EP1730608B1 - Method for modulating an atomic clock signal with coherent population trapping and corresponding atomic clock - Google Patents

Abstract

Method for modulating an atomic clock signal and a corresponding atomic clock. The laser beams (L1, L2) are pulse-modulated in amplitude to illuminate (A) an interactive medium. A detection (B) of the current pulse (Sr) and of the pulses (Sr-1 to Sr-p) preceding said current impulsion is performed. The pulses are superimposed (C) by linear combination to generate a compensated atomic clock signal (SHC) whereof the spectral width is minimized. The invention is applicable to atomic clocks with pulsed interrogation whereof the interactive medium consists of thermal or laser-cooled atoms.

Description

Atomic clocks with consistent population trapping, designated CPT clocks for "Coherent Population Trapping" are known from the state of the art.

In general, as well as represented in figure 1a , the atomic clocks use an interaction medium, generally formed by cesium or rubidium atoms excited by a radio-electric signal generated by a local oscillator LO and a synthesizer S at an excitation frequency and formed by a signal microwave at 6.8 GHz respectively 9.2 GHz for rubidium and cesium. The atoms of the interaction medium are excited between two energy levels e and f represented in figure 1b . This mode of excitation is called Rabi's interrogation mode if the interaction is continuous and Ramsey interrogation mode if the interrogation is based on two short interactions separated by a dead time.

The response signal from the interaction has an amplitude depending on the tuning to the resonance of the excitation signal. Detection of the response signal can be performed by optical absorption, magnetic selection, optical fluorescence or magnetic detection.

A system for controlling the local oscillator from the response signal makes it possible to obtain at the output of this oscillator a periodic signal S _u , having qualities of accuracy and frequency stability comparable to those of the resonance frequency. e → f.

Taking again the general principle of the servocontrol previously described, the CPT clocks also use an interaction medium illuminated by two laser waves and implement a continuous interrogation mode.

In a previous embodiment, the interaction medium consisting of sodium is spatially separated into two distinct interaction zones, separated by a distance of 30 cm.

The laser beams make it possible to generate a Raman transition resonance at 1 772 MHz, the central fringe of the Ramsey fringe pattern being reduced to a width of 650 Hz, thanks to an interaction produced in the interaction zones.

For a more detailed description of this type of atomic clock it will be useful to refer to the article entitled "Observation of Ramsey Fringes Using a Simulated, Resonance Raman Transition in a Sodium Atomic Beam" published by TE Thomas, PR Hemmer, and S. Ezekiel Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Mass. 02 139 and CC Leiby, Jr., RH Picard, and CR Willis, Rome Air Development Center, Hanscom Air Force Base, Massachusetts 01 731 PHYSICAL REVIEW LETTERS Volume 48, Number 13, 29 March 1982 .

We also know the article entitled " Coherent Population trapping with cold atoms "published by T.Zanon et al., 2003 IEEE International Frequency Control Symposium . This article discloses a method of generating a clock signal according to the preamble of claim 1 and an atomic clock according to the preamble of claim 7.

In general, CPT type atomic clocks implement a continuous mode interrogation, by means of two coherent laser waves in phase. Each laser wave is quasi-resonant with an optical transition of the atoms 2 → e and 2 → f and the difference between the frequencies of the two waves is close to the atomic reference frequency f → e. When the agreement f → e at resonance is satisfied, the atoms of the interaction medium are trapped in a coherent superposition of the states f and e corresponding to a black state. There is a decrease in the amplitude of the absorption of the laser waves, a decrease in the amplitude of the fluorescence signal. The coherent superposition of atomic states is also associated with a magnetization generating an electromagnetic wave oscillating at the frequency of the transition e → f in the microwave domain.

Fluorescence absorption or emission is minimal and the field of the electromagnetic wave emitted has a maximum amplitude at resonance. The atomic clock signal corresponds to the variation of the amplitude of the signal detected by absorption, fluorescence or microwave emission, as a function of the value of the frequency difference of the laser waves.

In all types of CPT atomic clock currently known, the interrogation of the interaction medium is continuous, the laser waves interacting continuously with the atoms of the interaction medium.

However, in the aforementioned types of atomic clocks, an excessive intensity of illumination of the interaction medium by the laser waves causes the widening of the resonance lines obtained, due to the optical saturation of the atoms of the interaction medium. .

This disadvantage leads to a degradation of the stability of the frequency of the atomic clock signal.

For this reason, in the currently existing CPT atomic clocks, the aforementioned technical problem is solved by simply reducing the intensity of illumination of the interaction medium by the laser beams used.

Such a measure does not solve the aforementioned technical problem because it actually results in an increased difficulty of detecting atomic clock signals of low amplitude, resulting from the interaction.

The above-mentioned low amplitude atomic clock signals are detected under degraded signal-to-noise ratio conditions, which again results in a degradation of the frequency stability of the atomic clock.

The object of the present invention is to remedy the technical problem of the optical saturation of the interaction media of the atomic clocks, in particular CPT or other clocks, while maintaining non-degraded signal-to-noise ratio conditions.

Another object of the present invention is, furthermore, through a specific processing of the response signal generated by the interrogation of the interaction medium in the current CPT atomic clocks, to obtain an increase in the contrast of the fringes of the interference in Ramsey mode and a decrease in slow amplitude variations or drifts of the atomic clock signal, generated in particular by the irreducible fluctuations of the operating parameters, such as the frequency and the amplitude of the interrogation lasers of the interaction medium.

