Andrew Kearsley
Colin Webb
The application of fiber Bragg gratings (FBGs) in modern optical communication networks is extremely widespread. The versatility of this type of device is illustrated by its use for stabilizing the wavelength of the semiconductor lasers used in communications and in the fiber lasers themselves. These gratings also are used as components for wavelength-division multiplexers/demultiplexers, add/drop multiplexers, as compensators for fiber dispersion, as well as gain-flattening devices for erbium-doped fiber amplifiers (see table on page 42).
Bragg`s law
It was in 1912 that Nobel Laureate Sir Lawrence Bragg realized the possibility of interpreting, by a very simple mathematical expression, the complex pattern of spots formed by the diffraction of x-rays by the regular array of atoms making up a crystal.
Bragg envisioned the three-dimensional (3-D) diffraction process as a combination of partial reflections from imaginary planes containing sheets of atoms. If these regularly spaced planes could provide reflected beams with exactly the right phase delay, the individual beams would add up to produce a strong beam at the angle given by the familiar law of reflection. For example, if the beam of x-rays of wavelength l is incident normal to a set of planes spaced at distance L from one another in a medium of refractive index n, a strongly diffracted beam will occur in exactly the reverse direction to the incident beam when:
l = 2nL.
This is a special case of Bragg`s law. Unlike the case of two-dimensional (2-D) arrays such as the diffraction grating used for spectrometers, a 3-D periodic array selects from all the incident angles and wavelengths presented to it just the ones satisfying both the law of reflection and the wavelength condition simultaneously.
The relevance of Bragg`s law to the propagation of light in optical fibers was realized as the result of a serendipitous discovery by Hill and coworkers at the Communications Research Center in Canada in 1978.1 They discovered that an optical fiber connected to the output of an argon-ion laser operating at 488 nm (blue/green) developed the ability over time to reflect light at this wavelength. This they attributed to Bragg reflection by a set of grating planes along the length of the fiber. The planes of altered refractive index had been imprinted at regular spacing as a result of the natural photosensitivity of the fiber at positions corresponding to the nodes of the standing wave pattern set up by the small reflection from the cleaved far end of the fiber.
The next key step was taken by Lam and Garside, who showed that the photosensitive response at 488 nm was actually a two-photon effect.2 This pointed the way to the use of ultraviolet light in the 240- to 260-nm region as a much more rapid and effective way of inscribing the gratings. Meltz and coworkers took the final step toward modern methods of fiber Bragg grating (FBG) fabrication by using a striped pattern of UV light to illuminate the core of the fiber from the side to imprint the grating.3
The structure of a Bragg grating inscribed in the core of a typical single-mode silica fiber containing a small doping of germanium (Ge), possibly with another co-dopant such as tin (Sn), is shown in Figure 1. For a grating reflecting at a wavelength of 1540.5 nm in the optical communications band, the Bragg law gives the spacing of grating planes as 525 nm when the fiber has a refractive index of 1.467. Light of this particular wavelength traveling down the fiber is strongly reflected, while light at other wavelengths is more completely transmitted (see Fig. 2).4
Making gratings
In the production of FBGs the pattern of bands of alternating high and low intensity of the UV light can be obtained by splitting the beam from a UV laser into two and recombining them so that interference fringes occur in the region of overlap. The fiber, stripped of its protective plastic coating (which would otherwise block the UV radiation) is stretched across the interference region so that the stripes of UV light imprint the grating structure into the fiber. Photons of UV in the range of 240 to 260 nm can change the linkages of the Ge-oxygen bonds, causing permanent changes in the local refractive index.
Various interferometers can be used to produce the UV fringe pattern, some of which, such as the Lloyds mirror arrangement, make strong demands on the spatial coherence of the UV laser. This is because the right-hand edge of the UV beam before the interferometer mirror is overlapped by the left-hand edge of the beam in the interference pattern.
