By Pat Gilliland and Carlos Jines
LC and MT-RJ connector geometries affect their suitability for use in SFF optical transceivers.
Originally designed to answer the need for smaller passive optical interconnects, LC and MT-RJ fiber optic connectors are now being adopted by transceiver original equipment manufacturers (OEMs) for their active optical interfaces. The LC connector is single-fiber and utilizes the traditional cylindrical ferrule and split-sleeve coupling approach. It addresses the need for high-density, configurable systems such as patch panels and switches.
 Figure 1. The LC connector angular displacement error (top) and the LC connector lateral displacement error (bottom). |
The MT-RJ connector originally started as the MT connector — a multifiber array connector with up to 12 positions. The MT connector utilizes a multifiber rectangular ferrule with two locating pins on the plug, and two corresponding bores on the mating connector. The evolution of the MT connector into the MT-RJ connector was undertaken to meet the needs of OEMs of local area network (LAN) network interface cards (NICs) and switches. The MT-RJ connector retains the rectangular ferrule, the locating pins, and the precision mating bores of the MT connector. However, the MT-RJ connector includes only two fiber positions.
Optical Interface Needs
In response to marketplace pressures for increased densities, OEMs of optical transceivers established the Multi-Sourcing Agreement (MSA) Committee in 1998. This committee was formed to develop, among other things, an interchangeable footprint for small form factor (SFF) optical transceivers that would support both LC and MT-RJ connectors. Because the footprint is interchangeable, the connector type can be chosen late in the manufacturing process.
 Figure 2. The MT-RJ connector mating components. |
Today, the MT-RJ connector is employed in multimode 1,300 nm light-emitting diode (LED) 10/100 Ethernet, asynchronous transfer mode (ATM) OC-3 and fiber distributed data interface (FDDI) transceivers. More recently, attempts have been made to incorporate the MT-RJ connector into 850 nm Gigabit Ethernet and 1.0625 Gbps Fibre Channel transceivers.
Some OEMs continue to support this connector for 850 nm Gigabit Ethernet and Fibre Channel applications, but others have shifted to the LC connector.
The LC connector has been used as a transceiver connector by a number of manufacturers. The LC interface is available on both multimode 850 nm and singlemode 1,300 and 1,550 nm laser-based transceivers, as well as on some multimode 1,300 nm LED transceivers. Gigabit Ethernet, Fibre Channel and ATM OC-48 transceivers are available with the LC interface.
Manufacturing Tolerances
 LC connectors |
As a manufacturer of both types of connectors, Stratos Lightwave supports the MT-RJ and LC form factors. When it comes to the manufacture of SFF optical transceivers, however, Stratos has chosen to utilize the LC connector. The decision was based on an analysis of the geometry issues surrounding MT-RJ and LC connectors. From this analysis, it was determined that the LC connector geometry is better-suited for use in SFF transceivers. This refers to the capabilities required for the precision molding and machining or grinding operations necessary to produce acceptable optical performance.
 MT-RJ connectors |
The analysis included an exploration of what would be required to mold precision receptacle bores. The technology available to machine and grind precision outside diameters on the LC ferrule as well as the locating pins on the MT-RJ connector was also assessed. Positional tolerance was the third key parameter analyzed.
In the LC connector, this positional tolerance is reflected in the concentricity requirement for the fiber opening in the ferrule. In the MT-RJ connector, this tolerance is the positional accuracy and manufacturing tolerance on the locating pin dimension and pin receptacle bores, as well as the two fiber openings in the rectangular ferrule. For this discussion, these molded tolerances are assumed to be the same for both LC and MT-RJ connectors.
LC Multimode Analysis
In multimode LC applications, the ferrule is made of molded polymer.
Misalignments, which lead to a loss of optical signal in mated pairs, can be caused by two mating problems: 1) angular displacement error — the axis of the fiber core being at an angle to the axis of the receiving bore, and 2) lateral displacement error — the center of the fiber core and ferrule not coinciding with the center of the receiving bore (see Figure 1).
Based on the assumption that both the ferrule and the receptacle bore are molded parts, each with a tolerance of 3.0 mm, geometric calculations show that the maximum possible angular displacement is 0.085°. This small computed angle would not lead to any significant optical coupling loss due to a reduction in the numerical aperture of a multimode fiber.
