Return-to-zero coding for optical satellite communication

Research project P 13998 Return-to-zero coding for optical satellite communication Peter J. WINZER 24.01.2000 Highly reliable optical communication links between satellites at data rates of several form the basis of future global broadband communication networks, owing primarily communication bandwidth and the low beam divergence found at optical constraints play an extremely important role in space, the design of receivers of utmost sensitivity is required - with the additional constraint of a reliable and economic design that can qualified with reasonable effort. Recent theoretical investigations have shown that on/off keying (OOK) with return-to-zero (RZ) coding represents a promising modulation scheme for use in optical space communication systems; there are strong indications that, non return-to zero coded OOK, RZ coding yields a sensitivity improvement of several dB, even if the receiver is only designed with the low bandwidth as required by NRZ. The archievable sensitivity gain should be exhausted at fairly moderate RZ duty cycles of about four already, which makes the concept particularly attractive from a technological point of view. Within this project, the sensitivity gain achievable by RZ coding that has been predicted using a comparatively simple analytical model shall be investigated by means of computer simulations, taking into account inter-symbol interference (ISI) and arbitrary ratios of signal-dependent and signal-independent noise. The simulations shall be accompanied and supplemented by an experimental verification using an optically preamplified receiver operated at a wavelength of 1.55 micron and at a data rate of 2.5Gbit/s. The usefulness of conventional, commercially available NRZ clock and data regenerators shall be investigated, and the data rate up to which RZ coding can be advantageously employed in practice shall be determined.

Optical communication links between satellites at data rates of several Gbit/s will form the basis of future global broadband communication networks. This owes primarily to the high communication bandwidth and the low beam divergence found at optical frequencies. One problem associated with optical intersatellite links is the generation of sufficient optical power levels at the transmitter in order to close the link: Neither can the signal be amplified between the transmitter and receiver (as is done in terrestrial fiber-optic links), nor are prime power resources abundantly available in space. Therefore emphasis has to be placed on designing receivers of as high a sensitivity as possible - with the additional constraint of a reliable and economic design that can be space qualified with reasonable effort. Optically preamplified direct detection with on/off-keying (OOK) modulation presents an optimum format for near-term realisation of optical intersatellite links. At the beginning of the project there were some indications that an appreciable sensitivity improvement can be obtained if the transmitter generates OOK with return-to-zero (RZ) coding. The RZ format would match nicely with one of the presumed properties of optical booster amplifiers to be implemented in the transmitter terminals, that is average-power limitation. Therefore we first studied the dynamic behavior of erbium doped fiber amplifiers (EDFAs). as they are encountered in transmitters of free-space laser communication terminals. We treated the gain dynamics of an EDFA theoretically and demonstrated that the amplifier is indeed average-power limited, provided that the data rate lies above a certain threshold rate. In case of optical power amplifiers we found this threshold to be typically 1Mbit/s, a value sufficiently below the typical data rate of some Gbit/s. These analytic results were substantiated by experiments. Then we investigated the sensitivity of optically preamplified direct detection receivers in case of RZ coded input signals and found an improvement of about 1-2 dB in terms of average input power compared to non-return-to- zero (NRZ) coding. We further showed that a pulse duty cycle of 33% is optimum with respect to sensitivity gain over NRZ. Our simulations and measurements revealed that, in case of RZ coding, the system is more tolerant regarding suboptimum parameters, such as optical and electrical filter bandwidth. Taking into account the finite extinction ratio of commercially available optical modulators, we identified modulation of a pulse train as the preferred method to generate an RZ coded waveform in practical implementations. With a carefully optimized receiver, we measured a sensitivity of only 1.4 dB above the quantum limit at a data rate of 10 Gbit/s. For the transmission scheme employed, this constituted a world record of sensitivity.