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0.48 NA Step-Index Multimode Fibers

  • Broad UV/VIS/NIR Spectral Range
  • Reduced Static Fatigue
  • Lower Microbend Losses

0.48 NA Step Index Multimode Fiber Cross Section

(Not to

Tefzel Buffer

Hard-Polymer Cladding

Pure Silica Core

Related Items

Stock Patch Cables (0.48 NA, Low OH Multimode Fiber)
Fiber TypeConnectorAvailable LengthsItem #
BFL48-400 SMA 1 m or 2 m M40L0x
BFL48-600 SMA 1 m or 2 m M41L0x
BFL48-1000 SMA 1 m or 2 m M71L0x


  • Broad UV/VIS/NIR Spectral Range:
    • 350 to 1200 nm (High OH)
    • 400 to 2200 nm (Low OH)
  • Reduced Static Fatigue, Lower Microbend Losses
  • Biocompatible Materials, Radiation Resistance
  • Sterilizable by ETO and Other Methods

Our 0.48 NA polymer-clad fibers offer high numerical apertures to suit a broad range of applications, from remote illumination to photodynamic therapy. The fiber is encased in a Tefzel buffer that has an operating temperature range of -40 to 150 °C.

The cladding material utilized to achieve the large NA of these fibers is a softer polymer than normally found in polymer clad step-index multimode fibers. Consequently, the cladding material has a higher probability of being removed from the fiber when the buffer is being stripped for normal connectorization. The difference between the indices of refraction of the core and the cladding determine the NA. Thus, without the cladding, the performance of this fiber is greatly diminished. To combat this problem, Thorlabs terminates to the buffer instead of terminating to the cladding. That is, the connector is epoxied directly onto the fiber buffer, thereby eliminating the need to strip the buffer, and the cladding, off of the fiber. The image in the upper right corner shows this difference.

Thorlabs offers SMA-terminated patch cables with our low-OH 0.48 NA multimode fibers (see table to the right). If you require another connector type, we also offer the ADAFCSMA1 FC/PC-to-SMA Mating Sleeve that can be used to convert an SMA connector to an FC/PC connector, as well as hybrid patch cables. Alternatively, we offer 0.39 NA fiber as bare fiber as well in patch cables with various connectors. Please contact Tech Support for further information.

Custom Fiber Patch Cables Optical Fiber Manufacturing
Alternate Numerical Aperture Step-Index Fibers
0.1 NA High-Power,
Small-Core Fibers
0.22 NA High- and
Low-OH Fibers
0.39 NA High-
and Low-OH Fibers
0.48 NA High- and
Low-OH Fibers
Item #Wavelength
Core /
CoatingBend RadiusProof Test
Short TermLong Term
BFH48-400 350 - 1200 nm High OH 400 μm ± 2% 430 μm ± 2% 730 μm ± 5% Pure Silica /
Hard Polymer
Tefzel 22 mm 65 mm 70 kpsi
BFL48-400 400 - 2200 nm Low OH
BFH48-600 350 - 1200 nm High OH 600 μm ± 2% 630 μm ± 2% 1040 μm ± 5% 32 mm 95 mm
BFL48-600 400 - 2200 nm Low OH
BFH48-1000 350 - 1200 nm High OH 1000 μm ± 2% 1035 μm ± 2% 1400 μm ± 5% 52 mm 155 mm
BFL48-1000 400 - 2200 nm Low OH
Power Handling Limitations Imposed by Optical Fiber
Click to Enlarge
Undamaged Fiber End
Power Handling Limitations Imposed by Optical Fiber
Click to Enlarge
Damaged Fiber End

Laser Induced Damage in Optical Fibers

The following tutorial details damage mechanisms in unterminated (bare) and terminated optical fibers, including damage mechanisms at both the air-to-glass interface and within the glass of the optical fiber. Please note that while general rules and scaling relations can be defined, absolute damage thresholds in optical fibers are extremely application dependent and user specific. This tutorial should only be used as a guide to estimate the damage threshold of an optical fiber in a given application. Additionally, all calculations below only apply if all cleaning and use recommendations listed in the last section of this tutorial have been followed. For further discussion about an optical fiber’s power handling abilities within a specific application, contact Thorlabs’ Tech Support.

