304 Characterization of Optical Devices optical power while having high peak optical powers at selected frequencies: first, the EDFA is usually limited by its total optical power; if finesse of the FP filter is F, the peak spectral density at peak transmission frequencies can be found as r FP ¼ rF =p, where r is the uniform power spectral density without FP filter. Sec- ond, a photodetector usually has limitations on the total input optical power to avoid saturation and damaging. If the free spectral range of the FP filter is FSR, the mixing between various discrete optical frequency components will generate a series of discrete RF frequency components at the photodetector with the fre- quency interval of FSR. In this application, the FSR is the actual sampling interval in the frequency domain. The RF spectral density measured at the spectrum analyzer can be approxi- mated as [17] S spÀsp ðf Þ % R 2 ð f ÞP 2 F X i B 0 p k 1 þ 1 f À k Á FSR Dv 2 ð3:3:10Þ where Dv is the FWHM width of the FP filter transmission peaks and B 0 is the optical bandwidth of each FP transmission peak. Compare Equation 3.3.10 with Equation 3.3.9; for the same total input optical power P i to the photodetec- tor, the peak RF spectral density is increased by F/p. For a typical fiber FP filter