JP7715375B2 - Measuring equipment - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. (1) Announced on May 11, 2020, at the website address https://www. osapublishing. org/abstract. cfm? uri=CLEO_AT-2020-JTu2G. 34. (2) Announced on May 12, 2020, at the online conference of the CLEO2020 conference (https://event.crowdcompass.com/cleo20), under presentation number JTu2G. 34.

The present invention relates to a measurement device that measures distance using a method similar to AMCW (amplitude-modulated continuous-wave), and in particular to a measurement device capable of continuous high-speed scanning.

Conventional measuring devices include those that use mechanical drive to deflect and sweep a laser beam (see, for example, Patent Documents 1 and 2). Mechanically driven measuring devices have the problem of little improvement in terms of sweep speed, vibration resistance, etc.

There are also technologies that perform beam scanning non-mechanically by combining a wavelength swept light source and a spatial dispersion device such as a diffraction grating (see, for example, Non-Patent Documents 1 and 2). However, the technologies in Non-Patent Documents 1 and 2 rely on mechanically driven wavelength swept light sources, and are unable to resolve the above-mentioned issues, such as sweep speed and vibration resistance.

JP 2008-18941 A Japanese Patent Application Laid-Open No. 2003-4850

Y. Zhai, et al. "Non-mechanical beam-steer lidar system based on swept-laser source." Optical Fiber Sensors. Optical Society of America, 2018. M. Okano, et al. "Swept Source Lidar: simultaneous FMCW ranging and nonmechanical beam steering with a wideband swept source." Optics Express 28.16 (2020): 23898-23915.

The present invention was made in consideration of the above background technology, and aims to provide a measuring device capable of continuous high-speed scanning.

To achieve the above objective, the measurement device of the present invention comprises a tunable laser that outputs a series of pulsed laser beams that have been non-mechanically wavelength-swept; a spatial dispersion device that spatially disperses the series of laser beams using non-mechanical optical properties and irradiates the target with the laser beam and causes the reflected light from the target to travel in the opposite direction; a receiving device that receives the reflected light via the spatial dispersion device; and a signal processing device that determines the distance to the target from the timing of detection of the reflected light by the receiving device and the timing of output of the laser beam from the tunable 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.

In the above-mentioned measurement device, a signal processing device acquires information about the distance and direction to the target based on the output timing of the laser light, the detection timing, and timing information for the series of laser light beams. By combining a wavelength-tunable laser, which serves as a wavelength-swept light source, with a spatial dispersion device, and performing wavelength sweeping and laser beam deflection entirely using a non-mechanical configuration, the sweep speed can be increased by several orders of magnitude compared to conventional mechanical configurations, and the vibration resistance of the device can be improved.

In a specific aspect of the present invention, in the above-mentioned measurement device, the tunable laser performs dispersion tuning using pulse modulation, outputting a series of laser light as a pulse train with a continuously changing repetition frequency. Here, dispersion tuning means controlling the oscillation wavelength by changing the modulation frequency in an active mode-locked state. Unlike sinusoidal modulation, pulse modulation contains high-frequency components, allowing for effective modulation with a narrow pulse width while maintaining a wavelength tunable band.

In another aspect of the present invention, a wavelength-tunable laser includes an optical amplifier that amplifies light, an intensity modulator that performs intensity modulation, and a high-dispersion medium that adjusts the wavelength dispersion. The high-dispersion medium enables pulses with high wavelength purity to be obtained.

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, making it possible to achieve a more compact measurement device.

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

In yet another aspect of the present invention, the spatial dispersion device includes a collimator that converts laser light into parallel light, and a dispersion section that spatially disperses the laser light that has passed through the collimator. In this case, precise spatial dispersion of the laser light is possible even if the output of the wavelength-tunable laser is diverging light.

In yet another aspect of the present invention, the dispersion section has one or more gratings. In this case, chromatic dispersion can be easily controlled.

In yet another aspect of the present invention, the signal processing device determines information regarding the emission direction of each laser beam from timing information of the pulse train, and calculates the phase difference between a reference signal obtained from the laser beam and a measurement signal of the reflected beam for each pulse constituting the pulse train to determine the distance to the target. By calculating the phase difference between the reference signal and the measurement signal for each pulse constituting the pulse train in relation to distance calculation, dynamic phase changes in the signal trace can be obtained. This enables continuous high-speed scanning.

To achieve the above objective, the measurement device of the present invention comprises a tunable laser that outputs a series of wavelength-swept laser beams as a pulse train; a spatial dispersion device that spatially disperses the pulse train using non-mechanical optical properties and irradiates the target with the pulse train, and causes the reflected light from the target to travel in the opposite direction; 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 timing information about the pulse train. The signal processing device calculates the phase difference between a reference signal obtained from the laser beam and a measurement signal of the reflected light for each pulse constituting the pulse train in order to determine the distance to the target.

In the above-mentioned measurement device, the signal processing device obtains information about the distance and direction to the target from the phase difference between the reference signal and the measurement signal and timing information of the pulse train. In this case, by combining a wavelength-tunable laser, which serves as a wavelength-swept light source, with a spatial dispersion device, the dynamic phase changes of the signal trace can be obtained by calculating the phase difference between the reference signal and the measurement signal for each pulse that makes up the pulse train in order to calculate the distance. This makes continuous high-speed scanning possible.

