JP7128516B2 - How to measure interference signals in dual comb spectroscopy - Google Patents

Description

The present invention relates to a method for measuring interference signals in dual comb spectroscopy.

Dual comb spectroscopy is one of Fourier transform spectroscopy using two optical frequency combs with different repetition frequencies, and is a spectroscopy that enables wide-wavelength, high-precision, and high-resolution measurements. Dual-comb spectroscopy is expected to replace Fourier transform infrared spectrometer (FTIR) as a new spectroscopic method because it can measure precise spectra at ultra-high speed.

Physical property information of a solid sample can be obtained as a phase spectrum by measurement using dual comb spectroscopy. In order to obtain the phase spectrum, a solid sample is placed on the path of one of the two optical frequency combs, and the interference waveform between the optical frequency comb that passes through the solid sample and the optical frequency comb that does not pass through is measured. get. By performing phase correction and coherent integration of the interferometric waveform in real time, the phase spectrum including the properties of the solid sample can be detected (see, for example, Non-Patent Document 1). Furthermore, physical property information of the solid sample can be obtained by Fourier analysis of the detected phase spectrum.

``Dual-comb spectroscopy for rapid characterization of complex optical properties of solids'', Akifumi Asahara, Akiko Nishiyama, Satoru Yoshida, Ken-ichi Kondo, Yoshiaki Nakajima and Kaoru Minoshima, Optics Letters, Vol. 41, No. 21, pp. 4971 -4974, 2016.

When acquiring a phase spectrum using dual comb spectroscopy, for example, one of two optical frequency combs with different repetition frequencies is used as a signal light pulse for carrying information on a solid sample, and the other The optical frequency comb is assumed to be a local optical pulse for causing multi-heterodyne interference with the signal optical pulse. A solid sample is placed on the path of the signal light pulse, and the phase delay of the light pulse in the time domain is measured when the signal light pulse passes through the solid sample or passes through the solid sample after internal reflection (multiple reflection). In order to measure the phase delay of a light pulse that occurs depending on the phase refractive index (simply called refractive index) of a solid sample, a standard light pulse, that is, light that does not pass through the solid sample and has no phase delay Detect the deviation on the time axis from the pulse.

In the measurement of interference signals using conventional dual comb spectroscopy, including the above-mentioned Non-Patent Document 1, it was essential to move the solid sample in order to measure the phase delay of the light pulse that passed through the solid sample. . Movement of the solid sample involves mechanically inserting and retracting the solid sample with respect to the path of the signal light pulse. Multi-heterodyne interference is caused between the signal light pulse and the local light pulse when the solid sample is inserted into the path of the signal light pulse and when the solid sample is retracted from the path of the signal light pulse, and an interference waveform is obtained. In order to improve the accuracy of phase detection, the signal light pulse is divided into two so as to travel different paths, and the solid sample is inserted and retracted from the path of the first signal light pulse as described above. Using the second signal light pulse that does not pass through the sample as a reference light pulse, the first signal light pulse and the reference light pulse are each caused to undergo multi-heterodyne interference with the local light pulse, and each interference waveform is acquired.

However, slow phase fluctuations occur when the solid sample is inserted and retracted from the path of the signal light pulse. The horizontal axis of FIG. 1 indicates the time scale t for observing the interference waveform between the signal light pulse and the local light pulse in dual comb spectroscopy in log scale. The time scale t represents the time from taking one measurement to taking the next. The vertical axis in FIG. 1 indicates the magnitude of phase fluctuation δφ (phase uncertainty) of the interference waveform on a log scale. As shown in FIG. 1, as the observation time _t increases from zero to the vicinity of the predetermined value ts, the signal-to-noise ratio decreases due to integration of the measured values, so the phase fluctuation δφ decreases (region R-1). . When the observation time _t further increases from the vicinity of the predetermined value ts, the phase fluctuation δφ increases (region R-3) due to the influence of the long-period phase drift of several seconds. Such a long-period phase drift is included in the phase fluctuation .delta..phi. of the measured value when the solid sample is inserted into and retracted from the path of the signal light pulse.

Further, when the reference light pulse is used as described above, phase detection is enhanced by removing the above-described phase fluctuation δφ through signal correction processing using the interference waveform of the reference light pulse and the local light pulse as a phase reference. Precision can be achieved. However, since the reference light pulse and the signal light pulse are branched into paths different from each other, the spatial region through which the reference light pulse and the signal light pulse pass differently and the influence of environmental fluctuations in the optical components for branching the paths are affected. , may be introduced into the measurements as phase fluctuations other than the phase fluctuation δφ. Therefore, even if the reference light pulse is used, a long-period phase drift is added to the phase fluctuation δφ of the measured value when the solid sample is placed on the path of the first signal light pulse.

