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Mid-Infrared Optical Fiber

  • Multimode MIR Fiber, Transmissive from the UV to 5.5 µm
  • Stable in Typical Lab Environments and Easy to Handle
  • Return Loss Less than 4% per Face

Multimode MIR Bare Fiber

Application Idea

A multimode ZrF4 patch cable can be used to propagate MIR light into a sample chamber for gas-phase spectroscopy. Our Optical Spectrum Analyzers provide detection over 350 nm to 12.0 µm (i.e., down to 833 cm-1).

Related Items

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 Click below for our selection of in-stock MIR cables.

Single Mode Fluoride Patch Cables

Multimode Fluoride Patch Cables

Multimode Fluoride Bundles

MIR Reflection/Backscatter Bundles

Indium Fluoride and Zirconium Fluoride Fiber Comparison
Click to Enlarge

Click for Raw Data
ZrFfiber has flatter attenuation than InFfiber in the MIR, while the InF3 fiber is transparent to longer wavelengths. Silica fiber, typically used in patch cables, is not MIR-transparent. For information on run-to-run variations, please see the Graphs tab.


  • Zirconium Fluoride (ZrF4) and Indium Fluoride (InF3) Fibers
  • Bare Fiber with 50 to 600 µm Core Sizes
  • Numerical Apertures of 0.20 or 0.26
  • See Our Specialty Optical Fiber Manufacturing Page for More Details
  • Single Mode MIR Fiber Available in Patch Cables

Thorlabs is pleased to extend its family of fiber products into the mid-infrared spectral region. Our IRphotonics® fibers, based upon zirconium fluoride (ZrF4) and indium fluoride (InF3) glasses, feature excellent mechanical flexibility, good environmental stability, and high transmission over the 285 nm - 4.5 µm spectral range or 310 nm - 5.5 µm spectral range, respectively. Like the rest of our fiber selection, fluoride fibers can be provided in a range of core diameters, cutoff wavelengths, and numerical apertures, suiting a variety of applications.

These fibers offer a flat attenuation curve in the MIR wavelength range (see the Graphs tab), aided by an extremely low hydroxyl ion (OH) content. They are fabricated using a proprietary technique that provides world-class purity, dimensional control, and strength. This technique gives us excellent control over the fibers' optical and mechanical properties, allowing a wide range of configurations to be drawn. The fluoride fiber is stable under typical environmental conditions and humidity.

Custom IRphotonics® MIR Fiber and Patch Cables

Several types of single mode and multimode fluoride patch cables are available from stock. We also offer bifurcated fiber bundles and reflection/backscatter proble bundles. Optical fibers with many other core sizes and configurations are currently under development.

If our standard offerings do not meet your needs, please contact Tech Support to discuss customization and potential fiber draws. Some of the many customization options we provide for MIR fibers and patch cables include:

  • MIR Optical Fibers with Lower Loss 
  • MIR Optical Fibers with Increased Power Handling Abilities 
  • Custom Patch Cables: Choose Components from Our Wide Selection of Standard Optical Fibers, Packaging Options, Connectors, and AR Coatings
  • Ruggedized Cabling Compatible with Harsh-Environment Applications
MIR Fiber Manufacturing


Multimode MIR Bare Fiber
Fiber Type Operating Wavelengtha Core Diameter Attenuationb NA Bend Radius
(Short Term / Long Term)
Operating Temperature
ZrF4 285 nm - 4.5 µm 50 µm ≤0.2 dB/m
(for 2.0 - 3.6 µm)
0.20 ± 0.02 ≥20 mm / ≥40 mm -55 to 90 °C
100 µmc ≥25 mm / ≥147 mm
200 µmc,d ≥40 mm / ≥80 mm
450 µmc,e ≥30 mm / ≥125 mm
600 µmc,e ≤0.25 dB/m
(for 2.0 - 3.6 µm)
≥75 mm / ≥160 mm
InF3 310 nm - 5.5 µm 50 µm ≤0.45 dB/m
(for 2.0 - 4.6 µm)
0.20 ± 0.02 ≥20 mm / ≥40 mm -55 to 90 °C
100 µmc 0.26 ± 0.02 ≥15 mm / ≥147 mm
  • The fiber’s operating wavelength range is defined as the region where the attenuation is <3 dB/m (>50% transmission per meter).
  • See the graph in the Overview tab.
  • Patch cables using these fibers are available from stock.
  • Reflection probe bundles using these fibers are available from stock.
  • Bifurcated fiber bundles using these fibers are available from stock.
Additional Specifications
Fiber Type ZrF4 InF3
Core Diameter 50 ± 2 µm 100 ± 2 µm 200 ± 10 µm 450 ± 15 µm 600 ± 20 µm 50 ± 2 µm 100 ± 2 µm
Cladding Diameter 140 ± 2.5 µm 192 ± 2.5 µm 290 ± 10 µm 540 ± 15 µm 690 ± 20 µm 160 ± 2 µm 192 ± 2.5 µm
Coating Diameter 270 ± 15 µm 270 ± 15 µm 355 ± 15 µm 650 ± 25 µm 770 ± 30 µm 270 ± 15 µm 287 ± 15 µm
Please contact Tech Support with your bare fiber and custom cable requests.

