JP7074311B2 - Optical distance measuring device and measuring method - Google Patents

Description

The present invention relates to an optical distance measuring device suitable for an environment recognition sensor or the like used in an automobile, an autonomous robot, or the like, and a measuring method.

The development of lidar (LiDAR: Laser Imaging Detection and Ranking) is progressing for the purpose of application to environment recognition sensors for automobiles and autonomous robots, and shape measurement at construction and civil engineering sites. The TOF (Time of Flyght) type rider, which has already been put into practical use, irradiates an object with an optical pulse, measures the distance from the time until it is reflected and returns, and spatially irradiates the optical pulse. It scans and generates three-dimensional distance data.

The TOF rider detects the reflected light from the object by direct detection. On the other hand, the FMCW (Frequency Modified Continuous Wave) method using coherent detection has the feature that it can detect reflected light with higher sensitivity and can measure the motion speed from the Doppler shift in addition to the distance to the object. There is. The FMCW radar in the millimeter wave region has been put into practical use as an in-vehicle collision prevention sensor. If the FMCW rider can be realized in the light wave region, it can be expected that the spatial resolution will be significantly improved. The current FMCW rider has a complicated device configuration and requires a high coherence laser light source, so that the field of application is limited.

14 (a), (b) and 14 (c) are diagrams illustrating the principle of the FMCW rider. The FMCW rider device of FIG. 14A includes a modulation signal generator 1, an injection current source 2, a semiconductor laser 3, beam splitters 5a and 5b, an optical circulator 6, a reflector 7, and a photodetector 12. To prepare for. Next, the operating principle of the FMCW rider will be described with respect to the case where the semiconductor laser 3 is used as the light source. The output of the modulation signal generator 1 that generates a triangular wave is input to the injection current source 2 to modulate the injection current of the semiconductor laser 3. The output light of the semiconductor laser 3 is divided into two, one of which is the probe light 9 and the other of which is the reference light 8. The reference light is light that is in phase with the probe light and serves as a reference for optical delay. A beam splitter 5a and 5b, an optical circulator 6, a reflector 7, and an object 11 constitute an interferometer. The probe light 9 is irradiated to the object 11, the reflected light 10 from the object 11 and the reference light 8 are combined and input to the photodetector 12 to detect the beat signal. FIG. 14B shows the waveform of the optical frequency of the semiconductor laser 3. The optical frequency of the semiconductor laser 3 periodically repeats up and down chirps corresponding to the triangular wave output from the modulation signal generator 1. A time difference occurs between the reference light 8 and the reflected light 10, and a beat signal having a frequency proportional to the time difference is generated at the output of the photodetector 12. FIG. 14C shows the reflected light 10 of the probe light, the optical frequency of the reference light, and the beat signal waveform. The optical frequencies of the reference light (solid line) and the reflected light (dotted line) are shown in the upper part of FIG. 14 (c) in correspondence with the beat signal. Therefore, if the frequency of the beat signal (hereinafter referred to as the beat frequency) is measured, the time difference, that is, the distance to the object 11 can be obtained. Both the up and down chirps are used in order to obtain the motion velocity from the Doppler shift when the object 11 is in motion. Here, the case where a semiconductor laser is used has been described, but the same measurement can be performed if the laser has a frequency modulation function.

The beat frequency f _FMCW measured by the FMCW rider can be expressed by the following equation.

Here, Δν is the chirp bandwidth, T _m = 1 / _fm is the modulation period, _fm is the modulation frequency, L is the distance to the object 11, and c is the optical velocity. In the equation (Equation 1), Δν / (T _m / 2) represents the frequency change per unit time, that is, the chirp rate. In order to calculate the distance from the beat frequency, it is necessary to obtain the chirp rate in advance.

The distance resolution δL in the FMCW rider can be expressed by the following equation.

The meaning of the resolution in the equation (Equation 2) is the ability to detect two adjacent reflection points separately. When there is one reflection point, the distance can be measured with higher accuracy. Since the distance resolution δL is inversely proportional to the chirp bandwidth Δν, a large chirp bandwidth is required to obtain high resolution. For example, the chirp bandwidths required to obtain a resolution of 10 cm and 1 cm are 1.5 GHz and 15 GHz, respectively. Even with one reflection point, the accuracy is inversely proportional to the chirp bandwidth.

In addition to the chirp bandwidth, the chirp linearity is also an important characteristic that determines the performance of the FMCW rider. The beat frequency of the equation (Equation 1) is based on the premise that the optical frequency increases (up chirp) or decreases (down chirp) in proportion to time. When the optical frequency changes non-linearly with time, the beat frequency, which should be a constant value, changes, which causes an error in the distance calculation.

A semiconductor laser operating in the eye-safe wavelength range is expected as a light source for a compact and low-cost FMCW rider because the frequency can be directly modulated by injection current modulation. However, it is known that the frequency modulation of the semiconductor laser is caused by the thermal effect and the frequency response characteristic is not flat, so that the non-linear chirp appears remarkably. It has been reported that when the frequency is modulated by a triangular wave, a frequency component not included in the modulated signal appears due to the nonlinear chirp (see Non-Patent Document 1).

