Avoiding Aliasing in Allan Variance: an Application to Fiber Link Data Analysis

Optical fiber links are known as the most performing tools to transfer ultrastable frequency reference signals. However, these signals are affected by phase noise up to bandwidths of several kilohertz and a careful data processing strategy is required to properly estimate the uncertainty. This aspect is often overlooked and a number of approaches have been proposed to implicitly deal with it. Here, we face this issue in terms of aliasing and show how typical tools of signal analysis can be adapted to the evaluation of optical fiber links performance. In this way, it is possible to use the Allan variance as estimator of stability and there is no need to introduce other estimators. The general rules we derive can be extended to all optical links. As an example, we apply this method to the experimental data we obtained on a 1284 km coherent optical link for frequency dissemination, which we realized in Italy

Distributed Raman optical amplification in phase coherent transfer of optical frequencies

We describe the application of Raman Optical-fiber Amplification (ROA) for the phase coherent transfer of optical frequencies in an optical fiber link. ROA uses the transmission fiber itself as a gain medium for bi-directional coherent amplification. In a test setup we evaluated the ROA in terms of on-off gain, signal-to-noise ratio, and phase noise added to the carrier. We transferred a laser frequency in a 200 km optical fiber link with an additional 16 dB fixed attenuator (equivalent to 275 km of fiber on a single span), and evaluated both co-propagating and counter-propagating amplification pump schemes, demonstrating nonlinear effects limiting the co-propagating pump configuration. The frequency at the remote end has a fractional frequency instability of 3e-19 over 1000 s with the optical fiber link noise compensation

Embedded Digital Phase Noise Analyzer for Optical Frequency Metrology

Digital signal processing (DSP) is supporting novel in-field applications of optical interferometry, such as in laser ranging and distributed acoustic sensing. While the highest performances are achieved with field-programmable gated arrays (FPGAs), their complexity and cost are often too high for many tasks. Here, we describe an alternative solution for processing optical signals in real-time, based on a dual-core 32-bit microcontroller. We implemented in-phase and quadrature (IQ) demodulation of optical beat-notes resulting from the interference of independent laser sources, phase noise analysis of deployed optical fibers covering intercity distances, and synchronization of remote acquisitions via optical trigger signals. The embedded architecture can efficiently accomplish a large variety of tasks in the context of optical signal processing, being also easily configurable, compact, and upgradable. These features make it attractive for applications that require long-term, remotely operated, and field-deployed instrumentation

Spectral purity transfer with 5 × 10−17 instability at 1 s using a multibranch Er:fiber frequency comb

In this work we describe the spectral purity transfer between a 1156 nm ultrastable laser and a 1542 nm diode laser by means of an Er:fiber multibranch comb. By using both the master laser light at 1156 nm and its second-harmonic at 578 nm, together with the 1542 nm slave laser, we investigate the residual noise between the main comb output, the octave-spanning output, and a wavelength conversion module including non-linear fibers, second-harmonic generation crystal and amplifiers. With an ultimate stability of the system at the level of 5E−17 at 1 s and accuracy of 3E−19, this configuration can sustain spectral transfer at the level required by the contemporary optical clocks with a simple and robust setup

Frequency transfer via a two-way optical phase comparison on a multiplexed fiber network

We performed a two-way remote optical phase comparison on optical fiber. Two optical frequency signals were launched in opposite directions in an optical fiber and their phases were simultaneously measured at the other end. In this technique, the fiber noise was passively cancelled, and we compared two optical frequencies at the ultimate 1E-21 stability level. The experiment was performed on a 47 km fiber that is part of the metropolitan network for Internet traffic. The technique relies on the synchronous measurement of the optical phases at the two ends of the link, that is made possible by the use of digital electronics. This scheme offers several advantages with respect to active noise cancellation, and can be upgraded to perform more complex tasks

Absolute frequency measurement of the 1S0 - 3P0 transition of 171Yb

We report the absolute frequency measurement of the unperturbed transition 1S0 - 3P0 at 578 nm in 171Yb realized in an optical lattice frequency standard. The absolute frequency is measured 518 295 836 590 863.55(28) Hz relative to a cryogenic caesium fountain with a fractional uncertainty of 5.4x10-16 . This value is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the International System of Units.Comment: This is an author-created, un-copyedited version of an article accepted for publication/published in Metrologia. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://dx.doi.org/10.1088/1681-7575/aa4e62. It is published under a CC BY licenc

Erbium-Doped LiYF4 as a Potential Solid-State Frequency Reference: Eligibility and Spectroscopic Assessment

Time and frequency metrology is a key enabler for both forefront science and innovation. At the moment, atomic frequency standards (AFSs) are based on atoms either in the vapor phase or trapped in magneto-optical lattices in a vacuum. Finding a solid-state material that contains atoms suitable to be used as a frequency reference would be an important step forward in the simplification of the setup of AFSs. Lanthanide-doped inorganic crystals, such as Er-doped LiYF4, have been studied for several decades, and their intrashell 4f transitions are usually identified as ultra-narrow. Nevertheless, a systematic characterization of these transitions and their linewidths with a correlation to the dopant’s concentration and isotopic purity at low temperatures is lacking. In this work, we studied Er-doped LiYF4 as a potential benchmark material for solid-state frequency references. We chose Er as it has a set of transitions in the telecom band. The influence of Er concentrations and isotope purity on the transition linewidth was systematically studied using high-resolution optical spectroscopy at 5 K. The results indicate that the material under study is an interesting potential candidate as a solid-state frequency reference, having transition linewidths as low as 250 MHz at ~1530 nm