Optical frequency measurement device and measurement method

Abstract

A control unit (24-1) stabilizes the frequency of a carrier-envelope offset signal of an optical frequency comb. A control unit (24-2) stabilizes the frequency of a beat signal produced by the interference between measured light from a measured light source (21) and the optical frequency comb. A detection unit (34) detects a frequency interval for each of four operations for which the frequency interval between modes of the optical frequency comb is different. An optical frequency calculation unit (30) calculates first through fourth approximate estimate values for a mode number for an initial operation on the basis of the frequency interval for each of the four operations and calculates the true mode number of the initial operation on the basis of the estimate values to calculate the frequency of the measured light.

Description

Optical frequency measuring device and measuring method

[0001] The present invention relates to an optical frequency measurement device and a measurement method that can measure optical frequencies with high accuracy by using an optically stabilized optical frequency comb.

[0002] An optical frequency comb is an optical signal with a comb-shaped spectrum consisting of a set of modes spaced at equal intervals on the frequency axis, and is used as a ruler on the frequency axis. As shown in Figure 15, the frequency spacing of the modes of an optical frequency comb is expressed as f ｒｅｐ , when the spacing of the optical frequency comb is swept to 0, the frequency closest to 0 (carrier envelope offset frequency) is f ｃｅｏ (-f ｒｅｐ / 2<f ｃｅｏ ≦f ｒｅｐ / 2) The frequency f of each mode of the optical frequency comb is ｎ can be written as equation (1) using a natural number n. ｎ = f ｃｅｏ + n × f ｒｅｐ ...(1)

[0003] For example, if the frequency of one mode is 200 THz, f ｒｅｐ = 100 MHz optical frequency comb, the comb mode number N is 200 THz / 100 MHz = 2 × 10 ６ By obtaining a beat between the laser to be measured and the optical frequency comb, the optical frequency f ｃｗ As shown in FIG. 15, the optical frequency f ｃｗ The beat frequency obtained by heterodyne interference between the mode with the frequency closest to f and the narrow linewidth laser is ｂｅａｔ (-f ｒｅｐ / 2<f ｂｅａｔ ≦f ｒｅｐ / 2), then the optical frequency f ｃｗ is expressed as equation (2) using the comb mode number N. ｃｗ = f ｃｅｏ + N × f ｒｅｐ +f ｂｅａｔ ... (2)

[0004] f ｒｅｐcan be varied to a certain extent by changing the cavity length of the laser. ｂｅａｔ Whether is positive or negative depends on f ｒｅｐ When changing f ｂｅａｔ Specifically, it can be determined by increasing or decreasing the absolute value of f ｒｅｐ When f is increased ｂｅａｔ When the absolute value of becomes smaller, f ｂｅａｔ is a positive value. ｒｅｐ When f is increased ｂｅａｔ When the absolute value of f increases, ｂｅａｔ is a negative value. ｃｅｏ Whether f is positive or negative also depends on ｒｅｐ When changing f ｃｅｏ Specifically, it can be determined by increasing or decreasing the absolute value of f ｒｅｐ When f is increased ｃｅｏ When the absolute value of becomes smaller, f ｃｅｏ is a positive value. ｒｅｐ When f is increased ｃｅｏ When the absolute value of f increases, ｃｅｏ is a negative value.

[0005] optical frequency f ｃｗ To determine the comb mode number N, it is necessary to determine the comb mode number N. However, it is difficult to determine the comb mode number N using only the optical frequency comb, and it is usually necessary to know the approximate frequency of the narrow linewidth laser using a high-precision optical wavemeter (see Non-Patent Document 1). For example, if you want to know whether N = 2,000,000 or 2,000,001, since there are seven significant digits, you can use the optical frequency f ｃｗ It is necessary to measure with an accuracy of seven digits. Commercially available high-precision wavemeters can determine wavelengths with an accuracy of 0.1 pm. In the communication wavelength band (1550 nm), wavelengths can be measured with approximately eight significant digits. Therefore, if a high-precision wavemeter is used, the comb mode number N can be determined.

[0006] On the other hand, high-precision wavemeters are expensive and require a separate calibration laser for wavelength calibration. High-precision wavemeters also have the problem that measurement accuracy decreases if the measured wavelength deviates from the calibrated wavelength. Furthermore, when converting the wavelength of an optical comb to the visible or infrared region, new wavemeters and reference light sources that meet the specifications are required, which is costly. Therefore, if an optical frequency comb with frequency stabilization can be realized, it would be possible to measure the optical frequency f ｃｗ It is desirable to be able to measure the following.

[0007] As described above, conventional optical frequency combs have a problem in that when measuring the frequency of a light source, the comb mode number N must be determined using a highly accurate wavemeter.

[0008] Shigeo Nagano et al., “3-4 Precision Frequency Measurement Using a Femtosecond Laser Optical Frequency Comb,” National Institute of Information and Communications Technology, Quarterly Bulletin of the National Institute of Information and Communications Technology, Vol. 56, Nos. 3 / 4, 2010, <https: / / www.nict.go.jp / publication / shuppan / kihou-journal / kihou-vol56no3_4 / kihou-vol56no3_4_0304.pdf>

[0009] The present invention has been made to solve the above-mentioned problems, and has as its object to provide a practical optical frequency measuring device and measuring method that can measure optical frequencies without using a wavemeter.

[0010] The optical frequency measurement device of the present invention includes a light source configured to generate an optical frequency comb in which modes are arranged at equal intervals on a frequency axis, a first detection unit configured to detect a carrier envelope offset signal of the optical frequency comb, a second detection unit configured to detect a beat signal generated by interference between light under measurement and the optical frequency comb, a third detection unit configured to detect the frequency interval of the modes, a first control unit configured to control the light source so that the frequency of the carrier envelope offset signal is stabilized at a first reference frequency, a second control unit configured to control the light source so that the frequency of the beat signal is stabilized at a second reference frequency, and an optical frequency calculation unit configured to calculate the frequency of the light under measurement, wherein the second control unit performs a first operation to first stabilize the frequency of the beat signal, and in the first operation, calculates a frequency of the light under measurement by stabilizing the frequency of a second mode having a number one smaller than the number of a first mode whose frequency is closest to the frequency of the light under measurement. a second operation for stabilizing the frequency of the beat signal so that the frequency of the measured light is closest to the frequency of a third mode having a number smaller than the second mode number; and a fourth operation for stabilizing the frequency of the beat signal so that the frequency of the measured light is closest to the frequency of a fourth mode having a number smaller than the third mode number, and the first control unit controls the light source so that the frequency of the carrier-envelope offset signal is stabilized in each of the first, second, third, and fourth operations, and the optical frequency calculation unit calculates approximate values ​​of first-, second-, third-, and fourth-order approximate mode numbers in the first operation based on the frequency interval in each of the first, second, third, and fourth operations, and calculates the true mode number in the first operation based on these approximate values, thereby calculating the frequency of the measured light.

