 is typically well below 1 nm, depends on both the length and the strength of the refractive index modulation. The narrowest bandwidth values are obtained for long gratings with weak index modulation. Large bandwidths may be achieved not only with short and strong gratings, but also with ape- riodic designs. 4.4.1.3 Thin-Film Interference Filters Thin-film filters offer another approach for fixed optical filtering [16]. These filters are simi- lar to fiber Bragg grating devices with the exception that they are fabricated by depositing alternating layers of low-index and high-index materials onto a substrate layer. Thin-film filter technology allows the shape of the filter to be designed on the system needs since almost every pass-band shape can be realized via deposition of a sufficiently high number of layers. The main limitation is on the filter bandwidth that cannot be too tight. Typical values of the thin-film filter bandwidth are 1 nm at a wavelength of 1.5 um. 4.4.2 Tunable Optical Filters 4.4.2.1 Etalon The etalon [16], or Fabry-Perot filter, consists of a single cavity formed by two paral- lel mirrors. Light from an input fiber enters the cavity and reflects a number of times between the mirrors. By adjusting the distance between the mirrors, the cavity resonates on the desired wavelength that passes through the cavity, while the other wavelengths are reflected. The varying transmission function of an etalon is caused by interference between the multiple reflections of light between the two reflecting surfaces. Constructive interference occurs if the transmitted beams are in phase, and this corresponds to a high-transmission peak of the etalon. If the transmitted beams are out-of-phase, destructive interference occurs and this corresponds to a transmission minimum. Whether the multiply-reflected beams are in-phase or not depends on the wavelength (2) of the light (in vacuum), the angle the light travels through the etalon (0), the thickness of the etalon (l), and the refrac- tive index of the material bet