JPH0756516Y2 - Frequency standard - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to improvement of stability of a frequency standard device using hyperfine spectrum transition of a standard material.

<Prior Art> As a frequency standard device that generates a highly stable frequency output, a device using hyperfine spectrum transition of atoms is known.

FIG. 4 shows a prior art of a frequency standard device according to the same applicant. In FIG. 4, reference numeral 30 denotes a semiconductor laser, the output light of which is incident on the absorption cell 31. The absorption cell 31 is filled with a standard substance that absorbs light of a specific frequency, for example, Rb ⁸⁷ . Reference numeral 32 is a photodetector, which converts the intensity of the transmitted light of the absorption cell 31 into an electric signal. A preamplifier 33 amplifies the output of the photodetector 32. Reference numeral 34 is a first lock-in amplifier which constitutes a first amplifying means, to which the output of the preamplifier 33 is inputted. Reference numeral 35 is a PID control unit, to which the output of the first lock-in amplifier 34 is input. 36 is an adder to which the output of the oscillator 37 and the output of the PID control unit 35 are input. The output of the oscillator 37 is also input to the first lock-in amplifier 34 as a reference signal. The oscillator 37 and the adder 36 constitute a first modulating means, and the PID control unit 35 and the adder 36 constitute a frequency stabilizing means. Reference numeral 38 is a second lock-in amplifier which constitutes a second amplifying means, to which the output of the preamplifier 33 is inputted. 39 is a PID control unit, to which the output of the second lock-in amplifier 38 is input. Reference numeral 40 is a crystal oscillator, which outputs a signal of 10 MHz, for example. This crystal oscillator 40
The output of the PID control unit 39 is input to. 41 is a phase modulator to which the output of the crystal oscillator 40 is input. The output of the oscillator 42 is also input to the phase modulator 41. The output of this oscillator 42 is also input to the second lock-in amplifier 38. 43 is a frequency synthesizer to which the output of the phase modulator 41 is input. Reference numeral 44 denotes a cavity resonator, which is arranged so as to surround the absorption cell 31. The output of the frequency synthesizer 43 is input to the cavity resonator 44. Oscillator 42 and phase modulator
41 constitutes the second modulating means, PID control section 39 constitutes the frequency stabilizing means, and frequency synthesizer 43 and cavity resonator 44 constitute the electromagnetic wave generating means.

