JPH01177719A - Frequency standard device - Google Patents

Abstract

PURPOSE:To improve the short period stability and frequency accuracy and to attain the small sized device by using only one absorption cell, modulating an output light of a semiconductor laser and an electromagnetic wave radiated to the absorption cell by different frequency and separating the result. CONSTITUTION:An output of an oscillator 37 control a drive current of a semiconductor laser 30 via an adder 36, an output light frequency of the laser 30 is modulated in a frequency fm1, made incident on the absorption cell 31, where it is absorbed by Rb<87> and the result is detected by a photodetector 32. The output of the detector 32 is subject to synchronizing detection by a lock-in amplifier 34 and only the frequency change signal of the laser 30 is separated. On the other hand, the output of a crystal oscillator 10 is modulated by a frequency fm2 at a phase modulator 41, multiplied by a frequency synthesizer 43, and a cavity resonator 44 radiates an electromagnetic wave of 6.8MHz to the absorption cell 31. The transmitted light is detected to the detector 32 and subject to synchronizing detection by a lock-in amplifier 38. Thus, only a change in the transmitted light intensity due to the electromagnetic wave radiated appears at the output of the amplifier 38.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a frequency standard that utilizes ultra-fine spectral transitions of a standard material, and particularly relates to a frequency standard that has a simple configuration.

<Prior Art> Frequency standards that generate highly stable frequency outputs that utilize ultrafine spectral transitions of atoms are known. FIG. 4 shows an example of such a frequency standard. In FIG. 4, light from a rubidium lamp 1 is irradiated onto a resonance cell 2 in which Rb87 is sealed. The transmitted light of this resonant cell 2 is detected by a photodetector 3, and its detection output is amplified by a signal amplifier 4 and input to a synchronous detector 5. On the other hand, the output of the voltage controlled crystal oscillator 6 is input to a frequency synthesizer 7 and its frequency is multiplied. The output of the zero frequency synthesizer 7, which is phase modulated by the output of the low frequency oscillator 8, is transmitted to the cavity resonator 1 surrounding the resonant cell 2 via the varactor diode 9.
0, and the resonance cell 2 is irradiated with electromagnetic waves. The synchronous detector 5 is operated by the output of the low frequency oscillator 8, and the output thereof is integrated by the integrator 11 and input to the voltage controlled crystal oscillator 6. The output of this voltage controlled crystal oscillator 6 is taken out as a frequency standard output. The rubidium lamp 1 is the drive part 1
Driven by 2. .

Next, the operation of this frequency standard will be explained based on FIG. FIG. 5 shows the energy unit of Rb87, whose fundamental fi5s is divided into two ultrafine units, F=1 and F=2. When light from rubidium lamp 1 is absorbed, R
b87 is excited from the F-1 unit to the 5P excitation unit,
From there, it falls into units of F=1 and F=2 with equal probability. F=
Rb87, which has fallen to the unit of 1, is excited again to the excitation level 5P by the light from the rubidium lamp 1. In this way F=
Rb87 in units of 1 gradually decreases and Rb87 in units of F=2 increases. In such a situation,
When an electromagnetic wave of 6.8 GHz, which is the energy difference between F=1 and F=2, is applied to the cavity resonator 10, vt b 87, which was in the unit of F=2, falls to the unit of F=1 due to stimulated emission, Absorption by the resonance cell 2 becomes stronger. When the transmitted light of the resonant cell 2 is detected by the photodetector 3 and the voltage-controlled crystal oscillator 6 is controlled so that the intensity of the transmitted light is minimized, that is, the absorbed light is maximized, the output frequency of the voltage-controlled crystal oscillator 6 is It is highly stable because it is regulated by ultrafine units of Rb87.

Note that the multiplication factor in the frequency synthesizer 7 is set to about 136, for example.
0 so that a frequency of 5 MHz can be obtained.

FIG. 6 shows the configuration of another frequency standard, in which a semiconductor laser is used as the light source.

