Why OPTOKON Expanded Beam connectors are essential in military and harsh environment applications

Why OPTOKON Expanded Beam connectors are essential in military and harsh environment applications

Traditional butt-contact fiber optic connectors have proven to be unreliable in harsh environment applications due to their sensitivity to dirt, dust, mud, water, oil and other contaminants. Expanded beam fiber optic connectors offer the solution. This article explains the benefits of expanded beam technology over butt-contact connectors and demonstrates how expanded beam connectors offer improved reliability in military and other harsh environment applications.

Expanded beam fiber optic connectors are specifically designed to operate in harsh environments. They employ a non-contact technique where the fiber is fully sealed behind a lens. The lens effectively enlarges the active area of the fiber, thus providing a connector with greatly reduced dirt sensitivity and therefore greater reliability when used in adverse conditions. 

The Expanded Beam Principle

Expanded beam fiber optic connectors utilize a lens to expand and collimate the light emitting from an optical fiber. This collimated light beam is transmitted through an air gap to a mating connector, where the light is collected and focussed by a second lens into a second optical fiber to complete the connection.

With 50/125 multimode fiber, the expanded and collimated light beam has an active area of around 150 times larger than the original optical fiber core. In single-mode expanded beam connectors the active area of the light beam is approximately 2000 times larger.

The effect of collimating and greatly increasing the beam diameter means that the connector is less sensitive to small particles of dust or other contamination which could completely obscure transmission in butt-contact type connectors.

Expanded Beam Connector Inserts

Multi-channel expanded beam connector inserts have been developed to offer a practical method of packaging the expanded beam concept. Typical expanded beam inserts comprise a stainless metal housing containing 1 to 16 spherical lenses (optical channels).

The OPTOKON expanded beam insert utilises a unique optical arrangement whereby the critical interface between the optical fiber and the lens is permanently sealed in a controlled atmosphere during manufacture. This ensures that no dust, moisture or other contaminants can migrate into the optical path during the product life.

The front face of the expanded beam insert has a mating surface, an alignment pin and socket and one or more spherical lenses which are epoxy sealed. The lenses have an extremely hard anti-reflection coating. The OPTOKON expanded beam insert has been carefully designed to avoid “dirt traps” and the flat front surfaces facilitate easy cleaning.

The expanded beam insert is sprung loaded when assembled in a connector shell. When two connectors are mated, the expanded beam inserts are aligned by the pin and socket arrangement and contact under spring pressure on their mating faces. The spherical lenses do not contact 

Expanded Beam Connector Shells

OPTOKON hermaphroditic type expanded beam connectors have been designed specifically for external harsh environment applications and are widely used in military tactical communications, outside broadcast, mining and many other industrial applications. Cable assemblies with hermaphroditic connectors can be easily daisy-chained and there is no requirement for male / female adaptors.

OPTOKON connector shell materials suit a wide range of environments including Aluminium Alloy, Stainless Steel and Nickel Aluminium Bronze. OPTOKON hermaphroditic tactical connectors are fully sealed (IP68) mated or unmated and are available with or without a dust-cap.

High Reliability in Extreme Environments

Whilst the connector shell design and the quality of the harness manufacture have an important bearing on the reliability of a harsh environment fiber optic connector, by far the most critical factor is its fundamental ability to withstand the harsh environment conditions both when the connector is mated (or with its protective cap fitted), and in its unmated and therefore unprotected state.

Butt-contact connectors must also withstand the cleaning process as damage can ofter occur if untrained personnel attempt to clean the connector or incorrect cleaning materials are used. In contrast, expanded beam connectors do not require disassembly or any special tools or cleaning materials to properly clean the connector. Damage cannot occur from the cleaning process.

1. Plug connector immersed in mud. The connector is fully sealed even without the dust cap fitted. 2. The mud will not cause damage to the expanded beam insert. A butt-contact connector would almost certainly already    be destroyed in this state.
3. Initial cleaning to remove mud and debris from the connector face can be done by rinsing in water. A hose, bucket of water, stream or muddy puddle will do. 4. Connector end-face is clean after rinsing.
5. The insert face is dried and wiped clean. Ideally a clean cloth or tissue is used but if these are not available the bottom of your coat will suffice. No damage to the insert surfaces or lenses will occur. 6. Connectors plugged and ready for service. The whole cleaning cycle was completed in under 5 minutes.  Insertion loss was measured between -0.90dB to -1.15dB (9/125 4 channels) with an Optical Time Domain Reflectometer.


Whilst butt-contact connectors have lower insertion loss than expanded beam connectors when the product is in perfect condition (typically -0.3dB), their optical performance will constantly degrade over the product life eventually leading to signal failure.

Regular maintenance of butt-contact connectors is required to maintain good optical performance. This involves disassembling the female connector parts, removing alignment sleeves (small parts which are easily lost in field conditions) and the use of special cleaning swabs, solvents and tools (including a ferrule inspection microscope).

Butt-contact connectors can only be serviced by specially trained technicians. If the ferrules have been damaged, cleaning will not restore the original optical performance. Damaged connector ferrules will require re-polishing and this can only be done using a fiber optic polishing machine in workshop conditions. Connectors with severely damaged ferrules will require complete re-termination or replacement.

OPTOKON Expanded beam connectors have been shown to withstand extreme environments without the need for special servicing or cleaning equipment. Protecting the optical fibers behind the lenses ensures that no damage or degradation can occur.

The non-contact design of the optical surfaces in expanded beam connectors results in higher insertion loss than butt-contact connectors (multimode typically -0.7dB, single-mode typically -1.0dB). However, the optical performance of expanded beam connectors is constant throughout the product life and the likelihood of product failure at a critical point is greatly reduced.