Square-Core Multimode Fiber

  • 0.39 NA, Multimode Fiber with 150 µm x 150 µm Square Core
  • 400 nm - 2200 nm Wavelength Range
  • Ideal for Imaging and Astronomical Spectroscopy Applications


Tefzel Buffer

Hard Polymer Cladding

Pure Silica Core

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Collimated output from a patch cable using FP150QMT (left) compared to M29L02 Ø200 µm core patch cable (right). A M625F2 Fiber-Coupled LED was used as the source.
Stock Patch Cables Available with this Fibera
Item # Fiber Used Description Length
M97L02 FP150QMT SMA Connectors 2 m
M101L02 FC/PC Connectors 2 m
M102L05 SMA Connectors 5 m
M103L05 FC/PC Connectors 5 m
  • The full web presentation for these patch cables can be found by clicking here.


  • Multimode, 0.39 NA, Step-Index Square-Core Fiber
  • 150 µm x 150 µm Pure Silica Core
  • Ø225 µm Hard Polymer Cladding
  • 400 - 2200 nm Wavelength Range, Low-OH Content
  • Ideal for Imaging and Spectroscopy Applications
  • Remove Coating Using T12S21 Stripping Tool

The FP150QMT 0.39 NA multimode optical fiber features a 150 µm x 150 µm square silica core, unlike most step-index fibers that have a circular core. The core is surrounded by a Ø225 µm circular polymer cladding and the fiber is coated in a Tefzel buffer. It has a specified wavelength range of 400 - 2200 nm; see the graph below for an attenuation curve and the Specs tab for complete specifications.

The square shape of the core promotes mode mixing in the fiber that results in an uniform spatial distribution, square beam shape, and a top-hat profile at the output. To maintain the square-beam shape at far-field distances, a collimator should be used to image the fiber core (see photo to the right). A well-defined square beam shape is useful for imaging applications, such as imaging onto rectangular CCD detectors. The shape of the beam profile may also improve coupling from laser diodes or LEDs, which have rectangular emitting surfaces. For astronomical spectroscopy applications, square-core fibers also reduce focal ratio degradation (FRD) and improve scrambling gain (see the Applications tab for more information).

This fiber is available from stock as fiber patch cables in several configurations (see table to the right for details). Different lengths or connector configurations are possible; please contact Tech Support with inquiries.

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Top-hat beam profile measured using lenses to expand the beam.
Optical Fiber Manufacturing
Item # Wavelength
Core Size Cladding
Core / Cladding Coating Stripping Tool Proof Test
FP150QMT 400 - 2200 nm Low OH 150 ± 10 µm x 150 ± 10 µm 225 ± 5 µm 500 ± 30 µm Pure Silica /
Hard Polymer
Tefzel T12S21 ≥50 kpsi

Item # NA Core Index @ 589.3 nm Cladding Index @ 589.3 nm Attenuation (Click for Plot) Core Offset Bend Radius Operating
(Tefzel Coating)
Short Term Long Term
FP150QMT 0.39 1.458965 1.4016 20 dB/km @ 803 nm (Max) 6 µm (Max) 20 mm 40 mm -40 to 85 °C
Beam profile measurements using lenses to expand the beam with 530 nm LED light launched from a SM fiber into the test fiber.

Square-core fibers are used in a number of applications, including: astronomy, laser machining, dermatological equipment, and biomedical imaging. The examples below illustrate specific advantages that these fibers have over traditional circular-core fibers.

Flat Beam Profile

The defining feature of a square-core fiber is that it produces an optical beam with a uniform intensity over the core area, rather than the Gaussian beam profile typically observed with circular-core fibers. This occurs because the shape of the core promotes mode mixing as light propagates through the fiber, which results in an even distribution of spatial modes in the output beam.

Square-core fibers are ideal for applications such as laser machining, where sharp corners and edge cuts can be created without beam shaping optics or masking, or imaging, where a square beam profile better fits the shape of a rectangular CCD array. Please note that the shape of the beam is not preserved once the beam leaves the fiber; a collimator is needed in order to image the fiber core and preserve the beam shape in free space. 

