OPTICAL MEASURING DEVICE

Abstract

An optical measuring device comprising: a wavelength scanning light source (1) that outputs scanned light having a wavelength that continuously changes with time; an optical irradiation system (3) that emits measurement output light caused by the scanned light from the wavelength scanning light source (1) toward a measurement object (8) into a space as measurement light, receives reflected light obtained by the measurement object (8) reflecting the measurement light, and outputs the reflected light as reflected measurement light; a circulating light path (4) that includes a loop part and outputs circulating reference light for each circulation, wherein reference output light caused by the scanned light from the wavelength scanning light source (1) circulates N (an integer equal to or greater than 0) times through the loop part;a measurement signal acquisition unit (5) that combines the reflected measurement light from the optical irradiation system (3) and the circulating reference light from the circulation light path (4), outputs an accurate measurement signal obtained by photoelectrically converting the combined interference light, and outputs a plurality of coarse measurement signals including electrical signals obtained using a plurality of circulation number measurement light beams having different refractive index dependencies with respect to a light path based on the tuned light;and a signal processing unit (7) that identifies the number of revolutions in the circulation light path (4) of the circulating reference light, wherein a light path length difference between the reflected measurement light and the circulating reference light is obtained by the precise measurement signal from the measurement signal acquisition unit (5), and a light path length difference between the reflected measurement light and the circulating reference light is obtained by the plurality of coarse measurement signals from the measurement signal acquisition unit (5);

Description

TECHNICAL FIELDThe present disclosure relates to an optical measurement device.BACKGROUND TO THE PRIOR ARTAs a method for measuring a distance from a light source to an object using light emitted from the light source, a method such as a pulse propagation method, a triangulation method, a confocal method, a white interference method, or a wavelength sampling interference method is known. Patent Literature 1 discloses a measurement device capable of expanding a measurement range without being limited by coherence specific to a light source, using, among other things, a wavelength-sensing interference method.The measurement apparatus disclosed in Patent Literature 1 includes a circulation reference optical system including a 2×2 optical fiber coupler, an optical fiber, and a light path retarder / selector disposed between a 1×2 fiber direction coupler (coupler) and a 1×2 interference optical fiber coupler, and includes a measurement light path in which a measurement light output reaches a beam splitter of the coupler and measurement light reflected from a measurement object reaches the 1×2 interference optical fiber coupler of the beam splitter, and a reference light path in which a reference light output reaches the circulation reference optical system of the coupler and circulating reference light reaches the 1×2 interference optical fiber coupler.REFERENCE LISTPATENT LITERATUREPatent Literature 1: JP 2001-41706 ASUMMARY OF THE INVENTIONTECHNICAL PROBLEMIn the measurement device disclosed in Patent Literature 1, the refractive index of an optical fiber is temperature-dependent. Therefore, the speed of light in the optical fiber fluctuates by several tens μm / min depending on the ambient temperature, and the optical path lengths of the measurement optical path and the reference optical path, in particular, the optical path length of the reference optical path for each round trip are likely to fluctuate because the reference optical path includes the circulation reference optical system, and the measurement accuracy is likely to be affected by a temperature change.The present disclosure has been made in view of the above points, and an object of the present disclosure is to provide an interference type wavelength scanning optical measurement device that can extend a measurement range, suppress the resistance to a temperature change, i.e., hardly affected by a change in the ambient temperature, and perform highly accurate measurement.SOLUTION OF PROBLEMAn optical measurement apparatus according to the present disclosure includes: a wavelength scanning light source that outputs tunable light having a wavelength that continuously changes with time; an irradiation optical system that outputs measurement output light caused by the tunable light from the wavelength scanning light source toward a measurement object into a space as measurement light, receives reflected light obtained by the measurement object that reflects the measurement light, and outputs the reflected light as reflected measurement light; a circulation light path that includes a loop part and outputs circulating reference light for each circulation, wherein reference output light caused by the tunable light from the wavelength scanning light source circulates N (an integer equal to or greater than 0) times through the loop part; a measurement signal acquisition unit that combines the reflected measurement light from the irradiation optical system and the circulating reference light from the circulation light path, outputs an accurate measurement signal obtained by photoelectrically converting the combined interference light, and outputs a plurality of coarse measurement signals including electric signals obtained using a plurality of circulation number measurement light beams having different refractive index dependencies with respect to a light path based on the adjusted light; and a signal processing unit that identifies the number of revolutions in the circulating light path of the circulating reference light, wherein a light path length difference between the reflected measurement light and the circulating reference light is obtained by the accurate measurement signal from the measurement signal acquiring unit, and a light path length difference between the reflected measurement light and the circulating reference light is obtained by the plurality of rough measurement signals from the measurement signal acquiring unit.ADVANTAGEOUS EFFECTS OF THE INVENTIONAccording to the present disclosure, even when a light source having low coherence and a narrow measurement range is used, the distance to a measurement object can be measured with high accuracy while the measurement range is widened and the measurement is hardly affected by a change in the ambient temperature.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a configuration diagram showing an optical measurement device according to a first embodiment. FIG. 2 is a schematic diagram of a spectrum caused by reflected measurement light and a spectrum caused by reflected light of the end surface. FIG. 3 is a schematic diagram illustrating a light path length caused by reflected measurement light, a light path length caused by circulating reference light in a first wavelength range, and a light path length caused by circulating reference light in a second wavelength range in the optical measurement device according to the first embodiment. FIG. 4 is a schematic diagram showing a spectrum caused by the circulating reference light in the first wavelength range and a spectrum caused by the circulating reference light in the second wavelength range in the optical measurement apparatus according to the first embodiment. FIG. 5 is a schematic diagram showing the light intensity of a part of the tunable light before the tunable light passes through an optical filter in the optical measurement device according to the first embodiment. FIG. 6 is a schematic diagram showing a light intensity obtained by dividing a part of the tunable light after the tunable light passes through the optical filter in the optical measurement device according to the first embodiment into k parts. FIG. 7 is a schematic diagram of a spectrum obtained by accurate measurement in the optical measurement device according to the first embodiment. FIG. 8 is a schematic diagram of a spectrum obtained by rough measurement in the optical measurement device according to the first embodiment. FIG. 9 is a schematic diagram illustrating that the slope of a frequency of the circulating reference light changes with time due to the wavelength dispersion for each round trip of a loop in a loop part in the optical measurement device according to the first embodiment. FIG. 10 is a schematic diagram illustrating that the inclination of a frequency of the reflected measurement light changes with time due to the wavelength dispersion at each round trip of a loop in a loop part, another example, in the optical measurement device according to the first embodiment. FIG. 11 is a configuration diagram showing an optical measurement device according to a second embodiment. FIG. 12 is a configuration diagram showing a coarse measurement signal acquisition unit in the optical measurement device according to the second embodiment. FIG. 13 is a schematic diagram illustrating a light path length caused by reflected measurement light, a light path length caused by circulating reference light of the P-wave, and a light path length caused by circulating reference light of the S-wave, in the optical measurement device according to the second embodiment. FIG. 14 is a schematic diagram showing a spectrum caused by the circulating reference light of the P-wave and a spectrum caused by the circulating reference light of the S-wave in the optical measurement device according to the second embodiment. FIG. 15 is a configuration diagram showing an optical measurement device according to a third embodiment. FIG. 16 is a configuration diagram showing an optical measurement device according to a fourth embodiment.DESCRIPTION OF THE EMBODIMENTSFirst Embodiment.An optical measurement apparatus according to a first embodiment will be described with reference to FIGS. 1 to 10.The optical measurement device according to the first embodiment is an optical measurement device of a wavelength-scanning interference method using a wavelength-scanning optical coherence tomography (Swept Source-OCT (SS-OCT)).The optical measurement device according to the first embodiment is an optical measurement device using a low coherence light source (hereinafter referred to as a wavelength scanning light source) having a short coherence length, for example, a coherence length of about 10 mm.The low coherence light source is cost effective, but has a narrow range of measurement.The optical measurement device according to the first embodiment expands the measurement range by arranging a circulation light path in a reference light path.The optical measurement device according to the first embodiment identifies the number of revolutions of the circulation light path in the reference output light using a plurality of coarse measurement signals including electric signals obtained using a plurality of circulation number measurement light beams having different refractive index dependencies with respect to a light path on the basis of adjusted light from a wavelength scanning light source.The optical measurement device according to the first embodiment obtains a plurality of coarse measurement signals using the wavelength dependency of a refractive index derived from a light propagation medium in the reference light path, the so-called wavelength dispersion, and identifies the number of revolutions of the circulation light path in the reference output light from the obtained plurality of coarse measurement signals.The optical measurement device according to the first embodiment generates a plurality of coarse measurement signals by utilizing the fact that an amount of shift of a beat frequency with respect to each of the light beams of different frequencies is proportional to the number of revolutions of the circulation light path and the wavelength dependency of a refractive index, and identifies the number of revolutions of the circulation light path in the reference output light using the