Dead-band-free, high-resolution microwave frequency measurement using a free-running triple-comb fiber laser

Microwave photonic solutions of frequency measurement have advantages in broad frequency coverage, but achieving high-resolution measurement remains a challenge. Those schemes based on optical frequency combs could achieve high-resolution measurement over a broad frequency range. Here, a dead-band-free, high-resolution microwave frequency measurement scheme based on undersampling the microwave signal by three pulse sequences generated from a triple-wavelength mode-locked fiber laser is proposed and demonstrated. The triple-wavelength ultrashort pulses generated in one laser cavity have slightly different repetition rates due to chromatic dispersion. This eliminates the needs of multiple mode-locked lasers and frequency control between them and drastically reduces the system complexity. The absolute frequency of the microwave signal can be determined based on three down-converted low-frequency beat notes of the microwave signal with the nearest comb lines without ambiguity. A 10
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relative measurement precision at a sampling speed of 100 Hz is demonstrated from 1 to 20 GHz, and the measurement accuracy remains within 0.3 Hz. Microwave signal with an RF power as low as −75 dBm can be measured with a 10 Hz precision at 10 GHz by using RF frontend amplifiers. The simple and compact triple-comb fiber laser would enable the development of low-complexity, high-performance microwave characterization instrument.