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Measurement of FRD at 530 nm
FP150QMT: 150 µm x 150 µm Square Core
FT200EMT: Ø200 µm Circular Core
FT300EMT: Ø300 µm Circular Core

Astronomy Applications

For customers interested in stellar and astronomical spectroscopy, square-core fibers offer several additional benefits over circular-core fibers.

Reduced Focal Ratio Degradation (FRD)
Multimode fiber optic cables are employed in astronomy applications, particularly for building multi-object spectroscopy (MOS) systems that simultaneously observe the spectrum of multiple objects within the field-of-view of the telescope. The small field of view of an optical fiber will only capture light from the target object with minimal noise from surrounding objects. Due to microbending and termination stress on the fiber when attaching a connector, the focal ratio (also known as the f/#) at the fiber output will be lower than the input and the beam angle will be larger at the output. This phenomenon is known as focal ratio degradation (FRD) and the wider output beam angle results in decreased spectral resolution and reduced light collection at the detector. FRD is calculated as the ratio of the input f/# to the output f/#. 

Thorlabs' square-core fibers are designed to minimize termination stress and reduce focal ratio degradation. To demonstrate this, we tested three fibers terminated with low-stress epoxy and cured for 4 hours at 40 °C. As seen in the graph to the right, a patch cable with FP150QMT square-core fiber has reduced FRD (i.e., a smaller difference between input and output focal ratios) compared to FT200EMT (Ø200 µm core) and FT300EMT (Ø300 µm core) fibers. 

Scrambling Gain for Different Fiber Typesa
Fiber Length Fiber Type Core Scrambling Gain
2 m FT200EMT Circular 42
FP150QMT Square 121
5 m FT200EMT Circular 235
FP150QMT Square 465
  • Measurements performed at 530 nm using patch cables manufactured with the same epoxy and termination method.

Scrambling Gain
For multimode fibers used in stellar spectroscopy, tiny movements of the star being observed can lead to shifts in the spectra measured, which is a source of measurement noise. Increased mode scrambling in a fiber can decrease the sensitivity of the fiber to these fluctuating movements. "Scrambling gain" quantifies the sensitivity of an optical fiber to these perturbations and is defined as the ratio of displacement for a point source at the fiber input to the beam displacement measured at the fiber output. A higher value of scrambling gain indicates a reduced effect of point source fluctuations on the fiber output.

There are several methods to improve the scrambling gain in an optical fiber. In general, using a longer fiber increases the scrambling gain; however, the overall transmission of the fiber is lower as well. Square-core fibers can improve the scrambling gain without using longer fibers. As seen in the table to the left, the scrambling gain for Thorlabs' patch cables made with square-core fibers is higher compared to similar circular-core fiber patch cables.

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Total Internal Reflection in an Optical Fiber

Guiding Light in an Optical Fiber

Optical fibers are part of a broader class of optical components known as waveguides that utilize total internal reflection (TIR) in order to confine and guide light within a solid or liquid structure. Optical fibers, in particular, are used in numerous applications; common examples include telecommunications, spectroscopy, illumination, and sensors.

One of the more common glass (silica) optical fibers uses a structure known as a step-index fiber, which is shown in the image to the right. Step-index fibers have an inner core made from a material with a refractive index that is higher than the surrounding cladding layer. Within the fiber, a critical angle of incidence exists such that light will reflect off the core/cladding interface rather than refract into the surrounding medium. To fulfill the conditions for TIR in the fiber, the angle of incidence of light launched into the fiber must be less than a certain angle, which is defined as the acceptance angle, θacc. Snell's law can be used to calculate this angle:

where ncore is the refractive index of the fiber core, nclad is the refractive index of the fiber cladding, n is the refractive index of the outside medium, θcrit is the critical angle, and θacc is the acceptance half-angle of the fiber. The numerical aperture (NA) is a dimensionless quantity used by fiber manufacturers to specify the acceptance angle of an optical fiber and is defined as:

In step-index fibers with a large core (multimode), the NA can be calculated directly using this equation. The NA can also be determined experimentally by tracing the far-field beam profile and measuring the angle between the center of the beam and the point at which the beam intensity is 5% of the maximum; however, calculating the NA directly provides the most accurate value.


