Thomas E. Darcie

Subcarrier multiplexing for multiple-access lightwave networks

This paper describes the applicability of subcarrier multiplexing to lightwave multiple-access networks. It is shown how currently available microwave and lightwave components can be used, by using subcarrier multiplexing, to provide high-capacity networks. For example, the proposed multiple-access network can support 1024 users at a continuous bit rate of 1.5 Mbit/s, per user.

Subcarrier multiplexing for lightwave networks and video distribution systems

Applications for subcarrier multiplexing include a variety of analog and digital lightwave transmission systems. An overview of the requirements and capabilities of these systems is presented by describing specific examples of the most popular system types. These examples include multiuser interactive local area networks and multichannel digital, FM, and AM-VSB (vestigal sideband) video distribution systems. Limitations imposed on each by the linearity of directly or externally modulated sources, receiver noise, and relative-intensity noise are discussed. >

Lightwave subcarrier CATV transmission systems

The design and performance of multichannel amplitude-modulated (AM) vestigial-sideband (VSB) lightwave cable television (CATV) systems are described. Requirements on linearity and noise are derived, and factors limiting the performance of the laser transmitters and receivers are discussed. For the high-performance lasers the carrier-to-noise ratio and composite second- and third-order distortions are acceptable for video trunk systems. Impairments because of fiber reflections and dispersion and mode partition fluctuations in the laser are discussed. Feedforward, feedback, and predistortion are discussed; difficulties with each prevent immediate application. The use of LiNbO/sub 3/ external modulators and high-power solid-state lasers is considered. The third-order distortion and insertion loss of the modulator more than counteract the high available laser power (100 mW), making this alternative unattractive unless a third-order linearizer can be implemented. >

Fiber-reflection-induced impairments in lightwave AM-VSB CATV systems

Impairments due to multiple fiber reflections are evaluated for amplitude-modulated (AM) vestigial-side-band (VSB) lightwave cable TV (CATV) systems using directly modulated distributed feedback (DFB) lasers or externally modulated diode-pumped YAG laser transmitters. For DFB lasers, the spectral broadening caused by laser chirp results in a Gaussian optical spectrum with a width near 5 GHz. Square-law detection of multipath signals from two -30-dB reflections results in an effective intensity noise near -150 dB/Hz. For externally modulated YAG lasers, the same multiple reflections result in interference that cannot be described by an effective RIN. Although the total interference power is comparable to that of the DFB lasers, the signal and interference are highly correlated. As a result of this correlation, multipath interference for YAG-based systems cannot be measured using the standard test procedure with unmodulated carriers. >

Distortion related to polarization-mode dispersion in analog lightwave systems

Analog transmission in single-mode fiber using chirped sources gives rise to nonlinear distortion when polarization-mode dispersion (PMD) is present. We investigate experimentally and theoretically two mechanisms for this distortion: for chirped sources, PMD in the presence of polarization-mode coupling results in second-order distortion that is proportional to the square of the modulation frequency; when polarization-dependent loss is present, an additional second-order distortion term occurs that is independent of modulation frequency. Both mechanisms give rise to distortion that is time varying due to the sensitivity of PMD to ambient temperature changes. Numerical examples indicate that these effects can limit the capacity of analog systems that use directly modulated semiconductor lasers. >

Wide-band lightwave distribution system using subcarrier multiplexing

A description is given of a bidirectional lightwave distribution system based on subcarrier multiplexing. Eight independent wideband channels (180 Mb/s/channel) are transmitted from the head-end on one 1.3- mu m wavelength laser as frequency-shift-keyed (FSK) subcarriers, between 2.6 and 4.7 GHz. Each receiving node uses a parallel-resonant p-i-n FET receiver, a microwave bandpass filter and a limiter-discriminator demodulator to select and demodulate the appropriate channel. A 45-Mb/s return channel is provided from each node to the head-end on a separate subcarrier frequency. Error-free bidirectional transmission is obtained without penalties from laser nonlinearities, optical reflections or interference, or interchannel interference from closely spaced subcarrier channels. >

Resonant p-i-n-FET receivers for lightwave subcarrier systems

A theoretical and experimental analysis of narrowband resonant direct-detection p-i-n-FET receivers for subcarrier multiple-access networks is described. It is shown how a small inductance can be used to optimize the coupling between the p-i-n and FET, over a range of microwave subcarrier frequencies, minimizing the frequency-dependent thermal noise and leaving shot-noise as the ultimate limitation. Shot-noise then establishes a fixed ratio of the total usable bandwidth to the minimum received power per channel, which for the binary FSK system considered is 6.1 GHz/ mu mW. A resonant p-i-n-FET receiver, designed to provide maximum sensitivity between 2.5 and 5.0 GHz, has been constructed. The measured signal-to-noise ratio is in excellent agreement with that predicted by the noise analysis. >

Laser mode partition noise in lightwave systems using dispersive optical fiber

Semiconductor laser mode partition noise (LMPN) can impair analog optical systems, but this phenomenon has not previously been well-characterized. Theoretical expressions for the noise spectra that result when light from a nearly single-mode or strongly multimode semiconductor laser is passed through a length of dispersive optical fiber are presented and have been tested experimentally. A widely used model is found to greatly overestimate the partition noise of a DFB laser; a model for the strongly multimode case is found to match experimental results for a multimode laser. It is observed that partition noise can be significant even for multimode lasers operating near the fiber dispersion minimum. >

Fiber optic device technology for broadband analog video systems

Current CATV broadband networks are described, and a simplified model of these AM systems is given. AM systems performance metrics are examined from a CATV perspective. Noise sources are summarized, and the effects of intermodulation noise and laser nonlinearity are discussed. It is shown that the rigorous signal quality objectives associated with the CATV trunking application can, in principle, be satisfied with lightwave technology. The successful transmission of 42 carriers over 12 km of fibre while maintaining a C/N of 52 dB and composite distortions of less than -70 dBc is reported.<<ETX>>

Differential frequency-deviation multiplexing for lightwave networks

In differential frequency-deviation multiplexing (DFDM) systems, information is transmitted as the relative optical frequency difference between optical pulses transmitted in successive time intervals. This difference is detected using a fiber interferometer and a photodiode. Individual channels are defined by a unique frequency deviation in the frequency-shift-keying (FSK)-modulated laser spectrum. Limitations on receiver sensitivity indicate that for 100-Mb/s channels, which require approximately 200-MHz bandwidth, the total usable bandwidth is limited, by thermal noise, to 3.7 GHz per microwatt of received power. A network is proposed which uses a passive star coupler to interconnect approximately 12 channels, at 100 Mb/s per channel. >