In recent years, the dissemination of optical frequency signals along phase-stabilized fibers has allowed the comparison of optical clocks at unprecedented levels of accuracy. This has a strong relevance in frequency metrology and boosted the measurement capabilities also in other research fields such as radioastronomy [1], geodesy [2] and fundamental physics [3]. Fiber-based frequency dissemination relies on the transmission of a continuous-wave, narrow-linewidth laser whose frequency is accurately measured with an atomic clock at the National Metrology Institute (NMI) where it is generated. Specific techniques are used to compensate for the acoustic and seismic noise and temperature variations, which degrade the phase stability of the transmitted signal. However, in existing implementations, it is not possible to detect and compensate for the birefringence variations of the optical fiber. These can induce variations in the state of polarization (SOP) of the transmitted signal, affecting the optical beatnotes at the receiving end. This not only degrades the achievable performances in terms of phase stability of the metrological signal, but can also lead to beatnote fading, with consequent failure of the entire system. This problem currently represents one of the strongest limitations to the reliability of fiber-based frequency dissemination.
In recent years, the dissemination of optical frequency signals along phase-stabilized fibers has allowed the comparison of optical clocks at unprecedented levels of accuracy. This has a strong relevance in frequency metrology and boosted the measurement capabilities also in other research fields such as radioastronomy [1], geodesy [2] and fundamental physics [3]. Fiber-based frequency dissemination relies on the transmission of a continuous-wave, narrow-linewidth laser whose frequency is accurately measured with an atomic clock at the National Metrology Institute (NMI) where it is generated. Specific techniques are used to compensate for the acoustic and seismic noise and temperature variations, which degrade the phase stability of the transmitted signal. However, in existing implementations, it is not possible to detect and compensate for the birefringence variations of the optical fiber. These can induce variations in the state of polarization (SOP) of the transmitted signal, affecting the optical beatnotes at the receiving end. This not only degrades the achievable performances in terms of phase stability of the metrological signal, but can also lead to beatnote fading, with consequent failure of the entire system. This problem currently represents one of the strongest limitations to the reliability of fiber-based frequency dissemination.
Frequency dissemination over optical fiber links relies on measuring the phase of fiber-delivered lasers. Phase is extracted from optical beatnotes and the detection fails in case of beatnotes fading due to polarization changes, which strongly limit the reliability and robustness of the dissemination chain. We propose a new method that overcomes this issue, based on a dual-polarization coherent receiver and a dedicated signal processing that we developed on a field programmable gated array. Our method allowed analysis of polarization-induced phase noise from a theoretical and experimental point of view and endless tracking of the optical phase. This removes a major obstacle in the use of optical links for those physics experiments where long measurement times and high reliability are required.
This thesis describes the realization of a facility for the high-accuracy frequency dissemination on a continental scale. This project was developed at the Optics division of the Italian National Metrology Institute (INRIM), which is in charge of the realization and operation of the atomic primary frequency standards, of their dissemination, and cooperates to the International Atomic Time generation. Frequency dissemination is now based on satellite techniques; however, the resolution provided by these methods does not cope with the accuracy and stability of newgeneration frequency standards. For instance, more than 10 days of measurement are needed to effectively compare primary standards, and years would be necessary to compare optical clocks via satellite links. On the other hand, it has been demonstrated that systems based on optical fibers can improve the resolution by about 5 orders of magnitude with respect to satellite techniques. This is the reason why many national metrology institutes in Europe are cooperating to develop a network for frequency dissemination and clocks comparisons all across the continent. This technique is based on the transmission of a narrow-linewidth laser along a standard telecom fiber. At the metrological laboratory, the frequency is measured with respect to a frequency standard, while at the remote link end it is extracted and used as a frequency reference. However, temperature changes, acoustic noise, air flows can degrade the stability of the delivered frequency, and must therefore be compensated. This is done through a Doppler cancellation technique, in which the light travels twice in the optical fiber in opposite directions. This is different from what happens in telecom links, which are inherently bidirectional. Dedicated instruments have therefore been developed. This thesis describes the realization of a phase-stabilized optical link between INRIM and the European Laboratory for Non Linear Spectroscopy in Florence, where an optical clock based on Sr atoms was under development at the time of measurements. This clock has been compared to the INRIM primary standard via the optical link. I worked on the realization of the ultrastable laser sources, on the development and characterization of the optical and electronics apparatus, and at the development of the metrological chain for the remote clocks comparison. This thesis also describes some novel techniques which we investigated in the framework of this project, and new applications of optical fiber links. During my studies I spent two months at the Laboratoire National d'Essais-Systemes de Référence Temps-Espace (LNE-SYRTE) in Paris, where I cooperated to the realization of an ultrastable laser source based on an optical delay line. This is reported in the thesis as well
The transfer of ultra-stable frequencies between distant laboratories is required by many applications from fundamental metrology to high-precision measurements. For that purpose, optical fibre links have been intensively studied over the last decade. They have demonstrated impressive results far beyond the GPS capabilities on distances up to 2000 km, thanks to an active compensation of the fiber propagation noise.Up to now, optical links has been mainly implemented between two labs, when future applications will require the development of metrological fiber networks. Towards this goal, we have developed a few techniques which allow us to distribute the ultrastable optical signal to many users simultaneously. We have designed repeater laser stations, which can be used to build a cascaded link or to distribute the ultrastable signal among two optical links [1]. These stations allow us also to compare the output end signals of two optical links. These versatile stations will be a key-component of the national metrological network currently being developed in France within the Refimeve+ project. To complete the network, we have demonstrated the extraction of the ultrastable signal from a main link and its seeding to a secondary link [2].Optical links are already been successfully used for clocks comparison. Recently we were able to compare the atomic clocks of the French and German National Metrology Institutes through two connected optical links of total length 1415-km [3]. We found that the two fully independent Sr lattice optical clocks at the two sites agreed to better than 5x10-17 limited only by the clocks uncertainties. A fractional statistical uncertainty of 3x10-17 was reached after only 1000 s averaging time, which is 10 times better, and more than four orders of magnitude faster than with any other existing frequency transfer method. The atomic fountain primary frequency standards from the two institutes were also compared and they were found to agree well within their combined uncertainties of a few 10-16 [4].
