Measuring device

Abstract

To provide a measurement device capable of a continuous rapid scan.SOLUTION: A measurement device 100 includes: a wavelength variable laser 10 for outputting a series of pulse-like laser beams IL wavelength swept non-mechanically; a space dispersion device 20 for applying a series of laser beams IL to a target OB by spatially dispersing the laser beams IL by non-mechanical optical characteristics, and reserving a reflection light RL from the target OB; a reception device 30 for receiving the reflection light RL through the space dispersion device 20; a signal processor 40 for determining the distance to the target OB from the timing when the reception device 30 detects the reflection light RL and the timing when the wavelength variable laser 10 outputs the laser beam IL. The signal processor 40 determines information on the direction of emission of each laser light IL from timing information of the series of laser beams IL.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a measuring device that performs distance measurement in a manner similar to amplitude-modulated continuous-wave (AMCW), and particularly to a measuring device capable of continuous high-speed scanning.

Background Art

[0002] As a conventional measuring device, there is one that deflects and sweeps a laser beam by mechanical drive (see, for example, Patent Documents 1 and 2). The measuring device driven by mechanical drive has a problem in that it lacks improvement in terms of sweep speed, vibration resistance, etc.

[0003] Also, there is a technique for performing non-mechanical beam scanning by combining a wavelength-sweeping light source and a spatial dispersion device such as a diffraction grating (see, for example, Non-Patent Documents 1 and 2). However, in the techniques of Non-Patent Documents 1 and 2, the wavelength-sweeping light source is based on mechanical drive, and the above problems such as sweep speed and vibration resistance have not been solved.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

[0006] The present invention has been made in view of the above background art, and an object thereof is to provide a measuring device capable of continuous high-speed scanning.

[0007] To achieve the above object, a measuring device according to the present invention includes a wavelength-variable laser that outputs a series of pulsed laser beams with non-mechanical wavelength sweeping, a spatial dispersion device that spatially disperses the series of laser beams by non-mechanical optical characteristics, irradiates an object, and reverses the reflected light from the object, a receiving device that receives the reflected light via the spatial dispersion device, and a signal processing device that determines the distance to the object from the detection timing of the reflected light by the receiving device and the output timing of the laser light from the wavelength-variable laser. The signal processing device determines information regarding the emission direction of each laser beam from the timing information of the series of laser beams.

[0008] In the above measuring device, the signal processing device acquires information regarding the distance and direction to the object based on the output timing, detection timing, and timing information of the series of laser beams of the laser light. At this time, by combining a wavelength-variable laser, which is a wavelength-sweeping light source, and a spatial dispersion device, and performing wavelength sweeping and laser beam deflection all using a non-mechanical configuration, the scanning speed can be increased by several orders of magnitude compared to the conventional mechanical configuration, and the vibration resistance of the device can be improved.

[0009] In a specific aspect of the present invention, in the above measurement device, the wavelength-variable laser performs dispersion tuning by pulse modulation and outputs a series of laser lights as a pulse train whose repetition frequency continuously changes. Here, dispersion tuning means controlling the oscillation wavelength by changing the modulation frequency in the active mode synchronous generation state. Different from sine wave modulation, pulse modulation includes high-frequency components, so modulation can be effectively performed with a narrow pulse width while securing a wavelength-variable band.

[0010] In another aspect of the present invention, the wavelength-variable laser includes an optical amplifier that amplifies light, an intensity modulator that performs intensity modulation, and a high-dispersion medium that dispersively adjusts the wavelength. With the high-dispersion medium, pulses with high wavelength purity can be obtained.

[0011] In yet another aspect of the present invention, the high-dispersion medium is a chirped fiber Bragg grating. In this case, the length of the resonator can be shortened, enabling miniaturization of the measurement device.

[0012] In yet another aspect of the present invention, the optical amplifier is a semiconductor optical amplifier.

[0013] In yet another aspect of the present invention, the spatial dispersion device includes a collimator that collimates the laser light into parallel light and a dispersion unit that spatially disperses the laser light that has passed through the collimator. In this case, even if the output of the wavelength-variable laser is divergent light, precise spatial dispersion of the laser light becomes possible.

[0014] In yet another aspect of the present invention, the dispersion unit has one or more gratings. In this case, wavelength dispersion can be easily controlled.

[0015] In yet another aspect of the present invention, the signal processing device determines information regarding the emission direction of each laser beam from the timing information of the pulse train, and calculates the phase difference between the reference signal obtained from the laser beam in units of pulses constituting the pulse train and the measurement signal of the reflected light in order to determine the distance to the target. By calculating the phase difference between the reference signal and the measurement signal in units of pulses constituting the pulse train with respect to distance calculation, it is possible to obtain the dynamic phase change of the signal trace. As a result, continuous high-speed scanning becomes possible.

[0016] To achieve the above object, a measuring device according to the present invention includes a wavelength-variable laser that outputs a series of wavelength-swept laser beams as a pulse train, a spatial dispersion device that spatially disperses the pulse train by non-mechanical optical characteristics and irradiates the target, and reflects the reflected light from the target, a receiving device that receives the reflected light via the spatial dispersion device, and a signal processing device that determines information regarding the emission direction of each laser beam from the timing information of the pulse train. The signal processing device calculates the phase difference between the reference signal obtained from the laser beam in units of pulses constituting the pulse train and the measurement signal of the reflected light in order to determine the distance to the target.

[0017] In the above measuring device, the signal processing device obtains information regarding the distance and direction to the target based on the phase difference between the reference signal and the measurement signal and the timing information of the pulse train. At this time, while combining the wavelength-variable laser, which is a wavelength-sweeping light source, and the spatial dispersion device, it is possible to obtain the dynamic phase change of the signal trace by calculating the phase difference between the reference signal and the measurement signal in units of pulses constituting the pulse train with respect to distance calculation. As a result, continuous high-speed scanning becomes possible.

[0018] In another aspect of the present invention, the signal processing device performs a multiplication process between the signal obtained by performing a Hilbert transform on the reference signal and the measurement signal. In this case, phase information is given to all frequency components of the reference signal by the Hilbert transform, and it is possible to avoid the phase error caused by the non-uniform amplitude.

