JPH04118533A - Optical frequency counter - Google Patents

Abstract

PURPOSE:To highly precisely identify an optical frequency by determining the beat signal between a plurality of oscillating frequency stabilized light sources having a medium absorbing a light of a determined wavelength as a frequency standard, and a light to be measured. CONSTITUTION:A light to be measured 110 is divided into two by an optical coupler 13, and emitted to optical couplers 14, 15 respectively. These lights are combined with oscillating frequency stabilized light sources 11, 12 therein, and photoelectrically converted in light receivers 16, 17. The beat signal is processed in an optical frequency judging circuit 18 to identify the optical frequency of the light to be measured, which is displayed on a light frequency display part 19. Thus, by finding out the beat signal with a plurality of oscillating frequency stabilized light sources using a medium absorbing a light of a determined wavelength as a frequency standard, the absolute frequency of the measured light can be highly precisely identified, and the oscillating frequency of a laser beam can be highly precisely measured.

Description

Detailed Description of the Invention "Field of Industrial Application" The present invention uses resonance lines and absorption lines of atomic or molecular gases as standards to measure the optical frequency of a light source in optical communication and optical measurement. The present invention relates to an optical frequency counter configured to measure the optical frequency of light to be measured by measuring a beat signal with the light.

"Prior Art" FIG. 7 is an explanatory diagram of a conventional optical frequency counter using a Michelson interferometer. The light to be measured l is split into two by a beam splitter 2, reflected by a mirror 3.4, and caused to interfere on the light receiving surface of a light receiver 5. At this time, while moving the mirror 3 in parallel, changes in the interference signal are read, and the optical frequency of the light to be measured is measured by subjecting the interference signal to Fourier transform processing.

``Problem to be Solved by the Invention'' However, in such a conventional optical frequency counter, the interferometer fluctuates due to disturbances such as temperature fluctuations during measurement. Therefore, the accuracy is limited to about 10 MHz. Furthermore, it was necessary to correct variations in optical path length using a He-Ne laser or the like. Furthermore, since the mirror is moved mechanically, there is a problem that measurement takes time.

The present invention has been made to solve these problems, and an object of the present invention is to provide a practical optical frequency counter that can measure optical frequencies with high accuracy, stability, and high speed.

"Means for Solving the Problem" In order to solve the problem, the present invention provides a plurality of oscillation frequency stabilized light sources each using a medium that absorbs light of a predetermined wavelength as a frequency reference and each having a different oscillation frequency; an optical coupler for coupling the output light from the oscillation frequency stabilized light source and the measured light; a photodetector for photoelectrically converting the light emitted from the optical coupler; and a beat output from the photodetector. The present invention is characterized by comprising an optical frequency determining means for identifying an optical frequency based on a signal.

"Operation" In the configuration system of the present invention, by determining the beat signal between the light to be measured and a plurality of oscillation frequency stabilized light sources whose frequency is a medium that absorbs light of a predetermined wavelength, the light to be measured can be measured with high precision. The absolute frequency of light can be identified.

"Embodiments" The present invention will be described in detail below based on embodiments shown in the drawings.

FIG. 1(a) is a block diagram showing an embodiment of the optical frequency counter of the present invention. In this example, FIG.
), an oscillation frequency stabilized light source 11.12 oscillating at the absorption wavelength of the light absorption medium, an optical coupler 13,
14.15, optical receivers 16.17, optical frequency determination circuit 18, and optical frequency display section 19.

Here, the optical coupler 13 has an input end for the light to be measured 110 and divides the light to be measured 110 incident from this input end into two, and the divided light to be measured 110 is transmitted to the optical coupler 13. The light is made to enter the optical coupler 14.15 from the two output ends. On the other hand, optical coupler 14.
The output ends of the oscillation frequency stabilized light sources 11 and 12 are respectively connected to the input ends of the oscillation frequency stabilized light sources 11 and 15, so that the light to be measured 110 and the output light of the oscillation frequency stabilized light sources 11 and 12 are combined.

Note that the output ends of the optical couplers 14 and 15 are connected to optical receivers 16 and 17, respectively.
．． At 17, the combined light is charged and converted. The output of the optical receivers 16 and 17 is supplied to an optical frequency determination circuit 18, and after identifying the frequency of the light to be measured, the optical frequency determination circuit 18 causes the optical frequency display section 19 to display the frequency of the light to be measured. It looks like this.

