JP2009049623A - Atomic oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an atomic oscillator equipped with an optical system which is made thin and facilitates repair and replacement of respective components. <P>SOLUTION: Disclosed is the optical system 1 of the atomic oscillator 100 whose oscillation frequency is controlled utilizing light absorption characteristics by quantum interference effect when two kinds of pieces of resonant light are made incident as coherent light beams differing in wavelength, the optical system comprising a gas cell 3 in which gaseous metal atoms are charged, a coherent light source 2 supplying the resonant light to metal atoms in the gas cell 3, a first light guide means 9 of guiding light emitted by the coherent light source 2 to the gas cell 3, a second light guide means 8 of guiding light 6 transmitted through the gas cell 3 to a photodetector 4, an optical detector (optical detecting means) 4 of detecting the transmitted light 6a guided by the light guide means 8, and a frequency control circuit 5 which controls the oscillation frequency with a detection signal from the optical detector 4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an atomic oscillator, and more particularly to a technique for thinning an optical system constituting the atomic oscillator.

An atomic oscillator using an alkali metal such as rubidium or cesium needs to keep atoms in a gas state when using energy transition of atoms, and therefore operates a gas cell in which atoms are hermetically sealed at a high temperature. . The principle of operation of an atomic oscillator is roughly divided into a double resonance method using light and microwaves, and a method using a quantum interference effect (hereinafter referred to as CPT: Coherent Population Trapping) by two types of laser light. In both cases, the amount of the light incident on the gas cell is detected by the detector provided on the opposite side to detect how much light has been absorbed by the atomic gas, so that the atomic resonance is detected and a reference signal from a crystal oscillator or the like is sent to the control system. Output is obtained in synchronization with atomic resonance. Here, in an atomic oscillator using CPT, a light emitting element, a gas cell, and a light receiving element are integrally formed to form an optical system (see Patent Document 1).
US6808064B2

However, in the configuration of the conventional optical system disclosed in Patent Document 1, the light emitting element 33, the passive optical element 34, the gas cell 35, and the light receiving element 30 are vertically arranged as shown in FIG. For this reason, the wire bonding 31 for electrically connecting the light receiving element 30 arranged on the uppermost surface becomes long and the mounting of the module becomes complicated, and the signal obtained from the light receiving element 30 is weak. There is a problem that the S / N characteristic is not good because of being easily received. In addition, since each component is stacked, the height of the optical system is increased, and there are problems with miniaturization and thinning. Further, since the optical system is adjusted after the components are stacked and the wire bonding 31 is completed, when the optical axes of the light emitting element 33 and the light receiving element 30 are deviated or when any of the parts is defective. The wire bonding 31 must be removed again and the parts must be assembled from the beginning, and it took time and trouble to repair and replace the parts.
In view of such problems, the present invention sequentially arranges a light emitting element, a gas cell, and a light receiving element along a plane direction on the same plane, and bends light by the first light guiding means and the second light guiding means. Another object of the present invention is to provide an atomic oscillator including an optical system that has a thin optical system and that facilitates repair and replacement of each component.
Another object is to shorten the wire bonding for electrically connecting the light receiving elements to facilitate the module mounting and to improve the S / N.

In order to solve such a problem, the present invention is an optical system of an atomic oscillator that controls an oscillation frequency by utilizing a light absorption characteristic due to a quantum interference effect when two types of resonant light as coherent lights having different wavelengths are incident. A gas cell encapsulating gaseous metal atoms, a coherent light source for supplying the resonance light to the metal atoms in the gas cell, a light detecting means for detecting the light transmitted through the gas cell, and light emission from the coherent light source First light guiding means for guiding the emitted light to the gas cell, and second light guiding means for guiding the light transmitted through the gas cell to the light detection means, the coherent light source, the gas cell, and the light The detection means is sequentially arranged along the surface direction on the same plane.
The atomic oscillator of the present invention utilizes the quantum interference effect of coherent light such as laser light. In this method, the frequencies of two resonant lights irradiated simultaneously in a three-level system (for example, a Λ-type level) in which two ground levels receive resonant light and are resonantly coupled with a common excitation level. Is exactly coincident with the energy difference between the ground level 1 and the ground level 2, the system becomes a superposition state of the two ground levels, and the excitation to the excited level 3 stops. CPT uses this principle to detect and use a state in which light absorption in the gas cell stops when the wavelength of one or both of the two resonance lights is changed. In the optical system of the present invention, the coherent light source, the gas cell, and the light detection means are sequentially arranged along the plane direction on the same plane, and the light is bent by the first light guide means and the second light guide means. The configuration. As a result, the optical system can be thinned and each component can be easily repaired and replaced.