plurality of generated coarse measurement signals.The optical measurement apparatus according to the first embodiment generates a plurality of coarse measurement signals using beat frequencies of circulation number measurement light beams in different wavelength ranges within a scanning range of tuned light from a wavelength scanning light source 1, and identifies the number of revolutions of the circulation light path in the reference output light using the plurality of generated coarse measurement signals. Note that the optical path length is proportional to a product of the length of a light propagation medium and a refractive index, the beat frequency is proportional to the optical path length, and a difference between optical path lengths having different wavelength dependencies of a refractive index is proportional to the number of revolutions of the circulation optical path.As illustrated in FIG. 1, the optical measurement device according to the first embodiment includes the wavelength scanning light source 1, a light division unit 2, an irradiation optical system 3, a circulation light path 4, a measurement signal acquisition unit 5, a measurement position correction signal generation unit 6, and a signal processing unit 7.Note that the measurement position correction signal generation unit 6 is illustrated as a separate component from the measurement signal acquisition unit 5 for convenience, but is an element of the measurement signal acquisition unit 5.The wavelength scanning light source 1 includes a laser light source and a scanning unit. The scanning unit continuously changes the wavelength of the single frequency laser light coming from the laser light source with time, and outputs tunable light, i.e., wavelength-tunable laser light.The wavelength scanning by the scanning unit may use a method for simultaneously scanning a plurality of wavelengths such as TRIOSA used in optical information communication.The tunable light is desirably linearly tunable with respect to time, and the time and wavelength are desirably at a ratio of 1:1.Even if the tuned light is non-linear in time, the measurement signal acquisition unit 5 and the signal processing unit 7 only need to compensate for the non-linearity. As a technique for compensating the nonlinearity, only the application of a well-known technique is required.When a plurality of (N) revolutions are defined as one cycle, the tuned light in each cycle is emitted from the scanning unit, and an emission time is longer than a time between the revolutions and shorter than two revolutions.The wavelength scanning light source 1 is a light source having a short coherence length, for example, having a coherence length of about 10 mm.The wavelength scanning light source 1 continuously changes a wavelength within a scanning range with respect to time and emits tunable light, for example, wavelength-tunable laser light having an average wavelength of 1550 nm and a scanning range of 100 nm with a wide scanning band.Note that in the wavelength scanning light source 1, the scanning unit may continuously change a plurality of wavelength ranges having different wavelength ranges within one scanning range in a time multiplexed manner and emit wavelength-tuned light that is laser light that becomes light in a plurality of wavelength ranges, for example, tuned light having a central wavelength of substantially 1550 nm and a scanning range of 100 nm with a wide scanning band obtained by sweeping 20 tuned light beams each having a wavelength of 1550 nm shifted by 5 nm as a center.The light dividing unit 2 receives as an input the modulated light from the wavelength scanning light source 1 via an optical fiber and divides the modulated light into measurement output light and reference output light. A division ratio between the measurement output light and the reference output light is set depending on various conditions, but it is desirable to set a large division ratio for the measurement output light so that the measurement object 8 can be measured even when the measurement object 8 has a low reflectance.The light division unit 2 is a coupler that is a 1×2 fiber direction coupler.The optical waveguide is a monomode fiber which is frequently used. An optical fiber connecting the components described below is also a single mode fiber.The irradiation optical system 3 receives as an input the measurement output light from the light division unit 2 via an optical fiber, irradiates the measurement output light as measurement light into a space toward the measurement object 8, receives the reflected light obtained by the measurement object 8 reflecting the measurement light, and outputs the reflected light as reflected measurement light.The irradiation optical system 3 includes an optical circulator, a condenser lens, and a port.The optical circulator outputs the measurement output light from the light division unit 2 to the condenser lens as measurement light, receives the reflected light obtained by the measurement object 8 reflecting the measurement light, and outputs the reflected light to the measurement signal acquisition unit 5 as measurement reflected light.The optical circulator and the light dividing unit 2 are connected to each other by an optical fiber, and the optical circulator and the measurement signal acquiring unit 5 are connected to each other by an optical fiber.The measurement light from the optical circuit is guided to the condenser lens via an optical circulator, and the measurement light condensed by the condenser lens is radiated into a space toward the measurement object 8 via an optical fiber from an end surface of the port located at an end of the optical fiber.The reflected light obtained by the measurement object 8 reflecting the measurement light being incident on the end face of the port and is output as the reflected measurement light to the measurement signal acquisition unit 5 through an optical fiber by the optical circulator.The measurement object 8 is preferably located in the vicinity of the focal point of the condenser lens in order to obtain a sufficient light intensity of the reflected light of the measurement object 8.The spatial scanning may be performed with light using a galvanometer mirror or the like.Through the circulation light path 4, the reference output light of the light division unit 2 circulates N times (an integer equal to or greater than 0), and the circulation light path 4 outputs circulating reference light at each circulation.The circulation optical path 4 includes a coupler 41 and a loop part 42 made of an optical fiber.The coupler 41 is an optical fiber coupler having two input ports and two output ports.The reference output light from the light dividing unit 2, which is input to an input terminal of the coupler 41, is branched to two output terminals. The reference circulating light that has been 0 times is output from one output terminal, and the circulating light for the loop part 42 is output from the other output terminal.The circulating light to the loop part 42 input to the other input terminal of the coupler 41 is branched into two output terminals. The reference circulating light that has circulated N times is output from one output terminal, and the circulating light to the loop part 42 is output from the other output terminal.That is, the coupler 41 outputs, from an output terminal, the circulating reference light obtained by passing the reference output light as it is and the circulating reference light to the measurement signal acquisition unit 5 for each time the reference output light circulates through the loop part 42 once to N times.The loop portion 42 is an optical fiber that connects the other output and the other input of the coupler 41 to each other.The optical fiber constituting the loop member 42 is a single mode fiber.The length of the optical fiber constituting the loop part 42 is, for example, 1.0 m with respect to 0.5 m of the length of the reference light path from the wavelength scanning light source 1 to the measurement signal acquisition unit 5 except for the loop part.Note that a dispersion shifted fiber can be used as the optical fiber constituting the loop part 42. By using the dispersion shifted fiber, the slope of a frequency versus time can be increased for each number of revolutions.Moreover, the optical fiber constituting the loop portion 42 may be covered with a heat insulating material. By covering the optical fiber constituting the loop part 42 with a heat insulating material, the influence of a temperature change of the loop part 42 can be further suppressed.The measurement signal acquisition unit 5 combines the reflected measurement light from the irradiation optical system 3 and the circulating reference light from the circulation light path 4, and outputs an accurate measurement signal obtained by photoelectrically converting the combined interference light.The signal processing unit 7 performs Fast Fourier Transform (FFT) on the accurate measurement signal and performs accurate measurement to obtain a light path length difference between the reflected measurement light and the circulating reference light based on a peak position of a spectrum in the interference light of the reflected measurement light and the circulating reference light in a wavelength range of the scanning range of the tuned light.Based on the tuned light, the measurement signal acquisition unit 5 outputs a plurality of coarse measurement signals including electric signals obtained using a plurality of circulation number measurement light beams having different refractive index dependencies with respect to a light path.The signal processing unit 7 performs Fast Fourier Transform (FFT) on the plurality of coarse measurement signals and performs coarse measurement to determine the number of revolutions in the loop part 42 using a light path length difference between the plurality of circulation number measurement light beams based on a peak position of a spectrum in the plurality of circulation number measurement light beams.The plurality of circulation number measurement light beams having different refractive index dependencies with respect to the reference light path, which is a light path, are light beams having a plurality of wavelength time dependencies based on the matched light.Specifically, the plurality of circulation number measurement light beams are correction reference light beams having different wavelengths obtained by dividing the circulating reference light and correction reference light beams having different wavelengths obtained by dividing the reflected measurement light by a measurement position correction signal obtained by converting the light divided into a plurality of different wavelengths within the scanning range of the tuned light into an electric signal.Each of the plurality of coarse measurement signals is a signal obtained by combining the correction reference light and the correction reflection light having a corresponding wavelength and photoelectrically converting the combined interference light.In the first embodiment, for example, when using tuned light having a center wavelength of 1550 nm and a scanning range of 100 nm, two light beams of light in a first wavelength range in a band of 1500 nm to 1550 nm and light in a second wavelength range in a band of 1550 nm to 1600 nm are used as correction reference light and correction reflection light, respectively.Note that the light in the first wavelength range and the light in the second wavelength range having a center wavelength of 1550 nm as a center are separated from each other. However, the light in the first wavelength range may be, for example, light in the range of 1500 nm to 1560 nm, the light in the second wavelength range may be light in the range of 1540 nm to 1600 nm, and the light in the first wavelength range and the light in the second wavelength range may illuminate in partially overlapping wavelength ranges. The light in the first wavelength range may be light in the range of 1500 nm to 1540 nm, the light in the second wavelength range may be light in the range of 1560 nm to 1600 nm, and the light in the first wavelength range and the light in the second wavelength range may be light in separate wavelength ranges.In addition, light in 20 kinds of wavelength ranges each having a band shifted