kHz-Resolution Frequency Measurement Based on Selective High-Order Modes in a Micro-Resonator

We propose a photonic microwave frequency measurement system with kHz resolution for the instantaneous measurement of the Doppler frequency shift (DFS) of a moving object. By joint use of an optical frequency comb (OFC) generator implemented by cascading an intensity modulator (IM) and a phase modulator (PM) and a high-Q metal-coated micro-disk resonator (MDR), an ultra-sensitive amplitude comparison function (ACF) for frequency-to-power mapping is established, which enables ultra-high sensitivity frequency measurement. The proposed technique is experimentally demonstrated. A Doppler signal is applied to the two modulators to generate multiple sidebands corresponding to an optical comb. By aligning the ±3-rd order sidebands with the steep slopes of two adjacent notches of the metal-coated MDR, an ACF is obtained. A frequency measurement sensitivity of 2.28 dB/MHz is experimentally achieved. With a power measurement resolution of 0.01 dB, the frequency measurement resolution is 4.37 kHz, which is the highest ever reported based on photonic techniques. It enables Doppler frequency shift measurement for an object moving at speeds between 40.34 to 1384 m/s.