EP2473886B1 - Device for atomic clock - Google Patents

Device for atomic clock Download PDF

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Publication number
EP2473886B1
EP2473886B1 EP10760897.8A EP10760897A EP2473886B1 EP 2473886 B1 EP2473886 B1 EP 2473886B1 EP 10760897 A EP10760897 A EP 10760897A EP 2473886 B1 EP2473886 B1 EP 2473886B1
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EP
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Prior art keywords
laser
photodetector
mirror
laser beam
reflected
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EP10760897.8A
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German (de)
French (fr)
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EP2473886A1 (en
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Steve Lecomte
Jacques Haesler
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Centre Suisse dElectronique et Microtechnique SA CSEM
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Centre Suisse dElectronique et Microtechnique SA CSEM
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Priority claimed from CH01122/10A external-priority patent/CH703410A1/en
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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
    • 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
    • G04F5/145—Apparatus for producing preselected time intervals for use as timing standards using atomic clocks using Coherent Population Trapping

Definitions

  • the present invention relates to the field of atomic clocks.
  • Miniature atomic clocks volume of one cm 3 or less
  • CPT coherent entrapment of population
  • Raman Raman
  • an atomic clock architecture based on a gas absorption cell.
  • These two physical principles do not require a microwave cavity to interrogate the reference atoms (typically Rubidium or Cesium) and thus eliminate the volume constraint associated with conventional cell-type atomic clocks.
  • the physical part of the clock which consists of the light source, the optical elements, the gas cell, the photodetector and all functions such as heating and magnetic field generation, will be the subject of considerations that follow.
  • VCSEL vertical cavity surface-emitting laser
  • microfabrication techniques for gas cells and vacuum encapsulation have made it possible to Massively reduce the volume and power consumption of these atomic clocks.
  • VCSEL lasers offer the possibility of combining the optical pumping function and the microwave interrogation of the reference atoms. This type of laser offers the following advantages: modulation of the injection current possible up to several gigahertz, low consumption, wavelength compatible with standard reference atoms (Rubidium or cesium), excellent service life, high temperature operation, low cost and ideal optical power.
  • Silicon microstructuring technologies coupled to the bonding / welding processes of a glass substrate (typically pyrex or quartz) on a silicon substrate make it possible to produce gas cells that are much smaller in size than is possible. achieve with the traditional technique of blowing and forming of glass tube.
  • the reduction of the dimensions of the gas cell is also accompanied by a reduction in the consumption required to heat the gas cell.
  • a detector can be placed before the passage of light in the cell and another after the double passage in the cell, but no photodetector can be positioned after a single passage of light in the cell.
  • This additional detector makes it possible to obtain a signal additional to that of the detector placed after the double pass.
  • This additional signal is useful for measuring and controlling clock parameters such as the temperature of the cell or the frequency of the laser source, for example.
  • the configurations described above are not applicable in a configuration of a Raman oscillator because the servocontrolling of the frequency of the laser source is performed by the same detector ensuring the detection of the return laser beam of the cell. .
  • the present invention therefore aims to provide an atomic clock device allowing a double pass through the cell and which allows easy servocontrolling of the laser frequency, for both a CPT oscillator and a Raman oscillator.
  • an atomic clock device comprising a laser source generating a laser beam, a quarter wave plate modifying the linear polarization of the laser beam in a circular polarization and vice versa, a gas cell placed on the polarization laser beam.
  • mirror a mirror returning the laser beam to the gas cell, a first photodetector, and means for preventing the reflected beam from reaching the laser source, characterized in that it comprises a second photodetector, placed behind the mirror, said mirror being semi-transparent and passing a portion of the laser beam, said second photodetector serving as the optical frequency servocontrol of the laser and / or the control of the temperature of the cell.
  • FIG. 1a illustrates the block diagram of the CPT atomic clock comprising a laser diode 102, a ⁇ / 4 (or quarter wave plate) blade 105, an (atomic) gas cell 106, an optional B magnetic field, a first photodetector 108, a control electronics (A) and a microwave oscillator (C).
  • the laser beam having passed through the gas cell 106 is picked up by the first photodetector 108 and is used by the control electronics to stabilize the laser frequency (B) and the frequency of the microwave oscillator (C).
  • a microwave divider ( ⁇ ) is used to generate the reference frequency requested by the end user of the device.
  • the figure 1b illustrates the block diagram of a closed loop Raman oscillator including a laser diode 102, a ⁇ / 4 blade (or quarter wave plate) 105, a (atomic) gas cell 106, an optional B magnetic field, a first photodetector 108, a microwave frequency divider ( ⁇ ), and a radio frequency amplifier (RF) (D).
  • the laser beam emitted by the laser diode 102 undergoes in the gas cell 106 a light-atom interaction which generates a complementary beam called Raman beam.
  • the two beams of light are picked up by the first photodetector 108 and the frequency beat of these two beams is amplified (D) and used as feedback on the laser to close the microwave loop of the Raman oscillator.
  • the Figures 2, 3 and 4 illustrate 3 different embodiments for simultaneously performing the double passage in the gas cell and the frequency control and the protection of the laser source to the reflections.
  • the common point of these various embodiments is the presence of a semi-transparent mirror 107 which passes a portion of the laser beam passed through the gas cell to reach a photodetector 109 serving for the optical frequency servocontrol of the laser and / or to control the temperature of the cell.
  • the figure 2 illustrates the first embodiment of the invention.
  • the laser source 102 produces a linearly polarized laser beam which is directed towards the polarizer 103, the transmission axis of which is oriented so as to let the laser beam pass, then to the separator 101 whose percentage of separation is predefined. Part of the beam is thus transmitted to the optional photodetector 108b.
  • the separator reflects the other part of the beam to 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. Part 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 3 illustrates the second embodiment of the invention. It differs from the first mode 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 3 ) 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 4 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 it is advantageous for the mirror 107 to be of concave shape, the concave shape being intended to focus 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 be realized on the modes of the Figures 2 and 3 providing the benefits described above.
  • 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 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 ratio of light intensity between the beams transmitted and reflected by the cube 101 is adjusted by an orientation. appropriate linear polarization axis of the 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 so that the incident linear polarization 113 is changed to a 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 Rubidium-87 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 may be integrated in the PBSC 101.