[0011] The optical frequency measurement method of the present invention includes a first step of controlling a light source of an optical frequency comb so that the frequency of a carrier envelope offset signal of the optical frequency comb, in which modes are arranged at equal intervals on a frequency axis, is stabilized at a first reference frequency; a second step of controlling the light source so that the frequency of a beat signal generated by interference between the optical frequency comb and the light under measurement is stabilized at a second reference frequency; a third step of detecting a frequency interval between the modes when the light source is controlled by the first and second steps; a fourth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency in a state in which the frequency of a second mode having a number one smaller than a first mode whose frequency is closest to the frequency of the light under measurement when the light source is controlled by the first and second steps; a fifth step of detecting a frequency interval between the modes when the light source is controlled by the first and fourth steps; and a fifth step of detecting a frequency interval between the modes when the light source is controlled by the first and fourth steps. a sixth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency in a state where the frequency of the measured light is closest to the frequency of the fourth mode having a number smaller than the third mode number; a seventh step of detecting a frequency interval of the modes when the light source is controlled by the first and sixth steps; an eighth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency in a state where the frequency of the measured light is closest to the frequency of the fourth mode having a number smaller than the third mode number; a ninth step of detecting a frequency interval of the modes when the light source is controlled by the first and eighth steps; and a tenth step of calculating, based on the frequency intervals detected in the third, fifth, seventh, and ninth steps, approximate values ​​of first, second, third, and fourth-order approximate mode numbers when the light source is controlled by the first and second steps, and calculating, based on the approximate values, a true mode number when the light source is controlled by the first and second steps, thereby calculating the frequency of the measured light.

[0012] According to the present invention, the first control unit stabilizes the frequency of the carrier envelope offset signal, the second control unit stabilizes the frequency of the beat signal in each of the first, second, third, and fourth operations, the third detection unit detects the mode frequency intervals in each of the first, second, third, and fourth operations, the optical frequency calculation unit calculates first-, second-, third-, and fourth-order approximate mode number estimates for the first operation based on the frequency intervals in each of the first, second, third, and fourth operations, and calculates the true mode number for the first operation based on these approximate values ​​to calculate the frequency of the light under measurement. This eliminates the need for a high-precision wavemeter, and therefore makes it possible to accurately measure the optical frequency of the light source under measurement without the need to purchase and maintain an expensive wavemeter.

[0013] FIG. 1 is a block diagram showing the configuration of an optical frequency measurement device according to a first embodiment of the present invention. FIG. 2 is a block diagram showing an example configuration of an optical frequency comb light source according to a first embodiment of the present invention. FIG. 3 is a block diagram showing another example configuration of an optical frequency comb light source according to a first embodiment of the present invention. FIGS. 4A and 4B are diagrams explaining a method for measuring the optical frequency of a light source under measurement under two different conditions. FIGS. 5A and 5B are diagrams explaining a method for measuring the optical frequency of a light source under measurement under two different conditions from those shown in FIGS. 4A and 4B. FIG. 6 is a diagram explaining, in an x-y coordinate system, a method for calculating a mode number according to a first embodiment of the present invention. FIG. 7 is a diagram explaining a method for calculating a mode number according to a first embodiment of the present invention. FIG. 8 is a flowchart explaining an optical frequency measurement method according to a first embodiment of the present invention. FIG. 9 is a flowchart explaining an optical frequency measurement method according to a first embodiment of the present invention. FIG. 10 is a diagram showing the error in the mode number and the error in the estimated value of the difference between the mode numbers. FIG. 11 is a diagram showing the range in which the mode number can be determined for the relative frequency fluctuation of the laser under measurement. Fig. 12A is a histogram showing the error in the difference between mode numbers when the frequency fluctuation of the laser under test is 35 kHz. Fig. 12B is a histogram showing the error in the mode number when the frequency fluctuation of the laser under test is 35 kHz. Fig. 13 is a diagram showing the number of times the error in the mode number becomes less than 0.1 when the frequency fluctuation of the laser under test is changed. Fig. 14 is a block diagram showing an example configuration of a computer that realizes the optical frequency measurement apparatus according to the first and second embodiments of the present invention. Fig. 15 is a diagram explaining an optical frequency comb.

[0014] [First Embodiment] An embodiment of the present invention will be described below with reference to the drawings. In the following description, optical frequency comb will be abbreviated as optical comb. FIG. 1 is a block diagram showing the configuration of an optical frequency measurement device according to this embodiment. The optical frequency measurement device includes an optical comb light source 22 that generates an optical comb in which modes are arranged at equal intervals on the frequency axis, a self-referencing interferometer 23, control units 24-1 and 24-2, a frequency counter 25, photodetectors (PDs) 27-29, an optical frequency calculation unit 30, and a beam splitter 31.

[0015] The beam splitter 31 and the PD 27 constitute a detector 32 that detects a beat signal generated by interference between the light under test and the optical comb. The self-referencing interferometer 23 and the PD 28 constitute a detector 33 that detects a carrier envelope offset (CEO) signal of the optical comb. The PD 29 and the frequency counter 25 constitute a detector 34 that detects the frequency interval of the optical comb mode.

[0016] In this embodiment, the optical frequency f ｃｗ An optical comb source 22 is used to measure the polarization state. FIG. 2 is a block diagram showing an example of the configuration of the optical comb source 22. The example in FIG. 2 shows a mode-locked fiber laser configuration. The optical comb source 22 includes a fiber loop 220, an Er-doped fiber 221 arranged in the fiber loop 220, a polarization controller 222 arranged in the fiber loop 220 and controlling the polarization propagating through the fiber loop 220 to an arbitrary polarization state, an isolator 223 arranged in the fiber loop 220, a semiconductor laser 224 serving as a pumping light source for pumping the Er-doped fiber 221, a wavelength division multiplexing (WDM) coupler 225 serving as a multiplexer for adding the pumping light output from the semiconductor laser 224 to the fiber loop 220, an output coupler 226 for extracting the light propagating through the fiber loop 220, and a delay line 227 for adjusting the resonator length of the optical comb source 22.

[0017] The isolator 223 is inserted to determine the propagation direction of the oscillating laser light. To pump the Er-doped fiber 221 in the fiber loop 220, light from an external semiconductor laser 224 is inserted into the fiber loop 220 via a WDM coupler 225. The delay line 227 is composed of a PZT (piezoelectric transducer) or stage for coarsely adjusting the optical path length of the fiber loop 220, and an EOM (electro-optic modulator) for finely adjusting the optical path length. The light propagating through the fiber loop 220 (optical comb) is extracted to the outside via an output coupler 226.

[0018] Although not explicitly shown in FIG. 2, in order to detect the CEO signal of the optical comb using the self-referencing interferometer 23, the optical comb extracted to the outside via the output coupler 226 must be amplified by an Er-doped fiber amplifier, and the wavelength of the amplified optical comb must be broadened to a one-octave band using a highly nonlinear fiber.

[0019] FIG. 3 is a block diagram showing another example of the optical comb source 22. The optical comb source 22 in the example shown in FIG. 3 is configured using a chirp-mirror mode-locked laser. The optical comb source 22 in this example includes a femtosecond laser 300 that outputs pump light, an acousto-optic modulator (AOM) 301 that intensity-modulates the light from the femtosecond laser 300, a lens 302, multiple chirp mirrors 303-305, a Ti:sapphire crystal 306 positioned between the two chirp mirrors 303 and 304, an output coupling mirror 307 for extracting light, and a delay line 308 for adjusting the resonator length of the optical comb source 22. In the configuration shown in FIG. 3, the delay line 308 is configured with a PZT or stage for coarsely adjusting the resonator length of the optical comb source 22 and an EOM for finely adjusting the optical path length.

