Page 440 - Fiber Bragg Gratings

9.2 Perfect Bragg gratings 417

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ized and apodized (cos profile) gratings. The unapodized grating is nomi-
nally a 100% reflection grating, and the apodized one has identical length
and refractive index modulation. The effect of apodization is to reduce the
effective length to approximately L/2. As a result, the FWFZ bandwidth
approximately matches the second zeroes of the unapodized grating. Note
that the reflectivity is also reduced (~halved). To generate an apodized
grating with the same bandwidth, the length has to be approximately
doubled, and the coupling constant has to be adjusted, so that an 8-
mm-long raised cosine apodized grating will have the same approximate
bandwidth and reflectivity.
In order to resolve the reduced side lobes for the apodized grating,
the spectrum analyzer linewidth should be selected to remove artifacts
and a false noise floor.

9.2 Perfect Bragg gratings

It is possible to make very high-quality uniform-period Bragg gratings.
This is because optical fiber has very uniform properties. The theoretically
calculated reflection, along with the measured spectrum, of a 30-mm-long
grating is shown in Fig. 9.10. The grating was fabricated by scanning a
phase mask with a UV beam [31.

Figure 9.10: Measured and computed reflection spectrum of a 29.5-mm-long
fiber Bragg grating, produced by the scanned phase-mask technique [4]. The
uniformity of the grating is indicated by the close agreement between the zeroes
of the theoretical and measured response.