The connector is one of the most important components in the development of fiber optics. Connectors are necessary to patch fibers to create a link and connect fibers to equipment. Fiber has been mostly the same single-mode fiber from 40 years ago, splicing is almost all fusion splicing developed around the same time and the connectors we use today were also developed in those early days of fiber optic applications.
An astounding variety of designs were used in the first years of fiber optic applications. Let’s take a look at the early development of fiber optic connectors in the 1970s and 1980s and examine the problems developers were trying to solve with their different designs.
But first, a review of the requirements for fiber optic connectors. A fiber optic connector is basically designed to join two fibers with some requirements similar to a splice:
- Low loss
- Low reflectance
- Stable over time and temperature
- Reliable over time
But fiber optic connectors also have some different requirements:
- Must be able to mate and unmate fibers
- Have repeatable loss over multiple matings
- Not wear out over multiple matings
- Be able to mate to transmitters, receivers and test equipment
The first three commercial fiber optic connectors took very different approaches. Here are the results.
The Deutsch connector
The Deutsch connector was a giant, heavy, stainless-steel connector. It held the fiber in place with a clamp, so no adhesives were required. The connector’s nose was spring-loaded. The connector was inserted in a fixture that pushed the nose back and cleaved the fiber like a splice. When the connector was inserted in an adapter, the nose was pushed back, allowing the fiber to fit into a plastic lens filled with index matching fluid.
Unfortunately, this did not produce a very low loss, even with the multimode fiber of the late 1970s. A good connection was 2 decibels (dB) or better, but 3 dB was not unusual. Later designs like this used a V-groove alignment method and improved on the loss.
AT&T Biconic connector
AT&T took a different approach with the Biconic connector. The connector ferrule was a cone molded from a glass-filled plastic. The alignment was made with a molded adapter with two cones facing each other, hence the name Biconic.
The Biconic connector was developed by Jack Cook at Bell Labs in Murray Hill, N.J. I visited him in his lab in 1977-78, marveling at the fact that he could squeeze a desk and a plastic molding machine in the tiny space.
At the time, he explained, he could not mold a hole so small (125 microns is only 0.005 inches) in the plastic, so he actually molded the connector around the fiber. After molding, he would cleave and polish the fiber to get a usable connector. It was much better than other connectors’ attempts, with a loss of less than 1 dB.
It would take another year before Cook could actually mold ferrules with holes. Then the connectors would be injected with epoxy, fibers inserted and the epoxy cured with heat. After curing the epoxy, the fiber would be cleaved and polished.
Improvements in the process allowed the Biconic to be used for single-mode fiber when it became available, but the termination process for single-mode was difficult and costs were high. In the mid 1980s, a single-mode Biconic pigtail sold for about $100.
But the Biconic connector worked very well, and millions were used in fiber optic networks. Some are still in use today.
The Amphenol SMA
Amphenol was a manufacturer of electrical and electronic connectors that had high-precision metal manufacturing capabilities. Their version of a fiber optic connector was simple: a drilled 1/8-inch diameter stainless steel ferrule held in place with a threaded nut from a microwave connector, called the “subminiature A,” or SMA. The fiber was glued in the ferrule with epoxy and polished.
The first versions were straight ferrules that were aligned in an adapter with a precision-drilled hole. For multimode fiber, the connector loss was acceptable, but it often required rotating the connectors when mating to get better fiber alignment. That would produce losses of about 1 dB or less. Later versions machined a smaller end on the ferrule and used a plastic sleeve to align the ferrules, giving slightly better loss.
The SMA connector was never precise enough for single-mode fiber, but it’s all-metal construction has given it the longest lifetime of any fiber optic connector. Many are still used in critical applications like process control or nuclear reactors as well as high-power laser welding and surgery.
Expanded beam connectors
The alignment problem with the smaller size of the core of single-mode fiber led to a few tries with expanded beam connectors. Instead of trying to align fibers with submicron precision, you could expand the light beam from the fiber to 1-2 mm with a lens, then mate it to a similar connector.
The larger beam of light simplified alignment, but coupling light in and out of lenses caused light loss also. Special types of lenses were tried. Even Kodak used their technology for molding camera lenses to try making fiber optic connectors. None of these designs proved to be low loss and all were high cost, but the technique is still used in military connectors today.
The ceramic ferrule connector
Telephone companies committed to single-mode fiber in 1983 and 1984. Single-mode fiber allowed longer and faster links than multimode fiber, making them much more cost effective.
The transition from multimode fiber with a 50-micron core to single-mode fiber with a 9-micron core required much higher precision connectors. Fortunately, a very simple solution solved the alignment problem. The Japanese telecom company NTT (Nippon Telephone and Telegraph) and Kyocera, a manufacturer of molded ceramic substrates for integrated circuits, developed a molded ceramic ferrule for connectors. The connectors were aligned by a springy split sleeve that held them tightly.
The ceramic ferrules could be molded so precisely that connectors using them could mate single-mode fibers with losses in the 0.5 dB range, better than many multimode connectors. In some cases, the limit on connection loss was the variations in the fibers, not the connectors. The hardness of the ceramic ferrule simplified polishing the fibers and did not wear during the mating process, making mating losses more consistent.
The ceramic ferrule design first appeared in the NTT FC connector, followed by the AT&T ST and a plastic connector from Japan called the SC. The FC connector used a screw-on locking nut, the ST used a bayonet latching nut and the SC was a simple push-pull latching connector.
Within a short period of time, most fiber optic connectors, single-mode and multimode, used the same ceramic ferrule.
Today, most connectors still use a similar ceramic ferrule. The original 2.5 mm ferrule is still in widespread use, especially in SC connectors, and a smaller 1.25 mm ferrule is used in the LC and several other connectors designed to be smaller for high-density patch panel and transceiver applications.
One of the most popular pages on the FOA Guide website is a page devoted to identifying fiber optic connectors. You can view it here: https://foa.org/tech/connID.htm.
For over 20 years starting in the early days of fiber optics (1980-2001), I started and ran a company making fiber optic test equipment (FOTEC.) When you make test equipment, you have more insight into a market than any other kind of company because you are expected to solve people’s problems.
At FOTEC, we told our customers that we would adapt our equipment to their application no matter what it was. Building fiber optic power meters was easy, except we had to adapt to every fiber optic connector made. Over those 20 years, we designed and built over 100 adapters for different fiber optic connectors. In the last 20 years, another couple of dozen designs have also appeared. That is a lot of connectors!