J. L. Gimlett

Effects of phase-to-intensity noise conversion by multiple reflections on gigabit-per-second DFB laser transmission systems

Large power penalties and bit-error-rate floors have been observed in some Gb/s systems using distributed feedback (DFB) lasers, which could be attributed to interferometric conversion of laser phase noise to intensity noise by multiple reflections at connectors and splices. The authors calculated the power spectral density of the interferometric noise and its impact on system performance as a function of both the magnitude and number of reflections, and they compare the theoretical predictions with experimental results. Their studies indicate that connectors and splices with return losses of more than about 25 dB are required for the reliable operation of Gb/s fiber transmission systems, even if optical isolators are used. >

Sensitivity penalty in multichannel coherent optical communications

The error rates and sensitivity penalties for multichannel coherent optical communications systems are evaluated for amplitude-shift keying (ASK), phase-shift keying (PSK), and frequency-shift keying (FSK) modulation, taking into account adjacent channel interference. Both time-domain and frequency-domain analysis are used, the latter being based on a Gaussian approximation. Both techniques yield similar results for sensitivity penalties below 1 dB. For FSK systems, larger values of the modulation index Delta do not necessarily lead to larger channel spacings. ASK and PSK systems both require larger channel spacings than FSK systems with Delta =1. The study was conducted for sources with linewidths narrow enough so that phase noise does not degrade the performance of receivers with matched filter demodulators. >

Dispersion penalty analysis for LED/single-mode fiber transmission systems

GREAT DEAL of attention has been focused recently on telecommunication systems based on single-mode fiber (SMF) and LED light sources for future deployment in the local network and subscriber loop. These systems combine the advantages of the low loss, large bandwidth, and upgrade potential of single-mode fiber with the high reliability and temperature stability, as well as the low cost of LED’s. Recent experiments have demonstrated the feasibility of LED-SMF ’ systems for transmission rates up to 560 Mbit/s and span lengths (at 140 Mbit/s) up to 50 km [1]-[lo]. Chromatic dispersion is a potential limitation at these bit rates and span lepgths, however, because of the broad spectral widths of LED’s. Even for a practical system of shorter transmission distance, the dispersion penalty remains an important consideration in the power budget since the total power coupled into single-mode fiber is relatively small for LED systems. It is thus important to have realistic estimates of dispersion penalties in LED-SMF system design. Penalties for chromatic dispersion in multimode fiber have been calculated for LED’s based on the assumptions of an optical receiver which has been re-equalized to compensate for the fiber dispersion, and an f receiver noise spectral density [ 111, [12]. In practical systems, however, these assumptions are often not applicable. In this paper we present a new calculation of dispersion penalties arising from intersymbol interference for LED-SMF systems with no re-equalization (the more commonly adopted scheme), and compare the results obtained with a treatment of the re-equalized system which considers both white, andf2 receiver noise spectral densities. Simple approximations of dispersion penalties are derived, and bitrate distance product limits are presented as a useful estimation tool for system feasibility studies. Single-mode fiber transmission experiments at 90, 140, and 560 Mbit/ s have been performed using 1.3- and 1.5-pm LED’s [3][6], and the measured dispersion penalties are compared with the results of the above analysis.

FSK heterodyne transmission experiments at 560 Mbit/s and 1 Gbit/s

Optical frequency-shift-keying (FSK) signals are obtained from directly modulated distributed feedback (DFB) semiconductor lasers. Experimental studies of the direct frequency modulation (FM) characteristics of the DFB lasers show a nonuniform FM response due to the competing effects of thermal modulation of the laser active region and carrier density modulation. Equalization of the signal current to the laser is employed to produce a flat FM response from 30 kHz to 1 GHz. Optical FSK transmission and heterodyne detection experiments at 560-Mbit/s and 1-Gbit/s are conducted at a wavelength of 1497 nm. Receiver sensitivities of -39 dBm at 560 Mbit/s and -37 dBm at 1 Gbit/s are obtained. Transmission through 100 km of single-mode fiber at 1 Gbit/s is achieved with no degradation in receiver sensitivity.

A 2-Gbit/s optical FSK heterodyne transmission experiment using a 1520-nm DFB laser transmitter

Optical FSK transmission at 2 Gbit/s using a directly modulated DFB laser at 1520-nm wavelength is reported. A receiver sensitivity of \bar{P} = -36.7 dBm ( \eta\bar{P} = -39.2 dBm) at 10-9BER was achieved for transmission over 101 km of single-mode fiber with no additional penalty attributable to the fiber. The effect of the nonlinear phase of the transmitter FM response, and the system performance for discriminator demodulation, including the impact of laser phase noise, is analyzed and compared with experimental results.

Ultrawide bandwidth optical receivers

Low-noise p-i-n/HEMT optical receivers with bandwidths in excess of 16 GHz have recently been developed using advanced microwave design and packaging techniques. Here, state-of-the-art receiver technology, including design tradeoffs and implementation techniques for this class of ultrawide bandwidth receiver, is discussed. Some directions for future research are explored. >

Silicon bipolar integrated circuits for multi-Gb/second optical communication systems

The authors discuss several important circuits for fiber-optic transmission, implemented in an advanced silicon bipolar integrated circuit technology. Specifically, the authors discuss the design considerations and measured performance of a 2:1 multiplexer, front end receiver, limiting amplifier, and decision circuit IC. Also discussed are three hybrid circuit modules: a 2:1 multiplexer, 1:2 demultiplexer, and parallel processing decision circuit. These ICs and hybrid circuit modules operate at multi-Gb/s data rates. The performance of these ICs indicates that advanced silicon bipolar integrated circuits with their high speed, functionality and low cost potential could play an important role in alleviating the electronic bottleneck in future multigigabit optical communication systems. >

Conversion of laser phase noise to intensity noise in single-mode fiber-optic components

We have recently observed that the Fabry-Perot cavities formed by index discontinuities in connectors and other fiber-optic components convert the intrinsic phase noise of semiconductor lasers into excess intensity noise which can significantly impact both direct detection and coherent communication systems. The level of this noise can be higher than −100 dB/Hz and can result in bit-error-rate (BER) floors.