JP2015082763A - Optical module and atomic oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical module that can increase stability of an atomic oscillator by controlling a polarization direction of polarized light that is emitted from a surface emitting laser and incident to a quarter-wave plate, with respect to the quarter-wave plate.SOLUTION: An optical module 100 includes: a surface emitting laser 10 that emits light; a polarization control unit 20 which includes a reflective polarizing element 22 irradiated with light emitted from the surface emitting laser 10, and a quarter-wave plate 24 that is disposed to set a fast axis rotated by 45 degrees from the polarization transmission axis of the polarizing element 22 and is irradiated with light transmitted through the polarizing element 22; a gas cell 30 that is filled with an alkaline metal gas and is irradiated with the light transmitted through the quarter-wave plate 24; a first photodetector 40 that detects intensity of the light transmitted through the gas cell 30; a second photodetector 50 that detects intensity of the light reflected by the polarizing element 22; a drive unit 60 that rotates the polarization control unit 20; and a control unit 70 that operates the drive unit 60 in accordance with luminous energy detected by the second photodetector 50.

Description

  The present invention relates to an optical module and an atomic oscillator.

In recent years, CPT (Coherent Population), one of the quantum interference effects
Atomic oscillators using trapping have been proposed, and miniaturization of devices and low power consumption are expected. An atomic oscillator using CPT uses an electromagnetically induced transmission phenomenon (EIT phenomenon: Electromagnetically Induced Transparency) in which absorption of coherent light stops when an alkali metal atom is irradiated with coherent light having two different wavelengths (frequencies). It is an oscillator.

For example, in Patent Document 1, in order to increase the probability of occurrence of the EIT phenomenon, resonant light emitted from a light source is converted into circularly polarized light by a λ / 4 plate, and atoms irradiated to a gas cell in which alkali metal atoms are enclosed are disclosed. An oscillator is disclosed. By irradiating the gas cell with circularly polarized light, an interaction occurs between the alkali metal atom and the circularly polarized light, and the probability that the alkali metal atom exists at the ground level of the magnetic quantum number m _F = 0 is increased. be able to. Thereby, the expression probability of the EIT phenomenon can be increased.

JP 2013-98606 A

  The light generated by the surface emitting laser has coherence and is suitable for obtaining a quantum interference effect. A surface emitting laser generally emits polarized light (polarized light).

  However, the polarization direction of the polarized light emitted from the surface emitting laser changes with time, for example. Therefore, when a surface emitting laser is applied as the light source of the atomic oscillator of Patent Document 1, the polarization direction of the polarized light with respect to the λ / 4 plate changes due to the change in the polarization direction of the polarized light emitted by the surface emitting laser, and λ / 4 The ratio of the circularly polarized component of the light that has passed through the plate changes (for example, it is converted into elliptically polarized light without being converted into circularly polarized light by the λ / 4 plate). Therefore, when the surface emitting laser is applied as the light source of the atomic oscillator of Patent Document 1, the ratio of the circularly polarized component of the light irradiated to the gas cell changes with time, and the stability of the atomic oscillator is lowered. is there.

  One of the objects according to some aspects of the present invention is to control the polarization direction of the polarized light emitted from the surface emitting laser and incident on the λ / 4 plate with respect to the λ / 4 plate, thereby improving the stability of the atomic oscillator. The object is to provide an optical module that can be enhanced. Another object of some aspects of the present invention is to provide an atomic oscillator including the optical module.

The optical module according to the present invention is
An optical module of an atomic oscillator,
A surface emitting laser that emits light;
A reflection type polarizing element irradiated with light emitted from the surface emitting laser, and a fast axis rotated by 45 degrees with respect to the polarization transmission axis of the polarizing element, and the light transmitted through the polarizing element A polarization controller having a λ / 4 plate to be irradiated;
A gas cell filled with an alkali metal gas and irradiated with light transmitted through the λ / 4 plate;
A first light detector for detecting the intensity of light transmitted through the gas cell;
A second light detection unit for detecting the intensity of light reflected by the polarizing element;
A driving unit for rotating the polarization control unit;
A control unit that operates the driving unit in accordance with the amount of light detected by the second light detection unit;
including.

  In such an optical module, the light emitted from the surface emitting laser is incident on the λ / 4 plate as the first polarized light that is separated by the polarizing element and polarized in the first direction. Therefore, the polarization direction of the polarized light incident on the λ / 4 plate is constant with respect to the λ / 4 plate (with respect to the fast axis of the λ / 4 plate) even if the polarization direction of the polarized light in the surface emitting laser changes. Direction inclined by 45 degrees). Thereby, for example, it is possible to prevent the ratio of the circularly polarized component of the light irradiated to the gas cell from changing due to the change in the polarization direction of the polarized light with time in the surface emitting laser.

