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Polarization-Maintaining Dispersion-Compensating Fiber Optic Patch Cables
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The PMDCF fiber used in these patch cables has a short section of PM1550-XP fiber spliced to each end to minimize loss when connecting to other PM patch cables.
These polarization-maintaining (PM) fiber optic patch cables incorporate dispersion-compensating fiber (DCF) for applications which require precise control over the dispersion in a system. As shown in the diagram above, each end of the DCF is spliced onto a short length of PM1550-XP fiber to minimize loss when connecting to other PM patch cables. Both ends are teminated with narrow key, ceramic-ferrule FC/APC connectors. These cables feature a high-quality polish, which leads to a typical return loss of 60 dB. Assembled in our facility, each cable is individually tested at 1550 nm to ensure its extinction ratio and insertion loss fall within specifications. Each patch cable is shipped with a data sheet that summarizes the results of this testing (click here for a sample data sheet).
Dispersion-free links can be built by combining a PM DCF cable with a compensated length patch cable that incorporates PM1550-XP fiber. For example, a PMDCFA2 cable can be connected with a P3-1550PM-FC-2 cable to create a dispersion-free fiber link with an approximate length of 2.7 m. Similarly, a PMDCFA5 cable can be connected to a P3-1550PM-FC-5 cable to create a dispersion-free fiber link with an approximate length of 6.2 m. For more information on dispersion in fiber and the use of DCF, see the DCF tab.
Two protective caps are included with each patch cable that shield the ferrule ends from dust and other hazards. Additional CAPF Plastic Fiber Caps and CAPFM Metal Threaded Fiber Caps for FC/APC-terminated ends are also sold separately.
PM patch cables like the ones sold on this page are available in various lengths and tubing. Please contact Tech Support for more information.
Dispersion in Optical Fiber
Chromatic dispersion, D, in an optical fiber occurs when the group velocity and phase velocity of an optical pulse depend on the optical wavelength/frequency. It is primarily the sum of two components, material dispersion and waveguide dispersion:
Material dispersion arises from the change in a material's refractive index with wavelength, which changes the propagation velocity of light as a function of wavelength. Waveguide dispersion is a separate effect, arising from the geometry of the fiber optic waveguide. Waveguide properties are also a function of wavelength; consequently, changing the wavelength affects how light is guided in a single-mode fiber. For example, decreasing the wavelength will increase the relative waveguide dimensions, causing a change in the distribution of light in the cladding and core.Another useful parameter is the dispersion coefficient, β, which is also called the phase constant or mode-propagation constant when featured in the nonlinear Schrodinger equation. If the optical pulse propagates along a fiber of length, L, then the associated phase shift is defined as:
β can be expanded to include higher-order nonlinear modes, βi. In particular, the second-order and third-order propagation constants are related to dispersion by:
Where dDfiber/dλ is known as the dispersion slope, which can be positive, negative, or zero, and written as:
Group velocity dispersion (GVD) is the temporal pulse broadening due to different group velocities, and it has significant influence on optical pulse widths on the order of picoseconds or shorter. The group velocity, vg, can be defined as the rate at which the entire pulse envelope will propagate:
Which allows the group velocity dispersion to be defined as:
There is no change in the shape of the temporal pulse when GVD equals zero, however there will always be temporal broadening when GVD is nonzero. When the GVD is greater than zero, the longer wavelength components will propagate faster than the shorter wavelengths; and when the GVD is less than zero, the longer wavelength components will propagate more slowly.
Polarization-mode dispersion (PMD) in typical single-mode fiber occurs as a result of birefringence in the fiber due to asymmetries in fiber stress and geometry. In the frequency domain, it presents itself as a linear change in a fixed input polarization with respect to frequency. In the time domain, it presents itself as the mean time delay of a pulse propagating along the fiber. The group delay is the difference between the mean arrival times at the fiber input and the fiber output.
Polarization-state pairs (PSP) are orthogonal pairs of polarization states at the optical fiber input. For polarization-maintaining fibers, these are the fiber’s fast and slow axes, which are treated separately and generally have different phase shifts and group delays. The differential group delay (DGD) is the difference in group delay between the orthogonal pairs of polarization states. DGD increases proportionally to the square root of the fiber length. Polarization-mode dispersion can be defined as a vector that has a magnitude equal to the DGD and points in the direction of the slow axis.
Since dispersion is inevitable in optical fibers, dispersion-compensating fibers (DCF) can be incorporated into optical systems. The overall dispersion of these fibers is opposite in sign and much larger in magnitude than that of standard fiber, so they can be used to cancel out or compensate for the dispersion of a standard single-mode or polarization-maintaining fiber. A negative dispersion slope enables effective cancellation of dispersion over a larger wavelength range, since the dispersion slope of standard fiber is usually positive. Generally, a short length of DCF is spliced into a longer length of standard fiber to compensate for dispersion, as shown in the example below.
The dispersion-compensating fiber should be selected to match the dispersion of a regular SM or PM fiber, not only at a single wavelength, but over the whole spectral range of the optical pulse. This means that the DCF should match not only the dispersion, D, but the dispersion slope, dDfiber/Dλ. The ratio of these two factors is called the relative dispersion slope. Similarly, the ratio β2/β3 can be used as another numerical parameter to optimize fiber selection. The more similar these parameters are for the DCF and the standard fiber, the less distorted and impaired the transmitted optical pulse will be at the spliced fibers’ output.
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Mating Between a Narrow-Key Mating Sleeve and Connector
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Mating Between a Wide-Key Mating Sleeve and Connector
FC/PC and FC/APC Patch Cable Key Alignment
FC/PC and FC/APC Patch Cables are equipped with either a 2.0 mm narrow or 2.2 mm wide alignment key that fits into a corresponding slot on a mated component. These keys and slots are essential to correctly align the cores of connected fiber patch cables and minimize the insertion loss of the connection.
As an example, Thorlabs designs and manufactures mating sleeves for FC/PC- and FC/APC-terminated patch cables to precise specifications that ensure good alignment when used correctly. To ensure the best alignment, the alignment key on the patch cable is inserted into the corresponding narrow or wide-key slot on the mating sleeve.
Wide-Key-Slot Mating Sleeves
Narrow-Key-Slot Mating Sleeves
Narrow-Key-Slot Mating Sleeve and Narrow Key Connector
Once a narrow key connector is inserted into a narrow-key-slot mating sleeve, the connector will not rotate. We therefore recommend these mating sleeves for FC/PC and FC/APC connectors with narrow keys.
Wide-Key-Slot Mating Sleeve and Narrow Key Connector
When a narrow key connector is inserted into a wide-key-slot mating sleeve, the connector has room to rotate. For narrow key FC/PC connectors, this is acceptable, but for narrow key FC/APC connectors, significant coupling losses will result.
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