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Dead-Band-Free, High-Resolution Microwave Frequency Measurement Using a Free-Running Triple-Comb Fiber Laser

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Dead-Band-Free, High-Resolution Microwave Frequency Measurement Using a Free-Running Triple-Comb Fiber Laser This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTQE.2017.2770098, IEEE Journal of Selected Topics in Quantum Electronics > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Dead-band-free, high-resolution microwave frequency measurement using a free-running triple-comb fiber laser Xin Zhao, Cui Li, Ting Li, Guoqing Hu, Ruixiao Li, Ming Bai, Takeshi Yasui, and Zheng Zheng, Member, IEEE Abstract—Microwave photonic solutions of frequency I. INTRODUCTION measurement have advantages in broad frequency recise determination of the frequency of microwave (MW) coverage, but achieving high-resolution measurement Psignals is of great importance, since such signals covering remains a challenge. Those schemes based on optical an increasing broad electromagnetic wave spectrum, from frequency combs could achieve high-resolution megahertz to even terahertz, have been prevalently used in our measurement over a broad frequency range. Here a information technology infrastructures, such as dead-band-free, high-resolution microwave frequency communications, remote sensing, radar, navigation and many measurement scheme based on undersampling the other areas. Electronic solutions for real-time MW signal microwave signal by three pulse sequences generated from measurement may be limited by the bandwidth of the a triple-wavelength mode-locked fiber laser is proposed acquisition frontend’s and are facing challenges in dealing with and demonstrated. The triple-wavelength ultrashort pulses unknown signals that may reside over an increasingly broad generated in one laser cavity have slightly different frequency range . Microwave photonic techniques have been repetition rates due to chromatic dispersion. This investigated as a promising solution for high-frequency eliminates the needs of multiple mode-locked lasers and microwave signal characterization and processing, due to their frequency control between them and drastically reduces potential in broadband operation and immunity to the system complexity. The absolute frequency of the electromagnetic interferences [1-3]. Numerous photonics microwave signal can be determined based on three schemes for MW frequency measurement had been proposed. down-converted low-frequency beat notes of the microwave Frequency-to-power conversion can be realized when the signal with the nearest comb lines without ambiguity. A -10 sideband of an optical CW carrier generated by MW 10 relative measurement precision at a sampling speed of modulation is filtered by narrow optical filters [4-6]. Optical 100 Hz is demonstrated from 1 GHz to 20 GHz, and the mixing method could also map the MW signal frequency to measurement accuracy remains within 0.3 Hz. Microwave optical power by mixing an optical signal modulated by MW signal with a RF power as low as -75dBm can be measured signal and its replica with a given time delay. The mixing with a 10Hz precision at 10 GHz by using RF frontend process could be realized by two cascaded modulators [7] or amplifiers. The simple and compact triple-comb fiber laser nonlinear optical process [8]. Similarly, the power fading of the would enable the development of low-complexity, modulated double-sideband optical signal propagating through high-performance microwave characterization instrument. a dispersive medium can provide information about the modulation frequency [9-11]. These methods can cover MW Index Terms—Mode locked lasers, optical fiber lasers, frequency measurement, optical pulses frequencies up to several tens of gigahertz, but their measurement resolution is no better than MHz level. Instead of the schemes based on using CW lasers, optical Manuscript received August1, 2017. This work was supported by the NSFC frequency combs could also be used in MW signal (61435002/61521091/61675014/61675015) and Fundamental Research Funds characteristics measurement. When a highly stable periodic for the Central Universities. pulse train generated by a mode-locked laser, often with a X. Zhao, C. Li, R. Li, T. Li, G. Hu, M. Bai and Z. Zheng are with the School relatively low repetition rate, is modulated by a high-frequency of Electronic and Information Engineering, Beihang University, Beijing, 100083, China (e-mail: [email protected]; [email protected]; MW signal, the MW signal is undersampled by the short pulse [email protected]; [email protected]; [email