  Furthermore, in such an optical module, even if the polarization direction of polarized light in the surface emitting laser changes, the control unit operates the drive unit to rotate the polarization control unit, thereby the amount of light irradiated to the gas cell. Can be made constant.

  Therefore, in such an optical module, it is possible to prevent a change in the ratio of the circularly polarized component of the light irradiated to the gas cell and the amount of light due to the change in the polarization direction of the polarized light in the surface emitting laser. Therefore, the stability of the atomic oscillator can be improved.

In the optical module according to the present invention,
The control unit may drive the driving unit so that the amount of light detected by the second light detection unit is minimized.

  In such an optical module, the amount of the first polarized light can be made constant, so that the amount of light applied to the gas cell can be made constant.

In the optical module according to the present invention,
The polarizing element may be a wire grid polarizer.

  In such an optical module, the polarizing element reflects the second polarized light whose electric field vibrates in the direction of the central axis of the wire of the polarizing element, and the first polarized light whose electric field vibrates in a direction orthogonal to the direction of the central axis. Can be transmitted.

In the optical module according to the present invention,
The polarizing element may be provided on the optical axis such that the normal line of the incident surface is inclined with respect to the optical axis of the light emitted from the surface emitting laser.

  In such an optical module, the second polarized light reflected by the polarizing element can be advanced in a direction inclined with respect to the optical axis.

In the optical module according to the present invention,
The second light detection unit has a light receiving surface that receives light reflected by the polarizing element,
The light receiving surface is annular when viewed from the direction of the optical axis of the light emitted from the surface emitting laser,
The optical axis may be located inside the light receiving surface when viewed from the direction of the optical axis of the light emitted from the surface emitting laser.

  In such an optical module, the second light detection unit can receive the second polarized light traveling in the direction inclined with respect to the optical axis.

In the optical module according to the present invention,
The light receiving surface may be provided with an opening through which light emitted from the surface emitting laser is transmitted.

  In such an optical module, the distance between the polarizing element and the light receiving surface can be reduced. Therefore, in such an optical module, the second polarized light reflected by the polarizing element can be received more reliably.

In the optical module according to the present invention,
The surface emitting laser may emit polarized light.

The atomic oscillator according to the present invention is
The optical module which concerns on this invention is included.

  Since such an atomic oscillator includes the optical module according to the present invention, the stability of the apparatus can be improved.

The block diagram which shows the atomic oscillator containing the optical module which concerns on this embodiment. The figure which shows the frequency spectrum of resonant light. The figure which shows the relationship of the (LAMBDA) type | mold 3 level model of an alkali metal atom, a 1st sideband, and a 2nd sideband. The figure which shows typically the polarization control part and drive part of the optical module which concern on this embodiment. The figure which shows typically the surface emitting laser and 2nd light detection part of the optical module which concern on this embodiment. Sectional drawing which shows typically the surface emitting laser and 2nd light detection part of the optical module which concern on this embodiment. Sectional drawing which shows typically the surface emitting laser and 2nd light detection part of the optical module which concern on the 1st modification of this embodiment. The block diagram which shows the atomic oscillator containing the optical module which concerns on the 2nd modification of this embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The embodiments described below do not unduly limit the contents of the present invention described in the claims. In addition, not all of the configurations described below are essential constituent requirements of the present invention.

1. Optical module 1.1. Configuration First, an optical module according to the present embodiment will be described with reference to the drawings. Here, an example in which the optical module according to the present embodiment is applied to an atomic oscillator will be described. FIG. 1 is a block diagram showing an atomic oscillator 1 including an optical module 100 according to this embodiment.

  As shown in FIG. 1, the atomic oscillator 1 includes an optical module 100, a center wavelength control unit 2, and a high frequency control unit 4.

  As shown in FIG. 1, the optical module 100 includes a surface emitting laser 10, a polarization control unit 20 having a reflective polarizing element 22 and a λ / 4 plate 24, a gas cell 30, a first light detection unit 40, The second light detection unit 50, the drive unit 60, and the control unit 70 are included.

  The surface emitting laser 10 is, for example, a vertical cavity surface emitting laser (VCSEL) in which a resonator is formed perpendicular to a semiconductor substrate.

  The surface emitting laser 10 emits polarized light. Here, the polarized light includes a case of linearly polarized light and a case of elliptically polarized light that can be regarded as substantially linearly polarized light. Linearly polarized light refers to light in which the direction of vibration of the electric field of light is within one plane. In addition, elliptically polarized light that can be regarded as substantially linearly polarized light means that the length of the major axis of the elliptically polarized light is sufficiently longer than the length of the minor axis of the elliptically polarized light. For example, it is elliptically polarized light in which the ratio of the major axis length a to the minor axis length b satisfies the relationship of a / b ≧ 10. Here, the major axis of elliptically polarized light refers to the major axis of the ellipse drawn by the tip of the vibration vector in elliptically polarized light in which the tip of the vibration vector of the electric field of light moves elliptically. The minor axis of elliptically polarized light refers to the minor axis of the ellipse. In FIG. 1, polarized light (linearly polarized light) is indicated by a solid white arrow, and light (circularly polarized light) in which the tip of the vibration vector of the electric field makes a circular motion is indicated by a dashed white arrow. Yes.