In another aspect of the present invention, the signal processing device multiplies the measurement signal by a signal obtained by performing a Hilbert transform on the reference signal. In this case, the Hilbert transform provides phase information to all frequency components of the reference signal, making it possible to avoid phase errors caused by uneven amplitudes.

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

FIG. 1 is a conceptual diagram illustrating a measurement device according to a first embodiment. (A) is a conceptual diagram showing a sweep waveform in a signal generator, (B) is a conceptual diagram showing a modulated waveform in a synthesizer, (C) is a conceptual diagram showing a modulated pulse waveform in a pulse generator, and (D) is a conceptual diagram showing an optical sweep pulse waveform emitted from a wavelength-tunable laser. 1 is a spectrum of an optical sweep pulse output from a wavelength tunable laser. FIG. 2 is a conceptual diagram illustrating a spatial dispersion device of the measurement apparatus shown in FIG. 1 . (A) is a conceptual diagram showing the change over time in wavelength of each output pulse output from a wavelength-tunable laser, and (B) is a conceptual diagram showing the emission angle corresponding to each output pulse of laser light emitted from a spatial dispersion device. 10A and 10B are diagrams illustrating a phase shift between a reference signal obtained from laser light and a measurement signal of reflected light. FIG. 2 is a conceptual diagram illustrating signal processing in a signal processing device. FIG. 1 is a conceptual diagram illustrating a specific example of a measurement device. (A) is the waveform of the measurement signal, (B) is the waveform of the reference signal, (C) is an enlarged portion of the waveform of (A), and (D) is an enlarged portion of the waveform of (B). (A) and (B) are diagrams illustrating frequency domain information of the measurement signal and the reference signal, respectively, and (C) and (D) are diagrams illustrating the main peaks in the modulation frequency range of 720 MHz to 726 MHz of (A) and (B). 8 is a conceptual diagram illustrating a specific example of a processing procedure in the signal processing device shown in FIG. 7. (A) is the amplitude of the lock-in data, (B) is the phase of the lock-in data, (C) is an enlarged view of a portion of (A), (D) is an enlarged view of a portion of (B), (E) is the RF spectrum of the lock-in data, and (F) is an enlarged view of a portion of (E). 10A and 10B show the amplitude and phase of the lock-in data after filtering. (A) shows the results of line scanning a planar sample at a span of non-ambiguous distance, and (B) shows the results of line scanning a planar sample at a 4 mm span with different averaging times. (A) is a schematic plan view of two metal plates used for distance measurement, and (B) is a diagram showing the results of line scanning. FIG. 10 is a conceptual diagram illustrating a measurement device according to a second embodiment. FIG. 10 is a conceptual diagram illustrating a measurement device according to a third embodiment.

First Embodiment
A measurement apparatus according to a first embodiment of the present invention will be described below with reference to FIG.

As shown in FIG. 1, the measurement device 100 includes a wavelength-tunable laser 10, a spatial dispersion device 20, a receiving device 30, and a signal processing device 40. The measurement device 100 also includes an output-side circulator 50 that optically splits the output light and the received light. The measurement device 100 is capable of scanning the laser light IL in the X-axis direction, and can perform one-dimensional measurement by scanning in the X-axis direction.

The tunable laser 10 outputs a series of pulsed laser light IL that has been non-mechanically wavelength swept. In other words, the tunable laser 10 outputs the series of wavelength-swept laser light IL as a pulse train. Here, the series of pulsed laser light IL refers to laser light whose pulses are changed over time. The tunable laser 10 performs dispersion tuning using pulse modulation. Here, dispersion tuning refers to controlling the oscillation wavelength by changing the modulation frequency in an active mode-locking state. Using a dispersion-tuned swept laser (DTSL), which employs a dispersion tuning method, as the wavelength-swept light source enables high-speed non-mechanical sweeping. In this embodiment, the tunable laser 10 is a ring-type laser.

The wavelength-tunable laser 10 utilizes the principle of dispersion tuning. By inserting a high-dispersion device into the resonator to increase the wavelength dispersion value, and then applying intensity modulation in this state, the laser light undergoes active mode-locked oscillation. The oscillation frequency can be controlled by varying the modulation frequency. In mode-locking, the longitudinal mode is synchronized by applying modulation in accordance with the round-trip time of the light within the resonator, and the oscillation wavelength can be swept by varying the modulation frequency. Because the wavelength-tunable laser 10 electrically controls the oscillation wavelength, there are no mechanical limitations, enabling high-speed, wide-band wavelength sweeping. The pulsed laser light output from the wavelength-tunable laser 10 has a sweep bandwidth of 50 nm or more at a sweep speed of 10 kHz, for example.

The tunable laser 10 comprises 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 tunable 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 tunable laser 10 is connected by an optical fiber 19.

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

Synthesizer 12 is an electrical circuit that generates a drive signal with a waveform corresponding to the ramp signal from signal generator 11 as a modulating signal. In other words, synthesizer 12 is a function generator that continuously sweeps the frequency of the ramp signal from signal generator 11 as a modulating signal. As shown in Figure 2(B), the waveform of the modulating signal from synthesizer 12 is a sine wave with a continuously changing frequency (chirped). During one sweep of the sweep waveform, if the ramp voltage of signal generator 11 is low, the frequency of the modulating signal from synthesizer 12 decreases, and if the ramp voltage of signal generator 11 is high, the frequency of the modulating signal from synthesizer 12 increases. The modulation frequency of the modulating signal is, for example, 788 MHz to 795 MHz or 716 MHz to 726 MHz.