[0020] The configuration of the optical frequency comb source 22 shown in Figures 2 and 3 is disclosed in Non-Patent Document 1. In order to stabilize the frequency of the optical frequency comb source 22 with respect to the light source under test 21, a beam splitter 31 combines the light under test from the light source under test 21 with the optical frequency comb from the optical frequency comb source 22. A photodiode 27 receives the combined light of the light under test and the optical frequency comb and converts it into a beat signal. The frequency of the beat signal is f ｂｅａｔ is.

[0021] On the other hand, the self-referencing interferometer 23 outputs interference light obtained by, for example, f-2f interferometry, which causes interference between a fundamental wave of a short wavelength component and a second harmonic wave of a long wavelength component. The PD 28 receives the interference light from the self-referencing interferometer 23 and converts it into a CEO signal. The frequency of the CEO signal is f ｃｅｏ is.

[0022] The control unit 24-1 is ｃｅｏ The reference frequency signal f is compared with the CEO signal to generate an error signal f ｃｅｏ -f ｒ and generates an error signal f ｃｅｏ -f ｒ so that f ｃｅｏ (so that the frequency of the reference frequency signal and the CEO signal are equal), f ｃｅｏ A control signal is generated to stabilize the

[0023] The control unit 24-2 is ｂｅａｔ The reference frequency signal is compared with the beat signal to obtain an error signal f ｂｅａｔ -f ｒ and generates an error signal f ｂｅａｔ -f ｒ so that f ｂｅａｔ (so that the frequencies of the reference frequency signal and the beat signal are equal) f ｂｅａｔ A control signal is generated to stabilize the

[0024] In general, f ｃｅｏ In order to frequency-stabilize f, the pumping light power of the optical frequency comb light source 22 is adjusted. ｂｅａｔIn order to stabilize the frequency of the optical comb light source 22, the control unit 24-1 adjusts the resonator length of the optical comb light source 22. Specifically, the control unit 24-1 outputs a control signal (current signal) to the lasers 224 and 300 to finely adjust the laser current and thus the pumping light power. The control unit 24-2 also outputs a control signal (voltage signal) to the EOMs in the delay lines 227 and 308 to finely adjust the voltage applied to the EOMs, thereby finely adjusting the resonator length of the optical comb light source 22.

[0025] f ｃｗ , f ｃｅｏ , f ｂｅａｔ Once this is determined, the frequency spacing f of the optical comb mode can be calculated using equation (2). ｒｅｐ The frequency interval f ｒｅｐ The PD 29 receives the optical comb and converts it into an electrical signal to measure the frequency interval f. The frequency counter 25 measures the frequency of the electrical signal output from the PD 29 to obtain the frequency interval f. ｒｅｐ The control units 24-1 and 24-2 and the frequency counter 25 need to receive an external reference signal such as a microwave signal 26 calibrated with a GPS signal.

[0026] Next, the measurement method of this embodiment will be described. Steps 1 to 3 are performed under three conditions to measure the frequency interval f ｒｅｐ０ , f ｒｅｐ１ , f ｒｅｐ２ In step 4, we consider the case where we measure the frequency interval f ｒｅｐ３ , ..., f ｒｅｐｓ This is expanded to the case where the optical frequency f of the light source 21 to be measured (s≧3) is measured. ｃｗ In order to obtain the above, in step 1, the optical frequency comb is frequency-stabilized with respect to the light source under test 21 under two different conditions, and the first-order approximation of the mode number N １ Ask for.

[0027] For example, as shown in FIG. 4A, ｃｗ For the reference value of , the control unit 24-1 controls the frequency f of the CEO signal. ｃｅｏ The control unit 24-2 stabilizes the frequency at the beat frequency f ｂｅａｔ The frequency counter 25 measures the frequency interval f of the optical comb at this time.ｒｅｐ０ Measure.

[0028] Next, the frequency interval of the optical comb, f ｒｅｐ The control unit 24-2 gradually increases the beat frequency f and then performs frequency stabilization under the same conditions. That is, the control unit 24-2 continuously changes the resonator length of the optical comb light source 22 by changing the control signal output to the EOM or PZT in the delay lines 227 and 308 of the optical comb light source 22. Next, the control unit 24-2 changes the beat frequency f ｂｅａｔ The frequency is stabilized when the frequency reaches +10 MHz. The frequency counter 25 measures the frequency interval f ｒｅｐ１ The state shown in FIG. 4A has a mode number that is one less than the state shown in FIG. 4B. Therefore, the relationship between the states shown in FIG. 4A and FIG. 4B is expressed by a natural number (mode number) N １ , N １ When expressed using -1, it becomes as shown in equation (3). ｃｗ = f ｃｅｏ +N １ f ｒｅｐ０ +f ｂｅａｔ = f ｃｅｏ + (N １ -1) f ｒｅｐ１ +f ｂｅａｔ ...(3)

[0029] Equation (3) is used as the mode number N １ When we solve for (4), we obtain equation (4).

[0030]

[0031] Mode number N １ Error δN １ is the frequency interval f of the optical comb ｒｅｐ The error of δ ｆｒｅｐ In this case, it is expressed by the formula (5).

[0032]

[0033] error δ ｆｒｅｐis determined by the frequency fluctuation of the light source 21 under test, the measurement accuracy of the frequency counter 25, and the accuracy of the microwave signal 26 calibrated with a GPS signal input as an external reference signal. When a commercially available frequency counter or a microwave signal calibrated with a GPS signal is used, the accuracy is 11 to 12 digits / s. For example, the optical frequency f ｃｗ is 200 THz, and the frequency interval of the optical comb is f ｒｅｐ When is 100 MHz, δf ｒｅｐ / f ｒｅｐ =10 －１２ As the error δN １ is estimated to be 4√2, which is greater than 1. Therefore, the mode number N １ is a first-order approximation.

[0034] Next, in step 2, in addition to the two conditions above that the beat frequency is the same but the frequency interval is different, the frequency interval of the optical comb is set to f ｒｅｐ１ and the frequency is stabilized again, and the second-order approximation mode number N ２ The control unit 24-2 changes the resonator length of the optical comb light source 22 by changing the position of the stage in the delay lines 227 and 308 of the optical comb light source 22 or the control signal output to the PZT. The control unit 24-2 determines the beat frequency f ｂｅａｔ The frequency is stabilized when the frequency reaches +10 MHz. The frequency counter 25 measures the frequency interval f ｒｅｐ２ is measured (Figure 5A).

[0035] The state shown in FIG. 4A is different from the state shown in FIG. 5A in that the mode number is k. ２ In this case, k ２ Since the relationship between the states shown in FIGS. 4A and 5A can be calculated by a natural number (mode number), N ２ , N ２ -k ２ When expressed using f, it becomes as shown in equation (6). ｃｗ = f ｃｅｏ +N ２ f ｒｅｐ０ +f ｂｅａｔ = f ｃｅｏ + (N ２ -k２ ) f ｒｅｐ２ +f ｂｅａｔ ...(6)

[0036] Equation (6) is used to calculate mode number N ２ By solving for , we obtain equation (7).

[0037]

[0038] Mode number N ２ Error δN ２ is shown in equation (8).

