Optical coherence distance measuring device

Abstract

An optical coherence distance measuring device (100) comprises a first lens (15) that condenses signal light (51A) emitted by an optical transmission component (18) on an object (19) to be measured, a second lens (16A) that is used for scanning at a first magnification, a third lens (16B) that is used for scanning at a second magnification higher than the first magnification, and a lens switching unit (16) that inserts the second lens (16A) between the first lens (15) and the optical transmission component (18) when scanning at the first magnification, and inserts the third lens (16B) between the first lens (15) and the optical transmission component (18) when scanning at the second magnification, characterized in that the distance between the first lens (15) and the object (19) is maintained in a state in which the signal light (51A) can be condensed on the surface of the object (19).

Description

Optical Interference Distance Measurement Device

[0001] The present disclosure relates to an optical interferometric distance measurement device.

[0002] An optical interferometric distance measurement device using optical coherence tomography (OCT) technology irradiates one of the beams of light branched within the device onto a measurement object, returns the light reflected by the measurement object back into the device, and causes the returned light to interfere with the other branched beam to obtain the optical path length and convert it into distance. Furthermore, Patent Document 1 discloses an optical sensor device that uses a wavelength sweep type in which the wavelength of the light output from the light source changes over time, making it easy to separate light reflected by the measurement object or scattered by areas other than the measurement object when there is a large amount of light included.

[0003] The optical sensor device described in Patent Document 1 can increase the resolution in the measurement plane direction by increasing the optical magnification of the optical system, but at the same time, there is a problem that the measurement range in the depth direction is narrowed. As a result, when it is desired to acquire a three-dimensional shape, it is necessary to scan in the depth direction in addition to scanning in the measurement plane, which results in an enormous measurement time.

[0004] Patent Literature 2 discloses an invention of an optical coherence tomography apparatus that switches optical magnification by replacing the objective lens of the optical system. The optical coherence tomography apparatus described in Patent Literature 2 can minimize the scanning range in the depth direction when performing high-magnification and high-resolution measurements by using depth information obtained without scanning through low-magnification and low-resolution measurements.

[0005] International Publication No. 2022 / 264393 Japanese Patent Application Laid-Open No. 2017-64378

[0006] However, the optical coherence tomography device described in Patent Document 2 has the problem that when switching the optical magnification, it is necessary to change the distance (working distance) between the object to be measured and the measuring device in addition to replacing the objective lens, which makes it impossible to perform rapid measurements.

[0007] An object of the present disclosure is to provide an optical interferometric distance measurement device that can switch optical magnification without changing the working distance between the measurement object and the measurement device.

[0008] The optical interferometric distance measurement device disclosed herein comprises a first lens that focuses signal light emitted by a light source on a measurement object, a second lens that is used for scanning at a first magnification, a third lens that is used for scanning at a second magnification that is higher than the first magnification, and a lens switching unit that inserts the second lens between the first lens and the light source when scanning at the first magnification, and inserts the third lens between the first lens and the light source when scanning at the second magnification, thereby switching between the second lens and the third lens, and is characterized in that the distance between the first lens and the measurement object is maintained while the signal light can be focused on the surface of the measurement object.

[0009] According to the present disclosure, it is possible to provide an optical interferometric distance measurement device that can switch optical magnification without changing the working distance between the measurement object and the measurement device.

[0010] 1A is a schematic diagram showing an example of the configuration of an optical interferometric distance measurement device according to the present embodiment; and FIG. 1B is a block diagram showing an example of the hardware configuration of a signal processing device according to the present embodiment. FIG. 1A is a schematic diagram showing the state of an optical sensor head in measurement at high magnification, and FIG. 1B is a schematic diagram showing the state of an optical sensor head in measurement at low magnification.

[0011] Hereinafter, optical interferometric distance measurement devices according to embodiments will be described with reference to the drawings. The following embodiments are merely examples, and the embodiments can be combined and modified as appropriate.

[0012] 1 is a schematic diagram showing an example of the configuration of an optical interferometric distance measurement device 100 according to this embodiment. The optical interferometric distance measurement device 100 includes a wavelength swept light source 1, an optical branching device 2, an optical circulator 3, a reference reflection point 4, an optical sensor head 5, an optical heterodyne receiver 6, an analog-to-digital converter (ADC) 7, a digital-to-analog converter (DAC) 8, a signal processing device 9, a reference clock 10, a branching device 11, a phase-locked loop (PLL) 12, and a switch 13. The wavelength swept light source 1, the optical branching device 2, the optical circulator 3, the reference reflection point 4, the optical sensor head 5, and the optical heterodyne receiver 6 are each connected by an optical transmission path such as an optical fiber.

