Page 284 - Fiber Bragg Gratings

6A The Mach-Zehnder interferometer band-pass filter 261

Figure 6.30: The Mach-Zehnder interferometer used as a band-pass filter.
UV trimming of the paths has been shown to be a powerful tool for rebalancing
the paths such that 100% of the light reflected from the gratings appears at the
output port on the left [53,43]. By adjusting the phase difference at the coupler
beyond the gratings, the output may be directed to either output port of the
coupler.

phase adjustment between the guides to balance the interferometer is
difficult without active control.
The first demonstration of a working band-pass device using the
principle of the Mach-Zehnder interferometer (MZI) with two identical
UV written gratings was in planar-Ge:silica waveguide form [53]. The
device was an MZI with overclad ridge waveguides, which had been photo-
sensitized using hot-hydrogen treatment [51]. "UV trimming" was used to
balance the interferometer after the gratings were written, demonstrating
this powerful technique also for the first time [53], This is shown Fig.
6.30. "UV trimming" relies on photoinduced change in the refractive index
to adjust the optical path-length difference. The 6-dB insertion loss for a
single-grating band-pass filter was overcome and reducer to —1.34 dB for
the fiber pigtailed device, by UV trimming; much of it comprised coupling
and intrinsic waveguide loss. The fiber gratings had a reflectivity of ~15
dB each and were well matched in wavelength. Approximately 10% of
the light was reflected into the input port. Although the insertion loss of
this MZI-BPF was not as low as later devices, the planar MZI has the
advantage of being extremely stable to environmental effects. Since the
demonstration, several groups used this scheme of UV trimming in fiber-
based MZIs to demonstrate band-pass filters with better extinction and