The performance requirements and device specifications of single mode couplers have been changing rapidly in recent years. This paper reviews the present state of the art in this technology and introduces new device concepts which rely on the fuse-taper technology in their fabrication process. Details will be given of the fabrication and application of single mode wavelength division multiplexers which have an insertion loss below 0.5 dB and have a 20 dB isolation over a 30 nm operating wavelength range. Wavelength division multiplexers with a narrow wavelength separation, <5 nm, will also be described in terms of their fabrication and application. Details of the utilization and performance of concatenated wavelength division multiplexers as filters for uni- and bi-directional communication will also be presented. Finally, techniques for reducing the wavelength sensitivity of the coupling ratio in single mode couplers will be discussed which result in the development of a broad band coupler, BBC.
Many fiber optic systems in operation today require low loss single-mode couplers to passively split or combine light between two or more single-mode fibers. At present, this need is best met by standard fused biconical taper couplers because of their ease of fabrication and very stable performance over a wide range of operating conditions. However, as single-mode fiber technology transitions from today's predominately long haul applications to tomorrow's local area networks (LANs) and subscriber loops, more sophisticated single-mode couplers will be required. For example, many of these systems will likely incorporate wavelength division multiplexing techniques and, as such, will require several types of wavelength dependent couplers. In anticipation of this growing need, three different types of single-mode, wavelength-dependent couplers have been developed at Gould. A discussion of the wavelength dependent coupling in these fused couplers is presented in this paper.
A tree configuration single-mode fiber wavelength division multiplexer (WDM) is proposed and demonstrated. This configuration is similar to tree topology single-mode star couplers which have been reported. Both theoretical analysis and experimental data show that a tree WDM can easily achieve a very high wavelength isolation over a wide wavelength range aroun 1300 nm and 1520 nm. The wavelength isolations measured in our experiment are greater than 28 dB both from 1255 nm to 1335 nm (80 nm-wide range, 41 dB at 1305 nm) and from 1490 nm to 1565 nm (75 nm-wide range). The insertion losses are 0.7 dB at 1300 nm and 1.1 dB at 1550 nm.
A theoretical investigation of the manufacturing tolerances necessary to fabricate fused 3x3 single-mode fiber optical couplers with acceptable port-to-port uniformity in the coupling ratios is reported. Arbitrary coupler geometries are assumed, i.e. the core-to-core distances may all be the same or they may all differ. Thus, there may be one, two or three different coupling constants determining the behavior of the coupler. By using the three equations relating the fields in the three cores to one another and an approximate expression for the coupling constant, coupling ratios and uniformities can be determined for arbitrary coupler geometries. The manufacturing tolerances on core-to-core separation can be determined once the required coupling ratio and uniformity are known.
We propose the use of holographic optical elements (HOEs) to make 1XN couplers for single-mode fibers. The main advantage of this approach is that each HOE is made with the particular fibers that it is intended for, and automatically adjusts to their locations, which need not be chosen in any particular manner. A number of possible techniques are considered. All require the use of a focusing element at some point in the process, but some do not use a lens in the final device; clearly that situation is preferred. At this time, a transmission hologram which generates a time-reversed replica of the output of the original N fibers looks most attractive. In theory, the overall efficiency of such a holographic coupler can approach 100%, regardless of the value of N. The fabrication technique is also virtually the same for all N. For these reasons, this method could be particularly attractive for large values of N, where more conventional techniques become less efficient, and more costly. In this paper we present theoretical and practical considerations concerning the fabrication of these devices. We also describe preliminary experiments which we have performed, and point to promising experimental techniques for future work.
The technology for utilizing micro-optic components with fibers has been established in various LAN, airborne and other telecommunication systems. Similar devices are being developed and tested for spectroscopic applications. Parameters and performance data for both are discussed in detail.
The passive linear bus is an attractive topology for a fiber optic Local Area Network (LAN). An effective use of this architecture requires the development of an asymmetric optical coupler. Fabrication and measurement of such a coupler is described in this paper.
