Page 78 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 57

The fiber is held at the intersection of the beams. This method was
originally successfully used to write gratings at visible wavelengths. The
interferometer is ideal for single-pulse writing of short gratings, and great
care has to be taken in the design of the optical mounts. Mechanical
vibrations and the inherently long path lengths in air can cause the
quality of the interferogram to change over a period of time, limiting
its application to short exposures. For low-coherence sources, the path
difference between the two interfering beams must be equalized; a simple
method is to introduce a mirror blank in one arm to compensate for the
path imbalance imposed by the beam splitter, as shown in Fig. 3.1. Note
that the in arriving at the fiber, the beam that is transmitted through
the beam splitter undergoes a 180° rotation so that they have different
spatial profiles. This is an important factor for spatially incoherent beams.
The interferometer shown in Fig. 3.1 has several beams paths in open
air. It is important that these are shielded from turbulence, since the
interference fringes formed at the fiber can drift if the paths of the two
beams change during the inscription time. As is common with all holo-
graphic arrangements, it is not sensible to mount mirrors, beam splitters,
or the fiber on flimsy platforms prone to disturbance, such as tall 10-mm
diameter mounting posts. The interferometer needs to be built on a sturdy
base, with stable optical mounts. This is especially true in cases that
require long (minutes to hours) exposures. It is common practice to enclose
the entire interferometer within a Perspex housing, which allows visual
and physical access to the setup, at the same time protecting the interfer-
ometer from constant path-length variations and the operator from acci-
dental exposure to UV radiation. Extreme care needs to be taken to
minimize exposure of personnel to high-energy UV radiation or long-term
exposure to low-power radiation. Adhering to safe operating practices is
essential when using UV radiation.
In principle, a diffraction grating used in reflection can replace the
50% beam splitter shown in Fig. 3.1. In this interferometer, two coherent
beams are required, so that reflection from a diffraction grating to divide
the input UV beam into two is equally feasible. However, a simpler compo-
nent, the transmission phase-grating, otherwise known as the phase
mask, is better suited to this application.

3.1.2 The phase mask
A major step toward easier inscription of fiber gratings was made possible
by the application of the phase mask as a component of the interferometer.