微波與光時間傳送技術及其在光電震盪器上的應用

Microwave and Optical Time Transfer Techniques and their Applications on Optoelectronic Oscillators

曾文宏(Wen-Hung Tseng)

指導教授 ： 馮開明

國立清華大學/電機資訊學院/光電工程研究所/博士(2012年)

摘要

本論文目的在研究精準的時間傳送技術，並將此技術延伸到光電震盪器的研發上。對於微波領域的時間傳送技術而言，衛星雙向傳時是目前國際上用來長距離精準比對時間與頻率標準的主要技術之一，本論文提出兩種新方法來提升衛星雙向傳時的精度。其一為利用衛星雙向傳時網絡上的大量比對數據來改善單一鏈路短期穩定度的方法，透過亞太地區衛星雙向傳時比對數據的分析，我們展示此方法可以改善氫原子鐘傳時比對的短期穩定度平均達22%，以日本情報通信研究機構(NICT)與台灣電信研究所(TL)的例行氫原子比對鏈路為例，在1小時平均時間的時間不穩定度從91皮秒(ps, 10^-12 s)降到72皮秒以下。另一項研究，目的不僅是要進一步提升衛星雙向傳時的精度並朝更有效利用衛星頻寬而設計，我們與日本情報通信研究機構合作利用軟體無線電系統的技術，產生雙虛擬電碼的展頻信號，來進行新式的衛星雙向傳時比對。首次跨國長距離的比對實驗進行了半年，數據顯示其短期穩定度優於國際度量衡局(BIPM)所計算的GPS 精密單點定位(Precise Point Positioning, PPP)技術結果，長期穩定度則與例行的衛星雙向傳時結果一致；分析從1秒到1天(即86400秒)所有平均時間的時間穩定度，此技術皆可優於75皮秒，這是國際上一個新的紀錄。由於雙虛擬電碼租用兩個分開20 MHz頻寬距離僅佔200 kHz的衛星頻道，此兩個窄的頻道物理上即可達到與20 MHz頻寬信號的相同精度，節省了昂貴的衛星租用費，故此方法已成為國際上高度重視的衛星雙向傳時新一代技術。
相較於微波領域對於頻寬的限制，利用光纖來傳遞時間信號已成為高精度傳時的發展趨勢。於是我們以共同路徑的光纖鏈路進行雙向傳時比對實驗，結果顯示25公里光纖傳送路徑受環境因素影響的時間延遲變化達2奈秒(ns, 10^-9 s)，透過雙向傳時方式消去後殘餘的峰值變化量僅有83皮秒，我們進一步分析這些殘餘的變化主要是兩端傳時設備沒有完美同步到參考信號所造成。由於共同路徑的光纖鏈路具有高度的對稱性，此系統傳時穩定度可優於7皮秒；而平均時間10^5 秒的頻率穩定度可達1.9×10^-16。
延伸微波及光傳時技術的研究，我們提出將時間傳送技術延伸應用到光電震盪器的研發上。光電震盪器是利用混合光路與微波元件的方式，來產生超低相位雜訊的高頻信號源；在這個前沿的研究課題中，多數的團隊都致力於降低相位雜訊的研究，少有針對長期穩定度的研究。鑒於典型光電震盪器採用長距離光纖扮演共振迴路，以達到高品質因子(Q-factor)的目的，我們提出利用傳時技術的展頻調變方式產生監控信號注入震盪器，可用於監測並穩定光纖共振迴路的用途上。相位雜訊的量測證實注入的監測信號並不會對震盪信號產生干擾；透過長期的光纖時延監測及震盪器頻率量測數據，本論文驗證該方法的理論基礎，及討論延伸的改善方法。論文的最後，針對光電震盪器參考信號注入式鎖定的特性進行研究，藉由監測光纖的時延，我們指出此注入式鎖定的光電震盪器的相位與其參考信號會因光纖時延的長期變動而改變，此現象不僅會劣化光電震盪器的特性，並可能導致失鎖。本論文提出利用監控光纖時延來穩定震盪器相位的方法，具有此功能的震盪器可望作為原子頻率標準的本地震盪器，實現10^-15等級頻率穩定度的原子鐘。
本論文的研究，對於微波領域的時間傳送技術及光電震盪器的發展上，都有開拓性的貢獻。

