Observatoire Cantonal de Neuchâtel      

Introduction

Historical context

Important dates of the Observatory

Atomic Clocks background and activities

Time and Meteorological services

Conclusion

 

Address and contact

 

Introduction

The cantonal Observatory of Neuchâtel (ON) is a research and development institute depending on the Public Economy Department of the Neuchâtel State (Switzerland), and employs about thirty collaborators.

The ON dedicates its R&D activity to high precision time measurement (atomic clocks) and to atmospheric studies by optical radar (lidar). The instruments which are developed in the frame of the research projects are aimed at a large number of applications in space as well as on earth: navigation, telecommunications, metrology, climate study, fundamental research.

The ON has also service activities: contribution to the international atomic time (TAI), to the dissemination of the exact time in Switzerland (with METAS – Swiss Federal Office of  Metrology), meteorological statements.

The ON is an "external support laboratory" of the European Space Agency (ESA) in the field of time and frequency. The ON also maintains connections with universities, high-schools, governmental research institutes and industries.

The scope of this document is to give a short overview of the ON activities.

 

Historical context

The creation of the Neuchâtel Observatory can be explained by the historic location of an important watch making industry in the area of Neuchâtel and in the nearby Jura. Instituted by a decree of the government of Neuchâtel in 1858, its original missions were:

 

*      To control, as an official institution, the watches produced by the watch making industry and to deliver certificates of the relevant quality.

*      To be a reference time center for the watchmakers and Swiss institutions.

*      To create scientific work capabilities.

 

Until 1967, the duration of a time unit (a second), and consequently the exact time, were defined with respect to the earth rotation. This rotation period was measured with astronomic instruments ensuring the observation of stars and the precise determination of their passage through zenith. This leads to a great utility of the Observatory.

 

Fig. 1: Historic cupola of the Observatory

(Hirsch pavilion – Telescope with Zeiss refracting device, 1912)

 

This historic telescope of the Observatory is still functional today. Its use is not part anymore of research activities. Nevertheless, the telescope use is managed by a group of amateur astronomers of Neuchâtel.

 

Important dates of the Observatory

1858

Foundation of the Observatory

1860

First chronometric controls

1864

Introduction of meteorology

1912

Introduction of seismology

1934

First diffusion of time signals from Observatory

1963

Construction of the first atomic clock of the Observatory (thallium)

1967

Replacement of  astronomical time reference by atomic time reference

1967

Start of the HGB 75 kHz time signal transmitter in Prangins near Geneva

1987

Creation of the "spin-off" Precitel for the manufacturing and marketing of the Vip-Line and the radio-synchronized modules developed at the Observatory

1988

Starting of the "rubidium activity" for the realization of miniaturized industrial and spatial atomic clocks

1989

Start of the "hydrogen maser activity" for radio-astronomy

1990

Renovation, extension and improvement of the scientific park of the Observatory

 

1991

Start of the "cold atoms activity", in order to realize a primary cesium clock for the Swiss Federal Office of Metrology (METAS)

1991

Start of the "lidar activity" (atmospheric research)

1993

Start of the "space maser activity"

1993

Nomination of the Observatory as external support laboratory of the European Space Agency (ESA)

1996

Creation of the "spin-off" Temex Neuchâtel Time for the manufacturing and marketing of the rubidium clocks developed at the Observatory

 

Today, the main activities of the Observatory are on the one hand directed towards the design and construction of innovative atomic clocks, especially in the space field, and on the other hand towards lidar activities in the frame of atmospheric observations. Nevertheless, services activities such as exact time dissemination and meteorological reports are also part of ON missions.

 

Atomic Clocks background and activities

ON is a unique institute which has developed so far all classical microwave atomic clocks: rubidium, cesium, hydrogen maser.

 

Hydrogen Masers

Maser stands for Microwave Amplification by Stimulated Emission of Radiation. The maser activity at ON started in 1989.

The first activity was devoted to the development of hydrogen masers for ground applications (c.f. fig.2b), i.e. Very Long Baseline Interferometry (VLBI) and deep-space tracking. For this first application ON has manufactured for radio-astronomical observatories 26 masers (from 1982 up to end of 2001). ON also has the responsibility of the after sales service and of maintenance contracts.

