JP7344541B2 - Composite optical resonator, temperature sensor, optical resonator device - Google Patents

Description

The present invention relates to a composite optical resonator, a temperature sensor, and an optical resonator device.

An optical resonator used in an atomic clock that utilizes the transition frequency of atoms or molecules has a cavity formed in a spacer and a pair of reflecting mirrors provided at both ends of the cavity. The resonant frequency depends on the distance between a pair of reflecting mirrors (resonator length), so in order to prevent frequency fluctuations, optical resonators are designed to prevent thermal expansion due to temperature fluctuations, atmospheric pressure changes, and is configured to eliminate mechanical vibration as much as possible.

In order to suppress the thermal expansion of the spacer, the spacer is made of an ultra-low thermal expansion material, the optical resonator is housed in a vacuum constant temperature chamber with a temperature adjustment function, and a thermal shield is provided between the vacuum constant temperature chamber and the optical resonator. The temperature is controlled in microkelvin units (Non-Patent Documents 1 to 3). Further, an optical resonator is disclosed in which a compensation member made of the same material as the spacer is attached to the reflecting mirror to suppress deflection of the reflecting mirror due to temperature fluctuations (Patent Document 1). Furthermore, a device has been disclosed that reduces the influence of thermal vibration by averaging the frequencies of the respective feedback lights by operating a plurality of optical resonators in parallel (Patent Document 2). Further, the optical resonator is mounted on a vibration isolation table to suppress mechanical vibration. Furthermore, a device has been disclosed that uses a plurality of optical resonators sharing a single spacer as an acceleration sensor and is mounted on a stand driven by negative feedback of the output to actively isolate vibrations (Patent Document 3). ).

German Patent Application No. 102008049367 Patent No. 6284176 Patent No. 6256876

J. Keller, S. Ignatovich, S. A. Webster, T. E. Mehlstaubler, "Simple vibration insensitive cavity for laser stabilization at the 10-16 level", Applied Physics B 116, 203 (2014), 2013 D.R. Leibrandt, J. C. Bergquist, T. Rosenband, "Cavity-stabilized laser with acceleration sensitivity below 10-12 g-1", Physical Review A 87, 023829 (2013), 2013 J. Alnis, A. Matveev, N. Kolachevsky, Th. Udem, T. W. Hansch, "Subhertz linewidth diode lasers by stabilization to vibrationally and thermally compensated ultralow-expansion glass Fabry-Perot cavities", Physical Review A 77, 053809 (2008) , 2008

However, in order to maintain a good frequency uncertainty (accuracy) of less than 10 ^-15 , optical resonators require temperature stability on the order of nanokelvin or less, which cannot be achieved with current standard temperature sensors. It is. In order to suppress temperature fluctuations that are less than the accuracy of such temperature sensors, the optical resonator is housed in a larger and more precise constant temperature chamber with a thick or double or more heat shielding material interposed. However, sufficient temperature control is not possible.

The present invention has been made in view of the above-mentioned problems, and provides a highly accurate temperature sensor on the order of nanokelvin, a composite optical resonator constituting this temperature sensor, and a frequency of less than 10 ^-15 of the optical resonator. An object of the present invention is to provide an optical resonator device that can achieve the accuracy of .

The composite optical resonator according to the present invention includes a plurality of optical resonators each including a cavity penetrating from one surface to the other surface of a spacer and reflecting mirrors provided at both ends of the cavity, such that the cavities are parallel to the spacer. In the provided composite optical resonator, the reflecting mirrors of some of the optical resonators and the reflecting mirrors of the other optical resonators have different coefficients of thermal expansion.

Further, another composite optical resonator according to the present invention includes an optical resonator including a cavity penetrating from one surface to the other surface of a spacer and reflecting mirrors provided at both ends of the cavity, the cavity being parallel to the spacer. A plurality of composite optical resonators are provided such that some of the optical resonators have a compensating member having a lower coefficient of thermal expansion than the reflecting mirror on a side opposite to the side of the reflecting mirror facing the cavity. It is provided by connecting to.

With this configuration, in the composite optical resonator, when the temperature changes, the amount of change in the resonator length differs between some optical resonators and other optical resonators. Therefore, temperature fluctuations can be detected by measuring the difference between the resonant frequencies of some of the optical resonators and the other optical resonators.

The temperature sensor according to the present invention includes any one of the above-mentioned composite optical resonators, a laser light source that introduces laser light into each optical resonator of the above-mentioned composite optical resonators, and a part of the above-mentioned optical resonators and another optical resonator. a frequency comparing means for measuring a resonance frequency difference that is a difference between resonance frequencies of the optical resonator, the temperature of the composite optical resonator when the resonance frequency difference reaches a predetermined value as a reference temperature; The configuration is such that the difference between the temperature of the composite optical resonator and the reference temperature is detected from the difference between the resonant frequency difference measured by the frequency comparison means and the predetermined value.

With this configuration, the temperature sensor includes a composite optical resonator in which some of the optical resonators have relatively high temperature sensitivity, and further, the resonant frequency of some of the optical resonators is adjusted to the resonance frequency of the other optical resonators. The temperature of the composite optical resonator can be detected with high precision by the frequency comparison means that calculates the difference from the frequency.

An optical resonator device according to the present invention includes any one of the above-mentioned composite optical resonators, a laser light source that introduces laser light into each optical resonator of the above-mentioned composite optical resonator, and some of the above-mentioned optical resonators. A frequency comparison means for measuring a resonance frequency difference, which is a difference between the resonance frequencies of the other optical resonators, and a temperature control means for controlling the temperature of the composite optical resonator, the temperature control means comprising: The configuration is such that feedback control of the temperature of the composite optical resonator is performed based on the resonance frequency difference measured by the frequency comparing means so that the resonance frequency difference becomes a constant value.

