I. Introduction
Optical processing of radio-frequency (RF) signals in the optical domain is an interesting alternative to traditional RF filters since microwave photonic filters are immune to electromagnetic interference, the signal losses are low and independent of frequency, they are lightweight and feature high processing bandwidth. Several approaches have been proposed to implement transversal filters. Some of the filter implementations are restricted to positive taps because the coefficients of the optical samples are proportional to the optical power. However, the implementation of negative coefficients is a main topic since it provides flexibility in the design of the transfer function of the filter, as well as allowing the suppression of the baseband resonance [1]–[7]. Negative coefficients can be performed in many different ways such as solutions based on the generation of phase shift by using special electrooptical modulators [1], [2] or differential detection [3]. All of these are not strictly all-optical since the RF processing combines the optical and RF domain. Concerning the use of all-optical RF processing, active and passive photonics proposals have been demonstrated. The first ones use active optical components such as semiconductor optical amplifiers (SOAs). Here, the negative taps are obtained by means of cross-gain modulation (XGM) in SOAs [4], while [5] proposes the use of the same XGM effect to get the negative tap, but by modulating the ASE instead of the continuous input probe wavelength. Another approach are the passive solutions such as those based on the use of an optical broadband source sliced by fiber Bragg gratings [6].