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0.10 NA Multimode Step Index Optical Fiber

  • 0.10 NA Fiber with Ø10 µm, Ø25 µm, or Ø105 µm Core
  • Undoped Core
  • Fluorine-Doped Cladding

0.10 NA Step Index Multimode Fiber Cross Section

Related Items

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Stock Patch Cables Available with These 0.10 NA MM Fibers
Item # Fiber Used Description Length
M65 FG010LDA SMA905 to SMA905 1 or 2 m
M64 FC/PC to FC/PC 1 or 2 m
M23 FC/PC to SMA905 1 m
M68 FG025LJA SMA905 to SMA905 1 or 2 m
M67 FC/PC to FC/PC 1 or 2 m
M39 FC/PC to SMA905 1 m
M96 FG105LVA SMA905 to SMA905 1 or 2 m
M94 FC/PC to FC/PC 1 or 2 m
M100 FC/PC to SMA905 1 m


  • Wavelength Range:
    • FG010LDA: 400 to 550 nm and 700 to 1000 nm
    • FG025LJA: 400 to 550 nm and 700 to 1400 nm
    • FG105LVA: 400 to 2100 nm
  • Ø10 µm, Ø25 µm, or Ø105 µm Core
  • Undoped Silica Core and Fluorine-Doped Cladding
  • Dual Acrylate Coating

Thorlabs 0.10 NA multimode fibers are protected with an enhanced coating material that guarantees long-term performance and reliability. The dual layer acrylate material is designed to strengthen the low-NA, small-core fiber, thus reducing losses related to micro-bending. Additionally, the coating is easy to strip and leaves no residue. Each fiber provides unsurpassed durability and transmission time stability. They can be used with any one of our multimode connectors with a Ø125 µm bore, as well as many fiber components designed for fiber with Ø125 µm cladding.

0.10 NA Multimode Fibers
Item # Core Size Cladding Size Coating Size Min. Bend Radius
(Short Terma / Long Termb)
FG010LDA Ø10 µm ± 3.0 µm Ø125 ± 2.0 µm Ø245 ± 10 µm 120 x Cladding Diameter /
240 x Cladding Diameter
FG025LJA Ø25 µm ± 3.0 µm
FG105LVA Ø105 µm ± 3.0 µm Ø250 ± 10 µm
  • Recommended geometric strain during installation is 100% of proof test level, based upon statistical analysis of fiber failures.
  • Recommended geometric strain is 50% of proof test level, based upon statistical analysis of fiber failures.

Custom Fiber Patch Cables
Optical Fiber Manufacturing
Alternate Numerical Aperture Step-Index Fibers
0.10 NA High-Power,
Small-Core Fibers
0.22 NA High- and
Low-OH Fibers
0.39 NA High- and
Low-OH Fibers
0.48/0.50 NA High- and
Low-OH Fibers
Item # FG010LDA FG025LJA FG105LVA
Optical Specifications
Wavelength Range  400 - 550 nm and 700 - 1000 nm   400 - 550 nm and 700 - 1400 nm           400 - 2100 nm          
Numerical Aperture 0.100 ± 0.015
Core Index Proprietarya
Cladding Index Proprietarya
Geometric Specifications
Core Diameter 10 ± 3.0 µm 25 ± 3.0 µm 105 ± 3.0 µm
Cladding Diameter 125 ± 2.0 μm
Coating Diameter 245 ± 10 μm 250 ± 10 μm
Core/Clad Concentricity <1.0 μm
Other Specifications
Coating Two-Layer Acrylate
Minimum Bend Radius
(Short Termb / Long Termc)
120 x Cladding Diameter / 240 x Cladding Diameter
Operating Temperature -60 to 85 °C -40 to 85 °C
Proof Test ≥100 kpsi
  • We regret that we cannot provide this proprietary information.
  • Recommended geometric strain during installation is 100% of proof test level, based upon statistical analysis of fiber failures.
  • Recommended geometric strain is 50% of proof test level, based upon statistical analysis of fiber failures.
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 Silica 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.

Unterminated Silica Fiber Maximum Power Densities
Type Theoretical Damage Threshold Practical Safe Value
(Average Power)
1 MW/cm2 250 kW/cm2
10 ns Pulsed
(Peak Power)
5 GW/cm2 1 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. To achieve good efficiency when coupling into a single mode fiber, a free-space beam of light must match the diameter given by the MDF. Thus, a portion of the light travels through the cladding when matching the MFD. The 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. For MM fibers, a free-space beam of light must be focused down to a spot of roughly 70 - 80% of the MFD to be coupled into the fiber with good efficiency.

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, the light must fill the MFD of the fiber. Thus, the effective diameter is Ø3 µm with an effective area of 7.07 µm2:

Area = πr2 = π(MFD/2)2 = π • 1.52 µm2 = 7.07 µm2

This can be extrapolated to a damage threshold of 17.7 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

7.07 µm2 • 2.5 mW/µm2 = 17.7 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.

Numerical Aperture

Numerical Aperture (NA), a measure of the acceptance angle of a fiber, is a dimensionless quantity.  For applications, it is most commonly expressed as

NA Equation 1

where θa  is the maximum 1/2 acceptance angle of the fiber, and ni  is the index of refraction of the material outside of the fiber. This material is typically air, making it equal to approximately 1.0, as shown in the figure below.

Fiber Acceptance Angles

Figure: A ray at the maximum 1/2 acceptance angle propagates into a fiber.

Numerical Aperture can also be defined in terms of the index of refraction of the fiber core and cladding. Due to Snell’s law, there is a critical angle above which all of the light at a fiber-cladding interface will experience total internal reflection. In turn, this means that there is a maximum acceptance angle at which light can enter the fiber. Following Snell’s law, the maximum acceptance angle can be determined:

NA Equation 2
NA Equation 3
NA Equation 4

nf  is the index of refraction of the fiber core, nc is the index of refraction of the cladding,
θc  is the critical angle for total internal reflection, and
θa  is the maximum 1/2 acceptance angle.