Number of Modes in an Optical Fiber

Each potential path that light propagates through in an optical fiber is known as a guided mode of the fiber. Depending on the physical dimensions of the core/cladding regions, refractive index, and wavelength, anything from one to thousands of modes can be supported within a single optical fiber. The two most commonly manufactured variants are single mode fiber (which supports a single guided mode) and multimode fiber (which supports a large number of guided modes). In a multimode fiber, lower-order modes tend to confine light spatially in the core of the fiber; higher-order modes, on the other hand, tend to confine light spatially near the core/cladding interface.

Using a few simple calculations, it is possible to estimate the number of modes (single mode or multimode) supported by an optical fiber. The normalized optical frequency, also known as the V-number, is a dimensionless quantity that is proportional to the free space optical frequency but is normalized to guiding properties of an optical fiber. The V-number is defined as:

where V is the normalized frequency (V-number), a is the fiber core radius, and λ is the free space wavelength. Multimode fibers have very large V-numbers; for example, a Ø50 µm core, 0.39 NA multimode fiber at a wavelength of 1.5 µm has a V-number of 40.8.

For multimode fiber, which has a large V-number, the number of modes supported is approximated using the following relationship.

In the example above of the Ø50 µm core, 0.39 NA multimode fiber, it supports approximately 832 different guided modes that can all travel simultaneously through the fiber.

Single mode fibers are defined with a V-number cut-off of V < 2.405, which represents the point at which light is coupled only into the fiber's fundamental mode. To meet this condition, a single mode fiber has a much smaller core size and NA compared to a multimode fiber at the same wavelength. One example of this, SMF-28 Ultra single mode fiber, has a nominal NA of 0.14 and an Ø8.2 µm core at 1550 nm, which results in a V-number of 2.404.


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Attenuation Due to Macrobend Loss

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Attenuation Due to Microbend Loss

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Beam profile measurement of FT200EMT multimode fiber and a former generation M565F1 LED (replaced by the M565F3) showing light guided in the cladding rather than the core of the fiber.

Sources of Attenuation

Loss within an optical fiber, also referred to as attenuation, is characterized and quantified in order to predict the total transmitted power lost within a fiber optic setup. The sources of these losses are typically wavelength dependent and range from the material used in the fiber itself to bending of the fiber. Common sources of attenuation are detailed below:

Because light in a standard optical fiber is guided via a solid material, there are losses due to absorption as light propagates through the fiber. Standard fibers are manufactured using fused silica and are optimized for transmission from 1300 nm to 1550 nm. At longer wavelengths (>2000 nm), multi-phonon interactions in fused silica cause significant absorption. Fluoride glasses such as ZrF4 and InF3 are used in manufacturing Mid-IR optical fibers primarily because they exhibit lower loss at these wavelengths. ZrF4 and InF3 fibers have a multi-phonon edge of ~3.6 µm and ~4.6 µm, respectively.

Contaminants in the fiber also contribute to the absorption loss. One example of an undesired impurity is water molecules that are trapped in the glass of the optical fiber, which will absorb light around 1300 nm and 2.94 µm. Since telecom signals and some lasers operate in that same region, any water molecules present in the fiber will attenuate the signal significantly.

The concentration of ions in the fiber glass is often controlled by manufacturers to tune the transmission/attenuation properties of a fiber. For example, hydroxyl ions (OH-) are naturally present in silica and absorb light in the NIR-IR spectrum. Therefore, fibers with low-OH content are preferred for transmission at telecom wavelengths. On the other hand, fibers with high-OH content typically exhibit increased transmission at UV wavelengths and thus may be preferred by users interested in applications such as fluorescence or UV-VIS spectroscopy. 

For the majority of fiber optics applications, light scattering is a source of loss that occurs when light encounters a change in the refractive index of the medium. These changes can be extrinsic, caused by impurities, particulates, or bubbles; or intrinsic, caused by fluctuations in the glass density, composition, or phase state. Scattering is inversely related to the wavelength of light, so scattering loss becomes significant at shorter wavelengths such as the UV or blue regions of the spectrum. Using proper fiber cleaning, handling, and storage procedures may minimize the presence of impurities on tips of fibers that cause large scattering losses.

Bending Loss
Losses that occur due to changes in the external and internal geometry of an optical fiber are known as bending loss. These are usually separated into two categories: macrobending loss and microbending loss.

