Multimode Fiber Optic Filter/Attenuator Mounts

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Diagram of the Beam Path Through the In-Line Fiber Filter Mount

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Ø1" Filters can Easily be
Removed and Replaced

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FOFMF Mounted to an Optical Table Using a BA2R Magnetic Base

Features

• Free-Space, Fiber-to-Fiber Coupling System Utilizing Off-Axis Parabolic (OAP) Mirrors
  • UV-Enhanced Aluminum Coating (250 nm - 450 nm) for UV Sources
  • Protected Silver Coating (450 nm - 20 µm) for Broadband Sources
• FC/PC or SMA Connectors
  • Mounts with Item # Prefix FOFMF Accept Both 2.2 mm Wide Key and 2.0 mm Narrow Key Connectors
  • Mounts with Item # Prefix FOFMS Accept SMA905 Connectors
• Two CFH2-F Ø1" Filter Holders Included With Each Mount
• Compatible with Macro and Micro Cuvettes (Not Included, See Specs Tab for Details)
• Bottom-Located 1/4"-20 (M6) Tap for Mounting to a Post or any Mounting Base
  
• Accessories Available Separately Below
  • Replacement Filter Holder Insert
  • Blank Insert
  • Variable Attenuator Insert
  • Light-Tight Cover

Thorlabs' In-Line Fiber Optic Filter Mounts are ideal for fiber-based applications that require light to be spectrally filtered or reduced in intensity. As shown in the diagram to the right, each mount is composed of two removable CFH2-F filter holders within a fixed, free-space fiber-to-fiber coupling system. The system utilizes two off-axis parabolic (OAP) mirrors to collimate the light out of the input fiber and couple it into the output fiber. The use of these mirrors provides high reflectance, high NA, and achromatic performance over the reflection band. For more information about the OAP mirrors, please see the Specs tab. The all-reflective design eliminates phase delays and absorption losses introduced by transmissive optics.

Either filter holder can be replaced with a 12.5 mm x 12.5 mm UV fused quartz cuvette (not included), making these mounts ideal for use in absorption or transmission spectroscopy measurements (see the Application tab for details). The system is light tight when two filter holders are used or with the use of the FOFM-CV light-tight cover.

For custom fiber bulkheads, mirror coatings, or filter holders, please contact Tech Support.

Fiber Compatibility
Thorlabs' In-Line Fiber Optic Filter Mounts are designed to work with multimode patch cables. In general, the mounts cannot be used with single-mode fibers because significant losses are incurred when light is coupled to the narrow core of these fibers in the absence of precision alignment. However, a single mode fiber can be used as the input of the mount provided that the output is connected to a multimode fiber.

Filter Holders, Blank Plates, and Variable Attenuation Insert
Each SM1-threaded (1.035"-40) filter holder can be used to mount various Ø1" filters up to 0.31" (8.0 mm) thick. The precision, spring-loaded, dovetail design of the removable holders ensures reproducible positioning. Two laser-engraved boxes on top of each holder are used for labeling and identification of the mounted optic. Additionally, Ø1/2" filters can be mounted using the SM1A6T Thread Adapter. Extra filter holders are sold separately below. 

If the included filter holder does not suit your application, the CFH2-B Blank Plate is also available separately below. This plate uses the same mounting method as the included CFH2-F and can be machined to suit your specific requirements. Alternatively, it can also be used as a manual shutter. The CFH2-V Variable Attenuation Insert can be inserted in place of one of the filter holders to partially or completely block light from passing. It features an adjuster capable of ±0.01 dB attenuation resolution (when used with Ø200 - Ø1000 µm Core, 0.22 - 0.50 NA fiber) along with the ability to quickly block the aperture via a quick-release mechanism.

Light-Tight Cover
A light-tight cover for the fiber optic filter mounts is available for applications that are particularly sensitive to light. This cover is compatible with cuvettes up to 50 mm (1.96") tall, including the stopper. Please note that the FOFM-CV cover is not compatible with the CFH2-V attenuator insert.

Mounting Options
A bottom-located 1/4"-20 (M6) tap is used to mount the in-line fiber filter mounts onto a Ø1/2" post or post holder base, such as the BA2R Magnetic Base (shown in the image above) or UBP2 Universal Base.

We also offer cuvette holders for applications such as free-space fluorescence measurements. 

Beam Diameter After Collimation

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Diagram of the Beam Path Through the In-Line Fiber Filter Mount

Beam diameter can be calculated very easily using the numerical aperture of the fiber (NA) and the reflected focal length of the OAP. To calculate the beam diameter in the small angle approximation, use the following equation:

Beam Diameter = 2 x NA (Fiber) x Reflected Focal Length

The table below lists the output beam diameter as a function of the reflected focal length of the mirror and the numerical aperture of the fiber. The clear aperture of the OAP should be larger than the desired beam output diameter.

Insertion Loss vs Wavelength

The graphs below compare the insertion loss vs wavelength for different fiber types. The shaded region denotes the specified wavelength range and insertion loss of the in-line fiber filter mount. Insertion loss at any given wavelength is also dependent upon the properties of the fiber used and is measured without filters or cuvettes.

