Optical Interferometric Distance Sensor

Abstract

To provide an optical interference ranging sensor for increasing measurement precision by suppressing return light between optical couplers.SOLUTION: An optical interference ranging sensor includes: a light source for projecting light while changing a wavelength continuously; multiple interferometers for generating interference light based on measurement light reflected from a measurement object by guiding supplied light to the measurement object and reference light following an optical path at least partially different from that of the measurement light; multiple stages of optical couplers that are connected in series, and receive light from the light source from a pre-stage and supply light to a corresponding interferometer from among the multiple interferometers and to a post-stage by branching; suppression means for suppressing supply of light from the post-stage in multiple stages of optical couplers to the pre-stage; and a processing part for calculating distance to a measurement object on the basis of multiple pieces of interference light generated by the multiple interferometers.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to an optical interferometric distance measuring sensor. [Background technology]

[0002] In recent years, optical distance measuring sensors that measure the distance to a measurement object without contact have become widespread. For example, an optical interferometric distance measuring sensor is known that generates interference light based on a reference light and a measurement light from light projected from a wavelength swept light source and measures the distance to the measurement object based on the interference light.

[0003] In particular, from the viewpoint of improving measurement accuracy, a multistage optical interferometer distance measuring sensor equipped with a plurality of interferometers that generate interference light is known. For example, Patent Document 1 discloses an optical coherence tomography imaging device equipped with a light beam controller, a branching means that branches a plurality of light beams from the light beam controller into object light and reference light, an irradiation means that irradiates a measurement object with the plurality of object light beams, and an interference means that causes interference between the object light and the reference light scattered from the measurement object and leads them to a light receiver.

[0004] Furthermore, Non-Patent Document 1 discloses an optical interferometric distance measuring sensor that branches light without using costly components such as circulators. Non-Patent Document 1 discloses an optical interferometric distance measuring sensor that includes a wavelength swept light source, multiple optical couplers, multiple interferometers corresponding to each optical coupler, and a light receiving unit. The multiple optical couplers included in the optical interferometric distance measuring sensor are connected in series and configured to sequentially supply a portion of the light from the wavelength swept light source from the optical coupler in the preceding stage to the coupler in the succeeding stage, while branching the other portion of the light and supplying it to the interferometer corresponding to each optical coupler. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2019 / 131298 [Non-patent literature]

[0006] [Non-Patent Document 1] Jesse Zheng, “Optical Frequency-Modulated Continuos-Wave (FMCW) Interferometry,” Springer, January 4, 2005, p. 154 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in a configuration in which light is split into interferometers by a plurality of optical couplers connected in series, there is a possibility that light may return from the latter stage to the former stage, which may result in a decrease in measurement accuracy.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical interferometric distance measuring sensor that improves measurement accuracy by suppressing return light between optical couplers. [Means for solving the problem]

[0009] An optical interferometric distance measuring sensor according to one embodiment of the present invention comprises a light source that emits light while continuously changing the wavelength, a plurality of interferometers that generate interference light based on measurement light reflected from the measurement object by guiding the supplied light to the measurement object and reference light that follows an optical path that is at least partially different from that of the measurement light, a plurality of serially connected optical couplers that receive light from the light source from an upstream stage and branch and supply the light to a corresponding interferometer among the plurality of interferometers and to a downstream stage, a suppression means for suppressing the supply of light from the downstream stage to the upstream stage in the plurality of optical couplers, and a processing unit that calculates the distance to the measurement object based on the plurality of interference lights generated by the plurality of interferometers.

[0010] According to this aspect, in a multi-stage optical coupler, the supply of light from the later stage to the earlier stage is suppressed, thereby suppressing the return light between the optical couplers and improving the measurement accuracy of the optical interferometric distance measuring sensor.

[0011] In the above aspect, the suppression means may include at least one of the optical couplers in the multiple stages, which is configured so that the amount of light branched to a subsequent stage is greater than the amount of light branched to a corresponding interferometer.

[0012] According to this aspect, in a multi-stage optical coupler, the supply of light from the later stage to the earlier stage is suppressed, and by suppressing the light returning from the later stage optical coupler to the earlier stage optical coupler, it is possible to improve the measurement accuracy of the optical interferometric ranging sensor.

[0013] In the above embodiment, when the ratio of the amount of light branched to the corresponding interferometer with respect to the amount of light branched to the subsequent stage for the ith optical coupler among the optical couplers in the multiple stages is Ri, R i+1 ≧R i It may be set so that:

[0014] According to this aspect, the variation in the amount of light received by each interferometer is reduced, and it is possible to improve the measurement accuracy of the optical interferometer distance measuring sensor.

[0015] In the above aspect, the interferometer may generate each interference light based on a first reflected light of the measurement light that is irradiated onto the measurement object and reflected by the measurement object, and a second reflected light of the reference light that is reflected by the reference surface.

[0016] According to this aspect, interference light is generated based on a first reflected light of the measurement light that is irradiated onto the measurement object and reflected by the measurement object, and a second reflected light of the reference light that is reflected by the reference surface. By setting the optical path length difference between the measurement light and the reference light to be different for each of the beams branched corresponding to the multiple spots, each peak can be appropriately detected, and the distance to the measurement object can be calculated with high accuracy based on the distance value corresponding to the detected peak.

[0017] In the above embodiment, the optical path length from the ith optical coupler to the reference surface of the corresponding interferometer is L CR,iLet the optical path length from the ith optical coupler to the (i+1)th optical coupler be L CC,i In this case, |L CR,i -(L CR,i+1 +L CC,i )| may be set to be equal to or greater than the first threshold.

[0018] According to this aspect, interference between the multiple interfering light beams generated by the multiple interferometers is suppressed, and it is possible to improve the measurement accuracy of the optical interferometric distance measuring sensor.

[0019] In the above aspect, the first threshold may be set based on a frequency band of a light receiving unit that converts the plurality of interference light beams into an electrical signal and supplies the electrical signal to a processing unit.

[0020] According to this aspect, it is possible to effectively suppress interference between a plurality of interfering light beams in accordance with the frequency band of the light receiving section, thereby improving the measurement accuracy of the optical interferometric distance measuring sensor.

[0021] In the above aspect, the suppression means may include a blocking unit connected between two optical couplers of the multiple stages of optical couplers and configured to guide light from the optical coupler in the previous stage to the optical coupler in the subsequent stage, but not guide light from the optical coupler in the subsequent stage to the optical coupler in the previous stage.

[0022] According to this aspect, it is possible to suppress return light from the optical coupler in the subsequent stage to the optical coupler in the preceding stage, thereby improving the measurement accuracy of the optical interferometric distance measuring sensor.

[0023] In the above embodiment, the optical path length from the ith optical coupler to the reference surface of the corresponding interferometer is L CR,i Let L be the optical path length from the ith optical coupler to the cutoff part connected to it in the subsequent stage. CI,i In this case, |L CR,i -L CI,i may be set to be equal to or greater than the first threshold.

[0024] According to this aspect, interference between the reflected light from the blocking portion and the interference light generated by the interferometer is suppressed, and it is possible to improve the measurement accuracy of the optical interferometric distance measuring sensor.

[0025] In the above aspect, the second threshold may be set based on a frequency band of a light receiving unit that converts the plurality of interference light beams into an electrical signal and supplies the electrical signal to a processing unit.