[0019] In yet another aspect of the present invention, filtering is performed on the lock-in data after the multiplication process. In this case, frequency components irrelevant to the lock-in data after the multiplication process can be removed.

Brief Description of the Drawings

[0020]

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Mode for Carrying Out the Invention

[0021] 〔First Embodiment〕 Hereinafter, with reference to FIG. 1 and the like, the measuring device according to the first embodiment of the present invention will be described.

[0022] As shown in FIG. 1, the measuring device 100 includes a wavelength-variable laser 10, a spatial dispersion device 20, a receiving device 30, and a signal processing device 40. Further, the measuring device 100 includes an output-side circulator 50 that optically branches the output light and the received light. The measuring device 100 can scan the laser beam IL in the X-axis direction and can perform one-dimensional measurement by scanning in the X-axis direction.

[0023] The wavelength-variable laser 10 outputs a series of pulsed laser beams IL that are non-mechanically wavelength-swept. In other words, the wavelength-variable laser 10 outputs a series of wavelength-swept laser beams IL as a pulse train. Here, the series of pulsed laser beams IL means laser beams whose pulses are changed in time series. The wavelength-variable laser 10 performs dispersion tuning by pulse modulation. Here, dispersion tuning means controlling the oscillation wavelength by changing the modulation frequency in the active mode synchronization state. By using a dispersion-tuned swept laser (DTSL) that employs a dispersion tuning method for the wavelength-swept light source, high-speed non-mechanical sweeping becomes possible. In this embodiment, the wavelength-variable laser 10 is a ring-type laser.

[0024] The wavelength-variable laser 10 utilizes the principle of dispersion tuning. By inserting a high-dispersion device into the resonator, the wavelength dispersion value is increased, and in this state, intensity modulation is applied to actively mode-lock the laser beam. At this time, by changing the modulation frequency, the oscillation frequency can be controlled. In mode locking, modulation is applied in accordance with the round-trip time of light in the resonator to synchronize the longitudinal modes, and by varying the modulation frequency, the oscillation wavelength can be swept. Since the wavelength-variable laser 10 electrically controls the oscillation wavelength, there are no mechanical limitations, and high-speed and wide-band wavelength sweeping are possible. The pulsed laser beam output from the wavelength-variable laser 10 has, for example, a sweeping band of 50 nm or more at a sweeping speed of 10 kHz.

[0025] The wavelength-variable laser 10 includes a signal generator 11, a synthesizer 12, a pulse generator 13, an internal optical amplifier 14, an intensity modulator 15, a high-dispersion medium 16, an internal circulator 17, and an output-side optical amplifier 18. In the wavelength-variable laser 10, the internal optical amplifier 14, the intensity modulator 15, the high-dispersion medium 16, and the internal circulator 17 function as a resonator. Each component of the wavelength-variable laser 10 is connected by an optical fiber 19.

[0026] The signal generator 11 is a synchronization circuit that outputs a synchronization signal indicating a continuous frequency change to the synthesizer 12. As shown in Fig. 2(A), the sweep waveform of the ramp signal (sweep signal) in the signal generator 11 is a linear waveform such as a sawtooth wave or a triangular wave, for example. The sweep frequency of the signal generator 11 is, for example, 10 kHz.

[0027] The synthesizer 12 is an electric circuit that forms a drive signal having a waveform corresponding to the ramp signal from the signal generator 11 as a modulation signal. That is, the synthesizer 12 is a function generator that continuously performs frequency sweeping using the ramp signal from the signal generator 11 as a modulation signal. As shown in Fig. 2(B), the waveform of the modulation signal in the synthesizer 12 is a sine wave whose frequency is continuously changed (chirped). In one sweep of the sweep waveform, when the ramp voltage of the signal generator 11 is low, the frequency of the modulation signal of the synthesizer 12 becomes low, and when the ramp voltage of the signal generator 11 is high, the frequency of the modulation signal of the synthesizer 12 becomes high. The modulation frequency of the modulation signal is, for example, 788 MHz to 795 MHz or 716 MHz to 726 MHz.

[0028] The pulse generator 13 generates a pulse signal corresponding to the modulation signal from the synthesizer 12. The pulse generator 13 can change the pulse width, for example, between 50 ps and 250 ps. As shown in Fig. 2(C), the waveform of the pulse signal in the pulse generator 13 is a pulse whose frequency is continuously changed (chirped). In one sweep of the sweep waveform, when the modulation signal of the synthesizer 12 changes from a low frequency to a high frequency, the wavelength of the pulse train generated by the pulse generator 13 changes from a long wavelength to a short wavelength. The modulation frequency of the pulse signal is, for example, 788 MHz to 795 MHz or 716 MHz to 726 MHz.

[0029] The internal optical amplifier 14 is, for example, a semiconductor optical amplifier (SOA), and performs optical amplification by stimulated emission by making light incident on the semiconductor element from the outside.

[0030] The intensity modulator 15 is, for example, an optical modulator using a lithium niobate crystal, and performs intensity modulation by receiving the pulse signal output from the pulse generator 13 and adjusting the transmittance of the laser beam. The intensity modulation of the laser beam is of a pulse type, and for example, frequencies of 788 MHz to 795 MHz or 716 MHz to 726 MHz are used. Different from sine wave modulation, pulse modulation contains high-frequency components, so effective modulation can be performed with a narrow pulse width while securing a wavelength variable band.

[0031] The high-dispersion medium 16 adjusts the dispersion of the wavelength. That is, the high-dispersion medium 16 adjusts the dispersion with respect to the wavelength. By using the high-dispersion medium 16, pulses with high wavelength purity can be obtained. Examples of the high-dispersion medium 16 include a chirped fiber Bragg grating (CFBG) and a pair of diffraction gratings. As a result, the length of the resonator can be shortened, and the miniaturization of the measuring device 100 can be achieved. In this embodiment, an example in which the high-dispersion medium 16 is a CFBG is shown. The dispersion of the CFBG is, for example, +10 ps / nm, and 65% of the light is reflected. The remaining 35% of the light is amplified by the output-side optical amplifier 18 and then output and used as the laser beam IL. Since the dispersion in the resonator is increased by the high-dispersion medium 16, the free spectral range (FSR) varies greatly with the wavelength, and selectivity of the oscillation wavelength occurs. From the high-dispersion medium 16, pulses having a wavelength of, for example, 1530 nm to 1580 nm or 1530 nm to 1600 nm that satisfy the resonance condition are swept and output.

