Photonic Microwave Bandpass Filter With Negative Coefficients Using a Polarization Modulator

A novel photonic microwave bandpass filter with negative coefficients implemented using a polarization modulator (PolM) is proposed and experimentally demonstrated. In the proposed filter, a lightwave from a tunable laser source (TLS) is sent to a PolM, with the incident lightwave adjusted to have its polarization direction aligned at an angle of plusmn45deg to the principal axes of the PolM. A microwave signal is applied to the PolM through its RF port. Thanks to the polarization modulation in the PolM, two complementary microwave signals with identical amplitudes carried by two optical carriers with orthogonal polarizations are generated. The two complementary optical microwave signals are then sent to an optical delay line consisting of one section or two sections of polarization-maintaining fiber (PMF), with the polarization directions aligned with the fast and slow axes of the PMF. A photonic microwave bandpass filter with two or four taps is obtained

I. Introduction

Processing of microwave and millimeter-wave signals in the optical domain with the advantageous features such as broad bandwidth, low loss, light weight, and immunity to electromagnetic interference offered by optics has been a topic of interest for over two decades [1], [2]. In general, photonic microwave filters are implemented based on a delay line structure, in which different time delays are generated using optical delay line devices. The major difficulty associated with the implementation of an optical delay line microwave filter is the optical interference which is very sensitive to environmental changes, leading to poor system stability. A straightforward solution to the problem is to use incoherent detection, such as using a low coherent light source. Photonic microwave filters based on incoherent detection have only positive coefficients, with lowpass filtering only. For many applications, such as in a radio-over-fiber link, bandpass filters are required. In the last few years, many approaches have been proposed to implement microwave bandpass filters. An approach to achieve bandpass filtering is to employ differential photodetection using a balanced photodetector (PD) [3]. Microwave filters with negative coefficients can also be achieved based on cross-gain modulation in a semiconductor optical amplifier [4], or carrier depletion effect in a laser diode [5], [6]. The use of a complementary light source obtained by filtering a broadband light source using fiber Bragg gratings (FBGs) can also achieve bandpass filtering with negative coefficients [7]. Photonic microwave bandpass filters can also be realized by biasing a pair of intensity modulators at the opposite slopes to achieve phase inversion [8], or by using a single dual-output intensity modulator with a double-pass modulation [9]. Recently, we demonstrated a microwave bandpass filter with negative coefficients based on optical phase modulation to intensity modulation conversion in an optical filter or chirped FBGs. By locating the phase-modulated optical signals at the opposite slopes of the transfer function of the optical filter or by passing the phase-modulated optical signals through chirped FBGs with group-delay responses of positive and negative slopes [10], [11], microwave bandpass filters with negative coefficients were generated.