  • the cell 106 may be filled with other types of alkaline metal vapor (rubidium-85, natural rubidium, cesium-133 for example) and other types of buffer gas (Xe, Ne for example).
  • the figure 6 illustrates the design of a device particularly suitable for the CPT clock according to the first embodiment.
  • the teaching of this embodiment can be adapted to the realization of other atomic clocks than that based on the scheme of the Raman oscillator ( figure 1b ).
  • the separation percentage of the separator 101 is predefined so as to have a majority transmission and a minority reflection of about 90% and 10% (+/- 10%) respectively.
  • the circularly polarized light beam 114 is mainly reflected by a mirror 107.
  • the exit window of the gas cell 106 is covered with metal (silver or gold, for example) to play the role of reflector.
  • the coating of the exit window of the gas cell 106 may be a dielectric mirror.
  • the transmission of the reflector 107 is chosen so that a small portion of the light is transmitted to the photodetector 109.
  • the retro-reflected light 115 passes through and interacts a second time with the atomic medium (double pass).
  • the beam passes through the quarter-wave plate 105 which transforms its circular polarization into a linear polarization 116, perpendicular to the transmission axis of the polarizer 103, and is mainly transmitted by the miniature separator cube 101.
  • transmitted light 117 reaches the photodetector 108a which records the absorption spectrum and more specifically the decrease of absorption due to the process of coherent population trapping (CPT).
  • the photodetector 108a is a silicon type photodetector. In other CPT embodiments, different types of photodetectors may be used.
  • the minority portion 119 of the beam 116 deflected by the separator 101 is attenuated by the polarizer 103 and thus does not disturb the laser.
  • the second photodetector 108b records the light beam 118 initially transmitted by the miniature divider cube 101. In this manner, the output power of the laser diode 102 can be measured and adjusted by a dedicated servocontrol loop.
  • the diaphragms 110 and 111 are used to prevent undesirable light from reaching the photodetectors if the size of the laser beam is larger than the dimensions of the faces of the miniature separator cube 101.
  • the light recorded by the photodetector 109 located after the mirror 107 can be used for different types of servocontrol such as laser frequency or cell temperature.
  • the figure 7 illustrates a double-pass optical design based on the second embodiment, with a straight geometry 200 (the digital coding starts at 200 for the design 200) which is very similar to the right-angle and double-pass design 100 (see Figure 5 ).
  • the main difference compared with the design 100 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 then modified 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 for laser power measurement.
  • the operating principle of the design 200 is the same as for the model 100.
  • the separation percentage of the separator cube is predefined inversely to that previously described (right-angle housing of the figure 6 ), namely a minority transmission and a majority reflection of approximately 10% and 90% respectively (+/- 10%).
  • the two-pass design and straight geometry thus obtained 200 (the digital coding starts at 200 for the design 200) is very similar to the right-angle and double-pass design 100 (see Figure 6 ).
  • the role of the separator 201 is thus reversed so that the minority portion of the beam from the laser diode 202 is transmitted rather than deflected. For its part, the retro-reflected beam 216 is then mainly deflected towards the photodetector 208a.
  • the main difference in the arrangement of the different elements compared to the design 100 lies in the position of the entity "gas cell 206, quarter wave plate 205, semi-transparent mirror 207 and photodetector 209".
  • the gas-cell entity is placed above the separator cube 201 and is thus located with respect to the laser 202.
  • the photodetector 208b is placed at right angles, where the beam of light emitted by the laser 202 is reflected by the splitter cube 201 and is used for the measurement of the laser power.
  • the operating principle of the design 200 is the same as for the model 100.
  • the figure 8b illustrates the schematic representation of the double-pass right geometry casing 200 of the embodiment of the Raman oscillator according to the first embodiment. All numerals correspond to model 100 of the Raman embodiment and begin with “ 2 " instead of "1".
  • the separation percentage of the separator cube is predefined opposite to that described above (CPT atomic clock of the figure 8a ), namely a minority reflection and a majority transmission of about 2% and 98% respectively (+/- 2%).
  • the figure 9 illustrates a device particularly suitable for a Raman oscillator 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 stimulated Raman scattered light beam (called the Raman beam) 114b are reflected by a mirror 107.
  • the mirror 107 is coated with silver, it is inclined (typically 2 to 20 degrees) and / or eccentric with respect to its axis of symmetry and the axis defined by the incident laser beam and is concave with a chosen focal length 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 the photodetector 109 is used to measure the absorption spectrum and to stabilize the optical frequency of the laser.
  • 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 blade 105 converts these circularly polarized beams of light 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 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 by a dedicated servocontrol loop.
  • the photodetector 121 records the retro-reflected beam 117 transmitted by the separator 101.
  • the diaphragms 110 and 111 are used to prevent unwanted light from reaching the photodetectors if their dimensions are larger than those of the miniature separator cube 101 .
  • the Figures 10 and 11 illustrate the third embodiment for the CPT atomic clock and the Raman oscillator, respectively, and which is not based on a separator cube, but on a simple double-pass geometry.
  • the light emitted by the laser source is linearly polarized, converted into circular polarization by a quarter-wave plate 105 before passing through the cell, reflection on the mirror, second passage in the cell, and detection on a photodetector 108a.
  • the mirror 107 is semi-transparent, with a second photodetector 109 placed behind the mirror.
  • the semi-transparent mirror 107 which allows the detection of light having interacted with the atoms of the cell by the photodetector 109.
  • This detection by a second photodetector is particularly favorable in the case of use of the device based on a Raman oscillator.
  • the photodetector 108a In the case of a Raman oscillator, the photodetector 108a has a very narrow bandwidth and centered around the resonance 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 that interacted with the atoms of the cell to adjust the optical frequency of the laser on the resonance peak, or to adjust the temperature of the cell.
  • a low cut-off frequency typically ⁇ 100 kHz
  • DC operation a low cut-off frequency
  • two detectors one for detecting the clock signal, the other for servocontrolling the optical frequency of the laser and / or controlling the temperature of the cell.
  • the ideal way to realize this second detection of a signal having interacted with the atoms of the cell is to use a semi-transparent mirror for the reflection and to place behind this mirror a photodetector 109.
  • the mirror 107 is also advantageous for the mirror 107 to be of concave shape as in FIG. figure 11 , the concave shape being intended to focus the light beam reflected on the photodetector 108a.
  • This arrangement is also interesting for a clock based on a CPT principle, because the photodetector located behind the semi-transparent mirror can be used for stabilizing the temperature of the cell containing the atoms or the frequency of the laser source.
  • 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.

