Measurement device and measurement method

Abstract

A measurement device according to the present invention comprises: a first laser light source 11 that emits a first laser beam B1 for which the peak frequency is a first frequency; a second laser light source 12 that emits a second laser beam B2 for which the peak frequency is a second frequency different from the first frequency; a first beam splitter 25 that causes a first light B6 to interfere with the second laser beam, the first light B6 including first Raman scattered light R1 generated by an object 16 when the object 16 is irradiated with the first laser beam; a first detector 21 that detects a first interference light between the first light and the second laser beam; and a first telescope 23 that is provided in the optical path of the first light and directed toward the object.

Description

Measuring device and measuring method

[0001] The present disclosure relates to a measurement device and a measurement method.

[0002] It is known to irradiate an object with laser light and detect Raman scattering occurring in the object. A Raman microscope is known that irradiates the object with excitation light and Stokes light, generates stimulated Raman scattering light, and causes interference between the light and a reference light, and performs heterodyne detection of the interference light (see, for example, Patent Document 1).

[0003] International Publication No. 2013 / 047698

[0004] For example, when detecting a potentially explosive object such as hydrogen gas, it is necessary to detect the object remotely for safety reasons. To achieve this, a laser beam is irradiated onto the object remotely. The Raman scattered light generated in the object is then detected remotely. However, Raman scattered light is weak, making it difficult to detect the Raman scattered light generated in the object remotely.

[0005] An object of the present disclosure is to provide a measurement device and a measurement method that are capable of remotely detecting Raman scattered light with high sensitivity.

[0006] According to an embodiment of the present disclosure, a measurement device includes a first laser light source that emits a first laser light having a peak frequency of a first frequency, a second laser light source that emits a second laser light having a peak frequency of a second frequency different from the first frequency, a first beam splitter that causes interference between the second laser light and first light including first Raman scattered light generated in an object when the first laser light is irradiated onto the object, a first detector that detects first interference light between the first light and the second laser light, and a first telescope that is provided in an optical path of the first light and directed toward the object.

[0007] According to an embodiment of the present disclosure, a measurement method includes irradiating an object with a first laser light having a peak frequency of a first frequency; irradiating a first beam splitter with a second laser light having a peak frequency of a second frequency different from the first frequency; causing interference between the first light, which includes first Raman scattered light generated in the object when the first laser light is irradiated onto the object and passes through a telescope directed towards the object, and the second laser light, by the first beam splitter; and detecting first interference light, which is interference light between the first light and the second laser light, by a first detector.

[0008] According to the present disclosure, Raman scattered light can be detected remotely with high sensitivity.

[0009] FIG. 1 is a block diagram of a measurement device according to a first embodiment. FIG. 2A is a diagram showing a detection signal versus time. FIG. 2B is a diagram showing the signal strength of the detection signal versus frequency. FIG. 2C is a diagram showing the signal strength of the detection signal versus frequency. FIG. 3A is a diagram explaining the role of a telescope. FIG. 3B is a diagram explaining the role of a telescope. FIG. 4A is a diagram showing laser light B1 and light B6 when the beam waist diameter is small. FIG. 4B is a diagram showing laser light B1 and light B6 when the beam waist diameter is large. FIG. 5 is a block diagram of a measurement device according to a second embodiment. FIG. 6A is a diagram showing signal strength versus frequency in a measurement method example 1 according to the second embodiment. FIG. 6B is a diagram showing signal strength versus frequency in a measurement method example 1 according to the second embodiment. FIG. 7 is a diagram showing signal strength versus frequency in a measurement method example 1 according to the second embodiment. FIG. 8A is a diagram showing signal strength versus frequency in a measurement method example 2 according to the second embodiment. FIG. 8B is an enlarged view of the vicinity of Raman scattered light R1 in FIG. 8A. FIG. 9 is a diagram explaining a calibration method in measurement method example 2 according to the second embodiment. FIG. 10 is a block diagram of a measurement apparatus according to a third embodiment. FIG. 11 is a block diagram of a measurement apparatus according to a fourth embodiment. FIG. 12 is a diagram showing signal strength versus frequency in the fourth embodiment. FIG. 13 is a block diagram of the vicinity of a detector of a measurement apparatus according to a fifth embodiment. 14A is a diagram showing signal strength versus time in the fifth embodiment. 14B is a diagram showing signal strength versus time in the fifth embodiment. FIG. 15A is a block diagram of a measurement apparatus according to a sixth embodiment. FIG. 15B is a schematic diagram showing how peak frequencies ω1(t) and ω2(t) change over time. FIG. 15C is a schematic diagram showing how peak frequencies ωR1(t) and ω2(t) change over time. FIG. 16 is a block diagram of a measurement apparatus according to a seventh embodiment. FIG. 17 is a diagram showing the signal strength of laser light versus wavelength and the transmittance of a wavelength-selective filter versus wavelength. FIG. 18 is a block diagram of a measurement apparatus according to an eighth embodiment. FIG. 19 is a block diagram of a measurement apparatus according to a ninth embodiment. FIG. 20 is a block diagram of a measurement apparatus according to a tenth embodiment. FIG. 21 is a block diagram of a measurement apparatus according to an eleventh embodiment.

[0010] Hereinafter, embodiments for carrying out the present disclosure will be described in detail with reference to the drawings. The following embodiments are examples for embodying the technical ideas of the invention, and the present disclosure is not limited to the described configurations and numerical values. In each drawing, the same components are given the same reference numerals, and duplicate explanations may be omitted as appropriate. The size, positional relationship, etc. of each component shown in each drawing may be exaggerated to facilitate understanding of the invention.

[0011] First Embodiment An example of a measurement device is shown in which the concentration of a specific substance (e.g., hydrogen gas) in a remotely located object such as a gas is measured by detecting Raman scattered light in the object and measuring the signal intensity of the Raman scattered light, thereby measuring the concentration of the specific substance.

[0012] In the first embodiment, heterodyne detection is used to detect Raman scattered light, and a telescope is provided facing the target. This allows for highly accurate detection of Raman scattered light from a remote target. FIG. 1 is a block diagram of a measurement device according to the first embodiment. As shown in FIG. 1, the measurement device 100 according to the first embodiment includes a first laser light source 11, a second laser light source 12, a first detector 21, telescopes 23 and 24, a beam splitter 25, and a signal processing circuit 30.

[0013] The first laser light source 11 emits a laser beam B1 (i.e., a first laser beam). The peak frequency of the laser beam B1 is a frequency ω1 (i.e., a first frequency). The telescope 24 (i.e., a second telescope) has a lens 24B corresponding to an eyepiece and a lens 24A corresponding to an objective lens, and is provided to face the target 16. As a result, the telescope 24 magnifies the laser beam B1 and irradiates it onto the target 16.

[0014] When the laser light B1 is irradiated onto the object 16, light B6 (i.e., first light) containing Raman scattered light R1 (i.e., first Raman scattered light) is generated in the object 16. The frequency of the Raman scattered light is shifted from the frequency of the laser light B1 by a Raman shift frequency specific to the material. Of the Raman scattered light, Stokes light, which has a higher intensity than anti-Stokes light, may be used for detection. The light B6 passes through a telescope 23 (i.e., a first telescope) and reaches a beam splitter 25. The telescope 23 has a lens 23B corresponding to an eyepiece and a lens 23A corresponding to an objective lens, and is arranged to face the object 16.

[0015] The second laser light source 12 emits laser light B2 (i.e., second laser light). The laser light B2 is local oscillator light. The peak frequency of the laser light B2 is a frequency ω2 (i.e., second frequency) different from the frequency ω1. The difference between the frequencies ω1 and ω2 may be, for example, 1 GHz or more and 200 THz or less. The beam splitter 25 (i.e., first beam splitter) causes interference between the laser light B2 and light B6. The first detector 21 detects the first interference light between the laser light B2 and light B6. The signal processing circuit 30 processes the output signal of the first detector 21. The signal processing circuit 30 may be, for example, a spectrum analyzer. The signal processing circuit 30 may perform lock-in detection or Fourier analysis on the output signal of the first detector 21 to calculate a spectrum of signal intensity versus frequency.

[0016] Next, heterodyne detection will be explained. Fig. 2A is a diagram showing a detection signal with respect to time. Fig. 2B and Fig. 2C are diagrams showing the signal strength of the detection signal with respect to frequency. When the first detector 21 is a photodiode, the detection signal is an output current. The signal strength is the power of the detection signal, which is the square of the amplitude of the detection signal.

[0017] Equation 1 is the detection signal I of the first detector 21. ｈｅｔｅｒｏ is the formula.

[0018] η represents the efficiency of converting light power into current. Ｓ denotes the electric field of light B6, and E ＬＯ denotes the electric field of the laser beam B2.Ｓ denotes the angular frequency of light B6, and ω ＬＯ denotes the angular frequency of the laser beam B2, and t denotes time.

[0019] 2A is a diagram showing the detection signal of the first detector 21 as a function of time t. ｈｅｔｅｒｏ Also, FIG. 2A shows the I ｈｅｔｅｒｏ For comparison, the current value when the Raman scattered light R1 is directly received by a photodiode without heterodyne detection is I ｄｉｒｅｃｔ The detection signal I ｈｅｔｅｒｏ In this case, the detection signal I ｈｅｔｅｒｏ The constant component A1 is expressed as η×[<E Ｓ ２ >+<E ＬＯ ２ The amplitude A2 of the beat signal in which the current fluctuates over time corresponds to η×[2<E Ｓ E ＬＯ The angular frequency of the beat signal is |ω ｓ -ω ＬＯ |It is.