Multimode Fluoride Patch Cables

Zirconium Fluoride Attenuation
Click to Enlarge

Click for Raw Data
This plot contains the measured attenuation from five independent draws of the Ø200 µm core ZrF4 fiber. This data is representative of our Ø100 µm, Ø200 µm, and Ø450 µm core fibers.
Zirconium Fluoride Attenuation
Click to Enlarge

Click for Raw Data
This plot contains the measured attenuation from five independent draws of the Ø600 µm core ZrF4 fiber.
Indium Fluoride Attenuation
Click to Enlarge

Click for Raw Data
This plot contains the measured attenuation from six independent draws of the Ø100 µm core InF3 fiber.
Mid-IR Optical Fiber Manufacturing Overview


  • Zirconium Fluoride (ZrF4) and Indium Fluoride (InF3) Fibers
  • Transmission in the Mid-IR Region up to 5.5 µm
  • Flexible Manufacturing Setups and Schedules Allow for Prototyping as well as Catalog Production
  • Custom Fiber Options:
    • Different Cross-Sectional Shapes, Such as Hexagonal, Square, and D-Shaped
    • Single Mode Fiber with Core Diameters <20 µm
    • Multimode Fiber with Core Diameters >20 µm
    • NAs Ranging from 0.10 to 0.35

Thorlabs' optical fiber draw facility produces and draws zirconium fluoride (ZrF4) and indium fluoride (InF3) fibers in addition to drawing silica fiber. ZrF4 and InF3 fiber feature high transmission over the 300 nm - 4.5 µm spectral range or 300 nm - 5.5 µm spectral range, respectively, with no material absorption peaks, excellent mechanical strength, and good environmental stability.

Fluoride fibers are ideal for transmission in the mid-IR wavelength range. Low attenuation in the MIR wavelength range is aided by an extremely low hydroxyl ion (OH) content. Fluoride fibers also have a lower refractive index and lower chromatic dispersion when compared to other fibers that offer transmission in the mid-IR range. Thorlabs' fluoride fibers are ideal for use in applications including mid-IR spectroscopy, fiber optic sensors, imaging, and fiber lasers.

Thorlabs' fluoride fibers are fabricated using a technique that provides world-class purity, dimensional control, and strength. The glass components are combined and melted in the controlled environment of a glove box for purity. Once the glass is melted, it is poured into the preform mold and cooled.

After preparation, the preform is loaded into the down-feed unit at the top of the tower and drawn into fiber. Fluoride glass fiber is drawn using preform techniques similar to that used for silica fibers (see the Draw Process tab for details). This technique is well developed and has proven to be very effective in controlling fiber parameters, such as fiber diameter, concentricity, and the refractive index profile. The drawing temperature range of fluoride glasses is lower than that of silica, significantly reducing the cooling time. Thus, our fluoride fiber draw tower is much shorter than our silica fiber towers. The diagram below to the right details the components on our fluoride fiber draw tower.

Thorlabs' team of mid-IR fiber researchers and engineers has many years of experience in fluoride glass research and development, production, and fiber draw. Our team is divided into two groups: one dedicated to production of catalog items and the second devoted to research and development and custom fiber manufacturing. Their knowledge and expertise, as well as flexible tower configurations and draw schedules, allow us to produce both catalog items as well as custom orders. For details on our custom fluoride fiber capabilities, please contact Tech Support.

