The feasibility of coherent optical fiber transmission systems, in which laser coherence is utilized to carry information, are studied from the view point of device and system consideration and expected performance. Modulation scheme comparison among optical ASK, FSK, and PSK, transmitter and receiver configurations, single-polarization transmission through fibers, problems in providing stable local oscillator waves, and wide-band photodetectors are discussed together with alternative technologies to realize high performance systems. A study on digital system impairments caused by FM quantum noise of laser oscillators, interferometric FM-AM conversion noise, IF frequency fluctuation, and optical fiber transmission turbulence show the feasibility of the systems. Repeater spacing is estimated by considering the transmitting and receiving signal levels, optical fiber loss, and fiber transmission capacity. Repeater spacing of 240 km is feasible by 400 Mbit/s PSK homodyne-detection system and 220 km by 400 Mbit/s FSK heterodyne-discrimination detection system. The regenerative repeater spacing can be expanded further by employing intermediate repeaters with direct optical signal amplification. Based on the performance of semiconductor laser amplifiers, such as traveling wave type, Fabry-Perot cavity type, and injection locked devices, it is estimated that regenerative repeater spacing of more than 104km is feasible with 50 km intermediate repeater spacing. These systems will find application in transoceanic optical fiber cable transmission as well as terrestrial long distance transmission systems.

Coherent optical fiber transmission systems

Coherent Optical Fiber Transmission Systems

This paper deals with the problem of interconnection of many high-speed bursty traffic users via an optical passive star coupler. Each user can tune its laser over a range of wavelengths, thus resulting in a wavelength division multiplexed communication. The total number of wavelengths over which user tunability exists could be much smaller than the number of users. Therefore, some form of random access sharing and packet switching may be necessary. We propose several protocols that require each user to have a tunable receiver. The information on "where" and "when" to tune the laser is confined to a control (setup) channel that users tune to when in idle mode. An interconnection between two users lasting for the length of a data packet is set up on the control channel by the transmitting user who informs the receiving user where to tune in order to receive the data packet. No centralized control or coordination is required among the users. After analyzing each protocol, we present the throughput/ delay versus the offered traffic and the delay versus throughput in a sequence of plots. We show that in typical applications an average throughput of up to 0.95 can be achieved at a reasonable average delay using one of these protocols. In our benchmark examples we present an optical local area network (LAN) with a total throughput of 100- Gbit/s in which every user has access to a 1-Gbit/s data rate and the network can support over 1000 users. The protocols can be used in a) large LAN's that do not require a large capacity, b) small LAN's (1 kin) that require a large capacity and c) large LAN's (tens of kilometers) that require a rather large capacity.

Protocols for very high-speed optical fiber local area networks using a passive star topology

Balanced phase-locked loops for optical homodyne receivers are investigated. When a balanced loop is employed in a communications system, a part of the transmitter power must be used for unmodulated residual carrier transmission. This leads to a power penalty. In addition, the performance of the balanced loops is affected by the laser phase noise, by the shot noise, and by the crosstalk between the data-detection- and phase-lock-branches of the receiver. The impact of these interferences is minimized if the loop bandwidth B is optimized. The value of B opt and the corresponding optimum loop performance are evaluated in this paper. Further, the maximum permissible laser linewidth \delta\nu is evaluated and found to be 5.9 \times 10^{-6} times R b , where R b (bit/s) is the system bit rate. This number corresponds to BER = 10^{-10} and power penalty of 1 dB (0.5 dB due to residual carrier transmission, and 0.5 dB due to imperfect carrier phase recovery). For comparison, decision-driven phase-locked loops require only \delta\nu = 3.1 \times 10^{-4}. R_{b} . Thus, balanced loops impose more stringent requirements on the laser linewidth than decision-driven loops, but have the advantage of simpler implementation. An important additional advantage of balanced loops is their capability to suppress the excess intensity noise of semiconductor lasers.

Balanced phase-locked loops for optical homodyne receivers: Performance analysis, design considerations, and laser linewidth requirements

The impact of laser phase noise on optical heterodyne communications receivers is analyzed in terms of the system signal-to-noise ratios and error rates. An ASK receiver, three FSK receivers and a DPSK receiver are investigated. The maximum permissible laser linewidth Δν is evaluated for each of these receivers and compared with previously published theoretical and experimental results. It is shown that Δν depends on the system data rate R and on the modulation/demodulation technique chosen. For example, DPSK receivers require at least Δν ^ 0.7% of R while FSK receivers with postdetection frequency discrimination require at least Δν ^ 1.9% of R if the mark-space separation 2 fd is equal to R. At the same time, ASK receivers with envelope postdetection processing, and FSK receivers with large frequency deviation are much more tolerant to phase noise: they only require Av<9%ofR.

Impact of Laser Phase Noise on Optical Heterodyne Communication Systems

The key results obtained from experimental systems studies over monomode fiber and cable links are given for both direct detection and coherent optical systems. Direct detection techniques have been studied at 1300 nm using conventional lasers and at 1500 nm with narrow line transmitters of the injection-locked and external cavity types. In the case of coherent optical systems, developments of the same transmitters have been used in conjunction with amplitude and phase shift key (PSK) modulation techniques to obtain improved performance. Increases in receiver sensitivity of up to 14 and 17 dB have been obtained using, respectively, heterodyne and homodyne detection techniques. These results approach theoretical predictions and offer the possibility of much greater transmission capacity and extended regenerator spacings for monomode systems.

Progress in monomode transmission techniques in the United Kingdom

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Progress in monomode transmission techniques in the United Kingdom