Page 101 - Fiber Bragg Gratings

80 Chapter 3 Fabrication of Bragg Gratings

through the phase mask with the ± 1 orders, but also due to the formation
of a damage grating which is no longer sinusoidal in amplitude [120].

3.1.8 Point-by-point writing

The period of a reflection grating operating at 1.5 /mi is —0.5 /mi, as per
Eq. (3.1.3). Since a diffraction-limited spot size of radiation at 244 nm is
—0.25 /mi, it is possible in principle to form a periodic refractive index
grating by illuminating a single spot at a time using a point-by-point
writing scheme. Technically, using positioning sensors linked to an inter-
ferometer, a grating of such periods can be written. This is only suitable for
short gratings, since it is difficult to control translation stage movement
accurately enough to make point-by-point writing of a first-order grating
routinely practical. Other methods, including the use of a phase mask or
the multiple-printing in-fiber-grating scheme, are better suited to writing
long first-order gratings. However, high-quality high-order gratings have
been demonstrated for N = 3 and 5 [57]. While excellent reflection gratings
can be written using other schemes, point-by-point writing is extremely
useful for fabricating gratings of long periods (>10 /mi). These gratings
couple light from one propagating polarization mode to another in the
backward [58] or forward direction as in a rocking filter [59-61] (discussed
in Chapters 4 and 6), to forward-propagating radiation modes (also see
Chapter 4), or from one guided mode to another [62-64]. Figure 3.18
shows the technique used for point-by-point writing of reflection gratings
and polarization couplers. For the simple reflection grating, the fiber is
illuminated by a tightly focused spot through a mask for the required
duration before being translated by a motorized micropositioner for the
next illumination. In this way, a reflection grating of any order may be
written. Naturally, the method benefits from the use of a pulsed laser,
since the motion of the fiber can be stepped without the need to control
the operation of the laser as well. However, the method is most useful for
long-period gratings, which do not require such a demanding positional
accuracy.

3.1.9 Gratings for mode and polarization conversion
Polarization mode converters may be fabricated in birefringent fibers
using this scheme. In this case, it is necessary to orient the birefringent
axes of the fiber at 45° to the illuminating beam. Two methods have been