JP2018197705A - Optical measurement device, and optical measurement method - Google Patents

Abstract

To provide an optical measurement device and optical measurement method that can conduct a measurement at high signal-to-noise ratio in comparison with a case where an intensity change in probe light resulting from a responce to stimulation to be applied to a sample is detected as a phase change in a detection signal.SOLUTION: An adder (201) is configured to add weight in accordance with intensity of an optical detection signal to be obtained by a weight adder (200) to a synchronization signal tuned by a chase controller (16) to derive a compensation synchronization signal. A multiplier (12) is configured to multiply the compensation synchronization signal by the optical detection signal, and thereby, even when phase noise of the optical detection signal is caused by intensity noise, a phase of the synchronization signal is modulated in correlation with the phase nose, which in turn the phase noise can be cancelled.SELECTED DRAWING: Figure 11

Description

  The present invention relates to an optical measurement device and an optical measurement method.

  Conventionally, a pump-probe method is known as an optical measurement method for applying a stimulus to a sample and measuring the response to the stimulus with light. As an example of measuring the molecular vibration of a sample by using this pump-probe method, an optical measurement method by a stimulated Raman scattering method is also known. In the stimulated Raman scattering method, each pulsed light that is used as pump light and probe light is used, and these pump light and probe light are overlapped in time and space. When the energy difference between the photons of the pump light and the probe light matches the molecular vibration energy, the intensity of the probe light changes. Therefore, the molecular vibration spectrum can be acquired by sweeping the photon energy of the pump light or the probe light. In addition, imaging by molecular vibration is possible by matching the photon energy difference between the pump light and the probe light with the energy of molecular vibration and spatially sweeping the light. In addition, an optical measurement method using a white pulse light source as probe light to measure a plurality of molecular vibrations of a sample is also known.

  However, when measuring multiple molecular vibrations of a sample using a white pulse light source, the intensity fluctuation (intensity noise) of white light is large, and a long integration time may be required to obtain a sufficient signal-to-noise ratio. . As a method to prevent signal-to-noise ratio degradation due to intensity noise of the light source, the reference light of the light source is prepared, and the intensity noise detected by subtracting or dividing the reference light detection signal from the probe light detection signal is removed. There is a way. However, in the case of white pulse light, this intensity noise may differ for each wavelength. In this case, unless the detection wavelength and wavelength width of the reference light are exactly the same as the probe light, the intensity noise cannot be removed efficiently. Therefore, when white light is detected by spectroscopy, it is necessary to prepare a spectroscope having the same characteristics for the probe light and the reference light, which is complicated. In order to solve this problem, prepare a reference light that is delayed in the incident time with respect to the probe light incident on the light detecting element, and share the probe light and the reference light separated by a common spectrometer. An optical measurement device that suppresses signal-to-noise ratio degradation caused by intensity noise of white light that differs from wavelength to wavelength is detected by detecting the intensity change of the probe light due to the sample measurement as a phase change of the detection signal. (See Patent Document 1).

JP2015-87385A

  However, when the intensity of light incident on the optical measurement device varies with each irradiation, a phase shift of the detected signal may be caused. For example, if the photodetection element has a light intensity dependency in its time response, or the electronic circuit element of the detection unit has a distortion characteristic, the phase may vary depending on the signal intensity. When phase variation occurs according to the signal intensity in this way, the intensity noise is converted into phase noise due to the phase variation characteristic, and the signal-to-noise ratio is reduced. Therefore, there is room for improvement in improving the signal-to-noise ratio.

  The present invention has been made in consideration of the above facts, and has a higher signal-to-noise ratio than the case of detecting the change in the intensity of the probe light as a result of the response to the stimulus applied to the sample as the phase change of the detection signal. It is an object of the present invention to provide an optical measurement apparatus and an optical measurement method that can be measured by the above method.

The optical measurement device of the first aspect is
A delay unit that delays the output light output from the light source for a predetermined time and outputs it as reference light;
A light receiving unit that irradiates the sample with the output light, the reference light, and the stimulation light, receives the measurement light from the sample, and outputs a light detection signal according to the intensity of the received light;
A removing unit that removes a noise component included in the photodetection signal by multiplying the compensation sync signal determined based on the photodetection signal and the sync signal synchronized with the light source, and the photodetection signal;
It has.

  The delay unit can output the output light as reference light by delaying the output light by π / 2 as the predetermined time.

  As the compensation synchronization signal, a signal obtained by adding the photodetection signal obtained by adding a predetermined weight to the synchronization signal can be used.

  As the compensation synchronization signal, a signal in which the phase of the synchronization signal is adjusted based on the instantaneous amplitude of the light detection signal can be used.

  As the compensation synchronization signal, a signal in which the phase of the synchronization signal is adjusted based on the power of the instantaneous amplitude of the light detection signal can be used.

  The predetermined weight can be determined based on an instantaneous amplitude of the light detection signal.

  The predetermined weight can be determined based on a power of an instantaneous amplitude of the light detection signal.

  The compensation synchronization signal can be determined so that the light detection signal and the instantaneous phase are orthogonal.

The removing unit is
A phase adjustment unit that adjusts the phase of the synchronization signal to obtain the compensation synchronization signal;
A multiplier for multiplying the compensation synchronization signal and the light detection signal;
A feedback unit that feeds back the output of the multiplication unit to the removal unit;
Can be included.

  The feedback unit further includes a filter that extracts a low frequency component having a frequency equal to or lower than a predetermined frequency from the output of the multiplication unit, and the low frequency component adjusts a weight when the photodetection signal is added to the synchronization signal or It can be configured to be fed back to the phase adjustment unit.

The light source is a white light source;
A spectroscope disposed in front of the light receiving unit;
Further comprising
The light receiving unit and the removing unit may be configured to be arranged for each wavelength separated by the spectroscope.

The optical measurement apparatus of the second aspect is
A delay unit that delays the output light output from the light source according to the synchronization signal for a predetermined time and outputs it as reference light;
A light receiving unit that irradiates the sample with the output light, the reference light, and the stimulation light, receives the measurement light from the sample, and outputs a light detection signal according to the intensity of the received light;
A suppression unit that multiplies the light detection signal and the synchronization signal to suppress an intensity noise component included in the light detection signal;
A subtracting unit for subtracting a light intensity signal related to phase noise determined based on the suppressed signal and the light detection signal from the signal in which the intensity noise component is suppressed by the suppressing unit;
It has.

  The delay unit can output the output light as reference light by delaying the output light by π / 2 as the predetermined time.

  The subtractor may determine a light intensity signal related to phase noise based on the instantaneous amplitude of the light detection signal.

  The subtracting unit may determine a light intensity signal related to phase noise based on a power of an instantaneous amplitude of the light detection signal.

The optical measurement method of the third aspect is
The output light output from the light source according to the synchronization signal is delayed for a predetermined time and output as reference light,
Irradiating the sample with the output light, the reference light, and the stimulation light, and receiving the measurement light from the sample, and outputting a light detection signal according to the intensity of the received light,
A compensation synchronization signal determined based on the light detection signal and the synchronization signal is multiplied by the light detection signal to remove a noise component contained in the light detection signal.

The optical measurement method of the fourth aspect is
The output light output from the light source according to the synchronization signal is delayed for a predetermined time and output as reference light,
Irradiating the sample with the output light, the reference light, and the stimulation light, and receiving the measurement light from the sample, and outputting a light detection signal according to the intensity of the received light,
Multiplying the photodetection signal and the synchronization signal to suppress an intensity noise component included in the photodetection signal,
A light intensity signal related to phase noise determined based on the suppressed signal and the light detection signal is subtracted from the suppressed signal.

  According to the present invention, it is possible to measure with a high signal-to-noise ratio compared to the case where the intensity change of the probe light resulting from the response to the stimulus applied to the sample is detected as the phase change of the detection signal. Play.

