Abstract
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.
© 2008 IEEE
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