Next, the operation of this frequency standard will be described with reference to FIG. 5 showing the energy level of Rb ⁸⁷ . The adder 36 has an oscillator 37
The output of the adder 36 is input to the semiconductor laser.
Since the drive current of 30 is controlled, the output light frequency of the semiconductor laser 30 is modulated at the frequency f _m1 (for example, 2 kHz) which is the output frequency of the oscillator 37. The output light of this semiconductor laser 30 enters the absorption cell 31 and is absorbed by Rb ⁸⁷ , and the intensity of the transmitted light is detected by the photodetector 32. The output of the photodetector 32 is synchronously detected at the frequency f _m1 by the first lock-in amplifier 34, and only the frequency change signal of the output light of the semiconductor laser 30 is separated. The PID control unit 35 controls the drive current of the semiconductor laser 30 via the adder 36 so that the output of the first lock-in amplifier 34 becomes constant, and as shown in FIG. The output frequency is controlled to be an optical frequency at which the Rb ⁸⁷ enclosed in the absorption cell 31 transits from the level of the ground state F = 1 to the excited state of 5P. Rb ⁸⁷ excited to 5P falls to the levels of F = 1 and F = with equal probability. The Rb ⁸⁷ in the F = 1 level is excited to the excitation level 5P again by the output light of the semiconductor laser 30. In this way, Rb ⁸⁷ in the level of F = 1 gradually decreases and Fb
Rb ^87, which becomes the level of = 2, increases. On the other hand, crystal oscillator 40
Is modulated by the phase modulator 41 at a frequency f _m2 (for example, 150 Hz), multiplied by 6.8 GHz by the frequency synthesizer 43, and added to the cavity resonator 44. The cavity resonator 44 is 6.8 GHz in the absorption cell 31.
Irradiate the electromagnetic waves of. The frequency of this electromagnetic wave is frequency f _m2
Is modulated by. In FIG. 5, F = of the ground state 5S
The Rb ^{87 at the} 2nd level drops to the F = 1 level by the stimulated emission by this electromagnetic wave, and the semiconductor laser by the absorption cell 31
The absorption of the output light of 30 increases and its transmitted light intensity decreases. The intensity of the transmitted light is detected by the photodetector 32 and is synchronously detected by the second lock-in amplifier 38 at the frequency f _m2 . Therefore, the output of the second lock-in amplifier 38 shows only the change in the transmitted light intensity resulting from the electromagnetic wave with which the absorption cell 31 is irradiated by the cavity resonator 44. PI
The D control unit 39 has the minimum output of the photodetector 32, that is, the F due to the electromagnetic wave irradiated to the absorption cell 31 by the cavity resonator 44.
The output frequency of the crystal oscillator 40 is controlled so that the stimulated emission from = 2 to F = 1 is maximized. That is, crystal oscillator
The output frequency of 40 is highly stable because it is locked by the energy difference between F = 2 and F = 1 of Rb ⁸⁷ . In this prior art, by making the frequencies f _m1 and f _m2 different,
Two pieces of information on the change in the output frequency of the crystal oscillator 40 and the change in the frequency of the output light of the semiconductor laser 30 are separated.

Since the light source of the above frequency standard is a semiconductor laser,
The spectrum width is narrower than that using a conventional rubidium lamp, short-term stability and frequency accuracy are improved,
Also, the life can be extended. Further, since one absorption cell is used and the frequencies of the electromagnetic wave and the output light of the semiconductor laser are modulated at different frequencies to separate these signals, the size and simplification can be greatly reduced. Especially when used as a secondary standard, the effect is great.

<Problems to be Solved by the Invention> However, such a frequency standard device has the following problems. That is, in the apparatus shown in FIG. 4, when the transition from F = 2 to F = 1 occurs due to the collision between Rb ⁸⁷ or Rb ⁸⁷ and the tube wall of the absorption cell 31, the relaxation time becomes short and the optical-microwave double The line width of the resonance line becomes large.
In addition, the line width becomes large due to the Doppler effect. Therefore, in order to prevent these and increase the Q, a buffer gas is usually enclosed in the absorption cell 31, and for this reason, the resonance line of the light used to control the frequency of the semiconductor laser 30 (ground state F = The absorptance of the optical frequency at which the transition from the 1 level to the 5P excited state is decreased, and the line width is also increased. As a result, the stability of the output frequency of the semiconductor laser 30 is lowered, and the stability of the frequency standard is lowered.

The present invention has been made in order to solve the above-mentioned problems, and an object thereof is to provide a frequency standard device which is miniaturized and has improved frequency stability.

<Means for Solving the Problem> The present invention irradiates an absorption cell in which a standard substance having a specific absorption spectrum is filled with light of a predetermined frequency, and
This is related to a frequency standard device that irradiates an electromagnetic wave to the absorption cell to generate a hyperfine spectrum transition of the standard substance, and generates a standard frequency based on the hyperfine spectrum transition. Its characteristic is a semiconductor laser. A first modulation means for modulating the output light of this semiconductor laser at a first frequency; a first absorption cell in which a standard substance having a specific absorption spectrum is enclosed; and the standard substance therein. And a second absorption cell filled with a buffer gas, a photodetector for detecting the light output from the semiconductor laser after passing through one of the first and second absorption cells and then through the other, and an oscillator. A second modulating means for modulating the output of the oscillator with a second frequency; and an electromagnetic wave generation for multiplying the output frequency of the second modulating means and applying an electromagnetic wave to the second absorption cell. A stage, first amplification means for synchronously detecting the output of the photodetector at a frequency related to the first frequency, and the frequency of the output light of the semiconductor laser based on the output of the first amplification means. Frequency stabilizing means for stabilizing; second amplifying means for synchronously detecting the output of the photodetector at a frequency related to the second frequency; and of the oscillator based on the output of the second amplifying means. And a frequency control means for controlling the output frequency.