Another object of the invention is, finally, the implementation of a method for generating a clock signal CPT and a corresponding clock CPT allowing a miniaturization of this type of clock for the production clocks in which the interaction cell does not exceed a volume of a few mm ³ .

The method for generating a coherent population-entrapped atomic clock signal, which is the subject of the present invention, uses a first and a second phase-coherent laser wave, each substantially resonant with an optical transition of the atomic atoms. an interaction medium. The coherent superposition of the atomic states corresponding to the coherent trapping of the atomic population makes it possible to generate a response signal exhibiting a amplitude resonant at the resonance and representative of the atomic clock signal corresponding to the amplitude variation of the detected signal as a function of the value of the frequency difference of the first and second in-phase coherent laser waves.

It is remarkable in that it consists at least in modulating in synchronism by successive pulses the intensity of the first and second laser waves, according to a given form factor between a high level and a low intensity level, the response signal generated during a current pulse depending on the atomic state generated during at least one pulse preceding this current pulse and the evolution of this atomic state during the duration of the low level of intensity separating these pulses.

The response signal is detected and superimposed by linear combination of the response signal generated during this current pulse and at least one pulse preceding this current pulse, to generate a resulting compensated atomic clock signal, the spectral width of which is minimized.

The pulsed interrogation atomic clock, which is the subject of the present invention, comprises at least one interrogation optical module making it possible to generate a coherent first and second phase-coherent laser beam, each substantially resonant with an optical transition of the atomic atoms. an interaction medium, an interaction cell comprising this interaction medium, illuminated in operation by the first and second coherent laser beams in phase, to generate a response signal having an extreme amplitude at resonance and corresponding to the amplitude variation of the detected signal as a function of the frequency difference of the first and second coherent laser beams in phase and a detection module of this response signal adapted to the wavelength and amplitude of the response signal .

It is notable in that it further comprises a pulse modulating block of the intensity of the first and second laser beams between a high level and a low intensity level. This modulation block is placed on the path of the first and second laser beams, upstream of the interaction cell, to synchronously generate a first and a second pulsed laser beam. The interaction between the first respectively the second laser beam and the interaction medium is substantially limited to the duration of each successive pulse corresponding to a high intensity level and the signal of The response generated during a current pulse depends on the atomic state generated during at least one pulse preceding this current pulse and the evolution of that atomic state during the low intensity period between these pulses.

In addition, the detection module comprises a summation module by linear combination of the response signal generated during this current pulse and the response signal generated during at least one pulse preceding this current pulse. The summation module by linear combination makes it possible to generate a resulting compensated atomic clock signal whose spectral width is minimized.

The method and the atomic clock with coherent trapping of population objects of the present invention find application to the industrial implementation of timepieces or frequency reference embedded very small footprint, used in particular in space applications.

• la figure 2a représente, à titre purement illustratif, un organigramme des étapes essentielles de mise en oeuvre du procédé objet de la présente invention ;
• la figure 2b représente, à titre purement illustratif, au point 1), un chronogramme des signaux d'impulsions de faisceau laser pulsé susceptibles d'être utilisés pour la mise en oeuvre du procédé objet de l'invention décrit en figure 2a, et, au point 2), un chronogramme du signal de réponse obtenu après détection en sortie de la cellule d'interaction ;
• la figure 3 représente, à titre purement illustratif, un schéma fonctionnel d'une horloge atomique CPT conforme à l'objet de la présente invention, permettant la mise en oeuvre du procédé décrit en liaison avec les figures 2a et 2b;
• la figure 4a représente, à titre illustratif, un schéma détaillé d'un module de traitement du signal de réponse après détection, dans un mode de réalisation préférentiel non limitatif, ce module de traitement du signal de réponse étant plus particulièrement adapté à l'exécution d'un traitement numérique dédié ;
• la figure 4b représente, à titre illustratif, un chronogramme d'exécution d'opérations sur des valeurs échantillonnées d'impulsions successives de signal de réponse, plus particulièrement sur une impulsion courante et au moins une impulsion précédant cette impulsion courante, les opérations conduites sur les valeurs échantillonnées précitées permettant en particulier d'améliorer sensiblement la pureté spectrale et le contraste du signal d'horloge atomique compensé résultant obtenu, suite à l'exécution de ces opérations ;
• la figure 4c représente, à titre illustratif, un diagramme amplitude-fréquence du désaccord Raman, désaccord de la différence de fréquence entre les deux ondes laser et du motif de franges de Ramsey obtenu en sortie du module de traitement dédié représenté en figure 3, après application d'une superposition par combinaison linéaire du signal de réponse engendré pendant une impulsion courante et au moins une impulsion précédant cette impulsion courante.

