JP7066138B2 - Light source system for atomic clocks - Google Patents

Description

The present invention relates to a light source system for an atomic clock.

The current time standard is defined using the microwave transition (9.2 GHz) of the cesium 133 ( ¹³³ Cs) atom. In recent years, research on next-generation atomic clocks (optical clocks) that surpass the accuracy of atomic clocks using Cs (Cs atomic clocks) by using optical frequencies (several 100 THz) has been conducted in Japan, Asia, Europe and the United States. It is carried out at research institutions such as. Optical lattice clocks mainly include optical lattice clocks and ion clocks, both of which have already been reported to have an accuracy two orders of magnitude higher than that of Cs atomic clocks, and research is being conducted toward the realization of more accurate clocks. (See Non-Patent Document 1).

By connecting such high-precision watches with an optical fiber network (see Non-Patent Document 2) and comparing the optical frequencies of the two watches, it has become possible to accurately detect even a height difference of 1 cm (see Non-Patent Document 2). See Non-Patent Document 3). By applying this and making the atomic clock portable and miniaturized, for example, research on a portable optical lattice clock for the purpose of altitude mapping is also underway (see Non-Patent Document 4). In this way, research on photon clocks is drawing attention not only in academic aspects but also in practical aspects.

By the way, the above-mentioned optical lattice clock system is a development of the cooling technology and the spectroscopic technology of neutral atoms. Normally, the optical system of an atomic clock system is constructed with a space between each part, so it is susceptible to disturbances from the outside world such as changes in temperature, humidity and atmospheric pressure, and vibration.

Here, the operating principle of the optical lattice clock disclosed in Non-Patent Document 1 will be briefly described. First, atoms are cooled and captured by Zeeman cooling and a magneto-optical trap using lasers of several wavelengths that have been combined. Next, the atoms are recaptured in the optical lattice potential by launching a laser for forming the optical lattice potential while gradually weakening the laser intensity. Next, the resonance spectrum is measured by irradiating the atom captured by the optical lattice potential with a clock laser pulse having a narrow line width that resonates with the clock transition of the atom.

Clock lasers are generally stabilized by a high finesse resonator made of low expansion (ULE) glass, but the distortion of the glass changes the resonator length and causes frequency drift. Therefore, as described above, the clock laser frequency is repeatedly adjusted so as to always match the resonance frequency of the atomic spectrum, and the clock is operated as a clock.

Next, the conventional atomic clock system will be described with reference to FIG. This system consists of a laser unit 401, a laser light modulation unit 402, and a measurement unit 403 that performs atomic cooling and spectroscopic measurement. The laser unit 401 usually includes several to 10 lasers having wavelengths from the near-ultraviolet region to the near-infrared region.

In the laser unit 401, the laser light is frequency-stabilized and emitted using an optical comb or the like. The laser light emitted from the laser unit 401 is modulated in intensity and frequency by the laser light modulation unit 402, and is pulsed. The laser light modulator 402 is composed of an acousto-optic modulator (AOM) 404, a mechanical shutter (MS) 405, and a combiner 406. AOM404 is used for intensity modulation and frequency modulation. Further, the AOM404 can turn on and off light at high speed on the order of several tens of ns by electrically blocking the applied radio frequency (RF), and can pulse CW (Continuous Wave) light.

However, even if RF is blocked, slight diffraction occurs and unnecessary leakage light (stray light) is generated. Since this stray light is a factor that lowers the accuracy of the atomic clock, the stray light is prevented from being irradiated to the atom by arranging the MS405 after the AOM404.

Since it is necessary to use several kinds of wavelengths at the same time on the same optical path in the magneto-optical trap, wavelength multiplexing is performed by a combiner 406 configured by using a dichroic mirror or a beam splitter. The modulated and coupled (wavelength-division-multiplexed) laser is finally carried in space or through an optical fiber to a measuring unit 403 such as a vacuum chamber.

As described above, in order to maintain the accuracy of the configured atomic clock system, it is necessary to periodically perform maintenance such as laser alignment in the laser unit 401 and the laser light modulation unit 402. Further, in order to construct the optical clock network as described above, it is necessary to transport and arrange a plurality of clocks at each point. However, there is a danger that the system will collapse due to vibration during transportation, and the environment in which it is placed is not always in a stable temperature and humidity environment, so it is necessary to consider a system that is resistant to vibration and disturbance.

