Using asynchronous optical sampling to measure timing jitter of electro-optic frequency combs
Authors: Ma Bo-Wen , Dai Wen , Meng Fei , Tao Jia-Ning , Wu Zi-Ling , Shi Yan-Qing , Fang Zhan-Jun , Hu Ming-Lie , Song You-Jian ,   State Key Laboratory of Precision Measurement Technology and Instruments, Tianjin University, Tianjin 300072, China,   Shanghai Radio Equipment Research Institute, Shanghai 201109, China,   Laboratory of Optical Frequency Standard, Time and Frequency Metrology Division, National Institute of Metrology, Beijing 100029, China
Published: July 2024

<sec>Electro-optic frequency combs (EOCs) are optical frequency combs constructed by phase modulation of single frequency lasers. The electro-optic modulated optical frequency combs have shown their unique advantages in many application fields due to their high repetition frequencies, high stabilities and other advantages, especially in precision measurement applications. Through accurate dispersion control, the electro-optical frequency combs can output ultra-short pulse laser sequences in the time domain, and their timing jitter characteristic is very important for precision measurement and other applications. This work presents a scheme to measure the timing jitter of the electro-optic combs directly in the time domain based on the principle of dual-comb asynchronous optical sampling method(ASOPS), which relies on temporal cross-correlation between the high repetition rate electro-optic combs and a low repetition rate passively mode-locked fiber laser. The ASOPS process allows timing jitter measurement in a magnified time scale where the timing jitter at a femtosecond level can be received and visualized by standard low speed electronics. We build a theoretical model for timing jitter measurement, conduct a numerical study to verify the model, and also construct an experimental system to characterize the period jitter of a 10-GHz electro-optic comb.</sec><sec>Firstly, the theoretical model for measuring timing jitter is established. In this work, the basic theory of measuring the timing jitter is discussed by analyzing the histogram directly in time domain through using the obtained ASOPS signal. Subsequently, numerical simulations are conducted to simulate the ASOPS process after establishing a sequence of Gaussian pulse train with quantum limited timing jitter. Another pulse train without timing jitter serves as a local oscillator. Through the square law optical detection after sum-frequency generation between LO and LUT, the ASOPS process can be realized and periodic jitter can be obtained directly through histogram statistical analysis. The simulation result is consistent with the theoretical result very well. Finally, an EOC system with cascaded modulators at a repetition rate of 10 GHz is designed and built, and a timing jitter measurement system is designed and built with an all-fiber configuration. The period jitter of 10-GHz EOC is measured by using a 161-MHz mode-locked fiber laser as local oscillator. Histogram analysis shows that the period jitter of the EOC is 3.86 fs.</sec><sec>This measurement technique does not require to use the intricate electrical phase-locked circuits or a high-speed photodetector to receive ultrashort pulses of EOC. Like the eye map analysis method commonly used in telecommunication, the histogram analysis can be used to determine the timing jitter approaching the quantum limit. This approach is easy to set up and operate, and it is anticipated to become a standard method of measuring period jitter of ultrashort pulse with high repetition frequency in a laboratory setting. It will be particularly useful for measuring timing jitters of the sources of novel high repetition rate optical frequency combs, such as micro-resonators and electro-optic frequency combs.</sec>