What is the difference between MT ferrules and standard connector ferrules? Unlike the cylindrical ferrules typically used to splice a single fiber, MT ferrules are designed to mate to multiple fibers, greatly increasing the number of fibers that can be mated per connection. An MT ferrule is a rectangular polymer (plastic) block with holes for multiple fibers, where a typical single fiber ferrule is ceramic and requires different finishing processes and materials. The fiber hole spacing ("pitch") of MT ferrules is standardized to ensure alignment with mating ferrules. Mated ferrule alignment is achieved by molding two guide pin holes into the MT ferrule itself. "
What is the difference between MT ferrules and standard connector ferrules? Unlike cylindrical ferrules typically used to splice a single fiber, MT ferrules are designed to mate to multiple fibers, greatly increasing the number of fibers that can be mated per connection. An MT ferrule is a rectangular polymer (plastic) block with holes for multiple fibers. The fiber hole spacing ("pitch") of MT ferrules is standardized to ensure alignment with mating ferrules. Mated ferrule alignment is achieved through two guide pin holes molded into the MT ferrule itself.
MPO connector for MT male ferrule
MPO connector with MT (male) ferrule
How many holes are there? The answer will depend on the equipment and protocols used in the fiber optic network.
Options for MT ferrule/MPO connectors from various companies range from 4 to 72 holes per ferrule. Therefore, a mating pair can provide connector splices of up to 72 fibers. The number of holes in a single row can range from 4 to 16, and there can be up to six rows in a ferrule. For example, the 72-hole version on the market has six rows of 12 holes. Another example is a 32-well product with two rows of 16 holes. The most commonly used configurations today are 12-hole (1 row of 12) or 24-well (2 rows of 12).
MT ferrules and MPO connectors are available for both singlemode and multimode fibers, the only difference being the tighter dimensional tolerances of the singlemode version. Singlemode applications typically require 8 degree polished MT endfaces. Single-molded ferrules can be purchased in "flat" form (cable assembly producers will need to polish the 8 degree angle into the ferrule) or "pre-angled" with the 8 degree angle already produced (this reduces polishing) time). Multimode applications often require “flat” (no angle) ferrule faces.
Make an MPO Type Assembly
The presence of multi-fiber and polymeric ferrule materials means that the assembly production process is substantially different from that used for traditional single-fiber ceramic or metal ferrules. The main differences are in polishing: the materials required; the number of steps involved; and the industry-defined finished geometry of the polished endface. Achieving a low-loss connection for multiple fibers in a mating is far more complex than using a single fiber connection. In short, for MPO connectors, all the challenges and difficulties of achieving a good termination are multiplied.
Some suppliers offer MT ferrules and they use different polymers. The properties of the polymer are related to the polishing procedure. At least three suppliers use variants of the thermoplastic polymer polyphenylene sulfide (PPS). The ceramic material used for ferrules is more uniform between suppliers, and ferrule connectors from different suppliers can use the same or very similar finish. For MPO connectors, there are large differences between the processes used by different assembly manufacturers.
All MT ferrule polishing processes have three main goals:
A. Removal of epoxy on end faces, which in some cases creates angles (single-mode only if required);
B. One or more steps to produce the desired fiber height;
C. Perform one or more steps to polish the fiber end face while maintaining the desired fiber height.
The number of polishing steps required to achieve these three goals can vary between MPO component manufacturers. For example, four polishing steps are recommended, one for the objective (A.), one for the objective (B.), and two for the objective (C.). To achieve the latter goal, Gaffer recommends using two polishing pads with different abrasives. Some other companies may use different abrasives and different number of steps for targets (B.) and (C.). In our experience, many assembly manufacturers typically employ a four-step process.
Details of polishing steps
For the objective (A.), to remove the epoxy and possibly create angulation problems, a 16 µm (grain size) SiC disc is recommended. This is similar to using a zirconia ferrule abrasive in the first polishing step. Regarding angles, assembly companies have two options: order pre-angled MT ferrules or create the angle in the first step of polishing. Pre-angled ferrules reduce polishing time and help ensure more consistent product geometry.
For the objective (B.), to shape the end face for correct fiber protrusion, a 3 µm silicon carbide polishing disc is recommended. This step is very different from any procedure used to manufacture components with cylindrical zirconia ferrules. For zirconia ferrules, the goal is to have the fiber end flush with the ceramic ferrule end. This is why diamond films are practical - diamond cuts glass and zirconia at similar speeds. In an MPO assembly, the fibers must protrude from the end face of the polymer sleeve: the surface of the MT sleeve is a plane, and the fibers must protrude beyond this surface. The extent of the protrusions or fiber height is typically 1.5 to 4.0 μm.
In addition, the goal was to have all fibers have the same amount of protrusion, and fiber height variation between different fibers in the range of hundreds of nanometers or tenths of a micrometer. The abrasive used for this polishing step must cut away (mechanically etch) some of the plastic ferrule without shortening the fibers. A 3 µm disc has a fiber height of approximately 3 µm. The final polishing step may reduce this height slightly, so the process in this step will be adjusted to achieve the final fiber height specification.
For the objective lens (C.), where the fiber end face is polished to remove scratches, pits and other defects, a two-step process using two different abrasives is recommended. This goal is something we see different approaches in different assembly manufacturers and use different numbers of polishing steps.
In the suggested two-step process, the abrasive is in the flocked pad, not the film. Flocked materials have tiny strands (microscopic in size, but paint the carpet!), and abrasive particles (1-μm or submicron) adhere to the strands in the pile. Specifically, the recommended flocking for flocking in the first polishing step is a 1 µm alumina abrasive, and a submicron abrasive of cerium oxide or silica in the second polishing step.
Flocked pads are designed for use with the same glass substrates and polishers as cylindrical ceramic ferrules. That is, MT polishing of flocked pads uses the same polishing motion. The number of MT ferrules that can be polished in one step depends on the machine and fixture used. There can be more than 12 MT ferrules in one fixture. There is an important difference from ceramic polishing: All films and polishing pads used for MT polishing are intended for one-time use only. (Discs used for ceramic ferrule polishing can be used multiple times; this is not the case with MT abrasives.)
Summary and Key Reminders
Producing components with MPO connectors presents different challenges than ceramic ferrules. Here are some general recommendations for engineers.
When polishing MT ferrules, use films and flocked pads made specifically for MT ferrules.
MT ferrules are not all made from the same material. Polishing techniques are not as uniform as cylindrical ceramic ferrules. This may be a reason to stay with an MT ferrule supplier.
All films and flocking pads should only be used once. This includes a first step using a 16 µm membrane, a second step using a 3 µm membrane, and a final step using a flocked pad. (Key point: MT ferrules have a much higher polishing cost per ferrule than traditional ceramic single fiber ferrules.
Finally, we noticed that the guide pin holes of the MT ferrules may become contaminated, so a mandatory cleaning step must be performed to clean these guide holes after polishing.
