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Dispersion Compensating Fiber 


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Dispersion Compensating Fiber 

Single Mode Fiber Cross Section
Click to Enlarge

Single Mode Fiber
Cross Section
Compatible Connector Supplies
FC/APC Connector30126K1
FC/PC Connector30126D1
Stripping ToolT06S13
Cleaving ToolS90R
FC/APC Connectorization KitCK05
FC/PC Connectorization KitCK03

Features

  • Dispersion Compensating Fiber for Telecom Wavelengths (1500 - 1625 nm)
    • DCF3 is for General Telecom Dispersion Compensation (Negative Dispersion and Low Positive Dispersion Slope)
    • DCF38 is Designed to Compensate Corning SMF-28e+ Fiber
  • Outer Jacket Available upon Request
  • Shipped from Stock with No Minimum Order

Thorlabs offers dispersion compensating optical fiber for custom solutions across a broad spectral range in the telecom region. DCF3 fiber is a non-zero dispersion-shifted fiber (NZ-DSF) with negative dispersion and low positive dispersion slope that is optimal for medium distances and wide band WDM systems. Optical dispersion across the entire C-band enables effective dispersion compensation and suppresses nonlinear impairments. DCF38 has dispersion designed specifically to match and compensate Corning SMF-28e+ or Vascade L1000 fiber. Please see the Dispersion Tutorial tab for more detailed information about dispersion compensating fibers.

Connectorization

Dispersion compensating fibers are fully compatible with typical connectors and termination tools. Loss is slightly higher than typical SM fibers at around 1 dB. Lower losses can be achieved by splicing. Various compatible connectors and tools are summarized in the table to the upper right.

Custom Fiber Patch Cables   Optical Fiber Manufacturing

Splicing

Splicers and tooling designed for Ø125 µm fiber can be used with this fiber. To achieve an optimized fuse, the program should use a shorter fusion time than with conventional SM fibers. This is due to excess diffusion of the core dopants, which alters the guiding properties of the fiber in the splice region. Alteriatively, DCF3 can be used as a bridge between DCF38 and SMF-28 to reduce splice losses. For more information, see "New Technique for Reducing the Splice Loss to Dispersion Compensating Fiber", Edvold B., Gruner-Nielsen, L., Optical Communication, 2, 245-248 (September 19, 1996).

Item #DCF3DCF38
DescriptionNegative Dispersion, NZ-DSF fiber with low, positive dispersion slopeHigh dispersion fiber with negative slope. Designed to be paired with Corning SMF-28e+ or Vascade L1000 Fiber
Dispersion Specifications
Dispersion [ps/(nm*km)]-4.30 to -1.60-49.00 to -30.00
Dispersion Slope [ps/(nm2*km)]0.043 to 0.065-0.155 to -0.075
Effective Area (µm2)4827
Polarization Mode Dispersion (ps/√km)≤0.05≤0.05
Index of Refraction1.470 at 1310 nm
1.469 at 1550 nm
1.476 at 1310 nm
1.474 at 1550 nm
General Specifications
Nominal Mode Field Diameter
@ 1550 nm (µm)
8.1 ± 0.46.01 ± 0.29
Numerical Aperture @ 1550 nm0.140.14
Cladding Diameter (µm)125.0 ± 1.0125.0 ± 1.0
Coating Diameter (µm)250 ± 5250 ± 5
Cutoff Wavelength (nm)≤1500≤1520
Attenuation @ 1550 nm (dB/km)≤0.220≤0.265
Attenuation Slope from 1530 - 1565 nm [dB/(nm*km)]-0.00034 to -0.00010-0.00040 to -0.00011
Standard SM Fiber Dispersion Diagram
Waveguide dispersion offsets chromatic dispersion to produce zero dispersion at 1.31 µm in step-index SM fiber (Click to Enlarge).

Dispersion in Optical Fiber

Chromatic dispersion is a property of optical fiber where different wavelengths of light propogate at different velocities. Chromatic dispersion is a function of wavelength, and is the sum of two components: material and waveguide dispersion. Material dispersion arises from the change in a material's refractive index with wavelength, which changes the propogation 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 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. In general:

Dispersionchromatic(λ) = Dispersionmaterial(λ) + Dispersionwaveguide(λ)

Since material and waveguide dispersion are wavelength dependent, the dispersion is a function of wavelength. The dispersion slope can be positive or negative.

