Impulsive coding in optical free-space links: optimum choice of the receive filter and impact of a transmit booster amplifier

摘要

Abstract: A theory for the signal-to-noise ratio of optical direct detection receivers employing return-to-zero coding (and possibly optical preamplification) is developed. The results are valid for both signal-independent noise limited and signal-dependent noise limited receivers, as well as for arbitrary optical pulse shapes and receive filter characteristics, taking into account intersymbol interference. Even if the same receiver bandwidth is used, return-to-zero coding is seen to yield higher signal-to-noise ratio than nonreturn-to-zero coding. Asymptotic expressions for the signal-to-noise ratio for very high and very low receiver bandwidths show that the full sensitivity enhancement potential of return-to-zero coding is exhausted at fairly moderate duty cycles. A realistic example taking into account inter-symbol interference shows that a receiver sensitivity gain (compared to nonreturn-to-zero coding) of e.g. 3.2 dB can be obtained in a signal-independent noise limited receiver with a bandwidth of 80% of the data rate, using a duty cycle of 3. For the signal-independent noise limited case, the sensitivity enhancement potential depends on the receive filter characteristics; we provide a design rule for filters with high sensitivity enhancement potential. Further, we investigate the role of rare-earth doped booster amplifiers in impulsively coded communication links: It is shown that, due to the average power limitation of these devices, a less powerful booster amplifier as with nonreturn-to-zero coding can be employed if certain conditions regarding the data rate and the return-to-zero duty cycle are met. !14