They will be better understood by reading the description and by observing the drawings below, in which, in addition to the Figures 1a and 1b relating to the prior art:

• the figure 2a represents, purely by way of illustration, a flowchart of the essential steps for implementing the method which is the subject of the present invention;
• the figure 2b represents, for purely illustrative purposes, in point 1), a timing diagram of pulsed laser beam pulse signals that may be used for the implementation of the method forming the subject of the invention described in FIG. figure 2a and, in point 2), a timing diagram of the response signal obtained after detection at the output of the interaction cell;
• the figure 3 represents, purely by way of illustration, a block diagram of an atomic clock CPT according to the subject of the present invention, allowing the implementation of the method described in connection with the Figures 2a and 2b ;
• the figure 4a represents, by way of illustration, a detailed diagram of a processing module of the response signal after detection, in a preferred non-limiting embodiment, this response signal processing module being more particularly adapted to the execution of a dedicated digital processing;
• the figure 4b represents, for illustrative purposes, a chronogram of execution of operations on sampled values of successive pulses of response signal, more particularly on a current pulse and at least one pulse preceding this current pulse, the operations conducted on the sampled values above, in particular making it possible to substantially improve the spectral purity and the contrast of the resulting compensated atomic clock signal obtained following the execution of these operations;
• the figure 4c represents, for illustrative purposes, an amplitude-frequency diagram of the Raman disagreement, disagreement of the frequency difference between the two laser waves and the Ramsey fringe pattern obtained at the output of the dedicated processing module represented in FIG. figure 3 , after applying a superposition by linear combination of the response signal generated during a current pulse and at least one pulse preceding this current pulse.

The method of generating a coherent population-entrapped nucleic acid clock signal of the present invention will now be described in connection with the Figures 2a and 2b .

In general, it will be recalled that, in accordance with the principles of the operating mode of the CPT atomic clocks, the method which is the subject of the present invention is implemented from a first laser wave L ₁ and a second laser wave L ₂ consistent in phase.

With reference to the figure 1b , each of the aforementioned laser waves is substantially in resonance with an optical transition of the atoms of an interaction medium, the laser waves L ₁ and L ₂ being deemed to be emitted at a frequency f ₁ and f ₂ and at their wavelength corresponding in vacuum or air, the frequency difference of the aforementioned laser waves being noted Δf ₁₂ . Preferably, the laser waves L1 and L2 are polarized either circularly or linearly orthogonally.

The coherent superposition of the atomic states corresponding to the coherent trapping of the atomic population as represented in FIG. figure 1b generates a response signal in the microwave domain having a amplitude at resonance and representative of the atomic clock signal corresponding to the amplitude variation of the detected response signal as a function of the value of the frequency difference Δf ₁₂ of the first and second coherent laser waves in phase L ₁ and L ₂ .

It is understood, in particular, that the mode of interaction of the first and second waves with the interaction medium corresponds to the continuous interaction mode known from the state of the art from the physical point of view.

However, and according to a particularly remarkable aspect of the method which is the subject of the invention, this consists at least in a step A of sequentially pulsing modulating the intensity of the first and second laser waves L ₁ , L ₂ according to a given form factor, between a high level and a low intensity level.

On the figure 2a in step A thereof, the laser waves L ₁ and L _{2 are represented which are} synchronously modulated by successive pulses, the successive pulses being deemed to have a rank r, r-1, ..., rp with respect to a growing time scale t.

By convention, the current pulse is deemed to have a rank r, the pulse immediately preceding this current pulse rank r-1 and successive previous pulses being deemed to have an earlier rank successively up to r-p.

It is furthermore understood that the laser waves L ₁ and L ₂ are superimposed on the same optical path, which of course makes it possible to obtain modulated laser wave pulses which are coherent and in phase under conditions which will be explained later in the description. .

Thus, it will be understood that the interaction between the first and second laser waves L ₁ , L ₂ and in particular the pulsed shape thereof and the interaction medium is substantially limited to the duration of each successive pulse S _r , S _r-1 to S _rp corresponding to a high level of intensity.

Consequently, the response signal generated during a current pulse, the pulse of rank r previously described, depends on the atomic state generated during at least one pulse preceding this current pulse, that is to say the previous pulses of rank r-1 to rp and the evolution of this atomic state during the duration of low level of intensity separating the aforementioned pulses.

Following the successive pulse modulation of the intensity of the first and second laser waves L ₁ , L ₂ and of course the illumination of the interaction medium by the laser wave pulses thus obtained, the object process of the invention consists particularly remarkably to detect in step B and superimpose by linear combination in step C the response signal generated during the current pulse response signal noted S _r of rank r corresponding to that of the illumination pulse of the same rank and at least one pulse preceding this current pulse to generate the resulting compensated atomic clock signal, whose spectral width is minimized.

On the figure 2a , the detection operation is represented in step B, the response signal being deemed to consist of the corresponding response signal S _r of rank r and the successive successive response signals S _r-1 to S _rp .

The linear combination superposition operation is represented in step C of the figure 2a and illustrated by the following linear combination formula: $${\mathrm{S}}_{\mathrm{HC}}\mathrm{=}{\displaystyle \underset{\mathrm{k}\mathrm{=}\mathrm{r}\mathrm{-}\mathrm{p}}{\overset{\mathrm{k}\mathrm{=}\mathrm{r}}{\mathrm{\Sigma }}}}{\mathrm{VS}}_{\mathrm{k}}{\mathrm{x\; S}}_{\mathrm{k}}$$

In the aforementioned formula, it is indicated that S _HC represents the resulting compensated atomic clock signal obtained by the aforementioned linear combination C _k designating a positive and / or negative weighting coefficient applied to each successive response signal pulse S _k .

