JP2009182562A - Optical system and atomic oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an atomic oscillator equipped with an optical system which facilitates module mounting by arranging a plurality of light receiving elements and light emitting elements side by side on the same side thereby reducing the length of a bonding wire for electrically connecting the plurality of light receiving elements, and which has a high EIT signal level thereby has an improved S/N. <P>SOLUTION: The optical system 1 comprises: a coherent light source 2 for emitting resonance light 3; a light guiding means 4 which is arranged at an outgoing side of the coherent light source 2 and guides the resonance light 3 to at least two optical paths 5 and 6; a gas cell 7 which is arranged an outgoing side of the light guiding means 4 and encapsulates gas-shaped metal atoms thereinto and transmits the resonance light 5 and 6 guided by the light guiding means 4 to this metal atomic gas; photodetectors (light detecting means) 10 and 11 for detecting the respective resonance light 5 and 6 transmitting the gas cell 7, respectively; a synthesizing circuit 10 for synthesizing signals detected from photodetectors 8 and 9; and a frequency control circuit 11 for controlling an oscillation frequency by an output signal of the synthesizing circuit 10. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical system for an atomic oscillator, and more particularly to a mounting technique for a light source and a light receiving element included in the 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 93, the gas cell 95, and the light receiving element 90 are vertically arranged as shown in FIG. For this reason, the bonding wire 91 for electrically connecting the light receiving element 90 arranged on the uppermost surface becomes long, and the mounting structure of the module becomes complicated, and the signal obtained from the light receiving element 90 is weak, so that the wire is used. There is also a problem that the S / N characteristic is not good because it is easily affected by superimposed noise.
In view of such problems, the present invention allows light emitted from a light emitting element to pass through a gas cell as a plurality of resonance lights by a light guide, guides the plurality of light passing through a mirror to a plurality of light receiving elements, and a plurality of light receiving elements. Is arranged on the same side as the light emitting element, thereby shortening the bonding wire for electrically connecting a plurality of light receiving elements to facilitate module mounting, and increasing the EIT signal level to improve the S / N. It is an object to provide an atomic oscillator including the above.

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 coherent light source that emits the resonance light, a light guide unit that is disposed on an emission side of the coherent light source and guides the resonance light to two optical paths, and an emission of each optical path guided by the light guide unit. A gas cell that is disposed on the side and encloses gaseous metal atoms and that allows the resonant light guided by the light guide means to pass through the metal atom gas, and a plurality of light beams that detect each transmitted light that has passed through the gas cell. And a light detection means.
The atomic oscillator of the present invention utilizes the quantum interference effect of coherent light such as laser light. In this method, two ground levels of two resonant lights irradiated simultaneously in a three-level system (for example, a Λ-type level system) in which two ground levels receive resonant light and are resonantly coupled with a common excitation level. When the frequency exactly matches the energy difference between the ground level 1 and the ground level 2, the three-level system becomes a superposition state of the two ground levels, and the excitation to the excitation 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 and the plurality of light detection means are mounted on the same side, and the light guide means is configured so that the transmitted light is received by the plurality of light detection means. This shortens the wire bonding, improves the S / N characteristics of the signal, and facilitates the mounting of the entire optical system.

The light guiding means is branched by the branching means for branching the resonant light emitted from the coherent light source into two optical paths, leading one of the resonant light to the gas cell and passing the other resonant light. And optical path changing means for guiding the other resonance light to the gas cell.
The resonant light emitted from the coherent light source is split into two optical paths by the branching unit. One optical path is incident on the gas cell, and the other optical path is converted by the optical path converting means and incident on the gas cell. Accordingly, since one coherent light source has substantially passed through the gas cell twice, a larger contrast of transmitted light can be obtained, and the S / N characteristics of the signal can be improved.
The plurality of light detection means and the coherent light source are juxtaposed on the same side on the same substrate.
In the optical system of the present invention, the coherent light source and the plurality of light detection means are mounted on the same side on the same substrate, and the light guide means is configured so that the transmitted light is received by the plurality of light detection means. As a result, the bonding wire is shortened, the signal S / N characteristic is improved, and the entire optical system can be easily mounted.

