Multiband radio-frequency (RF) communications are becoming increasingly important for first responders to obtain essential assistance from available wireless and satellite communications that operate in the megahertz to tens-of-gigahertz range. To pre-select the desired frequency and prevent interference, tunable multiband RF filters are in demand.Unfortunately, conventional RF electronic techniques lack reconfigurability and are limited to six passbands with inconsistent filter profiles, while microwave photonic (MWP) filters using conventional optical frequency combs are limited to generating just one frequency passband, or passbands with fixed free spectral range. Now, researchers at the University of Georgia (Athens, GA) have used a two-stage Lyot loop filter to spectrally slice a broadband light source and generate 12 selectable comb spacing combinations using just two sections of high-birefringence fiber, enabling up to 12 passbands in the MWP filter.1
The Lyot loop filter is essentially a bidirectional Lyot filter that consists of high-birefringence fiber, a polarizer, and an optical circulator. Since light travels in the Lyot loop filter twice, it can experience more-complex wavelength-dependent phase shifts, resulting in single or multiple (two to 12) interleaving optical combs. The particular RF passband depends on which optical frequency comb (one or multiple) is chosen from the Lyot filter. Both the passband center frequencies and simultaneous passband numbers are adjustable, and each passband has a sharp passband profile and good filter selectivity with up to 30 dB side-lobe suppression. The passband numbers and frequencies are flexibly tuned by adjusting the polarization state in the two-stage Lyot loop filter. The researchers say that this is the first demonstration of a MWP multiband filter with such a large passband count, reconfigurable capability, uniform filter profile, and high selectivity.
1. J. Ge and M. P. Fok, Sci. Rep., 5, 15882 (2015); doi:10.1038/srep15882.
Contact Mable P. Fok at [email protected].