Chirped in-fiber Bragg gratings for compensation

It is well known that chirped (aperiodic) refractiveindex Bragg gratings can be used for compensating for the dispersion of optical fiber waveguides.' When used in reflection mode (in contrast to transmission mode), such Bragg gratings exhibit large dispersion. As well, the sign of the dispersion in reflection mode is easily controlled. Together these two characteristics suggest the use of chirped Bragg gratings for dispersion compensation in optical-fiber systems. Theoretical calculations show that a relatively short Bragg grating device can compensate, over a useful bandwidth, for the dispersion of an optical fiber that is tens of kilometers long. In this Letter we discuss the optical characteristics of low-excess-loss (0.1-dB) linearly chirped Bragg gratings photoimprinted in the core of standard telecommunications fiber that has been H2 loaded to enhance its photosensitive response.3 The gratings are chirped in the sense that the optical pitch (i.e., effective refractive index neff X grating pitch A) varies linearly along the length of the structure. Our Bragg grating devices were designed to compensate, within an optical bandwidth approximately 0.4 nm wide, for the dispersion exhibited at 1549 nm [-19 ps/(nmkm)] by a 20-km length of standard telecommunication optical fiber optimized for use at 1300 nm (zero-dispersion wavelength). Further development of the technique should easily permit compensation of dispersion over narrower bandwidths (matched to 10-Gbit/s channels, for example) and significantly longer fiber strands. To make the devices, we use a novel doubleexposure photoimprinting method for precisely controlling the chirp. A schematic of the technique is shown in Fig. 1. In the first exposure, actinic radiation from either a KrF (249-nm) or an ArF (193-nm) excimer laser beam irradiates a segment of an optical fiber screened by a mask that is translated along the length of the fiber at constant velocity (see Fig. 1, top). The motion of the mask increases the fraction of the fiber segment that is exposed to excimer light, thereby varying linearly along the fiber length the total irradiation dose that is received by the fiber segment. Since the photoinduced refractive-index change is a function of the received dose, the first exposure photoinduces an effective refractive index, neff, that increases linearly along the length of the fiber segment. In the second exposure, we photoimprint a Bragg grating in the same segment of optical fiber by passing the actinic radiation through a zero-order nulled phase mask4 (see Fig. 1, bottom). The second exposure patterns a uniform (spatially periodic) refractive-index Bragg grating into the core of the waveguide. Although the grating is spatially periodic, its effective optical pitch is chirped because of the photoinduced index change produced in the first exposure. Using the double-exposure photoimprinting technique, we fabricated a 1.5-cm-long Bragg grating with a center resonant wavelength AO of 1549 nm and linear chirp of 0.03%. The chirped Bragg grating was fabricated in Corning SMF-28 optical fiber that had been photosensitized by H2 loading before irradiation. The measured reflectivity of the chirped grating as a function of offset from the center frequency is shown in Fig. 2. Using coupled-mode equations, we have calculated theoretically the reflection response of a grating with a linear chirp; the results are also plotted in Fig. 2. We fitted the theoretical curve to the experimental curve, using KL = 2, where K is the grating coupling coefficient and L is the grating length. The close agreement between the theoretical and the experimental curves suggests that the double-exposure chirping technique provides good control of the Bragg grating chirp. As an aid in the design of linearly chirped gratings for dispersion compensation, we note the following relationship between the percent chirp and the other parameters of the Bragg grating. For weak band-stop reflectivity (R < 50%) the full width at quarter-maximum of the spectral response curve of a linearly chirped Bragg grating is given by the percent chirp multiplied by the center Bragg resonant wavelength (AO) of the device. An alterna-