  High efficiency and a compact footprint are desired properties for
electro-optic modulators. In this paper, we propose, theoretically investigate
and experimentally demonstrate a recirculating phase modulator, which increases
the modulation efficiency by modulating the optical field several times in a
non-resonant waveguide structure. The 'recycling' of light is achieved by
looping the optical path that exits the phase modulator back and coupling it to
a higher order waveguide mode, which then repeats its passage through the phase
modulator. By looping the light back twice, we were able to demonstrate a
recirculating phase modulator that requires nine times lower power to generate
the same modulation index of a single pass phase modulator. This approach of
modulation efficiency enhancement is promising for the design of advanced
tunable electro optical frequency comb generators and other electro-optical
devices with defined operational frequency bandwidths.
