Photonic interferometry based optical carrier cancellation for optical interference noise reduction

In the coherent optical access network, two major optical interference noise (OIN) including Rayleigh back scattering (RBS) at fiber link and optical beating noise (OBI) at the uplink receiver need to considered. RBS noise is a nonlinearly back-scattered component due to non-uniformity in single-fiber loopback system when reflective-type modulator is used as an uplink modulator. Influence of this noise would be stronger for high power of the optical carrier and this carrier-induced reflecting component can beat with optical carrier at local oscillator (LO) due to square-law detection in uplink coherent receiver. In addition, OBI noise is unwanted beating term due to optical signal carrier-to-LO carrier beat by the same receiving principle. Thus, alive optical carrier causes OIN of both noises enabling to inevitable obstacles for uplink signal. Moreover, OIN becomes more severe as increasing fiber input power and transmission distance which are important requirements for the next generation passive optical network (PON). In this paper, a novel photonic interferometry-based optical carrier cancellation (OCC) technique is proposed to mitigate these OIN for multiple access in coherent PON. Our goal is to cancel out the optical carrier at optical network unit (ONU) which is main cause of OIN. Basic principle of the proposed technique is making a photonic interferometry consisted of two optical interferometric paths and generating a destructive interference through phase difference of 180 degrees between two paths. In-line optical phase shifter was used considering its advantages of cost-effectiveness and easy bias control properties. In addition, power imbalance and fluctuation of the state of polarization between two paths were minimized by using a variable optical attenuator and simple polarization control at ONU. Furthermore, transparency for modulation format was possible because the proposed technique is applied at ONU after uplink signal modulation. Through experimental demonstration for the proposed technique, optical carrier was cancelled out up to about 30 dB and this cancellation facilitates OIN mitigation at the overall uplink signal band. Under the severely OIN-induced condition of the high fiber input power more than minimally required fiber launch power, a remarkable performance improvement in terms of the noise floor and eye-pattern were verified as a proof of concept. The proposed technique shows a good tolerance to OIN-induced uplink transmission by taking advantages of the low-complexity, cost-effectiveness and transparency for modulation format of uplink signal.