Also, an optical system of an atomic oscillator that controls the oscillation frequency by utilizing the light absorption characteristics due to the quantum interference effect when two types of resonance light as coherent lights having different wavelengths are incident, and the gaseous metal atoms are An encapsulated gas cell; a coherent light source that supplies the resonance light to the metal atoms in the gas cell; and a light detection unit that detects light transmitted through the gas cell, wherein the coherent light source and the light detection unit are opposed to each other. It arrange | positions so that it may carry out.
In order to reduce the thickness of the optical system, it is necessary to reduce the number of parts interposed on the optical path. For this purpose, the light emitted from the coherent light source is directly incident on the light detection means via the gas cell. Therefore, in the present invention, the gas cell is arranged between the coherent light source and the light detecting means by arranging the coherent light source and the light detecting means so as to face each other. Thereby, the number of parts can be reduced and the optical system can be further thinned.

The coherent light source, the gas cell, and a substrate on which the light detection means are juxtaposed on the same plane, provided with an opening or a transparent portion at an appropriate position of the substrate, the coherent light source and / or the light detection means Is disposed on a surface opposite to the plane of the substrate, and emits light and receives light through the opening or the transparent portion.
In the case of providing a substrate on which the coherent light source, the gas cell, and the light detection means are juxtaposed on the same plane, the light emitting surface of the coherent light source and the light receiving surface of the light detection means face upward with respect to the substrate surface. In order to reduce the thickness of the optical system with this arrangement, the gas cell is arranged between the coherent light source and the light detection means, and the light emitted from the upper side is bent and incident on the gas cell to transmit the transmitted light from the gas cell. Furthermore, it is necessary to bend and guide to the light detection means. At this time, when the coherent light source and the light detection means are mounted on the surface of the substrate, the overall height is increased by the height of the component. Therefore, in the present invention, an opening or a transparent part is provided at an appropriate position of the substrate, and the coherent light source or / and the light detection means are arranged on the opposite surface of the same plane of the substrate. Thereby, the height of the components on the substrate can be made as low as possible.

In addition, a passive optical element that collects the light emitted from the coherent light source and corrects the light into parallel light is disposed between the first light guide unit and the gas cell.
In the optical system, a passive optical element such as a lens or a wave plate is used to collect light emitted from the coherent light source and correct it so as to become parallel light. This passive optical element may be disposed anywhere before entering the gas cell. Therefore, in the present invention, the passive optical element is disposed between the first light guide means and the gas cell. Thereby, even if it has a passive optical element, it can suppress that the whole optical system becomes high as much as possible.

Further, the passive optical element is arranged between the coherent light source and the first light guide means.
If the passive optical element is disposed between the first light guide means and the gas cell, the height of the optical system can be suppressed, but the width of the substrate may be increased. Therefore, in the present invention, the passive optical element is disposed between the coherent light source and the first light guide unit. Thereby, the magnitude | size of the width direction of a board | substrate can be suppressed.

Further, the first light guide means and the second light guide means are provided with an optical axis adjusting mechanism.
The light emitted from the coherent light source reaches the light detection means via the first light guide means and the second light guide means. At this time, when the optical axis is shifted due to an attachment error of each component, the light does not enter the gas cell, or the transmitted light from the gas cell does not reach the light detection unit. Therefore, in the present invention, the first light guide means and the second light guide means are provided with an optical axis adjustment mechanism. Thereby, the shift | offset | difference of an optical axis can be adjusted easily.