[0013] The wavelength swept light source 1 outputs light whose frequency changes over time (wavelength swept light) to the optical splitter 2. The wavelength swept light source 1 performs frequency sweep (wavelength sweep). Therefore, the wavelength swept light source 1 outputs light whose frequency changes over time to the optical splitter 2. For example, the wavelength swept light source 1 can be a laser light source whose wavelength can be controlled by controlling the resonator length, or a laser light source whose wavelength changes depending on the amount of injected current. By performing frequency sweep, the wavelength swept light source 1 may output light that alternately repeats continuous triangular wave-like up-chirps and down-chirps, may output light that repeats sawtooth wave-like up-chirps, may output light that repeats sawtooth wave-like down-chirps, or may output a pulsed up-chirp or down-chirp chirp pulse signal.

[0014] The optical splitter 2 splits the light output from the wavelength swept light source into signal light 21 and reference light 22. The optical splitter 2 outputs the split signal light 21 to the optical circulator 3 and outputs the split reference light 22 to the optical heterodyne receiver 6.

[0015] The optical circulator 3 outputs the signal light 21 branched by the optical brancher 2 to the reference reflection point 4, and outputs the reflected light 51B of the object 19 input from the reference reflection point 4 side or the internally reflected light by the reference reflection point 4 as reflected light 31 to the optical heterodyne receiver 6.

[0016] The signal light 21 branched by the optical branching device 2 is input to the reference reflection point 4 via the optical circulator 3. The reference reflection point 4 internally reflects the input signal light 21, partially reflecting it inside the reference reflection point 4. The internally reflected light that is internally reflected by the reference reflection point 4 is output to the optical heterodyne receiver 6 via the optical circulator 3. Furthermore, the signal light 21 that passes through the reference reflection point 4 without being internally reflected at the reference reflection point 4 is output to the optical sensor head 5. Examples of the reference reflection point 4 include a partially reflecting mirror or a connector end face.

[0017] The optical sensor head 5 emits signal light 51A, which is branched from the optical branching device 2 and passed through the reference reflection point 4, toward the object 19 to be measured, and receives reflected light 51B generated by reflection by the object 19. The optical sensor head 5 outputs the received reflected light 51B to the optical heterodyne receiver 6 as reflected light 31 via the reference reflection point 4 and the optical circulator 3. A first lens 15, which is an objective lens, is provided at the end of the optical sensor head 5 facing the object 19. Behind the first lens 15 on the reference reflection point 4 side, a second lens used for scanning at a first magnification (hereinafter referred to as "low magnification") and a third lens used for scanning at a second magnification (hereinafter referred to as "high magnification") higher than the first magnification are provided, and a lens switching unit 16 is provided to switch between the second lens and the third lens depending on the required magnification. In this embodiment, the first lens 15 is held at a constant distance from the object 19 together with the housing 17 of the optical interferometric distance measurement device 100. The lens switching unit 16 switches between the second lens and the third lens based on a command signal 123 output from the signal processing device 9. For example, when the command signal 123 instructs scanning at a low magnification, the lens switching unit 16 inserts the second lens between the first lens 15 and a light source (an optical transmission component 18 described later). When the command signal 123 instructs scanning at a high magnification, the lens switching unit 16 inserts the third lens between the first lens 15 and the light source (an optical transmission component 18 described later). The distance between the first lens 15 and the object 19 is defined, for example, as the distance from a flange position attached to the first lens 15 to the surface of the object 19 facing the optical interferometric distance measurement device 100. The distance between the first lens 15 and the object 19 is maintained in a state where the signal light 51A can be focused on the surface of the object 19 during scanning. In other words, the distance between the first lens 15 and the object 19 is the distance at which the first lens 15 focuses the signal light 51A on the surface of the object 19 during scanning. As a result, the distance between the first lens 15 and the object 19 is within the measurement range of the optical interferometric distance measurement device 100 during scanning.

[0018] The optical heterodyne receiver 6 performs heterodyne processing to combine the reference light 22 supplied from the wavelength swept light source 1 and the reflected light 31 received by the optical sensor head 5, as a result of branching by the optical branching device 2. The combined light obtained by the heterodyne processing is then photoelectrically converted by, for example, a photodiode (PD) to obtain a received signal (beat signal) as an electrical signal.