By convention, as will be described later in more detail with respect to an atomic clock CPT according to the subject of the present invention, the weighting coefficient C _k relative to the rank k = r of the current pulse can be taken equal to 1, the rank coefficients k = r, identified with respect to the current pulse for the previous pulses, can then be taken successively equal to different negative values for example in order to correct and compensate the signal of atomic clock finally got. The final summation rank by linear combination k = r can be determined experimentally or taken as a parameter.

With reference to the figure 2b it is indicated that the pulse modulation process of the laser waves L ₁ and L ₂ is advantageously carried out by pulse train, the frequency of the modulation pulses being between 0.2 Hz and 10 ⁴ Hz.

With reference to the figure 2b above and at the time axis t, the high intensity level of each pulse for a given pulse train has a duration h and the low intensity level has a duration b.

Under these conditions, the frequency range of the modulated laser wave pulses represented in point 1 of the figure 2b and finally response signal of successive rows r, r-1, rp is given by the value 1 / h + b for the different values of h and b and the form factor defined by the value h / h + b is then between 10 ^-6 and 10 ^-1 .

It will of course be understood that the pulses I of modulated laser waves represented in point 1) can be obtained by an electronic control signal having exactly the temporal and / or frequency characteristics of those represented in point 1) of FIG. figure 2b above.

With regard to the choice of the duration interval b separating the current pulse of rank r from the pulse preceding this current pulse or any previous pulse of rank r-1 to rp in a pulse train of modulation, one indicates that this duration b is less than the life time of the hyperfine coherence existing between the two clock levels.

With reference to the figure 1b it will be recalled that the two clock levels concerned are the levels e and f which determine the frequency of the resulting atomic clock signal and that this life time depends essentially on the interaction medium considered.

One of the remarkable aspects of the process that is the subject of the present invention is in particular that the latter is capable of being implemented from interaction media consisting either of atomic populations the cells contained by cold atoms and, in particular, cooled by laser.

The interrogation process is advantageously constituted by a Ramsey interrogation mode with at least two pulses.

With regard to the technique for implementing the aforementioned interaction media, it is indicated that the thermal atoms are delivered in the form of steam or jet. Obtaining the laser-cooled atoms involves interacting the thermal atoms with correctly tuned laser waves with respect to optical transitions of the atoms. The radiation pressure induced by the laser waves makes it possible to rapidly reduce the kinetic energy of the atoms. Thus samples of cooled atoms are obtained with very low erratic speeds, of the order of 1 cm / s, corresponding to a temperature of 10 ^-6 K, much lower than that of thermal atoms, of the order of a few hundred meters per second at a temperature of 300 K.

With regard to the mode of implementation of a laser cooling cell of the atoms allowing the interaction of one or two pulse modulated laser beams, such an embodiment, known from the state of the art. , will not be described in detail. For this purpose, reference may be made to the French patent application published under number 2 730 845 in the name of the CNRS.

In the cooling process, it is recalled that the kinetic energy of the atoms or the variation of kinetic energy thereof is proportional to the temperature decrease from the initial value 300 K to 10 ^-6 K, the coefficient of proportionality depending on the Boltzmann constant.

With regard to the process of detecting the response signal and in particular the successive response signal pulses S _r to S _rp , the aforementioned detection process is advantageously chosen from the group of detection processes comprising optical absorption, Optical fluorescence, microwave detection as a function of the frequency of the interrogation signal.

It will be understood that the method which is the subject of the present invention can be implemented in many situations taking into account the nature of the interaction medium chosen, the interrogation mode being however preferentially the Ramsey interrogation mode with at least two pulses, as mentioned previously in the description. The detection processes are then the detection processes by optical absorption, optical fluorescence, microwave detection as a function of the frequency of the aforementioned interrogation signal.

The table below establishes the type of atomic clock capable of implementing the method that is the subject of the present invention, indicating the atomic source used to enable the implementation of the method, the interrogation method or mode and the process. detecting the corresponding clock signal. TYPE OF ATOMIC CLOCK ATOMIC SOURCE INTERROGATION MODE CLOCK SIGNAL DETECTION CPT (coherent trapping of population on thermal atoms in cells) Thermal steam with or without buffer gas Optical interrogation (clock transition in the microwave domain) Continuous in existing devices Pulsed interrogation in this type of clock Optical absorption or microwave detection CPT (coherent trapping of population on cold atoms) Steam + laser cooling Optical interrogation (clock transition in the microwave domain) Pulsed type query. Optical absorption or microwave detection

With reference to the aforementioned table, it is pointed out that atomic clocks of CPT type allow the implementation of the method which is the subject of the invention according to the figure 2a .

A more detailed description of a pulsed interrogation atomic clock in accordance with the subject of the present invention will now be given in connection with the figure 3 and the following figures.

In general, it is indicated that the architecture of the pulsed interrogation atomic clock in accordance with the subject of the present invention corresponds to that represented in FIG. figure 3 .

In particular, such a clock comprises, in an optical section SO, an interrogation optical module 1 for generating a coherent first and second laser beam in the L ₁ , L ₂ phase. As mentioned earlier, each of the aforementioned laser beams is substantially in resonance with an optical transition of the atoms of an interaction medium.

