Microwave Delay Measurement Based On Optical Low Coherence Light Interferometry

The core of Optical Beam-forming Networks (OBFN),which is a research highlight in microwave photonics, is the True Time Delay (TTD) module. In TTD system, the measurement of microwave delay can be achieved through the measurement of time delay. In order to improve the performance of OBFN system, accurate measurement of the time delay of key optical devices is important.Optical Low Coherence Interferometry is a simple making,low cost and high precision method of time delay measuring. The interference patterns of reference light and signal light through the device under test (DUT) are acquired, then, the phase characteristic and relative time delay can be calculated. The paper introduces the principle of Optical Low Coherence Interferometry and analyzes various kinds of factors influencing the uncertainty of time delay. Main achievement as follows:Error Propagation in digital signal processing, which mainly includes Fourier transform and polynomial fitting, is analyzed through theoretical analysis and simulation calculation. Research shows that phase error is directly proportional to intensity noise and time delay error is directly proportional to phase error too. What's more, polynomial fitting to phase curve can immunize noise influence greatly.Different kinds of noise in measurement are analyzed too. Research shows that intensity noise mainly includes thermal noise,shot noise and beat noise of bandwidth light and the beat noise is the main intensity noise. Calibration error and instability of the intensity of reference light will also result in time delay error. However, the main time delay error come from the instability of the phase characteristic of DUT due to temperature variation and other environment factors.Experimentally, relative time delay of a section of photonic crystal fiber (PCF) about 20 meters from 1540nm to 1560nm is measured and the best measurement precision of sub-picoseconds is acquired.