<Operation> Since the first absorption cell does not contain a buffer gas, the absorption rate of the resonance line of light is increased, the line width is reduced, the stability of the output frequency of the semiconductor laser is improved, and the stability of the frequency standard device is improved. The degree improves.

<Example> The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a configuration block diagram showing an embodiment of a frequency standard device according to the present invention. The same parts as those in FIG. 4 are designated by the same reference numerals and the description thereof is omitted. In FIG. 1, reference numeral 311 denotes the first absorption cell in which only the reference material Rb ⁸⁷ is enclosed, 312
Is a second absorption cell in which, for example, a mixed gas of Ne and Ar is enclosed as a buffer gas together with the standard substance Rb ⁸⁷ . Here, the absorption cells 311 and 312 are adjacently and integrally formed. The cavity resonator 44 is the second absorption cell 31.
It is arranged so as to surround 2 and irradiate it with electromagnetic waves of 6.8 GHz. The output light of the semiconductor laser 30 is absorbed by the absorption cell 312.
After passing through the absorption cell 311 and is detected by the photodetector 32. FIG. 2 is a diagram showing a resonance absorption spectrum of light in the absorption cell 311.312. As shown in FIG. 2 (A), the absorption line width generated in the absorption cell 312 widens due to the influence of the buffer gas, but as shown in FIG. 2 (B), the absorption line width generated in the absorption cell 311 thereafter becomes the buffer line. Since there is no gas, the absorption rate of the resonance line of light increases and the line width decreases. As a result, the resonance absorption spectrum of the light incident on the photodetector 32 is the sum of the respective absorption spectra, as shown in FIG. 2C, and has a small line width portion a. Locking the output frequency of the semiconductor laser 30 to the resonance absorption spectrum of light based on the output of the photodetector 32 is the same as in the case of FIG. The output of the second lock-in amplifier 38 is separated by the cavity resonator 44, and only the change in the transmitted light intensity caused by the electromagnetic wave with which the second absorption cell 312 is irradiated appears. The output frequency of the crystal oscillator 40 is controlled so that the stimulated emission from F = 2 to F = 1 by the electromagnetic wave with which the second absorption cell 312 is irradiated is maximized. Other operations are the same as in the case of FIG.

According to the frequency standard device having such a configuration, by adding the absorption cell containing only the standard substance without the buffer gas, the absorption rate of the resonance line of light is increased and the line width can be reduced.
As compared with the case of FIG. 4, the stability of the output frequency of the semiconductor laser is improved, and as a result, the stability of the frequency standard is improved.

Although the injection current of the semiconductor laser is modulated as a means for modulating the frequency of the output light of the semiconductor laser in the above-mentioned embodiment, the invention is not limited to this, and an acousto-optic modulator may be used. In this way, the frequency of the output light of the semiconductor laser 30 is not modulated, so that it can be used as an optical frequency standard. Further, a waveguide phase modulator or a phase modulator using a bulk electro-optical element may be used instead of the acousto-optic modulator. This includes, for example, a vertical modulator, a horizontal modulator, a traveling waveform modulator and the like.

Further, in this embodiment, stimulated emission of the levels of F = 2 and F = 1 is used, but an optical-microwave double resonance signal due to absorption of electromagnetic waves may be used. This will be described with reference to FIG. Rb ^{87 at} the F = 2 level of the ground state 5S absorbs the output light of the semiconductor laser 30 and transits to the excited state of 5P,
With equal probability, it falls to the level of F = 1 and F = 2 of 5S. Here, the electromagnetic wave is applied to the absorption cell 31 so that the F = 1 level is transited to the F = 2 level. Since the configuration is the same as that in FIG. 1, detailed description will be omitted.