Dispersion-Shifted Fiber Dispersion Diagram
A fiber designed with more waveguide dispersion shifts the zero-dispersion wavelength to 1.55 µm (Click to Enlarge).

Dispersion-Shifted Fiber

In standard step-index single-mode fiber, the sum of the material and waveguide dispersion is zero near 1310 nm, which is called the zero-disperion wavelength. By varying the fiber's waveguide structure, the waveguide dispersion can be shifted up or down, thus changing the zero-dispersion point. Fiber in which the zero-dispersion wavelength has been changed is called zero dispersion-shifted fiber.

An initial strategy was to alter the waveguide structure to shift the zero-dispersion point to the signal wavelength of 1550 nm, creating zero-dispersion shifted fiber (see the diagram to the right). Unfortunately, fixing the dispersion problem is not so simple. When multiple optical channels pass through the same fiber at wavelengths where dispersion is very close to zero, they suffer from a type of crosstalk called four-wave mixing. The degradation is so severe that zero dispersion-shifted fiber cannot be used for dense-WDM systems. To avoid four-wave mixing, the zero-dispersion wavelength is moved outside the transmission band. So-called nonzero dispersion-shifted fibers have a dispersion that is low, but nonzero in the 1550 nm band (typically 0.1 to 6 ps/nm*km). Although dispersion is minimized, it is still present.

Dispersion-Compensating Fiber Dispersion Diagram
Only total dispersion is shown in this graph. (Click to Enlarge)

Dispersion-Compensating Fiber

Since dispersion is inevitable in optical fibers, dispersion-compensating fibers, such as those sold on this page, 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 the dispersion of a standard single-mode fiber, such as a nonzero dispersion-shifted 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 dispersion-compensating fiber is spliced into a longer length of standard fiber to compensate for dispersion, as in the example below.

Dispersion Compensation Schematic

Dispersion Management

Dispersion can cause various penalties in signal transmission in optical communications systems. Thus, dispersion management is a very important part of designing a fiber optic transmission system. The following table, provided by ITU* standards, which gives the maximum distances for different transmission bit rates and fiber types at around 1550 nm as limited by dispersion.

Bit rate per channel (Gbps)SDHSONETSSMFNZ-DSF
2.5 GbpsSTM-16 OC-48640 km4400 km
10 GbpsSTM-64 OC-19250-100 km300-500 km
40 GbpsSTM-256OC-7685 km20-30 km

*ITU: International Telecommunication Union
SDH: Syhcnronous Digital Hierarchy
SONET: Sychronous Optical Network
SSMF: Standard Single Mode Fiber

NZ-DSF: Non-Zero Dispersion Shifter Fiber
STM: SDH Level and Frame Format
OC: SONET Optical Carrier Level


There are different techniques to reduce the impact of chromatic dispersion, among them fiber with small dispersion, using fiber with negative dispersion, or dispersion compensating optics. Chromatic dispersion may or may not need to be compensated for in an optical system. Total fiber system dispersion can be estimated by:

CDtotal = CDfi + CDDCM + CDother

Where:
CDfi = total fiber chromatic dispersion
CDDCM = total chromatic dispersion of dispersion compensating systems
CDother = total chromatic dispersion due to other components

A dispersion limit, CDlimit, is provided by ITU standards providing the maximum allowable accumulated chromatic dispersion. In general, the relation CDlimit ≥ CDtotal  should be true. When CDlimit= CDtotal , a 1 dB decrease in signal strength as a function of bit rate will be present.

Bit Rate per Channel (Gbps)SDHSONETTotal Allowable Dispersion Coefficient at 1550 nm for a Given Link with SSMF (CDlimit)
2.5 GbpsSTM-16 OC-4812000 to 16000 ps/nm
10 GbpsSTM-64 OC-192800 to 1000 ps/nm
40 GbpsSTM-256OC-76860 to 100 ps/nm

Dispersion Compensating Planning Example

Transmitted Power: 4 dBm
Signal: 10 Gbps
CDlimit: ±1000 ps/nm
Length: 100 km
Fiber: Single Mode with Dispersion: 18.0 ps/(nm x km) at λ = 1550 nm

First, is dispersion compensation necessary? CDfi = Dispersion x Length = 18.00 ps/(nm x km) x 100 km = 1800 ps/nm. The dispersion limit for this system is CDlimit = ±1000 ps/nm, and so we need dispersion compensation. For this example, we need CDlimit - CDDCM ≥ CDfi.

