JP2004061126A - Optical frequency measuring apparatus and measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To fix the vertical mode of reference laser beams for generating beat light and to uniquely determine the optical frequency of the laser beams to be inspected. <P>SOLUTION: An optical frequency measuring apparatus comprises a multimode light source that is driven by a modulation signal having a frequency f<SB>m</SB>and generates an optical COM having a vertical mode interval f<SB>m</SB>, an optical synthesizing means for synthesizing the optical COM and the laser beams to be inspected, a photodetection means for converting one vertical mode of the synthesized optical COM and the beat light of the laser beams to be inspected to an electric signal, and a frequency counter for measuring a frequency (f) of the electric signal. The optical frequency measuring apparatus measures the optical frequency of the laser beams to be inspected. Additionally, the optical frequency measuring apparatus includes a means for changing the vertical mode interval of the optical COM by changing the frequency of the modulation signal, an optical frequency deviation means for the optical frequency of the entire vertical mode of the optical COM in the same direction, and an optical frequency counting circuit for determining the optical frequency of the laser beams to be inspected from the amount of change in the frequency (f) of the electrical signal measured according to the vertical mode interval and the optical frequency deviation of the vertical mode. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical frequency measuring device for measuring an optical frequency of laser light and a measuring method.
[0002]
[Prior art]
A conventional optical frequency characteristic device is obtained by inputting a test laser beam to a Michelson interferometer, sweeping the optical length of one optical path, and measuring the fringe of interference output (the number of peaks and valleys in optical power fluctuation). There is an apparatus that measures the optical frequency of a test laser beam. However, in order to increase the accuracy in this configuration, it is necessary to increase the size of the interferometer, and therefore, there have been problems that the scale of the apparatus becomes large, that the apparatus becomes weak against vibration, and that the weight becomes heavy. Further, it was necessary to calibrate the sweep length using the optical frequency reference light.
[0003]
To avoid this problem, the beat light obtained by combining the reference laser light whose oscillation frequency is stabilized and the test laser light is converted into an electric signal, the frequency is counted, and the frequency of the reference laser light is counted. A method has been proposed for measuring the optical frequency of the test laser light by adding to the above.
[0004]
Furthermore, a method has been proposed in which this technology is developed and a measurable band is extended by using a broadband light source as the reference laser beam. As the broadband reference light source, to generate optical comb of the optical frequency interval f _m (set of longitudinal modes) by the oscillation frequency [nu ₀ is the electro-optic modulator a single-mode reference laser light stabilized, further optical nonlinearity There has been proposed a configuration in which the spectrum width is increased by inputting to a medium. When this broadband reference light source is used, the optical frequency of the beat light obtained from the reference light in the optical frequency range close to the test laser light is measured.
[0005]
[Problems to be solved by the invention]
By the way, since it is not known which beat light frequency to be measured is the beat between the test laser light and a number of longitudinal modes, the optical frequency of the test laser light could not be determined.
[0006]
An object of the present invention is to provide an optical frequency measurement device and a measurement method that can determine the longitudinal mode of a reference laser beam that generates beat light and uniquely determine the optical frequency of a test laser beam.
[0007]
[Means for Solving the Problems]
Claim 1 is driven by the modulation signal of the frequency f _m, and a multimode light source for generating a comb of longitudinal mode interval f _m around the reference mode of the reference light frequency [nu _0, the light output from the multimode light source Optical multiplexing means for multiplexing the comb and the test laser light, and light for inputting the light multiplexed by the optical multiplexing means and converting one longitudinal mode of the optical comb and the beat light of the test laser light into an electric signal. An optical frequency measuring device for measuring the optical frequency of the test laser light, comprising: a detecting unit; and a frequency counter for measuring a frequency f of the electric signal output from the optical detecting unit. Means for changing the longitudinal mode interval of the comb, Optical frequency shifting means for shifting the optical frequency of all longitudinal modes of the optical comb in the same direction, and measurement according to the vertical mode interval and the optical frequency shift of the vertical mode Be done And an optical frequency counter circuit for determining the optical frequency of the test laser beam from the change in the frequency f of the electrical signal.
