Yann Le Coq

Optical to microwave clock frequency ratios with a nearly continuous strontium optical lattice clock

Optical lattice clocks are at the forefront of frequency metrology. Both the instability and systematic uncertainty of these clocks have been reported to be two orders of magnitude smaller than the best microwave clocks. For this reason, a redefinition of the SI second based on optical clocks seems possible in the near future. However, the operation of optical lattice clocks has not yet reached the reliability that microwave clocks have achieved so far. In this paper, we report on the operation of a strontium optical lattice clock that spans several weeks, with more than 80% uptime. We make use of this long integration time to demonstrate a reproducible measurement of frequency ratios between the strontium clock transition and microwave Cs primary and Rb secondary frequency standards.

Photonic microwave signals with zeptosecond-level absolute timing noise

Ultralow-noise microwave signals are generated at 12 GHz by a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. The microwave signals have a fractional frequency stability below 6.5 × 10–16 at 1 s and a timing noise floor below 41 zs Hz–1/2. Photonic synthesis of radiofrequency (RF) waveforms revived the quest for unrivalled microwave purity because of its ability to convey the benefits of optics to the microwave world1,2,3,4,5,6,7,8,9,10,11. In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 × 10−16 at 1 s and a timing noise floor below 41 zs Hz−1/2 (phase noise below −173 dBc Hz−1 for a 12 GHz carrier). This outperforms existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with the lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems12, telecommunications13 and time–frequency metrology2,14. The measurement methods developed here can benefit the characterization of a broad range of signals.

Quantum cascade laser frequency stabilization at the sub-Hz level

Quantum Cascade Lasers (QCL) are increasingly being used to probe the mid-infrared “molecular fingerprint” region. This prompted efforts towards improving their spectral performance, in order to reach ever-higher resolution and precision. Here, we report the stabilisation of a QCL onto an optical frequency comb. We demonstrate a relative stability and accuracy of 2x10-15 and 10-14, respectively. The comb is stabilised to a remote near-infrared ultra-stable laser referenced to frequency primary standards, whose signal is transferred via an optical fibre link. The stability and frequency traceability of our QCL exceed those demonstrated so far by two orders of magnitude. As a demonstration of its capability, we then use it to perform high-resolution molecular spectroscopy. We measure absorption frequencies with an 8x10-13 relative uncertainty. This confirms the potential of this setup for ultra-high precision measurements with molecules, such as our ongoing effort towards testing the parity symmetry by probing chiral species.

Accuracy evaluation of an optical lattice clock with bosonic atoms

We report what we believe to be the first accuracy evaluation of an optical lattice clock based on the S01-->P03 transition of an alkaline earth boson, namely, Sr88 atoms. This transition has been enabled by using a static coupling magnetic field. The clock frequency is determined to be 429228066418009(32)Hz. The isotopic shift between Sr87 and Sr88 is 62188135Hz with fractional uncertainty 5x10(-7). We discuss the necessary conditions to reach a clock accuracy of 10(-17) or less by using this scheme.

Spectral purity transfer between optical wavelengths at the 10 −18 level

We present an optical frequency comb-based scheme that transfers 4.5 × 10-16 fractional frequency stability from a 1062 nm wavelength laser to a 1542 nm laser. We demonstrate that this scheme does not hinder the transfer down to 3 × 10-18 at 1 s, one order of magnitude better than previous reported result. This exceeds, by more than one order of magnitude, the stability of any optical oscillator demonstrated to date, and satisfies the stability requirement for quantum projection noise-limited optical lattice clocks. We will finally describe our latest efforts to apply this technique for improving the performance the LNE-SYRTE Sr-based optical lattice clocks.