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Excel Spreadsheet with Raw Data
The above plot shows the typical insertion loss versus wavelength for the 450 nm - 250 µm wavelength range in-line fiber filter mounts when MIR multimode fiber patch cables are used with a mode-stripped fiber launch. This can then be compared to the performance of a low-OH silica fiber cable and a theoretical lossless fiber. This data was taken using Thorlabs' SLS202L Broadband Light Source and OSA201C and OSA207C Spectrometer.

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Excel Spreadsheet with Raw Data
The above plot shows the typical insertion loss versus wavelength for the 250 nm - 450 nm wavelength range in-line fiber filter mount when used with 0.22 NA solarization-resistant patch cables. This data was taken using a broadband light source and CCS200 Spectrometer. Because the data is measured using a spectrometer, the measured insertion loss is similar to that for a mode-stripped fiber launch.

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Excel Spreadsheet with Raw Data
The above plot compares the typical insertion loss versus wavelength for the 450 nm - 250 µm wavelength range FOFMS and the 250 nm - 450 nm wavelength range FOFMS-UV. Note that the insertion loss increases rapidly when approaching UV wavelengths for the FOFMS with low OH silica fiber. In comparison, the FOFMS-UV, with solarization-resistant fiber, maintains <3.5 dB down to 250 nm.

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Excel Spreadsheet with Raw Data
The above plot shows the typical insertion loss versus wavelength for the in-line fiber filter mount when used with various 0.22 NA silica fiber patch cables. Note that even for the 50 µm core fiber and fiber bundle, the insertion loss is still around 3 dB. This data was taken using Thorlabs' SLS201L Broadband Light Source and CCS200 Spectrometer. Because the data is measured using a spectrometer, the measured insertion loss is similar to that for a mode-stripped fiber launch.

Insertion Loss at Mode-Stripped and Overfilled Launch Conditions

The graphs below compare the performance of fibers used with the in-line filter mount under different launch conditions. A mode-stripped (70/70) fiber launch condition removes cladding modes that are lost in the in-line filter while an overfilled (>100%) fiber launch condition contains all propagating modes. Because measuring the input and output power using a power meter does not allow for discrimination of these mode types, a mode-stripped launch condition is a better measurement of the actual insertion loss of the in-line fiber filter.

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Excel Spreadsheet with Raw Data
The above data was taken to show the negative effects of overfilling the input fiber (defined above). In this plot, the pinhole used in the mode-stripped example to the left was removed and the LED output was sent directly into the fiber. The output from the fiber filter holder was then coupled into an S120C Power Meter.

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Excel Spreadsheet with Raw Data
The above data was taken using a mode-stripped fiber launch (defined above). To achieve these conditions a pinhole with a diameter 70% of the fiber core was placed after the output of an M470F3 Fiber-Coupled LED. The output from the fiber filter holder was then coupled into an S120C Power Meter. This plot shows similar results compared to using a single mode laser source or mandrel wrapping method.

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Excel Spreadsheet with Raw Data
The above data was taken to show the negative effects of overfilling the input fiber (defined above). In this plot, the pinhole used in the mode-stripped example to the left was removed and the LED output was sent directly into the fiber. The output from the fiber filter holder was then coupled into an S120VC Power Meter.

Click to Enlarge
Excel Spreadsheet with Raw Data
The above data was taken using a mode-stripped fiber launch (defined above). To achieve these conditions a pinhole with a diameter 70% of the fiber core was placed after the output of an M385F1 Fiber-Coupled LED. The output from the fiber filter holder was then coupled into an S120VC Power Meter. This plot shows similar results compared to using a single mode laser source or mandrel wrapping method.

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Absorption Spectrum of Methanol

Absorption Spectroscopy

Absorption spectroscopy is a technique used to characterize the optical properties of a material by measuring the amount of light absorbed by a sample at a given frequency or wavelength. As a broadband light source is sent through a sample, the sample absorbs a fraction of the incident radiation at certain frequencies or wavelengths. These regional decreases in the overall transmission curve describe the material’s unique absorption spectrum. This technique is commonly used in material analysis to gather details about the chemical composition of an unknown substance or the concentration of a molecule in a solution.

Shown to the right is the absorption spectrum of methanol obtained by using Thorlabs' FOFMS In-Line Fiber Optic Filter Mount along with an SLS202L Broadband Light Source, two 0.20 NA, Ø200 µm Core ZrF₄ MIR multimode fiber patch cables, and a CV10Q3500 3500 µL Capacity Quartz Cuvette. Quartz cuvettes are ideal due to their high transmission from the UV to the MIR spectrum. A similar example of this setup can be seen in the image at the top of the page.

As light enters the FOFMF filter system, it is collimated at the first protected silver OAP with a beam diameter of approximately 6 mm (see the Specs tab for details). It then passes through two slots that house a Ø1" optical filter or cuvette. In this case, we loaded a single cuvette with a sample of methanol. Once through the cuvette, the light is couple into the output fiber using the second protected silver OAP mirror and measured using a Fourier Transform Infrared Spectrometer (FTIR).