Fiber Optic Connectors: Common plug types & trends

As different as these above mentioned common fiber optic connectors - LC connector, SC connector, MTP^®/MPO connector, E-2000^® connector - look, they have a common contact principle as a basis. This principle is called physical contact. Very often appreviated as PC contact.

The optical fibers are fixed within a small precision ceramic pin, the so-called ferrule, and the end face is then polished during a special defined process. Inside the connector adapter, these ferrules are centered on each other and an exactly defined pressure force creates a direct physical contact between the optical fibers. Besides the tolerances of the ferrule the polishing process is highly important for the quality of the fiber connector. This principle exists in two versions: the so-called PC and APC connectors. In the PC connectors, the end faces are aligned straight against each other, while in the APC variant they are at an angle of 8 degrees to each other. APC connectors are characterized by high and stable return loss. This is an increasingly important parameter, especially for high-speed applications.

Already for many years, PC and APC connectors provide reliable performance:

• Low insertion loss (IL)
• High return loss (RL)
• Stable performance over many mating cycles

The factor of cleanliness & the right way to handle fiber-optic plug connectors

However, one challenge remains: the potential contamination of the connector end face! The possible effects are a higher insertion loss - especially with singlemode fibers - as well as a reduced return loss, what in worst case can result into a total link failure. Moreover, there is a risk of permanent damage of the polished end face if they are plugged in dirty. Then the complete connector need to be replaced. For all these reasons there are comprehensive guidelines for handling fiber optic connectors. The recommended procedure can be seen here in this flow chart:

For this reason, another concept for fiber optic connectors was developed some time ago, which makes use of the expanded beam principle. Therefore they are called expanded beam connectors. Here, collimating lenses are connected in front of the ferrule or directly in front of the fiber, which expand and parallelize the light beam on one side and focus it back into the fiber on the opposite side (see figure). What does this achieve? The beam expansion significantly reduces the influence of dirt particles. In addition, there is no physical contact of the end surfaces, which reduces the risk of damage to the end surfaces by dirt.

Through an extremely interesting further development of this concept, the expanded beam principle is implemented in a completely new way and with previously unknown precision. It was developed and patented by the company 3M^TM and therefore also bears the name 3M^TM EBO. EBO stands for expanded beam optical. Rosenberger OSI is one of the first companies to assemble the innovative 3M^™ Expanded Beam Optical Interconnect with fiber optic cables.

By means of a new delevoped EBO ferrule the light beam is at first expanded. Afterwords on a curved surface it is divirted by 90 degrees and aligned parallel by total internal reflection. The effect is comparable to that of a collimating deflection mirror. In the opposite connector (the bottom part of the illustration) the principle is reversed and the light is coupled back into the fibre. This principle is implemented with the EBO ferrule shown here. This is a precision injection-moulded part. In the rear area it has grooves for fibre positioning instead of holes as we know it from classical PC-type connectors. The cleaved fibres are bonded  via an automated process. No polishing of fibre ends are required, what is a very unique feature. Among many others the EBO ferrule offers thus two main advantages: first it realizes an absolute new optical principle with highest precision and second it allows the possibility to automize the termination process to an extend, which was until today in case of fiber optic connectors not possible.

The illustration above shows a terminated EBO ferrule. At a defined distance from the ferrule, a so-called clamping, the collet, is attached here (Figure 1). This assembly is then inserted into a so-called cassette, which is an important part of the fibre connector (Figure 2). As already indicated the fibers are capted here with a suspension what finally supports the mating process. This mating process is shown here in 3 phases. You can see how the EBO ferrules slide over each other and finally make the optical contact. The cassette can be seen as an ideal building block for integrating the EBO technology into many other connector environments with relatively little effort.

Basic properties of the EBO plug-in system:

• Suitable for both singlemode and multimode applications
• Less sensitive to dust and dirt particels, whats makes installation and handling much easier
• High reliability with good performance data
  - low insertion loss (IL)
  - high return loss (RL)
  - stable performance over a lot of mating cycles

• Low mating forces
• Flexible and scalable connector variants & options, which means it can be integrated into various connector environments 

Some connector options as an example: EBO-12 latch connector (already assembled by Rosenberger OSI)

A new innovative connector that belongs to the Very Small Form Factor (VSFF) category is the MDC (stands for Miniature Duplex Connector) from US Conec. It is a true push-pull duplex connector manufactured on the basis of 1.25 mm all-ceramic ferrule technology. Designed as a Media Dependent Interface (MDI) or Optical Interface for the new SFP-DD and QSFP-DD transceivers, it has the potential to replace the LC-Duplex mass connector.