That is, in the measurement of light pulses using the conventional dual comb spectroscopy, it is difficult to accurately measure the time delay of the signal light pulses of the optical frequency comb, which varies depending on whether or not they pass through the sample. There is a problem that the phase fluctuation .delta..phi. increases.

The present invention has been made in view of the above-mentioned circumstances, and is capable of accurately measuring phase information of each frequency mode of an optical frequency comb that changes while passing through a sample to be measured. A method for measuring interference signals in dual-comb spectroscopy that is unnecessary and can be measured at high speed is provided.

A method of measuring an interference signal in dual comb spectroscopy of the present invention is a method of measuring an interference signal in dual comb spectroscopy using a first optical frequency comb and a second optical frequency comb having repetition frequencies different from each other. of the single first optical frequency comb whose passage width in the width direction perpendicular to the traveling direction of the single first optical frequency comb is larger than the measurement width in the width direction of the portion to be measured of the sample a first step of arranging the part to be measured in a part of the region through which the passage width-adjusted single first optical frequency comb passes in the width direction at a sample placement position on the path; The passage width-adjusted single first optical frequency comb not passing through the part to be measured and the passage emitted from the part to be measured on the far side of the passage direction of the passage width-adjusted single first optical frequency comb a second step of collectively receiving a width-adjusted single first optical frequency comb; generating a first interference signal between the passwidth-adjusted single first optical frequency comb and the second optical frequency comb that have not passed through, and the passwidth adjustment received collectively in the second step a third step of generating a second interference signal between the passage width-adjusted single first optical frequency comb and the second optical frequency comb, which have passed through the part under test, among the single first optical frequency combs; , of the optical pulses of the first passwidth-adjusted single optical frequency comb that have not passed through the part to be measured among the first passwidth-adjusted single optical frequency combs received collectively in the second step; Using the position on the time axis as a reference position, the pass width-adjusted single first optical frequency comb that has passed through the part to be measured among the pass width-adjusted single first optical frequency combs received collectively in the second step. A fourth step of obtaining information on the position of the measurement target with respect to the reference position based on the first interference signal and the second interference signal, with the position on the time axis of the optical pulse of the optical frequency comb as the position of the measurement target. And prepare.

The method for measuring an interference signal in dual comb spectroscopy described above may further include a fifth step of calculating physical property information of the sample based on the information of the position to be measured with respect to the reference position acquired in the third step. good.

In the method for measuring an interference signal in dual comb spectroscopy described above, in the second step, the passage width-adjusted single first optical frequency comb emitted from the part to be measured travels along the traveling direction. the passage width-adjusted single first optical frequency comb transmitted through the section and the passage width-adjusted single first optical frequency comb multiple-reflected along the traveling direction between the incident surface and the exit surface of the measured section; and an optical frequency comb.

According to the present invention, it is possible to accurately measure the phase information of each frequency mode of the optical frequency comb that changes through the sample to be measured, does not require sample movement, and can be measured at high speed Dual comb spectroscopy A method for measuring an interfering signal in a method is provided.

FIG. 4 is a schematic diagram showing changes in phase fluctuation with respect to time for observing a signal light pulse; 1 is a schematic diagram of an optical frequency comb in the frequency domain and time domain; FIG. It is a schematic diagram for demonstrating the principle of dual comb spectroscopy. FIG. 3 is a schematic diagram for explaining a model of dual comb spectroscopy and the behavior of light pulses; 1 is a schematic diagram of a measuring apparatus for measuring a phase profile based on the method for measuring interference signals in dual comb spectroscopy of the present invention; FIG. 6 is a schematic diagram showing part of the measuring device shown in FIG. 5; FIG. 6 is an enlarged view of region 174 of the measuring device shown in FIG. 5; FIG. 6 is an enlarged view of a part of the measuring device shown in FIG. 5; FIG.

An embodiment of a method for measuring an interference signal in dual comb spectroscopy of the present invention (hereinafter sometimes simply referred to as "measurement method") will be described below with reference to the drawings.

[Explanation of principle]
As shown on the right side of FIG. 2, the optical frequency comb has a first optical frequency mode with an offset frequency f _CEO with respect to zero on the frequency axis in the frequency domain and and a plurality of second optical frequency modes spaced apart by a positive integer multiple of the repetition frequency f _rep . The frequency fm of the 0th to _m -th optical frequency modes on the frequency axis is expressed by the following equation (1).

Hereinafter, the first frequency mode and the second optical frequency mode are collectively referred to as an optical frequency mode (or multiple optical frequency modes). When the frequency distribution (spectrum) of the optical frequency comb is Fourier transformed and viewed in the time domain, an optical pulse train having a repetition period (1/f _rep ) appears as shown on the left side of FIG.