[0032] The internal circulator 17 makes the light that has passed through the intensity modulator 15 enter the high-dispersion medium 16 and guides the light reflected by the high-dispersion medium 16 to the internal optical amplifier 14.

[0033] In dispersion tuning, a highly dispersive medium 16 is intentionally inserted into the resonator, and active mode synchronization is generated by applying intensity modulation. By changing the modulation frequency in the state where active mode synchronization is generated, the oscillation wavelength can be controlled. That is, in dispersion tuning, the oscillation wavelength can be controlled without using a mechanical wavelength selection filter. As a result, high-speed and wide-band wavelength sweeping becomes possible. In dispersion tuning, the sweeping method is only to change the modulation frequency of mode synchronization (frequency modulation), and the spectral shape can also be controlled by appropriately setting the sweeping waveform (that is, the range and rate of modulation). The FSR, which is the frequency interval between longitudinal modes, depends on the refractive index of light in the optical fiber, and in a resonator with large wavelength dispersion, the FSR has modulation frequency dependence. When modulation is applied using a frequency signal that is an integer multiple of the FSR from outside the resonator, mode synchronization is applied only to the wavelength having the FSR corresponding to the modulation frequency and oscillation occurs. When this modulation frequency is changed, the oscillation wavelength changes proportionally with the modulation frequency and becomes a wavelength-variable laser beam IL. When a sweep is performed in which the modulation frequency changes linearly, the oscillation wavelength also changes linearly and the corresponding sweep is performed. The wavelength variable range of dispersion tuning is determined by the gain band of the gain medium, and its maximum value is determined by the variable range of the modulation frequency. The smaller the total dispersion amount and modulation frequency of the resonator, the larger the maximum value of the wavelength variable band. In a resonator with a large dispersion value, the pulse of mode synchronization becomes a chirped pulse. The spectral width of the pulse becomes narrower as the modulation frequency is higher and as the dispersion amount is larger.

[0034] Figure 2(D) shows the time variation of the optical sweep pulse output from the wavelength-variable laser 10. The laser light IL output from the wavelength-variable laser 10 is wavelength-swept from a long wavelength to a short wavelength based on the sweep signal (see Figure 2(A)) output from the signal generator 11. In one sweep of the sweep waveform, a pulse train composed of a plurality of pulses with different wavelengths in time series is generated. The pulse signal generated by the pulse generator 13 and the pulse signal output from the wavelength-variable laser 10 have a one-to-one relationship at a time interval t. The wavelength difference nΔ between adjacent pulses corresponds to the FSR, and the modulation frequency dependence of the FSR is determined by the resonator dispersion.

[0035] Figure 3 shows the spectrum of the dispersion-adjusted laser light IL output from the wavelength-variable laser 10. The output power of the laser light IL output from the resonator, specifically the high-dispersion medium 16, is, for example, 6 dBm or -2 dBm. The cavity length of the resonator is, for example, 11.9 m, the basic FSR is 17 MHz when the wavelength is 1550 nm, and the modulation frequency range is 788 MHz to 795 MHz. The sweep speed of the wavelength-variable laser 10 is, for example, 10 kHz, and the sweep wavelength is, for example, 1530 nm to 1580 nm. Also, if the cavity length of the resonator is, for example, 11.1 m, the basic FSR of the light with a wavelength of 1550 nm becomes approximately 18.6 MHz. In this case, at a modulation frequency of 716 MHz to 726 MHz, the sweep wavelength is, for example, 1530 nm to 1600 nm.

[0036] Returning to Figure 1, the output-side optical amplifier 18 is, for example, a fiber amplifier. Although detailed description is omitted, it has an erbium-doped fiber amplification section, an excitation light source, and a coupler that guides the light from the excitation light source to the fiber amplification section. Note that the output-side optical amplifier 18 may be an SOA or the like, similar to the internal optical amplifier 14. The power of the laser light IL amplified by the output-side optical amplifier 18 is, for example, 16 dBm or 14.6 dBm.

[0037] The output-side circulator 50 causes the laser beam IL that has passed through the wavelength-variable laser 10 to be incident on the spatial dispersion device 20, and causes the reflected beam RL that has passed through the spatial dispersion device 20 to be incident on the receiving device 30.

[0038] The spatial dispersion device 20 spatially disperses a series of laser beams IL or pulse trains by non-mechanical optical characteristics and irradiates the target OB, and causes the reflected beam RL from the target OB to travel in the reverse direction. By irradiating the spatial dispersion device 20 with wavelength-swept light, beam deflection that does not rely on machinery becomes possible. The laser scanning range via the spatial dispersion device 20 is, for example, 4° or 1 cm in the X-axis direction (lateral direction), and the resolution is 10 μm. When the power of the laser beam IL incident on the spatial dispersion device 20 is, for example, 16 dBm, the received power of the reflected beam RL reflected by the target OB after passing through the spatial dispersion device 20 is -50 dBm. The measurable distance of the measuring device 100 depends on the scanning power of the laser beam IL output from the wavelength-variable laser 10.

[0039] As shown in FIG. 4, the spatial dispersion device 20 includes a collimator 21, a dispersion unit 22, and a lens 23. Thereby, even if the output of the wavelength-variable laser 10 is a divergent light, precise spatial dispersion of the laser beam IL becomes possible. Note that the lens 23 may not be provided depending on the distance to the target OB.

[0040] The collimator 21 makes the laser beam IL into parallel light. The diameter of the laser beam collimated by the collimator 21 is, for example, 2 mm.