關鍵字

時間 ； 頻率 ； 光纖傳時 ； 衛星雙向傳時 ； 光電震盪器 ； 原子鐘 ； 衛星 ； 全球定衛系統

並列摘要

In this dissertation, we study the precise time transfer techniques and extend their applications into the study of optoelectronic oscillators. In the microwave domain, two-way satellite time and frequency transfer (TWSTFT) is one of the main techniques used to compare atomic time scales over long distances. As more and more TWSTFT measurements have been performed, the large number of point-to-point two-way time transfer links has grown to be a complex network. For future improvement of the TWSTFT performance, it is important to reduce measurement noise of the TWSTFT results. One method is using TWSTFT network time transfer. We propose a feasible method to improve the short-term stability by combining the direct and indirect links in the network. Through the comparisons of time deviation (TDEV), the results of network time transfer exhibit clear improved short-term stabilities. For the links used to compare 2 hydrogen masers, the average gain of TDEV at averaging times of 1 hour is 22%. As TWSTFT short-term stability can be improved by network time transfer, the network may allow a larger number of simultaneously transmitting stations. In the other work, to both improve the precision of TWSTFT and decrease the satellite link fee, a new software-defined modem with dual pseudo-random noise (DPN) codes has been developed. We demonstrate the first international DPN-based TWSTFT experiment over a period of 6 months. The results of DPN exhibit excellent performance, which is competitive with the Global Positioning System (GPS) precise point positioning (PPP) technique in the short-term and consistent with the conventional TWSTFT in the long-term. Time deviations of less than 75 ps (ps, 10^-12 s) are achieved for averaging times from 1 s to 1 day (i.e., 86400 s). Because the DPN-based system has advantages of higher precision and lower bandwidth cost, it is one of the most promising methods to improve international time-transfer links.
Because there is no bandwidth limit in an optical fiber link, a new trend is to perform the time transfer through the optical fiber link. Hence, we present a two-way time transfer experiment through a 25 km optical fiber link. The fiber link, which is constructed to a common-path configuration, is used to replace the satellite link. The resulting data exhibits the time deviation of less than 7 ps at one-day averaging time. The frequency stability on the order of 1.9×10^-16 at 10^5 s has been demonstrated.
In the final part of this dissertation, we point that the time transfer techniques are useful for the study of optoelectronic oscillators (OEOs). Based on optical fiber loops to act as a high-Q cavity, the OEOs are capable of generating stable radio-frequencies (RF). The long-term frequency stability of the OEO is then limited by the cavity variation that is mainly induced by temperature sensitivity of the optical fiber. In order to actively stabilize the OEO cavity, we employ the technique of RF transfer over optical fibers. We propose and experimentally demonstrate a dual-loop-OEO scheme to enhance the long-term stability with an injected probe signal to monitor the phase variation in the fiber loops. The experimental results show that the resulting spread-spectrum signal is useful in monitoring the fiber delay without observable interference. The relationships between the measured frequency and the monitored delay are theoretically and numerically discussed. We also estimate the long-term stability of the proposed OEO scheme with the cavity phase correction. The corrected result shows the long-term frequency stability of the proposed OEO is within 8.4×10^−8 at one day. Finally, we study the impact of fiber delay fluctuation on reference injection-locked OEOs. We demonstrate that the phase shift of a reference injection-locked OEO varies as the change of its fiber delay over a long period of time. The variation of the fiber delay is monitored using an injected probe signal and is compared with the phase shift. With actively stabilized fiber delays according to the monitored data, the long-term frequency stability of the reference injection-locked OEO is evaluated. In future progression, to act as a local oscillator for an atomic clock, a tunable OEO can generate an oscillation frequency corresponding to the desired atomic transition without the use of a synthesizer.

並列關鍵字

time ； frequency ； time transfer over optical fiber ； two-way satellite time and frequency transfer ； optoelectronic oscillator ； atomic clock ； satellite ； GPS

參考文獻

[1] D. W. Allan, N. Ashby, C. C. Hodge, “The science of timekeeping,” Hewlett Packard (Agilent) application note 1289, (88-pages), 1997.

[4] C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, T. Rosenband, “Frequency comparison of two high-accuracy Al+ optical clocks,” Phys. Rev. Lett.,104(7), 070802, 2010.

[5] A. Bauch, J. Achkar, S. Bize, D. Calonico, R. Dach, R. Hlavac, L. Lorini, T. Parker, G. Petit, D. Piester, K. Szymaniec, P. Uhrich, “Comparison between frequency standards in Europe and the USA at the 10-15 uncertainty level,” Metrologia, vol. 43, no. 1, pp. 109–120, Feb. 2006.

[6] D. Piester, A. Bauch, L. Breakiron, D. Matsakis, B. Blanzano, and O. Koudelka, “Time transfer with nanosecond accuracy for the realization of International Atomic Time,” Metrologia, vol. 45, no. 2, pp. 185–198, Apr. 2008.

[7] Bruce Warrington, "Two atomic clocks ticking as one," Science 336, 421, 2012.