t limited exclusively to the RIN spectrum. As a matter of fact, the same behavior is exhibited by the frequency noise (i.e., the pro- cess = Sq/St), whose power spectral density is given by [8] (5.9) Besides the presence of the laser characteristic resonance, the expression of the fre- quency noises spectrum allows also the other typical diode laser mechanism to be evidenced. As a matter of fact, it is made by two terms: the first is due to the phase of random emit- ted photons, and the second is proportional through r'on to a2 and represents the addi- tional phase noise due to the coupling between amplitude and phase through the carrier density [9]. The power spectral densities of the RIN and of the phase noise for a typical DFB laser emitting at 1550 nm are represented in Figures 5.7 [10] and 5.8 respectively. From the theory we have presented, it results that the same phenomenon causes both the excess RIN and the excess phase noise with respect to the quantum limit. Thus a relation- ship must exist between the RIN and the phase noise spectra. This relationship can be obtained by simplifying the denominator in the expressions of the two spectra and deriving an expression for the relation between them. This expression can be experimentally verified by passing from the phase noise to the linewidth that is one of the most commonly measured quantities in a laser. Telecommunication Networks RIN (dB/Hz) Bias current 19.5 mA 65.5 mA 112 mA 158 mA Frequency deviation from linewidth maximum (GHz) FIGURE 5.7 Power spectral density of the RIN for a typical DFB laser emitting at 1550nm. (After Aragon Photonics Application Note, Characterization of the main parameters of DFB using the BOSE. 2002, www.aragonphoton- ics.com [accessed: May 25, 2010].) Phase noise power spectrum (dBrad) Bias current 112 mA Frequency deviation from linewidth maximum (GHz) FIGURE 5.8 Power spectral density of the phase noise for a typical DFB laser emitting at 1550nm. (After Travagnin, M., J Opt. B. Quant. Semiclass. Opt., 2, L25, 2000.) A rigoro