It is a block diagram which shows an example of a structure of the typical optical measuring apparatus which concerns on 1st Embodiment. It is a block diagram which shows an example of a structure of the optical measuring device comprised using the white pulsed light which concerns on 1st Embodiment. It is a block diagram which shows an example of a structure of the optical measuring device comprised using the balance detection method which concerns on 1st Embodiment. It is a block diagram showing an example of composition of an optical measuring device which optically delays reference light to probe light concerning a 1st embodiment. It is a block diagram which shows the modification of a structure of the optical measuring device containing the light source which outputs the intensity | strength-modulated light based on 1st Embodiment. It is a block diagram which shows the other modification of a structure of the optical measuring device containing the light source which outputs the intensity | strength-modulated light based on 1st Embodiment. It is a block diagram which shows the modification of the structure regarding the irradiation of the light to a sample in the optical measuring device which concerns on 1st Embodiment. It is a block diagram which shows an example of a structure of the detection part applicable to the optical measuring device which concerns on 1st Embodiment. It is an image figure which shows an example of the relationship between the probe light and reference light in the complex plane which concerns on 1st Embodiment. It is an image figure which shows the characteristic regarding the phase delay of the photon detection element and electric circuit element which concern on 1st Embodiment. It is a block diagram which shows an example of a structure of the detection part which can improve the signal noise ratio which concerns on 1st Embodiment. It is an image figure which shows an example of the relationship between the optical detection signal in the complex plane which concerns on 1st Embodiment, and the synchronizing signal of probe light and reference light. It is a block diagram which shows an example of a structure of the detection part applicable to the optical measuring device which concerns on 2nd Embodiment. It is a block diagram which shows an example of a structure of the detection part applicable to the optical measuring device which concerns on 3rd Embodiment. It is a block diagram which shows an example of a structure of the detection part applicable to the optical measuring device which concerns on 4th Embodiment. It is a block diagram which shows an example of a structure of the detection part applicable to the optical measuring device which concerns on the 1st example of 5th Embodiment. It is a block diagram which shows an example of a structure of the detection part applicable to the optical measuring device which concerns on the 2nd example of 5th Embodiment. It is a block diagram which shows an example of a structure of the detection part applicable to the optical measuring device which concerns on the 3rd example of 5th Embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In the following description, components and processes having the same functions and functions may be given the same reference numerals throughout the drawings, and overlapping descriptions may be omitted as appropriate.

[First Embodiment]
First, prior to the description of the first embodiment, a prerequisite technique of the optical measurement device will be described.
FIG. 1 shows an example of the configuration of the optical measurement apparatus 1 under typical measurement conditions in which the intensity change (intensity modulation) of the probe light to be measured due to the sample measurement response is as small as about 10 ⁻³ to 10 ^−5. . This optical measuring device 1 is configured to measure the intensity of probe light using a lock-in detection method.

  Specifically, the optical measurement apparatus 1 includes a first light source 2 that emits probe light Pr, a second light source 3 that emits pump light Pp that is a stimulation source applied to the sample SP, and a photodetector as a light receiving unit. 4, a detection unit 5, a reference signal source 8 </ b> A, and an intensity modulator 8 </ b> B. The probe light Pr is incident on the photodetector 4 through the sample SP. The pump light Pp is reflected by the mirror 6, passes through the optical axis of the probe light Pr by the beam splitter 7, is irradiated onto the sample SP, and stimulates the sample SP. The photodetector 4 is connected to a detection unit 5, and the detection unit 5 is configured by a lock-in amplifier. A reference signal is input to the detection unit 5 from the reference signal source 8A. The intensity modulator 8B is provided on the emission side of the second light source 3 in order to modulate the intensity of the pump light Pp. The reference signal is input from the reference signal source 8A to the intensity modulator 8B, and intensity modulation of the reference signal is applied to the pump light Pp.

  In the first embodiment, an optical measurement apparatus that detects the change in the intensity of the probe light by using the stimulated Raman scattering method and obtains a molecular vibration spectrum and the like will be described. The light from the SP may be transmitted light or reflected light. In this case, transmitted light or reflected light may be used corresponding to the amount of light from the sample SP. In the following description, a case where measurement is performed using transmitted light from the sample SP will be described as an example.

  Further, there are the following changes in the intensity of the probe light Pr. The first is an intensity change corresponding to the response of the sample to be detected, that is, an intensity change of the probe light due to the sample measurement response. The second is a change in the intensity of the probe light when the first light source 2 is intensity-modulated to define the phase. For example, in pulsed light, it is a carrier having a phase of a pulse repetition period. The third is a change in intensity of the probe light due to intensity noise of the first light source 2 that cannot be controlled. The first embodiment reduces the intensity change of the probe light due to the intensity noise of the uncontrollable first light source 2 described in the third. Therefore, in the following description, the intensity change refers to the intensity change of the probe light due to the intensity noise of the uncontrollable first light source 2 described in the third. Moreover, when distinguishing and explaining these differences, each intensity change is distinguished and described.

The optical measurement device 1 configured using the lock-in detection method shown in FIG. 1 enables high-speed imaging. For example, if the intensity modulation of the probe light Pr is directly observed under a minute measurement condition of about 10 ⁻³ to 10 ⁻⁵ , the intensity modulation of the probe light is buried in the intensity noise of the light source, and a long integration time is required. It is difficult to apply to imaging that requires high-speed measurement. Therefore, by configuring the optical measuring device 1 using the lock-in detection method as shown in FIG. 1, it is possible to modulate the intensity of the pump light Pp and measure the intensity of the probe light Pr only around the modulation frequency. It becomes possible. That is, the frequency of the signal to be measured is the intensity modulation frequency of the pump light Pp, and the light intensity noise includes other irrelevant components. Therefore, by observing only the vicinity of the intensity modulation frequency of the pump light Pp, The noise ratio can be improved.

In the optical measurement apparatus 1 shown in FIG. 1, since the wavelength of the pump light Pp or the probe light Pr is constant, only single-molecule vibration can be observed, and it is difficult to obtain a molecular vibration spectrum. Therefore, it is possible to obtain a molecular vibration spectrum by using white pulse light as the probe light Pr, spectrally dividing the light from the sample SP, and detecting each spectral component that has been spectrally separated.
FIG. 2 shows an example of an optical measurement apparatus 1A that uses white pulsed light as the probe light Pr. As shown in FIG. 2, the optical measurement apparatus 1A includes a spectroscope 4S that splits a molecular vibration spectrum into n wavelength components, and detects photodetectors 4-1, ˜4-n (each detecting a molecular vibration spectrum). n is a natural number). In addition, detectors 5-1 to 5 -n are provided corresponding to the photodetectors 4-1 to 4 -n, respectively.

However, the white pulse light generally has a large intensity noise at every frequency, and a long integration time is required even when the lock-in detection method is used. A balance detection method is known as an example of a method for suppressing the influence of such light intensity noise.
FIG. 3 shows an example of an optical measurement apparatus 1B configured using a balance detection method. As shown in FIG. 3, the optical measurement apparatus 1B includes a beam splitter 9A that divides the probe light Pr into light that irradiates the sample SP and light that is used as the reference light Rf. The optical measurement device 1B includes a photodetector 4A that detects light that has passed through the sample SP, and a photodetector 4B that detects the reference light Rf. The reference light Rf divided by the beam splitter 9A is guided to the photodetector 4B by the mirror 9B. Further, the optical measurement device 1B includes a subtracter 4C that calculates a difference between output signals of the photodetector 4A and the photodetector 4B.

  In the optical measuring device 1B, the probe light Pr is divided and one light is used for measuring the sample SP, and the other light is used as the reference light Rf. The detection signal of the probe light Pr after measurement of the sample SP is a signal in which the intensity modulation component by the sample SP and the intensity noise component are combined. On the other hand, since the detection signal of the reference light Rf is not affected by the sample SP, only the intensity noise component is present. Therefore, by subtracting the detection signal of the reference light Rf from the detection signal of the probe light Pr, only the intensity noise component can be canceled without reducing the intensity modulation component by the sample SP.

  The optical measurement apparatus 1B shown in FIG. 3 is required to meet the following two conditions in order to function optimally. The first condition is that the intensity fluctuations of the probe light Pr and the reference light Rf are equal (correlate temporally). The second condition is a condition that the intensity ratio of the probe light Pr and the reference light Rf is 1. It has been difficult for the following three reasons to construct the optical measurement device 1B so that the intensity noise of the white pulse light can be canceled while adapting to such conditions.

