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Polarizing Optical Fiber![]()
"Bow-Tie" Style PZ Fiber Aligned Coiled Single Mode Unpolarized Splicing Area Polarized Light Splicing Area Polarized Light ![]() Please Wait ![]() Click to Enlarge The polarizing window for these PZ fibers is defined as the wavelength range between the 20 dB attenuation of the fast axis and the 3 dB attenuation of the slow axis. For available graphs for each fiber type, please see the Graphs tab. ![]() Click for Details Bow-Tie-Style PM Fiber Cross Section Features
Thorlabs' Polarizing (PZ) fibers, also known as Zing™ fibers, are specialty optical fibers in which one and only one polarization state is allowed to propagate. Light with any other polarization direction will experience significant optical loss and thus is not propagated through the fiber. To create this effect, our PZ fiber utilizes bow-tie geometry to create an extreme birefringence. This birefringence ensures that only light with the proper polarization direction is guided through the fiber, whereas all other polarization directions experience high loss. These PZ fibers offer a broad polarizing window (~100 nm), high extinction ratio (≥30 dB), and low attenuation. Polarization performance is specified using an Ø89 mm coil of roughly 5 m of fiber, which is about 16 turns; see the Specs tab for details. However, the polarizing window and extinction ratio can be tuned by coiling the PZ fiber (referred to as fiber deployment). Coiling the fiber to smaller diameters will narrow the polarization window and shift the window to lower wavelengths (see Graphs tab for more information). Note that performance cannot be guaranteed using coil diameters other than Ø89 mm. The polarizing window is defined between the 20 dB Fast Edge and 3 dB Slow Edge (see graph to the right). PZ fibers are all-fiber devices with several advantages over in-line polarizers, such as lower insertion loss, higher extinction ratio, and no complex component assemblies or bulky packaging (see the PZ Tutorial tab). This cost-effective fiber delivers a high extinction ratio (ER), a bandwidth so wide that it will polarize at the design operating wavelength (830 nm for HB830Z, 1064 nm for HB1060Z, and 1550 nm for HB1550Z) even if the fiber is stressed, an ER and insertion loss that is stable with temperature, and long-term reliability in use. Our PZ fiber cleaves, handles, and splices just as any other silica fiber would and is compatible with standard PM fiber systems (both PANDA and bow-tie). This fiber also terminates just like any other PM fiber that requires low-stress epoxy and alignment of the key to the axis. It is important to note that PZ fibers are not the same as Polarization-Maintaining (PM) fibers. While PM fibers maintain the linear polarization state when the polarization direction is aligned with the birefringence axis, they are also capable of propagating any polarization direction. Unlike PM fibers, PZ fibers suffer no polarization cross-talk, which makes them ideal for polarization-sensitive applications. These fibers can be ordered as connectorized patch cables using our custom cables configurator. Please note that adding tubing may change fiber performance.
![]() Click to Enlarge The polarizing window for these PZ fibers is defined as the wavelength range between the 20 dB attenuation of the fast axis and the 3 dB attenuation of the slow axis. For available graphs for each fiber type, please see the Graphs tab.
HB830Z
HB1060Z
HB1550Z
PZ Fiber OverviewPolarizing (PZ) fiber (i.e., Zing™ fiber) is a specialty optical fiber that will guide only one polarization direction, thus polarizing light that is propagated through the fiber. This form of single-polarization transmission carries several benefits over single mode (SM) or polarization-maintaining (PM) fiber. While PM fiber maintains the polarization direction that is aligned with the birefringence axis, cross talk occurs since the PM fiber is capable of guiding any polarization direction. SM fiber can be stressed to induce birefringence (see manual fiber polarization controllers), which causes the fiber to behave much like a wave plate. While the polarization axis can be manipulated in this case, the SM fiber does not polarize the light. In contrast, PZ fiber guides only one polarization direction; all other directions are unguided. As a result, PZ fibers will polarize the light guided through it, creating excellent suppression of unguided polarization directions. While in-line polarizers can provide between 20 and 30 dB suppression of unwanted polarization directions, PZ fibers can realize ≥30 dB suppression at the design wavelength. Additionally, by stressing the fiber, the polarization window can be manipulated and the user can realize suppression of over 35 dB. Because stress on a PZ fiber can alter its operation, "deployment" of the fiber becomes an essential quality. Deployment refers to how the fiber is laid out, whether it is straight, coiled, or randomly piled. Using PZ FiberIt is important to note that the deployment of the PZ fiber is key to its performance. Our PZ fiber has a very wide polarizing window, the width and center wavelength of which depends on how the fiber is deployed (see the Graphs Tab). In nominal usage of the fiber around its design wavelength, the PZ fiber will polarize for any deployment. For other usage, however, the user should ensure that the deployment shifts the polarization window such that the window overlaps the source. This method works best with a narrow linewidth souce such as a laser. For broadband sources, the PZ fiber needs to be coiled appropriately such that the width and center wavelength of the polarization window can overlap the source. It is advantageous to use a depolarizer at the input of the PZ fiber because it ensures the light is evenly polarized, avoiding power variations that can occur with all types of polarizers. The depolarizer can be made from two sections of SM fiber spliced at 45° (length will depend on the source). In this system, the PZ fiber is also typically coupled to the depolarized output at 45° to take full advantage of the depolarizing effect (Note: If using patch cables, one termination should be rotated 45° relative to the other where the depolarizer fiber meets the PZ fiber, by offsetting the key in either fiber at 45°). When the input light to the PZ fiber is depolarized, the light incident upon the fast and slow axes of the PZ fiber is equal, resulting in consistent 3 dB rejection and stable output power. The figure below shows an example system for using our polarizing fibers with an unpolarized light source. The chart below the figure lists the components in the system for each of our PZ fibers. ![]()
Coiling our PZ fiber to smaller diameters will create a narrower polarization window and blueshift the center wavelength. Coiling the PZ fiber can result in a better polarization extinction ratio, although it can lead to greater loss. If loss is too high, the coil is too tight; conversely, if the polarization extinction ratio is too low, the coil is not tight enough. For example, to achieve an extinction ratio of 35 dB with our HB1060Z PZ Fiber, 2 m of fiber is coiled into Ø5 cm loops. The diagram above demonstrates how this is implemented. Unpolarized light was sent into the PZ fiber, which is coiled to produce the desired effect. The PZ fiber was then spliced into a PM fiber, which goes out to the system. For additional performance stability, it is recommended to use 3 - 5 m of HB1060Z fiber or 4 - 10 m of HB830Z and HB1550Z. However, due to the high birefringence of the PZ fiber, the polarization window will still be broad, giving the user a wide variety of packaging and deployment options. ![]() Click to Enlarge The left image shows the intensity profile of the beam propagated through the fiber overlaid on the fiber itself. The right image shows the standard intensity profile of the beam propagated through the fiber with the MFD and core diameter called out. Definition of the Mode Field DiameterThe 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. The light can be approximated as a Gaussian power distribution as shown to the right, where the MFD is the diameter at which 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.
Laser-Induced Damage in Silica Optical FibersThe 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. ![]() Click to Enlarge Undamaged Fiber End ![]() Click to Enlarge Damaged Fiber End Damage at the Air / Glass InterfaceThere 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 FaceDamage 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![]() Click to Enlarge 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 MechanismsWhen 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 ThresholdIn 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 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. 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 FibersGeneral Cleaning and Operation Guidelines
Tips for Using Fiber at Higher Optical Power
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