This is the common way numerical aperture is defined for optical fibers. It is also important to note that these equations assume that a Gaussian beam is being outputted from the fiber.

Thorlabs offers multimode bare optical fiber with silica, zirconium fluoride (ZrF4), or indium fluoride (InF3) cores. The graph below is an attenuation comparison of our step-index silica core fibers. We also offer fluoride core fiber for higher transmission into the mid-infrared as well as graded-index fiber. The table below details all of Thorlabs' multimode bare optical fiber offerings.

Attenuation of Thorlabs' Silica Core Multimode Step-Index Fibers
Index Profile NA Fiber Type Item # Core Size Wavelength Range
Step Index 0.100 Fluorine-Doped Cladding,
Enhanced Coating
View These Fibers
FG010LDA Ø10 µm 400 to 550 nm and 700 to 1000 nm
FG025LJA Ø25 µm 400 to 550 nm and 700 to 1400 nm
FG105LVA Ø105 µm 400 to 2100 nm
(Low OH)
0.22 Glass-Clad Slilca
Multimode Fiber
View These Fibers
FG050UGA Ø50 µm 250 to 1200 nm
(High OH)
FG105UCA Ø105 µm
FG200UEA Ø200 µm
FG050LGA Ø50 µm 400 to 2400 nm
(Low OH)
FG105LCA Ø105 µm
FG200LEA Ø200 µm
High Power Double TECS /
Silica Cladding
Multimode Fiber
View These Fibers
FG200UCC Ø200 µm 250 to 1200 nm
(High OH)
FG365UEC Ø365 µm
FG550UEC Ø550 µm
FG910UEC Ø910 µm
FG200LCC Ø200 µm 400 to 2200 nm
(Low OH)
FG365LEC Ø365 µm
FG550LEC Ø550 µm
FG910LEC Ø910 µm
Solarization-Resistant Multimode
Fiber for UV Use
View These Fibers
FG105ACA Ø105 µm 180 to 1200 nm
Acrylate Coating
for Ease of Handling
FG200AEA Ø200 µm
FG300AEA Ø300 µm
FG400AEA Ø400 µm
FG600AEA Ø600 µm
UM22-100 Ø100 µm 180 to 1150 nm
Polyimide Coating
for Use up to 300 °C
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 to 1200 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 to 2200 nm
(Low OH)
FT300EMT Ø300 µm
FT400EMT Ø400 µm
FT600EMT Ø600 µm
FT800EMT Ø800 µm
FT1000EMT Ø1000 µm
FT1500EMT Ø1500 µm
0.50 High NA Multimode Fiber
View These Fibers
FP200URT Ø200 µm 300 to 1200 nm
(High OH)
FP400URT Ø400 µm
FP600URT Ø600 µm
FP1000URT Ø1000 µm
FP200ERT Ø200 µm 400 to 2200 nm
(Low OH)
FP400ERT Ø400 µm
FP600ERT Ø600 µm
FP1000ERT Ø1000 µm
0.20 Mid-IR Fiber with Zirconium Fluoride (ZrF4) Core
View These Fibers
Various Sizes Between
Ø50 µm and Ø600 µm
285 to 4.5 µm
0.20 or 0.26 Mid-IR Fiber with Indium Fluoride (InF3) Core
View These Fibers
Ø50 µm or Ø100 µm 310 to 5.5 µm
Graded Index 0.20 Graded-Index Fiber
for Low Bend Loss
View These Fibers
GIF50C Ø50 µm 750 to 1450 nm
0.275 GIF625 Ø62.5 µm 800 to 1350 nm

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Posted Comments:
Posted Date:2011-07-13 16:58:00.0
Response from Javier at Thorlabs to schaefer: Thank you very much for contacting us. Although the attenuation of the HPSC series fibers increases relatively quickly as the operating wavelength goes further into the UV, it would be recommendable to use a filter to block any transmission below 300 nm through the fiber. I will contact you directly for further support.
Posted Date:2011-07-13 11:31:42.0
I already have one of these fibers. I am using it with a Halogen lamp which does have some (smallish) UV-emissions. Am I right to assume that most of the UV (below 300nm) will be filtered out by the fiber anyway or do I need an additional filter for that?
Posted Date:2008-11-13 16:59:12.0
A response from Tyler at Thorlabs to stepenofwheele: Thank you for posting your question. We have updated the presentation to include the damage threshold for both CW and pulsed laser sources. I hope this information helps and if you have any further suggestions on how to improve our product presentation, please ask.
Posted Date:2008-11-10 10:13:50.0
What do you mean by high-power? What is the breakdown threshold or power rated at for cw and pulsed?
Posted Date:2008-08-25 18:31:41.0
Reply to srubin from Inge@thorlabs: The material of the core is specified as Undoped Pure Silica, and the cladding is Fluorine doped. We updated our web presentation to include this information. Thank you for your feedback.
Posted Date:2008-08-19 20:03:11.0
What material are the core and clad made of? Is this a Quartz fiber?
Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
FG010LDA Support Documentation
FG010LDANEW!Multimode Fiber, 0.10 NA, Ø10 µm Core
Per Meter
Volume Pricing
FG025LJA Support Documentation
FG025LJANEW!Multimode Fiber, 0.10 NA, Ø25 µm Core
Per Meter
Volume Pricing
FG105LVA Support Documentation
FG105LVANEW!Multimode Fiber, 0.10 NA, Ø105 µm Core
Per Meter
Volume Pricing
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