A 43-km optical link has been developed between LPL and LNE-SYRTE, to transfer a primary reference frequency in the RF or microwave range. The phase noise introduced by the fluctuations of the propagation delay is compensated using a delay line. The resolution has been improved by increasing the transferred frequency up to 9 GHz. This link was used to compare a molecular optical clock in LPL to a primary reference in SYRTE, which leads to a test of the stability of electron to proton mass ratio.
A 43-km optical link has been developed between LPL and LNE-SYRTE, to transfer a primary reference frequency in the RF or microwave range. The phase noise introduced by the fluctuations of the propagation delay is compensated using a delay line. The resolution has been improved by increasing the transferred frequency up to 9 GHz. This link was used to compare a molecular optical clock in LPL to a primary reference in SYRTE, which leads to a test of the stability of electron to proton mass ratio.
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].
We transferred the frequency of an ultrastable laser over 86 km of urban fiber. The link is composed of two cascaded 43 km fibers connecting two laboratories, Laboratoire National de Métrologie et d'Essais-Systèmes de Référence Temps-Espace (LNE-SYRTE) and Laboratoire de Physique des Lasers (LPL), in the Paris area. In an effort to realistically demonstrate a link of 172 km without using spooled fiber extensions, we implemented a recirculation loop to double the length of the urban fiber link. The link is fed with a 1542 nm cavity-stabilized fiber laser having a sub-Hz linewidth. The fiber-induced phase noise is measured and cancelled with an all fiber-based interferometer using commercial off-the-shelf pigtailed telecommunication components. The compensated link shows an Allan deviation of a few 10−16 at one second and a few 10−19 at 10,000 seconds.
The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. This thesis describes the development of ultra-stable optical links over distances from 86 km to 300 km. The long-term objective is to connect the main European National Metrology Institutes with optical links extending up to 1000 km. The optical link is based on the transfer of the optical phase of an ultra-stable laser emitting light of wavelength 1542 nm through an optical fiber of the telecommunication network. For that purpose, ultra-stable cavity-stabilized lasers and fiber-stabilized lasers have been developed. The fractional instability of cavity-stabilized laser is approximately 10-15 at 1s integration time. The fiber-stabilized lasers are robust, compact, simple, tunable and their frequency noise is comparable to that of the cavity-stabilized laser for Fourier frequencies higher than a few tens of Hertz. Optical links were first demonstrated on 86 km and 172 km using dedicated fibers connecting the laboratories LNE-SYRTE and LPL. Fractional frequency instability in the range of or below 10-19 at 104 s integration time has been obtained. For larger distances, the ultra-stable laser signal is transferred on the Internet network simultaneously with the data traffic but on a dedicated frequency channel, using wavelength division multiplexing. Multiplexed links were demonstrated over distances up to 300 km, with fractional frequency instabilities at the same level or better than with dedicated fibers.
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.
Abstract: we report the first demonstration of a long-distance ultra stable frequency dissemination in the microwave range. A 9.15 GHz signal is transferred through a 86-km urban optical link with a fractional frequency instability of 1.3×10-15 at 1 s integration time and below 10-18 at one day. The optical link phase noise compensation is performed with a round-trip method. To achieve such a result we implement light polarisation scrambling and dispersion compensation. This link outperforms all the previous radiofrequency links and compares well with recently demonstrated full optical links.