Description

IntroductionIntroduction

La présente invention concerne le domaine des horloges atomiques.The present invention relates to the field of atomic clocks.

Etat de l'ArtState of the art

Les horloges atomiques miniatures (volume d'un cm3 ou inférieur), à basse consommation électrique (inférieure au Watt) et qui permettent des applications portables sont des dispositifs rendus possibles par la combinaison des principes physiques CPT (piégeage cohérent de population) ou Raman avec une architecture d'horloge atomique basée sur une cellule d'absorption à gaz. Ces deux principes physiques ne nécessitent pas de cavité microonde pour interroger les atomes de référence (typiquement Rubidium ou Césium) et éliminent ainsi la contrainte de volume associée aux horloges atomiques traditionnelles de type cellule. La partie physique de l'horloge, qui est constituée de la source lumineuse, des éléments optiques, de la cellule à gaz, du photodétecteur et de toutes les fonctions telles que chauffage et génération de champ magnétique, va faire l'objet des considérations qui suivent. L'implémentation de technologies telles que les lasers de type semi-conducteur à émission de surface et cavité verticale (vertical cavity surface-emitting laser, VCSEL), les techniques de microfabrication pour les cellules à gaz et d'encapsulation sous vide ont permis de réduire massivement le volume et la consommation électrique de ces horloges atomiques. Les lasers VCSEL offrent la possibilité de combiner la fonction de pompage optique et l'interrogation microonde des atomes de référence. Ce type de laser offre les avantages suivants : modulation du courant d'injection possible jusqu'à plusieurs gigahertz, basse consommation, longueur d'onde compatible avec les atomes standards de référence (Rubidium ou Césium), excellente durée de vie, fonctionnement à haute température, bas coût et puissance optique idéalement adaptée. Les technologies de microstructuration du silicium couplées aux procédés de collage/soudage d'un substrat en verre (typiquement pyrex ou quartz) sur un substrat en silicium permettent de réaliser des cellules à gaz de dimensions beaucoup plus petites que ce qu'il est possible de réaliser avec la technique traditionnelle de soufflage et formage de tube en verre. La réduction des dimensions de la cellule à gaz est également accompagnée par une diminution de la consommation nécessaire pour chauffer la cellule à gaz.Miniature atomic clocks (volume of one cm 3 or less), with low power consumption (below Watt) and which allow portable applications are devices made possible by the combination of the physical principles CPT (coherent entrapment of population) or Raman with an atomic clock architecture based on a gas absorption cell. These two physical principles do not require a microwave cavity to interrogate the reference atoms (typically Rubidium or Cesium) and thus eliminate the volume constraint associated with conventional cell-type atomic clocks. The physical part of the clock, which consists of the light source, the optical elements, the gas cell, the photodetector and all functions such as heating and magnetic field generation, will be the subject of considerations that follow. The implementation of technologies such as vertical cavity surface-emitting laser (VCSEL), microfabrication techniques for gas cells and vacuum encapsulation have made it possible to Massively reduce the volume and power consumption of these atomic clocks. VCSEL lasers offer the possibility of combining the optical pumping function and the microwave interrogation of the reference atoms. This type of laser offers the following advantages: modulation of the injection current possible up to several gigahertz, low consumption, wavelength compatible with standard reference atoms (Rubidium or cesium), excellent service life, high temperature operation, low cost and ideal optical power. Silicon microstructuring technologies coupled to the bonding / welding processes of a glass substrate (typically pyrex or quartz) on a silicon substrate make it possible to produce gas cells that are much smaller in size than is possible. achieve with the traditional technique of blowing and forming of glass tube. The reduction of the dimensions of the gas cell is also accompanied by a reduction in the consumption required to heat the gas cell.

Différents arrangements de la partie physique d'une telle horloge ont été réalisés. La majorité des arrangements sont basés sur un passage unique du faisceau laser au travers de la cellule (voir S. Knappe, MEMS atomic clocks, Book chapter in Comprehensive Microsystems, vol. 3, p. 571 (2008), Ed. Elsevier ), d'autres tirent profit de cellules à gaz comportant des miroirs à l'intérieur de la cellule ou encore permettant un double passage du faisceau laser au travers de la cellule (voir documents US7064835 et EP0550240 ). Les arrangements avec double passage de la lumière au travers de la cellule ont l'avantage de doubler la longueur optique effective de la cellule et donc d'améliorer les performances de l'horloge atomique (en termes de consommation électrique et/ou de stabilité de fréquence). Néanmoins, ces arrangements double passage n'ont pas été implémentés pour des raisons d'instabilité du dispositif et en particulier à cause de perturbations du laser provoquées par la lumière rétro-réfléchie par les miroirs sur le laser.Different arrangements of the physical part of such a clock have been made. The majority of the arrangements are based on a single pass of the laser beam through the cell (see S. Knappe, MEMS atomic clocks, Book chapter in Comprehensive Microsystems, vol. 3, p. 571 (2008), Ed. Elsevier ), others take advantage of gas cells with mirrors inside the cell or allowing a double pass of the laser beam through the cell (see documents US7064835 and EP0550240 ). Arrangements with double passage of light through the cell have the advantage of doubling the effective optical length of the cell and thus improving the performance of the atomic clock (in terms of power consumption and / or stability of the cell. frequency). Nevertheless, these double-pass arrangements have not been implemented for reasons of instability of the device and in particular because of laser disturbances caused by the light reflected back by the mirrors on the laser.

