Fiber Optics

Fiber optics can be coupled into the vacuum space in a variety of ways. The fiber(s) can come in through either FC style connectors or compression fittings. Using FC connectors allows simpler changeout, but introduces losses at the junctions.

The fibers can come in through the base side panels or through used optical ports. Below is a table of the options.

P/N Style
4100-107-xx Single fiber FC connector feedthrough in 30mm port
4100-1130-xx Dual fiber FC connector feedthrough in 50mm port
4100-1200 Dual fiber compression feedthrough in base side panel
4100-1255 Dual fiber compression feedthrough in 50mm port

When ordering the FC style, specify the fiber type per the table below. Be aware there are typically signal strength losses at the junctions.

When using the compression fittings, the fiber is inserted in a Teflon sleeve then routed to the sample area. Be sure to respect the bond radius of the fiber or you will lose signal strength. Fibers coming in through the base area are more involved to route up to the sample. Fibers through side windows are easier to route, but must be removed when removing the housing for access.


Fiber optics can be coupled into the vacuum space using a feedthrough in an unused optical port or the base. The fiber connector mates on the outside, while another fiber connector mates within the side tube. The standard configuration has an FC connector on the inside and outside.

Window Feedthrough Assembly - 30mm Port 50mm Port


Connectorized Interface Connectors

There are several styles of interface connectors available:

Optical Port Size Fiber Optic Connectors
30mm One
50mm One or two

Fiber Types

There are several fiber types that can be supported. The standard styles are:

Style Core Diameter (um) Cladding Diameter (um)
1 9 125
2 50 125
3 62.5 125
4 50 170
5 400 430
6 400 440
7 910 1000
8 300 330
9 9 80

The fiber style should be specified when this option is ordered. If another fiber type is desired, the customer may provide about 3” of the desired fiber and we can make an appropriate connector with that fiber.


Montana Instruments can also provide a 1/8” Swagelok fitting on a window plate which can be used with a user supplied Teflon ferrule to make a compression fitting for direct feedthrough applications.

There are two fiber optic feedthrough ports on a base side panel. The covers can be removed and the fiber added. This image below shows the fiber optic side panel on a side panel doubler, which converts one base side panel location to allow two side panels.

The image below shows components disassembled. There is a small Teflon ferrule with predrilled hole used in each assembly. The hole size should match (be slightly larger than) your fiber OD. Available sizes are 125um, 250um and 343um. The Teflon ferrule is inserted into the cone shaped piece and then the fiber fed through the Teflon ferrule. The top ring is added and then the cap placed on. The cap should be tightened such that the fiber is clamped through the Teflon ferrule and the fiber can no longer be pulled through the Teflon ferrule. These compression fittings avoid having fiber FC/PC lossy connections.

The compression style fittings are available with two per base side panel or with three per 50mm window port feedthrough.


Montana Instruments can also provide various styles of brackets for holding your optical fibers. These are designed with special “V”-shaped grooves and felt compression pads to safely secure fibers at base temperature. They are typically mounted on Attocube positioners or fixed directly to the 4K platform and can accommodate most common fiber diameters. A few examples of these mounts are shown below.


How to install your fiber optic cable: The fiber optic cable inside is provided by the customer. Any unjacketed cable with the proper end connector and fiber size should be acceptable. Teflon coated cables are also OK. Note that cables with plastic sleeving may not be vacuum compatible. The cable shown in Figure 3 was made from a kit, which is a common way to make the cable. You must respect the minimum bend radius of the fiber. For installations where the fiber comes in through the side window port, there are two ways the fiber may be routed. The fiber may be wrapped around inside the housing for a service loop, then dropped down to pass through the thermal clamp to enter the radiation shield, then wrapped around the shield to enter a clip near the sample. Note that in this case, the fiber does not need thermal lagging, but this is a convenient place to hold the fiber, and allow the user to remove the radiation shield. The other approach is to bring the fiber directly into the radiation shield, either through the window port or through a user provided notch under the radiation lid. In this configuration, the user accesses the sample space and fiber by removing the radiation lid. The fiber is held in approximate place by the radiation lid or some VGE adhesive.