Phase-Locked Coherent Demodulator With Feedback and Sampling for Optically Phase-Modulated Microwave Links

A novel phase-locked coherent optical phase demodulator with feedback and sampling is presented and investigated for high-linearity microwave photonic links. The overall receiver concept is novel in terms that we use feedback loop for linear phase demodulation and optical sampling to achieve operation at high RF signal frequencies.

The phase-locked demodulator is to be used for RF signal demodulation of optically phase-modulated analog links. We aim at operating the demodulator for short-range applications since the demodulator only recovers the phase. A new time-domain numerical model is developed and the calculated results are in good agreement with measurements.

The effect of amplitude and timing jitter associated with the optical pulse source is also taken into account. Stochastic Euler scheme is used to solve stochastic differential equations associated with amplitude and timing jitter. Using the model, we investigate how loop gain, tracking phase-modulator nonlinearities and amplitude modulation influence the signal-to-intermodulation ratio (SIR) of the demodulated signal.

Furthermore, in order to alleviate nonlinearities associated with the tracking phase modulator, we report on a novel cancellation technique. The proposed cancellation technique is input RF signal power and frequency independent leading to a significant increase in dynamic range of the coherent demodulator.

This technique demonstrates that large values of the signal-to-intermodulation ratio of the demodulated signal can be obtained even though the tracking phase modulator is fairly nonlinear, and thereby relaxing the linearity requirements for the tracking phase modulator. The demodulator is capable of operating at high frequencies, by using optical sampling to downconvert the high-frequency input RF signal to the frequency range of the baseband loop.

Using the model the effect of optical sampling on the signal demodulation is investigated. The simulation results show that the operation of the sampling demodulator resembles the operation of the baseband demodulator for very short optical pulses (<2 ps). We also investigate how signal-to-noise-ratio of the demodulator is affected by timing and amplitude jitter of the pulsed optical source.

Finally, experimental demonstration of the sampling loop using the phase-locked demodulator is presented. We show that an improvement of 14.1 dB in spur-free-dynamic-range is obtained using the proposed phase-locked coherent demodulator compared to the open loop sampling.