JP2018169265A - Distance measuring device and distance measuring method - Google Patents

Abstract

To provide a distance measuring device and a distance measuring method with which it is possible to carry out long range and high accuracy distance measurement at low cost by a compact configuration.SOLUTION: A distance measuring device comprises: a first light source for emitting measurement light of a first repetition frequency; a second light source for emitting reference light of a second repetition frequency different from the first repetition frequency; an interference signal detection unit for detecting an interference signal of first reflected light of the measurement light reflected at a reference plane, second reflected light of the measurement light reflected at a measurement object and the reference light emitted from the second light source; an envelope waveform generation unit for generating the envelope waveform of the interference signal; a peak position detection unit for detecting, from the envelope waveform, a first peak position that corresponds to the first interference signal component of the first reflected light and reference light and a second peak position that corresponds to the second interference signal component of the second reflected light and reference light; and a distance determination unit for determining the distance to the measurement object, with the reference plane serving as a point of reference, on the basis of the first peak position and the second peak position.SELECTED DRAWING: Figure 8

Description

  The present invention relates to a distance measuring device and a distance measuring method for measuring a distance to an object to be measured.

  As a distance measuring device for measuring a distance to a measurement object, a device using a TOF (Time-of-Flight) method, a device using an optical frequency comb, and a device using a multiwavelength interferometer are well known. A distance measuring device using the TOF method emits pulse-modulated measurement light toward a measurement object, and measures based on the measurement result of measuring the time until the measurement light is reflected by the measurement object and returns. Measure the distance to the object. When the TOF method is used, since the device configuration is simple, the distance measuring device can be manufactured at a relatively low cost. In addition, since the distance measuring device using TOF does not require a mechanical drive unit, high-speed measurement is possible.

  A distance measuring device using an optical frequency comb includes an interferometer that uses an optical frequency comb as measurement light, and an object to be measured from a reference position based on an envelope waveform (envelope) and phase of an interference signal obtained by the interferometer Is measured (for example, see Patent Documents 1 and 2). For example, the phase is compared between the self-beat frequency of the measurement signal of the optical frequency comb and the self-beat frequency of the reference signal, whereby the distance is obtained by the matching method. Thus, when the optical frequency comb is used, the accuracy of distance measurement is 1 μm, and a relatively good accuracy can be obtained.

  A distance measuring apparatus using a multi-wavelength interferometer includes a multi-wavelength interferometer using a plurality of laser beams having different wavelengths, and measures a distance to a measurement object using beat wavelengths of the plurality of laser beams (non- Patent Document 1). When a multiwavelength interferometer is used, high reproducibility (2σ = 2 nm or less) is realized.

JP 2013-7591 A JP 2010-14549 A

“Multiwavelength Interferometry (MWLI) Technology”, [online], [Search on March 21, 2017], Internet <http://www.taylor-hobson.jp/multi-wavelength-interferometry.html>

  By the way, when the TOF method is used, there is a problem that the reference position for distance measurement becomes unclear because the configuration of the interferometer is not used. Further, the measurement accuracy of the TOF method is about 1 mm, and is not suitable for high-precision distance measurement.

  Further, in the case of using the optical frequency comb, the configuration for generating the optical frequency comb, that is, the configuration for accurately controlling the offset frequency and the repetition frequency of the mode-locked laser light is complicated, so that the cost of the apparatus is increased. The problem of enlargement occurs. In addition, when the optical frequency comb is used, the distance resolution is limited by the measurement resolution of the phase, so it is necessary to improve the distance resolution using an ultra-high frequency electric signal (for example, 75 GHz in order to obtain a resolution of 1 μm). is there. For this reason, there exists a problem that the phase stability of the electric circuit of a distance measuring device affects measurement accuracy.

  Furthermore, in a distance measuring device using a multi-wavelength interferometer, it is necessary to use a plurality of laser beams whose wavelengths are stabilized, so that there is a problem that the device is expensive and large. In addition, when using a multi-wavelength interferometer, it is difficult to increase the beat wavelengths of a plurality of laser beams, so that there is a problem that the measurement range is as short as 1 mm to 2 mm.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a distance measuring device and a distance measuring method capable of performing distance measurement with a long range and high accuracy with a low cost and a compact configuration. And

  A distance measuring device for achieving an object of the present invention is a first light source that emits measurement light having a first repetition frequency, and emits measurement light that is incident on a reference surface and a measurement object, respectively. A light source, a second light source that emits reference light having a second repetition frequency different from the first repetition frequency, a first reflected light of measurement light reflected by the reference surface, and a measurement reflected by the measurement object An interference signal detection unit that detects an interference signal between the second reflected light of the light and the reference light emitted from the second light source, and an envelope waveform generation unit that generates an envelope waveform of the interference signal detected by the interference signal detection unit And the first peak position corresponding to the first interference signal component of the first reflected light and the reference light and the second interference signal component of the second reflected light and the reference light from the envelope waveform generated by the envelope waveform generation unit A peak position detector for detecting a second peak position Provided on the basis of the first peak position and the second peak positions Peak position detection section detects the distance determiner for determining the distance of the reference plane to the measurement object on the basis, the.

  According to this distance measuring device, an envelope waveform of an interference signal is generated, and each peak position is detected from the envelope waveform, so that the distance to the measurement object with reference to the reference plane can be obtained without using an optical frequency comb. Can be measured.

  In the distance measuring device according to another aspect of the present invention, the envelope waveform generation unit generates an envelope waveform using Hilbert transform. Thereby, an envelope waveform of the interference signal is obtained.

