FAQ-Vacuum Optics


Question: What torque should I use to mount my CF Viewport?
Answer: A specific torque for mounting viewports cannot be given. This is due to the fact that the coefficient of friction of the screws and nuts that you use is not known to us, with this having a significant influence on the torque.
In general, the following recommendations for action are valid for viewports, which can also be taken from the enclosed handling brochure:
Only use special vacuum-annealed copper gaskets as well as washers for all screws when mounting CF viewports. Tighten opposing screws carefully and evenly in several passes with a torque wrench. Uneven tightening of screws leads to tensions on the viewport, which can cause leaks!
Question: There is dirt on my viewport. How can I clean it off?
Answer: You can clean standard viewports for monitoring with a clean tissue and some solvent. Viewports with sensitive optics for high precision or optical coatings should initially be cleaned by dry air blowing. If this doesn’t lead to your desired result, you can clean the optical surface with a lint-free optical tissue and some solvent. Please note, that too much solvent can produce stains. For viewports with highly sensitive surfaces we recommend special cleaning polymers qualified for high precision optics.
CAUTION: We generally advise against viewport cleaning in an ultrasonic bath. This can destroy the connection between flange and optical material.

Question: Can you provide standard viewports with AR-coatings?
Answer: Antireflection coatings are used to increase the overall transmission through the optic. There are three factors, which affect the transmission of a substrate: absorption, reflection and scattering. Coating a surface only adds up to a significant increase in transmission if the surface shows very low scattering. We can specify this for our high precision optics with high surface quality. Therefore, we generally advise against coating standard viewports. If you are interested in coated optics, we gladly recommend our “viewports for optical applications” which can be coated with your individual specification.

Question: Can I use your High Precision Optics Series (HiPO-series) in the UHV-range?
Answer: Our HiPO-series is designed for sealing the optic with a vacuum-suitable polymer ring. The He-leckage rate is tested down to 5E-10 mbar·l/s for 100% of the products after manufacturing. The final pressure, which can be reached in your application correlates to the throughput of your vacuum pump as well as to the conditioning process of your chamber. With an optimized set up you can use our HiPO-series in the pressure range of E-10mbar.

Question: Can VACOM provide customized viewports?
Answer: Yes! We are looking forward to developing your individually designed high quality viewport. Please contact your Sales Representatives with the following information:

  • What is the application of the viewport?
  • What is the wavelength range the viewport will be used in?
    • Do you have a preferred optical material?
  • Do you need a defined surface quality?
    • Flatness (wavefront distortion)
    • Parallelism
    • Surface defects
  • What minimum clear aperture do you need?
  • What are your requirements for transmission or more specifically do you wish an AR-coated optic?
    • For which wavelength (range) shall the AR-coating be optimized?
    • What minimum percentage of transmission do you wish to achieve?
  • Which maximum baking or minimum cooling temperatures are you expecting in your application?
  • In what pressure range are you working in, what is your preferred flange connection and what is your required He-leckage rate?
  • Do you have special requirements for magnetic properties of the viewport?
Question: We occasionally experience excess pressure in our application. Is this a problem for the viewports?
Answer: Generally our viewports are destined for vacuum applications and we do not endorse you for using them with excess pressure. Yet, the viewports will withstand controlled venting processes.

Question: Can you assemble viewports with other optical materials than listed in your product catalog?
Antwort. Depending on your individual requirements we are able to use several mounting technologies to install a great variety of optical materials into our viewports. If you are looking for a viewport with a specific optical material, please contact your Sales Representatives.

Optical Feedthroughs

Question: Is the feedthrough hermetically sealed if no cables are plugged in?
Answer: Yes, the feedthrough is internally sealed. The He-leakage rate of every single unit is measured after the manufacturing process.
When inspecting the feedthrough you might notice a bright spot in the center of the ferrule. What may look like a leakage, is the integrated translucent optical fiber.

Question: How is the feedthrough designed – is one side permanently attached to a cable?
Answer: No. Integrated in the feedthrough is a short piece of the optical fiber. On either side you can connect a glass fiber cable to the feedthrough. We offer the connection types FC/APC, FC/PC and FSMA. The great advantage of this design – in case of a broken cable you can simply attach a new cable instead of exchanging the entire fiber feedthrough.

Question: Is the feedthrough suited for applications with polarized light (PM)?
Answer: The integrated fibers (length < 30mm) are not polarization maintaining. However, they are joined in a low-stress process. If our feedthroughs are suitable for your PM-application primarily depends on your required level of polarization maintenance. For testing, we gladly lend you our feedthroughs for a small fee.