[0041] The dispersing unit 22 spatially disperses the laser beam IL that has passed through the collimator 21. Examples of the dispersing unit 22 include a grating (diffraction grating), a prism, an angular amplification lens, a photonic crystal, a phased array, and the like. By having the spatial dispersion device 20 include the dispersing unit 22, wavelength dispersion can be easily controlled. In the present embodiment, the dispersing unit 22 includes two gratings 22a and 22b. The dispersion amount of the dispersing unit 22 (specifically, the dispersion amount in one grating) is, for example, 0.04° / nm and has a diffraction efficiency of 60% to 80%. Note that in FIG. 4, an example of a reflective grating is given for the dispersing unit 22, but a transmissive grating may also be used. Further, the dispersing unit 22 may include one or three or more gratings.

[0042] The lens 23 condenses the light of each wavelength that has been dispersed while correcting chromatic aberration and the like. The lens 23 is, for example, an achromatic lens. The focal length of the lens 23 is, for example, 15 cm.

[0043] As shown in FIG. 5(A), when each output pulse (for example, the optical sweep pulses of pulse numbers 1 to 5,... having different wavelengths) or each output pulse train output from the wavelength-variable laser 10 is emitted from the spatial dispersion device 20, as shown in FIG. 5(B), the emission angle changes according to each output pulse. Information regarding the relationship between this output pulse and the emission angle is determined and stored in a signal processing device 40 described later, and is used for calculating the distance to the target OB.

[0044] The receiving device 30 receives the reflected light RL via the spatial dispersion device 20. The receiving device 30 includes a photodiode 31. The photodiode 31 detects the reflected light RL and outputs a measurement signal corresponding to the reflected light RL.

[0045] The signal processing device 40 determines the distance to the target OB based on the detection timing of the reflected light RL by the receiving device 30 and the output timing of the laser light IL or the pulse train from the wavelength-variable laser. Further, the signal processing device 40 determines information regarding the emission direction of each laser light IL from the timing information of a series of laser lights IL or pulse trains. The timing information of the laser light IL includes, for example, information necessary for arithmetic processing such as the order of output pulses and the wavelength difference based on experimental verification. Further, the signal processing device 40 calculates the phase difference between the reference signal RS obtained from the laser light IL in pulse units constituting the pulse train and the measurement signal MS of the reflected light RL in order to determine the distance to the target OB.

[0046] The signal processing device 40 includes a signal collection device 41 and a signal arithmetic processing device 42. Although details will be described later in the embodiments, as shown in FIG. 6, in the measurement device 100 of the present embodiment, the modulation frequency of the laser light IL chirps and the phase shift amounts Δφ1 to Δφ5 between the reference signal RS and the measurement signal MS change in time series. Therefore, in the signal processing in the signal processing device 40, in order to acquire the dynamic phase change of the signal trace whose repetition frequency continues to change (chirps), the chirped amplitude-modulated phase-shift measurement (CAMPS) technique is used.

[0047] FIG. 7 is a conceptual diagram for explaining the configuration of the signal processing device 40 that uses the CAMPS technique. The signal collection device 41 of the signal processing device 40 is a high-speed signal processing circuit including an A / D converter for digital data processing in the signal arithmetic processing device 42, but may be, for example, an oscilloscope. In that case, while making it possible to synchronize with frequency sweeping based on the signal of the signal generator 11, the measurement signal and the reference signal are synchronously captured so that they can be compared.

[0048] The signal arithmetic processing device 42 includes a computer or other arithmetic processing unit, and determines the distance OBD from the measuring device 100 to the target OB. Further, the signal arithmetic processing device 42 determines information regarding the emission direction of each laser beam IL from the timing information of a series of laser beams IL output from the wavelength-variable laser 10. In addition, information regarding the emission angle corresponding to each output pulse shown in FIG. 5(B) is pre-recorded in the signal arithmetic processing device 42 as spatial position information in the X-axis direction.

[0049] The signal arithmetic processing device 42 includes a Hilbert transform unit 42a, a multiplication unit 42b, a low-pass filter 42c, a phase detection unit 42d, and a phase / distance conversion unit 42e. The Hilbert transform unit 42a performs a Hilbert transform on the reference signal RS before the multiplication process in the multiplication unit 42b to obtain a Hilbert-transformed reference signal RSa. That is, the Hilbert transform unit 42a converts a real signal into a complex time signal by Hilbert transform in order to process adjacent phase information with respect to the reference signal RS. The Hilbert transform is an operation of giving a harmonic conjugate to a given real-valued function and converting it into a complex signal. By the Hilbert transform, a complex-valued function can be extended to the complex upper half plane, and the phase information of the signal can be obtained. The multiplication unit 42b multiplies the measurement signal MS and the Hilbert-transformed reference signal RSa to obtain lock-in data DT1. The low-pass filter 42c filters the lock-in data DT1 obtained by multiplication in the multiplication unit 42b to obtain lock-in data DT2 from which irrelevant frequency components are removed. Thereafter, the phase detection unit 42d detects a phase shift amount PS using the filtered lock-in data DT2. The phase / distance conversion unit 42e calculates the distance OBD from the measuring device 100 to the target OB based on the phase shift amount PS detected by the phase detection unit 42d.

[0050] The phase difference based on the phase shift amount PS between the reference signal RS and the measurement signal MS corresponds to the difference between the output timing of the laser beam IL and the detection timing of the reflected light RL, and provides information regarding the distance OBD from the measuring device 100 to the object OB. In the case where there is a circuit delay time between the reference signal RS output from the synthesizer 12 and the pulse signal output from the wavelength variable laser 10, a calibration means considering this delay time is used to calculate the phase difference between the reference signal RS and the measurement signal MS.

[0051] In the signal arithmetic processing device 42, the shape of the object OB can be obtained from the distance information calculated from the phase difference between the reference signal RS and the measurement signal MS described above and the spatial position information based on the emission angle of the laser beam IL described above.

[0052] (Embodiment) <Wavelength variable laser> Hereinafter, an embodiment of the wavelength variable laser 10 in the measuring device 100 will be mainly described.