Characterizing a fiber-based frequency comb with electro-optic modulator

We report on the characterization of a commercial core fiber-based frequency comb equipped with an intracavity free-space electro-optic modulator (EOM). We investigate the relationship between the noise of the pump diode and the laser relative intensity noise (RIN) and demonstrate the use of a low-noise current supply to substantially reduce the laser RIN. By measuring several critical transfer functions, we evaluate the potential of the EOM for comb repetition rate stabilization. We also evaluate the coupling to other relevant parameters of the comb. From these measurements, we infer the capabilities of the femtosecond laser comb to generate very-low-phase-noise microwave signals when phase-locked to a high-spectral-purity ultra-stable laser.

Advanced noise reduction techniques for ultra-low phase noise optical-to-microwave division with femtosecond fiber combs

We report what we believe to be the lowest phase noise optical-to-microwave frequency division using fiber-based femtosecond optical frequency combs: a residual phase noise of -120 dBc/Hz at 1 Hz offset from an 11.55 GHz carrier frequency. Furthermore, we report a detailed investigation into the fundamental noise sources which affect the division process itself. Two frequency combs with quasi-identical configurations are referenced to a common ultrastable cavity laser source. To identify each of the limiting effects, we implement an ultra-low noise carrier-suppression measurement system, which avoids the detection and amplification noise of more conventional techniques. This technique suppresses these unwanted sources of noise to very low levels. In the Fourier frequency range of ~200 Hz to 100 kHz, a feed-forward technique based on a voltage-controlled phase shifter delivers a further noise reduction of 10 dB. For lower Fourier frequencies, optical power stabilization is implemented to reduce the relative intensity noise which causes unwanted phase noise through power-to-phase conversion in the detector. We implement and compare two possible control schemes based on an acousto-optical modulator and comb pump current. We also present wideband measurements of the relative intensity noise of the fiber comb.

Ultrastable laser array at 633 nm for real-time dimensional metrology

We describe a laser system for very-high-accuracy dimensional metrology. A sealed-cavity helium–neon laser is offset locked to an iodine-stabilized laser in order to realize a secondary standard with higher power and less phase noise. Synchronous averaging is employed to remove the effect of the frequency modulation present on the iodine-stabilized laser. Additional lasers are offset locked to the secondary standard for use in interferometry. All servo loops are implemented digitally. The offset-locked lasers have intrinsic linewidths of the order of 2.5 kHz and exhibit a rms deviation from the iodine-stabilized laser below 18 kHz. The amplitude noise is at the shot-noise limit for frequencies above 700 kHz. We describe and evaluate the system in detail, and include a discussion of the noise associated with various types of power supplies.

Atomic fountains and optical clocks at SYRTE: status and perspectives

In this article, we report on the work done with the LNE–SYRTE atomic clock ensemble during the last 10 years. We cover the progress made in atomic fountains and in their application to timekeeping. We also cover the development of optical lattice clocks based on strontium and on mercury. We report on tests of fundamental physical laws made with these highly accurate atomic clocks. We also report on work relevant to a future possible redefinition of the SI second.

Accurate control of optoelectronic amplitude to phase noise conversion in photodetection of ultra-fast optical pulses

When illuminating a photodiode with modulated laser light, optical intensity fluctuations of the incident beam are converted into phase fluctuations of the output electrical signal. This amplitude to phase noise conversion (APC) thus imposes a stringent constraint on the relative intensity noise (RIN) of the laser carrier when dealing with ultra-low phase noise microwave generation. Although the APC vanishes under certain conditions, it exhibits random fluctuations preventing efficient long-term passive stabilization schemes. In this paper, we present a digital coherent modulation-demodulation system for automatic measurement and control of the APC of a photodetector. The system is demonstrated in the detection of ultra-short optical pulses with an InGaAs photodetector and enables stable generation of ultra-low phase noise microwave signals with RIN rejection beyond 50 dB. This simple system can be used in various optoelectronic schemes, making photodetection virtually insensitive to the RIN of the lasers. We utilize this system to investigate the impact of the radiofrequency (RF) transmission line at the output of the photodetector on the APC coefficient that can affect the accuracy of the measurement in certain cases.