  In the surface emitting laser 10, the polarization direction of polarized light (electric field vibration direction in linearly polarized light) changes with time. That is, in the surface emitting laser 10, the polarization plane rotates with time. Here, the polarization plane is a plane including the traveling direction of light and the vibration direction of the electric field. In elliptically polarized light that can be regarded as substantially linearly polarized light, the polarization direction of polarized light can refer to the direction of the major axis of elliptically polarized light.

FIG. 2 is a diagram illustrating a frequency spectrum of light emitted from the surface emitting laser 10. FIG. 3 is a diagram showing the relationship between the Λ-type three-level model of alkali metal atoms and the first sideband wave W1 and the second sideband wave W2. The light emitted from the surface emitting laser 10 includes a fundamental wave F having a center frequency f ₀ (= c / λ _{0, where} c is the speed of light and λ ₀ is the center wavelength of the laser light), as shown in FIG. It includes a first sideband W1 having a frequency f ₁ to the upper sideband with respect to the center frequency f _0, and the second sideband wave W2 having a frequency f ₂ to the lower sideband with respect to the center frequency f _0, the. Frequency _{f 1} of the first sideband W1 _{is f 1 = f 0 + f m} , the frequency _{f 2} of the second sideband wave W2 _is f _{2 =} f 0 -f _m.

As shown in FIG. 3, the energy difference between the frequency f ₁ of the first sideband wave W1 frequency difference between the frequency f ₂ of the second sideband wave W2 is the ground level GL1 and ground level GL2 alkali metal atom ΔE _This corresponds to the frequency corresponding to ₁₂ . Therefore, the alkali metal atom is, causing the first sideband W1 having a frequency _{f 1,} a second sideband wave W2 having a frequency _{f 2,} the EIT phenomenon by.

As shown in FIG. 1, the light emitted from the surface emitting laser 10 enters the polarizing element 22 (the light is irradiated). The polarizing element 22 converts the light emitted from the surface emitting laser 10 into a first polarized light L1 that is polarized in a first direction and a second polarized light L2 that is polarized in a second direction orthogonal to the first direction. To separate. Then, the polarizing element 22 transmits the first polarized light L1 and reflects the second polarized light L2. Here, the first polarized light L1 polarized in the first direction refers to linearly polarized light whose electric field vibration direction is the first direction. Similarly, the second polarized light L2 polarized in the second direction refers to linearly polarized light whose electric field vibration direction is the second direction.

  For example, the polarizing element 22 separates the light emitted from the surface emitting laser 10 into the first polarized light L1 and the second polarized light L2, transmits the first polarized light L1, and reflects the second polarized light L2. It is a wire grid polarizer. That is, the polarizing element 22 transmits the component polarized in the first direction and reflects the component polarized in the second direction of the light emitted from the surface emitting laser 10. Below, the polarizing element 22 is demonstrated as a wire grid polarizing plate. FIG. 4 is a diagram schematically illustrating the polarization control unit 20 and the driving unit 60.

  As illustrated in FIG. 4, the polarizing element 22 includes a plurality of wires 23. The material of the wire 23 is, for example, gold, silver, copper, platinum or the like. The plurality of wires 23 extend in parallel to each other. That is, in the plurality of wires 23, the central axes 23c passing through the center of one end of the wire 23 and the center of the other end are parallel to each other. The second direction described above is the extending direction of the wire 23 (the direction of the central axis 23c). The first direction described above is a direction orthogonal to the extending direction of the wire 23.

  In the polarizing element 22, when light (electromagnetic wave) whose electric field vibrates in the same direction as the extending direction of the wire 23, electrons on the wire 23 move by the incident electromagnetic wave. And the electron of the wire 23 radiates | emits electromagnetic waves so that the electromagnetic waves which permeate | transmit may be canceled. As a result, the electromagnetic wave of the component polarized in the same direction (second direction) as the extending direction of the wire 23 cannot pass through the polarizing element 22, and the polarizing element 22 is a component polarized in the second direction. The electromagnetic wave (second polarized light L2) is radiated as a reflected wave.

  On the other hand, the electrons of the wire 23 cannot move in the direction orthogonal to the extending direction of the wire 23. Therefore, the electromagnetic wave of the component polarized in the direction orthogonal to the extending direction of the wire 23 (first direction) can pass through the polarizing element 22, and the polarizing element 22 has the component polarized in the first direction. Electromagnetic waves (first polarized light L1) are radiated as transmitted waves. The axis orthogonal to the central axis 23 c of the wire 23 is the polarization transmission axis 22 t of the polarizing element 22.

