Fiber Bragg Gratings Filter WDM Signals

Breck Hitz

Fiber Bragg gratings are versatile wavelength filters for multiplexing and demultiplexing wavelength division multiplexing (WDM) signals. They also can compensate for chromatic dispersion that can degrade the quality of the WDM signal in an optical fiber.

Such a grating is, in fact, a fiber with a core that has a sinusoidal variation of refractive index (Figure 1). This variation affects the phase of light passing through the grating so that some wavelengths interfere constructively and some destructively.

Figure 1. A fiber Bragg grating is most efficient as a WDM signal filter when it reflects a single wavelength and transmits the other wavelengths.

In this regard, a fiber Bragg grating resembles a thin-film coating, but there are significant differences. In a thin-film filter, the changes in refractive index are discontinuous and on the order of 10^—1. In the grating, the variation in refractive index is continuous and much smaller. Moreover, a thin-film filter deals with waves in free space, while the grating’s waves are confined to a waveguide. Thus, the fiber Bragg grating is most efficient as a filter when it reflects a single wavelength and transmits the rest, while a thin-film filter reflects all wavelengths except the one filtered out.

Filtering

Because the selected wavelength reflects from the grating, it must work in conjunction with a circulator or other isolating component. A circulator, for instance, is a three-port device designed so that light entering the first port emerges from the second port, and light entering the second port emerges from the third. An optical add/drop multiplexer is an example of a fiber Bragg grating used to multiplex and demultiplex a WDM signal (Figure 2). A signal composed of many wavelengths enters Port 1 of the first circulator, emerges from Port 2 and enters the grating, where the selected wavelength, λ₂, is reflected and all others are transmitted. The reflected light enters Port 2 and emerges from Port 3 in a process that demultiplexes it from all the other wavelengths.

Figure 2. In an optical add/drop multiplexer, a WDM signal with many wavelengths enters Port 1 of the first circulator, emerges from Port 2 and enters the fiber Bragg grating, where the selected wavelength, l2, is reflected and all the others transmitted. The reflected light enters Port 2 of the first circulator and emerges from Port 3, essentially being dropped.

The “add” signal at λ₂ enters Port 1 of the second circulator and emerges from Port 2. It enters the grating, reflects from it and joins the other wavelengths transmitted through the grating. It has been multiplexed into the WDM signal.

In the early years of WDM, almost all multiplexing/demultiplexing was based on thin-film technology, and thin films are still the best solution when the separation between adjacent channels is 100 GHz or more. But as channel spacing drops below 100 GHz, thin-film multiplexers and demultiplexers become increasingly complex and expensive. Fiber Bragg gratings, on the other hand, can easily handle channel spacing of 25 GHz and perhaps even less. One drawback is their thermal sensitivity, which, although an order of magnitude greater than that of a thin-film filter, can be reduced by athermal packaging. This, however, adds a layer of complexity and expense. An advantage of the gratings is that they are fabricated in the fiber itself. There is no optical loss through coupling out of the fiber into another device.

Chromatic dispersion

Chromatic dispersion in optical fiber results from both the bulk dispersion of the glass and the waveguide dispersion of the fiber. The signal degrades as it travels because dispersion causes pulses to widen as they propagate down the fiber, eventually making it difficult to distinguish a 1 from a 0 in a signal. In the extreme case, the pulses level out to a flat, continuous signal.

Pulses widen because they are not perfectly monochromatic. Each pulse has a bandwidth that, according to the Fourier theorem, is inversely proportional to its temporal width. As the pulse travels down the fiber, the short-wavelength components get ahead of the long-wavelength components, causing the entire pulse to widen. This is analogous to a gym class taking a half-mile run; at the starting line the runners are all close together, but by the time they’ve gone half a mile, they’re spread out over many yards.

Interestingly, the problem of signal degradation due to chromatic dispersion scales as the square of the pulse frequency. The problem is 16 times worse at 10 GHz than it is at 2.5 GHz. Part of the reason is that, as mentioned above, a 10-GHz pulse has four times the bandwidth of a 2.5-GHz pulse, which means that the pulse widening is four times greater. But the 10-GHz pulse train has one-fourth the space between pulses that the 2.5-GHz pulse train has, so it has one-fourth the tolerance for pulse widening. Pulse widening up by a factor of four combined with a reduced tolerance for widening by a factor of four degrades the signal 16 times. Hence, as systems move to higher pulse frequencies, the importance of chromatic-dispersion compensation will increase dramatically.

For chromatic dispersion compensation applications, the grating in the fiber is not linearly sinusoidal as it is in a multiplexing or demultiplexing device. Rather, it is chirped, with spacing varying over the length of the grating (Figure 3).

Figure 3. A fiber Bragg grating (FBG) designed for chromatic-dispersion compensation differs from one designed for multiplexing and demultiplexingin that the fiber is not linearly sinusoidal. Instead, it is chirped, with varied spacing over the length of the grating.

In one example, the shorter wavelengths will reflect from the right-hand side of the grating, and longer wavelengths from the left-hand side. In a pulse that has traveled through a considerable length of fiber, the short wavelengths have moved ahead of the long wavelengths. Inject the pulse into this grating, and the short wavelengths must travel a greater distance before reflection than the long wavelengths. Both wavelengths emerge from the grating at relatively the same time, and the pulse has been compressed, (almost) to its original width.

In the runner analogy, suppose that each runner must touch a different post before returning to the starting line. If the fastest runner’s post is farthest from the starting line, and everybody else’s post is scaled according to his or her speed, all should return to the starting line at the same time.

Figure 4. Networks rely on a circulator to couple a pulse of a multiwavelength WDM signal into a fiber Bragg grating.

In practice, networks will use a circulator to couple the pulse into the grating (Figure 4). Because each channel in a WDM signal requires its own fiber Bragg grating, the signal must be demultiplexed and fed into a bank of gratings (Figure 5).

At present, there are drawbacks to such a solution. First, it works only with a particular set of channels. Adding channels or changing their wavelengths requires redesign of the dispersion compensator. Second, redesign of the compensator is necessary if the system’s dispersion changes (because a portion of the fiber is being replaced, for example).

Figure 5. Each channel in a WDM signal requires its own fiber Bragg grating, so the signal must be demultiplexed and fed into a bank of gratings.

Research continues to resolve such issues. Several R&D projects are working on making very long fiber Bragg gratings to allow a single device to compensate for dispersion across the entire C-band without demultiplexing the signal. Scientists also are exploring combining fiber Bragg grating compensators with heaters, so that the compensation can be thermally tuned to make up for small, short-term dispersion changes in the fiber.

Meet the author

Breck Hitz is executive director of the Laser and Electro-Optics Manufacturers’ Association (LEOMA). The material in this article is taken from LEOMA’s short course, “Understanding Fiberoptics Technology.”