MU fiber-optic connector system
Shin`ichi Iwano Nippon Telegraph & Telephone
The MU connector system features plugs with a 1.25-mm-diameter ferrule and a self-retentive mechanism for backplane applications.
This spring Nippon Telegraph & Telephone (NTT) started to install a new optical access system called the¥system, which the company expects to accelerate the "opticalization" of access networks toward fiber-to-the-home. Compact and high-performance optical connectors, which interconnect equipment and cables, are essential for the construction of advanced optical systems. A compact optical-fiber connector system called the MU has been developed by NTT to help realize compact and densely packaged systems. This connector system can cope with a variety of optical connections, including fiber-cable connection and optical backplane connection.
The MU connector system features compact plugs (see Photo 1) with a 1.25-mm-diameter ferrule and a self-retentive mechanism for backplane applications. It consists of compact optical plugs, adapters, backplane connectors, and simplified receptacles. A variety of tools and accessories were also developed to facilitate both operation and maintenance. Ultra-compact optical modules have also been using the MU connector.
The basic element of an optical connector is its ferrule, and if connectors are to be miniaturized, the first step is to miniaturize the ferrule. Conventional 2.5-mm-diameter ferrules, which have recently become the worldwide standard, limit possible optical-connector miniaturization. When you miniaturize optical connectors, it is important to design the connector shape, structure, and dimensions so they are suitable for the intended equipment environment. Of the many types of optical-fiber connectors, optical backplane connectors are severely restricted in terms of their dimensions and structure by the physical design of equipment. Optical backplane connectors connect optical components on a printed board and optical cables through a backplane and are used together with electrical and coaxial backplane connectors. The dimensions of the area on which the connectors are mounted depend on the design of the equipment. The height of the area is limited by such structures as reinforcing bars. As well, the width of the area is constrained by the distance between two printed boards.
Taking the various practical conditions into consideration, a ferrule diameter of 1.25 mm allows the largest number of plugs to be installed on the backplane. In addition, when considering physical contact (PC) stability in relation to ferrule miniaturization, the 1.25-mm ferrule demonstrates sufficient mechanical strength for actual use and achieves stable physical contact. Moreover, 1.25 is one of the R10 preferred numbers. Therefore, NTT selected this diameter as the most suitable for compact multiple optical connectors and developed the 1.25-mm diameter ferrule, which the company then adopted for the MU optical connector system. This 1.25-mm ferrule is now employed in other compact optical connectors worldwide.
Backplane connections
Optical connectors generally employ PC technology to achieve low insertion loss, high return loss, and high reliability. Two connecting plug ferrules are butted together by coil springs, which supply a ferrule compression force of about 10 Newtons. With conventional optical backplane connectors, the ferrule compression springs exert a pressing force directly onto a backplane. The total pressing force increases in proportion to the total number of optical plugs/jacks mounted on the backplane. This force may cause backplane deformation. This, in turn, restricts the number of available optical connectors in a plug-in unit. A mechanism which can absorb the ferrule pressing force and which is suitable for the plugging operation is indispensable for multiple optical backplane connectors designed to realize a large number of optical connections.
To realize both the plugging operation and the pressing force absorption, Ntt has developed a self-retentive mechanism for optical backplane connectors (see Fig. 1). The mechanism incorporates two types of latching mechanism and a double-shell backplane housing structure (see Fig. 1a). The connector housing consists of an outer and an inner backplane housing and a printed board housing. These three housings are coupled and released by a main latching mechanism and an inner latching mechanism.
When the printed board is inserted into the backplane, the main latch is locked and the printed board housing is coupled directly to the inner housing (see Fig. 1b). In this state the plugs and jacks are mated and optical connection is completed. The ferrule compression springs temporarily exert a pressing force onto the backplane. As the printed board housing is inserted still further, the inner latch is released. The inner housing becomes free from the outer housing mounted on the backplane, and is coupled firmly to the printed board housing. The ferrule compression force is blocked by the main latch (see Fig. 2c). Thus, no pressing force is exerted on the backplane, and backplane deformation does not occur. The latching sequence for the removal of the printed board is almost the reverse of the insertion procedure.
MU connector system
The MU connector system is based on MU plugs featuring the 1.25-mm-diameter ferrule. The system consists of adapter-type optical connectors for cable connection (MU-A series), backplane connectors equipped with the self-retentive mechanism (MU-B series--see Fig. 2), simplified receptacles for connecting plugs to LD/PD modules (MU-SR series), and a variety of tools and accessories for operation and maintenance.
