Atomic oscillator

An atomic oscillator includes a gas cell having alkali metal atoms sealed therein; alight source that irradiates the gas cell with light; and a light detecting unit that detects the quantity of light transmitted through the gas cell. The light source includes an optical oscillation layer having a first reflective layer, an active layer, and a second reflective layer laminated therein in this order, an electrical field absorption layer having a first semiconductor layer, a quantum well layer, and a second semiconductor layer laminated therein in this order, and a heat diffusion layer that is disposed between the optical oscillation layer and the electrical field absorption layer and has a higher thermal conductivity than that of the second reflective layer.

BACKGROUND

1. Technical Field

The present invention relates to an atomic oscillator.

2. Related Art

An atomic oscillators using transition energy of atoms as reference frequency is widely used as one of highest-precision oscillators in communication base stations or the like. Although there are several types as the atomic oscillator, a microwave double resonance type using a rubidium (Rb) lamp is most generally used.

In recent years, an atomic oscillator using a phenomenon called Coherent Population Trapping (CPT) that is one of quantum interference effects is suggested (for example, refer to JP-A-2015-62167), and reduced size and low power consumption of the atomic oscillator are expected compared with the related art. In the case of the CPT type, sidebands are used for development of a CPT phenomenon by superimposing a high-frequency signal using a coherent light source, such as a laser, as a light source. The CPT type atomic oscillator is an oscillator using an electromagnetically induced transparency (EIT) phenomenon in which if alkali metal atoms are irradiated with coherent light having two different kinds of wavelength (frequency), the absorption of the coherent light stops.

In order to develop the CPT phenomenon as the light source of the atomic oscillator, high-precision adjustment of the output wavelength of a laser element or the like is required. If an inflow current to the laser element or the like is changed, it is possible to adjust the output wavelength.

However, if the inflow current to the laser element is changed, the optical output of the laser element or the like varies simultaneously. Therefore, it is necessary to form a control loop of the atomic oscillator in consideration of this, and complicated control is required. Therefore, a light source that can control its output wavelength and optical output individually is needed.

SUMMARY

An advantage of some aspects of the invention is to provide an atomic oscillator that can control the output wavelength and optical output of a light source individually.

An atomic oscillator according to an aspect of the invention includes a gas cell having alkali metal atoms sealed therein; a light source that irradiates the gas cell with light; and a light detecting unit that detects the quantity of light transmitted through the gas cell. The light source includes an optical oscillation layer having a first reflective layer, an active layer, and a second reflective layer laminated therein in this order, an electrical field absorption layer having a first semiconductor layer, a quantum well layer, and a second semiconductor layer laminated therein in this order, and a heat diffusion layer that is disposed between the optical oscillation layer and the electrical field absorption layer and has a higher thermal conductivity than that of the second reflective layer.

In such an atomic oscillator, in a case where the central wavelength of light exited from the light source is changed by changing the quantity of a current to be flowed into the active layer, even if the optical output (the quantity of light) of the light exited from the light source shifts from a predetermined value, the optical output of the light exited from the light source can be returned to the predetermined value by changing a voltage to be applied to the electrical field absorption layer. Moreover, in such an atomic oscillator, even if the electrical field absorption layer (the quantum well layer) absorbs light to generate heat, this heat can be diffused to the outside via the heat diffusion layer, and this heat can be prevented from reaching the second reflective layer or the active layer. Accordingly, a temperature change in the light source caused by the heat generated in the electrical field absorption layer can be suppressed. Therefore, the central wavelength of the light source can be prevented from fluctuating with temperature, and the output wavelength (central wavelength) and the optical output of the light source can be individually (independently) controlled depending on the quantity of an inflow current to the active layer, and a voltage applied to the electrical field absorption layer.

In the atomic oscillator according to the aspect of the invention, the heat diffusion layer may be an i-type AlAs layer.

In such an atomic oscillator, the heat diffusion layer can be formed in a series of processes (by the same apparatus as an apparatus for forming the optical oscillation layer and the electrical field absorption layer) together with the optical oscillation layer and the electrical field absorption layer.

In the atomic oscillator according to the aspect of the invention, the heat diffusion layer may be an i-type GaAs layer.

In such an atomic oscillator, the heat diffusion layer can be formed in a series of processes together with the optical oscillation layer and the electrical field absorption layer.

The atomic oscillator according to the aspect of the invention may further include a contact layer provided between the heat diffusion layer and the first semiconductor layer, and a surface of the contact layer where the first semiconductor layer is disposed may be provided with an electrode for applying a voltage to the electrical field absorption layer.

In such an atomic oscillator, the contact resistance of the electrode can be reduced compared to a case where the electrode is in direct contact with the first semiconductor layer.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A preferred embodiment of the invention will be described below in detail with reference to the drawings. It should be noted that the embodiment to be described below does not unduly limit the contents of the invention set forth in the appended claims. Additionally, all the components to be describing below are not necessarily indispensable constituent elements of the invention.

First, an atomic oscillator according to the present embodiment will be described, referring to drawings.FIG. 1is a functional block diagram of an atomic oscillator1000according to the present embodiment.

