  Waveguide-based interferometric circuits are widely employed in optical
communications, sensing and computing applications. In particular, glass-based
devices are appealing due to the transparency and bio-compatibility of this
substrate, or where low-loss interfacing with fiber networks is required.
However, fast electro-optic phase modulation is hard to achieve in glass
materials. Here, we demonstrate an optical phase and intensity modulator in
glass, working in the megahertz range. This modulator exploits the elasto-optic
effect inside a mechanical microstructure, brought to oscillation at resonance,
and is entirely realized by femtosecond laser micromachining. In detail, we
demonstrate 23-dB optical intensity modulation at 1.17 MHz, with an internal
optical loss of the phase-modulator component as low as 0.04 dB.