  As shown in FIG. 1, the polarizing element 22 is provided on the optical axis A such that the normal line N of the incident surface 21 is inclined with respect to the optical axis A of the light emitted from the surface emitting laser 10. As a result, the second polarized light L2 reflected by the polarizing element 22 travels in a direction inclined with respect to the optical axis A. Here, the optical axis A is an axis passing through the center of the spread of the light emitted from the surface emitting laser 10. The incident surface 21 of the polarizing element 22 is a plane including the central axes 23 c of the plurality of wires 23.

  In the optical module 100, the light emitted from the surface emitting laser 10 is separated into the first polarized light L1 and the second polarized light L2 by the polarizing element 22, thereby being divided into two paths (path R1 and path R2). Divided. The path R <b> 1 is a light path from being separated by the polarizing element 22 to being detected by the first light detection unit 40 through the λ / 4 plate 24 and the gas cell 30. The path R1 is a light path for operating the atomic oscillator 1 as an oscillator using the EIT phenomenon. The path R <b> 2 is a light path from being separated by the polarizing element 22 to being detected by the second light detection unit 50. The path R2 is a light path for monitoring the amount of the second polarized light L2 (first polarized light L1).

The first polarized light L1 transmitted through the polarizing element 22 is incident on the λ / 4 plate 24 (irradiated with the first polarized light L1). The λ / 4 plate 24 is a wave plate that gives an optical path difference of ¼ wavelength (90 ° phase difference) between linearly polarized light components orthogonal to each other. The λ / 4 plate 24 converts the first polarized light L1 (linearly polarized light) into circularly polarized light. As shown in FIG. 4, the λ / 4 plate 24 is provided such that the fast axis 24 f is at an angle of 45 degrees with respect to the polarization transmission axis 22 t of the polarizing element 22. That is, λ
The / 4 plate 24 is provided with a fast axis 24f rotated by 45 degrees with respect to the polarization transmission axis 22t. Thereby, the λ / 4 plate 24 can convert the first polarized light L1 into circularly polarized light. Here, the fast axis (high-speed axis) means an axis having a small refractive index in the λ / 4 plate, and a slow axis (slow axis, axis having a large refractive index) of the λ / 4 plate. Are orthogonal axes.

  The λ / 4 plate 24 may be provided apart from the polarizing element 22, but it is preferable that the λ / 4 plate 24 be provided bonded to the polarizing element 22. Thereby, the polarization control unit 20 can be reduced in size, and the control of the driving unit 60 that rotates the polarization control unit 20 can be simplified. In the example shown in FIG. 1, the normal line of the incident surface 25 of the λ / 4 plate 24 is inclined with respect to the optical axis A. As the λ / 4 plate 24, for example, a crystal plate or a mica plate can be used.

  The gas cell 30 is a container in which gaseous alkali metal atoms (sodium atoms, rubidium atoms, cesium atoms, etc.) are sealed. When the gas cell 30 is irradiated with two light waves having a frequency (wavelength) corresponding to the energy difference between the two ground levels of the alkali metal atoms, the alkali metal atoms cause an EIT phenomenon. For example, if the alkali metal atom is a cesium atom, the frequency corresponding to the energy difference between the ground level GL1 and the ground level GL2 in the D1 line is 9.19263... GHz, so the frequency difference is 9.19263. When two light waves of GHz are irradiated, an EIT phenomenon occurs.

  The gas cell 30 is irradiated with light (circularly polarized light) transmitted through the λ / 4 plate 24. In the optical module 100, the light emitted from the surface emitting laser 10 is converted into circularly polarized light by the λ / 4 plate 24 through the polarizing element 22 and irradiated to the gas cell 30. Thereby, the expression probability of the EIT phenomenon can be increased.

  The first light detection unit 40 detects the intensity of light transmitted through the alkali metal atoms enclosed in the gas cell 30. The first light detection unit 40 outputs a detection signal corresponding to the intensity of light transmitted through the alkali metal atom. For example, a photodiode is used as the first light detection unit 40.

  The second light detection unit 50 detects the intensity of the light reflected by the polarizing element 22 (second polarized light L2). Here, FIG. 5 is a diagram schematically showing a state in which the surface emitting laser 10 and the second light detection unit 50 are viewed from the direction of the optical axis A. 6 is a cross-sectional view taken along the line VI-VI in FIG. 5 schematically illustrating the surface emitting laser 10 and the second light detection unit 50.

  The second light detection unit 50 may be provided monolithically (on the same substrate) as the surface emitting laser 10. As shown in FIG. 6, the surface emitting laser 10 is provided on the upper surface of the second light detection unit 50 (the surface on the side in which the light emitted from the surface emitting laser 10 travels). A region where the surface emitting laser 10 is not provided (an exposed region) on the upper surface of the second light detection unit 50 is a light receiving surface 52 that receives the second polarized light L <b> 2 reflected by the polarizing element 22. . As shown in FIG. 5, the light receiving surface 52 has an annular shape (specifically, a donut shape) when viewed from the direction of the optical axis A, and the optical axis A is received from the direction of the optical axis A. It is located inside the surface 52.