  In the distance measuring device according to another aspect of the present invention, the envelope waveform generation unit is based on a converted signal obtained by converting the interference signal into a Hilbert transform and a delayed signal obtained by delaying the interference signal in accordance with the delay of the converted signal caused by the Hilbert transform. Generate an envelope waveform.

  A distance measurement device according to another aspect of the present invention includes a repetition frequency acquisition unit that acquires a first repetition frequency of measurement light and a second repetition frequency of reference light, and the distance determination unit is detected by a peak position detection unit. The distance is determined based on the difference between the first peak position and the second peak position and the first repetition frequency and the second repetition frequency acquired by the repetition frequency acquisition unit. Thereby, the distance to the measuring object on the basis of the reference plane can be measured.

  In the distance measuring device according to another aspect of the present invention, the measurement light and the reference light are mode-locked laser light different from the optical frequency comb. This eliminates the need for a configuration for accurately controlling the offset frequency and the repetition frequency of the mode-locked laser beam, and eliminates the need to handle an ultra-high frequency electric signal as in the case of using an optical frequency comb. The apparatus has a low cost and a compact configuration.

  A distance measuring device according to another aspect of the present invention has an end face that reflects a part of the measurement light input from the first light source, emits the remainder of the measurement light toward the measurement object, and An end surface reflection type sensor head on which the first reflected light reflected by the measurement object is incident is provided, and the reference surface is an end surface of the sensor head. Thereby, the distance to the measuring object based on the end face of the sensor head can be measured.

  A distance measurement method for achieving the object of the present invention includes a first emission step of emitting measurement light having a first repetition frequency from a first light source and causing the measurement light to enter a reference plane and a measurement object, respectively. A second emission step of emitting a reference light having a second repetition frequency different from the first repetition frequency from the second light source; a first reflected light of the measurement light reflected by the reference surface; An interference signal detecting step for detecting an interference signal between the second reflected light of the measured light and the reference light emitted from the second light source, and an envelope waveform for generating an envelope waveform of the interference signal detected in the interference signal detecting step A first peak position corresponding to the first interference signal component of the first reflected light and the reference light and a second interference signal component of the second reflected light and the reference light from the generation waveform and the envelope waveform generated in the envelope waveform generation step; No. corresponding to A peak position detecting step for detecting the peak position; a distance determining step for determining a distance to the measurement object based on the reference plane based on the first peak position and the second peak position detected in the peak position detecting step; Have.

  The distance measuring device and the distance measuring method of the present invention can carry out long-range and high-precision distance measurement with a low-cost and compact configuration.

It is explanatory drawing for demonstrating the characteristic of a mode-locked laser beam. It is explanatory drawing for demonstrating the photoelectric conversion result of a mode lock laser beam. It is explanatory drawing for demonstrating the coherence of the pulse of the mode-locked laser beam radiate | emitted from the two mode-locked laser light sources. It is the graph which represented the electric field strength of both the measurement light combined with a prism, and reference light on the same time axis. It is the schematic of the distance measuring apparatus which measures the absolute distance to a measuring object. It is the graph which showed an example of the interference signal output to a signal processing part from a digitizer. It is explanatory drawing for demonstrating the subject at the time of detecting the peak position by a signal processing part. It is explanatory drawing for demonstrating the function of a signal processing part. It is explanatory drawing for demonstrating the production | generation of the envelope waveform by an envelope waveform generation part. It is a flowchart which shows the flow of the production | generation process of the envelope waveform by an envelope waveform generation part. It is a flowchart which shows the flow of the measurement process (distance measuring method of this invention) of the absolute distance to the measuring object by a distance measuring device.

[Characteristics of mode-locked laser light]
The distance measuring apparatus 10 (see FIG. 5) of the present embodiment performs distance measurement using two types of mode-locked laser light (MLL: Mode locked laser). First, the characteristics of the mode-locked laser beam will be described.

  FIG. 1 is an explanatory diagram for explaining the characteristics of a mode-locked laser beam. 1 is a graph representing the intensity of the mode-locked laser beam on the frequency axis, and 1B in FIG. 1 is a graph representing the electric field strength of the mode-locked laser beam on the time axis.

As shown in FIG. 1, the mode-locked laser beam is a laser beam having a plurality of frequencies whose phases are synchronized, in other words, a pulsed laser beam having a constant repetition frequency f _rep . The mode-locked laser light has optical frequencies arranged at every repetition frequency f _rep on the frequency axis (see reference numeral 1A), and an optical pulse is repeated at the frequency f _rep on the time axis (see reference numeral 1B). In mode-locked laser light, the virtual optical frequency start point (offset frequency) is called _fceo (carrier envelope offset).

The optical frequency comb is a mode-locked laser beam in which the repetition frequency f _rep and the offset frequency f _ceo are accurately controlled. Here, the “mode-locked laser beam” in the present specification refers to a mode-locked laser beam that is different from the optical frequency comb, that is, the repetition frequency f _rep and the offset frequency _f.sub.eo are not accurately controlled.

FIG. 2 is an explanatory diagram for explaining the photoelectric conversion result of the mode-locked laser beam. The mode-locked laser beam shown in the upper part of FIG. 2 is received by a photoelectric converter and subjected to photoelectric conversion (detection). When the frequency analysis is performed on the result of the photoelectric conversion, a peak is obtained at each repetition frequency f _{rep as} shown in the lower part of FIG. A self-beat signal whose frequency is observed is obtained.

