To build a network, fiber optic cables must obviously be connected, but there is always a risk that dust, dirt or other particles will find a way in between the two fibers being linked, particularly in harsh, outdoor environments.
Fibers are usually very fine, with the diameter of the core carrying the light often only around 50 microns (0.05mm) or even less. Even tiny particles can block enough of this light to cause a problem, with this light loss leading to transmission problems, lower bandwidth, or even a complete loss of signal.
How expanded beam works
To overcome this issue, one suitable approach is expanded beam (EB) technology. This uses two matching lenses, one at the end of the fiber from which the light beam is emerging, and the other at the end of the fiber it’s being connected to.
The first lens expands the emerging beam, and collimates it – which means that it makes all the rays of light parallel. The second lens than refocuses the light, reconstructing a beam in the second fiber of the same size of the original beam. This can be achieved by using lenses with a refractive index that is either constant, or varies through the lens. In both cases, the beam of light from the fiber passes through the lens’ focal point, and is therefore collimated.
The benefits of expanded beam
What’s the point of all this, you may be asking?
As the beam of light passing between the two lenses is expanded to have a much larger diameter than the beams inside the fibers, this means that any particle of dust or dirt is much less significant as a proportion of the light beam area. It therefore has less effect on blocking the light, with signal loss minimised.
Using an expanded beam also means that any misalignment of the two connecters has less of an effect, because the misalignment will be lower in proportion to the beam diameter. Having said that, it is still important that alignment is as accurate as possible, both in terms of lateral position and angular alignment with the centre axis.
Since the expanded beam connection is less prone to problems with dust and dirt, it is therefore less likely to cause issues in harsh or dirty environments. It also means that cleaning is easier and quicker. We call this the ‘bucket of mud’ test: you should be able to dunk your connectors in mud, then rinse them off with water, and mate them back up – and with a good connector, they’ll work fine straight away.
The expanded beam connection is contactless, with light passing over a small gap between the two lenses. This improves reliability and resistance to vibration, and can reduce mechanical wear, thus increasing lifetime of the connector.
Expanded beam connectors can be used with both multi-mode and single-mode fibers, and can provide a reliable solution for many applications. They have, however, typically been more expensive than other connectors, which has limited their usage outside telecom and data com use cases.