by 5 nm from light in a scanning range of 1500 nm to 1600 nm can be used.When the wavelength scanning light source 1 that emits tunable light having a center wavelength of 1550 nm and a scanning range of 100 nm with a wide scanning band is used, light in two wavelength ranges in the first wavelength range and the second wavelength range needs to be divided into two wavelength ranges of light in the first wavelength range and light in the second wavelength range based on the tunable light only by the measurement signal acquisition unit 5.When the wavelength scanning light source 1 which scans sweeping light in different wavelength ranges with 1550 nm as a center, for example, 20 scans sweeping light beams with a wavelength shifted by 5 nm in a time multiplexed manner and emits sweeping light with a center wavelength of substantially 1550 nm and a scanning range of 100 nm is used, it is only necessary to use light in different wavelength ranges divided into light in the first wavelength range and light in the second wavelength range at an emission stage of the wavelength scanning light source 1.In the first embodiment, the plurality of coarse measurement signals are a first coarse measurement signal obtained by combining the correction reference light in the first wavelength range and the correction reflection light in the first wavelength range and photoelectrically converting the combined interference light, and a second coarse measurement signal obtained by combining the correction reference light in the second wavelength range and the correction reflection light in the second wavelength range and photoelectrically converting the combined interference light.When the wavelength scanning light source 1 that continuously changes a wavelength within a scanning range with respect to time and outputs tunable light that is wavelength-tuned laser light is used, the measurement signal acquisition unit 5 includes a combination unit, a photoelectric conversion unit, and the measurement position correction signal generation unit 6.The combining unit combines the reflected measurement light of the irradiation optical system 3 and the circulating reference light of the circulation light path 4, and outputs the combined light, i.e., the interference light. The combining unit is a well-known unit that generates interference light by combining two light beams.The photoelectric conversion unit converts the interference light from the combining unit into an electric signal and outputs a measurement signal.Note that, for the light path of the measurement light from the light division unit 2 to the photoelectric conversion unit of the measurement signal acquisition unit 5, a fiber that maintains polarization and has two orthogonal polarization states is preferably used. By using the polarization maintaining fiber, an influence on the delay caused by a cause other than the inside of the measurement object 8 is made less likely, and the measurement can be performed under a condition where a delay variation in an air layer from the irradiation optical system 3 to the measurement object 8 is small.FIG. 2 shows a spectrum M caused by reflected measurement light indicated by a dark black mountain, and spectra S 0 to S N, caused by reflected light from end faces reflected by an end face from which measurement light is emitted into the terminal of the irradiation optical system 3, which is a so-called Fresnel reflection indicated by light black mountains. In Fig. 2, C represents a coherence length.The spectrum M caused by reflected measurement light is a spectrum obtained by the signal processing unit 7 performing fast Fourier transform on a measurement signal obtained by converting a beat frequency of the reflected measurement light obtained by the combining unit of the measurement signal acquisition unit 5 into an electric signal.Moreover, the spectra S 0 to S N, which are caused by the end surface reflected light, are spectra obtained by the signal processing unit 7 performing fast Fourier transform on a measurement signal obtained by converting a beat frequency of the measurement reflected light obtained by the combining unit of the measurement signal acquisition unit 5 into an electric signal.By performing a fast Fourier transform on an electric signal caused by interference light from the measurement signal acquisition unit 5, a frequency corresponding to a reflection position is obtained, and a peak is stronger the stronger the reflected light is.When the coherence length is short and the reference light path is longer, i.e., when the number of revolutions of the loop part 42 increases, the peak of each of the beat frequencies S 0 to S N caused by the end face reflected light is lower.FIG. 3 shows a light path length (beat frequency) caused by reflected measurement light, a light path length (beat frequency) caused by circulating reference light in the first wavelength range (band from 1500 nm to 1550 nm), and a light path length (beat frequency) caused by circulating reference light in the second wavelength range (band from 1550 nm to 1600 nm) caused by the combining unit of the measurement signal acquiring unit 5.The optical path length caused by the circulating reference light in the first wavelength range corresponds to an optical path length caused by the first circulation number measurement light, and the optical path length caused by the circulating reference light in the second wavelength range corresponds to an optical path length caused by the second circulation number measurement light.In FIG. 3, the vertical axis represents the number of revolutions of the circulation light path, the horizontal axis represents a light path length (beat frequency), the solid line represents a light path length caused by reflected measurement light, the broken line represents a light path length caused by circulating reference light in the first wavelength range, and the one-dot chain line represents a light path length caused by circulating reference light in the second wavelength range.Moreover, FIG. 3 illustrates that the reflected measurement light is received between the k-th round trip and the (k+1)-th round trip, and FIG. 3 is a diagram illustrating a case where a beat frequency f b1, which is caused by the circulating reference light of the k-th round trip and the reflected measurement light, is greater than a beat frequency f b2, which is caused by the reflected measurement light of the (k+1)-th round trip.As illustrated in FIG. 3, the optical path length is proportional to a refractive index at a wavelength in a refractive index of a light propagation medium, and therefore, the optical path length caused by the circulating reference light in the first wavelength range is shorter than the optical path length caused by the circulating reference light in the second wavelength range propagating through the same reference optical path.In a general single mode fiber, the slope of refractive index / wavelength is about -0.001 / 100 nm. For example, a light path length measured at a wavelength of 1500 nm is different from a light path length measured at a wavelength of 1600 nm, and the longer the wavelength, the shorter the light path length. Assuming that the loop length of the loop part 42 is 1 m, when the wavelength is increased from 1500 nm to 1600 nm, 1000 μm shifts as a reduced difference in the optical path length in a negative direction.Moreover, a light path length caused by the circulating reference light in the first wavelength range and a light path length caused by the circulating reference light in the second wavelength range become longer each time the circulating reference light circulates through the loop part 42 once, and a slope of the light path length caused by the circulating reference light in the first wavelength range is larger in number of revolutions than a slope of the light path length caused by the circulating reference light in the second wavelength range in number of revolutions.Therefore, a difference between the optical path length caused by the circulating reference light in the first wavelength range and the optical path length caused by the circulating reference light in the second wavelength range is proportional to the number of revolutions for each revolution, i.e., a shift amount.FIG. 4 shows a spectrum obtained by the signal processing unit 7 performing fast Fourier transform on a measurement signal obtained by converting a beat frequency obtained by the combining unit of the measurement signal acquiring unit 5 into an electric signal for the circulating reference light in the first wavelength range and the circulating reference light in the second wavelength range.In FIG. 4, a spectrum f bλ1 caused by the circulating reference light in the first wavelength range is represented by a dark black mountain, a spectrum f bλ2 caused by the circulating reference light in the second wavelength range is represented by a light black mountain, and FIG. 4 is a diagram of a case where a beat frequency f b1 is larger than a beat frequency f b2.In FIG. 4, the horizontal axis represents the number of revolutions, i.e., a measured distance, and the vertical axis represents the intensity of a spectrum.In FIG. 4, a spectrum f bλ11 and a spectrum f bλ21, which are located on the left side of the drawing, indicate spectra in the case where the number of revolutions is zero, and an interval between a peak position of the spectrum f bλ11 and a peak position of the spectrum f bλ21 indicates a shift amount, in other words, a difference in the optical path length difference in the case where the number of revolutions is zero.Moreover, a spectrum f bλ12 and a spectrum f bλ22, which are located on the right side in FIG. 4, indicate spectra in a case where the number of revolutions is N, and an interval between a peak position of the spectrum f bλ12 and a peak position of the spectrum f bλ22 indicates a shift amount, in other words, a difference in the optical path length difference in a case where the number of revolutions is N.Since the shift amount is proportional to the number of revolutions, the time at which reflected measurement light is received, in other words, the number of revolutions, can be determined by obtaining the shift amount.The optical measurement device according to the first embodiment performs rough measurement for determining the number of revolutions of the loop part 42 using a difference between a light path length caused by the reference circulating light in the first wavelength range and a light path length caused by the reference circulating light in the second wavelength range and a light path length caused by the reference circulating light in the second wavelength range, by utilizing the fact that a wavelength dispersion characteristic of the measurement light path and a wavelength dispersion characteristic of the reference light path are different due to the presence of a space between an end surface of the terminal of the irradiation optical system 3 and the measurement object 8 in the measurement light path.The measurement position correction signal generation unit 6 (hereinafter, abbreviated as a correction signal generation unit) generates a measurement position correction signal (hereinafter, abbreviated as a correction signal) used for cutting out a plurality of wavelengths at the time of rough measurement for each cycle on the basis of the tuned light of each cycle.In this example, the correction signal generation unit 6 generates a correction signal for cutting out the first wavelength range and the second wavelength range from the circulating reference light and the reflected measurement light.The modulated light of the wavelength scanning light source 1 undergoes linear fluctuation (jitter) in the direction of the time axis, i.e., with respect to time, every cycle, i.e., every pass.The correction signal is a signal for accurately cutting out a plurality of wavelengths even if the tuned light fluctuates due to jitter.The correction signal generator unit 6 comprises an optical filter 61 and an optical detector 62.The optical filter 61 receives as an input a part of the tuned light emitted from the wavelength scanning light source 1 via the light division unit 2, and filters out position correction light in the first wavelength range and position correction light in the