A second application was one of the objectives of an ESA contract starting in 1988 for the development of an advanced hydrogen maser (called supermaser). Top priorities were the maser reliability and lifetime. The 250 kg supermaser was successfully tested in 1995 with a tracking experiment of the Ulysses spacecraft at the ESA station of Kourou (Guyana) and is now part of the equipment that ON maintains for ESA as external support laboratory in the field of time and frequency. The supermaser is used extensively as a test bed for studies of new space maser technologies, as described below.

The Space Hydrogen Maser (SHM) is a state-of-the-art ultra-stable atomic frequency standard based on a sapphire loaded miniature microwave cavity.  It is capable of frequency stability in the order of 1x10-15 over averaging intervals of 1’000s to 10’000 s.  If used as a clock, this is equivalent to a time stability in the order of 1 to 10 ps over time intervals of 1'000 s to 10’000 s respectively.

The maser uses the hyperfine transition of atomic hydrogen at 1'420'405'751 Hz.  A plasma discharge dissociator first dissociates hydrogen molecules. A collimated beam of hydrogen atoms passes through a magnetic atomic state selector.  Atoms in the selected energy state enter the storage bulb via another collimator. The storage bulb is located inside a microwave cavity tuned to the hydrogen atom resonant frequency. If the flux of useful atoms is high enough, about 1012 atoms/s, the energy available from the stimulated emission of radiation is sufficient to overcome the microwave losses of the cavity, creating sustained atomic oscillation (active maser). There is another possible mode (passive maser), in which the atomic resonance needs to be interrogated with an externally generated microwave signal.

The linewidth of the atomic signal is determined by the average time the hydrogen atoms spend inside the storage bulb and by several atomic relaxation processes.  The power of the atomic signal is about 1x10-14 W, and the line width is less than 1 Hz at about 1420 MHz.

ON has already successfully designed, manufactured and tested a demonstration model of SHM, made up by a prototype physics package and a breadboard electronics package. All critical electrical specifications have been demonstrated. Furthermore the microwave cavity, (a complex electrical,  thermal,  and mechanical structure which is the heart of the system) and its shield assembly, survived two vibration tests. The first one for Radioastron program and the second for GPS program at qualification levels.

ON is presently developing two new innovative space hydrogen masers:

Firstly, a passive space maser (SPHM) for GalileoSat program:

Although based on the same physics principle as a passive hydrogen maser for ground applications it represents a novel technological development. The stringent requirements, for launch survival and unattended operation in space environment for ten years, called for improved design approaches in all sub-assemblies of the instrument. It will be used as a frequency generator in precise positioning, precise time keeping and other applications requiring a frequency source with outstanding frequency stability  performance for averaging time in the range between 1 and 100'000s. The mass budget objective is of 15kg and the expected relative frequency stability is about 1x10-14 for a 10'000s sampling time.

Secondly, an active space maser (SHM) for ACES program (Atomic Clock Ensemble in Space, c.f. fig. 2a):

ACES is a program to test the performance of a new type of atomic clock (laser cooled cesium clock - c.f. cesium clocks section) that relies on micro-gravity conditions to provide better performance than any comparable ground clock. Comparisons between the micro-gravity cesium clock and the SHM will take place on the International Space Station. It will allow the validation in space of the performance of this new generation clocks, providing an ultra high performance global time scale, and performing fundamental physics tests. The SHM will serve as a short term reference clock to verify the performance of the laser cooled cesium clock and as a local oscillator for the atomic resonance interrogation. The microwave and the optical links will allow laboratories on the ground to receive the data and to interact with the space-based systems.The mass budget objective for SHM is of 35kg and the expected relative frequency stability is about 1x10-15 for a 10'000s sampling time (one order of magnitude better than for SPHM).

These two projects are very challenging as they represent the state-of-the-art of masers capabilities in terms both of performance and compactness.

