  We report a very high precision interferometric sensor with resolution up to
~{\lambda}/1024, exploiting hollow photonic bandgap waveguide-based geometry
for the first time. Here sensing has been measured by a complete switching in
the direction of the outgoing beam, owing to transverse momentum oscillation
phenomena. Using a 1.32 {\mu}m source and core-width of 7.25 {\mu}m, a complete
switching cycle is obtained even due to a small change of ~1 nm in the
core-width. Using hollow-core photonic bandgap waveguide, Talbot effect,
revivals of the initial phase, oscillation in the transverse momentum along
with multi-mode interference served as the backbone of the design. The
ultra-sensitive multi-mode interferometric sensor based on photonic crystals
will certainly open up a paradigm shift in interferometer-based sensing
technologies toward device-level applications in photonic sensing/switching and
related precision measurement systems.
