Last Updated: August 5, 2022
The first connector to be accepted as a widely used standard in the early days of the fiber optic industry, in the late 1970s, was designed by Amphenol in CT as a variation of their coaxial electrical connect known as subminiature assembly, or SMA. In those days, fiber was multimode and the clad diameters were 125 um and up, to 230 and 1000 um. For the smallest sizes, Amphenol developed a unique and patented design that used four tightly machined metal rods in a larger hole in the connector ferrule. The rods were uniformly round enough to place the fiber in the center of the connector and allow for acceptable insertion losses of 1.25 - 2 dB per km at 850 nm.
The SMA was not ideally suited to the emerging use of optical fiber in the communications networks – telephone, cable TV, private data networks – for a number of reasons:
- It is suitable only for multimode and not for single-mode fibers, (issues of both insertions and later, return loss).
- It is not friendly to terminate in the field.
- It is not keyed, which means that there is no easy way to repeat the position of the fiber across multiple matings.
- Its screw-threads are not immune to vibration. There is a +/- 3/10000” tolerance on the polished ferrule length. If connectors are not polished within the long side of the length tolerance, one could crash another connector (damage the glass and the polish) or worse, a device like an LED, laser or detector.
- The polishing process itself is markedly different from those of standard communications connectors and therefore requires some different procedures, tools and films, in order to produce acceptable assemblies.
The first widely used connector in the data world, at least in North America, was the ST, sometimes called Snap and Twist. This connector dealt successfully with many of the limitations of the SMA. Ceramic ferrules could be molded and machined with the submicron tolerances required for 9-micron core single-mode fibers, field termination and polishing were a little easier, and the connector ferrules, and fibers, touched each other with the force of a spring, instead of the screw threads of the SMA. Japanese companies, in conjunction with NTT, developed a number of single fiber connectors that dominated communications fiber installations for decades, and still do: FC, SC and LC.
So who needs a fiber optic SMA today?
If you remove the communications applications and markets for fiber optic connectors, what is left?: something less than .1% of the market. What are the applications?
- Medical: laser delivery for ophthalmic (eye), dermatological (skin), and other surgeries. Data and video transmission over short distances, e.g., MRIs.
- Materials processing: laser delivery for cutting and welding thick and thin steel, PC boards, other materials and production processes. Marking of small parts in high volume.
- Spectroscopy: non-destructive testing, the measurement for medical, forensic, industrial, research.
- Sensing: pressure, temperature, magnetic, light, position, acceleration. Composite materials for aircraft, boats, wind turbines, vehicles.
- Inspection: parts counting, quality control.
This is just a smattering of what has been and is being done with SMA connectors throughout North America and Europe, and well as Japan, and more recently, Latin America and the rest of Asia, including China. What sets SMA connectors apart from the rest of the fiber world is that the connectors are used in so many different hole sizes; many different fiber sizes are used across such a myriad of applications. For an inventor, scientist, doctor, engineer, or production or marketing manager who finds an unusual or little used fiber in an odd size and wants to use it in a new or modified product that requires a novel fiber size, SMA connectors are readily available in small to moderate quantities in a matter of days, at a stable and predictable price.
There are other advantages that SMAs have, which are not shared by their more modern and recent connector style cousins:
- Doctors are familiar, and comfortable, working with SMAs on lasers and other medical equipment.
- There are many more accessories available for the SMA than any other connector: mating sleeves/adaptors in half and full-lengths, in a variety of threadings, nuts and materials; device receptacles sized to take several standards “can” (LED, laser and detector packages) configurations and dimensions; hardware for positioning on optical benches.
- Testing assemblies are largely limited to visual inspection and insertion loss, limiting time and expense related to testing equipment.
- Almost 40 years of reliable experience in a tremendous variety of applications.
Fiber Optic Center, Inc. (FOC), has stock on the shelf of SMA connectors in over 40 holes sizes from 127 to 2645 um, in volumes of as little as 10 to over 20,000 pieces, depending on design and hole size. On any given day, these can include: 144, 150, 172, 201, 205, 210, 220, 225, 231, 236, 240, 245, 250, 260, 270, 280, 284, 336, 340, 390, 410, 435, 440, 441, 455, 470, 480, 490, 510, 580, 612, 641, 660, 672, 690, 710, 720, 750, 770, 850, 1000, 1040, 1050, 1055, 1060, 1100, 1275, 1280, 1545, 1645, 1650, 1950, 2025, 2070, 2100, 2125 and 2150, as well as others.
The ripple effect of having such a readily available supply of specialty connectors has a positive impact on those considering using non-standard fiber sizes. Sometimes the effects are hard to predict. Years ago a small company was producing a laser for wart removal using large core fibers and SMA connectors. They called one day to order a small number of connectors in a smaller size than usual. A baby was born in the Netherlands with warts in his throat and was having trouble breathing. The plan was to downsize the laser and fiber to make it suitable for use on the infant. The new laser with appropriate fiber assemblies was delivered in a matter of a few days, the baby was operated on successfully, and survived.
Although the SMA connector is part of ancient history in the fiber industry, it nevertheless remains a relevant and key enabler for tomorrow’s instruments, products and procedures which use specialty fibers.
Additional resources from the FOC team include:
- Category Resource: FOC Interconnect Page
- View the Glossary, Acronyms, Military Specifications for Connectors
- Q&A Resource: email technical questions to AskFOC@focenter.com
Have questions about this article?
Contact FOC with questions at: (800) 473-4237 / 508-992-6464 or email: FiberOpticCenter@focenter.com and we will respond ASAP.