<title>Use Of Wavelength Division Multiplexing (WDM) In Duplex Data Links For Field Applications</title>

Abstract

Abstract A number of schemes for wavelength division multiplexing (WDM) have been reported in the literature in the past few years. However, most of them tend to be laboratory devices used in demonstrating the principle of operation. Recently a simple, low cost, and rugged wave­ length multiplexer (A-MUX) based on the interference filter has been devised at Lockheed. These multiplexers have also been used as the central elements for duplexed data links in field applications and have performed exceedingly well. Typically, these A-MUXs have inser­ tion loss of 2 dB and a crosstalk between channels of 33 dB at a channel separation of 22 nm. When used in 3-km duplex links, electrical crosstalk between 15 MHz video channels is less than -40 dB, and in 10 MBPS digital links (with contributions from system noise) BERs of less than 10-10 are realized. In this paper, the principle of operation, mechanical con­ struction, and test results of the A-MUX will be reported. Detailed test results on full duplex links (up to 3 km link length) will be presented. Use of A-MUX, directional coup­ lers, and optical switches for computer interconnects will be discussed. Finally, the use of integrated optic devices for such endeavors will be suggested.IntroductionOptical fibers, by virtue of their low attenuation and low dispersion properties, are ideal as the transmission medium for wideband communications, both short haul and long haul. During the past few years, intensive work at different laboratories has pushed forward the performance of fiber optic (FO) systems significantly. For instance, analog data links having bandwidths in the 650 MHz region were reported as early as 1978 [Ref. 1]. Experi­ mental digital systems having data rates in the 450 MBPS range have been operated by a number of telephone companies [Refs. 2,3]. It is generally agreed by workers in this field that operations in the 1 GHz analog [Ref. 4] and 1 GBPS digital regions can be readily obtained [Ref. 5], For many systems the optical fiber per se does not impose the upper bandwidth limit of ~ 1 GHz (particularly when one considers the use of 1.3-1.5 ym window in which the attenuation is very low and the dispersion can be made to be zero). Modulation electronics and optical sources (injection lasers), however, do become more troublesome at bandwidths above 1 GHz. To obtain substantially higher total system data throughput on a single fiber, other techniques must be studied.Wavelength division multiplexing (WDM) serves this purpose remarkably well. If a number of optical sources (say n) can be selected to have their output wavelengths separated and stabilized, and if a linear optical summing device can be made so that these outputs can be channeled into a given fiber, then the effective data capacity of the system is increased n-fold. This performance is achievable due to the fact that in linear summing, there is no intermodulation among signals of differing wavelengths; the fiber transmits all the n optical carriers independently, each one modulated to its full electronic bandwidth. Since a high data rate signal is generally composed from a number of lower data rate signals, the use of wavelength multiplexing (A-MUX) techniques is also highly desirable from the system's point of view. This is true because (a) it can eliminate many conventional electronic multiplexing (electronic MUX) and source modulation problems, (b) for a given power, the signal- to-noise ratio for each wavelength channel can be maintained relatively high, and (c) differ­ ent types of signals (analog, digital, timing, etc.) may be logically multiplexed onto separate wavelengths without compromise of system parameters such as timing requirement, functional performance, logistical similarity, direction of flow, etc. A system employing WDM to transmit a large number of signals is depicted schematically in Fig. 1.For these reasons, WDM has been studied quite extensively and reported in the recent literature. Fundamentally, two approaches are pursued: one based on diffraction gratings [Refs. 6,7], the other based on dielectric interference filters [Refs. 8,9]. We have inves­ tigated both approaches and implemented a rugged wavelength multiplexer for field use based on dielectric interference filters. In this paper, we shall first discuss the principles and applications of these two types of wavelength multiplexers, then report on the design of the filter-based multiplexer and the performance of a 3-km A-MUX system fielded in a military installation; and finally discuss other related topics such as computer inter­ connections, and the use of integrated optics.