JPH083071Y2 - Frequency standard - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a frequency standard device using hyperfine spectrum transition of a standard substance, and particularly to improvement of a frequency standard device having a simple structure.

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

FIG. 5 shows a prior art of a frequency standard device according to the same applicant. In FIG. 5, 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 . A photodetector 32 exchanges the intensity of the transmitted light of the absorption cell 31 with 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,
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. First with oscillator 37 and adder 36
The PID control unit 35 and the adder 36 constitute 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, the PID control section 39 constitutes the frequency stabilizing means, and the frequency synthesizer 43 and the cavity resonator 44 constitute the electromagnetic wave generating means.

Next, the operation of this frequency standard will be described with reference to FIG. 6 showing the energy level of Rb ⁸⁷ . The adder 36 has an oscillator
The output of 37 is input, and the drive current of the semiconductor laser 30 is controlled by the output of this adder 36. Therefore, the output optical frequency of the semiconductor laser 30 is the frequency f which is the output frequency of the oscillator 37.
Modulated at _m1 (eg 2kHz). 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. Since 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, the output of the semiconductor laser 30 as shown in FIG. The frequency is controlled to be an optical frequency at which the Rb ⁸⁷ enclosed in the absorption cell 31 transits from the ground state F = 1 level to the 5P excited state.
Rb ⁸⁷ excited to 5P has equal probability F = 1 and F =
It falls to the level of 2. 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.
Rb ⁸⁷ in the level of F = 1 in this manner Rb ⁸⁷ in the level of F = 2 becomes less and less is increased. On the other hand, the output of the crystal oscillator 40 is output by the phase modulator 41 at a frequency f _m2 (for example, 150 Hz).
Is modulated by the frequency synthesizer 43, multiplied by 6.8 GHz by the frequency synthesizer 43, and added to the cavity resonator 44. The cavity resonator 44 is attached to the absorption cell 31.
Irradiates 8 GHz electromagnetic wave. The frequency of this electromagnetic wave is the frequency
It is modulated by f _m2 . In FIG. 6, Rb ^{87 at} the F = 2 level in the ground state 5S drops to the F = 1 level due to stimulated emission by this electromagnetic wave, and absorption of the output light of the semiconductor laser 30 by the absorption cell 31 increases. The transmitted light intensity decreases. The intensity of this transmitted light is detected by the photodetector 32 and the second
The lock-in amplifier 38 of 1 _performs synchronous detection 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.

<Problems to be solved by the device> However, such a frequency standard device has the following problems. That is, the light incident on the absorption cell 31 in FIG. 5 is absorbed by Rb ⁸⁷ atoms as it travels through the absorption cell 31, and the optical power density gradually decreases. Therefore, a non-uniform distribution of optical power density occurs in the optical axis direction of the absorption cell 31. Generally, when the optical power density changes, the resonance frequency of the atom changes.Therefore, a non-uniform distribution of the resonance frequency is created in the absorption cell 31, a non-uniform spread of the resonance absorption spectrum line width occurs, and the stability of the atomic frequency standard is improved. Will fall.

<Object of the Invention> An object of the present invention is to provide a frequency standard device that can be downsized and have improved frequency stability.

<Means for Solving the Problems> The present invention irradiates light having a predetermined frequency to an absorption cell in which a standard substance having a specific absorption spectrum is encapsulated, and irradiates an electromagnetic wave to the absorption cell to detect the above-mentioned standard substance. In a frequency standard device that causes a fine spectrum transition and generates a standard frequency based on the hyperfine spectrum transition, a semiconductor laser, and a first modulator that modulates output light of the semiconductor laser at a first frequency, An absorption cell in which a standard substance having a specific absorption spectrum is enclosed, and an output light of the semiconductor laser is converged to irradiate the absorption cell to make the distribution of the optical power density in the absorption cell uniform in the optical axis direction. Optical means, a photodetector for detecting the transmitted light of the absorption cell, an oscillator, second modulating means for modulating the output of the oscillator at a second frequency, and Electromagnetic wave generating means for multiplying the output of the modulating means to apply an electromagnetic wave to the absorption cell; first amplifying means for synchronously detecting the output of the photodetector at a frequency related to the first frequency; 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 second detecting means for synchronously detecting the output of the photodetector at a frequency related to the second frequency. An amplification means and a frequency control means for controlling the output frequency of the oscillator based on the output of the second amplification means are provided, and the first and second frequencies are made different.

<Operation> Since the output light of the semiconductor laser is converged light by the optical means, it is possible to cancel the change in the optical power density caused by the absorption in the absorption cell.

<Example> A detailed description will be given below with reference to the drawings of the present invention.

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. 5 are designated by the same reference numerals and the description thereof will be omitted. Reference numeral 45 denotes a lens which makes the output light of the semiconductor laser 30 incident to be a convergent light and makes the output light enter the absorption cell. Here, a convex lens is used.

Consider a case where the z-axis is taken in the optical axis direction and the convergent light 46 is applied to the absorption cell 31 as shown in FIG. the light intensity at z = 0
_Assuming that I _o is the absorption coefficient in the absorption cell and k is the length in the optical axis direction of the absorption cell, the light intensity I is I (z) = I _o e ^-kz (where O ≦ Z ≦ l). 1) Therefore, when the angle formed by the converged light with the optical axis is θ, the optical power density ρ is given by the following equation.