[0039]

[0040] From equation (8), the error δN ２ The size of k ２ is inversely proportional to the value of f ｒｅｐ２ The measurement point of f ｒｅｐ０ The further away from f, the more significant figures there are in the denominator of equation (7). ｒｅｐ２ The measurement point of f ｒｅｐ０ The further away from ２ The next problem is to find the unknown mode number difference k ２ The key is to specifically identify the following.

[0041] Therefore, in step 3, the difference k of the mode number from the initial state ２ From equation (7), k ２ To find the mode number N ２ is required, but here we use equation (9) to obtain k ２ Approximate value (real value) of k ｅｘｐ２ The value of mode number N １ It is estimated from k ２ Ha K ｅｘｐ２ is defined as the natural number with the smallest difference from

[0042]

[0043]

[0044] The method of steps 1 to 3 can be explained in the xy coordinate system as shown in Figure 6. The mode number N obtained in step 1 １ is the coordinate (fｒｅｐ０ , 0) and point A at coordinates (f ｒｅｐ１ , f ｒｅｐ１ ) point D １ A straight line AD connecting １ is the slope of f ｒｅｐ１ -f ｒｅｐ０ Since is small, the straight line AD １ The slope of the mode number N described in step 2 is not accurate. ２ is the distance between point A and coordinate (f ｒｅｐ２ , f ｒｅｐ２ ) Point B ２ A straight line AB connecting ２ The slope of k ２ The unknown natural number k ２ In step 3, the line AD １ and the line x = f ｒｅｐ２ The intersection point with C ２ , frequency interval f ｒｅｐ２ Among the grid points, the line x = f ｒｅｐ２ Point C on the top ２ The point closest to ２ Point C ２ The coordinates of (f ｒｅｐ２ , k ｅｘｐ２ f ｒｅｐ２ ), point D ２ The coordinates of (f ｒｅｐ２ , k ２ f ｒｅｐ２ ) Mode number N ２ is a straight line AD ２ In this way, f ｒｅｐ２ -f ｒｅｐ０ (or the difference in mode number k ２ ) increases the accuracy of the initial mode number N. The remaining issue is how to accurately infer the difference in mode numbers.

[0045] Therefore, in step 4, in addition to the above three conditions that the beat frequency is the same but the frequency interval is different, the frequency interval of the optical comb is set to f ｒｅｐ２ Let us consider the case where the mode number N is increased to be larger than 1 and stabilized again, and measurement is performed at more multiple value points. ｉ Derive an equation for (i≧3).

[0046] The control unit 24-2 changes the resonator length of the optical comb light source 22 by significantly changing the position of the stage in the delay lines 227 and 308 of the optical comb light source 22 or the control signal output to the PZT. ｂｅａｔ The frequency is stabilized when the frequency reaches +10 MHz. The frequency counter 25 measures the frequency interval f ｒｅｐｉ is measured (Figure 5B).

[0047] Using the recurrence formula, the difference in mode numbers k ｉ Approximate value (real value) of k ｅｘｐｉ The value of k is defined by equation (11). ｉ is the estimated value k ｅｘｐｉ The i-th order approximate mode number N is defined as the natural number with the smallest difference from ｉ is defined by equation (13).

[0048]

[0049]

[0050]

[0051] k ｉ Error δk ｉ The magnitude of k is given by equation (14). ｉ Tok ｉ－１ It is determined by the ratio of k ｉ If we gradually increase the error δk ｉ does not become large.

[0052]

[0053] Specifically, the error δk ｉ Under the condition that the magnitude of k is not larger than 1 / 10 ｉ In other words, k ｉ It is important to increase the value stepwise, from 1 to 2 to 4 to 10, etc., rather than increasing it all at once from 1 to 1000.

[0054] The algorithm for the calculation method is shown in Figure 7. The numbers in parentheses in Figure 7 indicate the numbers in the above formula. The approximate value N of the mode number is calculated using formula (4). １After calculating the difference in mode numbers k ２ The estimated value of k ｅｘｐ２ Calculate the difference in mode numbers k using equation (10). ２ After calculating the value, the approximate value N of the mode number is calculated using equation (7). ２ Furthermore, the difference in mode numbers k is calculated using equation (11). ３ The estimated value of k ｅｘｐ３ and calculate the difference in mode numbers k using equation (12). ３ After calculating the value, the approximate value N of the mode number is calculated using equation (13). ３ By repeatedly using such a recurrence formula, the i-th order approximate mode number N ｉ Calculate N ｉ converges to a certain natural number, the true mode number N can be determined.

[0055] After the initial mode number N is determined by the method of this embodiment, the frequency interval f ｒｅｐｉ The mode number for ｉ Here, k ０ = 0, k １ If we define it as 1, the frequency interval is f ｒｅｐｉ The optical frequency f of the light source 21 to be measured when ｃｗｉ (i=0, 1, 2, . . . , s) can be determined with high accuracy from equation (15).

[0056]

[0057] 8 and 9 are flowcharts explaining the optical frequency measurement method of this embodiment. The flowchart is divided into a measurement step and a calculation step. First, the measurement step will be explained. First, the control unit 24-1 adjusts the pump light power by outputting a control signal to the lasers 224 and 300 of the optical frequency comb light source 22 (step S100 in FIG. 8). At this time, the initial value of the control signal (current signal) is set to the optical frequency f ｃｗ Reference value and mode number N １ With respect to the reference value of the CEO signal frequency f ｃｅｏ ga f ｃｅｏThe control unit 24-1 changes the control signal to finely adjust the pump light power, thereby adjusting the frequency f ｃｅｏ = +10 MHz (step S101 in FIG. 8).

[0058] Next, the control unit 24-2 adjusts the resonator length of the optical comb light source 22 by outputting a predetermined control signal to the EOMs in the delay lines 227 and 308 of the optical comb light source 22 (step S102 in FIG. 8). At this time, the initial value of the control signal (voltage signal) is set to the optical frequency f ｃｗ Reference value and mode number N １ and the beat frequency f ｂｅａｔ ga f ｂｅａｔ The control unit 24-2 changes the control signal to finely adjust the resonator length, thereby adjusting the beat frequency f ｂｅａｔ = +10 MHz (step S103 in FIG. 8). ｂｅａｔ The reference frequency of f ｃｅｏ The reference frequencies may be the same or different.

[0059] The frequency counter 25 measures the beat frequency f ｂｅａｔ = +10 MHz, CEO signal frequency f ｃｅｏ When stabilized at +10 MHz, the frequency interval of the optical comb is f ｒｅｐ = f ｒｅｐ０ is measured (step S104 in FIG. 8).

[0060] Next, the control unit 24-2 performs the processes in steps S102 and S103 to obtain the beat frequency f ｂｅａｔ The stabilization is released from the state where the frequency is stabilized at f = +10 MHz, and the control signal output to the EOM or PZT in the delay lines 227 and 308 is continuously changed to continuously shorten the resonator length of the optical comb light source 22 (step S105 in FIG. 8). At this time, the rate of change of the resonator length is ｂｅａｔ = +10MHz, then the beat frequency f ｂｅａｔThe control unit 24-2 is configured to search for a state where the beat frequency f ｂｅａｔ = +10 MHz, the frequency is stabilized (step S106 in FIG. 8).