FIG. 3 is an explanatory diagram for explaining the coherence of the pulses of the mode-locked laser light emitted from the two mode-locked laser light sources 3 and 4. As shown in FIG. 3, the mode-locked laser light source 3 (shown as “MLL1” in the figure) _emits a mode-locked laser beam having the first repetition frequency f _rep1 toward the prism 5 as the measurement light L1. The measurement light L 1 passes through the prism 5 and then enters the measurement corner cube prism 6. The measurement light L 1 incident on the measurement corner cube prism 6 is reflected toward the prism 5 by the measurement corner cube prism 6. The reflected measurement light L1 is incident on the prism 5 and then reflected from the prism 5 toward the prism 7.

On the other hand, the mode-locked laser light source 4 (indicated as “MLL2” in the _drawing ) _emits a mode-locked laser beam having the second repetition frequency f _rep2 (≠ f _rep1 ) toward the prism 7 as reference light L2. Thereby, the measurement light L1 (reflected light) and the reference light L2 are combined in the prism 7, and an interference signal between the measurement light L1 and the reference light L2 enters the photoelectric converter 8 and is photoelectrically converted. As the photoelectric converter 8, for example, a charge coupled device (CCD) type or a complementary metal oxide semiconductor (CMOS) type image sensor or a photodiode is used (a differential photoelectric converter 20 and photoelectric converters 21 and 22 described later are also used). The same).

FIG. 4 is a graph showing the electric field strengths of both the measurement light L1 (reflected light) and the reference light L2 combined by the prism 7 on the same time axis. As shown in FIG. 4, the optical pulse of the measurement light L1 (reflected light) is shifted from the reference light L2 by slightly shifting the first repetition frequency f _rep1 of the measurement light L1 and the second repetition frequency f _rep2 of the reference light L2. The same effect as when scanning with a light pulse can be obtained. In the present invention, this effect is used to perform distance measurement. The symbol “q” in the figure indicates the scanning interval [| c / f _rep1 −c / f _rep2 | for each sample of the optical pulse when the optical pulse of the measuring light L1 is scanned with the optical pulse of the reference light L2. , C: speed of light].

[Configuration of Distance Measuring Device of this Embodiment]
FIG. 5 is a schematic diagram of the distance measuring device 10 that measures the absolute distance D to the measuring object 9. As shown in FIG. 5, the distance measuring device 10 includes a mode-locked laser light source 12 (shown as “MLL1” in the figure), a mode-locked laser light source 13 (shown as “MLL2” in the figure), a coupler 15, , Fiber circulator 16, sensor head 17, coupler 18, coupler 19, differential photoelectric converter 20, single-ended photoelectric converters 21 and 22, mixer 23, and low-pass filter 24 (in the figure). “LPF”), a frequency counter 25, a digitizer 26, a signal processing unit 27, a display unit 28, and a storage unit 29.

The mode-locked laser light source 12 (corresponding to the first light source of the present invention) is connected to the coupler 15 via the optical fiber cable F1. The mode-locked laser light source 12 _emits a mode-locked laser beam having a first repetition frequency f _rep1 as measurement light L1. The measurement light L1 is input to the coupler 15 via the optical fiber cable F1.

The mode-locked laser light source 13 (corresponding to the second light source of the present invention) is connected to the coupler 18 via the optical fiber cable F2. The mode-locked laser light source 13 emits a mode-locked laser beams having different second repetition frequency _{f rep2} the first repetition frequency _{f rep1} as the reference light L2. The reference light L2 is input to the coupler 18 via the optical fiber cable F2.

  The coupler 15 has one end connected to the mode-locked laser light source 12 via the optical fiber cable F1, and the other end connected to the fiber circulator 16 via the optical fiber cable F3 and via the optical fiber cable F4. It is connected to the photoelectric converter 21. The coupler 15 divides the measurement light L1 input from the optical fiber cable F1 into two, outputs one of the measurement light L1 to the fiber circulator 16 via the optical fiber cable F3, and outputs the other of the measurement light L1 to the optical fiber cable F3. It outputs to the photoelectric converter 21 via the fiber cable F3.

  The fiber circulator 16 is connected to the coupler 15 via the optical fiber cable F3, is connected to the sensor head 17 via the optical fiber cable F5, and is connected to the coupler 19 via the optical fiber cable F6. ing. The fiber circulator 16 is, for example, a non-reciprocating and unidirectional device, and has three ports. The sensor head 17 receives the measurement light L1 input from the optical fiber cable F3 via the optical fiber cable F5. Output to. Further, the fiber circulator 16 outputs first reflected light R1 and second reflected light R2, which will be described later, input from the sensor head 17 via the optical fiber cable F5 to the coupler 19 via the optical fiber cable F6.

  The sensor head 17 is an end surface reflection type head, reflects a part of the measurement light L1 input from the optical fiber cable F5 toward the fiber circulator 16, and directs the rest of the measurement light L1 toward the measurement object 9. Exit. The measurement light L1 emitted toward the measurement object 9 is reflected by the measurement object 9, and enters the sensor head 17 as the first reflected light R1.