second wavelength range.The optical filter 61 receives as an input a part of the tunable light having a central wavelength of 1550 nm and a scanning range of 100 nm as illustrated in FIG. 6, and filters out position correction light in the first wavelength range in a band of 1500 nm to 1500 nm and position correction light in the second wavelength range in a band of 1550 nm to 1600 nm.Note that, as illustrated in FIG. 6, the position correction light only needs to be composed of k position correction light beams composed of λ 1 to λ k beams obtained by dividing the scanning range of the tunable light into 1 / k (k is an integer equal to or greater than 2).For example, when k is 20, the optical filter 61 may cut out position correction light beams in the first wavelength range λ 1 to the twentieth wavelength range λ 20 having a bandwidth of 5 nm in a band of 1500 nm to 1600 nm.By increasing the number of position correction light beams, the accuracy of determination of the number of revolutions is increased.The optical filter 61 uses a gas cell which transmits only a certain wavelength.As the optical filter 61, an element capable of obtaining an absorption spectrum corresponding to a molecular vibration mode, such as a hydrogen cyanide (HCN) gas cell, or an element transmitting only a certain wavelength by a Mach-Zehnder (MZ) interferometer, such as an etalon, may be used.The photodetector (PD) 62 converts the position correction light in the first wavelength range and the position correction light in the second wavelength range from the optical filter 61 into electrical signals, and outputs a first correction signal and a second correction signal to the measurement signal acquisition unit 5.Note that when 20 position correction light beams are cut out by the optical filter 61, the position correction light beams are converted into electrical signals, and the first correction signal to the second correction signal are output to the measurement signal acquisition unit 5.The correction signal generating unit 6 generates the correction signal using a part of the tuned light emitted from the wavelength scanning light source 1 and input via the light distribution unit 2. However, it is only necessary to obtain a scanning characteristic of the wavelength scanning light source as the correction signal, and therefore the correction signal generating unit 6 can obtain the correction signal using reference circulating light that circulates zero times (does not circulate) as reference output light from the circulation light path 4.The measurement signal acquisition unit 5 synchronizes the tuned light of the wavelength scanning light source 1 with the first correction signal and the second correction signal of the correction signal generation unit 6, filters correction reference light in the first wavelength range and correction reference light in the second wavelength range from the circulating reference light of the circulating light path 4 as circulation number measurement light, and filters correction reflection light in the first wavelength range and correction reflection light in the second wavelength range from the reflected measurement light of the irradiation optical system 3.The measurement signal acquisition unit 5 outputs a first rough measurement signal obtained by combining the correction reference light in the first wavelength range and the correction reflection light in the first wavelength range and photoelectrically converting the combined interference light, and a second rough measurement signal obtained by combining the correction reference light in the second wavelength range and the correction reflection light in the second wavelength range and photoelectrically converting the combined interference light, to the signal processing unit 7.Note that in a case where the wavelength scanning light source 1 scans 20 modulated light beams each having a wavelength shifted by 5 nm from 1550 nm as a center and emits modulated light having a center wavelength of substantially 1550 nm and a scanning range of 100 nm with a wide scanning band, and in a case where an accurate measurement signal is obtained, light having a center wavelength of substantially 1550 nm and a scanning range of 100 nm with a wide scanning band is used as measurement reflected light from the irradiation optical system 3 and as reference circulating light from the circulation light path 4.In a case where rough measurement signals in the first wavelength range and the second wavelength range are obtained, it is only necessary to use ten reflected measurement light beams from the irradiation optical system 3 and ten circulating reference light beams from the circulation light path 4, the reflected measurement light beams and the circulating reference light beams being obtained by dividing the wavelengths of the 20 tuned light beams into two parts including an upper part and a lower part.At this time, the tunable light in the first wavelength range output from the wavelength scanning light source 1 is synchronized with the correction reference light in the first wavelength range and the correction reflection light in the first wavelength range input to the measurement signal acquisition unit 5, and the tunable light in the second wavelength range output from the wavelength scanning light source 1 is synchronized with the correction reference light in the second wavelength range and the correction reflection light in the second wavelength range input to the measurement signal acquisition unit 5.In this case, the correction signal generator unit 6 is not required.The signal processing unit 7 performs fast Fourier transform on an accurate measurement signal including an electric signal obtained by combining the reflected measurement light from the irradiation optical system 3 and the circulating reference light from the circulating light path 4 in the same band as the band of the scanning range of the scanning light emitted from the wavelength scanning light source 1 in the measurement signal acquisition unit 5, and performs accurate measurement to obtain a light path length difference between the reflected measurement light and the circulating reference light on the basis of a peak position of a spectrum in interference light between the reflected measurement light and the circulating reference light in the wavelength range of the scanning range of the tuned light.In the accurate measurement, the signal processing unit 7 acquires a beat frequency using the reflected measurement light and the circulating reference light in a wide wavelength range in the same band as the scanning range of the tuned light. Therefore, the half width of the beat frequency is inversely proportional to the wavelength range used for the fast Fourier transform, but accurate measurement can be performed with high accuracy.For example, when the center wavelength is 1550 nm and the scanning range is 100 nm, the half width of a obtained beat frequency is about 10 μm, so that distance measurement can be performed with sufficiently high accuracy of about 1 μm.A spectrum obtained as a result of accurate measurement by the signal processing unit 7 is shown in FIG. 7. In FIG. 7, when the number of revolutions of the reflected measurement light illustrated in FIG. 3 is k times or (k+1) times, a spectrum on the left indicates that the reflected measurement light is on the k-th side of the circulating reference light, and a spectrum on the right indicates that the reflected measurement light is on the (k+1)-th side of the circulating reference light.That is, it is possible to measure a light path length difference between the reflected measurement light and the circulating reference light, i.e., a distance based on a peak position of a spectrum in the interference light between the reflected measurement light in the wavelength range and the circulating reference light in the scanning range of the tunable light.Note that the number of revolutions k of the circulating reference light cannot be determined by the accurate measurement alone.For example, it is assumed that the length of the reference light path from the wavelength scanning light source 1 to the measurement signal acquisition unit 5 except for the loop part 42 is 0.5 m, the length of a loop of the loop part 42 is 1.0 m, and a measurement light path passing through the measurement object 8 is 1.8 m. A result obtained by the accurate measurement by the signal processing unit 7 is 0.3 m (=1.8-1.5), which is a difference in the light path length between the reflected measurement light and the circulating reference light.The signal processing unit 7 performs fast Fourier transform on the first coarse measurement signal synchronized with the tuned light emitted from the wavelength scanning light source 1 by the first correction signal from the correction signal generating unit 6 in the measurement signal acquiring unit 5, and obtains a light path length difference in the first wavelength range on the basis of a peak position of a spectrum in the interference light caused by the correction reference light in the first wavelength range.The signal processing unit 7 performs fast Fourier transform on the second coarse measurement signal synchronized with the adjusted light emitted from the wavelength scanning light source 1 by the second correction signal from the correction signal generating unit 6 in the measurement signal acquiring unit 5, and obtains a light path length difference in the second wavelength range on the basis of a peak position of a spectrum in the interference light caused by the correction reference light in the second wavelength range.FIG. 8 shows a spectrum obtained by performing a fast Fourier transform of the first coarse measurement signal and the second coarse measurement signal by the signal processing unit 7.As shown in FIG. 8, a shift amount, which is a peak interval between a spectrum caused by a light path length difference in the first wavelength range and a spectrum caused by a light path length difference in the second wavelength range, is proportional to the number of revolutions.Therefore, a relationship between the number of revolutions and the displacement amount is stored in a table in advance, or a linear relationship between the number of revolutions and the displacement amount is stored.When the slope of a wavelength with respect to the temperature of a light propagation medium in the reference light path is non-linear, an influence of a temperature change is taken into consideration in the table or the linear relationship.FIG. 8 is a similar diagram to FIG. 4, and in FIG. 8, a spectrum f bλ11, which is caused by a light path length difference in the first wavelength range, and a spectrum f bλ21, which is caused by a light path length difference in the second wavelength range, which are located on the left side of the drawing, indicate spectra in a case where the number of revolutions is zero, and an interval between a peak position of the spectrum f bλ11 and a peak position of the spectrum f bλ21 indicates a shift amount, in other words, a difference in the light path length difference in a case where the number of revolutions is zero.Moreover, a spectrum f bλ12 and a spectrum f bλ22, which are located on the right side in FIG. 8, indicate spectra in a case where the number of revolutions is N, and an interval between a peak position of the spectrum f bλ12 and a peak position of the spectrum f bλ22 indicates a shift amount, in other words, a light path length difference in a case where the number of revolutions is N.When the first rough measurement signal is a rough measurement signal including an electric signal obtained by combining the reflected measurement light (correction reflection light) from the irradiation optical system 3 and the circulating reference light (correction reference light) from the circulation light path 4 in the first wavelength range, and the second rough measurement signal is a rough measurement signal including an electric signal obtained by combining the reflected measurement light (correction reflection light) from the irradiation optical system 3 and the circulating reference light (correction reference light) from the circulation light path 4 in the second wavelength range, obtaining a light path length difference in the first wavelength range