Macrobend loss is typically associated with the physical bending of an optical fiber; for example, rolling it in a tight coil. As shown in the image to the right, guided light is spatially distributed within the core and cladding regions of the fiber. When a fiber is bent at a radius, light near the outer radius of the bend cannot maintain the same spatial mode profile without exceeding the speed of light. Instead, the energy is lost to the surroundings as radiation. For a large bend radius, the losses associated with bending are small; however, at bend radii smaller than the recommended bend radius of a fiber, bend losses become very significant. For short periods of time, optical fibers can be operated at a small bend radius; however, for long-term storage, the bend radius should be larger than the recommended value. Use proper storage conditions (temperature and bend radius) to reduce the likelihood of permanently damaging the fiber; the FSR1 Fiber Storage Reel is designed to minimize high bend loss.

Microbend loss arises from changes in the internal geometry of the fiber, particularly the core and cladding layers. These random variations (i.e., bumps) in the fiber structure disturb the conditions needed for total internal reflection, causing propagating light to couple into a non-propagating mode that leaks from the fiber (see the image to the right for details). Unlike macrobend loss, which is controlled by the bend radius, microbend loss occurs due to permanent defects in the fiber that are created during fiber manufacturing.

Cladding Modes
While most light in a multimode fiber is guided via TIR within the core of the fiber, higher-order modes that guide light within both the core and cladding layer, because of TIR at the cladding and coating/buffer interface, can also exist. This results in what is known as a cladding mode. An example of this can be seen in the beam profile measurement to the right, which shows cladding modes with a higher intensity in the cladding than in the core of the fiber. These modes can be non-propagating (i.e., they do not fulfill the conditions for TIR) or they can propagate over a significant length of fiber. Because cladding modes are typically higher-order, they are a source of loss in the presence of fiber bending and microbending defects. Cladding modes are also lost when connecting two fibers via connectors as they cannot be easily coupled between optical fibers.

Cladding modes may be undesired for some applications (e.g., launching into free space) because of their effect on the beam spatial profile. Over long fiber lengths, these modes will naturally attenuate. For short fiber lengths (<10 m), one method for removing cladding modes from a fiber is to use a mandrel wrap at a radius that removes cladding modes while keeping the desired propagating modes.


Launch Conditions

Underfilled Launch Condition
For a large multimode fiber which accepts light over a wide NA, the condition of the light (e.g., source type, beam diameter, NA) coupled into the fiber can have a significant effect on performance. An underfilled launch condition occurs when the beam diameter and NA of light at the coupling interface are smaller than the core diameter and NA of the fiber. A common example of this is launching a laser source into a large multimode fiber. As seen in the diagram and beam profile measurement below, underfilled launches tend to concentrate light spatially in the center of the fiber, filling lower-order modes preferentially over higher-order modes. As a result, they are less sensitive to macrobend losses and do not have cladding modes. The measured insertion loss for an underfilled launch tends to be lower than typical, with a higher power density in the core of the fiber. 

Diagram illustrating an underfilled launch condition (left) and a beam profile measurement using a FT200EMT multimode fiber (right).

Overfilled Launch Condition
Overfilled launches are defined by situations where the beam diameter and NA at the coupling interface are larger than the core diameter and NA of the fiber. One method to achieve this is by launching light from an LED source into a small multimode fiber. An overfilled launch completely exposes the fiber core and some of the cladding to light, enabling the filling of lower- and higher-order modes equally (as seen in the images below) and increasing the likelihood of coupling into cladding modes of the fiber. This increased percentage of higher-order modes means that overfilled fibers are more sensitive to bending loss. The measured insertion loss for an overfilled launch tends to be higher than typical, but results in an overall higher output power compared to an underfilled fiber launch. 

Diagram illustrating an overfilled launch condition (left) and a beam profile measurement using a FT200EMT multimode fiber (right).

There are advantages and disadvantages to underfilled or overfilled launch conditions, depending on the needs of the intended application. For measuring the baseline performance of a multimode fiber, Thorlabs recommends using a launch condition where the beam diameter is 70-80% of the fiber core diameter. Over short distances, an overfilled fiber has more output power; however, over long distances (>10 - 20 m) the higher-order modes that more susceptible to attenuation will disappear. 

