Page 96 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 75

incident at the surface of the mirror at a shallow angle is reflected across
the path of the beam. Interference occurs in the region of overlap of the
reflected and unreflected parts of the incident beams. The interferometer
is therefore extremely simple and easy to use. However, since half of the
incident beam is reflected, interference fringes appear in a region of length
equal to half the width of the beam. Secondly, since half the beam is folded
onto the other half, interference occurs, but the fringes may not be of high
quality. In the Lloyd arrangement, the folding action of the mirror limits
what is possible. It requires a source with a coherence length equal to at
least the path difference introduced by the fold in the beam. Although a
phase plate may be used over half the beam to compensate, experimentally
this is not straightforward. Ideally, the intensity profile and coherence
properties should be constant across the beam; otherwise, the fringe visi-
bility will be impaired and the imprinted grating will be nonuniform.
Since most sources tend to have a Gaussian beam profile, it is difficult
to produce fringes which have a uniform transverse profile. The grating
profile remains half-Gaussian, unless the beam is expanded to provide a
more uniform profile. The Gaussian intensity profile of the fringes intro-
duces a chirp in the imprinted grating. Diffractive effects at the edge of
the mirror may also cause a deterioration of the fringes closest to it.
The Lloyd arrangement with the single mirror is easy to tune. How-
ever, the fiber axis should be placed orthogonal to the plane of the mirror
so that the grating is not slanted.
Replacement of the mirror by a prism in the Lloyd arrangement
results in a more stable interferometer. This is shown in Figure 3.15. The
UV writing beam is now directed at the apex of a UV-transmitting silica
right-angled prism such that the beam is bisected, as in the Lloyd mirror.
Both halves of the UV beam are therefore refracted and no longer travel
in air paths, which can change with time.
The interferometer thus becomes intrinsically stable and was used
to produce the first photoinduced fiber Bragg gratings in the 1500-nm
wavelength window [51]. The interferometer has been used to demon-
strate a distributed-feedback dye laser. Interfering the pump beams in
the dye at the appropriate angle to create a Bragg reflector (absorption
grating) caused the dye to emit laser radiation [52].
The advantages and disadvantages of this interferometer are similar
to those of the Lloyd mirror. However, there are two further points of
interest. Owing to the shallow angle subtended by the UV beam on the
hypotenuse, the prism face has to be much larger than the beam width,