In addition, atomic clock systems tend to be very complex and large, so it takes a lot of time to build a single device. For the above reasons, a robust atomic clock system that is small, easy to carry, and requires no maintenance in any environment is desired.

I. Ushijima et al., "Cryogenic optical lattice clocks", Nature Photonics, vol. 9, pp. 185-189, 2015. F. Riehle, "Optical clock networks", Nature Photonics, vol. 11, pp. 25-31, 2017. T. Takano et al., "Geopotential measurements with synchronously linked optical lattice clocks", Nature Photonics, vol. 10, pp. 662-666, 2016. S. B. Koller et al., "Transportable Optical Lattice Clock with 7 × 10-17 Uncertainty", Physical Review Letters, vol. 118, no. 7, 073601, 2017. A. Nakao et al., "Integrated waveguide-type red-green-blue beam combiners for compact projection-type displays", Optics Communications, vol. 330, pp. 45-48, 2014. J. Sakamoto et al., "High-efficiency multiple-light-source red-green-blue power combiner with optical waveguide mode coupling technique", Proc. Of SPIE, vol. 10126, 101260M, 2017. K. Higuma et al., "X-cut lithium niobate optical singlesideband modulator", Electronics Letters, vol. 37, no. 8, pp. 515-516, 2001. Shinji Mino et al., "High-speed multi-valued optical modulator using PLC-LN hybrid integration technology", Research and development of optical component technology for 100 Gbit / s digital coherent communication, NTT Technology Journal, pp. 57-61, March 2011 ..

In order for the above-mentioned atomic clock to operate stably at all times, it is necessary to reduce the laser intensity and frequency fluctuation as much as possible. The causes of these fluctuations are mainly due to changes in temperature, humidity, and atmospheric pressure, and the effects of vibration. The influence of these disturbances is considered to be proportional to the length of the optical path length from the laser unit 401 to the measurement unit 403 of the atomic clock system described with reference to FIG.

The light source of a conventional atomic clock is generally arranged in space, and various parts such as an optical element such as a mirror and a lens, and a post stand are mainly used. As a result, the size from the laser unit 401 to the laser light modulation unit 402 is at least the size of several optical surface plates for fixing the above-mentioned optical components. This means that the total optical path length from the laser unit 401 to the measurement unit 403 is finally very long.

In order to operate this system stably, it is necessary to seal the entire optical surface plate to control the temperature, or to reduce the size of the optical system to shorten the optical path length and reduce the influence of the above-mentioned disturbance. However, although the temperature can be controlled locally, it is difficult to control the temperature uniformly as a whole when the size of the optical system is large.

Further, as described above, in order to construct an atomic clock network using a plurality of atomic clocks, it is important to make the atomic clock smaller and simpler. The configuration of a conventional atomic clock as described above is very complicated, and the entire device is also large. For this reason, a large vehicle is used to transport the atomic clock, and there is a risk that the optical path may be displaced due to vibration during transportation. In addition, it takes a lot of time to build atomic clocks one by one, and it is essential to simplify the entire device.

As a technique for solving the above-mentioned problems, for example, an optical element coupled with an optical fiber can be used. AOMs, MSs, and optical combiners in the visible range are generally commercially available, and by connecting these elements with optical fibers and using them, the effects of disturbance due to environmental changes can be reduced, and the size and size can be reduced. There is also an advantage in terms of simplification. However, in general, an optical fiber in the visible region has a problem that the propagation loss of laser power is large as compared with the communication wavelength band, and the connection loss between optical fibers is large because the size of the core diameter is small. There is also a problem of frequency fluctuation due to fluctuation of the optical fiber length called optical fiber noise and intensity fluctuation due to vibration of the optical fiber.

The present invention has been made to solve the above problems, and an object of the present invention is to simplify a light source system for an atomic clock that operates more stably to a smaller size.

The light source system for an atomic clock according to the present invention is a light source system for an atomic clock for generating a laser beam used for an atomic clock, and modulates a laser unit that emits a laser beam and a laser beam emitted from the laser unit. The laser light modulation unit is provided with a laser light modulation unit, and the laser light modulation unit is composed of a planar light wave circuit.

In one configuration example of the light source system for an atomic clock, the laser light modulation unit includes a first modulation unit that modulates the frequency of the laser light emitted from the laser unit and a second modulation unit that modulates the intensity, and the first modulation unit is provided . The unit is a carrier suppression single sideband modulator.