The pulsed interrogation atomic clock further comprises an interaction cell 3 comprising the aforementioned interaction medium.

By notion of interaction cell 3, it is indicated that the interaction cell can be constituted in a conventional manner by a transparent envelope to the laser beam L ₁ , L ₂ and of course, by any device generating the interaction medium, c i.e. thermal atoms and / or laser cooled.

The interrogation module 1 generates the two laser beams L ₁ and L ₂ whose difference in frequency is equal to the resonance frequency, the microwave frequency at 9.2 GHz for cesium and 6.8 GHz for rubidium. for example.

In the case of cesium, the frequencies of the laser diodes are in the neighborhood of 852 nm for the D _{2 line} and 894 nm for the D _{1 line} .

The aforementioned laser lines can be used for a CPT interaction as described above in the description.

Thanks to their greater hyperfine difference in the excited state, the transitions of the line D ₁ appear more interesting because they make it possible to reduce, on the one hand, the losses of atoms due to leakage on adjacent transitions, and, on the other hand, light shifts.

It is furthermore possible to use rubidium atoms for which the D _{2 line} is at 780 nm and the D ₁ line at 795 nm, the corresponding frequencies f ₂ and f ₁ being easily accessible with commercial laser diodes. .

Different processes can be implemented to generate two radiations, that is to say the laser beams L ₁ and L ₂ , which induce the coherent entrapment of the atomic population of the interaction medium. The frequency difference between the laser beams L ₁ and L ₂ is equal to the clock frequency, that is to say the frequency of the atomic clock signal. The phase difference between the phase of the laser beams L ₁ and L ₂ must have fluctuations as low as possible in order to avoid any destruction of the interference phenomenon. The required transmit power for laser beams is in the milliwatt range.

In a specific implementation mode, it is indicated that the interrogation optics can be made from a single laser source to which a frequency modulation at several GHz of the modulation type is applied. sidebands, the distance between the sidebands corresponding to the clock frequency. The two previously mentioned lines are thus provided with phase coherence as good as that of the modulation signal.

The two lines or laser beams L ₁ and L ₂ are then physically superimposed in a conventional manner so that the latter follow the same optical path and are subjected to the same successive phase shifts until they are applied to the interaction medium.

Regarding the interaction cell 3, it is indicated that it can be implemented from a pyrex or quartz enclosure.

In addition, buffer gases may be added to eliminate Doppler line broadening by placing in the Lamb-Dicke regime. The magnetic and thermal environment is tightly controlled to avoid any variation in frequency shift that would affect the accuracy and long-term stability of the atomic clock thus formed.

The pulsed interrogation atomic clock comprises, in a detection section SD, also a module 4 for detecting the response signal, the response signal being defined as the signal delivered by the interaction medium of the cell 3 after illumination of the interaction medium by laser beams L ₁ and L ₂ . The detection module 4 is of course adapted to the wavelength and the amplitude of the response signal to deliver an electronic version of the response signal.

More specifically, the module 4 for detecting the response signal may consist of modules implementing the detection processes as described in the above-mentioned table.

According to a particularly remarkable aspect of the pulsed interrogation atomic clock object of the present invention, it comprises a module 2 pulse modulation of the intensity of the first and second laser beams L ₁ and L ₂ between a high level and a low level of intensity.

Of course, as represented on the figure 3 , the modulation module 2 is placed in the optical section SO on the path of the first and second laser beam upstream of the interaction cell 3 for generating in synchronism a first and a second pulsed laser beam for illuminating the interaction medium contained in the cell 3, according to the figure 2a .

Due to the illumination of the aforementioned interaction medium by the first and the second laser beams or pulsed radiofrequency signal, the interaction between the aforementioned laser beams and the interaction medium is substantially limited to the duration of each corresponding successive pulse. at a high level of intensity.

Consequently, the response signal generated during a current pulse of rank r for example depends on the atomic state generated during at least one pulse preceding this current pulse, that is to say pulses of rank r-1 to rp previously mentioned in the description, and, of course, the evolution of this atomic state during the duration of low intensity energy level separating these pulses.

In addition, as shown on the figure 3 the detection module of the response signal 4 can be followed by a processing module 5, the processing module receiving the electronic version of the response signal and performing summation processing by linear combination of the response signal generated during the first response signal. current pulse and during at least one pulse preceding this current pulse, that is to say during the successive previous pulses. The linear combination processing module 5 thus makes it possible to generate a resulting compensated atomic clock signal whose spectral width is minimized and to construct a correction signal S _c making it possible to drive the frequency of a local oscillator 6.

On the figure 3 , the processing module 5 in fact delivers the correction signal Sc to the module 6 implanted in an analog section SA and constituted for example by a local oscillator LO and a synthesizer S delivering, on the one hand, a periodic signal servocontrolled in frequency S _u , for use as a frequency reference for an external user, and, secondly, a control signal S _co of the interrogation optical module 1.

This control signal S _co may for example consist of a frequency reference for performing the control of the modulation process in lateral bands previously mentioned in the description. to obtain the two laser beams L ₁ and L ₂ , from a single laser source, for example. It is indicated that the above-mentioned control signal S _co may also make it possible to provide control of the wavelength and / or the frequency of the laser beams L ₁ and L ₂ at the chosen value.

The mode of implementation of this servo control process will not be described in detail because it corresponds to an implementation mode known from the state of the art.