 

 

Fig. 2a: Overall design of SHM maser

 

Fig.2b: Physics package (essentially composed of the microwave cavity and its shields) ofanON hydrogen maser (type EFOS-C) for ground applications

 

 

Cesium Fountain Clock

ON is developing a laser cooled cesium continuous fountain clock, the new generation primary time standard, under a contract of the Swiss Federal Office of  Metrology (METAS).

As the accuracy of an atomic clock is crucially dependent on the time spent by the atoms in the interaction region, the idea in this type of clock is to cool down cesium atoms before sending them as a slow atomic beam into the microwave cavity in order to enhance this interaction time. Its principle of operation and the apparatus are presented hereafter (c.f. fig. 3a).

The lower part of the vessel vacuum is separated in two compartments: source and detection. They are efficiently isolated from each other for stray light and cesium atoms by appropriate traps and getters, while being connected to the same ion pump. Cold atoms are produced in the source part by capture  and cooling from a thermal cesium vapour at the intersection of three pairs of mutually orthogonal  laser beams. Following transverse cooling, the longitudinal and transverse temperatures in the atomic beam are 80 mK and 5 mK, respectively. During their ballistic flight, the atoms pass twice through the same cavity in which an oscillating magnetic field is maintained. The apogee is 0.3 m above the cavity, yielding a 0.5 s interaction time and an atomic line Q of 1010.

 


 

 

1-      cooling and launching laser beam

 

2-      transverse cooling laser  beam

 

3-      Magnetic shields

 

4-      Vacuum chamber

 

5-      Parabolic flight path of the continuous cold atoms fountain

 

6-      Microwave cavity

 

7-      Rotating optical trap

 

8-      Detection laser beam

Fig. 3a: Primary standard based on a continuous fountain of cold cesium atoms

Fig. 3b: Apparatus for research on continuous beams of  laser-cooled cesium atoms

 

Due to the continuous beam operation, the accuracy of the clock is not affected by atomic collisions, and its stability is not degraded by aliasing effects from the quartz local oscillator. The continuous fountain at ON is presently the only alternative to a number of similar pulsed fountains under development around the world, of which three have been evaluated as primary standards. The laboratory prototype clocks is currently in operation and under evaluation. The relative accuracy objective is < 1x10-15. When fully evaluated (end 2002), it will provide uniquely meaningful comparisons with other standards worldwide.

 

Laser cooling research

The development of cold atom sources is of great importance in the perspective of the innumerable applications of laser cooling of atoms in basic and applied research, such as atomic physics (Bose-Einstein condensation, cold collisions, high resolution spectroscopy, spectroscopy of rare isotopes), atom optics, atom interferometry, metrology (atomic frequency standards, inertial sensors for acceleration and rotation rate measurements), nanotechnology (atomic lithography for the fabrication of structures controlled at the atomic level). These are  some of the fields that have already benefited, or are expected to benefit in the near future, from laser cooling of atoms.

The development of continuous sources is motivated by the advantages of the continuous availability of a measurement signal in all metrological applications (particularly true in the field of time and  frequency metrology and which leads to a  better stability).

Research now is focussed on production of intense beams of slow (few m/s) and cold atoms (50-100 mK longitudinal temperature). The further reduction of the transverse temperature (presently 5 mK) requires new cooling techniques downstream of the atomic source. The recoil-induced resonance technique  is used as a diagnostic method for local, and directional velocity-field measurements (c.f. fig. 3b).

These fundamental physical and metrological studies will also be an important contribution to the development of a new generation of clocks for use in space (c.f. hydrogen masers section related to SHM). Such clocks are expected to be more accurate (micro gravity) and might further enhance the already spectacular range of fundamental experiments and applications of space atomic frequency standards (navigation systems such as GPS and Galileo, communication, fundamental physics, radio-astronomy and cosmology).

 

Rubidium Clocks

The possibility of  realizing and industrialising compact atomic frequency standards appeared in the early sixties. The rubidium atom was preferred for practical reasons. Due to its structure, the rubidium cell clock is a secondary standard (i.e. its frequency should be calibrated with a primary source). Its frequency stability performance is in the range limited below by a quartz oscillator and above by a cesium clock (excellent short-term frequency stability < 10-11 t-1/2  for 1 s < t < 10’000 s). This type of atomic clock is principally used for positioning, navigation and telecommunication systems, and its volume can be in the order of 0,2 dm3.

