In different ways, three components may be called optical feedthroughs, since they combine a part that is transparent to light with a metal part that attached to a vacuum chamber:
- Fiber-Optics Feedthroughs
Viewports (sometimes called sight-glasses) are used to:
- Allow the operator view a process (checking plasma initiation in sputter deposition)
- Initiate chemical or physical action using specific wavelengths (pulse laser deposition)
- Make measurements of emissions occurring in a process (optical emission spectrometry)
- Monitor the effects of specific wavelengths (ellipsometry)
Optical disks for the first application are commercial-grade, fully annealed alkali borosilicate glass such as 7056 or Kodial. For other applications, special optically polished disks of MgF2, fused silica, quartz, sapphire, germanium, etc. are used. Choosing an optical material that has a high transmission at the wavelengths of interest typically yields the best results. Keep in mind that what is not transmitted will be reflected or absorbed (and absorption heats the optical material).
Kodial viewports can withstand repeated bakeout cycles to 350°C. However, to avoid thermal expansion stress, the heating rate should not exceed 2-3° per minute. In addition, covering the viewport externally with multiple layers of aluminum foil during heating reflects thermal radiation from the chamber back through the glass.
All viewports are fragile and should be handled and mounted with extreme caution. Fully-annealed copper gaskets are often used to mount ConFlat® flanged viewports. With any copper gasket sealing a viewport, it is recommended to finger-tighten the bolts, strictly adhere to the cross-flange tightening pattern, and never rotate any particular bolt more than 1/8 turn at a time.
Never scratch the viewing area—a weakened viewport may implode (or more easily explode under adverse pressure conditions).
These feedthroughs provide the interface between ultrahigh vacuum and UV/IR fiber optics. There are a variety of single mode and multi mode options available with industry standard connection types such as SMA, FC/PC, and FC/APC, with both ends of the feedthrough terminating in the chosen connector type.
The fibers have very low signal loss and high radiation immunity. They are used in medical, industrial, and research applications that require optical interfacing with ultrahigh or high vacuum systems.