The performance characteristics of any multimode fiber optic component, be it connector, coupler, multiplexer, or switch, are dependent on the modal intensity profile at the input of the device. It follows that any component that restructures the modal distribution from its input ports to its output ports can affect the overall system performance by modifying the operation of downstream parts. Specifically, commonly available multimode couplers are known to modify the input modal distribution as a result of the coupling mechanism; typically, the coupled fiber includes an overabundance of higher order modes while the straight through fiber suffers from a depletion of these higher order modes. The modal profiles of the coupler output ports can be actively monitored during the manufacturing process, thus allowing this information to be fed back to optimize the resulting device. The results of such measurements, which show the evolution of the coupling process during fabrication, will be presented and discussed. This capability raises questions concerning quality control measures, specifications, and the actual influence of a single component's capability on different systems' overall performance.
Designers of electro-optic systems must possess strengths in a variety of fields. It is not unusual for a systems engineer to be proficient in mechanics, dynamics, electronics, sensors and economics. However, an optical expert of the highest caliber can have difficulty in assessing the advantage of a multi-mode optical fiber in even a simple system application. The data available on multi-mode optical fibers frequently is not presented in a form which can be readily interpreted into system concepts or parameters. The conventional presentations can be highly analytical or very philosophical. An analogy would be to require electronic engineers to design circuits using transistor data only specified in terms of mobility, volume and recombination parameters. The typical analytical approach is developed from rather sophisticated single-mode waveguide theory. An analytical extension to multi-mode optical fiber is precise, but can be difficult to interpret and apply to a specific multi-mode application. The philosophical approach allows one to feel that an understanding of fiber optics exists. However, an inability to demonstrate this understanding usually occurs when the designer is pressed to apply values to necessary system parameters. The theory of multi-mode fiber optic components is presented herein in terms of simple component and system concepts which can be comprehended by the layman. Each concept is accompanied with illustrations and sufficient equations so that a designer can develop the needed mathematical expressions to make trade-offs in system parameters and to predict system performance. The emphasis is to describe fiber transmission properties in terms which better assist the designer of successful systems.
A low-cost two wavelength multi/demultiplexer was fabricated for multimode local area network applications in the transportation vehicle environment. Polymer optical waveguides, a Y-splitter, and intra waveguide filter were combined to form the optical circuit using photofabrication techniques amenable to mass production methodology. A 6 dB insertion loss was attained which was 3 dB greater than the expected 50/50 splitter loss.
Optical communication systems require components that allow for optical switching to access networks or to permit redundancy of active components. The component presented here has been developped to be used as a bypass switch in a network within an hostile environment.
An innovative 2 x 2 Optical Switch has been designed by GTE Fiber Optic Products as an electromechanical bypass product for Data Communication, Local Area Network, and Telecommunication applications. The switch provides a means for easily transferring optical signals from one fiber to another in a two by two or quad port configuration. GTE's design differs from others on the market in that it uses expanded beam technology with a sliding prism oriented to reduce lateral alignment sensitivity. This tolerance of misalignment results in notably consistent throughput - a feature not always found in electromechanical switches. The design calls for two input fibers on one side of the switch with two output fibers opposite. Because a loopback fiber is unnecessary, optical throughput is similar for all four beam paths. Other advantages include beam separation flexibility and a more rugged package. Using this technology, both latching and fail safe designs have been developed. Optical switches are increasingly being used to provide system protection and to aid in network testing during installations and troubleshooting. In Local Area Networks, particularly those of a ring configuration, 2 x 2 switches are also gaining acceptance as a technologically simple and smart way of isolating or alternately connecting parts of a fiber network without introducing high losses.