With this configuration, the optical resonator device can maintain the temperature of the composite optical resonator at a predetermined temperature without relying solely on passive temperature maintaining means such as a heat shielding material, or without installing a separate temperature sensor in contact with the composite optical resonator. can be managed with high precision.

Another optical resonator device according to the present invention includes any one of the above-described composite optical resonators, a laser light source that introduces a laser beam into each optical resonator of the composite optical resonator, and a part of the optical resonator. a frequency comparison means for measuring a difference in resonance frequency between the resonant frequency of the optical resonator and the other optical resonator; and a wavelength shifter for shifting the wavelength of light resonating in one of the optical resonators of the composite optical resonator. The wavelength shifting means is configured to perform feedforward control to cancel fluctuations in the resonant frequency of the one optical resonator based on the resonant frequency difference measured by the frequency comparing means.

With such a configuration, the optical resonator device can compensate for drifts in the resonant frequency caused by minute fluctuations in temperature.

According to the present invention, a highly accurate temperature sensor on the nanokelvin order, a composite optical resonator constituting this temperature sensor, and an optical resonator device capable of realizing frequency accuracy of less than 10 ^-15 of the optical resonator is obtained.

FIG. 1 is a block diagram illustrating the structure of an optical resonator device according to an embodiment of the present invention. FIG. 1 is an external view schematically illustrating the structure of a composite optical resonator according to an embodiment of the present invention. FIG. 2B is a partial cross-sectional view taken along line IIB-IIB in FIG. 2A. 2 is a block diagram illustrating the structure of a wavelength stabilizing device of the optical resonator device shown in FIG. 1. FIG. 2 is a block diagram illustrating the structure of a tuner of the optical resonator device shown in FIG. 1. FIG. FIG. 3 is a partial cross-sectional view schematically exaggerating deformation due to temperature change of the composite optical resonator according to the present invention. 3 is a graph showing the temperature dependence of the resonant frequency of each optical resonator in a composite optical resonator.

Embodiments for implementing a composite optical resonator, a temperature sensor, and an optical resonator device according to the present invention will be described with reference to the drawings. The composite optical resonator and its elements shown in the drawings may be exaggerated in size, positional relationship, etc., or simplified in shape for clarity of explanation. In addition, elements having the same structure are denoted by the same reference numerals.

[Optical resonator device]
As shown in FIG. 1, an optical resonator device 100 according to an embodiment of the present invention includes a composite optical resonator 10 having a plurality of optical resonators 11, 12, and 13, and a laser beam in the optical resonators 11, 12, and 13. A laser light source 3 that introduces light, a frequency comparator (frequency comparison means) 7 that measures the difference (resonant frequency difference) between the resonant frequency f2 of the optical resonator 12 and the resonant frequency f1 of the optical resonator 13, and a composite light A vacuum constant temperature chamber 8 is provided with a temperature control device (temperature control means) 81 that controls the temperature of the resonator 10 to a predetermined temperature, and a wavelength shifter (wavelength shift means) 9, and the light resonant in the optical resonator 11 is , frequency drift (fluctuation) due to temperature fluctuation is canceled out by the wavelength shifter 9 and output. The optical resonator device 100 further includes an optical isolator 61, beam splitters 62, 63, 65 that split the laser light L0 emitted by the laser light source 3, mirrors 64, 66, and stabilizes the oscillation wavelength of the laser light source 3. It includes a wavelength stabilizing device 4 and tuners 5, 5 that stabilize the wavelengths of the laser beams L1, L2 introduced into the optical resonators 13, 12, respectively. Further, a vacuum constant temperature chamber 8 accommodates a composite optical resonator 10 in its vacuum container and is mounted on a vibration isolation table (not shown). The light L0 _corr output by the optical resonator device 100 is input to an external optical frequency counter (not shown) or the like. In FIG. 1 and FIGS. 3 and 4 described later, solid arrows represent electrical signals, hatched arrows represent light, and white arrows represent control of the environment (temperature and pressure), respectively. Each element will be explained in detail below.

(composite optical resonator)
The composite optical resonator 10 includes a plurality of optical resonators each having a cavity penetrating from the upper surface to the lower surface of the spacer and reflecting mirrors provided at both ends of the cavity so that the cavities are parallel to each other. That is, each of these optical resonators is a Fabry-Perot resonator whose optical path is a cavity, and they share one spacer. As shown in FIG. 2A, the composite optical resonator 10 according to this embodiment includes three optical resonators 11, 12, and 13 arranged in a line. For this purpose, the composite optical resonator 10 includes a spacer 1 in which three cavities 1c are formed, reflective mirrors 21 and 22, reflective mirrors 23 and 24, and reflective mirrors 21 and 22 provided facing each other at both ends of each cavity 1c of the spacer 1. It includes mirrors 25, 26 and a compensating member 27 joined to each of the reflecting mirrors 21, 22. These members constituting the composite optical resonator 10 are preferably made of a material with low thermal expansion, and a material that does not easily deteriorate over time.