Further, although Rb ⁸⁷ is used as the standard substance to be enclosed in the absorption cells 311 and 312 in the above-mentioned examples, Rb ⁸⁵ and Cs ¹³³ may be used instead. In the case of Rb ⁸⁵ , the electromagnetic wave emitted is 3 GHz, Cs
^In case of ¹³³ , it becomes 9.2GHz.

Further, in FIG. 1, the cavity resonator 44 may be arranged so as to surround both the absorption cells 311 and 312.

Further, the two absorption cells 311 and 312 may not be integrated, but may be separated.

Contrary to FIG. 1, the transmitted light from the first absorption cell 311 is changed to the second
The light may be incident on the absorption cell 312.

Further, since the first absorption cell 311 has a high light absorptivity, its optical path length can be made shorter than that of the second absorption cell 312.

Further, as a means for irradiating the electromagnetic wave to the absorption cell, in addition to the cavity resonator, any antenna such as a general antenna that can output the electromagnetic wave can be used.

Further, the temperature may be controlled as a means for changing the frequency of the output light of the semiconductor laser.

Furthermore, the signals of the frequencies f _m1 and f _m2 , which are the outputs of the oscillators 37 and 42, are input to the first lock-in amplifier 34 and the second lock-in amplifier 38. You may

<Advantages of the Invention> As described above, according to the present invention, it is possible to realize a frequency standard device that can be downsized and have improved frequency stability with a simple configuration.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration block diagram showing an embodiment of a frequency standard device according to the present invention, FIG. 2 is a diagram for explaining the operation of the device of FIG. 1, and FIG. 3 is an electromagnetic wave. FIG. 4 is a diagram showing the energy level of Rb ⁸⁷ in the case of absorption, FIG. 4 is a structural block diagram showing a conventional frequency standard, and FIG. 5 is a diagram showing the energy level of Rb ⁸⁷ in the case of stimulated emission. 30 ... Semiconductor laser, 32 ... Photodetector, 34 ... First amplifying means, 35, 39 ... PID control section, 36 ... Adder, 37, 42 ... Oscillator, 38
... second amplifying means, 40 ... crystal oscillator, 41 ... phase modulator,
43 ... Frequency synthesizer, 44 ... Cavity resonator, 311 ... First absorption cell, 312 ... Second absorption cell.

Claims (1)

[Scope of utility model registration request] 1. A light having a predetermined frequency is applied to an absorption cell in which a standard substance having a specific absorption spectrum is encapsulated, and an electromagnetic wave is applied to the absorption cell to cause a hyperfine spectrum transition of the standard substance. In a frequency standard that generates a standard frequency based on this ultrafine spectrum transition, a semiconductor laser, a first modulation means for modulating the output light of this semiconductor laser at a first frequency, and a specific absorption spectrum inside And a second absorption cell in which the standard substance and a buffer gas are enclosed, and the output light of the semiconductor laser is the first and second absorption cells. A photodetector that detects light that has passed through one and then the other, an oscillator, a second modulator that modulates the output of the oscillator at a second frequency, and Electromagnetic wave generation means for multiplying the output frequency of the second modulation means to apply an electromagnetic wave to the second absorption cell; and first amplification for synchronously detecting the output of the photodetector at a frequency related to the first frequency. Means, frequency stabilizing means for stabilizing the frequency of the output light of the semiconductor laser on the basis of the output of the first amplifying means, and synchronization of the output of the photodetector with a frequency related to the second frequency. A frequency standard device comprising: second amplifying means for detecting; and frequency control means for controlling an output frequency of the oscillator based on an output of the second amplifying means.