The properties of glass optical fibers are not significantly different from those found in a form of bulk or sheet glass. A notable exception with smooth glass fibers is that the strength degrades quickly with small surface cracks and flaws that can result from handling and material processing. The strength of glass optical fibers is governed by mechanical properties similar to those of crystalline solids. Glass is elastic up to the fracture point and will fail in tension before it does in compression. Glass is considered a brittle material because no plastic flow or permanent deformation takes place when tensile fracture occurs. Since glass is a strong elastic material it will obey Hooke's Law over a wide range of stresses, therefore the stress in an optical fiber only depends upon two elastic constants: Young's modulus, E, and the bending strain, ε. Under bending the cross sectional area will also neck down and a decrease by the factor known as Poisson's Ratio6, μ. The Poisson effect is important in calculating stress, but is usually ignored by the fiberoptic industry. This paper addresses the use of Poisson's Ratio in the stress equations for both bending and twisting strains.
The chromatic dispersion characteristics of several heavy metal fluoride fibers were evaluated as a function of wavelength, core diameter and index difference for a single-mode step-index fiber structure. The results show that, by appropriate choice of core diameter and index difference for each material, the total dispersion can be reduced to zero over a broad wavelength range. It has been demostrated that it is possible to have zero-dispersion fluoride fibers at the minimun loss wavelength of 2.5 μm, but further study of other core profiles as well as the role of the extrinsic losses is necessary for a complete design study of these single-mode fibers.
The paper deals with Multicore Optical Fibres /MOFs/. Such fibres have two or more single mode or multimode or simultaneously both kinds of cores distributed in the cladding. The individual cores could be naturally coupled or not. We have made a few families of microoptical components of these fibres. These components have been used for construction of the following devices, circuits and equipment: hybrid integrated active optoelectronic devices, fiber optic network nodes, microoptics for links with multifactor flow, buses, signal processing - local and distributed, and various kinds of simple links. In the paper we outline the fibering techniques for multicore filaments, some of their theory and define basic kinds of operations. Then we present kinds of multicore optical fibre microcomponents and some results of our own laboratory experiments. The work concludes with a digest of existing as well as potential and ultimate applications of MOF microoptics and MOFs themselves. In other words, the paper deals with new possibilities of optical signal transmission, distribution and more complex processing like optical direct multiplexing, demultiplexing, mixing, interfacing, interfering, coupling in MOF microdevices for IO and COFOCS.
Topologies of the following optical fibre networks are presented: parallel and in series multiloop nodes,with single or multiple coupled loops, with branched loops, multiport-multiloop hybrids. Some properties of a loop coupler have been investigated in an optical fibre data link. Analysis of a laboratory model of optical fibre pulse sequencer consisting of three or four loop couplers has been presented.
The relationships between optical launch conditions (application) and fiber optic connector losses are developed. An analysis of fiber optic connection losses which covers eight factors such as ferrule hole eccentricity and hole diameter, fiber numerical aperture, and core diameter is given. The analysis is statistical in nature and based on a Monte Carlo computational method. The results of precise dimensional measurements of connector components are included in the evaluation. Specific examples for 62/125μm fiber connections are cited and the evaluation is extended to multi-connector LAN systems. The results of the analysis are compared with actual test results achieved with the connectors, and the use of the program to predict performance results in new or specific LAN applications is explained.
The achievement of low fiber optic interconnection losses depends upon precise fiber alignment, and on the quality of the fiber end faces. This paper is concerned with end face quality as obtained through the use of fiber cleaving tools. High quality end faces require careful design and use of cleaving tools. The factors affecting end face quality are addressed, including scribing, tensile force and twist angle. Deviations from optimal conditions produce a number of different types of defects. A basic requirement for low interconnection loss is that the end face be nearly perpendicular to the fiber axis. The fiber end angle relative to the ideal plane can be minimized by attention to the methods used for clamping the fiber and for applying the tensile force. In addition to the end face angle, a number of other defects are identified, including hackle, mist, cracks, lips, breakover, and chips. These defects are related to the manner in which the scribing and cleaving are performed. The effects of the defect, as wen as the influence of index-matching media and fiber fusion on losses, are discussed. Some characteristics of commercial cleaving tools are given. Fiber Optic Test Procedures are being developed by the Electronic Industries Association (EIA) to identify and quantify fiber end face defects. Generic and Sectional Specifications are also being written for cleaving tools by the EIA FO-6.1 Subcommittee. These are major steps toward defining standards for quality of cleaved end faces.