Pulse generator 13 generates a pulse signal corresponding to the modulation signal from synthesizer 12. Pulse generator 13 can vary the pulse width, for example, between 50 ps and 250 ps. As shown in Figure 2(C), the waveform of the pulse signal from pulse generator 13 is a pulse with a continuously changing frequency (chirped). During one sweep of the sweep waveform, when the modulation signal from synthesizer 12 changes from a low frequency to a high frequency, the wavelength of the pulse train generated by 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.

The internal optical amplifier 14 is, for example, a semiconductor optical amplifier (SOA), which amplifies light by stimulated emission when light is incident on the semiconductor element from the outside.

The intensity modulator 15 is an optical modulator that uses, for example, lithium niobate crystal, and performs intensity modulation by adjusting the transmittance of the laser light upon receiving the pulse signal output from the pulse generator 13. The intensity modulation of the laser light is pulsed, and frequencies of, for example, 788 MHz to 795 MHz or 716 MHz to 726 MHz are used. Unlike sinusoidal modulation, pulse modulation contains high-frequency components, allowing for effective modulation with a narrow pulse width while maintaining a wavelength tunable band.

The high-dispersion medium 16 adjusts the wavelength dispersion. In other words, the high-dispersion medium 16 adjusts the wavelength dispersion. The high-dispersion medium 16 enables pulses with high wavelength purity to be obtained. Examples of the high-dispersion medium 16 include chirped fiber Bragg gratings (CFBGs) and diffraction grating pairs. This allows the length of the resonator to be shortened, thereby enabling the measurement device 100 to be miniaturized. In this embodiment, the high-dispersion medium 16 is a CFBG. The dispersion of a CFBG is, for example, +10 ps/nm, and it reflects 65% of the light. The remaining 35% of the light is amplified by the output-side optical amplifier 18, output, and used as laser light IL. Because the high-dispersion medium 16 increases the dispersion in the resonator, the resonant frequency interval (FSR: Free Spectral Range) varies significantly depending on the wavelength, resulting in selectivity of the oscillation wavelength. The high-dispersion medium 16 outputs a sweeping pulse with a wavelength of, for example, 1530 nm to 1580 nm or 1530 nm to 1600 nm that satisfies the resonance conditions.

The internal circulator 17 directs the light that has passed through the intensity modulator 15 into the high-dispersion medium 16, and guides the light reflected by the high-dispersion medium 16 to the internal optical amplifier 14.

In dispersion tuning, a highly dispersive medium 16 is intentionally inserted into the resonator and intensity-modulated to generate active mode locking. The oscillation wavelength can be controlled by varying the modulation frequency while active mode locking is occurring. In other words, dispersion tuning allows for control of the oscillation wavelength without using a mechanical wavelength-selective filter. This enables high-speed, wideband wavelength sweeping. In dispersion tuning, the sweeping method simply involves changing the mode-locking modulation frequency (frequency modulation). The spectral shape can also be controlled by appropriately setting the sweep waveform (i.e., modulation range and rate). The FSR, which is the frequency spacing of the longitudinal modes, depends on the refractive index of light in the optical fiber. In resonators with large chromatic dispersion, the FSR depends on the modulation frequency. When the resonator is modulated externally using a frequency signal that is an integer multiple of the FSR, mode locking occurs and oscillation occurs only at wavelengths with an FSR corresponding to the modulation frequency. Varying this modulation frequency causes the oscillation wavelength to change proportionally with the modulation frequency, resulting in wavelength-tunable laser light IL. When a linear sweep of the modulation frequency is performed, the oscillation wavelength also changes linearly, resulting in a corresponding sweep. The wavelength tunable range of dispersion tuning is determined by the gain bandwidth of the gain medium, and its maximum value is determined by the variable range of the modulation frequency. The smaller the total dispersion of the resonator and the modulation frequency, the larger the maximum wavelength tunable range. In a resonator with a large dispersion value, the mode-locked pulse becomes a chirped pulse. The linewidth of the pulse spectrum narrows as the modulation frequency and the dispersion increase.

Figure 2(D) shows the time variation of the optical sweep pulse output from the wavelength-tunable laser 10. The laser light IL output from the wavelength-tunable laser 10 is wavelength-swept, changing 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 consisting of multiple pulses with wavelengths that vary in a time series is generated. The pulse signal generated by the pulse generator 13 and the pulse signal output from the wavelength-tunable laser 10 have a one-to-one relationship at 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 cavity dispersion.

Figure 3 shows the spectrum of the dispersion-adjusted laser light IL output from the wavelength-tunable 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 fundamental FSR is 17 MHz for a wavelength of 1550 nm, and the modulation frequency range is 788 MHz to 795 MHz. The sweep speed of the wavelength-tunable laser 10 is, for example, 10 kHz, and the sweep wavelength is, for example, 1530 nm to 1580 nm. Furthermore, if the cavity length of the resonator is, for example, 11.1 m, the fundamental FSR of light with a wavelength of 1550 nm is approximately 18.6 MHz. In this case, the modulation frequency is 716 MHz to 726 MHz, and the sweep wavelength is, for example, 1530 nm to 1600 nm.