[0061] The frequency counter 25 measures the beat frequency f ｂｅａｔ = +10 MHz, CEO signal frequency f ｃｅｏ When stabilized at +10 MHz, the frequency interval of the optical comb is f ｒｅｐ = f ｒｅｐ１ is measured (step S107 in FIG. 8).

[0062] Next, the control unit 24-2 performs the processes in steps S105 and S106 to obtain the beat frequency f ｂｅａｔ The control unit 24-2 cancels the stabilization at f = +10 MHz and shortens the resonator length of the optical comb light source 22 by changing the stage position in the delay lines 227 and 308 of the optical comb light source 22 or the control signal output to the PZT (step S108 in FIG. 8). After changing the control signal, the control unit 24-2 further changes the control signal slightly to fine-tune the resonator length, thereby reducing the beat frequency f ｂｅａｔ = +10 MHz (step S109 in FIG. 8).

[0063] The frequency counter 25 measures the beat frequency f ｂｅａｔ = +10 MHz, CEO signal frequency f ｃｅｏ When stabilized at +10 MHz, the frequency interval of the optical comb is f ｒｅｐ = f ｒｅｐ２ is measured (step S110 in FIG. 8).

[0064] Next, the control unit 24-2 performs the processes in steps S108 and S109 to obtain the beat frequency f ｂｅａｔ The control unit 24-2 cancels the stabilization at f = +10 MHz and shortens the resonator length of the optical comb light source 22 by changing the stage position in the delay lines 227 and 308 of the optical comb light source 22 or the control signal output to the PZT (step S111 in FIG. 8). After changing the control signal, the control unit 24-2 further changes the control signal slightly to fine-tune the resonator length, thereby reducing the beat frequency f ｂｅａｔ= +10 MHz (step S112 in FIG. 8).

[0065] The frequency counter 25 measures the beat frequency f ｂｅａｔ = +10 MHz, CEO signal frequency f ｃｅｏ When stabilized at +10 MHz, the frequency interval of the optical comb is f ｒｅｐ = f ｒｅｐｉ (i=3, ..., s) (s≧3) is measured (step S113 in FIG. 8). In this way, the processing of steps S111 to S113 is performed (s-2) times (s≧3) up to i=3, ..., s.

[0066] Next, the calculation step will be described. The optical frequency calculation unit 30 calculates the frequency interval f measured by the frequency counter 25. ｒｅｐ０ , f ｒｅｐ１ Based on this, the approximate value N of the mode number is calculated by equation (4). １ (Step S115 in FIG. 9). Next, the optical frequency calculation unit 30 calculates N １ and the frequency interval f measured by the frequency counter 25 ｒｅｐ０ , f ｒｅｐ２ Based on this, the difference in mode number k is calculated using equation (9). ２ The estimated value of k ｅｘｐ２ (Step S116 in FIG. 9). The optical frequency calculation unit 30 calculates the natural number k ２ (Step S117 in FIG. 9). Furthermore, the optical frequency calculation unit 30 calculates the value of the calculated natural number k ２ and the frequency interval f measured by the frequency counter 25. ｒｅｐ０ , f ｒｅｐ２ Based on this, the approximate value N of the mode number is calculated by equation (7). ２ is calculated (step S118 in FIG. 9).

[0067] Next, the optical frequency calculation unit 30 calculates the approximate value N of the mode number calculated in step S118 or step S121 described later. ｉ－１ (i=3, . . . , s) and the frequency interval f measured by the frequency counter 25 ｒｅｐ０ , f ｒｅｐｉ Based on this, the difference in mode number k is calculated using equation (11). ｉ The estimated value of kｅｘｐｉ (Step S119 in FIG. 9). The optical frequency calculation unit 30 calculates the natural number k ｉ (Step S120 in FIG. 9). Furthermore, the optical frequency calculation unit 30 calculates the value of the calculated natural number k ｉ and the frequency interval f measured by the frequency counter 25. ｒｅｐ０ , f ｒｅｐｉ Based on this, the approximate value N of the mode number is calculated by equation (13). ｉ (Step S121 in FIG. 9) In this way, the processing of steps S119 to S121 is performed (s-2) times (s≧3) for i=3, ..., s.

[0068] The value obtained by equation (13) is a real number. The optical frequency calculation unit 30 determines whether the difference between the real number obtained by equation (13) and the nearest natural number is within 1 / 10 (step S123 in FIG. 9 ). If the difference is within 1 / 10, the optical frequency calculation unit 30 sets the natural number nearest to the real number obtained by equation (13) as the value of mode number N (step S124 in FIG. 9 ).

[0069] If the difference is greater than 1 / 10, the process returns to step S111. In this case, the control unit 24-2 determines the beat frequency f ｂｅａｔ The controller 24-2 releases the stabilization at the beat frequency f = +10 MHz and changes the resonator length of the optical comb light source 22 so that it becomes even shorter (step S111). ｂｅａｔ = +10 MHz (step S112). In this manner, the processes of steps S111 to S121 are repeated until the value of mode number N can be calculated.

[0070] In the process of step S123, it is determined whether the difference between the real number obtained by equation (13) and the nearest natural number is within 1 / 10, but since this is only a guide to ensure that the value of N is correctly obtained, it may be 1 / 4 or the like. ｉ If it is observed that the mode number approaches a certain natural number N, the natural number may be determined as the mode number N.

[0071] When the value of the mode number N is calculated by the processing of steps S115 to S124, the optical frequency calculation unit 30 calculates the value of the mode number N by combining the calculated mode number N and the natural number k calculated by the processing of steps S115 to S121. ｉ and the frequency interval f measured by the frequency counter 25. ｒｅｐ０ , f ｒｅｐ１ , f ｒｅｐ２ , ..., f ｒｅｐｓ and a known frequency f ｂｅａｔ , f ｃｅｏ (=+10 MHz) based on the optical frequency f ｃｗ At this time, the optical frequency calculation unit 30 calculates the optical frequency f ｃｗｉ Calculate the optical frequency f ｃｗｉ By calculating the average value of ｃｗ The final value of is then determined (step S125 in FIG. 9).

[0072] In this embodiment, a highly accurate wavemeter is not required, so there is no need to purchase and maintain an expensive wavemeter. ｃｗ It is possible to measure with high accuracy.

[0073] [Second Example] Next, a second example of the present invention will be described. This example is a specific example of the first example. Here, a product manufactured by Neoark (repetition rate of around 100 MHz) was used as the optical comb light source 22, and an ultra-high stability laser manufactured by Menlo was used as the light source to be measured 21. In order to improve the measurement accuracy, a so-called Λ-type counter was used as the frequency counter 25, and measurements were made with a gate time of 10 seconds. Specifically, a 53230A manufactured by Keysight was used as the frequency counter 25. The frequency interval f ｒｅｐ The frequency fluctuation is 0.24 mHz.