Dual comb spectroscopy is Fourier transform spectroscopy using two optical frequency combs (a first optical frequency comb and a second optical frequency comb) with different repetition frequencies. Let repetition frequencies of the two optical frequency combs be repetition frequencies f _rep1 and f _rep2 , respectively. Using the repetition frequency f _rep1 as a reference, the repetition frequency f _rep2 is expressed by the following equation (2).

Note that Δf _rep in equation (2) represents the repetition frequency difference between the two optical frequency combs.

As shown in FIG. 3, in the dual comb spectroscopy, among the two optical frequency combs 1 and 2 having a repetition frequency difference Δf _rep , only the optical frequency comb 1 (first optical frequency comb, Signal shown in FIG. 3) pass through the solid sample 5 (sample, Sample shown in FIG. 3), and the optical frequency comb 2 (second optical frequency comb, Local; LO shown in FIG. 3) does not pass through the solid sample 5 . By passing through the solid sample 5 , the spectrum of the optical frequency comb 1 changes under the influence of the physical property information of the solid sample 5 . That is, the optical frequency comb 1 changes to the optical frequency comb 3 by passing through the solid sample 5 . The repetition frequency of optical frequency comb 3 is equal to the repetition frequency f _rep1 of optical frequency comb 1 . On the other hand, the envelope formed by the multiple frequency modes of the optical frequency comb 3 is different from the envelope formed by the multiple frequency modes of the optical frequency comb 1 . Physical property information of the solid sample 5 is reflected in the difference between these envelopes.

By performing multi-heterodyne detection on the optical frequency combs 2 and 3, information on the amplitude and phase of the optical frequency comb 3 can be obtained as a radio frequency comb 4 (Radio Frequency Comb; RF frequency comb shown in FIG. 3). The repetition frequency of the RF frequency comb 4 is equal to the repetition frequency difference Δf _rep . Also, the envelope formed by the multiple frequency modes of the RF frequency comb 4 reflects the envelope formed by the multiple frequency modes of the optical frequency comb 3 . Therefore, the physical property information of the solid sample 5 reflected on the optical frequency comb 3 can be detected by the RF frequency comb 4 having a repetition frequency lower than that of the optical frequency comb 3 .

Physical property information that can be derived by dual-comb spectroscopy includes, for example, thickness L, group index n _g (ω), and phase index n _p (ω). ω represents the angular frequency of the optical frequency comb 1 passing through the solid sample 5, and is represented by 2πf (f is the frequency of the optical frequency comb 1 corresponding to the angular frequency ω). The group index n _g (ω) and the phase index n _p (ω) are each a function of the angular frequency ω.

Thickness L represents the geometrical thickness of solid sample 5 . The group refractive index n _g (ω) is a refractive index corresponding to the propagation velocity of the optical pulse of the optical frequency comb, and is expressed by the following equation (3).

The phase refractive index n _p (ω) is included in the complex refractive index n _c in the following equation (4). i is the imaginary unit and κ represents the absorption coefficient of the solid sample 5 .

For example, when the extinction coefficient κ is 0 or more and 0.001 or less and is sufficiently small, such as the extinction coefficient of silica glass, only the real phase refractive index n _p (ω) is used as the complex refractive index _nc . Consider.

In order to acquire the above physical property information of the solid sample 5, a model shown in FIG. 4 is assumed. As shown in the upper part of FIG. 4, the solid sample 5 has an incident surface 6 and an exit surface 7 parallel to each other. The entrance surface 6 and the exit surface 7 are arranged so as to be substantially perpendicular to the course of the optical frequency comb 1 . When the optical frequency comb 1 enters the solid sample 5 from the incident surface 6 , part of the optical frequency comb 1 travels inside the solid sample 5 and exits from the exit surface 7 as the transmitted optical frequency comb 3 . The remaining part of the optical frequency comb 1 passes through the inside of the solid sample 5, is reflected by the exit surface 7, then goes back to the entrance surface 6, and after being reflected by the entrance surface 6, is multiple-reflected. The comb 3 is emitted from the emission surface 7 . The rest of the optical frequency comb 1 is sequentially emitted from the solid sample 5 as the optical frequency comb 3 that is more multiple-reflected inside the solid sample 5 between the entrance surface 6 and the exit surface 7 . In this embodiment, as shown in FIG. 4, the optical frequency comb 3 transmitted through the solid sample 5 and the optical frequency comb 3 multiple-reflected the solid sample 5 twice are handled.

As shown in the upper part of FIG. 4, when the optical pulse 8 of the optical frequency comb 1 passes through the solid sample 5, after a certain time has passed, the transmitted optical pulse 9 of the optical frequency comb 3 (“Transmitted pulse") and a multiple reflected light pulse 10 ("First echo" shown in FIG. 4) of the optical frequency comb 3 multiple-reflected (multiple-reflected twice) at the exit surface 7 and the entrance surface 6. FIG.