"BORNOPTIQUE" developped by Amphenol-Socapex is a small sized remountable splice allowing a simple and rapid (thus performing ≤ 0.3 dB) connection of 2 fibers with a core diameter greater than or equal to 50 urn. All parts of this component are manufactured by injection molding, which explains that "BORNOPTIQUE" can work as mixing agent into the optical distribution frames. First we present the operating principle of "BORNOPTIQUE" along with the associated tool kit, and then will describe its characteristics (attenuation, mechanical and thermal characteristics).
Two types of emergency optical fiber cable systems and an easy use connector have been developed. The first is a 250 m length of 12-fiber cable which is used when an optical fiber cable is damaged at one point due to construction work as gas, water lines or roads. The second is a 1 km length of 4-fiber cable which is used during large disasters such as typhoons, earthquakes or floods. The easy use connector is capable of being joined in 5 minutes without using a special tool.
An optical fiber coupler based on the semi-linear passive phase conjugate mirror is analyzed. The threshold coupling strength is determined from theoretical considerations. An estimate of 50% for the minimum overall coupling efficiency is made assuming there is good mode matching between the incident light and the fiber. The coupler should exhibit a high degree of alignment insensitivity providing for simple construction.
Data are presented for an ultra-low-loss multimode SMA connector. Histogram data showing insertion loss data below 0.6 dB will be presented for 50/125 and 100/140 fiber. These results were achieved via the use of ultra precision ceramic ferrules, controlled critical connector length, and a new termination procedure. These data were collected in conjunction with a new precision adapter. This interconnection system showed very low variability in: remating, rotation, and temperature cycling.
The development of a single mode optical rotary joint based on expanded beam technology is described. The trade offs of expanded beam versus butt coupling design are discussed and the loss mechanisms for the two approaches are summarized. A V-groove and ball bearing alignment technique utilized to orient the two halves of the rotary joint is described and mechanical tolerances for it are summarized. This design is shown to yield < 2.0 dB ± .5 dB total loss in rotation with improved performance expected as assembly techniques are improved.
We compare the basic detector structures used in receivers for optical communication system applications. The three types of detectors most commonly considered for the direct detection of digital signals are the photoconductor, the p-i-n detector and the avalanche photodiode (APD). A comparison of these structures from a noise (and hence, sensitivity) and bandwidth perspective indicates that reversed biased detectors such as p-i-n photodiodes and APDs afford the greatest advantages, although the latter structure can be extremely difficult to fabricate.
The hydride vapor phase epitaxy (VPE) technique has been used to grow In(x) Ga(1-x) As on InP substrates with 0<x<0.8. The "double barrel" VPE reactor along with relevant growth parameters will be discussed. The effects of temperature, flow rates and reactor geometry on compositional and thickness uniformity of grown layers will be described. An automatic probing station has been used to evaluate devices (i.e. p-i-n photodetectors) across the whole wafer. Device characteristics will be correlated with the compositional and thickness variations of grown layers on that wafer. Finally, detectors manufactured from the above wafers will be discussed, i.e.: (a) detectors with extended spectral response from 0.8 microns to 2.55 microns, as opposed to standard In(0.53) Ga(0.44) As detectors which respond in the range of 1.0 - 1.7 microns; (b) operational characteristics of large area In(0.53) Ga(0.44) As detectors with diameters of 1.0, 2.0 and 3.0mm respectively, will be presented and discussed.
Room-temperature Ga0.82 InO.18As0.17Sb0.83/GaSb photodiodes have been fabricated for use at wavelengths to 2.3μpm. Back-illuminated heterojunction p-n photodiodes show high external quantum efficiency (65%) for wavelengths between 1.8 and 2.3 μm. Front-illuminated pin homojunction photodiodes have an impulse response of 110 ps and respond to pseudorandom modulation at rates as high as 4 Gbit/s. Double heterostructure Ga0.841n0.16As0.15Sb0.85/A10.34Ga0.66As0.04Sb0.96 lasers have pulsed threshold current densities as low as 3.5 kA/cm2 at room temperature and operate cw at temperatures of up to 235K.