Of course, as well as represented further in figure 3 , the pulsed interrogation atomic clock, object of the present invention, is provided with a control unit 7 which can be constituted by a microcomputer connected by a bus link to all the modules such as the module of pulse modulation 2, the module 4 for detecting the response signal, and of course the processing module 5 and the module 6 acting as local oscillator LO and / or synthesizer S.

It is understood in particular that the control unit 7 makes it possible to ensure the synchronization of all the aforementioned modules as well as the control of the generated modulation pulse trains, from an electronic control signal, for example, developed by the control unit 7, to control the modulation module 2.

With regard to the modulus 2 for pulse modulating the intensity of the first and second laser beams L ₁ , L ₂ indicates that the latter may be constituted by an acousto-optical modulator, an electro-optical modulator or finally by any other component for modulating the intensity of a light signal whose response time is short enough to ensure such modulation.

More specifically, it is indicated that the low level of intensity corresponds to a substantially zero intensity of each of the laser beams or the radio frequency signal, these being totally absorbed by the modulation module 2 previously mentioned.

A more detailed description of the summation processing module 5 by linear combination of the response signal will now be given in connection with the figure 4a and the figure 4b .

In general, it will be understood that the above-mentioned processing module 5 receives the response signal in the form of an electronic signal delivered by the detection module 4.

To ensure the processing of the successive pulses S _r received, the processing module 5 can, as represented in FIG. figure 4a , advantageously comprising a sampling module 50 of the response signal generated during the interaction of the current pulse and at least one pulse preceding this current pulse, the aforementioned sampling module 50 being triggered in synchronism with the control of the module 2 for modulating the laser beams L ₁ and L ₂ .

The sampling module 50 is preferably followed by a module 51 for storing the sampled values of the response signal generated during the interaction of each of the aforementioned pulses.

Finally, the storage module 51 can be followed by a module 52 making it possible to calculate a linear combination of the stored sampled values making it possible to generate the compensated atomic clock signal S _HC previously mentioned in the description. From the latter, a module 53 formed for example by an integrator makes it possible to deliver the correction signal S _c to the module 6 constituted by the local oscillator LO and the synthesizer S, for example.

The synthesizer S makes it possible to generate a microwave signal whose frequency is close to the resonant frequency of the transition e → f.

Finally, the control unit 7 may advantageously be constituted by a workstation or a microcomputer comprising a control program of the assembly, so as to ensure the synchronization of the modulation module 2, the detection module 4 of the response signal from the processing module 5 previously described in connection with the figure 4a and, of course, the module 6 constituted by the local oscillator and the synthesizer previously described.

In particular, in a non-limiting embodiment, it is indicated that the control unit 7 may advantageously be programmed to ensure, by means of control software, a reading of the sampled values stored in the storage module 51 at given times. .

In particular, under these conditions, the control unit 7 can then comprise a program for sorting the sampled values stored for for each of the pulses S _r and S _{rp, determining} the maximum and / or minimum values of these sampled values for each of the aforementioned successive pulses.

Thus, in a non-limiting embodiment of the atomic clock which is the subject of the present invention, it is indicated that a treatment process may advantageously consist, as represented in point 2 of FIG. figure 4b for the current pulse S _r of rank r, to determine the sampled value of this pulse which has the maximum value, this maximum value being denoted M _r then for the successive pulses of prior rank r-1 to rp, to be determined in each of these the minimum of the corresponding sampled values in its successive pulses.

Thus, the corresponding minima are denoted m _r-1 for the previous impulse immediately preceding the current impulse, this anterior impulse being of rank r-1, then the successive values m _r-2 to m _rp for previous previous impulses of rank r-2 to rp.

According to a preferred non-limiting embodiment of the pulsed interrogation atomic clock object of the present invention, it is indicated that the linear combination of the sampled values can then consist in adding the maximum of the sampled values for the current rank pulse. r and to subtract the successive minimum values of the previous pulses of rank r-1 to rp, as represented on the figure 4b , or an average value thereof.

It is understood that the sorting program can then perform the sorting with respect to the origin of each of the pulses, these origins being noted successively or, O _r-1 , O _rp .

Thus, thanks to the implementation of the processing process performed by the processing module 5 represented in FIG. figure 3 , 4a and 4b it is understood, in particular, that the maximum M _r of the current pulse of rank r makes it possible to obtain the maximum amplitude value for the detected response signal while the subtraction of the successive sampled values representative of the local minima for the latter, on the contrary, makes it possible to subtract a sampled value representative of the drifts and perturbations introduced by the interaction medium contained in the cell 3, in order to obtain a compensated atomic clock signal whose spectral width is thus minimized and whose contrast can to be significantly improved, thanks to the removal of continuous or slowly variable components representative of the drift of the entire system.

Of course, and with the aim of increasing the speed of processing and obtaining real-time response for the digital part of the processing module, the modules 51, 52 and 53 can be replaced by a programmed dedicated signal processor. for this purpose.

Theoretical and experimental justifications relating to the performances obtained thanks to the implementation of the method and of a pulsed interrogation atomic clock in accordance with the subject of the present invention will now be given below in connection with the figure 4c .

