Optically preamplified receiver with low quantum limit

Summary form only given. We clarify that the sensitivity of direct detection optically preamplified receivers using on/off keying can never be lower than the well-known quantum limit of 38.4 photons/bit, achievable with a matched optical filter and no post-detection filtering only. In this Comment, we (i) clarify that the (standard) quantum limit of 38.4 photons/bit cannot be beaten by any receiver of this type, and (ii) give the correct results for the receiver considered by Tafur in ibid., vol. 35, p. 1182, 1999. The following reasoning can be applied to prove that no direct detection optically preamplified receiver can perform better than 38.4 photons/bit: the quantum limit - the ideal performance for a given class of optical receivers and a given modulation format is defined as the minimum number of photons per bit required on average at the receiver input to obtain a certain bit error probability (BEP)