As shown in the lower part of FIG. 4, the light pulse 8 becomes a reference light pulse 11 after a certain period of time has elapsed without passing through the solid sample 5 . Note that "without passing through the solid sample 5" means "passing through a solid sample (virtual sample indicated by a dashed line in FIG. 4) 12 having the same thickness L as the solid sample 5 and the refractive index n _air of the atmosphere. has the same meaning as A phase spectrum φ ₁ (ω) of the transmitted light pulse 9 with respect to the reference light pulse 11 is expressed by the following equation (5).

N1 in equation (5) represents the _phase offset coefficient of the transmitted light pulse 9 . A phase spectrum φ ₂ (ω) of the multiple-reflected light pulse 10 with respect to the reference light pulse 11 is expressed by the following equation (6). c represents the speed of light in vacuum. n _air represents the refractive index of the atmosphere.

From the equations (5) and (6), the thickness L of the solid sample 5 is expressed by the following equation (7). Expression (7) is an expression showing the relative relationship between the thickness L and the slopes of the _{phase spectra φ 1 ( ω) and φ 2 (} ω). This means that the thickness L is obtained based on

Here, by differentiating both sides of the equation (5) with respect to the angular frequency ω, the following equation (8) is obtained.

By transforming the equation (3), the following equation (9) is obtained, and substituting the equation (9) into the equation (8) leads to the equation (10) described later.

When formula (10) is arranged with respect to the group refractive index n _g (ω), the group refractive index n _g (ω) of the solid sample 5 is expressed by the following formula (11). Formula (11) is a formula showing the relative relationship between the group refractive index n _g (ω) and the slope of the _{phase spectrum φ 1 (} ω). It means that the rate n _g (ω) is determined.

However, in equation (5), the phase spectrum φ ₁ (ω) is simplified and the phase offset factor N is not considered. As in the equations (7) and (11), when the inclination of the phase spectra φ ₁ (ω) and φ ₂ (ω) with respect to the angular frequency ω should be noted, the phase offset coefficient N does not necessarily have to be considered. good. As shown in FIG. 4, considering the phase offset coefficient N, the correct phase spectrum Φ(ω) can be expressed as the following equation (12).

In heterodyne detection based on dual-comb spectroscopy, the difference in the second term on the right-hand side of Eqs. (5) and (6) cannot be obtained (i.e., not visualized) and the intrinsic phase absolute spectrum Φ(ω) is Remainders of division by 2π are obtained as phase spectra φ ₁ (ω) and φ ₂ (ω). Therefore, even if the phase spectra φ ₁ (ω) and φ ₂ (ω) are not corrected by the phase offset 2πN based on the measurement results based on the dual comb spectroscopy, the thickness L of the solid sample 5 and the group The refractive index n _g (ω) is easily and directly derived.

The phase refractive index n _p (ω) of the solid sample 5 is represented by the following equation (13) and includes the phase offset coefficient N as a parameter.

That is, in order to accurately derive the phase refractive index n _p (ω) of the solid sample 5, the thickness L, the phase spectrum φ ₁ (ω), and the phase offset coefficient N must be accurately obtained by predetermined methods. .

[Method for measuring interference signals in dual comb spectroscopy of the present invention]
In the measuring method of the present invention, the phase spectra φ ₁ (ω) and φ ₂ (ω) are accurately obtained based on the phase difference on the time axis between the transmitted light pulse 9 and the multiple reflected light pulse 10 based on dual comb spectroscopy. can be measured.

As an example of a measuring device for accurately measuring the phase spectra φ ₁ (ω) and φ ₂ (ω) based on the measuring method of the present invention, there is a dual comb spectroscopic measuring device 210 shown in FIG. As shown in FIG. 5, the dual comb spectrometer 210 includes optical frequency comb output units 210A and 210B, a frequency control unit 290, and a continuous wave laser as a configuration for generating the two optical frequency combs 1 and 2. (hereinafter referred to as a CW laser) 292 and a frequency stabilizer 294 are provided.

The optical frequency comb output section 210A outputs the optical frequency comb 1 . The optical frequency comb output unit 210B outputs the optical frequency comb 2. FIG. The frequency control section 290 inputs a reference signal for controlling the offset frequency difference Δf _CEO of each of the optical frequency combs 1 and 2 to each of the optical frequency comb output sections 210A and 210B. A CW laser 292 synchronizes the phases of the optical frequency combs 1 and 2 . The frequency stabilizer 294 controls continuous wave light (hereinafter referred to as CW light) emitted from the CW laser 292 and beat signals of the optical frequency combs 1 and 2, respectively.