The first reason is that the intensity fluctuation of white light is different for each wavelength. For the first reason, the detection center wavelength and the detection wavelength width of the probe light Pr and the reference light Rf must be the same in order to satisfy the first condition when observing the probe light Pr and the reference light Rf by spectroscopy. Required. Further, in order to satisfy the first condition, it is required to make the characteristics of the two spectrometers prepared by the probe light Pr and the reference light Rf the same, and the configuration is complicated and technically difficult. Therefore, there is room for improvement in order to cancel the intensity noise as much as possible.
The second reason is that when the characteristics of the optical element through which the probe light Pr and the reference light Rf pass have wavelength dependence, the intensity ratio of the probe Pr and the reference light Rf is different for each wavelength. For the second reason, for example, when the split ratio of the probe light Pr and the reference light Rf by the beam splitter 9A differs for each wavelength, it is difficult to satisfy the second condition at all wavelengths. As a result, even if the intensity noise can be canceled as much as possible at the specific wavelength, the cancellation of the intensity noise may be insufficient at other wavelengths. This is a factor that limits application to spectroscopic measurement.
The third reason is that the intensity ratio between the probe light Pr and the reference light Rf is affected by the sample SP. For the third reason, even if the second condition is solved at a certain moment, if the state at the time of measurement of the sample SP changes, the transmittance of the probe light Pr that has passed through the sample SP with respect to the reference light Rf changes, and the intensity ratio becomes Will fluctuate. Therefore, when analyzing the temporal change of the sample SP and in imaging where the measurement position of the sample SP changes from moment to moment, the intensity noise is removed to the maximum only at a specific time and measurement position, but at other times and other times. Intensity noise is not sufficiently removed at the measurement position.

An optical measurement device that optically delays either the probe light Pr or the reference light Rf in order to suppress the influence of the intensity noise of the light while solving the problems due to these three reasons will be described.
FIG. 4 shows an example of an optical measuring device 1C that optically delays the reference light Rf with respect to the probe light Pr.

  As shown in FIG. 4, the optical measurement apparatus 1C includes a beam splitter 9A that divides the probe light Pr into light used for measuring the sample SP and light used as the reference light Rf. Further, in the optical measuring device 1C, mirrors 9B and 9C are provided in order on the side of the beam splitter 9A where the reference light Rf divided into the reference light Rf is superimposed in order to superimpose the reference light Rf on the probe light Pr. It has been. A beam splitter 9D is provided on the reflection side of the mirror 9C and on the optical axis of the probe light Pr. By approaching or separating the mirrors 9B and 9C with respect to or away from the optical axis of the probe light Pr, the reference light Rf is generated with respect to the probe light Pr by an optical delay due to a change in the optical path length of the reference light Rf with respect to the probe light Pr. The time of incidence on the photodetector 4 can be delayed.

  In the first embodiment, a case where the reference light Rf is delayed by π / 2 with respect to the probe light Pr will be described as an example. Note that the delay of the reference light Rf with respect to the probe light Pr is not limited to π / 2. For example, it may be an integral multiple of the period of the synchronization signal (π / 2 + mπ) or a part of the period (π / m). That is, the reference light Rf only needs to have a phase different from that of the probe light Pr, and preferably the probe light Pr and the reference light Rf may be delayed in a distinguishable manner. Therefore, it is more preferable to delay the reference light Rf by π / 2 with respect to the probe light Pr.

  The optical measurement device 1C includes a detection unit 5A and a lock-in amplifier 5B as detection units. The photodetector 4 is connected to the detection unit 5A, and the detection unit 5A is connected to the lock-in amplifier 5B. Here, the intensity of the probe light Pr is modulated, and a synchronization signal synchronized with the periodic intensity modulation of the probe light source Pr can be acquired. As shown in FIG. 4, in the optical measuring device 1C according to the first embodiment, the light output from the first light source 2, that is, the probe light Pr is intensity-modulated in accordance with the synchronization signal from the synchronization signal source 5C. Will be explained. The probe light Pr is intensity-modulated by the first light source 2 by the synchronization signal from the synchronization signal source 5C, and the synchronization signal is input to the detection unit 5A. Therefore, the synchronization signal is input to the detection unit 5A from the synchronization signal source 5C. The lock-in amplifier 5B receives a reference signal from the reference signal source 8A for modulating the intensity of the pump light Pp.

  In the first embodiment, a case will be described in which the intensity-modulated probe light Pr output from the first light source 2 to which a synchronization signal is given is used. However, the probe light Pr gives the synchronization signal to the first light source 2 and has an intensity. It is not limited to modulating. For example, a light source that outputs intensity-modulated light such as a passive mode-locked pulse laser may be used. In this case, for example, as shown in FIG. 5, a synchronization signal detector 5D that detects a synchronization signal of the intensity-modulated probe light Pr output from the first light source 2 such as a passive mode-locked pulse laser is provided. What is necessary is just to comprise so that the synchronizing signal detected by signal detector 5D may be input into the detection part 5A. In addition, as shown in FIG. 6, the synchronization signal may be detected from the signal from the photodetector 4 by providing a synchronization signal detector 5E in the detection unit 5A.

  In the first embodiment, an optical measurement device that irradiates the sample SP with the probe light Pr and the reference light Rf will be described. However, the present invention is not limited to irradiating the sample SP with the probe light Pr and the reference light Rf. . For example, as shown in FIG. 7, the sample SP may be irradiated with only the probe light Pr, and the reference light Rf may be superimposed on the downstream side of the sample SP. The probe light Pr or the probe light Pr and the reference light Rf selected as the light to be irradiated on the sample SP is the intensity ratio of the measurement light that is the probe light Pr and the reference light Rf obtained from the sample SP, that is, the degree of change in transmittance. It can be set according to the size. Specifically, the sample SP may be irradiated with the probe light Pr and the reference light Rf when the intensity ratio of the measurement light (degree of change in transmittance) exceeds a threshold value determined in advance through experiments or the like. The sample SP may be irradiated with only the probe light Pr when it is equal to or less than a predetermined threshold value.

FIG. 8 shows an example of the configuration of the detection unit 5A shown in FIG.
As shown in FIG. 8, the detection unit 5A includes an amplifier 11, a multiplier 12, a synchronization signal unit 13, a low-pass filter 14, an integrator 15, a phase controller 16, a low-pass filter 17, and an output unit 18. In the detection unit 5A of the optical measurement apparatus 1C, the detection signal of the photodetector 4 is amplified by the amplifier 11 and output to one input side of the multiplier 12. A synchronization signal output from the synchronization signal unit 13 is input to the other input side of the multiplier 12 via the phase controller 16. The synchronization signal unit 13 extracts and outputs the synchronization signal from the synchronization signal source 5C. The control side of the phase controller 16 is connected to the output side of the multiplier 12 in order from the output side of the multiplier 12 via a low-pass filter 14 and an integrator 15. The output unit 18 outputs the signal from the low-pass filter 17 to the lock-in amplifier 5B.

The optical measuring device 1C shown in FIGS. 4 and 8 has the following functions.
The first function is a function of delaying the time at which the reference light Rf is incident on the photodetector 4 with respect to the probe light Pr by using an optical delay (change of the optical path length).
The second function is a function of detecting the probe light Pr and the reference light Rf with the same photodetector 4. Since the time at which the probe light Pr and the reference light Rf enter the photodetector 4 is different, the output is distinguished from the probe light and the reference light Rf depending on the phase to be detected. By using this same photodetector 4, the same spectrometer can be used for the probe light Pr and the reference light Rf, and the detection center wavelength and the detection wavelength width can be maintained in common. Thereby, the subject by the said 1st reason can be eliminated.
The third function is a function that takes a difference while adjusting the weight of the detection signal derived from the probe light Pr and the reference light Rf by adjusting the phase to be detected. Thereby, even if the intensity of the probe light Pr and the reference light Rf is optically different, it can be compensated at the time of detection. Thereby, the subject by the said 2nd reason can be eliminated.
The fourth function is a function that feedback-controls the weight at an appropriate response speed so that the fluctuation of the light intensity, that is, the difference becomes zero. Thereby, even if the state of the sample SP changes during measurement and the intensity ratio between the probe light Pr and the reference light Rf changes, the weight is automatically optimized, and the problem due to the third reason can be solved. .