The atomic oscillator1000, as illustrated inFIG. 1, includes the light source100, a gas cell102, light detecting means (light detecting unit)104, a optical output variable unit106, a central wavelength variable unit108, a high-frequency generating unit110, an absorption detecting unit112, an EIT detecting unit114, and a control unit120. The control unit120has an optical output control unit122, a central wavelength control unit124, and a high-frequency control unit126. The atomic oscillator1000causes an EIT phenomenon in alkali metal atoms using a resonance light pair (first light and second light) having two different frequency components.

The light source100generates the first light and the second light having mutually different frequencies, and irradiates alkali metal atoms sealed in the gas cell102with the first light and the second light. The detailed configuration of the light source100will be described below.

Here,FIG. 2is a view illustrating a frequency spectrum of resonance light.FIG. 3is a view illustrating a relationship between a Λ-type three-level model of alkali metal atoms, and a first sideband wave (first light) W1and a second sideband wave (second light) W2. The light L exited from the light source100includes a fundamental mode F having a central frequency f0(=c/λ0: c is the speed of light, and X0is the central wavelength of laser light), the first sideband wave W1having a frequency f1in an upper sideband with respect to the central frequency f0, and the second sideband wave W2having a frequency f2in a lower sideband with respect to the central frequency f0, which are illustrated inFIG. 2. The frequency f1of the first sideband wave W1is f1=f0+fm, and the frequency f2of the second sideband wave W2is f2=f0−fm.

As illustrated inFIG. 3, a frequency difference between the frequency f1of the first sideband wave W1and the frequency f2of the second sideband wave W2coincides with a frequency equivalent to an energy difference ΔE12of a ground level GL1and a ground level GL2of the alkali metal atoms. Therefore, the alkali metal atoms cause an EIT phenomenon due to the first sideband wave W1having the frequency f1and the second sideband wave W2having the frequency f2.

Here, the EIT phenomenon will be described. It is known that an interaction between alkali metal atoms and light can be explained with a Λ-type three-level system model. As illustrated inFIG. 3, the alkali metal atoms have two ground levels, and if the first sideband wave W1having a wavelength (frequency f1) equivalent to an energy difference between the ground level GL1and an excitation level, or the second sideband wave W2having a wavelength (frequency f2) equivalent to an energy difference between the ground level GL2and the excitation level radiates the alkali metal atoms respectively and independently, optical absorption is caused. On the contrary, as illustrated inFIG. 2if the alkali metal atoms are simultaneously irradiated with the first sideband wave W1and the second sideband wave W2in which a frequency difference f1- f2exactly coincides with the frequency equivalent to the energy difference ΔE12between the ground level GL1and the ground level GL2, a superposition state of the two ground levels, that is, a quantum interference state is brought about. As a result, a transparency phenomenon (EIT phenomenon) in which excitation to the excitation level stops and the first sideband wave W1and the second sideband wave W2are transmitted through the alkali metal atoms. A high-precision oscillator can be built by using this EIT phenomenon and detecting and controlling a steep change in optical absorption behavior when the frequency difference f1-f2between the first sideband wave W1and the second sideband wave W2deviates from the frequency equivalent to the energy difference ΔE12between the ground level GL1and the ground level GL2.

The gas cell102encloses gaseous alkali metal atoms (sodium atoms, rubidium atoms, cesium atoms, or the like) in a container. The cesium atoms are heated to, for example, about 80° C., and is turned into gas. If this gas cell102is irradiated with two light waves (the first light and the second light) having a frequency (wavelength) equivalent to an energy difference between two ground levels of the alkali metal atoms, the alkali metal atoms cause the EIT phenomenon. For example, if the alkali metal atoms are cesium atoms, a frequency equivalent to the energy difference between the ground level GL1and the ground level GL2in line D1is 9.19263 . . . GHz. Thus, if two light waves of which the frequency difference is 9.19263 . . . GHz, the EIT phenomenon is caused.

The light detecting unit104detects the quantity (intensity) of light of light (transmitted through the alkali metal atoms sealed in the gas cell102) transmitted through the gas cell102. The light detecting unit104outputs a detection signal according to the quantity of the light transmitted through the alkali metal atoms. As the light detecting unit104, for example, a photodiode is used.

On the basis of the signal from the optical output control unit122, the optical output variable unit106applies a voltage between the electrodes60and62(refer toFIG. 5to be described below) of the light source100, and changes the optical output (quantity of light) of the light source100. The optical output variable unit106may be configured to include the power source4(refer toFIG. 8to be described below) that applies a voltage between the electrodes60and62.

On the basis of a signal from the central wavelength control unit124, the central wavelength variable unit108applies a voltage between the electrodes30and32(refer toFIG. 5) of the light source100, allows a current to flow into the active layer22, and changes the central wavelength of the light L exited from the light source100. Accordingly, the central wavelength of a resonance light pair (the first light and the second light) included in the light L can be changed. The central wavelength variable unit108may be configured to include the power source2(refer toFIG. 8) that applies a voltage between electrodes30and32.

On the basis of a signal from the high-frequency control unit126, the high-frequency generating unit110supplies a high-frequency signal between the electrodes30and32of the light source100to generate a resonance light pair. The high-frequency generating unit110may be realized by a dedicated circuit.

The absorption detecting unit112, for example, detects a minimum value (the bottom of absorption) of the signal intensity of a detection signal output from the light detecting unit104when the central wavelength of the light L has been changed. The absorption detecting unit112may be realized by a dedicated circuit.

The EIT detecting unit114synchronously detect the detection signal output from the light detecting unit104, and detects the EIT phenomenon. The EIT detecting unit114may be realized by a dedicated circuit.