  The end surface of the sensor head 17 on the emission end side (or the incident end side) of the measurement light L functions as a reference surface 17a (also referred to as a reference surface) that reflects part of the measurement light L1 toward the fiber circulator 16. To do. For this reason, a part of the measurement light L1 described above is reflected by the reference surface 17a to become the second reflected light R2. Thereby, the absolute distance D to the measuring object 9 with the reference surface 17a as the reference position can be measured. The first reflected light R1 and the second reflected light R2 are input from the sensor head 17 to the fiber circulator 16 via the optical fiber cable F5, and further input from the fiber circulator 16 to the coupler 19 via the optical fiber cable F6. The

  One end of the coupler 18 is connected to the mode-locked laser light source 13 via the optical fiber cable F2, and the other end is connected to the coupler 19 via the optical fiber cable F7. It is connected to the converter 22. The coupler 15 divides the reference light L2 input from the optical fiber cable F2 into two parts, outputs one of the reference lights L2 to the coupler 19 via the optical fiber cable F7, and outputs the other of the reference lights L2 to the optical fiber cable F2. It outputs to the photoelectric converter 22 via the fiber cable F8.

  The optical fiber cables F6 and F7 described above are connected to one end of the coupler 19, and two optical fiber cables F9 are connected to the other end of the coupler 19. The coupler 19 combines the first reflected light R1 and the second reflected light R2 input from the optical fiber cable F6 and the reference light L2 input from the optical fiber cable F7, and combines the first reflected light R1 and the first reflected light R1. The interference signal SG (interference light) between the two reflected light R2 and the reference light L2 is generated. Further, the coupler 19 divides the generated interference signal SG into two and outputs them to the two optical fiber cables F9. Here, when there is an intensity difference between the first reflected light R1 and the reference light L2, the coupler 19 uses the first reflected light R1 and the reference light L2 as non-interfering signals, and simultaneously with the interference signal SG, the second reflected light R2 and the reference light. The non-interfering signal having the intensity of L2 is divided into two and output to the two optical fiber cables F9. Similarly, when there is an intensity difference between the second reflected light R2 and the reference light L2, the coupler 19 uses the second reflected light R2 and the reference light L2 as non-interfering signals, and simultaneously with the interference signal SG, the second reflected light R2 and the reference light L2. The non-interfering signal having the intensity of 2 is divided into two and output to the two optical fiber cables F9. At this time, as one of the basic properties of the coupler 19, the interference signal SG is output to the two optical fiber cables F9 in a form in which the intensity of light is inverted, and the first reflected light R1 and the first reflected light R1 which are non-interference signals are transmitted. The two reflected light R2 and the reference light L2 are output to the two optical fiber cables F9 in the form of the same light intensity. The optical fiber cable F9 individually outputs the interference signal SG input from the coupler 19 toward the differential photoelectric converter 20.

The interference signal SG includes a first interference signal component SG1 that is an interference signal component of the first reflected light R1 and the reference light L2, and a second interference signal component SG2 that is an interference signal component of the second reflected light R2 and the reference light L2. And are included. The first interference signal component SG1 is the measuring light L1 frequency of the difference between the second repetition frequency _{f rep2} the first repetition frequency _{f rep1} the reference light L2 (first reflected light _{R1) [| f rep1 -f rep2} |] The same interference waveform (pulse interference waveform) is repeated at (Hz) (see FIG. 6). Similarly, the second interference signal component SG2 has the same interference waveform (pulse) at the frequency [| f _rep1 −f _rep2 |] (Hz) of the difference between the first repetition frequency f _rep1 and the second repetition frequency f _rep2. (Interference waveform) is repeated (see FIG. 6).

  The differential photoelectric converter 20 corresponds to the interference signal detector of the present invention, and includes two photodetectors that individually detect the interference signals SG respectively emitted from the two optical fiber cables F9, and 2 And a differential output unit that outputs a differential signal by taking a difference between outputs of the two photodetecting elements. Since the differential photoelectric converter 20 is a known technique, detailed description thereof is omitted. The differential photoelectric converter 20 converts the interference signal SG emitted from the two optical fiber cables F9 into an electric signal (differential signal) and outputs the electric signal to the digitizer 26. Since the differential photoelectric converter 20 outputs an intensity difference between the two input lights, the first reflected light R1, the second reflected light R2, and the reference light L2 that are non-interfering signals are extracted by extracting the above-described interference signal SG. By eliminating the above, it is possible to reduce the noise of the interference signal SG.

The photoelectric converter 21 photoelectrically converts (detects) the measurement light L1 input from the coupler 15 via the optical fiber cable F4, and outputs the detection signal MG1 of the measurement light L1 to the mixer 23 and the frequency counter 25, respectively. The frequency of the detection signal MG1 is f _rep1 (Hz).

The photoelectric converter 22 photoelectrically converts (detects) the reference light L2 input from the coupler 18 via the optical fiber cable F8, and detects the detection signal MG2 of the reference light L2 to the mixer 23, the frequency counter 25, and the digitizer 26, respectively. Output. The frequency of the detection signal MG2 is f _rep2 (Hz). The detection signal MG2 input to the digitizer 26 is used as a sampling clock in the digitizer 26.

The mixer 23 is a signal having a sum frequency of the respective frequencies based on the detection signal MG1 of the measurement light L1 input from the photoelectric converter 21 and the detection signal MG2 of the reference light L2 input from the photoelectric converter 22. And a signal having a difference frequency. As described above, the frequency of the detection signal MG1 is f _rep1 (Hz), and the frequency of the detection signal MG2 is f _rep2 (Hz). Therefore, the mixer 23 _generates a signal having a sum frequency [f _rep1 + f _rep2 ] (Hz) and a signal having a difference frequency [| f _rep1 −f _rep2 |] (Hz), Output to the low-pass filter 24.

