EP2738628B1 - Wristwatch with atomic oscillator - Google Patents

Wristwatch with atomic oscillator Download PDF

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Publication number
EP2738628B1
EP2738628B1 EP14157063.0A EP14157063A EP2738628B1 EP 2738628 B1 EP2738628 B1 EP 2738628B1 EP 14157063 A EP14157063 A EP 14157063A EP 2738628 B1 EP2738628 B1 EP 2738628B1
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EP
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Prior art keywords
laser
atomic oscillator
wristwatch
oscillator
cell
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German (de)
French (fr)
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EP2738628A2 (en
EP2738628A3 (en
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Laurent Balet
Jacques Haesler
Steve Lecomte
David Ruffieux
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Rolex SA
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Rolex SA
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    • G—PHYSICS
    • G04—HOROLOGY
    • G04F—TIME-INTERVAL MEASURING
    • G04F5/00—Apparatus for producing preselected time intervals for use as timing standards
    • G04F5/14—Apparatus for producing preselected time intervals for use as timing standards using atomic clocks

Definitions

  • the present invention relates to an oscillator for a wristwatch and a wristwatch as such comprising such an oscillator. It also relates to a method for transmitting a time reference signal for an atomic oscillator wristwatch.
  • the search for precision is one of the engines of technical innovation in watchmaking. This accuracy is largely determined by the performance of an oscillator whose oscillation frequency generates a time signal that determines the time base exploited by the mechanism of a wristwatch to finally indicate the time on a display .
  • a first solution of the state of the art consists of a mechanical oscillator, based on a flywheel, called pendulum, coupled to a spiral spring type.
  • the stability of a mechanical oscillator is of the order of one second per day, despite the efforts of innovation based on the choice of particular materials, as is for example described in the documents EP0886195 or EP1422436 .
  • a second state-of-the-art solution consists of a quartz oscillator, which can reach an accuracy of one second per month, or even one second a year, using more complex thermo-compensated devices to avoid drifts caused by variations. temperature, as described in the document WO2008125646 .
  • a third solution relatively theoretical because difficult to achieve in practice, is envisaged in the documents EP1852756 or EP1906271 , from an atomic oscillator, based on the effect known by its Anglo-Saxon name of "Coherent Population Trapping" (CPT), which makes it possible to measure a light intensity transmitted through a mixture of atoms such as cesium or Rubidium.
  • CPT Coherent Population Trapping
  • the invention seeks to achieve an oscillator for a wristwatch that achieves high accuracy, while respecting the severe constraints of very small volume and low power available within a wristwatch.
  • the invention is based on an atomic oscillator for a wristwatch which is based on a Raman effect beat detection system to obtain a high accuracy time reference.
  • the chosen solution is based on the use of an atomic oscillator based on the Raman effect, which is based on the irradiation of reference atoms at an optical resonant frequency which induces the emission of photons with an optical frequency shifted by the hyperfine frequency of the atom of reference.
  • the combination of the two resulting signals provides a detectable beat, the signal frequency of which serves as the time base for the wristwatch.
  • the figure 1 schematically illustrates the optical part of a Raman atomic oscillator according to one embodiment of the invention. It comprises a laser diode 1, which can be low-consumption and of the VCSEL type, which emits a linearly polarized beam 11, a quarter-wave plate 2 which polarises the light coming from the laser according to an incident beam of circular polarization 12.
  • This beam 12 passes through a cell 3 comprising selected atoms, such as cesium or rubidium with a buffer gas, optionally placed in a magnetic field B.
  • the incident signal 12 is combined with the second signal 13 generated by the Raman effect, as explained above.
  • the combination of the two signals is detected by a photodetector 4 which allows the recovery of the signal comprising the atomic time base, originating from the cesium or rubidium atoms.
  • This output signal 14 is analyzed by an electronic signal processing device of the microwave frequency divider type 5 to generate the signal frequency necessary for the time base. Exit 15 finally represents this time base, exploited by a wristwatch as will be explained later.
  • An optional radio frequency amplifier 6 is positioned at the output of the photodetector 4.
  • a part of the output signal 14 is used to modulate the injection current of the laser, by a microwave injection at the level of the laser 1, represented by the arrow 7. This makes it possible to achieve a signal-on-noise output 14 of better quality and easier to operate. This principle is equivalent to an amplitude modulation of the laser.
  • the cell 3 has been positioned within a magnetic field B, which makes it possible to remove the degeneracy of the Zeeman sub-states of the atoms.
  • it could be in a zero magnetic field, to obtain a superposition of energy levels, and a high signal, and a simplified oscillator.
  • the figure 2 functionally represents a Raman atomic oscillator according to one embodiment of the invention. It comprises a power supply device and DC / DC converter 21, a processing center 23 which can be a processor or a low-power electronics, the main functions of which comprise all or some of the following functions: setting the operating frequency of the laser 1 and its injection current, control of the temperature of the cell 3 and the laser 1, management of the intermittent mode of the laser, correction of the frequency of the atomic oscillator as a function of the temperature, setting of an oscillator additional low precision as quartz based. The implementation of these functions will be detailed later.
  • the oscillator then comprises a DC current source 24 for the laser 1, a DC current source 25 for heating the laser 1, a current source 26 of the solenoid for the generation of the magnetic field B 36, a current source 27 for heating the cell 3, which cooperates with an associated heater 37, to which a temperature sensor can be additionally added.
  • a fast photodetector 4 comprises a DC output for returning a signal proportional to the light intensity received to the processing center 23. It furthermore comprises an RF output for a signal which is first amplified by a signal chain.
  • amplification 32 then a delay line and phase shifter 33 to be fed back to a diplexer 34 (bias tee) which combines the RF signal with the DC laser injection current from the current source 24.
  • a frequency divider 5 Part of the amplified RF signal is processed by a frequency divider 5 before returning to the processing center 23.
  • a user frequency signal 22 is obtained (for example 32 kHz, or 1 pulse per second, etc. .).
  • the implementation of this oscillator is made from low-power components for an implementation compatible with a wristwatch environment.
  • the CPT type atomic clocks all use a complex architecture and include a device for local oscillator correction, called by its English name “Voltage Controlled Oscillator” (VCO), as well as a control electronics of the oscillator, representing in total a high power consumption.
  • VCO Voltage Controlled Oscillator
  • the Raman atomic oscillator described above has the advantage of greater simplicity for a greatly reduced consumption.
  • a laser beam incident at a first frequency interacts with an atom vapor, thereby stimulating, by light-atom interaction, the emission of a second Raman beam having a second frequency.
  • the beat between the first frequency and the second frequency produces a third frequency: the beat frequency, which is used as a time base.
  • the vapor comprises for example Rubidium-85 and where the laser is of the vertical cavity semiconductor type and With surface emission emitting a light beam at a wavelength of around 780 nm or 794 nm, the beat frequency is of the order of 3GHz with a bandwidth around a hundred kHz. This beat frequency is generally very low and has a very low spectral content.
  • the detection of these beat frequencies at the output of the oscillator for their use in a wristwatch is a delicate technical problem, in particular to limit consumption.
  • the system includes a generator for providing the signal (i PD ) as a current, and a parallel resonance circuit for varying the impedance of the generator output as a function of the frequency of the generated signal and for converting the current in tension.
  • the system further includes an amplification stage to further increase the gain by minimally degrading the system noise to allow the detection of a signal of very low amplitude.
  • the generator is the photodetector 4 mentioned above, stimulated by electromagnetic radiation.
  • a simple inductor L1 is included in the realization of the parallel resonance circuit and the photodetector is of the PD photodiode type.
  • the photodiode PD is polarized through the inductor L1 connected to a voltage source. This makes it possible to maintain the photodiode PD at a desired voltage by supplying the current necessary for the photodiode PD to work properly.
  • the signal to be detected has a spectral content centered around a predetermined frequency ⁇ C which is of the order of a few gigahertz and very narrow (of the order of 10 -4 ⁇ ⁇ C ).
  • the signal to be detected i PD appears in the form of a current on a node N) which connects the inductance L1 to the photodiode PD.
  • This node N is electrically coupled to the input of the MAMP amplifier and the amplified signal appears at the output of the MAMP amplifier.
  • N node thus configured has a parasitic capacitance C IN .
  • This parasitic capacitance C IN forms with the inductor L1 the parallel resonance circuit.
  • the figure 5 shows the difference in gain versus frequency for both types of amplifier.
  • a broadband transimpedance amplifier of the state of the art can cover a large frequency range, but leads to high consumption and noise comparatively high, since the noise is all the more important that the bandwidth is wide.
  • the proposed solution selects with a resonant element a signal centered around a central frequency that is significantly lower than the cutoff frequency typical of the photodetector technology used.
  • the gain characteristic shows a very narrow bandwidth, compatible with the narrow spectral content of the signal (of the order of 10 -4 ⁇ ⁇ C ), which strongly reduces the noise compared to a transimpedance amplifier. Consumption is very low because the system has no active elements.
  • node N Since node N has a very high impedance, it is sufficient to use a simple low noise common source MOS type amplifier to further increase the gain by minimally degrading the system noise to allow the detection of a signal. very low amplitude.
  • the amplifier has a resistive load on the output.
  • the load at the output of the amplifier is provided by a second inductor L2. whose value is chosen to maximize the gain for a signal at the predetermined frequency ⁇ C.
  • the input of the amplifier can be coupled in AC mode with the node N, ie, with a DC coupling capacitance, and the input of the amplifier can therefore be biased by a voltage source Vb through a resistor Rb so that the input of the amplifier is at an optimum voltage.
  • the value of the parasitic capacitance C IN or the value of the inductance L1 may vary from one batch to another or from one room to another . This would have the effect of moving the resonance frequency of the resonance circuit out of the appropriate frequency band to detect a signal at the predetermined frequency. For this reason, it is proposed to act on the value of the capacity of the node N by adjusting this capacity.
  • the resonance circuit comprises an electromechanical resonator of the bulk wave resonator type or Bulk Acoustic Wave (BAW), as illustrated by FIG. figure 4 .
  • the volume wave resonator (BAW) allows an even more selective filtering and it has, at the anti-resonance, a high real impedance while allowing to neutralize the parasitic capacitance C IN of the node N.
  • the electromechanical resonator makes it possible to achieve a quality factor greater than 300.
  • the photodiode is polarized by means of an adaptive circuit whose output stage is a source of current CCS controlled so as to guarantee a fixed polarization voltage on the diode at low frequency.
  • a feedback of the RF signal detected on the optical frequency of the laser, in order to control the emission frequency of the laser, is always recommended in the state of the art to obtain a stable and high-precision atomic oscillator, in particular for the atomic clocks of type CPT.
  • the temperature of the cell was also lowered to below the melting temperature of Rubidium (39.3 ° C).
  • a temperature drop of 90 ° C to 35 ° C corresponds to a decrease in the saturation vapor pressure by two orders of magnitude ( ⁇ 10 -4 torr at 10 -6 torr).
  • the stability depends on the temperature of the cell but remains acceptable up to a temperature of 35 ° C. Indeed, at a temperature of 40 ° C, the Raman oscillator always works to satisfaction with a stability of one second every 16 years, which is remarkable. At 35 ° C, the Raman signal is still present and sufficiently stable.
  • the atomic oscillator without heating the cell according to an alternative embodiment, functioning for example only when the temperature around the cell is sufficient, for example around 35 ° C., preferably around 40 ° C.
  • the atomic oscillator can operate at a temperature less than or equal to 40 ° C, or even lower than or equal to 35 ° C. It is also possible to reduce the operating temperature by using Cs instead of Rb in the cell, the cesium melting temperature being even lower than that of Rubidium (28.5 ° C instead of 39.3 ° C).
  • This operating range is more accurately illustrated by the figure 6 for the case of natural Rubidium.
  • This figure shows the optical absorption curve 50 of the Rubidium, by the signal obtained on the photodiode 6, as a function of the injection current of the laser.
  • the favorable current range is located in zone 52, which represents a portion of the peak of greater absorption 51, at a distance from the two maximum values Vmax and Vmin of this peak.
  • the laser injection current must be chosen between 2.25760 mA and 2.25824 mA.
  • V1 is 15% Vmax-Vmin above Vmin and V2 at 67% Vmax-Vmin above Vmin.
  • This first phase of the ignition process can be performed before each ignition of the oscillator, in order to obtain the highest possible accuracy, which makes it possible to modify the previous values in time as a function of possible drift of the device or measurement conditions.
  • this phase is performed only once to calibrate the device and the data are stored to be repeated at each ignition.
  • this method comprises a preliminary step of measuring the optical power of the laser, since the frequency of the oscillator may depend on the optical power interacting with the atoms. This can be done by measuring the optical power by means of a photodiode of the device and comparing the photovoltage thus generated with a reference stable voltage source. Adjusting the laser injection current and the laser temperature then makes it possible to obtain the optical power and the optical frequency of the oscillator.
  • this method comprises a preliminary step of warming the gas cell and the laser, since the operation of the oscillator depends on the temperature, as mentioned previously. There is a correlation between the frequency of the closed-loop Raman oscillator and the temperature of the cell. This property makes it possible to control the frequency during the on and off phases of the oscillator by the sole measurement of the temperature.
  • the Raman oscillator comprises a temperature control.
  • it comprises a temperature sensor, which may be a photodiode, and a heating device for increasing the temperature if it is under a set temperature.
  • the previously described steps of the ignition process are managed automatically by the oscillator, on the basis of the hardware and software resources of the processing center 23 mentioned above, in particular under the control of the microprocessor.
  • the previous atomic oscillator is thus implemented within a wristwatch.
  • the Raman oscillator is used intermittently, in addition to a conventional oscillator of the state of the art, for example quartz.
  • the atomic oscillator transmits a time base which allows the calibration of the quartz oscillator, its correction, and allows to greatly increase its accuracy over time.
  • This intermittent operation of the atomic oscillator has the advantage of an additional controlled consumption compared to a conventional wristwatch.
  • the ignition of this oscillator is controlled by the method explained above, this first implementation in a wristwatch is very efficient.
  • the ignition period of the atomic oscillator is chosen according to a compromise between the consumption and the precision of the wristwatch: the more this oscillator is used, the more accurate the clock but the higher the consumption will be.
  • the Raman oscillator is used alone to replace the usual conventional oscillator, as a single time base, and therefore according to a permanent operation. In this embodiment, the highest accuracy is achieved, but through greater energy consumption.
  • the atomic oscillator described above is also made in a compact and compact structure, to facilitate insertion into a wristwatch.
  • the Figures 7 to 14 thus describe several embodiments of the optical part of the atomic oscillator, making it possible to reach a volume compatible with integration into a wristwatch. For this, all these achievements are based on a double pass of the laser beam in the cell, which allows to achieve a large total length of the laser beam in a small volume.
  • FIGS. 7 to 9 illustrate three different embodiments making it possible to simultaneously perform a double pass in the gas cell 106 and a protection of the laser source 102 towards the reflections.
  • a common feature of these various embodiments is the presence of a semitransparent mirror 107 which passes a portion of the laser beam passed through the gas cell 106 to reach a photodetector 109 for servocontrol of the cell temperature.
  • these embodiments could be simplified by removing this photodetector 109 and using a non-transparent mirror.
  • These three embodiments differ in the means used to direct the beam towards the cell and the photodetectors, and in the means used to prevent the beam reflected by the mirror from disturbing the laser source.
  • the figure 7 illustrates the first embodiment of the invention.
  • the laser source 102 produces a linearly polarized laser beam which is directed towards a polarizer 103, the transmission axis of which is oriented so as to let the laser beam pass, then to a separator 101 whose percentage of separation is predefined. Part of the beam is thus transmitted to an optional photodetector 108b.
  • the separator reflects the other part of the beam towards a quarter-wave plate 105.
  • the linear polarization is denoted "P" for the part parallel to the transmission axis of the polarizer (transmitted part) and "S" for the perpendicular part to the transmission axis of the polarizer (part absorbed by the polarizer).
  • the part “P” is symbolized by solid circles and the part “S” by lines.
  • the role of the blade 105 is to change the linear polarization of the laser beam into a circular polarization and this blade is oriented relative to the polarizer so as to generate a circular polarization. Indeed, the interaction between the light and the atoms of the gas cell 106 is optimal when it is performed with a circular polarization beam. A portion of the beam leaving the gas cell 106 is then reflected by a mirror 107, which reverses the direction of its circular polarization, and thus passes through the gas cell 106 a second time. When leaving the gas cell 106, the beam reaches the quarter wave plate 105.
  • this beam is then partially transmitted and reaches the photodetector 108a.
  • Another part of this beam is deflected by the separator 101 and is strongly attenuated by the polarizer 103 because its polarization is perpendicular to that of the transmission axis of the polarizer 103, the laser source 102 thus being protected from retro-reflections.
  • a small portion of the beam passed through the gas cell 106 is transmitted by the mirror 107 and picked up by the photodetector 109.
  • the figure 8 illustrates the second embodiment. It differs from the first mode described above by the use of a separator 101 which reflects the beam in a first polarization and passes the beam in a second polarization. Thus the beam leaving the laser source 102 is separated according to its polarization and the same principle applies to the reflected beam. It is thus not necessary to place a polarizer between the separator 101 and the laser source because the reflected beam is fully transmitted to the photodetector 108a.
  • the linear polarization is denoted "P" for the part parallel to the polarization axis of the separator (part transmitted in the right angle configuration of the figure 8 ) and "S” for the part perpendicular to the polarization axis of the separator (part deflected at 90 °).
  • P linear polarization
  • S the part perpendicular to the polarization axis of the separator
  • the figure 9 illustrates the third embodiment of the invention.
  • the deflection of the laser beam is provided by the semi-transparent mirror 107 which is disposed at an angle not perpendicular to the axis of the laser beam.
  • the reflected beam does not reach the laser source 102 but is directed directly on the photodetector 108a.
  • the mirror 107 is of concave shape for focusing the light beam reflected on the photodetector 108a. A small portion of the beam having passed through the gas cell 106 is transmitted by the mirror 107 and picked up by the photodetector 109.
  • This concave shape of the mirror can also be used on the two embodiments of the Figures 7 and 8 providing the benefits described above.
  • FIG. figure 10 A more complete exemplary embodiment corresponding to the second embodiment is illustrated in FIG. figure 10 .
  • the separator 101 is in the form of a polarizing beam splitter cube (PBSC).
  • PBSC polarizing beam splitter cube
  • the optical assembly is based on a miniature separator cube 101 whose sides are preferably less than or equal to 1 mm, the cube 101 acting as a separator. In a standard mode, the volume of the cube is typically 1 mm 3 .
  • the light beam of the laser diode 102 arrives on one of the sides of the cube 101.
  • the laser diode is of the vertical cavity and surface emission semiconductor (VCSEL) type emitting a diverging beam. of light at 795 nm.
  • VCSEL vertical cavity and surface emission semiconductor
  • other types of laser diodes having wavelengths typically ranging from 780 nm to 894 nm may be used for a gas cell 106 containing Rubidium or cesium. This choice is dictated by the atomic composition of the gas cell.
  • a collimating lens may be added in front of the laser diode to produce a non-diverging laser beam.
  • the light produced 112 by the laser 102 has a linear polarization and is attenuated by an absorbent neutral filter 104a.
  • a different type of filter can be used in other embodiments. The presence of this filter is not necessary for the invention.
  • a half wave plate 104b may be used to change the angle of the linear polarization of the laser source. In combination with the miniature cube 101, the half wave plate 104b plays the role of a variable attenuator. In other embodiments, the use of the half-wave plate 104b may be omitted and the light intensity ratio between the beams transmitted and reflected by the cube 101 is adjusted by an appropriate orientation of the polarization axis. linear light emitted by the laser relative to the separator cube.
  • a quarter-wave plate 105 is placed at the cube outlet against the face from which the laser beam deflected by the separator 101, or at right angles from the incident beam to the cube.
  • the fast axis of the quarter-wave plate 105 is oriented such that the incident linear polarization 113 is changed to circular polarization 114 in a first direction of rotation.
  • the quarter-wave plate 105 is oriented such that the incident linear polarization 113 is changed to a circular polarization in a reverse direction of rotation to the first.
  • the circular polarization laser beam 114 passes through the gas cell 106 and reaches the mirror 107. The latter only returns the ray partially and a portion of the ray passes through the mirror 107 to go towards the photodetector 109.
  • the gas cell is made of glass-silicon-glass by MEMS (electromechanical microsystem) techniques with an internal volume of typically 1 mm 3 and filled with an absorbent medium of atomic vapor type of alkali metal (Rubidium or cesium), and a buffer gas mixture.
  • the gas cell is filled with natural Rubidium and a mixture of nitrogen and argon as a buffer gas.
  • other types of cells may be filled with different buffer gases.
  • a cylindrical miniature cell can be used.
  • the gas cell can be integrated in the PBSC 101.
  • the cell 106 can be filled with other types of alkaline metal vapor (rubidium-85, rubidium-87, cesium-133 for example) and other types of buffer gas (Xe, Ne for example).
  • the figure 11 illustrates a double-pass optical design based on the second embodiment corresponding to the figure 8 , with a straight geometry that is very similar to the right-angle and double-pass design depicted on the figure 10 .
  • the main difference lies in the position of the entity "gas cell 206, quarter-wave plate 205, semitransparent mirror 207 and photodetector 209" and photo-detector 208b.
  • the gas cell 206 is placed above the PBSC 201 and is therefore located vis-à-vis the laser 202.
  • the polarization light beam P 213 transmitted by the PBSC and modified in circular polarization beam by the quarter-wave plate 205 interacts with the atomic medium.
  • the S-polarization light beam 217 is reflected by the PBSC 201 and the right-angle photodetector 208b is used to measure the laser power.
  • the operating principle of this embodiment is the same as for the previous model.
  • the figure 12 illustrates the schematic representation of the double-pass straight geometry case of the embodiment of the Raman oscillator according to the first embodiment, corresponding to the figure 7 .
  • the digital coding starts at 201 for this design, keeping the same tens and units as those used on Figures 7 to 9 for the same elements.
  • a separator cube 201 is used, whose percentage of separation is predefined so as to have a minority reflection and a majority transmission, of about 2% and 98% respectively (+/- 2%).
  • the retro-reflected beam 216 is then mainly deflected towards the photodetector 208a.
  • the gas cell entity 206 is placed above the separator cube 201 and is therefore located with respect to the laser 202.
  • the photodetector 208b is placed at right angles, where the light beam 212 emitted by the laser 202 is reflected 218 by the separator cube 201 and is used for example for the measurement of laser power.
  • the operating principle of this design remains similar to the previous descriptions.
  • the figure 13 illustrates a device according to the first embodiment and right angle geometry.
  • the separation percentage of the separator 101 is predefined so as to have a minority transmission and a majority reflection of about 2% and 98% respectively (+/- 2%).
  • the incident light beam 114a and the light beam generated by the scattering Raman stimulated Raman 114b are reflected by a mirror 107.
  • the mirror 107 is coated with silver, is inclined (typically 2 to 20 degrees) and / or eccentric to the mirror.
  • the exit window of the gas cell 106 is concave, coated with silver (or other metal, such as gold) and acts as a reflector. In other embodiments, the coating of the exit window of the mirror can be made of dielectric layers.
  • the retro-reflected light beams 115 pass through and interact a second time with the atomic medium (double pass).
  • the quarter wave plate 105 transforms these circularly polarized light beams into linear polarization light beams 116.
  • These light beams are mainly deflected 119 (incident and Raman) and reach the first photodetector 108a which records the frequency beat between the beam. incident beam and the Raman beam.
  • the first photodetector 108a is a high-speed semiconductor photodetector (silicon or gallium arsenide) which is positioned at the focus of the concave mirror 107. In other embodiments Raman, different types of high-speed photodetectors may be used.
  • the second photodetector 108b records the light 118 coming directly from the laser 102 and initially transmitted by the miniature divider cube 101. In this manner, the output power of the laser diode 102 can be measured and adjusted.
  • the photodetector 121 records the retro-reflected beam 117 transmitted by the separator 101.
  • the diaphragms 110 and 111 are used to prevent undesirable light from reaching the photodetectors if their dimensions are larger than those of the miniature separator cube 101.
  • the figure 14 illustrates the third embodiment of the Raman oscillator, not based on a splitter cube but on a simple double-pass geometry.
  • the light emitted by the laser source 102 is linearly polarized, converted into circular polarization by a quarter-wave plate 105 before passing through the cell 106, reflection on the mirror 107, second passage in the cell, and detection on a first photodetector 108a .
  • the mirror 107 is semi-transparent, with a second photodetector 109 placed behind the mirror. This use of the semi-transparent mirror 107 allows the detection of light having interacted with the atoms of the cell by the photodetector 109.
  • a polarizer 103 in front of the laser source 102 and with a transmission axis parallel to the polarization of the beam emitted by the laser source 102.
  • the photodetector 108a, 208a serves to detect the beat induced by the Raman effect of the gas present in the cell 106, 206, and is therefore a photodetector adapted for the detection of microwaves.
  • the first photodetector 108a has a very narrow bandwidth and centered around the resonant frequency of the atoms to maximize its signal detection efficiency.
  • the high atomic resonance frequency typically> 1GHz
  • This specification is not compatible with a detection of the signal having interacted with the atoms of the cell to adjust for example the temperature of the cell, which is implemented by the photodetector 109, 209 and / or the photodetector 108b , 208b.
  • a low cut-off frequency typically ⁇ 100 kHz
  • a DC operation are indicated. This is why it is preferable to have at least two detectors, one 108a serving for the detection of the clock signal, the other 109 for controlling the temperature of the cell.
  • the ideal way to achieve this second detection of a signal having interacted with the atoms of the cell is to use a semi-transparent mirror 107 for reflection and to place behind this mirror a photodetector 109, as has been illustrated. It is also advantageous if the mirror 107 is of concave shape, as illustrated in FIG. figure 14 , the concave shape being intended to focus the light beam reflected on the photodetector 108a. As a note, these last photodetectors are optional.