  Specifically, in FIG. 8, the pulsed light emitted from the first light source 2 with a predetermined period (for example, a repetition frequency of 76.3 MHz) is divided, with one light being the probe light Pr and the other light being the reference light Rf. Each reaches the photodetector 4 through the sample SP. The reference light Rf is optically delayed by increasing the optical path length. For example, the reference light Rf is spatially overlapped with the probe light Pr again after being delayed by ¼ of the pulse repetition period. On the other hand, the pump light Pp is spatially overlapped with the probe light Pr and the reference light Rf. However, it is superimposed only on the probe light Pr in terms of time. When these three lights are incident on the sample SP, stimulation by the pump light Pp is detected only by the probe light Pr, and intensity modulation m (t) is given. The intensity modulation m is a function of the intensity modulation of the given pump light Pp and the time due to the temporal change of the sample SP. Therefore, the fundamental wave component of the probe light Pr can be expressed as a × A (t) × (1 + m (t)) × cos (ωt). Further, the reference light Rf can be expressed as b × A (t) × sin (ωt). However, a and b are constants proportional to the intensity ratio, which are optically determined by an optical element such as a beam splitter. A (t) is an instantaneous amplitude including intensity noise that fluctuates every moment. ω is the pulse repetition angular frequency. Therefore, the detection signal in which the probe light Pr and the reference light Rf are detected by the same photodetector 4 can be expressed by the following equation (1).

A detection signal from the photodetector 4 is input to the multiplier 12. On the other hand, a synchronization signal with the pulse light source is prepared. This phase is adjusted by the phase controller 16. Here, the synchronization signal is expressed by the following equation (2).

The result of multiplying the synchronization signal and the detection signal by the multiplier 12 and removing the high frequency component by the low-pass filter 17 is proportional to the relationship expressed by the following equation (3). The proportional signal is output from the output unit 18.

Here, the phase φ is adjusted so as to satisfy the following expression (4).

  When adjusted in this way, the additive noise component A (t) × {a × cos (φ) −b × sin (φ)} of the first term in the expression (3) as an output is canceled and should be detected. Only the signal a × A (t) × m (t) × cos (φ) is output. In this way, the signal to noise ratio is improved. Here, even if a and b are not equal due to optical factors, they can be compensated by adjusting the phase φ so as to satisfy the equation (4). Further, when the expression (4) is not satisfied, a direct current component included in A (t) × {a × cos (φ) −b × sin (φ)} appears in the output, and becomes 0 as an error signal. As described above, φ can be feedback-controlled. Thereby, even if the coefficients a and b dynamically change, the condition of the expression (4) is automatically satisfied. At this time, if the response frequency of feedback control is made smaller than the intensity modulation frequency of the pump light Pp by sufficiently reducing the cutoff frequency of the low-pass filter 14, A (t) × m (t) × cos (φ) is canceled out. It is possible to detect without being.

  FIG. 9 shows an example of the relationship between the probe light Pr and the reference light Rf in the complex plane. In FIG. 9, the component of the detection signal of the probe light Pr is indicated by a component with the arrow X direction as an axis, and the component of the detection signal of the reference light Rf is indicated by a component with the arrow Y direction as an axis. FIG. 9A shows an example when there is no signal from the sample SP, and FIG. 9B shows an example when the signal from the sample SP is present.

  Therefore, the detection signal represented by the expression (3) is in the direction in which the signal of the probe light Pr and the signal of the reference light Rf are combined (in the example of FIG. 9, the direction in which the X direction and the Y direction are combined). . On the other hand, the synchronization signal is in the direction orthogonal to the paper surface and the detection signal (in the example of FIG. 9, the direction obtained by combining the X direction and the reverse direction of the Y direction, θ + φ = π / 2). The extraction of the low frequency component by multiplying the detection signal and the synchronization signal is to take the inner product of the detection signal and the synchronization signal, which is equivalent to projecting the detection signal to the synchronization signal. The frequency of the low frequency component corresponds to a frequency equal to or lower than a predetermined frequency.

  Here, a case where there is no signal from the sample SP is considered (see FIG. 9A). At this time, although the magnitude of the detection signal between the probe light Pr and the reference light Rf changes due to the intensity noise of the light source, the intensity ratio of these signals is constant, so the declination angle θ of the superimposed detection signals is It is constant. Therefore, the component projected on the synchronization signal remains zero. This means that no intensity noise is detected.

  On the other hand, when a signal from the sample SP appears (see FIG. 9B), only the intensity of the probe light Pr is modulated by m (t), while the intensity of the reference beam Rf is not modulated. Therefore, the deflection angle of the superimposed detection signals changes by δθ. A component projected onto the synchronization signal appears by this change. That is, while the intensity noise of the light source is not output as a projection component, the intensity modulation by the sample SP is output, so that the signal to noise ratio is improved.

  Here, if the feedback frequency of the phase of the synchronization signal is sufficiently larger than the intensity modulation frequency of the pump light Pp, the superimposed light detection signal and the synchronization signal are always in a state of being orthogonal. In this case, A (t) × m (t) × cos (φ) appearing after the low-pass filter 17 is also canceled and is not output. However, the phase of the superimposed light detection signal is modulated by intensity modulation by the sample SP, and the phase φ is feedback-controlled so as to be orthogonal to this, so that information on the phase modulation is included in this control signal. On the other hand, intensity noise does not modulate the phase. That is, by observing the feedback control signal, the signal-to-noise ratio is improved, and it is possible to detect intensity modulation by the sample SP. In addition, in this case, it is possible to cancel A (t) included in A (t) × m (t) × cos (φ) that contributes to multiplication.

  However, in the optical measuring device 1C shown in FIGS. 4 and 8, it is assumed that the time relationship between the incident pulse light and the detected signal is constant. That is, it is assumed that even if the light intensity varies from pulse to pulse, no phase shift of the detected signal is caused. However, the time response of a photodetection element such as an actual photodiode is dependent on light intensity. In addition, the coupling capacitor to the signal line connecting between the amplifier and the electric circuit element, and other electric circuit elements have distortion characteristics, and the phase may vary depending on the signal strength due to the distortion characteristics.

FIG. 10 shows characteristics related to the phase delay of the light detection element and the electric circuit element. FIG. 10A shows an example of the relationship between the light intensity and the phase. FIG. 10B shows an example of the relationship between the signal of the electronic circuit and the phase.
In the example shown in FIG. 10A, the relationship between the light intensity and the phase when pulse light having a stable repetition frequency of 76.3 MHz is detected by a photodiode is shown. The phase is delayed as the detected light intensity increases. That is, when the intensity fluctuates, the phase fluctuates and the intensity noise is converted into phase noise.

For the sake of simplicity, the case where the signal from the sample SP is not detected, that is, the case where m = 0 will be described. The phase delay is approximated to be proportional to the instantaneous amplitude. Therefore, when the proportionality constant is k and the phase noise derived from intensity noise is −k × A (t), the above equation (1) can be expressed by the following equation (5).

The expression (3) showing the signal of the low frequency part obtained by multiplying the signal by the expression (5) and the synchronizing signal by the expression (2) can be expressed by the following expression (6).

Here, when the phase φ of the synchronization signal is selected as shown by the equation (4), the output can be expressed by the following equation (7).

  The right side of equation (7) is for k << 1. That is, when the intensity noise is converted into phase noise with the proportionality constant k, the noise represented by the equation (7) is added to the output. This will be explained in terms of a complex plane (see FIG. 9). Since the detection signals of the probe light Pr and the reference light Rf rotate counterclockwise by k × A (t) in proportion to the intensity, the probe light Pr and the reference light The detection signal superimposed with Rf also becomes longer in proportion to A (t) and rotates counterclockwise by k × A (t). By this rotation, the amplitude noise component A (t) is projected onto the synchronization signal vector and output. However, since the output is proportional to both the rotation and the instantaneous amplitude, the output noise is proportional to the square of A (t) as shown in the equation (7). Since A (t) is intensity noise of the light source and fluctuates stochastically, it is difficult to cancel even if the phase is controlled by feedback based on the past output.

  The above describes the case where the phase shift is linearly proportional to the intensity. However, there may be a case where it is non-linearly proportional. For example, FIG. 10B shows a phase shift when an electrical signal having a constant phase and a different amplitude is applied to the multiplier 12. In FIG. 10B, the horizontal axis represents the amplitude of the input signal, and the vertical axis represents the phase shift. As shown in FIG. 10B, it can be understood that although the phase shift is almost linear up to an amplitude of 0.2 V, the nonlinearity is remarkable from 0.2 V or more.

-Generation of compensated synchronization signal In consideration of the above facts, the present inventor paid attention to the fact that the phase of the light detection signal is modulated according to the light detection signal intensity, and added the light detection signal to the synchronization signal. By obtaining the compensation synchronization signal, the optical measurement apparatus that can suppress the intensity noise can be reached.