The low-pass filter 24 cuts a signal having a “sum frequency” from signals input from the mixer 23 and outputs only a signal having a “difference frequency” to the digitizer 26. A signal having the difference frequency [| f _rep1 −f _rep2 |] (Hz) is used as a trigger signal that serves as a reference when the digitizer 26 described later captures the interference signal SG.

The frequency counter 25 functions as a repetition frequency acquisition unit of the present invention. The frequency counter 25 detects the first repetition frequency f _rep1 of the measurement light L1 based on the detection signal MG1 input from the photoelectric converter 21, and also detects the reference light L2 based on the detection signal MG2 input from the photoelectric converter 22. A second repetition frequency f _rep2 is detected. Then, the frequency counter 25 outputs a repetition frequency detection result that is a detection result of the first repetition frequency f _rep1 and the second repetition frequency f _rep2 to the signal processing unit 27.

The digitizer 26 samples and digitizes (digitizes) the interference signal SG input from the differential photoelectric converter 20. Specifically, the digitizer 26 starts sampling on the basis of the trigger signal input from the low-pass filter 24, that is, the trigger signal repeated at [| f _rep1 −f _rep2 |] (Hz) at the difference frequency. At this time, the digitizer 26 performs sampling using the frequency “f _rep2 ” (Hz) of the detection signal MG2 input from the photoelectric converter 22 as a sampling clock. Accordingly, the digitizer 26 samples and digitizes the interference signal SG according to the sampling clock before the next trigger in response to a new trigger input.

Here, the interference waveform of the first interference signal component SG1 and the interference waveform of the second interference signal component SG2 are each repeated at a frequency [| f _rep1 −f _rep2 |] (Hz) (see FIG. 6). For this reason, by setting the interval of the trigger signal input to the digitizer 26 to the difference frequency [| f _rep1 −f _rep2 |] (Hz) as described above, both the interference waveforms can be reliably sampled and wasteful. Sampling can be prevented. Thereby, the digitizer 26 outputs the digitized interference signal SG to the signal processing unit 27 every time a trigger signal is input.

  FIG. 6 is a graph showing an example of the interference signal SG output from the digitizer 26 to the signal processing unit 27. The horizontal axis of the graph is the number of sampling of the interference signal SG by the digitizer 26 [sample (N)], which is substantially equivalent to time.

As shown in FIG. 6, in the interference signal SG, as described above, the interference waveform of the first interference signal component SG1 and the interference waveform of the second interference signal component SG2 have the frequency [| f _rep1 −f _rep2 | ] Repeated every Hz. Then, the first peak position P1 corresponding to the peak position of the envelope waveform 36 (see FIG. 7) of the interference waveform of the first interference signal component SG1 and the peak position of the envelope waveform 36 of the interference waveform of the second interference signal component SG2. The sampling number ΔN between the corresponding second peak position P2 corresponds to the absolute distance D from the reference surface 17a described above to the measurement object 9. For this reason, the signal processing unit 27 individually detects the peak positions P1 and P2 of the envelope waveform 36 of both interference waveforms, and further detects the sampling number ΔN between the peak positions P1 and P2 based on the result of detecting the absolute distance. D is obtained.

  FIG. 7 is an explanatory diagram for explaining a problem when the signal processing unit 27 detects the peak position, and the interference waveform of the first interference signal component SG1 (second interference signal component SG2) is enlarged. FIG. Also, the horizontal axis in the figure is the sampling number [sample (N)], and the vertical axis in the figure is the signal intensity of the interference signal SG (the first interference signal component SG1 and the second interference signal component SG2).

Reference numeral 35A (upper stage) in FIG. 7 represents an interference waveform when an optical frequency comb in which the repetition frequencies f _rep1 and f _rep2 and the offset frequency f _ceo are accurately controlled is used as the measurement light L1 and the reference light L2. . In this case, the relative position between the peak position u of the interference waveform of the first interference signal component SG1 and the first peak position P1 of the envelope waveform 36 of the first interference signal component SG1 can be stabilized and obtained in advance. The relative position between the peak position u of the interference waveform of the second interference signal component SG2 and the second peak position P2 of the envelope waveform 36 of the second interference signal component SG2 can be stabilized and obtained in advance. Therefore, by analyzing the interference signal SG and detecting two peak positions u, the first peak position P1 of the envelope waveform 36 of the first interference signal component SG1 and the envelope waveform 36 of the second interference signal component SG2 are detected. The second peak position P2 can be detected.

On the other hand, reference numeral 35B (lower stage) in FIG. 7 shows a case where a mode-locked laser beam in which the repetition frequencies f _rep1 , f _rep2 and the offset frequency f _ceo are not accurately controlled is used as the measurement light L1 and the reference light L2. Interference waveform. In this case, the relative position between the peak position u of the interference waveform of the first interference signal component SG1 and the first peak position P1 of the envelope waveform 36 of the first interference signal component SG1 is not stable and can be obtained in advance. In addition, the relative position between the peak position u of the interference waveform of the second interference signal component SG2 and the second peak position P2 of the envelope waveform 36 of the second interference signal component SG2 is not stable and cannot be obtained in advance. For this reason, even if two peak positions u are detected by frequency analysis of the interference signal SG, there is an error between the peak positions P1 and P2, and therefore the absolute distance D is accurately determined from each peak position u. It is not required.