On the basis of an average (DC component) of the quantity of light of the detection signal output from the light detecting unit104, the optical output control unit122controls the optical output variable unit106, thereby controlling a voltage to be applied to the electrical field absorption layer59(refer to Fig,5) of the light source100(applied to the quantum well layer52) so as to change the quantity of absorbed light in the electrical field absorption layer59(in the quantum well layer52). Accordingly, the optical output control unit122can change the optical output (the quantity of light) of the light source100. The optical output control unit122may control the optical output variable unit106, on the basis of a moving average of the quantity of light of the detection signal. The optical output control unit122controls a voltage to be applied to the electrical field absorption layer59such that the optical output exited from the light source100becomes constant (for example, the DC component of the detection signal output from the light detecting unit104becomes constant). The optical output control unit122may be configured to include an auto power control (APC) circuit.

On the basis of a signal from the absorption detecting unit112, the central wavelength control unit124controls the central wavelength variable unit108, thereby controlling a current to be flowed into the active layer22(refer toFIG. 5) of the light source100so as to change the optical output (the quantity of light) and the wavelength (central wavelength) of the light L exited from the light source100.

The high-frequency control unit126inputs a signal to generate a high-frequency signal, to the high-frequency generating unit110, on the basis of a signal from the EIT detecting unit114.

It should be noted that the control unit120may be configured to be realized by a dedicated circuit so as to perform the above control. Additionally, the control unit120may be configured to function as, for example, a computer by a central processing unit (CPU) executing a control program stored in a storage, such as a read only memory (ROM) or a random access memory (RAM), so as to perform the above control.

Next, the operation of the atomic oscillator1000will be described. First, the initial operation when starting the atomic oscillator1000in a stopped state will be described.

The high-frequency control unit126inputs a signal to the high-frequency generating unit110, and inputs a high-frequency signal from the high-frequency generating unit110to the light source100. In this case, the frequency of the high-frequency signal is slightly shifted such that the EIT phenomenon does not occur. For example, in a case where cesium is used as the alkali metal atoms of the gas cell102, the frequency is shifted from the value of 4.596 . . . GHz.

Next, the central wavelength control unit124controls the central wavelength variable unit108to sweep the central wavelength of the light L. In this case, since the frequency of the high-frequency signal is set such that the EIT phenomenon does not occur, the EIT phenomenon does not occur. The absorption detecting unit112detects the minimum value (the bottom of absorption) of the intensity of a detection signal to be output in the light detecting unit104when the central wavelength of the light L has been swept. The absorption detecting unit112, for example, uses a point where a change in intensity of a detection signal with respect to the central wavelength of the light L, as the bottom of absorption.

If the absorption detecting unit112detects the bottom of absorption, the central wavelength control unit124controls the central wavelength variable unit108to fix (lock) the central wavelength. That is, the central wavelength control unit124fixes the central wavelength of the light L to a wavelength equivalent to the bottom of absorption.

Next, the optical output control unit122controls the optical output variable unit106to change the optical output of the light source100, on the basis of the DC component of the detection signal output from the light detecting unit104. Specifically, the optical output control unit122changes the optical output of the light source100such that the DC component of the detection signal has a predetermined value.

Next, the high-frequency control unit126controls the high-frequency generating unit110to match the frequency of the high-frequency signal with a frequency at which the EIT phenomenon occurs. Thereafter, the high-frequency control unit proceeds to a loop operation, and an EIT signal is detected by the EIT detecting unit114.

Next, the loop operation of the atomic oscillator1000will be described.

The EIT detecting unit114synchronously detects the detection signal output from the light detecting unit104, and the high-frequency control unit126performs control such that the frequency of the high-frequency signal generated from the high-frequency generating unit110becomes a frequency equivalent to half of ΔE12of the alkali metal atoms in the gas cell102, on the basis of the signal input from the EIT detecting unit114.

The absorption detecting unit112synchronously detects the detection signal output from the light detecting unit104, and the central wavelength control unit124controls the central wavelength variable unit108such that the central wavelength of the light L becomes a wavelength equivalent to the minimum value (the bottom of absorption) of the intensity of a detection signal to be output in the light detecting unit104, on the basis of the signal input from the absorption detecting unit112.

The optical output control unit122controls the optical output variable unit106on the basis of the DC component of the detection signal output from the light detecting unit104. Specifically, in a case where the DC component of the detection signal becomes smaller than a predetermined value, the optical output control unit122controls the optical output variable unit106such that the DC component of the detection signal has a predetermined value. Even if the central wavelength of the light L deviates from the wavelength equivalent to the bottom of absorption through the control of the optical output control unit122, the central wavelength of the light L can be matched with the wavelength equivalent to the bottom of absorption through the control of the central wavelength control unit124. Moreover, even if the DC component of the detection signal deviates from the predetermined value through the control of the central wavelength control unit124, the DC component of the detection signal can be returned to the predetermined value through the control of the optical output control unit122.

In the atomic oscillator1000, control may be performed such that the temperature (driving temperature) of the light source100becomes constant.

