Clément Courde

Time and frequency distribution improvement in Calern/Geoazur laboratory for T2L2 campaigns

The Time Transfer by Laser Link (T2L2)[1) experiment aim to synchronise remote ultra stable clocks over large-scaled distances using two laser ranging stations. T2L2 ultimate time transfer capability can only be demonstrate with a picosecond range ground mastering. We focus this year in knowledge and equipment improvement to perform a T2L2 time transfer with accuracy and stability of a few picoseconds. A deep analysis of signals stability has been carried out this year in the time and frequency laboratory in Plateau de Calern. The aim was to better understand the limits and hardware configuration and to enhance time and frequency distribution for T2L2 experiment. We showed phase noise and stability problem on our H-maser distribution. Final measures were conducted in October in collaboration with SYRTE. Then a complete equipment reorganisation was done. This paper focus on the time and frequency laboratory characterization before and after the reorganisation. We introduce our new equipments and present our new H-maser time scale and discuss the performances obtained.

T2L2 : Ground to ground Time Transfer

T2L2 (Time Transfer by Laser Link), developed by both CNES and OCA permits the synchronization of remote ultra stable clocks over intercontinental distances. The principle is derived from laser telemetry technology with a dedicated space equipment deigned to record arrival times of laser pulses at the satellite. Using laser pulses instead of radio frequency signals used in classical time transfer techniques (GPS, TWSTFT), T2L2 permits to realize some links between distant clocks with time stability of a few picoseconds and accuracy better than 100 ps.

Accuracy validation of T2L2 time transfer in co-location

The Time Transfer by Laser Link (T2L2) experiment has been developed in close collaboration between Centre National dEtudes Spatiales and Observatoire de la Côte dAzur. The aim is to synchronize remote ultra-stable clocks over large-scale distances using two laser ranging stations. This ground to space time transfer has been derived from laser telemetry technology with dedicated space equipment designed to record arrival time of laser pulses on board the satellite. For 3 years, specific campaigns have been organized to prove T2L2 performance. In April 2012, we performed a 2-week campaign with our two laser ranging stations, Métrologie Optique and French Transportable Laser Ranging Station, to demonstrate the T2L2 time transfer accuracy in co-location. We have compared three independent time transfer techniques: T2L2, GPS, and direct measurement, with both an event timer and an interval counter. The most important result obtained in this campaign was a mean agreement between T2L2 and a direct comparison better than 200 ps. This is the first major step to validate the uncertainty budget of the entire T2L2 experiment. This paper focuses on this campaign setup and the obtained results.

A sub-ns comparison between GPS common view and T2L2

T2L2 (Time Transfer by Laser Link) permits the synchronization of remote ultra stable clocks over intercontinental distances. The principle is based upon laser telemetry technology with a network of laser stations on ground and dedicated space equipment designed to record arrival time of laser pulses at the satellite. T2L2 allows realization of some links between distant clocks with time stability of a few picoseconds and accuracy better than 100 ps. The instrumental metrology associated with such performance needs to be designed with utmost care. This requirement concerns all the instrumentation directly linked with the specific T2L2 equipment as well as the instrumentation doing the link between the laboratory reference and the T2L2 ground segment. Several campaigns were done to demonstrate both the ultimate time accuracy and time stability capabilities of T2L2. The paper is focused on the current high accuracy equipment that has been designed for the picosecond metrology and on some recent campaigns involving global calibrations of both laser stations and GNSS equipment. Results obtained during two months of comparisons between GPS in common view and T2L2 at three European laboratories show some differences below 300 ps with a standard deviation better than 500 ps. This is the first time that two different techniques of time transfer independently calibrated are in agreement at sub-ns level over continental distances.