  FIG. 11 shows a configuration of a detection unit 20 that can further improve the signal-to-noise ratio by further suppressing the intensity noise by the compensation synchronization signal as an example of the detection unit applicable to the optical measurement apparatus. The detection unit 20 shown in FIG. 11 is an example of adding a weight c to the synchronization signal and adding it to the light detection signal in the detection unit 5A (see also FIG. 8) in the optical measurement apparatus 1C shown in FIG.

  Specifically, an adder 201 is provided between the output side of the phase controller 16 and the other input side of the multiplier 12, and is output from the synchronization signal unit 13 to one input side of the adder 201. The synchronization signal adjusted by the phase controller 16 is input later. An input side of a weight adder 200 that outputs an output signal obtained by adding a weight c to the input signal is connected between the output side of the amplifier 11 and one input side of the multiplier 12. The output side of the weight adder 200 is connected to the other input side of the adder 201. The output side of the adder 201 is connected to the other input side of the multiplier 12.

  By configuring in this way, instead of the synchronization signal adjusted by the phase controller 16, the compensated synchronization signal generated by adding the light detection signal to which the weight c is added is used. Even when the phase noise is caused by intensity noise, the phase of the synchronization signal is modulated in correlation with the phase noise, so that the phase noise can be canceled.

FIG. 12 shows an example of the relationship between the light detection signal in the complex plane and the synchronization signal of the probe light Pr and the reference light Rf in the detection unit 20 shown in FIG.
In FIG. 12, the light detection signals are only shown by superimposing the signals derived from the probe light and the reference light. It is assumed that this signal is phase-shifted in a direction delayed by k × A (t) according to the instantaneous amplitude A (t). The synchronization signal vector is substantially orthogonal to this. As described above, the output signal is obtained by projecting the photodetection signal onto the synchronization signal vector. In the following description, a photodetection signal is added to the synchronization signal vector with an appropriate weight c, and this is referred to as a compensation synchronization signal vector. By appropriately selecting the weight c, the compensation synchronization signal can always be orthogonal to the phase-modulated photodetection signal. Therefore, even if the phase noise of the light detection signal is caused by the intensity noise, the phase noise can be canceled by modulating the phase of the synchronization signal in correlation with the phase noise.

That is, the superimposed light detection signal can be expressed by the following equation (8) by appropriately selecting the initial phase.

Here, m ′ (t) is a phase modulation caused by the sample and is a signal to be observed.

This phase modulation is determined by the ratio of the instantaneous amplitude of the probe light including the intensity modulation by the sample measurement and the instantaneous amplitude of the reference light. The synchronization signal can be expressed by the above-described equation (2). However, since the initial phase of the superimposed photodetection signal is zero, Dcos (ωt) is obtained. Therefore, the compensation synchronization signal can be expressed by the following equation (9) by adding the photodetection signal of equation (8) with the weight c to the synchronization signal.

In this case, the output signal can be expressed by the following equation (10). Equation (10) is the product of Equation (8) and Equation (9), and the low frequency component is extracted (that is, equivalent to the inner product).

However, | m ′ (t) |, | kA (t) | << 1. The first term is a signal to be detected, and the second term is noise due to phase noise caused by intensity noise (amplitude noise).

  It can be seen that this phase noise can be canceled by selecting c = k × D. On the other hand, adding the light detection signal does not reduce the gain of the signal (first term) derived from the measurement of the sample SP. In this way, the signal to noise ratio can be further improved.

[Second Embodiment]
Next, a second embodiment will be described. Although 1st Embodiment demonstrated the case where a phase shift was linear with respect to an intensity | strength, the technique of an indication is not limited to when a phase shift is linear with respect to an intensity | strength. In the second embodiment, the disclosed technique is applied when the phase shift is nonlinear with respect to the intensity. In addition, since 2nd Embodiment is the structure similar to 1st Embodiment, it attaches | subjects the same code | symbol to the same part, and abbreviate | omits detailed description.

  FIG. 13 shows an example of the configuration of the detection unit 21 applicable to the optical measurement device according to the second embodiment. The detection unit 21 illustrated in FIG. 13 is an example in which the detection unit 20 illustrated in FIG. 11 performs control of intensity modulation on the weight adder 200 to generate a compensation synchronization signal.

  Specifically, the output side of the low-pass filter 17 and the input side of the output unit 18 are connected between the output side of the amplifier 11 and one input side of the multiplier 12 via the detector 210 and the intensity modulator 211. Connected between. Note that the detector 210 is denoted as DET in FIG. The detector 210 receives the output signal of the amplifier 11 and outputs the instantaneous amplitude signal of the amplifier 11, that is, the instantaneous amplitude A (t) of the light detection signal of the photodetector 4. As an example of the detector 210, an envelope detector and a synchronous detector can be used. The intensity modulator 211 uses at least the instantaneous amplitude A (t) of the light detection signal output from the amplifier 11 to generate an intensity modulation signal having a correlation with the nonlinear phase shift on the control side of the weight adder 200. Configured to output. That is, in the second embodiment, the weight c in the weight adder 20 is determined using the instantaneous amplitude A (t).

  The intensity modulator 211 can also add the output signal of the low-pass filter 17 to the input (connection relationship indicated by a dotted line in FIG. 13) and determine the weight c using the output signal of the low-pass filter 17 as well. In the second embodiment, a case will be described in which the weight c is determined without using the output signal of the low-pass filter 17 (that is, without the connection relationship indicated by the dotted line in FIG. 13). Details regarding the determination of the weight c using the output signal of the low-pass filter 17 will be described later.

  With this configuration, even when a phase shift is caused nonlinearly, the magnitude of the weight c is controlled by the intensity modulation signal having a correlation with the nonlinear phase shift, and amplitude noise The phase shift due to (phase noise) can be canceled.

Specifically, in the first embodiment, the case where k is a constant and the phase shift is linear with respect to the intensity has been described. On the other hand, for example, as shown in FIG. 10B, when the constant k of the multiplier 12 has non-linearity that depends on the instantaneous amplitude A (t) of the photodetection signal, the phase is expressed by the following equation (11), for example. The shift is expressed by a function f.

When the phase shift is caused nonlinearly in this way, the weight c generated by the weight adder 200 is intensity-modulated based on the result of the function f (A (t)) / A (t), and the adder 201 By adding, it is possible to generate a compensation synchronization signal when the phase shift is caused nonlinearly. That is, when the phase shift is caused nonlinearly, the equation (8) indicating the photodetection signal can be expressed by the following equation (12).

At this time, the compensation synchronization signal can be expressed by the following equation (13).

Here, c (A (t)) is determined as follows. By taking the product of Equation (12) and Equation (13) and extracting the low frequency component, it can be expressed by the following Equation (14).

Therefore, as shown by the following equation (15), the phase shift (phase noise) due to the amplitude noise can be canceled by selecting the weight c.

Usually, this function f can be described by a power function and can be expressed by the following equation (16).

In this case, the weight c can be obtained by the following equation (17).

  As shown in the equation (17), the weight c can be obtained from a fixed constant Dki and an instantaneous amplitude A (t). That is, the detector 210 obtains the instantaneous amplitude A (t), and the intensity modulator 211 obtains the weight c from the instantaneous amplitude A (t) and the constant Dki. Therefore, the phase shift (phase noise) due to the amplitude noise can be canceled by using the signal in which the photodetection signal is controlled by the weight c determined by multiplying the power of the instantaneous amplitude A (t) by the constant Dki.

  As described above, in the second embodiment, even when the phase shift is caused nonlinearly, the magnitude of the weight c is controlled by the intensity modulation signal having a correlation with the nonlinear phase shift. A phase shift (phase noise) due to amplitude noise can be canceled.

[Third Embodiment]
Next, a third embodiment will be described. In the first embodiment and the second embodiment, a case has been described in which a compensation synchronization signal is obtained by adding a photodetection signal with an appropriate weight to the synchronization signal. In the third embodiment, the disclosed technique is applied when a phase of a synchronization signal is directly modulated to obtain a compensation synchronization signal. That is, in the third embodiment, a compensated synchronization signal is obtained by correlating the phase of the synchronization signal with the instantaneous amplitude. In addition, since 3rd Embodiment is the structure similar to 1st Embodiment and 2nd Embodiment, the same code | symbol is attached | subjected to the same part and detailed description is abbreviate | omitted.