1.2. Light Source

First, the light source100of the atomic oscillator1000according to the present embodiment will be described, referring to drawings.FIG. 4is a plan view schematically illustrating a light source100according to the present embodiment.FIG. 5is a sectional view, taken along line V-V ofFIG. 4, schematically illustrating the light source100according to the present embodiment.FIG. 6is a sectional view, taken along line VI-VI ofFIG. 4, schematically illustrating the light source100according to the present embodiment.FIG. 7is a sectional view, taken along line VII-VII ofFIG. 4, schematically illustrating the light source100according to the present embodiment.FIG. 8is a circuit diagram for explaining the light source100according to the present embodiment.

The light source100, as illustrated inFIGS. 4 to 7, includes a substrate10, a first reflective layer20, an active layer22, a second reflective layer24, a current constriction layer26, a first electrode30, a second electrode32, a heat diffusion layer40, a first contact layer50, a first semiconductor layer51, a quantum well layer52, a second semiconductor layer53, a second contact layer54, a third electrode60, a fourth electrode62, and insulating layers70,72, and74.

The substrate10is, for example, a first conductivity type (for example, n-type) GaAs substrate.

The first reflective layer20is provided on the substrate10. The first reflective layer20is a first conductivity type semiconductor layer. The first reflective layer20is a distribution Bragg reflector (DBR) mirror in which a high refractive-index layer and a low refractive-index layer having a lower refractive index than the high refractive-index layer are alternately laminated. The high refractive-index layer is, for example, an n-type Al0.12Ga0.88As layer. The low refractive-index layer is, for example, an n-type Al0.9Ga0.1As layer. The number (the number of pairs) of lamination of the high refractive-index layer and the low refractive-index layer is, for example, 10 pairs or more to 50 pairs or less, and specifically, 40.5 pairs.

The active layer22is provided on the first reflective layer20. The active layer22has, for example, a multiplex quantum well (MQW) structure in which quantum well structures constituted of an i-type In0.06Ga0.94As layer and an i-type Al0.3Ga0.7As layer are superimposed on each other in three layers.

The second reflective layer24is provided on the active layer22. The second reflective layer is a second conductivity type (for example, p-type) semiconductor layer. The second reflective layer24is a distribution Bragg reflector (DBR) mirror in which a high refractive-index layer and a low refractive-index layer having a lower refractive index than the high refractive-index layer are alternately laminated. The high refractive-index layer is, for example, a p-type Al0.12Ga0.88As layer. The low refractive-index layer is, for example, a p-type A10.9Ga0.1As layer. The number (the number of pairs) of lamination of the high refractive-index layer and the low refractive-index layer is, for example, 3 pairs or more to 40 pairs or less, and specifically, 20 pairs.

The second reflective layer24, the active layer22, and the first reflective layer20constitute an optical oscillation layer29. The optical oscillation layer29is a laminated body in which the first reflective layer20, the active layer22, and the second reflective layer24are laminated in this order. The optical oscillation layer29constitutes a perpendicular resonator type pin diode. As illustrated inFIG. 8, if a forward voltage of the pin diode3is applied between the electrodes30and32electrically connected to the power source2, the re-coupling of an electron and a positive hole occurs in the active layer22, and light emission occurs. The light generated in the active layer22is bounced back and forth (multi-reflected) between the first reflective layer20and the second reflective layer24, induced emission occurs in that case, and intensity is amplified. Then, if optical gain exceeds optical loss, laser oscillation occurs, and laser light is exited in a vertical direction (in a laminated direction of the active layer22and the first reflective layer20) from an upper surface of the second contact layer54. The wavelength of this laser light is, for example, 800 nm or more to 950 nm or less, and specifically, 852 nm or 895 nm.

The current constriction layer26is provided between the first reflective layer20and the second reflective layer24. In an example illustrated inFIG. 5, the current constriction layer26is provided on the active layer22. The current constriction layer26is an insulating layer in which an opening is formed, and this opening is provided with the second reflective layer24. A planar shape (a shape as seen from the laminated direction of the active layer22and the first reflective layer20) of the current constriction layer26is ring-shaped. The current constriction layer26can prevent a current to be flowed into a perpendicular resonator by the electrodes30and32from widening in a planar direction (a direction orthogonal to the laminated direction of the active layer22and the first reflective layer20).

The current constriction layer26, the second reflective layer24, the active layer22, and the first reflective layer20constitute a columnar section28. A planar shape of the columnar section28is, for example, circular.

The first electrode30is provided under the substrate10. The first electrode30is provided, for example, on a lower surface of a layer (a substrate10in the example illustrated inFIG. 5) that comes in ohmic contact with the first electrode30. The first electrode30is electrically connected to the first reflective layer20. As the first electrode30, for example, an electrode in which a Cr layer, an AuGe layer, an Ni layer, and an Au layer are laminated in this order from the substrate10side is used. The first electrode30is one electrode to flow a current into the active layer22.

The second electrode32is disposed on the second reflective layer24. The second electrode32is electrically connected to the second reflective layer24. As the second electrode32, for example, an electrode in which a Cr layer, a Pt layer, a Ti layer, a Pt layer, and an Au layer are laminated in this order from the second reflective layer24side is used. The second electrode32is the other electrode to flow a current into the active layer22.