Les documents US7064835 (Symmetricom), US5340986 (Wong ) et US2009/128820 (Seiko, fig. 6) décrivent l'utilisation d'un élément séparateur afin de diriger le faisceau réfléchi vers le photodétecteur. La lumière émise par le laser est polarisée linéairement, convertie en polarisation circulaire par une lame quart-d'onde avant passage dans la cellule, réflexion sur le miroir, deuxième passage dans la cellule, et détection sur un photodétecteur.The documents US7064835 (Symmetricom) US5340986 (Wong ) and US2009 / 128820 (Seiko, Fig. 6 ) describe the use of a separator element to direct the reflected beam to the photodetector. The light emitted by the laser is polarized linearly, converted into circular polarization by a quarter-wave plate before passage in the cell, reflection on the mirror, second passage in the cell, and detection on a photodetector.

Les configurations décrites ci-dessus présentent des inconvénients pour réaliser un oscillateur CPT. En effet, un détecteur peut être placé avant le passage de la lumière dans la cellule et un autre après le double passage dans la cellule, mais aucun photodétecteur ne peut être positionné après un seul passage de la lumière dans la cellule. Ce détecteur additionnel permet d'obtenir un signal supplémentaire à celui du détecteur placé après le double passage. Ce signal supplémentaire est utile pour mesurer et contrôler des paramètres de l'horloge tels que la température de la cellule ou la fréquence de la source laser par exemple. De plus, les configurations décrites ci-dessus sont peu applicables dans une configuration d'un oscillateur Raman du fait que l'asservissement de la fréquence de la source laser est effectué par le même détecteur assurant la détection du faisceau laser de retour de la cellule.The configurations described above have disadvantages for producing a CPT oscillator. Indeed, a detector can be placed before the passage of light in the cell and another after the double passage in the cell, but no photodetector can be positioned after a single passage of light in the cell. This additional detector makes it possible to obtain a signal additional to that of the detector placed after the double pass. This additional signal is useful for measuring and controlling clock parameters such as the temperature of the cell or the frequency of the laser source, for example. In addition, the configurations described above are not applicable in a configuration of a Raman oscillator because the servocontrolling of the frequency of the laser source is performed by the same detector ensuring the detection of the return laser beam of the cell. .

Brève description de l'inventionBrief description of the invention

La présente invention vise donc à proposer un dispositif pour horloge atomique permettant un double passage dans la cellule et qui permet un asservissement aisé de la fréquence laser, tant pour un oscillateur CPT que pour un oscillateur Raman.The present invention therefore aims to provide an atomic clock device allowing a double pass through the cell and which allows easy servocontrolling of the laser frequency, for both a CPT oscillator and a Raman oscillator.

Ce but est atteint par un dispositif pour horloge atomique comprenant une source laser générant un faisceau laser, une lame quart d'onde modifiant la polarisation linéaire du faisceau laser en une polarisation circulaire et inversement, une cellule à gaz placée sur le faisceau laser de polarisation circulaire, un miroir renvoyant le faisceau laser vers la cellule à gaz, un premier photodétecteur, ainsi que des moyens pour empêcher le faisceau réfléchi d'atteindre la source laser, caractérisé en ce qu'il comprend un second photodétecteur, placé derrière le miroir, ledit miroir étant semi-transparent et laissant passer une partie du faisceau laser, ledit second photodétecteur servant à l'asservissement en fréquence optique du laser et/ou à l'asservissement de la température de la cellule.This object is achieved by an atomic clock device comprising a laser source generating a laser beam, a quarter wave plate modifying the linear polarization of the laser beam in a circular polarization and vice versa, a gas cell placed on the polarization laser beam. mirror, a mirror returning the laser beam to the gas cell, a first photodetector, and means for preventing the reflected beam from reaching the laser source, characterized in that it comprises a second photodetector, placed behind the mirror, said mirror being semi-transparent and passing a portion of the laser beam, said second photodetector serving as the optical frequency servocontrol of the laser and / or the control of the temperature of the cell.

Brève description des figuresBrief description of the figures

L'invention sera mieux comprise grâce à la description détaillée qui va suivre en se référant aux dessins annexés dans lesquels :

  • Figure 1(a): Schéma de principe de l'oscillateur CPT
  • Figure 1(b): Schéma de principe de l'oscillateur Raman
  • Figure 2: Premier mode de réalisation à double passage avec filtre polarisant
  • Figure 3: Second mode de réalisation à double passage avec cube polarisant
  • Figure 4: Troisième mode de réalisation à double passage avec miroir oblique
  • Figure 5: Présentation schématique éclatée du dispositif de l'invention basé sur le second mode de réalisation à double passage et une géométrie à angle droit
  • Figure 6: Présentation schématique selon le premier mode de réalisation à double passage de la conception du dispositif de l'invention basé sur le concept de l'horloge atomique CPT avec géométrie à angle droit
  • Figure 7: Présentation schématique éclaté du dispositif de l'invention basé sur le second mode de réalisation à double passage et avec une géométrie droite
  • Figure 8a et 8b: Présentation schématique selon le premier mode de réalisation à double passage de la conception du dispositif de l'invention à géométrie droite pour l'horloge atomique CPT (8a) et l'oscillateur Raman (8b)
  • Figure 9: Présentation schématique selon le premier mode de réalisation à double passage de la conception du dispositif de l'invention basé sur le concept de l'oscillateur Raman avec géométrie à angle droit
  • Figure 10: Présentation schématique selon le troisième mode de réalisation à double passage de la conception du dispositif de l'invention basé sur le concept de l'horloge atomique CPT sans cube séparateur placé entre la source laser et la cellule
  • Figures 11: Présentation schématique selon le troisième mode de réalisation à double passage de la conception du dispositif de l'invention basé sur le concept de l'oscillateur Raman sans cube séparateur placé entre la source laser et la cellule
The invention will be better understood from the following detailed description with reference to the accompanying drawings in which:
  • Figure 1 (a) : Schematic diagram of the CPT oscillator
  • Figure 1 (b) : Schematic diagram of the Raman oscillator
  • Figure 2 : First double-pass embodiment with polarizing filter
  • Figure 3 : Second double-pass embodiment with polarizing cube
  • Figure 4 : Third double-pass embodiment with oblique mirror
  • Figure 5 : Exploded schematic presentation of the device of the invention based on the second embodiment double pass and a right angle geometry
  • Figure 6 : Schematic presentation according to the first embodiment of double pass of the design of the device of the invention based on the concept of the atomic clock CPT with geometry at right angles
  • Figure 7 : Schematic exploded presentation of the device of the invention based on the second embodiment with double passage and with a straight geometry
  • Figure 8a and 8b : Schematic presentation according to the first double-pass embodiment of the design of the device of the invention with a right geometry for the atomic clock CPT (8a) and the oscillator Raman (8b)
  • Figure 9 : Schematic presentation according to the first double-pass embodiment of the device design of the invention based on the concept of the Raman oscillator with right-angle geometry
  • Figure 10 : Schematic presentation according to the third embodiment of double pass of the design of the device of the invention based on the concept of the atomic clock CPT without separator cube placed between the laser source and the cell
  • Figures 11 : Schematic presentation according to the third embodiment of double pass of the design of the device of the invention based on the concept of the Raman oscillator without separator cube placed between the laser source and the cell