ρ (z) = I / πr 2 = I o e -kz / π (R-ztanθ) 2 ......
(2) However, the optical power in the beam is uniform. Where z =
It is assumed that the optical power densities at O and z = 1 are equal.
That is, from ρ (O) = ρ (l) (3), I _o / πR ² = I _O e ^-kl / π (R-ltan θ) ² (4). If equation (4) is transformed and solved for θ, θ = tan ⁻¹ R (1 ± e ^{−kl / 2} ) / l (5) Since it is not preferable to focus in the absorption cell 31,
Excluding from equation (5), θ = tan ⁻¹ R (1-e ^{−kl / 2} ) / l (6) Therefore, if θ is determined using equation (6), z
The optical power densities at = 0 and z = 1 can be made equal. The adjustment of θ can be performed by changing the focal length of the lens 45. For example, the absorption in the absorption cell 31 is 50
%, That is, when e ^−kl = 0.5, the extreme value of ρ (z) is ρ = 0.97ρ (0) = 0.97ρ (l). Therefore, the rate of change of the optical power density is greatly improved from 50% in the case of parallel light to 3% in the case of convergent light.

According to the frequency standard device having such a configuration, the convergent light is applied to the absorption cell, so that as the light incident on the absorption cell advances, the optical power decreases due to absorption of atoms, but the light beam diameter decreases. Since it goes, compared to the case of parallel light,
The distribution of the optical power density in the optical axis direction becomes uniform. As a result, the line width of the resonance absorption spectrum becomes narrower and the frequency stability is improved.

Moreover, since the convergent light is used, the photodetector can be small,
Fast response and low price.

Further, in this invention, one absorption cell is provided, and the output light of the semiconductor laser and the electromagnetic wave irradiating the absorption cell are modulated at different frequencies and separated. Therefore, by using a semiconductor laser as the light source, the spectrum width becomes narrower than that using a conventional rubidium lamp, so that short-term stability and frequency accuracy are improved, and the life can be extended. Also, by using one absorption cell, the absorption cell is irradiated with electromagnetic waves, and the frequencies of the electromagnetic waves and the output light of the semiconductor laser are modulated at different frequencies to separate these signals. Becomes possible. Especially when used as a secondary standard, the effect is great.

Although a convex lens is used as the optical means for converging the output light of the semiconductor laser in the above embodiment, the present invention is not limited to this, and any optical means can be used. For example, FIG. 3 shows a case where the concave mirror 47 is used to converge.

Further, in the above-mentioned embodiment, the injection current of the semiconductor laser is modulated as a means for modulating the frequency of the output light of the semiconductor laser, but 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 levels of F = 2 and F = 1 was used, but light-microwave 2 by absorption of electromagnetic waves was used.
The multiple resonance signal 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 5P excited state, and falls to the F = 1 and F = 2 levels of the 5S with equal probability. Here, the electromagnetic wave is applied to the absorption cell 31 and F = from the level of F = 1.
Transition to the 2nd 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 substance to be sealed in the absorption cell 31 in the above-described examples, Rb ⁸⁵ and Cs ¹³³ may be used instead. For Rb ⁸⁵ , the electromagnetic wave emitted is 3 GHz, and for Cs ¹³³ , it is 9.2 GHz.

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.

Further, the first lock-in amplifier 34 and the second lock-in amplifier 38 have frequencies f _m1 and f _m2 which are the outputs of the oscillators 37 and 42.
Although the signal of is input, the frequency may be an integral multiple thereof.

<Effects 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.

[Brief description of drawings]

FIG. 1 is a structural lock 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 a modification of the device of FIG. Fig. 4 is a block diagram showing the structure, Fig. 4 is a diagram showing the energy level of Rb ⁸⁷ in the case of electromagnetic wave absorption, Fig. 5 is a block diagram showing the structure of a conventional frequency standard device, and Fig. 6 is Rb ^{87 in} the case of stimulated emission. It is a figure which shows the energy level of. 30 ... Semiconductor laser, 31 ... Absorption cell, 32 ... Photodetector, 34 ... First amplification means, 35, 39 ... PID control section, 36 ...
... adder, 37, 42 ... oscillator, 38 ... second amplifying means, 4
0 …… Crystal oscillator, 41 …… Phase modulator, 43 …… Frequency synthesizer, 44 …… Cavity resonator, 45,47 …… Optical means.

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 enclosed, and an electromagnetic wave is applied to the absorption cell to cause a hyperfine spectrum transition of the standard substance. In a frequency standard device that generates a standard frequency based on this ultrafine spectral transition, a semiconductor laser, a first modulator that modulates the output light of this semiconductor laser at a first frequency, and a specific absorption spectrum inside An absorption cell in which a standard substance is enclosed, optical means for converging the output light of the semiconductor laser and irradiating the absorption cell to make the distribution of the optical power density in the optical axis direction uniform in the absorption cell; and the absorption cell A photodetector for detecting the transmitted light of the, an oscillator, a second modulator for modulating the output of the oscillator at a second frequency, and an output of the second modulator for multiplication. Electromagnetic wave generating means for applying an electromagnetic wave to the absorption cell, first amplifying means for synchronously detecting the output of the photodetector at a frequency related to the first frequency, and based on the output of the first amplifying means Frequency stabilizing means for stabilizing the frequency of the output light of the semiconductor laser; second amplifying means for synchronously detecting the output of the photodetector at a frequency related to the second frequency; and And a frequency control means for controlling the output frequency of the oscillator based on the output of the amplifying means, wherein the first and second frequencies are made different from each other.