The second electrode32, as illustrated inFIG. 4, has a contacting part32a, a lead-out part32b, and a pad part32c. The contacting part32ais in contact with the second reflective layer24. In the example illustrated inFIG. 4, a planar shape of the contacting part32ais a shape obtained by cutting off a portion of a ring shape. A planar shape of the lead-out part32bis, for example, linear. The lead-out part32bconnects the contacting part32aand the pad part32ctogether. The lead-out part32band the pad part32care provided on an insulating layer70. The pad part32cis connected to external wiring or the like serving as an electrode pad. In the illustrated example, a planar shape of the pad part32cis circular. The insulating layer70is provided so as to surround the columnar section28in contact with, for example, a side surface of the columnar section28. The insulating layer70is, for example, a polyimide layer or a silicon oxide layer.

The second electrode32, the second reflective layer24, the active layer22, the first reflective layer20, and the first electrode30constitute a vertical cavity surface emitting laser (VCSEL).

The heat diffusion layer40is disposed on the second reflective layer24. The heat diffusion layer40is disposed between the second reflective layer24and the first contact layer50(between the optical oscillation layer29and an electrical field absorption layer59). A planar shape of the heat diffusion layer40is, for example, circular. In the plan view (as seen in the laminated direction of the active layer and the first reflective layer20), the area of heat diffusion layer40is smaller than the area of an upper surface of the second reflective layer24, and heat diffusion layer40is provided inside an outer edge of the second reflective layer24. The thermal conductivity of the heat diffusion layer40is higher than the thermal conductivity of the second reflective layer24. Specifically, the thermal conductivity of the heat diffusion layer40is higher than the thermal conductivity of the high refractive-index layer that constitutes the second reflective layer24, and is lower than the thermal conductivity of the low refractive-index layer that constitutes the second reflective layer24. The heat diffusion layer40is, for example, an i-type AlAs layer, or an i-type GaAs layer. For example, the thermal conductivity of i-type GaAs is 0.55 W/(cm·K).

The first contact layer50is disposed on the heat diffusion layer40. The first contact layer50is provided between the heat diffusion layer40and the first semiconductor layer51. In the example illustrated inFIG. 4, a planar shape of the first contact layer50is circular. In the plan view, the area of the first contact layer50and the area of the heat diffusion layer40are, for example, the same as each other. The first contact layer50is, for example, a p-type GaAs layer.

The first semiconductor layer51is provided on the first contact layer50. A planar shape of the first semiconductor layer51is, for example, circular. In the plan view, the area of the first semiconductor layer51is smaller than the area of an upper surface of the first contact layer50, and the first semiconductor layer51is provided inside an outer edge of the first contact layer50. The first semiconductor layer51is, for example, a p-type Al0.3Ga0.7As layer.

The quantum well layer52is provided on the first semiconductor layer51. The quantum well layer52has a multiplex quantum well (MQW) structure in which three quantum well structures constituted of an i-type GaAs well layer and an i-type Al0.3Ga0.7As barrier layer are superimposed on each other.

The second semiconductor layer53is provided on the quantum well layer52. The second semiconductor layer53is, for example, an n-type Al0.3Ga0.7As layer. The semiconductor layers51and53are layers that have a greater band gap and a smaller refractive index than the quantum well layer52.

The second semiconductor layer53, the quantum well layer52, and the first semiconductor layer51constitute the electrical field absorption layer59. The electrical field absorption layer59is a laminated body in which the first semiconductor layer51, the quantum well layer52, and the second semiconductor layer53are laminated in this order. The electrical field absorption layer59constitutes the pin diode (pin photodiode). As illustrated inFIG. 8, if a backward voltage of the pin diode5is applied between the electrodes60and62electrically connected to a power source4, light can be absorbed in the quantum well layer52. Accordingly, light (laser light generated in a vertical cavity surface emitting laser) generated in the optical oscillation layer29can be absorbed. The quantity of absorption of light in the quantum well layer52can be adjusted depending on the magnitude of a voltage to be applied to the electrical field absorption layer59.

It should be noted that the pin photodiode configured to include the electrical field absorption layer59may take out or may not take out a photocurrent caused by an electron and a positive hole that are excited by absorbing light in the quantum well layer52, to an external circuit as a signal.

Here, if a voltage is applied to the electrical field absorption layer59, the absorption wavelength (absorption peak wavelength) of the electrical field absorption layer59shifts to a longer wavelength side due to the quantum confined Stark effect compared to the case where no voltage is applied. Therefore, in a state where no voltage is applied to the electrical field absorption layer59, the absorption peak wavelength in the electrical field absorption layer59is set closer to a shorter wavelength side than the oscillation wavelength in the optical oscillation layer29(vertical cavity surface emitting laser). Then, by applying a voltage to the electrical field absorption layer59, the absorption peak wavelength of the electrical field absorption layer59is shifted, and the light generated in the optical oscillation layer29is absorbed.

For example, in a case where the oscillation wavelength in the optical oscillation layer29is 852 nm, the absorption peak wavelength (quantum well layer52) of the electrical field absorption layer59reaches 800 nm in a state where no voltage is applied. In this case, as the quantum well layer52, there is used quantum well layer having a multiplex quantum well (MQW) structure in which three quantum well structures constituted of a GaAs well layer with a thickness of 4 nm and an Al0.3Ga0.7As barrier layer with a thickness of 10 nm are superimposed on each other.

The second contact layer54is provided on the second semiconductor layer53. In the example illustrated inFIG. 4, a planar shape of the second contact layer54is circular. A material for the second contact layer54is, for example, a n-type GaAs layer.