Question: How much power can be put through the feedthrough?
Answer: For using single-mode fibers with a small core security measures should be considered when working with cw-lasers exceeding average powers of 100 mW. Multi-mode fibers with significantly larger core diameters enable working with powers around 1 W.
There are no general limits for pulsed lasers. Please contact our technical representatives to inform you which feedthrough will work with your laser system.
When testing our fiber feedthroughs for maximum throughput, please make sure that all end faces of the fibers (feedthrough and connectors!) are perfectly clean.

Question: Are your feedthroughs suitable for applications in cryogenic environment?
Answer: Customers have already used our feedthroughs at temperatures down to 77K. Feedthroughs are usually placed best in the warmest areas of cryogenic systems. In that case feedthroughs can be connected to the area of lowest temperature with our vacuum fiber cables. 

Question: Is your feedthrough a VACOM in-house production – are modifications possible?
Answer: Yes, our optical fiber feedthroughs (OFD) are manufactured and tested in our own VACOM Production and Technology Center. For batches from at least 6 pieces up (according to the grade of modification) we can adapt our design or integrate custom fibers in order to meet your requirements. Any adaptions will be made subsequently to an in-house proofing of feasibility.

Question: What kind of materials does the feedthrough consist of?
Answer: All metallic components of the feedthrough are made out of stainless steel 1.4404 (316L). The inside parts consist of Zirconium Dioxide (ferrules) and Fused Silica (fibers). The feedthrough also contains a small amount of low-outgassing epoxy (confirmed by NASA and ASTM E595). The same materials are used in the connectors of the vacuum fiber cables.

Question: Can I use your feedthrough in liquids or at overpressure?
Answer: No. Please only use our optical fiber feedthroughs for vacuum applications.

Question: None of the standard feedthroughs in your portfolio meets my operating wavelength. How wide is the wavelength range in which the fibers are usable?
Answer: Even though fibers come with a distinct nominal wavelength (range), their actual transmitting ranges highly depend on the actual application, so that some fibers become suitable for quite wider fields than designated for. Our technical support will assist you in finding your best solution.

 Transmission range optical material
Question: Is the fiber integrated in VACOM’s standard MM50 feedthrough a gradient-index- or step-index-type?
Answer: As standard the MM50 feedthrough comes with a gradient-index-fiber. If desired, we can readily build a modified version with a step-index-type.

Question: Is it possible to hermetically seal an existing fiber-cable/-bundle into a chamber wall?
Answer: Depending on cable type or bundle configuration we can provide this kind of connection. Please contact us to evaluate the feasibility for your specific configuration.

Question: Can I use your standard atmospheric-fiber-cables inside a vacuum-chamber if my vacuum requirements are rather low?
Answer: In principal, yes. However, please take into account that outgassing of the polymer-buffer in vacuum environment will accelerate aging effects. Additionally the tube will act as a reservoir that is rather difficult to pump.

Question: Are the vacuum fiber cables equipped with a protective coating or tube?
Answer: Our UHV-suitable cables only consist of the fiber and the respective coating (polyimide or metallic). Buffers (stainless steel, PTFE or PEEK) can be added according to the application requirements. Please note, that the applied cladding or any added buffer will limit the achievable vacuum pressure and maximum operating temperatures.

Question: Can I get drawings or STEP-files of your feedthroughs?
Answer: Yes, please contact your Sales Representatives for further information.

Question: Are the vacuum fiber cables available with heat or radiation resistant metallization?
Answer: The single mode fibers SM633 and SM1310/1550 are available with aluminum-coating. The maximum operating temperature of the aluminum-coated fiber cables is 400°C.

Question: What limitations do I have to consider when mounting the weldable feedthrough version?
Answer: Please stick to the maximum operating temperatures and heating and cooling rates given in the product data sheets of the feedthroughs. Make sure, that all safety caps need to be fully tightened in order to avoid infiltration of gases, particles or liquids.

Question: There is dirt on my feedthrough - transmission losses are high. How can I clean it?
Answer: Please clean the end faces of the fibers with commercially available cleaning supplies for optical plug-connectors. For persistent cases we offer you to clean your parts in our in-house cleaning department in a cleanroom environment. Corresponding to the cleaning procedures we can provide you with associated measuring results and reports, respectively.

Here we answer frequently asked questions. If you have any questions on our products, you can also contact your Sales Representatives.

Here you can download the brochure for handling of viewports.