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Description

IntroductionIntroduction

La présente invention concerne un oscillateur pour montre-bracelet ainsi qu'une montre-bracelet en tant que telle comprenant un tel oscillateur. Elle concerne aussi un procédé d'émission d'un signal de référence de temps pour montre-bracelet par oscillateur atomique.The present invention relates to an oscillator for a wristwatch and a wristwatch as such comprising such an oscillator. It also relates to a method for transmitting a time reference signal for an atomic oscillator wristwatch.

Etat de l'Art State of the Art

La quête de la précision est l'un des moteurs de l'innovation technique en horlogerie. Cette précision est en grande partie déterminée par la performance d'un oscillateur dont la fréquence d'oscillation génère un signal de temps qui détermine la base de temps exploitée par le mécanisme d'une montre-bracelet pour finalement indiquer l'heure sur un afficheur.The search for precision is one of the engines of technical innovation in watchmaking. This accuracy is largely determined by the performance of an oscillator whose oscillation frequency generates a time signal that determines the time base exploited by the mechanism of a wristwatch to finally indicate the time on a display .

Une première solution de l'état de la technique consiste en un oscillateur mécanique, reposant sur un volant d'inertie, appelé balancier, couplé à un ressort de type spirale. La stabilité d'un oscillateur mécanique est de l'ordre de une seconde par jour, malgré les efforts d'innovation reposant sur le choix de matériaux particuliers, comme cela est par exemple décrit dans les documents EP0886195 ou EP1422436 .A first solution of the state of the art consists of a mechanical oscillator, based on a flywheel, called pendulum, coupled to a spiral spring type. The stability of a mechanical oscillator is of the order of one second per day, despite the efforts of innovation based on the choice of particular materials, as is for example described in the documents EP0886195 or EP1422436 .

