Chromatic dispersion of short optical pulses traversing optical fibers is a fundamental problem when it comes to optical transport. Signal distortion, if not properly compensated, will lead to inter-symbol interference that may result in data loss or traffic interruption. This paper describes the technologies used to compensate for chromatic dispersion and how to implement reliable optical connections between equipment nodes that could extend more than 100km apart.
Dispersion, by definition, is the separation or light by refraction into individual components of different wavelengths. Dispersion results in most materials because a material’s index of refraction depends on the wavelength of the light passing through it. Therefore, different wavelengths entering a fiber-optic cable will fan out into different paths. This phenomenon is called chromatic dispersion.
Chromatic dispersion is due to an inherent property of silica fiber. The speed of a light wave is dependent on the refractive index of the medium in which it is propagating. The refractive index varies with wavelength. Therefore, different wavelength channels will travel at slightly different speeds in the fiber. A pulse of light from a laser has some spectral width, so each of its wavelength components will travel at slightly different speeds in the fiber. This phenomenon results in a spreading of the transmission pulse as it travels through the fiber, thus distorting the light.
A common method of controlling dispersion is the use of dispersion-compensating modules (DCMs) placed at periodic intervals. DCMs usually come in one of two types. The first type uses a dispersion-compensating fiber (DCF), which is simply a spool of a special type of fiber that has a very large negative dispersion. Typically, DCF dispersion can be in the range of –80 ps/(nm x km). For example, a 20km length of DCF can compensate for the dispersion in a 100km length of fiber.
One drawback of this technology is that the DCF by design has a high insertion loss. This downside, combined with the fact that the length of the DCF is proportional to the length of the transport fiber, leads to bulky terminal components in addition to the high insertion loss. The result is higher cost.
The second type of DCM is a fiber Bragg grating (FBG). By employing the unique properties of FBG, insertion-loss issues can effectively be addressed. FGB provides a cost-effective alternative to the incumbent DCF technology.
Dispersion compensation using FBG is based on the introduction of wavelength-specific time delays by way of chirped FBG. By combining a FBG with a standard optical circulator, a highly effective dispersion-compensation module (DCM) can be achieved. Compensation results from letting the “fast” wavelengths of the pulse reflect farther away in the FBG than the “slow” wavelengths, which reflect closer to the circulator. The exact reflection position for each wavelength is governed by the precise photo-induced refractive-index changes in the fiber. These changes are controlled, down to single nanometers, by a highly sophisticated manufacturing method.
Accurate control over the FBG chirp is the key to precise dispersion compensation. By using state-of-the-art direct-write FBG manufacturing techniques, dispersion characteristics can be made to precisely mimic the dispersion properties of the fiber span.
Two main types of FBG-based dispersion compensators are commercially available today: multichannel (or channelized) and continuous. The channelized version provides channel-spacing-specific compensation. The continuous type provides continuous compensation throughout the C or L band. Therefore, the continuous type offers total channel-plan independence, an important feature when considering higher bit rates, dense channel spacing and upgradability.
Fiber Bragg Grating vs. Dispersion-Compensating Fibers
Insertion loss is a major cost driver for optical networks, and the biggest drawback to using DCFs for dispersion compensation is the amount of insertion loss they add. Insertion loss also drives the requirements for the number of erbium-doped fiber amplifiers (EDFAs) needed. EDFAs, however, actually add wavelength-dependent dispersion, negatively affecting system performance as their numbers increase.
FBG-DCM diminishes the insertion-loss obstacle, as it has less than half the insertion loss as DCF. Another benefit of the FBG-DCM is its resilience when using high optical power. In contrast to DCFs, which display nonlinearities at moderate optical power levels, the FBG-DCM can tolerate the highest optical power in optical networks without inducing nonlinearity.
Accurate dispersion compensation becomes more stringent when increasing the bit rate. Typically, the chromatic dispersion for a 10Gbps transmission line is above 100 ps/nm. But when considering optical transport at 40Gbps, this tolerance typically falls well below 100 ps/nm.
DCM for Higher-Rate Systems
To overcome the severe dispersion requirements for high-bit-rate transport, a number of strategies have been developed. One method to increase dispersion tolerance is to move away from simple digital encoding formats and start employing more dispersion-tolerant formats such as differential quadrature phase-shift keying (DQPSK). DQPSK serves in many 40Gbps networks. Its format transmits two encoded bits per symbol, so the symbol rate is half the bit rate with somewhat reduced system complexity. DQPSK is tolerant to chromatic dispersion and polarization-mode dispersion, and it has a high spectral efficiency, enabling its use in long-haul transmission. The configuration of a DQPSK system is less complex compared with that of a QPSK system, but it’s larger in size and consumes more power in the optical transceivers.
For long-haul 100Gbps, the industry is adopting dual-polarization quadrature PSK (DP-QPSK) modulation with a coherent receiver. DP-QPSK uses two independent optical signals of exactly the same frequency, but with dual polarization, reducing the symbol rate by a factor of two. The QPSK modulation allows an additional factor-of-two reduction in symbol rate.
Furthermore, using new modulation schemes will increase the tolerance to chromatic dispersion. As a result, system vendors and operators are turning to tunable dispersion compensators (T-DCMs) for future systems. T-DCMs allow operators to basically use 10Gbps design rules for 40Gbps networks, since they have the potential to increase the dispersion tolerance tenfold. As such, the original 10Gbps link can remain largely intact. In addition, the T-DCM will also handle time-varying dispersion changes induced by normal temperature variations along the fiber.
FBG-based technology has proven very suitable for T-DCM. FBG-based adaptive dispersion compensation is readily available, and tunable FBGs are under consideration as the technology of choice in numerous 40Gbps and 100Gbps optical systems.
Leading article image courtesy of www.ictas.vt.edu under a Creative Commons license
About the Author
Gene Norgard is Vice President of Operations at Sorrento Networks. He manages the operations of Sorrento Networks International and is directly responsible for manufacturing, repair and development of the GigaMux product line. He has been involved with engineering development and manufacturing for more than 30 years, the last 20 as a senior executive. Gene was previously VP of Engineering at Sorrento Networks; before that, he was Director of Engineering at Charles Industries and was a founder and vice president of Oasys Telecom.