arXiv subjects

Y. Le Coq

Publications and source records attributed to Y. Le Coq.

34 records · Page 2Linked to original sources

arXiv2011

An Optical Fibre Pulse Rate Multiplier for Ultra-low Phase-noise Signal Generation

In this letter we report on an all optical-fibre approach to the synthesis of ultra-low noise microwave signals by photodetection of femtosecond laser pulses. We use a cascade of Mach-Zehnder fibre interferometers to realize stable and efficient repetition rate multiplication. This technique increases the signal level of the photodetected microwave signal by close to 18 dB, that in turn, allows us to demonstrate a phase noise level of 118 dBc/Hz at 1 Hz and -160 dBc/Hz at 10 MHz from 12 GHz signal. The noise floor of the fiber multiplier and photodetection system alone is around -164 dBc/Hz at the same offset frequency, which is very close to the fundamental shot noise floor

physics.optics↗

arXiv2011

Amplitude to phase conversion of InGaAs pin photo-diodes for femtosecond lasers microwave signal generation

When a photo-diode is illuminated by a pulse train from a femtosecond laser, it generates microwaves components at the harmonics of the repetition rate within its bandwidth. The phase of these components (relative to the optical pulse train) is known to be dependent on the optical energy per pulse. We present an experimental study of this dependence in InGaAs pin photo-diodes illuminated with ultra-short pulses generated by an Erbium-doped fiber based femtosecond laser. The energy to phase dependence is measured over a large range of impinging pulse energies near and above saturation for two typical detectors, commonly used in optical frequency metrology with femtosecond laser based optical frequency combs. When scanning the optical pulse energy, the coefficient which relates phase variations to energy variations is found to alternate between positive and negative values, with many (for high harmonics of the repetition rate) vanishing points. By operating the system near one of these vanishing points, the typical amplitude noise level of commercial-core fiber-based femtosecond lasers is sufficiently low to generate state-of-the-art ultra-low phase noise microwave signals, virtually immune to amplitude to phase conversion related noise.

physics.optics↗

arXiv2011

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 -120dBc/Hz at 1 Hz offset from a 11.55GHz carrier frequency. We furthermore report a detailed investigation into the fundamental noise sources which afflicts 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 circumvents the detection and amplification noise of more conventional techniques. This technique now allows the suppression of these unwanted sources of noise to very low levels. In the Fourier frequency range of ~200 Hz to 100 kHz, a fed-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 in order 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 on the relative intensity noise of the fiber comb.

physics.optics↗

arXiv2010

An Ultra-Stable Referenced Interrogation System in the Deep Ultraviolet for a Mercury Optical Lattice Clock

We have developed an ultra-stable source in the deep ultraviolet, suitable to fulfill the interrogation requirements of a future fully-operational lattice clock based on neutral mercury. At the core of the system is a Fabry-Pérot cavity which is highly impervious to temperature and vibrational perturbations. The mirror substrate is made of fused silica in order to exploit the comparatively low thermal noise limits associated with this material. By stabilizing the frequency of a 1062.6 nm Yb-doped fiber laser to the cavity, and including an additional link to LNE-SYRTE's fountain primary frequency standards via an optical frequency comb, we produce a signal which is both stable at the 1E-15 level in fractional terms and referenced to primary frequency standards. The signal is subsequently amplified and frequency-doubled twice to produce several milliwatts of interrogation signal at 265.6 nm in the deep ultraviolet.

physics.atom-ph↗

arXiv2010

Sub-100 attoseconds optics-to-microwave synchronization

We use two fiber-based femtosecond frequency combs and a low-noise carrier suppression phase detection system to characterize the optical to microwave synchronization achievable with such frequency divider systems. By applying specific noise reduction strategies, a residual phase noise as low as -120 dBc/Hz at 1 Hz offset frequency from a 11.55 GHz carrier is measured. The fractional frequency instability from a single optical-to-frequency divider is 1.1E-16 at 1 s averaging down to below 2E-19 after only 1000 s. The corresponding rms time deviation is lower than 100 attoseconds up to 1000 s averaging duration.

physics.optics↗

arXiv2009

Ultra-Low Noise Microwave Extraction from Fiber-Based Optical Frequency Comb

In this letter, we report on all-optical fiber approach to the generation of ultra-low noise microwave signals. We make use of two erbium fiber mode-locked lasers phase locked to a common ultra-stable laser source to generate an 11.55 GHz signal with an unprecedented relative phase noise of -111 dBc/Hz at 1 Hz from the carrier.The residual frequency instability of the microwave signals derived from the two optical frequency combs is below 2.3 10^(-16) at 1s and about 4 10^(-19) at 6.5 10^(4)s (in 5 Hz bandwidth, three days continuous operation).

physics.optics↗

arXiv2009

Ultrastable lasers based on vibration insensitive cavities

We present two ultra-stable lasers based on two vibration insensitive cavity designs, one with vertical optical axis geometry, the other horizontal. Ultra-stable cavities are constructed with fused silica mirror substrates, shown to decrease the thermal noise limit, in order to improve the frequency stability over previous designs. Vibration sensitivity components measured are equal to or better than 1.5e-11 per m.s^-2 for each spatial direction, which shows significant improvement over previous studies. We have tested the very low dependence on the position of the cavity support points, in order to establish that our designs eliminate the need for fine tuning to achieve extremely low vibration sensitivity. Relative frequency measurements show that at least one of the stabilized lasers has a stability better than 5.6e-16 at 1 second, which is the best result obtained for this length of cavity.

physics.optics↗

arXiv2009

Ultra-low noise microwave generation with fiber-based optical frequency comb and application to atomic fountain clock