Une seconde solution de l'état de la technique consiste en un oscillateur à quartz, qui peut atteindre une précision de une seconde par mois, voire une seconde par an en utilisant des dispositifs plus complexes thermo-compensés pour éviter les dérives causées par les variations de température, comme cela est décrit dans le document WO2008125646 . Enfin, une troisième solution, relativement théorique car délicate à réaliser en pratique, est envisagée dans les documents EP1852756 ou EP1906271 , à partir d'un oscillateur atomique, basé sur l'effet connu par sa dénomination anglo-saxonne de « Coherent Population Trapping » (CPT), qui permet de mesurer une intensité lumineuse transmise au travers un mélange d'atomes comme du Césium ou du Rubidium. Cette solution permet en théorie d'obtenir un oscillateur plus précis que celui des deux premières solutions. Toutefois, ces documents ne donnent pas d'information sur la réalisation concrète d'un oscillateur atomique au sein d'une montre-bracelet. Par exemple, l'oscillateur atomique est utilisé de manière intermittente sans explication sur la réalisation concrète et stable d'un tel principe. Il n'est pas non plus précisé comment atteindre une consommation et un volume compatibles avec une implémentation dans une montre-bracelet.A second state-of-the-art solution consists of a quartz oscillator, which can reach an accuracy of one second per month, or even one second a year, using more complex thermo-compensated devices to avoid drifts caused by variations. temperature, as described in the document WO2008125646 . Finally, a third solution, relatively theoretical because difficult to achieve in practice, is envisaged in the documents EP1852756 or EP1906271 , from an atomic oscillator, based on the effect known by its Anglo-Saxon name of "Coherent Population Trapping" (CPT), which makes it possible to measure a light intensity transmitted through a mixture of atoms such as cesium or Rubidium. This solution theoretically makes it possible to obtain a more precise oscillator than that of the first two solutions. However, these documents do not give information on the concrete realization of an atomic oscillator within a wristwatch. For example, the atomic oscillator is used intermittently without explanation on the concrete and stable realization of such a principle. It is also not specified how to achieve a consumption and volume compatible with an implementation in a wristwatch.

Ainsi, l'invention cherche à atteindre un oscillateur pour montre-bracelet qui permette d'atteindre une grande précision, tout en respectant les contraintes sévères de volume très restreint et de faible puissance disponible au sein d'une montre-bracelet.Thus, the invention seeks to achieve an oscillator for a wristwatch that achieves high accuracy, while respecting the severe constraints of very small volume and low power available within a wristwatch.

Brève description de l'invention Brief description of the invention

A cet effet, l'invention repose sur un oscillateur atomique pour montre-bracelet qui repose sur un système de détection de fréquences de battement obtenues par effet Raman pour obtenir une référence de temps de grande précision.For this purpose, the invention is based on an atomic oscillator for a wristwatch which is based on a Raman effect beat detection system to obtain a high accuracy time reference.

L'invention est précisément définie par les revendications.The invention is precisely defined by the claims.

Brève description des figuresBrief description of the figures

Ces objets, caractéristiques et avantages de la présente invention seront exposés en détail dans la description suivante de modes de réalisation particuliers faits à titre non-limitatif en relation avec les figures jointes parmi lesquelles :

  • La figure 1 représente un schéma de principe d'un oscillateur atomique pour montre-bracelet selon un mode de réalisation de l'invention.
  • La figure 2 représente un schéma fonctionnel de l'oscillateur atomique pour montre-bracelet selon un mode de réalisation de l'invention.
  • La figure 3 représente un schéma électrique équivalent d'un système de détection optoélectronique selon un mode de réalisation de la présente invention.
  • La figure 4 représente un schéma électrique équivalent d'un détecteur optoélectronique selon un autre mode de réalisation de la présente invention.
  • La figure 5 représente schématiquement les courbes de gain g en fonction de la fréquence ω, les deux axes étant logarithmiques, pour un amplificateur à transimpédance classique (trait plein), un amplificateur à transimpédance doté d'un élément permettant d'augmenter la bande passante (« inductor peaking » ou « high frequency gain boosting », traitillé), et un système de détection selon l'invention (pointillé).
  • La figure 6 représente le spectre d'absorption d'un gaz en fonction du balayage en courant d'injection laser avec l'oscillateur atomique en boucle ouverte.
  • La figure 7 représente un premier mode de réalisation d'un oscillateur atomique à double passage.
  • La figure 8 représente un second mode de réalisation d'un oscillateur atomique à double passage.
  • La figure 9 représente un troisième mode de réalisation d'un oscillateur atomique à double passage.
  • La figure 10 représente une vue schématique éclatée d'un oscillateur atomique basé sur le second mode de réalisation à double passage et une géométrie à angle droit.
  • La figure 11 représente une vue schématique éclatée d'un oscillateur atomique basé sur le second mode de réalisation à double passage et une géométrie droite.
  • La figure 12 représente une vue schématique d'un oscillateur atomique basé sur le premier mode de réalisation à double passage.
  • La figure 13 représente une vue schématique d'un oscillateur atomique basé sur le premier mode de réalisation à double passage avec géométrie à angle droit.
  • La figure 14 représente une vue schématique d'un oscillateur atomique basé sur le troisième mode de réalisation à double passage.
These objects, features and advantages of the present invention will be set forth in detail in the following description of particular embodiments given as a non-limiting example in relation to the appended figures among which:
  • The figure 1 represents a block diagram of an atomic oscillator for a wristwatch according to one embodiment of the invention.
  • The figure 2 represents a block diagram of the atomic oscillator for a wristwatch according to one embodiment of the invention.
  • The figure 3 represents an equivalent circuit diagram of an optoelectronic detection system according to an embodiment of the present invention.
  • The figure 4 represents an equivalent circuit diagram of an optoelectronic detector according to another embodiment of the present invention.
  • The figure 5 schematically represents the gain curves g as a function of the frequency ω, the two axes being logarithmic, for a conventional transimpedance amplifier (solid line), a transimpedance amplifier provided with an element making it possible to increase the bandwidth ("inductor peaking "or" high frequency gain boosting ", etched), and a detection system according to the invention (dotted).
  • The figure 6 represents the absorption spectrum of a gas as a function of laser injection current scanning with the open-loop atomic oscillator.
  • The figure 7 represents a first embodiment of a double-pass atomic oscillator.
  • The figure 8 represents a second embodiment of a double-pass atomic oscillator.
  • The figure 9 represents a third embodiment of a double pass atomic oscillator.
  • The figure 10 is an exploded schematic view of an atomic oscillator based on the second double-pass embodiment and a right-angle geometry.
  • The figure 11 represents an exploded schematic view of an atomic oscillator based on the second double-pass embodiment and a right geometry.
  • The figure 12 is a schematic view of an atomic oscillator based on the first double-pass embodiment.
  • The figure 13 is a schematic view of an atomic oscillator based on the first double-pass embodiment with right-angle geometry.
  • The figure 14 is a schematic view of an atomic oscillator based on the third double pass embodiment.

La solution retenue repose sur l'utilisation d'un oscillateur atomique basé sur l'effet Raman, qui repose sur l'irradiation d'atomes de référence à une fréquence optique de résonance qui induit l'émission de photons avec une fréquence optique décalée de la fréquence hyperfine de l'atome de référence. La combinaison des deux signaux résultants permet d'obtenir un battement détectable, dont la fréquence du signal sert de base de temps à la montre-bracelet.The chosen solution is based on the use of an atomic oscillator based on the Raman effect, which is based on the irradiation of reference atoms at an optical resonant frequency which induces the emission of photons with an optical frequency shifted by the hyperfine frequency of the atom of reference. The combination of the two resulting signals provides a detectable beat, the signal frequency of which serves as the time base for the wristwatch.

La figure 1 illustre schématiquement la partie optique d'un oscillateur atomique à effet Raman selon un mode de réalisation de l'invention. Il comprend une diode laser 1, qui peut être à basse consommation et de type VCSEL, qui émet un faisceau polarisé linéairement 11, une lame quart d'onde 2 qui polarise la lumière provenant du laser selon un faisceau incident de polarisation circulaire 12. Ce faisceau 12 traverse une cellule 3 comprenant des atomes choisis, comme du Césium ou du Rubidium avec un gaz tampon, placée de manière optionnelle dans un champ magnétique B. En sortie de cette cellule 3, le signal incident 12 est combiné avec le second signal 13 généré par l'effet Raman, comme explicité ci-dessus. La combinaison des deux signaux est détectée par un photodétecteur 4 qui permet la récupération du signal comprenant la base de temps atomique, provenant des atomes de Césium ou Rubidium. Ce signal de sortie 14 est analysé par un dispositif électronique de traitement du signal, de type diviseur de fréquence microonde 5 pour générer la fréquence du signal nécessaire à la base de temps. La sortie 15 représente finalement cette base de temps, exploitée par une montre-bracelet comme cela sera explicité par la suite. Un amplificateur radiofréquence 6, optionnel, est positionné en sortie du photodétecteur 4.The figure 1 schematically illustrates the optical part of a Raman atomic oscillator according to one embodiment of the invention. It comprises a laser diode 1, which can be low-consumption and of the VCSEL type, which emits a linearly polarized beam 11, a quarter-wave plate 2 which polarises the light coming from the laser according to an incident beam of circular polarization 12. This beam 12 passes through a cell 3 comprising selected atoms, such as cesium or rubidium with a buffer gas, optionally placed in a magnetic field B. At the output of this cell 3, the incident signal 12 is combined with the second signal 13 generated by the Raman effect, as explained above. The combination of the two signals is detected by a photodetector 4 which allows the recovery of the signal comprising the atomic time base, originating from the cesium or rubidium atoms. This output signal 14 is analyzed by an electronic signal processing device of the microwave frequency divider type 5 to generate the signal frequency necessary for the time base. Exit 15 finally represents this time base, exploited by a wristwatch as will be explained later. An optional radio frequency amplifier 6 is positioned at the output of the photodetector 4.

En remarque, de manière optionnelle mais avantageuse, une partie du signal de sortie 14 est utilisé pour moduler le courant d'injection du laser, par une injection micro-onde au niveau du laser 1, représentée par la flèche 7. Cela permet d'atteindre un rapport signal-sur-bruit en sortie 14 de meilleure qualité et plus facile à exploiter. Ce principe équivaut à une modulation en amplitude du laser.As a remark, optionally but advantageously, a part of the output signal 14 is used to modulate the injection current of the laser, by a microwave injection at the level of the laser 1, represented by the arrow 7. This makes it possible to achieve a signal-on-noise output 14 of better quality and easier to operate. This principle is equivalent to an amplitude modulation of the laser.

En remarque, la cellule 3 a été positionnée au sein d'un champ magnétique B, qui permet de lever la dégénérescence des sous-états Zeeman des atomes. En variante, elle pourrait se trouver dans un champ magnétique nul, permettant d'obtenir une superposition des niveaux d'énergie, et un signal élevé, ainsi qu'un oscillateur simplifié.As a remark, the cell 3 has been positioned within a magnetic field B, which makes it possible to remove the degeneracy of the Zeeman sub-states of the atoms. Alternatively, it could be in a zero magnetic field, to obtain a superposition of energy levels, and a high signal, and a simplified oscillator.

La figure 2 représente de manière fonctionnelle un oscillateur atomique à effet Raman selon un mode de réalisation de l'invention. Il comprend un dispositif d'alimentation et convertisseur DC/DC 21, un centre de traitement 23 qui peut être un processeur ou une électronique à basse puissance, dont les fonctions principales comprennent tout ou partie des fonctions suivantes : fixation de la fréquence de fonctionnement du laser 1 et de son courant d'injection, contrôle de la température de la cellule 3 et du laser 1, gestion du mode intermittent du laser, correction de la fréquence de l'oscillateur atomique en fonction de la température, calage d'un oscillateur supplémentaire de moindre précision comme à base de quartz. La mise en oeuvre de ces fonctions sera détaillée par la suite. L'oscillateur comprend ensuite une source de courant DC 24 pour le laser 1, une source de courant DC 25 pour le chauffage du laser 1, une source de courant 26 du solénoïde pour la génération du champ magnétique B 36, une source de courant 27 pour le chauffage de la cellule 3, qui coopère avec un dispositif de chauffage 37 associé, auquel peut être de plus ajouté un capteur de température.The figure 2 functionally represents a Raman atomic oscillator according to one embodiment of the invention. It comprises a power supply device and DC / DC converter 21, a processing center 23 which can be a processor or a low-power electronics, the main functions of which comprise all or some of the following functions: setting the operating frequency of the laser 1 and its injection current, control of the temperature of the cell 3 and the laser 1, management of the intermittent mode of the laser, correction of the frequency of the atomic oscillator as a function of the temperature, setting of an oscillator additional low precision as quartz based. The implementation of these functions will be detailed later. The oscillator then comprises a DC current source 24 for the laser 1, a DC current source 25 for heating the laser 1, a current source 26 of the solenoid for the generation of the magnetic field B 36, a current source 27 for heating the cell 3, which cooperates with an associated heater 37, to which a temperature sensor can be additionally added.