When considering a clock clock atom clock CPT thermal atoms in which the interaction medium is free of buffer gas, the width of the oscillation line obtained for the clock signal, width to 3dB with respect to the The maximum amplitude at the top of the oscillation is a few kHz for a central frequency of the order of a few GHz. Such a linewidth is too important to be compatible with the use of such atomic clocks as a reference clock. This can be explained by the fact that in the absence of a buffer gas, the atoms of the interaction medium are subjected to an excessive rapid erratic displacement which widens the resonance phenomenon by Doppler effect and limitation of the transit time and finally, the quality of the radio-electric resonator thus formed.

When, on the contrary, a buffer gas is used in this same type of clock, the Lamb-Dicke regime is reached and the linewidth of the atomic clock signal is mainly limited by the relaxation of the coherence in the ground state. and enlargement due to laser saturation. Line widths of the order of 100 Hz have so far been obtained. Short-term stabilities of the user signal frequency S _u of about 5 to 10 ^-12 after 1 second of integration have been measured with optical or microwave detection of the aforementioned clock signal. Long-term stability is essentially limited by the frequency fluctuations induced by collisions with the buffer gas. The corresponding frequency shift with respect to the Raman mismatch is directly related to the buffer gas pressure which is itself a function of the temperature of the interaction medium and therefore of the cell.

The line width Δf _CPT of the resonance signal and the clock signal in a clock of this type at a value given by the relation (1). $${\mathrm{.DELTA.f}}_{\mathrm{CPT}}={\mathrm{.DELTA.f}}_{\mathrm{TT}}+{\mathrm{.DELTA.f}}_{\mathrm{collision}}+{\mathrm{.DELTA.f}}_{\mathrm{Doppler}}+{\mathrm{.DELTA.f}}_{\mathrm{saturation}}$$

• Δf_TT décrit l'élargissement dû au temps de transit limité des atomes du milieu d'interaction à travers les faisceaux laser.

In this relationship:

• Δf _TT describes the expansion due to the limited transit time of the atoms of the interaction medium through the laser beams.

For a continuous interrogation, Δf _TT varies as 1 / T where T denotes the interaction time between an atom and the laser waves.

• Δf_collision est l'élargissement de la raie résultant de l'amortissement de la cohérence dû aux collisions entre atomes ;
• Δf_Doppler est l'élargissement par effet Doppler du premier ordre ;
• Δf_saturation est l'élargissement par saturation lié aux intensités réelles des faisceaux laser illuminant le milieu d'interaction.

For pulsed interrogation according to the implementation of the pulsed interrogation method and clock object of the present invention, Δf _TT varies as 1 / 2b where b denotes the dead time between two consecutive pulses of a pulse train ;

• Δf _collision is the widening of the line resulting from the damping of the coherence due to collisions between atoms;
• Δf _Doppler is the first order Doppler broadening;
• Δf _saturation is the _saturation enlargement related to the real intensities of the laser beams illuminating the interaction medium.

• Δf_Doppler et Δf_TT sont négligeables en raison de la présence du gaz tampon ;
• Δf_saturation peut être réduit en ajustant la puissance laser mais au détriment du rapport signal à bruit, ainsi que mentionné précédemment dans l'introduction à la description pour les dispositifs de l'art antérieur à interrogation continue ;
• Δf_collision est la source prédominante de l'élargissement de la raie constitutive du signal d'horloge atomique obtenu.

For an atomic clock CPT whose interaction medium consists of thermal atoms in the form of a vapor:

• Δf _Doppler and Δf _TT are negligible due to the presence of the buffer gas;
• _Saturation can be reduced by adjusting the laser power but at the expense of the signal-to-noise ratio, as previously mentioned in the introduction to the description for the devices of the prior art with continuous interrogation;
• Δf _collision is the predominant source of broadening of the constituent line of the atomic clock signal obtained.

The figure 4c illustrates the mode of implementation of the method that is the subject of the present invention by virtue of a pulsed interrogation atomic clock in which the interaction medium is constituted by thermal atoms of cesium in the presence of a buffer gas, formed by nitrogen. It represents the amplitude of compensated clock signal S _HC as a function of the disagreement of the difference in the frequencies Δf ₁₂ of the two laser waves.

The x-axis of the figure 4c is graduated in kHz with respect to a 0 value origin of the Raman mismatch. The distance δ represents the disagreement introduced due to the presence of the buffer gas. This frequency bias can be reduced by using two buffer gases, nitrogen and argon for example, inducing collisional movements of opposite sign.

With reference to the above-mentioned figure, it can be seen that for the maximum amplitude measured in millivolts on the ordinate axis, the width of the oscillations remains as low as 25 Hz thanks to the implementation of the treatment and, of course, the pulse modulation of the laser beams L ₁ and L ₂ used. When, on the contrary and according to a particularly remarkable aspect of the method and pulsed interrogation atomic clock in accordance with the subject of the present invention, the interaction medium is constituted by atoms cooled by laser, the speed of the atoms is reduced under the conditions previously mentioned in the description, that is to say at erratic speeds about 1000 times lower than those of thermal atoms.

Under these conditions, it is then possible to obtain long interaction times between the illumination laser beams and the interaction medium without the use of a buffer gas, thereby canceling the displacement δ previously mentioned in connection with the figure 4c resonance and frequency expansion due to collisions.