[0026] According to this aspect, it is possible to effectively suppress interference between reflected light from the blocking section and interference light generated by the interferometer depending on the frequency band of the light receiving section, thereby improving the measurement accuracy of the optical interferometric ranging sensor. [Effects of the Invention]

[0027] According to the present invention, it is possible to provide an optical interferometric distance measuring sensor that improves measurement accuracy by suppressing return light between optical couplers. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a schematic external view illustrating an overview of a displacement sensor 10 according to the present disclosure. [Figure 2] 1 is a flowchart showing a procedure for measuring a measurement object T by a displacement sensor 10 according to the present disclosure. [Figure 3] 1 is a functional block diagram showing an overview of a sensor system 1 in which a displacement sensor 10 according to the present disclosure is used. [Figure 4] 1 is a flowchart showing a procedure for measuring a measurement object T by a sensor system 1 that uses a displacement sensor 10 according to the present disclosure. [Figure 5A] 1 is a diagram for explaining the principle of measurement of a measurement object T by a displacement sensor 10 according to the present disclosure. [Figure 5B] 10A and 10B are diagrams for explaining another principle by which the measurement object T is measured by the displacement sensor 10 according to the present disclosure. [Figure 6A] FIG. 2 is a perspective view showing a schematic configuration of a sensor head 20. [Figure 6B]2 is a perspective view showing a schematic configuration of a collimator lens holder arranged inside the sensor head 20. FIG. [Figure 6C] 2 is a cross-sectional view showing the internal structure of the sensor head 20. FIG. [Figure 7] FIG. 2 is a block diagram for explaining signal processing in a controller 30. [Figure 8] 10 is a flowchart showing a method for calculating a distance to a measurement object T, which is executed by a processing unit 59 in the controller 30. [Figure 9A] FIG. 1 is a diagram showing how a waveform signal (voltage vs. time) is frequency-converted into a spectrum (voltage vs. frequency). [Figure 9B] FIG. 10 is a diagram showing how a spectrum (voltage vs. frequency) is distance-transformed into a spectrum (voltage vs. distance). [Figure 9C] FIG. 10 is a diagram showing how values ​​(distance value, SNR) corresponding to peaks are calculated based on a spectrum (voltage vs. distance). [Figure 10] 1 is a schematic diagram showing the general configuration of an optical interferometric distance measuring sensor 100 according to a first embodiment of the present invention. [Figure 11] FIG. 10 is a schematic diagram showing the general configuration of an optical interferometric distance measuring sensor 200 according to a second embodiment of the present invention. [Figure 12] 10A and 10B are diagrams showing variations of an interferometer that generates interference light using measurement light and reference light. DETAILED DESCRIPTION OF THE INVENTION

[0029] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Note that each embodiment described below is merely a specific example for carrying out the present invention and is not intended to limit the scope of the present invention. Furthermore, to facilitate understanding of the description, the same components in each drawing will be designated by the same reference numerals wherever possible, and duplicate descriptions may be omitted.

[0030] [Displacement sensor overview] First, an overview of the displacement sensor according to the present disclosure will be described. 1 is a schematic external view showing an overview of a displacement sensor 10 according to the present disclosure. As shown in Fig. 1, the displacement sensor 10 includes a sensor head 20 and a controller 30, and measures the displacement of a measurement object T (the distance to the measurement object T).

[0031] The sensor head 20 and the controller 30 are connected by an optical fiber 40, and an objective lens 21 is attached to the sensor head 20. The controller 30 also includes a display unit 31, a setting unit 32, an external interface (I / F) unit 33, an optical fiber connection unit 34, and an external storage unit 35, and further includes a measurement processing unit 36 ​​inside.

[0032] The sensor head 20 irradiates the measurement object T with light output from the controller 30 and receives the light reflected from the measurement object T. The sensor head 20 has an internal reference surface that reflects the light output from the controller 30 and received via the optical fiber 40 and causes it to interfere with the light reflected from the measurement object T described above.

[0033] The sensor head 20 is provided with an objective lens 21, which is detachable. The objective lens 21 can be replaced with an objective lens having an appropriate focal length depending on the distance between the sensor head 20 and the measurement target T, or a variable-focus objective lens may be used.

[0034] Furthermore, when installing the sensor head 20, guide light (visible light) may be irradiated onto the measurement object T, and the sensor head 20 and / or the measurement object T may be installed so that the measurement object T is appropriately positioned within the measurement area of ​​the displacement sensor 10.

[0035] The optical fiber 40 is connected to and extends from an optical fiber connection unit 34 arranged in the controller 30, connecting the controller 30 and the sensor head 20. As a result, the optical fiber 40 is configured to guide light emitted from the controller 30 to the sensor head 20, and further guide return light from the sensor head 20 to the controller 30. The optical fiber 40 is detachable from the sensor head 20 and the controller 30, and various optical fibers having different lengths, thicknesses, characteristics, etc. can be used.

[0036] The display unit 31 is configured with, for example, a liquid crystal display or an organic EL display, etc. The display unit 31 displays the set value of the displacement sensor 10, the amount of received return light from the sensor head 20, and measurement results such as the displacement of the measurement object T measured by the displacement sensor 10 (the distance to the measurement object T).

[0037] The setting unit 32 performs settings necessary for measuring the measurement target T, for example, by a user operating a mechanical button, a touch panel, or the like. All or part of these necessary settings may be set in advance, or may be set from an externally connected device (not shown) connected to the external I / F unit 33. The externally connected device may be connected by wire or wirelessly via a network.

[0038] Here, the external I / F unit 33 is configured with, for example, Ethernet (registered trademark), RS232C, analog output, etc. The external I / F unit 33 may be connected to another connected device so that necessary settings can be made from the external connected device, or the measurement results measured by the displacement sensor 10, etc., may be output to the external connected device.

[0039] Furthermore, the controller 30 may perform settings necessary for measuring the measurement object T by importing data stored in the external storage unit 35. The external storage unit 35 is, for example, an auxiliary storage device such as a USB (Universal Serial Bus) memory, and stores settings and the like necessary for measuring the measurement object T in advance.

[0040] The measurement processing unit 36 ​​in the controller 30 includes, for example, a wavelength swept light source that emits light while continuously changing the wavelength, a light receiving element that receives the returned light from the sensor head 20 and converts it into an electrical signal, and a signal processing circuit that processes the electrical signal. The measurement processing unit 36 ​​performs various processes using a control unit, a memory unit, etc., based on the returned light from the sensor head 20 so that the displacement of the measurement object T (the distance to the measurement object T) is ultimately calculated. These processes will be described in detail later.

[0041] 2 is a flowchart showing a procedure for measuring the measurement target T by the displacement sensor 10 according to the present disclosure. As shown in FIG. 2, the procedure includes steps S11 to S14.

[0042] In step S11, the sensor head 20 is installed. For example, guide light is irradiated from the sensor head 20 onto the measurement target T, and the sensor head 20 is installed in an appropriate position based on the guide light.

[0043] Specifically, the amount of received light returning from the sensor head 20 is displayed on the display unit 31 of the controller 30, and the user may adjust the orientation of the sensor head 20 and the distance (height position) from the measurement object T while checking the amount of received light. Basically, if the light from the sensor head 20 can be irradiated perpendicularly to the measurement object T (at an angle closer to perpendicular), the amount of reflected light from the measurement object T will be large and the amount of received light returning from the sensor head 20 will also be large.

[0044] Furthermore, depending on the distance between the sensor head 20 and the object T to be measured, the objective lens 21 may be replaced with one having an appropriate focal length.

[0045] Furthermore, if appropriate settings cannot be made when measuring the measurement object T (for example, the amount of light received required for measurement cannot be obtained, or the focal length of the objective lens 21 is inappropriate), an error or incomplete settings may be displayed on the display unit 31 or output to an externally connected device to notify the user.

[0046] In step S12, various measurement conditions are set when measuring the measurement object T. For example, the user sets the inherent calibration data (such as a function for correcting linearity) of the sensor head 20 by operating the setting unit 32 in the controller 30.

[0047] Various parameters may also be set. For example, a sampling time, a measurement range, and a threshold for determining whether a measurement result is normal or abnormal may be set. Furthermore, a measurement cycle may be set according to the characteristics of the measurement object T, such as the reflectance and material of the measurement object T, and a measurement mode may be set according to the material of the measurement object T.

[0048] These measurement conditions and various parameters are set by operating the setting unit 32 in the controller 30, but they may also be set from an externally connected device or by importing data from the external memory unit 35.

[0049] In step S13, the sensor head 20 installed in step S11 measures the measurement target T in accordance with the measurement conditions and various parameters set in step S12.

[0050] Specifically, in the measurement processing unit 36 ​​of the controller 30, light is projected from the wavelength swept light source, and the light returning from the sensor head 20 is received by a light receiving element, and frequency analysis, distance conversion, peak detection, etc. are performed by a signal processing circuit to calculate the displacement of the measurement object T (the distance to the measurement object T). Specific details of the measurement processing will be described later.

[0051] In step S14, the measurement results measured in step S13 are output. For example, the displacement of the measurement object T (distance to the measurement object T) measured in step S13 is displayed on the display unit 31 of the controller 30 or output to an externally connected device.

[0052] In addition, whether the displacement of the measurement object T (distance to the measurement object T) measured in step S13 is within a normal range or abnormal based on the threshold set in step S12 may also be displayed or output as a measurement result. Furthermore, the measurement conditions, various parameters, measurement mode, etc. set in step S12 may also be displayed or output.