Ces différents composants permettent le fonctionnement du laser 1 qui agit sur le dispositif optique 10 de l'oscillateur dont une représentation simplifiée a été présentée en référence à la figure 1. Dans cette réalisation, l'ensemble formé par le générateur de champ magnétique optionnel B 36, le dispositif de chauffage 37 et la cellule 3 est positionné dans une enceinte permettant d'atteindre leur blindage magnétique. En variante, une partie seulement de ces composants peut être intégrée au sein de ce blindage. En variante encore, ce champ magnétique peut être nul et l'oscillateur simplifié, comme explicité précédemment. En sortie, un photodétecteur 4 rapide comprend une sortie DC pour renvoyer un signal proportionnel à l'intensité lumineuse reçue vers le centre de traitement 23. Il comprend de plus une sortie RF pour un signal qui est d'abord amplifié par une chaine d'amplification 32 puis une ligne à retard et déphaseur 33 pour être réinjecté sur un diplexeur 34 (bias tee) qui permet de combiner le signal RF avec le courant DC d'injection laser provenant de la source de courant 24. Une partie du signal RF amplifié est traité par un diviseur de fréquence 5 avant son retour vers le centre de traitement 23. En sortie de ce centre de traitement, un signal à fréquence d'utilisateur 22 est obtenu (par exemple de 32 kHz, ou 1 pulse par seconde, etc.). Enfin, la mise en oeuvre de cet oscillateur est réalisée à partir de composants à faible consommation, pour une implémentation compatible avec un environnement de montre-bracelet.These various components allow the operation of the laser 1 which acts on the optical device 10 of the oscillator, a simplified representation has been presented with reference to the figure 1 . In this embodiment, the assembly formed by the optional magnetic field generator B 36, the heater 37 and the cell 3 is positioned in an enclosure for reaching their magnetic shielding. Alternatively, only part of these components can be integrated within this shield. In another variant, this magnetic field may be zero and the oscillator simplified, as explained previously. At the output, a fast photodetector 4 comprises a DC output for returning a signal proportional to the light intensity received to the processing center 23. It furthermore comprises an RF output for a signal which is first amplified by a signal chain. amplification 32 then a delay line and phase shifter 33 to be fed back to a diplexer 34 (bias tee) which combines the RF signal with the DC laser injection current from the current source 24. Part of the amplified RF signal is processed by a frequency divider 5 before returning to the processing center 23. At the output of this processing center, a user frequency signal 22 is obtained (for example 32 kHz, or 1 pulse per second, etc. .). Finally, the implementation of this oscillator is made from low-power components for an implementation compatible with a wristwatch environment.

En remarque, les horloges atomiques de type CPT utilisent toutes une architecture complexe et comprennent un dispositif de correction de l'oscillateur local, appelé par sa dénomination anglo-saxonne « Voltage Controlled Oscillator » (VCO), ainsi qu'une électronique de contrôle de l'oscillateur, représentant au total une forte consommation de puissance. L'oscillateur atomique de type Raman décrit précédemment présente l'avantage d'une plus grande simplicité pour une consommation fortement réduite.As a reminder, the CPT type atomic clocks all use a complex architecture and include a device for local oscillator correction, called by its English name "Voltage Controlled Oscillator" (VCO), as well as a control electronics of the oscillator, representing in total a high power consumption. The Raman atomic oscillator described above has the advantage of greater simplicity for a greatly reduced consumption.

Dans un tel oscillateur à effet Raman, un faisceau laser incident à une première fréquence interagit avec une vapeur d'atomes, stimulant ainsi, par une interaction lumière-atome, l'émission d'un second faisceau par effet Raman ayant une deuxième fréquence. Comme cela a été mentionné, le battement entre la première fréquence et la deuxième fréquence produit une troisième fréquence : la fréquence de battement, qui est exploitée comme base de temps. Dans le cas où la vapeur comprend par exemple du Rubidium-85 et où le laser est de type semi-conducteur à cavité verticale et à émission de surface émettant un faisceau de lumière à une longueur d'onde se situant aux alentours de 780 nm ou de 794 nm, la fréquence de battement est de l'ordre de 3GHz avec une bande passante autour d'une centaine de kHz. Cette fréquence de battement est en général de très bas niveau et a un contenu spectral très réduit. La détection de ces fréquences de battement en sortie de l'oscillateur pour leur exploitation dans une montre-bracelet est un problème technique délicat, en particulier pour limiter la consommation.In such a Raman oscillator, a laser beam incident at a first frequency interacts with an atom vapor, thereby stimulating, by light-atom interaction, the emission of a second Raman beam having a second frequency. As mentioned, the beat between the first frequency and the second frequency produces a third frequency: the beat frequency, which is used as a time base. In the case where the vapor comprises for example Rubidium-85 and where the laser is of the vertical cavity semiconductor type and With surface emission emitting a light beam at a wavelength of around 780 nm or 794 nm, the beat frequency is of the order of 3GHz with a bandwidth around a hundred kHz. This beat frequency is generally very low and has a very low spectral content. The detection of these beat frequencies at the output of the oscillator for their use in a wristwatch is a delicate technical problem, in particular to limit consumption.

Pour répondre à ce problème technique, il est proposé un système de détection d'un signal (iPD) haute fréquence (ωC) à bande étroite, ledit système ayant une basse consommation de courant. Le système comprend un générateur pour fournir le signal (iPD) sous forme d'un courant, et un circuit de résonance parallèle pour faire varier l'impédance de la sortie du générateur en fonction de la fréquence du signal généré et pour convertir le courant en tension. Le système comprend en plus un étage d'amplification pour augmenter encore le gain en dégradant de façon minimale le bruit du système pour permettre la détection d'un signal de très faible amplitude. Le générateur est le photodétecteur 4 mentionné précédemment, stimulé par de la radiation électromagnétique.To answer this technical problem, it is proposed a detection system of a signal (i PD ) high frequency (ω C ) narrow band, said system having a low power consumption. The system includes a generator for providing the signal (i PD ) as a current, and a parallel resonance circuit for varying the impedance of the generator output as a function of the frequency of the generated signal and for converting the current in tension. The system further includes an amplification stage to further increase the gain by minimally degrading the system noise to allow the detection of a signal of very low amplitude. The generator is the photodetector 4 mentioned above, stimulated by electromagnetic radiation.

Selon un mode de réalisation du système de détection, représenté sur la figure 3, une simple inductance L1 est comprise dans la réalisation du circuit de résonance parallèle et le photodétecteur est du type photodiode PD. La photodiode PD est polarisée au travers de l'inductance L1 connectée à une source de tension. Ceci permet de maintenir la photodiode PD à une tension désirée en fournissant le courant nécessaire pour que la photodiode PD fonctionne correctement. Il est à noter que le signal à détecter a un contenu spectral centré autour d'une fréquence prédéterminée ωC qui est de l'ordre de quelques gigahertz et très étroit (de l'ordre de 10-4×ωC).According to an embodiment of the detection system, represented on the figure 3 a simple inductor L1 is included in the realization of the parallel resonance circuit and the photodetector is of the PD photodiode type. The photodiode PD is polarized through the inductor L1 connected to a voltage source. This makes it possible to maintain the photodiode PD at a desired voltage by supplying the current necessary for the photodiode PD to work properly. It should be noted that the signal to be detected has a spectral content centered around a predetermined frequency ω C which is of the order of a few gigahertz and very narrow (of the order of 10 -4 × ω C ).

Le signal à détecter iPD apparaît sous la forme d'un courant sur un noeud N) qui relie l'inductance L1 à la photodiode PD. Ce noeud N est couplé électriquement à l'entrée de l'amplificateur MAMP et le signal amplifié apparaît à la sortie de l'amplificateur MAMP. Le noeud N ainsi configuré a donc une capacité parasite CIN. Cette capacité parasite CIN forme avec l'inductance L1 le circuit de résonance parallèle. La valeur de l'inductance est déterminée de sorte que sa réactance inductive à la fréquence du signal à détecter soit égale à la réactance capacitive de la capacité parasite CIN. En d'autres termes ωC×L1=1/(ωC×CIN). Sous ces conditions, on a un filtre passe-bande avec un facteur de qualité Q et une largeur à mi-hauteur de 1/Q. Avec une inductance L1 intégrée au circuit, on atteint un facteur de qualité Q de 10 environ, alors que l'on obtient un facteur de qualité Q de 50 environ avec une inductance L1 externe au circuit. La résistance parallèle équivalente Rp a une valeur de ω×L×Q. Grâce à un facteur de qualité Q élevé, on peut réaliser un gain important sans la consommation qui lui serait normalement associée. Sans la présente invention, on utiliserait un amplificateur transimpédance large bande avec 10GHz de bande passante à la place de celui qui est proposé. Typiquement, ce genre d'amplificateur consomme autour d'un watt, alors que l'amplificateur proposé ci-dessus consomme moins de deux milliwatts.The signal to be detected i PD appears in the form of a current on a node N) which connects the inductance L1 to the photodiode PD. This node N is electrically coupled to the input of the MAMP amplifier and the amplified signal appears at the output of the MAMP amplifier. N node thus configured has a parasitic capacitance C IN . This parasitic capacitance C IN forms with the inductor L1 the parallel resonance circuit. The value of the inductance is determined so that its inductive reactance at the frequency of the signal to be detected is equal to the capacitive reactance of the parasitic capacitance C IN . In other words ω C × L1 = 1 / (ω C × C IN ). Under these conditions, there is a bandpass filter with Q quality factor and a half-height width of 1 / Q. With an inductor L1 integrated in the circuit, a quality factor Q of about 10 is reached, whereas a quality factor Q of about 50 with an inductance L1 external to the circuit is obtained. The equivalent parallel resistance Rp has a value of ω × L × Q. Thanks to a high quality factor Q, you can achieve a significant gain without the consumption that would normally be associated with it. Without the present invention, a broadband transimpedance amplifier with 10 GHz bandwidth would be used in place of the proposed one. Typically, this kind of amplifier consumes around a watt, while the amplifier proposed above consumes less than two milliwatts.

La figure 5 montre bien la différence du gain en fonction de la fréquence pour les deux types d'amplificateur. Un amplificateur transimpédance large bande de l'état de la technique permet de couvrir une grande plage de fréquence, mais entraîne une forte consommation et un bruit comparativement élevé, vu que le bruit est d'autant plus important que la bande passante est large. Contrairement à l'amplificateur à transimpédance large bande, la solution proposée sélectionne avec un élément résonant un signal centré autour d'une fréquence centrale qui est nettement plus faible que la fréquence de coupure typique de la technologie de photodétecteur utilisée. La caractéristique de gain montre une bande passante très étroite, compatible avec le contenu spectral étroit du signal (de l'ordre de 10-4×ωC), ce qui diminue fortement le bruit par rapport à un amplificateur transimpédance. La consommation est très faible car le système ne comporte pas d'éléments actifs.
Puisque le noeud N a une impédance très élevée, il suffit d'utiliser un simple amplificateur de type MOS à source commune à faible bruit pour augmenter encore le gain en dégradant de façon minimale le bruit du système pour permettre la détection d'un signal de très faible amplitude. Dans un mode de réalisation, l'amplificateur a une charge résistive sur la sortie. Dans un autre mode de réalisation, profitant du fait que le signal à détecter a un contenu spectral très réduit, qui peut être d'une seule fréquence non-modulée, la charge à la sortie de l'amplificateur est assurée par une deuxième inductance L2 dont la valeur est choisie pour maximiser le gain pour un signal à la fréquence prédéterminée ωC.The figure 5 shows the difference in gain versus frequency for both types of amplifier. A broadband transimpedance amplifier of the state of the art can cover a large frequency range, but leads to high consumption and noise comparatively high, since the noise is all the more important that the bandwidth is wide. Unlike the broadband transimpedance amplifier, the proposed solution selects with a resonant element a signal centered around a central frequency that is significantly lower than the cutoff frequency typical of the photodetector technology used. The gain characteristic shows a very narrow bandwidth, compatible with the narrow spectral content of the signal (of the order of 10 -4 × ω C ), which strongly reduces the noise compared to a transimpedance amplifier. Consumption is very low because the system has no active elements.
Since node N has a very high impedance, it is sufficient to use a simple low noise common source MOS type amplifier to further increase the gain by minimally degrading the system noise to allow the detection of a signal. very low amplitude. In one embodiment, the amplifier has a resistive load on the output. In another embodiment, taking advantage of the fact that the signal to be detected has a very low spectral content, which may be of a single unmodulated frequency, the load at the output of the amplifier is provided by a second inductor L2. whose value is chosen to maximize the gain for a signal at the predetermined frequency ω C.

L'entrée de l'amplificateur peut être couplée en mode AC avec le noeud N, i.e., avec une capacité de couplage CC, et l'entrée de l'amplificateur peut donc être polarisée par une source de tension Vb à travers une résistance Rb de façon à ce que l'entrée de l'amplificateur soit à une tension optimale. Dans la fabrication d'un circuit selon la présente invention, il se peut que la valeur de la capacité parasite CIN ou la valeur de l'inductance L1 varie d'un lot à l'autre ou d'une pièce à l'autre. Ceci aurait l'effet de déplacer la fréquence de résonance du circuit de résonance en dehors de la bande de fréquence adéquate pour détecter un signal à la fréquence prédéterminée. Pour cette raison, il est proposé d'agir sur la valeur de la capacité du noeud N en ajustant cette capacité. Ceci peut se faire de différentes manières, par exemple par l'emploi d'une capacité ajustable (trim capacitor) ou par l'emploi de plusieurs capacités que l'on peut connecter ou déconnecter au noeud N, par exemple par le dépôt ciblé de métal lors de la fabrication. Ceci peut être également accompli par un système de laser-trimming où le noeud (N) est connecté à une capacité dont la valeur est ajustée par ablation laser au moment du test de l'ensemble.The input of the amplifier can be coupled in AC mode with the node N, ie, with a DC coupling capacitance, and the input of the amplifier can therefore be biased by a voltage source Vb through a resistor Rb so that the input of the amplifier is at an optimum voltage. In the manufacture of a circuit according to the present invention, the value of the parasitic capacitance C IN or the value of the inductance L1 may vary from one batch to another or from one room to another . This would have the effect of moving the resonance frequency of the resonance circuit out of the appropriate frequency band to detect a signal at the predetermined frequency. For this reason, it is proposed to act on the value of the capacity of the node N by adjusting this capacity. This can be done in various ways, for example by using an adjustable capacitor (trim capacitor) or by the use of several capacities that can be connected or disconnected at the node N, for example by the targeted deposition of metal during manufacture. This can also be accomplished by a laser-trimming system where the node (N) is connected to a capacity whose value is adjusted by laser ablation at the time of the test of the whole.

