Page 246 - Fiber Bragg Gratings

References 223

in a phase mask. However, the symmetric stretch apodization method
combines the two and is flexibly applied to any length of grating. SAM
is excellent for long gratings, since the stretch is fixed by the period of
the grating, and therefore the strain changes inversely with length. This
is shown in Fig. 5.17. For a 1-mm grating, the fiber has to be strained by
—0.1%, which is acceptable, but drops to an insignificant 0.01% for a 10-
mm grating. It is, however, very important to ensure that the stretch is
symmetric; otherwise, the apodization will not be satisfactory. Once the
interferometer is aligned, then any length of grating may be apodized. If
the grating is not symmetrically placed between the stretchers, then the
piezoelectric movement can be adjusted to stretch one end of the fiber
more than the other, easily compensating for the misalignment. Finally,
certain types of fiber show a photosensitivity that is a function of applied
strain [27]. However, the strain used for the apodization of fibers is only
a small fraction of that reported in Ref. [27] and should not pose a problem
for gratings longer than a millimeter. For long moire grating formation,
the applied strain will also remain low enough to not to cause nonlinearity
in the photosensitivity.

References

1 "Caecilla," Encyclopedia Britannica, Micropaedia, Vol. 2, p. 715.
2 Hill K. O., "Aperiodic distributed-parameter waveguides for integrated optics,"
Appl. Opt. 13, 1853-1856 (1974).
3 Matsuhara M. and Hill K. O., "Optical-waveguide band-rejection niters: de-
sign," Appl. Opt. 13, 2886-2888 (1974).
4 Mizrahi V. and Sipe J. E., "Optical properties of photosensitive fiber phase
gratings," J Lightwave Technol. 11(10), 1513-1517 (1993).
5 Meltz G., Morey W. W., and Glenn W. H, "Formation of Bragg gratings in
optical fibres by transverse holographic method," Opt. Lett. 14(15), 823 (1989).
6 Kashyap R., McKee P. F., and Armes D., "UV written reflection grating struc-
tures in photosensitive optical fibres using phase-shifted phase-masks," Elec-
tron. Lett. 30(23), 1977-1978 (1994).
7 Pakulski G., Moore R., Maritan C., Shepard F., Fallahi M., Templeton I., and
Champion G., "Fused silica masks for printing uniform and phase adjusted
gratings for distributed feedback lasers," Appl. Phys. Lett. 62(3), 222 (1993).