Therefore, it is the lateral displacement error that creates signal loss. The maximum lateral displacement of 4.4 mm is computed as the sum of the concentricity with one-half the difference between the receptacle maximum outside diameter and the ferrule minimum inside diameter. In a multimode fiber, 4.4 mm of lateral offset predicts a maximum optical coupling loss of approximately 0.5 dB for 50.0/125.0 mm multimode fiber and 0.4 dB for 62.5/125.0 mm multimode fiber.
LC Singlemode Analysis
In LC singlemode applications, the ferrule is made of Zirconia or Alumina ceramic. This material can be ground and polished to a high degree of accuracy when using state-of-the-art machinery. Similarly, the bores can be ground and polished using the same high-precision equipment. The result is a reduction in the angular displacement error to 0.041° maximum. Because this is less than half of the 0.085° for multimode, there would be no optical coupling loss associated with an angular displacement error.
Because of the precision manufacturing technology available for ceramic ferrules, the lateral displacement error for singlemode ceramic LC connectors is also reduced, to just 2.85 mm, leading to a predicted maximum optical coupling loss of 1.4 dB for a 9.0 mm singlemode fiber.
MT-RJ Multimode Analysis
The multimode MT-RJ connector is manufactured of molded plastic with mating components (see Figure 2). The primary loss mechanism is the lateral displacement error of the fibers in the plugs relative to the fibers in the receptacle. Here, the lateral displacement error can be caused by a number of factors including tolerances in the diameters of the mating pins, tolerances in the diameters of the alignment holes and tolerances in the locations of the fibers, both in the plug and the receptacle. In other words, lateral displacement in the MT-RJ connector is caused by the statistical sum of a number of possible dimensional inaccuracies (see Figure 3).
 Figure 3. Sectionalized view of the MT-RJ connector showing dimensions and tolerances. |
Analysis of these tolerances indicates a potential lateral displacement error of 5.0 mm. This number is comparable to that of a multimode LC connector, and would indicate a comparable optical coupling loss. Furthermore, additional optical coupling losses in the MT-RJ connector can be caused by air gaps between fiber end faces. These air gaps can become significant when the delicate MT-RJ mating mechanism is subjected to side loading.
MT-RJ Singlemode Analysis
The singlemode MT-RJ connector, also made of molded plastic, does not afford all the opportunities for increased manufacturing precision as does the ceramic singlemode LC connector. However, Stratos has been able to reduce the lateral displacement error in the singlemode MT-RJ connector to 4.5 mm. Although this number may appear to be an improvement over the multimode MT-RJ connector, the core of the singlemode fiber still has a diameter of only 9.0 mm. Therefore, the 4.5 mm lateral displacement error leads to a larger singlemode optical coupling loss, calculated to be in excess of 4.0 dB (see Figure 4).
Conclusion
 Figure 4. The MT-RJ connector lateral displacement error (singlemode). |
For both LC and MT-RJ fiber optic connectors, the primary cause of optical coupling loss is the lateral displacement error of mated pairs. Multimode LC and MT-RJ connectors provide comparable levels of optical performance. However, in singlemode applications, LC connectors provide significantly better optical performance.
For multimode transceivers employing LEDs as the active optical element, and where cost is the primary issue, MT-RJ connectors may represent a preferable choice even though a greater optical coupling loss would be predicted because of the greater lateral displacement error. In transceivers employing lasers as the active optical element, and in singlemode transceivers, the significantly better optical performance of LC connectors is likely to make it the connector of choice.
PAT GILLILAND is Director of Product Development, and CARLOS JINES is Mechanical Engineer, Stratos Lightwave LLC, 7444 W. Wilson Ave., Chicago, IL 60706; (800) 323-6858; Fax: (708) 867-9621; Web site: www.stratoslightwave.com.
SPEC SHEET
End Applications:
SFF optical transceivers
Related Products:
LC and MT-RJ connectors, ferrules
Main Point:
When it comes to the manufacture of SFF optical transceivers, LC connectors are likely to be chosen. This is based on analysis of the geometry surrounding MT-RJ and LC connectors. For both connectors, the primary cause of optical coupling loss is the lateral displacement error of mated pairs. Multimode LC and MT-RJ connectors provide comparable levels of optical performance. However, in singlemode applications, LC connectors provide better optical performance.