In FIG. 6, the output light of the semiconductor laser 13 is divided into two parts by a half mirror 14, and the two parts are input into an Rb87 resonant cell 15 and an Rb87 absorption cell 16, respectively. Rb8
7 The transmitted light of the absorption cell 16 is detected by the photodetector 17, and the detected output is inputted to the frequency control means 18. The 0 frequency control means 18 determines that the frequency of the output light of the semiconductor laser 13 matches the absorption line of Rb87. control its frequency as desired. The transmitted light of the Rb87 resonance cell 15 is detected by a photodetector 19 and inputted to a frequency control means 20. The zero frequency control means 20 adjusts the output of the crystal oscillator 21 so that the intensity of the light detected by the photodetector 19 is minimized. Control frequency. The output 1 of the crystal oscillator 21 is taken out as a standard frequency, and the frequency is multiplied by the frequency multiplier 22.
The signal is input to a cavity resonator 23 surrounding the resonance cell 15 . Since the operating principle is the same as that shown in FIG. 4, the explanation will be omitted.

<Problems to be Solved by the Invention> However, such frequency standards have the following problems. The frequency standard shown in FIG. 4 requires a large amount of power to excite the rubidium lamp 1, and has the disadvantage that the lamp life is short. Furthermore, since the output light of the rubidium lamp 1 has a wide spectral width, it has the disadvantage that it is inferior to the laser excitation method shown in FIG. 6 in terms of short-term stability and frequency accuracy.

The frequency standard shown in FIG. 6 uses a semiconductor laser as a light source, so it has a long life, and since the spectral width of the output light of the semiconductor laser is narrow, it has excellent short-term stability and frequency accuracy. However, since two Rb87 cells are required, the structure becomes complicated and the device becomes large.

<Object of the invention> An object of the invention is to provide a frequency standard that can be miniaturized.

Means for Solving the Problems> In order to solve the above problems, the present invention irradiates light of a predetermined frequency to an absorption cell containing a standard substance having a specific absorption spectrum, and In a frequency standard device that applies an electromagnetic wave to produce an ultra-fine spectral transition in this reference material and generates a standard frequency based on this ultra-fine spectral transition, the output light of a semiconductor laser is modulated into a first frequency by a first modulating means. The output light from this semiconductor laser is irradiated onto an absorption cell containing a standard substance having a specific absorption spectrum.

Electromagnetic waves are applied to this absorption cell by electromagnetic wave generating means. This electromagnetic wave is produced by modulating the output of the oscillator with a second frequency different from the first frequency using a second modulating means, and multiplying this modulated output. Further, the transmitted light intensity of the absorption cell is detected by a photodetector, the output of this photodetector is synchronously detected at a frequency related to the first frequency by a first lock-in amplifier, and based on the output, the The frequency of the output light of the semiconductor laser is stabilized by a frequency stabilizing means, and the output of the photodetector is synchronously detected at a frequency related to the second frequency by a second lock-in amplifier, and based on this output, The output frequency of the oscillator is controlled by a frequency control means.

<Embodiment> FIG. 1 shows an embodiment of the frequency standard according to the present invention.
In FIG. 1, 30 is a semiconductor laser, and its output light is incident on an absorption cell 31. In FIG.

The absorption cell 31 is filled with a standard substance that absorbs light at a specific frequency, such as Rb 87. A photodetector 32 converts the intensity of the light transmitted through the absorption cell 31 into an electrical signal. Reference numeral 33 denotes a front-end amplifier, which amplifies the output of the photodetector 32. 34 is a first lock-in amplifier, into which the output of the preamplifier 33 is input. 35 is a PID control unit, into which the output of the first lock-in amplifier 34 is input. 36 is an adder, which outputs the output of the oscillator 37 and the PID
iii The output of the IJ control section 35 is input. This oscillator 37
The output of is also input to a first lock-in amplifier 34, and a zero oscillator 37 and an adder 36 constitute a first modulation means.
The PID control section 35 and the adder 36 constitute frequency stabilization means. 38 is a second lock-in amplifier, to which the output of the preamplifier 33 is input. 39 is a PID control unit, into which the output of the second lock-in amplifier 38 is input. 40 is a crystal oscillator, which outputs a signal of, for example, 10 MHz. This crystal oscillator 40 has PIDIIJI1
The output of the first section 39 is input. 41 is a phase modulator, into which the output of the crystal oscillator 40 is input. The output of an oscillator 42 is also input to this phase modulator 41 .