[0008]
The optical frequency counter circuit, when the longitudinal mode interval by shifting the frequency of the modulation signal is varied from f _m to f _{m +} Delta] f _m, the variation of the longitudinal mode spacing Delta] f m _(including the sign), the electrical signal the means for counting the ratio n = Δf / Δf m with variation Delta] f of the frequency f (including sign) _(including the sign), and the change in the frequency f of the longitudinal mode of the optical frequency shift and an electric signal code S _M in the ratio means for determining _(if positive, -1 if it is negative), the reference light frequency [nu _0, and f _m, and n, on the basis of the S _M, the optical frequency of the test laser beam the _{ν X ν X = ν 0 +} S M nf m -S M f
(Claims 3 and 11).
[0009]
Claim 2 is driven by the modulation signal of the frequency f _m, and a multimode light source for generating a comb of longitudinal mode interval f _m around the reference mode of the reference light frequency [nu _0, the light output from the multimode light source Optical multiplexing means for multiplexing the comb and the test laser light, and light for inputting the light multiplexed by the optical multiplexing means and converting one longitudinal mode of the optical comb and the beat light of the test laser light into an electric signal. An optical frequency measuring device for measuring the optical frequency of the test laser light, comprising: a detecting unit; and a frequency counter for measuring a frequency f of the electric signal output from the optical detecting unit. Means for changing the longitudinal mode interval of the comb; optical frequency shifting means for shifting the optical frequency of the test laser light input to the optical multiplexing means; and, depending on the vertical mode interval and the optical frequency shift of the test laser light. Measured And an optical frequency counter circuit for determining the optical frequency of the test laser beam from the change in the frequency f of the electric signal.
[0010]
The optical frequency counter circuit, when the longitudinal mode interval by shifting the frequency of the modulation signal is varied from f _m to f _{m +} Delta] f _m, the variation of the longitudinal mode spacing Delta] f m _(including the sign), the electrical signal means for counting the change in Delta] f of the frequency f ratio (including the sign) n = Δf / Δf m _(including the sign), the change in the frequency f of the optical frequency shift and an electric signal of a test laser beam means for determining the ratio of the coded S _{T (if} positive 1, if negative -1) with a reference light frequency [nu _0, and f _m, and n, on the basis of S _T, of the test laser beam an optical frequency _{ν X ν X = ν 0 -S} T nf m + S T f
(Claims 4 and 12).
[0011]
The multi-mode light source includes reference light frequency control means for controlling the reference light frequency ν ₀ of the reference mode to be constant irrespective of a change in the longitudinal mode interval (claim 5).
[0012]
The multi-mode light source includes a mode-locked laser driven by a modulation signal, and an optical non-linear medium that receives an optical comb output from the mode-locked laser and widens its spectral width (claim 6). The multi-mode light source includes a light source that outputs a single-mode light having a reference light frequency ν ₀ , an optical modulation unit that is driven by a modulation signal, modulates the single-mode light to generate an optical comb, and an optical modulation unit. And an optical non-linear medium for inputting an optical comb output from the optical comb and widening its spectrum width.
[0013]
The optical frequency shifting means in the optical frequency measuring device according to claim 1, wherein the optical frequency shift means is provided between the multi-mode light source and the optical multiplexing means in accordance with the shift frequency signal output from the shift frequency signal generator. The optical frequency shift means for shifting the optical frequency is inserted (claim 8). Further, when the multi-mode light source includes the optical nonlinear medium according to claim 6 or claim 7, the optical frequency of the optical comb is provided before the optical nonlinear medium in accordance with the shift frequency signal output from the shift frequency signal generator. (Embodiment 9). Further, when the multi-mode light source includes the reference light frequency control means, the reference frequency of the optical frequency reference is shifted according to the shift frequency signal output from the shift frequency signal generator. Claim 10).
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows a first embodiment of the optical frequency measuring device of the present invention. In the figure, a multimode light source 1, the modulation signal is driven by the modulation signal of a frequency f _m which is output from the generator 2 to generate a comb of longitudinal mode interval f _m around the reference mode of the reference light frequency [nu ₀ . When the frequency of the modulated signal to Delta] f _m shifted from _{f m} to _{f m} + Delta] f _m, it varies longitudinal mode interval of the optical frequency comb from _{f m} to _{f m} + Delta] f _m. However, the reference light frequency ν ₀ in the reference mode does not change even if the vertical mode interval is changed.
[0015]
This optical comb is input to the optical coupler 4 via the optical frequency shift means 3, multiplexed with the test laser light, and input to the light detection means 5. The light detecting means 5 converts one longitudinal mode of the optical comb and the beat light of the test laser light into an electric signal, and the frequency f thereof is measured by the frequency counter 6.