Returning to Figure 1, the output-side optical amplifier 18 is, for example, a fiber amplifier, and although detailed description will be omitted, it includes an erbium-doped fiber amplifier, a pumping light source, and a coupler that guides light from the pumping light source to the fiber amplifier. Note that, like the internal optical amplifier 14, the output-side optical amplifier 18 may also be an SOA or the like. The power of the laser light IL amplified by the output-side optical amplifier 18 is, for example, 16 dBm or 14.6 dBm.

The output-side circulator 50 directs the laser light IL that has passed through the wavelength-tunable laser 10 into the spatial dispersion device 20, and directs the reflected light RL that has passed through the spatial dispersion device 20 into the receiving device 30.

The spatial dispersing device 20 spatially disperses a series of laser light IL or pulse trains using non-mechanical optical properties, irradiates the target OB, and reverses the reflected light RL from the target OB. Irradiating wavelength-swept light onto the spatial dispersing device 20 enables beam deflection without relying on machinery. The laser scanning range via the spatial dispersing device 20 is, for example, 4° or 1 cm in the X-axis direction (horizontal direction), with a resolution of 10 μm. If the power of the laser light IL incident on the spatial dispersing device 20 is, for example, 16 dBm, the received power of the reflected light RL reflected by the target OB after passing through the spatial dispersing device 20 is -50 dBm. The measurable distance of the measurement device 100 depends on the sweep power of the laser light IL output from the wavelength-tunable laser 10.

As shown in Figure 4, the spatial dispersion device 20 has a collimator 21, a dispersion section 22, and a lens 23. This enables precise spatial dispersion of the laser light IL even if the output of the wavelength-tunable laser 10 is diverging light. Note that the lens 23 may not be provided depending on the distance to the target OB.

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

The dispersion section 22 spatially disperses the laser light IL that has passed through the collimator 21. Examples of the dispersion section 22 include a grating (diffraction grating), a prism, an angle amplification lens, a photonic crystal, and a phase array. The spatial dispersion device 20 includes the dispersion section 22, which makes it easy to control wavelength dispersion. In this embodiment, the dispersion section 22 has two gratings 22a and 22b. The dispersion amount of the dispersion section 22 (specifically, the dispersion amount per grating) is, for example, 0.04°/nm, and the dispersion efficiency is 60% to 80%. While Figure 4 shows an example of a reflective grating for the dispersion section 22, a transmissive grating may also be used. The dispersion section 22 may also have one or three or more gratings.

Lens 23 focuses the dispersed light of each wavelength while correcting chromatic aberration and other issues. Lens 23 is, for example, an achromatic lens. The focal length of lens 23 is, for example, 15 cm.

As shown in Figure 5(A), when each output pulse (e.g., optical sweep pulses with pulse numbers 1 to 5, ...) or each output pulse train output from the wavelength-tunable laser 10 is emitted from the spatial dispersion device 20, the emission angle changes depending on the output pulse, as shown in Figure 5(B). Information regarding the relationship between this output pulse and the emission angle is determined and stored in the signal processing device 40, which will be described later, and is used to calculate the distance to the target OB.

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

The signal processing device 40 determines the distance to the target OB from the detection timing of the reflected light RL by the receiving device 30 and the output timing of the laser light IL or pulse train from the wavelength-tunable laser. The signal processing device 40 also determines information regarding the emission direction of each laser light IL from timing information of the series of laser light IL or pulse train. The timing information of the laser light IL includes, for example, experimentally supported information necessary for calculation processing such as the order of output pulses and wavelength differences. Furthermore, to determine the distance to the target OB, the signal processing device 40 calculates the phase difference between the reference signal RS obtained from the laser light IL and the measurement signal MS of the reflected light RL for each pulse constituting the pulse train.

The signal processing device 40 has a signal collecting device 41 and a signal calculation processing device 42. Although details will be described later in an example, as shown in Fig. 6, in the measurement device 100 of this embodiment, the modulation frequency of the laser light IL chirps, and the phase shift amounts Δφ ₁ to Δφ ₅ between the reference signal RS and the measurement signal MS change over time. Therefore, in the signal processing in the signal processing device 40, chirped amplitude-modulated phase-shift measurement (CAMPS) technology is used to acquire dynamic phase changes of a signal trace whose repetition frequency keeps changing (chirping).

Figure 7 is a conceptual diagram illustrating the configuration of a signal processing device 40 that uses CAMPS technology. The signal acquisition device 41 of the signal processing device 40 is a high-speed signal processing circuit that includes an A/D converter for digital data processing in the signal calculation processing device 42, but it may also be an oscilloscope, for example. In this case, it allows synchronization with the frequency sweep based on the signal from the signal generator 11, and captures the measurement signal and reference signal in a comparable synchronized manner.

The signal processing device 42 includes a computer or other processing unit, and determines the distance OBD from the measurement device 100 to the object OB. The signal processing device 42 also determines information regarding the emission direction of each laser beam IL from timing information of the series of laser beams IL output from the wavelength-tunable laser 10. The signal processing device 42 also pre-records information regarding the emission angle corresponding to each output pulse shown in Figure 5(B) as spatial position information in the X-axis direction.

