-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Fiber Optic Interconnect Overview
December 31, 1969 |Estimated reading time: 3 minutes
Optical fibers guide most Internet and long distance communication. The fibers link optoelectronic modules comprising light sources, detectors, optical components, integrated circuits, SMT passives, substrates, alignment aids, housings, adhesives, solders and thermal management elements.
By Leo M. Higgins III
Optical fibers often carry single data streams, or single multiplexed data streams. Optical fibers in newer, higher-bandwidth networks can carry data in multiple wavelength (up to 160 in development) light beams, using a technique called wavelength division multiplexing (WDM). Each wavelength can be encoded to carry independent, or independently multiplexed, data streams in a single fiber.
The three main elements of a silicate glass optical fiber are the waveguide core, the glass cladding and an external organic polymer buffer, resulting in a typical diameter of 125 micrometers. The fibers are drawn at high temperatures from preforms fabricated by various vapor phase processes. Eliminating most impurities and hydroxyl ions in the silicate glass permits efficient transmission of ever-longer wavelengths (greater than 1,550 nm).
Fibers used in optical networks are characterized as multimode (MMF) or single mode (SMF). Light travels down the core axis in SMF, rarely being incident on the core-clad interface. The light transmitted in an MMF comprises many modes, which propagate down the fiber because of total internal reflection (TIR) from the core-clad interface. Modes experiencing a high incidence of TIR events over a propagation distance are high-order modes, while those experiencing a lower incidence are low-order modes. Most MMF fibers are made by varying silica dopant levels in the core to provide a refractive index that decreases radially from the core axis. This Graded Index MMF reduces pulse degradation from modal dispersion, allowing longer distance transmission.
Optical communication requires light to be launched into the fibers. The LED or laser diode output must be coupled into the 8 to 9 micrometer SMF and 50 to 62.5 micrometer MMF cores. Light sources emit photons in paths or modes that diverge as they exit the source. High-performance optical transmitters require this diverging beam to couple with high efficiency, typically no better than approximately 82 percent, into the fiber's core.
The refractive index difference between the core and clad controls the ability of non-axial modes to launch down the core. Modes that launch into the core or cladding at less than a critical acceptance angle (a) to the axis of the core will escape into the cladding where they soon are lost. The core-clad index difference affects the acceptance angle for modes that will launch successfully into the core and experience TIR at the first incidence with core-clad interface. Only these modes will propagate down the fiber. Because angle a also is influenced by the indices of the two materials in contact at the core endface, angle a can be changed by altering the medium in contact with the endface (typically air, vacuum or refractive index-matched polymer).
Lengths of fiber ("pigtails") commonly are bonded to optoelectronic modules. Pigtail ends often are spliced to other fibers by fusing the end pairs. Fusion involves buffer removal, cleaning, cleaving, precision alignment, heating/fusion, splice coating and testing. The goal is to form a low loss (typically 0.02 to 0.04 dB), high strength splice. Fibers also may be coupled by mechanical means. Mechanical splicing generally requires that fiber ends be bonded in glass, ceramic, metal, or organic polymer sleeves or ferrules, and then the ferruled fiber ends are polished. Defects in the fusion splice, or anywhere on the glass surface under the buffer coating, can cause crack propagation if the fiber bend radius is too small, or tensile stress is too high. Also, bending a fiber to less than a 25 mm radius may cause some light modes to be incident on the core-clad interface at less than the critical angle for TIR, allowing escape into the cladding.
In a high-performance laser transmitter or receiver module, the fiber is aligned with the active device and typically is bound in place with welded micro-clips, solder glass, metal solder or organic adhesives. With MMF applications, reduction in assembly and fiber handling costs are driving pigtail elimination. It is becoming common to assemble connectors on the ends of a length of fiber and to form a receptacle for the fiber on the active (or passive) device module, enabling mechanical connection of the fiber. To ensure low loss connector coupling, it is necessary to assemble and polish the fiber end, and join this end to a male connector element. The fiber must be aligned precisely to the internal and external features of the male connector elements. The female receptacle on the module must be manufactured similarly with accurate dimension control.
Leo M. Higgins III, Ph.D., may be contacted at Siemens Dematic Electronic Assembly Systems, 11921 North Mopac Expressway, Suite 120, Austin, TX 78759; (512) 997-0602; Fax: (512) 997-0621; E-mail: leo.higgins@siemens.com.