Laser-Induced Damage in Silica Optical Fibers

The following tutorial details damage mechanisms relevant to unterminated (bare) fiber, terminated optical fiber, and other fiber components from laser light sources. These mechanisms include damage that occurs at the air / glass interface (when free-space coupling or when using connectors) and in the optical fiber itself. A fiber component, such as a bare fiber, patch cable, or fused coupler, may have multiple potential avenues for damage (e.g., connectors, fiber end faces, and the device itself). The maximum power that a fiber can handle will always be limited by the lowest limit of any of these damage mechanisms.

While the damage threshold can be estimated using scaling relations and general rules, absolute damage thresholds in optical fibers are very application dependent and user specific. Users can use this guide to estimate a safe power level that minimizes the risk of damage. Following all appropriate preparation and handling guidelines, users should be able to operate a fiber component up to the specified maximum power level; if no maximum is specified for a component, users should abide by the "practical safe level" described below for safe operation of the component. Factors that can reduce power handling and cause damage to a fiber component include, but are not limited to, misalignment during fiber coupling, contamination of the fiber end face, or imperfections in the fiber itself. For further discussion about an optical fiber’s power handling abilities for a specific application, please contact Thorlabs’ Tech Support.

Power Handling Limitations Imposed by Optical Fiber
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Undamaged Fiber End
Power Handling Limitations Imposed by Optical Fiber
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Damaged Fiber End

Damage at the Air / Glass Interface

There are several potential damage mechanisms that can occur at the air / glass interface. Light is incident on this interface when free-space coupling or when two fibers are mated using optical connectors. High-intensity light can damage the end face leading to reduced power handling and permanent damage to the fiber. For fibers terminated with optical connectors where the connectors are fixed to the fiber ends using epoxy, the heat generated by high-intensity light can burn the epoxy and leave residues on the fiber facet directly in the beam path.

Estimated Optical Power Densities on Air / Glass Interfacea
Type Theoretical Damage Thresholdb Practical Safe Levelc
(Average Power)
~1 MW/cm2 ~250 kW/cm2
10 ns Pulsed
(Peak Power)
~5 GW/cm2 ~1 GW/cm2
  • All values are specified for unterminated (bare), undoped silica fiber and apply for free space coupling into a clean fiber end face.
  • This is an estimated maximum power density that can be incident on a fiber end face without risking damage. Verification of the performance and reliability of fiber components in the system before operating at high power must be done by the user, as it is highly system dependent.
  • This is the estimated safe optical power density that can be incident on a fiber end face without damaging the fiber under most operating conditions.

Damage Mechanisms on the Bare Fiber End Face

Damage mechanisms on a fiber end face can be modeled similarly to bulk optics, and industry-standard damage thresholds for UV Fused Silica substrates can be applied to silica-based fiber. However, unlike bulk optics, the relevant surface areas and beam diameters involved at the air / glass interface of an optical fiber are very small, particularly for coupling into single mode (SM) fiber. therefore, for a given power density, the power incident on the fiber needs to be lower for a smaller beam diameter.

The table to the right lists two thresholds for optical power densities: a theoretical damage threshold and a "practical safe level". In general, the theoretical damage threshold represents the estimated maximum power density that can be incident on the fiber end face without risking damage with very good fiber end face and coupling conditions. The "practical safe level" power density represents minimal risk of fiber damage. Operating a fiber or component beyond the practical safe level is possible, but users must follow the appropriate handling instructions and verify performance at low powers prior to use.

Calculating the Effective Area for Single Mode Fibers
The effective area for single mode (SM) fiber is defined by the mode field diameter (MFD), which is the cross-sectional area through which light propagates in the fiber; this area includes the fiber core and also a portion of the cladding. To achieve good efficiency when coupling into a single mode fiber, the diameter of the input beam must match the MFD of the fiber.