The output of this oscillator 42 is also supplied to the second lock-in amplifier 3.
8 is also input. 43 is a frequency synthesizer, into which the output of the phase modulator 41 is input. A cavity resonator 44 is arranged to surround the absorption cell 31. This cavity resonator 44 has a second modulation means with an oscillator 42 to which the output of the frequency synthesizer 43 is input, and a phase modulator 41.
The PID control section 39 constitutes a frequency stabilizing means, and the frequency synthesizer 43 and the cavity resonator 44 constitute an electromagnetic wave generating means.

Next, the operation of this embodiment will be explained. The output of the oscillator 37 is input to the adder 36, and the drive current of the semiconductor laser 30 is controlled by the output of the adder 36, so that the output optical frequency of the semiconductor laser 30 is the output frequency of the oscillator 37, which is the frequency f, 1 (for example, 2 kHz>). The output light of this semiconductor laser 30 enters an absorption cell 31 and is absorbed by Rb 87, and the transmitted light intensity is detected by a photodetector 32. The output is synchronously detected at the frequency f1 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 drive current of the semiconductor laser 30 is increased to i9HM+ via the adder 36 so that the output of
The optical frequency is controlled to cause a transition from the ground state of b87 to the excited state of 5P. On the other hand, the crystal oscillator 40
The output of
H2), multiplied by frequency synthesizer 43 to 6.8 GH2, and applied to cavity resonator 44. Cavity resonator 4
4 irradiates the absorption cell 31 with electromagnetic waves of 6.8 GHz.

The frequency of this electromagnetic wave is modulated at frequency fn2.

As explained in FIG. 5, Rb87 at the F=2 level of the ground state 5S falls to the F=1 unit due to stimulated emission,
The absorption of the output light from the semiconductor laser 30 by the absorption cell 31 increases, and the intensity of the transmitted light decreases. The intensity of this transmitted light is detected by a photodetector 32 and sent to the second lock-in amplifier 3.
8 (from this, synchronous detection is performed at frequency f12. Therefore, only the change in transmitted light intensity caused by the electromagnetic wave irradiated to the absorption cell 31 by the cavity resonator 44 is separated into the output of the second lock-in amplifier 38. The PID control unit 39 determines that the output of the second lock-in amplifier 38 is the minimum, that is, that the stimulated emission from F=2 to F=1 due to the electromagnetic wave irradiated to the absorption cell 31 by the cavity resonator 44 occurs. The output frequency of the crystal oscillator 40 is controlled so as to reach the maximum frequency.

That is, the output frequency of the crystal oscillator 40 is F of Rb87.
It is highly stable because it is locked by the unit energy difference from =2 to F=1. In this example, the frequency is 1 and f
By making 1□ different, two pieces of information, 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.

Another embodiment is shown in FIG. 2. The same elements as in FIG. 1 are designated by the same reference numerals and their explanations will be omitted. This embodiment uses an acousto-optic modulator as means for modulating the frequency of output light from a semiconductor laser. In FIG. 2, reference numeral 45 denotes an acousto-optic modulator, into which the output light from the semiconductor laser 30 is incident, and the output light is incident into the absorption cell 31. 46
is an oscillator and oscillates a signal of frequency f. This signal is applied via switch 47 to acousto-optic modulator 45 . Switch 47 is driven by the output of oscillator 37. The acousto-optic modulator 45 changes the frequency of the output light from the semiconductor laser 30 to f only when the switch 47 is on. Shift tall. Therefore, the frequency of the output light from the semiconductor laser 30 is modulated by the frequency f,1. Since the operation is the same as in FIG. 1, the explanation will be omitted. In this way, the frequency of the output light from the semiconductor laser 30 is not modulated, so it can also be used as an optical frequency standard. Further, instead of the acousto-optic modulator 45, a waveguide type phase modulator or a phase modulator using a bulk type electro-optic element may be used. There are utensils etc.