  FIG. 14 shows an example of the configuration of the detection unit 22 applicable to the optical measurement device according to the third embodiment. 14 detects the instantaneous output signal of the amplifier 11, that is, the light detection signal of the photodetector 4 when the phase controller 16 controls the phase of the synchronization signal in the detection unit 5A shown in FIG. It is an example which produces | generates a compensation synchronizing signal using amplitude A (t).

  Specifically, an adder 221 is provided between the output side of the integrator 15 and the control side of the phase controller 16 so that a signal indicating the addition result is input to the control side of the phase controller 16. Further, the output side of the amplifier 11 and one input side of the multiplier 12 are connected to the adder 221 via the detector 210 and the direct phase modulation controller 220. That is, the direct phase modulation controller 220 outputs a signal for directly modulating the phase to the control side of the phase controller 16 using the instantaneous amplitude A (t) of the light detection signal output from the amplifier 11. Configured to do. Specifically, the output signal of the integrator 15 is input to one input side of the adder 221, and the output of the direct phase modulation controller 220 is input to the other input side of the adder 221. A signal is input. That is, in the third embodiment, the phase of the synchronization signal is correlated with the instantaneous amplitude using the instantaneous amplitude A (t).

  The direct phase modulation controller 220 adds the output signal of the low-pass filter 17 to the input (connection relation indicated by a dotted line in FIG. 14), and also uses the output signal of the low-pass filter 17 to change the phase of the synchronization signal to the instantaneous amplitude. However, in the third embodiment, a case will be described in which the correlation is performed without using the output signal of the low-pass filter 17 (that is, without the connection relationship indicated by the dotted line in FIG. 14). Details regarding correlating the phase of the synchronization signal with the instantaneous amplitude using the output signal of the low-pass filter 17 will be described later.

Here, when the phase of the synchronization signal is directly modulated to obtain a compensation synchronization signal, the compensation synchronization signal by phase modulation Ψ (A (t)) can be expressed by the following (18).

The result of multiplying Expressions (12) and (18) can be expressed by the following Expression (19).

The low frequency component in the equation (19) is the second term, and is the following equation (20).

Here, -f (A (t)), which is the second term of the function sin in the equation (20), indicates phase noise caused by amplitude noise. Therefore, phase noise can be canceled by modulating the phase controller 16 as shown in the following equation (21).

  That is, similarly to the second embodiment, the phase modulation Ψ (A (t)) can be fixedly obtained using the instantaneous amplitude A (t). Therefore, the direct phase modulation controller 220 uses the instantaneous amplitude A (t) obtained by the detector 210 to obtain a signal for directly modulating the phase of the synchronization signal, and correlates the phase of the synchronization signal with the instantaneous amplitude. Thus, the phase shift (phase noise) due to amplitude noise can be canceled.

  As described above, in the third embodiment, by directly modulating the phase of the synchronization signal, the compensation synchronization signal can be obtained and the phase noise can be canceled.

[Fourth Embodiment]
Next, a fourth embodiment will be described. In the first to third embodiments, a case where a photodetection signal is added to the synchronization signal with an appropriate weight to obtain a compensation synchronization signal, and a case where the phase of the synchronization signal is directly modulated to obtain a compensation synchronization signal are described. did. In the fourth embodiment, the disclosed technique is applied when noise is directly subtracted from the output. In addition, since 4th Embodiment is the structure similar to 1st Embodiment to 3rd Embodiment, it attaches | subjects the same code | symbol to the same part, and abbreviate | omits detailed description.

  FIG. 15 shows an example of the configuration of the detection unit 23 applicable to the optical measurement apparatus according to the fourth embodiment. The detection unit 23 shown in FIG. 15 uses the output signal of the amplifier 11, that is, the instantaneous amplitude A (t) of the photodetection signal of the photodetector 4 in the detection unit 5A shown in FIG. Is an example in which noise is canceled by subtracting directly from the output signal.

  Specifically, a subtracter 231 is provided between the output side of the low-pass filter 17 and the input side of the output unit 18. The subtractor 231 has an input side connected to the output side of the low-pass filter 17, a subtraction input side connected to the output side of the subtraction controller 230, and an output side connected to the input side of the output unit 18. The output side of the amplifier 11 and one input side of the multiplier 12 are connected to a subtracter 231 via a detector 210 and a subtraction controller 230. Specifically, the subtracting controller 230 outputs a signal indicating phase noise to the subtracting input side of the subtractor 231 using the instantaneous amplitude A (t) of the light detection signal output from the amplifier 11. Configured to do. That is, in the fourth embodiment, a signal indicating phase noise obtained using the instantaneous amplitude A (t) is directly subtracted from the output signal.

  Here, the subtracting controller 230 adds the output signal of the low-pass filter 17 to the input (connection relation indicated by a dotted line in FIG. 15), and obtains a signal indicating phase noise using the output signal of the low-pass filter 17 as well. However, in the fourth embodiment, a case will be described in which a signal indicating phase noise is obtained without using the output signal of the low-pass filter 17 (that is, without a connection relationship indicated by a dotted line in FIG. 15). Details regarding obtaining a signal indicating phase noise using the output signal of the low-pass filter 17 will be described later.

In the fourth embodiment, a case where a synchronization signal is given by the following equation (22) will be described as an example of a configuration in which phase modulation is not performed on the synchronization signal.

When the synchronization signal is given by the equation (22), the output of the low-pass filter 17 can be derived by multiplying the equations (12) and (22), and can be expressed by the following equation (23).

The second term in the equation (23) corresponds to a signal indicating a low frequency component and can be approximately expressed by the following equation (24).

In this equation (24), the second term on the right side corresponds to a signal indicating phase noise caused by amplitude noise. Therefore, the noise derived from the phase noise can be canceled by subtracting the signal represented by the second term on the right side of the equation (24) shown by the following equation (25) from the output of the low-pass filter 17. . That is, the subtracting controller 230 derives a signal indicating phase noise using the instantaneous amplitude A (t) of the light detection signal output from the amplifier 11 and outputs the signal to the subtracting input side of the subtractor 231. .

  That is, similarly to the second embodiment, the signal according to the equation (25) can be fixedly obtained using the instantaneous amplitude A (t). Therefore, the subtraction controller 230 uses the instantaneous amplitude A (t) obtained by the detector 210 to obtain a signal indicating phase noise obtained using the instantaneous amplitude A (t), and outputs a signal indicating phase noise as an output signal. By directly subtracting from, phase shift (phase noise) due to amplitude noise can be canceled.

  Thus, in the fourth embodiment, noise can be canceled by directly subtracting a signal indicating phase noise caused by amplitude noise from the output signal.

[Fifth Embodiment]
Next, a fifth embodiment will be described.
In the first to fourth embodiments, it is assumed that the characteristic variation due to the individual difference of the elements and the environmental variation is constant. However, for example, an actual element such as the photodetector 4 and an electronic circuit element such as the multiplier 12 have individual differences as characteristics that cause noise, cases depending on environmental fluctuations such as temperature and humidity, and detection wavelengths. May depend on. In the fifth embodiment, the disclosed technique is applied when characteristic variations due to individual differences of elements and environmental variations occur.
In addition, since 5th Embodiment is the structure similar to 1st Embodiment to 4th Embodiment, it attaches | subjects the same code | symbol to the same part, and abbreviate | omits detailed description.
In the fifth embodiment, an example of feedback control in the case where the phase shift is linearly proportional to the amplitude will be described in order to simplify the description.

First, a first example of the fifth embodiment will be described.
In the first example of the fifth embodiment, the weight c in the weight adder 20 is determined using the instantaneous amplitude A (t) in order to cancel the phase shift (phase noise) due to amplitude noise (see FIG. 13). This is an example in which feedback control is also performed using the output signal of the low-pass filter 17.

When the noise included in the output signal fluctuates due to individual differences of elements and environmental fluctuations in the detection unit of the optical measurement device, the weight c is automatically optimized by feedback control or the like to stably cancel the noise. Can be considered. However, an error signal that appears when it is not optimal for feedback control is required. Focusing on the second term of the expression (10) indicating the output signal described above, when the weight c is not optimal (c ≠ kD), the DC component proportional to the magnitude of the square A ² (t) of the instantaneous amplitude is appear. However, since the DC component of the output signal is used as an error signal for feedback control of the phase of the synchronization signal in the first term on the right side of equation (3), it is difficult to use it for feedback control of the weight c.

