What Fiber Optic Connectors Are Used for Non Standard Fiber Sizes?

The question I seem to be asked over and over is “What fiber optic connectors are used when I have non-standard size multimode and singlemode fiber”? The frequency of this question led me to write this blog. It can be very frustrating when installers and technicians are faced with this situation, the proper fiber has been identified, but what good is it to me if I cannot install connectors. Fortunately, there are answers and I hope to relieve some of the angst that you may have.

 

Most telecommunications projects utilize standard equipment and fibers that are readily available, but what happens when this is not the case? The standard LC, SC, ST and FC style optical fiber connectors with ferrule holes at or around 126um will suffice 99% of the time but, nonstandard applications such as medical, automotive, high power and others utilize specialty fibers where the standards will not work.

 

Non-Standard (Specialty, Large Core) Fiber

 

As most of us know, standard singlemode optical cable is made with a 9um optical glass core and a 125um cladding (9/125) with multimode standards being 50/125 and the old North American standard of 62.5/125. Many non telecommunication optical applications utilize non-standard fibers. Following is a list of just a few of these fibers:

 

100/140 – This fiber is identifiable by its green jacket color and has a typical attenuation of about 4 dB/km.

 

200/230 - 200/230 will typically have a blue jacket and has a standard attenuation of 6dB/km. You may notice that as the fiber cores increase in size, so does the standard attenuation.

 

960/1000 – This fiber is actually manufactured using plastic instead of the glass we usually find in fiber. Commonly black jacketed, this fiber is popular for optical audio cables; its 300 dB/km attenuation relegates it to short distance transmissions.

 

Large core fibers are also available: 300/330, 400/440, 500/550/ 600/660, 800/880 and many others too numerous to mention.

 

Precision-Drilled Connectors

 

Specialty fibers will not accept the standard 126um fiber connectors, so the technician must search out alternative solutions. Let’s first address the components that make up a fiber connector:

 

The Fiber Connector:

 

Strain Relief Boot

 

The strain relief boot allows the fiber exiting the connector to maintain its bend radius. A connector without a boot would kink the fiber causing attenuation (loss) or possibly a break in the fiber itself. It is important that the boot not be glued into place, gluing the boot will hinder the spring inside the connector, so the boot should be slip fit onto the connector body.

 

Connector Body

 

The body of the connector holds the ferrule in place and allows the connector to be crimped to the fiber and body via the use of a crimp sleeve. Connectors that only crimp to the fiber and not the body (like an ST) will allow the ferrule to piston when force is applied to the fiber, this is why LC, SC and FC connectors are dual crimp and have the advantage of non-optical disconnection.

 

The Ferrule

 

The most important component of any connector is the Ferrule. In the past, ferrules were made from stainless steel but due to performance issues most of today’s ferrules are made from ceramic (Zirconia). The ferrule’s primary function is to hold the fiber precisely to allow for the transmission of optical signal. Most standard ferrules have a hole size of 126um.

 

When specialty/uncommon (large core) fibers are used, many times ferrules with the larger hole sizes are not available. When larger size holes are needed, the ferrule must be drilled to accommodate these sizes.

 

When drilling a ceramic connector ferrule, issues occur that effect the hole tolerance and concentricity. During the drilling process, the ceramic material will chip and flake making the ferrule unusable. Because of these issues, the only ferrule type that can be consistently drilled is stainless alloy.

 

When drilling stainless alloy ferrules, sizes range typically from 250um to as large as 1550um, these sizes step in 10um increments (example 310um, 320um, 330um etc). When manufacturing these drilled ferrules, specifications for hole tolerance, concentricity, length and diameter are all measured and have pass/fail criteria. Some companies offer two options for their drilled ferrules, standard and premium. A standard drilled optical connector has a hole tolerance specification of -10µm/+50µm and a concentricity value of +/- 50µm. Premium drilled connectors will have tighter tolerances; so if higher performance (low attenuation) is required the premium product is the answer. Premium drilled optical ferrules have a hole tolerance specification of -4 µm/+10 µm and a concentricity value of +/- 25 µm. The premium ferrule will allow for better light transmission, which makes this the most popular of all drilled connectors.

 

Understand that the ferrule cannot be drilled while inserted inside of the connector body. All ferrules are drilled before the connecter is manufactured. Once the ferrule is drilled and passes all specifications then it is installed into the finished connector. Many times (like in medical devices) the ferrule is the only thing that is installed onto the fiber, leaving the body and strain relief boot out.

 

It is important to also note that the older SMA905 and SMA906 connectors can be drilled and are commonly used in military, medical, aerospace and research facilities where higher power lasers and heat dissipation are required. The SMA connector uses a larger 3mm ferrule, compared to the typical 2.5mm for SC, FC and ST and the 1.25mm ferrules used for LC connectors.

 

Installation of the optical connector

 

Once you have identified the correct connector style (ST, FC, SC, LC) and hole drill size the next question is “How to install these connectors?” When considering standard fiber optic connectors (dozens of manufacturers) it really boils down to only three options on how to install the connector on the optical fiber. These options are:

 

Hand/Machine Polishing

 

All fiber optic connectors are manufactured using this epoxy/polish procedure. The move in the fiber industry over the last decade has been to shy away from this process. Labor, consumables, skill level and overall quality is deeming this process obsolete for field connector terminations. The thought process is to let the industry manufacturing professionals handle the task.

 

Mechanical Connectors

 

A mechanical connector is manufactured and machine polished by the connector manufacturer with a small piece of fiber inserted into the connector, this fiber is precision cleaved inside the back of the ferrule and the end is then machine polished. The field installer simply cleaves his fiber and inserts it into the back of the mechanical connector and clamps it into place. Using mechanical connectors drastically reduces labor, and the required skill level of the technician. These connectors are more expensive than epoxy style connectors but the savings in labor costs tend to outweigh the expense.

 

Mechanical Splice

 

Fusion Spliced Connector

 

Most people believe that fusion splicers are used to lengthen and repair fiber optic cables. While this is true, the most common use of a fusion splicer is to attach premade pigtails or the newer Splice on Connectors (SOC). Although the perception is that the investment in the fusion splicer makes this process expensive, the reality is that fusion splicing a connector is the least expensive, lowest labor, highest quality way to install a factory manufactured connector.

 

Now that we have identified the field installation processes for fiber connectors how do we apply this to large core/specialty/Non Standard drilled connectors? Reality is that there are really only two options you have. Fusion splicers cannot splice these specialty fibers and there are no mechanical connectors made today that can be used with these larger core fibers. This leaves us with field installing these connectors using the epoxy/hand polishing procedure or purchasing the cable with the connectors installed by a fiber optic manufacturer. The obvious choice is having your cables manufactured by a company that is a reputable fiber assembly house. These pre-terminated cables will be professionally manufactured and tested; your job is to simply install the cable.

 

Remember that when using specialty/large core fibers there are solutions to your connectorization needs and most of the time the answer is a precision drilled fiber optic connector.