Many fibre optic links have to go through a panel, separating two volumes which need to be hermetically isolated (such as a vacuum, liquid, gas, temperature or radiations etc).
In most cases the fibre optic connection is made of two male connectors with a female coupling part. The coupling part between two connectors is a precision connection, which allows the alignment of the connecting ferrules of optical patchcords or cables. In a hermetic feedthough this coupling part is made longer, to allow the connection of patchcords through a determined thickness of panel or plate.
Within this coupling part, fibre is optically bonded inside an interconnecting ferrule to make a precisely aligned hermetic optical path. The fibre is selected to match the fibres that are to be connected at each end of the feedthrough and the interface can be either FC, or SMA. There are three types of SMA feedthough which are SMA standard, SMA compact, and SMA simplified.
The hermeticity between the feedthough itself and the panel is obtained by a classical flange and gasket solution and the feedthough obtains hermeticity of 10-8 torrs (helium).
At LASER COMPONENTS we can supply hermetic feedthoughs with standard FC or SMA interfaces. FC interfaces are used for single mode or graded index multimode fibre. SMA interfaces are used for large core silica fibres HCS, or all-silica and silica with polyimide coating. These small diameter fibres require a high precision, which is obtained by ceramic ferrule centre parts. A large flange allows the integration of two O-ring gaskets for the external hermeticity.
Connectable hermetic feedthoughs do inherently induce insertion losses which can rule them out of some applications where photon budget is tight. In this case feedthroughs without fibre discontinuity are to be considered. These are referred to as a ‘Monolithic Fibre Feedthrough’. Depending on the level of hermeticity required the sealing of the fibre will be different. For hermeticity up to 10-2 the sealing will be made on the fibre coating. For hermeticity up to 10-8 or even 10-9 the sealing will be made on the silica cladding or on the polyimide.
