Page 286 - Fiber Bragg Gratings

6.4 The Mack-Zehnder interferometer band-pass filter 263

may be used to monitor the reflection while optical trimming is under-
taken in the region closest to port 1, to minimize the reflection at the
Bragg wavelength [53]. Lastly, the output ports need to be balanced by
trimming on the far side of the gratings. The power on the long-wavelength
side of the Bragg wavelength can be steered to either port 3 or 4 as
necessary. In their device Cullen et al. [52] reported an insertion loss of
only 0.5 dB, and 0.35 dB for a standard fiber device. With 99% reflectors,
strong radiation loss was noted on the blue side of the Bragg wavelength.
The long-term stability of the GMZI-BPF depends on the stability of
the substrate and the uniformity of the stress and temperature gradients.
Silica for the substrate is a good choice since it is better matched to the
properties of the fiber. However, the Bragg wavelength in Ge: doped fiber
shifts by —16 pm/°C. Compensation of the drift can be countered by the
use of packaging with an effective negative thermal expansion coefficient
[54-56,47,48]. Athermalization using a /3-eucrytite glass substrate has
been shown to reduce the temperature sensitivity of fiber Bragg gratings
to ~0.0022 nm/°C, and the drift in the wavelength was measured to be
0.02 nm after thermal cycling 60 times over a temperature excursion from
-40°C to +85°C [56]. The glass is based on a stuffed derivative of the
crystalline phase, /3-quartz (LiC^A^Os^SiC^). Normally, this phase has
a large negative thermal expansion coefficient along the c-axis but forms
a poor glass. Nucleated with TiC^ and adjusting the glass composition to
form /3-eucrytite results in a stable glass melt. Heat treatment at above
1200°C forms the glass ceramic with the appropriate crystalline micro-
structure for a negative thermal expansion coefficient.
The principle of athermalization of delay through a fiber using a
tube of oriented liquid-crystalline polymer [54] has also been used to
athermalize gratings. A measured dA Eragg/dT of 0.01 nm/°C for the un-
compensated FBG was reduced to 0.13 nm/100°C after compensation,
with no significant hysteresis during the temperature cycling from -40
to +80 °C [47]. Use of such materials as substrates for the GMZI-BP
should result in robust devices.

6.4.1 Optical add-drop multiplexers based on the
GMZI-BPF
Changing the phase in one of the arms of the MZI between the coupler
and the gratings routes the reflected light to either the "drop" or the
"input" port. Any method that can reliably alter the phase can be used