[0074] The results are shown in Table 1. This table shows the frequency interval f ｒｅｐ０ , f ｒｅｐ１ , f ｒｅｐ２ , ..., f ｒｅｐ１２ (Steps S100 to S113 in FIG. 8), the approximate value N of the mode number calculated by the formula (4) １(Step S115 in FIG. 9), the approximate value k of the difference in mode numbers calculated by equation (9) ｅｘｐ２ (Step S116 in FIG. 9), the natural number k calculated by equation (10) ２ (Step S117 in FIG. 9), the approximate value N of the mode number calculated by the formula (7) ２ (Step S118 in FIG. 9), the approximate value k of the difference in mode numbers calculated by equation (11) ｅｘｐｉ (i=3, 4, . . . , 12) (step S119 in FIG. 9), the natural number k calculated by equation (12) ｉ (Step S120 in FIG. 9), the approximate value N of the mode number calculated by the formula (13) ｉ (Step S121 in FIG. 9) is listed.

[0075]

[0076] Approximate value k of the difference in mode numbers ｅｘｐ２ , k ｅｘｐｉ Considering the natural number k ２ , k ｉ Error δk ２ , δk ｉ It can be seen that is very small and can be calculated accurately. Mode number N ｉ The error δN ｉ , and the estimated value k ｅｘｐｉ natural number k ｉ Error δk ｉ is shown in Figure 10. 90 in Figure 10 is the error δN ｉ , 91 is the error δk ｉ According to FIG. 10, the natural number k ｉ Error δk ｉ It can be seen that the value fluctuates at a low level.

[0077] Next, the approximate value of the mode number N ｉ Considering the natural number k ｉ The larger the value of N, the smaller the error from the estimated N = 1,945,407. ｉ The reason why the error of becomes small is that the error is a natural number k ｉ This tendency of inverse proportion can be confirmed from the characteristic shown by 90 in FIG. ｉ Since approaches a specific natural number, the approximate value Nｉ Therefore, according to this embodiment, the mode number N can be determined with high accuracy.

[0078] The frequency interval f measured by the frequency counter 25 ｒｅｐ０ , f ｒｅｐ１ , f ｒｅｐ２ , ..., f ｒｅｐ１２ The calculation results of step S125 for the values ​​of f in Table 2 are shown in Table 2. ｃｗｉ is the value of the optical frequency calculated by equation (15) with mode number N=1,945,407.

[0079]

[0080] From the results in Table 2, f ｃｗｉ By calculating the average value of f and rounding it off to the nearest hundred, ｃｗ = 194.346218835 THz.

[0081] In the first and second embodiments, the fluctuation of the optical frequency of the light source 21 to be measured was about 1 Hz / s. Here, we consider how much fluctuation of the optical frequency the present invention can be applied to.

[0082] In the present invention, even if the frequency fluctuation of the laser under test becomes large to some extent, the mode number can be determined. ｉ is smaller than 1 / 10, so from equations (5) and (8), the natural number k ｉ The condition for can be expressed by equation (16).

[0083]

[0084] Such a natural number k ｉ The condition for existence is that the repetition frequency f ｒｅｐ Variable frequency range Δf ｒｅｐ is determined by a natural number k ｉ The condition for can be expressed by equation (17): From equation (17), the upper limit of the relative frequency fluctuation of the laser under test is given by equation (18).

[0085]

[0086]

[0087] For example, the approximate value of the mode number N １ is 2 x 10 ６ , variable frequency range Δf ｒｅｐ is 500 kHz, frequency interval f ｒｅｐ When is 100 MHz, δf ｒｅｐ / f ｒｅｐ is approximately 1.8 x 10 －１０ The optical frequency f ｃｗ is about 200 THz, fluctuations in the optical frequency can be tolerated up to 35 kHz.

[0088] The above discussion is based on the difference in mode number k ｉ The condition can be easily understood by visualizing it in a graph. １ is 2 x 10 ６ , frequency interval f ｒｅｐ is 100 MHz, variable frequency range Δf ｒｅｐ The vertical axis of FIG. 11 represents the relative frequency fluctuation δf of the laser under test. ｒｅｐ / f ｒｅｐ The horizontal axis represents the difference k in mode numbers.

[0089] In the present invention, the error δN ｉ < 1 / 10 and the tuning frequency limit of the laser under test. ｉ The line 111 indicates the limit of the tunable frequency of the laser under test. The arrow 112 indicates the region where the mode number N can be determined. The present invention is based on the relative frequency fluctuation δf ｒｅｐ / f ｒｅｐ = 1.8 x 10 －１０ It can handle optical frequencies up to f ｃｗ It can be seen that this method is effective if the fluctuations are of a certain magnitude.

[0090] To verify the accuracy of our argument, we performed a simulation with 10,000 measurement trials. Figure 12A shows the frequency fluctuation δf of the laser under test. ｃｗ Approximate value k of the difference in mode number when is 35 kHzｅｘｐｉ and the true value k ｉ The difference δk = k ｅｘｐｉ -k ｉ The histogram shows the frequency fluctuation δf ｃｗ When δk = 35 kHz, the error δk < 0.005, and the value of the mode number difference k could be determined with 100% probability. This result shows that the method for estimating the mode number difference using the recurrence formula is very reliable.

[0091] FIG. 12B shows the frequency fluctuation δf of the laser under test. ｃｗ Approximate value of mode number N when is 35 kHz ｉ and the difference between the true value N and δN = N ｉ -N is shown in the histogram. Frequency fluctuation δf ｃｗ When the frequency is 35 kHz, the probability that the error δN<0.1 is in the range indicated by the arrow 120 in Fig. 12B is about 70%. Furthermore, the probability that the error δN<0.2 is 90% or more.

[0092] FIG. 13 shows the frequency fluctuation δf of the laser under test. ｃｗ This represents the number of times that the error δN<0.1 occurs when the frequency is changed from 1 kHz to 100 kHz. ｃｗ = 10 kHz, the probability that the error δN<0.1 is almost 100%. ｃｗ When the frequency fluctuation δf is 35 kHz, the probability that the error δN is less than 0.1 is about 70%. ｃｗ As the error δN<0.1 increases, the probability of the error δN<0.1 gradually decreases. This result confirms that it is possible to determine the mode number N even if the frequency fluctuation of the laser under test is several tens of kHz.

[0093] The control units 24-1 and 24-2, frequency counter 25, and optical frequency calculation unit 30 of the optical frequency measurement device described in the first and second embodiments can be realized by a computer equipped with a CPU (Central Processing Unit), a storage device, and an interface, and a program that controls these hardware resources. An example configuration of this computer is shown in Figure 14.

[0094] The computer includes a CPU 200, a storage device 201, and an interface device (I / F) 202. The I / F 202 is connected to the hardware components of the control units 24-1 and 24-2 and the hardware component of the frequency counter 25. In such a computer, a program for implementing the optical frequency measurement method of the present invention is stored in the storage device 201. The CPU 200 executes the processing described in the first and second embodiments in accordance with the program stored in the storage device 201. As described in the second embodiment, the frequency counter 25 may be a device separate from the control units 24-1 and 24-2.

[0095] Some or all of the above embodiments can be described as, but are not limited to, the following supplementary notes.