The signal calculation processing device 42 includes a Hilbert transformer 42a, a multiplier 42b, a low-pass filter 42c, a phase detector 42d, and a phase-distance converter 42e. The Hilbert transformer 42a performs a Hilbert transform on the reference signal RS before the multiplication process in the multiplier 42b, generating a Hilbert-transformed reference signal RSa. In other words, the Hilbert transformer 42a converts a real signal into a complex time signal using a Hilbert transform to process adjacent phase information regarding the reference signal RS. The Hilbert transform is an operation that converts a given real-valued function into a complex signal by applying a harmonic conjugate to it. The Hilbert transform allows the complex-valued function to be extended to the complex upper half-plane, thereby obtaining phase information of the signal. The multiplier 42b multiplies the measurement signal MS by 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 multiplier 42b to remove irrelevant frequency components, thereby obtaining lock-in data DT2. The phase detector 42d then detects the phase shift amount PS using the filtered lock-in data DT2. The phase-distance converter 42e calculates the distance OBD from the measurement device 100 to the object OB based on the phase shift amount PS detected by the phase detector 42d.

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 light IL and the detection timing of the reflected light RL, and provides information regarding the distance OBD from the measurement device 100 to the object OB. If there is a circuit-related delay time between the reference signal RS output from the synthesizer 12 and the pulse signal output from the wavelength-tunable laser 10, a calibration method that takes this delay time into account is used to calculate the phase difference between the reference signal RS and the measurement signal MS.

The signal calculation processing device 42 can determine the shape of the target OB using distance information calculated from the phase difference between the reference signal RS and measurement signal MS described above, and spatial position information based on the emission angle of the laser light IL described above.

(Example)
<Tunable wavelength laser>
Hereinafter, an embodiment of the measurement device 100 will be mainly described, focusing on the wavelength tunable laser 10.

Referring to FIG. 1, tunable laser 10 is an actively mode-locked fiber laser including the previously described elements 11, 12, 13, 14, 15, 16, 17, and 18, which is mode-locked by modulating the optical intensity in the cavity at a specific modulation frequency. Of these elements, elements 14, 15, 16, and 17 function as dispersion tuning sections. A large amount of chromatic dispersion is introduced into the cavity of tunable laser 10, causing the FSR at each wavelength to be different. This allows only wavelengths whose FSR corresponds to the modulation frequency to be mode-locked. Given a laser cavity of a specific length, the FSR of this cavity is given by:
where c is the speed of light in a vacuum, l is the length of the laser cavity, and n is the refractive index within the cavity. By applying intensity modulation at an integer multiple of the FSR, the harmonic mode-locking condition can be satisfied. Given the central modulation frequency _fm0 and the central wavelength _λ0 , the swept laser wavelength can be expressed as follows:
where f _m is the swept modulation frequency and D is the total dispersion within the cavity. By sweeping the modulation frequency, a pulsed optical output with swept wavelength is achieved.

The maximum wavelength tuning range Δλ _max is mainly determined by the gain bandwidth. However, when the modulation frequency changes more than 1 FSR, two wavelengths of adjacent harmonic modes may appear simultaneously, so the maximum tuning range of a single wavelength output is given by
where _fFSR0 is the FSR at the center wavelength _λ0 . According to this equation, it is desirable to reduce the intracavity total dispersion D and the center modulation frequency _fm0 to achieve a wide tuning range. However, reducing the intracavity total dispersion D and the center modulation frequency _fm0 leads to laser wavelength instability and linewidth broadening. Therefore, there is a trade-off between the wavelength tuning range and the instantaneous linewidth. For non-mechanical beam scanners, the wavelength tuning range determines the maximum lateral scan angle, and the instantaneous linewidth broadening can be said to limit the lateral resolution. Versatile non-mechanical scanners can be realized by modifying the parameters of the laser source.

The tunable laser 10 of the embodiment has the same configuration as that shown in Figure 1. The total cavity length of the tunable laser 10 of the embodiment is 11.1 m. This means that the fundamental FSR of light with a wavelength of 1550 nm is approximately 18.6 MHz. This allows a mode-locked pulse output with wavelengths from 1530 nm to 1600 nm to be obtained at a modulation frequency of 716 MHz to 726 MHz. For 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 to reduce the spectral linewidth of the laser. In the embodiment, the pulse width of the pulses generated by the pulse generator 13 is 100 ps, so 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 approximately 40%.

The tunable laser 10 inserts an internal optical amplifier (SOA) 14 into the cavity to provide broadband optical gain over a 100 nm range, and employs a highly dispersive medium 16, a CFBG, as both the output element and the dispersive element. The CFBG has a chromatic dispersion of 10 ps/nm and a reflectivity of 65%, with 35% of the cavity power passing through the CFBG as output. The CFBG significantly shortens the cavity length, effectively improving wavelength sweep speed. The above optoelectronic approach produces pulsed optical output with an average power of -2 dBm and a pulse width of approximately 100 ps. The output wavelength of the tunable laser 10 is swept at a frequency of 10 kHz, and the instantaneous spectral full width at half maximum (FWHM) is approximately 0.3 nm.

However, for laser detection applications, higher optical power levels are required, so the output is amplified by an erbium-doped fiber amplifier (EDFA) such as output optical amplifier 18.

