When utilizing fiber optic technology for high-speed data communications, maintaining the integrity of the optical signal during transmission over long distances is essential to ensure maximum performance. A fundamental technical characteristic that arises during fiber-based signal transmission is chromatic dispersion (CD), a phenomenon that can severely degrade signal quality. In this article, we will explore CDs including their harmful effects on signal transduction, along with their presentation Dispersion compensation fiber (DCF)a special type of optical fiber designed to mitigate the effects of CDs, successfully overcomes this challenge.
What is chromatic dispersion?
Chromatic dispersionIt refers to the phenomenon in which different wavelengths (colors) of light travel at different speeds when transmitted through optical fibers. This occurs because the refractive index of the fiber material varies with wavelength, a property known as “material dispersion.” In addition, waveguide dispersion, resulting from the fiber geometry, also contributes to chromatic dispersion. Unfortunately, the CD characteristic produces effects that are detrimental to achieving optical signal transmission performance.
What causes chromatic aberration?
There are two main causes of chromatic dispersion:
Material dispersion: Caused by the inherent properties of the fiber material (usually silica glass), the refractive index changes with wavelength.
Waveguide dispersion: results from the physical structure of the fiber that affects how light modes propagate.
Effects of chromatic dispersion on optical signals
When chromatic dispersion occurs, what happens to the optical signal? The two recognized effects of this phenomenon are:
Pulse broadening: As light pulses travel through an optical fiber, color dispersion causes them to spread out over time.
Intersymbol Interference (ISI): Spread pulse interference causes network devices to have difficulty distinguishing between bits, leading to increased error rates.
This chart is provided by Cisco Systems Blogs
Negative effects of chromatic dispersion on the transmission of optical signals
Because of these noticeable effects, chromatic dispersion can significantly impair optical communication system performance in several ways:
Signal degradation: Pulse broadening reduces the clarity of the transmitted signal, making it difficult for the receiver to interpret the data accurately.
Limited Bandwidth: Dispersion limits the maximum data rate that can be transmitted over a given distance.
Increased bit error rate (BER): Overlapping pulses increases the probability of bit errors, necessitating complex error correction methods.
Reduce effective transmission distance: Without dispersion compensation, CD limits how far a signal can travel before it becomes unintelligible to network devices.
With chromatic dispersion negatively impacting optical signal performance as well as overall network performance, it creates a technical hurdle that network operators must address. One of the primary solutions to successfully reduce or even eliminate chromatic dispersion is the use of optical fibers that compensate for dispersion.
What is Dispersion Compensated Fiber (DCF)?
Dispersion compensation fiber, often abbreviated as DCF, is a type of specialized optical fiber that is designed and manufactured to exhibit negative chromatic dispersion characteristics over a specific wavelength range. It is used to effectively counter the positive chromatic dispersion value accumulated in standard single-mode fiber during signal transmission. By introducing DCF into the fiber band, the overall dispersion can be significantly reduced or even effectively neutralized, maintaining signal quality and enhancing system performance.
Design and manufacture of DCF
Dispersion-compensated fibers are designed and engineered to have dispersion properties that are inconsistent with those of standard transmission fibers. Simply put, their negative dispersion properties conflict with the positive dispersion properties of optical fibers. The main elements of its design include:
Negative Dispersion Coefficient: This is achieved by changing the refractive index profile to produce negative dispersion at the operational wavelength.
Custom refractive index profile: This is achieved by using dopant materials such as germanium or fluorine to modify the core and cladding indices.
It should be noted that DCF is designed to have a smaller core size and mode field diameter (MFD) compared to standard single-mode optical fibers. While standard G.652.D single-mode fiber has an MFD of around 9µm, the DCF may only be 4~5µm. Despite the significant difference, fibers can be directly spliced together using a high-quality splicer, although the link loss will be higher due to mode field mismatch. Another approach developed involves special “transitional” fibers designed for use between DCF and SMF and/or highly specialized fusion splicers that provide core taper capability.
DCF manufacturing techniques
There are several manufacturing techniques that companies that design and manufacture DCFs often use, including:
Modified Chemical Vapor Deposition (MCVD): A process in which layers of doped silica are deposited inside a hollow substrate tube.
External vapor deposition (OVD): Silica soot is deposited on a rotating rod and later incorporated into a solid mold.
Plasma-enhanced chemical vapor deposition (PECVD): Uses plasma to deposit doped layers at low temperatures.
