
modal birefringence with wavelength, which contributes to the second term of
group birefringence in Eqn (15.13) [42]. The modal birefringence of this type of
fiber is 10
3
, one order of magnitude lower than the group birefringence, which is
quite different from conventional fibers. It is also notable that despite the high
birefringence, the polarization-maintaining properties of such fibers have not yet
been demonstrated because of the involvement of surface modes [43, 44]. How-
ever, with a recently developed technique based on a resonant coupling mechan-
ism [45], the higher-order modes may be suppressed to achieve high extinction
ratio and polarization-maintaini ...