[0053] Referring to FIG. 1, the wavelength variable laser 10 is an actively mode-locked fiber laser including the aforementioned elements 11, 12, 13, 14, 15, 16, 17, 18, and is mode-locked by modulating the light intensity in the cavity at a specific modulation frequency. Among the above elements, the elements 14, 15, 16, 17 function as a dispersion tuning unit. A large amount of wavelength dispersion is introduced into the cavity of the wavelength variable laser 10 so that the FSRs of the respective wavelengths are different. Thereby, only the wavelength whose FSR corresponds to the modulation frequency can be mode-locked. When a laser cavity of a specific length is given, the FSR of this cavity is as follows. TIFF of 0007715375000001.tif12166 Here, c is the speed of light in a vacuum, l is the length of the laser cavity, and n is the refractive index in the cavity. By applying intensity modulation that is an integer multiple of the FSR, the harmonic mode-locking condition can be satisfied. Let the center modulation frequency be f m0 and the center wavelength be λ0, then the swept laser wavelength can be expressed by the following equation. TIFF0007715375000002.tif13166 Here, f m is the sweep modulation frequency, and D is the total dispersion in the cavity. By sweeping the modulation frequency, pulsed optical output with wavelength sweeping is realized.

[0054] The maximum tuning range Δλ of the wavelength max is mainly determined by the gain bandwidth. However, when the change in the modulation frequency exceeds 1 FSR, two wavelengths of adjacent harmonic modes may appear simultaneously. Therefore, the maximum tuning range of the single-wavelength output is expressed as follows. TIFF0007715375000003.tif12165 Here, f FSR0 is the FSR of the central wavelength λ0. According to this formula, to achieve a wide tuning range, it is desirable to reduce the total dispersion D in the cavity and the central modulation frequency f m0 . However, reducing the total dispersion D in the cavity and the central modulation frequency f m0 will make the laser wavelength unstable and cause the linewidth to expand. Therefore, there is a trade-off relationship between the wavelength tuning range and the instantaneous linewidth. In the case of a non-mechanical beam scanner, it can be said that the maximum lateral scanning angle is determined by the above wavelength tuning range, and the lateral resolution is limited by the spread of the instantaneous linewidth. A multi-purpose non-mechanical scanner can be realized by changing the parameters of the laser light source.

[0055] The wavelength tunable laser 10 of the embodiment is the same as the configuration shown in FIG. 1. The total length of the cavity of the wavelength tunable laser 10 of the embodiment is 11.1 m. This means that the basic FSR of light with a wavelength of 1550 nm is about 18.6 MHz. Thus, a mode-locked pulse output with a wavelength of 1530 nm to 1600 nm can be obtained at modulation frequencies of 716 MHz to 726 MHz. In the case of high-speed wavelength sweeping, the signal generator 11 provides a 10 kHz ramp signal to sweep the modulation frequency every 0.1 milliseconds. The pulse generator 13 is used to convert the sine wave generated by the synthesizer 12 into a short pulse train in order to reduce the spectral linewidth of the laser. In the embodiment, since the pulse width of the pulse generated by the pulse generator 13 is 100 ps, when the modulation frequency is 716 MHz to 726 MHz, the duty cycle is 0.0716 to 0.0726. Using the pulse generator 13 can improve the linewidth of the laser output by about 40%.

[0056] The wavelength tunable laser 10 inserts an SOA, which is an internal optical amplifier 14, into the cavity to provide broadband optical gain in a range of 100 nm, and employs a CFBG, which is a high-dispersion medium 16, not only as an output element but also as a dispersion element. The wavelength dispersion of the CFBG is 10 ps / nm, the reflectivity is 65%, and 35% of the cavity power passes through the CFBG as output. Since the CFBG can significantly shorten the cavity length, it is effective in improving the wavelength sweeping speed. Due to the above optoelectronic efforts, a pulsed optical output with an average power of -2 dBm and a pulse width of about 100 ps can be obtained. The output wavelength of the wavelength tunable laser 10 is swept at a frequency of 10 kHz, and the full width at half maximum (FWHM) of the instantaneous spectrum is about 0.3 nm.

[0057] However, in the application of laser detection, a higher optical power level is required, so the output is amplified by an erbium-doped fiber amplifier (EDFA), such as the output-side optical amplifier 18.

[0058] <Measuring device> Hereinafter, a specific example of the measuring device 100 will be described with reference to FIG. 8. FIG. 8 shows an overall non-mechanical spectrum scanning laser detection system as an example of the measuring device 100. Reference numeral 10A indicates a dispersion tuning unit corresponding to the internal optical amplifier 14, the intensity modulator 15, the high dispersion medium 16, and the internal circulator 17 shown in FIG. 1. The spatial dispersion device 20 is composed of a collimator 21, a beam splitter 24 (specifically, a pellicle beam splitter (BP145B3 manufactured by Thorlabs)), and a dispersion unit 22 (specifically, a telecommunication transmission type diffraction grating (T-966C-27x10-94 manufactured by LightSmyth)). In the embodiment, without providing the output side circulator 50, the laser light IL and the reflected light RL are separated by the beam splitter 24. The receiving device 30 is composed of a lens 32 and a photodiode 31 (specifically, an InGaAs variable gain type avalanche photodiode (APD, APD450C)). The measuring device 100 of the embodiment has an experimental system configuration, and the signal collection device 41 is an oscilloscope that integrates a plurality of sweeps and is synchronized with the signal generator 11.

[0059] In the measuring device 100, the optical output with an average power of -2 dBm from the wavelength variable laser 10 is amplified to 14.6 dBm by the EDFA which is the output side optical amplifier, and in the spatial dispersion device 20, it is collimated into free space using the collimator 21 with a beam diameter of 2 mm. Using the highly efficient telecommunication transmission type diffraction grating which is the dispersion unit 22, the wavelength-swept light is diffracted with a spatial dispersion of about 0.09° / nm and a diffraction efficiency of 94%. The relationship between the spatial dispersion and the scanning angle of the dispersion unit 22 can be calculated as follows. TIFF0007715375000004.tif8165 Here, Δλ is the wavelength adjustment range, and d is the dispersion of a single diffraction grating. Using this formula, the scan angle can be estimated to be 3.5°. [

[0060] When the optical beam (laser beam IL) output from the measuring device 100 hits the surface of the object OB, two types of reflection, namely specular reflection and diffuse reflection, may occur. Since diffuse light reflection is dominant in the return light (reflected light RL) from most indoor objects, the latter diffuse light reflection is more important for laser detection applications. According to Lambert's cosine law, the reflected power per steradian observed from an ideal diffuse reflection surface can be expressed as follows. TIFF0007715375000005.tif14165 Here, P T is the root mean square (RMS) radiant power from the light source, ρ is the diffuse reflectivity of the surface, and θ is the deflection angle between the direction of the incident light and the normal of the surface. In the experiment, a coaxial configuration with θ = 0 was used to maximize the reflected light power. The reflected light RL from the object OB returns to the beam splitter 24 through the same optical path and is converted into an electrical signal by the photodiode 31. In the measuring device 100 of the embodiment, since a single pixel detector can be used instead of a detector array, it is compatible with a high-sensitivity avalanche photodiode (APD) or a photomultiplier tube (PMT). The oscilloscope, which is the signal collection device 41, synchronizes the measurement signal and the reference signal, collects data from the receiving device 30, which is the photodetector, and performs post-data processing.