We demonstrate the use of a fiber-based femtosecond laser locked onto an ultra-stable optical cavity to generate a low-noise microwave reference signal. Comparison with both a liquid Helium cryogenic sapphire oscillator (CSO) and a Ti:Sapphire-based optical frequency comb system exhibit a stability about $3\times10^{-15}$ between 1 s and 10 s. The microwave signal from the fiber system is used to perform Ramsey spectroscopy in a state-of-the-art Cesium fountain clock. The resulting clock system is compared to the CSO and exhibits a stability of $3.5\times10^{-14}τ^{-1/2}$. Our continuously operated fiber-based system therefore demonstrates its potential to replace the CSO for atomic clocks with high stability in both the optical and microwave domain, most particularly for operational primary frequency standards.

physics.atom-ph↗

arXiv2008

Sr lattice clock at 1x10^{-16} fractional uncertainty by remote optical evaluation with a Ca clock

Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over km-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1x10-16 fractional level, surpassing the best current evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation-induced frequency shifts.

physics.atom-ph↗

arXiv2008

Theoretical tools for atom laser beam propagation

We present a theoretical model for the propagation of non self-interacting atom laser beams. We start from a general propagation integral equation, and we use the same approximations as in photon optics to derive tools to calculate the atom laser beam propagation. We discuss the approximations that allow to reduce the general equation whether to a Fresnel-Kirchhoff integral calculated by using the stationary phase method, or to the eikonal. Within the paraxial approximation, we also introduce the ABCD matrices formalism and the beam quality factor. As an example, we apply these tools to analyse the recent experiment by Riou et al. [Phys. Rev. Lett. 96, 070404 (2006)].

cond-mat.other↗

arXiv2007

Measurement of excited-state transitions in cold calcium atoms by direct femtosecond frequency-comb spectroscopy

We apply direct frequency-comb spectroscopy, in combination with precision cw spectroscopy, to measure the ${\rm 4s4p} ^3P_1 \to {\rm 4s5s} ^3S_1$ transition frequency in cold calcium atoms. A 657 nm ultrastable cw laser was used to excite atoms on the narrow ($γ\sim 400$ Hz) ${\rm 4s^2} ^1S_0 \to {\rm 4s4p} ^3P_1$ clock transition, and the direct output of the frequency comb was used to excite those atoms from the ${\rm 4s4p} ^3P_1$ state to the ${\rm 4s5s} ^3S_1$ state. The resonance of this second stage was detected by observing a decrease in population of the ground state as a result of atoms being optically pumped to the metastable ${\rm 4s4p} ^3P_{0,2}$ states. The ${\rm 4s4p} ^3P_1 \to {\rm 4s5s} ^3S_1$ transition frequency is measured to be $ν= 489 544 285 713(56)$ kHz; which is an improvement by almost four orders of magnitude over the previously measured value. In addition, we demonstrate spectroscopy on magnetically trapped atoms in the ${\rm 4s4p} ^3P_2$ state.

physics.atom-ph↗

arXiv2006

Kilohertz-resolution spectroscopy of cold atoms with an optical frequency comb

We have performed sub-Doppler spectroscopy on the narrow intercombination line of cold calcium atoms using the amplified output of a femtosecond laser frequency comb. Injection locking of a 657-nm diode laser with a femtosecond comb allows for two regimes of amplification, one in which many lines of the comb are amplified, and one where a single line is predominantly amplified. The output of the laser in both regimes was used to perform kilohertz-level spectroscopy. This experiment demonstrates the potential for high-resolution absolute-frequency spectroscopy over the entire spectrum of the frequency comb output using a single high-finesse optical reference cavity.

physics.atom-ph↗

arXiv2005

Beam quality of a non-ideal atom laser

We study the propagation of a non-interacting atom laser distorted by the strong lensing effect of the Bose-Einstein Condensate (BEC) from which it is outcoupled. We observe a transverse structure containing caustics that vary with the density within the residing BEC. Using WKB approximation, Fresnel-Kirchhoff integral formalism and ABCD matrices, we are able to describe analytically the atom laser propagation. This allows us to characterize the quality of the non-ideal atom laser beam by a generalized M$^2$ factor defined in analogy to photon lasers. Finally we measure this quality factor for different lensing effects.

cond-mat.other↗

arXiv2001

Atom laser divergence

We measure the angular divergence of a quasi-continuous, rf-outcoupled, free-falling atom laser as a function of the outcoupling frequency. The data is compared to a Gaussian-beam model of laser propagation that generalizes the standard formalism of photonic lasers. Our treatment includes diffraction, magnetic lensing, and interaction between the atom laser and the condensate. We find that the dominant source of divergence is the condensate-laser interaction.

cond-mat↗

arXiv2000

Multi frequency evaporative cooling to BEC in a high magnetic field

We demonstrate a way to circumvent the interruption of evaporative cooling observed at high bias field for $^{87}$Rb atoms trapped in the (F=2, m=+2) ground state. Our scheme uses a 3-frequencies-RF-knife achieved by mixing two RF frequencies. This compensates part of the non linearity of the Zeeman effect, allowing us to achieve BEC where standard 1-frequency-RF-knife evaporation method did not work. We are able to get efficient evaporative cooling, provided that the residual detuning between the transition and the RF frequencies in our scheme is smaller than the power broadening of the RF transitions at the end of the evaporation ramp.

physics.atom-ph↗

arXiv2000

RF-induced evaporative cooling and BEC in a high magnetic field

We present the design of our iron-core electromagnet for BEC, and how to solve the specific experimental problems raised by this technique. After presenting the experimental set-up, we address the interruption of runaway evaporative cooling when the Zeeman effect is not linear. We present the ways to circumvent this problem, use of multiple RF frequencies, sympathetic cooling and show some applications of these high magnetic fields (cavity coupling, high confinement).

physics.atom-ph↗