Response times of ~60 and 25 ps (FWHM), respectively, have been measured for photoconductive detectors fabricated in GaAs layers grown by molecular beam epitaxy on silicon substrates and silicon-on-sapphire substrates. Photoconductive detectors, which can be readily combined with GaAs logic devices such as MESFETs to provide high-speed optical to electrical conversion, could be used in optical interconnects that are integrated with Si circuits on monolithic GaAs/Si wafers. Transconductance values of 120 mS/mm have been obtained for MESFET's fabricated in GaAs layers grown on silicon-on-sapphire substrates.
The influence of PIN photodetector characteristics on receiver performance for both digital and microwave analog systems is examined, and the implications for device design and packaging are discussed.
A novel photodetector for long-wavelength fiber optical communication systems is presented and analyzed. The Ge/GaAs Heterostructure Avalanche Photodiode (HAPD) consists of a thin epitaxial p+-n-n+ or p+-n-v-n+ Ge structure grown on either semi-insulating or n-GaAs substrates. The use of a Ge/GaAs heterostructure improves the performance of the HAPD relative to conventional Ge avalanche photodiodes. The ease of fabrication and projected low cost make this photodiode particularly attractive for monolithic integration. Several methods for integrating the HAPD with other electronic and optoelectronic devices are described.
The push for high bit rates (above 1 Gb/s) and long unrepeatered fiber lengths (beyond 50 km) for transmission has spurred recent device research. Lasers with high bandwidth and/or high output power, and detectors with improved bandwidth, gain and noise characteristics all increase the system gain, or margin, permitting greater speeds and distances.
This paper presents data concerning the speed, reliability, coupling and package stability of 1.3 and 1.55 μm edge-emitting light emitting diodes (ELED). Two device geometries are considered; a continuous 10 μm wide stripe and a truncated 6 μm wide stripe (TS). In the latter, the p-side stripe contact is restricted to a fraction of the total cavity length ("pump"region), as shown in Fig. 1.
High performance surface emitting LEDs for use in integrated optical transmitters and receivers for data link applications at 200 Mbit/s are reported. Average optical power coupled to 62.5/125 um multimode fiber at 75mA peak drive current as high as -13dBm has been obtained, together with a rise and fall time of 1.5ns and 2.5ns respectively. Optical loss budget of more than 20dB for the transmitter/receiver pair has been demonstrated.
The present generation of optical transmission systems is based on the 1.3um wavelength range where conventional singlemode fibers have low attenuation and dispersion close to zero. The minimum attenuation of these fibers, however, lies close to 1.55um. At this wavelength transmission systems using semiconductor laser sources with launch powers of 1mW and conventional PIN-FET receivers could potentially operate with inter repeater spacings in excess of 100km. Improvements in transmitter power or receiver sensitivity would increase this spacing still further. The introduction of systems operating at 1.55um, however, has been inhibited by the dispersive nature of conventional silica fiber at this wavelength. Typical dispersion is 18ps/nm Km. The characteristics of the source therefore have an important effect on the overall system performance limiting the distance/bandwidth product that can be achieved. This paper describes the development of a range of 1.55um laser sources. It analyses their characteristics as components and shows their impact on system performance parameters.
High performance 1.3 um lasers and LEDs have been developed for optical communications systems. The lasers exhibit low threshold currents, excellent high speed and spectral characteristics, and high reliability. The surface emitting LEDs provide launched powers greater than -15 dBm into 62.5 um core fiber with rise and fall times suitable for operation to 220 Mb/s.
The fabrication and performance characteristics of InGaAsP semiconductor diode lasers and LEDs are described. Measured L-I, spectra, far-field angle, small-signal modulation bandwidth, as well as the optical power coupled into multimode and single-mode fibers are presented. Their applications in high-speed optical communications are discussed.