Selon un autre mode de réalisation de la présente invention, le circuit de résonance comprend un résonateur électromécanique de type résonateur à onde de volume ou Bulk Acoustic Wave (BAW), comme illustré par la figure 4. Le résonateur à onde de volume (BAW) permet un filtrage encore plus sélectif et il présente, à l'anti-résonance, une impédance réelle élevée tout en permettant de neutraliser la capacité parasite CIN du noeud N. Selon un mode de réalisation, le résonateur électromécanique permet d'atteindre un facteur de qualité supérieur à 300. Dans ce mode de réalisation, la photodiode est polarisée à l'aide d'un circuit adaptif dont l'étage de sortie est une source de courant CCS contrôlée de façon à garantir une tension de polarisation fixe sur la diode en basse fréquence.According to another embodiment of the present invention, the resonance circuit comprises an electromechanical resonator of the bulk wave resonator type or Bulk Acoustic Wave (BAW), as illustrated by FIG. figure 4 . The volume wave resonator (BAW) allows an even more selective filtering and it has, at the anti-resonance, a high real impedance while allowing to neutralize the parasitic capacitance C IN of the node N. According to one embodiment, the electromechanical resonator makes it possible to achieve a quality factor greater than 300. In this embodiment, the photodiode is polarized by means of an adaptive circuit whose output stage is a source of current CCS controlled so as to guarantee a fixed polarization voltage on the diode at low frequency.

Un autre problème technique rencontré pour la mise en oeuvre de l'oscillateur à effet Raman au sein d'une montre-bracelet est d'atteindre une stabilité suffisante, lui permettant un fonctionnement précis sur une durée satisfaisante. Ce problème est résolu par le fonctionnement décrit précédemment en relation avec la figure 1 et représenté de manière fonctionnelle par la figure 2.Another technical problem encountered for the implementation of the Raman oscillator within a wristwatch is to achieve sufficient stability, allowing it to operate accurately over a satisfactory period. This problem is solved by the operation described above in relation to the figure 1 and functionally represented by the figure 2 .

Une rétroaction du signal RF détecté sur la fréquence optique du laser, afin d'asservir la fréquence d'émission du laser, est toujours préconisée dans l'état de la technique pour obtenir un oscillateur atomique stable et de haute précision, en particulier pour les horloges atomiques de type CPT. Dans le cas présent, il a été constaté qu'il était quasiment impossible de maîtriser le fonctionnement de l'oscillateur Raman de façon répétable et fiable en boucle fermée vis-à-vis de la fréquence optique du laser. La détection synchrone pour la stabilisation de la fréquence d'un laser n'est pas adéquate dans le cas d'un oscillateur Raman en boucle fermée.A feedback of the RF signal detected on the optical frequency of the laser, in order to control the emission frequency of the laser, is always recommended in the state of the art to obtain a stable and high-precision atomic oscillator, in particular for the atomic clocks of type CPT. In the present case, it has been found that it is almost impossible to control the operation of the Raman oscillator in a repeatable and reliable closed-loop manner with respect to the optical frequency of the laser. Synchronous detection for frequency stabilization of a laser is not adequate in the case of a closed-loop Raman oscillator.

De manière surprenante, il a été possible de faire fonctionner l'oscillateur Raman sans asservissement en fréquence optique du laser, c'est-à-dire avec un asservissement en fréquence nul, ou autrement dit sans contrôle actif de la fréquence optique du laser, soit un fonctionnement en boucle ouverte vis-à-vis de la fréquence du laser.Surprisingly, it was possible to operate the Raman oscillator without servocontrolling the optical frequency of the laser, that is to say with zero frequency control, or in other words without active control of the optical frequency of the laser. either an open loop operation with respect to the frequency of the laser.

Des tests de stabilité ont été effectués selon le principe précédent, qui ont démontré une grande stabilité. A une température de 87,5 °C, l'oscillateur Raman varie d'une seconde tous les 160 ans et fonctionne de façon stable pendant plusieurs jours au moins en continu.Stability tests were performed according to the previous principle, which showed great stability. At a temperature of 87.5 ° C, the Raman oscillator varies by one second every 160 years and runs steadily for several days at least continuously.

La température de la cellule, d'une longueur active de 5 mm, a aussi été abaissée jusque sous la température de fusion du Rubidium (39.3°C). Une baisse de température de 90°C à 35°C correspond à une diminution de la pression de vapeur saturante de deux ordres de grandeur (∼10-4 torr à 10-6 torr). La stabilité dépend de la température de la cellule mais reste acceptable jusqu'à une température de 35 °C. En effet, à une température de 40°C, l'oscillateur Raman fonctionne toujours à satisfaction avec une stabilité de une seconde tous les 16 ans, ce qui est remarquable. A 35°C, le signal Raman est toujours présent et suffisamment stable. Cette constatation inattendue permet d'envisager un oscillateur atomique sans chauffage de la cellule selon une variante de réalisation, fonctionnant par exemple uniquement quand la température autour de la cellule est suffisante, par exemple autour de 35°C, préférentiellement autour de 40°C. Ainsi, selon un mode de réalisation, l'oscillateur atomique peut fonctionner à une température inférieure ou égale à 40 °C, voire inférieure ou égale à 35 °C. Il est aussi envisageable de diminuer la température de fonctionnement en utilisant du Cs au lieu du Rb dans la cellule, la température de fusion du Césium étant encore plus basse que celle du Rubidium (28.5°C au lieu de 39.3°C).The temperature of the cell, with an active length of 5 mm, was also lowered to below the melting temperature of Rubidium (39.3 ° C). A temperature drop of 90 ° C to 35 ° C corresponds to a decrease in the saturation vapor pressure by two orders of magnitude (~ 10 -4 torr at 10 -6 torr). The stability depends on the temperature of the cell but remains acceptable up to a temperature of 35 ° C. Indeed, at a temperature of 40 ° C, the Raman oscillator always works to satisfaction with a stability of one second every 16 years, which is remarkable. At 35 ° C, the Raman signal is still present and sufficiently stable. This unexpected finding makes it possible to envisage an atomic oscillator without heating the cell according to an alternative embodiment, functioning for example only when the temperature around the cell is sufficient, for example around 35 ° C., preferably around 40 ° C. Thus, according to one embodiment, the atomic oscillator can operate at a temperature less than or equal to 40 ° C, or even lower than or equal to 35 ° C. It is also possible to reduce the operating temperature by using Cs instead of Rb in the cell, the cesium melting temperature being even lower than that of Rubidium (28.5 ° C instead of 39.3 ° C).

Un problème technique supplémentaire est rencontré lors de la mise en route de l'oscillateur. En effet, la solution explicitée précédemment montre comment obtenir un fonctionnement stable et performant de l'oscillateur lorsqu'il est en régime de croisière, à partir des dispositifs décrits en rapport avec les figures 1 et 2. Un fonctionnement entièrement en boucle ouverte, c'est-à-dire sans le retour 7 de la figure 1, serait une variante de réalisation envisageable moins performante car le signal obtenu serait relativement faible et spectralement moins pur.An additional technical problem is encountered when starting the oscillator. Indeed, the solution explained above shows how to obtain a stable and efficient operation of the oscillator when it is in cruise mode, from the devices described in connection with the Figures 1 and 2 . A fully open loop operation, that is to say without the return 7 of the figure 1 , would be an alternative embodiment feasible less efficient because the signal obtained would be relatively weak and spectrally less pure.

Pour cela, il a été constaté qu'il existe une plage réduite de courant d'injection du laser, soit une plage de fréquence correspondante, à proximité du pic d'absorption optique du gaz de la cellule, qui permet lorsqu'on débute une irradiation laser sur la cellule en boucle ouverte puis qu'on passe en régime fermé tel que décrit précédemment d'obtenir une mise en résonance de l'oscillateur pour atteindre le régime de fonctionnement optimal décrit précédemment. Ainsi, par le choix judicieux du courant d'injection du laser lors de l'allumage du laser puis la mise en circuit fermé vis-à-vis du courant d'injection du laser tel qu'explicité ci-dessus, l'oscillateur atteint naturellement son régime de fonctionnement optimal. Ce phénomène permet un auto-allumage de l'oscillateur, et rend possible son utilisation de manière intermittente.For this, it has been found that there is a reduced range of laser injection current, ie a corresponding frequency range, close to the optical absorption peak of the cell gas, which allows when starting a laser irradiation on the open-loop cell and then going into closed mode as described above to obtain a resonance of the oscillator to achieve the optimal operating regime described above. Thus, by the judicious choice of the injection current of the laser during the ignition of the laser and the closed circuit with respect to the injection current of the laser as explained above, the oscillator reaches of course its optimal operating regime. This phenomenon allows self-ignition of the oscillator, and makes it possible to use it intermittently.

Cette plage de fonctionnement est plus exactement illustrée par la figure 6 pour le cas du Rubidium naturel. Cette figure montre la courbe 50 d'absorption optique du Rubidium, par le signal obtenu sur la photodiode 6, en fonction du courant d'injection du laser. La plage de courant favorable est située dans la zone 52, qui représente une portion du pic de plus grande absorption 51, à une certaine distance des deux valeurs maximale Vmax et Vmin de ce pic. En choisissant une plage réduite [V1 ; V2], suffisamment éloignée de ces valeurs, on en déduit une plage de courant [i1, i2] favorable. Les considérations précédentes permettent la mise en oeuvre du procédé d'allumage suivant d'un oscillateur pour montre-bracelet à effet Raman, qui fait partie du procédé d'émission d'un signal de temps par oscillateur atomique selon l'invention.This operating range is more accurately illustrated by the figure 6 for the case of natural Rubidium. This figure shows the optical absorption curve 50 of the Rubidium, by the signal obtained on the photodiode 6, as a function of the injection current of the laser. The favorable current range is located in zone 52, which represents a portion of the peak of greater absorption 51, at a distance from the two maximum values Vmax and Vmin of this peak. By choosing a reduced range [V1; V2], sufficiently far from these values, we deduce a favorable current range [i1, i2]. The preceding considerations allow the implementation of the following ignition method of an oscillator for wristwatch with Raman effect, which is part of the method of transmitting an atomic oscillator time signal according to the invention.

Une première phase consiste en une recherche du courant d'injection optimal i du laser, c'est à dire la fourchette i1 à i2. Cette première phase comprend les étapes suivantes :

  • mise en boucle ouverte de l'oscillateur à effet Raman ;
  • balayage de la fréquence du laser et identification du point Vmax d'absorption maximale et du courant d'injection Imax correspondant ainsi que du point Vmin d'absorption minimale du pic 51 associé et du courant d'injection Imin correspondant ;
  • détermination d'un courant d'injection ILD entre i1 et i2 en ajoutant une certaine valeur seuil delta à Imin ou en la retranchant à Imax. Par exemple, une valeur proche de i1 peut être choisie.
A first phase consists of a search for the optimal injection current i of the laser, ie the range i1 to i2. This first phase includes the following steps:
  • open looping of the Raman oscillator;
  • scanning the laser frequency and identifying the maximum absorption point Vmax and the corresponding Imax injection current and the minimum absorption point Vmin of the associated peak 51 and the corresponding Imin injection current;
  • determining an ILD injection current between i1 and i2 by adding a certain delta threshold value to Imin or subtracting it from Imax. For example, a value close to i1 can be chosen.

A titre d'exemple, pour le Rubidium et le laser VCSEL utilisé pour les expériences, le courant d'injection du laser doit être choisi entre 2.25760 mA et 2.25824 mA. V1 se trouve 15% de Vmax-Vmin au-dessus de Vmin et V2 à 67% de Vmax-Vmin au-dessus de Vmin.For example, for the Rubidium and the VCSEL laser used for the experiments, the laser injection current must be chosen between 2.25760 mA and 2.25824 mA. V1 is 15% Vmax-Vmin above Vmin and V2 at 67% Vmax-Vmin above Vmin.

Cette première phase du procédé d'allumage peut être réalisée avant chaque allumage de l'oscillateur, afin d'obtenir la plus grande précision possible, ce qui permet de modifier les valeurs précédentes dans le temps en fonction d'une éventuelle dérive du dispositif ou des conditions de mesure. En variante, cette phase n'est réalisée qu'une fois pour étalonner le dispositif et les données sont mémorisées pour être reprises à chaque allumage.This first phase of the ignition process can be performed before each ignition of the oscillator, in order to obtain the highest possible accuracy, which makes it possible to modify the previous values in time as a function of possible drift of the device or measurement conditions. Alternatively, this phase is performed only once to calibrate the device and the data are stored to be repeated at each ignition.

Le procédé d'allumage met de plus en oeuvre les étapes suivantes d'allumage concret du laser et de l'oscillateur ;

  • mise en boucle fermée de l'oscillateur, en ajoutant la rétroaction 7 explicitée précédemment ;
  • ajustement du courant d'injection du laser à la valeur ILD identifiée par la première phase ;
  • vérification de l'obtention du phénomène de résonance de l'oscillateur en sortie ;
  • en cas de non résonance, légère modification de la valeur du courant d'injection ILD dans la plage [i1 ; i2] selon un pas prédéfini, et répétition de cette étape jusqu'à obtenir le phénomène de résonance.
The ignition process further implements the following steps of concrete ignition of the laser and the oscillator;
  • closed looping of the oscillator, adding the feedback 7 explained previously;
  • adjusting the laser injection current to the ILD value identified by the first phase;
  • verifying the obtaining of the resonance phenomenon of the oscillator at the output;
  • in case of non-resonance, slight modification of the value of the injection current ILD in the range [i1; i2] in a predefined step, and repetition of this step until the resonance phenomenon.

Selon une variante avantageuse, ce procédé comprend une étape préalable de mesure de la puissance optique du laser, car la fréquence de l'oscillateur peut dépendre de la puissance optique interagissant avec les atomes. Cette opération peut se faire en mesurant la puissance optique au moyen d'une photodiode du dispositif et en comparant la photo-tension ainsi générée avec une source de tension stable de référence. L'ajustement du courant d'injection du laser et de la température du laser permet ensuite d'obtenir la puissance optique et la fréquence optique nominales de l'oscillateur.According to an advantageous variant, this method comprises a preliminary step of measuring the optical power of the laser, since the frequency of the oscillator may depend on the optical power interacting with the atoms. This can be done by measuring the optical power by means of a photodiode of the device and comparing the photovoltage thus generated with a reference stable voltage source. Adjusting the laser injection current and the laser temperature then makes it possible to obtain the optical power and the optical frequency of the oscillator.