between the correction reflection light and the correction reference light in the first wavelength range and a light path length difference in the second wavelength range between the correction reflection light and the correction reference light in the second wavelength range.A shift amount between the optical path length difference in the first wavelength range and the optical path length difference in the second wavelength range is obtained, and the number of revolutions with respect to the obtained shift amount is obtained from the obtained shift amount, and the relationship between the number of revolutions and the shift amount is stored as a table or the stored linear relationship between the number of revolutions and the shift amount.The signal processing unit 7 performs rough measurement using the first rough measurement signal and the second rough measurement signal from the measurement signal acquisition unit 5, and determines the number of revolutions through which the reflected measurement light of the irradiation optical system 3 is obtained.For example, it is assumed that the length of the reference light path from the wavelength scanning light source 1 to the measurement signal acquisition unit 5 except for the loop part 42 is 0.5 m, the length of a loop of the loop part 42 is 1.0 m, and a measurement light path passing through the measurement object 8 is 1.8 m.In the rough measurement by the signal processing unit 7, it is determined from the shift amount between the optical path length difference in the first wavelength range and the optical path length difference in the second wavelength range and the relationship between the number of revolutions and the shift amount in the table that the number of revolutions is between the number of revolutions 1 (optical path length of the circulating reference light: 1.5 m) and the number of revolutions 2 (optical path length of the circulating reference light: 2.5 m) and on one side of the number of revolutions 1.As a result, a distance measurement of 1.8 m obtained from the sum of 0.3 m from the accurate measurement and 1.5 m in the number of revolutions 1 from the rough measurement can be performed.The signal processing unit 7 increases the speed by parallel processing of the Fast Fourier Transform of the accurate measurement signal in the accurate measurement and the Fast Fourier Transform of each of the first coarse measurement signals and the second coarse measurement signal in the rough measurement.As illustrated in FIG. 9, the optical measurement device according to the first embodiment can identify the number of revolutions by rough measurement because a slope of a frequency of the circulating reference light with respect to time changes due to the wavelength dispersion for each revolution of the loop in the loop part 42 by utilizing the fact that a wavelength dispersion characteristic of the measurement light path and a wavelength dispersion characteristic of the reference light path are different due to the presence of a space between an end surface of the terminal of the irradiation optical system 3 and the measurement object 8 in the measurement light path.Note that by using an optical fiber different from a monomode fiber used for another path, e.g., a dispersion-shifted fiber, as the optical fiber constituting the loop part 42, the pitch in the number of revolutions can be increased, and the accuracy of identifying the number of revolutions can be further improved.Moreover, as illustrated in FIG. 10, since the inclination of a frequency of the measurement reflected light with respect to time changes due to the wavelength dispersion for each round of the loop in the loop part 42, the number of revolutions can be obtained through rough measurement by utilizing the fact that the thickness of an air layer between the measurement object 8 and an end surface of the terminal of the irradiation optical system 3 is proportional to a distance to the measurement object 8.Next, the operations in the accurate measurement and the rough measurement in the optical measurement device according to the first embodiment will be described.First, the process of accurate measurement will be described.When the reflected measurement light is input to the measurement signal acquisition unit 5, the measurement signal acquisition unit 5 combines the input reflected measurement light and the circulating reference light before and after a time point at which the reflected measurement light is input, and outputs an accurate measurement signal converted into an electric signal to the signal processing unit 7.The signal processing unit 7 performs Fast Fourier Transformation of the accurate measurement signal, detects a light path difference between the reflected measurement light and the circulating reference light based on a peak position of a spectrum in interference light between the reflected measurement light and the circulating reference light in a wavelength range of the scanning range of the tuned light, and detects a distance obtained by the reflected measurement light and the circulating reference light.Meanwhile, in the rough measurement, when the reflected measurement light is input to the measurement signal acquisition unit 5, the measurement signal acquisition unit 5 combines the correction reflection light, which is the reflected measurement light in the first wavelength range, in the input reflected measurement light synchronized with the first wavelength range of the tuned light emitted from the wavelength scanning light source 1 by the first correction signal from the correction signal generation unit 6 and the correction reference light, which is the circulating reference light in the first wavelength range in the circulating reference light before and after a time point at which the measurement output light is input, and outputs the first rough measurement signal converted into an electric signal to the signal processing unit 7.Moreover, the measurement signal acquisition unit 5 combines the correction reflection light, which is the reflected measurement light in the second wavelength range, in the input measurement reflection light synchronized with the second wavelength range of the tunable light emitted from the wavelength scanning light source 1 by the second correction signal from the correction signal generation unit 6 and the correction reference light, which is the circulating reference light in the second wavelength range in the circulating reference light before and after a time point at which the measurement output light is input, and outputs the second rough measurement signal converted into an electric signal to the signal processing unit 7.Note that when the wavelength scanning light source 1 that outputs each of the tuned light beams wavelength-tuned in the first wavelength range and the second wavelength range having different wavelength ranges by the scanning unit within the scanning range in a time-multiplexed manner is used, the measurement signal acquisition unit 5 combines each of the correction reflection light beams that are incoming reflected measurement light beams in the first wavelength range and the second wavelength range synchronized with the tuned light beams in the first wavelength range and the second wavelength range emitted from the wavelength scanning light source 1, and each of the correction reference light beams that are circulating reference light beams in the first wavelength range and the second wavelength range before and after a time point, to which the measurement reflection light beams in the first wavelength range and the second wavelength range are input, and outputs the measurement signal of the first wavelength range and the second coarse measurement signal converted into electrical signals to the signal processing unit 7.The correction reflection light and the correction reference light constitute the circulation number measurement light.The signal processing unit 7 performs Fast Fourier Transformation of the first coarse measurement signal, obtains a light path length difference in the first wavelength range between the reflected measurement light and the circulating reference light in the first wavelength range based on a peak position of a spectrum in the interference light caused by the circulating reference light in the first wavelength range, performs Fast Fourier Transformation on the second coarse measurement signal, and obtains a light path length difference in the second wavelength range between the reflected measurement light and the circulating reference light in the second wavelength range based on a peak position of a spectrum in the interference light caused by the circulating reference light in the second wavelength range.The signal processing unit 7 acquires a shift amount between the optical path length difference in the first wavelength range and the optical path length difference in the second wavelength range, and acquires, from the acquired shift amount, the number of revolutions with respect to the acquired shift amount and the ratio between the number of revolutions and the shift amount stored as a table or the stored linear ratio between the number of revolutions and the shift amount.The signal processing unit 7 acquires a distance from the measurement object 8 from the distance obtained by the reflected measurement light and the reference circulating light obtained by the accurate measurement and the number of revolutions obtained by the rough measurement, and outputs the obtained distance.As described above, in the accurate measurement, by using the reflected measurement light and the circulating reference light in the same band as the wide scanning range of the wavelength scanning light source 1 and using all the wide bands for the fast Fourier transform, there are no variations in the peak position as illustrated in FIG. 7, and high accuracy can be maintained in the distance measurement.In the rough measurement, by performing a fast Fourier transform using the reflected measurement light and the circulating reference light in a band narrower than the scanning range as illustrated in FIG. 8, accurate information on the shift amount can be obtained although the spectrum is broader.Since the optical measurement device according to the first embodiment includes the circulation light path having the loop part, the measurement range can be widened even when a low coherence light source having a narrow measurement range is used as the wavelength scanning light source 1. In addition, the measurement signal acquisition unit 5 that outputs, on the basis of the tuned light, a plurality of coarse measurement signals including electric signals obtained using a plurality of circulating number of measurement light beams having different refractive index dependencies with respect to a light path, and the signal processing unit 7 that identifies the number of revolutions of the circulating reference light in the circulating light path 4 in which the light path length difference between the reflected measurement light and the circulating reference light among the plurality of coarse measurement signals is obtained from the measurement signal acquisition unit 5 are included, and therefore the measurement is hardly affected by a change in the ambient temperature, and a distance to a measurement object can be measured with high accuracy.Moreover, the optical measurement device according to the first embodiment obtains a plurality of coarse measurement signals for identifying the number of revolutions by the reflected measurement light and the circulating reference light in wavelength ranges corresponding to the wavelength ranges obtained by dividing the scanning range of the tuned light from the wavelength scanning light source 1 into a plurality of parts, and therefore does not increase the deterioration of the measurement temporal resolution and the complexity of a hardware configuration as the optical measurement device.Second Embodiment.An optical measuring apparatus according to a second embodiment will be described with reference to Figs. 11 to 14.The optical measurement apparatus according to the second embodiment is different from the optical measurement apparatus according to the first embodiment in that the optical measurement apparatus according to the second embodiment obtains