5 and 6, the optical frequency comb output units 220 of the optical frequency comb output units 210A and 210B, the function generator (FG) 64 of the offset frequency control unit 218, the FG 74 of the repetition frequency control unit 222, the piezo Principal portions such as the (PZT) element 230 are illustrated, and the illustration of the configuration other than the principal portions is omitted. Each of the optical frequency comb output units 210A and 210B is composed of a semiconductor laser (LD), a large number of optical elements, etc., as shown in FIG.

The optical frequency comb output units 210A and 210B each include an optical frequency comb light source 212, an optical interference unit 214, a beat signal detection unit 216, an offset frequency control unit 218, an optical frequency comb output unit 220, and a repetition frequency control unit 222. ing.

The optical frequency comb light source 212 is, for example, a loop-type fiber laser. The optical frequency comb light source 212 includes an erbium-doped fiber (EDF) 24 and an LD 26 . An optical isolator 34, an optical coupler 32, a PZT element 230 capable of changing the cavity length of the fiber laser, and a polarization controller 28 are operated by the EDF 24 along the direction of light emitted from the EDF 24 (clockwise direction in FIG. 6). Concatenated.

The optical frequency comb output from the optical coupler 32 is supplied to the optical interference section 214 and the optical frequency comb output section 220 . Between the optical coupler 32 and the optical interference unit 214 and the optical frequency comb output unit 220, a polarization controller 38 and an EDF amplifier 40 are provided in order from the side closer to the optical coupler 32. FIG. The EDF amplifier 40 has an EDF 39 , an excitation LD 41 and an optical coupler 43 . Each component up to the optical coupler 32 and the optical interference unit 214 and each component up to the optical coupler 32 and the optical frequency comb output unit 220 are connected by an optical fiber 36 . A high-nonlinear fiber (HNLF) 42 is arranged between the EDF amplifier 40 and the optical interference section 214 . The HNLF 42 emits an optical frequency comb having a wider band than before it is incident.

The optical interference unit 214 includes, in order from the side closer to the optical frequency comb light source 212, a fiber collimator 44, a condenser lens 46, a λ/2 wavelength plate 48, a periodically-poled lithium niobate (PPLN) 50, An optical bandpass filter 52 is provided. The PPLN 50 emits a component in which the wideband optical frequency comb (2f) and the second harmonic (2×1f) newly generated by the PPLN 50 overlap.

The broadband optical comb and the second harmonic interfere with each other at the beat signal detector 216 . The beat signal detector 216 detects the beat signal of the broadband optical comb and the second harmonic. Light emitted from the PPLN 50 is detected by the photodetector 54 of the beat signal detector 216 . The electrical signal output from the photodetector 54 is transmitted to the offset frequency control section 218 via the electrical cable 56 and transmitted to the repetition frequency control section 222 via the electrical cable 58 .

The offset frequency control unit 218 includes a high frequency bandpass filter 61 , a high frequency amplifier 62 , a function generator (FG) 64 , a frequency converter (Double Balanced Mixer (DBM) 66 ), and a loop filter 68 . The offset frequency control section 218 applies feedback to the current applied to the excitation LD 26 by the loop filter 68 when the frequency of the reference signal transmitted from the FG 64 is changed. That is, by controlling the frequency of the reference signal transmitted from the FG 64, the offset frequency f _CEO of the optical comb emitted from the optical frequency comb light source 212 is controlled.

The repetition frequency control section 222 includes a high frequency bandpass filter 71 , a high frequency amplifier 72 , an FG 74 , a DBM 76 and a loop filter 78 . The repetition frequency control section 222 applies feedback to the PZT element 230 through the loop filter 78 when the frequency of the reference signal transmitted from the FG 74 is changed. That is, by controlling the frequency of the reference signal emitted from the FG 74, the repetition frequency f _rep of the optical frequency comb emitted from the optical frequency comb light source 212 is controlled.

The frequency stabilization unit 294 shown in FIG. 5 includes a frequency control unit 290 composed of a program or the like incorporated in a computer, FGs 130 and 132, DBMs 108 and 118, and PID controllers 110 and 120. The optical frequency comb 1 emitted from the optical frequency comb output section 210A enters the optical coupler 104 via the optical coupler 102 . CW light emitted from the CW laser 292 enters the optical coupler 104 via the optical coupler 112 . The optical frequency comb 1 and the CW light combined by the optical coupler 104 are received by the light receiving unit 106 such as a photodetector and converted into an electric signal. An electrical signal emitted from the light receiving section 106 is input to the DBM 108 and combined with a reference signal from the FG 130 . The output from DBM 108 is input to PID controller 110 . The output from PID controller 110 is fed back to the input current value to CW laser 292 .

