Passive fiber-optic splitters keep pace with advanced network needs
Field deployment data and qualification tests confirm that complex optomechanical splitters are contributing to network reliability
David R. Maack and
LAUra b. slyk
porta systems corp.
Mature, reliable and economical passive splitters and wavelength-division multiplexers, which are essential for redirecting fiber-optic signals, endow the development of advanced lightwave networks. Broadband video, graphical interfaces and local area networks will continue to involve increased system complexity, optical branching, bidirectional transmission and wavelength-multiplexed signals.
To this end, splitter performance and reliability are keeping up with market requirements. Manufacturing processes are resulting in increased yields and improved reliability. For example, fused-biconic technology for fabricating higher port-count devices is reducing the number of components needed in a fiber-optic system. New products and breakthroughs, such as variable ratio splitters, unitary geometry and narrowband wavelength-division multiplexers, are becoming readily available.
In addition, implementation of major reliability programs, along with ISO-9000 registration by the leading splitter manufacturers, demonstrates market commitment and quality recognition.
In fact, long-term testing by several organizations, such as Bell Communications Research and British Telecommunications, and extensive field deployment data, confirm that passive optical splitters are successfully serving long-distance telecommunications, local-loop distribution, cable-TV, local area network and specialized communications applications.
Passive fiber-optic splitters are bidirectional, m¥n branching devices that divide the entering light signals in accordance with predetermined ratios. Each input/output signal pair has a characteristic power transfer function or insertion loss that depends on both wavelength and polarization. Specific devices are created by controlling the coupling ratio (and thus, the insertion loss) and wavelength sensitivity during the manufacturing process.
Common types include the wideband coupler or splitter and the wavelength-division multiplexer. The coupler divides the input power with minimum wavelength sensitivity over a range of wavelengths, typically the 1310- and 1550-nanometer windows. The multiplexer is a special 1¥n splitter that divides the signals into separate wavelength bands rather than power ratios. In operation, each connected output fiber captures all the power in one of the bands.
Common wideband couplers employ 1ٴ, 2ٴ, 1ٸ, 1ٺ, 2ٺ, 1䂄 and 2䂄 configurations. The more dominant 1ٴ and 2ٴ couplers are applied in a variety of splitting ratios. Larger port counts are usually for power dividers. The most common multiplexer configuration is the 1ٴ type, where output leg 1 passes the 1310-nm band signal, and output leg 2 passes the 1550-nm band signal.
Other important optical parameters for splitters are reflection, directivity and isolation. Reflection is the amount of input power reflected back into the same fiber. Directivity is the amount of input power reflected back into the other fibers on the same side of the splitter. Isolation, a multiplexer parameter, measures the amount of alternate fiber signal or crosstalk. Like insertion loss, each parameter is a function of both wavelength and polarization and must be carefully specified to ensure optimum component performance.
Three major construction technologies are used for making passive optical splitters and wavelength-division multiplexers--discrete micro-optic, planar and fused-biconic taper techniques. Micro-optic technology fabricates splitters by mounting small, but discrete, optical components on a stable platform similar to a standard optical bench. The advantages of micro-optic devices are the wide range and diversity of produced devices, including narrowband multiplexers and specialized devices.
On the debit side of micro-optic devices, the light signals must leave and then re-enter the optical fiber. This path creates alignment and cleanliness issues that generally require labor-intensive manufacturing techniques. Maintaining long-term stability and reliability are other shortcomings.
In planar fabrication technology, devices are made using ion-exchange or photolithography techniques that replicate solid-state circuit methods. Various splitting components are available, including 2䂄 and 2䂔 units. Ultimately, the per-unit cost for the expected high volumes will become advantageous for planar technology, especially for higher port devices. A difficult manufacturing problem involves a low-loss method for attaching the optical fibers to the chip and then passing the market`s qualification and reliability requirements.
The fused-biconic tapered technology directly bonds or melts the fibers together so that the final splitter can be mounted in small diameter (approximately 3-millimeter) stainless-steel tubes. This technology produces small, low-cost, high-performance devices. A tough fabrication obstacle involves the small and delicate final coupling region. However, when properly mounted and packaged, these devices meet long-term stability and reliability requirements.