Selon une autre variante de réalisation avantageuse, ce procédé comprend une étape préalable de mise en température de la cellule de gaz et du laser, car le fonctionnement de l'oscillateur dépend de la température, comme cela a été mentionné précédemment. Il existe une corrélation entre la fréquence de l'oscillateur Raman en boucle fermée et la température de la cellule. Cette propriété permet de maîtriser la fréquence lors des phases de marche et d'arrêt de l'oscillateur par la seule mesure de la température.According to another advantageous embodiment, this method comprises a preliminary step of warming the gas cell and the laser, since the operation of the oscillator depends on the temperature, as mentioned previously. There is a correlation between the frequency of the closed-loop Raman oscillator and the temperature of the cell. This property makes it possible to control the frequency during the on and off phases of the oscillator by the sole measurement of the temperature.

Ainsi, selon le mode de réalisation choisi, l'oscillateur à effet Raman comprend un asservissement en température. Pour cela, il comprend un capteur de température, qui peut être une photodiode, et un dispositif de chauffage pour augmenter la température si elle est sous une température de consigne.Thus, according to the embodiment chosen, the Raman oscillator comprises a temperature control. For this purpose, it comprises a temperature sensor, which may be a photodiode, and a heating device for increasing the temperature if it is under a set temperature.

Les étapes décrites précédemment du procédé d'allumage sont gérées automatiquement par l'oscillateur, sur la base des moyens matériel (hardware) et logiciel (software) du centre de traitement 23 mentionné précédemment, notamment sous le pilotage du microprocesseur.The previously described steps of the ignition process are managed automatically by the oscillator, on the basis of the hardware and software resources of the processing center 23 mentioned above, in particular under the control of the microprocessor.

L'oscillateur atomique précédent est ainsi implémenté au sein d'une montre-bracelet.The previous atomic oscillator is thus implemented within a wristwatch.

Selon une première réalisation de montre-bracelet, l'oscillateur à effet Raman est utilisé de manière intermittente, en complément d'un oscillateur conventionnel de l'état de la technique, par exemple à quartz. Dans cette réalisation, l'oscillateur atomique transmet une base de temps qui permet le calage de l'oscillateur à quartz, sa correction, et permet d'augmenter fortement sa précision dans le temps. Ce fonctionnement intermittent de l'oscillateur atomique présente l'avantage d'une consommation supplémentaire maitrisée par rapport à une montre-bracelet conventionnelle. Comme l'allumage de cet oscillateur est maîtrisé par le procédé explicité ci-dessus, cette première implémentation dans une montre-bracelet est très performante. La période d'allumage de l'oscillateur atomique est choisie en fonction d'un compromis entre la consommation et la précision de la montre-bracelet : plus cet oscillateur est utilisé, plus l'horloge sera précise mais plus la consommation sera élevée. Lorsque l'oscillateur supplémentaire de moindre précision est corrigé par l'oscillateur atomique, ce dernier est éteint. Selon une seconde réalisation de montre-bracelet, l'oscillateur à effet Raman est utilisé seul en remplacement de l'oscillateur habituel conventionnel, comme unique base de temps, et donc selon un fonctionnement permanent. Dans cette réalisation, la plus grande précision est obtenue, mais par l'intermédiaire d'une plus grande consommation énergétique.According to a first embodiment of a wristwatch, the Raman oscillator is used intermittently, in addition to a conventional oscillator of the state of the art, for example quartz. In this embodiment, the atomic oscillator transmits a time base which allows the calibration of the quartz oscillator, its correction, and allows to greatly increase its accuracy over time. This intermittent operation of the atomic oscillator has the advantage of an additional controlled consumption compared to a conventional wristwatch. As the ignition of this oscillator is controlled by the method explained above, this first implementation in a wristwatch is very efficient. The ignition period of the atomic oscillator is chosen according to a compromise between the consumption and the precision of the wristwatch: the more this oscillator is used, the more accurate the clock but the higher the consumption will be. When the additional oscillator of less precision is corrected by the atomic oscillator, the latter is off. According to a second embodiment of a wristwatch, the Raman oscillator is used alone to replace the usual conventional oscillator, as a single time base, and therefore according to a permanent operation. In this embodiment, the highest accuracy is achieved, but through greater energy consumption.

L'oscillateur atomique décrit précédemment est de plus réalisé selon une structure compacte et peu encombrante, pour faciliter son insertion dans une montre-bracelet. Les figures 7 à 14 décrivent ainsi plusieurs modes de réalisation de la partie optique de l'oscillateur atomique, permettant d'atteindre un volume compatible avec l'intégration dans une montre-bracelet. Pour cela, toutes ces réalisations sont basées sur un double passage du faisceau laser dans la cellule, ce qui permet d'atteindre une longueur totale importante du faisceau laser dans un petit volume.The atomic oscillator described above is also made in a compact and compact structure, to facilitate insertion into a wristwatch. The Figures 7 to 14 thus describe several embodiments of the optical part of the atomic oscillator, making it possible to reach a volume compatible with integration into a wristwatch. For this, all these achievements are based on a double pass of the laser beam in the cell, which allows to achieve a large total length of the laser beam in a small volume.

Les figures 7 à 9 illustrent trois modes de réalisation différents permettant de réaliser simultanément un double passage dans la cellule à gaz 106 et une protection de la source laser 102 envers les réflexions. Un point commun de ces différents modes de réalisation est la présence d'un miroir semi-transparent 107 qui laisse passer une partie du faisceau laser ayant traversé la cellule à gaz 106 afin d'atteindre un photodétecteur 109, servant à l'asservissement de la température de la cellule. En variante, ces modes de réalisation pourraient être simplifiés en supprimant ce photodétecteur 109 et en utilisant un miroir non transparent.The Figures 7 to 9 illustrate three different embodiments making it possible to simultaneously perform a double pass in the gas cell 106 and a protection of the laser source 102 towards the reflections. A common feature of these various embodiments is the presence of a semitransparent mirror 107 which passes a portion of the laser beam passed through the gas cell 106 to reach a photodetector 109 for servocontrol of the cell temperature. Alternatively, these embodiments could be simplified by removing this photodetector 109 and using a non-transparent mirror.

Ces trois modes de réalisations diffèrent dans le moyen utilisé pour diriger le faisceau vers la cellule et les photodétecteurs, et dans le moyen utilisé pour empêcher le faisceau réfléchi par le miroir de venir perturber la source laser.These three embodiments differ in the means used to direct the beam towards the cell and the photodetectors, and in the means used to prevent the beam reflected by the mirror from disturbing the laser source.

La figure 7 illustre le premier mode de réalisation de l'invention. La source laser 102 produit un faisceau laser polarisé linéairement et qui est dirigé vers un polariseur 103, dont l'axe de transmission est orienté de manière à laisser passer le faisceau laser, puis vers un séparateur 101 dont le pourcentage de séparation est prédéfini. Une partie du faisceau est ainsi transmise vers un photodétecteur optionnel 108b. Le séparateur réfléchit l'autre partie du faisceau vers une lame quart d'onde 105. La polarisation linéaire est notée « P » pour la partie parallèle à l'axe de transmission du polariseur (partie transmise) et « S » pour la partie perpendiculaire à l'axe de transmission du polariseur (partie absorbée par le polariseur). Dans les figures, la partie « P » est symbolisée par des cercles pleins et la partie « S » par des traits. Le rôle de la lame 105 est de changer la polarisation linéaire du faisceau laser en une polarisation circulaire et cette lame est orientée par rapport au polariseur de façon à générer une polarisation circulaire. En effet, l'interaction entre la lumière et les atomes de la cellule à gaz 106 est optimale lorsqu'elle est réalisée avec un faisceau de polarisation circulaire. Une partie du faisceau sortant de la cellule à gaz 106 est ensuite réfléchie par un miroir 107, ce qui inverse le sens de sa polarisation circulaire, et traverse ainsi une seconde fois la cellule à gaz 106. En sortant de la cellule à gaz 106, le faisceau atteint la lame quart d'onde 105. Selon le pourcentage de séparation prédéfini du séparateur 101, ce faisceau est ensuite en partie transmis et atteint le photodétecteur 108a. Une autre partie de ce faisceau est déviée par le séparateur 101 et est fortement atténuée par le polariseur 103 car sa polarisation est perpendiculaire à celle de l'axe de transmission du polariseur 103, la source laser 102 étant ainsi protégée des rétro-réflexions. Une faible partie du faisceau ayant traversé la cellule à gaz 106 est transmise par le miroir 107 et captée par le photodétecteur 109.The figure 7 illustrates the first embodiment of the invention. The laser source 102 produces a linearly polarized laser beam which is directed towards a polarizer 103, the transmission axis of which is oriented so as to let the laser beam pass, then to a separator 101 whose percentage of separation is predefined. Part of the beam is thus transmitted to an optional photodetector 108b. The separator reflects the other part of the beam towards a quarter-wave plate 105. The linear polarization is denoted "P" for the part parallel to the transmission axis of the polarizer (transmitted part) and "S" for the perpendicular part to the transmission axis of the polarizer (part absorbed by the polarizer). In the figures, the part "P" is symbolized by solid circles and the part "S" by lines. The role of the blade 105 is to change the linear polarization of the laser beam into a circular polarization and this blade is oriented relative to the polarizer so as to generate a circular polarization. Indeed, the interaction between the light and the atoms of the gas cell 106 is optimal when it is performed with a circular polarization beam. A portion of the beam leaving the gas cell 106 is then reflected by a mirror 107, which reverses the direction of its circular polarization, and thus passes through the gas cell 106 a second time. When leaving the gas cell 106, the beam reaches the quarter wave plate 105. According to the predefined separation percentage of the separator 101, this beam is then partially transmitted and reaches the photodetector 108a. Another part of this beam is deflected by the separator 101 and is strongly attenuated by the polarizer 103 because its polarization is perpendicular to that of the transmission axis of the polarizer 103, the laser source 102 thus being protected from retro-reflections. A small portion of the beam passed through the gas cell 106 is transmitted by the mirror 107 and picked up by the photodetector 109.

La figure 8 illustre le second mode de réalisation. Il diffère du premier mode décrit ci-dessus par l'utilisation d'un séparateur 101 qui réfléchit le faisceau selon une première polarisation et laisse passer le faisceau selon une seconde polarisation. Ainsi le faisceau sortant de la source laser 102 est séparé selon sa polarisation et le même principe s'applique au faisceau réfléchi. Il n'est ainsi pas nécessaire de placer un polariseur entre le séparateur 101 et la source laser du fait que le faisceau réfléchi est entièrement transmis vers le photodétecteur 108a. La polarisation linéaire est notée « P » pour la partie parallèle à l'axe de polarisation du séparateur (partie transmise dans la configuration à angle droit de la figure 8) et « S » pour la partie perpendiculaire à l'axe de polarisation du séparateur (partie déviée à 90°). Dans la figure 10, la partie « P » est symbolisée par des traits et la partie « S » par des cercles pleins. Une faible partie du faisceau ayant traversé la cellule à gaz 106 est transmise par le miroir 107 et captée par le photodétecteur 109.The figure 8 illustrates the second embodiment. It differs from the first mode described above by the use of a separator 101 which reflects the beam in a first polarization and passes the beam in a second polarization. Thus the beam leaving the laser source 102 is separated according to its polarization and the same principle applies to the reflected beam. It is thus not necessary to place a polarizer between the separator 101 and the laser source because the reflected beam is fully transmitted to the photodetector 108a. The linear polarization is denoted "P" for the part parallel to the polarization axis of the separator (part transmitted in the right angle configuration of the figure 8 ) and "S" for the part perpendicular to the polarization axis of the separator (part deflected at 90 °). In the figure 10 , the part "P" is symbolized by lines and the part "S" by solid circles. A small portion of the beam passed through the gas cell 106 is transmitted by the mirror 107 and picked up by the photodetector 109.

La figure 9 illustre le troisième mode de réalisation de l'invention. Sur cette figure, la déviation du faisceau laser est assurée par le miroir semi-transparent 107 qui est disposé selon un angle non perpendiculaire par rapport à l'axe du faisceau laser. Ainsi, le faisceau réfléchi n'atteint pas la source laser 102 mais est dirigé directement sur le photodétecteur 108a. Avantageusement, le miroir 107 est de forme concave pour focaliser le faisceau de lumière réfléchi sur le photodétecteur 108a. Une faible partie du faisceau ayant traversé la cellule à gaz 106 est transmise par le miroir 107 et captée par le photodétecteur 109. Cette forme concave du miroir peut aussi être utilisée sur les deux modes de réalisation des figures 7 et 8 apportant les avantages décrits ci-dessus.The figure 9 illustrates the third embodiment of the invention. In this figure, the deflection of the laser beam is provided by the semi-transparent mirror 107 which is disposed at an angle not perpendicular to the axis of the laser beam. Thus, the reflected beam does not reach the laser source 102 but is directed directly on the photodetector 108a. Advantageously, the mirror 107 is of concave shape for focusing the light beam reflected on the photodetector 108a. A small portion of the beam having passed through the gas cell 106 is transmitted by the mirror 107 and picked up by the photodetector 109. This concave shape of the mirror can also be used on the two embodiments of the Figures 7 and 8 providing the benefits described above.

Un exemple de réalisation plus complet correspondant au deuxième mode de réalisation est illustré à la figure 10. Le séparateur 101 est réalisé sous la forme d'un cube à sélection de polarisation (polarizing beam splitter cube, PBSC). Ce cube permet d'implémenter une double traversée de la cellule à gaz 106, qui multiplie par deux l'interaction entre la lumière du laser et le milieu atomique. On obtient un meilleur signal atomique et ainsi une meilleure stabilité de la fréquence de l'oscillateur atomique.A more complete exemplary embodiment corresponding to the second embodiment is illustrated in FIG. figure 10 . The separator 101 is in the form of a polarizing beam splitter cube (PBSC). This cube makes it possible to implement a double crossing of the gas cell 106, which doubles the interaction between the laser light and the atomic medium. We obtain a better atomic signal and thus a better stability of the frequency of the atomic oscillator.