The CW light emitted from the CW laser 292 also enters the optical coupler 114 via the optical coupler 112 . The optical frequency comb 2 emitted from the optical frequency comb output section 210B enters the optical coupler 114 via the optical coupler 122 . The optical frequency comb 2 and the CW light combined by the optical coupler 114 are received by the light receiving unit 116 such as a photodetector and converted into an electric signal. An electrical signal emitted from the light receiving section 116 is input to the DBM 118 and combined with a reference signal from the FG 132 . The output from DBM 118 is input to PID controller 120 . The output from PID controller 120 is fed back to the amount of displacement of PZT element 230 in optical frequency comb light source 212 of optical frequency comb output section 210B.

In the dual comb spectrometer 210, in order to cause the optical frequency comb 2 to follow the optical frequency comb 1 and generate a phase-synchronized dual comb, first, the repetition frequency f _rep1 of the optical frequency comb 1 and the offset frequency f _CEO1 is stabilized to the frequency of the reference signal emitted from FGs 64 and 74 of optical frequency comb output 210A. Next, the beat signal of the optical frequency comb 1 and the CW light output from the optical frequency comb output section 210A is stabilized with respect to the reference signal from the FG130. Next, after stabilizing the offset frequency _fCEO2 of the optical frequency comb 2 output from the optical frequency comb output unit 210B, the beat signal between the CW light and the optical frequency comb 2 is detected, and the detected beat signal is Stabilize against the reference signal from FG118. By such a procedure, the repetition frequencies f _rep1 and f _rep2 of the optical frequency combs 1 and 2 are made to follow the frequency of the CW light. That is, the optical frequency comb 2 follows the optical frequency comb 1, and a dual comb phase-synchronized with each other is obtained.

In addition, the frequency control unit 290 sets the six parameters of the offset frequency difference Δf _CEO , the offset frequency f _CEO1 , the offset frequency f _CEO2 , the repetition frequency difference Δf _rep , the repetition frequency f _rep1 , and the repetition frequency f _rep2 at arbitrary integer ratios. The four parameters of the offset frequencies f _CEO1 and f _CEO2 and the repetition frequencies f _rep1 and f _rep2 are controlled so that the indicated relative relationships hold.

In the dual comb spectrometer 210 , the optical frequency comb 1 controlled as described above and emitted from the fiber collimator 141 is folded back by the folding mirror 171 and expanded by the beam expander 172 . As shown in FIG. 7, the passage width of the optical frequency comb 1 expanded by the beam expander 172 (that is, the size of the optical frequency comb 1 in the direction (width direction) perpendicular to the path of the optical frequency comb 1) 301 is It is larger than the measurement width 19 in the width direction of the portion 17 to be measured of the solid sample 5, and is approximately twice the measurement width 19, for example.

The solid sample 5 has a uniform thickness L, and the entrance plane 6 and the exit plane 7 are perpendicular to the path of the optical frequency comb 1 and parallel to each other. The measured portion 17 means a portion that needs to be irradiated with the optical frequency comb 1 in order to obtain physical property information of the solid sample 5 . If the portion to be measured 17 is an arbitrary portion of the solid sample 5 (that is, no matter which portion of the solid sample 5 is irradiated with the optical frequency comb 1, the transmitted light pulse 9 and the multiple reflected light pulse 10 to be measured can be obtained. ), if the size in the width direction of the optical frequency comb 1 in the solid sample 5 is larger than the size that allows the optical frequency comb 1 to be substantially expanded, the passage width 301 is increased to an appropriate size by the beam expander 172, and the width A solid sample 5 can be inserted on one side of a directionally spread optical frequency comb (passwidth adjusted single first optical frequency comb) 311 .

As shown in FIG. 7, at a sample placement position 320 on the path of the optical frequency comb 311, the part to be measured 17 is placed in a part of the region through which the optical frequency comb 311 passes in the width direction. As a result, the light emitted from the optical frequency comb 312 and the measured portion 17 that does not pass through the measured portion 17 in the width direction is on the far side (that is, forward) in the traveling direction of the optical frequency comb 311 from the sample placement position 320 . An optical frequency comb 321 appears in a state in which the optical frequency comb 313 is spatially divided.

As shown in FIG. 5 , the optical frequency comb 321 emitted from the sample placement position 320 to the far side in the traveling direction of the optical frequency comb 311 enters the beam expander 176 . The passage width of the optical frequency comb 321 is about the same as the passage width of the optical frequency comb 2 by the beam expander 176 (that is, the size of the optical frequency comb 2 in the direction (width direction) perpendicular to the path of the optical frequency comb 2). reduced to size. After passing through the beam expander 176 , the optical frequency comb 321 enters the beam splitter 146 , is reflected, and travels along the same path as the optical frequency comb 2 .