Damage at the Free Space-to-Fiber Interface

There are several potential damage mechanisms that can occur at the free space-to-fiber interface when coupling light into a fiber. These come into play whether the fiber is used bare or terminated in a connector.

Silica Optical Fiber Maximum Power Densities
TypeTheoretical Damage ThresholdPractical Safe Value
(Average Power)
1 MW/cm2250 kW/cm2
10 ns Pulsed
(Peak Power)
5 GW/cm21 GW/cm2

Unterminated (Bare) Fiber
Damage mechanisms in bare optical fiber can be modeled similarly to bulk optics, and industry-standard damage thresholds for UV Fused Silica substrates can be applied to silica-based fiber (refer to the table to the right). The surface areas and beam diameters involved at the air-to-glass interface are extremely small compared to bulk optics, especially with single mode (SM) fiber, resulting in very small damage thresholds.

The effective area for SM fiber is defined by the mode field diameter (MFD), which is the effective cross-sectional area through which light propagates in the fiber. A free-space beam of light must be focused down to a spot of roughly 80% of this diameter to be coupled into the fiber with good efficiency. MFD increases roughly linearly with wavelength, which yields a roughly quadratic increase in damage threshold with wavelength. Additionally, a beam coupled into SM fiber typically has a Gaussian-like profile, resulting in a higher power density at the center of the beam compared with the edges, so a safety margin must be built into the calculated damage threshold value if the calculations assume a uniform density.

Multimode (MM) fiber’s effective area is defined by the core diameter, which is typically far larger than the MFD in SM fiber. Kilowatts of power can be typically coupled into multimode fiber without damage, due to the larger core size and the resulting reduced power density.

It is typically uncommon to use single mode fibers for pulsed applications with high per-pulse powers because the beam needs to be focused down to a very small area for coupling, resulting in a very high power density. It is also uncommon to use SM fiber with ultraviolet light because the MFD becomes extremely small; thus, power handling becomes very low, and coupling becomes very difficult.

Example Calculation
For SM400 single mode fiber operating at 400 nm with CW light, the mode field diameter (MFD) is approximately Ø3 µm. For good coupling efficiency, 80% of the MFD is typically filled with light. This yields an effective diameter of Ø2.4 µm and an effective area of 4.52 µm2:

Area = πr2 = π(MFD/2)2 = π • 1.22 µm2 = 4.52 µm2

This can be extrapolated to a damage threshold of 11.3 mW. We recommend using the "practical value" maximum power density from the table above to account for a Gaussian power distribution, possible coupling misalignment, and contaminants or imperfections on the fiber end face:

250 kW/cm2 = 2.5 mW/µm2

4.25 µm2 • 2.5 mW/µm2 = 11.3 mW

Terminated Fiber
Optical fiber that is terminated in a connector has additional power handling considerations. Fiber is typically terminated by being epoxied into a ceramic or steel ferrule, which forms the interfacing surface of the connector. When light is coupled into the fiber, light that does not enter the core and propagate down the fiber is scattered into the outer layers of the fiber, inside the ferrule.

The scattered light propagates into the epoxy that holds the fiber in the ferrule. If the light is intense enough, it can melt the epoxy, causing it to run onto the face of the connector and into the beam path. The epoxy can be burned off, leaving residue on the end of the fiber, which reduces coupling efficiency and increases scattering, causing further damage. The lack of epoxy between the fiber and ferrule can also cause the fiber to be decentered, which reduces the coupling efficiency and further increases scattering and damage.

The power handling of terminated optical fiber scales with wavelength for two reasons. First, the higher per photon energy of short-wavelength light leads to a greater likelihood of scattering, which increases the optical power incident on the epoxy near the end of the connector. Second, shorter-wavelength light is inherently more difficult to couple into SM fiber due to the smaller MFD, as discussed above. The greater likelihood of light not entering the fiber’s core again increases the chance of damaging scattering effects. This second effect is not as common with MM fibers because their larger core sizes allow easier coupling in general, including with short-wavelength light.

Fiber connectors can be constructed to have an epoxy-free air gap between the optical fiber and ferrule near the fiber end face. This design feature, commonly used with multimode fiber, allows some of the connector-related damage mechanisms to be avoided. Our high-power multimode fiber patch cables use connectors with this design feature.

