The recently developed digital coherent receiver enables us to employ a variety of spectrally efficient modulation formats such as $M$ -ary phase-shift keying and quadrature-amplitude modulation. Moreover, in the digital domain, we can equalize all linear transmission impairments such as group-velocity dispersion and polarization-mode dispersion of transmission fibers, because coherent detection preserves the phase information of the optical signal. This paper reviews the history of research and development related to coherent optical communications and describes the principle of coherent detection, including its quantum-noise characteristics. In addition, it discusses the role of digital signal processing in mitigating linear transmission impairments, estimating the carrier phase, and tracking the state of polarization of the signal in coherent receivers.

Fundamentals of Coherent Optical Fiber Communications

Polarization-maintaining fibers and their applications are reviewed. The classification of high-birefringent fibers and low-birefringent fibers and their fabrication methods and characteristics are discussed in Section II. Analytical methods and numerical methods for fiber design on the birefringence are presented in Section III. Degradation factors of polarization maintenance expressed as crosstalk or mode-coupling parameters caused by internal origins such as structural imperfections, wavelength, and nonlinear effects, and by external origins such as temperature fluctuations, mechanical perturbations, and electromagnetic effects, are discussed in Section IV. Characterization methods on beat length, mode coupling, stress distribution, and mechanical strength are presented in Section V. Applications to the fiber devices and nonlinear effects, and splicing methods for the polarization-maintaining fibers are described in Sections VI and VII.

Polarization-maintaining fibers and their applications

Origins and control of polarization effects in single-mode fibers (A)

The polarization state of light in single-mode fibers is very sensitive to any perturbation which is not symmetric about the fiber axis. While this is a source of noise, drift, or signal fading in some applications, it can also be exploited in novel guided-wave devices. The basic birefringences that couple the two modes and change the polarization state along the fiber are reviewed. The three cases of uniform, phase-matched, and random coupling are considered. Polarization preservation in both low- and high-birefringence fibers is achieved by reducing this coupling. In addition to polarization-state changes, bireftingent fibers can quickly reduce the polarization degree of nonmonochromatic light if both modes are excited, a characteristic that greatly simplifies evaluation of the degree of polarization preservation in these fibers. Current evaluations of the birefringence and the polarization-holding ability of state-of-the-art fibers are discussed, and it is concluded that fibers with good polarization-holding properties are becoming available.

Origins and control of polarization effects in single-mode fibers

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

Guidelines for the design of a circular-polarisation-maintaining single-mode fibre cable are given. The use of circular polarisation allows use of low birefringence fibres and there is no need for orienting the fibres at splices. Quarter-wave plates at input and output of the fibres achieve the desired linear-circular polarisation conversion, and vice versa.

Polarisation-maintaining single-mode fibre cable design

We report a very simple method for measuring the mode spot size in single-mode fibres on the same apparatus as used for attenuation measurements without additional fibre handling. Measurements as a function of wavelength lead to the equivalent step index parameters allowing one to predict directly the splice, microbending and curvature losses.

Fundamental mode spot-size measurement in single-mode optical fibres

We report for the first time experimental evaluation of the material dispersion in a fluoride glass. Very interesting features are observed as the wavelength of zero material dispersion is around 1.7 μm and the disperison is about only 45 ps/nm × km at a 4 μm wavelength.

Material dispersion evaluation in a fluoride glass

A frequency domain determination of polarisation mode dispersion in single mode fibres is described, which allows one to obtain directly the group delay difference for long fibres. The results of such measurements are shown on a birefringent fibre with elliptical cross-section which maintains an extinction ratio between linearly polarised eigenstates greater than 10 dB over 1 km.

Polarisation mode dispersion measurements in long single mode fibres

The splicing and cabling added loss are evaluated for 4 μm-wavelength single-mode fibres. It is shown that splice offset and microbending losses are critical and that high index differences are required. Added losses are small as 0.01 dB/km can be obtained with careful fibre design and handling.

L. Jeunhomme

Papers

Polarisation-maintaining single-mode fibre cable design

Fundamental mode spot-size measurement in single-mode optical fibres

Material dispersion evaluation in a fluoride glass

Polarisation mode dispersion measurements in long single mode fibres

Single-mode-fibre design for 4 μm band operation