The optical resonator 11 is a reference optical resonator that stabilizes the oscillation wavelength of the laser light source 3 at its resonant frequency, and is used as a main optical resonator that is used as a frequency reference. For this purpose, the optical resonator 11 is configured to have particularly excellent stability of the resonance frequency. Furthermore, the optical resonator 11 is configured such that its temperature sensitivity is lower than at least one of the other optical resonators, that is, lower than the optical resonator 12. Such an optical resonator 11 is composed of a single cavity 1c formed at the center of the spacer 1, a pair of reflecting mirrors 21 and 22 facing each other at both ends of the cavity 1c, and compensation members 27 and 27. The optical resonator 12 and the optical resonator 13 are used in combination as a temperature sensor. For this purpose, the optical resonator 12 is configured to have higher temperature sensitivity than the optical resonator 13. Further, it is preferable that the optical resonator 13 is configured to have lower temperature sensitivity. The optical resonator 12 is composed of one cavity 1c and a pair of reflecting mirrors 23 and 24 facing each other at both ends of the cavity 1c. Similarly, the optical resonator 13 is composed of one cavity 1c and a pair of reflecting mirrors 25 and 26 facing each other at both ends of the cavity 1c.

The spacer 1 is a rotating body that has a biconical truncated (barrel-shaped) outer shape and has a support portion projecting like a brim at a thicker diameter portion. The axis of rotation of this rotating body is called the central axis of the spacer 1. In the spacer 1, three cavities 1c are formed which penetrate in the direction of the central axis from one surface to the other surface. The cavity 1c has a cylindrical shape. The cavities 1c are formed by lining up three cavities 1c at equal intervals in one direction, and the central axis of the central one coincides with the central axis of the spacer 1. The shape of the spacer 1 preferably has rotational symmetry including the cavity 1c. Furthermore, since the central axis of the spacer 1 is at a position that is not particularly susceptible to mechanical vibrations, the central cavity 1c is applied to the optical resonator 11. Note that the inside of the cavity 1c is in a high vacuum and communicates with a through hole (not shown) formed in the spacer 1 so as to be open at the outer peripheral surface of the spacer 1 for vacuum suction. The spacer 1 is made of a material with a particularly small coefficient of thermal expansion (ultra-low thermal expansion material). Specifically, examples include ULE (Ultra-Low-Expansion, registered trademark, Corning Inc.) glass whose zero expansion rate (Zero-Crossing temperature) is close to room temperature, and Zerodur (registered trademark, Schott Inc.). , ULE glass is particularly preferred because of its low creep.

In the optical resonator 11, the reflecting mirror 21 and the reflecting mirror 22 are joined to the spacer 1 by optical contact so as to close the openings at both ends of the cavity 1c. As shown in FIG. 2B, the reflecting mirrors 21 and 22 each include a reflecting film 2a forming a reflecting surface and a substrate 2b supporting the reflecting film 2a. The reflective mirrors 21 and 22 are formed by forming a reflective film 2a on a region of the substrate 2b that faces the cavity 1c. The reflective film 2a is, for example, a dielectric multilayer film of SiO ₂ and Ta ₂ O ₅ . It is preferable that the reflecting mirrors 21 and 22 include a substrate 2b with low thermal noise in order to suppress fluctuations in resonance frequency due to Brownian motion. An example of such a substrate 2b is quartz glass. Further, the reflecting mirror 21 is a plane mirror, and the reflecting mirror 22 is a concave (spherical) mirror, and the substrate 2b is processed into the respective shapes. Alternatively, the reflecting mirror 21 and the reflecting mirror 22 may be a combination of a convex mirror and a concave mirror, or both may be concave mirrors.

The reflecting mirrors 23 and 25 have the same shape as the reflecting mirror 21, and the reflecting mirrors 24 and 26 have the same shape as the reflecting mirror 22, respectively, and like the reflecting mirrors 21 and 22, they are designed to close the openings at both ends of the cavity 1c. It is joined to the spacer 1. The reflecting mirrors 23 and 24 of the optical resonator 12 each include a reflecting film 2a forming a reflecting surface and a substrate 2c supporting the reflecting film 2a. The reflecting mirrors 25 and 26 of the optical resonator 13 each include a reflecting film 2a forming a reflecting surface and a substrate 2d supporting the reflecting film 2a. The reflective mirrors 23 and 24 and the reflective mirrors 25 and 26 have different coefficients of thermal expansion. For this reason, the substrates 2c and 2d, which are members that occupy the majority of each of the reflecting mirrors 23, 24 and 25, 26, are formed of materials having different coefficients of thermal expansion. Specifically, the substrate 2c of the reflecting mirrors 23 and 24 of the optical resonator 12, which has a higher temperature sensitivity, is made of a material with a higher coefficient of thermal expansion. Further, the substrate 2d having a low coefficient of thermal expansion preferably has the same coefficient of thermal expansion as the spacer 1, that is, it is preferably formed of the same material as the spacer 1. For example, when the spacer 1 is made of ULE glass, the substrates 2d of the reflecting mirrors 25 and 26 are also made of ULE glass, whereas the substrates 2c of the reflecting mirrors 23 and 24 are made of quartz glass or BK7 (borosilicate glass). Acid crown glass). Note that it is sufficient that at least one of the reflecting mirror 23 and the reflecting mirror 24 of the optical resonator 12 has a higher coefficient of thermal expansion than the reflecting mirrors 25 and 26. For example, the reflecting mirror 24 may have a substrate 2c with a high coefficient of thermal expansion, and the reflecting mirror 23 may have the same substrate 2d as the reflecting mirror 25 of the optical resonator 13. With such a configuration, the optical resonator 12 has lower temperature sensitivity than when both the reflecting mirrors 23 and 24 have the substrate 2c. The optical resonator 12 has reflecting mirrors 23 and 24 designed depending on the required temperature sensitivity, as will be described later.