Combined Damage Thresholds
As a general guideline, for short-wavelength light at around 400 nm, scattering within connectors typically limits the power handling of optical fiber to about 300 mW. Note that this limit is higher than the limit set by the optical power density at the fiber tip. However, power handing limitations due to connector effects do not diminish as rapidly with wavelength when compared to power density effects. Thus, a terminated fiber’s power handling is "connector-limited" at wavelengths above approximately 600 nm and is "fiber-limited" at lower wavelengths.

The graph to the right shows the power handling limitations imposed by the fiber itself and a surrounding connector. The total power handling of a terminated fiber at a given wavelength is limited by the lower of the two limitations at that wavelength. The fiber-limited (blue) line is for SM fibers. An equivalent line for multimode fiber would be far above the SM line on the Y-axis. For terminated multimode fibers, the connector-limited (red) line always determines the damage threshold.

Please note that the values in this graph are rough guidelines detailing estimates of power levels where damage is very unlikely with proper handling and alignment procedures. It is worth noting that optical fibers are frequently used at power levels above those described here. However, damage is likely in these applications. The optical fiber should be considered a consumable lab supply if used at power levels above those recommended by Thorlabs.

Damage Within Optical Fibers

In addition to damage mechanisms at the air-to-glass interface, optical fibers also display power handling limitations due to damage mechanisms within the optical fiber itself. Two categories of damage within the fiber are damage from bend losses and damage from photodarkening.

Bend Losses
Bend losses occur when a fiber is bent to a point where light traveling in the core is incident on the core/cladding interface at an angle higher than the critical angle, making total internal reflection impossible.Under these circumstances, light escapes the fiber, often in one localized area. The light escaping the fiber typically has a high power density, which can cause burns to the fiber as well as any surrounding furcation tubing.

A special category of optical fiber, called double-clad fiber, can reduce the risk of bend-loss damage by allowing the fiber’s cladding (2nd layer) to also function as a waveguide in addition to the core. By making the critical angle of the cladding/coating interface higher than the critical angle of the core/clad interface, light that escapes the core is loosely confined within the cladding. It will then leak out over a distance of centimeters or meters instead of at one localized spot within the fiber, minimizing damage. Thorlabs manufactures and sells 0.22 NA double-clad multimode fiber, which boasts very high, megawatt range power handling.

A second damage mechanism within optical fiber, called photodarkening or solarization, typically occurs over time in fibers used with ultraviolet or short-wavelength visible light. The pure silica core of standard multimode optical fiber can transmit ultraviolet light, but the attenuation at these short wavelengths increases with the time exposed to the light. The mechanism that causes photodarkening is largely unknown, but several strategies have been developed to combat it. Fibers with a very low hydroxyl ion (OH) content have been found to resist photodarkening. Other dopants, including fluorine, can also reduce photodarkening.

Germanium-doped silica, which is commonly used for the core of single mode fiber for red or IR wavelengths, can experience photodarkening with blue visible light. Thus, pure silica core single mode fibers are typically used with short wavelength visible light. Single mode fibers are typically not used with UV light due to the small MFD at these wavelengths, which makes coupling extremely difficult.

Even with the above strategies in place, all fibers eventually experience photodarkening when used with UV light, and thus, fibers used with these wavelengths should be considered consumables.

Tips for Maximizing an Optical Fiber's Power Handling Capability

With a clear understanding of the power-limiting mechanisms of an optical fiber, strategies can be implemented to increase a fiber’s power handling capability and reduce the risk of damage in a given application. All of the calculations above only apply if the following strategies are implemented.

One of the most important aspects of a fiber’s power-handling capability is the quality of the end face. The end face should be clean and clear of dirt and other contaminants that can cause scattering of coupled light. Additionally, if working with bare fiber, the end of the fiber should have a good quality cleave, and any splices should be of good quality to prevent scattering at interfaces.

The alignment process for coupling light into optical fiber is also important to avoid damage to the fiber. During alignment, before optimum coupling is achieved, light may be easily focused onto parts of the fiber other than the core. If a high power beam is focused on the cladding or other parts of the fiber, scattering can occur, causing damage.

Additionally, terminated fibers should not be plugged in or unplugged while the light source is on, again so that focused beams of light are not incident on fragile parts of the connector, possibly causing damage.