Where and how to apply DCF in a network or test setup
Because a fiber network and the deployment of devices across the network will vary depending on its architecture and intended performance goals, there are different ways engineering teams can deploy dispersion compensation fiber in a real network environment or in a test lab. These methods include the following:
Precompensation: Place the DCF before the primary transmission fiber to preset the signal at the beginning of the extension.
Postcompensation: Place the DCF after the primary transmission fiber to adjust for the accumulated dispersion at the end of the extension.
Distributed Compensation: Integrating DCF segments across the transmission path.
Using DCF, the negative effects of chromatic dispersion such as signal degradation from pulsing and increased bit error rate (BER) are mitigated, enabling improved signal reach and integrity.
In terms of installing or implementing a DCF, the most common solution methods are usually either in a rack-mounted structure or in a modular, modular format. It is common to see/hear them referred to as dispersion compensators, dispersion compensating fiber modules, or other similar names.
Above: DCF in a 1RU rack chassis
Determine the appropriate length of DCF to compensate for the length of SMF
Since most DCFs manufactured provide much higher negative dispersion specifications compared to positive dispersion specifications for transmission fibers on a physical length/distance basis (per meter or per kilometer), a much shorter length of DCF will be needed to compensate for the length of the transmission fiber. Additionally, not all DCFs offer the same negative dispersion performance specifications between different manufacturers. In some cases, performance specifications can vary slightly between different reels from the same manufacturer.
To calculate the appropriate length of DCF needed to compensate for the length of the transmission fiber, the positive dispersion specifications of the transmission fiber and the negative dispersion specifications of the DCF must be known. It then becomes a mathematical scenario to determine the appropriate length of DCF needed to compensate for the length of the transmission fiber.
Here is a basic example in which we will calculate the approximate length of DCF needed to compensate for a 50 km length of standard single-mode G.652.D fiber:
Fiber type and dispersion value
50 km of G.652.D single-mode fiber; (+) 18 HP/Nm * km @ 1550 Nm
Dispersion compensation fiber. (-) 144 PS/Nm*km at 1550 Nm
the accounts
SMF: Total dispersion value for 50 km (50*18) = 900
Resulting DCF length (900/144) = 6.25 km
As shown above, in this scenario, a DCF length of 6.25 km would be needed to fully mitigate the chromatic dispersion of the 50 km transmission fiber. If the negative dispersion of the DCF were higher (i.e. -160 vs. -144) and thus more compensated on a per kilometer basis, a shorter DCF length would be needed. Alternatively, if the DCF dispersion value is lower (i.e. -110 vs. -144) then as expected, a longer DCF length will be needed. For more accurate calculations, having the exact CD value for each fiber allows the greatest degree of DCF length accuracy.
Applications of DCF in testing fiber optics and communications networks
Telecommunications and data center service providers
Because the chromatic dispersion of an optical signal builds up with increasing distance, CD attenuation is important for almost any type of medium-to-long-distance fiber extension, including metro networks, long-haul regional and terrestrial networks, and long-haul submarine networks.
Multi-channel WDM transmission systems
In addition, DCF is critical in WDM systems where multiple channels are transmitted simultaneously, each at different wavelengths, so enabling devices to interpret data more easily while reducing bit error rates is essential.
Optical fiber testing
Network Simulation and Latency Testing: When fiber network simulators are used to replicate the optical and latency characteristics of full fiber spans, DCF can be incorporated into spans to mimic real-world scenarios or to create variable performance conditions during R&D, device certification, and system demonstration efforts.
There are some scenarios that require a more specialized DCF solution, for example, simulating the optical time delay of high-bandwidth fiber RF communications. This test application requires dispersion compensation for the fiber, but since it is a time delay, it must also be the exact physical length to produce the exact delay value. Essentially, it is a dispersion-compensated optical delay line, and therefore requires more advanced calculation to produce an accurate length of fiber with the right mix of SMF and DCF to achieve the intended delay value without color dispersion.
Calibration of Test Equipment and Networks: Ensure that equipment tests CD correctly and/or performs accurately under various dispersion conditions.
DCF solutions are customized for your test lab or network
Do you have additional questions about dispersion compensation fibers or need a DCF for your test lab or network application? Leveraging over two decades of experience designing and supplying custom DCF solutions to entities across numerous sectors, the team of fiber experts at M2 Optics are here to help you in any way.