A much simpler arrangement is to use a phase mask. This is a plane-transmission diffraction grating specially designed to produce only two orders of diffraction, say +1 and -1. There is no need for the fiber to be held in close contact with the phase mask if the UV laser has a good coherence length. The interference pattern extends beyond the phase mask for the distance of the coherence length of the laser.
Three types of laser are used industrially for the production of FBGs. These are the frequency-doubled argon-ion laser, the krypton fluoride (KrF) excimer laser, and the frequency-doubled copper laser. In choosing a laser for industrial FBG production, the manufacturer would look for a stable UV output power, good beam pointing stability, long coherence length, and low running costs. Our experience with the frequency-doubled copper laser has shown that it can easily satisfy all of these requirements (see Fig. 3).
Stabilizing lasers
For stabilizing and controlling the wavelength of individual semiconductor laser chips to the precise tolerances needed for wavelength-division multiplexing (WDM), one end facet of the laser stripe is anti-reflection coated and pigtailed with a length of single-mode fiber incorporating an FBG. The laser cavity has a high Q-factor only at the precise wavelength defined by the FBG and thus its oscillation is locked to this wavelength value (see Fig. 4). This offers real advantages over earlier production techniques in which the Bragg grating was etched directly into the side of the semiconductor chip since the optical fiber pigtails are more easily reproducible than those etched directly onto the semiconductor chip.
For the very stringent demands of dense-wavelength division multiplexing (DWDM) the FBG offers real advantages over dielectric-coated band-pass thin-film filters. Thin-film filters can be used where the channel spacing is 200 GHz or greater, but for the close channel spacing of 100 GHz or lower, only the FBG provides filtering characteristics sharp enough to separate adjacent channels.
Fiber Bragg gratings also are ideal components for the construction of interferometer-based WDM devices.
In a balanced Mach-Zehnder multiplexer/demultiplexer arrangement a signal comprising N channels at wavelengths l1, ... lk, ... lN, arrives at Port 1 of the device containing two identical Bragg gratings resonant at lk (see Fig. 5). The phase shifts introduced by the FBGs are trimmed with UV light during manufacture to balance the two arms of the interferometer so that only the signal at lk exits at Port 2, while all other wavelengths exit at Port 4. This device also can be used as an "add" rather than "drop" device-in this case the signal at lk is inserted at Port 3 and all other wavelengths at Port 1. The combined output is available at Port 4.
The device was used to drop/insert a single wavelength channel from/into a multiple-wavelength transmission link with 100-GHz channel spacing at 1550 nm. Measurements showed that the extraction/coupling efficiency was 99.4%, with an excess loss < 0.5 dB and an adjacent channel isolation > 20 dB with a return loss > 23 dB.5
We have discussed only two of the applications for FBGs but these sturdy and versatile devices have already found many more uses, and will be manufactured in huge numbers in the coming revolution in optical communications.
REFERENCES
1. K. O. Hill et al., Appl. Phys. Lett., 32, 647 (1978).
2. D. K. W. Lam and B. K. Garside, Appl. Opt., 20, 440 (1981).
3. G. Meltz, W. W. Morey and W. H. Glenn, Opt. Lett., 14, 823 (1989).
4. L. Zhang and Y. Lin, Aston University, private communication (1999).
5. F. Bilodeau et a.l, IEEE Photon. Tech. Lett. 7, 388 (1995).
ANDREW KEARSLEY is managing director of Oxford Lasers Ltd, Abingdon Science Park, Barton Lane, Abingdon OX14 3YR, England; e-mail [email protected]. COLIN WEBB is chairman of Oxford Lasers Ltd and professor of laser physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU England; e-mail: [email protected]FIGURE 5. The Mach-Zehnder wavelength division multiplexer/demultiplexer is based on two identical FBGs that are resonant at (k. Wavelengths input at Port 1 emerge at Port 4 with the exception of (k, which has been "dropped" and emerges from Port 2.