Bend losses, discussed above, can cause localized burning in an optical fiber when a large amount of light escapes the fiber in a small area. Fibers carrying large amounts of light should be secured to a steady surface along their entire length to avoid being disturbed or bent.

Additionally, choosing an appropriate optical fiber for a given application can help to avoid damage. Large-mode-area fibers are a good alternative to standard single mode fibers in high-power applications. They provide good beam quality with a larger MFD, thereby decreasing power densities. Standard single mode fibers are also not generally used for ultraviolet applications or high-peak-power pulsed applications due to the high spatial power densities these applications present.

The table below details all of Thorlabs' multimode bare optical fiber offerings.

Index ProfileNAFiber TypeItem #Core SizeWavelength Range
Step Index 0.1 Double Clad, Enhanced Coating
View These Fibers
HPSC10 Ø10 µm 400 - 550 nm and
700 - 1400 nm
HPSC25 Ø25 µm
0.22 Glass-Clad Slilca
Multimode Fiber
View These Fibers
FG050UGA Ø50 µm 250 - 1200 nm
(High OH)
FG105UCA Ø105 µm
FG200UEA Ø200 µm
FG050LGA Ø50 µm 400 - 2400 nm
(Low OH)
FG105LCA Ø105 µm
FG200LEA Ø200 µm
High Power Double TECS /
Silica Cladding
Multimode Fiber
View These Fibers
FG200UCC Ø200 µm 190 - 1200 nm
(High OH)
FG365UEC Ø365 µm
FG550UEC Ø550 µm
FG910UEC Ø910 µm
FG200LCC Ø200 µm 400 - 2200 nm
(Low OH)
FG365LEC Ø365 µm
FG550LEC Ø550 µm
FG910LEC Ø910 µm
Solarization-Resistant Multimode
Fiber for UV Use
View These Fibers
UM22-100 Ø100 µm 180 - 1150 nm
(High OH)
UM22-200 Ø200 µm
UM22-300 Ø300 µm
UM22-400 Ø400 µm
UM22-600 Ø600 µm
0.39 High Power TECS Cladding
Multimode Fiber
View These Fibers
FT200UMT Ø200 µm 300 - 1100 nm
(High OH)
FT300UMT Ø300 µm
FT400UMT Ø400 µm
FT600UMT Ø600 µm
FT800UMT Ø800 µm
FT1000UMT Ø1000 µm
FT1500UMT Ø1500 µm
FT200EMT Ø200 µm 400 - 2200 nm
(Low OH)
FT300EMT Ø400 µm
FT400EMT Ø400 µm
FT600EMT Ø600 µm
FT800EMT Ø600 µm
FT1000EMT Ø1000 µm
FT1500EMT Ø1500 µm
0.48 High NA Multimode Fiber
View These Fibers
BFH48-400 Ø200 µm 300 - 1200 nm
(High OH)
BFH48-600 Ø400 µm
BFH48-1000 Ø600 µm
BFL48-400 Ø200 µm 400 - 2200 nm
(Low OH)
BFL48-600 Ø400 µm
BFL48-1000 Ø600 µm
Graded Index 0.20 Graded-Index Fiber
for Low Bend Loss
View These Fibers
GIF50C Ø50 µm 750 - 1450 nm
0.275 GIF625 Ø62.5 µm 800 - 1350 nm
Click the Support Documentation icon document icon or Part Number below to view the available support documentation
Part Number Product Description
BFH48-1000 Support Documentation BFH48-1000 : Multimode Fiber, 0.48 NA, High OH, Ø1000 µm Core
BFH48-400 Support Documentation BFH48-400 : Multimode Fiber, 0.48 NA, High OH, Ø400 µm Core
BFH48-600 Support Documentation BFH48-600 : Multimode Fiber, 0.48 NA, High OH, Ø600 µm Core
Part Number Product Description
BFL48-1000 Support Documentation BFL48-1000 : Multimode Fiber, 0.48 NA, Low OH, Ø1000 µm Core
BFL48-400 Support Documentation BFL48-400 : Multimode Fiber, 0.48 NA, Low OH, Ø400 µm Core
BFL48-600 Support Documentation BFL48-600 : Multimode Fiber, 0.48 NA, Low OH, Ø600 µm Core

