Double Coupler Amplified Recirculating Delay Line Microwave Photonic Bandpass Filter

Theoretical and experimental investigations on a double coupler amplified recirculating delay line (ARDL) microwave photonic bandpass filter are presented. These show that the system involves both coherent and incoherent summation of optical signals. This enables a higher order filter transfer function to be realized, which results in a high-performance frequency response that cannot be achieved by the conventional single coupler ARDL structure. Experimental results are presented that demonstrate a sharp, high skirt selectivity and high stopband rejection level bandpass filter response.

I. Introduction

Microwave photonic signal processors are attractive because of their high time-bandwidth product capability and high reconfigurability. A simple photonic signal processing structure can be implemented by an optical coupler connected in the way that one of the coupler outputs is looped around and connected to one of the inputs, and an optical amplifier placed in the loop to compensate the loss [1], [2]. While this amplified recirculating delay line (ARDL) structure can achieve a sharp bandpass filter response, the skirt selectivity and the stopband rejection level of the filter response are low. Extensions on the conventional ARDL structure by including an additional optical coupler inside the loop have been investigated. Various configurations of the double coupler ARDL signal processor with different frequency response characteristics have been reported [3]–[5]. Experimental results on the double coupler ARDL signal processors presented in [3] and [4] showed that they have no performance improvement in terms of the skirt selectivity and the stopband rejection level in comparison to the conventional single coupler ARDL. Binh et al. presented the theoretical studies of a Z-shaped double coupler ARDL [5]. Incoherent summation of optical signals was used throughout the analysis.