  Therefore, the signal processing unit 27 generates an envelope waveform 36 of the interference signal SG based on the interference waveform (signal waveform) of the interference signal SG. The envelope waveform 36 includes envelope waveforms 36 of both the first interference signal component SG1 and the second interference signal component SG2. Then, the signal processing unit 27 individually detects the first peak position P1 corresponding to the first interference signal component SG1 and the second peak position P2 corresponding to the second interference signal component SG2 from the generated envelope waveform 36. Thus, the absolute distance D is obtained.

  FIG. 8 is an explanatory diagram for explaining the function of the signal processing unit 27. As shown in FIG. 8, the signal processing unit 27 functions as an envelope waveform generation unit 41, a peak position detection unit 42, and a distance determination unit 43.

  The envelope waveform generation unit 41 generates an envelope waveform 36 of the interference signal SG based on the interference signal SG input from the digitizer 26. Hereinafter, the generation of the envelope waveform 36 will be described in detail.

  FIG. 9 is an explanatory diagram for explaining generation of the envelope waveform 36 by the envelope waveform generation unit 41. FIG. 10 is a flowchart showing a flow of processing for generating the envelope waveform 36 by the envelope waveform generating unit 41.

  As shown in the upper part of FIG. 9 and FIG. 10, the envelope waveform generation unit 41 performs a Hilbert transform on the interference signal SG and generates a converted signal ST having a phase difference of 90 ° with respect to the original interference signal SG. (Step S1). In addition, since the specific method of Hilbert transform is a well-known technique, specific description is abbreviate | omitted here.

  Further, the envelope waveform generation unit 41 generates a delayed signal SD obtained by delaying the interference signal SG according to the temporal delay of the converted signal ST by the Hilbert transform (step S2).

  Then, the envelope waveform generation unit 41 squares the generated conversion signal ST and delay signal SD (step S3 and step S4), and then adds the squared signals (step S5).

  Next, the envelope waveform generation unit 41 obtains the square root of the signal added in step S5 (step S6). As a result, as shown in the lower part of FIG. 9, an envelope waveform 36 of the interference signal SG, that is, an envelope waveform 36 of both the first interference signal component SG1 and the second interference signal component SG2 is generated (step S7).

  Returning to FIG. 8, the peak position detector 42 calculates the first peak position P1 and the second peak position P2 from the envelope waveform 36 of the interference signal SG (first interference signal component SG1 and second interference signal component SG2). Detect each. For example, the peak position detection unit 42 detects the first peak position P <b> 1 and the second peak position P <b> 2 by performing waveform analysis of the envelope waveform 36 and detecting two local maximum values in the envelope waveform 36. Note that a method for detecting a local maximum value by waveform analysis is a known technique, and various known methods may be employed.

  Further, the peak position detection unit 42 has data (point) intervals corresponding to the peak positions P1 and P2 of the envelope waveform 36 due to, for example, a long sampling interval of the interference signal SG. In the case of being open, values between individual data may be interpolated. Then, the peak position detection unit 42 outputs the detection results of the peak positions P1 and P2 to the distance determination unit 43.

Based on the detection results of the peak positions P1 and P2 input from the peak position detection unit 42 and the repetition frequency detection results (f _rep1 and f _rep2 ) input from the frequency counter 25, the distance determination unit 43 The absolute distance D from 17a to the measuring object 9 is determined.

  Specifically, the distance determination unit 43 determines the number of samples ΔN between the first peak position P1 and the second peak position P2 based on the detection results of the peak positions P1 and P2 input from the peak position detection unit 42 (see FIG. 6). This sampling number ΔN is a value indicating the absolute distance D as described above.

In addition, the distance determination unit 43 converts the “sampling number” into an actual “distance” based on the repetition frequency detection result input from the frequency counter 25 [c / f _rep1 −c / f _rep2 ]. Calculate (m / Sample). The speed of light c needs to be corrected because it changes depending on the refractive index of the medium through which the light passes. For example, in an optical wave length measuring device, the refractive index of air is obtained by measuring air temperature, atmospheric pressure, and humidity, and the measurement accuracy with respect to the geometric distance is improved by correcting the speed of light.

  The distance determination unit 43 determines the absolute distance D by converting the previously detected sampling number ΔN into a distance using the conversion coefficient. This completes the measurement of the absolute distance D. The distance determination unit 43 outputs the measurement result of the absolute distance D to the display unit 28 and the storage unit 29, respectively. Thereby, the display of the measurement result of the absolute distance D by the display unit 28 and the storage of the measurement result of the absolute distance D by the storage unit 29 are executed.

[Operation of distance measuring device]
FIG. 11 is a flowchart showing the flow of the measurement process (the distance measurement method of the present invention) of the absolute distance D to the measurement object 9 by the distance measurement device 10 having the above configuration.

As shown in FIG. 11, when the user sets the measurement object 9 at a predetermined position of the distance measuring device 10 and then performs a measurement start operation using an operation unit (not shown), the mode-locked laser light sources 12 and 13 are respectively connected. Operate. Thereby, the measurement light L1 having the first repetition frequency f _rep1 is emitted from the mode-locked laser light source 12, and the reference light L2 having the second repetition frequency f _rep2 is emitted from the mode-locked laser light source 13 (Step S10, this) Equivalent to the first emission step and the second emission step of the invention).

  The measurement light L1 emitted from the mode-locked laser light source 12 is divided into two by the coupler 15, one of which is input to the sensor head 17 via the fiber circulator 16 and the other is input to the photoelectric converter 21.

