Fiber-Optic Adapters Help Systems Communicate

Along the way, performance has improved, too.  The Deutsch 1000 achieved about 3 dB insertion loss, compared to 0.25 dB for the LC connector.

Figure 1:  Fiber-optic connectors (newest to oldest, from top) – LC, SC, Biconic, Deutsch 1000 (image courtesy theFOA.org).

The table below includes some of the more common connector types:

For highest performance (i.e. lowest loss), the tip is then polished to produce a slightly convex surface, called “physical contact” polish. This surface profile means that when the fibers are mated they touch only at their cores, allowing transmission with low loss.

To secure and align the fiber within the connector, the fiber end is embedded in a ferrule made of ceramic (zirconia), stainless alloy or plastic. The ferrule is commonly 2.5 mm in diameter, although 1.25 mm is used in the LC and other connectors.

Fiber-optic adapters

Although fiber-optic connectors get most of the attention, fiber-optic adapters are also a critical part of an optical installation. Fiber-optic adapters enable two connectors to be joined and aligned with minimal losses, no matter whether the connectors are the same type or of different styles (hybrid adapters).

There are numerous adapters to interface between different fiber-optic connectors as can be seen from the samples above.

Figure 2: Hybrid adapter examples (top → bottom): ST → FC, SC → FC, SC → ST (image courtesy theFOA.org).

Adapters are most often used to mate two connectors with ferrules of the same size – 2.5 mm, for example. It is more difficult to align ferrules of different sizes with sufficient precision; hybrid patch cables are more popular for this application.

To minimize losses, the alignment between two fiber-optic cables must be very precise. This is achieved by two primary components – the ferrule within each connector and the alignment sleeve within the adapter. An alignment sleeve in the adapter brings together the ferrules of the two mating connectors so that the optical signal can pass from one fiber to the other with minimal losses.

A comparison of adapter alignment sleeves

Adapter alignment sleeves are made of a variety of materials at various price and performance points.

Polymer sleeves represent the lowest-cost and lowest-performance option. They are generally recommended only for multimode use and limited insertions – 10 or less. When combined with a very hard ceramic ferrule, multiple insertions will scrape off plastic in the form of dust, which forms a film on the end of the fiber and acts as a filter to increase losses. 

Phosphor bronze sleeves are more durable (up to 500 insertions) and work well with ceramic ferrules, but offer lower precision compared to ceramic sleeves. Bronze alignment sleeves are commonly used in multimode applications where precision alignment is not as critical. They are about twice the cost of polymer sleeves.

Zirconia ceramic sleeves are hardest, so offer the highest performance, and are most expensive, up to three times that of polymer. For single-mode applications, zirconia ceramic is required.

Key adapter specifications

Each component in the fiber-optic system contributes to the overall performance. In addition, it’s important that both the adapter and each connector – and all three may be from different manufacturers – adhere to common mechanical standards to ensure precise alignment. The following are the key fiber-optic adapter performance specifications:

Applicable standard – e.g. TIA/EIA FOCIS-3 (see below)

Split-sleeve material – Phosphor bronze, zirconia ceramic or polymer.

Insertion loss – the loss of signal power resulting from the insertion of the adapter into the fiber-optic line; expressed in dB.

Return loss – loss of power in the signal returned or reflected by a refractive index discontinuity in a  fiber-optic line, e.g. due to a mismatch with the terminating load or a device inserted in the line; expressed in dB.

Applicable mechanical standards for connectors and adapters

Various standards have been issued by the Telecommunications Industry Association (TIA) part of the Electronic Industries Alliance (EIA) that cover fiber-optic connectors and adapters.

A FOCIS (Fiber-Optic Connector Intermateability Standard) document is a TIA/EIA component standard to ensure that same-type connectors from different manufacturers will mate together without problems. FOCIS documents have the format TIA-604-XX, where XX refers to the FOCIS number.

According to the TIA, “Intermateability standards define the minimum physical attributes of mating connector components. Fully dimensioned components are not within the scope or intent of FOCIS. The requirements of FOCIS have been selected with the objectives of ensuring that any combination of plugs and adapters conforming to the requirements of FOCIS will mechanically intermate and that intermated connector assemblies will meet their common level of performance. The common level of performance of an intermated connector assembly is the least demanding set of performance requirements in the separate performance specifications of each of the products in the assembly.”

Each FOCIS number covers a different connector type, as shown below.

Fiber-optic adapters from Arrow

Arrow offers over 1,600 fiber-optic adapters, connectors, assemblies and tools from many leading manufacturers including TE Connectivity, Molex and Panduit. For example, Molex has a wide range of fiber-optic adapters, including die-cast SC to LC hybrid adapters with EMI shielding, 8-port LC adapters with both phosphor bronze and ceramic sleeves, snap-in ESCON to ST adapters for multi-mode use, and many others.