MU plug--The MU plugs are simplex, duplex, and flat-duplex miniature plugs. They are equipped with a push/pull mechanism and have a designed based on SC connector technology. The simplex and duplex plugs are commonly used with adapters, backplane connectors, and simplified receptacles. The 1.25-mm diameter zirconia ferrules enabled the desired plug size and cost. The simplex plug is 4.4 mm high and 6.6 mm wide. The plugs can be installed 4.5 mm apart, center to center. The ferrule has a rounded square-shaped flange and is symmetrical in relation to its axis by a 90 rotation. This makes it possible to rotate the ferrule by 90 in the plug for tuning; we have achieved a low insertion loss of <0.5 dB for random connection with singlemode fiber. The ferrule ends are polished by using the advanced PC (AdPC) polishing technique. This method realizes a high return loss of >40 dB and allows the plugs to be applied to systems requiring low reflection such as analog transmission systems.
MU-A series (adapter-type connectors)--The MU adapters are simplex, duplex, 8-port, and flat-duplex adapters. They use a miniature zirconia split sleeve that was developed to realize a low gauge-retention force and to improve mating durability. The adoption of a reverse spring-latching structure gives the adapters a plug-latching strength of >70 N. In addition, the plug-insertion hole in the adapter is asymmetrical along its axis, thus preventing a plug from being inserted with the wrong orientation. The width of the simplex, duplex, and 8-port adapter housings is 9 mm or less. This dimension is suitable for printed boards that are separated by a distance of 15 mm in a plug-in unit. The flat-duplex adapter is 6.6 mm high, which is suitable for a card-type package.
Adapter housings are assembled by interlocking two half-plastic housings, and do not need the ultrasonic welding that is commonly used in SC adapter assembly. This reduces their assembly cost. The adapters are mounted by using metal plates. The adapters can be used for a variety of mountings simply by replacing the mounting plates. Two types of metal mounting plates were originally prepared. One is for printed-board mounting, the other for panel mounting.
MU-B series (optical backplane connectors)--The MU optical backplane connectors are plug-in-type connectors equipped with the newly developed self-retentive mechanism. The connectors comprise MU plugs, a backplane housing, a printed board housing, sleeve holders, and j-plugs (see Fig. 2b). Eight- and 2-port housings were originally developed. Four-port housing is now also available. Two 8-port housings or four 2-port housings can be mounted in an area about 100 mm high. The j-plug structure is the same as that of an MU plug without an outer shell.
The 8-port backplane housing is 41.6 mm high and 13.2 mm wide, and the 8-port printed board housing is 43 mm high and 9.9 mm wide. The plugs and jacks are installed 4.5 mm apart, center to center. The density of this connector is four times higher than conventional DS backplane connectors. The pressing force to the backplane is reduced drastically both by adopting the self-retentive mechanism and by reducing the ferrule-compression force (about 6 N for the backplane connectors). In addition, the connectors can cope with variations in backplane thickness of 2.4 to 3.2 mm by changing mounting pins.
Backplane connector performance depends on the design of the guiding and housing floating mechanisms. To realize compactness and a reduction in part number, the MU backplane connectors use both housing guide and element guide mechanisms. The backplane housing is float-mounted on the backplane; the printed board housing is fixed to the printed board. This was undertaken to design the self-retentive mechanism and to avoid dynamic interference between the printed board connector and other components mounted on the printed board. In addition, a sleeve holder can be attached to or removed from the j-plug from the front-end of the printed board housing. This makes it easier to clean the ferrule endfaces.
MU-SR series (simplified receptacles)--In optical access systems, the cost of optical connection is in urgent need of reduction. Plugs arranged inside equipment are seldom handled after manufacture. Therefore, the strength to resist cable pulling, torsion, and bending, and to ensure mating durability, for example, is not required to be as great as that of conventional plugs. In addition, thin fibers such as 0.25-mm diameter UV-coated fibers are used for optical wiring inside equipment. The MU simplified receptacles were developed for such applications. Fiber connection is achieved by an MU plug and a simplified receptacle. Simplex, duplex, flat-duplex, and 16-port receptacles have been developed for a variety of applications.
The simplex simplified receptacles consist of an MU simplified plug, a plastic molded receptacle housing, and a narrow-diameter zirconia sleeve. The simplified plug is like the MU ferrule itself. It consists of only two parts: a 1.25-mm- diameter cylindrical ferrule with a molded flange and an optional reinforcement boot for fiber. The plug is symmetrical in relation to its axis by 90 rotation, and is equipped with a 90 tuning mechanism. Moreover, the plug weighs only about 0.2 g is about 3 mm in diameter making it suitable for thin-fiber applications.