[0019] On the other hand, the optical heterodyne receiver 6 combines the reference light 22 branched by the optical brancher 2 with the reflected light 31, which is internally reflected light generated when the signal light 21 branched by the optical brancher 2 is internally reflected at the reference reflection point 4, and further acquires an internal received signal as an electrical signal by photoelectrically converting the combined light. The optical heterodyne receiver 6 outputs the acquired received signal and signal 61, which is the internal received signal, to the ADC 7.

[0020] The reference clock 10 generates a reference clock signal and outputs the generated reference clock signal to the branching device 11. The branching device 11 branches the reference clock signal generated by the reference clock 10 to the signal processing device 9 and the PLL 12.

[0021] The PLL 12 generates second clock signals 121 and 122 in synchronization with the reference clock signal branched by the branching device 11. The PLL 12 outputs the generated second clock signal 121 to the DAC 8, and also outputs the generated second clock signal 122 to the switch 13.

[0022] The DAC 8 generates a first clock signal 81 used by the ADC 7 in synchronization with the second clock signal 121 generated by the PLL 12. The DAC 8 outputs the generated first clock signal 81 to the switch 13. Details of the first clock signal will be described later.

[0023] In this embodiment, the DAC 8 generates the first clock signal 81 for the ADC 7 in synchronization with the second clock signal 121 generated by the PLL 12. However, the optical interferometric distance measurement device 100 may further include a separate circuit for generating a clock, and the DAC 8 may generate the first clock signal 81 for the ADC 7 in synchronization with the clock generated by that circuit. Furthermore, the frequency of the first clock signal 81 does not need to be synchronized with the frequency of the second clock signal 121.

[0024] The switch 13 switches the clock signal of the ADC 7 to either the first clock signal 81 generated by the DAC 8 or the second clock signal 122 generated by the PLL 12. For example, when the optical interferometric distance measurement device 100 acquires first frequency fluctuation reference signal data (described later), the switch 13 switches the clock signal of the ADC 7 to the second clock signal 122 generated by the PLL 12. For example, when the optical interferometric distance measurement device 100 acquires measurement data on an object 19 (described later), the switch 13 switches the clock signal of the ADC 7 to the first clock signal 81 generated by the DAC 8.

[0025] The ADC 7 samples the internal reception signal acquired by the optical heterodyne receiver 6 in synchronization with the second clock signal 121 generated by the PLL 12 and acquired via the switch 13. The ADC 7 outputs a reception signal 71, which is the internal reception signal converted into a digital signal, to the signal processing device 9.

[0026] Based on the internal reception signal converted into a digital signal by the ADC 7, the signal processing device 9 calculates first frequency fluctuation reference signal data that serves as a reference for frequency fluctuation of the light output from the wavelength swept light source 1. The signal processing device 9 stores the calculated first frequency fluctuation reference signal data in a storage unit 44 (described later) and outputs the calculated first frequency fluctuation reference signal data 91 to the DAC 8.

[0027] The signal processing device 9 calculates the first frequency fluctuation reference signal data as follows: The signal processing device 9 performs a Hilbert transform on the internal reception signal converted into a digital signal by the ADC 7, thereby obtaining the instantaneous frequency f ｒｅｆ(t) is calculated, and the calculated instantaneous frequency f ｒｅｆ By multiplying (t) by K (K is a positive integer), the frequency component Kf ｒｅｆ The first frequency fluctuation reference signal data (t) is calculated.

[0028] In this embodiment, DAC 8 converts the first frequency variation reference signal data calculated by signal processing device 9 into an analog signal in synchronization with second clock signal 121 generated by PLL 12, thereby generating the first frequency variation reference signal, which is the above-mentioned first clock signal 81. DAC 8 outputs the generated first frequency variation reference signal to switch 13.

[0029] The ADC 7 samples the received signal acquired by the optical heterodyne receiver 6 in synchronization with the first frequency variation reference signal generated by the DAC 8 and acquired via the switch 13. The ADC 7 outputs the received signal 71 converted into a digital signal to the signal processing device 9. The ADC 7 samples the received signal in synchronization with the first frequency variation reference signal, thereby compensating for the nonlinearity of the wavelength swept light.