Description détailléedetailed description

La figure la illustre le schéma de principe de l'horloge atomique CPT comprenant une diode laser 102, une lame □/4 (ou lame quart d'onde) 105, une cellule à gaz (atomique) 106, un champ magnétique B optionnel, un premier photodétecteur 108, une électronique de contrôle (A) et un oscillateur micro-onde (C). Le faisceau laser ayant traversé la cellule à gaz 106 est capté par le premier photodétecteur 108 et est utilisé par l'électronique de contrôle pour stabiliser la fréquence du laser (B) et la fréquence de l'oscillateur microonde (C). Un diviseur microonde (÷) permet de générer la fréquence de référence demandée par l'utilisateur final du dispositif.FIG. 1a illustrates the block diagram of the CPT atomic clock comprising a laser diode 102, a □ / 4 (or quarter wave plate) blade 105, an (atomic) gas cell 106, an optional B magnetic field, a first photodetector 108, a control electronics (A) and a microwave oscillator (C). The laser beam having passed through the gas cell 106 is picked up by the first photodetector 108 and is used by the control electronics to stabilize the laser frequency (B) and the frequency of the microwave oscillator (C). A microwave divider (÷) is used to generate the reference frequency requested by the end user of the device.

La figure 1b illustre le schéma de principe d'un oscillateur Raman en boucle fermée comprenant une diode laser 102, une lame □/4 (ou lame quart d'onde) 105, une cellule à gaz (atomique) 106, un champ magnétique B optionnel, un premier photodétecteur 108, un diviseur de fréquence microonde (÷), et un amplificateur radiofréquence (RF) (D). Le faisceau laser émis par la diode laser 102 subit dans la cellule à gaz 106 une interaction lumière-atome qui génère un faisceau complémentaire appelé faisceau Raman. Les deux faisceaux de lumière sont captés par le premier photodétecteur 108 et le battement de fréquences de ces deux faisceaux est amplifié (D) et utilisé comme rétroaction sur le laser pour fermer la boucle microonde de l'oscillateur Raman.The figure 1b illustrates the block diagram of a closed loop Raman oscillator including a laser diode 102, a □ / 4 blade (or quarter wave plate) 105, a (atomic) gas cell 106, an optional B magnetic field, a first photodetector 108, a microwave frequency divider (÷), and a radio frequency amplifier (RF) (D). The laser beam emitted by the laser diode 102 undergoes in the gas cell 106 a light-atom interaction which generates a complementary beam called Raman beam. The two beams of light are picked up by the first photodetector 108 and the frequency beat of these two beams is amplified (D) and used as feedback on the laser to close the microwave loop of the Raman oscillator.

Les figures 2, 3 et 4 illustrent 3 modes de réalisation différents permettant de réaliser simultanément le double passage dans la cellule à gaz et l'asservissement de fréquence ainsi que la protection de la source laser envers les réflexions. Le 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 afin d'atteindre un photodétecteur 109 servant à l'asservissement en fréquence optique du laser et/ou à l'asservissement de la température de la cellule.The Figures 2, 3 and 4 illustrate 3 different embodiments for simultaneously performing the double passage in the gas cell and the frequency control and the protection of the laser source to the reflections. The common point of these various embodiments is the presence of a semi-transparent mirror 107 which passes a portion of the laser beam passed through the gas cell to reach a photodetector 109 serving for the optical frequency servocontrol of the laser and / or to control the temperature of the cell.

Ces trois modes de réalisations différent 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 different embodiments in the means used to direct the beam to 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 2 illustre le premier mode de réalisation de l'invention. La source laser 102 produit un faisceau laser polarisé linéairement et qui est dirigée vers le polariseur 103, dont l'axe de transmission est orienté de manière à laisser passer le faisceau laser, puis vers le séparateur 101 dont le pourcentage de séparation est prédéfini. Une partie du faisceau est ainsi transmise vers le photodétecteur optionnel 108b. Le séparateur réfléchi 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 réfléchi 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 rétros-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 2 illustrates the first embodiment of the invention. The laser source 102 produces a linearly polarized laser beam which is directed towards the polarizer 103, the transmission axis of which is oriented so as to let the laser beam pass, then to the separator 101 whose percentage of separation is predefined. Part of the beam is thus transmitted to the optional photodetector 108b. The separator reflects the other part of the beam to 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. Part 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 3 illustre le second mode de réalisation de l'invention. Il diffère du premier mode 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 3) et « S » pour la partie perpendiculaire à l'axe de polarisation du séparateur (partie déviée à 90°). Dans la figure 3, 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 3 illustrates the second embodiment of the invention. It differs from the first mode 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 3 ) and "S" for the part perpendicular to the polarization axis of the separator (part deflected at 90 °). In the figure 3 , 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 4 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. Dans le cas de l'oscillateur Raman, il est avantageux que le miroir 107 soit de forme concave, la forme concave étant destinée à 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 être réalisée sur les modes des figures 2 et 3 apportant les avantages décrits ci-dessus.The figure 4 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. In the case of the Raman oscillator, it is advantageous for the mirror 107 to be of concave shape, the concave shape being intended to focus 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 be realized on the modes of the Figures 2 and 3 providing the benefits described above.