To reach the positive limit:
CDDCM ≤ 1000 ps/nm - 1800 ps/nm = -800 ps/nm
To reach the negative limit:
CDDCM ≥ -1000 ps/nm - 1800 ps/nm = -2800 ps/nm

Thus, we need -2800 ps/nm ≤ CDDCM ≤ -800 ps/nm. Our DCF38 fiber has dispersion -38.0 ps/(nm x km), so we can use two 13.2 km segments for a total CDDCM of: CDDCM = 2 x 13.2 km x -38.0 ps/ (nm x km) = -1003.2 ps/nm.

Our total dispersion is then CDtot = -1003.2 ps/nm + 1800 ps/nm = 796.8 ps/nm, which is below the dispersion compensation limit.

Power Handling Limitations Imposed by Optical Fiber
Click to Enlarge
Undamaged Fiber End
Power Handling Limitations Imposed by Optical Fiber
Click to Enlarge
Damaged Fiber End

Laser Induced Damage in 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.

Silica Optical Fiber Maximum Power Densities
TypeTheoretical Damage ThresholdPractical Safe Value
CW
(Average Power)
1 MW/cm2250 kW/cm2
10 ns Pulsed
(Peak Power)
5 GW/cm21 GW/cm2

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. A free-space beam of light must be focused down to a spot of roughly 80% of this diameter to be coupled into the fiber with good efficiency. 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.

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.

Example Calculation
For SM400 single mode fiber operating at 400 nm with CW light, the mode field diameter (MFD) is approximately Ø3 µm. For good coupling efficiency, 80% of the MFD is typically filled with light. This yields an effective diameter of Ø2.4 µm and an effective area of 4.52 µm2:

Area = πr2 = π(MFD/2)2 = π • 1.22 µm2 = 4.52 µm2

This can be extrapolated to a damage threshold of 11.3 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

4.25 µm2 • 2.5 mW/µm2 = 11.3 mW

Terminated 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
As a general guideline, for short-wavelength light at around 400 nm, scattering within connectors typically limits the power handling of optical fiber to about 300 mW. Note that this limit is higher than the limit set by the optical power density at the fiber tip. However, power handing limitations due to connector effects do not diminish as rapidly with wavelength when compared to power density effects. Thus, a terminated fiber’s power handling is "connector-limited" at wavelengths above approximately 600 nm and is "fiber-limited" at lower wavelengths.

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.

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 one localized area. The light escaping the fiber typically has a high power density, which can cause burns to the fiber 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 damage. Thorlabs manufactures and sells 0.22 NA double-clad multimode fiber, which boasts very high, megawatt range power handling.

Photodarkening
A second damage mechanism within optical fiber, called photodarkening or solarization, typically occurs over time in fibers used with ultraviolet or short-wavelength visible light. The pure silica core of standard multimode optical fiber can transmit ultraviolet light, but the attenuation at these short wavelengths increases with the time exposed to the light. The mechanism that causes photodarkening is largely unknown, but several strategies have been developed to combat it. Fibers with a very low hydroxyl ion (OH) content have been found to resist photodarkening. Other dopants, including fluorine, can also reduce photodarkening.

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.

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Posted Comments:
Poster: pbui
Posted Date: 2014-06-02 05:25:25.0
Unfortunately, Thorlabs does not currently have any dispersion compensating fibers that meet your required specs.
Poster: avendaedu
Posted Date: 2014-05-24 16:03:09.53
Hello I want to know the length of the reel of DCF 38, does yours have another DCF fiber with a higuer Dispersion, something around -90ps/nm*km? I appreciate your support
Poster: mchen
Posted Date: 2014-01-28 17:16:05.367
What are the dispersion and the loss of DCF38 at 2 micron? Do you have DCF designed for 2 micron? Thanks a lot, Mike
Poster: tcohen
Posted Date: 2012-06-20 09:32:00.0
Response from Tim at Thorlabs: The cutoff wavelengths for these fibers are =1500nm and =1520nm. Below this, many modes will propagate and there will be much more dispersion. I would like to talk to you about the length of fiber needed and the dispersion requirements for your system so that we can see what solution would be best for your application. I will contact you directly to get more information.
Poster: tttang
Posted Date: 2012-06-17 03:52:52.0
Do you have low-dispersion optical fiber for Ti:sapphire fs laser (800nm)? or would you be able to provide some information of these dispersion compensating fiber at 800nm? Thank you very much. -Tom.
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