[0053] [System overview including displacement sensors] Fig. 3 is a functional block diagram showing an overview of a sensor system 1 that uses a displacement sensor 10 according to the present disclosure. As shown in Fig. 3, the sensor system 1 includes the displacement sensor 10, a control device 11, a control signal input sensor 12, and an external connection device 13. Note that the displacement sensor 10 is connected to the control device 11 and the external connection device 13 by, for example, a communication cable or an external connection cord (including, for example, an external input line, an external output line, a power line, etc.), and the control device 11 and the control signal input sensor 12 are connected by a signal line.

[0054] 1 and 2, the displacement sensor 10 measures the displacement of the measurement object T (the distance to the measurement object T). The displacement sensor 10 may then output the measurement results and the like to the control device 11 and the externally connected device 13.

[0055] The control device 11 is, for example, a PLC (Programmable Logic Controller), and gives various instructions to the displacement sensor 10 when the displacement sensor 10 measures the measurement target T.

[0056] For example, the control device 11 may output a measurement timing signal to the displacement sensor 10 based on an input signal from a control signal input sensor 12 connected to the control device 11, or may output a zero reset command signal (a signal for setting the current measurement value to 0) or the like to the displacement sensor 10.

[0057] The control signal input sensor 12 outputs an on / off signal to the control device 11, which indicates the timing at which the displacement sensor 10 measures the measurement object T. For example, the control signal input sensor 12 may be installed near a production line on which the measurement object T moves, and may output an on / off signal to the control device 11 upon detecting that the measurement object T has moved to a predetermined position.

[0058] The externally connected device 13 is, for example, a PC (Personal Computer), and various settings can be made to the displacement sensor 10 by the user operating it.

[0059] As specific examples, the measurement mode, operation mode, measurement period, material of the measurement object T, etc. are set.

[0060] The measurement mode can be set to either an "internal synchronous measurement mode" in which measurement is started periodically within the control device 11, or an "external synchronous measurement mode" in which measurement is started in response to an input signal from outside the control device 11, or the like.

[0061] As the setting of the operation mode, an "operation mode" in which the measurement object T is actually measured, or an "adjustment mode" in which measurement conditions for measuring the measurement object T are set, etc., can be selected.

[0062] The measurement period is the period for measuring the measurement object T, and can be set according to the reflectance of the measurement object T. However, even if the reflectance of the measurement object T is low, the measurement object T can be measured appropriately by lengthening the measurement period and setting the measurement period appropriately.

[0063] For the measurement object T, a "rough surface mode" suitable for when diffuse reflection is a relatively large component of the reflected light, a "specular surface mode" suitable for when specular reflection is a relatively large component of the reflected light, or a "standard mode" that is intermediate between these two is selected.

[0064] In this way, by making appropriate settings depending on the reflectance and material of the measurement object T, the measurement object T can be measured with higher accuracy.

[0065] 4 is a flowchart showing a procedure for measuring a measurement target T by the sensor system 1 that uses the displacement sensor 10 according to the present disclosure. As shown in FIG. 4, this procedure is a procedure for the external synchronization measurement mode described above, and includes steps S21 to S24.

[0066] In step S21, the sensor system 1 detects a measurement target T, which is an object to be measured. Specifically, the control signal input sensor 12 detects that the measurement target T has moved to a predetermined position on the production line.

[0067] In step S22, the sensor system 1 issues a measurement instruction to measure the measurement object T detected in step S21 using the displacement sensor 10. Specifically, the control signal input sensor 12 outputs an on / off signal to the control device 11 to instruct the timing of measuring the measurement object T detected in step S21, and the control device 11 outputs a measurement timing signal to the displacement sensor 10 based on the on / off signal to instruct the displacement sensor 10 to measure the measurement object T.

[0068] In step S23, the measurement object T is measured by the displacement sensor 10. Specifically, the displacement sensor 10 measures the measurement object T based on the measurement instruction received in step S22.

[0069] In step S24, the sensor system 1 outputs the measurement result obtained in step S23. Specifically, the displacement sensor 10 displays the result of the measurement process on the display unit 31, or outputs the result to the control device 11 or the externally connected device 13 via the external I / F unit 33.

[0070] 4, the procedure for the external synchronization measurement mode in which the measurement object T is measured by detecting the measurement object T with the control signal input sensor 12 has been described, but the present invention is not limited to this. For example, in the internal synchronization measurement mode, instead of steps S21 and S22, a measurement timing signal is generated based on a preset cycle to instruct the displacement sensor 10 to measure the measurement object T.

[0071] Next, the principle of measurement of the measurement target T by the displacement sensor 10 according to the present disclosure will be described. 5A is a diagram illustrating the principle of measurement of a measurement object T by a displacement sensor 10 according to the present disclosure. As shown in FIG. 5A, the displacement sensor 10 includes a sensor head 20 and a controller 30. The sensor head 20 includes an objective lens 21 and a plurality of collimating lenses 22a to 22c, and the controller 30 includes a wavelength swept light source 51, an optical amplifier 52, a plurality of isolators 53 and 53a to 53b, a plurality of optical couplers 54 and 54a to 54e, an attenuator 55, a plurality of light receiving elements (e.g., photodetectors (PD)) 56a to 56c, a multiplexing circuit 57, an analog-to-digital (AD) conversion unit (e.g., an analog-to-digital converter) 58, a processing unit (e.g., a processor) 59, a balanced detector 60, and a correction signal generation unit 61.

[0072] The wavelength swept light source 51 emits a laser beam with a swept wavelength. For example, if a method of modulating a VCSEL (Vertical Cavity Surface Emitting Laser) with current is applied as the wavelength swept light source 51, mode hopping is unlikely to occur due to a short cavity length, wavelength change is easy, and it can be realized at low cost.

[0073] The optical amplifier 52 amplifies the light emitted from the wavelength swept light source 51. The optical amplifier 52 may be, for example, an erbium-doped fiber amplifier (EDFA), and may be, for example, an optical amplifier dedicated to 1550 nm.

[0074] The isolator 53 is an optical element that transmits incident light in one direction, and may be disposed immediately after the wavelength swept light source 51 to prevent the influence of noise caused by returning light.

[0075] In this way, the light emitted from the wavelength swept light source 51 is amplified by the optical amplifier 52, passes through the isolator 53, and is branched to the main interferometer and the sub interferometer by the optical coupler 54. For example, the optical coupler 54 may be configured to branch the light to the main interferometer and the sub interferometer at a ratio of 90:10 to 99:1.

[0076] The light branched to the main interferometer is further branched by the first-stage optical coupler 54a into the direction of the sensor head 20 and the direction of the second-stage optical coupler 54b.

[0077] The light branched by the first-stage optical coupler 54a toward the sensor head 20 passes from the tip of the optical fiber through the collimator lens 22a and the objective lens 21 in the sensor head 20 and is irradiated onto the measurement object T. Then, the tip (end face) of the optical fiber serves as a reference surface, and the light reflected by the reference surface interferes with the light reflected by the measurement object T, generating interference light that returns to the first-stage optical coupler 54a and is then received by the light-receiving element 56a and converted into an electrical signal.

[0078] The light branched by the first-stage optical coupler 54a toward the second-stage optical coupler 54b travels through the isolator 53a to the second-stage optical coupler 54b, where it is further branched by the second-stage optical coupler 54b toward the sensor head 20 and the third-stage optical coupler 54c. As in the first stage, the light branched from the optical coupler 54b toward the sensor head 20 passes from the tip of the optical fiber through the collimator lens 22b and the objective lens 21 in the sensor head 20 and is irradiated onto the measurement object T. The tip (end face) of the optical fiber serves as a reference surface, and the light reflected by the reference surface and the light reflected by the measurement object T interfere with each other to generate interference light. This interference light returns to the second-stage optical coupler 54b and is branched by the optical coupler 54b toward the isolator 53a and the light-receiving element 56b, respectively. The light branched from the optical coupler 54b toward the light-receiving element 56b is received by the light-receiving element 56b and converted into an electrical signal. On the other hand, the isolator 53a is an example of a suppression means and a blocking unit, and transmits light from the upstream optical coupler 54a to the downstream optical coupler 54b and blocks light from the downstream optical coupler 54b to the upstream optical coupler 54a, thereby blocking the light branched from the optical coupler 54b toward the isolator 53a.

