Dispersion Compensating Fiber (DCF) is a straightforward technology, based on optical fibers that have a dispersion coefficient with opposite sign compared to standard single mode fiber. Today, it is possible to make DCFs that have a dispersion coefficient four to eight times that of standard single mode fiber. Even higher has been demonstrated. However, this level of dispersion is achieved by reducing the diameter of the fiber core, which in turn increases the fiber’s transmission loss of the fiber. For example, commercial DCFs for 100 to 120 km standard single mode fiber have about 10 dB loss.

Figure 1. Principle of the DCM-FB

DCMs based on FBGs are devised by basically assembling an optical circulator and a chirped FBG. The basic principle is shown in figure 1. Compression of a dispersed pulse is accomplished by letting the “fast” wavelengths of the pulse be reflected late in the grating while the “slow” wavelengths are reflected early. Losses are typically quite low, consisting only of circulator and grating reflection losses.
For a long time, the only commercially available DCMs were based on short Bragg gratings, created by exposing the fiber through a phase mask. By using a very complex phase mask design, it is possible to create multi-channel DCMs. The limitation of the technology is found in the design of the phase mask, which allows for a relatively narrow operating bandwidth centered around each ITU frequency, before serious ripple impairments start to occur, affecting both insertion loss and group delay. The bandwidth of such gratings is typically +/-15GHz. This represents a serious problem for 40Gbps systems, as the bandwidth of the signal is about as wide at the filter operating bandwidth itself. But even at 10 Gbps, such channelized approach has serious limitations for cascaded links due to windowing effects that result in a narrowing of the effective channel bandwidth.
Two other alternatives for dispersion compensation are etalon filters, virtually imaged phased arrays and AWG. They do also suffer from the disadvantages mentioned above beside limitations in maximum dispersion/bandwidth values. They have also a relatively high loss that is comparable with DCFs.

Other solutions like electronic dispersion compensators, EDC are evolving but the capability to do longer distances are still not good enough. A disadvantage is that EDCs compensate on a per channel basis, after demultiplexing the wavelengths. However, they will surely play an important role in future system solutions especially in combination with other dispersion compensation solutions.

Proximion’s DCM on the other hand is completely free from the channel-related limitations affecting the other technologies (except EDC). The dispersion compensation is obtained by creating an optical delay line in the fiber using a very long, continuously chirped grating. Thanks to a highly flexible grating manufacturing process, the dispersion level and dispersion slope can be tailored to match any fiber type, by controlling the chirp of the grating. Although the parameters for Proximion’s standard products are chosen to fit standard G.652 or G.655 fiber, Proximion can produce DCMs for any span length and fiber based on customer requests. Even DCM having a positive dispersion can easily be produced.

Figure 2. Compensation for virtually any fiber can be done. Standard versions are highlighted

There is no channel partitioning of the bandwidth, and the DCM operates continuously over the whole C-band. The standard products extend from 1527nm to 1565nm, but custom ranges can be obtained, extending all the way to the L-band. No temperature control is required for the component, as there is no need to prevent the filter’s operating bandwidth to glide out of the ITU channel. This is illustrated in figure 3. The slope of the group delay i.e. the dispersion, is the same regardless if the grating shifts in wavelength with temperature changes and the DCM-FB is thus temperature independent.

Figure 3. Principle why DCM-FB is temperature insensitive

DCF has a loss that is approximately linear with the compensated length whereas in FBGs loss is more or less constant. The advantages DCM-FB are significant especially when compensating for long fibers. A DCF for 100-120 km SMF has about 10 dB while DCM-FB only has about one third of this.

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