ON is involved in the development of gas-cell rubidium atomic frequency standards since 1987. The expertise of ON includes the use of  discharge lamp or  laser diode, as well as the utilisation of wall-coating. In addition, rubidium prototype clocks for industrial and for space applications were fully engineered and manufactured.

The lamp-pumpedrubidium gas-cell standards realised in ON from 1987 to 1996 are based on the so-called "integrated-filter technique" which allowed a substantial miniaturisation of the physics package and of the driving electronics, without compromising the performances. In addition, a "magnetron-type" microwave resonator was developed, which permitted a further reduction of the clock size, and for which ON holds a patent. The activity included both technological aspects (such as feasibility studies, material studies, thermal and mechanical design, etc.) and scientific studies related to the analysis of the main physical phenomena occurring in the resonance cell (temperature coefficient, light-shift, microwave power shift, etc.). A number of scientific publications and conference communications report on this activity, which was concluded in 1996 with the realization of 10 prototypes of an industrial version (RUSO-Rubidium Ultra Stable Oscillator) and 5 (completely space qualified) flight models (S-RUSO, space type). A technological transfer to industry took place in 1996. A spin-off company, Tekelec Neuchâtel Time (today named Temex Neuchâtel Time), started the industrialisation, the production and the marketing of the industrial RUSO. The director of the company is the former associate director of the ON. This company is highly successful.

In parallel to these developments, ON was involved in the field of laser-pumped gas-cell frequency standards. From 1987 to 1991, as AlGas laser diodes tunable around 800 nm became easily available, preliminary studies were realized in order to determine the potential of this technology for replacing the discharge lamp. The main issues and the main problems were identified and investigated (optimisation of resonance signal, requirements on laser noise, degradation of performance due to light shift, reliability and lifetime of diodes, etc.). From 1991 to 1995, these investigations were realised in the frame of a PhD thesis research, where systematic experiments were made and a theoretical model developed. Based on this expertise, ON is currently developing a demonstration instrument for the European Space Agency, in view of the second generation of frequency standards for the satellite navigation system "Galileo". In parallel, a more basic research is conducted for developing new strategies in order to limit the degradation due to light-shift and to the spectral properties of the optical and microwave spectra. Finally, the novel approaches which employ the so-called "coherent dark state" of the atoms (instead of intensity optical pumping) is also investigated.

During the last 15 years, different R&D activities at ON have involved frequency stabilized laser diodes. In addition to the above mentioned research related to gas-cell atomic frequency standards, laser diodes have been employed for cesium beam frequency standards, cesium atomic fountain, laser cooling, high spectral resolution lidars, miniature lidars for airborne and space applications.

 

 

Fig. 4a: View of an industrial lamp-pumped rubidium, developed at ON and now produced by Temex Neuchatel Time (demonstration unit). The (magenta) light is emitted from the discharge lamp (on the right size), passes through the resonance cell (left) and the resonance signal is detected by a photodetector placed behind the cell. Some of the electronics printed circuit are visible. The full clock was less than ¼ liter in volume, and has been further miniaturised since then.

 

Fig. 4b: View of the complete ON space-qualified lamp-pumped rubidium on its base-plate (1996). This unit should have flown on the Russian "Radioastron" satellite (space radio-telescope).

 

 

 

Lidar background and activities

Lidar (Light Detection and Ranging) is a widely used optical technique of remote sensing. The sensed target can be - among others - a reflecting solid object or the atmosphere containing backscattering aerosols. Since range and angular resolution is normally required, a laser source capable of fast modulation or switching must be used. Typically, high peak power (»1-10 MW) short pulsed (»10ns) lasers are used, but many applications call for compact lasers sources (preferably solid state/semiconductor) in which maximum instantaneous power is significantly lower.

Atmospheric lidar activity carried by ON (started in 1991), includes investigation of lidar technology for space application, development of atmospheric lidar instruments and their application in specific studies and campaigns.