[0020] FIG. 2B shows the detection signal I ｈｅｔｅｒｏ This figure shows the signal intensity versus frequency. The frequency is 2π times the angular frequency. Let ωR1 be the frequency of peak R1P of Raman scattered light R1, and ω2 be the frequency of the peak of laser light B2. When the frequency is ωR1-ω2, the peak R1H of the interference light occurs. The intensity of Raman scattered light R1 can be detected from the intensity A2 of peak R1H and the known intensity of laser light B2.

[0021] In the case of heterodyne detection, the height A2 of the peak R1H ２ is <E ｓ E ＬＯ > ２ The signal intensity of the laser light B2 can be several tens of mW, whereas the intensity of the Raman scattered light can be nW or less. When the Raman scattered light R1 is directly received by a photodiode without heterodyne detection, the current value I ｄｉｒeｃｔ is η×<E Ｓ ２ >, but because the Raman scattered light is weak, as shown in Figure 2A, × < EＳ ２ On the other hand, when heterodyne measurement is used, the current amplitude A2 is as shown in Equation 1, η×<E Ｓ E ＬＯ >. ＬＯ Is E ｓ is orders of magnitude larger than Ｓ E ＬＯ > is <E Ｓ ２ >It is orders of magnitude larger, making it easier to detect light B6.

[0022] In FIG. 2C , R1 represents the frequency distribution of the Raman scattered light to be detected, and B0 represents the frequency distribution of background light, which becomes detection noise. When attempting to detect Raman scattered light R1, signals within the detector's resolution are integrated and output. If background light B0 is present, not only the Raman scattered light R1 but also the signal intensity of the background light B0 is integrated within the range of the resolution RB. As a result, the signal intensity of the Raman scattered light R1 is buried in the signal intensity of the background light B0. RB is the resolution of a bandpass filter, for example, when detecting Raman scattered light R1 using an optical bandpass filter without heterodyne detection. For example, the resolution of a bandpass filter made of a dielectric multilayer film is typically 1 GHz or higher. RA is the resolution when heterodyne detection is used. The resolution RA is determined by the signal processing method of the signal processing circuit 30; for example, a spectrum analyzer can achieve a sufficient measurement time resolution RA of 1 kHz or less. In heterodyne detection, the resolution RA can be reduced, which prevents the signal intensity of the Raman scattered light R1 from being buried in the signal intensity of the background light B0, thereby improving the S / N ratio (ratio of signal to noise).

[0023] Next, the telescope will be described. Figures 3A and 3B are diagrams illustrating the role of the telescope. As shown in Figure 3A, laser light B2 is emitted from the second laser light source 12, reflected by the beam splitter 25, and irradiated onto the first detector 21. The laser light B2 has a width (or diameter). The virtual laser light B2X is the laser light B2 viewed from the beam splitter 25 toward the second laser light source 12, and then reflected from the beam splitter 25 toward the object 16. When the light B6 from the object 16 is outside the range of the virtual laser light B2X, as in the case of light B6X, the light B6X does not interfere with the laser light B2. On the other hand, when the light B6 from the object 16 is within the range of the virtual laser light B2X, as in the case of light B6Y, the light B6Y interferes with the laser light B2.

[0024] As shown in FIG. 3B, the beam area of ​​the virtual laser light B2X at the beam waist B2W where the beam diameter of the virtual laser light B2X is smallest is the beam waist area S(0). The distance between the beam waist B2W and the object 16 is L. The rate at which the light B6 isotropically scattered from the object 16 interferes with the laser light B2 within the range of the virtual laser light B2X is S(0) / (4πL ２ ) That is, it is the ratio of S(0) to the surface area of ​​the whole sphere with radius L.

[0025] 4A and 4B are diagrams showing laser light B1 and light B6 when the beam waist diameter is small and large, respectively. As shown in FIG. 4A, when the beam waist diameter is small, the beam waist area S(0) is small. On the other hand, only light that falls within the beam waist range can interfere with laser light B2. As a result, the proportion of light B6 that can be detected is small.

[0026] As shown in Fig. 4B, when the beam waist diameter is larger than that of Fig. 4A, the beam waist area S(0) is large, and the proportion of light B6 that can interfere with laser light B2 and be detected increases.

[0027] In this way, by increasing the beam waist area S(0) of the virtual laser light B2X, the detection sensitivity can be improved. When the telescope 23 is provided facing the object 16, the telescope 23 expands the light on the object 16 side relative to the first detector 21 side. This makes it possible to increase the beam waist area S(0) of the virtual laser light B2X. Therefore, the detection sensitivity of the light B6 can be improved.

[0028] The telescope 24 does not have to be provided. However, as shown in FIG. 4A , if the divergence angle of the laser light B1 is large and the beam area at the position of the object 16 is larger than the object 16, the proportion of the laser light B1 irradiated onto the object 16 will be reduced. As a result, since the intensity of the Raman scattered light is proportional to the intensity of the light B1 irradiated onto the object 16, the intensity of the light B6 generated at the object 16 will be reduced. Therefore, if the telescope 24 is provided facing the object 16, the telescope 24 will expand the light on the object 16 side rather than the first laser light source 11 side. This reduces the divergence angle, allowing the laser light B1 to be irradiated onto the object 16 in a concentrated manner. Therefore, the intensity of the light B6 generated at the object 16 will be increased. This improves the detection sensitivity of the light B6.

[0029] The magnification power of the telescopes 23 and 24 may be, for example, 2 to 100. The magnification power of the telescope 23 is preferably smaller than the magnification power of the telescope 24. This allows the range irradiated with the laser light B1 to be sufficiently contained within the field of view.

[0030] The telescopes 23 and 24 may be refracting telescopes. Alternatively, the telescopes 23 and 24 may be reflecting telescopes formed by reflecting mirrors instead of lenses. For the sake of compactness, the telescopes 23 and 24 are preferably refracting telescopes.

[0031] As described above, according to the first embodiment, the beam splitter 25 causes the light B6 and the laser light B2 to interfere with each other. The first detector 21 detects the first interference light between the light B6 and the laser light B2. This enables heterodyne detection, making it possible to detect weak Raman scattered light from the remote object 16 with high sensitivity. Furthermore, the telescope 23 is provided in the optical path of the light B6, facing the object 16. This increases the beam waist area of ​​the virtual laser light B2X, making it possible to detect weak Raman scattered light from the remote object 16 with high sensitivity.

[0032] The telescope 24 is provided on the optical path of the laser light B1 leading to the target 16, facing the target 16. This allows the laser light B1 to be irradiated in a concentrated manner onto the target 16, thereby enabling high-sensitivity detection of Raman scattered light. The telescope 24 does not necessarily have to be provided. A single telescope may be used as the telescopes 23 and 24. The telescopes 23 and 24 may be any telescope that can expand the beam, and the configuration thereof is not limited to that shown in FIG. 1.

[0033] Second Embodiment Fig. 5 is a block diagram of a measurement device according to a second embodiment. As shown in Fig. 5, a measurement device 101 according to the second embodiment includes, in addition to the measurement device 100 of the first embodiment, a fourth laser light source 14, a mirror 27A, a beam splitter 28A, and a switching unit 31. The third laser light source 13 will be described in an embodiment described later.

[0034] The fourth laser light source 14 emits laser light B4 (i.e., fourth laser light). The peak frequency of laser light B4 is frequency ω4 (i.e., fourth frequency), which is different from frequencies ω1 and ω2. The mirror 27A reflects the laser light B4 and irradiates the reflected laser light B4 onto the beam splitter 28A. The beam splitter 28A aligns the optical axes of the laser lights B1 and B4 and irradiates one of the laser lights B1 and B4 onto the object 16. When the laser light B4 irradiates the object 16, light B7 (i.e., second light) is generated at the object 16. The beam splitter 25 causes light B6 and laser light B2 to interfere with each other, and also causes light B7 and laser light B2 to interfere with each other. The first detector 21 detects first interference light between light B6 and laser light B2 and third interference light between light B7 and laser light B2.

[0035] The switching unit 31 electrically or mechanically switches between the first period, the second period, and the third period. The first period is a period during which the laser beam B1 is irradiated onto the object 16, but the laser beam B4 is not irradiated onto the object 16. The second period is a period during which the laser beam B4 is irradiated onto the object 16, but the laser beam B1 is not irradiated onto the object 16. The third period is a period during which the laser beams B1 and B4 are not irradiated onto the object 16. The other configurations are the same as those in the first embodiment, and therefore description thereof will be omitted.

[0036] (Measurement Method Example 1) Figures 6A, 6B, and 7 are diagrams showing signal strength versus frequency in Measurement Method Example 1 of the second embodiment. Figure 6A shows signal strength versus frequency in a first period. Figure 6B shows signal strength versus frequency in a second period. Figure 7 is a diagram in which Figures 6A and 6B are superimposed.

[0037] 6A , peak B1P is the peak of the light of laser light B1 that returns to first detector 21 (i.e., Rayleigh scattered light in and around object 16). Peak B1P has a frequency of ω1. Peak B2P is the peak of the light of laser light B2 that enters first detector 21. Peak B2P has a frequency of ω2. Peak R1P is the peak of Raman scattered light R1 that occurs in object 16 when laser light B1 is irradiated onto object 16.

[0038] The laser light B1 contains spontaneously emitted light in addition to stimulated emission light. The spontaneously emitted light has a broad spectrum. Light B1S is the spontaneously emitted light contained in the laser light B1 that returns to the first detector 21 (i.e., Rayleigh scattered light in and around the object 16). Background light B0 is ambient light present in the environment and is not caused by the laser lights B1 and B2. The difference between the frequency ω1 of peak B1P of the laser light B1 and the frequency ωR1 of peak R1P of the Raman scattered light R1 is the frequency ΔωR of the Raman shift in the object 16.

