ween the reflecting surfaces (n). The phase difference between each succeeding reflection is given by 8: 5=(20) (4.92) If both surfaces have a reflectance R, the transmittance function of the etalon is given by the following: (4.93) where F=R/(1 - R)2 is the coefficient of finesse. Telecommunication Networks FIGURE 4.45 Periodic transmission of an etalon for different values of the finesse. Wavelength The transmission of an etalon as a function of wavelength is plotted in Figure 4.45 for different values of the finesse coefficient. A high-finesse etalon shows sharper peaks and lower transmission minima than a low-finesse etalon. The etalon is probably the most used optical filter and many types of etalons exist for different applications. Air-spaced etalons have fused silica substrates and an air gap between the cavity mir- rors; the outside-facets are antireflection coatings to prevent extraneous interference pat- terns from forming. Spacers determine the parallelism of the mirrors and the etalon's FSR. Solid state etalons typically have fused silica substrates and a solid material, transpar- ent at the desired wavelengths, to form the etalon body. Dielectric (or, rarely, metallic) coatings provide the reflectivity necessary for the required finesse. The faces are ground, polished, and figured typically to better than 1/100 flatness to achieve the desired control of the transfer function. Solid state etalons cannot be tuned by moving the mirrors, but tuning can be achieved with other means, for example, changing the material refraction index. Both air spaced and solid etalons exist in miniaturized versions for telecommunication applications. To overcome the limit of the thinness of a solid etalon and yet retain its mechanical strength, Deposited etalons exist, realized by means of layers deposited on a substrate. 4.4.2.2 Mach Zehnder Interferometer In an integrated Mach Zehnder interferometer (MZI) [16], a splitter splits the incoming wave into two waveguides and a combiner recombines the signals at the outputs 