Further, the first light guide means and the second light guide means are mirrors, diffraction gratings, corner cube prisms, optical fibers, or optical waveguides.
In the optical system of the present invention, the coherent light emitted from the light source needs to be transmitted through the gas cell, and the transmitted light needs to be received by the light detection means. For this purpose, it is necessary to reflect, diffract, refract, or guide the transmitted light. A mirror for reflection, a diffraction grating for diffraction, a corner cube prism for refraction, and an optical fiber or optical waveguide for light guide are optimal. This makes it possible to select an optimum light guide means for the optical system depending on the purpose and configuration.

The coherent light is laser light.
Ordinary light is light in which various wavelengths are mixed and the phase is random. On the other hand, laser light has good monochromaticity in wavelength and is light with a uniform phase. Coherence is defined as a measure of the stability of the wavelength and phase of light. Light with good coherence, that is, with stable wavelength and phase, can cause a quantum interference effect. In that respect, laser light is optimal.

The gaseous metal atom is rubidium or cesium.
If cesium atoms are used, a highly accurate atomic oscillator can be realized. In addition, rubidium atoms are widely spread easily. Therefore, any metal atom can be selected in consideration of the required performance and cost of the atomic oscillator.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, the components, types, combinations, shapes, relative arrangements, and the like described in this embodiment are merely illustrative examples and not intended to limit the scope of the present invention only unless otherwise specified. .
FIG. 1 is a configuration diagram of a main part of an optical system of an atomic oscillator according to an embodiment of the present invention. This optical system 1 is an optical system 1 of an atomic oscillator 100 that controls an oscillation frequency by using a light absorption characteristic due to a quantum interference effect when two types of resonant light as coherent lights having different wavelengths are incident. A gas cell 3 in which gaseous metal atoms are sealed, a coherent light source 2 for supplying resonance light to the metal atoms in the gas cell 3, and a first light guiding means 9 for guiding the light emitted from the coherent light source 2 to the gas cell 3 The second light guide means 8 that guides the transmitted light 6 that has passed through the gas cell 3 to the light detector 4, and the light detector (light detection means) that detects the transmitted light 6a guided by the second light guide means 8. 4 and a frequency control circuit 5 that controls the oscillation frequency based on a signal detected from the photodetector 4. Since the gist of the present invention is the configuration of the optical system that constitutes the atomic oscillator, a detailed description of the frequency control of the atomic oscillator is omitted.

  That is, the atomic oscillator 100 of this embodiment uses quantum interference of coherent light such as laser light. In this system, in a system in which two ground levels receive resonance light and are resonantly coupled with a common excitation level, the frequencies of the two resonance lights irradiated simultaneously are precisely the ground level 1 and the ground level. When the energy difference of level 2 coincides, the atom is in a superposition state of two ground levels, and excitation to excitation level 3 stops. CPT uses this principle to detect and use a state where light absorption in the gas cell stops when the wavelength of one or both of the two resonance lights is changed (details will be described later). .

  FIG. 2 is an example for explaining a three-level system of atoms by the CPT method. The rubidium and cesium ground levels used in the atomic oscillator are divided into two kinds of ground levels by the hyperfine structure due to the nuclear spin-electron spin interaction. These ground level atoms absorb light and excite to higher energy levels. A state in which two ground levels receive light and are resonantly coupled to a common excitation level as shown in FIG. 2 is called two-photon resonance. In FIG. 2, since the ground level 1 (23) and the ground level 2 (24) have slightly different levels of energy, the resonant light also has a wavelength slightly different from that of the resonant light 1 (20) and the resonant light 2 (22), respectively. Different. When the frequency difference (wavelength difference) between the resonant light 1 (20) and the resonant light 2 (22) irradiated at the same time exactly matches the energy difference between the ground level 1 (23) and the ground level 2 (24), The system shown in FIG. 2 enters a superposition state of two ground levels, and excitation to the excitation level 21 stops. The CPT utilizes this principle to absorb light in the gas cell 3 (that is, to the excitation level 21) when the wavelength of one or both of the resonant light 1 (20) and the resonant light 2 (22) is changed. This is a method of detecting and using a state where the conversion is stopped.

