Microwave photonic filters based on optical fiber cladding-mode couplers
Student thesis: Master's Thesis
Related Research Unit(s)
Photonic signal processing can overcome the inherent bottleneck caused by the limited sampling speed in conventional electrical signal processors and realize highly adaptive and reconfigurable operation. One of the important applications of photonic signal processing is microwave signal filtering. A microwave photonic filter, which functions as an ordinary microwave filter in a radio-frequency system, is expected to provide microwave frequency selectivity, high stop-band attenuation, and a large free spectral range (FSR). Although various microwave photonic filters have been demonstrated, there are still problems that have not been fully solved. For example, it is difficult to tune the notch depth and the FSR and produce an FSR in the gigahertz range with conventional fiber ring based microwave photonic filters. There is practically no existing microwave photonic filter that can provide stable, low-noise coherence-free operation and, at the same time, an FSR that can be tuned continuously and precisely over a wide range. In this study, we propose two new microwave photonic filters formed with optical fiber cladding-mode couplers, which can overcome many of the problems associated with the conventional filters. An optical fiber cladding-mode coupler consists of two parallel touching bare fibers placed in a low-index groove. It operates on the principle of evanescent-field coupling between the cladding modes of the same order in the two bare fibers. The cladding mode in the input fiber of the coupler is excited with a long-period fiber grating (LPFG), while the cladding mode in the output fiber of the coupler is extracted with a matching LPFG. A cladding-mode coupler offers several distinct features, including a tunable coupling efficiency, wavelength selectivity, and the possibility of translating one of the fibers through the coupling region without affecting the coupling efficiency. The first proposed microwave photonic filter is a fiber ring resonator, where an output port of a cladding-mode coupler is connected to its input port to form a bare fiber loop. The loop length of the resonator is in the range of 10 cm, which offers an FSR in the gigahertz range. On the other hand, the loop length of a conventional fiber ring resonator based on a commercial directional coupler is typically several tens of centimeters or longer, which limits the FSR to several hundred megahertz. In the other extreme, the loop length of a microfiber ring resonator is of the order of 1 mm, which corresponds to an FSR of several tens of gigahertz or higher. Therefore, our proposed fiber ring resonator can fill the FSR gap between a conventional fiber ring resonator and a microfiber ring resonator and also allow the FSR and the notch depth to be tuned easily. Furthermore, the use of LPFGs allows wavelength selection and tuning and is compatible with the wavelength-division-multiplexing (WDM) technology. Using a standard single-mode fiber and a pair of CO2 laser-written LPFGs, we successfully demonstrate a cladding-mode fiber ring filter that has an FSR of ~1.3 GHz and a notch depth of ~17 dB. The second proposed microwave photonic filter is a Mach-Zehnder interferometer, where input signals at two different wavelengths are separated by a cladding-mode coupler together with a pair of matching LPFGs. This filter allows the time delay of a wavelength channel to be tuned by sliding a fiber through the cladding-mode coupler and, therefore, offers continuous and precise FSR tuning over a wide range. On the other hand, the conventional method of tuning the FSR of a filter is based on changing the operation wavelength of the signal passing through a fixed-length dispersive fiber delay line, which requires an expensive wavelength-tunable laser or a spectrum-sliced broadband source and can offer only a limited tuning range. Furthermore, the use of two widely separated wavelengths in our proposed filter guarantees coherence-free operation and therefore low-noise operation. Our experimental Mach-Zehnder filter has a notch depth larger than 30 dB and a precisely tunable FSR up to ~10 GHz (limited by the measurement equipment). The largest FSR that can be achieved with our filter is actually limited by the length-difference resolution (~0.2 mm) of the experimental setup, which is ~1 THz.
- Photonics, Microwave filters