In one configuration example of the light source system for an atomic clock, the first modulation unit turns on / off the laser beam to be output in addition to the frequency modulation.

In one configuration example of the light source system for an atomic clock, the first modulation unit is composed of an optical waveguide composed of an electro-optical crystal.

In one configuration example of the light source system for an atomic clock, the second modulation unit is composed of an optical switch having a Mach-Zehnder interference structure.

In one configuration example of the light source system for an atomic clock, the second modulation unit is composed of a Mach Zender interference structure using an optical waveguide composed of electro-optical crystals.

In one configuration example of the light source system for an atomic clock, the laser light modulation unit includes a combine unit arranged on the output side of the second modulation unit.

As described above, according to the present invention, since the laser light modulator is composed of a planar light wave circuit, there is an excellent effect that a light source system for an atomic clock that operates more stably can be simplified to a smaller size. can get.

FIG. 1 is a configuration diagram showing a configuration of a light source system for an atomic clock according to an embodiment of the present invention. FIG. 2 is a configuration diagram showing a configuration of a light source system for an atomic clock according to the first embodiment of the present invention. FIG. 3 is a configuration diagram showing a configuration of a light source system for an atomic clock according to a second embodiment of the embodiment of the present invention. FIG. 4 is a configuration diagram showing the configuration of a light source system for an atomic clock.

Hereinafter, the light source system for an atomic clock according to the embodiment of the present invention will be described with reference to FIG. This light source system for an atomic clock includes a laser unit 101 that emits laser light whose frequency is stabilized at a plurality of frequencies, and a laser light modulation unit 102 that is composed of a planar light wave circuit. The planar light wave circuit can be composed of, for example, an optical waveguide made of a quartz-based material.

This light source system for atomic clocks is used to generate the laser light used in atomic clocks, such as to stabilize atomic clocks. For example, this light source system for atomic clocks uses a laser beam for cooling and capturing atoms used in an optical lattice clock by a Zeeman cooling and magneto-optical trap, a laser beam for forming an optical lattice potential, or a clock laser. Used to generate. The measuring unit 103 cools the atom, traps it, and measures the resonance spectrum (spectral measurement).

The laser light modulation unit 102 includes a first modulation unit 104 that modulates the frequency of the laser light emitted from the laser unit 101, and a second modulation unit 105 that modulates the intensity.

The first modulation unit 104 can be composed of, for example, a carrier wave suppression single sideband (SSB-SC) modulator. The SSB-SC modulator suppresses the carrier wave and the sideband wave on one side by appropriately adjusting the phase of the carrier wave and the sideband wave generated by applying the RF frequency to the electro-optical crystal with the applied DC voltage. , A frequency modulator that extracts and uses only the sideband wave on the other side (see Non-Patent Document 7).

Similar to the acousto-optic modulator (AOM) described above, the SSB-SC modulator can be switched at high speed by blocking the applied RF, and can pulse CW light. .. In AOM, the diffraction angle depends on the modulation frequency, and the laser optical path shifts when the frequency is modulated. Therefore, for example, when the diffracted light is optical fiber coupled, a coupling loss occurs. On the other hand, since the SSB-SC modulator passes on the same optical path even when frequency modulation is performed, the optical path does not shift, so that problems such as coupling loss such as AOM do not occur.

The SSB-SC modulator can be composed of a planar light wave circuit using an optical waveguide made of an electro-optical crystal such as a lithium niobate (LN) crystal. A technique for connecting an LN crystal and a planar light wave circuit in a planar light wave circuit in a communication wavelength band has been reported (see Non-Patent Document 8).

The second modulation unit 105 can be configured by an optical switch. The optical switch is composed of a Mach-Zehnder interferometer structure (Mach-Zehnder type interferometer). A heater is placed on one arm of the Mach-Zehnder type interferometer, and the optical path length of one arm is controlled by temperature control. The intensity can be modulated at the speed of ms. Further, when one optical switch is used in combination with a maximum of 30 dBm and two optical switches are used in combination, a sufficient extinction ratio of a maximum of 60 dBm can be obtained, so that it can be used as a substitute for MS. Optical switches are a technique often used in planar optical wave circuits in the communication wavelength band, and can be realized even in the visible range by designing the core diameter so as to satisfy the single mode condition.