The third electrode layer60is provided on the first contact layer50. The third electrode60is provided on a surface where the first semiconductor layer51of the first contact layer50is disposed. The third electrode60is electrically connected to the first semiconductor layer51. For example, the third electrode60comes in ohmic contact with the first contact layer50. A material for the third electrode60is, for example, the same as a material for the second electrode32. The third electrode60is one electrode for applying a voltage to the electrical field absorption layer59.

The third electrode60, as illustrated inFIG. 4, has a contacting part60aand a pad part60c. The contacting part60ais in contact with the first contact layer50. In the example illustrated inFIG. 4, the contacting part60ahas a shape obtained by cutting off a part of a ring shape in the plan view, and is provided so as to surround the second contact layer54.

The pad part60cof the third electrode60is connected to, for example, the contacting part60a. The pad part60chas a first portion601and a second portion602. In the plan view, the area of the first portion601is greater than the area of the pad part32cof the second electrode32and the area of the pad part62cof the fourth electrode62. In the plan view, the area of the second portion602is greater than the area of the pad part32cand the area of the pad part62c. In an illustrated example, planar shapes of the first portion601and the second portion602are substantially quadrangular. In the plan view, the first portion601and the second portion602may be provided point-symmetrically with respect to the center of the second contact layer54.

The pad part60cof the third electrode60is provided on the insulating layer72. The pad part60cis connected to external wiring or the like serving as an electrode pad. The insulating layer72, as illustrated inFIG. 7, is provided on the insulating layer70in contact with side surfaces of the heat diffusion layer40and the first contact layer50. A material for the insulating layer72is, for example, the same as a material for the insulating layer70.

The fourth electrode62is provided on the second contact layer54. The fourth electrode62is electrically connected to the second semiconductor layer53. For example, the fourth electrode62comes in ohmic contact with the second contact layer54. A material for the fourth electrode62is, for example, the same as a material for the first electrode30. The fourth electrode62is the other electrode for applying a voltage to the electrical field absorption layer59.

The fourth electrode62, as illustrated inFIG. 4, has a contacting part62a, a lead-out part62b, and a pad part62c. The contacting part62ais in contact with the second contact layer54. In the example illustrated inFIG. 4, a planar shape of the contacting part62ais ring-shaped. A planar shape of the lead-out part62bis, for example, linear. The lead-out part62bconnects the contacting part62aand the pad part62ctogether. The lead-out part62band the pad part62care provided on an insulating layer74. The pad part62cis connected to external wiring or the like serving as an electrode pad. In the illustrated example, a planar shape of the pad part62cis circular. The insulating layer74, as illustrated inFIG. 6, is provided on the insulating layer72in contact with side surfaces of the electrical field absorption layer59and the second contact layer54. A material for the insulating layer74is, for example, the same as a material for the insulating layer70.

Although not illustrated, the insulating layer72may be provided so as to surround the heat diffusion layer40and the first contact layer50, or the insulating layer74may be provided so as to surround the electrical field absorption layer59and the second contact layer54.

Additionally, although the AlGaAs-based light source has been described above, for example, a GaInP-based, ZnSSe-based, InGaN-based, AlGaN-based, InGaAs-based, GaInNAs-based, or GaAsSb-based semiconductor material may be used for the light source, according to oscillation wavelength.

1. 3. Method for Manufacturing Light Source

Next, a method for manufacturing the light source100according to the present embodiment will be described, referring to drawings.FIGS. 9 to 11are sectional views schematically illustrating a process of manufacturing the light source100according to the present embodiment.

As illustrated inFIG. 9, the first reflective layer20, the active layer22, the oxidized layer26athat is oxidized and partially serves as the current constriction layer26, the second reflective layer24, the heat diffusion layer40, the first contact layer50, the first semiconductor layer51, the quantum well layer52, the second semiconductor layer53, and the second contact layer54are epitaxially grown in this order on the substrate10. The epitaxial growing method includes, for example, a metal organic chemical vapor deposition (MOCVD) method, and a molecular beam epitaxy (MBE) method.

As illustrated inFIG. 10, the second contact layer54, the second semiconductor layer53, the quantum well layer52, the first semiconductor layer51, the first contact layer50, the heat diffusion layer40, the second reflective layer24, the oxidized layer26a, the active layer22, and the first reflective layer20are patterned in a predetermined shape. Patterning is performed by, for example, photolithography or etching. The second contact layer54, the second semiconductor layer53, the quantum well layer52, and the first semiconductor layer51may be patterned in the same process (for example, simultaneously). The first contact layer50and the heat diffusion layer40may be patterned in the same process. The second reflective layer24, the oxidized layer26a, the active layer22, and the first reflective layer20may be patterned in the same process. The order of patterning the respective layers is not limited particularly. The columnar section28can be formed by the present process.

As illustrated inFIG. 11, the current constriction layer26is formed by oxidizing a portion of the oxidized layer26a. The oxidized layer26ais, for example, an AlxGa1-xAs (x≥0.95) layer. For example, by charging the substrate10in that each layer is formed in a steam atmosphere of about 400° C., the current constriction layer26is formed by oxidizing the oxidized layer26afrom a side surface. In the present process, a side surface of the heat diffusion layer40is covered with a resist (not illustrated) or the like such that the heat diffusion layer40is not oxidized.

As illustrated inFIG. 5, the insulating layer70is formed around the columnar section28. In other words, in a plan view of the columnar section28, the columnar section28is surrounded by the insulating layer70. The insulating layer70is formed, for example, by film formation using a spin coating method or a CVD method, or by patterning. Patterning is performed by, for example, photolithography or etching.