Next, the oscillation frequency stabilized light source 11.12 is
), a semiconductor laser 111, an optical coupler 11
2. Optical frequency modulator 113, optical frequency reference absorption cell 1
14, a light receiver 115, and a feedback circuit 116. The optical coupler 112 is inserted between the semiconductor laser 111 and the optical frequency modulator 113, and its input end is connected to the semiconductor laser 1.
The output ends of the optical frequency modulator 1130 are connected to the input ends of the optical frequency modulator 1130, respectively.

The optical frequency modulator 113 applies predetermined frequency modulation to the output light of the semiconductor laser 111. The modulated light transmitted through the optical frequency standard absorption cell 114 is photoelectrically converted by the photoreceiver 115, and a feedback circuit 116 generates an error signal from the center frequency of the absorption line, thereby causing the oscillation frequency of the semiconductor laser 111 to follow the frequency standard. The injection current controlled in this manner is supplied to the semiconductor laser 111 to stabilize the oscillation frequency of the semiconductor laser 111. Although an example using an external optical frequency modulator has been shown here, the same effect can be obtained by using means for direct modulation of a semiconductor laser.

Next, the operation of the optical frequency counter of this embodiment is shown in Fig. 1 (a).
). The light to be measured 110 is connected to the optical coupler 13
The light is divided into two parts by 14° and 15°, respectively, and is incident on optical couplers 14 and 15, respectively. Here, each oscillation frequency stabilized light source 1
1.12 and is photoelectrically converted at the photoreceptor 16.17. The beat signal is processed by the optical frequency determination circuit 18, the optical frequency of the light to be measured is identified, and the optical frequency display section 19
will be displayed.

For example, in the device configuration shown in FIG.
1, InGaAsP oscillates at a wavelength of 1.536 μm.
A distributed feedback semiconductor laser (DFB type LD) is used, and acetylene gas < l 2 is used as the light absorption medium.
c 2H2> and isotopically substituted acetylene gas (13C2H
2) was used.

FIG. 2 is a diagram showing the absorption characteristics of acetylene gas and isotope-substituted acetylene gas. The cell length is 10 cm and the pressure is 760 Torr. Of these, 1. 536μm
The oscillation frequency of the semiconductor laser was stabilized using two nearby absorption lines.

FIG. 3 shows detailed characteristics of the two absorption lines used. The center wavelength of each absorption line is 1536.0
49nm (light frequency 195306GHz), 1535
．． 977 nm (optical frequency 195315 GHz),
The frequency difference is 9 GHz. Using these two absorption lines, we can calculate the fluctuation in the center oscillation wavelength of the two semiconductor lasers by I
The optical frequency was kept below 0-0-5n (IMHz).

Output light 1 of these two oscillation frequency stabilized light sources 11 and 12
17 and the light to be measured 110 are multiplexed by an optical coupler 14°15, and an InGaAsP system MQW with a frequency band of 10 GHz is combined.
High-speed avalanche photodiode (APD) structure
) to obtain the beat signal.

The optical frequency of the light to be measured is determined based on the appearance frequency of the beat signal.

For example, if the optical frequency of the first oscillation frequency stabilized light source 11 is flO1 and the optical frequency of the second oscillation frequency stabilized light source 12 is f20, then only the first light receiver 16 is transmitted as shown in FIG. 4(a). When the beat signal fll appears, the measured light 1
The optical frequency f+ of 10 is fl=flO−fll・
...(1). Similarly, when the light appears on both the first light receiver 16 and the second light receiver 17 as shown in FIG. 4(b), the optical frequency f2 of the measured light is f2=f10+f12=f20-f22. ...
...(2) becomes. Similarly, when the light appears only on the second light receiver 17 as shown in FIG. 4(C), the optical frequency f3 of the light to be measured
is f3=f20+f23...
(3) becomes.

With this configuration, the optical frequency ranges from 195,296 GHz to 195 GHz.
325GHz (wavelength 1.5359μm to 1.5361
It was possible to measure the optical frequency of the measured light up to (μm) with an accuracy of IMHz.

Furthermore, as shown in FIG. 5, even if the output light of the oscillation frequency stabilized light sources 11 and 12 is switched using the optical switch 51,
A similar effect can be obtained. In this case, the output ends of the oscillation frequency stabilized light sources 11.12 are respectively connected to the input ends of the optical switch 51, and the optical switch 51 internally switches the optical path to output the oscillation frequency stabilized light sources 11.12. It is configured to output one output light to the optical coupler 13.