[0079] The light branched by the second-stage optical coupler 54b toward the third-stage optical coupler 54c travels through the isolator 53b to the third-stage optical coupler 54c, where it is further branched toward the sensor head 20 and the attenuator 55. Similar to the first and second stages, the light branched from the optical coupler 54c toward the sensor head 20 passes through the collimator lens 22c and the objective lens 21 from the tip of the optical fiber in the sensor head 20, and is then irradiated onto the measurement target T. The light reflected from the tip (end face) of the optical fiber and the light reflected from the measurement target T interfere with each other, generating interference light. The interference light returns to the third-stage optical coupler 54c, and is branched from the optical coupler 54c toward the isolator 53b and the light-receiving element 56c. The isolator 53b is an example of a suppression means, and suppresses the supply of light from the subsequent-stage optical coupler 54c to the previous-stage optical coupler 54b. The light branched from the optical coupler 54c toward the light receiving element 56c is received by the light receiving element 56c and converted into an electrical signal.

[0080] The light branched in the direction of the attenuator 55 by the third-stage optical coupler 54c is not used for measuring the measurement object T, and is therefore attenuated so as not to be reflected back.

[0081] In this way, the main interferometer has three optical paths (three channels), each with an optical path length difference that is twice the distance (round trip) from the tip (end face) of the optical fiber of the sensor head 20 to the object to be measured T, and generates three interference lights corresponding to the optical path length difference.

[0082] As described above, the light receiving elements 56a to 56c receive the interference light from the main interferometer and generate electrical signals according to the amount of light received.

[0083] The multiplexing circuit 57 multiplexes the electrical signals output from the light receiving elements 56a to 56c.

[0084] The AD conversion unit 58 receives the electrical signal from the multiplexing circuit 57 and converts the electrical signal from an analog signal to a digital signal (AD conversion). Here, the AD conversion unit 58 performs AD conversion based on the correction signal from the correction signal generation unit 61 in the sub-interferometer.

[0085] In order to correct the nonlinearity of the wavelength when the wavelength swept light source 51 is swept, the secondary interferometer acquires an interference signal and generates a correction signal called a K clock.

[0086] Specifically, the light branched to the sub-interferometer by optical coupler 54 is further branched by optical coupler 54d. Here, the optical paths of the branched light are configured to have an optical path length difference, for example, by using optical fibers of different lengths between optical coupler 54d and optical coupler 54e, and interference light corresponding to the optical path length difference is output from optical coupler 54e. Then, balance detector 60 receives the interference light from optical coupler 54e and amplifies the optical signal and converts it into an electrical signal while removing noise by taking the difference with a signal of the opposite phase.

[0087] The optical coupler 54d and the optical coupler 54e may both branch light at a ratio of 50:50.

[0088] The correction signal generator 61 determines the nonlinearity of the wavelength during the sweep of the wavelength swept light source 51 based on the electrical signal from the balance detector 60, generates a K clock according to the nonlinearity, and outputs it to the AD converter 58.

[0089] Due to the nonlinearity of the wavelength when the wavelength swept light source 51 is swept, the waves of the analog signal input to the AD converter 58 in the main interferometer are not spaced at equal intervals. The AD converter 58 performs AD conversion (sampling) by correcting the sampling time based on the K clock described above so that the waves are spaced at equal intervals.

[0090] As described above, the K clock is a correction signal used to sample the analog signal of the main interferometer, and therefore needs to be generated at a higher frequency than the analog signal of the main interferometer. Specifically, the optical path length difference between the optical couplers 54d and 54e in the sub interferometer may be made longer than the optical path length difference between the tip (end face) of the optical fiber in the main interferometer and the measurement object T, or the frequency may be multiplied (e.g., eight times) by the correction signal generator 61 to increase the frequency.

[0091] The processing unit 59 acquires the digital signal that has been AD converted while the nonlinearity has been corrected by the AD conversion unit 58, and calculates the displacement of the measurement object T (the distance to the measurement object T) based on the digital signal. Specifically, the processing unit 59 converts the frequency of the digital signal using a fast Fourier transform (FFT), and calculates the distance by analyzing the frequency converted digital signal. Detailed processing in the processing unit 59 will be described later.

[0092] Since the processing unit 59 is required to perform high-speed processing, it is often realized by an integrated circuit such as an FPGA (field-programmable gate array).

[0093] Moreover, here, the multiplexing circuit 57 is arranged before the AD conversion unit 58, but it may be arranged after the AD conversion unit 58. The outputs from the multiple PDs 56a to 56c are each AD converted and then multiplexed by the multiplexing circuit 57.

[0094] Also, here, three optical paths are provided in the main interferometer, and the sensor head 20 irradiates measurement light from each optical path onto the measurement object T, and based on the interference light (return light) obtained from each, the distance to the measurement object T and the like are measured (multi-channel). The number of channels in the main interferometer is not limited to three, and may be one, two, or four or more.

[0095] 5B is a diagram illustrating another principle by which the measurement target T is measured by the displacement sensor 10 according to the present disclosure. As shown in FIG. 5B, the displacement sensor 10 includes a sensor head 20 and a controller 30. The sensor head 20 includes an objective lens 21 and a plurality of collimating lenses 22a to 22c, and the controller 30 includes a wavelength swept light source 51, an optical amplifier 52, a plurality of isolators 53 and 53a to 53b, a plurality of optical couplers 54 and 54a to 54j, an attenuator 55, a plurality of light receiving elements (e.g., photodetectors (PD)) 56a to 56c, a multiplexing circuit 57, an analog-to-digital (AD) conversion unit (e.g., an analog-to-digital converter) 58, a processing unit (e.g., a processor) 59, a balanced detector 60, and a correction signal generation unit 61. The displacement sensor 10 shown in FIG. 5B differs from the configuration of the displacement sensor 10 shown in FIG. 5A mainly in that it includes optical couplers 54f to 54j. The principle of this different configuration will be described in detail below in comparison with FIG. 5A.

[0096] The light emitted from the wavelength swept light source 51 is amplified by an optical amplifier 52, passes through an isolator 53, and is branched by an optical coupler 54 to the main interferometer side and the sub-interferometer side. The light branched to the main interferometer side is further branched by an optical coupler 54f into measurement light and reference light.

[0097] As described in FIG. 5A, the measurement light is passed through the collimator lens 22a and the objective lens 21 by the first-stage optical coupler 54a, irradiated onto the measurement object T, and reflected by the measurement object T. Here, in FIG. 5A, the tip (end face) of the optical fiber is used as a reference surface, and light reflected from the reference surface interferes with light reflected by the measurement object T to generate interference light, but in FIG. 5B, no reference surface is provided for reflecting light. That is, in FIG. 5B, no light is reflected by the reference surface as in FIG. 5A, and therefore the measurement light reflected by the measurement object T returns to the first-stage optical coupler 54a.

[0098] Similarly, light branched from the first-stage optical coupler 54a toward the second-stage optical coupler 54b passes through the collimator lens 22b and the objective lens 21 by the second-stage optical coupler 54b, is irradiated onto the measurement object T, is reflected by the measurement object T, and returns to the second-stage optical coupler 54b. Light branched from the second-stage optical coupler 54b toward the third-stage optical coupler 54c passes through the collimator lens 22c and the objective lens 21 by the third-stage optical coupler 54c, is irradiated onto the measurement object T, is reflected by the measurement object T, and returns to the third-stage optical coupler 54c.

[0099] On the other hand, the reference light split by the optical coupler 54f is further split by the optical coupler 54g to optical couplers 54h, 54i, and 54j.

[0100] In the optical coupler 54h, the measurement light output from the optical coupler 54a and reflected by the measurement object T interferes with the reference light output from the optical coupler 54g, generating interference light that is received by the light-receiving element 56a and converted into an electrical signal. In other words, the measurement light and the reference light are split by the optical coupler 54f, and interference light is generated according to the difference in optical path length between the optical path of the measurement light (the optical path from the optical coupler 54f, via the optical coupler 54a, the collimator lens 22a, and the objective lens 21, reflected by the measurement object T, and reaching the optical coupler 54h) and the optical path of the reference light (the optical path from the optical coupler 54f, via the optical coupler 54g, to reach the optical coupler 54h). The interference light is received by the light-receiving element 56a and converted into an electrical signal.

[0101] Similarly, in the optical coupler 54i, interference light is generated according to the difference in optical path length between the optical path of the measurement light (the optical path from the optical coupler 54f, through the optical couplers 54a and 54b, the collimator lens 22b, and the objective lens 21, reflected by the measurement object T, and reaching the optical coupler 54i) and the optical path of the reference light (the optical path from the optical coupler 54f, through the optical coupler 54g, and reaching the optical coupler 54i), and the interference light is received by the photodetector 56b and converted into an electrical signal.