The optical system further includes optical path changing means for guiding each transmitted light that has passed through the gas cell to the plurality of light detecting means.
The plurality of lights that have passed through the gas cell pass in parallel to the gas cell. In this state, light cannot be received by the light detection means. Therefore, the present invention further includes optical path changing means for guiding light that has passed through the gas cell to a plurality of light detecting means. Thereby, the light emitted from the gas cell can be guided to a plurality of light detection means.
Further, a synthesis circuit for synthesizing output signals from the plurality of light detection means is provided.
A plurality of light detection means output signals having levels corresponding to the received light. In order to feed back this signal to the frequency control circuit of the atomic oscillator, it is necessary to combine them as one signal. Therefore, the present invention includes a synthesis circuit that synthesizes output signals from a plurality of light detection means. Thereby, an EIT signal having a large level can be obtained.
The branching means is a beam splitter or a half mirror.

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 easy to use because the microwave transition frequency is lower than 6.8 GHz and 9.2 GHz of cesium atoms. Therefore, any metal atom can be selected in consideration of the required performance and convenience of the atomic oscillator.
In addition, a passive optical element that collects light emitted from the coherent light source and performs processing for correcting the light into parallel light is disposed between the coherent light source and the light guide unit.
In the optical system, a passive optical element such as a lens or a wave plate is used to collect light emitted from a coherent light source and correct it to become parallel light, or to change the polarization state. 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 coherent light source and the gas cell. Thereby, light can be accurately incident on the light guide means.
Further, the optical system according to any one of claims 1 to 9 is provided.
By adopting a structure that passes through the gas cell a plurality of times, it is possible to provide an optical system that obtains a larger EIT signal, so that a high-performance atomic oscillator with improved S / N can be provided.

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 coherent light source 2 that emits the resonant light 3, a light guide unit 4 that is disposed on the output side of the coherent light source 2 and guides the resonant light 3 to at least two optical paths 5 and 6, and a gas that is disposed on the output side of the light guide unit 4. In the metal atom gas, the gas cell 7 that allows the resonant light 5 and 6 guided by the light guide 4 to pass therethrough and the transmitted light 8 and 9 that passes through the gas cell 7 are detected. Photodetectors (photodetection means) 10 and 11, a synthesis circuit 12 for synthesizing signals detected from the photodetectors 8 and 9, and a frequency control circuit 13 for controlling the oscillation frequency based on an output signal of the synthesis circuit 12. The Ete is configured. Also, 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 the present embodiment uses the quantum interference effect of coherent light such as laser light. In this method, two ground levels of two resonant lights irradiated simultaneously in a three-level system (for example, a Λ-type level system) in which two ground levels receive resonant light and are resonantly coupled with a common excitation level. When the frequency exactly matches the energy difference between the ground level 1 and the ground level 2, the three-level system becomes a superposition state of the two ground levels, and the excitation to the 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). . The transmitted lights 8 and 9 passing through the gas cell 7 in a state where the light absorption is stopped are called EIT signals. In the optical system 1 of the present embodiment, the coherent light source 2 and the plurality of photodetectors 10 and 11 are mounted on the same side of the same substrate, and the transmitted lights 8 and 9 are received by the plurality of photodetectors 10 and 11. The light guide 4 was configured as described above. This shortens the wire bonding, improves the S / N characteristics of the signal, and facilitates the mounting of the entire optical system.
The photodetectors 10 and 11 output signals having levels corresponding to the received light. In order to feed back this signal to the frequency control circuit 13 of the atomic oscillator, it is necessary to combine them as one signal. Therefore, the present invention includes a synthesis circuit 12 that synthesizes output signals from the photodetectors 10 and 11. Thereby, an EIT signal having a large level can be obtained.
Further, the optical system 1 of the present invention needs to split the resonance light 3 emitted from the coherent light source 2 into two. A beam splitter and a half mirror are optimal for branching. Thereby, the resonance light 3 can be branched into a plurality of lights 5 and 6.