[0039] During the first period, the signal processing circuit 30 measures the signal intensity SR1 at the frequency ωR1. This allows the intensity of the Raman scattered light R1 from the object 16 to be measured. However, the light at the frequency ωR1 detected by the first detector 21 contains light B1S and background light B0. As a result, the intensity of the Raman scattered light R1 cannot be measured with high accuracy.

[0040] 6B , in the second period, the fourth laser light source 14 emits laser light B4. Peak B4P is the peak of light of the laser light B4 that returns to the first detector 21. The frequency of peak B4P is ω4. Peak R4P is the peak of Raman scattered light R4 generated in the object 16 when the laser light B4 is irradiated onto the object 16. Light B4S is the spontaneous emission light contained in the laser light B4 that returns to the first detector 21. The difference between the frequency ω4 of peak B4P of the laser light B4 and the frequency ωR4 of peak R4P of the Raman scattered light R4 is the frequency ΔωR of the Raman shift in the object 16.

[0041] 6A, 6B, and 7, the signal processing circuit 30 measures a signal strength SR1 at a frequency ωR1 in a first period. The signal processing circuit 30 measures a signal strength B4S at a frequency ωR1 in a second period. The signal processing circuit 30 measures a signal strength SO at a frequency ωR1 in a third period.

[0042] The influence of light B1S on signal strength SR1 can be calibrated from signal strength B4S. The signal processing circuit 30 may use the value obtained by subtracting S4S from signal strength SR1 as the calibrated signal strength. The signal processing circuit 30 may estimate the magnitude of the signal strength of light B1S at frequency ωR1 based on signal strength B4S, and use the difference between signal strength SR1 and the estimated signal strength of light B1S as the calibrated signal strength.

[0043] The signal processing circuit 30 can calibrate the influence of background light B0 in the signal strength SR1 from the signal strength S0. The signal processing circuit 30 may use the value obtained by subtracting B4S and S0 from the signal strength SR1 as the calibrated signal strength. Note that the switching unit 31 does not need to switch to the third period, and the signal processing circuit 30 does not need to perform calibration using the signal strength S0. Calibration of the signal strength SR1 may be performed by an external computer.

[0044] The preferred range of each frequency when performing calibration in measurement method example 1 will be described. In order to electrically process the interference signal between light B6 and laser light B2 using signal processing circuit 30, the absolute value |ω2-ωR1| of the difference ω2-ωR1 between the frequency ωR1 of Raman scattered light R1 in light B6 and the peak frequency ω2 of laser light B2 in FIG. 6A is preferably 30 GHz or less, more preferably 10 GHz or less, and even more preferably 5 GHz or less or 1 GHz or less. From the perspective of heterodyne detection, the absolute value |ω2-ωR1| is greater than 0 Hz. For example, if the target object 16 is hydrogen molecules at room temperature, the half-width at half maximum of Raman scattered light R1 is approximately 50 MHz, so the absolute value |ω2-ωR1| is preferably 50 MHz or more, and more preferably 100 MHz or more.

[0045] The absolute value |ω2-ωR1| corresponds to the absolute value |ΔωR-(ω1-ω2)| of the difference between the difference ω1-ω2 between the peak frequency ω1 of the laser light B1 and the frequency ω2 of the laser light B2 and the Raman shift frequency ΔωR of the object 16.

[0046] The first detector 21 detects interference light with the laser beam B2. Therefore, if the absolute value |ω4-ω2| of the difference between the frequency ω4 of the laser beam B4 and the frequency ω2 of the laser beam B2 in FIG. 7 falls within the band D1 of the first detector 21, the peak Rayleigh scattered light of the laser beam B4 will affect the detection of the Raman scattered light R1. Therefore, |ω4-ω2| is set outside the band D1. In FIG. 7, if ω4 falls outside the range D1A of ±D1 centered on the frequency ω2, |ω4-ω2| will also fall outside the band D1. From the viewpoint of preventing the peak Rayleigh scattered light of the laser beam B4 from affecting the detection of the Raman scattered light R1, |ω4-ω2| is preferably 1.5 times or more, and more preferably 2 times or more, of D1. This prevents the peak Rayleigh scattered light of the laser beam B4 from affecting the detection of the Raman scattered light R1.

[0047] The absolute value |ωR4-ωR1| of the difference between the frequency ωR4 of the peak R4P of the Raman scattered light R4 due to the laser light B4 and the frequency ωR1 of the peak R1P of the Raman scattered light R1 due to the laser light B1 is equal to the absolute value |ω4-ω1| of the difference between the frequency ω4 of the laser light B4 and the frequency ω1 of the laser light B1. If the Raman scattered light R4 gets too close to the Raman scattered light R1, it will affect the detection of the Raman scattered light R1. From this perspective, |ω4-ω1| is set to be equal to or greater than the linewidth R1W of the Raman scattered light R1. Note that the linewidth R1W is twice the half-width at half maximum of the Raman scattered light R1. From the perspective of preventing the Raman scattered light R4 from affecting the detection of the Raman scattered light R1, |ω4-ω1| is preferably 1.5 times or more, and more preferably twice or more, the linewidth R1W.

[0048] It is preferable that |ω4-ω1| satisfies the above range for the band D1 and the line width R1W, while the frequencies ω4 and ω1 are as close as possible. When the peak R4P of the Raman scattered light R4 due to the laser light B4 and the peak R1P of the Raman scattered light R1 due to the laser light B1 approach each other, the difference in intensity between the light B1S and the light B4S at the same frequency becomes smaller. As a result, the smaller the difference in intensity between the light B1S and the light B4S, the more accurate the calibration. Therefore, |ω4-ω1| may be 1000 times or less, preferably 100 times or less, the line width R1W.

[0049] (Measurement Method Example 2) Fig. 8A is a diagram showing signal intensity versus frequency in Measurement Method Example 2 of the second embodiment. Fig. 8B is an enlarged view of the vicinity of Raman scattered light R1 in Fig. 8A. In Measurement Method Example 2, irradiation with laser light B1 and irradiation with laser light B4 may be performed simultaneously. Furthermore, switching unit 31 may be used to switch between the first period, the second period, and the third period.

[0050] 8A and 8B, the frequency ω4 of the peak B4P of the laser light B4 is closer to the frequency ω2 than to the frequency ω1. In the example measurement method 2, the signal processing circuit 30 measures the signal strength SR1 at the frequency ωR1 and the signal strength S4P at the frequency ω4.

[0051] 9 is a diagram illustrating a calibration method in measurement method example 2 of the second embodiment. As shown in FIG. 9, a reflector 38, which is a random scatterer, is assumed to be present around the object 16. The beam waist area of ​​the beam waist B2W of the virtual laser light B2X is S(0). The distance from the beam waist B2W to the object 16 is L1, and the distance from the beam waist B2W to the reflector 38 is L2. The solid angle seen from the object 16 to the range A0 of the virtual laser light B2X within the reflector 38 is Ω. The power of the Raman scattered light per unit concentration of a specific substance is PR1, and the concentration of the specific substance in the object 16 is n.

[0052] At this time, the signal intensity SR1 of the Raman scattered light R1 in the first detector 21 is expressed by Equation 2.

[0053]

[0054] In Equation 2, PR1×n is the total power of the Raman scattered light R1 in the object 16. The Raman scattered light R1 is emitted isotropically in all directions from the object 16. The first term in the parentheses corresponds to the proportion of the Raman scattered light R1 emitted from the object 16 that directly reaches the first detector 21. The first term is an equation obtained by dividing the beam waist area S(0) by the surface area of ​​a global sphere with a radius of L1. The second term in the parentheses corresponds to the proportion of the Raman scattered light R1 emitted by the object 16 that reaches the reflector 38 from the object 16, is scattered by the reflector 38, and reaches the first detector 21. The first proportion of the Raman scattered light R1 that falls on a range A0 within the reflector 38 is Ω / 4π. The second proportion of the light reflected in the range A0 that reaches the first detector 21 is calculated by dividing the beam waist area S(0) by the surface area of ​​a hemisphere with a radius of L2 and multiplying the result by the reflectivity R. The second term is calculated by multiplying the first proportion by the second proportion.

[0055] Since the reflector 38 changes depending on the measurement conditions, the second term in the parentheses of Equation 2 also changes depending on the measurement conditions. In this case, even if the concentration n is calculated based on the signal strength SR1, the error will be large.

[0056] Assuming that the beam waist area of ​​the virtual laser light B2X and the beam waist area of ​​the laser light B4 are the same S(0), and the power of the laser light B4 is P4, the signal intensity S4P of the laser light B4 at the first detector 21 is given by the following equation (3):

[0057]

[0058] In Equation 3, the power P4 of the laser light B4 is multiplied by the third proportion of the light returning from the reflector 38 to the first detector 21. The third proportion is the same as the second proportion in Equation 2. Therefore, if the third proportion can be calculated from the signal strength S4P, the influence of the second term in the parentheses can be calibrated from the signal strength SR1. This improves the measurement accuracy of the concentration n. The above calibration method for SR1 is one example, and SR1 can be calibrated using various methods using SP4.

[0059] Furthermore, the intensity of the spontaneous emission light emitted by the first laser light source 11 at the frequency ωR1 is measured in advance. If the rate at which the spontaneous emission light returns to the first detector 21 from the reflector 38 is considered to be similar to Equation 3, the intensity of the light B1S at the frequency ωR1 can also be calculated. This makes it possible to calibrate the light B1S. As in Measurement Method Example 1, it is also possible to calibrate the background light B0.

[0060] The preferred range of the peak frequency ω2 of laser beam B2 is the same as in Measurement Method Example 1. The first detector 21 detects interference light between the return light of laser beam B4 and laser beam B2. Therefore, in FIG. 8B, the absolute value |ω4 - ω2| of the difference between the frequency ω4 of laser beam B4 and the frequency ω2 of laser beam B2 is preferably located within the band D1 of the first detector 21. In other words, |ω4 - ω2| is set to be equal to or less than band D1. In FIG. 8B, the frequency ω4 is located within a range D1A of ±D1 centered on the frequency ω2. It is preferable that |ω4 - ω2| is equal to or less than 0.8 times D1. |ω4 - ω2| may be, for example, 50 MHz or more and 30 GHz or less.

