Out-of-band suppression improved tunable microwave photonic Hilbert transformer based on optical spectral shaping

Abstract

An out-of-band suppression improved microwave photonic Hilbert transformer (MPHT) with adjustable bandwidth and central frequency based on optical spectral shaping is theoretically analyzed and experimentally demonstrated. The MPHT consists of a broadband optical source, a Mach–Zehnder interferometer (MZI), a dispersion compensating fiber and a photodetector. The MZI incorporates an intensity modulator in one arm and a programmable optical filter (POF) and a variable optical delay line in the other. The frequency response of the MPHT is determined by the Fourier transformation of the optical spectral shaping response of the POF. By tailoring the optical phase profile, a π phase shift can be introduced into the center of the MPHT. By shaping the optical amplitude profile, a MPHT with adjustable bandwidth and improved sidelobe suppression can be achieved. In the experiments, MPHTs with bandwidths varying from 100 MHz to 1 GHz and the central frequency varying from 2.5 GHz to 16.5 GHz are demonstrated. The out-of-band suppression is more than 20 dB, which is the best result ever reported for a MPHT. The impulse response of the MPHT is verified and the result proves that the proposed MPHT can be applied to microwave analytic signal processing systems.