Therefore, attention is paid to the high-frequency component of the signal represented by the square A ² (t) of the instantaneous amplitude. The instantaneous amplitude A (t) is random noise, and the AC component of the square A ² (t) is also random noise. However, the frequency component includes a frequency component in which the square A ² (t) of the instantaneous amplitude is higher than the instantaneous amplitude A (t). That is, the upper limit frequency f _2up included in the square A ² (t) is twice as high as the upper limit frequency f _1up included in the instantaneous amplitude A (t) (f _2up = 2f _1up ). Therefore, a signal indicating the instantaneous amplitude square A ² (t) is generated from the light detection signal, components from the frequency f _1up to the frequency f _2up are extracted, and the extracted component signal and the output signal are multiplied. A (DC) error signal can be generated from A ² (t) (c−kD). On the other hand, the signal component DA (t) m ′ (t) to be detected and A (t) {acos (φ) −bsin (φ )} includes only the following component frequency f _1up, since it is uncorrelated, is not affected by these signals. Therefore, only the error signal of the weight c can be obtained. Based on the error signal obtained in this way, the weight c can be feedback controlled.

FIG. 16 shows an example of the configuration of the detection unit 24 applicable to the optical measurement device according to the first example of the fifth embodiment. The detection unit 24 illustrated in FIG. 16 is an example that performs feedback control of the weight c.
As shown in FIG. 16, the detection unit 24 includes a multiplier 240, a high-pass filter 241, a multiplier 242, and an integrator 243 as an example of the intensity modulator 211 shown in FIG.

  Specifically, the output side of the amplifier 11 and one input side of the multiplier 12 are connected to the input side of the detector 210. The output side of the detector 210 is commonly connected to the two input sides of the multiplier 240. The output side of the multiplier 240 is connected to one input side of the multiplier 242 via the high pass filter 241. The other input side of the multiplier 242 is connected between the output side of the low-pass filter 17 and the input side of the output unit 18. The output side of the multiplier 242 is connected to the control side of the weight adder 200 via the integrator 243.

Next, the operation of the detection unit 24 according to the first example of the fifth embodiment will be described.
As shown in FIG. 16, first, the detector 210 outputs a signal indicating the instantaneous amplitude A (t) using the photodetection signal. A signal indicating the instantaneous amplitude A (t) is input to both input sides of the multiplier 240, and the multiplier 240 outputs a signal obtained by squaring the instantaneous amplitude A (t). Next, the high-pass filter 241 has a cutoff frequency f _1up , and among the AC components of the signal indicating the square A ² (t) of the instantaneous amplitude A (t), a frequency component _{equal to} or higher than the cutoff frequency f _{1up is supplied} to the multiplier 242. Output to one input side. The output signal of the low-pass filter 17 is input to the other input side of the multiplier 242, and the multiplier 242 multiplies these input signals and outputs an error signal having a weight c, which is a multiplication result, to the integrator 243. To do. The integrator 243 is configured to integrate the input error signal of the weight c (integration control) and output the integration result to the control side of the weight adder 200. With this configuration, the weight c can be feedback controlled. The feedback cutoff frequency is sufficiently smaller than the modulation frequency of the pump light Pp, and small enough to ignore the correlation between the instantaneous amplitude A (t) and the high-frequency component of the instantaneous amplitude square A ² (t). To do.

Thus, in the optical measuring device according to the first example of the fifth embodiment, only the error signal of the weight c is obtained using the high-frequency component of the signal represented by the square A ² (t) of the instantaneous amplitude, The weight c can be feedback controlled based on the obtained error signal. As a result, the weight c can be automatically optimized even when the noise included in the output signal varies due to individual differences between elements and environmental variations.

Note that the feedback control in the first example of the fifth embodiment has been described for the case where the phase shift is linearly proportional to the amplitude, but can be applied to the case where the phase shift corresponds to the amplitude nonlinearly. Of course there is. Specifically, the error signal of the component proportional to the m-th order is obtained by deriving the product of the output and the high-frequency component of the (m + 1) _-order frequency f _1up of the instantaneous amplitude A (t) (m + 1) times. Can do. For example, the error signal for the weight c ₂ for canceling the coefficient k ₂ generates a high frequency having a frequency larger than twice the frequency f _1up of the signal indicating the cube A ³ (t) of the instantaneous amplitude, and derives an output and a product. It is obtained by doing.

That is, in the equation (16), when the error signal for each of the coefficient k ₁ and the coefficient k ₂ is generated considering the coefficient k ₁ and the coefficient k ₂ at the same time, the error signal for the weight c ₁ of the coefficient k ₁ is A ² (t) can be derived from the product of the 2 f _1up signal and the output of 2f _1up . The error signal for the weight c ₂ can be derived from the product of the signal of A ³ (t) from 2f _1up to 3f _1up and the output. When these are generalized, the (c _m ) error signal of the component proportional to the m-th order can be derived from the product of the m × f _1up signal and the output of (m−1) f _{1up of} A ^m (t). In this case, the instantaneous amplitude A (t) is not a single frequency (because it includes noise) but a wide band and includes the frequency f _1up from the direct current component (DC).

Next, a second example of the fifth embodiment will be described.
The second example of the fifth embodiment is a case where the phase of the synchronization signal is directly modulated using the instantaneous amplitude A (t) in order to cancel the phase shift (phase noise) due to the amplitude noise (see FIG. 14). This is an example of feedback control using the output signal of the low-pass filter 17.

  The method of canceling phase shift (phase noise) due to amplitude noise can also be applied when directly modulating the phase of the synchronization signal. That is, when the phase of the synchronization signal is directly modulated, the function f (x) in the above equation (21) is approximated as a sum of powers, and the coefficient ki is estimated by feedback.

Specifically, the coefficient ki may be estimated in the same manner as in the first example of the fifth embodiment, with the left side of the above expression (21) as the following expression (26).

FIG. 17 shows an example of the configuration of the detection unit 25 applicable to the optical measurement device according to the second example of the fifth embodiment. The detection unit 25 illustrated in FIG. 17 is an example that performs feedback control when the phase of the synchronization signal is directly modulated.
As illustrated in FIG. 17, the detection unit 25 includes a multiplier 240, a high-pass filter 241, a multiplier 242, an integrator 243, and an amplifier 244 as an example of the direct phase modulation controller 220 illustrated in FIG. . The direct phase modulation controller 220 shown in FIG. 17 is obtained by adding an amplifier 244 to the configuration of the intensity modulator 211 shown in FIG. The amplifier 244 operates as a controller that outputs a signal for directly modulating the phase of the synchronization signal.

Specifically, the output side of the multiplier 242 is connected to the control side of the amplifier 244 via the integrator 243. The input side of the amplifier 244 is connected to the output side of the detector 210, and the output side of the amplifier 244 is connected to the adder 221. That is, the amplifier 244 directly modulates the phase by controlling the signal indicating the instantaneous amplitude A (t) of the light detection signal using the signal indicating the square A ² (t) of the instantaneous amplitude A (t). Is output to the other addition side of the adder 221. That is, in the second example of the fifth embodiment, the phase of the synchronization signal is directly modulated using a signal indicating the instantaneous amplitude A (t) and the square A ² (t) of the instantaneous amplitude A (t).

Next, the operation of the detection unit 25 according to the second example of the fifth embodiment will be described.
The multiplier 242 includes a signal having a frequency component equal to or higher than the cutoff frequency f _1up among the AC components of the signal indicating the square A ² (t) of the instantaneous amplitude A (t) by the input high-pass filter 241 and the low-pass filter 17. The output signal is multiplied and the multiplication result is output to the integrator 243 as an error signal. The integrator 243 integrates the input error signal (integration control) and outputs the integration result to the control side of the amplifier 244.

As described above, in the optical measurement apparatus according to the second example of the fifth embodiment, an error signal is obtained using the high-frequency component of the signal represented by the square A ² (t) of the instantaneous amplitude, and the obtained error is obtained. Based on the signal, feedback control can be performed to a signal that directly modulates the phase of the synchronization signal.

  The feedback control in the second example of the fifth embodiment has been described for the case where the phase shift is linearly proportional to the amplitude. However, the phase shift is similar to the feedback control in the first example of the fifth embodiment. It is also applicable to the case where corresponds to nonlinearity with respect to amplitude.

Next, a third example of the fifth embodiment will be described.
In the third example of the fifth embodiment, in order to cancel phase shift (phase noise) due to amplitude noise, noise derived from phase noise is subtracted from the output using instantaneous amplitude A (t) (see FIG. 15), which is an example of feedback control using the output signal of the low-pass filter 17.

