Page 285 - Fiber Bragg Gratings

262 Chapter 6 Fiber Grating Band-pass Filters

insertion loss [43,52]. For proper operation, the output coupler needs to
be balanced, requiring trimming on the RHSs of the gratings.
Indeed, it is simple to observe that the device can be used as an add
multiplexer at the dropped wavelength if the same wavelength is locally
injected at the port marked "Insert" (RHS, bottom). This wavelength
will be routed through to the Add port performing the basic Add-Drop
multiplexer function. Many of these MZIs may be cascaded to perform a
multiple-wavelength band-pass function.
Cullen et al. [52] demonstrated a compact GMZI-BPF in fiber form.
The device, based on two 50:50 splitting fused fiber couplers fabricated
in boron-germania codoped fibers (Core-cladding index difference Arc =
3
7 X 10~ and core diameter of 7 /um), with 1-meter tails. The two pieces
of fiber were first tapered and fused to a constant diameter of 100 yum
over a length of 20 mm. A 3-dB coupler was formed by further tapering
one end of the fused region, until the desired splitting ratio of 50% was
achieved. When the second coupler is made, if the path lengths in the
two arms are identical, 100% of the light will appear in the crossed state,
i.e., in port 4 when port 1 is excited. Allowing for fabrication loss and
slight imbalance, between 95 and 99% of the light was available at port
4 after the second coupler was fabricated under the same conditions. The
finished device had ~5 mm of space in the parallel fiber section between
the couplers for the inscription of the gratings and for UV trimming. The
advantage of such a structure is the relative stability of the MZI, since
the couplers and the fused fiber regions are so close together. Any ambient
temperature fluctuations affect both fibers equally. This was established
by a measured change in the output power of the MZI of <0.05 dB over
a temperature excursion of —20 to +60°C, with a wavelength window of
40 nm. It is necessary to mount the fibers on a mechanical support in
order to proceed with grating inscription. A silica microscope slide is ideal
for this application, since it enables the device to be supported, handled,
aligned in the interferometer. For this device, gratings of 3-mm length
were written in both arms under identical conditions using an intracavity
CW frequency-doubled argon ion laser. The grating reflectivity can be
monitored accurately by the size of the dip from the transmitted level at
a few nanometers on the long-wavelength side of the Bragg wavelength
(to avoid the radiation loss region on the blue side, as well as the side
lobes on the red side of the grating spectrum). Once the two gratings
are written using identical conditions, the device can be balanced by
examining the reflection in port 1. Ideally, either a coupler or a circulator