Sur la figure 10, l'ensemble optique est basé sur un cube séparateur miniature 101 dont les côtés sont de préférence inférieurs ou égaux à 1 mm, le cube 101 faisant office de séparateur. Selon un mode standard, le volume du cube est de typiquement 1 mm3. Le faisceau de lumière de la diode laser 102 arrive sur l'un des côtés du cube 101. Selon un mode de réalisation, la diode laser est de type semi-conducteur à cavité verticale et à émission de surface (VCSEL) émettant un faisceau divergeant de lumière à 795 nm. Dans d'autres modes de réalisation, d'autres types de diodes laser ayant des longueurs d'ondes variant typiquement de 780 nm à 894 nm peuvent être utilisés, pour une cellule à gaz 106 contenant du Rubidium ou du Césium. Ce choix est dicté par la composition atomique de la cellule à gaz. Selon un mode de réalisation, une lentille de collimation peut être ajoutée devant la diode laser pour produire un faisceau laser non-divergeant.On the figure 10 , the optical assembly is based on a miniature separator cube 101 whose sides are preferably less than or equal to 1 mm, the cube 101 acting as a separator. In a standard mode, the volume of the cube is typically 1 mm 3 . The light beam of the laser diode 102 arrives on one of the sides of the cube 101. According to one embodiment, the laser diode is of the vertical cavity and surface emission semiconductor (VCSEL) type emitting a diverging beam. of light at 795 nm. In other embodiments, other types of laser diodes having wavelengths typically ranging from 780 nm to 894 nm may be used for a gas cell 106 containing Rubidium or cesium. This choice is dictated by the atomic composition of the gas cell. According to one embodiment, a collimating lens may be added in front of the laser diode to produce a non-diverging laser beam.

Selon un mode standard, la lumière produite 112 par le laser 102 a une polarisation linéaire et est atténuée par un filtre neutre absorbant 104a. Un type différent de filtre peut être utilisé dans d'autres modes de réalisation. La présence de ce filtre n'est pas nécessaire à l'invention. Une lame demi-onde 104b peut être utilisée pour modifier l'angle de la polarisation linéaire de la source laser. En combinaison avec le cube miniature 101, la lame demi-onde 104b joue le rôle d'un atténuateur variable. Dans d'autres modes de réalisation, l'utilisation de la lame demi-onde 104b peut être omise et le rapport d'intensité lumineuse entre les faisceaux transmis et réfléchis par le cube 101 est ajusté par une orientation appropriée de l'axe de polarisation linéaire de la lumière émise par le laser par rapport au cube séparateur. Une lame quart d'onde 105 est placée en sortie de cube contre la face d'où sort le faisceau laser dévié par le séparateur 101, soit à angle droit du faisceau incident au cube. L'axe rapide de la lame quart d'onde 105 est orienté de telle sorte que la polarisation linéaire incidente 113 est modifiée vers une polarisation circulaire 114 selon un premier sens de rotation. Dans d'autres modes de réalisation, la lame quart d'onde 105 est orientée de telle sorte que la polarisation linéaire incidente 113 est modifiée vers une polarisation circulaire selon un sens de rotation inverse au premier. Le rayon laser de polarisation circulaire 114 traverse la cellule à gaz 106 et parvient sur le miroir 107. Ce dernier ne renvoie le rayon que partiellement et une partie du rayon traverse le miroir 107 pour se diriger vers le photodétecteur 109. Selon un mode standard, la cellule à gaz est réalisée en verre-silicium-verre par des techniques MEMS (microsystème électromécanique) avec un volume intérieur de typiquement 1 mm3 et remplie avec un milieu absorbant de type vapeur atomique de métal alcalin (Rubidium ou Césium), et un mélange de gaz tampon. Selon un mode standard, la cellule à gaz est remplie avec du Rubidium naturel et un mélange d'azote et d'argon comme gaz tampon. Dans d'autres formes de réalisations, d'autres types de cellules peuvent être remplies avec des gaz tampons différents. Selon un mode particulier, une cellule miniature cylindrique peut être utilisée. Selon un autre mode particulier, la cellule à gaz peut être intégrée dans le PBSC 101. La cellule 106 peut être remplie avec d'autres types de vapeur métallique alcaline (rubidium-85, rubidium-87, césium-133 par exemple) et d'autres types de gaz tampon (Xe, Ne par exemple).According to a standard mode, the light produced 112 by the laser 102 has a linear polarization and is attenuated by an absorbent neutral filter 104a. A different type of filter can be used in other embodiments. The presence of this filter is not necessary for the invention. A half wave plate 104b may be used to change the angle of the linear polarization of the laser source. In combination with the miniature cube 101, the half wave plate 104b plays the role of a variable attenuator. In other embodiments, the use of the half-wave plate 104b may be omitted and the light intensity ratio between the beams transmitted and reflected by the cube 101 is adjusted by an appropriate orientation of the polarization axis. linear light emitted by the laser relative to the separator cube. A quarter-wave plate 105 is placed at the cube outlet against the face from which the laser beam deflected by the separator 101, or at right angles from the incident beam to the cube. The fast axis of the quarter-wave plate 105 is oriented such that the incident linear polarization 113 is changed to circular polarization 114 in a first direction of rotation. In other embodiments, the quarter-wave plate 105 is oriented such that the incident linear polarization 113 is changed to a circular polarization in a reverse direction of rotation to the first. The circular polarization laser beam 114 passes through the gas cell 106 and reaches the mirror 107. The latter only returns the ray partially and a portion of the ray passes through the mirror 107 to go towards the photodetector 109. According to a standard mode, the gas cell is made of glass-silicon-glass by MEMS (electromechanical microsystem) techniques with an internal volume of typically 1 mm 3 and filled with an absorbent medium of atomic vapor type of alkali metal (Rubidium or cesium), and a buffer gas mixture. In a standard mode, the gas cell is filled with natural Rubidium and a mixture of nitrogen and argon as a buffer gas. In other embodiments, other types of cells may be filled with different buffer gases. In a particular embodiment, a cylindrical miniature cell can be used. According to another particular embodiment, the gas cell can be integrated in the PBSC 101. The cell 106 can be filled with other types of alkaline metal vapor (rubidium-85, rubidium-87, cesium-133 for example) and other types of buffer gas (Xe, Ne for example).

La figure 11 illustre une conception à double passage optique basée sur le deuxième mode de réalisation correspondant à la figure 8, avec une géométrie droite qui est très similaire à la conception à angle droit et double passage représenté sur la figure 10. La différence principale réside dans la position de l'entité « cellule à gaz 206, lame quart d'onde 205, miroir semi-transparent 207 et photodétecteur 209 » et du photodétecteur 208b. Dans le modèle de la figure 11, la cellule à gaz 206 est placée au dessus du PBSC 201 et est donc située vis-à-vis du laser 202. De cette manière, le faisceau de lumière de polarisation P 213 transmis par le PBSC puis modifié en faisceau de polarisation circulaire par la lame quart d'onde 205 interagit avec le milieu atomique. Le faisceau de lumière de polarisation S 217 est réfléchi par le PBSC 201 et le photodétecteur 208b, placé à angle droit, est utilisé pour la mesure de la puissance laser. A part ces différences, le principe de fonctionnement de cette réalisation est le même que pour le modèle précédent.The figure 11 illustrates a double-pass optical design based on the second embodiment corresponding to the figure 8 , with a straight geometry that is very similar to the right-angle and double-pass design depicted on the figure 10 . The main difference lies in the position of the entity "gas cell 206, quarter-wave plate 205, semitransparent mirror 207 and photodetector 209" and photo-detector 208b. In the model of figure 11 , the gas cell 206 is placed above the PBSC 201 and is therefore located vis-à-vis the laser 202. In this way, the polarization light beam P 213 transmitted by the PBSC and modified in circular polarization beam by the quarter-wave plate 205 interacts with the atomic medium. The S-polarization light beam 217 is reflected by the PBSC 201 and the right-angle photodetector 208b is used to measure the laser power. Apart from these differences, the operating principle of this embodiment is the same as for the previous model.

La figure 12 illustre la représentation schématique du boîtier à géométrie droite à double passage du mode de réalisation de l'oscillateur Raman selon le premier mode de réalisation, correspondant à la figure 7. Le codage numérique commence à 201 pour cette conception, en conservant les mêmes dizaines et unités que celles utilisées sur les figures 7 à 9 pour les mêmes éléments. Un cube séparateur 201 est utilisé, dont le pourcentage de séparation est prédéfini de sorte d'avoir une réflexion minoritaire et une transmission majoritaire, d'environ 2% et 98% respectivement (+/- 2%). Le faisceau rétro-réfléchi 216 est alors majoritairement dévié vers le photodétecteur 208a. Dans ce modèle, l'entité cellule à gaz 206 est placée au dessus du cube séparateur 201 et est donc située vis-à-vis du laser 202. Le photodétecteur 208b est placé à angle droit, où le faisceau de lumière 212 émis par le laser 202 est réfléchi 218 par le cube séparateur 201 et est utilisé par exemple pour la mesure de la puissance laser. Le principe de fonctionnement de cette conception reste similaire aux descriptions précédentes.The figure 12 illustrates the schematic representation of the double-pass straight geometry case of the embodiment of the Raman oscillator according to the first embodiment, corresponding to the figure 7 . The digital coding starts at 201 for this design, keeping the same tens and units as those used on Figures 7 to 9 for the same elements. A separator cube 201 is used, whose percentage of separation is predefined so as to have a minority reflection and a majority transmission, of about 2% and 98% respectively (+/- 2%). The retro-reflected beam 216 is then mainly deflected towards the photodetector 208a. In this model, the gas cell entity 206 is placed above the separator cube 201 and is therefore located with respect to the laser 202. The photodetector 208b is placed at right angles, where the light beam 212 emitted by the laser 202 is reflected 218 by the separator cube 201 and is used for example for the measurement of laser power. The operating principle of this design remains similar to the previous descriptions.

La figure 13 illustre un dispositif selon le premier mode de réalisation et géométrie à angle droit. Le pourcentage de séparation du séparateur 101 est prédéfini de manière à avoir une transmission minoritaire et une réflexion majoritaire d'environ 2% et 98% respectivement (+/- 2%). Après son interaction avec les atomes de la vapeur de métal alcalin, le faisceau de lumière incident 114a et le faisceau de lumière généré par la diffusion Raman stimulée (appelé faisceau Raman) 114b sont réfléchis par un miroir 107. Dans un mode de réalisation Raman standard, le miroir 107 est revêtu d'argent, il est incliné (typiquement de 2 à 20 degrés) et/ou excentré par rapport à son axe de symétrie et l'axe défini par le faisceau laser incident et est concave avec une longueur focale choisie pour focaliser les faisceaux de lumière rétro-réfléchis 115 (faisceaux incident et Raman) sur le photodétecteur 108a. Le miroir 107 a une transmission typique de quelques pourcents. Ces pourcents de lumière transmise atteignant la surface du photodétecteur 109 sont utilisés pour mesurer le spectre d'absorption. Dans une réalisation Raman différente, la fenêtre de sortie de la cellule à gaz 106 est concave, revêtue d'argent (ou d'un autre métal, comme par exemple l'or) et joue le rôle de réflecteur. Dans d'autres réalisations, le revêtement de la fenêtre de sortie du miroir peut être fait de couches diélectriques.The figure 13 illustrates a device according to the first embodiment and right angle geometry. The separation percentage of the separator 101 is predefined so as to have a minority transmission and a majority reflection of about 2% and 98% respectively (+/- 2%). After its interaction with the atoms of the alkali metal vapor, the incident light beam 114a and the light beam generated by the scattering Raman stimulated Raman 114b are reflected by a mirror 107. In a standard Raman embodiment, the mirror 107 is coated with silver, is inclined (typically 2 to 20 degrees) and / or eccentric to the mirror. its axis of symmetry and the axis defined by the incident laser beam and is concave with a focal length chosen to focus the retro-reflected light beams 115 (incident beams and Raman) on the photodetector 108a. Mirror 107 has a typical transmission of a few percent. These percent of transmitted light reaching the surface of photodetector 109 is used to measure the absorption spectrum. In a different embodiment Raman, the exit window of the gas cell 106 is concave, coated with silver (or other metal, such as gold) and acts as a reflector. In other embodiments, the coating of the exit window of the mirror can be made of dielectric layers.

Les faisceaux de lumière rétro-réfléchis 115 (incident et Raman) passent à travers et interagissent une seconde fois avec le milieu atomique (double passage). La lame quart d'onde 105 transforme ces faisceaux de lumière polarisés circulairement en faisceaux de lumière de polarisation linéaire 116. Ces faisceaux de lumière sont majoritairement déviés 119 (incident et Raman) et atteignent le premier photodétecteur 108a qui enregistre le battement de fréquences entre le faisceau incident et le faisceau Raman. Dans un mode de réalisation Raman standard, le premier photodétecteur 108a est un photodétecteur de type semi-conducteur à grande vitesse (silicium ou arséniure de gallium) qui est positionné au foyer du miroir concave 107. Dans d'autres modes de réalisation Raman, différents types de photodétecteurs à grande vitesse peuvent être utilisés. Le second photodétecteur 108b enregistre la lumière 118 provenant directement du laser 102 et transmise initialement par le cube séparateur miniature 101. De cette manière, la puissance de sortie de la diode laser 102 peut être mesurée et réglée. En option, le photodétecteur 121 enregistre le faisceau rétro-réfléchi 117 transmis par le séparateur 101. Les diaphragmes 110 et 111 sont utilisés pour éviter qu'une lumière indésirable n'atteigne les photodétecteurs si leurs dimensions sont supérieures à celles du cube séparateur miniature 101.The retro-reflected light beams 115 (incident and Raman) pass through and interact a second time with the atomic medium (double pass). The quarter wave plate 105 transforms these circularly polarized light beams into linear polarization light beams 116. These light beams are mainly deflected 119 (incident and Raman) and reach the first photodetector 108a which records the frequency beat between the beam. incident beam and the Raman beam. In a standard Raman embodiment, the first photodetector 108a is a high-speed semiconductor photodetector (silicon or gallium arsenide) which is positioned at the focus of the concave mirror 107. In other embodiments Raman, different types of high-speed photodetectors may be used. The second photodetector 108b records the light 118 coming directly from the laser 102 and initially transmitted by the miniature divider cube 101. In this manner, the output power of the laser diode 102 can be measured and adjusted. Optionally, the photodetector 121 records the retro-reflected beam 117 transmitted by the separator 101. The diaphragms 110 and 111 are used to prevent undesirable light from reaching the photodetectors if their dimensions are larger than those of the miniature separator cube 101.

