Photonic reconfigurable microwave filter with negative coefficients - Electronics Letters

Introduction: Optically realised microwave filters offer advantages such as large operating bandwidth and immunity to microwave interference. Several researchers have investigated microwave photonic filters to enable tunable and reconfigurable systems. Capmany et al. [1] proposed a filter system that employs an array of tunable lasers. The filter can be both tuned and reconfigured by changing the wavelength separation and optical power, respectively. Vidal et al. [2] proposed a modification that replaced the array of tunable lasers with a single multiwavelength source. Discrete tuning is achieved using a switched dispersion matrix. This is a more economical solution but the filter taps are limited to positive coefficients. This restricts the filters to only lowpass and also restricts the flexibility in the design of the lowpass filters. Several techniques have been proposed to implement negative coefficients. Two-tap bandpass filters using all optical techniques have been demonstrated in [3–5]. Electro-optic techniques have been reported in [6] and [7]. However, these require separate arrays of tunable lasers to implement each of the set of positive and negative coefficients. In this Letter, we propose a modification to the filter proposed in [2]. The implementation allows for both positive and negative taps using a single wavelength selective element. A four-tap version of the filter is demonstrated.