A slight stress change in the Bragg fiber grating at 6nm
If the strength of the index modulation in a grating is constant over some length, and suddenly drops to zero outside that range, the reflection spectrum exhibits side lobes, in particular if the peak reflectance is high (see Figure 2). Some fiber Bragg gratings are fabricated such that the planes of constant refractive index are not normal to the fiber axis, as usual, but are tilted against the axis by some angle (often a few degrees). If that tilt is strong enough, the coupling to backward core modes may become quite weak; instead, one has a coupling of core modes to cladding mo. As with silica fibers, one usually uses ultraviolet light, but the physical mechanisms are somewhat different.
Read More
