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Chapter 7:
When asked to describe the most important detector used in laser systems, most people will not mention the human eye (Fig. 7.1). With a dynamic range still unmatched by technology, the human eye is also the most common detector in or near the system. Laser safety officers spend many hours training employees on how to protect their eyes from laser damage, as the optical gain of the eye can increase the irradiance on the retina by a factor of 100,000 in comparison with that incident on the cornea, easily resulting in localized damage or even complete blindness using surprisingly low-power or low-energy lasers. While the detector technologies developed to date are also susceptible to damage from low-power lasers, the purpose of this chapter is to review the options available for photon detection, not damage protection (aka laser hardening). These options include a choice of detector materials, detector types, detector geometry, and FPA technologies. For example, a laserscanning microscope used to obtain images of biomedical samples typically scans a single-pixel, high-sensitivity detector to collect power over the FOV of the specimen. A flash laser radar system, on the other hand, relies on an FPA to collect both power and time-of-flight information for each pixel in the array. One of the most basic choices for detector selection, then, is single-pixel versus FPA geometry - a choice that is influenced as much by the detector options as it is by the other subsystems (laser, optics, and beam scanning) that must be included in the overall system design. In this chapter, we first review the available single-pixel types for photon detection: PIN photodiodes (Section 7.1), avalanche photodiodes (Section 7.2), and photomultiplier tubes (Section 7.3). In Section 7.4, we then look at the unique aspects of incorporating PIN and avalanche photodiodes into laser-system FPAs. Section 7.5 closes the chapter with a review of single-pixel and FPA noise limitations, photon sensitivity, and detector selection.
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