As an example, SM400 single mode fiber has a mode field diameter (MFD) of ~Ø3 µm operating at 400 nm, while the MFD for SMF-28 Ultra single mode fiber operating at 1550 nm is Ø10.5 µm. The effective area for these fibers can be calculated as follows:

SM400 Fiber: Area = Pi x (MFD/2)2 = Pi x (1.5 µm)2 = 7.07 µm= 7.07 x 10-8 cm2

 SMF-28 Ultra Fiber: Area = Pi x (MFD/2)2 = Pi x (5.25 µm)2 = 86.6 µm= 8.66 x 10-7 cm2

To estimate the power level that a fiber facet can handle, the power density is multiplied by the effective area. Please note that this calculation assumes a uniform intensity profile, but most laser beams exhibit a Gaussian-like shape within single mode fiber, resulting in a higher power density at the center of the beam compared to the edges. Therefore, these calculations will slightly overestimate the power corresponding to the damage threshold or the practical safe level. Using the estimated power densities assuming a CW light source, we can determine the corresponding power levels as:

SM400 Fiber: 7.07 x 10-8 cm2 x 1 MW/cm2 = 7.1 x 10-8 MW = 71 mW (Theoretical Damage Threshold)
     7.07 x 10-8 cm2 x 250 kW/cm2 = 1.8 x 10-5 kW = 18 mW (Practical Safe Level)

SMF-28 Ultra Fiber: 8.66 x 10-7 cm2 x 1 MW/cm2 = 8.7 x 10-7 MW = 870 mW (Theoretical Damage Threshold)
           8.66 x 10-7 cm2 x 250 kW/cm2 = 2.1 x 10-4 kW = 210 mW (Practical Safe Level)

Effective Area of Multimode Fibers
The effective area of a multimode (MM) fiber is defined by the core diameter, which is typically far larger than the MFD of an SM fiber. For optimal coupling, Thorlabs recommends focusing a beam to a spot roughly 70 - 80% of the core diameter. The larger effective area of MM fibers lowers the power density on the fiber end face, allowing higher optical powers (typically on the order of kilowatts) to be coupled into multimode fiber without damage.

Damage Mechanisms Related to Ferrule / Connector Termination

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Plot showing approximate input power that can be incident on a single mode silica optical fiber with a termination. Each line shows the estimated power level due to a specific damage mechanism. The maximum power handling is limited by the lowest power level from all relevant damage mechanisms (indicated by a solid line).

Fibers terminated with optical connectors have additional power handling considerations. Fiber is typically terminated using epoxy to bond the fiber to a ceramic or steel ferrule. When light is coupled into the fiber through a connector, light that does not enter the core and propagate down the fiber is scattered into the outer layers of the fiber, into the ferrule, and the epoxy used to hold the fiber in the ferrule. If the light is intense enough, it can burn the epoxy, causing it to vaporize and deposit a residue on the face of the connector. This results in localized absorption sites on the fiber end face that reduce coupling efficiency and increase scattering, causing further damage.

For several reasons, epoxy-related damage is dependent on the wavelength. In general, light scatters more strongly at short wavelengths than at longer wavelengths. Misalignment when coupling is also more likely due to the small MFD of short-wavelength SM fiber that also produces more scattered light.

To minimize the risk of burning the epoxy, fiber connectors can be constructed to have an epoxy-free air gap between the optical fiber and ferrule near the fiber end face. Our high-power multimode fiber patch cables use connectors with this design feature.

Determining Power Handling with Multiple Damage Mechanisms

When fiber cables or components have multiple avenues for damage (e.g., fiber patch cables), the maximum power handling is always limited by the lowest damage threshold that is relevant to the fiber component. In general, this represents the highest input power that can be incident on the patch cable end face and not the coupled output power.

As an illustrative example, the graph to the right shows an estimate of the power handling limitations of a single mode fiber patch cable due to damage to the fiber end face and damage via an optical connector. The total input power handling of a terminated fiber at a given wavelength is limited by the lower of the two limitations at any given wavelength (indicated by the solid lines). A single mode fiber operating at around 488 nm is primarily limited by damage to the fiber end face (blue solid line), but fibers operating at 1550 nm are limited by damage to the optical connector (red solid line).

In the case of a multimode fiber, the effective mode area is defined by the core diameter, which is larger than the effective mode area for SM fiber. This results in a lower power density on the fiber end face and allows higher optical powers (on the order of kilowatts) to be coupled into the fiber without damage (not shown in graph). However, the damage limit of the ferrule / connector termination remains unchanged and as a result, the maximum power handling for a multimode fiber is limited by the ferrule and connector termination. 

Please note that these are rough 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, these applications typically require expert users and testing at lower powers first to minimize risk of damage. Even still, optical fiber components should be considered a consumable lab supply if used at high power levels.

