Parity Time Symmetric Optoelectronic Oscillator With Low Phase Noise and High Sidemode Suppression Ratio

This paper presents the investigation of the phase noise of a parity-time (PT) symmetric optoelectronic oscillator (OEO). The finding leads to the development of a low phase noise and high sidemode suppression PT symmetric OEO. The high-performance PT symmetric OEO has the same structure as the conventional single-loop OEO. PT symmetry is achieved by higher order modulation in a Mach Zehnder modulator and controlling the gain of an RF amplifier inside the OEO loop to ensure the magnitude of the open loop gains of the gain and loss loop in decibels are the same. Simulation results show around 20 dB phase noise suppression can be achieved by simply using a low relative intensity noise (RIN) laser diode and a low phase noise RF amplifier in the OEO loop. This avoids using a special technique or a long fibre coil for phase noise suppression. This allows the PT symmetric OEO to have a simple and compact structure while having a high phase noise performance. An OEO with a short loop length also has the advantage of large sidemode suppression. The performance of the PT symmetric OEO is evaluated experimentally. Experimental results show a 10 GHz RF signal generated by the PT symmetric OEO implemented by a single physical loop with a 1 km long loop length has a phase noise of -139 dBc/Hz at a 10 kHz offset frequency and a sidemode suppression ratio of 74 dB. Over an hour measurement period, the oscillation mode has less than 8.8 kHz frequency drift and is always 68 dB above the sidemodes. Both the phase noise and sidemode suppression performance are around 20 dB better than the reported PT symmetric OEOs that have more complex structures and longer loop lengths. Results also show suppressing the phase noise by optimising the system components used in the OEO loop reduces the sidemode amplitudes.