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Optical Phase/Frequency Demodulation Using Polarization-Maintaining Fiber Bragg Gratings

Conventional techniques for demodulating phase/frequency-modulated (PM/FM) optical signals either require a sophisticated frequency-stable laser or employ a discriminator with poor characteristics. Here, we present a simple, compact, and robust technique featuring high linearity over a wide bandwidth and low background noise. Our technique exploits the reflection characteristics of fiber Bragg gratings written in polarization-maintaining fibers to create a frequency discriminator, which is able to convert PM/FM signals into intensity-modulated (IM) signals. A simple theoretical analysis is presented to highlight the advantages of the scheme. Experimentally, the transfer characteristics of the demodulator are characterized, yielding a linear FM-to-IM conversion slope of 1.238 mV/Hz in a 0.2-nm (∼25 GHz) bandwidth. Highly linear operation up to an FM-modulation index of 1.2 has been achieved. Background-free operation is shown by demonstrating negligible carrier leakage and a 10-dB IM rejection. Finally, high fidelity in demodulating arbitrary PM/FM optical signals is demonstrated with a correlation coefficient of 0.99 and a confidence of almost 100%. This simple yet efficient technique can potentially find applications in optical communications, microwave photonics and radar/lidar technologies.