Intrinsic Damage Threshold

In addition to damage mechanisms at the air / glass interface, optical fibers also display power handling limitations due to damage mechanisms within the optical fiber itself. These limitations will affect all fiber components as they are intrinsic to the 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 a localized area. The light escaping the fiber typically has a high power density, which burns the fiber coating 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 the risk of damage. Thorlabs manufactures and sells 0.22 NA double-clad multimode fiber, which boasts very high, megawatt range power handling.

A second damage mechanism, called photodarkening or solarization, can occur in fibers used with ultraviolet or short-wavelength visible light, particularly those with germanium-doped cores. Fibers used at these wavelengths will experience increased attenuation over time. The mechanism that causes photodarkening is largely unknown, but several fiber designs have been developed to mitigate it. For example, fibers with a very low hydroxyl ion (OH) content have been found to resist photodarkening and using other dopants, such as fluorine, can also reduce photodarkening.

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

Preparation and Handling of Optical Fibers

General Cleaning and Operation Guidelines
These general cleaning and operation guidelines are recommended for all fiber optic products. Users should still follow specific guidelines for an individual product as outlined in the support documentation or manual. Damage threshold calculations only apply when all appropriate cleaning and handling procedures are followed.

  1. All light sources should be turned off prior to installing or integrating optical fibers (terminated or bare). This ensures that focused beams of light are not incident on fragile parts of the connector or fiber, which can possibly cause damage.

  2. The power-handling capability of an optical fiber is directly linked to the quality of the fiber/connector end face. Always inspect the fiber end prior to connecting the fiber to an optical system. The fiber end face should be clean and clear of dirt and other contaminants that can cause scattering of coupled light. Bare fiber should be cleaved prior to use and users should inspect the fiber end to ensure a good quality cleave is achieved.

  3. If an optical fiber is to be spliced into the optical system, users should first verify that the splice is of good quality at a low optical power prior to high-power use. Poor splice quality may increase light scattering at the splice interface, which can be a source of fiber damage.

  4. Users should use low power when aligning the system and optimizing coupling; this minimizes exposure of other parts of the fiber (other than the core) to light. Damage from scattered light can occur if a high power beam is focused on the cladding, coating, or connector.

Tips for Using Fiber at Higher Optical Power
Optical fibers and fiber components should generally be operated within safe power level limits, but under ideal conditions (very good optical alignment and very clean optical end faces), the power handling of a fiber component may be increased. Users must verify the performance and stability of a fiber component within their system prior to increasing input or output power and follow all necessary safety and operation instructions. The tips below are useful suggestions when considering increasing optical power in an optical fiber or component.

  1. Splicing a fiber component into a system using a fiber splicer can increase power handling as it minimizes possibility of air/fiber interface damage. Users should follow all appropriate guidelines to prepare and make a high-quality fiber splice. Poor splices can lead to scattering or regions of highly localized heat at the splice interface that can damage the fiber.

  2. After connecting the fiber or component, the system should be tested and aligned using a light source at low power. The system power can be ramped up slowly to the desired output power while periodically verifying all components are properly aligned and that coupling efficiency is not changing with respect to optical launch power.

  3. Bend losses that result from sharply bending a fiber can cause light to leak from the fiber in the stressed area. When operating at high power, the localized heating that can occur when a large amount of light escapes a small localized area (the stressed region) can damage the fiber. Avoid disturbing or accidently bending fibers during operation to minimize bend losses.

  4. Users should always choose the appropriate optical fiber for a given application. For example, large-mode-area fibers are a good alternative to standard single mode fibers in high-power applications as they provide good beam quality with a larger MFD, decreasing the power density on the air/fiber interface.

  5. Step-index silica single mode fibers are normally not used for ultraviolet light or high-peak-power pulsed applications due to the high spatial power densities associated with these applications.

