Radio-frequency spectrometry based on laser speckle imaging

We demonstrate a microwave photonic spectrometer based on laser speckle pattern imaging that has some significant advantages over state-of-the-art electronic technology, including high resolution, multifrequency detection over broad and reconfigurable bandwidths. The spectrometer operates by modulating the radio-frequency (RF) signal on a frequency-stabilized continuous wave (CW) laser using an electro-optic intensity modulator. The modulated CW laser travels through a 100-m long high numerical aperture multimode optical fiber before collection and recording on a camera. To calibrate the spectrometer, an RF tone generator is stepped over the desired operational range of the spectrometer with a frequency step size on the order of the desired frequency resolution while the speckle pattern images are recorded and stored. An unknown signal under test is then generated and recovered from the calibration set using regression analysis techniques. The spectrometer exhibits a 5-MHz resolution with a single-tone recovery accuracy of greater than 99.9% over a 17-GHz frequency range and a mean recovery error below 1 MHz over smaller ranges. We also incorporated a high-speed camera, operated at an 800-kHz frame rate, permitting data collection at speeds commensurate with the best real-time spectrum analyzers. We present the results of our analysis of the spectrum recovery process with the goal of increasing the spectral recovery rate. In addition, in the interest of miniaturization, we present the results of simulations of 50-mu m wide and 40-cm long multimode waveguides in silicon nitride and silicon, which suggest that the 100-m long multimode fiber can be replaced by short waveguides without degradation in performance.