  FIG. 3 is a diagram schematically showing the configuration of the optical system 1a according to the first embodiment of the present invention. In this optical system 1 a, a light emitting element (coherent light source) 2, a gas cell 3, and a light receiving element (light detection means) 4 are sequentially arranged along the surface direction on the substrate 12, and each element is connected to the substrate 12 by wire bonding 14. Is electrically connected. Then, a predetermined distance is secured by the spacer 15, and a mirror (first light guide unit) 16 that bends coherent light emitted from the light emitting element 2 on the spacer 15 and the light bent by the mirror 16 are collected. In this case, the mirror 16 and the passive optical element 17 are integrated into a single module. The light 11 b emitted from the passive optical element 17 passes through the gas cell 3, and the transmitted light is bent by the mirror (second light guide unit) 19 and received by the light receiving element 4. The mirror 19 and the light receiving element 4 are integrated into one module. Accordingly, three modules are mounted on the substrate 12. Note that a surface emitting laser (VCSEL) is often used as the light emitting element 2 and a photodiode is often used as the light receiving element. Therefore, the optical system 1 of the present embodiment has a configuration in which the coherent light source 2, the gas cell 3, and the photodetector 4 are sequentially arranged along the plane direction on the same plane and the transmitted light is bent by the mirror 16 and the mirror 19. did. Thereby, the optical system 1 can be reduced in thickness, and each module can be easily repaired and replaced. The optical system uses a passive optical element 17 that can condense light emitted from the coherent light source 2, make it parallel light, or change the polarization state. The passive optical element 17 may be disposed anywhere as long as it is before entering the gas cell 3. Therefore, in the present embodiment, the passive optical element 17 is disposed between the mirror 16 and the gas cell 3. Thereby, even if it has the passive optical element 17, it can suppress that the optical system 1a whole becomes high as much as possible.

The schematic operation will be described with reference to FIG. The coherent light 11 a emitted from the light emitting element 2 is bent by 90 degrees by the mirror 16, corrected to the condensed, parallel light, and polarized light 11 b by the passive optical element 17 and enters the gas cell 3. The gas cell 3 operates so that light absorption stops when one or both of the wavelengths of the coherent light 11a having two wavelengths are changed. The transmitted light that has passed through the gas cell 3 is bent by 90 degrees by the mirror 19 and received by the light receiving element 4 as transmitted light 11c.
The coherent light in this embodiment uses laser light. Laser light has good monochromaticity in wavelength and has a uniform phase. Coherence is defined as a measure of the stability of the wavelength and phase of light. Light having good coherence, that is, light having a stable wavelength and phase can cause a quantum interference effect. In that respect, laser light is optimal. The gaseous metal atom used in the gas cell 3 is rubidium or cesium. For example, a cesium atom used in a primary atom standard can be used to realize a highly accurate atomic oscillator. The rubidium atom used in the secondary standard is easily and widely used, so it can be used to realize a small and low-priced atomic oscillator. Therefore, what is used for the metal atom may be selected according to the purpose of use. In this embodiment, rubidium and cesium are used. However, any atom having a three-level system may be used.

FIG. 4 is a diagram schematically showing the configuration of the optical system 1b according to the second embodiment of the present invention. The same components will be described with the same reference numerals as in FIG. 4 differs from FIG. 3 in that an opening or a transparent portion 10 is provided at an appropriate position of the substrate 12, and the light emitting element 2 and / or the light receiving element 4 are disposed on the opposite surface of the same plane of the substrate 12. Alternatively, light is emitted and received through the transparent portion 10.
That is, when the substrate 12 on which the light emitting element 2, the gas cell 3, and the light receiving element 4 are arranged on the same plane is provided, the light emitting surface of the light emitting element 2 and the light receiving surface of the light receiving element 4 face upward with respect to the substrate surface. become. In order to reduce the thickness of the optical system with this arrangement, the gas cell 3 is arranged between the light emitting element 2 and the light receiving element 4, and the light emitted upward is bent and incident on the gas cell 3. The transmitted light needs to be further bent and guided to the light receiving element 4. At this time, when the light emitting element 2 and the light receiving element 4 are mounted on the surface of the substrate 12, the overall height is increased by the height of the component. Therefore, in the present embodiment, an opening or a transparent portion 10 is provided at an appropriate position on the substrate 12, and the light emitting element 2 and / or the light receiving element 4 are arranged on the opposite side of the same plane of the substrate 12. Thereby, the height of the component on the board | substrate 12 can be made as low as possible.
The same effect can be obtained even when the substrate 12 is a glass plate.