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

In the measurement device 100, the optical output from the wavelength-tunable laser 10, with an average power of −2 dBm, is amplified to 14.6 dBm by an EDFA, which is the output-side optical amplifier 18, and then collimated into free space using a collimator 21 with a beam diameter of 2 mm in a spatial dispersing device 20. A high-efficiency telecom transmission grating, which is the dispersing section 22, diffracts the wavelength-swept light with a spatial dispersion of approximately 0.09°/nm and a diffraction efficiency of 94%. The relationship between the spatial dispersion of the dispersing section 22 and the scanning angle can be calculated as follows:
where Δλ is the wavelength tuning range and d is the dispersion of a single grating. Using this formula, the scan angle can be estimated to be 3.5°.

When the light beam (laser light IL) output from the measurement device 100 strikes the surface of the object OB, two types of reflection can occur: specular reflection and diffuse reflection. The latter type is more important in laser detection applications because it dominates the return light (reflected light RL) from most indoor objects. According to Lambert's cosine law, the reflected power per steradian observed from an ideal diffuse-reflecting surface can be expressed as follows:
where P _T is the root-mean-square (RMS) radiant power from the light source, ρ is the diffuse reflectance of the surface, and θ is the deflection angle between the incident light direction and the surface normal. 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 along the same optical path and is converted into an electrical signal by the photodiode 31. The measurement device 100 of the embodiment can use a single pixel detector instead of a detector array, making it compatible with high-sensitivity avalanche photodiodes (APDs) or photomultiplier tubes (PMTs). The signal acquisition device 41, an oscilloscope, synchronizes the measurement signal with a reference signal to acquire data from the photodetector receiving device 30 and perform post-data processing.

Lambert's cosine law also suggests that diffuse light is reflected in all directions. The closer the object OB is to the measurement device 100, the more reflected light RL can be detected. The measurement device 100 can detect measurable reflected light RL 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. Longer detection distances can be achieved by using higher optical power. Because the wavelength of the tunable laser 10 is continuously adjustable, the lateral resolution is primarily determined by the beam spot size and instantaneous linewidth. The main factor limiting the lateral resolution of the measurement device 100 is the 2 mm beam size. Improvements are expected by using collimators and lens systems with smaller beam sizes.

Chirp Intensity Modulation Phase Shift Measurement (CAMPS)
To explain the CAMPS procedure, we use data from a non-mechanical line scan performed on a planar sample as an example. The sample used is a metal plate with an anodic coating. Figures 9(A) to 9(D) and 10(A) to 10(D) show time-domain and frequency-domain information for the raw measurement signal and reference signal. Figure 9(A) shows the waveform of the measurement signal, and Figure 9(C) shows an enlarged waveform of a portion of Figure 9(A). Figure 9(B) shows the waveform of the reference signal, and Figure 9(D) shows an enlarged waveform of a portion of Figure 9(B). Figures 10(A) and 10(B) show frequency-domain information (RF spectra) for the measurement signal and reference signal, respectively, and Figures 10(C) and 10(D) show the main peak in the modulation frequency range of 720 MHz to 726 MHz in Figures 10(A) and 10(B).

As shown in Figures 9(A) to 9(D), the two waveforms of the measurement signal and the reference signal are synchronized so that they have the same instantaneous frequency. Unlike the reference signal or reference data, the amplitude of the measurement signal or signal data is not constant because the sample surface does not necessarily provide a constant reflected light RL at different locations.

As shown in Figures 10(B) and 10(D), the reference signal or reference data has the highest peak at a modulation frequency of 720 MHz to 726 MHz, with the other frequency components remaining nearly 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 measurement signal or signal data, the waveform is a pulse wave rather than a sine wave such as a DC component or _2fm component, and therefore multiple peaks of equal intensity are present.

In the measurement device 100 of this embodiment, unlike a standard amplitude-modulated continuous wave (AMCW) whose modulation frequency is constant, the modulation frequency is also modulated, so instead of using a mixer, post-data processing is performed by the computer of the signal processing device 40 to obtain the dynamic phase changes of the signal trace. The normalized continuous intensity I _sgn (t) of the measurement signal, the normalized continuous intensity I _ref (t) of the reference signal, and the continuous relative phase Δφ(t) can be calculated as follows:
Here, 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 domain to remove other irrelevant frequency components. However, because the modulation frequency is not constant and the measurement signal is a pulse wave rather than a sine wave (see Figure 9(C)), the filter must cover the entire main frequency range from 720 MHz to 726 MHz, as shown in Figures 10(C) and 10(D), but this is 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 they can hardly be removed by the filter. To solve this problem, we use the CAMPS technique for data processing of high-speed chirp AM phase detection.

Fig. 11 is a diagram illustrating 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:
where A(t) represents the envelope of the continuous amplitude of the measurement signal. The signal processing device 40 utilizes a lock-in technique to extract the phase shift between the measurement signal and the reference signal by applying a low-pass filter to their multiplication. Compared with applying band-pass filters to the measurement signal and the measurement reference separately, this method can provide higher accuracy because it is not affected by side lobes that overlap in the frequency domain caused by the sweep repetition rate. To avoid phase errors caused by uneven amplitudes due to the Kramers-Kronig relation, the reference signal RS is first Hilbert transformed in the Hilbert transformer 42a of the signal processing device 42 to provide phase information for all frequency components. The Hilbert transformed reference signal RSa is then multiplied by the measurement signal MS in the multiplier 42b to obtain lock-in data DT1. The lock-in data can be expressed as follows:
where 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.