Un exemple de réalisation plus complet correspondant au deuxième mode de réalisation est illustré à la figure 5. 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'horloge atomique.A more complete exemplary embodiment corresponding to the second embodiment is illustrated in FIG. figure 5 . The separator 101 is in the form of a polarizing beam splitter cube (PBSC). This cube allows to implement a double crossing of the gas cell 106 which multiplies by two 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 clock.

Sur la figure 5, 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 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 5 , 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 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 réfléchi 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-87 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 naturel, 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 ratio of light intensity between the beams transmitted and reflected by the cube 101 is adjusted by an orientation. appropriate linear polarization axis of the 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 so that the incident linear polarization 113 is changed to a 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 Rubidium-87 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 may be integrated in the PBSC 101. The cell 106 may be filled with other types of alkaline metal vapor (rubidium-85, natural rubidium, cesium-133 for example) and other types of buffer gas (Xe, Ne for example).

La figure 6 illustre la conception d'un dispositif particulièrement adapté pour l'horloge CPT selon le premier mode de réalisation. L'enseignement de ce mode de réalisation peut être adapté à la réalisation d'autres horloges atomiques que celle basée sur le schéma de l'oscillateur Raman (figure 1b). Selon un mode de réalisation standard (géométrie à angle droit), le pourcentage de séparation du séparateur 101 est prédéfini de manière à avoir une transmission majoritaire et une réflexion minoritaire d'environ 90% et 10% (+/- 10%) respectivement.The figure 6 illustrates the design of a device particularly suitable for the CPT clock according to the first embodiment. The teaching of this embodiment can be adapted to the realization of other atomic clocks than that based on the scheme of the Raman oscillator ( figure 1b ). According to a standard embodiment (right angle geometry), the separation percentage of the separator 101 is predefined so as to have a majority transmission and a minority reflection of about 90% and 10% (+/- 10%) respectively.

Après son interaction avec les atomes de la vapeur de métal alcalin, le faisceau de lumière 114 polarisé circulairement est majoritairement réfléchi par un miroir 107. Dans un mode de réalisation CPT standard, la fenêtre de sortie de la cellule à gaz 106 est recouverte de métal (argent ou or, par exemple) pour jouer le rôle de réflecteur. Dans un autre mode de réalisation, le revêtement de la fenêtre de sortie de la cellule à gaz 106 peut être un miroir diélectrique. La transmission du réflecteur 107 est choisie de manière à ce qu'une faible partie de la lumière soit transmise vers le photodétecteur 109. La lumière rétro-réfléchie 115 passe à travers et interagit une seconde fois avec le milieu atomique (double passage). En sortie de cellule, le faisceau traverse la lame quart d'onde 105 qui transforme sa polarisation circulaire en polarisation linéaire 116, perpendiculaire à l'axe de transmission du polariseur 103, et est majoritairement transmis par le cube séparateur miniature 101. Ce faisceau de lumière transmis 117 atteint le photodétecteur 108a qui enregistre le spectre d'absorption et plus spécifiquement la diminution d'absorption due au processus de piégeage cohérent de population (CPT). Dans un mode de réalisation CPT standard, le photodétecteur 108a est un photodétecteur de type silicium. Dans d'autres modes de réalisation CPT, différents types de photodétecteurs peuvent être utilisés. La partie minoritaire 119 du faisceau 116 dévié par le séparateur 101 est atténuée par le polariseur 103 et ne perturbe ainsi pas le laser. Le second photodétecteur 108b enregistre le faisceau de lumière 118 transmis initialement par le cube séparateur miniature 101. De cette manière, la puissance de sortie de la diode laser 102 peut être mesurée et réglée par une boucle d'asservissement dédiée. Les diaphragmes 110 et 111 sont utilisés pour éviter qu'une lumière indésirable n'atteigne les photodétecteurs si la taille du faisceau laser est supérieure aux dimensions des faces du cube séparateur miniature 101. La lumière enregistrée par le photodétecteur 109 situé après le miroir 107 peut être utilisée pour différents types d'asservissement tels que fréquence du laser ou température de la cellule.After its interaction with the atoms of the alkali metal vapor, the circularly polarized light beam 114 is mainly reflected by a mirror 107. In a standard CPT embodiment, the exit window of the gas cell 106 is covered with metal (silver or gold, for example) to play the role of reflector. In another embodiment, the coating of the exit window of the gas cell 106 may be a dielectric mirror. The transmission of the reflector 107 is chosen so that a small portion of the light is transmitted to the photodetector 109. The retro-reflected light 115 passes through and interacts a second time with the atomic medium (double pass). At the output of the cell, the beam passes through the quarter-wave plate 105 which transforms its circular polarization into a linear polarization 116, perpendicular to the transmission axis of the polarizer 103, and is mainly transmitted by the miniature separator cube 101. transmitted light 117 reaches the photodetector 108a which records the absorption spectrum and more specifically the decrease of absorption due to the process of coherent population trapping (CPT). In a standard CPT embodiment, the photodetector 108a is a silicon type photodetector. In other CPT embodiments, different types of photodetectors may be used. The minority portion 119 of the beam 116 deflected by the separator 101 is attenuated by the polarizer 103 and thus does not disturb the laser. The second photodetector 108b records the light beam 118 initially transmitted by the miniature divider cube 101. In this manner, the output power of the laser diode 102 can be measured and adjusted by a dedicated servocontrol loop. The diaphragms 110 and 111 are used to prevent undesirable light from reaching the photodetectors if the size of the laser beam is larger than the dimensions of the faces of the miniature separator cube 101. The light recorded by the photodetector 109 located after the mirror 107 can be used for different types of servocontrol such as laser frequency or cell temperature.

La figure 7 illustre une conception à double passage optique basée sur le deuxième mode de réalisation, avec une géométrie droite 200 (le codage numérique commence à 200 pour la conception 200) qui est très similaire à la conception à angle droit et double passage 100 (voir Figure 5). La différence principale comparée à la conception 100 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 200 de la figure 7, 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 la conception 200 est le même que pour le modèle 100. The figure 7 illustrates a double-pass optical design based on the second embodiment, with a straight geometry 200 (the digital coding starts at 200 for the design 200) which is very similar to the right-angle and double-pass design 100 (see Figure 5 ). The main difference compared with the design 100 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 200 of the figure 7 , 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 then modified 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 for laser power measurement. Apart from these differences, the operating principle of the design 200 is the same as for the model 100.