The compensation member 27 is provided to suppress changes in the resonant frequency of the optical resonator 11 due to thermal fluctuations. As described above, the substrate 2b of the reflecting mirrors 21 and 22 is preferably made of quartz glass, which has a higher coefficient of thermal expansion than the ULE glass forming the spacer 1. Therefore, in order to suppress deformation due to the difference in thermal expansion between the reflecting mirrors 21 and 22 and the spacer 1, a material having a lower coefficient of thermal expansion than the substrate 2b is made on the side of the reflecting mirrors 21 and 22 opposite to the joint surface with the spacer 1. The compensating member 27 is joined. The compensation member 27 is joined to the reflecting mirrors 21 and 22 (substrate 2b) by optical contact, similar to the joining between the spacer 1 and the reflecting mirrors 21 and 22. It is preferable that the compensation member 27 has the same thermal expansion coefficient as the spacer 1, that is, it is preferably made of the same material as the spacer 1. The compensation member 27 is formed in the shape of a circular column having a through hole on an extension of the cavity 1c, avoiding the optical path of the laser beam introduced into the optical resonator 11. Further, it is preferable that the compensation member 27 has a sufficiently large thickness and a sufficiently large joint area with the reflecting mirrors 21 and 22 in order to sandwich the reflecting mirrors 21 and 22 together with the spacer 1 from both sides and suppress the deflection thereof.

It is preferable that laser light is introduced into the optical resonators 11, 12, 13 from the side of reflecting mirrors 21, 23, 25, which are plane mirrors, and the optical path is preferably in the direction of gravity. Therefore, as shown in FIG. 2A, the composite optical resonator 10 is installed in the vacuum container of the vacuum constant temperature chamber 8 with the penetration direction of the cavity 1c facing the vertical direction. At this time, the composite optical resonator 10 is supported by pillars attached to an annular pedestal, for example, at three or four equally spaced points on a collar-shaped support provided at the vertical center of the spacer 1. Points other than the points are non-contact. In order to reduce the influence of mechanical vibration etc. on the optical resonators 11, 12, 13, the support point is preferably located at a long distance from the cavity 1c. Further, the composite optical resonator 10 is preferably used near room temperature (for example, 25° C.).

(laser light source, optical element)
The laser light source 3 is a light source of laser light introduced into the optical resonators 11, 12, and 13. The laser light source 3 can slightly change the oscillation wavelength by, for example, changing the supplied current. As shown in FIG. 1, the laser light L0 emitted by the laser light source 3 passes through an optical isolator 61, is split by beam splitters 62 and 63 and a mirror 64, and then passes through a wavelength stabilizer 4 or a tuner 5. and introduced into optical resonators 11, 12, and 13. The laser beam L0 is further split by a beam splitter 65 and output to the outside. The optical isolator 61 prevents the oscillation operation of the laser light source 3 from being disturbed by returned light.

(wavelength stabilization device)
The wavelength stabilizing device 4 stabilizes the oscillation wavelength of the laser light source 3 using the PDH method (Pound-Drever-Hall method), which locks the oscillation wavelength of the laser light source 3 to the resonant wavelength (resonant frequency f0) of the optical resonator 11. For example, as shown in FIG. 3, the wavelength stabilizing device 4 includes an optical modulator 42 that modulates the phase of laser light, an oscillator 43, a half-wave plate (λ/2 plate, HWP) 47, and a PBS ( a polarizing beam splitter) 48, a quarter wavelength plate (λ/4 plate, QWP) 49, a photodetector (PD) 44, a synchronous detector 45, and a PID (Proportional-Integral-Differential). (integral/differential) controller 46.

The optical modulator 42 phase-modulates the laser beam L0 emitted from the laser light source 3 and transmitted through the optical isolator 61 based on the signal from the oscillator 43 to generate sideband waves. The modulated laser beam has its light intensity adjusted by a half-wave plate 47 and a PBS 48, and enters the optical resonator 11 via a quarter-wave plate 49. The light L _out 0 output from the optical resonator 11 passes through a quarter-wave plate 49 and is input to the photodetector 44 by the PBS 48 . The photodetector 44 converts the light L _out 0 into an electrical signal. The synchronous detector 45 synchronously detects this electrical signal with the signal from the oscillator 43 and outputs a feedback signal, which is smoothed by the PID controller 46. The smoothed feedback signal S _Fb is fed back to the laser light source 3. This feedback loop allows the output wavelength of the laser light source 3 to be locked to the resonance spectrum of the optical resonator 11.

(tuner)
The tuner 5 locks the laser beam L0 emitted from the laser light source 3 to the resonant wavelength (resonant frequency f2) of the optical resonator 12, thereby obtaining wavelength-stabilized laser beam L2. Further, the tuner 5 locks the laser beam L0 to the resonant wavelength (resonant frequency f1) of the optical resonator 13, thereby obtaining a wavelength-stabilized laser beam L1. For example, as shown in FIG. 4, the tuner 5 includes a wavelength shifter 51 that makes the wavelength variable, an optical modulator 52 that modulates the phase of the laser beam, an oscillator 53, a half-wave plate 47, and a PBS 48. , a quarter wavelength plate 49, a photodetector (PD) 54, a synchronous detector 55, a PID controller 56, a VCO (voltage controlled oscillator) 57, and an offset voltage source 58. . Although FIG. 4 shows the tuner 5 that introduces the laser beam L2 into the optical resonator 12, the tuner 5 that introduces the laser beam L1 into the optical resonator 13 has a similar configuration.