[0096] (Supplementary Note 1) An optical frequency measurement device of the present invention includes a light source configured to generate an optical frequency comb in which modes are arranged at equal intervals on a frequency axis, a first detection unit configured to detect a carrier envelope offset signal of the optical frequency comb, a second detection unit configured to detect a beat signal generated by interference between light under measurement and the optical frequency comb, a third detection unit configured to detect a frequency interval of the modes, a first control unit configured to control the light source so that the frequency of the carrier envelope offset signal is stabilized at a first reference frequency, a second control unit configured to control the light source so that the frequency of the beat signal is stabilized at a second reference frequency, and an optical frequency calculation unit configured to calculate the frequency of the light under measurement, wherein the second control unit performs a first operation to first stabilize the frequency of the beat signal, and in the first operation, stabilizes the frequency of a second mode having a number one smaller than the number of a first mode whose frequency is closest to the light under measurement, and At least four operations are sequentially performed: a second operation for stabilizing the frequency of the beat signal so that the frequency of the measured light is closest to the frequency of the actual light; a third operation for stabilizing the frequency of the beat signal so that the frequency of the measured light is closest to the frequency of a third mode having a number smaller than the second mode number; and a fourth operation for stabilizing the frequency of the beat signal so that the frequency of the measured light is closest to the frequency of a fourth mode having a number smaller than the third mode number; the first control unit controls the light source so that the frequency of the carrier envelope offset signal is stabilized in each of the first, second, third, and fourth operations; and the optical frequency calculation unit calculates approximate values ​​of first-, second-, third-, and fourth-order approximate mode numbers in the first operation based on the frequency intervals in the first, second, third, and fourth operations, and calculates the true mode number in the first operation based on these approximate values, thereby calculating the frequency of the measured light.

[0097] (Supplementary Note 2) In the optical frequency measurement device according to Supplementary Note 1, the optical frequency calculation unit calculates the number of the first approximate mode in the first operation as N １ , the number of the second-order approximate mode is N２ , the third and fourth approximate mode numbers are N ｉ (i is a natural number equal to or greater than 3), and the frequency interval detected during the first operation is defined as f ｒｅｐ０ , the frequency interval detected during the second operation is f ｒｅｐ１ , the frequency interval detected during the third operation is f ｒｅｐ２ , the frequency interval detected during the fourth operation is f ｒｅｐｉ , the difference between the mode number during the second operation and the mode number during the first operation is 1, the difference between the mode number during the third operation and the mode number during the first operation is k ２ , the difference between the mode number during the fourth operation and the mode number during the first operation is k ｉ When the above f ｒｅｐ０ , f ｒｅｐ１ Based on the N １ Calculate the approximate value of N １ The estimated value of and the f ｒｅｐ０ , f ｒｅｐ２ and the difference in the number of modes based on k ２ Calculate the calculated k ２ and the f ｒｅｐ０ , f ｒｅｐ２ Based on the above, ２ Calculate the approximate value of N ｉ－１ and the f ｒｅｐ０ , f ｒｅｐｉ and the difference in the number of modes based on k ｉ Calculate the calculated k ｉ and the f ｒｅｐ０ , f ｒｅｐｉ Based on the above, ｉ Calculate the approximate value of N ｉ Calculating the true mode number N during the first operation based on the estimated value of ｉ and the f ｒｅｐ０ , f ｒｅｐ１ , f ｒｅｐ２ , f ｒｅｐｉ The frequency of the light to be measured is calculated based on the above.

[0098] (Supplementary Note 3) In the optical frequency measurement device described in Supplementary Note 1, the first control unit adjusts the power of the excitation light of the light source to stabilize the frequency of the carrier envelope offset signal, and the second control unit adjusts the resonator length of the light source to stabilize the frequency of the beat signal.

[0099] (Supplementary Note 4) The optical frequency measurement method of the present invention includes a first step of controlling a light source of an optical frequency comb so that the frequency of a carrier envelope offset signal of the optical frequency comb, in which modes are arranged at equal intervals on a frequency axis, is stabilized at a first reference frequency; a second step of controlling the light source so that the frequency of a beat signal generated by interference between the optical frequency comb and the light under measurement is stabilized at a second reference frequency; a third step of detecting a frequency interval between the modes when the light source is controlled by the first and second steps; a fourth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency in a state in which the frequency of a second mode having a number one smaller than a first mode whose frequency is closest to the frequency of the light under measurement when the light source is controlled by the first and second steps; a fifth step of detecting a frequency interval between the modes when the light source is controlled by the first and fourth steps; and a fifth step of detecting a frequency interval between the modes when the light source is controlled by the first and fourth steps. a sixth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency in a state where the frequency of the light under measurement is closest to the second reference frequency; a seventh step of detecting a frequency interval between the modes when the light source is controlled by the first and sixth steps; an eighth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency in a state where the frequency of the light under measurement is closest to the frequency of a fourth mode having a number smaller than the third mode number; a ninth step of detecting a frequency interval between the modes when the light source is controlled by the first and eighth steps; and a tenth step of calculating, based on the frequency intervals detected in the third, fifth, seventh, and ninth steps, approximate values ​​of first, second, third, and fourth-order approximate mode numbers when the light source is controlled by the first and second steps, and calculating, based on the approximate values, a true mode number when the light source is controlled by the first and second steps, thereby calculating the frequency of the light under measurement.

[0100] (Supplementary Note 5) In the optical frequency measuring method according to Supplementary Note 4, the tenth step is to set the number of the first approximate mode in the first operation to N １ , the number of the second-order approximate mode is N ２ , the third and fourth approximate mode numbers are N ｉ (i is a natural number equal to or greater than 3), and the frequency interval detected in the third step is defined as f ｒｅｐ０ , the frequency interval detected in the fifth step is f ｒｅｐ１ , the frequency interval detected in the seventh step is f ｒｅｐ２ , the frequency interval detected in the ninth step is f ｒｅｐｉ , the difference between the mode number during the second operation and the mode number during the first operation is 1, the difference between the mode number during the third operation and the mode number during the first operation is k ２ , the difference between the mode number during the fourth operation and the mode number during the first operation is k ｉ When the above f ｒｅｐ０ , f ｒｅｐ１ Based on the N １ Calculate the approximate value of N １ The estimated value of and the f ｒｅｐ０ , f ｒｅｐ２ and the difference in the number of modes based on k ２ Calculate the calculated k ２ and the f ｒｅｐ０ , f ｒｅｐ２ Based on the above, ２ Calculate the approximate value of N ｉ－１ and the f ｒｅｐ０ , f ｒｅｐｉ and the difference in the number of modes based on k ｉ Calculate the calculated k ｉ and the f ｒｅｐ０ , f ｒｅｐｉと Based on the N ｉ and calculate the true mode number N when the light source is controlled by the first and second steps, and calculate the approximate value of N ｉ Calculation is performed based on the estimated value of N and the k ｉ and the f ｒｅｐ０ , f ｒｅｐ１ , f ｒｅｐ２ , fｒｅｐｉ and calculating the frequency of the light to be measured based on the frequency of the light to be measured.

[0101] (Supplementary Note 6) In the optical frequency measurement method described in Supplementary Note 4, the first step includes a step of stabilizing the frequency of the carrier-envelope offset signal by adjusting the power of the excitation light of the light source, and the second, fourth, sixth, and eighth steps include a step of stabilizing the frequency of the beat signal by adjusting the resonator length of the light source.

[0102] The present invention can be applied to techniques for measuring optical frequencies.

[0103] 21...Light source to be measured, 22...Optical comb light source, 23...Self-referencing interferometer, 24-1, 24-2...Control unit, 25...Frequency counter, 26...Microwave signal composed of GPS, 27 to 29...Photodetector, 30...Optical frequency calculation unit, 31...Beam splitter, 32 to 34...Detection unit.