[0102] In the optical coupler 54j, interference light is generated according to the difference in optical path length between the optical path of the measurement light (the optical path from the optical coupler 54f, through the optical couplers 54a, 54b, and 54c, the collimator lens 22c, and the objective lens 21, reflected by the object to be measured T, and reaching the optical coupler 54j) and the optical path of the reference light (the optical path from the optical coupler 54f, through the optical coupler 54g, and reaching the optical coupler 54j), and the interference light is received by the photodetector 56c and converted into an electrical signal.

[0103] In this way, the main interferometer has three optical paths (three channels) and generates three interference lights according to the optical path length difference between the measurement light reflected by the measurement object T and input to the optical couplers 54h, 54i, and 54j, and the reference light input to the optical couplers 54h, 54i, and 54j, respectively, via the optical couplers 54f and 54g.

[0104] The optical path length difference between the measurement light and the reference light may be set to be different for each of the three channels, for example, the optical path lengths of the optical coupler 54g and the optical couplers 54h, 54i, and 54j may be set to be different.

[0105] Then, based on the interference light obtained from each, the distance to the measurement object T and the like are measured (multi-channel).

[0106] [Sensor head structure] Here, the structure of the sensor head used in the displacement sensor 10 will be described. FIG. 6A is a perspective view showing the schematic configuration of the sensor head 20, FIG. 6B is a perspective view showing the schematic configuration of a collimator lens holder arranged inside the sensor head 20, and FIG. 6C is a cross-sectional view showing the internal structure of the sensor head.

[0107] 6A, in the sensor head 20, the objective lens 21 and the collimator lens are stored in the objective lens holder 23. For example, the size of the objective lens holder 23 is such that the length of one side surrounding the objective lens 21 is about 10 mm, and the length in the optical axis direction is about 22 mm.

[0108] 6B, the collimating lens unit 24 is configured by fixing the collimating lens 22 to a collimating lens holder using an adhesive. The collimating lens unit 24 is configured so that the spot diameter can be adjusted by inserting an optical fiber into the holder. For example, the size of the collimating lens 22 is about 2 mm in diameter.

[0109] 6C, three collimating lenses 22a to 22c are held by collimating lens holders to form collimating lens units 24a to 24c, and three optical fibers are inserted into the collimating lens units 24a to 24c so as to correspond to the three collimating lenses 22a to 22c, respectively. Alternatively, each of the three optical fibers may be held by a collimating lens holder.

[0110] These optical fibers and collimator lens units 24a to 24c, together with the objective lens 21, are held by an objective lens holder 23 to form the sensor head 20.

[0111] Here, as shown in FIG. 6C, the three collimator lens units are arranged with offsets from one another in order to form different optical path length differences at positions in the optical axis direction of the sensor head 20.

[0112] Furthermore, the objective lens holder 23 and the collimator lens units 24a to 24c that constitute the sensor head 20 may be made of a metal (for example, A2017) that is strong and can be processed with high precision.

[0113] Fig. 7 is a block diagram for explaining signal processing in the controller 30. As shown in Fig. 7, the controller 30 includes a plurality of light receiving elements 71a to 71e, a plurality of amplifier circuits 72a to 72c, a multiplexing circuit 73, an AD conversion unit 74, a processing unit 75, a differential amplifier circuit 76, and a correction signal generation unit 77.

[0114] As shown in Figure 5, the controller 30 splits the light emitted from the wavelength swept light source 51 into a main interferometer and a sub interferometer using an optical coupler 54, and calculates the distance to the measurement object T by processing the main interference signal and the sub interferometer signal obtained from each.

[0115] The plurality of light receiving elements 71a to 71c correspond to the light receiving elements 56a to 56c shown in FIG. 5A, and receive the main interference signal from the main interferometer and output it as a current signal to the amplifier circuits 72a to 72c, respectively.

[0116] The plurality of amplifier circuits 72a to 72c convert the current signals into voltage signals (IV conversion) and amplify them.

[0117] The multiplexing circuit 73 multiplexes the voltage signals output from the amplifier circuits 72a to 72c and outputs the result to the AD conversion unit 74 as one voltage signal.

[0118] AD conversion section 74 corresponds to AD conversion section 58 shown in FIG. 5A, and converts the voltage signal into a digital signal (AD conversion) based on K clocks from correction signal generation section 77, which will be described later.

[0119] Processing unit 75 corresponds to processing unit 59 shown in FIG. 5A, converts the digital signal from AD conversion unit 74 into a frequency using FFT, analyzes it, and calculates the distance value to measurement object T.

[0120] The multiple light receiving elements 71d to 71e and the differential amplifier circuit 76 correspond to the balanced detector 60 shown in Figure 5A, and each receive the interference light in the sub-interferometer, one of which outputs an interference signal with an inverted phase, and by taking the difference between the two signals, noise is removed, while the interference signal is amplified and converted into a voltage signal.

[0121] 5A, the correction signal generating unit 77 binarizes the voltage signal using a comparator, generates a K clock, and outputs it to the AD converting unit 74. Since the K clock needs to be generated at a higher frequency than the analog signal of the main interferometer, the clock generating unit 77 may multiply the frequency (for example, by 8 times) to increase the frequency.

[0122] 7, the multiplexing circuit 73 is arranged before the AD conversion unit 74, but may be arranged after the AD conversion unit 74. The outputs of the plurality of light receiving elements 71a to 71c and the plurality of amplifier circuits 72a to 72c are each AD converted, and then multiplexed by the multiplexing circuit 73.

[0123] 8 is a flowchart showing a method for calculating the distance to the measurement object T, which is executed by the processing unit 59 in the controller 30. As shown in FIG. 8, the method includes steps S31 to S35.

[0124] In step S31, processing unit 59 performs frequency conversion on the waveform signal (voltage vs. time) into a spectrum (voltage vs. frequency) using the following FFT: Fig. 9A is a diagram showing how the waveform signal (voltage vs. time) is frequency converted into a spectrum (voltage vs. frequency).

number

[0125] In step S32, the processing unit 59 performs distance conversion on the spectrum (voltage vs. frequency) to convert it into a spectrum (voltage vs. distance). Fig. 9B is a diagram showing how the spectrum (voltage vs. frequency) is converted into a spectrum (voltage vs. distance).

[0126] In step S33, processing unit 59 calculates values ​​(distance value, SNR) corresponding to the peak based on the spectrum (voltage vs. distance). Fig. 9C is a diagram showing how values ​​(distance value, SNR) corresponding to the peak are calculated based on the spectrum (voltage vs. distance).

[0127] (1) Calculate the peak value of the voltage. Specifically, for the voltage shown in FIG. 9C, the pair of the distance value and the voltage value at the distance where the differential value of the voltage changes from positive to negative (D x ,V x ) and sort the sets in descending order of voltage value. (D1,V1),(D2,V2),(D3,V3),...,(D n ,V n )

[0128] (2) Combinations exceeding the number of multi-heads are excluded. For example, as shown in FIG. 5A, the displacement sensor 10 has three optical paths in the main interferometer, and the sensor head 20 irradiates the measurement object T with measurement light from each optical path, and receives the interference light (return light) obtained from each (number of multi-heads = 3). If there are four or more peaks, peaks exceeding three are due to noise and can be excluded from the calculation. When the number of multi-heads is 3, the results are (D1, V1), (D2, V2), and (D3, V3).

[0129] (3) Sort by distance. For example, sorting by distance from shortest to longest would be (D3, V3), (D1, V1), (D2, V2).

[0130] (4) Obtain the peak-to-peak voltage. Specifically, D 31 Voltage V 31and obtain the intermediate distance D between D1 and D2. 12 Voltage V 12 Then, the average voltage Vn = (V 31 +V 12 ) / 2.

[0131] (5) Calculate the SNR of each signal. Specifically, SN1 = V1 / V n , SN2=V2 / V n , SN3=V3 / V n This becomes:

[0132] In this way, values ​​(distance value, SNR)=(D1, SN1), (D2, SN2), (D3, SN3) corresponding to the peaks are calculated based on the spectrum (voltage vs. distance).