[0030] The signal processing device 9 generates and outputs a command signal 123 for switching between the second lens and the third lens in order to obtain a received signal 71 sufficient to calculate measurement data related to the object 19. The signal processing device 9 calculates the measurement data related to the object 19 based on the received signal 71 converted into a digital signal by the ADC 7. When calculating the measurement data related to the object 19, the signal processing device 9 determines whether the received signal 71 contains sufficient information for calculating the measurement data related to the object 19, and if the information related to the horizontal direction (i.e., the direction parallel to) the surface of the object 19 is insufficient, the signal processing device 9 generates and outputs to the lens switching unit 16 a command signal 123 for switching to the third lens. When calculating the measurement data related to the object 19, the signal processing device 9 determines whether the received signal 71 contains sufficient information for calculating the measurement data related to the object 19, and if the information related to the vertical direction (the direction perpendicular to) the surface of the object 19 is insufficient, the signal processing device 9 generates and outputs to the lens switching unit 16 a command signal 123 for switching to the second lens.

[0031] When the frequency of the wavelength swept light output by the wavelength swept light source 1 exhibits linearity, the time delay between the reference light 22 and the reflected light 31 reflected by the object 19 is constant, and the frequency of the beat signal obtained by combining the reference light 22 and the reflected light 31 is also constant. Therefore, the frequency spectrum based on the beat signal exhibits a sharp peak at a specific frequency. The signal processing device 9 can calculate position information of the object 19 to be measured based on the calculation result of a fast Fourier transform (FFT) that includes the specific frequency.

[0032] The signal processing device 9 outputs the calculated measurement data 92 to an output unit 14 located outside the optical interferometric distance measurement device 100. Examples of the measurement data calculated by the signal processing device 9 include information indicating the distance from the optical interferometric distance measurement device 100 to the object 19, or information indicating the position of the object 19. The output unit 14 is an output device such as a display device such as an LCD (liquid crystal display), a printer, or a plotter.

[0033] 2 is a block diagram showing an example of a hardware configuration of a signal processing device 9 according to this embodiment. As shown in Fig. 2, the signal processing device 9 is configured by a computer in which a CPU (Central Processing Unit) 41, which is a processing element (processor), a main memory 42, an input / output interface (I / O interface) 43, and a storage unit 44 are each connected to a system bus 45. The signal processing device 9 may be configured by a plurality of computers connected via a network, or may be configured by a processing circuit.

[0034] The CPU 41 is an integrated circuit (IC) that performs arithmetic processing. In addition to the CPU 41, an arithmetic element such as a digital signal processor (DSP), a graphics processing unit (GPU), a network processor, or a field programmable gate array (FPGA) may be used. By executing the optical interferometric distance measurement program according to this embodiment, the CPU 41 calculates first frequency fluctuation reference signal data from the internal reception signal converted into a digital signal by the ADC 7, and also functions as a calculation function for calculating information indicating the distance from the optical interferometric distance measurement device 100 to the object 19 or information indicating the position of the object 19, and a lens switching control function for generating and outputting a command signal 123 for switching between the second lens and the third lens in order to obtain a reception signal 71 sufficient to calculate information indicating the distance from the optical interferometric distance measurement device 100 to the object 19 or information indicating the position of the object 19. As a result, by executing the optical interferometric distance measurement program, the CPU 41 functions as a calculation unit and a lens switching control unit. The optical interferometric distance measurement program is provided, for example, on a recording medium on which it is recorded.

[0035] The main memory 42 is configured by a volatile storage device such as a RAM (Random Access Memory) or a non-volatile storage device such as a ROM (Read Only Memory). The storage unit 44 is configured by a non-volatile storage device such as a HDD (Hard Disk Drive) or a flash memory.

[0036] The I / O interface 43 is a port to which the ADC 7, DAC 8, splitter 11, output unit 14, lens switching unit 16, etc. are connected.

[0037] 3A is a schematic diagram showing the state of the optical sensor head 5 during high-magnification measurement suitable for horizontal scanning of the surface of an object 19. In high-magnification measurement, signal light 21 passes through the reference reflection point 4 and is emitted from an optical transmission component 18 provided at the end of an optical fiber connected to the reference reflection point 4. The signal light 21 is collected by a third lens 16B and a first lens 15 and irradiated onto the object 19. The first lens 15 is focused on the surface of the object 19 and collects the signal light 51A emitted from the optical transmission component 18 onto the object 19. The third lens 16B emits the signal light 51A emitted from the optical transmission component 18 toward the first lens 15 and contributes to the collection of the signal light 51A onto the object 19 by the first lens. The third lens 16B is a flat optical element made of optical glass or resin having a predetermined refractive index, or an air gap. The first lens 15 is an optical element made of optical glass or resin having a predetermined refractive index, molded into a spherical or aspherical shape. The third lens 16B may also be an optical element made of optical glass or resin having a predetermined refractive index, molded into a spherical or aspherical shape.

