High-accuracy optical time delay measurement is essential to optical beamforming networks, multi-antenna GNSS-overfiber systems, and clock synchronization networks. To meet the demand of high-performance measurement, a time delay measurement system via microwave phase shift analysis is proposed, in which a modulator bias controller (MBC) is employed to stabilize the bias point of Mach-Zehnder modulator. However, because the MBC is easy to lose lock in long-term use, the stability of the measurement system is limited. To overcome the above problem, we propose an enhanced measurement system, in which the intensity modulation is achieved using a phase modulator and an optical Hilbert transformer. To verify the performance of the proposed system, a proof-of-concept experiment is carried out. The measurement results show that an accuracy of ±0.05 ps is obtained.
A novel method to perform high-resolution and wideband optical vector analysis (OVA) by using fixed low-frequency detection is proposed and demonstrated. In the proposed OVA, an optical superheterodyne structure is employed to down-convert the frequency-sweeping probe signal into a fixed low-frequency photocurrent. An electrical low-speed and high-sensitivity receiver is used to extract the complex amplitude of the photocurrent accurately, which can improve the sensitivity and dynamic range of the measurement system. Besides, by using the asymmetrical double-sideband modulation (AODSB), the measurable frequency range will be expanded to twice the bandwidth of the electro-optic modulator and microwave synthesizer. Moreover, the high-speed photodetector and wideband phase-magnitude detector are omitted, which can greatly reduce the hardware cost. In an experiment, the electrical receiver works at 199 MHz and 201 MHz, respectively. The measurement range is 80 GHz, and the resolution is 200 kHz.
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