[0133] 8, in step S34, the processing unit 59 corrects the distance values ​​among the values ​​(distance values, SNRs) corresponding to the peaks calculated in step S33. Specifically, as shown in Fig. 6C, the three collimator lens units 24a to 24c (collimator lenses 22a to 22c and the optical fibers) are arranged with a deviation from each other in the optical axis direction of the sensor head 20, and therefore the processing unit 59 corrects the distance values ​​D1, D2, and D3 corresponding to the peaks, respectively, in accordance with the deviation amounts (for example, h1, h2, h3, etc.).

[0134] As a result, the values ​​corresponding to the peaks (corrected distance value, SNR)=(D1+h1, SN1), (D2+h2, SN2), (D3+h3, SN3).

[0135] In step S35, processing unit 59 averages the distance values ​​among the values ​​(corrected distance values, SNR) corresponding to the peaks calculated in step S34. Specifically, processing unit 59 preferably averages the corrected distance values ​​having an SNR equal to or greater than a threshold among the values ​​(corrected distance values, SNR) corresponding to the peaks, and outputs the averaged calculation result as the distance to measurement object T.

[0136] Next, the present invention will be described in detail as a specific embodiment, focusing on its more characteristic configurations, functions, and properties. Note that the optical interferometric distance measuring sensor shown below corresponds to the displacement sensor 10 described with reference to Figures 1 to 9, and all or part of the basic configurations, functions, and properties included in the optical interferometric distance measuring sensor are common to the configurations, functions, and properties included in the displacement sensor 10 described with reference to Figures 1 to 9.

[0137] First Embodiment 10 is a schematic diagram showing the overall configuration of an optical interferometric distance measuring sensor 100 according to a first embodiment of the present invention. As shown in FIG. 10, the optical interferometric distance measuring sensor 100 includes a wavelength swept light source 110, optical couplers 120a to 120c, an attenuator 122, interferometers 130a to 130c, light receiving units 140a to 140c, and a processing unit 150. When it is not necessary to distinguish between the optical couplers 120a to 120c, they may be simply referred to as optical couplers 120. When it is not necessary to distinguish between the interferometers 130a to 130c, they may be simply referred to as interferometers 130. When it is not necessary to distinguish between the light receiving units 140a to 140c, they may be simply referred to as light receiving units 140. The optical interferometer ranging sensor 100 shown in FIG. 10 is configured as a multi-stage optical interferometer ranging sensor, and as an example, is configured as a three-stage optical interferometer ranging sensor having three interferometers, but the number of interferometers (i.e., the number of stages) may be two, four or more.

[0138] The wavelength swept light source 110 is connected to the first port a1 of the optical coupler 120 directly or indirectly via other components (optical amplifier 52, isolator 53, optical coupler 54, etc.), and emits light while continuously changing the wavelength.

[0139] Optical couplers 120a to 120c are connected in series to form a three-stage configuration: optical coupler 120a forms the first stage corresponding to interferometer 130a, optical coupler 120b forms the second stage corresponding to interferometer 130b, and optical coupler 120c forms the third stage corresponding to interferometer 130c.

[0140] Each optical coupler 120 has 2x2 (four) ports, and light input to one port at one end is output to two ports at the other end at a predetermined branching ratio. Specifically, the first-stage optical coupler 120a has a first port a1, a second port a2, a third port a3, and a fourth port a4. Light input to the first port a1 or the second port a2 is output to the third port a3 and the fourth port a4 at a predetermined branching ratio. Furthermore, light input to the third port a3 or the fourth port a4 is output to the first port a1 and the second port a2 at a predetermined branching ratio.

[0141] The second-stage optical coupler 120b has a first port b1, a second port b2, a third port b3, and a fourth port b4. Light input to the first port b1 or the second port b2 is output to the third port b3 and the fourth port b4 at a predetermined branching ratio. Light input to the third port b3 or the fourth port b4 is output to the first port b1 and the second port b2 at a predetermined branching ratio.

[0142] The third-stage optical coupler 120c has a first port c1, a second port c2, a third port c3, and a fourth port c4. Light input to the first port c1 or the second port c2 is output to the third port c3 and the fourth port c4 at a predetermined branching ratio. Light input to the third port c3 or the fourth port c4 is output to the first port c1 and the second port c2 at a predetermined branching ratio.

[0143] A first port a1 of the first-stage optical coupler 120a is connected to the wavelength swept light source 110, and light whose wavelength changes continuously is input from the wavelength swept light source 110 directly or indirectly.

[0144] The first-stage optical coupler 120a branches light input from the wavelength swept light source 110 to the first port a1 at a predetermined branching ratio and outputs the branched light to the third port a3 and the fourth port a4. The light output from the third port a3 of the first-stage optical coupler 120a is input to the first-stage interferometer 130a. The light output from the fourth port a4 of the first-stage optical coupler 120a is input to the first port b1 of the second-stage optical coupler 120b.

[0145] The second-stage optical coupler 120b branches the light input to the first port b1 from the first-stage optical coupler 120a to the third port b3 and the fourth port b4 at a predetermined branching ratio. The light output from the third port b3 of the second-stage optical coupler 120b is input to the second-stage interferometer 130b. The light output from the fourth port b4 of the second-stage optical coupler 120b is input to the first port c1 of the third-stage optical coupler 120c.

[0146] The third-stage optical coupler 120c branches the light input from the second-stage optical coupler 120b to the first port c1 at a predetermined branching ratio and outputs the branched light to the third port c3 and the fourth port c4. The light output from the third port c3 of the third-stage optical coupler 120c is input to the third-stage interferometer 130c. The light output from the fourth port c4 of the third-stage optical coupler 120c is input to the attenuator 122.

[0147] The interferometers 130a to 130c have sensor heads 131a to 131c, respectively. The sensor heads 131a to 131c have objective lenses 132a to 132c, respectively. Each of the sensor heads 131a to 131c may have a collimating lens disposed between the tip of the optical fiber and the objective lens 132a to 132c.

[0148] Light input from the third port a3 of the first-stage optical coupler 120a to the first-stage interferometer 130a is input to the sensor head 131a via the optical fiber. A portion of the light input to the sensor head 131a is irradiated as measurement light onto the measurement object T via the objective lens 132a and reflected by the measurement object T. The measurement light reflected by the measurement object T is then collected by the objective lens 132a of the sensor head 131a and input to the sensor head 131a. Another portion of the light input to the sensor head 131a is reflected as reference light by a reference surface provided at the tip of the optical fiber. The measurement light and the reference light interfere with each other at the reference surface of the sensor head 131a, generating first interference light corresponding to the optical path length difference between the measurement light and the reference light. The first interference light is output from the interferometer 130a and input to the third port a3 of the optical coupler 120a.

[0149] The light input from the third port b3 of the second-stage optical coupler 120b to the second-stage interferometer 130b is input to the sensor head 131b via the optical fiber. A portion of the light input to the sensor head 131b is irradiated as measurement light onto the measurement object T via the objective lens 132b and reflected by the measurement object T. The measurement light reflected by the measurement object T is then collected by the objective lens 132b of the sensor head 131b and input to the sensor head 131b. Another portion of the light input to the sensor head 131b is reflected as reference light by a reference surface provided at the tip of the optical fiber. The measurement light and the reference light interfere with each other at the reference surface of the sensor head 131b, generating second interference light corresponding to the optical path length difference between the measurement light and the reference light. The second interference light is output from the interferometer 130b and input to the third port b3 of the optical coupler 120b.

[0150] The light input from the third port c3 of the third-stage optical coupler 120c to the third-stage interferometer 130c is input to the sensor head 131c via the optical fiber. A portion of the light input to the sensor head 131c is irradiated as measurement light onto the measurement object T via the objective lens 132c and reflected by the measurement object T. The measurement light reflected by the measurement object T is then collected by the objective lens 132c of the sensor head 131c and input to the sensor head 131c. Another portion of the light input to the sensor head 131c is reflected as reference light by a reference surface provided at the tip of the optical fiber. The measurement light and the reference light interfere with each other at the reference surface of the sensor head 131c, generating third interference light corresponding to the optical path length difference between the measurement light and the reference light. The third interference light is output from the interferometer 130c and input to the third port c3 of the optical coupler 120c.

[0151] The attenuator 122 attenuates the light input from the fourth port c4 of the optical coupler 120c to reduce the light reflected back to the optical coupler 120c. By reducing the reflected light, the influence of phase noise can be reduced, and the optical interferometric distance measuring sensor 100 can measure the distance to the measurement target T with higher accuracy.