[0061] Lambert's cosine law also implies that the diffused light is reflected in all directions. The closer the object OB is brought to the measuring device 100, the more reflected light RL can be detected. The measuring device 100 can detect the reflected light RL measurable at a location up to 30 cm away from the object OB. Considering the scan angle, the maximum scan range at 30 cm is approximately 1.8 cm. By using a higher light power, a longer detection distance can be achieved. Since the wavelength of the wavelength-variable laser 10 can be continuously adjusted, the lateral resolution is mainly determined by the size of the beam spot and the instantaneous line width. The main factor limiting the lateral resolution in the measuring device 100 is the beam size of 2 mm. Improvement is expected by using a collimator or a lens system with a small beam size.

[0062] <Chirp Intensity Modulation Phase Shift Measurement (CAMPS)> Here, to explain the CAMPS procedure, data from a non-mechanical line scan performed on a planar sample is taken as an example. For the sample, a metal plate with an anodic coating is used. Figures 9(A) to 9(D) and Figures 10(A) to 10(D) show information in the time domain and frequency domain of the measured signal, which is the raw signal, and the reference signal. Figure 9(A) is the waveform of the measured signal, and Figure 9(C) is an enlarged waveform of a part of Figure 9(A). Figure 9(B) is the waveform of the reference signal, and Figure 9(D) is an enlarged waveform of a part of Figure 9(B). Figures 10(A) and 10(B) show the frequency domain information (RF spectrum) of the measured signal and the reference signal, respectively, and Figures 10(C) and 10(D) show the main peaks in the range of modulation frequencies from 720 MHz to 726 MHz of Figures 10(A) and 10(B).

[0063] As shown in Figures 9(A) to 9(D), the two waveforms of the measured signal and the reference signal are synchronized so that the instantaneous frequencies are the same. Different from the reference signal or reference data, since the surface of the sample does not always provide a constant reflected light RL at different locations, the amplitude of the measured signal or signal data is not constant.

[0064] As shown in Figures 10(B) and 10(D), for the reference signal or reference data, the peak of the modulation frequency from 720 MHz to 726 MHz is the highest, and the other frequency components are almost constant. The second highest peak is more than 10 dB lower than the main peak. However, as shown in Figures 10(A) and 10(C), in the case of the measured signal or signal data, since the waveform is a pulse wave rather than a sine wave such as the DC component or the 2f m component, there are multiple peaks with equivalent intensities.

[0065] In the measuring device 100 of the present embodiment, unlike a standard intensity-modulated continuous wave (AMCW) with a constant modulation frequency, since the modulation frequency is also modulated, instead of using a mixer, post-data processing is executed by a computer of the signal processing device 40 to obtain the dynamic phase change of the signal trace. The normalized continuous intensity I of the measurement signal sgn (t), the normalized continuous intensity I of the reference signal ref (t), and the continuous relative phase Δφ(t) can be calculated as follows. TIFF0007715375000006.tif19165Here, φ sgn (t) and φ ref (t) are the continuous phase angles of the measurement signal and the reference signal, respectively. φ sgn (t) and φ ref (t) can be easily obtained by applying a band-pass filter to the temporal train in the main frequency region and removing other irrelevant frequency components. However, since the modulation frequency is not constant and the measurement signal is a pulse wave instead of a sine wave (see Fig. 9(C)), as shown in Figs. 10(C) and 10(D), the filter needs to cover the entire main frequency region from 720 MHz to 726 MHz, but it becomes too wide to completely remove noise. The side lobes of the sweep repetition rate in the frequency domain overlap with each other while the modulation frequency is sweeping, so it is almost impossible to remove them with a filter. Here, in order to solve this problem, the CAMPS technique is used for the data processing of high-speed chirp AM phase detection.

[0066] Fig. 11 is a diagram for explaining a specific example of the processing procedure in the signal processing device 40 shown in Fig. 7. Since the amplitude of the intensity trace is not uniform, the normalized continuous intensity of the measurement signal I sgn (t) needs to be expressed as follows. TIFF0007715375000007.tif8165Here, A(t) represents the envelope of the continuous amplitude of the measurement signal. In the signal processing device 40, using the lock-in technique, the phase shift between the measurement signal and the reference signal is extracted by applying a low-pass filter to their multiplication. Compared with the case of applying a band-pass filter separately to the measurement signal and the measurement reference, this method can provide higher accuracy because it is not affected by the side lobes that overlap in the frequency domain caused by the sweep repetition rate. To avoid the phase error caused by the non-uniform amplitude due to the Kramers-Kronig relationship, the reference signal RS is first Hilbert-transformed by the Hilbert transform unit 42a of the signal arithmetic processing device 42 to give phase information to all frequency components. Next, the Hilbert-transformed reference signal RSa is multiplied by the measurement signal MS in the multiplication unit 42b to obtain the lock-in data DT1. The lock-in data can be expressed as follows. TIFF0007715375000Q08.tif19165Here, the first term is the phase shift between the measurement signal MS and the Hilbert-transformed reference signal RSa (reference signal), and the second term is the unwanted high-frequency noise.

[0067] Figure 12(A) shows the amplitude of the lock-in data DT1, and Figure 12(C) is an enlarged view of a part of Figure 12(A). Figure 12(B) shows the phase of the lock-in data DT1, and Figure 12(D) is an enlarged view of a part of Figure 12(B). Figure 12(E) shows the RF spectrum of the lock-in data DT1, and Figure 12(F) is an enlarged view of a part of Figure 12(E).