FIG. 5 is a diagram schematically showing the configuration of an optical system 1c according to the third embodiment of the present invention. The same components will be described with the same reference numerals as in FIG. 5 is different from FIG. 4 in that the passive optical element 17 is disposed between the light emitting element 2 and the mirror 16. That is, when the passive optical element 17 is arranged between the mirror 16 and the gas cell 3 as shown in FIG. 3, the height of the optical system 1 can be suppressed, but the width of the substrate 12 may be increased. Therefore, in the present embodiment, the passive optical element 17 is disposed between the light emitting element 2 and the mirror 16. Thereby, the size of the substrate 12 in the width direction can be suppressed.
The light emitted from the light emitting element 2 reaches the light receiving element 4 via the mirrors 16 and 19. At this time, when the optical axis is shifted due to an attachment error of each component, the light does not enter the gas cell 3 or the transmitted light from the gas cell 3 does not reach the light receiving element 4. Therefore, by providing the mirrors 16 and 19 with an optical axis adjusting mechanism, the optical axis shift can be easily adjusted.
In the conventional example, it is not known whether the optical axis is shifted unless all the components are assembled. However, in this configuration, the mirror and the passive optical element module can be finally mounted on the substrate while adjusting the optical axis.

FIG. 6 is a diagram schematically showing the configuration of an optical system 1d according to the fourth embodiment of the present invention. The same components will be described with the same reference numerals as in FIG. The optical system 1d includes a gas cell 3 in which gaseous metal atoms are sealed, a light emitting element 2 that supplies resonance light to the metal atoms in the gas cell 3, a light receiving element 4 that detects transmitted light that has passed through the gas cell 3, The light emitting element 2 and the light receiving element 4 are disposed so as to face each other. In order to arrange the light emitting element 2 and the light receiving element 4 so as to face each other, small boards 12 a and 12 b are prepared in which respective components are orthogonal to the board 12.
That is, in order to reduce the thickness of the optical system 1d, it is necessary to reduce the number of parts interposed on the optical path. For this purpose, the light emitted from the light emitting element 2 is configured to be directly incident on the light receiving element 4 through the gas cell 3. Therefore, in the present embodiment, the light emitting element 2 and the light receiving element 4 are disposed so as to face each other, and the gas cell 3 is disposed between the light emitting element 2 and the light receiving element 4. Thereby, the number of parts can be reduced and the optical system 1d can be further reduced in thickness.

In the above description, the mirror is described as the light guide unit, but the light guide unit includes a mirror, a diffraction grating, a corner cube prism, an optical fiber, or an optical waveguide. That is, in the optical system of the present invention, the coherent light emitted from the light source needs to pass through the gas cell 3 and the transmitted light needs to be received by the light receiving element 4. For this purpose, it is necessary to reflect, diffract, refract, or guide the transmitted light. A mirror for reflection, a diffraction grating for diffraction, a corner cube prism for refraction, and an optical fiber or optical waveguide for light guide are optimal. This makes it possible to select an optimum light guide means for the optical system depending on the purpose and configuration.
Similar to a prism, a diffraction grating is an optical element that extracts light of a specific wavelength from light mixed with various wavelengths. Originally, a large number of thin slits were arranged in parallel, and when light was incident on the slits, the light that passed through the slits was diffracted and spread, and interfered with each other so that only light of a specific wavelength could be seen at a specific angle. Current diffraction gratings use the fact that thousands of grooves are formed in parallel on a mirror-finished metal plate to interfere with reflected light. By rotating the diffraction grating and selecting an incident angle and a reflection angle, light having a specific wavelength can be extracted.

