Page 320 - Fiber Bragg Gratings

6.10 Mode converters 297

Figure 6.52: The transmission characteristics of the polarization coupling
Mach-Zehnder interferometer (Courtesy S. Kannellopoulos, from: Kanellopoulos
S. E., Handerek V. A. and Rogers A. J., "Compact Mach-Zehnder fibre interferome-
ter incorporating photo-induced gratings in elliptical core fibers," Opt. Lett. 18(12),
1013-1015, 1993. [106]).

ences the output state, from which a phase sensitivity of—0.22 rad/°C was
calculated over a temperature range of 150°C.

6.10 Mode converters

Mode converters work on a principle similar to that of the polarization
converter. There are several types of mode-converting filters: those that
couple one guided mode to another co- or counterpropagating guided mode,
as well as those which couple a guided (or cladding) mode to a co- or
counterpropagating cladding mode (or guided mode). Coupling of a guided
mode to a co- or counterpropagating cladding mode has been partially
covered in Section 6.8 on side-tap and long-period grating band-pass fil-
ters, and also in Chapter 4. Guided-guided co- and counterpropagating
mode coupling devices will be discussed in this section. Other devices
based on concatenated couplers for mode reconversion will also be dis-
cussed.

6.10.1 Guided-mode intermodal couplers

The theory of guided-mode coupling to both copropagating and counter-
propagating guided/radiation modes can be found Chapter 4. Equations