[0068] It should be noted that there seems to be a minor error in the "TIFF0007715375000Q08.tif" in the original text, which is likely a typo and should probably be "TIFF0007715375000008.tif". The translation has been done based on the provided text as accurately as possible.Since the DC peak of the RF spectrum represents homodyne information between the reference signal RS and the measurement signal MS, the lock-in data DT1 is filtered by the low-pass filter 42c of the signal arithmetic processing unit 42, and high-frequency noise is removed. Specifically, after performing a fast Fourier transform (FFT), a digital low-pass filter is applied to remove high-frequency noise. Here, instead of a rectangular window function, a Blackman window function is used as the data low-pass filter. The latter may cause undesirable ripple due to discontinuities. The filtered time waveform is obtained after performing an inverse FFT (iFFT).

[0069] FIGS. 13(A) and 13(B) are diagrams showing the amplitude and phase of the lock-in data after filtering. The continuous relative phase Δφ(t) can be calculated as follows. TIFF0007715375000009.tif13166 Here, S real (t) and S img (t) are the real part and the imaginary part of the lock-in data DT1 using the low-pass filter 42c, respectively. To correspond to different FSRs of the sweep repetition rate, the obtained phase shift is inclined. When the modulation frequency is swept from 726 MHz to 720 MHz, the unambiguous distance (uncertain distance) changes from 20.66 cm to 20.83 cm.

[0070] The phase shift between the measurement signal MS and the Hilbert-transformed reference signal RSa (reference signal) can be obtained after phase detection of the lock-in data DT1 and can be converted into the distance shown in the following equation. TIFF0007715375000010.tif13165 Here, d(t) is the relative distance, Δφ(t) is the continuous relative phase, f m (t) is the modulation frequency, t is the time of a single sweep changing from 0 to 0.1 milliseconds, and N is the number of cycles of the unambiguous distance.

[0071] <Verification experiment> Figure 14(A) is a diagram showing the result of line scanning a planar sample with an anodic coating over the span of the unambiguous distance (maximum continuous detection range), and Figure 14(B) is a diagram showing the result of line scanning the planar sample over a 4-mm span with different averaging numbers.

[0072] After distance conversion and background correction, the relative distance information of the planar sample can be obtained from the phase shift. Since the sample is placed about 20 cm away from the scanner, the corresponding lateral scanning range is about 12 mm. Considering the 20-cm axial span, the result shows a straight line as expected as shown in Figure 14(A). An error of less than 1 mm is observed in the millimeter span. By averaging the results over multiple scans, random noise can be minimized. As shown in Figure 14(B), averaging more than 8 times is sufficient to remove most of the noise.

[0073] As shown in Figure 14(B), in addition to the random noise that can be suppressed by averaging, there are reproducible variations. Since the averaged result suggests a systematic error with an amplitude of about 1 mm that does not exist in the sample, this can be regarded as a systematic error. By subtracting the highly averaged (96 times) result from the scanning results at the same location, the systematic error can be corrected and the accuracy of the detection system can be evaluated.

[0074] Figure 15(A) is a schematic plan view of two metal plates used for distance measurement, and Figure 15(B) is a diagram showing the result of line scanning.

[0075] In the example, the distance (specifically, the relative distance d1) was measured for the sample OB1 in which two metal plates were arranged to form a stepped surface. As shown in Fig. 15(A), the depth of the step 71 of the sample OB1 is 13 mm, and the laser beam IL, which is the scanning light, is appropriately irradiated so as to cover the step 71 of the sample OB1. Fig. 15(B) shows the result of the line scan with an averaging number of 32 times. From this result, a clear difference in depth is observed, which agrees well with the 13-mm step depth of the sample OB1. The region of the edge of the step 71 causes a gentle slope with a width of 2 mm corresponding to the beam size. The beam size is the main factor that limits the lateral resolution of the current system. In the future, it can be improved by reducing the beam size or increasing the detection distance.

[0076] As described above, using the dispersion-tuned laser and the new CAMPS technology, we have succeeded in demonstrating and proposing an overall non-mechanical spectral scanning type laser rangefinder. Thanks to the dispersion-tuned laser with high-speed wavelength sweep pulse output, neither a mechanical scanning device nor an additional external intensity modulator for amplitude modulation is included. By using the CAMPS technology, the dispersion-tuned laser can be utilized to continuously restore the phase shift information from the chirped amplitude modulation signal. The results of the distance measurement show that the proposed system can achieve a high-speed continuous line scan with an axial resolution of 10 kHz and approximately 1 mm as a proof of concept. The current limitation of the detection range in the system is the transmission power and the beam size. By improving the power efficiency of the free space system and replacing the collimator with a smaller beam size, an expansion of the detection range and an improvement in the lateral resolution can be expected.

[0077] In the measurement device 100 described above, the signal processing device 40 acquires information regarding the distance and direction to the target OB based on the output timing, detection timing, and timing information of a series of laser beams IL. At this time, by combining the wavelength-variable laser 10, which is a wavelength-sweeping light source, and the spatial dispersion device 20, and performing wavelength sweeping and laser beam deflection using all non-mechanical configurations, the sweeping speed can be increased by several orders of magnitude compared to conventional mechanical configurations, and the vibration resistance of the device can be improved.

[0078] The wavelength-variable laser 10 outputs a series of laser beams as a pulse train. The signal processing device 40 determines information regarding the emission direction of each laser beam IL from the timing information of the pulse train, and calculates the phase difference between the reference signal RS obtained from the laser beam IL (actually, the synthesizer 12) and the measurement signal MS of the reflected light RL in units of pulses constituting the pulse train in order to determine the distance to the target OB. By calculating the phase difference between the reference signal RS and the measurement signal MS in units of pulses constituting the pulse train for distance calculation, it is possible to acquire the dynamic phase change of the signal trace in which the repetition frequency continuously changes (chirps). As a result, continuous high-speed scanning becomes possible.