The duplex and 16-port housings consist of an outer and an inner shell. The outer shell has a rather simple structure, and the inner shell has duplex latching mechanisms for both MU plugs and the simplified plugs. This separated receptacle configuration is intended to allow adaptability to a variety of applications. Receptacles with four or eight ports, for example, can be easily realized using this design (see photo 2).
Accessories--To facilitate the operation and maintenance of MU plugs in dense systems, various tools for operations such as plug handling and cleaning have been developed. They are designed so that insertion, removal, or cleaning can be undertaken by means of a one-way push/pull operation. The plug-handling tool makes it possible to insert or remove MU plugs easily from backplane housings or adapters even when they are densely installed. The use of this tool prevents the plug from touching the housing or adapter. This means the ferrule end is protected from staining or dust accretion during the insertion operation. The cleaning tools are used for cleaning the ferrule endfaces of jacks that are installed inside a printed board housing.
The diameter and shape of MU plugs are different from those of conventional connectors. A sensor attachment makes it possible to measure the optical power of a variety of connector plugs--such as FC, SC, DS, and MU plugs--simply by changing the attachment sleeve. MU-type termination plugs have also been developed. These are inserted into a connector`s open end, and terminate the open end without reflection (return loss of >40 dB).
Performance
Fabricated using PC technology and containing a tuning mechanism, the connectors have typical average insertion and return losses for a set of random concatenations with singlemode fiber of 0.07 and 50.3 dB, respectively. A variety of mechanical and environmental tests results also reveal a low insertion loss of <0.5 dB and a return loss of >40 dB. The insertion loss change and return loss in these mechanical and environmental tests are <0.2 dB and >40 dB, respectively. For example, the insertion loss change was <0.1 dB and the return loss was >48 dB during 1000 insertion/removal cycles. Additionally, there was very little optical degradation as the result of wear debris during insertion/removal-cycle tests. The results for the simplified receptacles are similar to those described above.
MU interface optical modules
Optical communications systems have recently been employed in various fields such as local area networks, optical interconnection, Synchronous optical Network/Synchronous Digital hierarchy, Asynchronous Transfer Mode (ATM), and wavelength-division multiplexing systems. These systems require compact and high-performance optical modules.
Figure 3a shows compact optical transmitter and receiver modules equipped with an MU plug interface. The optical transmitter module (at the top of the figure), developed by F. Ishitsuka, et al., of NTT Opto-Electronics Laboratories, performs at 2.5 Gbits/sec with a volume of <6.2 cc. The coupling efficiency is more than 50% and the dissipation power was <1.8W (the LD-driver IC dissipated only 0.33W). The optical receiver module (at the bottom of the figure), developed by H. Ichino, et al., of NTT Optical Network Systems Labs has a size of 2 cc in volume achieved through the use of IC technology and an MU configuration. This module provides a minimum sensitivity of 18.8 dBm for 2.5-Gbit/sec operation with an error rate of 10-11 and a total power consumption of 0.64W.
Compact optical active connectors with an MU connector interface have also been developed. The active connector shown in Fig. 3b, developed by S. Sasaki, et al., of NTT Opto-Electronics Labs, has optical-to-electrical or electrical-to-optical conversion modules and an MU duplex-plug interface, and is capable of replacing the conventional electrical-cable connector. It can be used for 620-Mbit/sec data transmission, ATM switching systems, and high-speed computer networks.
The MU connector system has been successfully installed in NTT`s communication network since 1993, and the application area and yield of the MU connector system have increased greatly during this time. Last year, we began to transfer MU connector technology worldwide. The standardization of MU connector systems has already been completed by worldwide standardization organizations such as the JIS, CECC, IEEE, and IEC. Accordingly, the price of the MU connector is rapidly decreasing. The MU connector system is expected to find application in advanced optical transmission, exchange, and subscriber systems and other optical communications systems. u
References
S. Iwano, et al., "Compact and self-retentive multi-ferrule optical backpanel connector," J. Lightwave Technol., vol. 10, No. 10, pp. 1356-1362, 1992.
R. Nagase, et al., "MSC-type optical connector with small zirconia ferrule," IEEE Trans. Photon. Technol. Lett., vol. 3, No. 11, pp. 1045-1047, 1991.
S. Iwano, et al., "MU-type optical fiber connector system," NTT review, vol. 9, No. 2, pp. 63-70, 1997.
Shin`ichi Iwano is senior research engineer, supervision, at Nippon Telegraph & Telephone Opto-Electronics Laboratories (Tokyo).