The main realizations and research topics of the ON lidar group are:

 

*      PRN-CW (Pseudo Random Noise - Continuous Wavelength) lidar (c.f. fig.5) technical feasibility and development completed under an ESA/ESTEC contract.

*      Realization of micro-joule lidars for airborne application and airborne measurement campaigns in polar and tropical stratosphere and high troposphere (contribution to EU projects APE-POLECAT and APE-THESEO).

*      Ground based backscatter lidar realization and studies of sub-polar stratosphere (contribution to EU projects ELSA/ EASOE, MOANA/SESAME, SAONAS, EARLINET).

*      Development of stimulated Raman shifters for use as transmitters in ultra-violet DIAL (Differential Absorption Lidar) for ozone detection.

*      Numerical study of spontaneous Raman effect in ultra-violet Raman-DIAL instrument for ozone detection.

*      Backscatter lidar measurement of the variation of the PBL (Primary Boundary Layer) stratification during Foehn (FORM-Mesoscale Alpine Program).

 

Fig.4: ON lidar developed for demonstration of pseudo random noise modulation (ESA contract). This technique is used to preserve range resolution when using continuous low power laser sources. Its dimensions are 51x32x17 cm.

 

ON will soon participate with its airborne lidar instruments MAL-1 (Miniaturized Aerosol Lidar - c.f. fig. 5b) and MAL-2 to the validation of ENVISAT (Environmental Satellite - ESA) with the support of a Russian Mjasishchev M-55 research aircraft (c.f. fig.5a). These lidars are short-range depolarisation backscatter lidars. The objectives for their installation on this high altitude research aircraft is to detect optically thin clouds in the vicinity of the aircraft at stratospheric and high tropospheric altitudes in the tropics. Depending on the altitude and the geographical location of the flight, those clouds may be: cirrus clouds, polar stratospheric clouds, tropical high troposphere/tropopause laminar clouds, aerosol layers in the high troposphere and the lower stratosphere. MAL-1 and MAL-2 are self-contained single unit instruments. MAL-1 is installed to detect clouds and aerosols above the aircraft, while MAL-2 is installed to probe for clouds and aerosols below the aircraft.

These two instruments have already participated to the following flight campaigns:

 

* ETC (Extensive Test Campaign, Forli, Italy Dec. 1998-Jan. 1999).

* APE-THESEO (Airborne Platform for Earth observation - Third European Stratospheric Experiment on ozone, Seychelles, Feb.-March 1999).

* APE-GAIA (Airborne Platform for Earth observation - Geophysica Aircraft in Antarctica, Argentina, Ushuaia, Sept.-Oct. 1999).

 

 

 

 

Fig. 5a: A view of the Russian Mjasishchev M-55 “Geophysica” high altitude research aircraft

 

 

 

Fig. 5b: The lidar MAL-1 in its pressurised box and supporting frame, ready for installation on “Geophysica”. The power supply for ground test is seen left from the lidar.

 

 

 

Time and Meteorological services

Time services

The ON has always had close collaborations with the "Bureau International de l'Heure" (created in 1919, and is connected since 1987 to the "Bureau Internationnal des Poids et Mesures - BIPM). ON has participated to the elaboration of the Universal Astronomical Time and afterwards to the International Atomic Time (TAI) with its cesium clocks ensemble. Up to 1982, ON was the only furnisher of a reference time scale in Switzerland, and since 1985, its clocks are connected to Swiss Federal Institute of Metrology (METAS) for the elaboration of a unique time scale in Switzerland (UTC-CH).

The time dissemination is ensured by a 20 kW transmitter through all central Europe (HGB 75 kHz, in Prangins, near  Geneva).

 

Meteorological services

Since 1864, the ON is a meteorological station of the Swiss observation network for the Swiss Institute of Meteorology (altitude: 488m). The daily statements concern temperature, pression, humidity, pluviometry and winds measurement.

 

Conclusion

This short overview of the ON activies points out the expertise acquired during the last fifteen years in the field of time and frequency standards, and the more recent but also very promising activities in the lidar field. Furthermore, ON is also a well known Swiss institute for its time and meteorological services.