Further, the second modulation unit 105 can be configured from an LN modulator. In the LN modulator, instead of the heater of the Mach-Zehnder type interferometer as an optical switch, an optical waveguide composed of an electro-optical crystal such as an LN crystal is inserted in one arm or both arms. By applying a DC voltage to the electro-optical crystal, intensity modulation can be performed at high speed. Although voltage correction is required for the temperature drift of the electro-optical crystal, it has the advantage of being able to respond at a higher speed than an optical switch. Similar to the SSB-SC modulator, it can be sufficiently realized if the applied technology of the LN crystal to the visible light plane light wave circuit is improved.

Further, the laser light modulation unit 102 includes a wave combination unit 106 arranged on the output side of the second modulation unit 105. The combined wave section 106 is composed of a coupled optical waveguide and is used for optical wave-guided lasers having different wavelengths, and the coupling efficiency can be 90% or more (see Non-Patent Documents 5 and 6).

The first modulation unit 104, the second modulation unit 105, and the combine wave unit 106 are formed on the same substrate constituting the planar light wave circuit, and are optically connected by an optical waveguide constituting the light wave circuit. Further, the modulator constituting the first modulation unit 104, the optical switch constituting the second modulation unit 105, and the LN modulator are formed in a part of the optical waveguide constituting the optical wave circuit.

Hereinafter, a more detailed description will be given using examples.

[Example 1]
First, the light source system for an atomic clock according to the first embodiment will be described with reference to FIG. 2, taking the case where the present invention is applied to an optical lattice clock as an example. This atomic clock light source system includes a laser unit 101, a laser light modulation unit 102 composed of a planar light wave circuit, and a measurement unit 103. Further, the laser light modulation unit 102 includes a first modulation unit 104, a second modulation unit 105, and a combine wave unit 106.

In the first embodiment, the laser unit 101 includes five first laser 111, second laser 112, third laser 113, fourth laser 114, and fifth laser 115. Further, the laser unit 101 includes a frequency stabilizing unit 116 that stabilizes the oscillation frequency of each of the lasers. The frequency stabilizing unit 116 stabilizes the oscillation frequency of each laser by generating an optical frequency comb.

The first modulation unit 104 includes SSB-SC modulators 141 and 142. The second modulation unit 105 includes optical switches 151, 152, 153, 154, 155 composed of a two-stage Mach-Zehnder type interferometer. The combined wave section 106 includes coupled optical waveguides 161, 162.

The laser light from the first laser 111 and the laser light from the second laser 112 are modulated by the second modulation unit 105 and coupled (combined) by the combine unit 106. The laser light from the third laser 113 is modulated by the first modulation unit 104 and the second modulation unit 105, and is coupled by the combine unit 106. The laser light from the fourth laser 114 is modulated by the second modulation unit 105. These lasers are used as a light source for a magneto-optical trap in the measuring unit 103. The laser light from the fifth laser 115 is modulated by the first modulation unit 104 and the second modulation unit 105. This laser is mainly used as a clock laser light source in the measuring unit 103.

The laser beam of the first laser 111 and the laser beam of the second laser 112 are intensity-modulated by the optical switch 151 and the optical switch 152, and then incident on the coupled optical waveguide 161. In the coupled optical waveguide 161 the laser light of the first laser 111 and the laser light of the second laser 112 that have passed through the optical switch 151 and the optical switch 152 are coupled.

The frequency of the third laser 113 is modulated by the SSB-SC modulator 141, and then the intensity is modulated by the optical switch 153. The laser light of the third laser 113 that has passed through the optical switch 153 is optically combined with the first laser 111 and the second laser 112 that are optically combined by the coupled optical waveguide 161 and the coupled optical waveguide 162. These three optical combined laser beams are carried to the measuring unit 103 by the optical fiber 107a.

The laser light of the fourth laser 114 is intensity-modulated by the optical switch 154 and carried to the measuring unit 103 by the optical fiber 107b. The frequency of the laser light of the fifth laser 115 is modulated by the SSB-SC modulator 142, the light intensity is modulated by the optical switch 155, and the light is carried to the measuring unit 103 by the optical fiber 107c.

[Example 2]
Next, the light source system for an atomic clock according to the second embodiment will be described with reference to FIG. Also in the second embodiment, the case where the present invention is applied to an optical lattice clock will be described as an example. This atomic clock light source system includes a laser unit 101, a laser light modulation unit 102 composed of a planar light wave circuit, and a measurement unit 103. Further, the laser light modulation unit 102 includes a first modulation unit 104, a second modulation unit 105a, and a combine wave unit 106. The first modulation unit 104 and the combine wave unit 106 are the same as those in the first embodiment described above.