The wavelength shifter 51 includes an acousto-optic element (AOM) and the like, and shifts the laser beam L0 to a wavelength based on the correction signal S _corr 2. The optical modulator 52 phase-modulates the wavelength-shifted laser light based on the signal from the oscillator 53 to generate sideband waves. The modulated laser beam has its light intensity adjusted by a half-wave plate 47 and a PBS 48, and enters the optical resonator 12 via a quarter-wave plate 49. The light L _out 2 output from the optical resonator 12 passes through a quarter-wave plate 49 and is input to the photodetector 54 by the PBS 48 . Photodetector 54 converts the light L _out 2 into an electrical signal. The synchronous detector 55 synchronously detects this electric signal with the signal from the oscillator 53 and outputs a feedback signal, which is smoothed by the PID controller 56. The VCO 57 converts the smoothed feedback signal into a correction signal S _corr 2 that includes an alternating current signal with a frequency corresponding to its intensity. The frequency offset can be set by adjusting the voltage of the offset voltage source 58. The tuner 5 outputs a signal S _corr 2 (or signal S _corr 1) that reflects the wavelength fluctuation of the laser beam with reference to the resonant wavelength of the optical resonator 12 (or optical resonator 13). Here, the modulation for the PDH method is performed not only by using the independent optical modulator 52 but also by using the modulated laser light from the laser light source 3 stabilized by the wavelength stabilizing device 4. You can also do it. Furthermore, instead of the VCO 57, a synthesizer type signal generator can be used.

(frequency comparator)
The frequency comparator 7 measures the difference (resonance frequency difference) Δf _2-1 (=|f2−f1|) between the resonant frequency f1 of the optical resonator 13 and the resonant frequency f2 of the optical resonator 12, and outputs an electrical signal. Output as . The frequency comparator 7 measures the difference between the resonant frequency f2, which is relatively easy to change due to temperature fluctuations, and the resonant frequency f1, so that the amount of change in the resonant frequency f2 can be detected with high accuracy. As will be described later, in the composite optical resonator 10, the resonance frequency difference Δf _2-1 between the optical resonators 12 and 13 is several hundred MHz. In the optical resonator device 100, in order to maintain the frequency accuracy of the optical resonator 11 and the optical resonator 13 to less than 10 ^-15 , the frequency comparator 7 converts the resonance frequency difference Δf _2-1 to a resolution of less than 1 Hz. Measure with. Here, the correction signals S _corr 1 and S _corr 2 (see FIGS. 1 and 4) output by the tuner 5 that stabilizes the wavelength of the laser beams L1 and L2 introduced into the optical resonators 13 and 12 have respective frequencies is the frequency difference between the resonant frequencies f1, f2 and the resonant frequency f0 of the optical resonator 11. Therefore, the frequency difference (absolute value) between the correction signals S _corr 1 and S _corr 2 matches the resonance frequency difference Δf _2-1 between the optical resonators 12 and 13. Therefore, as shown in FIG. 1, in the optical resonator device 100, the correction signals S _corr 1 and S _corr 2 are inputted from the tuner 5 to the frequency comparator 7. Such a frequency comparator 7 includes a frequency mixer 71 such as a double balanced mixer (DBM), a low frequency filter (LPF) 72, and a frequency-voltage converter (FVC) 73. The frequency mixer 71 mixes the signal S _corr 1 and the signal S _corr 2, and the LPF 72 selects the sideband signal component on the low frequency side. This selected signal has a resonance frequency difference Δf _2-1 in frequency, and the FVC 73 converts it into a voltage signal S _diff corresponding to the frequency and outputs it.

(vacuum constant temperature chamber)
The vacuum constant temperature chamber 8 maintains its interior at a set temperature and is in a vacuum state at a predetermined pressure. For this purpose, as shown in FIG. 1, the vacuum constant temperature chamber 8 includes a vacuum container (not shown) that isolates the inside from the outside environment, a temperature control device 81 that controls the temperature inside the vacuum container, and a vacuum exhaust system that evacuates the inside of the vacuum container. A vacuum evacuation system 82 and the like is provided. In the optical resonator device 100, the vacuum constant temperature chamber 8 accommodates the composite optical resonator 10 in a vacuum container, maintains its temperature at a set temperature, and places the cavity 1c of the composite optical resonator 10 in a high vacuum. The vacuum container has a structure compatible with high vacuum and is made of aluminum alloy, stainless steel, etc., and also introduces laser beams L0, L2, and L1 from the outside into the optical resonators 11, 12, and 13 of the composite optical resonator 10. In order to do this, a window is provided with an AR coating (Anti-reflection coating) or the like. The vacuum evacuation system 82 includes a high vacuum pump, a pressure gauge, a flow meter, and the like. The temperature control device 81 includes a thermoelectric cooling system including a Peltier device, a control unit for controlling the thermoelectric cooling system, a temperature sensor using a platinum resistance temperature detector, and the like. Further, the temperature control device 81 includes a casing made of, for example, an aluminum alloy and having ventilation holes installed in a vacuum container in order to maintain a uniform temperature in the entire composite optical resonator 10. The composite optical resonator 10 is housed in the housing and heated and cooled by a thermoelectric cooling system attached to the outer surface of the housing. Alternatively, the temperature control device 81 may heat and cool the vacuum container from the outside. Further, the vacuum constant temperature chamber 8 may include a heat shielding material inside the vacuum container.

