Optical Communications, 2nd Edition

2.5 OPTICAL FILTERS

In most space communication receivers, optical filters are used in the front end during the light focusing, as was indicated in Figure 1.4. An optical filter is a material or element placed in the path of the optical beam to control the transmissivity of various wavelengths. As with any front end filter used in communications, its objective is to remove as much unwanted light as possible during the reception of the source light field. Thus optical filters are designed with properly selected passbands (high transmissivity) and reject bands (low transmissivity) with respect to optical wavelengths. In fiber systems, receiver filters are used primarily in frequency-division multiplexed systems, where multiple signals are separated over a wavelength band and transmitted simultaneously over the fiber. The filters are used to remove the undesired portions of the signal spectrum, as well as the noise. These systems are considered in Chapter 8.

The basic types of optical filters are the absorption filters, interference filters, birefringent filters, and atomic resonance filters [8]. Absorption filters are generally coatings of material connected to or painted on lens or photodetector surfaces to absorb (or scatter or reflect) incident light at certain wavelengths while passing others. The filtering characteristics depend on the type of material and thickness of coating. Figure 2.12 shows some basic filter materials and their transmissivity. Note that the filter bands ...