as DWDM operations, narrow band width filtering is used and thus ASE-ASE beating occurs to a small extent only during the 0 bits. The Optical Noise Figure The figure of merit to quantify an amplifier's noise performance is called its optical noise figure, and it should be as low as possible. The noise figure allows one amplifier to be com- pared with another. It is dimensionless and defined as the ratio of the optical signal-to-noise ratios at its input and output: F = SNRin/SNRout (6.2) Given that the amplifier adds its own noise, F must be greater than unity. Indeed, it follows from quantum mechanical considerations (Desurvire, 1994; Bristiel, 2006; Haus, 1998) that the lowest noise figure is 2 (i.e., 3 dB). Such performance is an ideal case, known as the "quantum limit," and amplifier manufacturers always strive to design devices that come as close as possible to it. Chapter 6 Optical Fiber Transmission A semiclassical treatment of an optical amplifier, such as an EDFA, allows the optical noise figure to be quantified in terms of its gain, G = Pout/Pin. Assuming that the signal is narrowband, an optical filter of bandwidth BO (Hz) is placed before the detector to elimi- nate ASE-ASE beat noise; assuming that all of the photon numbers are reasonably large (Desurvire, 1994), F=(1/G).(1+PAsE/hvsBO) (6.3) PASE is the exiting amplified spontaneous emission power (watts) that is forward-propa- gating with respect to the signal of frequency, Vs, and it is measured within Bo, as defined by the optical filter; h is Planck's constant (6.63 X Hz ¹. The first term on the right of equation 6.3 of magnitude 1/G is due to shot noise, which arises because of the corpuscular nature of light. The number of photon arrivals fluctuates from one time interval to the next, and the effect is more pronounced as the signal power decreases (i.e., at lower photon numbers). The quantities G and PASE can be provided from amplifier models or measured in a laboratory. In all practical amplifiers G and PASE are wavelength dependent and SO F v