In the FMCW rider, the method of suppressing or reducing the influence of such a non-linear chirp can be roughly divided into two. One is to control the modulation of the semiconductor laser to obtain the desired linear chirp. The other is to process the detected beat signal to eliminate the influence of the non-linear chirp.

A device and a method for optically detecting the frequency modulation of a laser and controlling an error with a reference signal by negatively feeding back to the laser have been reported as follows (see Patent Documents 1 to 3). A homodyne or heterodyne interferometer for detecting the frequency modulation of the laser is prepared, frequency-modulated light is input, and the beat frequency is detected from the interferometer output. The beat frequency should be a constant value, but it fluctuates due to the influence of the non-linear chirp. The detected beat frequency is compared with the frequency of the reference signal source to generate an error signal, and the injection current of the laser is controlled by negative feedback to suppress the non-linear chirp.

A method of monitoring a frequency-modulated signal or a laser output to control a modulated signal generator and correcting a detected beat signal to calculate a distance has been reported as follows (see Patent Document 4). .. The phase of the laser output light is mathematically modeled, the parameters included in the model are estimated from the monitor results, control and signal processing are performed, and the distance is calculated.

In the millimeter wave FMCW radar, a device that optically detects the millimeter wave frequency and suppresses the influence of the non-linear chirp by signal processing has been reported as follows (see Patent Document 5). The millimeter wave signal to be transmitted is converted into an optical signal, the beat signal is detected by a homodyne interferometer, and the pulse signal is converted. The pulse signal contains information on the non-linear chirp, and the influence of the non-linear chirp can be suppressed by AD-converting the beat signal generated by the reflection from the object using this pulse signal as a clock.

Japanese Unexamined Patent Publication No. 2000-11132 U.S. Patent Application Publication No. 2010/0085992 U.S. Patent Application Publication No. 2012/0106579 U.S. Patent Application Publication No. 2009/0135403 Japanese Patent Publication No. 2008-514910

H. Tsuchida, "Waveform measurement technology for phase / frequency-modulated lights based on heterodyne interferometry," Optics Express. 25, no. 5, pp. 4793-4799 (March 2017)

In an actual FMCW rider, there is a problem that the change in optical frequency changes non-linearly with time, which causes an error in distance calculation.

In the above-mentioned method of suppressing or reducing the influence of the non-linear chirp (see Patent Documents 1 to 3), since the non-linear chirp is suppressed by the negative feedback control of the laser, it is necessary to generate an error signal in real time. Therefore, it is necessary to provide a homodyne or heterodyne interferometer. Further, in the method of controlling the modulation signal generator and correcting the detection signal (see Patent Document 4), an optical device such as an interferometer is required. Further, although Patent Document 5 is a technique relating to a millimeter wave radar device, it can also be applied to an FMCW rider in an optical region. However, since it is necessary to generate the clock of the AD converter in real time, a homodyne interferometer is required.

As described above, in the prior art, a device for detecting and controlling frequency modulation is required separately from the interferometer for distance measurement, and the configuration of the device becomes complicated. If a compact and low-cost rider system can be realized as an optical distance measuring device without complicating the device configuration, it can be expected to be applied to the consumer field such as in-vehicle collision prevention and pedestrian detection sensor.

The present invention is intended to solve the above-mentioned problems in the FMCW rider, and enables accurate distance measurement by eliminating the influence of the non-linear charp of the laser without using an additional device such as an interferometer. It is an object of the present invention to provide an optical distance measuring device and a measuring method.

The present invention has the following features in order to achieve the above object.

