In single-mode systems and fiber sensors a single-mode, four-port, directional coupler is often required to transfer light between two fibers. The requirements for the coupler are low loss, a stable splitting ratio and, in some cases, it should maintain a given state of polarization. In this paper the analysis and behavior of single-mode fiber couplers made by the fused-taper technique are reviewed.

In an earlier paper we reported the development of a fusion technique for fabricating environmentally stable, single-mode couplers. During the past year we have fabricated couplers in a production environment and have studied the reliability of the coupler fabrication process. These couplers were designed to have a nominal coupling ratio of 50%. We found that a significant fraction exhibited excess losses of less than 0.05dB. Further, through the use of an automated test facility, we have investigated the performance of the couplers under varying ambient conditions. The test conditions included changes in temperature, variations in the polarization of the input light and variations in the wavelength of the source. In this paper we report the findings of our studies and discuss the implications with regard to the reliability of the fiber optic coupler. This data will allow the designer of a fiber optic system which utilizes single-mode couplers to predict the stability of that system in various environments.

A newly developed manufacturing technique for single mode fiber couplers relying upon improved microelectronics technologies of silicon is presented. The high accuracy needed for such couplers is obtained from curved V-grooves precisely etched on a silicon wafer, the flatness of which has been enhanced. Use of conventional polishing technique is then made. The proposed technique allows reliable production with a good reproductibility from batch to batch. Experimental results are presented as well as their capability of wave-length multiplexing and their polarization behavior.

Two different types of polarization-holding directional couplers have been made using fused D fiber. Insertion losses are as low as an estimated 0.1 db with polarization holding between 15 and 22 dB depending on the type of coupler.

Similarly as multimode fiber systems, practical single--mode systems require demountable connectors for terminating components, system reconfiguration, testing and maintenance. Since these single-mode systems are usually large - capacity long-haul systems, the connectors, as well as the other components, must have high performance characteristics so that the span lengths can be maximized. The first concern for the connector designer then is to minimize the insertion loss of the connectors. Since single-mode fibers have core diameters on the order of 5 to 10 microns, compared with 50 to 100 microns for multimode fibers, single-mode connector designs must provide fiber-to-fiber alignments to submicron precision. High-performance single-mode connectors, in both factory and field installed versions, have been reported that provide lateral offsets of less than 1 micron and angular misalignments of less than 0.5 degrees. For typical single-mode fibers, these alignments result in insertion losses on the order of 0.5 dB. Since reflections from connectors, and/or other components, may affect the longitudinal mode spectrum and noise characteristic of laser transmitters and cause system degradation, the return loss of the connector is also of concern to the designer. It has previously been reported that fiber-to-fiber end-face contact can substantially reduce reflections without the use of index matching, resulting in return losses of about 30 d2. Using Gaussian field theory, we will present the combined effect of lateral offsets, longitudinal offsets, and angular misalignments between two butt-jointed single-mode fibers having unequal mode-field radii. The expected insertion loss of connectors based on a statistical model of the effect of typical misalignments and fiber mismatches will be presented and compared with empirical measurements of single-mode connectors. Good agreement between the theoretical model and experimental data is realized.

An externally trimmed core-to-core alignment single-mode optical fiber connector has been developed. Average insertion loss and repeatability at 1.3 μm wavelength are 0.5 dB and 0.2 dB, respectively. The practical use of the external trimming technology and enclosed triangular cross section alignment sleeve, which is produced by press forming, has made it possible to achieve prospects for mass production of single-mode optical fiber connector in the factory.

There is a trend toward increasing use of single mode transmission, particularly in telecommunications where high data bit rates are transmitted for long distances. Inter-connections of multimode fibers can be made in a number of ways, using ferrules, v-grooves, elastomeric splices, etc. However, the connection of single mode fibers, which have core diameters of 4 to 13 μm, requires more precise alignment than do the multimode fibers having core diameters of 50 μm or more. At TRW, we have adapted the four rod alignment guide concept for single mode fiber inter-connections. The principle of this OPTAGUIDE* alignment guide is presented. The single mode connectors and splices use the four rod scheme with an index matching material to eliminate or reduce the losses incurred through fiber end roughness or angularity. We are able to produce demountable connectors for 80/4.4 pm fibers having typical insertion losses of 1.0dB. The main factors in obtaining this result are the naturally precise fiber alignment provided by the alignment guide, and the ability of several manufacturers to maintain tight diametral and core offset tolerances. The single mode OPTALIGN* SM Connectors have been subjected to performance and environmental tests including repeated matings, temperature cycle and vibration. The results of these tests are described in this paper. A feature of the OPTALIGN* SM Connectors is the relative ease and speed of attachment to fiber optic cable in the field, without the use of epoxy or polishing procedures. The alignment guide concept has also been applied to permanent single mode splices. The splicing procedure is simple to perform in the field without expensive or delicate equipment. Construction and assembly procedures of the demountable connectors and permanent splices will be described with the aid of diagrams and photographs.

