Optical Transfer Delay Measurement Based on Multi-frequency Phase-Derived Ranging

Optical transfer delay (OTD) measurement with high speed and accuracy is essential for many systems over fiber such as distributed coherent aperture radar system and base station time synchronization. In this article, a simple OTD measurement based on multi-frequency is proposed. Two microwaves with little frequency difference are converted into an optical signal by cascade modulation to generating multifrequency microwave signals via sidebands beating after photodetection. The OTD of the device under test (DUT) is converted into the phase shift with different frequencies which can be obtained with fast Fourier transform (FFT) in digital domain. In addition, the integer ambiguity can be removed by these selected frequencies in one single measurement to improve the measurement speed. In the proof-of-concept experiments, the delay accuracy can reach ±0.2 picoseconds while the measurement speed achieving one millisecond per measurement.