Reconfigurable microwave photonics filters with negative coefficients based on a quantum dash mode-locked laser

We propose and demonstrate a reconfigurable microwave photonics (MWP) filter with negative coefficients using a quantum dash (QDash) mode-locked laser (MML). Thanks to the large number of comb lines $(\sim 50)$ generated by the QDash-MLL, we can easily tailor the optical frequency comb (OFC) spectrum and the filter responses. The negative taps are enabled through a balanced photodiode (BPD). In the experiment, we realized the flat-top and triangular responses. The proposed MWP filter shows a great tunability. Moreover, the readily packaged QDash-MLL with the large number of comb lines is a promising OFC source for developing integrated and reconfigurable MWP applications.

I. Introduction

Microwave photonics (MWP) filters are key building blocks in radio frequency (RF) signal processing [1]. The implementation of MWP filters usually involves an optical frequency comb (OFC) source (to provide multiple taps) in conjunction with a broadband dispersive medium (to set the time delay between adjacent taps). For example, a reconfigurable MWP filter was demonstrated by adjusting the power levels of a laser source array and using a linearly chirped fiber Bragg grating as the dispersive medium [1]. However, the coefficient of each tap is positive so that this filter cannot achieve the passband responses that needs negative coefficients [1]. To realize the passband responses, a phase shift must be implemented on part of the taps (to provide negative coefficients) [2,3,4]. For example, we can consider the balanced photodetector (BPD) that contains a phase shift between the two channels [3] or two electro-optic (EO) modulators working at the positive or the negative linear slope of the signal’s modulation transfer function [4].