Figure 12(A) shows the amplitude of the lock-in data DT1, and Figure 12(C) is an enlarged view of a portion 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 portion 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 portion of Figure 12(E).

Since the DC peak in the RF spectrum represents the 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 processing unit 42 to remove high-frequency noise. Specifically, after performing a fast Fourier transform (FFT), a digital low-pass filter is applied to remove high-frequency noise. Here, a Blackman window function is used as the data low-pass filter rather than a rectangular window function, which may cause undesired ripples due to discontinuities. The filtered time waveform is obtained after performing an inverse FFT (iFFT).

13(A) and 13(B) show the amplitude and phase of the lock-in data after filtering. The continuous relative phase Δφ(t) can be calculated as follows:
where S _real (t) and S _img (t) are the real and imaginary parts, respectively, of the lock-in data DT1 obtained using low-pass filter 42c. The resulting phase shift is ramped because the sweep repetition rate corresponds to a different FSR. As 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.

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 converted into a distance as shown in the following equation:
where 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 varying from 0 to 0.1 ms, and N is the number of cycles of the unambiguous distance.

<Verification experiment>
FIG. 14(A) shows the results of line scanning an anodic coated planar sample at a span of non-ambiguous distance (maximum continuous detection range), and FIG. 14(B) shows the results of line scanning a planar sample at a 4 mm span with different averaging times.

After distance transformation and background correction, relative distance information for the planar sample can be obtained from the phase shift. The sample is placed approximately 20 cm away from the scanner, so the corresponding lateral scan range is approximately 12 mm. Considering an axial span of 20 cm, the results are linear as expected, as shown in Figure 14(A). Errors of less than 1 mm are observed over millimeter spans. Random noise can be minimized by averaging the results over multiple scans. Averaging more than 8 times is sufficient to remove most of the noise, as shown in Figure 14(B).

As shown in Figure 14(B), in addition to random noise that can be suppressed by averaging, there is reproducible variation. The averaged results suggest a systematic error of approximately 1 mm amplitude that is not present in the sample, and therefore can be considered a systematic error. By subtracting the highly averaged (96 times) results from the scan results of the same location, the systematic error can be corrected and the accuracy of the detection system can be evaluated.

Figure 15(A) is a schematic plan view of two metal plates used for distance measurement, and Figure 15(B) shows the results of line scanning.

In this example, distance (specifically, relative distance d1) was measured for sample OB1, which consisted of two metal plates arranged side by side to form a stepped surface. As shown in Figure 15(A), the depth of step 71 in sample OB1 was 13 mm, and the scanning light, laser light IL, was appropriately irradiated to cover step 71 of sample OB1. Figure 15(B) shows the results of line scanning with 32 averaging cycles. The difference in depth is clearly observed, which is consistent with the 13 mm step depth of sample OB1. The edge region of step 71 produces a gentle slope with a width of 2 mm, corresponding to the beam size. The beam size is the main factor limiting the lateral resolution of the current system. In the future, this can be improved by reducing the beam size or increasing the detection distance.

As described above, we have successfully demonstrated and proposed an entirely non-mechanical spectral scanning laser rangefinder using a dispersion-tuned laser and novel CAMPS technology. Thanks to the dispersion-tuned laser with a fast wavelength-swept pulse output, neither a mechanical scanning device nor an additional external intensity modulator for amplitude modulation is included. CAMPS technology can utilize the dispersion-tuned laser to continuously recover phase shift information from the chirped amplitude-modulated signal. The range measurement results demonstrate that the proposed system, as a proof-of-concept, can achieve high-speed continuous line scanning at 10 kHz and approximately 1 mm axial resolution. The detection range limitations of current systems are the transmit power and beam size. By improving the power efficiency of the free-space system and replacing the collimator with a smaller beam size, it is expected that the detection range and lateral resolution will be extended.

In the measurement device 100, the signal processing device 40 acquires information regarding the distance and direction to the target OB from the output timing of the laser light IL, the detection timing, and timing information for a series of laser light IL signals. By combining the wavelength-tunable laser 10, which serves as a wavelength-swept light source, with the spatial dispersion device 20, the wavelength sweep and laser beam deflection are all performed using a non-mechanical configuration, enabling the sweep speed to be several orders of magnitude faster than conventional mechanical configurations and improving the device's vibration resistance.

The tunable laser 10 outputs a series of laser beams as a pulse train, and the signal processing device 40 determines information regarding the emission direction of each laser beam IL from timing information of the pulse train. To determine the distance to the target OB, the signal processing device 40 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 for each pulse constituting the pulse train. By calculating the phase difference between the reference signal RS and the measurement signal MS for each pulse constituting the pulse train, it is possible to obtain dynamic phase changes in a signal trace whose repetition frequency is constantly changing (chirping). This enables continuous high-speed scanning.

Second Embodiment
The measurement device according to the second embodiment will be described below. Note that the measurement device according to the second embodiment is a modification of the first embodiment, and parts that are not particularly described are the same as those in the first embodiment.