  The method of canceling phase shift due to amplitude noise (phase noise) can also be applied when subtracting noise derived from phase noise from the output. That is, when subtracting the noise derived from the phase noise from the output, the function f (x) in the above equation (25) is approximated as the sum of powers and ki is estimated by feedback.

Specifically, the coefficient ki may be estimated by using the above expression (25) as the following expression (27) as in the first example of the fifth embodiment.

FIG. 18 shows an example of the configuration of the detection unit 26 applicable to the optical measurement apparatus according to the third example of the fifth embodiment. The detection unit 26 illustrated in FIG. 18 is an example that performs feedback control when noise derived from phase noise is subtracted from the output.
As shown in FIG. 18, the detection unit 26 includes a subtraction controller 230 having the same configuration as the direct phase modulation controller 220 shown in FIG. In the subtracting controller 230 shown in FIG. 18, the amplifier 244 operates as a controller that outputs a signal for subtracting noise derived from phase noise from the output.

Specifically, the input side of the amplifier 244 is connected to the output side of the multiplier 240, and the output side of the amplifier 244 is connected to the subtractor 231. In other words, the amplifier 244 performs control by using a signal indicating the square A ² (t) of the instantaneous amplitude A (t) of the light detection signal, and subtracts a signal for subtracting noise derived from phase noise from the output. The unit 231 is configured to output to the input side for subtraction. That is, in the third example of the fifth embodiment, the noise derived from the phase noise is subtracted from the output using signals indicating the instantaneous amplitude A (t) and the square A ² (t) of the instantaneous amplitude A (t).

Next, the operation of the detection unit 25 according to the third example of the fifth embodiment will be described.
The multiplier 242 includes a signal having a frequency component equal to or higher than the cutoff frequency f _1up among the AC components of the signal indicating the square A ² (t) of the instantaneous amplitude A (t) by the input high-pass filter 241 and the low-pass filter 17. The output signal is multiplied and the multiplication result is output to the integrator 243 as an error signal. The integrator 243 integrates the input error signal (integration control) and outputs the integration result to the control side of the amplifier 244.

Thus, in the optical measuring device according to the third example of the fifth embodiment, an error signal is obtained using the high frequency component of the signal represented by the square A ² (t) of the instantaneous amplitude, and the obtained error is obtained. Based on the signal, feedback control can be performed on a signal that subtracts noise derived from phase noise.

  Although the feedback control in the third example of the fifth embodiment has been described for the case where the phase shift is linearly proportional to the amplitude, the phase shift is similar to the feedback control in the first example of the fifth embodiment. The present invention can also be applied to cases where the shift corresponds nonlinearly to the amplitude.

  Although the present invention has been described using the embodiment, the technical scope of the present invention is not limited to the scope described in the embodiment. Various modifications or improvements can be added to the above-described embodiment without departing from the gist of the invention, and embodiments to which such modifications or improvements are added are also included in the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Optical measuring device 1A, 1B, 1C Optical measuring device 2, 3 Light source (light source)
4 Light detector (light receiving part)
4A, 4B Photodetector 4C Subtractor 4S Spectrometer 5 Detection unit 5A Detection unit 5B Lock-in amplifier 5C Synchronization signal source 5D Synchronization signal detector 5E Synchronization signal detector 6 Mirror 7 Beam splitter 8A Reference signal source 8B Intensity modulator 9A Beam splitter 9B, 9C Mirror 9D Beam splitter 11 Amplifier 12 Multiplier 13 Synchronization signal unit 14 Low pass filter 15 Integrator 16 Phase controller 17 Low pass filter 18 Output unit 20 Detection unit 21, 22, 23, 24 Detection unit 200 Weight adder 201 Adder 210 Detector 211 Intensity Modulator 220 Direct Phase Modulation Controller 221 Adder 230 Subtraction Controller 231 Subtractor (Subtraction Unit)
240 Multiplier 241 High Pass Filter 242 Multiplier 243 Integrator 244 Amplifier Pp Pump Light (Stimulation Light)
Pr probe light (output light)
Rf Reference light (reference light)
SP sample

Claims (17)

A delay unit that delays the output light output from the light source for a predetermined time and outputs it as reference light;
A light receiving unit that irradiates the sample with the output light, the reference light, and the stimulation light, receives the measurement light from the sample, and outputs a light detection signal according to the intensity of the received light;
A removing unit that removes a noise component included in the photodetection signal by multiplying the compensation sync signal determined based on the photodetection signal and the sync signal synchronized with the light source, and the photodetection signal;
An optical measuring device. The optical measurement apparatus according to claim 1, wherein the delay unit delays the output light by π / 2 as the predetermined time and outputs the delayed light as reference light. The optical measurement apparatus according to claim 1, wherein the compensation synchronization signal is a signal obtained by adding the photodetection signal obtained by adding a predetermined weight to the synchronization signal. The optical measurement apparatus according to claim 1, wherein the compensation synchronization signal uses a signal in which a phase of the synchronization signal is adjusted based on an instantaneous amplitude of the light detection signal. The optical measurement apparatus according to claim 1, wherein the compensation synchronization signal uses a signal in which a phase of the synchronization signal is adjusted based on a power of an instantaneous amplitude of the light detection signal. The optical measurement apparatus according to claim 3, wherein the predetermined weight is determined based on an instantaneous amplitude of the light detection signal. The optical measurement apparatus according to claim 3, wherein the predetermined weight is determined based on a power of an instantaneous amplitude of the light detection signal. The optical measurement apparatus according to claim 1, wherein the compensation synchronization signal is determined so that the light detection signal and an instantaneous phase are orthogonal to each other. The removing unit is
A phase adjustment unit that adjusts the phase of the synchronization signal to obtain the compensation synchronization signal;
A multiplier for multiplying the compensation synchronization signal and the light detection signal;
A feedback unit that feeds back the output of the multiplication unit to the removal unit;
The optical measuring device according to claim 1, comprising: The feedback unit further includes a filter that extracts a low frequency component having a frequency equal to or lower than a predetermined frequency from the output of the multiplication unit, and the low frequency component adjusts a weight when the photodetection signal is added to the synchronization signal or The optical measurement apparatus according to claim 9, which is fed back to the phase adjustment unit. The light source is a white light source;
A spectroscope disposed in front of the light receiving unit;
Further comprising
The optical measurement device according to any one of claims 1 to 10, wherein the light receiving unit and the removing unit are arranged for each wavelength separated by the spectroscope. A delay unit that delays the output light output from the light source for a predetermined time and outputs it as reference light;
A light receiving unit that irradiates the sample with the output light, the reference light, and the stimulation light, receives the measurement light from the sample, and outputs a light detection signal according to the intensity of the received light;
A suppression unit that suppresses an intensity noise component included in the light detection signal by multiplying the light detection signal and a synchronization signal synchronized with the light source;
A subtracting unit that subtracts a light intensity signal related to phase noise determined based on the photodetection signal from a signal in which the intensity noise component is suppressed by the suppressing unit;
An optical measuring device. The optical measurement apparatus according to claim 12, wherein the delay unit outputs the output light as reference light by delaying the output light by π / 2 as the predetermined time. The optical measurement device according to claim 12 or 13, wherein the subtraction unit determines a light intensity signal related to phase noise based on an instantaneous amplitude of the light detection signal. The optical measurement device according to claim 12 or 13, wherein the subtraction unit determines a light intensity signal related to phase noise based on a power of an instantaneous amplitude of the light detection signal. The output light output from the light source is delayed for a predetermined time and output as reference light,
Irradiating the sample with the output light, the reference light, and the stimulation light, and receiving the measurement light from the sample, and outputting a light detection signal according to the intensity of the received light,
A compensation synchronization signal determined based on the light detection signal and a synchronization signal synchronized with the light source is multiplied by the light detection signal to remove a noise component included in the light detection signal.
Optical measurement method. The output light output from the light source is delayed for a predetermined time and output as reference light,
Irradiating the sample with the output light, the reference light, and the stimulation light, and receiving the measurement light from the sample, and outputting a light detection signal according to the intensity of the received light,
Multiplying the light detection signal and a synchronization signal synchronized with the light source to suppress an intensity noise component included in the light detection signal,
An optical measurement method comprising: subtracting a light intensity signal related to phase noise determined based on the light detection signal from the suppressed signal.