In a recent paper (E. Desurvire, 1999, Opt. Fiber Technol., vol. 5, no. 1, 82), we presented a new model for describing nonideal (or realistic) linear optical amplifiers through a three-dimensional quantum beamsplitter (QBS) representation. This quantum model unifies the well-known properties of the passive attenuator and the ideal lossless optical amplifier into a single comprehensive picture. While such a representation stems from the straightforward cascading of the two fundamental QBS types (i.e., attenuator followed by ideal amplifier), we have emphasized that the cascading can only be made in such order, while the reverse one (attenuation following ideal amplification) is not applicable. We have also shown that realistic optical amplifiers can be represented by discrete or infinite chains of attenuators/ amplifier sequences, but always in the loss/gain order. The infinite-chain representation raises a question about the relevance of a specific ordering. The purpose of these comments is to clarify the issue and complement the discussion. In particular, we introduce an "elementary noise figure" (ENF) parameter, corresponding to the noise figure of the QBS element; we show that the ENF can be made arbitrarily small (e.g., 10-50 mdB for k = 1000). Its incremental deviation from the nonphysical noiseless limit (F = 1) is shown to decrease in inverse proportion to the number of elements in the QBS chain, (C) 2000 Academic Press.
