DETERMINATION OF THE NOISE FACTOR OF A RAMAN AMPLIFIER

To set up a soliton optical-fiber transmission system it is necessary to ensure that the amplitude of the transmitted signals remains unchan ged. This problem can be successfully solved by using an amplifier bas ed on the Raman effect in the fiber [1]. Moreover, amplifiers operatin g on this principle can also be used in general-purpose communications lines in order to increase the length of the regeneration section [2] . Many characteristics of Raman amplifiers have been investigated in d etail at the present time. In [3], for example, the Raman gain and rel ated parameters were considered. Various mechanisms of Raman amplifica tion on two Stokes components were analyzed in [4, 5]. Analytical expr essions were also given there describing the distortions of the transm itted signals. An estimate of the noise power at the output of Raman a mplifiers is given in [6]. For many practical problems it is the noise factor of the amplifier, namely, NF = (SNR(in)/(SNR(out)), that is of interest, since it can be used to estimate the change in the signal-t o-noise ratio (SNR) at the output of the amplifier for Raman amplifica tion. Approximate analytic expressions are obtained below which descri be the change in the pumping wave and the amplified optical radiation along the fiber, taking the noise into account. These expressions are used to determine the noise factor of a Raman amplifier.