Optical Bridge

The concept of an optical bridge circuit, like a classical Wheatstone bridge, is employed in many sensors and the optical sensor is a good example of that. One such use shown in.

Four-quadrant photodetector: (A) focusing an object on the sensor; (B) connection of the sensing elements to difference amplifiers; (C) sensor in a packaging. (From Advanced Photonix, Inc. Camarillo, CA.)

Afour-quadrant photodetector consists of four light detectors connected in a bridgelike circuit. The object must have an optical contrast against the background. Consider a positioning system of a spacecraft (Fig.1). An image of the Sun or any other sufficiently bright object is focused by an optical system (a telescope) on a four-quadrant photodetector. The opposite parts of the detector are connected to the corresponding inputs of the difference amplifiers (Fig. 1). Each amplifier produces the output signal proportional to a displacement of the image from the optical center of the sensor along a corresponding axis. When the image is perfectly centered, both amplifiers produce zero outputs. This may happen only when the optical axis of the telescope passes through the object.