Page 106 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 85

times longer. However, shadow masks are easily available, and the inscrip-
tion requires simple contact printing as with the phase mask, but without
the complexity of interferometry. Point-by-point writing is also possible.
Long-period amplitude masks have been patterned on dielectric mir-
rors to reduce the problem of optical damage with excimer lasers. In this
application, a dielectric mirror coated with photoresist was first exposed
to an amplitude pattern using a UV laser. The photoresist was developed
to expose the dielectric mirror, which was then etched in 5% HF solution
in deionized water. The resultant mirror had stripes at the required
period for the LPG. Exposure through this mirror only allows the UV
radiation to be transmitted through the regions where the mirror has
been etched away. These mirrors were shown to withstand 200 mJ/cm 2
per pulse over several pulses, thus making them better suited for use
than chrome-coated silica masks with an average damage threshold of
2
-50-100 mJ/cm [131].

3.1.12 Ultralong-fiber gratings

To circumvent the limitations of the finite length of the phase mask,
several techniques have been proposed to fabricate gratings of arbitrary
length >200 mm [49, 73-75]. The simplest method is to sequentially
inscribe gratings in a fiber from a phase mask of length L, to result in a
grating with a length equal to the number of sequential inscriptions X L
[74]. This is a powerful technique, which has special applications in the
fabrication of long chirped gratings and is discussed in Section 3.1.15.
A technique based on the principle of inscribing small, more elemen-
tary gratings to create a longer one has also been reported [73]. The
principle of inscription may be understood as follows: A short (4 mm)
interference pattern is printed periodically in a continuously but slowly
moving fiber. Using a pulsed laser (20-ns pulses), a 4-mm long section is
imprinted in the fiber at any one time. The velocity of the fiber is such
that within the pulse width, it may be regarded as being stationary. When
the fiber has moved a few integral numbers of grating periods, a second
pulse arrives, imprinting yet another grating partially overlapped with
the previous grating but adding a few extra periods to the length. A
mini-Michelson interferometer operating at 633 nm is attached to the
moving fiber platform to track its position relative to the interference
fringes. The latter is undertaken by the use of serrodyne control [73]. The
resulting gratings have the narrowest reported bandwidths of 0.0075 nm,