[0016]
The optical frequency shift means 3 shifts all modes of the optical comb in the same direction by driving with the shift frequency signal output from the shift frequency signal generator 7. The optical frequency shift amount corresponding to the frequency of the shift frequency signal is defined as Δν ₀ . As the optical frequency shift means 3, a frequency shifter using an acousto-optic modulator (AOM) or an optical modulator utilizing an electro-optic effect can be used.
[0017]
In the case of the multi-mode light source 1 'having a reference light frequency control means for controlling the reference light frequency ν ₀ to be constant, the light frequency shift means 3 is not used as in the second embodiment shown in FIG. The shift frequency signal output from the shift frequency signal generator 7 is input to the reference optical frequency control means of the multi-mode light source 1 ', and all the modes of the optical comb are shifted in the same direction by shifting the reference frequency of the optical frequency reference. It may be shifted. A specific configuration example will be described later.
[0018]
The optical frequency shift by the optical frequency shift means 3 is not performed on the optical comb output from the multimode light source 1 but on the test laser light as in the third embodiment shown in FIG. May go.
[0019]
Optical frequency counting circuit 8 receives the frequency f _m, type Delta] f _m, the optical frequency shift amount .DELTA..nu ₀ corresponding shift frequency signal generator 7 to shift the frequency signal of the modulation signal from the modulation signal generator 2, optical frequency f measured by the frequency counter 6, enter a Delta] f, and calculates an optical frequency [nu _X of the test laser beam.
[0020]
Hereinafter, by observing the change of the longitudinal mode interval of the optical comb and the direction of the optical frequency shift, the longitudinal mode of the reference laser light for generating the beat light is determined, and the optical frequency of the test laser light is uniquely determined. The method will be described with reference to FIGS.
[0021]
First, the combined light of the optical comb and the test laser light (optical frequency ν _X ) is input to the light detecting means 5, and the frequency f of the electric signal obtained by photoelectrically converting the beat light is measured by the frequency counter 6. This frequency f corresponds to the optical frequency difference between one longitudinal mode of the optical comb and the test laser light.
[0022]
Next, the frequency f _m of the modulation signal for driving the multimode light source 1 is taken as to increase Δf _m f _{m +} Δf _m, calculates the increase amount Delta] f of the frequency f by measuring the frequency f + Delta] f by the frequency counter 6 . Here, as shown in FIG. 4B, assuming that the longitudinal mode for generating the test laser light and the beat light is the nth (ν _n ) counting from the reference mode of the reference light frequency ν ₀ .
| Δf | = | nΔf _m |
Relationship exists. At this time, also determines the sign of Δf / Δf _m. Incidentally Delta] f _m is positive in the example of FIG. 4, since Delta] f is negative, the sign of Delta] f / Delta] f _m becomes negative.
[0023]
Next, when the optical frequency shift means 3 gives the optical frequency shift amount Δν ₀ , the sign of Δf / Δν ₀ is determined from the change direction of the measured frequency f, f + Δf. Incidentally, in the example of FIG. 4, since Δν ₀ is positive and Δf is negative, the sign of Δf / Δν ₀ is negative. The sign of Δf / Δν ₀ can be determined whether the optical frequency shift means 3 gives the optical frequency shift statically or sinusoidally.
[0024]
From these results, the magnitude relationship between ν ₀ , ν _n , and ν _X (one of FIGS. 5A to 5D) is determined, and the optical frequency ν _X of the test laser beam is determined based on the magnitude relationship. I do. In the following description, but shows the case where the optical comb provide light frequency shift .DELTA..nu _0, when giving the optical frequency deviation .DELTA..nu ₀ to test the laser light may be considered with its sign reversed.
[0025]
5 (a) is a _{ν n <ν X <ν 0} , Δf / Δf m> 0, the Δf / Δν _{0 <0.} 5 (b) is a _{ν X <ν n <ν 0} , Δf / Δf m <0, the Δf / Δν _{0> 0.} In each case, the optical frequency v _n of the longitudinal mode that generates the beat light is
_{ν n = ν 0 -nf m =} ν 0 -f m | Δf / Δf m |
It becomes.
