The use of optical fiber has revolutionized the communications industry during the few years in which it has been available, and still further advances are just over the horizon. Figure 1 shows five generations of optical fiber communication systems recognized today.1 The increasing capabilities of each generation are characterized by significant increases in repeater spacing and channel capacity. The most advanced of these concepts is the use of heterodyne or homodyne detection of digital signals.

This paper reports recent progress in the development of three categories of optical sources with linewidths, Ay, sufficiently narrow to be used in heterodyne and homodyne optical fiber communication systems: First, single-longitudinal-mode semiconductor lasers with relatively broad linewidths of several tens of MHz, suitable for amplitude-shift-keying and frequency-shift-keying heterodyne with envelope detection. Second, semi-conductor lasers with weak optical feedback having Lv = 1 MHz. Such narrow linewidth sources would be required for differential-phase-shift-keying heterodyne detection. Third, semiconductor lasers with strong optical feedback, and 1523 nm He-Ne lasers, both having Av , ▵ν≤15kHz ,Such very narrow linewidth sources would be required for PSK homodyne detection.

Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase shift Keying (PSK) optical heterodyne detection transmission systems have been developed using 1.3 μm and 1.55 μm wavelength DFB/DBR laser diodes. Receiver sensitivity improvement as much as 7-10 dB over direct detection was realized experimentally at 100-200 Mb/s using 50-105 km single mode fibers. The spectral linewidth characteristics of the laser diodes have been studied. Intermediate frequency beat with a few 10 MHz spectral linewdith has been realized by using DFB-DC-PBH LDs. When external cavity loaded DBR-DC-PBH LDs were used, the beat spectral linewidth has been reduced to less than 1 MHz. The intermediate beat frequency fluctuation was stabilized within a few MHz by controlling the local oscillator laser diode injection current.

The performance of coherent optical receivers is affected by the phase noise of the laser transmitter and the laser local oscillator, and by the shot noise due to the detectors employed in the receiver. In this paper, the performance of coherent optical receivers is analyzed while taking into account both aforementioned noise sources. Further, the maximum permissible laser linewidth tv for coherent optical communications systems is evaluated. Both heterodyne and homodyne systems are considered. It is shown that the value of ▵ν depends on the system bit-rate Rb and on the modulation/demodulation technique employed: for heterodyne receivers with noncoherent post detection processing (used in conjunction with ASK, FSK and DPSK modulation formats), Av<(0.007-0.09)Rb; for heterodyne receivers with coherent post detection processing (used in conjunction with the PSK modulation format), ▵ν≤7.4.1010-4rb and for homodyne receivers, ▵ν≤8.2.10--Rb.

A simulation model has been developed to evaluate ISI in coherent optical communication systems. Eye pattern results on ISI and jitter and the variation of bit error rate for different coherent modulation-demodulation schemes are presented. The results confirm earlier observations that coherent optical communication systems enhance spacing between adjacent repeaters by improving receiver sensitivity.

Network architectures that utilize coherent optical technology are, analyzed. Applications of this rapidly maturing technology are shown to increase network capabilities - throughput, variety of services, etc. - and alleviate some of the protocol and switching difficulties found in traditional network architectures. The designs presented and analyzed in this paper are concerned with multiple - access and broadcasting networks.

An experimental study of the polarization properties of conventional low-birefringence nominally circular-core installed single mode fiber cables is reported. The results of the measurements indicate (1) The orthogonality of the two launched polarization components is preserved to within 2°, (2) the total power distributed between the two linear orthogonal components, when normalized with respect to the launched power, is effectively constant over time, and (3) given a linearly polarized input, the state of polarization varies over a time scale of minutes with time, but the degree of polarization exceeded 90% and remained stable for periods as long as 65 hours.

Coherent optical waveguide transmission systems are expected to offer improved system performance. Spurious optical radiation outside the signal band can be eliminated homodyning or heterodyning techniques and improvements on the order of 10 - 30 dB 1,2 can be realized depending upon the detection scheme utilized. Polarization mode dispersion (PMD) and cross-coupling between modes may result in waveguide bandwidth-length products that are impractical for coherent telecommunication systems. It is important to determine the measurements and conditions of measurement required to ensure waveguide compatibility with coherent transmission systems.

A low-loss, wide band, compact, polarization preserving, fiber optic directional coupler (PPFDC) is developed for use in future coherent optical fiber transmission systems or gyroscopes. The PPFDC is made of two 200 μm diameter sapphire ball lenses, a half-mirror and four conical alumina-ceramic ferrules with flat reference plane to preserve polarization at the connection of the fibers. Built-in thermal stress change in the fiber is minimized, because the fiber is polished only for the short length within the straight ferrule. The optical and mechanical axes are tilted, so cross-talk light from the fiber endfaces or lens surfaces is avoided. The optical directivity of the PPFDC is over 50 dB and return loss is over 40 dB in the 1.3 μm wavelength range. The excess loss is under 1 dB in the 1.2 to 1.4 μm wavelength range. The extinction ratio is over 25 dB. The length of the coupling region of the PPFDC is about 1 mm including the two ball lenses and half mirror. Spectral line widths of a fiber interfaced DFB laser diode are measured by a self delayed homodyne detection scheme, using the PPFDC. Experimental results of a novel polarization preserving fiber-optic gyroscope using a phase modulating PPFDC showed reasonable sensitivity and stability. These experimental results verified the above mentioned characteristics of the PPFDC.