Link calibration against receiver calibration time transfer uncertainty when using the Global Positioning System

We present a direct comparison between two different techniques for the relative calibration of time transfer between remote time scales when using the signals transmitted by the Global Positioning System (GPS). In the remote sites, the local measurements are driving either the computation of the hardware delays of the local GPS equipment with respect to a given reference GPS station, a “receiver” calibration, or the computation of a global hardware offset between two distribution reference points of the remote time scales, a “link” calibration. Both techniques do not require the same measurements on site, and we discuss the uncertainty budget computation differences. We report on one calibration campaign organized during Autumn 2013 between Observatoire de Paris (OP), Paris, France, Observatoire de la Côte dAzur (OCA), Plateau de Calern, France, and NERC Space Geodesy Facility (SGF), Herstmonceux, United Kingdom. We show the different ways to compute uncertainty budgets, leading to improvement factors of 1.2 to 1.5 on the hardware delay uncertainties when comparing the relative link calibration to the relative receiver calibration.

Intensity interferometry revival on the Côte d’Azur

Recent advances in photonics have revived the interest in intensity interferometry for astronomical applications. The success of amplitude interferometry in the early 1970s, which is now mature and producing spectacular astrophysical results (e.g. GRAVITY, MATISSE, CHARA, etc.), coupled with the limited sensitivity of intensity interferometry stalled any progress on this technique for the past 50 years. However, the precise control of the optical path difference in amplitude interferometry is constraining for very long baselines and at shorter wavelengths. Polarization measurements are also challenging in amplitude interferometry due to instrumental effects. The fortuitous presence of strong groups in astronomical interferometry and quantum optics at Université Côte d’Azur led to the development of a prototype experiment at Calern Observatory, allowing the measure of the temporal correlation g(2)(τ, r=0) in 2016 and of the spatial correlation g(2)(r) in 2017 with a gain in sensitivity (normalized in observing time and collecting area) of a factor ~100 compared to Hanbury Brown and Twiss’s original Narrabri Interferometer. We present possible ways to further develop this technique and point to. possible implementations on existing facilities, such as CTA, the VLTI ATs or the summit of Maunakea, which offer a unique scientific niche.

Absolute distance measurements using two-mode laser telemetry

A novel laser ranging method is described that uses a two-mode laser source, and detection of the phase of the return beam. The design eliminates the cyclic error usually associated with phase measurements and provides unambiguous, absolute distance determination. Measurements of an ≈ 8m path are obtained at a beat frequency of 13 GHz. We analyse the ≈1 μm stability of the data obtained with this preliminary implementation, and expect that an improved version will allow accuracies well below 1 μm, for the kilometer-scale distances involved in satellite formation flight.

Calibration of the TWSTFT link between OCA and OP using a GPS link calibration

Three independent time transfer techniques are in operation in Observatoire de la Côte dAzur (OCA): GPS, TWSTFT and T2L2. In Autumn 2013, a GPS receiver relative calibration campaign has been carried out. The result of this campaign, primarily intended to compare GPS with T2L2, have allowed the calibration of the TWSTFT link between OCA and OP. We computed for OCA the value of CALR = 7111.9 ns, with an uncertainty u = 2.8 ns at k = 2. From this calibration we caracterise the others links with the triangle closure technique.

Remote steering of OCA local time scale using UTC(OP)

In October 2012, the Géoazur TF laboratory in OCA has been reorganized to improve signal distribution stability. In particular we have implemented a new time scale TA(OCA) based on H-maser T4Science. The H-maser frequency is steered with an HROG-10 microphase stepper. The correction applied is computed every day taking into account the frequency of the free running maser and the actual time difference between TA(OCA) and UTC(OP). The main idea is to keep the time difference between our local time scale TA(OCA) and UTC below 50 ns, in order to time tag the SLR laser pulses and to have an error below 1 mm for distance measurement between satellite and laser station.

On recent progress in all-fibered pulsed optical sources from 20 GHz to 2 THz based on multiple four wave mixing approach

In this paper, we report recent progress on the design of all-fibered ultra-high repetition-rate pulse sources for telecommunication applications around 1550 nm. Based on the nonlinear compression of an initial beat-signal in optical fibers through a multiple four-wave mixing process, we theoretically and experimentally demonstrate that this simple technique allows an efficient and accurate design of versatile pulse sources having repetition rates and pulse durations ranging from 20 GHz up to 2 THz and from 10 ps up to 110 fs, respectively.