amplifiers, FRAs, and SOAs-fall into this category (Srivastava & Sun, 2002; Dutta, 2003; Kasamatsu, Yano, & Ono, 2002; Zimmerman & Spiekman, 2004; Torres & Guzmán, 2007; Aozasa, 2007; Morito et al., 2003). Arguments based on the Heisenberg uncertainty principle show that phase-insensitive amplifiers always add noise to a signal (Desurvire, 1994). Therefore, when a noisy signal enters such an amplifier, three main things happen: The incident signal experiences gain, G. The incident noise also experiences gain, G. The amplifier adds some noise of its own. Consequently, the amplifier reduces the signal-to-noise ratio (SNR) of any signals that pass through it. The noise produced by the main optical amplifier types is continuous wave and broadband (and therefore incoherent). In EDFAs, EDWAs, TDFAs, and SOAs this noise is due to amplified spontaneous emission, while in fiber Raman amplifiers its cause is ampli- fied spontaneous scattering. To simplify the terminology, we use the expression "Amplified Spontaneous Emission" (ASE) in all instances. To a good approximation, the ASE produced by all fiber amplifiers is unpolarized, but there can be over 1 dB of polarization dependence from SOAs unless preventive measures are taken. The ASE is present during both the logical 1 and logical 0 bits and is carried to the detector along with the signal (Olsson, 1989). Normally, there is an optical filter at the detector (such as the de-multiplexer in a WDM system), and during the 1s it allows only the modulated signal plus a small spectral slice of the ASE to enter. In such circumstances, there is a coherent superposition of the signal with the ASE at the (square-law) detector and the dominant noise component is due to what is called signal-ASE beat noise. Should an optical filter not be present, there will be additional noise components because of the coherent superposition of every infinitesimal spectral slice of the ASE with every other one-the resulting term is ASE-ASE beat noise. However, in many practical situa- tions, such 