In the second embodiment, the second modulation unit 105a includes LN modulators 151a, 152a, 153a, 154a, 155a.

Also in the second embodiment, the laser light from the first laser 111 and the laser light from the second laser 112 are modulated by the second modulation section 105a and coupled (combined) by the combine section 106. The laser light from the third laser 113 is modulated by the first modulation unit 104 and the second modulation unit 105a, and is coupled by the combine unit 106. The laser beam from the fourth laser 114 is modulated by the second modulation unit 105a. The laser coupled by the combine wave unit 106 and the laser modulated by the second modulation unit 105a are used as a light source for a magneto-optical trap in the measurement unit 103. The laser light from the fifth laser 115 is modulated by the first modulation unit 104 and the second modulation unit 105a. The laser modulated by the second modulation unit 105a is mainly used as a clock laser light source in the measurement unit 103.

In the second embodiment, the laser beam of the first laser 111 and the laser beam of the second laser 112 are intensity-modulated by the LN modulator 151a and the LN modulator 152a, and then incident on the coupled optical waveguide 161. In the coupled optical waveguide 161 the laser light of the first laser 111 and the laser light of the second laser 112 that have passed through the LN modulator 151a and the LN modulator 152a are optically combined.

Further, the intensity of the third laser 113 is modulated by the LN modulator 153a after the frequency is modulated by the SSB-SC modulator 141. The laser light of the third laser 113 that has passed through the LN modulator 153a is optically combined with the first laser 111 and the second laser 112 photo-merged by the coupled optical waveguide 161 and by the coupled optical waveguide 162. These three optical combined laser beams are carried to the measuring unit 103 by the optical fiber 107a.

The laser light of the fourth laser 114 is intensity-modulated by the LN modulator 154a and carried to the measuring unit 103 by the optical fiber 107b. The laser light of the fifth laser 115 is frequency-modulated by the SSB-SC modulator 142, then its intensity is modulated by the LN modulator 155a, and is carried to the measuring unit 103 by the optical fiber 107c.

As described above, according to the present invention, since the laser light modulation unit is composed of a planar light wave circuit, it becomes possible to simplify the light source system for an atomic clock that operates more stably to a smaller size. The present invention applies a planar light wave circuit for visible light to a light source system for atomic clocks that uses a large number of lasers with wavelengths from the near-ultraviolet region to the near-infrared region, including atomic clocks such as ion clocks. It can be applied to the entire spectroscopic experiment and has a very high ripple effect. For example, the present invention can be applied to a light source system such as a laser beam for polarizing atomic spins and a laser beam for observing the number of atoms in the ground state and excited state between clock transitions. Further, the light source according to the present invention is stable and easy to mass-produce, and as a result, it is easy to increase the feasibility for research requiring a plurality of devices such as an optical clock network.

It should be noted that the present invention is not limited to the embodiments described above, and many modifications and combinations can be carried out by a person having ordinary knowledge in the art within the technical idea of the present invention. That is clear.

101 ... Laser unit, 102 ... Laser light modulation unit, 103 ... Measurement unit, 104 ... First modulation unit, 105 ... Second modulation unit, 106 ... Combined wave unit.

Claims (4)

A light source system for atomic clocks for generating laser light used in atomic clocks.
The laser unit that emits the laser beam and
A laser light modulation unit that modulates the laser light emitted from the laser unit is provided.
The laser light modulation unit is composed of a planar light wave circuit.
The laser light modulation unit includes a first modulation unit that modulates the frequency of the laser light emitted from the laser unit and a second modulation unit that modulates the intensity.
The first modulation unit is a light source system for an atomic clock, characterized in that it is a carrier wave suppression single sideband modulator . In the light source system for an atomic clock according to claim 1 ,
The second modulation unit is a light source system for an atomic clock, characterized in that it is composed of an optical switch having a Mach-Zehnder interference structure. In the light source system for an atomic clock according to claim 1 ,
The second modulation unit is a light source system for an atomic clock, characterized in that it is composed of a Mach Zender interference structure using an optical waveguide composed of an electro-optical crystal. In the light source system for an atomic clock according to any one of claims 1 to 3 .
The laser light modulation unit is a light source system for an atomic clock, characterized in that the laser light modulation unit includes a wave combination unit arranged on the output side of the second modulation unit.

Images