MIR Fiber Draw Tower
Schematic of Our Mid IR Fiber Draw Tower

Thorlabs' Mid-IR Fiber Capabilities

Thorlabs Lab Fact: Modifying Beam Profiles with Multimode Fibers

We present laboratory measurements demonstrating how the output beam profile from multimode fiber can be affected by the beam entry angle. In some applications, an alternative beam distribution such as a top hat or donut are desired instead of the inherent Gaussian distribution provided by typical optics. Here we investigated the effect of changing the input angle of a focused laser beam into a multimode fiber patch cable. Focusing the light normal to the fiber face produced a near-Gaussian output beam profile (Figure 1) and increasing the angle resulted in top hat- (Figure 2) and donut-shaped (Figure 3) beam profiles. These results demonstrate how multimode fibers can be used to change the shape of a beam profile.

For our experiment we used an M38L01 Ø200 µm, 0.39 NA, Step-Index Fiber Patch Cable (Bare Fiber Item # FT200EMT) as the test fiber into which we launched the focused laser beam. The input light was set incident at 0°, 11°, and 15° to the input face of the multimode fiber to create the initial, top hat, and donut profiles, respectively. Each time the angle was changed, the alignment of the input fiber was optimized while the output power was monitored with a power meter to ensure maximum coupling was achieved. Images were then acquired with a 9 second exposure and the shape of the beam profile was evaluated. Note that during the exposure, a 1500 grit diffuser was manually rotated between the coupling optics (before the fiber under test) to reduce the spatial coherence and create a clean output beam profile.

Assuming a ray tracing model, there are two general types of rays that propagate along a multimode fiber: (a) meridional rays, which pass through the central axis of the fiber after each reflection, and (b) skew rays, which never pass through the central axis of the fiber. The figures below illustrate the three basic ray propagation scenarios observed during the experiment. Figures 4 and 6 depict meridional and skew ray propagation through multimode fiber, respectively, and the associated theoretical beam distribution at the fiber output. As illustrated in Figure 6, skew rays propagate in a helical path along the fiber that is tangent to the inner caustic of the path with radius r. Figure 5 depicts the beam propagation and beam distribution from a combination of meridional and skew rays. By changing the input angle of the light launched into a multimode fiber, we were able to modify the proportion of light rays propagating as meridional rays vs. skew rays, and consequently, modify the output from a near-Gaussian distribution (primarily meridional rays, see Figure 1) to a top hat (mixture of meridional and skew rays, see Figure 2) to a donut (primarily skew rays, see Figure 3). The beam profiles shown in Figures 4 through 6 were obtained at a distance of 5 mm from the fiber end face. These results demonstrate the ability to use a standard multimode fiber patch cable as a relatively inexpensive method to modify an input Gaussian profile into a top hat and donut profile with minimal loss. For details on the experimental setup employed and these summarized results, please click here.

Gaussian Beam Profile
Figure 1. Near-Gaussian Beam Profile
Obtained at 0° Input Angle (Normal to Fiber Face)
Donut Beam Profile
Figure 3. Donut Beam Profile
Obtained at 15° Input Angle
Top Hat Beam Profile
Figure 2. Top Hat Beam Profile
Obtained at 11° Input Angle
Meridional Ray
Click to Enlarge

Figure 4. Meridional Ray Propagation
Corresponding to Near-Gaussian Output Profile
Skew Ray
Click to Enlarge

Figure 6. Skew Ray Propagation
Corresponding to Donut Profile
Meridional and Skew Rays
Click to Enlarge

Figure 5. Meridional and Skew Ray Propagation
Corresponding to Top Hat Profile

Posted Comments:
todd  (posted 2017-03-03 11:36:46.65)
What is the fluoride fiber buffer material? Is the buffer strippable? Once I know this I will figure out how much fiber I will need a quote for.
tfrisch  (posted 2017-03-13 02:41:45.0)
Hello, thank you for contacting Thorlabs. The buffer is acrylate, and I will contact you directly on how the recommended handling differs from silica fibers.
ilindsay  (posted 2015-08-27 15:41:39.307)
Hi. Can you comment on the end preparation of your mid-IR (fluoride) fibers, e.g. differences from SiO2 fibers in terms of cleaving and polishing techniques in the case of applications where connectors are not appropriate?
besembeson  (posted 2015-09-29 08:59:55.0)
Response from Bweh at Thorlabs USA: We recommend Thorlabs Vytran products, such as the LDC-400 ( for cleaving bare fiber. Polishing is only relevant when terminating fiber with a connector and it is different with these mid-IR fibers. I will follow-up with you for further guidance with these if needed.
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Last Edited: Jun 23, 2014 Author: Dan Daranciang