As shown in Figure 16, the measurement device 110 of this embodiment is formed by stacking multiple measurement devices 100 shown in Figure 1 in the Y-axis direction. In the measurement device 110, the gratings 122a and 122b of the dispersion section 122 of the spatial dispersion device 20 are each arranged in an array.

By stacking multiple measuring devices 100 in a measuring device 110, scanning in the Y-axis direction becomes possible, and two-dimensional measurements can be performed in combination with individual X-axis scans of the measuring devices 100.

Third Embodiment
The measurement device according to the third embodiment will be described below. Note that the measurement device according to the third embodiment is a modification of the first embodiment, and parts that are not particularly described are the same as those of the first embodiment.

As shown in FIG. 17 , in the measurement apparatus 120 of this embodiment, the spatial dispersion device 20 has a collimator 21, a virtually imaged phased array (VIPA) 25, and an array-shaped dispersion section 222. The VIPA 25 generates spatially distinguishable output light according to the wavelength of the input light, and is composed of a cylindrical lens 25a and a glass plate 25b. By incorporating the VIPA 25 into the spatial dispersion device 20, the measurement apparatus 120 functions as an automatic, non-mechanical two-dimensional scanner.

The measuring devices 110, 120 shown in Figures 16 and 17 can be applied to three-dimensional measurement. For example, the measuring devices 110, 120 could be applied to three-dimensional laser scanners used for visual inspection of industrial products. Object shape measurement is in demand in a wide variety of fields, and is applied to mold shape measurement, visual inspection of processed products, solder shape inspection on printed circuit boards, and more. Until now, inspection of component surfaces for fine scratches, cracks, etc. has been performed by engineers using their sense of touch or vision. However, the practical application of the measuring devices 110, 120 of the above-described embodiments is expected to accelerate the realization of smart factories. Furthermore, measuring devices are also used in autonomous vehicles. However, most known autonomous driving measuring devices are mechanically driven and cost more than a single automobile. Using a non-mechanical measuring device such as the measuring device of the above-described embodiment could revolutionize three-dimensional measurement in the autonomous driving industry.

〔others〕
Although the present invention has been described above based on the embodiments, the present invention is not limited to the above embodiments.

In the above embodiment, wavelength modulation may be achieved by directly modulating the drive current of the internal optical amplifier 14.

In the above embodiment, the pulse modulation method, wavelength sweep method, etc. can be changed as appropriate. The modulation frequency, sweep speed, etc. can also be changed as appropriate. The sweep waveform can also be changed as appropriate, such as linear, nonlinear, upsweep, or downsweep.

In the above embodiment, the configurations of the wavelength tunable laser 10, spatial dispersion device 20, receiving device 30, signal processing device 40, etc. can be modified as appropriate.

In the above embodiment, even if the wavelength sweep light source of the wavelength tunable laser 10 is configured to include mechanical drive, CAMPS technology can be used for signal processing when the modulation frequency of the laser light IL chirps and the amount of phase shift between the reference signal RS and the measurement signal MS changes over time.

10...Tunable wavelength 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 section, 22a, 22b, 122a, 122b...Grating, 23...Lens, 30...Receiver, 31...Photodiode, 40...Signal processing device, 41...Signal collection device, 42...Signal calculation processing device, 42a...Hilbert transform section, 42b...Multiplication section, 42c...Low-pass filter, 42d...Phase detection section, 42e...Phase/distance conversion section, 50...Output side circulator, 100, 110, 120...Measuring device, IL...Laser light, OB, OB1...Object, RL...Reflected light

Claims (8)

a wavelength-tunable laser that outputs a series of laser beams that have been non-mechanically swept in wavelength as a pulse train;
a spatial dispersion device that spatially disperses the pulse train using non-mechanical optical properties, irradiates the pulse train onto a target, and causes reflected light from the target to travel in the opposite direction;
a receiving device that receives the reflected light via the spatial dispersion device;
a signal processing device that determines the distance to the target based on the timing of detection of the reflected light by the receiving device and the timing of output of the pulse train ;
Equipped with
The signal processing device determines information regarding the emission direction of each laser beam from timing information of the pulse train , and in order to determine the distance to the target, multiplies a signal obtained by performing a Hilbert transform on a reference signal obtained from the laser beam for each pulse constituting the pulse train by the measurement signal of the reflected beam, and calculates a phase difference between the reference signal and the measurement signal after the multiplication . The measurement device according to claim 1 , wherein the tunable laser performs dispersion tuning by pulse modulation, and outputs the series of laser beams as the pulse train whose repetition frequency keeps changing. The measurement device described in either claim 1 or 2, wherein the tunable laser has an optical amplifier that amplifies light, an intensity modulator that performs intensity modulation, and a high-dispersion medium that adjusts the wavelength dispersion. The measurement device of claim 3, wherein the highly dispersive medium is a chirped fiber Bragg grating. The measurement device described in any one of claims 3 and 4, wherein the optical amplifier is a semiconductor optical amplifier. The measurement apparatus according to any one of claims 1 to 5, wherein the spatial dispersion device includes a collimator that converts the laser light into parallel light, and a dispersion section that spatially disperses the laser light that has passed through the collimator. The measurement device described in claim 6, wherein the dispersion section has one or more gratings. 8. The measurement device according to claim 1 , wherein filtering is performed on the lock-in data after the multiplication process.