(wavelength shifter)
The wavelength shifter 9 has a similar configuration to the wavelength shifter 51 of the tuner 5, and shifts the laser light L0 to a wavelength based on the input signal. In the optical resonator device 100, when outputting the laser beam L0, which is the light that resonates in the optical resonator 11, the wavelength shifter 9 compensates for the frequency drift of the laser beam L0 due to temperature fluctuations. For this purpose, the wavelength shifter 9 shifts the laser light L0 to a wavelength based on the signal S _diff output by the frequency comparator 7, and outputs it as light L0 _corr .

[How to operate temperature sensor and optical resonator device]
A method of operating an optical resonator device according to the present invention and a temperature sensor according to the present invention will be explained. First, the temperature dependence of the resonant frequency of the optical resonator in the composite optical resonator will be explained with reference to FIGS. 5 and 6.

The resonant frequency f ₀ of the optical resonator depends on the resonator length (optical path length) l. The relationship between the change in the resonator length and the change in the resonant frequency is expressed by the following equation (1). dl is the amount of change in the resonator length, and df is the amount of change in the resonant frequency. The resonator length l of the optical resonator is the distance between the reflecting surfaces of a pair of reflecting mirrors (reflecting mirrors 21 and 22 in the optical resonator 11). Further, when the spacer thermally expands (contracts) due to temperature fluctuation, the resonator length changes as shown in equation (2) below. α is the coefficient of thermal expansion of the spacer, and dT is the amount of temperature change of the spacer.
df/f ₀ =dl/l...(1)
dl=αl・dT...(2)

Therefore, the optical resonator 13 consisting of the spacer 1 and the reflecting mirrors 25 and 26 (substrate 2d) made of an ultra-low thermal expansion material has a small However, the resonator length changes, and as a result, the resonant frequency f1 changes as shown by the solid line in FIG. Note that the thermal expansion coefficients of the spacer 1 and the substrate 2d are positive in the temperature range shown in FIG. On the other hand, as shown in FIG. It bends and deforms so that the joint surface is concave. In this way, the optical resonator 12 has a larger amount of change in resonator length than the optical resonator 13 because the spacer 1 expands and the reflecting mirrors 23 and 24 deform due to temperature fluctuations. On the other hand, in the optical resonator 11, which is equipped with reflecting mirrors 21 and 22 having a substrate 2b with a high coefficient of thermal expansion, like the optical resonator 12, the reflecting mirrors 21 and 22 are deformed by the compensating member 27 with a low coefficient of thermal expansion. is suppressed. In addition, in FIG. 5, the top and bottom are reversed to FIG. 2B, and the side of reflecting mirrors 21, 23, and 25 which are plane mirrors is shown. As a result, as shown by the broken line in FIG. 6, the resonant frequency f2 of the optical resonator 12 has greater temperature dependence than the resonant frequency f1 of the optical resonator 13, that is, the optical resonator 12 has higher temperature sensitivity.

Furthermore, the larger the coefficient of thermal expansion of the substrate 2c is relative to the spacer 1 and the substrate 2d, the larger the difference in temperature dependence between the resonance frequencies f1 and f2. Furthermore, when both of the reflecting mirrors 23 and 24 of the optical resonator 12 have the substrate 2c, the temperature sensitivity is higher than when one of them has the substrate 2d. The temperature dependencies df1/dT and df2/dT of the resonance frequencies f1 and f2 should be calculated in advance by simulation based on the materials and dimensions of the spacer 1, the reflecting mirrors 25 and 26, and the reflecting mirrors 23 and 24. Can be done. The temperature dependence of the resonance frequency f2 is preferably greater than the temperature dependence of the resonance frequency f1 by 5 Hz or more per μK, and more preferably by 10 Hz or more.

Here, in the composite optical resonator 10, even if the optical resonators 11, 12, and 13 are designed to have the same shape, there may be slight errors in the length of each resonator due to the processing accuracy of the spacer 1 and the reflecting mirrors 21 to 26. has. Therefore, the resonance frequencies f0, f2, and f1, which are several hundred THz, have variations within a range of about several hundred MHz. In FIG. 6, f1<f2. In this case, the resonance frequency difference Δf _2-1 (=|f2−f1|) between the optical resonators 13 and 12 increases proportionally as the temperature increases. On the other hand, in the case of f1>f2, the higher the temperature, the smaller it becomes.

From this, the temperature T of the composite optical resonator 10 can be derived by measuring the resonance frequency difference Δf _2-1 between the optical resonators 13 and 12. For example, the temperature of the composite optical resonator 10 at which the resonance frequency difference Δf _2-1 reaches a certain predetermined value (hereinafter referred to as a reference value) Δf _ref is set as the reference temperature T _ref . In FIG. 6, at this time, f1=f ₀ and f2=f ₀ +Δf _ref (f1<f2). Note that the resonant frequency difference Δf _2-1 when the composite optical resonator 10 is at a certain temperature (reference temperature) T _ref may be set as the reference value Δf _ref , or the resonant frequency f0 of the optical resonator 11 may be set as the reference value. The reference value Δf _ref and the reference temperature T _ref may be set as follows. Based on the difference (Δf _2-1 - Δf _ref ) including the magnitude relationship between the resonance frequency difference Δf _2-1 and the reference value Δf _ref , based on the temperature dependence df1/dT and df2/dT of the resonance frequencies f1 and f2. , the temperature T of the composite optical resonator 10 can be calculated as the difference (TT _ref ) from the reference temperature T _ref . In the optical resonator device 100, as described above _, the frequency comparator 7 outputs the voltage signal S _diff corresponding to the resonant frequency difference Δf _2-1 . The difference (TT _ref ) between the temperature T of 10 and the reference temperature T _ref can be calculated.