a first coarse measurement signal and a second coarse measurement signal by correction reflection light and correction reference light obtained by splitting the reflected measurement light and the circulating reference light into two polarized light beams orthogonal to each other, respectively, wherein the optical measurement apparatus according to the first embodiment obtains the first circulation number measurement light and the second circulation number measurement light having different refractive index dependencies with respect to a light path to obtain the first coarse number measurement signal and the second coarse number measurement signal by reflected measurement light and circulating reference light in wavelength ranges corresponding to the wavelength ranges, which are obtained by dividing the scanning range of the tuned light of the wavelength scanning light source 1 into a plurality of parts, and the other points are the same or similar.In Figs. 11 to 14, the same reference numerals as in Figs. 1 to 10 denote the same or corresponding parts.The optical measurement apparatus according to the second embodiment obtains a first coarse measurement signal and a second coarse measurement signal by utilizing a polarization dependency of a refractive index of a light propagation medium in two polarized light beams, i.e., light in a polarization mode P and light in a polarization mode S orthogonal to the polarization mode P, the so-called birefringence, and identifying the number of revolutions of a circulation light path.The optical measurement device according to the second embodiment obtains a first coarse measurement signal and a second coarse measurement signal by utilizing the fact that a difference between a beat frequency (optical path length) of light in a polarization mode P and a beat frequency (optical path length) of light in a polarization mode S, a so-called shift amount, is proportional to the number of revolutions of a circulation optical path and birefringence, and identifies the number of revolutions of the circulation optical path in the reference output light by using the obtained first coarse measurement signal and second coarse measurement signal.Note that the optical path length is proportional to the product of the length of a light propagation medium and a refractive index, the beat frequency is proportional to the optical path length, and a difference between optical path lengths having different kinds of birefringence is proportional to the number of revolutions of the circulation optical path.As illustrated in FIG. 11, the optical measurement device according to the second embodiment includes a wavelength scanning light source 1, a light division unit 2, an irradiation optical system 3, a circulation light path 4, a measurement signal acquisition unit 5, and a signal processing unit 7.The measurement signal acquisition unit 5 includes a rough measurement signal acquisition unit 9 illustrated in FIG. 12.The optical measurement device according to the second embodiment will be described below, wherein the center of gravity on the measurement signal acquisition unit 5 is located in the optical measurement device according to the first embodiment, in particular on the coarse measurement signal acquisition unit 9.The description of the wavelength scanning light source 1, the light division unit 2, the irradiation optical system 3, and the circulation light path having the same configurations as those of the optical measurement device according to the first embodiment is omitted as much as possible.Note that it is advantageous to use a polarization maintaining optical fiber as optical fibers for a measurement optical path and a reference optical path. By using the polarization-maintaining optical waveguide, the birefringence is stabilized over time and spatially over the entire length of the polarization-maintaining optical waveguide.The measurement signal acquisition unit 5 combines the reflected measurement light from the irradiation optical system 3 and the circulating reference light from the circulation light path 4, and outputs an accurate measurement signal obtained by photoelectrically converting the combined interference light.As illustrated in FIG. 12, the rough measurement signal acquisition unit 9 includes a reflected light beam splitter 91, a reference light beam splitter 92, a P-wave combination unit 93, an S-wave combination unit 94, a P-wave balance detector 95, and an S-wave balance detector 96.The measurement signal acquisition unit 5 uses an integrated coherent receiver (ICR) which is an optical integrated device generally used for a receiver in the field of optical information communication.The reflected light beam splitter 91 splits the measurement reflected light of the irradiation optical system 3 into P-wave correction reflected light, i.e., measurement reflected light in a polarization mode P (hereinafter referred to as P-wave), and S-wave correction reflected light, i.e., measurement reflected light in a polarization mode S (hereinafter referred to as S-wave).The reference light beam splitter 92 splits the circulating reference light from the circulating light path 4 into P-wave correction reference light, which is circulating reference light of the P-wave, and P-wave correction reference light, which is circulating reference light of the S-wave.The P-wave combining unit 93 combines the reflected measurement light of the P-wave from the reflected light beam splitter 91 and the circulating reference light of the P-wave from the reference light beam splitter 92, and outputs the combined light, i.e., the P-wave interference light having a beat frequency.The S-wave combining unit 94 combines the reflected measurement light of the S-wave from the reflected light beam splitter 91 and the circulating reference light of the S-wave from the reference light beam splitter 92, and outputs the combined light, i.e., the S-wave interference light having a beat frequency.The P-wave balance detector 95 converts the P-wave interference light from the P-wave combining unit 93 into an electric signal and outputs a first measurement signal (P-wave).The S-wave balance detector 96 converts the S-wave interference light from the S-wave combining unit 94 into an electric signal and outputs a second measurement signal (S-wave).The P-wave balance detector 95 and the S-wave balance detector 96 are made up of balanced photodiodes (BPD) that convert the P-wave interference light and the S-wave interference light into electrical signals.FIG. 13 shows a light path length (beat frequency) caused by the circulating reference light of the P-wave and a light path length (beat frequency) caused by the circulating reference light of the S-wave.In FIG. 13, the vertical axis represents the number of revolutions of the circulating light path, the horizontal axis represents a light path length (beat frequency), the solid line represents a light path length caused by reflected measurement light, the broken line represents a light path length caused by the circulating reference light of the P-wave, and the one-dot chain line represents a light path length caused by the circulating reference light of the S-wave.Moreover, FIG. 13 illustrates that the reflected measurement light is received between the k-th circulation and the (k+1)-th circulation, and FIG. 13 is a diagram illustrating a case where a beat frequency f b1 of the circulating reference light of the k-th circulation and the reflected measurement light is greater than a beat frequency f b2 of the reflected measurement light of the (k+1)-th circulation.The optical path length caused by the circulating reference light of the P-wave is shorter than the optical path length caused by the circulating reference light of the S-wave.The optical path length caused by the circulating reference light of the P-wave and the optical path length caused by the circulating reference light of the S-wave become longer each time the circulating reference light once circulates through the loop part 42, and a slope of the optical path length caused by the circulating reference light of the P-wave with respect to the number of revolutions is greater than a slope of the optical path length caused by the circulating reference light of the S-wave with respect to the number of revolutions.Therefore, a difference between the optical path length caused by the circulating reference light of the P-wave and the optical path length caused by the circulating reference light of the S-wave for each round, i.e., a shift amount, is proportional to the number of round trip.FIG. 14 shows a spectrum obtained by the signal processing unit 7 by fast Fourier transformation of a first (P-wave) measurement signal and a second (S-wave) measurement signal.In FIG. 14, a spectrum f bP caused by the circulating reference light of the P wave is indicated by a dark black mountain, a spectrum f bS caused by the circulating reference light of the S wave is indicated by a light black mountain, and FIG. 14 is a diagram of a case where a beat frequency f b1 is larger than a beat frequency f b2.In FIG. 14, the horizontal axis represents the number of revolutions, i.e., a measured distance, and the vertical axis represents the intensity of a spectrum.In FIG. 14, a spectrum f bP1 and a spectrum f bP1, which are located on the left side of the drawing, indicate spectra in the case where the number of revolutions is zero, and an interval between a peak position of the spectrum f bP1 and a peak position of the spectrum f bS1 indicates a shift amount, in other words, a difference in the optical path length difference in the case where the number of revolutions is zero.Moreover, a spectrum f bP2 and a spectrum f bS2, which are located on the right side in FIG. 14, indicate spectra in a case where the number of revolutions is N, and an interval between a peak position of the spectrum f bP2 and a peak position of the spectrum f bS2 indicates a shift amount, in other words, a difference in the optical path length difference in a case where the number of revolutions is N.Since the shift amount is proportional to the number of revolutions, the time at which reflected measurement light is received, in other words, the number of revolutions, can be determined by obtaining the shift amount.Since birefringence is determined by a light propagation medium in the measurement light path and the reference light path and the shift amount is proportional to the number of revolutions, a relationship between the shift amount and the number of revolutions is acquired in advance, and the relationship between the number of revolutions and the shift amount is stored in a table as can be seen from FIGS. 13 and 14.Next, the operations in the accurate measurement and the rough measurement in the optical measurement device according to the second embodiment will be described.First, the process of accurate measurement will be described.When the reflected measurement light is input to the measurement signal acquisition unit 5, the measurement signal acquisition unit 5 combines the input reflected measurement light and the circulating reference light before and after a time point at which the reflected measurement light is input, and outputs an accurate measurement signal converted into an electric signal to the signal processing unit 7.The signal processing unit 7 performs Fast Fourier Transformation of the accurate measurement signal, detects a light path length difference between the reflected measurement light and the circulating reference light based on a peak position of a spectrum in the interference light of the reflected measurement light and the circulating reference light in a wavelength range of the scanning range of the tuned light, and detects a distance obtained by the reflected measurement light and the circulating reference light.Meanwhile, in the measurement signal acquisition unit 5, in the coarse measurement, when the reflected measurement light is input to the measurement signal acquisition unit 5, the measurement signal acquisition unit 9 combines the P-wave measurement light in the reflected measurement light and the circulating reference light in the circulating reference light before and after a time point at which the reflected measurement light is input, and outputs a first coarse measurement signal