[0026]
FIG. 5 (c) is a _{ν 0 <ν n <ν X} , Δf / Δf m <0, the Δf / Δν _{0 <0.} Figure 5 (d) is <a _{ν X <ν n, Δf /} Δf m> 0, Δf / Δν 0> ν 0 is 0. In either case _{ν n = ν 0 + nf m} = ν 0 + f m | Δf / Δf m |
It becomes.
[0027]
Therefore, the optical frequency counter circuit _{8, Δf / Δf m> 0} , the case of FIGS. 5 (a) if Δf / Δν _{0 <0,} the optical frequency [nu _X of the test laser beam,
_{ν X = ν n + f =} ν 0 -f m | Δf / Δf m | + f
Asking.
[0028]
_{Δf / Δf m <0, Δf} / Δν 0> If 0 is the case of FIG. 5 (b), the optical frequency [nu _X of the test laser beam,
_{ν X = ν n -f = ν} 0 -f m | Δf / Δf m | -f
Asking.
[0029]
Δf / Δf _m <0, if Δf / Δν _{0 <0} is the case of FIG. 5 (c), the optical frequency [nu _X of the test laser beam,
_{ν X = ν n + f =} ν 0 + f m | Δf / Δf m | + f
Asking.
[0030]
Δf / Δf _m> 0, if Delta] f / .DELTA..nu _0> 0 is the case of FIG. 5 (d), the optical frequency [nu _X of the test laser beam,
_{ν X = ν n -f = ν} 0 + f m | Δf / Δf m | -f
Asking.
[0031]
(Configuration example of multi-mode light source 1)
FIG. 6 shows a configuration example of the multi-mode light source 1. In the figure, the multi-mode light source 1 is input to the nonlinear optical fiber 13 outputs light of the active mode-locked laser 11 driven by the modulation signal of the frequency f _m and amplified by an optical amplifier 12, non-linear effects (supercontinuum generation Etc.) to generate a broadband optical comb having a spectrum of several hundred nm (Reference: Electronics Letters, Vol. 30, No. 10, pp. 790-791). Longitudinal mode interval f _m of the optical comb can be varied according to the frequency of the modulation signal for driving.
[0032]
In addition, as the active mode-locked laser 11, a semiconductor mode-locked laser (MLLD), an erbium-doped optical fiber mode-locked ring laser, or the like can be used. When using this MLLD, the mode locking frequency (longitudinal mode interval) is determined by the effective resonator length. A few percent is allowed for detuning (the difference between the driving frequency and the optimum driving frequency). Even if the driving frequency is changed by several percent, there is no significant change in the pulse width, the output power, and the like.
[0033]
In addition, one of the longitudinal modes output from the active mode-locked laser 11 (the reference mode of the reference optical frequency ν ₀ ) is split via the optical coupler 14 and the optical bandpass filter 15, and the optical frequency of the absorption line is reduced. The light is input to a light detection circuit 17 via an optical frequency reference 16 having a calibrated standard gas. The light oscillation frequency control circuit 18 controls the oscillation frequency of the active mode-locked laser 11 according to the output of the light detection circuit 17, and forms a feedback loop so that the reference light frequency ν ₀ in the reference mode is synchronized with the gas absorption line. Is done. This makes it possible to control the reference light frequency ν ₀ in the reference mode to be very constant.
[0034]
The optical frequency shift means 3 is connected between the output side of the multi-mode light source 1 and inserted between the active mode-locked laser 11 (optical coupler 14) and the optical amplifier 12 constituting the multi-mode light source 1. You may. FIG. 7 shows a configuration example in this case.
[0035]
(Configuration example of multi-mode light source 1 ')
FIG. 8 shows a configuration example of the multi-mode light source 1 '. In the figure, a multimode light source 1 ′ has a configuration in which an optical frequency reference control circuit 19 for controlling a reference light frequency ν ₀ is added to the multimode light source 1 shown in FIG. By inputting the shift frequency signal to the optical frequency reference control circuit 19 and controlling the temperature or pressure of the gas used as the optical frequency reference 16, the reference frequency of the optical frequency reference 16 is shifted. This is a multimode light source 1 'corresponding to the second embodiment shown in FIG.