[0061] If peak B4P is too close to peak R1P, it will affect the detection of Raman scattered light R1. From this perspective, the absolute value of the difference between frequencies ω4 and ωR1, |ω4 - ωR1|, is preferably equal to or greater than the linewidth R1W of the Raman scattered light R1. |ω4 - ωR1| is preferably equal to or greater than 1.5 times the linewidth R1W, and more preferably equal to or greater than 2 times. |ω4 - ω1| may be, for example, 75 MHz or greater and 30 GHz or less. Note that ωR1 is ω1 - ΔωR or ω1 + ΔωR. Because the Stokes light has a greater intensity than the anti-Stokes light, ωR1 is preferably ω1 - ΔωR.

[0062] Third Embodiment The third embodiment is an example of a measurement device that detects two types of Raman scattered light from an object 16. Fig. 10 is a block diagram of the measurement device according to the third embodiment. As shown in Fig. 10, the measurement device 102 according to the third embodiment includes a third laser light source 13, a second detector 22, and beam splitters 26 and 29 in addition to the components of the measurement device 101 of the second embodiment.

[0063] The third laser light source 13 emits laser light B3 (i.e., third laser light). Laser light B3 is local oscillator light. The peak frequency of laser light B3 is frequency ω3 (i.e., third frequency), which is different from frequencies ω1, ω2, and ω4. Beam splitter 29 splits light B6 and B7. Portions of the split light B6 and B7 reach first detector 21 via beam splitter 25. Other portions of the split light B6 and B7 reach beam splitter 26. Beam splitter 26 (i.e., second beam splitter) causes interference between light B6 and laser light B3, and also causes interference between light B7 and laser light B3. Second detector 22 detects second interference light between light B6 and laser light B3 and fourth interference light between light B7 and laser light B3.

[0064] An example will be described in which the measurement device 102 of the third embodiment is used to detect Raman scattered light from ortho-hydrogen and Raman scattered light from para-hydrogen in hydrogen gas. Hydrogen gas contains ortho-hydrogen, in which the nuclear spins of the two hydrogen atoms are parallel, and para-hydrogen, in which the nuclear spins are opposite. At room temperature, the ratio of ortho-hydrogen to para-hydrogen in hydrogen gas is approximately 3:1. When hydrogen is liquefied and stored, ortho-hydrogen changes to the more stable para-hydrogen. At this time, heat corresponding to the energy difference between ortho-hydrogen and para-hydrogen is generated. This causes the liquefied hydrogen to evaporate. Therefore, it has been considered to convert ortho-hydrogen to para-hydrogen using a catalyst before liquefying the hydrogen, and then liquefy the hydrogen. While the above example is merely an example, there is a demand for measuring the ratio of ortho-hydrogen to para-hydrogen in hydrogen gas.

[0065] If the rotational quantum number of a hydrogen molecule is J and the vibrational quantum number is v, the ground state of para-hydrogen is v=0, J=0, and the excited states of para-hydrogen include v=0, J=2 and v=1, J=0. The ground state of ortho-hydrogen is v=0, J=1, and the excited states of ortho-hydrogen include v=0, J=3 and v=1, J=1.

[0066] The energy difference between the ground state and the excited state is the Raman shift. The Raman shift frequencies ΔωRA of parahydrogen are 10.624 THz and 124.747 THz, etc. The Raman shift frequencies ΔωRB of orthohydrogen are 17.595 THz and 124.570 THz, etc. The Raman shift frequency can vary by several hundred MHz depending on the temperature and density of the hydrogen and disturbances from other molecules. Therefore, the Raman shift frequency is not limited to the above example. In the above example, the value on the order of 1 GHz, which is the value to the third decimal place, can vary depending on the measurement environment.

[0067] In the measuring device 102 of the third embodiment, when the laser beam B1 is irradiated onto the object 16, the Raman scattered light generated in the object 16 includes Raman scattered light R1A due to para-hydrogen (i.e., first Raman scattered light) and Raman scattered light R1B due to ortho-hydrogen (i.e., second Raman scattered light). Therefore, the beam splitter 25 causes interference between the Raman scattered light R1A and the laser beam B2. The first detector 21 detects first interference light between the Raman scattered light R1A and the laser beam B2. The beam splitter 26 causes interference between the Raman scattered light R1B and the laser beam B3. The second detector 22 detects second interference light between the Raman scattered light R1B and the laser beam B3. As described above, the Raman scattered light R1A due to para-hydrogen and the Raman scattered light R1B due to ortho-hydrogen can be detected.

[0068] In order for the first detector 21 to detect the Raman scattered light R1A, the absolute value of the difference between the frequency ω1 and ω2 and the frequency ΔωRA of the Raman shift due to parahydrogen, |(ω1-ω2)-ΔωRA|, is preferably 30 GHz or less, more preferably 10 GHz or less, and even more preferably 5 GHz or less or 1 GHz or less. From the viewpoint of heterodyne detection, |(ω1-ω2)-ΔωRA| is preferably greater than 0 Hz, more preferably 50 MHz or more, and even more preferably 100 MHz or more.

[0069] Similarly, in order for the second detector 22 to detect the Raman scattered light R1B, the absolute value of the difference between the frequency ω1 and ω3 and the frequency ΔωR of the Raman shift due to ortho-hydrogen, |(ω1-ω3)-ΔωR|, is preferably 30 GHz or less, more preferably 10 GHz or less, and even more preferably 5 GHz or less or 1 GHz or less. From the viewpoint of heterodyne detection, |(ω1-ω3)-ΔωR| is preferably greater than 0 Hz, more preferably 50 MHz or more, and even more preferably 100 MHz or more.

[0070] In the third embodiment, an example of Raman scattered light R1A and R2A caused by parahydrogen and orthohydrogen has been described, but the Raman scattered light R1A and R1B may be any Raman scattered light having different Raman shift frequencies.

[0071] As in measurement method example 1 of the second embodiment, the signal processing circuit 30 may calibrate the Raman scattered light R1A using a first interference light between light B6 and laser light B2 and a third interference light between light B7 and laser light B2. Furthermore, the signal processing circuit 30 may calibrate the Raman scattered light R1B using a second interference light between light B6 and laser light B3 and a fourth interference light between light B7 and laser light B3.

[0072] Fourth Embodiment The fourth embodiment is an example of a measurement device that uses a calibration laser beam for each of two Raman scattered lights and performs measurement in the same manner as in measurement method example 2 of the second embodiment.

[0073] Fig. 11 is a block diagram of a measurement device according to the fourth embodiment. As shown in Fig. 11, a measurement device 103 according to the fourth embodiment includes a fifth laser light source 15 and a beam splitter 28B in addition to the components of the measurement device 102 according to the third embodiment.

[0074] The fifth laser light source 15 emits laser light B5 (i.e., fifth laser light). The peak frequency of laser light B5 is frequency ω5 (i.e., fifth frequency), which is different from frequencies ω1, ω2, ω3, and ω4. The mirror 27A reflects the laser light B5. The beam splitter 28B aligns the optical axes of laser light B4 and B5 and irradiates them onto the beam splitter 28A. The beam splitter 28A aligns the optical axes of laser light B1, B4, and B5 and irradiates them onto the object 16. When the laser light B5 irradiates the object 16, light B8 (i.e., third light) is generated at the object 16. The beam splitter 26 causes light B6 and laser light B3 to interfere with each other, and also causes light B8 and laser light B3 to interfere with each other. The second detector 22 detects second interference light between light B6 and laser light B3 and fifth interference light between light B8 and laser light B3. The other configurations are the same as those of the third embodiment, and therefore the description thereof will be omitted.

[0075] 12 is a diagram showing signal intensity versus frequency in the fourth embodiment. As shown in FIG. 12, when laser beam B1 is irradiated onto object 16, Raman scattered light R1A and R1B are generated. The difference between the frequency ω1 of peak B1P of laser beam B1 and the frequency ωR1A of peak R1AP of Raman scattered light R1A is the frequency ΔωRA of the Raman shift of para-hydrogen. The difference between the frequency ω1 and the frequency ωR1B of peak R1BP of Raman scattered light R1B is the frequency ΔωRB of the Raman shift of ortho-hydrogen.

[0076] The frequency ω2 of peak B2P of laser beam B2 and the frequency ω4 of peak B4P of laser beam B4 are located closer to frequency ωR1A than to frequency ωR1B. The frequency ω3 of peak B3P of laser beam B3 and the frequency ω5 of peak B5P of laser beam B5 are located closer to frequency ωR1B than to frequency ωR1A. The frequency of peak R4AP of Raman scattered light R4A generated when laser beam B4 is irradiated onto object 16 is ωR4A. The frequency of peak R5AP of Raman scattered light R5A generated when laser beam B5 is irradiated onto object 16 is ωR5A.

[0077] The signal processing circuit 30 calibrates the signal intensity of the interference light between the Raman scattered light R1A and the laser light B2 using the signal intensity of the interference light between the return light of the laser light B4 and the laser light B2. Similarly, the signal processing circuit 30 calibrates the signal intensity of the interference light between the Raman scattered light R1B and the laser light B3 using the signal intensity of the interference light between the return light of the laser light B5 and the laser light B3. This makes it possible to measure the concentrations of, for example, para-hydrogen and ortho-hydrogen with high accuracy.

[0078] The preferred ranges of the frequencies ω2 and ω4 are the same as those in Measurement Method Example 2 of the second embodiment. The Raman shift frequency ΔωR in the second embodiment corresponds to either the Raman shift frequency ΔωRA or ΔωRB in the fourth embodiment.