[0038] 3B is a schematic diagram showing the state of the optical sensor head 5 during measurement at low magnification, which is suitable for scanning in a direction perpendicular to the surface of the object 19. In measurement at low magnification, signal light 21 passes through the reference reflection point 4 and is emitted from the optical transmission component 18 provided at the end of the optical fiber connected to the reference reflection point 4, and is collected by the second lens 16A and the first lens 15 to illuminate the object 19. The first lens 15 is focused on the surface of the object 19 and collects the signal light 51A emitted from the optical transmission component 18 on the object 19. The second lens 16A emits the signal light 51A emitted from the optical transmission component 18 toward the first lens 15, contributing to the collection of the signal light 51A on the object 19 by the first lens. The second lens 16A is a flat optical element made of optical glass or resin having a predetermined refractive index, or an air gap. The second lens 16A may be an optical element made of optical glass or resin having a predetermined refractive index and molded into a spherical or aspherical shape. By using a flat optical element for at least one of the second lens 16A and the third lens 16B, the cost of the optical interferometric distance measurement device 100 can be reduced, and the error in the observed distance measurement value between the high magnification mode and the low magnification mode can be reduced.

[0039] Reflected light 51B generated when signal light 51A is reflected by an object 19 is incident on the optical transmission component 18, and the reflected light 51B incident on the optical transmission component 18 is input to the optical heterodyne receiver 6 via the reference reflection point 4 and the optical circulator 3. In this embodiment, in order to align the positions of the interference spectra between the signal light and the reference light in the high magnification mode and the low magnification mode, the optical path length at the center of the optical axis of the second lens 16A through the first lens 15 is the same as the optical path length at the center of the optical axis of the third lens 16B through the third lens 16B and the first lens 15. With this configuration, errors in the observed ranging distance values ​​are suppressed between the high magnification mode and the low magnification mode.

[0040] The lens switching unit 16 switches between the second lens 16A and the third lens 16B based on a command signal 123 output from the signal processing device 9. The signal processing device 9 also outputs reflected light from the surface of the second lens 16A or the third lens 16B to the optical heterodyne receiver 6 as reflected light 31, and determines whether the lens switching unit 16 has completed switching based on an interference signal between the reflected light 31 from the surface of the second lens 16A or the third lens 16B and the reference light 22. Specifically, the signal processing device 9 detects and determines the phase difference of the interference signal obtained by heterodyne processing of the reflected light 31 with the reference light 22 using FFT processing. This determination allows instantaneous determination of lens switching.

[0041] This embodiment is characterized in that the distance between the first lens 15 and the object 19 is maintained at a predetermined constant value, and the optical path length through the second lens 16A and the first lens 15 at the optical axis center of the second lens 16A is the same as the optical path length through the third lens 16B and the first lens 15 at the optical axis center of the third lens 16B.

[0042] In this embodiment, assuming the above-mentioned conditions of distance and optical path length, the second lens 16A and the third lens 16B, which are provided on the light source side of the first lens 15 (i.e., on the optical transmission component 18 side), are switched by moving them in a direction perpendicular to the optical axis of the signal light 51A. By switching between the second lens 16A and the third lens 16B, a beam expander-like effect is achieved that changes the spot diameter of the signal light 51A irradiated onto the object 19, making it possible to switch between a high magnification state suitable for scanning the surface of the object 19 in the horizontal direction and a low magnification state suitable for scanning the surface of the object 19 in the vertical direction (depth direction).

[0043] The above-mentioned distances, the refractive indexes of the first lens 15, the second lens 16A, and the third lens 16B, the curvatures of the surfaces of the first lens 15 and the second lens 16A, and the thicknesses of the first lens 15, the second lens 16A, and the third lens 16B are optimized in advance so that the spot diameter of the signal light 51A irradiated on the object 19 is suitable for scanning at a desired magnification. As a result, the magnification during scanning can be changed by switching between the second lens 16A and the third lens 16B by moving them in a direction perpendicular to the optical axis of the signal light 51A. Furthermore, by configuring the second lens 16A and the third lens 16B to be made of the same material with the same refractive index and by configuring the thickness of the second lens 16A and the third lens 16B to be the same at the optical axis center, it is possible to reduce errors in the observed ranging distance values ​​between the high magnification mode and the low magnification mode.