Until recently, fused-biconic tapered technology devices were available mostly in 1ٴ and 2ٴ configurations. Higher port counts were assembled by concatenating 1ٴ and 2ٴ components and assembling them in a larger structure. This approach, however, mitigated some of the fused-biconic tapered technology cost, size and reliability advantages. Consequently, fabrication techniques have been developed that fuse larger numbers of fibers simultaneously in a unitary geometry technique such that the final package size is similar to that of the 2ٴ coupler. High-performance 1ٶ and 1ٸ configurations are available, with 1ٺ technology expected to be introduced soon.
Applications and trends
Long-distance telecommunications systems are using large quantities of wavelength-division multiplexers to increase the signal capacity of cables that had reached their maximum bandwidth capacity. The use of these wavelength-division multiplexers provides an economical solution over the installation of new transmission lines. The multiplexers pass multiple signals at different carrier wavelengths. The signals are then combined onto a single fiber and separated into either unidirectional or bidirectional mode. The common configuration incorporates the unidirectional mode with one wavelength band centered at 1310 nm and a second band at 1550 nm. Field reliability data on these devices include more than 400 million device-hours of operation.
Local loop distribution or fiber in the loop appears to be a promising application for large-scale implementation of wavelength-division multiplexers and wideband couplers. Analysts view FITL as a two-step deployment program, but with substantial overlap. Initially, they forecast that fiber-to-the-curb architecture will be deployed. Then, after technical obstacles are overcome and economic justification is secured, fiber-to- the-home technology will be installed. Splitters are integral components of both architectures because they are installed abundantly throughout passive distribution networks and passive optical networks. In practice, splitters are placed inside curbside pedestals, on the sides of houses and in hostile environmental locations.
The implementation of FITL has been slower than anticipated. One holdback was the lack of sufficient splitter reliability and long-term field experience. Accumulated data demonstrates that splitters are easier to manufacture and are meeting demanding optical performance requirements.
The splitters that must meet the stringent qualification demands of such organizations as Bellcore and British Telecommunications, however, are not so easy to fabricate. These demands have therefore emphasized the importance of reliability programs.
Other factors affecting splitter deployment in FITL architectures include the costs of installing fiber and upgrading older copper networks. The Public Utility Commissions in most states have not allowed the rate increases that would pay for these initial capital investments, thereby driving network providers to search for new revenue sources.
An interim concept to reduce initial costs and accelerate FITL is hybrid fiber/coaxial-cable architecture. Recent developments with new interactive demands, however, may push the economic advantage back to FTTC, which would result in higher deployment of splitters.
The cable-TV industry, a major user of broadband copper cable, markedly increased the deployment of optical fiber and splitters in 1994. The use of splitters in cable-TV networks significantly reduces the quantity of expensive lasers and associated electronics. Splitters also enable cable-TV networks to carry bidirectional transmission, interactive broadband and voice communications.
Cable-TV system architecture calls for complex concatenated wideband splitter arrays with a multitude of different power split ratios. Each array is a custom package of splitters, racks, housings and connector interfaces. It offers network flexibility in managing and administrating the optical fiber and associated splitter assemblies. Cable-TV insertion loss requirements require the careful matching of high-performance splitters, splices and connectors to ensure optical network performance.
Another key application for splitters is in local area networks. The amount of fiber in building riser cables can be reduced by placing splitters in closets and combining the signals from groups of workstations. Additional fiber and lasers can be saved by using splitters for bidirectional rather than unidirectional transmission. Moreover, to increase network efficiency, all-optical ring and star architectures can be built with splitters.
Local area networks often use multimode splitters to accommodate multimode fiber components and the short distances between lasers and detectors.
Various specialized applications such as gyroscopes for missiles, airplanes and automobile navigational systems; power delivery systems for laser fusion, metal fabrication and laser angioplasty; and temperature, pressure and electrical sensors, also use splitters.
Qualification and reliability
Strong industry indicators of the acceptance of passive fiber-optic splitters are the issuance of major specification documents by important telecommunications bodies. Typical documents include:
Bellcore TA-NWT-001209: Generic Requirements for Fiber Optic Branching Components.
Bellcore TA-NWT-001221: General Reliability Assurance Requirements for Fiber Optic Branching Devices.
British Telecom RC8937: Specification for Passive Optical Splitters.
British Telecom RC8938: Specification for Passive Optical Wavelength Division Multiplexers.
In general, these documents specify performance requirements, qualification tests and reliability criteria. Performance requirements comprise the optical, mechanical and dimensional characteristics that the passive devices must meet. u
David R. Maack is director for special projects, research, development and engineering, and Laura B. Slyk is manager for technical services at Porta Systems Corp. in Hopkinton, MA.