1276 IEEE JOURNAL zyxwvutsrqpo ON SELECTED AREAS IN COMMUNICATIONS. VOL. 8. NO. zyxwv 7. SEPTEMBER 1990 zyx Design Techniques for Subcarrier Multiplexed Broadcast Optical Networks STUART D. WALKER, MINGXING LI, ANTHONY C. BOUCOUVALAS, MEMBER, IEEE, DAVID G. CUNNINGHAM, AND ALISTAIR N. COLES zyxwv Abstract-We describe design techniques which enable the perfor- mance of subcarrier multiplexed FM broadcast optical networks to be optimized at the planning stage. At the transmitter node, criteria for maximizing the subscriber carrier-to-noise ratio (CNR) are presented. At the subscriber node, it is shown that under the majority of circum- stances, the optimum direct detection receiver consists of a low-noise Ill-V avalanche photodiode and high-impedance front-end preampli- fier. Sensitivities approaching -40 dB (1 mW) for 16.5 dB CNR in 36 MHz subscriber bandwidth are predicted for a 60-channel system. Pre- liminary experimental tests on an optical-feedback subcarrier receiver showed -34 dB (1 mW) sensitivity at 1.55 pm for 16.5 dB CNR in a 36 MHz bandwidth centered on 1.2 GHz with a 12%optical modulation index single-channel FM test signal. I. INTRODUCTION UBCARRIER multiplexed (SCM) broadcast optical S networks, as shown schematically in Fig. 1, have been the subject of much recent interest as they offer a cost- effective near-term means of information dispersal in lo- cal and metropolitan area networks (e.g., [l]). The cur- rent status of such systems has recently been summarized by Olshansky et al. [2]. Key enabling technologies are broad-band modulation transfer function semiconductor lasers, high-speed p-i-n, and avalanche photodiodes (APD's) [3] together with low-cost microwave preampli- fication and signal-processing modules developed for sat- ellite television-receive-only (TVRO) [4] applications in the 3.7-4.2 GHz band. The transparency of SCM networks to varying data for- mats offers a mixed-traffic handling capability which rep- resents a significant advantage over conventional digital- multiplexed systems. As SCM techniques are essentially analog, many of the design issues inherent in existing an- alog frequency division multiplex (FDM) cable and sat- ellite systems, again, become important. For example, it is well known that the laser transmitter is a nonlinear de- vice which introduces intermodulation and harmonic dis- tortion contributions from both intrinsic and light output versus current input curve nonlinearities (e.g., 151). These impairments, together with laser relative intensity noise Manuscript received July 24. 1989; revised January 20. 1990. S. D. Walker and M. Li are with the Department of Electronic Systems Engineering, University of Essex. Wivenhoe Park, Colchester. Essex. CO4 3SQ. England. A. C. Boucouvalas. zyxwvutsrqpo D. G. Cunningham. and A. N. Coles are with Hew- lett Packard Laboratories, Stoke Gifford. Bristol. BSI2 6 QZ. England. IEEE Log Number 9036375. ODtiCd n Power Splitter ~ u , O Datasource , - Reception zyxw ci' Fig. 1. Schematic subcarrier broadcast optical network. (RIN), mode-partition, chirp-induced, and transmission path modal noise can prescribe unacceptable upper limits to the achievable subscriber CNR. This paper attempts to provide design principles which identify quantitative limits to SCM broadcast network performance and in some cases indicate optimum perfor- mance criteria. Three main areas are addressed. First, the influence of laser overall nonlinearity, RIN, channel im- pairments, and receiver noise on subscriber CNR is as- sessed. It is shown that two alternative optimization strat- egies are available which allow either the per-channel optical modulation index (OMI) for a prespecified CNR or the OM1 for maximum CNR to be determined. Due regard to the overall system impairments and received op- tical power levels must be given. Second, the issue of receiver design is reviewed. It is found that 111-V APD's and integrating low-noise front ends offer performance advantages under optimum OM1 conditions, Finally, some experimental results are presented which show the exis- tence of an optimum OM1 for a commercially-available Fabry-Perot laser (ORTEL DP1SOlC) in a preliminary carrier to two-tone third-order intermodulation noise test. Additionally, a novel 111-V APD based integrating opti- cal-feedback subcarrier receiver design is described which simultaneously offers -34 dB ( 1 mW) sensitivity and 29 dB optical dynamic range for 16.5 dB CNR in a 36 MHz bandwidth centered on 1.2 GHz with a 12% OMI, FM test signal. 11. LASER-DEPENDENT CNR LIMITATIONS This section is concerned with an assessment of how laser-related effects such as total distortion and RIN to- gether with (where appropriate) mode-partition, modal, chirp, and clipping noise can set upper limits to the achievable CNR. The contributions from nonlinearities are dealt with separately from RIN and other effects ini- tially and then subsequently combined. 0733-8716/90/0900-1276$01 .OO zyxwvut 0 1990 IEEE