Dependence of optical amplifier noise figure on relative-intensity-noise

The noise figure of an optical amplifier is given by F=2n_sp (n_sp⩾1 is the spontaneous emission factor) when the dominant noise is signal-spontaneous beat noise. A general expression for noise figure is derived, considering (in addition to the signal-spontaneous beat noise) the effects of spontaneous-spontaneous noise, relative intensity noise (RIN), receiver shot noise, receiver thermal noise, and the interaction of these terms. When the amplifier gain is sufficiently large that shot and thermal noise may be neglected, it is shown that there is an optimum input power to minimize F. The normalized minimum noise figure ratio, F_min/2n_sp, is a function only of the product (RIN)B_o, where B_o is the effective bandwidth of the optical amplifier. For the noise figure not to be significantly degraded from the signal-spontaneous noise limit of 2n_sp, it is required that (RIN)B_o≪1