Present fiber performance properties are discussed with a particular emphasis on tradeoffs relating to system economics. The discussion concentrates on the fiber attenuation rate and information carrying capacity. Both state-of-the-art as well as typical performance levels are indicated.

There have been profound changes in optical waveguide technology over the past 10 years. Rapid advances have been made in optical performance characteristics--such as attenuation and bandwidth. There have also been significant improvements in the types of coatings used, fiber tolerances, fiber strength, lengths available, etc. At the same time, the demand for optical waveguides has increased rapidly and fiber production has moved from the R/D laboratory to full-scale manufacturing plants. Fiber prices have dropped dramatically and fiber types have been designed and optimized for specific market segments. In the future, performance/cost trade-offs will be appropriate and fibers optimized for specific market applications will become the norm. Economic trade-offs will take place as users and manufacturers work together to optimize the total system to be most cost-effective.

The performance characteristics of photodetectors and optical sources for use in optical fiber communication systems are discussed. Particular emphasis is placed on long wavelength devices. Desirable device requirements and their applications are indicated. The usefulness of wavelength division multiplexing in fiber systems is also discussed.

This paper presents a review of recent research and development in high radiance LEDs intended for use as sources in optical fiber communications systems. Development in the last decade has resulted in reliable devices for use in the 0.8 to 0.9 μm wavelength region based on the Al_xGa_i-_xAs-GaAs double heterostructure material system. Both high-power and wide-bandwidth LEDs have been produced. Near-optimum performance of small area surface emitters (Burrus diodes) as well as edge emitters has been obtained. The paper will describe new trends in improving LED-to-fiber coupling efficiency, methods for reducing the nonlinearity of LEDs for analog applications, degradation mechanisms and device life. Recent achievement in long wavelength LEDs and PIN photodiodes has led to the development of a second generation of LED-fiber systems for operation at 1.3 μm wavelength, with data rates as high as a few hundred Mb/s and repeater spacings (or data link spans) of many kilometers.

The intended fiber-optic applications for both short- and long-wavelength single-mode diode lasers are discussed. Major types of single-mode devices are grouped and described in two sections: low-power (3-7 mW/facet) lasers and high-power (10-40 mW/facet) lasers. The low-power laser section contains principles of mode stabilization, and a treatment in parallel of AlGaAs and InGaAsP buried-mesa, non-planar-substrate and laterally-lossy devices. The high-power laser section contains the various means of achieving high-power reliable single-mode operation and a description and comparison of major types of high-power AlGaAs devices. Emphasis is placed on the performance of the constricted double-heterojunction large-optical-cavity (CDH-LOC) laser. A brief discussion of single-mode stabilization and device dynamic behavior is also presented.

Recent experimental and theoretical results on a new class of low noise avalanche photo-diodes are reviewed. A large enhancement of the impact ionization rates ratio (a/(3=10) has been demonstrated in AlGaAs/GaAs superlattices and graded band gap detector structures. In addition two novel photodiodes ("staircase" and "channeling" APDs) where only electrons multiply, have been proposed. The staircase APD is the solid state analog of the photo-multiplier with discrete dynodes. It has a low operating voltage (5-20 volts) and a lower excess noise factor, in the ideal case, than that of an ideal conventional APD The channeling APD is instead the solid state analog of a channeltron photomultiplier and has the unique feature of an ultrahigh a/β, ratio (≈∞) compatible with high gain (>100).

The role of long wavelength systems (1.0μm < λ < 10.0pm) in fiber optics communications is evaluated. For high-bit rate optical telecommunications at 1.3pm or 1.5μm, GaInAs p-i-n detectors have emerged as the preferred choice because of their low noise, excellent sensitivity, and high temperature stability. Ge and HgCdTe photodiodes offer nearly equivalent performance and a somewhat more advanced production technology. Beyond 2μm, HgCdTe would seem to be the clear choice for photodetector applications. Avalanche photodiodes in long wavelength optical fiber linksmay find uses in situations involving high bit-rate transmission using low-cost components. Although avalanche gain will always improve the system sensitivity, the LED4iPD combination is only half as sensitive as a laser/p-i-n system, as well as being bandwidth-limited. Non-telecommunications applications involving_ data-base and fiber-guidance systems are discussed and a prospective look is taken at the uses, such as power transmission, of ultra low-loss fibers in the 2 to 10μm region of the optical spectrum.

An electrooptic A/D converter which utilizes optical sampling and which is capable of converting analog signals with bandwidths up to 414 MHz is under development. A device has been fabricated and individual bit channels tested using a beat-frequency technique. The operational principles, converter structure and testing results are summarized.

The evolution of fiber optic technology has provided an alternative multiplexing technique called wavelength division multiplexing (WDM). In WDM, each channel is optically modulated onto a light source of a distinct wavelength. All of the wavelengths are then focused onto a single fiber by a wavelength multiplexer device. At the receiving end, the composite optical signal is spectrally filtered or dispersed into its constituent wavelengths by a demultiplexer, and each channel is detected by a photodetector.

A series of tests was performed to evaluate off-the-shelf components for the design of a fiber optic channel for the Nascom System Improvements Project in support of the NASA Goddard Space Flight Center. This paper describes the results of this series of tests, establishing operational performance of active and passive fiber optic components at data rates up to 150 Mb/s. These tests determine the transmission characteristics of the fiber optic transmitters and receivers and the effects of data rate, bit pattern sensitivity, and vestigial optical energy on the performance of these devices. Tests were also performed to evaluate the capability of fused biconical couplers to Function properly at these high data rates.

Under contract from the Canadian Department of National Defence, Sperry Univac and Canstar Communications are developing a 10-megabit data link for military shipboard application. The system is based on a passive optical fiber network connecting 32 electro-optic bus access modules, each module being capable of servicing eight bus users. The key aspect of the project was to determine the optimum network topology based on considerations of optical loss, system security, simplicity of design, and replaceability of key components, and to demonstrate the capability of achieving the requirements with existing technology in electro-optic transmitters and receivers and in fiber optic access couplers. The proposed paper will discuss the overall system requirements, the selection of the optimum passive topology and the trade-offs associated with the alternatives, as well as outline the measured performance of a prototype system resulting from the initial phase of development.

A data distibution evaluation system (DDES) has been developed for use by NASA Langley Research Center to evaluate fiber optic data bus systems. The background leading to the system, a general description of the system, and some plans for use of the system are given. The DDES is a three station system with each station containing a microprocessor and a micoprogrammable bus control interface unit (BCIU). The systems are interconnected by both dual twisted pair buses (MIL-STD-1553B) and by dual 55 micron fiber optic data buses (proposed DOD-STD-XXFO). Use of writeable control store for the microcode of the BCIU allows varying protocols to be evaluated using only software changes. Use of removable fiber optic transmitter/receivers allows evaluation of different modulation techniques and transmission media by making front-end hardware changes. The three DDES stations are used alternately as system controller, remote terminal, and bus monitor, with automatic rotation of the functions among the stations. As delivered, the fiber optic portion of the system is connected using a transmissive star coupler and uses bi-level manchester encoding. Planned experiments include modification of the hardware to compare the four modulation schemes proposed for MIL-STD-XXFO, and evaluation of a four port wavelength division multiplexing (WDM) system currently under development. An overview of MIL-STD-XXFO and of the wavelength division multiplexing system is given along with the planned use of the DDES to evaluate their performance.