[0079] 〔Second Embodiment〕 Hereinafter, the measurement device according to the second embodiment will be described. Note that the measurement device according to the second embodiment is a modification of the first embodiment, and parts not particularly described are the same as those of the first embodiment.

[0080] As shown in FIG. 16, the measurement device 110 of this embodiment is obtained by stacking a plurality of the measurement devices 100 shown in FIG. 1 in the Y-axis direction. In the measurement device 110, the gratings 122a and 122b of the dispersion unit 122 among the spatial dispersion devices 20 are each in an array form.

[0081] By stacking a plurality of measurement devices 110 with a plurality of measurement devices 100, scanning in the Y-axis direction becomes possible, and two-dimensional measurement can be performed in combination with the individual X-axis direction scanning of the measurement device 100.

[0082] 〔Third Embodiment〕 Hereinafter, a measuring device according to the third embodiment will be described. The measuring device according to the third embodiment is a modification of the first embodiment and the like, and parts not particularly described are the same as those in the first embodiment and the like.

[0083] As shown in FIG. 17, in the measuring device 120 of the present embodiment, the spatial dispersion device 20 includes a collimator 21, a virtually imaged phased array 25 (VIPA: Virtually Imaged Phased Arrays), and an array-shaped dispersion unit 222. The VIPA 25 generates output light that can be spatially distinguished according to the wavelength of the input light, and is composed of a cylindrical lens 25a and a glass plate 25b. The measuring device 120 functions as an automatic non-mechanical two-dimensional scanner by incorporating the VIPA 25 into the spatial dispersion device 20.

[0084] The measuring devices 110 and 120 shown in FIGS. 16 and 17 can be applied to three-dimensional measurement. For example, the measuring devices 110 and 120 can be considered for application to a three-dimensional laser scanner used for appearance inspection of industrial products. Object shape measurement is in demand in various fields and is applied to shape measurement of molds, appearance inspection of processed products, solder shape inspection on printed circuit boards, etc. So far, inspections of fine scratches, cracks, etc. on the surface of parts have been performed by the tactile sense and vision of engineers, but the practical application of the measuring devices 110 and 120 of the above embodiments is expected to accelerate the realization of a smart factory. Also, although measuring devices are used in autonomous vehicles, the most well-known measuring devices for autonomous driving are based on mechanical drive and are also costly for one or more vehicles. By using the non-mechanical measuring device according to the above embodiment, there is a possibility of revolutionizing three-dimensional measurement in the autonomous driving industry.

[0085] 〔Others〕 Although the present invention has been described in accordance with the above embodiments, the present invention is not limited to the above embodiments.

[0086] In the above embodiment, the driving current of the internal optical amplifier 14 may be directly modulated to perform wavelength modulation.

[0087] In the above embodiment, the method of pulse modulation, the method of wavelength sweeping, etc. can be appropriately changed. Also, the modulation frequency, the sweeping speed, etc. can be appropriately changed. Further, the sweeping waveform can be appropriately changed such as linear, non-linear, upward sweeping, downward sweeping, etc.

[0088] In the above embodiment, the configurations of the wavelength-variable laser 10, the spatial dispersion device 20, the receiving device 30, the signal processing device 40, etc. can be appropriately changed.

[0089] In the above embodiment, even when the wavelength-sweeping light source of the wavelength-variable laser 10 has a configuration including mechanical driving, the CAMPS technique can be used for signal processing when the modulation frequency of the laser light IL chirps and the phase shift amount between the reference signal RS and the measurement signal MS changes in time series.

Explanation of Reference Signs

[0090] 10... wavelength-variable laser, 11... signal generator, 12... synthesizer, 13... pulse generator, 14... internal optical amplifier, 15... intensity modulator, 16... high-dispersion medium, 17... internal circulator, 18... output-side optical amplifier, 19... optical fiber, 20... spatial dispersion device, 21... collimator, 22, 122, 222... dispersion part, 22a, 22b, 122a, 122b... grating, 23... lens, 30... receiving device, 31... photodiode, 40... signal processing device, 41... signal collection device, 42... signal arithmetic processing device, 42a... Hilbert transform part, 42b... multiplication part, 42c... low-pass filter, 42d... phase detection part, 42e... phase-distance conversion part, 50... output-side circulator, 100, 110, 120... measuring device, IL... laser light, OB, OB1... object, RL... reflected light

Claims

1. A wavelength-variable laser that outputs a series of laser lights with non-mechanical wavelength sweeping as a pulse train, a spatial dispersion device that spatially disperses the pulse train by non-mechanical optical characteristics and irradiates an object, and reverses the reflected light from the object, a receiving device that receives the reflected light via the spatial dispersion device, a signal processing device that determines the distance to the object from the detection timing of the reflected light by the receiving device and the output timing of the pulse train, comprising: The signal processing device determines information regarding the emission direction of each laser light from the timing information of the pulse train, and performs a multiplication process between a signal obtained by performing a Hilbert transform on a reference signal obtained from the laser light in units of pulses constituting the pulse train and the measurement signal of the reflected light in order to determine the distance to the object, and calculates a phase difference between the reference signal and the measurement signal after the multiplication process. A measuring device.

2. The wavelength-variable laser according to claim 1, wherein the wavelength-variable laser performs dispersion tuning by pulse modulation and outputs the series of laser lights as the pulse train whose repetition frequency continuously changes.

3. The measuring device according to any one of claims 1 and 2, wherein the wavelength-variable laser includes an optical amplifier that amplifies light, an intensity modulator that performs intensity modulation, and a high-dispersion medium that dispersively adjusts the wavelength.

4. The measuring device according to claim 3, wherein the high-dispersion medium is a chirped fiber Bragg grating.

5. The measuring device according to any one of claims 3 and 4, wherein the optical amplifier is a semiconductor optical amplifier.

6. The measuring device according to any one of claims 1 to 5, wherein the spatial dispersion device includes a collimator that makes the laser light parallel light and a dispersion unit that spatially disperses the laser light that has passed through the collimator.

7. The measuring device according to claim 6, wherein the dispersion unit has one or more gratings.

8. The measuring device according to any one of claims 1 to 7, wherein filtering is performed on the lock-in data after the multiplication process.

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