Photonic instantaneous frequency measurement of wideband microwave signals

We propose a photonic system for instantaneous frequency measurement (IFM) of wideband microwave signals with a tunable measurement range and resolution based on a polarization-maintaining fiber Bragg grating (PM-FBG). Firstly, in order to be insensitive to laser power fluctuation, we aim at generating two different frequency to amplitude characteristics so that we can normalize them to obtain an amplitude comparison function (ACF). Then we encode these two different wavelengths in two perpendicular polarizations by using the PM-FBG which shows different transmission profiles at two polarizations. The ACF is capable of being adjusted by tuning polarization angle, therefore the measurement range and resolution are tunable. By theoretical analyses and simulated verification, a frequency measurement range of 0~17.2 GHz with average resolution of ±0.12 GHz can be achieved, which signifies a wide measurement range with relatively high resolution. Our system does not require large optical bandwidth for the components because the wavelength spacing can be small, making the system affordable, stable, and reliable with more consistent characteristics due to the narrowband nature of the optical parts. PM-FBG with high integration can be potentially used for more polarization manipulating systems and the use of a single-polarization dual-wavelength laser can simplify the architecture and enhance the stability.