3 Signal power distribution along a line with EDFAs and with hybrid amplifiers. Wavelength (nm) FIGURE 4.44 Spectral distribution of SN0 and NF of a high-performance hybrid amplifier. (After Optiwave White Papers. Hybrid amplifiers [Online], 2009, http://www.optiwave.com/literature/article_whitepaper.htm (Accessed: May 15, 2010).) noise ratio at the amplifier output if the input signal is in a coherent state. It is not difficult to compare these figures with an equivalent two stages EDFA amplifier and recognize the advantage of the hybrid solution in having a small overall noise factor. 4.4 Optical Filters The concept of filter in optics is almost the same as that in electronics: a passive and linear system used generally to admit into a system only the signals in a determined bandwidth. There are a great number of optical filters used for a wide variety of applications [16]. Here we will briefly review the most important filters used in telecommunication equipment. Optical filter can be classified in two classes: periodic and nonperiodic filters. Telecommunication Networks In some filters, the transfer function, or the shape of the filter passband, repeats itself after a certain period. The period of such devices is called free spectral range (FSR). The FSR usually depends on various physical parameters in the device, such as cavity or waveguide lengths. The finesse of a periodic filter is a measure of the width of the transfer function. It is the ratio of FSR to the filter 3 dB bandwidth. For all optical filters that are placed on the signal path, the attenuation is a fundamental characteristic, besides dispersion and passband shape. 4.4.1 Fixed Wavelength Optical Filters 4.4.1.1 Grating Filters One implementation of a fixed filter is the diffraction grating [16]. A bulk diffraction grat- ing is essentially a flat layer of transparent material (e.g., glass or plastic) with a row of parallel grooves cut into it. The grating separates light into its wavelengt ths component by reflecting light incident with the gr