  The second light detection unit 50 outputs a detection signal corresponding to the intensity of the second polarized light L2. For example, a photodiode is used as the second light detection unit 50. The detection signal of the second light detection unit 50 is output to the control unit 70.

The drive unit 60 rotates the polarization control unit 20 having the polarizing element 22 and the λ / 4 plate 24. Specifically, the driving unit 60 rotates the polarization control unit 20 around the normal line N of the incident surface 21 of the polarizing element 22. The polarizing element 22 and the λ / 4 plate 24 are bonded to each other, for example, and the polarizing element 22 and the λ / 4 plate 24 are rotated in a state where the fast axis 24f is inclined 45 degrees with respect to the polarization transmission axis 22t. . Here, the light quantity of the first polarized light L1 and the light quantity of the second polarized light L2 are determined by the separation ratio of the polarized lights L1 and L2 in the polarizing element 22. Therefore, the light amount of the first polarized light L1 (the light amount of the second polarized light L2) can be changed by the drive unit 60 rotating the polarization control unit 20 and rotating the polarization transmission axis 22t. In the case where the polarization controller 20 is rotated around the normal line N, the light receiving surface 52 is not necessarily provided in a ring shape.

  As shown in FIG. 4, the drive unit 60 includes a gear 62 that transmits power to the polarization control unit 20. The drive unit 60 rotates the gear 62 with a motor (not shown) to rotate the polarization control unit 20. Gear edges are provided at the edge of the polarization controller 20. That is, the polarization controller 20 constitutes a gear (spur gear). As the polarization controller 20 rotates, the polarization transmission axis 22t of the polarization element 22 and the fast axis 24f of the λ / 4 plate 24 rotate. The drive unit 60 is controlled by the control unit 70.

  The control unit 70 operates the driving unit 60 in accordance with the light amount detected by the second light detection unit 50. Specifically, when the light amount of the second polarized light L2 detected by the second light detection unit 50 changes, the control unit 70 returns the light amount of the second polarized light L2 before the change to the drive unit. 60 is operated. Thereby, since the light quantity of the 1st polarized light L1 can also be returned to the light quantity before changing, the quantity of the light irradiated to the gas cell 30 can be made constant.

  For example, the control unit 70 operates the driving unit 60 so that the light amount of the second polarized light L2 detected by the second light detection unit 50 becomes a predetermined light amount. The predetermined light amount is, for example, the light amount when the light amount of the second polarized light L2 is minimized. That is, the light amount of the second polarized light L2 when the light amount of the first polarized light L1 is maximized. The predetermined light amount is not limited to the light amount when the light amount of the second polarized light L2 is minimized, and can be set as appropriate.

  Specifically, the control unit 70 operates the driving unit 60 to rotate the polarization control unit 20 to monitor the light amount of the second polarized light L2 while changing the polarization transmission axis 22t and the fast axis 24f, and the second When the light amount of the polarized light L2 reaches a predetermined light amount, the operation of the drive unit 60 is stopped. Thereby, the light quantity of the 2nd polarized light L2 can be made into predetermined light quantity.

The center wavelength control unit 2 generates a drive current having a magnitude corresponding to the detection signal output from the first light detection unit 40, supplies the drive current to the surface emitting laser 10, and the center wavelength λ of light emitted from the surface emitting laser 10 Control ₀ . The center wavelength λ _{0 of the} light emitted from the surface emitting laser 10 is finely adjusted and stabilized by the feedback loop passing through the surface emitting laser 10, the gas cell 30, the first light detection unit 40, and the center wavelength control unit 2.

Based on the detection result output from the first light detection unit 40, the high-frequency control unit 4 determines that the difference in wavelength (frequency) between the first sideband W1 and the second sideband W2 is an alkali metal atom enclosed in the gas cell 30. The frequency is controlled to be equal to the frequency corresponding to the energy difference between the two ground levels. The high frequency control unit 4 generates a modulation signal having a modulation frequency f _m (see FIG. 2) corresponding to the detection result output by the first light detection unit 40.

Due to the feedback loop passing through the surface emitting laser 10, the gas cell 30, the first light detection unit 40, and the high frequency control unit 4, the frequency difference between the first sideband W1 and the second sideband W2 is two ground levels of alkali metal atoms. The feedback control is applied so as to match the frequency corresponding to the energy difference very accurately. As a result, the modulation frequency f _m is very since a stable frequency can be a modulation signal and the atomic oscillator 1 of the output signal (clock output).

