection cooperative optical beam tracking

The system under consideration is a cooperative spatial tracking system between two stations for laser beam communications, using a quadrant photodetector at each station. After determining the equilibrium points of the cooperative system for the case of periodical relative motion, the authors concentrate on the stability of the transient behavior around the equilibrium points for the case of uniform angular relative motion. This case corresponds to an assumption that the steady-state motion is slow with respect to transient phenomena, and is applicable to currently foreseen intersatellite and deep-space optical communications. The analysis is aimed at determining the combined effect of the basic system parameters, such as propagation delay time, tracking loop gains, DC servomotors time constant, and point-ahead velocity error, on the stability and the transient behavior of the overall tracking system. The stability conditions and the transient response around the steady-state trajectory provide a tool for evaluating the consistency of the design parameters for a given propagation delay. >

Coordinate measurement of the energetic center of multimodal light beams by a quadrant photodetector

An extensive class of applied optics problems associated with the probing of optically inhomogeneous media, optical location, etc., as well as laser investigations and development requires knowledge of the spatially energetic characteristics of light beams. The most important is the location of the energetic center (EC) of the beam r 0 that characterizes the beam direction as a whole [i]. Existing determinations of the EC reduce to fixing certain characteristic points of the energy distribution function f(~) Over the beam transverse section (the extrema of the function or its derivatives, points corresponding to a given intensity level relative to the maximum, etc.), most often with subsequent mathematical processing. In addition, the presence of several determinations leaves an undesirable arbitrari~ ness in the description of the different beams, in the case of irregular transverse distributions characterized by asymmetry, the presence of many extrem~ and significant variations in the intensity over the beam section, one of the definitions [I] is unique. The definition: of the EC as the "center of gravity" of the energy distribution [2]

Optical video disks with undulating tracks

The signal components of a video signal (luminance, color, and sound) are modulated on a main carrier and several subcarriers and then recorded on the video master disk. Apart from the signal distortion that can arise during master and disk manufacture, the optical readout of the disk also yields a nonlinear transfer of the signal. The result of nonlinearities is intermodulation between signal components. Intermodulation products affect the quality of the final TV picture. In this paper a method is described which reduces the contribution of the optical readout system to the intermodulation. An optical coding is introduced such that two signal components hardly influence one another. The spacing of the pits in the track direction carries the luminance information, while the undulation of the track carries the color or sound information. A quadrant photodetector positioned in the far field of the video disk restores the luminance and color or sound bands with a very low amount of intermodulation.