Page 287 - Fiber Bragg Gratings

264 Chapter 6 Fiber Grating Band-pass Filters

to switch the OADM. The parameters that can be altered are temperature
[40,41] or strain to alter the phase difference between the arms of the
MZI, or the Bragg wavelength of the gratings. To alter output state, the
phase in one arm can be tuned reliably and requires a 10-mm length of
fiber to be heated by~13°C. With strain, a fiber extension of l/4A Bragg is
required to switch the OADM.
Temperature tuning of the gratings requires a 65°C change to shift
the fiber Bragg grating wavelength by —0.8 nm. If the channels are
spaced 1.6 nm apart, then a channel may be dropped by tuning the Bragg
wavelength to match the channel wavelength and deselected by detuning.
Tuning both gratings simultaneously by either strain or temperature
while maintaining interferometric stability is not easy, so, this device
requires careful engineering.
The GMZI-BPF can be used as an optical add-drop multiplexer
(OADM). If the phase in one of the arms can be controlled actively, e.g.,
by a piezoelectric stretcher, then a wavelength may be either switched to
the drop port or reflected back to the source. The insert function is operated
in a similar manner by the use of a second piezoelectric stretcher on the
RHS of one of the gratings in the MZI. The "drop" and "add" ports have
fiber-coupler taps to monitor the state of the output and to control the
piezoelectric stretchers to switch the GMZI-BPF using phase-locked loops
[40]. A disadvantage of this scheme is that it always blocks the transmis-
sion of the channel, whether it is dropped or not, and it must be reinserted
for forward transmission.
Mizuochi and Kitayama [57] combined a set of GMZI-BPFs to perform
a two wavelength OADM function, which is shown in Fig. 6.31. The basic
element of the device is a double GMZI-BPF with four identical gratings
in the two MZIs, as well as an additional highly reflecting grating as an
"isolator" between the two MZIs, shown in the top half of Fig. 6.31. The
function of the additional grating is to prevent light at the grating Bragg
wavelength from crossing from one MZI to the other, increasing isolation.
This is particularly important because light inserted into the OADM can
cross from one MZI to the other (from left to right and the reverse) to
cause in-band coherent beat noise [58] (see Section 6.5). Light arriving
from the left in the top half at port Al is dropped at the Bragg
wavelength A £ and routed to the second GMZI-BPF in the lower half
of the figure, containing gratings at another wavelength, A,. The dropped
wavelength, A t therefore appears at Cl. Similarly, light injected in C2
at the wavelength A; uses the "add" part of the top GMZI-BPF on the