Optical frequency comb 321,2 is received by photodetector 160 . As shown in FIG. 8, in the optical frequency comb 321 traveling from the beam splitter 146, the optical frequency combs 312 and 313 are adjacent in the width direction and pass through different regions. A condenser lens 165 is arranged on the course of the optical frequency combs 321 and 2 between the beam splitter 146 and the photodetector 160 . A light-receiving section 162 of the photodetector 160 is arranged at the far side of the optical frequency combs 321 and 2 from the condenser lens 165 and at a position separated from the condenser lens 165 by the focal length of the condenser lens 165 . The optical frequencies 312 and 313 and the optical frequency comb 2 are received in the same area of the light receiving section 162 .

A first interference signal resulting from the interference of optical frequency combs 312 and 2 at photodetector 160 is produced as RF frequency comb 342 . A second interference signal resulting from the interference of optical frequency combs 313,2 at photodetector 160 is generated as RF frequency comb 343 . That is, in this embodiment, the first interference signal and the second interference signal (not shown) are acquired as the RF frequency combs 342 and 343 .

In the data processing unit 98, based on the RF frequency combs 342 and 343 transmitted from the photodetector 160 and the above equations (5) to (13), the thickness L of the solid sample 5, the group refractive index n _g (ω ) and phase refractive index n _p (ω).

That is, the measuring method of this embodiment includes at least the first to third steps, and further includes a fourth step. In the first step, a region through which the optical frequency comb 311 passes in the width direction at a sample placement position 320 on the path of the optical frequency comb 311 whose passage width in the width direction is greater than the measurement width 19 of the part 17 to be measured. A portion to be measured 17 is arranged in a part of the . In the second step, the optical frequency comb 312 that does not pass through the part to be measured 17 and the optical frequency comb 313 emitted from the part to be measured 17 are located on the far side of the traveling direction of the optical frequency combs 311 and 321 from the sample placement position 320. are collectively received by the photodetector 160 . In the third step, a first interference signal between the optical frequency comb 312 and the optical frequency comb 2 among the optical frequency combs 321 collectively received in the second step is generated as the RF frequency comb 342 . In the third step, a second interference signal between the optical frequency comb 313 and the optical frequency comb 2 among the optical frequency combs 321 collectively received in the second step is generated as the RF frequency comb 343 . In the fourth step, the position of the optical pulse 11 of the optical frequency comb 312 on the time axis is set as a reference position, and the positions of the transmitted light pulse 9 and the multiple reflected light pulse 10 of the optical frequency comb 313 on the time axis are set as the positions to be measured. , and RF frequency combs 342 and 343, the data processing unit 98 acquires information on the position of the object to be measured with respect to the reference position as a phase spectrum φ _DC (ω). In the fifth step, physical property information of the solid sample 5 is calculated based on the information of the measurement target position with respect to the reference position.

According to the measurement method of this embodiment, the optical frequency combs 312 and 313 are spatially divided in the single optical frequency comb 321, and the difference between the optical frequency combs 312 and 313 is passed through the solid sample 5 at the sample placement position 320. You can only ask whether or not In other words, the difference between the light pulse 11, which is the reference light pulse, and each of the transmitted light pulse 9 and the multiple-reflected light pulse 10 can be only whether or not they passed through the solid sample 5 at the sample arrangement position 320. can. In addition, the transmitted light pulse 9 and the multiple reflected light pulse 10 have different paths in that they pass through the solid sample 5 at the sample placement position 320 or undergo multiple reflections. 311,321. Furthermore, once the solid sample 5 is arranged, mechanical operations such as inserting it into the path of the optical frequency comb 311 or withdrawing it from the path are unnecessary. Therefore, the phase spectra φ ₁ (ω) and φ ₂ (ω) shown in FIG. is canceled, and the phase fluctuation .delta..phi. shown in FIG. 1 can be suppressed closer to the region R-2 than in the conventional measurement method.

Since the phase spectra φ ₁ (ω) and φ ₂ (ω) can be accurately measured by the measurement method of the present embodiment as described above, the phase spectra φ ₁ (ω) and φ ₂ (ω) are derived based on Accuracy of physical property information of the solid sample 5 can be improved. The above-mentioned effects can be obtained by using an optical frequency comb in which the position of each of a plurality of optical frequency modes on the frequency axis and the position of the optical pulse on the time axis are accurate, and the phase spectrum of the optical pulse (that is, the (Information above) is very effective as described above.

According to the measurement method of the present embodiment, in the second step, the optical frequency comb 321 emitted from the part to be measured 17 passes through the part to be measured 17 along the traveling direction of the optical frequency comb 311. 1, and an optical frequency comb 313-2 that is multiple-reflected along the traveling direction between the entrance surface 6 and the exit surface 7 of the measured part 17. FIG.