Sur la figure 8a et selon un mode de réalisation CPT à géométrie droite selon le premier mode de réalisation, le pourcentage de séparation du cube séparateur est prédéfini de manière inverse à celle décrite précédemment (boîtier à angle droit de la figure 6), à savoir une transmission minoritaire et une réflexion majoritaire d'environ 10% et 90% respectivement (+/- 10%). La conception à double passage et géométrie droite ainsi obtenue 200 (le codage numérique commence à 200 pour la conception 200) est très similaire à la conception à angle droit et double passage 100 (voir Figure 6). Le rôle du séparateur 201 est ainsi inversé afin que la partie minoritaire du faisceau provenant de la diode laser 202 soit transmise plutôt que déviée. Pour sa part, le faisceau rétro-réfléchi 216 est alors majoritairement dévié vers le photodétecteur 208a. La différence principale dans l'arrangement des différents éléments comparée à la conception 100 réside dans la position de l'entité « cellule à gaz 206, lame quart d'onde 205, miroir semi-transparent 207 et photodétecteur 209 ». Dans le modèle 200 de la figure 8a, l'entité cellule à gaz 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 émis par le laser 202 est réfléchi par le cube séparateur 201 et est utilisé pour la mesure de la puissance laser. A part ces différences, le principe de fonctionnement de la conception 200 est le même que pour le modèle 100. On the figure 8a and according to a right-angle embodiment CPT according to the first embodiment, the separation percentage of the separator cube is predefined inversely to that previously described (right-angle housing of the figure 6 ), namely a minority transmission and a majority reflection of approximately 10% and 90% respectively (+/- 10%). The two-pass design and straight geometry thus obtained 200 (the digital coding starts at 200 for the design 200) is very similar to the right-angle and double-pass design 100 (see Figure 6 ). The role of the separator 201 is thus reversed so that the minority portion of the beam from the laser diode 202 is transmitted rather than deflected. For its part, the retro-reflected beam 216 is then mainly deflected towards the photodetector 208a. The main difference in the arrangement of the different elements compared to the design 100 lies in the position of the entity "gas cell 206, quarter wave plate 205, semi-transparent mirror 207 and photodetector 209". In the model 200 of the figure 8a , the gas-cell entity is placed above the separator cube 201 and is thus located with respect to the laser 202. The photodetector 208b is placed at right angles, where the beam of light emitted by the laser 202 is reflected by the splitter cube 201 and is used for the measurement of the laser power. Apart from these differences, the operating principle of the design 200 is the same as for the model 100.

La figure 8b illustre la représentation schématique du boîtier à géométrie droite 200 à double passage du mode de réalisation de l'oscillateur Raman selon le premier mode de réalisation. Toutes les références numériques correspondent au modèle 100 du mode de réalisation Raman et commencent par "2" au lieu de "1". Dans le cas de l'oscillateur Raman, le pourcentage de séparation du cube séparateur est prédéfini de manière inverse à celle décrite ci-dessus (horloge atomique CPT de la figure 8a), à savoir une réflexion minoritaire et une transmission majoritaire d'environ 2% et 98% respectivement (+/- 2%).The figure 8b illustrates the schematic representation of the double-pass right geometry casing 200 of the embodiment of the Raman oscillator according to the first embodiment. All numerals correspond to model 100 of the Raman embodiment and begin with " 2 " instead of "1". In the case of the Raman oscillator, the separation percentage of the separator cube is predefined opposite to that described above (CPT atomic clock of the figure 8a ), namely a minority reflection and a majority transmission of about 2% and 98% respectively (+/- 2%).

La figure 9 illustre un dispositif particulièrement adapté pour un oscillateur Raman 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 et pour stabiliser la fréquence optique du laser. 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 9 illustrates a device particularly suitable for a Raman oscillator 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 alkali metal vapor atoms, the incident light beam 114a and the stimulated Raman scattered light beam (called the Raman beam) 114b are reflected by a mirror 107. In a standard Raman embodiment the mirror 107 is coated with silver, it is inclined (typically 2 to 20 degrees) and / or eccentric with respect to its axis of symmetry and the axis defined by the incident laser beam and is concave with a chosen focal length 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 the photodetector 109 is used to measure the absorption spectrum and to stabilize the optical frequency of the laser. 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 par une boucle d'asservissement dédiée. 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 blade 105 converts these circularly polarized beams of light 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 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 by a dedicated servocontrol loop. Optionally, the photodetector 121 records the retro-reflected beam 117 transmitted by the separator 101. The diaphragms 110 and 111 are used to prevent unwanted light from reaching the photodetectors if their dimensions are larger than those of the miniature separator cube 101 .

Les figures 10 et 11 illustrent le troisième mode de réalisation pour l'horloge atomique CPT et l'oscillateur Raman, respectivement, et qui n'est pas basé sur un cube séparateur, mais sur une simple géométrie à double passage. La lumière émise par la source laser est polarisée linéairement, convertie en polarisation circulaire par une lame quart d'onde 105 avant passage dans la cellule, réflexion sur le miroir, deuxième passage dans la cellule, et détection sur un photodétecteur 108a. Le miroir 107 est semi-transparent, avec un deuxième photodétecteur 109 placé derrière le miroir.The Figures 10 and 11 illustrate the third embodiment for the CPT atomic clock and the Raman oscillator, respectively, and which is not based on a separator cube, but on a simple double-pass geometry. The light emitted by the laser source is linearly polarized, converted into circular polarization by a quarter-wave plate 105 before passing through the cell, reflection on the mirror, second passage in the cell, and detection on a photodetector 108a. The mirror 107 is semi-transparent, with a second photodetector 109 placed behind the mirror.

