Yann Kersale

An ultra-stable laser frequency locked on a silicon cryogenic cavity is presently under development at the FEMTO-ST lab. The laser will present a remarkable frequency stability of σ y (τ)=3×10 ⁻¹⁷ . We are presenting the status of the development of this experiment.

The stabilization of lasers on ultra-stable optical cavities by the Pound-Drever-Hall (PDH) technique is a widely used method. The PDH method relies on the phase-modulation of the laser, which is usually performed by an electro-optic modulator (EOM). When approaching the 10⁻¹⁶ fractional frequency stability level, this technology requires an active control of the residual amplitude modulation (RAM) generated by the EOM in order to bring the frequency stability of the laser down to the thermal noise limit of the ultra-stable cavity. In this article, we report on the development of an active system of RAM reduction based on a free space EOM, which is used to perform PDH-stabilization of a laser on a cryogenic silicon cavity. A minimum RAM instability of 1.4 × 10⁻⁷ is obtained by employing a digital servo that stabilizes the EOM DC electric field, the crystal temperature and the laser power. Considering an ultra-stable cavity with a finesse of 2.5 × 10⁵, this RAM level would contribute to the fractional frequency instability at the level of about 5 × 10⁻¹⁹, well below the state of the art thermal noise limit of a few 10⁻¹⁷.

Progress made in optical references, including ultrastable Fabry–Perot cavities, optical frequency combs, and optical atomic clocks, has driven the need for ultrastable optical fiber networks. Telecom-wavelength ultrapure optical signal transport has been demonstrated on distances ranging from the laboratory scale to the continental scale. In this article, we present a Doppler-cancellation setup based on a digital phase-locked loop (PLL) for ultrastable optical signal dissemination over fiber. The optical phase stabilization setup is based on a usual heterodyne Michelson-interferometer setup, while the software-defined radio (SDR) implementation of the PLL is based on a compact commercial board embedding a field-programmable gate array and analog-to-digital and digital-to-analog converters. Using three different configurations, including an undersampling method, we demonstrate a 20-m-long fiber link with residual fractional frequency instability as low as 10⁻¹⁸ at 1000 s and optical phase noise of −70 dBc/Hz at 1 Hz with a telecom frequency carrier.

The ytterbium atom is widely used in the fields of atomic physics, cavity quantum electrodynamics, quantum information processing, and optical frequency standards. There is however a strong dispersion among the reported values of the ¹S₀ → ¹P₁ transition frequency. In this article, we present two independent measurements of the absolute frequency of this transition performed with two different wavemeters using atomic fluorescence spectroscopy. The cancellation of Doppler shifts is obtained by fine tuning the angle between the probe laser and the atomic beam. The resulting ¹⁷⁴Yb isotope transition frequency is estimated to be 751 526 537 ± 27 MHz.

In this paper, we present the current development status of a cavity stabilized laser at 1550 nm. The expected thermal noise limit of the silicon Fabry-Perot cavity is 3×10 ^-17 at 17 K in terms of fractional frequency instability. We cooled it to its thermal expansion turning point, measured at 18.1 K, with a pulse-tube based cryocooler. Thanks to the thermal filtering and the temperature control of the cavity, the temperature induced fractional frequency instability is below the thermal noise up to 1000 s of integration time. We also discuss the vibration induced limitations, the thermal characterization of the cryocooler and the digital servo implementation and performances.

We report on the development of a simple-architecture fiber-based frequency distribution system used to transfer high frequency stability 100 MHz signals. This work is focused on the emitter and the receiver performances that allow the transmission of the radio-frequency signal over an optical fiber. The system exhibits a residual fractional frequency stability of 1 × 10⁻¹⁴ at 1 s integration time and in the low 10⁻¹⁶ range after 100 s. These performances are suitable to transfer the signal of frequency references such as those of a state-of-the-art hydrogen maser without any phase noise compensation scheme. As an application, we demonstrate the dissemination of such a signal through a 100 m long optical fiber without any degradation. The proposed setup could be easily extended for operating frequencies in the 10 MHz–1 GHz range.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.