[0152] The optical element connected to the end of the optical fiber connected to the optical coupler 120c is not limited to the attenuator 122, but may be another optical element. For example, an isolator may be connected, or the tip of the optical fiber may be processed to use a coreless fiber or the like. Even in these cases, it is preferable to reduce the reflected light to the optical coupler 120c by applying, for example, fusion splicing and APC polishing, and thereby reduce the influence of the above-mentioned phase noise.

[0153] The light receiving units 140a to 140c include light receiving elements 141a to 141c and AD conversion units 142a to 141c, respectively. The light receiving elements 141a to 141c are, for example, photodetectors that receive light output from the second ports a2 to c2 of the optical couplers 120a to 120c and convert the received light into an electrical signal. The AD conversion units 142a to 142c convert the electrical signal from an analog signal to a digital signal.

[0154] The light receiving sections 140a to 140c correspond to the optical couplers 120a to 120c, respectively, and receive the light output from the second ports a2 to c2 of the optical couplers 120a to 120c, respectively.

[0155] As described above, the first interference light generated by the first-stage interferometer 130a is output from the interferometer 130a and input to the third port a3 of the optical coupler 120a. The first-stage optical coupler 120a then branches the first interference light input to the third port a3 to the first port a1 and the second port a2 at a predetermined branching ratio and outputs the branched light. The light receiving unit 140a receives the light output from the second port a2 of the optical coupler 120a, generates a digital signal based on the received light, and supplies the digital signal to the processing unit 150.

[0156] As described above, the second interference light generated by the second-stage interferometer 130b is output from the interferometer 130b and input to the third port b3 of the optical coupler 120b. The second-stage optical coupler 120b then branches the second interference light input to the third port b3 to the first port b1 and the second port b2 at a predetermined branching ratio and outputs the branched light. The light receiving unit 140b receives the light output from the second port b2 of the optical coupler 120b, generates a digital signal based on the received light, and supplies the digital signal to the processing unit 150.

[0157] As described above, the third interference light generated by the third-stage interferometer 130c is output from the interferometer 130c and input to the third port c3 of the optical coupler 120c. The third-stage optical coupler 120c then branches the third interference light input to the third port c3 to the first port c1 and the second port c2 at a predetermined branching ratio and outputs the branched light. The light receiving unit 140c receives the light output from the second port c2 of the optical coupler 120c, generates a digital signal based on the received light, and supplies the digital signal to the processing unit 150.

[0158] The processing unit 150 calculates the distance to the measurement object T based on each digital signal converted by the light receiving units 140a to 140c. For example, the processing unit 150 is a processor realized by an integrated circuit such as an FPGA, and performs frequency conversion on each input digital signal using FFT, and calculates the distance to the measurement object T based on the frequency conversion.

[0159] The optical interferometric distance measuring sensor 100 according to the first embodiment includes a suppression unit that suppresses the supply of light from a subsequent stage to a previous stage in the multi-stage optical couplers 120. In particular, the optical interferometric distance measuring sensor 100 according to the first embodiment includes, as the suppression unit, an optical coupler 120 configured to split a larger amount of light into a subsequent stage than the amount of light split into a corresponding interferometer 130. For example, the first-stage optical coupler 120a may be an example of the suppression unit, and the ratio of the amount of light split into the corresponding interferometer 130c to the amount of light split into the subsequent stage (the second-stage optical coupler 120b) may be 10:90. Furthermore, the second-stage optical coupler 120b may be an example of the suppression unit, and the ratio of the amount of light split into the corresponding interferometer 130b to the amount of light split into the subsequent stage (the third-stage optical coupler 120c) may be 15:85. Furthermore, for example, the third-stage optical coupler 120c may be an example of a suppression means, and the ratio of "amount of light branched to corresponding interferometer 130c: amount of light branched to subsequent stage (attenuator 122)" may be "20:80." This suppresses the supply of light from the subsequent stage to the previous stage in a multi-stage optical coupler, thereby suppressing light returning from the subsequent stage coupler to the previous stage optical coupler, thereby improving the measurement accuracy of the optical interferometric distance measuring sensor.

[0160] In the optical interferometer ranging sensor 100 according to the first embodiment, the optical coupler 120 may be set so that the ratio of the amount of light branched to the corresponding interferometer 130 is relatively higher for an optical coupler 120 farther from the wavelength swept light source 110. That is, if the number of stages of optical couplers 120 included in the optical interferometer ranging sensor 100 is i (i is any natural number between 1 and n, where n is the number of stages of optical couplers 120 included in the optical interferometer ranging sensor 100), and if the branching ratio, which is the ratio of the amount of light branched to the corresponding interferometer 130 to the amount of light branched to the subsequent stage for the i-th stage optical coupler 120, is Ri, then R i+1 ≧R i For example, in the above example, R1 is "10 / 90", R2 is "15 / 85", and R3 is "20 / 80". Therefore, R3 ≥ R2 ≥ R1, and R i+1 ≧R i is satisfied (where i=1, 2, 3). This reduces the variation in the amount of light received by each interferometer, making it possible to improve the measurement accuracy of the optical interferometer distance measuring sensor.

[0161] In the optical interferometer distance measuring sensor 100 according to the first embodiment, the optical path length from the optical coupler 120 in the i-th stage to the reference surface of the corresponding interferometer 130 is defined as L CR,i Let L be the optical path length from the optical coupler 120 in the i-th stage to the optical coupler 120 in the (i+1)-th stage. CC,i In this case, |L CR,i -(L CR,i+1 +L CC,i )| may be set to be equal to or greater than a predetermined threshold (first threshold) (where "||" is a symbol indicating an absolute value). CR,i and L CC,i may be a value obtained by multiplying the spatial length of the optical path by the refractive index. This suppresses interference between the multiple interfering light beams generated by the multiple interferometers, thereby improving the measurement accuracy of the optical interferometer distance measuring sensor.

[0162] In particular, |L CR,i -(L CR,i+1 +L CC,iThe threshold value (first threshold value) for || may be determined based on the frequency band of the light receiving unit 140. This makes it possible to effectively suppress interference between multiple interfering light beams according to the frequency band of the light receiving unit, thereby improving the measurement accuracy of the optical interferometric distance measuring sensor.

[0163] Second Embodiment 11 is a schematic diagram showing the outline of the configuration of an optical interferometric ranging sensor 200 according to the second embodiment of the present invention. The optical interferometric ranging sensor 200 according to the second embodiment includes isolators 221a and 221b in addition to the components included in the optical interferometric ranging sensor 100 according to the first embodiment described above.

[0164] The isolators 221a and 221b are an example of a suppression means for suppressing the supply of light from a subsequent stage to a previous stage in a multi-stage optical coupler, and are particularly an example of a blocking unit configured to guide light from a previous stage optical coupler to a subsequent stage optical coupler but not guide light from a subsequent stage optical coupler to a previous stage optical coupler. As will be described in detail below, the isolators 221a and 221b are configured to guide light from the previous stage optical coupler 120 to a subsequent stage optical coupler but not guide light from a subsequent stage optical coupler to a previous stage optical coupler. This makes it possible to suppress return light from a subsequent stage optical coupler to a previous stage optical coupler, thereby improving the measurement accuracy of the optical interferometer distance measuring sensor.

[0165] The isolator 221a is optically connected between the fourth port a4 of the first-stage optical coupler 120a and the first port b1 of the second-stage optical coupler 120b. The isolator 221a is configured to guide light from the first-stage optical coupler 120a to the second-stage optical coupler 120b, but not to guide light from the second-stage optical coupler 120b to the first-stage optical coupler 120a. Therefore, the isolator 221a blocks the light that is output from the first port b1 and directed toward the first-stage optical coupler 120a, out of the second interference light input from the third port b3 to the optical coupler 120b as described above.

[0166] The isolator 221b is optically connected between the fourth port b4 of the second-stage optical coupler 120b and the first port c1 of the third-stage optical coupler 120c. The isolator 221b is configured to guide light from the second-stage optical coupler 120b to the third-stage optical coupler 120c, but not to guide light from the third-stage optical coupler 120c to the second-stage optical coupler 120b. Therefore, the isolator 221b blocks the light output from the first port c1 and directed toward the second-stage optical coupler 120cb, out of the third interference light input to the optical coupler 120c from the third port c3 as described above.