protected]; sequence. When detected and filtered by a low-pass filter, a Corresponding Author: [email protected]). Z. Zheng is also with the down-converted copy of the MW wave with the same Collaborative Innovation Center of Geospatial Technology, 129 Luoyu Road, amplitude and phase characteristics is generated [12]. From Wuhan 430079, China T. Yasui is with the Graduate School of Science and Technology, another point of view, the down-converted signal corresponds Tokushima University, 2-1 Minami-Josanjima, Tokushima 770-8506, Japan to the beat note between the MW signal and the closest and JST, ERATO MINOSHIMA Intelligent Optical Synthesizer (IOS), 2-1, neighboring frequency comb line. If two pulse trains with Minami-Josanjima, Tokushima, Tokushima 770-8506, Japan (e-mail: slightly different repetition rates are used to sample the same [email protected]). MW signal, the aliasing caused by the undersampling could be 1077-260X (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSTQE.2017.2770098, IEEE Journal of Selected Topics in Quantum Electronics > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 resolved under certain assumptions, and the absolute frequency total length of fiber in the cavity is estimated to 6.5m, and the of the MW signal can be recovered. Such a scheme had been total dispersion is estimated to be ~103fs/nm. Because of the successfully demonstrated for real-time frequency relatively large anomalous dispersion, the laser is expected to measurement of continuous-wave THz radiation using dual operate in the soliton regime. By introducing the ILP with PMF THz combs, and very high measurement accuracy and pigtails into the ring fiber laser, birefringence-induced Lyot precision had been achieved [13]. Similarly, two sets of combs filtering effects can provide periodic spectral filtering within with slightly different comb tooth spacings generated by the gain profile of EDF and could enable simultaneously mode electro-optic modulation of light from a CW laser have been locking at more than one wavelengths, when the intra cavity proposed to down-convert MW signals for characterizations polarization state is properly adjusted [19, 22]. Based on the [14-16]. width of the EDF gain window around 1550nm and the length Recently, single-cavity, dual-comb mode-locked fiber lasers of PMF, i.e. the amount of birefringence introduced, it is have been proposed and demonstrated, which can generate two expected that around three filtered gain windows could be asynchronous pulse sequences simultaneously by leveraging multiplexing in the dimensions of center wavelength, present with relatively equalized peak gains. Therefore, polarization state, or mode-locking mechanism [17-21]. simultaneous triple-wavelength mode-locked ultrashort pulse Among these schemes, dual-wavelength mode-locked laser generation could be realized in such a compact and could generate dual pulsed trains with different center cost-effective platform. Due to the difference in their center wavelengths by gain tilt tuning [17] or birefringence-induced wavelengths and the intra cavity dispersion of the fiber laser, Lyot filtering effects [19, 21]. Due to the cavity dispersion, the the repetition rates of the triple-wavelength pulses (frep,i, i=1,2,3) repetition rates of the dual-wavelength pulse trains are slightly would be slightly different, and therefore triple-comb sampling different. These kinds of lasers have emerged as an attractive, signals could be generated from a single mode-locked fiber low-complexity dual-comb laser source, due to the inherent laser. As all pulses propagate through the same cavity and share mutual coherence between the dual pulse trains. Besides their almost the same disturbance and noises, such as temperature application in asynchronous optical sampling and spectroscopy fluctuation and pump power RIN noise, it had been [19], THz frequency measurement with a free-running, demonstrated that good mutual coherence and stable repetition dual-wavelength laser had been demonstrated [22]. rate difference between the multi-wavelength pulses can be However, in the current dual-comb frequency measurement maintained without active feedback control of the laser [19]. schemes, ‘dead bands’ exist due to ambiguity in determining Yet, as the laser is operated in a free-running mode, these the relative position between the beat signal and comb teeth repetition rates themselves would still drift over time. It is [22]. Our preliminary results had shown that using triple-comb necessary to monitor those repetition rates in real time for instead of dual-comb could eliminate such
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