converted into an electric signal to the signal processing unit 7.Moreover, the measurement signal acquisition unit 5 combines the reflected S-wave measurement light in the incoming reflected measurement light and the circulating S-wave reference light in the circulating reference light before and after a time point at which the reflected measurement light is input, and outputs a second coarse measurement signal converted into an electrical signal to the signal processing unit 7.The signal processing unit 7 performs a Fast Fourier Transform of the first coarse measurement signal, and determines a light path length difference between the reflected measurement light of the P wave and the circulating reference light of the P wave on the basis of a peak position of a spectrum in the interference light caused by the circulating reference light of the P wave, performs a Fast Fourier Transform of the second coarse measurement signal, and determines a light path length difference between the reflected light of the S wave measurement and the circulating reference light of the S wave measurement on the basis of a peak position of a spectrum in the interference light caused by the circulating reference light of the S wave.The signal processing unit 7 acquires a shift amount between the P-wave optical path length difference and the S-wave optical path length difference, and acquires the number of revolutions with respect to the acquired shift amount from the acquired shift amount, and the relationship between the number of revolutions and the shift amount is stored as a table.The signal processing unit 7 acquires a distance from the measurement object 8 from the distance obtained by the reflected measurement light and the reference circulating light obtained by the accurate measurement and the number of revolutions obtained by the rough measurement, and outputs the obtained distance.As described above, in the accurate measurement, by using the reflected measurement light and the circulating reference light in the same band as the wide scanning range of the wavelength scanning light source 1 and using all the wide bands for the fast Fourier transform, there are no variations in the peak position, and high accuracy can be maintained in the distance measurement.In the rough measurement, for the rough measurement, the first rough measurement signal caused by a P wave and the second rough measurement signal caused by an S wave are subjected to fast Fourier transformation using the reflected measurement light of the P wave, the circulating reference light of the P wave, the reflected measurement light of the S wave, and the circulating reference light of the S wave, whereby information on the shift amount can be accurately obtained.Since the optical measurement device according to the second embodiment includes the circulation light path having the loop part, the measurement range can be widened even when a light source having low coherence and narrow measurement range is used as the wavelength scanning light source 1.In addition, the measurement signal acquisition unit 5 that outputs the first coarse measurement signal caused by a P wave and the second coarse measurement signal caused by an S wave including electric signals by the circulating reference light of the P wave and the circulating reference light of the S wave, respectively, by utilizing the polarization dependence of a refractive index of a light propagation medium on two polarized light beams having different refractive index dependencies with respect to a light path, i.e., light in a polarization mode P and light in a polarization mode S, so-called birefringence, and the signal processing unit 7 that identifies the number of revolutions of the circulating reference light in the circulating light path 4 are the following:, in which a light path length difference between the reflected measurement light and the circulating reference light is obtained from the coarse measurement signal of the P wave and the coarse measurement signal of the S wave from the measurement signal acquisition unit 5, and therefore the measurement is hardly affected by a change in the ambient temperature, and a distance to a measurement object can be measured with high accuracy.Moreover, the optical measurement device according to the second embodiment obtains the first rough measurement signal and the second rough measurement signal for identifying the number of revolutions through a P wave and an S wave in each of the reflected measurement light and the difference of the circulating reference light, and therefore does not increase deterioration in the time resolution of measurement and the complexity of a hardware configuration as the optical measurement device.Third Embodiment.An optical measurement apparatus according to a third embodiment will be described with reference to FIG. 15.The optical measuring apparatus according to the third embodiment is different from the optical measuring apparatus according to the first embodiment in that the optical measuring apparatus according to the third embodiment uses a common optical path interference system and the other points are the same or similar.In Fig. 15, the same reference numerals as in Figs. 1 to 10 denote the same or corresponding parts.In the optical measurement apparatus according to the first embodiment, an optical fiber used in a light path in which measurement output light is emitted as measurement light from the light division unit 2 through the irradiation optical system 3 toward the measurement object 8 and the emitted measurement light is reflected from the measurement object 8, passes through the irradiation optical system 3, and reaches the measurement signal acquisition unit 5 as the reflected measurement signal is different from an optical fiber used in a reference light path in which the reference output light reaches the measurement signal acquisition unit 5 as the circulating reference light from the light division unit 2 through the circulation light path 4.In the optical measuring apparatus according to the third embodiment, a common optical path interference system is used, and a common optical fiber is used for a measurement optical path and a reference optical path.Note that in FIG. 15, the reflected measurement light and the circulating reference light are separately illustrated, but they are separately illustrated for convenience, and a common optical fiber is used for the reflected measurement light and the circulating reference light.As illustrated in FIG. 15, the optical measurement apparatus according to the third embodiment includes, similarly to the optical measurement apparatus according to the first embodiment, a wavelength scanning light source 1, a light division unit 2, an irradiation optical system 3, a circulation light path 4, a measurement signal acquisition unit 5 having a measurement position correction signal generation unit 6, and a signal processing unit 7.Since the optical measuring apparatus according to the third embodiment is different from the optical measuring apparatus according to the first embodiment in that the optical measuring apparatus according to the third embodiment uses a common optical path interference system as described above, the measurement optical path and the reference optical path are mainly described.The measurement light path will be described.The tuned light obtained from the light division unit 2 that divides the tuned light of the wavelength scanning light source 1 is input to a coupler 41 of the circulation light path 4 via a common optical fiber. The tuned light supplied to the coupler 41 is directly supplied to the irradiation optical system 3 via a common optical fiber as measurement output light.The measurement output light input to the irradiation optical system 3 is irradiated as measurement light into a space toward the measurement object 8. The irradiation optical system 3 receives reflected light obtained from the measurement object 8 reflecting the measurement light, and outputs the reflected light as reflected measurement light from the irradiation optical system 3 to the measurement signal acquisition unit 5 via a common optical fiber.The reference light path will be described.The tuned light obtained from the light division unit 2 that divides the tuned light of the wavelength scanning light source 1 is input to the coupler 41 of the circulation light path 4 via a common optical fiber. The tuned light supplied to the coupler 41 is directly supplied to the irradiation optical system 3 via a common optical fiber as a circulating reference light which has been circulated 0 times.Moreover, the tuned light input to the coupler 41 circulates through a loop part 42 and is supplied to the irradiation optical system 3 via a common optical fiber as circulating reference light each time the tuned light passes through the loop part 42 from one time to N times.The circulating reference light from the circulating light path 4 is output to the measurement signal acquisition unit 5 via a common optical fiber via the irradiation optical system 3 every time the measurement output light passes through the loop part 42 from zero to N times.The modulated output light of the light division unit 2 is supplied to the measurement signal acquisition unit 5 as reflected measurement light and circulating reference light via the common optical path interference system as described above.The measurement signal acquisition unit 5 operates similarly to the measurement signal acquisition unit 5 in the first embodiment with the reflected measurement light and the circulating reference light, and outputs an accurate measurement signal, a first (first wavelength range) coarse measurement signal, and a second (second wavelength range) coarse measurement signal.The signal processing unit 7 that has received the accurate measurement signal from the measurement signal acquisition unit 5 performs accurate measurement, similarly to the signal processing unit 7 in the first embodiment, and obtains a distance obtained by the reflected measurement light and the circulating reference light.The signal processing unit 7 that has obtained the first rough measurement signal and the second rough measurement signal from the measurement signal acquisition unit 5 performs rough measurement, similarly to the signal processing unit 7 in the first embodiment, and determines the number of revolutions.The signal processing unit 7 acquires a distance from the measurement object 8 from the distance obtained by the reflected measurement light and the reference circulating light obtained by the accurate measurement and the number of revolutions obtained by the rough measurement, and outputs the obtained distance.The optical measurement device according to the third embodiment has a similar effect to the optical measurement device according to the first embodiment, and includes the measurement light path and the reference light path formed by the common light path interference system, and therefore can suppress an influence of temperature variation of the common light guide on measurement of a distance to the measurement object 8.Fourth Embodiment.An optical measurement apparatus according to a fourth embodiment will be described with reference to FIG. 16.The optical measurement device according to the fourth embodiment is different from the optical measurement device according to the second embodiment in that the optical measurement device according to the fourth embodiment uses a common optical path interference system and the other points are the same or similar.In Fig. 16, the same reference numerals as in Fig. 11 denote the same or corresponding parts.In the optical measurement apparatus according to the second embodiment, an optical fiber used in a measurement light path in which measurement output light is emitted as measurement light from the light division unit 2 through the irradiation optical system 3 toward the measurement object 8 and the emitted measurement light is reflected from the measurement object 8, passes through the irradiation optical system 3, and reaches the measurement signal acquisition unit 5 as the reflected measurement signal is different from an optical fiber used in a reference light path in which reference output light from the light division unit 2 to the