Note that the output light is connected to the light to be measured 110 by the optical coupler 13.
The optical receiver 16 performs photoelectric conversion, and the output is supplied to the optical frequency determination circuit 18. After the optical frequency determination circuit 18 identifies the frequency of the light to be measured, it is displayed on the optical frequency display section 19. The frequency of the light to be measured is displayed.

Further, as shown in FIG. 6, a plurality of oscillation frequency stabilized light sources 11°12.60
．． . . 63 are connected, and these oscillation frequency stabilized light sources 11, 12, 60 . . . 63 can be switched and used to measure the optical frequency of the light to be measured over a wider range of wavelengths.

In particular, if the acetylene gas and isotope-substituted acetylene gas shown in Figure 3 are used, the thickness will decrease from 1.50 μm to 1.56 μm.
By using absorption lines localized over a wide range of
I over a wide range from 1.50I1m to 1.56pm
It is possible to measure optical frequencies with high precision of MHz.

Furthermore, even if ammonia gas, methane gas, carbon dioxide, or the like is used as the light-absorbing gas, the oscillation wavelength can be stabilized using the same operating principle as the above function.

The present invention has been specifically explained above based on examples, but
It goes without saying that the present invention is not limited to the embodiments described above, and can be modified in various ways without departing from the spirit thereof.

"Effects of the Invention" As explained above, the present invention provides a plurality of oscillation frequency stabilized light sources using a medium that absorbs light of a predetermined wavelength as a frequency reference and each having a different oscillation frequency, and the oscillation frequency stabilized light source. An optical coupler for coupling the output light from the optical coupler with the measured light, a photodetector for photoelectrically converting the light emitted from the optical coupler, and an optical frequency identified by the beat signal output from this photodetector. Since the present invention is characterized by comprising an optical frequency determining means for determining a light frequency of a predetermined wavelength, it is possible to obtain a beat signal with a plurality of oscillation frequency stabilized light sources using a medium that absorbs light of a predetermined wavelength as a frequency reference. The absolute frequency of the measurement light can be identified with high precision, and the oscillation frequency of the laser light can be measured with extremely high precision.This makes it possible to identify the optical frequency of the wavelength standard light source in coherent optical communication and the light source in optical measurement. There are advantages available for measurement.

[Brief explanation of drawings]

1 to 4 are shown to explain the first embodiment of the present invention, and FIG. 1(a) is a block diagram showing the configuration of the optical frequency counter of the present invention, and FIG. ) is a block diagram showing the configuration of the oscillation frequency stabilized light source, Figure 2 is a diagram showing the light absorption characteristics of acetylene gas and isotope-substituted acetylene gas, and Figure 3 is a diagram showing the light absorption characteristics of acetylene gas and isotope-substituted acetylene around 1.536 μm. A diagram showing the light absorption characteristics of gas, FIGS. 4(a), (b), and (c) are explanatory diagrams for determining the optical frequency, and FIG. 5 is a second embodiment of the optical frequency counter of the present invention. FIG. 6 is a configuration block diagram showing a third embodiment of the optical frequency counter of the present invention. FIG. 7 is a configuration block diagram showing a conventional optical frequency counter using a conventional optical interferometer. It is. 110... Light to be measured, 2... Beam splitter, 3
゜4... Mirror 5, 16, 17, 115... Light receiver, 11.12... Oscillation frequency stabilized light source, 13, 1
4゜15.112... Optical coupler, 18... Optical frequency determination circuit, 19... Optical frequency display section, 21.31.
・・Optical switch, 111・・Semiconductor laser, 113・
... Optical frequency modulator, 114 ... Absorption cell, 116.
... Feedback circuit, 117... Output light, 51... Optical switch, 61... Optical switch. Beat signal of optical receiver 16 (Fig. 4)

Claims (1)

[Claims] A plurality of oscillation frequency stabilized light sources using a medium that absorbs light of a predetermined wavelength as a frequency reference and each having a different oscillation frequency; It is characterized by comprising an optical coupler, a photodetector that photoelectrically converts the light emitted from the optical coupler, and an optical frequency determining means that identifies an optical frequency based on a beat signal output from the photodetector. Optical frequency counter.