Passive elimination of polarization sensitivity of fiber-optic microwave modulators

The polarization sensitivity of a bidirectional fiber-optic modulator is passively eliminated by incorporating the modulator in an orthoconjugate loop mirror or in an in-line fiber loop. We describe and analyze these fiber loop configurations which allow remote interrogation of polarization-sensitive devices with either one or two conventional singlemode fibers. For bulk single-drive modulators, the polarization sensitivity is reduced to ±0.15 dB out to 2 GHz. By utilizing a balanced, dual-drive feed to a bidirectional traveling-wave modulator, residual polarization sensitivity of ±2 dB is demonstrated for ultrawideband operation to 25 GHz and ±0.3 dB for narrowbands