  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 interaction between the nuclear spin I and the total angular momentum J of the electrons. 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 according to the first embodiment of the present invention. In the optical system 50, the light emitting element 30 and the light receiving elements 37 and 38 are arranged on the back surface of the substrate 31, and each element is electrically connected to the substrate 31 by a bonding wire 46. And the light which passed the passive optical element 32 is provided with the passive optical element 32 which condenses the coherent light 39 light-emitted from the light emitting element 30 on the surface of the board | substrate 31, converts into parallel light, or changes a polarization state. A beam splitter (branching means) 33 for branching and a mirror (optical path changing means) 34 for reflecting the light 41 that has passed through the beam splitter 33 are arranged. The light 40a that has passed through the beam splitter 33 and the light 42a that has undergone optical path conversion by the mirror 34 pass through the gas cell 35 to become light 40b and light 42b, respectively, and are optically converted by the mirror 36 and received by the light receiving elements 38 and 37. Is done. Note that a surface emitting laser (VCSEL) is often used as the light emitting element 30, and a photodiode is often used as the light receiving elements 37 and 38. Since the light emitting element 30 and the light receiving elements 37 and 38 are mounted on the back surface of the substrate 31 in this example, openings 31a are provided in the substrate 31, respectively.

Next, the schematic operation will be described with reference to FIG. The coherent light 39 emitted from the light emitting element 30 is condensed by the passive optical element 32, subjected to processing such as correction to parallel light, and branched to the resonance lights 40 a and 41 by the beam splitter 33. The resonance light 40 a enters the gas cell 35 and becomes light 40 b emitted from the gas cell 35, undergoes optical path conversion by the mirror 36 and is received by the light receiving element 38. The resonant light 41 becomes light 42 a whose optical path is changed by the mirror 34 and enters the gas cell 35, and the light 42 b emitted from the gas cell 35 is optically changed by the mirror 36 and received by the light receiving element 37. Therefore, the resonance light 39 is equivalent to passing through the gas cell 35 twice, so that a larger contrast can be obtained and the S / N characteristics of the signal can be improved.
The coherent light 39 of 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 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 used in the gas cell 35 is rubidium or cesium. By using the cesium atom used in the primary atom standard, a highly accurate atomic oscillator can be realized. The rubidium atom used in the secondary standard is convenient because the microwave transition frequency is lower than that of the cesium atom. By using this, a small and low-priced atomic oscillator can be realized. 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, but any atom may be used as long as it has a three-level system such as a Λ-type system.

In addition, the optical system 50 uses a passive optical element 32 such as a lens or a wave plate that collects the light emitted from the light emitting element 30 and corrects it to become parallel light or changes the polarization state of the light. Is done. The passive optical element 32 may be disposed anywhere as long as it is before entering the gas cell 35. Therefore, in the present embodiment, the passive optical element 32 is disposed between the light emitting element 30 and the beam splitter 33. Thereby, light can be accurately incident on the beam splitter 33.
According to the present invention, for example, the passive optical element 32, the beam splitter 33, and the mirror 34 are integrated into one module A (corresponding to the light guide means 4 in FIG. 1), and the mirror 36 is formed as one module B. Thus, the optical system 50 can be configured by mounting the module A, the gas cell 35 (corresponding to the gas cell 7 in FIG. 1), and the module B on the substrate 31. In addition, this modularization allows repairs to be performed in module units, thereby reducing manufacturing costs and maintenance costs.