measurement signal acquisition unit 5 moves as the circulating reference light through the circulation light path 4.In the optical measuring apparatus according to the fourth embodiment, a common optical path interference system is used, and a common optical fiber is used for a measurement optical path and a reference optical path.Note that in FIG. 16, the reflected measurement light and the circulating reference light are separately illustrated, but are separately illustrated for simplicity, and a common optical fiber is used for the reflected measurement light and the circulating reference light.As illustrated in FIG. 16, the optical measurement apparatus according to the fourth embodiment includes, similarly to the optical measurement apparatus according to the second embodiment, a wavelength scanning light source 1, an irradiation optical system 3, a circulation light path 4, a measurement signal acquisition unit 5 including a coarse measurement signal acquisition unit 9, and a signal processing unit 7.Since the optical measuring apparatus according to the fourth embodiment is different from the optical measuring apparatus according to the second embodiment in that the optical measuring apparatus according to the fourth embodiment uses a common optical path interference system as described above, the measurement optical path and the reference optical path are mainly described.The measurement light path will be described.The tuned light of the wavelength scanning light source 1 is fed to a coupler 41 of the circulating light path 4 via a common optical fiber. The tuned light supplied to the coupler 41 is directly input to the irradiation optical system 3 as measurement output light via a common optical fiber.The measurement output light input to the irradiation optical system 3 is irradiated as measurement light into a space toward the measurement object 8. The irradiation optical system 3 receives reflected light obtained from the measurement object 8 reflecting the measurement light, and outputs the reflected light as reflected measurement light from the irradiation optical system 3 to the measurement signal acquisition unit 5 via a common optical fiber.The reference light path will be described.The tuned light of the wavelength scanning light source 1 is fed to a coupler 41 of the circulating light path 4 via a common optical fiber. The tuned light supplied to the coupler 41 is directly supplied to the irradiation optical system 3 via the common optical fiber as a reference circulating light which has been circulated 0 times.In addition, the tuned light supplied to the coupler 41 circulates through a loop part 42 and is supplied to the irradiation optical system 3 via the common optical fiber as circulating reference light each time the tuned light circulates through the loop part 42 from one time to N times.The reference circulating light from the circulating light path 4 is output to the measurement signal acquisition unit 5 via a common optical fiber via the irradiation optical system 3 for every round trip from zero to N times.The tunable light of the wavelength scanning light source 1 is supplied to the measurement signal acquisition unit 5 as reflected measurement light and circulating reference light via the common optical path interference system as described above.The measurement signal acquisition unit 5 operates similarly to the measurement signal acquisition unit 5 in the second embodiment by the incoming reflected measurement light and the circulating reference light, and outputs an accurate measurement signal, a first (P-wave) coarse measurement signal, and a second (S-wave) coarse measurement signal.The signal processing unit 7 that has received the accurate measurement signal from the measurement signal acquisition unit 5 performs the accurate measurement similarly to the signal processing unit 7 in the second embodiment, and obtains the distance obtained by the reflected measurement light and the circulating reference light.The signal processing unit 7 that has received the first rough measurement signal and the second rough measurement signal from the measurement signal acquisition unit 5 performs rough measurement, similarly to the signal processing unit 7 in the second embodiment, and determines the number of revolutions.The signal processing unit 7 acquires a distance from the measurement object 8 from the distance obtained by the reflected measurement light and the reference circulating light obtained by the accurate measurement and the number of revolutions obtained by the rough measurement, and outputs the obtained distance.The optical measurement device according to the fourth embodiment has a similar effect to the optical measurement device according to the second embodiment, and includes the measurement light path and the reference light path formed by the common light path interference system, and therefore can suppress an influence of temperature variation of the common light guide with respect to measurement of a distance to the measurement object 8.Note that the embodiments can be freely combined with each other, that each individual element in each embodiment can be changed, or that each individual element in each embodiment can be omitted.INDUSTRIAL APPLICABILITYThe optical measurement device according to the present disclosure is suitable for an optical measurement device that measures a distance to a measurement object in a processing device and a semiconductor test device.LIST OF REFERENCE CHARACTERS1: Wavelength scanning light source, 2: Light division unit, 3: Irradiation optical system, 4: Circulation light path, 5: Measurement signal acquisition unit, 6: Measurement position correction signal generation unit, 7: Signal processing unit, 8: Measurement object, 9: Coarse measurement signal acquisition unitReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 2001-41706 A

[0004]

Claims

An optical measurement apparatus comprising: a wavelength scanning light source for outputting tunable light having a wavelength continuously changing with time; an irradiation optical system for emitting measurement output light caused by the tunable light from the wavelength scanning light source toward a measurement object into a space as measurement light, for receiving reflected light obtained by the measurement object reflecting the measurement light, and for outputting the reflected light as reflected measurement light; a circulation light path including a loop part for outputting circulating reference light for each circulation, wherein reference output light caused by the tunable light from the wavelength scanning light source circulates N (an integer equal to or greater than 0) times through the loop part; a measurement signal acquisition unit for combining the reflected measurement light from the irradiation optical system and the circulating reference light from the circulation light path, outputting an accurate measurement signal obtained by photoelectrically converting the combined interference light, and outputting a plurality of coarse measurement signals including electric signals obtained using a plurality of circulation number measurement light beams having different refractive index dependencies with respect to a light path based on the adjusted light; and a signal processing unit to identify the number of revolutions in the circulating light path of the circulating reference light, wherein a light path length difference between the reflected measurement light and the circulating reference light is obtained by the accurate measurement signal from the measurement signal acquiring unit, and a light path length difference between the reflected measurement light and the circulating reference light is obtained by the plurality of rough measurement signals from the measurement signal acquiring unit.The optical measurement device according to claim 1, wherein the plurality of circulation number measurement light beams having different refractive index dependencies with respect to the light path are light beams having multiple wavelength time dependencies based on the tunable light.The optical measurement apparatus according to claim 1, wherein the plurality of circulation number measurement light beams having different refractive index dependencies with respect to the light path are correction reference light beams having different wavelengths obtained by dividing the circulating reference light, and correction reflection light beams having different wavelengths obtained by dividing the reflected measurement light by a measurement position correction signal obtained by converting light beams divided into a plurality of different wavelengths within a scanning range of the tuned light into electrical signals, and each of the plurality of coarse measurement signals is a signal obtained by combining the correction reference light and the correction reflection light having a corresponding wavelength and photoelectrically converting the combined interference light.The optical measurement apparatus according to claim 1, wherein the plurality of circulation number measurement light beams having different refractive index dependencies with respect to the light path are correction reference light beams having different wavelengths obtained by dividing the circulating reference light, and correction reflection light beams having different wavelengths obtained by dividing the reflected measurement light by a measurement position correction signal obtained by converting light beams divided into a plurality of different wavelengths within a scanning range of circulating reference light that has been circulated zero times from the circulation light path into electrical signals, and each of the plurality of coarse measurement signals is a signal obtained by combining the correction reference light and the correction reflection light having a corresponding wavelength and photoelectrically converting the combined interference light.The optical measurement apparatus according to claim 1, wherein the tuned light output from the wavelength scanning light source and having a wavelength that continuously changes with time is tuned light scanned to a laser light wavelength in a plurality of wavelength ranges by continuously changing a plurality of wavelength ranges having different wavelength ranges within a scanning range in a time division manner, the plurality of circulation number measurement light beams having different refractive index dependencies with respect to the light path are correction reference light beams having different wavelengths caused by circulating reference light with respect to each of the light beams in the plurality of plurality of wavelength ranges output from the wavelength scanning light source and correction reflection light beams having different wavelengths, which are caused by reflected measurement light with respect to each of the light beams in the plurality of wavelength ranges, and each of the plurality of coarse measurement signals is a signal obtained by combining the correction reference light and the correction reflection light with a corresponding wavelength and photoelectrically converting the combined interference light.The optical measurement apparatus according to claim 1, wherein the plurality of circulation number measurement light beams having different refractive index dependencies with respect to the light path are correction reflection light and correction reference light separated into two polarized light beams orthogonal to the reflected measurement light and the circulating reference light, respectively, and each of the plurality of coarse measurement signals is a signal obtained by combining the correction reference light and the correction reflection light with a corresponding polarization and photoelectrically converting the combined interference light.The optical measurement apparatus according to any one of claims 1 to 6, wherein a light path of the measurement output light, the reference output light, and a light path of the circulating reference light to the irradiation optical system are a common light path, and a light path of the reflected measurement light and a light path of the circulating reference light from the irradiation optical system are a common light path.

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