1.2. Operation Next, the operation of the optical module 100 will be described with reference to FIG.

  The surface emitting laser 10 emits polarized light. The polarized light emitted from the surface emitting laser 10 enters the polarizing element 22. The light incident on the polarizing element 22 is separated into the first polarized light L1 and the second polarized light L2 whose directions of polarization are perpendicular to each other. As described above, the light emitted from the surface emitting laser 10 is separated into the first polarized light L1 and the second polarized light L2 by the polarizing element 22, and thus is divided into two paths (path R1 and path R2). . Hereinafter, each route R1, R2 will be described.

  First, the route R1 will be described.

  The first polarized light L 1 that has passed through the polarizing element 22 is incident on the λ / 4 plate 24. The first polarized light L 1 is converted into circularly polarized light by the λ / 4 plate 24 and enters the gas cell 30. Here, the light emitted from the surface emitting laser 10 is two light waves (first sideband wave W1 and second sideband wave W2) having a frequency (wavelength) corresponding to the energy difference between the two ground levels of the alkali metal atom. ), And an alkali metal atom causes an EIT phenomenon. The intensity of the light transmitted through the gas cell 30 is detected by the first light detection unit 40.

The center wavelength control unit 2 and the high frequency control unit 4 match the frequency difference between the first sideband wave W1 and the second sideband wave W2 very accurately with the frequency corresponding to the energy difference between the two ground levels of the alkali metal atom. Thus, feedback control is performed. In atomic oscillator 1, utilizing the EIT phenomenon, the frequency difference _f 1 -f ₂ between the first sideband wave W1 and the second sideband wave W2 corresponds to the energy difference Delta] E ₁₂ between the ground level GL1 and ground level GL2 By detecting and controlling a steep change in light absorption behavior when deviating from the frequency, a highly accurate oscillator can be produced.

  Next, the route R2 will be described.

  The second polarized light L <b> 2 reflected by the polarizing element 22 enters the second light detection unit 50. The second light detection unit 50 detects the intensity of the second polarized light L2.

  Here, the polarizing element 22 has a characteristic in which the separation ratio of the first polarized light L1 and the second polarized light L2 is different according to the inclination angle of the polarization direction of the incident polarized light with respect to the transmission axis 22t. Therefore, for example, when the polarization direction of the polarized light emitted from the surface emitting laser 10 changes, the separation ratio of the first polarized light L1 and the second polarized light L2 changes in the polarizing element 22. Thereby, the quantity of the light irradiated to the gas cell 30 will change.

  Therefore, when the light amount of the second polarized light L2 detected by the second light detection unit 50 is changed, the control unit 70 operates the drive unit 60 so as to return to the light amount of the second polarized light L2 before the change. Let Thereby, since the light quantity of the 1st polarized light L1 can also be returned to the light quantity before changing, the quantity of the light irradiated to the gas cell 30 can be made constant.

  An example of the processing of the control unit 70 will be specifically described.

  When the intensity of the detection signal output from the second light detection unit 50 changes from the set value, the control unit 70 operates the drive unit 60 to rotate the polarization control unit 20. Here, the set value is, for example, the intensity of the detection signal of the second light detection unit 50 when the light amount of the second polarized light L2 is minimized. By rotating the polarization control unit 20, the direction of the polarization transmission axis 22t changes. Thereby, the separation ratio of the polarized light L1 and L2 of the polarizing element 22 changes, and the light quantity of the second polarized light L2 changes.

The control unit 70 monitors the intensity of the detection signal output from the second light detection unit 50 (the amount of light of the second polarized light L2), and drives the drive unit 60 until the intensity of the detection signal reaches a set value (minimum value). Is operated to rotate the polarization controller 20. Then, the control unit 70 ends the process when the intensity of the detection signal reaches a set value (minimum value).

  In addition, when the control unit 70 rotates the polarization control unit 20 while monitoring the intensity of the detection signal output from the second light detection unit 50, the rotation of the polarization control unit 20 (for example, clockwise rotation) is accompanied. When the intensity of the detection signal increases, a process of rotating the polarization controller 20 in the reverse direction (for example, counterclockwise rotation) may be performed.

  In the above-described example, the polarizing element 22 has been described as a wire grid polarizing plate. However, the polarizing element 22 may be a polarizing beam splitter. In this case, the polarization element 22 may transmit p-polarized polarized light and reflect s-polarized polarized light, or may transmit s-polarized polarized light and reflect p-polarized polarized light. If the polarizing element 22 can transmit the first polarized light polarized in the first direction and reflect the second polarized light polarized in the second direction orthogonal to the first direction, the wire grid polarized light can be used. It is not limited to a plate or a polarizing beam splitter, and may be constituted by, for example, an alternating laminate of two layers having different refractive index anisotropies.

  The optical module 100 has the following features, for example.