C'est l'utilisation du miroir semi-transparent 107 qui permet la détection de lumière ayant interagi avec les atomes de la cellule par le photodétecteur 109. Cette détection par un deuxième photodétecteur est particulièrement favorable dans le cas d'une utilisation du dispositif basée sur un oscillateur Raman. Dans le cas d'un oscillateur Raman, le 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 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 la fréquence optique du laser sur le pic de résonance, ou pour ajuster la température de la cellule. Dans ce cas-là, une fréquence de coupure basse (typiquement < 100kHz), voire un fonctionnement DC, sont indiqués. Il est donc préférable de disposer de deux détecteurs, l'un servant à la détection du signal d'horloge, l'autre à l'asservissement en fréquence optique du laser et/ou 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 pour la réflexion et de placer derrière ce miroir un photodétecteur 109. It is the use of the semi-transparent mirror 107 which allows the detection of light having interacted with the atoms of the cell by the photodetector 109. This detection by a second photodetector is particularly favorable in the case of use of the device based on a Raman oscillator. In the case of a Raman oscillator, the photodetector 108a has a very narrow bandwidth and centered around the resonance 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. This specification is not compatible with a detection of the signal that interacted with the atoms of the cell to adjust the optical frequency of the laser on the resonance peak, or to adjust the temperature of the cell. In this case, a low cut-off frequency (typically <100 kHz), or even DC operation, are indicated. It is therefore preferable to have two detectors, one for detecting the clock signal, the other for servocontrolling the optical frequency of the laser and / or controlling the temperature of the cell. The ideal way to realize this second detection of a signal having interacted with the atoms of the cell is to use a semi-transparent mirror for the reflection and to place behind this mirror a photodetector 109.

Pour l'oscillateur Raman, il est également avantageux que le miroir 107 soit de forme concave comme à la figure 11, la forme concave étant destinée à focaliser le faisceau de lumière réfléchi sur le photodétecteur 108a. For the Raman oscillator, it is also advantageous for the mirror 107 to be of concave shape as in FIG. figure 11 , the concave shape being intended to focus the light beam reflected on the photodetector 108a.

Cet arrangement est également intéressant pour une horloge basée sur un principe CPT, car le photodétecteur situé derrière le miroir semi-transparent peut servir à des fins de stabilisation de la température de la cellule contenant les atomes ou de la fréquence de la source laser.This arrangement is also interesting for a clock based on a CPT principle, because the photodetector located behind the semi-transparent mirror can be used for stabilizing the temperature of the cell containing the atoms or the frequency of the laser source.

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. To prevent the beams retro-reflected by the mirror from disturbing the laser source 102, it is also It is 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 suivantes :

  • 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

Claims (9)

  1. A device for an atomic clock comprising a laser source (102) generating a laser beam, a quarter-wave plate (105) modifying the linear polarization of the laser beam into a circular polarization and vice versa, a gas cell (106) passed through by the laser beam with circular polarization, a mirror (107) sending the laser beam back toward the gas cell, and a first photodetector (108a), as well as means (103, 101, 107) for preventing the reflected beam from reaching the laser source (102), characterized in that it comprises a second photodetector (109), placed behind the mirror (107), said mirror being semitransparent and allowing a portion of the laser beam to pass, said second photodetector (109) being used to control the optical frequency of the laser and/or to control the temperature of the cell (106).
  2. The device as claimed in claim 1, characterized in that the means for preventing the reflected beam from reaching the laser source (102) comprise a splitter (101) placed between the laser source (102) and the mirror (107) and being used to deflect and allow a portion of the laser beam to pass according to a predefined percentage, as well as a polarizer (103) placed between the output of the laser beam and the splitter in order to protect the laser source from the back-reflections from the various optical elements making up the device.
  3. The device as claimed in claim 1, characterized in that the means for preventing the reflected beam from reaching the laser source (102) comprise a splitter (101) placed between the laser source (102) and the mirror (107) and being used to deflect and allow the laser beam to pass depending on the polarization of said beam in such a way that the polarization of the beam from the laser source (102) via the splitter (101) and arriving on the quarter-wave plate (105) is linear according to the first angle and is modified by the quarter-wave plate (105) into circular polarization, and so that the circular polarization of the beam reflected by the mirror (107) and passing a second time through the gas cell (106) is modified into linear polarization according to the second angle by the quarter-wave plate (105), the splitter (101) directing the back-reflected beam to the first photodetector (108a).
  4. The device as claimed in claim 1, characterized in that the means for preventing the reflected beam from reaching the laser source (102) comprise means for inclining the mirror (107) according to an angle that is not perpendicular to the axis of the laser beam, the reflected beam thus being deflected from the axis of the beam emitted by the laser source.
  5. The device as claimed in the claims 1 to 4, characterized in that the mirror (107) is of concave form, so as to focus the reflected light beam on the first photodetector (108a).
  6. The device as claimed in the claims 1 to 4, characterized in that the mirror (107) is of concave form and the axis of symmetry of which is off-center relative to that defined by the incident laser beam so as to focus the reflected light beam on the photodetector (108a) and prevent the reflected beam from reaching the laser source (102).
  7. The device as claimed in claim 2 or 3, characterized in that it comprises a third photodetector (108b) placed after the splitter (101) so that a portion of the laser beam reaches said third photodetector (108b) without having passed through the gas cell (106).
  8. The device as claimed in claim 2, 3 or 7 characterized in that it comprises a diaphragm (110) placed between the splitter and the gas cell (106), this diaphragm reducing the size of the laser beam.
  9. The device as claimed in one of claims 2, 3, 7 or 8, characterized in that it comprises a second diaphragm (111) placed between the splitter (101) and the gas cell (106), this diaphragm reducing the size of the laser beam.
EP10760897.8A 2009-09-04 2010-09-01 Device for atomic clock Active EP2473886B1 (en)

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CH01122/10A CH703410A1 (en) 2010-07-09 2010-07-09 Device for enabling double passage of laser beam into gas cell of coherent-population-trapping atomic clock, has photodetector controlling optical frequency of laser beam and/or controlling temperature of gas cell
PCT/CH2010/000215 WO2011026252A1 (en) 2009-09-04 2010-09-01 Device for an atomic clock
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US8816779B2 (en) 2014-08-26
EP2473886A1 (en) 2012-07-11
US20120256696A1 (en) 2012-10-11
WO2011026252A1 (en) 2011-03-10

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