WHAT ARE MT - RJ CONNECTOR?
The MT-RT connector utilizes the same rectangular plastic ferrule technology as the MTP array-style connector first developed by NTT, with a single ferrule body housing two fibers at a 750-um pitch (Fig. 3.1).
These ferrules are available in both single-mode and multimode tolerances, with the lowercost multimode version typically comprised of a glass-filled thermoplastic and the critically tolerance single-mode version comprised of a glass-filled thermoset material.
Unlike the thermoplastic multimode ferrules, which can be manufactured using the standard injection mold process, the thermoset single-mode ferrules must be transfer molded, which is generally a slower but more accurate process.
By design the alignment of two MT-RJ ferrules is achieved by mating a pair of metal guide pins with a corresponding pair of holes in the receptacle (Fig. 3.2).
This feature makes the MT-RJ the only Small Form Factor connector with a distinct male and female connector. As a general rule, wall outlets, transceivers, and internal patch panel connectors will retain the guide pins (thus making their gender male) and the interconnecting jumpers will have no pins (female).
In the event that two jumper assemblies require mating mid-span a special cable assembly with one male end and one female end must be used. However, some unique designs do allow for the insertion and extraction of guide pins in the field, affording the user the ability to change the connector’s gender as required.
Latching of the MT-RJ connector is modeled after the copper RJ-45 connector, whereby a single latch arm positioned at the top of the connector housing is positively latched into the coupler or transceiver window.
Although this latch design is similar in all the MT-RJ connector designs, individual latch pull strengths may vary depending on the connector material, arm deflection, and the relief angles built into the mating receptacles.
For this reason it is recommended to evaluate connector pull strengths as a complete interface, depending on the specific manufacturer’s connector, coupler, or transceiver design, as the coupling performances may vary. MT-RJ connectors are typically assembled on 2.8-mm round jacketed cable housing two optical fibers in one of three internal configurations.
The first construction style consists of the two optical fibers encapsulated within a ribbon at a 750-um pitch (Fig. 3.3).
This approach is unique to the MT-RJ connector and designed specifically to match the fiber spacing to the pitch of the ferrule for ease of fiber insertion. Although this construction style may be ideal for a MT-RJ termination, it can cause some difficulty when manufacturing a hybrid assembly, and availability may also be an issue based on its uniqueness.
A second design which is more universal, utilizes a single 900-um buffer to house two 250-um fibers (Fig. 3.4).
This construction is more conducive to hybrid cable manufacturing but the fibers will naturally maintain a 250-um pitch, thus making fiber insertion rather difficult.
The third design is considered a standard construction and is used across the industry (Fig. 3.5).
In this configuration each individual fiber is buffered with a PVC coating. The coating thickness is typically 900 um, but as in the previous case this does cause a mismatch of the fiber to ferrule pitch. To compensate for this, some connector designs incorporate a fiber transition boot, which gradually reduces the fiber pitch to 750 um, while others simply use a non-standard buffer coating of 750 um.
In general the assembly and polish of the MT-RJ factory-style connector is considerably more difficult than the other small form factor connectors. Typical MT-RJ designs have a minimum of eight individual components that must be assembled after the ferrule has been polished, allowing for a number of handling concerns (Fig. 3.6).
As with the case of most MT-style ferrules, the perpendicularity or flatness of the ferrule endface with reference to the ferrule’s inner shoulder is critical and this cannot be accomplished if the connector is pre-assembled. Another unique requirement of the MT-RJ polish involves fiber protrusion.
Although the ferrule endface is considered to be flat, depending on the polishing equipment, fixtures, and even contamination some angularity may occur. Therefore it is recommended that the fibers themselves protrude 1.2 to 3.0 ums from the ferrule surface in order to guarantee fiber-to-fiber contact.
For the reasons previously mentioned, the factory-style MT-RJ connector is not a good candidate for field assembly and polishing, so a number of similar yet unique Field Installable Connectors have been developed.
All of these field solutions utilize a pre-polished ferrule assembly, which is mated with two cleaved fibers and aligned by v-grooves, with the entire interface filled with index matching gel to compensate for any possible air gaps. One of the major differences between the various solutions is in the mechanism used to open and close the spring clip that maintains constant pressure on the two sandwiched halves of the channeled interface.
In one design a cam, which is integrated into the connector body, is used to separate the halves while in other designs a separate hand tool is required. The other general difference between the field solutions is the application design - a number of the connectors are designed to be just that - a field connector with a distinct gender that can be terminated onto distribution style cable - while others are designed to be male receptacles only that must be wall or cabinet mounted. Because of the inherent difference between the MT-RJ fields solutions one solution may be better suited to a given application than another.
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