[0079] Similarly, the absolute value |ω5-ω3| of the difference between the frequency ω5 of laser beam B5 and the frequency ω3 of laser beam B3 is preferably located within the band D2 of the second detector 22. That is, |ω5-ω3| is set to be equal to or smaller than band D2. |ω5-ω3| is preferably equal to or smaller than 0.8 times D2. The absolute value |ω5-ωR1B| of the difference between the frequencies ω5 and ωR1B is preferably equal to or larger than the linewidth of the Raman scattered light R1B. |ω5-ωR1B| is preferably equal to or larger than 1.5 times the linewidth, and more preferably equal to or larger than 2 times the linewidth.

[0080] Fifth Embodiment The fifth embodiment is an example in which a balanced detector 104 is used as the detector. FIG. 13 is a block diagram of the vicinity of the detector 104 of a measurement device according to the fifth embodiment. The measurement device using the detector 104 may be any of the measurement devices 100 to 103 described above, or any of the measurement devices 105 to 110 described below. As shown in FIG. 13, the first detector 21 includes a light-receiving element 21A (i.e., a first light-receiving element), a light-receiving element 21B (i.e., a second light-receiving element), and a circuit 21C. The beam splitter 25 has a reflecting surface 25A that reflects the laser beam B1 and a transmitting surface 25B that transmits the laser beam B2. The beam splitter 25 partially reflects the laser beam B2 and partially transmits the light B6, which is then irradiated onto the light-receiving element 21A. The beam splitter 25 partially transmits the laser beam B2 and partially reflects the light B6, which is then irradiated onto the light-receiving element 21B.

[0081] The light receiving element 21A is arranged on the reflective surface 25A side of the beam splitter 25. The light receiving element 21B is arranged on the transmissive surface 25B side of the beam splitter 25. The reflection to transmission ratio of the beam splitter 25 is 1:1. The circuit 21C calculates the difference between the detection signal S21A of the first interference light at the light receiving element 21A and the detection signal S21B of the first interference light at the light receiving element 21B, and outputs the calculation result as a signal S21C.

[0082] 14A and 14B are diagrams showing detection signals with respect to time in the fifth embodiment. Fig. 14A shows detection signals S21A and S21B, and Fig. 14B shows signal S21C. When photodiodes are used as the light receiving elements 21A and 21B, the detection signals correspond to currents.

[0083] 14A, the signal obtained is the sum of a component A1 whose signal intensity is constant over time and a beat signal whose amplitude is A2. The phases of the detection signals S21A and S21B are inverted because the phases of the light transmitted through the beam splitter 25 and the light reflected from it are inverted.

[0084] 14B, since the component A1 is subtracted from the signal S21C, a beat signal with an amplitude of 2×A2 remains, which can suppress the influence of fluctuations in the component A1 due to, for example, fluctuations in the intensity of the laser beam B2.

[0085] In any of the first to fourth embodiments, the first detector 21 may be a balanced detector, and the second detector 22 may be a balanced detector.

[0086] Sixth Embodiment The sixth embodiment is an example in which a measurement device is used to measure the distance to an object 16. Fig. 15A is a block diagram showing the measurement device according to the sixth embodiment. As shown in Fig. 15A, the measurement device 105 according to the sixth embodiment includes a control circuit 32 in addition to the measurement device 100 of the first embodiment.

[0087] The control circuit 32 applies a modulation signal to the first laser light source 11, which temporally modulates the frequency ω1 of the laser light B1. The signal processing circuit 30 calculates the distance to the object 16 based on the detection signal due to the first interference light in the first detector 21 and the modulation signal. This makes it possible to measure the distance to the source of the Raman scattered light. Note that since the object 16 may be spatially distributed in the traveling direction of the laser light, the calculated distance is only an estimate of the position of the object 16.

[0088] A delay signal that takes into account the distance to the object 16 may be further applied to at least one of the first laser light source 11 and the second laser light source 12. The delay signal may be included in the modulation signal from the control circuit 32. Alternatively, a delay generator may be disposed between the control circuit 32 and the first laser light source 11 or between the control circuit 32 and the second laser light source 12 to apply the delay signal.

[0089] The distance may be measured using a time-of-flight (TOF) method in which the laser beam B1 is pulsed light. Alternatively, a frequency-modulated continuous wave (FMCW) method in which the wavelength of the laser beam B1 is swept may be used. While the first embodiment is described as including the control circuit 32, the control circuit 32 may be included in any of the second to fifth embodiments.

[0090] The method for calculating the distance to the target object 16 is not limited to the above method, and may be the method described below. The method described below can be considered a type of FMCW method. As shown in FIG. 15A , the control circuit 32 may apply a modulation signal that temporally modulates the laser light B2 not only to the first laser light source 11 but also to the second laser light source 12. This allows predetermined modulation to be applied to both the laser light B1 and the laser light B2. Equation 4 is an example showing the changes over time t in the peak frequency ω1 of the laser light B1 and the peak frequency ω2 of the laser light B2. In Equation 2, ω1(t) and ω2(t) represent the time changes of the peak frequencies ω1 and ω2 for one period, respectively.

[0091]

[0092] Equation 4 indicates that the peak frequencies ω1(t) and ω2(t) are modulated by a sawtooth wave. The peak frequencies ω1(t) and ω2(t) are synchronized. In ω1(t) and ω2(t), ω10 is the frequency when time is t = 0, and b represents the rate of change over time (i.e., the slope). ΔωR in Equation 4 is the frequency of the Raman shift, and X is any frequency other than 0 GHz.

[0093] FIG. 15B is a schematic diagram showing how the peak frequencies ω1(t) and ω2(t) change over time. FIG. 15C is a schematic diagram showing how the peak frequency ωR1(t) of the Raman scattered light R1 and the peak frequency ω2(t) of the laser light B2 change over time. The time T shown in FIGS. 15B and 15C represents a period. In FIG. 15B, a modulation signal is applied in synchronization so that the difference between the peak frequency ω2(t) and the peak frequency ω1(t) at any time t is (ΔωR+X) Hz. Note that X corresponds to ω2-ωR1 in FIG. 6A and is a frequency that can be set arbitrarily other than 0 GHz, and may be, for example, between 1 GHz and 30 GHz.

[0094] By measuring how much the beat signal deviates from X, the distance from the measurement device 105 to the object 16 can be calculated. An example of a calculation method will be described with reference to FIG. 15C . For example, when the distance to the object 16 is L, the Raman scattered light R1 includes a delay of Δt = 2L / c. As a result, the frequency of the beat signal detected by heterodyne detection of the first detector 21, i.e., the difference frequency (ωR1(t) - ω2(t)) between the Raman scattered light R1 and the laser light B2, is shifted by b × Δt from X GHz, and (ωR1(t) - ω2(t)) is detected. Since X and b are known, Δt can be determined. Furthermore, the distance L to the object can be estimated from Δt and the speed of light c.

[0095] As shown in Figures 15B and 15C, the control circuit 32 applies modulation signals that temporally modulate the laser beams B1 and B2, and synchronizes them so that the difference between the frequency ω1 of the laser beam B1 and the frequency ω2 of the laser beam B2 is constant.

[0096] Seventh Embodiment The seventh embodiment is an example in which a wavelength-selecting filter is provided in the optical path of the laser light B1. FIG. 16 is a block diagram showing a measurement device according to the seventh embodiment. As shown in FIG. 16, a measurement device 106 according to the seventh embodiment includes, in addition to the measurement device 100 of the first embodiment, a wavelength-selecting filter 33 in the optical path of the laser light B1. The wavelength-selecting filter 33 may be, for example, a filter coated with a dielectric multilayer film or a diffraction grating using Bragg reflection. The other configurations are the same as those of the first embodiment, and therefore description thereof will be omitted.

[0097] 17 is a diagram showing the signal intensity of laser light versus wavelength and the transmittance of a wavelength-selective filter versus wavelength. As shown in FIG. 17, amplified spontaneous emission light B1E is also generated, overlapping the optical path of laser light B1. When the amplified spontaneous emission light B1E is irradiated onto the target object 16 or another object, Rayleigh scattered light and Raman scattered light are generated by the amplified spontaneous emission light B1E.

[0098] Therefore, the transmittance of the wavelength selection filter 33 is set so that it is high at the wavelength λ1 corresponding to the frequency ω1 of the relaxation light B1D and low at the wavelength λR1 corresponding to the frequency ωR1 of the Raman scattered light R1. This reduces the component of the amplified spontaneous emission light B1E near the frequency ωR1 of the Raman scattered light R1. Therefore, it is possible to reduce the influence of the Rayleigh scattered light due to the amplified spontaneous emission light B1E on the detection of the Raman scattered light R1.

[0099] Although an example in which the wavelength-selective filter 33 is provided has been described in the first embodiment, a wavelength-selective filter may also be provided in any of the second to seventh embodiments. A filter that selectively transmits light of wavelengths corresponding to frequencies ω1 to ω5 corresponding to the peak frequencies of the laser beams may be provided in the optical path of at least one of the laser beams B1 to B5. From the viewpoint of suppressing Rayleigh scattering and the like caused by the object 16 and its surroundings, it is preferable to provide the wavelength-selective filter 33 in the optical path of at least one of the laser beams B1, B4, and B5 to the object 16. Furthermore, it is preferable that the transmittance of the wavelength corresponding to the frequency ωR1 of the Raman scattered light R1 in the wavelength-selective filter 33 be lower than the transmittance of wavelengths corresponding to frequencies ω1, ω4, and ω5 corresponding to the peak frequencies of the laser beams.