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Posted Comments:
Poster: jlow
Posted Date: 2014-07-29 03:34:58.0
Response from Jeremy at Thorlabs: We do not have test data on the scintillation effect on these optical fiber. This could be due the hard polymer cladding. It would be interesting to try different type of fiber and see if the effect is still present.
Poster: marcel.haebi
Posted Date: 2014-05-31 11:51:12.9
dear gentlemen!first, we intended to use the BFH48-400 only as an optical fiber for light transmission from a doped fiber to a photodiode in order to measure the cross section of a proton beam. Later on, instead of using a doped fiber we were shooting with the proton beam of 18 MeV directly to the optical fiber BFH48-400 and we could also measure a clear signal, i.e. kind of a scintillating effect took place inside the BFH48-400. Do you have any information about impurities of the optical fiber? or is there another explanation why could detect a signal from just an optical fiber?Best regards
Poster: stephan.leo
Posted Date: 2014-05-12 16:02:32.007
Sehr geehrte Damen und Herren, wir benötigen diese BFH48-400 mit Länge 55mm, an den Enden gekappt und poliert. Ist das in dieser Spezifikation kurzfristig lieferbar? Unsere Kunden-Nr. ist 323316. Mit freundlichen Grüßen Stephan Leo Purchasing Qioptiq Photonics GmbH (Feldkirchen)
Poster: cdaly
Posted Date: 2013-07-03 13:20:00.0
Response from Chris at Thorlabs: Thank you for using our web feedback. No this fiber itself does not come with SMA connectors, though we do provide stock SMA version found here: We can customize these as well. If you would like a custom please contact us at Or request a quote with the new fiber patch cable calculator located here:
Poster: michail.lukaschek
Posted Date: 2013-07-03 17:44:43.447
Sehr geehrtre Damen und Herren, ist bei der Faser BFH48-600 kein, ein oder beide Enden mit SMA-Stecker konfektioniert? Ich bräuchte eine Ende mit SMA. Viele Grüße Michail Lukaschek
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0.48 NA, High-OH Multimode Optical Fiber

Item #Wavelength
Core /
BFH48-400 350 - 1200 nm High OH 0.48 ± 0.02 400 µm ± 2% 430 µm ± 2% 730 µm ± 5% Pure Silica /
Hard Polymer
Tefzel T21S31
BFH48-600 600 µm ± 2% 630 µm ± 2% 1040 µm ± 5% T28S46
BFH48-1000 1000 µm ± 2% 1035 µm ± 2% 1400 µm ± 5% M44S63
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal/Imperial Price Available / Ships
BFH48-400 Support Documentation
BFH48-400 Multimode Fiber, 0.48 NA, High OH, Ø400 µm Core
Per Meter
Volume Pricing
BFH48-600 Support Documentation
BFH48-600 Multimode Fiber, 0.48 NA, High OH, Ø600 µm Core
Per Meter
Volume Pricing
BFH48-1000 Support Documentation
BFH48-1000 Multimode Fiber, 0.48 NA, High OH, Ø1000 µm Core
Per Meter
Volume Pricing

0.48 NA, Low-OH Multimode Optical Fiber

Item #Wavelength
Core /
BFL48-400 400 - 2200 nm Low OH 0.48 ± 0.02 400 µm ± 2% 430 µm ± 2% 730 µm ± 5% Pure Silica /
Hard Polymer
Tefzel T21S31
BFL48-600 600 µm ± 2% 630 µm ± 2% 1040 µm ± 5% T28S46
BFL48-1000 1000 µm ± 2% 1035 µm ± 2% 1400 µm ± 5% M44S63
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal/Imperial Price Available / Ships
BFL48-400 Support Documentation
BFL48-400 Multimode Fiber, 0.48 NA, Low OH, Ø400 µm Core
Per Meter
Volume Pricing
BFL48-600 Support Documentation
BFL48-600 Multimode Fiber, 0.48 NA, Low OH, Ø600 µm Core
Per Meter
Volume Pricing
BFL48-1000 Support Documentation
BFL48-1000 Multimode Fiber, 0.48 NA, Low OH, Ø1000 µm Core
Per Meter
Volume Pricing
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