Optical feedthroughs are used to transfer light into a sealed environment. Optical feedthroughs are based on high technology glass-to-metal bonding techniques and they overcome all the disadvantages of other mechanical feedthroughs caused by unreliable epoxy bonding. An optical feedthrough has standard ST connectors on both sides and no moving parts or loose optical cables. An optical feedthrough is a stand-alone part designed to be mated with any temperature sensor and extension cable and it is optically mated with the outside. There are many types of optical feedthroughs. Examples include a fiber optic feedthrough and a window feedthrough. A fiber optics feedthrough has high electrical resistance and is not affected by electromagnetic fields. A window feedthrough uses a wide variety of optical glasses providing high transmittance and freedom from bubbles and inclusions and has post sealing optical processing for best optical properties. Other optical feedthroughs are commonly available.
There are several ways in which optical feedthroughs function. Optical feedthroughs are designed for installation on transformer tank walls and provide a long-term leak-free interface without any maintenance. A fiber optic feedthrough uses metal coated optical fibers which are brazed to a stainless steel tube to make a vacuum tight seal and then brazed to an SMA connector on the vacuum side. A fiber optic feedthrough is available in core diameters of 400µm, 600µm and 1000µm and is offered in UV (800-1200 nm) and IR (500-2600nm) transmission ranges. A multimode cable includes acrylate coated optical fibers, polyimide coated optical fibers. A multimode cable is radiation resistant. A singlemode cable includes standard single mode fibers in a range of wavelengths which are available with a variety of coatings. Singlemode optical fibers with acrylate coating are suitable for general use and the polyimide coated singlemode fibers are suitable for higher temperature use. In window feedthrough, high performance optical glasses can be hermetically bonded to a metal frame suitable for welding or soldering to the body of a hermetic package. Optical feedthroughs for metal housings are achieved by using a fiber or fiber stub threaded through the sidewalls of the package, followed by solder sealing.