As illustrated inFIGS. 6 and 7, the insulating layers72and74are formed on the insulating layer70. The insulating layers72and74are formed, for example, by film formation using a spin coating method or a CVD method, or by patterning. Patterning is performed by, for example, photolithography or etching.

As illustrated inFIG. 5, the first electrode30is formed under the substrate10, the second electrode32is formed on the second reflective layer24, the third electrode60is formed on the first contact layer50, and the fourth electrode62is formed on the second contact layer54. The electrodes30,32,60, and62are formed, for example, by the combination of a vacuum vapor deposition method, a lift-off method, and the like. It should be noted that the order in which the electrodes30,32,60, and62are formed is not limited particularly.

The light source100can be manufactured by the above process.

The atomic oscillator1000has, for example, the following features.

In the atomic oscillator1000, the light source100has the optical oscillation layer29in which the first reflective layer20, the active layer22, and the second reflective layer24are laminated in this order, and the electrical field absorption layer59in which the first semiconductor layer51, the quantum well layer52, and the second semiconductor layer53are laminated in this order. Therefore, in the atomic oscillator1000, in a case where the central wavelength of light (light exited from the upper surface of the second contact layer54) exited from the light source100is changed by changing the quantity of a current to be flowed into the active layer22, even if the optical output (the quantity of light) of the light exited from the light source100deviates from a predetermined value, the optical output of the light exited from the light source100can be returned to the predetermined value by changing a voltage to be applied to the electrical field absorption layer59(to the quantum well layer52).

Moreover, the light source100of the atomic oscillator1000has the heat diffusion layer40that is disposed between the optical oscillation layer29and the electrical field absorption layer59, and has a higher thermal conductivity than that of the second reflective layer24. Therefore, even if the electrical field absorption layer59(the quantum well layer52) absorbs light to generate heat, this heat can be diffused to the outside via the heat diffusion layer40, and this heat can be prevented from reaching the second reflective layer24or the active layer22. Specifically, the heat generated in the electrical field absorption layer59is released to the outside via the first contact layer50, the heat diffusion layer40, the contacting part60a, and the pad part60c. Accordingly, in the atomic oscillator1000, a temperature change in the light source100caused by the heat generated in the electrical field absorption layer59can be suppressed. Therefore, in the atomic oscillator1000, the central wavelength of the light source100can be prevented from fluctuating with temperature, and the output wavelength and the optical output of the light source100can be individually (independently) controlled depending on the quantity of an inflow current to the active layer22, and a voltage applied to the electrical field absorption layer59. In the atomic oscillator1000, for example, in order to make the central wavelength of the light source100constant, it is necessary to control the driving temperature of the light source100in units of tens of mK, and the control of temperature can be made easy by providing the heat diffusion layer40.

Moreover, even in a case where the light source is driven with the quantity of a current to be flowed into the active layer being constant and the driving temperature of the light source being constant, the output wavelength and the optical output of the light source may vary in the long term. Even in this case, in the atomic oscillator1000, the output wavelength and the optical output of the light source100can be individually controlled by the light source100, and the long-term stability of the atomic oscillator1000can be improved.

In the light source100of the atomic oscillator1000, the heat diffusion layer40is an i-type AlAs layer or an i-type GaAs layer. Therefore, the heat diffusion layer40can be formed in a series of processes (for example, in the same MOCVD apparatus) together with the optical oscillation layer29and the electrical field absorption layer59.

In the light source100of the atomic oscillator1000, the third electrode60for applying a voltage to the electrical field absorption layer59is provided on the surface where the first semiconductor layer51of the first contact layer50is disposed. Therefore, in the atomic oscillator1000, the contact resistance of the third electrode60can be reduced compared to a case where the third electrode60is in direct contact with the first semiconductor layer51.

2. Modification Example of Atomic Oscillator

2.1. First Modification Example

Next, an atomic oscillator according to a first modification example of the present embodiment will be described, referring to drawings.FIG. 12is a sectional view schematically illustrating a light source100of an atomic oscillator2000according to a first modification example of the present embodiment.

Hereinafter, in the atomic oscillator2000according to the first modification example of the present embodiment, members having the same functions as those of the constituent members of the atomic oscillator1000according to the present embodiment will be designated by the same reference signs, and the detailed description thereof will be omitted. The same applies to an atomic oscillator according to a second modification example of the present embodiment to be described below.

The light source100of the atomic oscillator2000, as illustrated inFIG. 12, is different from the light source100of the above-described atomic oscillator1000in that this light source has the heat insulating layer42.

The heat insulating layer42is provided on the second reflective layer24. The heat insulating layer42is provided between the second reflective layer24and the heat diffusion layer40. A planar shape of the heat insulating layer42is, for example, circular. In the plan view, the area of the heat insulating layer42and the area of the heat diffusion layer40are, for example, the same as each other. The thermal conductivity of the heat insulating layer42is lower than the thermal conductivity of the second reflective layer24. Specifically, the thermal conductivity of the heat insulating layer42is lower than the thermal conductivity of the high refractive-index layer that constitutes the second reflective layer24, and is lower than the thermal conductivity of the low refractive-index layer that constitutes the second reflective layer24. The heat insulating layer42is, for example, an aluminum oxide layer (Alx0ylayer). For example, the thermal conductivity of Al2O3is 0.3 W/(cm·K).