A thorough understanding of single-mode splice loss mechanisms is necessary to accurately predict the resultant splice loss of selected fiber pairs. Given a fusion splicing technique, which yields low variations of extrinsic splice losses, the intrinsic loss factors may be examined. In this paper, the method of calculating intrinsic losses due to fiber mode field radius mismatch and core lateral offset will be reviewed, and the theoretical losses of 10,000 randomly selected fiber pairs are presented. Splice loss data for arc fusion spliced pairs are correlated with the theoretical loss predictions.

High strength, low loss chlorine-hydrogen flame fusion (single mode optical) fiber splicing is discussed. This particular technology, developed at AT&T Bell Laboratories, is well suited to undersea cable applications. The strength and loss data initially achieved at one location has been reproduced at another AT&T Bell Laboratories facility. In addition, it is shown that a technique is available to produce predictably high loss splices for the purpose of placing line buildouts in a fiber span. The low loss splices and optical line buildouts do not require signal monitoring during production.

Reflections from connector end faces located near the transmitter generate intensity noise in the laser output which was found to be a function of the resonance frequency of the laser. The intensity noise introduces only an insignificant power penalty for buried-heterostructure lasers operating with a resonance frequency above 4 GHz, but could make transmission impossible in certain types of stripe geometry lasers operating with a resonance frequency below 3 GHz. Another potential system degradation may be due to modal noise which occurs mostly in laser pigtails or short jumper cables with overmoded single-mode fibers carrying both the fundamental and the higher order modes. The power penalty due to modal noise becomes negligible when the fiber section between two connectors is sufficiently long, or is deployed with a bend to sufficiently attenuate the LP₁₁ mode.

Modified fused biconical taper couplers have been developed with improved power and modal uniformity characteristics. While poor power uniformity results in wide receiver dynamic range requirements, poor modal uniformity causes more subtle problems affecting the interaction between the coupler and other system components. Data are shown for 16-port transmission star couplers, giving an indication of problems that occur in systems and showing the improved performance of a modified fused biconical taper coupler.

The temperature and vibration effects on the performance of multimode fused couplers is presently of concern to designers of fiber optic systems. To learn more about the performance of biconical taper couplers in certain environments, temperature and vibration tests were made on multiport star couplers according to some of the procedures in three military standard tests. An operating temperature test and a thermal shock test was made from +20°C to +85°C and from -20°C to +90°C, respectively. A vibration test was performed according to MIL-STD-5422. The three star couplers tested were fabricated at Aetna Telecommunications Laboratories and have 100/140 um core/clad diameter graded index fiber. The results of measurements made during temperature and vibration tests indicate that multiport fused couplers are stable, rugged and reliable components when properly packaged and assembled.

A new directional coupler and fiber optic connecting device are presented. Both components combine high performance with low cost. The devices are designed for standard low loss 200 micron plastic clad silica (PCS) fiber. The paper describes the design of a new connecting device (LANsplice) which features low loss, small dimensions, ease of installation, visible connection and inspectability of insertion loss after assembly. Mean insertion loss is 0.3 dB, and the device can be remated many times. By using index matching gel, Fresnel reflections are strongly suppressed. The influence of connection loss on reflected power is discussed, and the results of environmental tests (temperature, humidity and shock) are presented. The fiber optic directional coupler (LANtap) features mean excess loss of 0.24 dB for 1:1 couplers and 0.15 dB for 3:1 and 10:1 couplers. The mode selectivity of fused couplers is sometimes indicated as a drawback, however better understanding of this effect leads to a simple rule of thumb to overcome this in local area networks. Measurement results on a bi-directional link on a single fiber and on a fail-safe LAN with passive optical bypass are discussed. It is shown that both designs are quite insensitive to variations of coupler parameters and to the mode selectivity of this coupler. Results of environmental tests are presented.

A low cost, clamp-on coupler has been developed for use with fiber optic local area networks that employ a ring topology. The coupler takes the place of both connectors and optical relays required in ring networks. Macrobends in the optical fiber enable the pro-cessors to extract and launch signals from the ring without cutting the fiber.

A fiber optic data bus, using frequency division multiplexing (FDM) is discussed. The use of FDM is motivated by the need to avoid central control of the bus operation. A major difficulty of such a data bus is introduced by the couplers. An efficient low loss access coupler with an asymmetric structure is presented, and manufacturing processes for the coupler are proposed.

A commercial miniature monochromator, adapted with GRIN lens optics, has been characterized as a mechanically tunable wavelength demultiplexer for use in fiber optic transmission systems. The device is capable of selectively demultiplexing a single channel from a plurality of wavelength multiplexed channels over a broad spectral range (750 to 900 nm), with an insertion loss less than 3.5 dB, and an optical crosstalk rejection greater than 20 dB at wavelengths ±6 nm from the transmission peak of the tuned channel.