Claims

1. A light source configured to generate an optical frequency comb in which modes are equally spaced on a frequency axis; a first detector configured to detect a carrier envelope offset signal of the optical frequency comb; a second detector configured to detect a beat signal generated by interference between the light under measurement and the optical frequency comb; a third detector configured to detect the frequency interval of the modes; a first controller configured to control the light source so that the frequency of the carrier envelope offset signal is stabilized at a first reference frequency; a second controller configured to control the light source so that the frequency of the beat signal is stabilized at a second reference frequency; and an optical frequency calculator configured to calculate the frequency of the light under measurement, the second control unit sequentially performs at least four operations, including a first operation for initially stabilizing the frequency of the beat signal, a second operation for stabilizing the frequency of the beat signal so that the frequency of the light under measurement becomes closest to the frequency of a second mode having a number one smaller than the number of a first mode whose frequency is closest to the frequency of the light under measurement in the first operation, a third operation for stabilizing the frequency of the beat signal so that the frequency of the light under measurement becomes closest to the frequency of a third mode having an even smaller number than the number of the second mode, and a fourth operation for stabilizing the frequency of the beat signal so that the frequency of the light under measurement becomes closest to the frequency of a fourth mode having an even smaller number than the number of the third mode; and the first control unit controls the light source so that the frequency of the carrier envelope offset signal is stabilized in each of the first, second, third, and fourth operations. the optical frequency calculation unit calculates approximate values ​​of first, second, third, and fourth-order approximate mode numbers during the first operation based on the frequency intervals during each of the first, second, third, and fourth operations, and calculates a true mode number during the first operation based on these approximate values, thereby calculating the frequency of the light to be measured.

2. In the optical frequency measuring device according to claim 1, the optical frequency calculation section calculates the number of the first approximate mode in the first operation as N １ , the number of the second-order approximate mode is N ２ , the third and fourth approximate mode numbers are N ｉ (i is a natural number equal to or greater than 3), and the frequency interval detected during the first operation is defined as f ｒｅｐ０ , the frequency interval detected during the second operation is f ｒｅｐ１ , the frequency interval detected during the third operation is f ｒｅｐ２ , the frequency interval detected during the fourth operation is f ｒｅｐｉ , the difference between the mode number during the second operation and the mode number during the first operation is 1, the difference between the mode number during the third operation and the mode number during the first operation is k ２ , the difference between the mode number during the fourth operation and the mode number during the first operation is k ｉ When the above f ｒｅｐ０ , f ｒｅｐ１ Based on the N １ Calculate the approximate value of N １ The estimated value of and the f ｒｅｐ０ , f ｒｅｐ２ and the difference in the number of modes based on k ２ Calculate the calculated k ２ and the f ｒｅｐ０ , f ｒｅｐ２ Based on the above, ２ Calculate the approximate value of N ｉ－１ and the f ｒｅｐ０ , f ｒｅｐｉ and the difference in the number of modes based on k ｉ Calculate the calculated k ｉ and the f ｒｅｐ０ , f ｒｅｐｉ Based on the above, ｉ Calculate the approximate value of N ｉ Calculating the true mode number N during the first operation based on the estimated value of ｉ and the f ｒｅｐ０ , f ｒｅｐ１ , f ｒｅｐ２ , f ｒｅｐｉ and calculating the frequency of the light to be measured based on the above.

3. An optical frequency measuring device according to claim 1, wherein the first control unit adjusts the power of the excitation light of the light source to stabilize the frequency of the carrier envelope offset signal, and the second control unit adjusts the resonator length of the light source to stabilize the frequency of the beat signal.

4. A first step of controlling a light source of an optical frequency comb so that the frequency of a carrier envelope offset signal of the optical frequency comb, whose modes are arranged at equal intervals on the frequency axis, is stabilized at a first reference frequency; a second step of controlling the light source so that the frequency of a beat signal generated by interference between the optical frequency comb and the light under measurement is stabilized at a second reference frequency; a third step of detecting the frequency interval of the modes when the light source is controlled by the first and second steps; a fourth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency in a state where the frequency of a second mode having a number one smaller than the number of a first mode whose frequency is closest to the frequency of the light under measurement when the light source is controlled by the first and second steps; and a fifth step of detecting the frequency interval of the modes when the light source is controlled by the first and fourth steps. a sixth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency when the frequency of the measured light is closest to the frequency of a third mode having a number smaller than the number of the second mode; a seventh step of detecting the frequency interval of the modes when the light source is controlled by the first and sixth steps; an eighth step of controlling the light source so that the frequency of the beat signal is stabilized at the second reference frequency when the frequency of the measured light is closest to the frequency of a fourth mode having a number smaller than the number of the third mode; and a ninth step of detecting the frequency interval of the modes when the light source is controlled by the first and eighth steps. and a tenth step of calculating, based on the frequency intervals detected in the third, fifth, seventh, and ninth steps, approximate values ​​of first, second, third, and fourth order approximate mode numbers when the light source is controlled by the first and second steps, and calculating, based on the approximate values, a true mode number when the light source is controlled by the first and second steps, thereby calculating the frequency of the measured light.

5. The optical frequency measuring method according to claim 4, wherein the tenth step determines the number of the first approximate mode in the first operation as N １ , the number of the second-order approximate mode is N ２ , the third and fourth approximate mode numbers are N ｉ (i is a natural number equal to or greater than 3), and the frequency interval detected in the third step is defined as f ｒｅｐ０ , the frequency interval detected in the fifth step is f ｒｅｐ１ , the frequency interval detected in the seventh step is f ｒｅｐ２ , the frequency interval detected in the ninth step is f ｒｅｐｉ , the difference between the mode number during the second operation and the mode number during the first operation is 1, the difference between the mode number during the third operation and the mode number during the first operation is k ２ , the difference between the mode number during the fourth operation and the mode number during the first operation is k ｉ When the above f ｒｅｐ０ , f ｒｅｐ１ Based on the N １ Calculate the approximate value of N １ The estimated value of and the f ｒｅｐ０ , f ｒｅｐ２ and the difference in the number of modes based on k ２ Calculate the calculated k ２ and the f ｒｅｐ０ , f ｒｅｐ２ Based on the above, ２ Calculate the approximate value of N ｉ－１ and the f ｒｅｐ０ , f ｒｅｐｉ and the difference in the number of modes based on k ｉ Calculate the calculated k ｉ and the f ｒｅｐ０ , f ｒｅｐｉと Based on the N ｉ and calculate the true mode number N when the light source is controlled by the first and second steps, and calculate the approximate value of N ｉ Calculation is performed based on the estimated value of N and the k ｉ and the f ｒｅｐ０ , f ｒｅｐ１ , f ｒｅｐ２ , f ｒｅｐｉ and calculating a frequency of the light to be measured based on the result of the calculation.

6. An optical frequency measurement method according to claim 4, wherein the first step includes a step of stabilizing the frequency of the carrier envelope offset signal by adjusting the power of the excitation light of the light source, and the second, fourth, sixth, and eighth steps include a step of stabilizing the frequency of the beat signal by adjusting the resonator length of the light source.

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