[0044] In the present embodiment, the second lens 16A and the third lens 16B are moved in a direction perpendicular to the optical axis of the signal light 51A, but the second lens 16A or the third lens 16B may be moved parallel to the optical axis of the signal light 51A to adjust the focus of the signal light 51A on the object 19. When the second lens 16A and the third lens 16B are moved in a direction perpendicular to the optical axis of the signal light 51A, the lens switching unit 16 may be moved in a rotational direction to move the second lens 16A and the third lens 16B in a direction perpendicular to the optical axis of the signal light 51A, in addition to moving the lens switching unit 16 in the vertical direction.

[0045] As described above, according to this embodiment, in order to switch between the high magnification mode and the low magnification mode, the lens switching unit 16 that switches between the second lens 16A and the third lens 16B is provided, and the distance between the first lens 15 and the object 19 to be measured is kept constant, thereby enabling high-speed measurement without moving the object to be measured (without changing the working distance) or the optical interferometric distance measurement device 100. Furthermore, even if the object to be measured is large, there is no risk of the optical interferometric distance measurement device 100 interfering with the object to be measured.

[0046] Furthermore, in this embodiment, by configuring the optical path length through the second lens 16A and the first lens 15 at the optical axis center of the second lens 16A to be the same as the optical path length through the third lens 16B and the first lens 15 at the optical axis center of the third lens 16B, the error in the observed ranging distance value can be reduced between high magnification mode and low magnification mode.

[0047] By configuring either the second lens 16A or the third lens 16B to be a flat optical element, it is possible to reduce the cost of the optical interferometric distance measurement device 100 and further reduce the error in the observed ranging distance value between high magnification mode and low magnification mode.

[0048] By configuring the second lens 16A and the third lens 16B to be made of the same material with the same refractive index and by configuring the optical axis center thickness of the second lens 16A to be the same as the optical axis center thickness of the third lens 16B, the error in the observed ranging distance value can be further reduced between high magnification mode and low magnification mode.

[0049] By determining whether the lens switching unit 16 is switching between the second lens 16A and the third lens 16B based on the interference signal between the reflected light from either the surface of the second lens 16A or the surface of the third lens 16B and the reference light 22, the lens switching can be determined more quickly, and even objects that are large in the depth direction can be measured quickly.

[0050] 1 wavelength swept light source, 2 optical branching device, 3 optical circulator, 4 reference reflection point, 5 optical sensor head, 6 optical heterodyne receiver, 7 ADC, 8 DAC, 9 signal processing device, 10 reference clock, 11 branching device, 12 PLL, 13 switch, 14 output unit, 15 first lens, 16 lens switching unit, 16A second lens, 16B third lens, 17 housing, 18 optical transmission component, 19 object, 41 CPU, 42 main memory, 43 I / O interface, 44 memory unit, 51A signal light, 51B reflected light, 100 optical interferometric distance measurement device.

Claims

1. An optical interferometric distance measurement device comprising: a first lens that focuses signal light emitted by a light source on a measurement object; a second lens that is used for scanning at a first magnification; a third lens that is used for scanning at a second magnification higher than the first magnification; and a lens switching unit that inserts the second lens between the first lens and the light source when scanning at the first magnification, and inserts the third lens between the first lens and the light source when scanning at the second magnification, thereby switching between the second lens and the third lens, wherein the optical interferometric distance measurement device maintains the distance between the first lens and the measurement object while the signal light can be focused on the surface of the measurement object.

2. An optical interferometric distance measurement device as described in claim 1, characterized in that the optical path length through the second lens and the first lens at the optical axis center of the second lens is the same as the optical path length through the third lens and the first lens at the optical axis center of the third lens.

3. The optical interferometric distance measuring device according to claim 2, wherein either the second lens or the third lens is a flat optical element.

4. An optical interferometric distance measurement device as described in claim 2 or claim 3, characterized in that the second lens and the third lens are each made of the same material, and the thickness of the optical axis center of the second lens is the same as the thickness of the optical axis center of the third lens.

5. An optical interferometric distance measurement device as described in claim 2 or claim 3, characterized in that the lens switching unit determines whether to switch between the second lens and the third lens based on an interference signal between reflected light from either the surface of the second lens or the surface of the third lens and a reference light branched from the signal light.

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