[0100] Eighth Embodiment The eighth embodiment is an example in which laser beams B1 and B2 are obtained from the same master laser light source. Fig. 18 is a block diagram showing a measurement device according to the eighth embodiment. As shown in Fig. 18, a measurement device 107 according to the eighth embodiment includes a master laser light source 34, an acousto-optic element 35, and a mirror 27A in addition to the measurement device 100 of the first embodiment. The measurement device 107 does not include the first laser light source 11 and the second laser light source 12 individually.

[0101] The master laser light source 34 serves as both the first laser light source 11 and the second laser light source 12. The acousto-optic element 35 is disposed in the optical path of the master laser light BM emitted by the master laser light source 34. The acousto-optic element 35 is an AOM (Acousto-Optic Modulator). Light that passes through the acousto-optic element 35 becomes laser light B1. Light that is diffracted by the acousto-optic element 35 becomes laser light B2. The mirror 27A reflects the laser light B2, and irradiates the reflected laser light B2 onto the beam splitter 25. The other configurations are the same as those in the first embodiment, and therefore description thereof will be omitted.

[0102] When a modulated electrical signal SM is applied to the acousto-optical element 35, a diffraction grating of acoustic waves excited by the modulated electrical signal SM is formed in the crystal within the acousto-optical element 35. As a result, the diffracted light receives or imparts energy from the acoustic waves, causing the frequency to shift. As a result, the frequency of the light passing through the acousto-optical element 35 is the same as that of the master laser light BM, and the frequency of the light diffracted by the acousto-optical element 35 shifts from that of the master laser light BM in accordance with the modulated electrical signal SM.

[0103] Therefore, the diffracted light of the master laser beam BM whose frequency has been shifted by the acousto-optic element 35 is set as one of the laser beams B1 and B2. The transmitted light of the master laser beam BM whose frequency has not been shifted by the acousto-optic element 35 is set as the other of the laser beams B1 and B2. This makes it possible to reduce fluctuations in the difference in frequency between the laser beams B1 and B2.

[0104] Ninth Embodiment The ninth embodiment is an example in which a transfer resonator is used to apply feedback of the frequency of laser light to a laser light source. Fig. 19 is a block diagram showing a measurement device according to the ninth embodiment. As shown in Fig. 19, a measurement device 108 according to the ninth embodiment includes a transfer resonator 36, a light receiving element 37A, a feedback circuit 37B, mirrors 27A and 27B, and beam splitters 28A to 28C in addition to the components of the measurement device 100 of the first embodiment.

[0105] Beam splitter 28A splits laser beam B1 and irradiates beam splitter 28C with split laser beam B1A. Beam splitter 28B splits laser beam B2 and irradiates mirror 27B with split laser beam B2A. Mirror 27B reflects laser beam B2A and irradiates beam splitter 28C with the reflected laser beam B2A. Mirror 27A reflects laser beam B2 and irradiates beam splitter 25 with the reflected laser beam B2. Beam splitter 28C aligns the optical axes of laser beams B1A and B2A and irradiates transfer resonator 36 with laser beams B1A and B2A.

[0106] The transfer resonator 36 resonates the laser beams B1A and B2A. The transfer resonator 36 has transmittance peaks at specific frequency intervals. The frequencies at which the transmittance peaks are set as target frequencies for the laser beams B1 and B2. The light-receiving element 37A detects the light transmitted through the transfer resonator 36. The feedback circuit 37B controls the first laser light source 11 and the second laser light source 12 to adjust the frequency ω1 of the laser beam B1 and the frequency ω2 of the laser beam B2 so that the output signal of the light-receiving element 37A is maximized. For example, the frequencies of the laser beams B1 and B2 can be adjusted by adjusting the temperatures of the first laser light source 11 and the second laser light source 12. In this way, the feedback circuit 37B adjusts at least one of the frequency ω1 of the laser beam B1 and the frequency ω2 of the laser beam B2 based on the output light from the transfer resonator 36. This allows the frequencies ω1 and ω2 to be controlled with a simple configuration. In particular, fluctuations in the difference between ω1 and ω2 can be suppressed. The intensities of the laser beams B1A and B2A are preferably smaller than the intensities of the laser beams B1 and B2 passing through the beam splitters 28A and 28B. This ensures the power of the laser beam B1 irradiating the object 16. It also ensures the power of the laser beam B2, which is the local oscillator light for heterodyne measurement.

[0107] Tenth Embodiment The tenth embodiment is an example that uses an acousto-optic element and a transfer resonator. Fig. 20 is a block diagram showing a measurement device according to the tenth embodiment. As shown in Fig. 20, a measurement device 109 according to the tenth embodiment includes a master laser light source 34, an acousto-optic element 35, a mirror 27C, and a beam splitter 28D in addition to the measurement device 108 of the ninth embodiment. The measurement device 109 does not include the first laser light source 11 and the fourth laser light source 14 individually.

[0108] The master laser light source 34 serves as both the first laser light source 11 and the fourth laser light source 14. The acousto-optic element 35 is disposed in the optical path of the master laser light BM emitted by the master laser light source 34. The beam splitter 28A splits the master laser light BM and irradiates the split laser light BMA onto the beam splitter 28C. The diffracted light, in which the frequency of the master laser light BM has been shifted by the acousto-optic element 35, is designated as one of the laser lights B1 and B4. The transmitted light, in which the frequency of the master laser light BM has not been shifted by the acousto-optic element 35, is designated as the other of the laser lights B1 and B4. The mirror 27C reflects the laser light B4 and irradiates the reflected laser light B4 onto the beam splitter 28D. The beam splitter 28D aligns the optical axes of the laser lights B1 and B4 and irradiates them onto the target 16.

[0109] The beam splitter 28C aligns the optical axes of the laser beams BMA and B2A, and irradiates the laser beams BMA and B2A onto the transfer resonator 36. The frequency at which the transmittance in the transfer resonator 36 reaches its peak is set as the target frequency of the master laser beam BM and the laser beam B2. The feedback circuit 37B controls the master laser light source 34 and the second laser light source 12 to maximize the output signal of the light receiving element 37A, thereby controlling at least one of the frequency ωM of the master laser beam BM and the frequency ω2 of the laser beam B2. The other configurations are the same as those in the ninth embodiment, and therefore description thereof will be omitted.

[0110] The acousto-optic element 35 can precisely control the frequency difference between the diffracted light and the transmitted light when the frequency difference between the diffracted light and the transmitted light is small. For example, the acousto-optic element 35 can easily achieve a frequency difference of less than 10 GHz. The transfer resonator 36 can relatively increase the interval between transmittance peaks. For example, the transfer resonator 36 can easily achieve a frequency difference of 10 GHz or more. In measurement method 1 of the second embodiment, as shown in FIG. 7 , the frequency difference ω1-ω2 between the laser beams B1 and B2 is larger than the frequency difference ω1-ω4 between the laser beams B1 and B4. In this case, the master laser light source 34 serves as both the first laser light source 11 and the fourth laser light source 14. The acousto-optic element 35 is used to generate the laser beams B1 and B4, which have a small frequency difference ω1-ω4, and the transfer resonator 36 is used to control the frequencies of the laser beams B1 and B2, which have a large frequency difference ω1-ω2. This allows for accurate control of the frequency difference between the laser beams B1 and B2 and the frequency difference between the laser beams B1 and B4.

[0111] Eleventh Embodiment The eleventh embodiment is an example that uses an acousto-optic element and a transfer resonator. Fig. 21 is a block diagram showing a measurement device according to the eleventh embodiment. As shown in Fig. 21, the measurement device 110 according to the eleventh embodiment includes a master laser light source 34, an acousto-optic element 35, a mirror 27C, and a beam splitter 28D in addition to the measurement device 108 of the ninth embodiment. The measurement device 110 does not include the second laser light source 12 and the fourth laser light source 14 individually.

[0112] The master laser light source 34 serves as both the second laser light source 12 and the fourth laser light source 14. The acousto-optic element 35 is disposed in the optical path of the master laser light BM emitted by the master laser light source 34. The beam splitter 28B branches the master laser light BM and irradiates the branched laser light BMA onto the mirror 27B. The diffracted light, in which the frequency of the master laser light BM has been shifted by the acousto-optic element 35, is designated as one of the laser lights B2 and B4. The transmitted light, in which the frequency of the master laser light BM has not been shifted by the acousto-optic element 35, is designated as the other of the laser lights B2 and B4. The mirror 27C reflects the laser light B4, and irradiates the reflected laser light B4 onto the beam splitter 28D. The beam splitter 28D aligns the optical axes of the laser lights B1 and B4 and irradiates them onto the telescope 24.

[0113] The beam splitter 28C aligns the optical axes of the laser beams BMA and B1A, and irradiates the laser beams BMA and B1A onto the transfer resonator 36. The frequency at which the transmittance of the transfer resonator 36 reaches its peak is set as the target frequency of the master laser beam BM and the laser beam B1. The feedback circuit 37B controls the master laser light source 34 and the first laser light source 11 to maximize the output signal of the light receiving element 37A, thereby controlling at least one of the frequency ωM of the master laser beam BM and the frequency ω1 of the laser beam B1. This makes it possible to control the frequencies ωM and ω1 with a simple configuration. The other configurations are the same as those of the ninth embodiment, and therefore description thereof will be omitted.

[0114] In measurement method 2 of the second embodiment, as shown in FIG. 8A , the frequency difference ω1-ω2 between laser beams B1 and B2 is larger than the frequency difference ω2-ω4 between laser beams B2 and B4. In this case, master laser light source 34 serves as both second laser light source 12 and fourth laser light source 14. An acousto-optic element 35 is used to generate laser beams B2 and B4, which have a small frequency difference ω2-ω4, and a transfer resonator 36 is used to control the frequencies of laser beams B1 and B2, which have a large frequency difference ω1-ω2. This makes it possible to accurately control the frequency difference between laser beams B1 and B2 and the frequency difference between laser beams B2 and B4.