  In the optical module 100, the surface emitting laser 10, the reflective polarizing element 22 irradiated with the light emitted from the surface emitting laser 10, and the fast axis 24 f rotate 45 degrees with respect to the polarization transmission axis 22 t of the polarizing element 22. And the polarization controller 20 having the λ / 4 plate 24 irradiated with the light transmitted through the polarizing element 22, and the light sealed with the alkali metal gas and transmitted through the λ / 4 plate 24. Gas cell 30, a first light detector 40 that detects the intensity of light transmitted through the gas cell 30, a second light detector 50 that detects the intensity of light reflected by the polarizing element 22, and a polarization controller And a control unit 70 that operates the drive unit 60 in accordance with the amount of light detected by the second light detection unit 50. In this manner, in the optical module 100, the light emitted from the surface emitting laser 10 is incident on the λ / 4 plate 24 as the first polarized light L1 that is separated by the polarizing element 22 and polarized in the first direction. Therefore, even if the polarization direction of the polarized light in the surface emitting laser 10 is changed, the polarization direction of the polarized light incident on the λ / 4 plate 24 is set to a fixed direction (with respect to the λ / 4 plate 24). (A direction inclined by 45 degrees with respect to the fast axis 24f). Thereby, for example, it is possible to prevent the ratio of the circularly polarized component of the light irradiated to the gas cell 30 from changing due to the polarization direction of the polarized light changing with time in the surface emitting laser 10.

  Further, in the optical module 100, even when the polarization direction of polarized light in the surface emitting laser 10 changes, the control unit 70 operates the driving unit 60 to rotate the polarization control unit 20, thereby irradiating the gas cell 30. The amount of light can be made constant.

  As described above, in the optical module 100, by controlling the polarization direction of the first polarized light L 1 emitted from the surface emitting laser 10 and incident on the λ / 4 plate 24 with respect to the λ / 4 plate 24, Due to the change in the polarization direction of the polarized light, it is possible to prevent the ratio of the circularly polarized component of the light irradiated to the gas cell 30 and the amount of light from changing. As a result, in the optical module 100, the stability of the atomic oscillator 1 can be improved.

  Further, in the optical module 100, the driving unit 60 does not rotate the surface emitting laser 10 but rotates the polarization control unit 20. Therefore, the optical module 100 can be reduced in size as compared with, for example, the case where the surface emitting laser 10 is rotated.

In the optical module 100, the control unit 70 drives the drive unit 60 so that the amount of light detected by the second light detection unit 50 is minimized. Thereby, since the light quantity of the 1st polarized light L1 can be made constant, the quantity of the light irradiated to the gas cell 30 can be made constant.

  In the optical module 100, the polarizing element 22 is a wire grid polarizing plate. Therefore, the polarizing element 22 reflects the second polarized light L2 whose electric field oscillates in the direction of the central axis 23c of the wire 23 of the polarizing element 22, and the first polarized light whose electric field oscillates in a direction orthogonal to the direction of the central axis 23c. The light L1 can be transmitted.

  In the optical module 100, the polarizing element 22 is provided on the optical axis A with the normal line N of the incident surface 21 inclined with respect to the optical axis A of the light emitted from the surface emitting laser 10. Therefore, the second polarized light L2 reflected by the polarizing element 22 can be advanced in a direction inclined with respect to the optical axis A.

  In the optical module 100, the light receiving surface 52 of the second light detection unit 50 is annular when viewed from the direction of the optical axis A, and the optical axis A is located inside the light receiving surface 52 when viewed from the direction of the optical axis A. positioned. Therefore, the second light detection unit 50 can receive the second polarized light L2 that travels in a direction inclined with respect to the optical axis A.

2. Modification 2.1. First Modification Next, an optical module according to a first modification of the present embodiment will be described with reference to the drawings. FIG. 7 is a diagram schematically showing the surface-emitting laser 10 and the second light detection unit 50 of the optical module 200 according to the first modification of the present embodiment, and corresponds to FIG.

  Hereinafter, in the optical module 200 according to the first modification of the present embodiment, members having the same functions as those of the optical module 100 according to the present embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. To do. The same applies to the optical module according to the second modification of the present embodiment shown below.

  In the optical module 100 described above, the surface emitting laser 10 is provided on the second light detection unit 50 as shown in FIG. On the other hand, in the optical module 200, as shown in FIG. 7, a second light detection unit 50 is provided above the surface emitting laser 10 (on the side in which the light emitted from the surface emitting laser 10 travels). Yes. The second light detection unit 50 is provided between the surface emitting laser 10 and the polarizing element 22.

  In the optical module 200, the light receiving surface 52 of the second light detection unit 50 is provided with an opening 54 that transmits light emitted from the surface emitting laser. The shape of the opening 54 when viewed from the direction of the optical axis A is, for example, a circle.

  In the optical module 200, the distance between the polarizing element 22 and the light receiving surface 52 can be reduced as compared with the optical module 100. Therefore, the optical module 200 can receive the second polarized light L2 reflected by the polarizing element 22 more reliably.