• Δf_Doppler et Δf_TT sont négligeables grâce aux faibles vitesses des atomes froids, refroidis par laser ;
• Δf_collision est également négligeable lorsque la densité d'atomes froids est suffisamment faible.

Thus, for a pulsed interrogation clock, atomic clock CPT with cold atoms, the aforementioned parameters are then processed as follows:

• Δf _Doppler and Δf _TT are negligible due to the low velocities of cold atoms, cooled by laser;
• Δf _collision is also negligible when the density of cold atoms is sufficiently low.

As such, the rubidium atom appears more interesting than the cesium atom because the collisional displacement is at least 50 times lower.

Thus, it can be seen that it is the saturation broadening Δf _saturation that limits the linewidth of an atomic clock whose interaction medium consists of atoms cooled by laser.

When, moreover, the interrogation process is carried out in accordance with the method that is the subject of the present invention, that is to say by pulsed interrogation, it is then possible to very significantly reduce the contribution of the saturation effect. while continuing to detect the signals of sufficient intensity, that is to say with a satisfactory signal-to-noise ratio.

Claims (10)

1. A method for generating an atomic clock signal with coherent population trapping from a first and a second phase-coherent laser wave, each substantially in resonance with an optical transition of the atoms of an interactive medium, the coherent superimposition of the atomic states corresponding to the coherent population trapping of atoms producing a response signal having a resonance-extremal amplitude and representing the atomic clock signal corresponding to the variation in amplitude of the signal detected as a function of the value of the difference in frequency of the first and the second phase-coherent laser wave, characterized in that said method consists at least in:

   - modulating in synchronization by successive pulses the intensity of the first and the second laser wave, by a shape factor determined between a high level and a low level of intensity, the interaction between the first or the second laser wave respectively and the interactive medium being substantially limited to the duration of each successive pulse corresponding to a high level of intensity, said response signal produced during a current pulse depending on the atomic state produced during at least one pulse preceding this current pulse and on the development of this atomic state for the duration of a low level of intensity separating said pulses; and

   - detecting and superimposing by linear combination said response signal produced during said current pulse and at least one pulse preceding this current pulse to produce a resultant compensated atomic clock signal, the spectral width of which is minimized.
2. The method according to claim 1, characterized in that the pulse modulation is carried out by pulse trains, the frequency of the modulation pulses being between 0.2 Hz and 10⁴ Hz.
3. The method according to either claim 1 or claim 2, characterized in that the modulation pulses have a shape factor of between 10^-6 and 10^-1.
4. The method according to any one of claims 1 to 3, characterized in that the duration of a low level of intensity separating said current pulse from said pulse preceding this current pulse is shorter than the lifetime of the hyperfine coherence existing between two clock levels.
5. The method according to any one of claims 1 to 4, characterized in that said interactive medium is formed by a plurality of thermal or laser-cooled atoms.
6. The method according to any one of claims 1 to 5, characterized in that the step consisting in detecting said clock signal is chosen as one of the detection processes from among the group of detection processes comprising optical absorption, optical fluorescence, microwave detection, as a function of the difference in frequency of the first and the second phase-coherent laser waves.
7. An atomic clock with pulsed interrogation, comprising at least:

   - an optical interrogation means (1) allowing the production of a first and a second phase-coherent laser beam (L1, L2), each substantially in resonance with an optical transition of the atoms of an interactive medium (3);

   - an interactive cell (3) comprising said interactive medium, illuminated in operation by the first and the second phase-coherent laser beam, to produce a response signal having a resonance-extremal amplitude and corresponding to the variation in amplitude of the signal detected as a function of the difference in frequency of the first and the second phase-coherent laser beam;

   - means (4) for detecting said response signal, said detection means being adapted to the wavelength and to the amplitude of the response signal,
   characterized in that said atomic clock further comprises:

   - means (2) for pulse-modulating the intensity of the first and the second laser beam between a high level and a low level of intensity, said modulation means being placed on the path of said first and second laser beams upstream of said interactive cell (3) to produce in synchronization a first and a second pulsed laser beam, the interaction between the first or the second laser beam respectively and the interactive medium being substantially limited to the duration of each successive pulse corresponding to a high level of intensity, said response signal produced during a current pulse being dependent on the atomic state produced during at least one pulse preceding this current pulse and on the development of this atomic state for the duration of a low level of intensity energy separating said pulses, and in that

   - said detection means further comprise means (5) for adding by linear combination the response signal produced during this current pulse and the response signal produced during at least one pulse preceding this current pulse, said means for adding by linear combination allowing the production of a resultant compensated atomic clock signal, the spectral width of which is minimized.
8. The atomic clock according to claim 7, characterized in that said means for pulse-modulating the intensity of the first and the second laser beam between a high level of intensity and a low level comprise at least one acousto-optic modulator.
9. The atomic clock according to either claim 7 or claim 8, characterized in that said detection means further comprise:

   - means for sampling the response signal produced during the interaction of the current pulse and at least one pulse preceding this current pulse; and

   - means for storing sampled values of the response signal produced during the interaction of each of said pulses.
10. The atomic clock according to claim 9, characterized in that said detection means further comprise:

    - means for reading the values sampled at predetermined instants stored in said storage means; and

    - means for calculating a linear combination of said stored sampled values allowing the production of said compensated atomic clock signal.

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