[0115] As in the tenth and eleventh embodiments, the master laser light source 34 may serve as both the fourth laser light source 14 and either one of the first laser light source 11 and the second laser light source 12. In this case, the laser light B4 is diffracted light in which the frequency of the master laser light BM has been shifted by the acousto-optic element 35. Either one of the laser lights B1 and B2 is transmitted light in which the frequency of the master laser light BM has not been shifted by the acousto-optic element 35. This allows the peak frequency ω4 of the laser light B4 to be lower than the peak frequency ω1 of the laser light B1 or the peak frequency ω2 of the laser light B2. Furthermore, ω1-ω4 or ω2-ω4 can be controlled with high precision.

[0116] The intensity of Raman scattered light is proportional to the fourth power of the wavelength of the excitation light. Therefore, a shorter wavelength of laser light B1 is preferable. Furthermore, Japanese Industrial Standard JIS C6802 and International Electrotechnical Commission standard IEC60825-1 have high allowable power limits for laser light with wavelengths of 400 nm or less or 1400 nm or more. Therefore, the wavelengths of laser light B1 to B5 are preferably 400 nm or less, with a margin of error of 380 nm being more preferable.

[0117] The first laser light source 11 to the fifth laser light source 15 may be semiconductor laser elements. The first laser light source 11 to the fifth laser light source 15 preferably include semiconductor laser elements such as distributed feedback laser elements or distributed Bragg reflector laser elements capable of emitting laser light with a narrow spectral linewidth. The material of the semiconductor laser elements may be a nitride semiconductor.

[0118] The linewidth (half width at half maximum) of the distributed feedback laser element or the distributed Bragg reflector laser element may be, for example, 100 kHz to 50 MHz, or 1 MHz to 10 MHz. Alternatively, the first laser light source 11 to the fifth laser light source 15 may be external cavity laser devices including semiconductor laser elements.

[0119] The first to fifth laser light sources 11 to 15 and the master laser light source 34 may each include a semiconductor laser element and one or more collimating lenses arranged on the optical axis of the laser light. The collimating lenses may collimate at least one of the fast axis and the slow axis of the laser light. The laser light B1, B4, and BM emitted from the first laser light source 11, the fourth laser light source 14, and the master laser light source 34 can be efficiently incident on the telescope 24. The laser light B2 emitted from the second laser light source 12 can be efficiently incident on the first detector. The laser light B3 emitted from the third laser light source 13 can be efficiently incident on the second detector 22. Note that in this specification, the term "collimated" refers to a deviation within a range of ±5° from strict parallelism.

[0120] The first detector 21 and the second detector 22 may detect the interference light using photodiodes. The first detector 21 and the second detector 22 may include a two-dimensional detector having a plurality of pixels, and the distribution of the object 16 may be visualized from the intensity of the interference light of each pixel. The first detector 21 and the second detector 22 may have an imaging system for the object 16, and may form an image of the first interference light on the two-dimensional detector.

[0121] When the distribution of the object 16 is visualized, the position of the object 16 to be detected may be changed by scanning the laser light B1 with a galvanometer mirror.

[0122] Although the embodiments of the present disclosure have been described using hydrogen molecules as an example, the present disclosure is not limited to this. The object 16 may be something other than hydrogen molecules. The object 16 may be, for example, oxygen molecules (O ２ ), nitrogen molecule (N ２ ), nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO ２ ), sulfur (S), sulfur oxides (SOx), carbon disulfide (CS ２ ), benzene, nitrobenzene, toluene, other benzene derivatives, liquid oxygen, liquid nitrogen, liquid helium, methane, ammonia, or the like.

[0123] The measuring devices 100-103 and 105-110 may be located, for example, 10 m to 100 m away from the measurement position of the object 16. The measuring devices 100-103 and 105-110 may be located 100 m or more away from the measurement position of the object 16 as long as they are capable of detecting Raman scattered light. In this way, the measuring devices 100-103 and 105-110 are capable of remote measurement. The measuring devices 100-103 and 105-110 may be mounted on a mobile object such as a drone, and remote measurement may be performed by controlling the position of the mobile object. Furthermore, at least one of the measuring devices 100-103 and 105-110 may be remotely controlled from outside the space where it is located to measure the object 16 in an unmanned environment. This enables safer measurement. Measured data is transmitted to an observer in a remote location via wireless communication or the like. For example, unmanned measurement becomes possible by pointing a telescope in advance at a position where the target substance 16 is likely to be generated or leak, or at a position where the concentration of the target substance 16 should be measured. Measurement may be performed periodically at predetermined time intervals, or continuously for a predetermined period of time.

[0124] The above describes in detail preferred embodiments, but the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

[0125] The present disclosure includes the following configuration: (Item 1) A measurement device comprising: a first laser light source that emits a first laser light having a peak frequency of a first frequency; a second laser light source that emits a second laser light having a peak frequency of a second frequency different from the first frequency; a first beam splitter that causes interference between the second laser light and first light including first Raman scattered light generated in an object when the first laser light is irradiated onto the object; a first detector that detects first interference light between the first light and the second laser light; and a first telescope that is provided in an optical path of the first light and directed toward the object. (Item 2) The measurement device according to Item 1, wherein an absolute value of a difference between the first frequency and the second frequency and a frequency of Raman shift caused by the object is greater than 0 Hz and less than or equal to 30 GHz. (Item 3) The measurement device according to item 1, comprising: a third laser light source that emits third laser light having a peak frequency at a third frequency different from the first frequency and the second frequency, a second beam splitter that causes interference between the first light and the third laser light, and a second detector that detects second interference light between the first light and the third laser light, wherein the target is hydrogen gas, the first light includes Raman scattered light caused by the hydrogen gas, an absolute value of a difference between the first frequency and the second frequency and a frequency of Raman shift caused by para-hydrogen is greater than 0 Hz and not more than 30 GHz, and an absolute value of a difference between the first frequency and the third frequency and a frequency of Raman shift caused by ortho-hydrogen is greater than 0 Hz and not more than 30 GHz. (Item 4) The measurement device according to any one of items 1 to 3, further comprising: a second telescope that is provided toward the target, on an optical path of the first laser light until it reaches the target.(Item 5) The measurement device according to any one of Items 1 to 4, comprising: a fourth laser light source that emits fourth laser light having a peak frequency that is a fourth frequency different from the first frequency and the second frequency; an absolute value of a difference between the fourth frequency and the second frequency being outside the band of the first detector; an absolute value of the difference between the fourth frequency and the first frequency being equal to or greater than a linewidth of the first Raman scattered light; the first beam splitter causes interference between the first light and the second laser light, and causes interference between the second light and second light generated when the fourth laser light is irradiated onto the object; and the first detector detects the first interference light and third interference light between the second light and the second laser light. (Item 6) The measurement device according to Item 5, comprising: a master laser light source that serves as the fourth laser light source and either one of the first laser light source and the second laser light source; and an acousto-optic element arranged in an optical path of master laser light emitted by the master laser light source, wherein the fourth laser light is diffracted light of the master laser light, the frequency of which has been shifted by the acousto-optic element; and either the first laser light or the second laser light is transmitted light of the master laser light, the frequency of which has not been shifted by the acousto-optic element. (Item 7) The measurement device according to Item 5 or 6, comprising a switching unit that electrically or mechanically switches between a first period in which the first laser light is irradiated onto the object and the fourth laser light is not irradiated onto the object, and a second period in which the fourth laser light is irradiated onto the object and the first laser light is not irradiated onto the object. (Item 8) The measurement device according to any one of Items 1 to 4, comprising: a fourth laser light source that emits fourth laser light having a peak frequency that is a fourth frequency different from the first frequency and the second frequency; an absolute value of a difference between the fourth frequency and the second frequency is equal to or less than a band of the first detector; the first beam splitter causes interference between the first light and the second laser light, and causes interference between the second light generated when the fourth laser light is irradiated onto the object and the second laser light; and the first detector detects the first interference light and fourth interference light between the second light and the second laser light.(Item 9) A laser beam source comprising: a third laser light source that emits third laser light having a third frequency whose peak frequency is different from the first frequency and the second frequency; a fourth laser light source that emits fourth laser light having a fourth frequency whose peak frequency is different from the first frequency, the second frequency, and the third frequency; a fifth laser light source that emits fifth laser light having a fifth frequency whose peak frequency is different from the first frequency, the second frequency, the third frequency, and the fourth frequency; a second beam splitter that causes interference between the first light and the third laser light, and causes interference between the third light generated when the fifth laser light is irradiated onto the object and the third laser light; and a second detector that detects second interference light between the first light and the third laser light, and fifth interference light between the third light and the third laser light, 3. The measuring device according to item 1 or 2, wherein the first beam splitter causes the first light to interfere with the second laser light, and causes the second laser light to interfere with second light generated when the fourth laser light is irradiated onto the object; the first detector detects the first interference light and third interference light between the second light and the second laser light; an absolute value of a difference between the fourth frequency and the second frequency is equal to or less than a band of the first detector; and an absolute value of a difference between the fifth frequency and the third frequency is equal to or less than a band of the second detector. (Item 10) The measurement device described in any one of Items 1 to 9, wherein the first beam splitter has a reflective surface that reflects the second laser light and a transmissive surface that transmits the second laser light, and the first detector comprises: a first light-receiving element arranged on the reflective surface side of the first beam splitter; a second light-receiving element arranged on the transmissive surface side of the first beam splitter; and a circuit that calculates a difference between a detection signal of the first interference light at the first light-receiving element and a detection signal of the first interference light at the second light-receiving element.(Item 11) The measurement device according to any one of items 1 to 5, 8, and 9 not citing items 6 and 7, comprising: a master laser light source that serves as both the first laser light source and the second laser light source; and an acousto-optic element arranged in an optical path of master laser light emitted by the master laser light source, wherein one of the first laser light and the second laser light is diffracted light in which the frequency of the master laser light is shifted by the acousto-optic element, and the other of the first laser light and the second laser light is transmitted light in which the frequency of the master laser light is not shifted by the acousto-optic element. (Item 12) The measurement device according to any one of items 1 to 11, comprising: a transfer resonator that resonates the first laser light and the second laser light; and a feedback circuit that adjusts at least one of the first frequency and the second frequency based on output light from the transfer resonator. (Item 13) The measurement device according to any one of Items 1 to 12, comprising: a control circuit that applies a modulation signal that temporally modulates the first laser light; and a signal processing circuit that calculates a distance to the object based on a detection signal from the first interference light in the first detector and the modulation signal. (Item 14) The measurement device according to Item 13, wherein the control circuit applies the modulation signal that temporally modulates the second laser light, and the first and second laser lights are synchronized so that a difference in frequency between the first and second laser lights is constant. (Item 15) The measurement device according to any one of Items 1 to 14, wherein the wavelengths of the first and second laser lights are 400 nm or less. (Item 16) The measurement device according to any one of Items 1 to 15, comprising a wavelength selection filter provided in at least one of the optical path of the first laser light and the optical path of the second laser light. (Item 17) The measurement device according to any one of Items 1 to 16, wherein the first laser light source and the second laser light source include semiconductor laser elements, and the semiconductor laser elements are distributed feedback laser elements or distributed Bragg reflector laser elements.(Item 18) A measurement method including: irradiating an object with a first laser light having a peak frequency of a first frequency; irradiating a first beam splitter with a second laser light having a peak frequency of a second frequency different from the first frequency; causing interference between the first light, which includes first Raman scattered light generated in the object when the first laser light is irradiated onto the object and passes through a telescope directed towards the object, and the second laser light, by the first beam splitter; and detecting first interference light which is interference light between the first light and the second laser light by a first detector.