Optical clocks are considered as promising candidates for redefining the second in the International System of Units. Compared with microwave clocks, optical clocks are powerful tools for the fundamental research such as the constancy of the fundamental constants, the validity of Einstein’s theory of general relativity, and the predictions of quantum electrodynamics. Recently two research groups have demonstrated the optical clocks with an unprecedented precision level of 10-18, which is two orders better than the present primary frequency standard. Using two Sr optical clocks and three Cs fountain clocks, SYRTE group has demonstrated the definition of second with optical clocks.#br#For redefining the second with optical clocks in the future, the optical clocks from the remote laboratories should have a high precision and the frequency of the optical clocks need to be transferred over a long distance, with extremely high precision. Unfortunately the conventional means of frequency transfer such as two-way satellite time and frequency transfer can reach a 10-16 level in one day which is far below the requirement for an optical clocks. Various methods have been developed to transfer optical frequency signal via optical fibers. Especially a research group from Germany has achieved a frequency transfer stability of 10-19 level in hundreds of seconds with a fiber length of 1840 km.#br#We demonstrate the recent development of optical frequency transfer over a 70-km fiber spool at National Time Service Center. The measurement shows that the compensation for the fiber noise is close to the limitation induced by the fiber delay for the Fourier frequency from 1 Hz to 250 Hz. The transfer stability (Allan deviation) of the fiber link is 1.2×10-15 in 1 s averaging time, and 1.4×10-18 in 10000 s. A preliminary test of the optical frequency transfer over a 100-km spooled fiber is achieved with a stability of roughly one order worse than the 71 km result, 5×10-15 in 1 s.#br#We demonstrate a new scheme of remote compensation for optical frequency transfer via fibers against conventional local compensation method. This new scheme has the advantage of great simplification of the local site, which can find applications in massive extension of star network. The key feature is that we transfer the mixture of the round-trip signal and local reference to the remote user’s end via an auxiliary fiber. At remote site, the fiber noise is measured and compensated by AOM2 accordingly.#br#Transfer stabilities of 13×10-15 in 1 s averaging time and 4.8×10-18 in 10000 s are achieved with the remote fiber noise compensation via a 25 km fiber spool. The demonstrated transfer stability is comparable to that obtained by the local fiber noise compensation method.#br#The future star fiber network of optical frequency transfer can benefit from this method, because the simpler local setup is required and even can be shared in the central site for multitudinous remote users.
In recent years, the use of optical frequency dissemination through telecommunication fibers has steadily grown both in the context of stateof-the-art metrology like optical clocks and in the use of ultrastable reference signals for precision measurements in other fields of physics or earth science.Evaluating the performance of the fiber link can be difficult, as outliers and dysfunctioning regions lead to missing data.In this article, we discuss these challenges and investigate three different approaches to dealing with the missing data of a 1410-km-long optical link.We find a benefit in replacing the missing data with simulated noise with a statistical behavior similar to that of the original signal.
The continued remarkable advances in optical clocks and oscillators has led to a parallel strong development of optical clock networks [1]. Such networks have the potential to support a wide range of applications from basic time/frequency dissemination, to clock-based geodesy, to tests of general relativity [1]. To support optical clocks/oscillators at their highest accuracy and precision, these networks must rely on optical signals between the nodes to carry the timing signals. Networks based on the reciprocal transmission across fiber optics have allowed frequency comparisons between remote clocks across Europe and Japan [2, 3]. However, there are many potential applications, both terrestrial and space-based, for which the clocks will not be connected via an optical fiber and the signals must be sent across free space optical links. To this end, we have been developing techniques for the free-space transfer of time and frequency [4-6]. Moreover, rather than pursuing only frequency comparison, we have developed an approach that allows time comparison and even real-time synchronization between nodes. Such a capability can support the wide variety of proposed applications of mobile optical clock networks.