  A part of the measurement light L1 input to the sensor head 17 is reflected by the reference surface 17a, and the rest of the measurement light L is emitted from the sensor head 17 toward the measurement target 9, and the measurement target 9 The light is reflected and enters the sensor head 17. As a result, the first reflected light R1 reflected by the measurement object 9 and the second reflected light R2 reflected by the reference surface 17a are input from the sensor head 17 to the coupler 19 via the fiber circulator 16. .

  On the other hand, the reference light L 2 emitted from the mode-locked laser light source 13 is divided into two by the coupler 18, and one is input to the coupler 19 and the other is input to the photoelectric converter 22.

  The first reflected light R1, the second reflected light R2, and the reference light L2 input to the coupler 19 are combined by the coupler 19 to become an interference signal SG. The interference signal SG is divided into two by the coupler 19 and then emitted toward the differential photoelectric converter 20. As a result, the interference signal SG is detected by the differential photoelectric converter 20 and converted into an electrical signal (differential signal), and then output from the differential photoelectric converter 20 toward the digitizer 26 (step S11, main signal). Equivalent to the interference signal detection step of the invention).

  Upon receiving the measurement light L1, the photoelectric converter 21 photoelectrically converts the measurement light L1 and outputs a detection signal MG1 of the measurement light L1 to the mixer 23 and the frequency counter 25, respectively. The photoelectric converter 22 that has received the input of the reference light L2 photoelectrically converts the reference light L2, and outputs a detection signal MG2 of the reference light L2 to the mixer 23, the frequency counter 25, and the digitizer 26, respectively.

Receiving the detection signals MG1 and MG2, the mixer 23 generates a signal having the aforementioned sum frequency and a signal having the difference frequency, and outputs both signals to the low-pass filter 24. Then, only a signal having a difference frequency [| f _rep1 −f _rep2 |] (Hz) is output from the low pass filter 24 to the digitizer 26 as a trigger signal.

Further, the frequency counter 25 that has received the detection signals MG1 and MG2 detects the first repetition frequency f _rep1 of the measurement light L1 from the detection signal MG1, and at the same time detects the second repetition frequency f _rep2 of the reference light L2 from the detection signal MG2. Is detected. Then, the frequency counter 25 outputs the repetition frequency detection result to the distance determination unit 43 of the signal processing unit 27.

The digitizer 26 is input from the differential photoelectric converter 20 based on the trigger signal input from the low-pass filter 24 and the frequency “f _rep2 ” (Hz) of the detection signal MG2 input as a sampling clock from the photoelectric converter 22. The interference signal SG to be sampled is sampled and digitized. Thus, every time a trigger signal is input to the digitizer 26, the digitized interference signal SG is output from the digitizer 26 to the signal processing unit 27.

  The envelope waveform generation unit 41 that has received the input of the interference signal SG from the digitizer 26 generates the envelope waveform 36 of the interference signal SG using the Hilbert transform, as described in steps S1 to S7 of FIGS. (Step S12, corresponding to the envelope waveform generation step of the present invention).

  Next, the peak position detection unit 42 performs interpolation (interpolation) as necessary at portions corresponding to the peak positions P1 and P2 in the envelope waveform 36 (steps S13 and S14), and then Waveform analysis is performed to detect two maximum values in the envelope waveform 36. As a result, the first peak position P1 and the second peak position P2 shown in FIG. 6 described above are detected by the peak position detection unit 42, and the detection results of the peak positions P1 and P2 are output to the distance determination unit 43. (Steps S15 and S16, corresponding to the peak position detection step of the present invention).

The distance determination unit 43 that has received the detection results of the peak positions P1 and P2 and the detection result of the repetition frequency detects the sampling number ΔN between the first peak position P1 and the second peak position P2 from the former (see FIG. 6). (Step S17), and the conversion coefficient [c / f _rep1 −c / f _rep2 ] (m / Sample) described above is obtained from the latter. Note that the conversion coefficient may be obtained when a frequency detection result is repeatedly input from the frequency counter 25.

  Next, the distance determining unit 43 determines the absolute distance D by converting the sampling number ΔN into the distance using the conversion factor (step S18, corresponding to the distance determining step of the present invention). This completes the measurement of the absolute distance D. Then, the distance determination unit 43 outputs the distance measurement result of the absolute distance D to the display unit 28 and the storage unit 29 (step S19). Thereby, the measurement result of the absolute distance D is displayed on the display unit 28 and stored in the storage unit 29.

[Effect of this embodiment]
As described above, the distance measuring apparatus 10 of the present embodiment includes the first reflected light R1 reflected by the reference surface 17a, the second reflected light R2 reflected by the measurement object 9, and the reference light L2. By generating an envelope waveform 36 of the interference signal and detecting each peak position P1, P2 from the envelope waveform 36, the absolute distance D to the measurement object 9 with the reference plane 17a as a reference can be measured. For this reason, in this embodiment, the absolute distance D can be measured without using an optical frequency comb. This eliminates the need for a configuration for accurately controlling the offset frequency and the repetition frequency of the mode-locked laser beam, and eliminates the need to handle an ultra-high frequency electric signal as in the case of using an optical frequency comb. The device 10 has a low cost and a compact configuration. Moreover, since the distance measuring device 10 employs the configuration of an interferometer, it is possible to measure the distance with high accuracy. Furthermore, since the distance measuring device 10 does not need to employ the configuration of a multi-wavelength interferometer, the distance measuring device 10 can cope with a long range measurement. As a result, distance measurement with a long range and high accuracy can be performed with a low-cost and compact configuration.

