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Polarization-Maintaining Single Mode Optical Fiber
Bow-Tie PM Fiber
PANDA PM Fiber
Thorlabs offers both PANDA and Bow-Tie Single Mode Polarization-Maintaining (PM) fiber. These two fibers are named based on the stress rods used. Stress rods run parallel to the fiber's core and apply stress that creates birefringence in the fiber's core, allowing polarization-maintaining operation. PANDA stress rods are cylindrical, while bow-tie uses trapezoidal prism stress rods, as shown in the images above. For the average user, these two fiber types are interchangeable. PANDA fibers have historically been used in telecom applications, as it is easier to maintain uniformity in their cylindrical stress rods over very long lengths when manufacturing.
We also offer specialized PM fibers. Our photosensitive fiber can be exposed to UV light to create a Fiber Bragg Grating, our dispersion-compensating fiber corrects for chromatic dispersion, and our bend- and temperature-insensitive PM fiber is ideal for use in fiber optic gyroscopes (FOG).
PANDA Fibers, Pure Silica Core, 350 - 680 nm
PANDA Fibers, 460 - 2200 nm
Photosensitive PANDA Fiber, 980 nm
PANDA, Dispersion-Compensating Fiber, 1510 - 1620 nm
Bow-Tie Fibers, 980 - 1550 nm
Bend- and Temperature-Insensitive Bow-Tie Fiber, 800 - 1000 nm
Definition of the Mode Field Diameter
The mode field diameter (MFD) is one measure of the beam width of light propagating in a single mode fiber. It is a function of wavelength, core radius, and the refractive indices of the core and cladding. While much of the light in an optical fiber is trapped within the fiber core, a small fraction propagates in the cladding. For a Gaussian power distribution, the MFD is the diameter where the optical power is reduced to 1/e2 from its peak level.
Measurement of MFD
The MFD is then determined using Petermann's second definition, which is a mathematical model that does not assume a specific shape of power distribution. The MFD in the near field can be determined from this far-field measurement using the Hankel Transform.
The image above shows the intensity profile of the beam propagated through the fiber overlaid on the fiber itself. The image to the right shows the standard intensity profile of the beam propagated through the fiber with the MFD and core diameter called out.
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.
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Undamaged Fiber End
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Damaged Fiber End
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.
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 and Multimode Fibers
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 µm2 = 7.07 x 10-8 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)
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)
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.
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.
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.
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.
General Cleaning and Operation Guidelines
Tips for Using Fiber at Higher Optical Power
These pure silica core polarization-maintaining fibers are designed for wavelengths from 350 to 680 nm. Their pure silica core provides protection from photodarkening, which makes them ideal for use at short wavelengths. These fibers use PANDA-type stress rods for polarization-maintaining operation.
These polarization-maintaining fibers are designed for single-mode transmission in the visible, NIR, and telecom wavelength ranges. They have PANDA-type stress rods for polarization-maintaining operation.
PS-PM980 photosensitive 970 - 1550 nm polarization maintaining fiber is designed to perform all functions of a 980 nm PM fiber but with enhanced photosensitivity for fabrication of gratings. Portions of this fiber that are exposed to UV light will have their refractive index changed, thus allowing the construction of a Fiber Bragg Grating or other types of devices with periodic changes in refractive index.
This fiber is designed for use in 980 nm pump diodes, couplers and multiplexers. Using one fiber that provides excellent photosensitivity, as well as polarization maintaining attributes, substantially reduces writing time thus lowering costs.
Thorlab’s PMDCF Dispersion-Compensating fibers (DCF) corrects for both the chromatic dispersion and dispersion slope of standard PM optical fiber in the 1510 to 1620 nm wavelength range. Sub-picosecond pulses are transmitted with low loss and no pulse broadening caused by chromatic dispersion, all while maintaining linear polarization. The fiber has PANDA stress rod supports that run parallel to the fiber's core and apply stress that creates a birefringence in the fiber's core which enables polarization-maintaining operation, and is specially designed for slow-axis light propagation.
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Bow-Tie PM Fiber Cross Section
These polarization-maintaining fibers use bow-tie stress members. They are commonly used for sensor applications, polarization multiplexing of EDFA lasers, and laser pigtailing.
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Bow-Tie PM Fiber Cross Section
This polarization-maintaining fiber is optimized for fiber optic gyroscope (FOG) applications. It is designed for optimal performance over a wide temperature range and with a small coil radius. Extinction ratios of 29.5 dB at -40 °C and 28.5 dB at -60 °C are typical for this fiber.