D. Holleville

Long-distance frequency transfer over an urban fiber link using optical phase stabilization

We transferred the frequency of an ultra-stable laser over 86 km of urban fiber. The link is composed of two cascaded 43-km fibers connecting two laboratories, LNE-SYRTE and LPL in 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.

Development of a strontium optical lattice clock for the SOC mission on the ISS

The ESA mission “Space Optical Clock” project aims at operating an optical lattice clock on the ISS in approximately 2023. The scientific goals of the mission are to perform tests of fundamental physics, to enable space-assisted relativistic geodesy and to intercompare optical clocks on the ground using microwave and optical links. The performance goal of the space clock is less than 1 × 10-17 uncertainty and 1 × 10-15 τ-1/2 instability. Within an EU-FP7-funded project, a strontium optical lattice clock demonstrator has been developed. Goal performances are instability below 1 × 10-15 τ-1/2 and fractional inaccuracy 5 × 10-17. For the design of the clock, techniques and approaches suitable for later space application are used, such as modular design, diode lasers, low power consumption subunits, and compact dimensions. The Sr clock apparatus is fully operational, and the clock transition in 88Sr was observed with linewidth as small as 9 Hz.

Stability of the compact cold atom clock HORACE

HORACE is a compact cold cesium atom clock which is being developed in LNE-SYRTE for space applications and onboard systems. The operation of this clock is different from fountains since the laser cooling, the microwave interrogation and the detection are sequentially performed inside the spherical microwave cavity. We recently achieved short term stability of 5.5 10-13 tau-0.5 reaching the 10-14 level at 3000 sec. We report in this paper recent developments and improvements, particularly on the cooling sequence. We also study the main limitations.

Ultra-stable clock laser system development towards space applications.

The increasing performance of optical lattice clocks has made them attractive for scientific applications in space and thus has pushed the development of their components including the interrogation lasers of the clock transitions towards being suitable for space, which amongst others requires making them more power efficient, radiation hardened, smaller, lighter as well as more mechanically stable. Here we present the development towards a space-compatible interrogation laser system for a strontium lattice clock constructed within the Space Optical Clock (SOC2) project where we have concentrated on mechanical rigidity and size. The laser reaches a fractional frequency instability of 7.9u2009×u200910−16 at 300u2009ms averaging time. The laser system uses a single extended cavity diode laser that gives enough power for interrogating the atoms, frequency comparison by a frequency comb and diagnostics. It includes fibre link stabilisation to the atomic package and to the comb. The optics module containing the laser has dimensions 60u2009×u200945u2009×u20098u2009cm3; and the ultra-stable reference cavity used for frequency stabilisation with its vacuum system takes 30u2009×u200930u2009×u200930u2009cm3. The acceleration sensitivities in three orthogonal directions of the cavity are 3.6u2009×u200910−10/g, 5.8u2009×u200910−10/g and 3.1u2009×u200910−10/g, where gu2009≈u20099.8u2009m/s2 is the standard gravitational acceleration.

Reaching a few 10 −13 τ −1/2 stability level with the compact cold atom clock HORACE

HORACE is a compact cold cesium atom clock which is being developed in LNE-SYRTE for space applications and onboard systems. The operation of this clock is different from fountains since the laser cooling, the microwave interrogation and the detection are sequentially performed inside the spherical microwave cavity. The entire simplified operation sequence is described. A short term relative frequency stability of 2.2 10-13 tau-1/2 is achieved. Preliminary results on mid term show that a level of 4 10-15 is reached after 5 103s of integration. Limitations are investigated.

Development of a compact cold atom clock

HORACE is a compact cold atom clock where the atoms are cooled inside the microwave interrogation cavity. About 10/sup 8/ atoms can be cooled at kinetic temperatures as low as 2.5 /spl mu/K. We report, for the first time, a Ramsey pattern observed with a 14 Hz linewidth and fringe contrast better than 80%. Since this clock is designed for space applications, some properties are extrapolated to micro-gravity operation.

A new design of ECLD for compact atomic clocks

Cold atom clocks would take great benefits of microgravity environment. In Relation with the clock developments, we work on miniature laser-cooling optical benches. In this article we describe a new design of compact external cavity laser diode, which exhibits up to 80 mW at the output of the ECLD for a diode current of 100 mA and a line width of about 150 kHz.

Ultra-stable optical cavities: Design and experiments at LNE-SYRTE

We present the design of two different geometries of optical cavities (vertically and horizontally mounted). The Finite Element Modeling analysis is used to predict and decrease the influence of vibrations on the cavity stabilized lasers. Inspired by the results of simulation, two cavities have been constructed. Vibration sensitivity coefficients measured are equal or better than 1.4x10-11/(m.s-2) for all spatial directions. A preliminary measurement of the two independent cavity stabilized lasers shows a relative frequency stability of about 10-15 @ 1s.

Frequency tripled 1.5 µm telecom laser diode stabilized to iodine hyperfine line in the 10−15 range

We report on telecom laser frequency stabilization to narrow iodine hyperfine line in the green range of the optical domain, after a frequency tripling process using two nonlinear PPLN crystals. We have generated up to 300 mW optical power in the green (P_3ω), from 800 mW of infrared power (P_ω). This result corresponds to an optical conversion efficiency η = P_3ω/P_ω ~ 36 %. To our knowledge, this is the best value ever demonstrated for a CW frequency tripling process. We have used a narrow linewidth iodine hyperfine line (component a₁ of the ¹²⁷I₂ R 35 (44-0) line) to stabilize the IR laser yielding to frequency stability of 4.8×10^-14τ^-1/2 with a minimum value of 6×10^-15 reached after 50 s of integration time. The whole optical setup is very compact and mostly optically fibered. This approach opens the way for efficient and elegant architecture development for space applications as one of several potential uses.

A compact setup for double-modulation coherent population trapping clock

We demonstrate an atomic clock based on double-modulation CPT. With a high contrast CPT signal observed in this scheme, we get a preliminary result of the frequency stability: 4×10-13 at 1 second. It shows the possibility to implement an atomic clock with a compact and robust setup while still maintaining its high performance.