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Polarization-Maintaining Single Mode Optical Fiber
“Bow-Tie” Style PM Fiber
“Panda” Style PM Fiber
Thorlabs offers both panda and bow-tie style Single Mode Polarization-Maintaining (PM) fiber. The two styles 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 styles are interchangeable. Panda style 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, and our bend- and temperature-insensitive PM fiber is ideal for use in fiber optic gyroscopes (FOG).
Panda-Style Fibers, Pure Silica Core, 350 - 680 nm
Panda-Style Fibers, 460 - 2200 nm
Photosensitive Panda-Style Fiber, 980 nm
Bow-Tie-Style Fibers, 980 - 1550 nm
Bend- and Temperature-Insensitive Bow-Tie-Style 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.
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Undamaged Fiber End
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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 (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.
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
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The limiting factor with optical fiber terminated in a connector is free-space light entering the 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
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.
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.
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.
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.
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Bow-Tie-Style 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-Style 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.