As described above, the phase spectra φ ₁ (ω) and φ ₂ (ω) can be collectively and accurately measured by the measurement method of this embodiment. In this embodiment, the transmitted light pulse 9 and the multiple reflected light pulse 10 illustrated in FIG. 4 are measured. The power decreases as the number of multiple reflections between the exit surface 7 and the entrance surface 6 increases, but the number of times between the exit surface 7 and the entrance surface 6 increases, such as 4 times, 6 times, and so on. , and the phase spectrum φ _DC (ω) of those light pulses can be measured. Therefore, based on the phase spectrum φ _DC (ω) of the light pulses emitted from the exit surface 7 of the solid sample 5, including the transmitted light pulse 9 and the multiple reflected light pulse 10, with respect to the reference light pulse 11, the thickness L, the group refraction Physical property information of the solid sample 5 including the index φ _g (ω) and the phase refractive index φ _p (ω) can be obtained with high accuracy.

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the gist of the present invention described in the claims.・Changes are possible.

For example, in the embodiments described above, the optical frequency comb 1 was described as being expanded by the beam expander 172 . However, the method of expanding the optical frequency comb 1 to have the passage width 301 is not particularly limited, and it may be done with an optical component other than the beam expander 172 . Alternatively, the exit port of the fiber collimator 141 may be processed to be larger than the passage width 301 , and the optical frequency comb 311 having the passage width 301 may be directly emitted from the fiber collimator 141 .

Further, in the above-described embodiments, a solid sample whose absorption coefficient is negligibly small is assumed as a target for acquiring physical property information. However, by introducing appropriate formulas and models considering the absorption coefficient into the above contents, the method for deriving the phase refractive index according to the present invention enables the derivation of the phase refractive index of a general-purpose sample. Moreover, the sample used for the measurement is not limited to a solid sample unless the anisotropy of the phase refractive index is large.

In addition, although the measurement object of the above-described embodiment is the phase refractive index of the solid sample 5, the measuring method of the present invention is not limited to the phase refractive index of the solid sample 5, and the dielectric constant and conductivity of the solid sample 5 are measured. Can be targeted. That is, the measurement method of the present invention can be widely applied to measurements that can be obtained based on the interference signal of the optical frequency comb.

1... Optical frequency comb (first optical frequency comb)
2 Optical frequency comb (second optical frequency comb)
5 Solid sample (sample)
Reference numerals 17 ... measured part 19 ... measurement width 301 ... passage width 311 ... optical frequency comb (pass width adjusted single first optical frequency comb)
312... Optical frequency comb (pass width adjusted single first optical frequency comb that does not pass through the part to be measured)
313... Optical frequency comb (pass width adjusted single first optical frequency comb emitted from the part to be measured)

Claims (3)

A method for measuring an interference signal in dual comb spectroscopy using a first optical frequency comb and a second optical frequency comb having repetition frequencies different from each other, comprising:
On the path of the single first optical frequency comb whose passage width in the width direction orthogonal to the traveling direction of the single first optical frequency comb is larger than the measurement width in the width direction of the portion to be measured of the sample a first step of arranging the part to be measured in a part of the region through which the passage width-adjusted single first optical frequency comb passes in the width direction at the sample arrangement position of
the passage width-adjusted single first optical frequency comb that does not pass through the part to be measured, and the subject to be measured, on the far side of the passage direction of the passage width-adjusted single first optical frequency comb from the sample arrangement position; a second step of collectively receiving the passage width-adjusted single first optical frequency comb emitted from the unit;
Of the pass width-adjusted single first optical frequency combs received collectively in the second step, the pass width-adjusted single first optical frequency comb that does not pass through the part to be measured and the second The first interference signal with the optical frequency comb is generated, and the passage width-adjusted single first optical frequency comb received in the second step has passed through the part to be measured. a third step of generating a second interference signal of a single first optical frequency comb and said second optical frequency comb;
The time of the optical pulse of the first passwidth-adjusted single optical frequency comb that does not pass through the part to be measured among the first passwidth-adjusted single optical frequency combs that are collectively received in the second step. Using the position on the axis as a reference position, the passage width-adjusted single first light beam that has passed through the part to be measured among the passage width-adjusted single first optical frequency combs collectively received in the second step A fourth step of obtaining information on the position of the measurement target with respect to the reference position based on the first interference signal and the second interference signal, with the position of the optical pulse of the frequency comb on the time axis as the position of the measurement target; ,
A method for measuring interference signals in dual comb spectroscopy comprising: Further comprising a fifth step of calculating physical property information of the sample based on the information of the position to be measured with respect to the reference position acquired in the third step,
A method for measuring interference signals in dual comb spectroscopy according to claim 1 . In the second step,
The pass width-adjusted single first optical frequency comb emitted from the part to be measured,
the passage width-adjusted single first optical frequency comb that passes through the part under test along the traveling direction;
the passage width-adjusted single first optical frequency comb that is multiple-reflected along the traveling direction between the incident surface and the exit surface of the measured part,
The method for measuring an interference signal in dual comb spectroscopy according to claim 1 or 2.

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