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Optical signal processing based on an optoelectronic oscillator employing a polarization-dependent phase modulator

We propose and experimentally validate a compact and flexible structure of optical signal processing employing a polarization-dependent phase modulator (PM) based optoelectronic oscillator (OEO). The key point of the proposed design is to make good use of the polarization-dependent feature of the LiNbO3 crystal of the PM to realize special phase modulation to intensity modulation conversion. The phase modulation indexes along the principal direction and orthogonal direction of the PM are different. Thus an intensity signal can be obtained after the lights in the two orthogonal directions are combined at a polarization (Pol) after the PM. A polarization controller (PC) is inserted between the PM and Pol to introduce a static phase shift. One part of the special phase modulated data signal is introduced to the OEO loop to form a stable injected oscillation. While the other part is sent to a branch to do serial-to-parallel conversion or modulation format conversion. Non return-to-zero (NRZ) to return-to-zero (RZ) format conversion with tunable duty cycle which can be adjusted by tuning the phase shift introduced by the PC continuously is achieved in the branch. The receiver sensitivity of the converted RZ signal at the bit error rate (BER) of 10−9 has an improvement of about 2 dB compared to the original NRZ signal.