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
- Glass-Metal Adapters
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. The fiber used in both the feedthrough and cable is high purity 600µm silica, the so-called 'dry' or 'low OH' variety, which has an extended transmission range over conventional silica fibers. The fiber is clad in doped silica and coated with aluminum making it a multi-mode step index fiber.
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.
The body of the fiber optics feedthrough is limited to a maximum bakeout of 250°C and are ultrahigh vacuum compatible when mounted on a CF flange. When mounted on a KF flange, the O-ring material limits the bakeout temperature and the feedthrough is compatible the high vacuum applications.
Both ends of the feedthrough terminate in knurled SMA connectors. For air-side cables (protected by stainless steel sheath) both end are terminated in a knurled SMA connector. Vacuum-side cables (not sheathed) are terminated with:
- Both ends terminated by knurled SMA connector<, or/li>
- One end terminated by knurled SMA connector, the other cut, polished, and capped with a protective hat.
Connections between feedthrough and cable assemblies utilize an air-side or vacuum-side SMA threaded, double-ended coupler (one for each cable connection made). Connections between two cable assemblies are also made using the double-ended coupler (one for each two cables connected)
Glass-metal (GM) seals are transition pieces between metal and glass tubes. In constructing such a joint, the important characteristic is the coefficients of expansion of glass and metal. If they are similar (Kovar-7052 glass), the seal is called "matched". If they differ (SS-7740 Pyrex or SS-quartz), the seal is called "mismatched". For the latter, a graded glass seal (many glass rings each with a slightly different expansion coefficient fused together) is used. One end of the graded seal almost matches the SS expansion coefficient while the other end matches 7740 (or quartz). Both matched and mismatched seals have proven reliable in vacuum service.
GM seals are available in a number of different forms any of which can terminate, at the metal tube end, in a variety of standard vacuum flanges:
- glass tube to metal tube
- glass tube with domed end to metal tube
- glass tube to metal tube with bellows
- glass tube center piece between two metal tube ends
- metal tube center piece between two glass tube ends
- metal bellows center piece between two glass tube ends
We strongly recommend these cautions when constructing a system containing glass-metal seals.
- Where glass is used as a transition piece between two fixed (metal tube) points in a vacuum system, make sure the glass tube is annealed after clamping it in position. Even the smallest misalignment may cause the glass to shatter under the additional stress of atmospheric pressure when evacuated.
- Do not use a GM seal with a metal bellows as a substitute for annealing (see above). Under vacuum all bellows contract. The more flexible the bellows, the less resistance it has to contraction, which puts the actual joint between glass and metal under tension and will almost certainly cause premature failure.
- A bellows glass-metal seal is appropriate if a (domed) glass tube must be aligned in a support cradle. Use only a thicker wall, hydraulically formed bellows for this application to avoid crushing the bellows beyond its limit.
- For safety, cover the direct line of sight between glass-metal seals and workers (or visitors) with a suitable transparent rigid plastic to stop glass particles released during an implosion.
Note: The glass industry standard dimensions are millimeters (at least the OD). Metal tubes (in the USA) are dimensioned in inches.