La figure 14 illustre le troisième mode de réalisation de l'oscillateur Raman, non basé sur un cube séparateur mais sur une simple géométrie à double passage. La lumière émise par la source laser 102 est polarisée linéairement, convertie en polarisation circulaire par une lame quart d'onde 105 avant passage dans la cellule 106, réflexion sur le miroir 107, deuxième passage dans la cellule, et détection sur un premier photodétecteur 108a. Le miroir 107 est semi-transparent, avec un deuxième photodétecteur 109 placé derrière le miroir. Cette utilisation du miroir semi-transparent 107 permet la détection de lumière ayant interagi avec les atomes de la cellule par le photodétecteur 109. Pour éviter que les faisceaux rétro-réfléchis par le miroir ne perturbent la source laser 102, il est aussi avantageux de placer un polariseur 103 devant la source laser 102 et avec un axe de transmission parallèle à la polarisation du faisceau émis par la source laser 102.The figure 14 illustrates the third embodiment of the Raman oscillator, not based on a splitter cube but on a simple double-pass geometry. The light emitted by the laser source 102 is linearly polarized, converted into circular polarization by a quarter-wave plate 105 before passing through the cell 106, reflection on the mirror 107, second passage in the cell, and detection on a first photodetector 108a . The mirror 107 is semi-transparent, with a second photodetector 109 placed behind the mirror. This use of the semi-transparent mirror 107 allows the detection of light having interacted with the atoms of the cell by the photodetector 109. In order to prevent the beams retro-reflected by the mirror from disturbing the laser source 102, it is also advantageous to place a polarizer 103 in front of the laser source 102 and with a transmission axis parallel to the polarization of the beam emitted by the laser source 102.

En option, on peut également utiliser les éléments suivants :

  • un filtre neutre 104 placé entre la source laser 102 et la lame quart d'onde 105 afin d'ajuster la puissance du faisceau laser
  • un filtre réflectif incliné 104 placé entre la source laser 102 et la lame quart d'onde 105 afin de réfléchir une partie du faisceau laser et d'ajuster sa puissance
  • un troisième photodétecteur 108b placé de manière à enregistrer la lumière réfléchie par le filtre réflectif incliné 104 pour l'asservissement de la puissance optique du laser 102.
Optionally, you can also use the following elements:
  • a neutral filter 104 placed between the laser source 102 and the quarter wave plate 105 in order to adjust the power of the laser beam
  • an inclined reflective filter 104 placed between the laser source 102 and the quarter-wave plate 105 to reflect a portion of the laser beam and adjust its power
  • a third photodetector 108b positioned to record the light reflected by the inclined reflective filter 104 for servocontrolling the optical power of the laser 102.

En remarque, dans ces réalisations décrites en rapport avec les figures 7 à 14, le photodétecteur 108a, 208a a pour fonction de détecter le battement induit par l'effet Raman du gaz présent dans la cellule 106, 206, et est donc un photodétecteur adapté pour la détection des micro-ondes. Le premier photodétecteur 108a a une bande passante très étroite et centrée autour de la fréquence de résonance des atomes afin de maximiser son efficacité de détection du signal. La fréquence de résonance atomique élevée (typiquement >1GHz) a pour conséquence d'avoir un photodétecteur 108a de petit taille. Ce cahier des charges n'est pas compatible avec une détection du signal ayant interagi avec les atomes de la cellule pour ajuster par exemple la température de la cellule, ce qui est mis en oeuvre par le photodétecteur 109, 209 et/ou le photodétecteur 108b, 208b. Pour ces derniers, une fréquence de coupure basse (typiquement < 100kHz), voire un fonctionnement DC sont indiqués. C'est pourquoi il est préférable de disposer d'au moins deux détecteurs, l'un 108a servant à la détection du signal d'horloge, l'autre 109 à l'asservissement de la température de la cellule. Le moyen idéal de réaliser cette deuxième détection d'un signal ayant interagi avec les atomes de la cellule est d'utiliser un miroir semi-transparent 107 pour la réflexion et de placer derrière ce miroir un photodétecteur 109, comme cela a été illustré. Il est également avantageux que le miroir 107 soit de forme concave, comme illustré à la figure 14, la forme concave étant destinée à focaliser le faisceau de lumière réfléchi sur le photodétecteur 108a. En remarque, ces derniers photodétecteurs sont optionnels.In note, in these achievements described in connection with the Figures 7 to 14 , the photodetector 108a, 208a serves to detect the beat induced by the Raman effect of the gas present in the cell 106, 206, and is therefore a photodetector adapted for the detection of microwaves. The first photodetector 108a has a very narrow bandwidth and centered around the resonant frequency of the atoms to maximize its signal detection efficiency. The high atomic resonance frequency (typically> 1GHz) has the consequence of having a small photodetector 108a. This specification is not compatible with a detection of the signal having interacted with the atoms of the cell to adjust for example the temperature of the cell, which is implemented by the photodetector 109, 209 and / or the photodetector 108b , 208b. For the latter, a low cut-off frequency (typically <100 kHz), or even a DC operation are indicated. This is why it is preferable to have at least two detectors, one 108a serving for the detection of the clock signal, the other 109 for controlling the temperature of the cell. The ideal way to achieve this second detection of a signal having interacted with the atoms of the cell is to use a semi-transparent mirror 107 for reflection and to place behind this mirror a photodetector 109, as has been illustrated. It is also advantageous if the mirror 107 is of concave shape, as illustrated in FIG. figure 14 , the concave shape being intended to focus the light beam reflected on the photodetector 108a. As a note, these last photodetectors are optional.

Claims (18)

  1. Method of emission of a time signal within a wristwatch by atomic oscillator, characterized in that it comprises a step of detecting beat frequencies obtained by Raman effect and in that it comprises a method for priming the atomic oscillator comprising:
    - a first phase of searching for the optimal injection current of the laser in open-loop of the atomic oscillator,
    - a second phase of priming the atomic oscillator comprising the placing of the atomic oscillator in closed-loop by the feedback of the microwave signal received at the output of the cell to the injection current of the laser.
  2. Method of emission of a time signal within a wristwatch by atomic oscillator according to the preceding claim, characterized in that the first phase of searching for the optimal injection current of the laser comprises the following steps:
    - Placing of the atomic oscillator in open-loop;
    - Scanning of the frequency of the laser and identification of the point (Vmax) of maximum absorption and of the corresponding injection current (Imax) as well as of the point (Vmin) of minimum absorption of the absorption peak (51) associated with the point (Vmax) of maximum absorption and of the corresponding injection current (Imin);
    - Determination of an initial injection current (ILD) by adding a certain threshold value delta to the injection current (Imin) corresponding to the point (Vmin) of minimum absorption of the absorption peak (51) or by deducting it from the injection current (Imax) corresponding to the point (Vmax) of maximum absorption, so as to lie in the interval [Imin; Imax] defined by the injection currents (Imin, Imax) corresponding to the points (Vmin, Vmax) of minimum and maximum absorption, distant from the bounds (Imin, Imax).
  3. Method of emission of a time signal within a wristwatch by atomic oscillator according to one of Claims 1 or 2, characterized in that the second phase of priming the atomic oscillator comprises the following steps:
    - Placing of the oscillator in closed-loop by the feedback of the microwave signal received at the output of the cell for the control of the injection current of the laser;
    - Adjusting of the injection current of the laser to a predetermined value (ILD);
    - Verification of the obtaining of the phenomenon of resonance of the oscillator at output;
    - In case of non-resonance of the oscillator, slight modification of the value of the injection current (ILD) according to a predefined notch, and repetition of this step until the phenomenon of resonance is obtained.
  4. Method of emission of a time signal within a wristwatch by atomic oscillator according to one of the preceding claims, characterized in that it comprises the following additional steps:
    a. dispatching of a laser beam (11; 112; 212) issuing from a laser source through a cell (3; 106; 206);
    b. detection of a frequency of beats obtained between the beam issuing from the laser source and transmitted within the cell and the beam induced by Raman effect within the atoms of the cell (3; 106; 206).
  5. Method of emission of a time signal within a wristwatch by atomic oscillator according to the preceding claim, characterized in that it comprises a step of feedback of the microwave signal received at the output of the cell to the injection current of the laser and in that it comprises a zero servocontrol of the frequency of the laser.
  6. Method of emission of a time signal within a wristwatch by atomic oscillator according to one of Claims 1 to 5, characterized in that it comprises a step of adjusting the power of the laser.
  7. Method of emission of a time signal within a wristwatch by atomic oscillator according to one of Claims 1 to 6, characterized in that it comprises a temperature-servocontrol of the atomic oscillator.
  8. Method of emission of a time signal within a wristwatch by atomic oscillator according to the preceding claim, characterized in that it comprises an operating of the atomic oscillator at a temperature of less than or equal to 40 °C, or an operating at a temperature of less than or equal to 35 °C.
  9. Method of emission of a time signal within a wristwatch by atomic oscillator according to one of Claims 1 to 8, characterized in that it comprises a measurement of the temperature of the atomic oscillator making it possible to correct the time signal emitted by the atomic oscillator as a function of the temperature.
  10. Wristwatch, characterized in that it comprises an atomic oscillator which comprises a system for detecting beat frequencies obtained by Raman effect, and in that it comprises a processing centre (23) configured to implement a method of emission of a time signal according to one of the preceding claims.
  11. Wristwatch according to the preceding claim, characterized in that its atomic oscillator comprises a laser source (1; 102; 202), a cell (3; 106; 206) comprising Caesium or Rubidium and disposed so as to receive a laser beam (11; 112; 212) issuing from the laser source, and a system for detecting beat frequencies, which comprises a photodetector (4; 108a; 208a) and an amplifier, which is disposed in such a way as to receive the laser beam issuing from the cell (3; 106; 206) so as to detect a beat frequency obtained between the beam issuing from the laser source and transmitted within the cell and the beam induced by Raman effect within the atoms of the cell (3; 106; 206).
  12. Wristwatch according to one of Claims 10 or 11, characterized in that it comprises an additional oscillator of lesser precision and in that the atomic oscillator operates in an intermittent manner, so as to adjust this additional oscillator.
  13. Wristwatch according to one of Claims 10 to 12, characterized in that its atomic oscillator comprises a zero servocontrol of the frequency of its laser.
  14. Wristwatch according to one of Claims 10 to 13, characterized in that it comprises a source of current (24) for the laser of its atomic oscillator, a diplexer (34), and a feedback link from the frequency detection system to the diplexer (34) which makes it possible to combine the signal detected by the detection system with the source of current (24) of the injection current of the laser.
  15. Wristwatch according to the preceding claim, characterized in that the system for detecting frequencies is a system for detecting a signal (iPD) corresponding to the beats induced by the Raman effect, with narrow spectral content centred around a central frequency (ωC), comprising at least one first inductive element (L1) which is connected to the photodetector (4; 108a; 208a) and a parasitic capacitance (CIN) parallel with the photodetector, forming a resonant circuit for selecting the signal to be detected, the said resonant circuit having a resonant frequency which corresponds to the central frequency (ωC).
  16. Wristwatch according to one of Claims 10 to 15, characterized in that it comprises at least one mirror (107; 207) for reflecting a laser beam and allowing it at least one second pass through a cell (3; 106; 206) before reaching the system for detecting frequencies.
  17. Wristwatch according to one of Claims 10 to 16, characterized in that it comprises a shielded enclosure (38) in which is disposed the cell (3; 106; 206) comprising Caesium or Rubidium so as to allow operation at zero magnetic field at the level of the said cell.
  18. Wristwatch according to one of Claims 10 to 17, characterized in that it comprises a heating device.
EP14157063.0A 2011-03-09 2011-03-09 Wristwatch with atomic oscillator Active EP2738628B1 (en)

Priority Applications (1)

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EP14157063.0A EP2738628B1 (en) 2011-03-09 2011-03-09 Wristwatch with atomic oscillator
EP11405232.7A EP2498151B1 (en) 2011-03-09 2011-03-09 Wristwatch with atomic oscillator

Related Parent Applications (2)

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EP11405232.7A Division-Into EP2498151B1 (en) 2011-03-09 2011-03-09 Wristwatch with atomic oscillator
EP11405232.7A Division EP2498151B1 (en) 2011-03-09 2011-03-09 Wristwatch with atomic oscillator

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EP2738628A2 EP2738628A2 (en) 2014-06-04
EP2738628A3 EP2738628A3 (en) 2014-08-27
EP2738628B1 true EP2738628B1 (en) 2016-01-06

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CN105372981B (en) * 2015-12-04 2017-10-13 兰州空间技术物理研究所 A kind of caesium CPT atomic clock physical systems
CN110928174B (en) * 2019-12-17 2022-01-11 中国科学院国家授时中心 Atomic clock frequency discrimination signal detection system
JP2023148751A (en) * 2022-03-30 2023-10-13 日本電気株式会社 atomic oscillator
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Publication number Publication date
EP2498151A1 (en) 2012-09-12
EP2498151B1 (en) 2014-09-24
JP2012189588A (en) 2012-10-04
JP6054613B2 (en) 2016-12-27
EP2738628A2 (en) 2014-06-04
US8922283B2 (en) 2014-12-30
CN102736510B (en) 2015-09-30
EP2738628A3 (en) 2014-08-27
CN102736510A (en) 2012-10-17
US20120229222A1 (en) 2012-09-13

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