2.2. Second Modification Example Next, an optical module according to a second modification example of the present embodiment will be described with reference to the drawings. FIG. 8 is a block diagram showing an atomic oscillator 1 configured to include an optical module 300 according to a second modification of the present embodiment.

In the optical module 100 described above, the incident surface 25 of the λ / 4 plate 24 is inclined with respect to the optical axis A as shown in FIG. On the other hand, in the optical module 300, the incident surface 25 of the λ / 4 plate 24 is orthogonal to the optical axis A as shown in FIG. That is, the normal line (not shown) of the incident surface 25 is parallel to the optical axis A.

  In the optical module 300, the first polarized light L1 transmitted through the polarizing element 22 can be more reliably converted to circularly polarized light.

  The above-described embodiments and modifications are merely examples, and the present invention is not limited to these. For example, it is possible to appropriately combine each embodiment and each modification.

  In the above description, the case where the λ / 4 plate 24 has the fast axis 24f inclined by 45 degrees with respect to the polarization transmission axis 22t of the polarizing element 22 has been described, but the fast axis 24f of the λ / 4 plate 24 is The tilt angle may be in the range of 44 degrees to 46 degrees with respect to the polarization transmission axis 22t. That is, the angle formed by the polarization transmission axis 22t and the fast axis 24f may be in a range of 44 degrees to 46 degrees. Even in such a case, the light transmitted through the λ / 4 plate 24 can be made into circularly polarized light or elliptically polarized light that can be regarded as substantially circularly polarized light, and the probability of occurrence of the EIT phenomenon can be increased. Even in such a case, it is possible to prevent the ratio of the circularly polarized component of the light irradiated to the gas cell 30 from changing due to the change in the polarization direction of the polarized light in the surface emitting laser 10. Therefore, in the optical module 100, the stability of the atomic oscillator 1 can be improved.

  The present invention includes configurations that are substantially the same as the configurations described in the embodiments (for example, configurations that have the same functions, methods, and results, or configurations that have the same objects and effects). In addition, the invention includes a configuration in which a non-essential part of the configuration described in the embodiment is replaced. In addition, the present invention includes a configuration that exhibits the same operational effects as the configuration described in the embodiment or a configuration that can achieve the same object. Further, the invention includes a configuration in which a known technique is added to the configuration described in the embodiment.

DESCRIPTION OF SYMBOLS 1 ... Atomic oscillator, 2 ... Center wavelength control part, 4 ... High frequency control part, 10 ... Surface emitting laser, 20 ... Polarization control part, 21 ... Incident surface, 22 ... Polarizing element, 22f ... Polarization transmission axis, 23 ... Wire, 23c: central axis, 24: λ / 4 plate, 24f: fast axis, 25: incident surface, 30: gas cell, 40: first light detecting unit, 50: second light detecting unit, 52: light receiving surface, 54: aperture Part, 60 ... drive part, 62 ... gear, 70 ... control part, 100, 200, 300 ... optical module

Claims (8)

An optical module of an atomic oscillator,
A surface emitting laser that emits light;
A reflection type polarizing element irradiated with light emitted from the surface emitting laser, and a fast axis rotated by 45 degrees with respect to the polarization transmission axis of the polarizing element, and the light transmitted through the polarizing element A polarization controller having a λ / 4 plate to be irradiated;
A gas cell filled with an alkali metal gas and irradiated with light transmitted through the λ / 4 plate;
A first light detector for detecting the intensity of light transmitted through the gas cell;
A second light detection unit for detecting the intensity of light reflected by the polarizing element;
A driving unit for rotating the polarization control unit;
A control unit that operates the driving unit in accordance with the amount of light detected by the second light detection unit;
An optical module comprising:   The optical module according to claim 1, wherein the control unit drives the driving unit so that the amount of light detected by the second light detection unit is minimized.   The optical module according to claim 1, wherein the polarizing element is a wire grid polarizing plate.   4. The polarizing element according to claim 1, wherein the polarizing element is provided on the optical axis so that a normal line of an incident surface is inclined with respect to an optical axis of light emitted from the surface emitting laser. The optical module according to any one of the above. The second light detection unit has a light receiving surface that receives light reflected by the polarizing element,
The light receiving surface is annular when viewed from the direction of the optical axis of the light emitted from the surface emitting laser,
The said optical axis is located inside the said light-receiving surface seeing from the direction of the optical axis of the light inject | emitted from the said surface emitting laser, The any one of Claim 1 thru | or 4 characterized by the above-mentioned. The optical module as described.   The optical module according to claim 5, wherein an opening for transmitting light emitted from the surface emitting laser is provided on the light receiving surface.   The optical module according to claim 1, wherein the surface emitting laser emits polarized light.   An atomic oscillator comprising the optical module according to claim 1.