The corner cube prism is a prism that turns back 180 ° in the incident direction regardless of the direction of the light incident on the prism using total internal reflection of three orthogonal surfaces.
The optical fiber is a wire for optical transmission that transmits a signal not by an electrical signal but by light using a semiconductor laser, an LED, or the like as a light source. The optical fiber has a double structure in which a high refractive index core (diameter of about 50 to 60 microns) made of quartz is covered with a low refractive index cladding layer. It is transmitted with total reflection only inside. However, in an actual optical fiber cable, the outermost coating is further applied to protect the wire. The optical fiber cable is characterized in that the signal is not easily attenuated and can be transmitted over a long distance (several tens to several hundreds km) without relay.
An optical waveguide is a circuit that is processed so as to guide an optical signal by providing a light path on a substrate by utilizing a difference in refractive index of light. The optical waveguide is a so-called light wiring board in that light flows through the circuit so that electrons flow through the electric circuit. An optical waveguide can be said to be similar to an optical fiber in that it uses a light refractive index to guide light with high straightness, but it is not a three-dimensional fiber structure like an optical fiber, but generally has a planar structure. Yes.

It is a principal part block diagram of the optical system of the atomic oscillator which concerns on embodiment of this invention. It is a figure explaining the 3 level system of an atom by a CPT system. It is the figure which modeled the structure of the optical system 1a which concerns on the 1st Embodiment of this invention. It is the figure which modeled the structure of the optical system 1b which concerns on the 2nd Embodiment of this invention. It is the figure which modeled the structure of the optical system 1c which concerns on the 3rd Embodiment of this invention. It is the figure which modeled the structure of the optical system 1d which concerns on the 4th Embodiment of this invention. It is a figure which shows the structure of the conventional optical system.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Optical system, 2 Coherent light source, 3 Gas cell, 4 Photo detector, 5 Frequency control circuit, 6 Transmitted light, 8, 9 Light guide means, 100 Atomic oscillator

Claims (9)

An optical system of an atomic oscillator that controls an oscillation frequency by utilizing a light absorption characteristic due to a quantum interference effect when two types of resonance light as coherent light having different wavelengths are incident,
A gas cell in which gaseous metal atoms are sealed; a coherent light source that supplies the resonance light to the metal atoms in the gas cell; a light detection means that detects light transmitted through the gas cell; and light emitted from the coherent light source. A first light guide means for guiding the light to the gas cell, and a second light guide means for guiding the light transmitted through the gas cell to the light detection means,
An atomic oscillator, wherein the coherent light source, the gas cell, and the light detection means are sequentially arranged along a plane direction on the same plane. An optical system of an atomic oscillator that controls an oscillation frequency by utilizing a light absorption characteristic due to a quantum interference effect when two types of resonance light as coherent light having different wavelengths are incident,
A gas cell in which gaseous metal atoms are sealed, a coherent light source that supplies the resonance light to the metal atoms in the gas cell, and a light detection means that detects light transmitted through the gas cell,
An atomic oscillator characterized in that the coherent light source and the light detection means are arranged to face each other.   A substrate on which the coherent light source, the gas cell, and the light detection means are juxtaposed on the same plane; an opening or a transparent portion is provided at an appropriate position of the substrate; and the coherent light source and / or the light detection means The atomic oscillator according to claim 1, wherein the atomic oscillator is disposed on a surface opposite to the flat surface of the substrate and emits and receives light through the opening or the transparent portion.   The passive optical element which condenses the light radiated | emitted from the said coherent light source, and correct | amends it to parallel light was arrange | positioned between the said 1st light guide means and the said gas cell, The Claim 1 or 3 characterized by the above-mentioned. Atomic oscillator.   The atomic oscillator according to claim 1, wherein the passive optical element is disposed between the coherent light source and the first light guide unit.   6. The atomic oscillator according to claim 1, wherein the first light guide means and the second light guide means are provided with an optical axis adjusting mechanism.   The first light guiding unit and the second light guiding unit are mirrors, diffraction gratings, corner cube prisms, optical fibers, or optical waveguides, respectively. 6. An atomic oscillator according to 6.   The atomic oscillator according to claim 1, wherein the coherent light is laser light.   The atomic oscillator according to any one of claims 1 to 8, wherein the gaseous metal atom is rubidium or cesium.

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