  FIG. 4 is a diagram schematically showing a configuration of an optical system 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 the number of light receiving elements is increased from two to three. In order to realize this, two beam splitters are provided. That is, in the optical system 51, the light emitting element 30 and the light receiving elements 37, 38, 45 are juxtaposed on the back surface of the substrate 31, and each element is electrically connected to the substrate 31 by the bonding wire 46. And the light which passed the passive optical element 32 is provided with the passive optical element 32 which condenses the coherent light 39 light-emitted from the light emitting element 30 on the surface of the board | substrate 31, converts into parallel light, or changes a polarization state. Are provided with a beam splitter (branching means) 33a for branching and a mirror 34 for reflecting the light 43 that has passed through the beam splitter (branching means) 33b. Further, the light 44a whose optical path has been changed by the mirror (optical path changing means) 34 and the light 40a and 42a whose optical paths have been changed by the beam splitter 33a and the beam splitter 33b pass through the gas cell 35 to become light 44b, 40b and 42b, respectively. The optical path is changed by the mirror 36 and received by the light receiving elements 37, 38 and 45. Note that a surface emitting laser (VCSEL) is often used as the light emitting element 30, and a photodiode is often used as the light receiving elements 37, 38, and 45.

That is, the resonance light 39 emitted from the light emitting element 30 is branched into light 40a, 44a, 42a by the beam splitters 33a, 33b, passes through the gas cell 35 and becomes transmitted light 40b, 44b, 42b, and the light receiving elements 38, 37, 45 receives light. Signals received by the light receiving elements 38, 37 and 45 are combined by the combining circuit 12. Therefore, this is equivalent to the resonance light 39 passing through the gas cell 35 three times. Thereby, a larger contrast can be obtained and the S / N characteristic of the signal can be further improved.
As described above, by arranging the beam splitters in series, the resonance light can be branched infinitely in theory, but the number is actually limited to a limited number. In the above description, the beam splitter is used as the light guiding means, but a half mirror may be used.

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. 1 is a diagram schematically illustrating a configuration of an optical system according to a first embodiment of the present invention. It is the figure which modeled the structure of the optical system which concerns on the 2nd Embodiment of this invention. It is a figure which shows the structure of the conventional optical system currently disclosed by patent document 1. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Optical system, 2 Coherent light source, 3 Resonant light, 4 Light guide means, 7 Gas cell, 8, 9 Transmitted light, 10, 11 Photo detector, 12 Synthesis circuit, 13 Frequency control circuit, 100 Atomic oscillator

Claims (10)

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 coherent light source that emits each of the resonance lights;
A light guiding means arranged on the emission side of the coherent light source and guiding each resonance light to two optical paths;
A gas cell disposed on the exit side of each optical path guided by the light guide means and enclosing gaseous metal atoms, and allowing the resonant light guided by the light guide means to pass through the metal atom gas;
A plurality of light detection means for detecting each transmitted light that has passed through the gas cell;
An optical system of an atomic oscillator characterized by comprising:   The light guiding means branches the resonance light emitted from the coherent light source into two optical paths, guides one resonance light to the gas cell, and passes the other resonance light, and the branching means branches the resonance light. The optical system of the atomic oscillator according to claim 1, further comprising: an optical path changing unit that guides the other resonance light to the gas cell.   The optical system of the atomic oscillator according to claim 1 or 2, wherein the plurality of light detection means and the coherent light source are juxtaposed on the same side of the same substrate.   The optical system of the atomic oscillator according to any one of claims 1 to 3, further comprising optical path changing means for guiding each transmitted light that has passed through the gas cell to the plurality of light detecting means.   5. The atomic oscillator optical system according to claim 1, further comprising a synthesis circuit that synthesizes output signals from the plurality of light detection means.   6. The optical system of an atomic oscillator according to claim 1, wherein the branching unit is a beam splitter or a half mirror.   The optical system for an atomic oscillator according to claim 1, wherein the coherent light is laser light.   2. The optical system of an atomic oscillator according to claim 1, wherein the gaseous metal atom is rubidium or cesium.   9. A passive optical element that collects light emitted from the coherent light source and corrects the light into parallel light is disposed between the coherent light source and the light guide unit. The optical system of the atomic oscillator according to one item.   An atomic oscillator comprising the optical system according to claim 1.

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