  In the present embodiment, since a part of the measurement light L1 is reflected by the reference surface 17a of the sensor head 17, distance measurement using the reference surface 17a as a reference position becomes possible. Thereby, highly accurate distance measurement becomes possible.

[Others]
In the said embodiment, although 2nd reflected light R2 is produced | generated by reflecting a part of measurement light L1 with the reference surface 17a of the sensor head 17, this invention is not limited to this. For example, a prism for dividing the measurement light L1 into two is provided in the optical path of the measurement light L1 output from the coupler 15, and one of the measurement light L1 is output toward the sensor head 17, and the other of the measurement light L1 is a mirror and a corner cube. You may produce | generate 1st reflected light R1 and 2nd reflected light R2 by radiating | emitting toward reflectors, such as a prism.

  In the above embodiment, the distance measuring device 10 using the optical fiber cables F1 to F9 has been described as an example, but the present invention is also applied to a device using another light guide member or a device not using the light guide member. be able to. Also, couplers 15, 18, 19, fiber circulator 16, sensor head 17, differential photoelectric converter 20, photoelectric converters 21, 22, mixer 23, low-pass filter 24, frequency counter 25 provided in the distance measuring device 10. The digitizer 26, the signal processing unit 27, and the like may be replaced with those having the same function. Further, a plurality of the mixer 23, the low-pass filter 24, the frequency counter 25, the digitizer 26, and the signal processing unit 27 may be integrated.

  In the above embodiment, mode-locked laser light is used as the measurement light L1 and the reference light L2. However, distance measurement can be similarly performed even when an optical frequency comb is used.

  The present invention not only measures the absolute distance D to the measurement object 9, but also the absolute distance D such as a shape measuring device that measures the shape of the measurement object 9 based on the measurement result of the absolute distance D, for example. The present invention is also applicable to an apparatus that performs various measurements (measurements) based on measurement results.

  DESCRIPTION OF SYMBOLS 9 ... Measuring object, 10 ... Distance measuring device 12, 13 ... Mode lock laser light source, 17 ... Sensor head, 17a ... Reference plane, 20 ... Differential photoelectric converter, 21, 22 ... Photoelectric converter, 25 ... Frequency Counter 26, Digitizer 27 Signal processing unit 36 Envelope waveform 41 Envelope waveform generation unit 42 Peak position detection unit 43 Distance determination unit

Claims (7)

A first light source that emits measurement light having a first repetition frequency, the first light source that emits the measurement light incident on a reference surface and a measurement object, and
A second light source that emits reference light having a second repetition frequency different from the first repetition frequency;
A first reflected light of the measurement light reflected by the reference surface, a second reflected light of the measurement light reflected by the measurement object, and the reference light emitted from the second light source, An interference signal detector for detecting the interference signal of
An envelope waveform generation unit that generates an envelope waveform of the interference signal detected by the interference signal detection unit;
From the envelope waveform generated by the envelope waveform generation unit, a first peak position corresponding to the first interference signal component of the first reflected light and the reference light, and the second interference signal component of the second reflected light and the reference light A peak position detector for detecting a second peak position corresponding to
A distance determination unit that determines a distance to the measurement object based on the reference plane based on the first peak position and the second peak position detected by the peak position detection unit;
A distance measuring device comprising:   The distance measuring device according to claim 1, wherein the envelope waveform generation unit generates the envelope waveform using Hilbert transform.   The envelope waveform generation unit generates the envelope waveform based on a converted signal obtained by converting the interference signal into a Hilbert transform and a delayed signal obtained by delaying the interference signal according to a delay of the converted signal due to the Hilbert transform. Item 3. The distance measuring device according to Item 2. A repetition frequency acquisition unit that acquires the first repetition frequency of the measurement light and the second repetition frequency of the reference light, respectively.
The distance determination unit includes a difference between the first peak position and the second peak position detected by the peak position detection unit, and the first repetition frequency and the second repetition frequency acquired by the repetition frequency acquisition unit. The distance measuring device according to claim 1, wherein the distance is determined based on the distance.   The distance measuring device according to any one of claims 1 to 4, wherein the measurement light and the reference light are mode-locked laser light different from an optical frequency comb. It has an end surface that reflects a part of the measurement light input from the first light source, and the rest of the measurement light is emitted toward the measurement object and reflected by the measurement object. An end face reflection type sensor head on which the first reflected light is incident;
The distance measuring device according to claim 1, wherein the reference surface is the end surface of the sensor head. A first emission step of emitting measurement light having a first repetition frequency from a first light source and causing the measurement light to enter a reference surface and a measurement object, respectively;
A second emission step of emitting a reference light having a second repetition frequency different from the first repetition frequency from the second light source;
A first reflected light of the measurement light reflected by the reference surface, a second reflected light of the measurement light reflected by the measurement object, and the reference light emitted from the second light source, An interference signal detection step of detecting an interference signal of
An envelope waveform generation step for generating an envelope waveform of the interference signal detected in the interference signal detection step;
From the envelope waveform generated in the envelope waveform generation step, a first peak position corresponding to the first interference signal component of the first reflected light and the reference light, and the second interference signal component of the second reflected light and the reference light A peak position detecting step for detecting a second peak position corresponding to
A distance determining step for determining a distance to the measurement object based on the reference plane based on the first peak position and the second peak position detected in the peak position detecting step;
A distance measuring method.

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