[0036]
Further, instead of the active mode-locked laser 11 of the multi-mode light sources 1,1 ', a light source for outputting a single mode light of the reference light frequency [nu _0, the optical comb by modulating a single-mode light by the modulation signal generator It is also possible to use light modulation means (for example, an electro-optic modulator) for causing the light to be modulated. The vertical mode interval f _m of multimode light source, since it is determined by the modulation signal frequency of the optical modulation means, it is possible to change the longitudinal mode interval f _m by changing its frequency.
[0037]
【The invention's effect】
As described above, according to the present invention, the longitudinal mode of the reference laser light for generating the beat light is determined, and the optical frequency of the test laser light can be uniquely determined. The optical frequency of the test laser beam can be measured with high accuracy.
[Brief description of the drawings]
FIG. 1 is a diagram showing a first embodiment of an optical frequency measurement device according to the present invention.
FIG. 2 is a diagram showing a second embodiment of the optical frequency measurement device of the present invention.
FIG. 3 is a diagram showing a third embodiment of the optical frequency measurement device of the present invention.
FIG. 4 is a view for explaining a specific principle of the optical frequency measurement device of the present invention.
FIG. 5 is a diagram illustrating a calculation pattern of an optical frequency of a test laser beam.
FIG. 6 is a diagram showing a configuration example of a multi-mode light source 1.
FIG. 7 is a diagram showing another configuration example of the multi-mode light source 1.
FIG. 8 is a diagram showing a configuration example of a multi-mode light source 1 '.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Multimode light source 2 Modulation signal generator 3 Optical frequency shift means 4 Optical coupler 5 Optical detection means 6 Frequency counter 7 Shift frequency signal generator 8 Optical frequency counting circuit 11 Active mode locked laser 12 Optical amplifier 13 Nonlinear optical fiber 14 Optical coupler 15 Optical bandpass filter 16 Optical frequency reference 17 Optical detection circuit 18 Optical oscillation frequency control circuit 19 Optical frequency reference control circuit

Claims (12)

Is driven by the modulation signal of the frequency f _m, and a multimode light source for generating a comb of longitudinal mode interval f _m around the reference mode of the reference light frequency [nu _0,
Optical multiplexing means for multiplexing the optical comb and the test laser light output from the multimode light source,
Light detecting means for inputting the light multiplexed by the optical multiplexing means and converting one longitudinal mode of the optical comb and beat light of the test laser light into an electric signal;
A frequency counter for measuring the frequency f of the electric signal output from the light detection means, an optical frequency measurement device for measuring the optical frequency of the test laser light,
Means for changing the longitudinal mode interval of the optical comb by changing the frequency of the modulation signal,
Optical frequency shift means for shifting the optical frequencies of all longitudinal modes of the optical comb in the same direction,
An optical frequency counting circuit that determines an optical frequency of the test laser light from a change in the frequency f of the electric signal measured according to the optical frequency shift of the longitudinal mode interval and the longitudinal mode. Optical frequency measuring device. Is driven by the modulation signal of the frequency f _m, and a multimode light source for generating a comb of longitudinal mode interval f _m around the reference mode of the reference light frequency [nu _0,
Optical multiplexing means for multiplexing the optical comb and the test laser light output from the multimode light source,
Light detecting means for inputting the light multiplexed by the optical multiplexing means and converting one longitudinal mode of the optical comb and beat light of the test laser light into an electric signal;
A frequency counter for measuring the frequency f of the electric signal output from the light detection means, an optical frequency measurement device for measuring the optical frequency of the test laser light,
Means for changing the longitudinal mode interval of the optical comb by changing the frequency of the modulation signal,
Optical frequency shift means for shifting the optical frequency of the test laser light input to the optical multiplexing means,
An optical frequency counting circuit that determines an optical frequency of the test laser light from a change in a frequency f of the electrical signal measured according to the vertical mode interval and an optical frequency shift of the test laser light. An optical frequency measuring device characterized by the above-mentioned. The optical frequency measurement device according to claim 1,
The optical frequency counting circuit,
The longitudinal mode spacing by shifting the frequency of the modulation signal is varied from f _m to f _{m +} Delta] f _m, the longitudinal mode spacing variation Delta] f m _(including the sign), the frequency f of the electric signal means for counting the change in Delta] f ratio (including the sign) n = Δf / Δf m _(including the sign),