Although stimulated emission in units of F=2 and F=1 was used in this example, it is also possible to use a light-microwave double resonance signal by absorption of electromagnetic waves. I will explain. Rb87 in unit of F=2 in ground state 5S
absorbs the output light of the semiconductor laser 30, transitions to the 5P excited state, and falls to the F=1 and F=2 levels of 58 with equal probability. Here, the absorption cell 31 is irradiated with electromagnetic waves to cause a transition from the F=1 level to the F=2 level. Since the configuration is the same as in FIGS. 1 and 2, detailed explanation will be omitted.

Further, in this example, Rb87 was used as the substance sealed in the absorption cell 31, but in addition, Rb85, c51
In the case of Rb85, the electromagnetic wave to be irradiated is 3GH2, and in the case of C5133, it is 9.2GHz.

In addition to the cavity resonator, any means capable of outputting electromagnetic waves, such as a general antenna, can be used as a means for irradiating the absorption cell with electromagnetic waves.

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

Furthermore, the frequency f which is the output of the oscillator 37.42 is applied to the first lock-in amplifier 34 and the second lock-in amplifier 38.
Although the signal f is input as Ill·m2, a frequency that is an integral multiple of the signal may be used.

<Effects of the Invention> As specifically explained above based on the embodiments, in the present invention, the output light of the semiconductor laser and the electromagnetic waves irradiated to the absorption cell are modulated and separated at different frequencies using one absorption cell. I did it like that. Therefore, by using a semiconductor laser as the light source, the spectral width becomes narrower than that using a rubidium lamp, so short-term stability and frequency accuracy are improved, and the service life can be extended. Furthermore, by using a single absorption cell, the electromagnetic waves are irradiated onto the absorption cell, and the frequencies of the electromagnetic waves and the output light of the semiconductor laser are modulated at different frequencies to separate these signals, making it significantly smaller and simpler. becomes possible.

However, if the frequency of the light irradiated to the absorption cell changes, the resonant frequency will shift and the frequency accuracy will decrease slightly, but since the number of absorption cells can be reduced to one, it can be significantly downsized and simplified. It is particularly effective when used as a secondary standard.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of a frequency standard according to the present invention, FIG. 2 is a block diagram showing another embodiment, and FIG.
5 and 5 are characteristic curve diagrams showing absorption characteristics, and FIGS. 4 and 6 are block diagrams showing the configuration of a conventional frequency standard. 30... Semiconductor laser, 31... Absorption cell, 32...
...Photodetector, 34...First lock-in amplifier, 3
5゜39... PID control unit, 36... Adder, 37
, 42° 46... Oscillator, 38... Second lock-in amplifier, 40... Crystal oscillator, 41... Phase modulator, 43... Frequency synthesizer, 44... Cavity resonance vessel,
45...Acousto-optic modulator, 47...Switch. Figure 3: 41! l

Claims (1)

[Claims] Irradiating light of a predetermined frequency to an absorption cell containing a standard substance having a specific absorption spectrum, and applying electromagnetic waves to the absorption cell to cause an ultrafine spectral transition of the standard substance, A frequency standard device that generates a standard frequency based on this ultrafine spectral transition includes a semiconductor laser, a first modulation means that modulates the output light of the semiconductor laser at a first frequency, and a first modulation means that modulates the output light of the semiconductor laser at a first frequency. an absorption cell in which a standard substance that is irradiated and has a specific absorption spectrum is sealed therein; a photodetector that detects the transmitted light of the absorption cell; an oscillator; and a second frequency that modulates the output of the oscillator at a second frequency. a second modulating means, an electromagnetic wave generating means for multiplying the output of the second modulating means and applying an electromagnetic wave to the absorption cell, and synchronously detecting the output of the photodetector at a frequency related to the first frequency. a first lock-in amplifier; a frequency stabilizing means for stabilizing the frequency of the output light of the semiconductor laser based on the output of the first lock-in amplifier;
a second lock-in amplifier that performs synchronous detection at a frequency related to the frequency of the second lock-in amplifier; and a frequency control means that controls the output frequency of the oscillator based on the output of the second lock-in amplifier; A frequency standard device characterized in that the second frequency is made different.