chapter 9, Consideration of Optics and Interferometer Alignment

Excerpt

In this chapter, the optical system's throughput for FT spectrometers is discussed first, and with the aim to help the designer select the proper size and “speed” of the focusing and the collimating optics, the diameter of the aperture, and the area of the detector element. Lenses and mirrors, the two types of focusing and collimating optics, are also briefly discussed. Finally, the alignment procedure for the interferometer is explained.

9.1 System Throughput

Optical throughput was defined in Sec. 1.3. It is revisited here and considered from an overall system perspective. In FT spectrometers, optical throughput is limited by either the interferometer's collimating optics or the detector's focusing optics.

In the detector subsystem, throughput is limited by the area of the detector and the “speed,” quantified by the f/#* (f-number) of the detector optics. Generally, the light is collected using a fast detector lens (f/# ≈ 1) in order to “squeeze” the beam into the smallest detector possible because smaller detectors have better noise characteristics and are usually less expensive. The throughput of the detector optics can then be calculated to be