As described above, according to the composite optical resonator 10 including the optical resonators 13 and 12 that share the spacer 1 and have different coefficients of thermal expansion of the reflecting mirrors, by measuring the resonant frequency difference Δf _2-1 , the composite The temperature T of the optical resonator 10 can be detected as a difference from the reference temperature T _ref . In particular, by measuring the resonance frequency difference Δf _2-1 at less than 1 Hz, the optical resonator device 100 can be used as a temperature sensor on the nano-Kelvin order.

In addition, by calculating the temperature dependence df0/dT of the resonant frequency f0 for the optical resonator 11 in advance in the same manner as for the optical resonators 12 and 13, from the resonant frequency difference Δf _2-1 , the composite optical resonator 10 It is possible to estimate the amount of variation (drift) in the resonant frequency f0 due to temperature variation. Therefore, the optical resonator device 100 is configured to input the signal S _diff output by the frequency comparator 7 to the wavelength shifter 9, and the wavelength shifter 9 converts the laser beam L0 outputted to the outside into the voltage of the signal S _diff . Shift to the wavelength according to the wavelength. The light L0 _corr whose wavelength has been shifted by the wavelength shifter 9 has a resonant frequency at the estimated reference temperature T _ref . In this way, the optical resonator device 100 uses the optical resonator 12 and the optical resonator 13, which have different temperature sensitivities, to adjust the temperature of the resonant light for the optical resonator 11, which has lower temperature sensitivity than the optical resonator 12. Enables feedforward control that cancels frequency drift due to fluctuations. As a result, the long-term stability of the resonant frequency of the optical resonator 11 can be improved.

[Modified example]
In the optical resonator device 100, when the frequency comparator 7 calculates the resonance frequency difference Δf _2-1 ≠Δf _ref , it is detected that T≠T _ref . Therefore, by driving the temperature control device 81 according to (Δf _2-1 - Δf _ref ) and increasing or decreasing the temperature inside the vacuum container of the vacuum constant temperature chamber 8, compensation is made to Δf _2-1 =Δf _ref . That is, the temperature T of the composite optical resonator 10 can be corrected to the reference temperature T _ref . For this purpose, the optical resonator device 100 may be configured to input the signal S _diff output by the frequency comparator 7 to the temperature control device 81 . A control section of the temperature control device 81 drives the thermoelectric cooling system according to the voltage of the signal S _diff to raise, lower, or maintain the temperature. In this way, the optical resonator device 100 enables highly accurate feedback control of temperature by using the optical resonator 12 and the optical resonator 13, which have different temperature sensitivities. For the resonator 11, the long-term stability of the resonant frequency can be improved. Therefore, the optical resonator device 100 can directly output the laser light L0 as light that resonates in the optical resonator 11 without frequency drift due to temperature fluctuations, and the wavelength shifter 9 becomes unnecessary.

As described above, when the temperature control device 81 raises or lowers the temperature to compensate for the reference temperature T _ref , each resonance frequency of the optical resonators 11, 12, and 13 changes to the value at the reference temperature T _ref . , that is, it may take some time to adjust to Δf _2-1 =Δf _ref . Therefore, in the optical resonator device 100, feedforward control of the frequency of the laser beam L0 by the wavelength shifter 9 and feedback control of the temperature by the temperature control device 81 may be used together. For this purpose, the optical resonator device 100 is configured to input the signal S _diff output by the frequency comparator 7 to the wavelength shifter 9 and the temperature control device 81 . With such a configuration, the optical resonator device 100 outputs the laser beam L0 with the frequency drift compensated for by the wavelength shifter 9 in the short term, while in the medium to long term, the temperature control device 81 compensates for the frequency drift of the laser beam L0. It is possible to obtain laser light L0 that is not present.

When the composite optical resonator 10 is used only as a temperature sensor, it may have a structure in which only two optical resonators 12 and 13 are provided. Further, the optical resonator 13 may be configured to include a compensating member 27 like the optical resonator 11 to suppress deformation of the reflecting mirrors 25 and 26 instead of using the substrate 2d made of an ultra-low thermal expansion material. Further, for this reason, the optical resonator 11 can be used in place of the optical resonator 13 in combination with the optical resonator 12 as a temperature sensor. In this way, when the composite optical resonator 10 includes two optical resonators 11 and 12 (or optical resonators 12 and 13), it may be configured to include one dummy optical resonator, or may include a spacer. Two cavities 1c may be formed symmetrically about the central axis. Further, in the optical resonator 12, the temperature sensitivity may be adjusted to be low by bonding the compensation member 27 to one of the reflecting mirrors 23 and 24 having the substrate 2c having a high coefficient of thermal expansion.

In the optical resonator device 100, when detecting the temperature fluctuation of the composite optical resonator 10 by measuring the resonance frequency difference between the optical resonator 11 and the optical resonator 12, the laser beam L2 introduced into the optical resonator 12 is It is sufficient to input only the wavelength-stabilized correction signal S _corr 2 of the tuner 5 to the FVC 73 of the frequency comparator 7 and convert it into the signal S _diff . As described above, the frequency of the correction signal S _corr 2 is the frequency difference between the resonant frequency f2 of the optical resonator 12 and the resonant frequency f0 of the optical resonator 11.