[0126] This application claims priority from basic patent application No. 2024-072912, filed with the Japan Patent Office on April 26, 2024, the entire contents of which are incorporated herein by reference.

[0127] 11: First laser light source 12: Second laser light source 13: Third laser light source 14: Fourth laser light source 15: Fifth laser light source 16: Object 21: First detector 21A, 21B, 37A: Light receiving element 21C: Circuit 22: Second detector 23, 24: Telescope 25, 26: Beam splitter 25A: Reflecting surface 25B: Transmitting surface 30: Signal processing circuit 31: Switching unit 32: Control circuit 33: Wavelength selection filter 34: Master laser light source 35: Acousto-optic element 36: Transfer resonator 37B: Feedback circuit 38: Reflector

Claims

1. A measurement device comprising: a first laser light source that emits a first laser light having a peak frequency of a first frequency; a second laser light source that emits a second laser light having a peak frequency of a second frequency different from the first frequency; a first beam splitter that causes interference between the second laser light and first light including first Raman scattered light generated in an object when the first laser light is irradiated onto the object; a first detector that detects first interference light between the first light and the second laser light; and a first telescope that is provided in the optical path of the first light and directed toward the object.

2. The measuring device according to claim 1, wherein the absolute value of the difference between the difference between the first frequency and the second frequency and the frequency of the Raman shift caused by the object is greater than 0 Hz and less than or equal to 30 GHz.

3. The measuring device according to claim 1, comprising: a third laser light source that emits third laser light having a peak frequency at a third frequency different from the first frequency and the second frequency; a second beam splitter that causes interference between the first light and the third laser light; and a second detector that detects second interference light between the first light and the third laser light, wherein the object is hydrogen gas, the first light includes Raman scattered light caused by the hydrogen gas, the absolute value of the difference between the first frequency and the second frequency and the frequency of the Raman shift caused by parahydrogen is greater than 0 Hz and not greater than 30 GHz, and the absolute value of the difference between the difference between the first frequency and the third frequency and the frequency of the Raman shift caused by orthohydrogen is greater than 0 Hz and not greater than 30 GHz.

4. A measurement device according to any one of claims 1 to 3, further comprising a second telescope arranged in the optical path of the first laser light leading to the object and directed toward the object.

5. A measuring device according to any one of claims 1 to 4, comprising a fourth laser light source that emits fourth laser light having a peak frequency that is a fourth frequency different from the first frequency and the second frequency, wherein the absolute value of the difference between the fourth frequency and the second frequency is outside the band of the first detector, wherein the absolute value of the difference between the fourth frequency and the first frequency is equal to or greater than the linewidth of the first Raman scattered light, wherein the first beam splitter causes interference between the first light and the second laser light, and causes interference between the second light and the second laser light generated when the fourth laser light is irradiated onto the object, and wherein the first detector detects the first interference light and third interference light between the second light and the second laser light.

6. The measurement device according to claim 5, comprising: a master laser light source that serves as both the fourth laser light source and one of the first and second laser light sources; and an acousto-optic element arranged in the optical path of the master laser light emitted by the master laser light source, wherein the fourth laser light is diffracted light in which the frequency of the master laser light has been shifted by the acousto-optic element, and either the first laser light or the second laser light is transmitted light in which the frequency of the master laser light has not been shifted by the acousto-optic element.

7. A measuring device as described in claim 5 or 6, comprising a switching unit that electrically or mechanically switches between a first period in which the first laser light is irradiated onto the object and the fourth laser light is not irradiated onto the object, and a second period in which the fourth laser light is irradiated onto the object and the first laser light is not irradiated onto the object.

8. A measuring device according to any one of claims 1 to 4, comprising a fourth laser light source that emits fourth laser light having a peak frequency that is a fourth frequency different from the first frequency and the second frequency, wherein the absolute value of the difference between the fourth frequency and the second frequency is equal to or less than the bandwidth of the first detector, wherein the first beam splitter causes interference between the first light and the second laser light, and causes interference between the second light generated when the fourth laser light is irradiated onto the object and the second laser light, and wherein the first detector detects the first interference light and fourth interference light between the second light and the second laser light.

9. A laser beam source comprising: a third laser light source that emits third laser light having a third frequency whose peak frequency is different from the first frequency and the second frequency; a fourth laser light source that emits fourth laser light having a fourth frequency whose peak frequency is different from the first frequency, the second frequency, and the third frequency; a fifth laser light source that emits fifth laser light having a fifth frequency whose peak frequency is different from the first frequency, the second frequency, the third frequency, and the fourth frequency; a second beam splitter that causes the first light to interfere with the third laser light and causes the third laser light to interfere with third light generated when the fifth laser light is irradiated onto the object; and a second detector that detects second interference light between the first light and the third laser light and fifth interference light between the third light and the third laser light, wherein the first beam splitter causes the first light to interfere with the second laser light and causes the second light generated when the fourth laser light is irradiated onto the object to interfere with the second laser light, 3. The measurement device according to claim 1, wherein the first detector detects the first interference light and a third interference light between the second light and the second laser light, an absolute value of a difference between the fourth frequency and the second frequency is equal to or less than a band of the first detector, and an absolute value of a difference between the fifth frequency and the third frequency is equal to or less than a band of the second detector.

10. A measurement device as described in any one of claims 1 to 9, wherein the first beam splitter has a reflective surface that reflects the second laser light and a transparent surface that transmits the second laser light, and the first detector comprises: a first light-receiving element arranged on the reflective surface side of the first beam splitter; a second light-receiving element arranged on the transparent surface side of the first beam splitter; and a circuit that calculates the difference between the detection signal of the first interference light at the first light-receiving element and the detection signal of the first interference light at the second light-receiving element.

11. A measurement device according to any one of claims 1 to 5, claim 8, claims 6 and 7, comprising: a master laser light source that serves as both the first laser light source and the second laser light source; and an acousto-optic element arranged in the optical path of master laser light emitted by the master laser light source, wherein one of the first laser light and the second laser light is diffracted light in which the frequency of the master laser light has been shifted by the acousto-optic element, and the other of the first laser light and the second laser light is transmitted light in which the frequency of the master laser light has not been shifted by the acousto-optic element.

12. A measurement device according to any one of claims 1 to 11, comprising: a transfer resonator that resonates the first laser light and the second laser light; and a feedback circuit that adjusts at least one of the first frequency and the second frequency based on the output light from the transfer resonator.

13. A measuring device as described in any one of claims 1 to 12, comprising: a control circuit that applies a modulation signal that temporally modulates the first laser light; and a signal processing circuit that calculates the distance to the object based on a detection signal from the first interference light in the first detector and the modulation signal.

14. The measurement device according to claim 13, wherein the control circuit applies the modulation signal that temporally modulates the second laser light, and synchronizes the first laser light and the second laser light so that the difference in frequency between the first laser light and the second laser light is constant.

15. A measuring device according to any one of claims 1 to 14, wherein the wavelengths of the first laser light and the second laser light are 400 nm or less.

16. A measurement device according to any one of claims 1 to 15, comprising a wavelength selection filter provided in at least one of the optical path of the first laser light and the optical path of the second laser light.

17. The measurement device according to any one of claims 1 to 16, wherein the first laser light source and the second laser light source include semiconductor laser elements, and the semiconductor laser elements are distributed feedback laser elements or distributed Bragg reflector laser elements.

18. A measurement method comprising: irradiating an object with a first laser light having a peak frequency of a first frequency; irradiating a first beam splitter with a second laser light having a peak frequency of a second frequency different from the first frequency; causing interference between the first light, which includes first Raman scattered light generated in the object when the first laser light is irradiated onto the object and passes through a telescope directed towards the object, and the second laser light, by the first beam splitter; and detecting first interference light, which is interference light between the first light and the second laser light, by a first detector.

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