The heat insulating layer42is formed, for example, by oxidizing an AlAs layer. A process of oxidizing an AlAs layer to form the heat insulating layer42may be performed simultaneously with a process of forming the current constriction layer26. The AlAs layer used as the heat insulating layer42is formed by, for example, an MOCVD method.

The light source100of the atomic oscillator2000has the heat insulating layer42having a lower thermal conductivity than that of the second reflective layer24between the second reflective layer24and the heat diffusion layer40. Therefore, in the atomic oscillator2000, even if the electrical field absorption layer59absorbs light to generate heat, the heat insulating layer42can insulate this heat, and can prevent this heat from reaching the second reflective layer24or the active layer22.

2.2. Second Modification Example

Next, an atomic oscillator according to a second modification example of the present embodiment will be described, referring to drawings.FIG. 13is a sectional view schematically illustrating a light source100of an atomic oscillator3000according to a second modification example of the present embodiment.FIG. 14is a plan view schematically illustrating the light source100of an atomic oscillator3000according to a second modification example of the present embodiment. It should be noted that illustration of members other than the contacting part62aof the fourth electrode62, the heat diffusion layer40, and the heat insulating layer42, is omitted inFIG. 14for convenience.

The light source100of the atomic oscillator3000, as illustrated inFIGS. 13 and 14, is different from the light source100of the above-described atomic oscillator1000in that this light source has the heat insulating layer42.

In the light source100of the atomic oscillator3000, in the plan view, the area of the heat insulating layer42is smaller than the area of the heat diffusion layer40. The heat insulating layer42, in the plan view, is provided inside the outer edge of the heat diffusion layer40. A space6is provided between the second reflective layer24and the heat diffusion layer40. In the illustrated example, in the plan view, a diameter R1of the heat insulating layer42has the same size as the external diameter of the contacting part62aof the fourth electrode62, and the diameter R1is greater than an internal diameter R2of the contacting part62a. In the plan view, the area of the heat insulating layer42is greater than the area of an opening162defined by the contacting part62a, and the opening162is provided inside an outer edge of the heat insulating layer42. Moreover, in the illustrated example, in the plan view, the diameter R1of the heat insulating layer42is greater than the internal diameter of the opening provided in the current constriction layer26.

The heat insulating layer42can be adjusted in the diameter R1, for example, by being selectively etched with hydrogen fluoride (HF). When the heat insulating layer42is etched with the hydrogen fluoride, the current constriction layer26is protected by a resist or the like.

In the light source100of the atomic oscillator3000, in the plan view, the area of the heat insulating layer42is smaller than the area of the heat diffusion layer40. Therefore, in the atomic oscillator3000, the space6is provided between the second reflective layer24and the heat diffusion layer40. Accordingly, in the atomic oscillator3000, even if the electrical field absorption layer59absorbs light to generate heat, the heat insulating layer42and the space6can insulate this heat, and can prevent this heat from reaching the second reflective layer24or the active layer22.

In the light source100of the atomic oscillator3000, in the plan view, the area of the heat insulating layer42is greater than the area of the opening162defined by the contacting part62a, and the opening162is provided inside the outer edge of the heat insulating layer42. Therefore, in the atomic oscillator3000, the light generated in the active layer22and exited from the upper surface of the second contact layer54can be prevented from passing through a boundary between the heat insulating layer42and the space6. Accordingly, in the atomic oscillator3000, scattering or loss of light in the boundary between the heat insulating layer42and the space6can be suppressed.

It should be noted that, as illustrated inFIG. 15, the low thermal conductivity layer43having a lower thermal conductivity than that of the heat insulating layer42may be provided around the heat insulating layer42. In other words, in a plan view of the heat insulating layer42, the heat insulating layer42may be surrounded by the low thermal conductivity layer43. The low thermal conductivity layer43is provided between the second reflective layer24and the heat diffusion layer40. The low thermal conductivity layer43is, for example, a polyimide layer. For example, the thermal conductivity of polyimide is 0.018 W/(cm·K). The low thermal conductivity layer43is formed by, for example, a CVD method or a spin coating method. By providing the low thermal conductivity layer43, shock resistance can be improved compared with a case (a case illustrated inFIG. 13) where the space6is provided between the second reflective layer24and heat diffusion layer40. Moreover, even if the electrical field absorption layer59absorbs light to generate heat, the heat insulating layer42and the low thermal conductivity layer43can insulate this heat, and can prevent this heat from reaching the second reflective layer24or the active layer22.

The invention may be provided by omitting a partial configuration in a range having the features and the effects described in the present application or combining each embodiment or the modification examples.

The invention includes substantially the same configuration as the configuration described in the embodiment (for example, a configuration having the same functions, methods, and results as those of the configuration described in the embodiment, or a configuration having the same object and effects as those of the configuration described in the embodiment). Additionally, the invention includes a configuration in which parts that are not essential to the configuration described in the embodiment are substituted. Additionally, the invention includes a configuration in which the same functional effects as those of the configuration described in the embodiment can be exhibited or a configuration in which the same object as that of the configuration described in the embodiment can be achieved.

Additionally, the invention includes a configuration in which well-known techniques are added to the configuration described in the embodiment.

The entire disclosure of Japanese Patent Application No. 2015-210824, filed Oct. 27, 2015 is expressly incorporated by reference herein.