Quad-cells and position-sensitive detectors play an important role in measuring optical beam deflection as might be encountered in image stabilization and active or adaptive optics systems. These are deceptively simple devices that can be constructed in several ways using discrete components or camera systems. This chapter will consider several different systems but will be predominantly concerned with the discrete components for the optically active photodiode-based subsystems. This will keep the complexity to a minimum and allow us to keep our focus on the electronics rather than the processing of the information from these devices.
Position-sensitive detectors and quad-cells can be constructed from discrete semiconductor photodiodes, and both are photosensitive. Both devices are regularly used as the sensor of a feedback-controlled positioning system that can be used to position a beam or to keep a beam of light focused on a single point.
In this chapter we will be concentrating on the electrical configuration, signal amplification, and filtering of quad-cells and lateral position sensitive detectors and how to process their outputs into a useable form using the analog electronics that we have been working with. In general, analog electronics allows us to keep as much of the signal information as possible while performing mathematical operations as fast as possible. These features are often highly desirable in high-speed, low-light optical systems, making them a good choice for many applications.
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