Posted Comments:
Thomas Freir  (posted 2021-06-23 06:31:42.037)
What specific material is the 'hard polymer cladding' made of? Would it also be possible to find out the refractive indices of both the core and the cladding?
YLohia  (posted 2021-06-28 02:28:45.0)
Hello, thank you for contacting Thorlabs. The coating material that is used as the cladding for the FP150QMT fiber is proprietary so, unfortunately, we cannot share that information. The refractive indices of both the core and cladding are specified in the "Specs" tab.
user  (posted 2019-05-15 11:58:18.28)
I'd like to know the modal dispersion in the fiber. Do you have any examples of output waveform of this fiber for optical pulse inputs? Particularly, around 1500nm. Thx!
YLohia  (posted 2019-05-20 10:22:15.0)
Hello, thank you for contacting Thorlabs. Unfortunately, we do not have this data. I have reached out to you directly to discuss your application further.
justin13120  (posted 2017-08-06 00:23:19.697)
I'd like to ask if the light position signal emission from the FP150QMT could captured by the PSD(position sensitive detector). Due to the modal dispersion in multimode fiber, we knew that the light signal refracted to many directions, and caused phase dispersion. However, your displayed output light profile seems to not having this problem, so that's why I ask the question. If it's free, please contact my e-mail in any time, tks!
tfrisch  (posted 2017-08-16 03:01:48.0)
Hello, thank you for contacting Thorlabs. I will reach out to you directly about detecting the speckle pattern. A camera may be helpful for this as a PSD will only indicate where the center is.

Thorlabs offers multimode bare optical fiber with silica, zirconium fluoride (ZrF4), or indium fluoride (InF3) cores. The table below details all of Thorlabs' multimode bare optical fiber offerings. Attenuation plots can be found by clicking the graph icons in the column to the right.

Index Profile NA Fiber Type Item # Core Size Wavelength Range Attenuation
(Click for Graph)
Step Index 0.100 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 Standard Glass-Clad Silica
View These Fibers
FG050UGA Ø50 µm 250 to 1200 nm
(High OH)
FG105UCA Ø105 µm
FG200UEA Ø200 µm
FG400UEA Ø400 µm
FG050LGA Ø50 µm 400 to 2400 nm
(Low OH)
FG105LCA Ø105 µm
FG200LEA Ø200 µm
FG400LEA Ø400 µm
Polyimide Coating
View These Fibers
FG200UEP Ø200 µm 250 to 1200 nm
(High OH)
FG400UEP Ø400 µm
FG200LEP Ø200 µm 400 to 2400 nm
(Low OH)
FG400LEP Ø400 µm
Solarization Resistant 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 850 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
High Power Double TECS /
Fluorine-Doped Silica Cladding,
View These Fibers
FG200UCC Ø200 µm 250 to 1200 nm
(High OH)
FG273UEC Ø273 µm
FG365UEC Ø365 µm
FG550UEC Ø550 µm
FG910UEC Ø910 µm
FG200LCC Ø200 µm 400 to 2200 nm
(Low OH)
FG273LEC Ø273 µm
FG365LEC Ø365 µm
FG550LEC Ø550 µm
FG910LEC Ø910 µm
0.39 High Power TECS Cladding
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
Square Core
View These Fibers
FP150QMT 150 µm x 150 µm 400 to 2200 nm
(Low OH)
0.50 Hard Polymer Cladding
View These Fibers
FP200URT Ø200 µm 300 to 1200 nm
(High OH)
FP400URT Ø400 µm
FP600URT Ø600 µm
Ø1000 µm
FP1500URT Ø1500 µm
FP200ERT Ø200 µm 400 to 2200 nm
(Low OH)
FP400ERT Ø400 µm
FP600ERT Ø600 µm
FP1000ERT Ø1000 µm
FP1500ERT Ø1500 µm
0.20 Zirconium Fluoride (ZrF4) Core for Mid-IR
View These Fibers
Various Sizes Between
Ø100 µm and Ø600 µm
285 nm to 4.5 µm
0.26 Indium Fluoride (InF3) Core for Mid-IR
View These Fibers
Ø100 µm 310 nm to 5.5 µm
Graded Index 0.20 Graded Index for Low Bend Loss
View These Fibers
GIF50C Ø50 µm 800 to 1600 nm
0.275 GIF625 Ø62.5 µm 800 to 1600 nm

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available
FP150QMT Support Documentation
FP150QMT0.39 NA Square-Core Multimode Optical Fiber, 150 µm x 150 µm Core, Low OH
Per Meter
Volume Pricing

* Orders for in-stock bare fiber placed by 3:00 PM ET are eligible for same-day shipment.