We propose and demonstrate, theoretically and experimentally, a novel achromatic optical phase shiftermodulator based on a frequency-domain optical delay line configured to maintain zero group delay as variable phase delay is generated by means of tilting a mirror. Compared with previously reported phase shiftermodulators, e.g., based on the Pancharatnam (geometric) phase, our device is high speed and polarization insensitive and produces a large, bounded phase delay that, uniquely, is

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... e mapped to a measurable parameter, the tilt angle.