An electromagnetic modal theory is presented for characterizing a tilt between the axes of two parabolic-index multimode fibers. The power loss, modal noise and distortion are analyzed by approximating the fiber modes by Laguerre-Gaussian functions. Using a uniform power distribution, this analysis corroborates a previously reported geometrical optics result that fiber axis tilt at the splice is approximately equivalent to an axial offset. It is shown that this equivalence applies to the prediction of loss values only, and that it becomes less accurate as the fiber mode volume decreases. For a given loss, the modal noise and distortion results for tilt offset are shown to usually be higher than those due to axial or longitudinal offsets, both for coherent as well as for partially coherent illumination.

The techniques for predicting total transmission system losses in a fiber optic cable installation involving multiple splices are relatively well known, and significant work has been done in the field of laboratory measurement techniques for quantifying cable (fiber) characteristics and performance characteristics of splices and connectors. The shift of this technology from the laboratory to the actual environment of a field installation, however, poses some significantly different problems to the user of fiber optic cables and splices. The basic instrument available for field use in testing optical links is the Optical-Time-Domain-Reflectometer (OTDR). An OTDR can be used to check the loss profile (dB/km) of a fiber within a cable prior to the cable installation, and to locate installation induced faults in the cable. The capability of the OTDR above to characterize splice losses in an actual installation can be significantly degraded depending on the location of the splice with respect to cable faults, the length of the cable (fiber) run, the characteristics of the individual fibers being spliced, and the ability of the OTDR operator to interpret the-data the instrument is presenting. By following some specific procedures regarding the sequence of installation and testing, and through better understanding of the performance of optical fibers and splices where actually installed in the outside plant environment, installation crews can significantly reduce installation time and time lost because of faults in an individual link that must be reworked. Addi-tional testing with instruments specifically intended for optical power measurement can provide a realistic basis for evaluation of the overall performance of a completely installed link and will give the transmission engineer a solid reference base for use in system set-up in addition to evaluation of system deterioration. Records made through OTDR measurements and optical power instrument readings can be used to quickly locate "accident" faults and evaluate system performance after restoration.

The interconnection of fiber-optic transmission lines to rotating devices requires the development of a continuously rotatable, low-loss, bidirectional optical rotary joint. This paper discusses the physical properties and optical performance of a rotary joint that will accommodate a single optical fiber with graded index profile and 50-micrometer core size. The device was developed for the Naval Research Laboratory by ITT, Electro-Optical Products Division to mechanically decouple optical fibers exiting from a rotating cable-winch system. For this specific application, a rotary joint can be used at each end of the winch axle, thereby allowing access to a total of two optical fibers. Since many uses of a rotary joint require the transfer of both electrical and optical power, the optical joint was incorporated into the housing of a 7-band, 10 Ampere (each) electrical slip-ring assembly provided by Electro-Tec Corporation. mhe nrecision bearings in the slip-ring housing provided a mechanical runout of less than .013 mm and an angular variation no greater than 0.2 degrees. Beam expansion and collimation techniques were used to reduce the dependence of optical loss on longitudinal and transverse offset at the rotational interface. Two quarter-pitch SELFOC rod lenses of 1 mm diameter, obtained from NSG, America, were used to achieve the beam expansion and collimation. The optical leads to the rotary joint were in the form of an unterminated single-fiber cable of approximately one meter length having a KEVLAR aramid sheath and a polyurethane overjacket. The maximum optical insertion loss at any rotational position was 1.3 dB at 830 nm and less than 1.7 dB at 1300 nm. The variation of insertion loss caused by device rotation was less than 0.25 dB at both wavelengths. At the conclusion of an endurance test of 310,000 revolutions at a 30 RPM rate, no degradation in performance could be observed.

Connection of optical fibers underwater and at deep ocean locations present a variety of problems which are not generally found on the surface of the earth. Among many others, these may be grouped into two general categories. Problems intrinsic of the environment such as those presented be seawater, its reflective index, the presence of silt and turbidity and its corrosive effect upon both fibers and lenses. Problems caused by the difficulties encountered by underwater operators, either human or robots, in effecting the basically delicate operations in coupling and uncoupling. A variety of solutions and configurations developed for underwater use are described including cases in which the signal is transmitted from one fiber to another at the same pressure level and cases where the signal is transmitted across a pressure barrier.

The use of plastic clad silica as a low cost, large core, high N.A. radiation hard, low loss optical fiber has been tempered by several unique termination constraints. These constraints are caused by the soft and relatively thick silicone cladding material which contributes to mechanical instability, low shear strength, poor concentricity, and higher losses when using termination techniques designed for glass/glass fiber systems. Specific methods developed for PCS usually require the user to accept a tradeoff between optical loss and mechanical stability of the termination. The results of this work indicates there are broad areas of real world optical system usage where these termination trade-offs are more than compensated for by the advantages. PCS fiber is an extremely viable and cost effective candidate for low bandwidth, short length (less than 1 Km) systems using inexpensive E-0 devices and connectors. And finally, because of some of its unique properties, it is highly probable PCS will be required for many of the Fiber Optic Sensors of the future.

Times Fiber Communications, Inc. has undertaken the application of fiber optic technology to the subscriber feed portion of cable television distribution systems. In other words, we are currently bringing fiber optics into the home. We have installed systems in various cities throughout the country over the past two years.