[0167] Reflected light may occur from the isolators 221a and 221b to the optical coupler 120 at the previous stage, and this reflected light may further interfere with the interference light received by the optical coupler 120 at the previous stage from the corresponding interferometer 130. Therefore, in the optical interferometer distance measuring sensor 200 according to the second embodiment, in order to prevent the reflected light from the isolators 221a and 221b to the optical coupler 120 at the previous stage from interfering with the interference light generated by the interferometer 130, for example, the following setting may be made. That is, the optical path length from the optical coupler at the i-th stage to the reference surface of the corresponding interferometer is defined as L CR,i Let L be the optical path length from the ith optical coupler to the connected downstream isolator. CI,i In this case, |L CR,i -L CI,i may be set so that | is equal to or greater than a predetermined threshold (second threshold). CR,i and L CI,i may be a value obtained by multiplying the spatial length of the optical path by the refractive index. This suppresses interference between the reflected light from the isolator (blocking part) and the interference light generated by the interferometer, making it possible to improve the measurement accuracy of the optical interferometer distance measuring sensor.

[0168] In particular, |L CR,i -L CI,iThe threshold (second threshold) for | may be determined based on the frequency band of the light receiving unit 140. This makes it possible to effectively suppress interference between the reflected light from the isolator (blocking unit) and the interference light generated by the interferometer according to the frequency band of the light receiving unit, thereby improving the measurement accuracy of the optical interferometric distance measuring sensor.

[0169] [Modified interferometer] In the above-described embodiment, the optical interferometer distance measuring sensor 100 uses a Fizeau interferometer that generates reference light by using the tip of an optical fiber as a reference surface in each of the interferometers 130a to 130c, but the interferometer is not limited to this.

[0170] FIG. 12 shows variations of an interferometer that generates interference light using measurement light and reference light. In FIG. 12(a), in optical paths A to C branched by branching unit 121, the tips (end faces) of the optical fibers are used as reference surfaces, and the tips of the optical fibers are positioned offset in the optical axis direction so that the optical path length difference is different. This is the configuration of interferometer 120 of optical interferometer distance measuring sensor 100 according to the present embodiment described above (Fizeau interferometer), and the reference surface may be configured to reflect light due to the difference in refractive index between the optical fiber and air (Fresnel reflection). Alternatively, the tips of the optical fibers may be coated with a reflective film, or the tips of the optical fibers may be anti-reflective coated and a separate reflective surface such as a lens surface may be disposed thereon.

[0171] In FIG. 12(b), optical paths A to C branched by branching unit 121 are formed with measurement optical paths Lm1 to Lm3 that guide measurement light to measurement object T and reference optical paths Lr1 to Lr3 that guide reference light, and a reference surface is disposed at the end of each reference optical path Lr1 to Lr3 (Michelson interferometer). The reference surface may be formed by coating a reflective film on the tip of an optical fiber, or by applying an anti-reflective coating to the tip of an optical fiber and disposing a separate reflective surface such as a lens surface. In this configuration, the optical path lengths of the measurement optical paths Lm1 to Lm3 are made the same, and optical path length differences are provided among the reference optical paths Lr1 to Lr3, so that the optical path length differences are different among the optical paths A to C. Since the optical path lengths of the measurement optical paths Lm1 to Lm3 can be made the same, the optical design of the sensor head can be simplified.

[0172] 12(c), optical paths A to C branched by branching unit 121 are formed as measurement optical paths Lm1 to Lm3 that guide measurement light to measurement object T and reference optical paths Lr1 to Lr3 that guide reference light, and balance detectors are arranged in the reference optical paths Lr1 to Lr3 (Mach-Zehnder interferometers). In this configuration, the optical path lengths of the measurement optical paths Lm1 to Lm3 are made the same, and optical path length differences are provided in the reference optical paths Lr1 to Lr3, so that the optical path length differences are different among the optical paths A to C. Because the optical path lengths of the measurement optical paths Lm1 to Lm3 can be made the same, the optical design of the sensor head can be simplified.

[0173] As described above, the interferometer is not limited to the Fizeau interferometer described in this embodiment, but may be, for example, a Michelson interferometer or a Mach-Zehnder interferometer. Any interferometer may be applied as long as it is possible to generate interference light by setting the optical path length difference between the measurement light and the reference light, or a combination of these or other configurations may be applied. [Explanation of symbols]

[0174] 1...sensor system, 10...displacement sensor, 11...control device, 12...sensor for control signal input, 13...external connection device, 20...sensor head, 21...objective lens, 22, 22a to 22c, collimating lens, 23...objective lens holder, 24, 24a to 24c...collimating lens unit, 30...controller, 31...display unit, 32...setting unit, 33...external interface (I / F) unit, 34...optical fiber connection unit, 35...external memory unit, 36...measurement processing unit, 40...optical fiber, 51...wavelength swept light source, 52...optical amplifier, 53, 53a to 53b...isolator, 54, 54a to 54e...optical coupler, 55...attenuator, 56a to 56c...light receiving element, 57...multiplexing circuit, 58...AD conversion unit, 59...processing unit, 60...balanced die detector, 61...correction signal generating unit, 71a to 71e...light receiving elements, 72a to 72c...amplifying circuit, 73...wave combining circuit, 74...AD conversion unit, 75...processing unit, 76...differential amplifier circuit, 77...correction signal generating unit, 100, 200...optical interferometric distance measuring sensor, 110...wavelength sweep light source, 120a to 120c...optical coupler, 122...attenuator, 130a to 130c...interferometer, 131a to 1 31c...sensor head, 132a to 132c...objective lenses, 140a to 140c...light receiving units, 141a to 141c...light receiving elements, 142a to 142c...AD conversion units, 150...processing units, 221a, 221b...isolators, a1 to a4, b1 to b4, c1 to c4...ports of optical couplers, T...measurement object, Lm1 to Lm3...measurement optical paths, Lr1 to Lr3...reference optical paths

Claims

1. a light source that projects light while continuously changing the wavelength; a plurality of interferometers that generate interference light based on measurement light reflected from a measurement object by guiding the supplied light to the measurement object and reference light that follows an optical path at least partially different from that of the measurement light; a plurality of optical couplers connected in series, each of which receives light from the light source at a front stage and branches the light to a corresponding one of the plurality of interferometers and a rear stage; a suppression means for suppressing the supply of light from a subsequent stage to a previous stage in the multi-stage optical coupler; a processing unit that calculates a distance to the measurement object based on the frequencies of the plurality of interference lights generated by the plurality of interferometers, The suppression means is an optical interferometric distance measuring sensor that includes at least one of the optical couplers in the multiple stages, which is configured so that the amount of light branched to the subsequent stage is greater than the amount of light branched to the corresponding interferometer.

2. When the ratio of the amount of light branched to the corresponding interferometer with respect to the amount of light branched to the subsequent stage for the ith optical coupler among the optical couplers in the multiple stages is Ri, R i+1 ≧R i The optical interferometric distance measuring sensor according to claim 1 , wherein the optical interferometric distance measuring sensor is set so that:

3. the interferometer generates interference light beams based on a first reflected light beam of the measurement light beam that is irradiated onto the measurement object and reflected by the measurement object, and a second reflected light beam of the reference light beam that is reflected by a reference surface; 3. The optical interferometric distance measuring sensor according to claim 1.

4. The optical path length from the ith optical coupler to the corresponding reference surface of the interferometer is L CR,i Let L be the optical path length from the optical coupler in the i-th stage to the optical coupler in the (i+1)-th stage. CC,i In this case, |L CR,i - (L CR,i+1 +L CC,i 4. The optical interferometric distance measuring sensor according to claim 3, wherein the difference (i.e., ...

5. The optical interferometric distance measuring sensor according to claim 4 , wherein the first threshold value is set based on a frequency band of a light receiving unit that converts the plurality of interference light beams into an electrical signal and supplies the electrical signal to the processing unit.

6. An optical interferometric ranging sensor described in any one of claims 3 to 5, wherein the suppression means includes a blocking section connected between two of the multiple stages of optical couplers and configured to guide light from the optical coupler in the previous stage to the optical coupler in the subsequent stage, but not guide light from the optical coupler in the subsequent stage to the optical coupler in the previous stage.

7. The optical path length from the ith optical coupler to the corresponding reference surface of the interferometer is L CR,i Let L be the optical path length from the i-th optical coupler to the cutoff part connected to it in the subsequent stage. CI,i In this case, |L CR,i -L CI,i The optical interferometric distance measuring sensor according to claim 6 , wherein | is set to be equal to or greater than a second threshold value.

8. The optical interferometric distance measuring sensor according to claim 7 , wherein the second threshold value is set based on a frequency band of a light receiving unit that converts the plurality of interference light beams into an electrical signal and supplies the electrical signal to the processing unit.

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