In the optical resonator device 100 shown in FIG. 1, the laser light L0 emitted by one laser light source 3 is branched and introduced into the optical resonators 11, 12, 13, but if two or more laser light sources 3 are used, Three units may be provided, each provided with a wavelength stabilizing device 4, and independent laser beams may be introduced. In this case, the frequency comparator 7 includes, for example, a photodetector instead of the frequency mixer 71. The optical resonator device 100 splits the output lights L _out 1 and L _out 2 that are input from the optical resonators 13 and 12 to the wavelength stabilization device 4 or the tuner 5 using a beam splitter (not shown), respectively. It is configured to also be input to the photodetector of the frequency comparator 7. Then, the output lights L _out 1 and L _out 2 are caused to interfere on the photodetector to extract an electrical signal having a beat frequency.

The composite optical resonator 10 may include two or more optical resonators with high temperature sensitivity. For example, the optical resonator 13 is made to have a structure with high temperature sensitivity like the optical resonator 12, and the difference in resonance frequency with the optical resonator 11 is measured. In such a composite optical resonator 10, temperature fluctuations are detected by measuring the difference between the average of the resonant frequencies of the optical resonators 12 and 13 (see Patent Document 2) and the resonant frequency of the optical resonator 11. . Alternatively, the resonant frequency difference between the optical resonator 13 and the optical resonator 11 and the resonant frequency difference between the optical resonator 12 and the optical resonator 11 may be measured. Further, in this case, the optical resonator 12 and the optical resonator 13 may have three levels of temperature sensitivity including the optical resonator 11, so that the optical resonator 12 and the optical resonator 13 have different temperature sensitivities. Such a configuration is obtained by making the substrates 2b, 2c, and 2d have three different thermal expansion coefficients. Alternatively, the reflecting mirrors 23 and 24 of the optical resonator 12 are both configured to have a high coefficient of thermal expansion, and only one of the reflecting mirrors 25 and 26 of the optical resonator 13 is configured to have a high coefficient of thermal expansion, or a compensating member 27 is provided. . Further, the composite optical resonator 10 may include four or more optical resonators and may be used for purposes other than temperature sensors (for example, see Patent Document 3).

Although the embodiments for implementing the composite optical resonator, temperature sensor, and optical resonator device according to the present invention have been described above, the present invention is not limited to these embodiments, and the claims Various changes are possible within the range shown in .

100 Optical resonator device 10 Composite optical resonator 1 Spacer 11, 12, 13 Optical resonator 1c Cavity 21, 23, 25 Reflector 22, 24, 26 Reflector 27 Compensating member 2a Reflector film 2b, 2c, 2d Substrate 3 Laser Light source 4 Wavelength stabilizer 5 Tuner 7 Frequency comparator (frequency comparison means)
8 Vacuum constant temperature chamber 81 Temperature control device (temperature control means)
9 Wavelength shifter (wavelength shifting means)

Claims (6)

A plurality of optical resonators each having a cavity penetrating from one surface of the spacer to the other surface and reflecting mirrors provided at both ends of the cavity are provided in the spacer so that the cavities are parallel to each other. A composite optical resonator that measures a resonant frequency difference that is a difference between the resonant frequencies of some of the optical resonators and other optical resonators by introducing a laser beam,
A composite optical resonator, wherein the reflecting mirrors of some of the optical resonators and the reflecting mirrors of the other optical resonators have different coefficients of thermal expansion. A plurality of optical resonators each having a cavity penetrating from one surface of the spacer to the other surface and reflecting mirrors provided at both ends of the cavity are provided in the spacer so that the cavities are parallel to each other. A composite optical resonator that measures a resonant frequency difference that is a difference between the resonant frequencies of some of the optical resonators and other optical resonators by introducing a laser beam,
A composite optical resonator characterized in that some of the optical resonators are provided with a compensating member having a coefficient of thermal expansion lower than that of the reflecting mirror, connected to the opposite side of the reflecting mirror to the side facing the cavity. . 2. The composite optical resonator according to claim 1, wherein a substrate of a reflecting mirror of said part of the optical resonator is formed of the same material as said spacer. A composite optical resonator according to any one of claims 1 to 3, a laser light source for introducing the laser light into each optical resonator of the composite optical resonator, and measuring the resonance frequency difference. comprising frequency comparison means;
The temperature of the composite optical resonator when the resonance frequency difference reaches a predetermined value is set as a reference temperature, and the temperature of the composite optical resonator is calculated from the difference between the resonance frequency difference measured by the frequency comparison means and the predetermined value. A temperature sensor that detects a difference from the reference temperature. A composite optical resonator according to any one of claims 1 to 3, a laser light source for introducing the laser light into each optical resonator of the composite optical resonator, and measuring the resonance frequency difference. comprising a frequency comparison means and a temperature control means for controlling the temperature of the composite optical resonator,
The temperature control means is an optical resonator device that performs feedback control of the temperature of the composite optical resonator based on the resonance frequency difference measured by the frequency comparison means so that the resonance frequency difference becomes a constant value. A composite optical resonator according to any one of claims 1 to 3, a laser light source for introducing the laser light into each optical resonator of the composite optical resonator, and measuring the resonance frequency difference. comprising a frequency comparing means and a wavelength shifting means for shifting the wavelength of the light resonating in one of the optical resonators of the composite optical resonator,
The wavelength shifting means is an optical resonator device that performs feedforward control to cancel fluctuations in the resonant frequency of the one optical resonator based on the resonant frequency difference measured by the frequency comparing means.

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