Means for determining the sign S _M (1 for positive, −1 for negative) of the ratio between the optical frequency shift of the longitudinal mode and the change in the frequency f of the electrical signal;
And the reference optical frequency [nu _{0, f} and _m, and n, on the basis of the _{S M,} said test laser light frequency [nu _X the _{ν X = ν 0 + S M} nf m -S M f
An optical frequency measurement device comprising: The optical frequency measurement device according to claim 2,
The optical frequency counting circuit,
The longitudinal mode spacing by shifting the frequency of the modulation signal is varied from f _m to f _{m +} Delta] f _m, the longitudinal mode spacing variation Delta] f m _(including the sign), the frequency f of the electric signal means for counting the change in Delta] f ratio (including the sign) n = Δf / Δf m _(including the sign),
Means for determining the sign _ST (1 if positive, -1 if negative) of the ratio between the optical frequency shift of the test laser light and the change in the frequency f of the electrical signal;
And the reference optical frequency [nu _0, and _{f m,} n and, based on _{S T,} the optical frequency [nu _X of the test laser beam _{ν X = ν 0 -S T nf} m + S T f
An optical frequency measurement device comprising: The optical frequency measuring device according to claim 1 or 2,
The optical frequency measurement device according to claim 1, wherein the multimode light source includes a reference light frequency control unit that controls the reference light frequency ν ₀ in the reference mode to be constant regardless of a change in the vertical mode interval. The optical frequency measuring device according to claim 1 or 2,
The multi-mode light source,
A mode-locked laser driven by the modulation signal,
An optical frequency measuring device comprising: an optical non-linear medium that receives an optical comb output from the mode-locked laser and widens the spectrum width thereof. The optical frequency measuring device according to claim 1 or 2,
The multi-mode light source,
A light source that outputs single-mode light having the reference light frequency ν ₀ ,
Driven by the modulation signal, optical modulation means for generating an optical comb by modulating the single mode light,
An optical non-linear medium for inputting an optical comb output from the optical modulation means and expanding a spectrum width thereof; The optical frequency measurement device according to claim 1,
The optical frequency shift means inserts an optical frequency shift means for shifting the optical frequency of the optical comb according to the shift frequency signal output from the shift frequency signal generator between the multimode light source and the optical multiplexing means. An optical frequency measurement device having a configuration as described above. The optical frequency measurement device according to claim 1,
When the multimode light source includes the optical nonlinear medium described in claim 6 or 7, the optical frequency shift means responds to a shift frequency signal output from a shift frequency signal generator before the optical nonlinear medium. An optical frequency measuring device, characterized in that the optical frequency shifting means for shifting the optical frequency of the optical comb is inserted. The optical frequency measurement device according to claim 1,
The optical frequency shift means, when the multi-mode light source includes the reference optical frequency control means according to claim 5, changes the reference frequency of the optical frequency reference according to the shift frequency signal output from the shift frequency signal generator. An optical frequency measurement device having a shift configuration. An optical frequency measuring method in the optical frequency counting circuit of the optical frequency measuring device according to claim 1,
The longitudinal mode spacing by shifting the frequency of the modulation signal is varied from f _m to f _{m +} Delta] f _m, the longitudinal mode spacing variation Delta] f m _(including the sign), the frequency f of the electric signal a step of counting the change in Delta] f ratio (including the sign) n = Δf / Δf m _(including the sign),
Determining the sign S _M (1 if positive, -1 if negative) of the ratio between the optical frequency shift of the longitudinal mode and the change in the frequency f of the electrical signal;
And the reference optical frequency [nu _{0, f} and _m, and n, on the basis of the _{S M,} said test laser light frequency [nu _X the _{ν X = ν 0 + S M} nf m -S M f
Calculating the optical frequency according to the following formula: An optical frequency measuring method in the optical frequency counting circuit of the optical frequency measuring device according to claim 2,
The longitudinal mode spacing by shifting the frequency of the modulation signal is varied from f _m to f _{m +} Delta] f _m, the longitudinal mode spacing variation Delta] f m _(including the sign), the frequency f of the electric signal a step of counting the change in Delta] f ratio (including the sign) n = Δf / Δf m _(including the sign),
Wherein determining a ratio of the coded S _T between the change in the frequency f of the electric signal to the optical frequency deviation of the test laser light _(if positive 1, if negative -1),
And the reference optical frequency [nu _0, and _{f m,} n and, based on _{S T,} the optical frequency [nu _X of the test laser beam _{ν X = ν 0 -S T nf} m + S T f
The optical frequency measurement method.

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