(1) The frequency-modulated laser, the light detector, and the output light of the laser are divided into two, one of which is the reference light and the other of which is the probe light. The reflected light of the above, the interferometer that combines the reference light and is incident on the optical detector, the beat frequency detection device that detects the frequency of the beat signal generated from the optical detector, and the beat frequency detection device. Provided with an arithmetic processing device that obtains the average value of the detected beat frequencies and calculates the distance to the object based on the relationship between the distance calibrated in advance and the average beat frequency using the average value. An optical distance measuring device characterized by.
(2) The beat frequency detection device obtains the in-phase component I (t) and the orthogonal component Q (t) of the beat signal, and obtains the beat from the inverse positive tangent tan ^-1 {Q (t) / I (t)}. The optical distance measuring apparatus according to (1), wherein the instantaneous phase of a signal is calculated and the beat frequency is obtained from the time derivative of the instantaneous phase.
(3) The beat frequency detection device obtains the time of the zero cross point by intercalation from an AD converter that converts the beat signal into a digital signal and two sample values that sandwich the zero cross point of the digital signal, and the zero cross. The optical distance measuring apparatus according to (1), comprising an arithmetic processing unit for obtaining the beat frequency from the time of a point.
(4) The optical distance measuring device according to any one of (1) to (3), wherein the frequency modulation waveform of the laser is a triangular wave.
(5) The output light of the frequency-modulated laser is divided into two, one of which is the reference light and the other of which is the probe light, and the probe light is irradiated to the object to obtain the reflected light from the object and the reference light. The combined wave is incident on the optical detector, the frequency of the beat signal generated from the optical detector is detected, the average value of the frequencies of the beat signal is obtained, and the average value is used to obtain the distance calibrated in advance. An optical distance measuring method characterized in that the distance to the object is calculated based on the relationship with the average beat frequency.
(6) In the detection of the beat signal frequency, the in-phase component I (t) and the orthogonal component Q (t) of the beat signal are obtained, and the reverse positive tangent tan ^-1 {Q (t) / I (t)} is used. The optical distance measuring method according to (5), wherein the instantaneous phase of the beat signal is calculated and the beat frequency is obtained from the time derivative of the instantaneous phase.
(7) In the detection of the beat signal frequency, the beat signal is converted into a digital signal, the time of the zero cross point is obtained by interpolation from two sample values sandwiching the zero cross point of the digital signal, and the time of the zero cross point is obtained. The optical distance measuring method according to (5), wherein the beat frequency is obtained from the above.
(8) The optical distance measuring method according to any one of (5) to (7), wherein the frequency modulation waveform of the laser is a triangular wave.

In the optical distance measuring device and measuring method of the present invention, the average value of the beat signal frequencies generated by the FMCW rider is measured, and the distance is calculated based on the relationship between the pre-calibrated distance and the average frequency. Therefore, an optical system for monitoring the frequency modulation of the laser and an electronic circuit for controlling the frequency modulation are unnecessary. As a result, the device configuration can be remarkably simplified, and miniaturization and cost reduction can be realized.

Further, when a semiconductor laser in which a non-linear chirp is remarkably generated by direct frequency modulation is used, the influence of the non-linear chirp can be suppressed only by signal processing, so that a smaller and more accurate device can be realized.

It is a figure explaining the optical distance measuring apparatus which concerns on this invention. It is a figure which shows the result of the theoretical calculation about an ideal triangular wave. (A) shows the waveforms of the optical frequency and the beat frequency, and (b) shows the average beat frequency and the relative error. It is a figure which shows the result of the theoretical calculation about the triangular wave which has a nonlinear chirp. (A) shows the waveforms of the optical frequency and the beat frequency, and (b) shows the average beat frequency and the relative error. It is a figure explaining the beat frequency detection apparatus which constitutes the optical distance measuring apparatus of 1st Embodiment. It is a figure explaining the optical distance measuring apparatus of 1st Embodiment and the apparatus which calibrates the rate of change of the average beat frequency with respect to a distance. It is a figure which shows the waveform of the beat frequency when the manual variable optical delay line is used which concerns on 1st Embodiment. (B) is an enlarged view of a part (a part surrounded by a dotted line) of (a). It is a figure which shows the distance measurement value and the measurement error at the time of using the manual variable optical delay line which concerns on 1st Embodiment. It is a figure which shows the calibration result when the optical fiber patch cord which concerns on 1st Embodiment is used as an optical delay line. (A) shows the waveform of the beat frequency, and (b) shows the relationship between the fiber length and the average beat frequency. It is a figure explaining the beat frequency detection apparatus which constitutes the optical distance measuring apparatus of 2nd Embodiment. (A) shows a beat frequency detector, (b) shows a waveform of a beat signal output from a photodetector, and (c) shows a partially enlarged view of (b). It is a figure explaining the optical distance measuring apparatus of 2nd Embodiment, and the apparatus which calibrates the rate of change of the average beat frequency with respect to the distance. It is a figure which shows the waveform at the time of using the manual variable optical delay line which concerns on 2nd Embodiment. (A) shows the waveform of the beat signal, and (b) shows the waveform of the beat frequency. It is a figure which shows the distance measurement value and the measurement error at the time of using the manual variable optical delay line which concerns on 2nd Embodiment. It is a figure which shows the calibration result when the optical fiber patch cord which concerns on 2nd Embodiment is used as an optical delay line. (A) shows the waveform of the beat frequency, and (b) shows the relationship between the fiber length and the average beat frequency. It is a figure explaining the principle of FMCW rider in the prior art. (A) shows the FMCW rider device, (b) shows the waveform of the optical frequency of the semiconductor laser, and (c) shows the optical frequency of the reflected light and the reference light, and the beat signal waveform.

Hereinafter, embodiments of the present invention will be described in detail. In the present invention, the distance to an object is measured by using a device including a frequency-modulated laser, an interferometer, a photodetector, a beat frequency detection device, and an arithmetic processing device.

FIG. 1 is a diagram illustrating a basic configuration of an optical distance measuring device according to an embodiment of the present invention. The optical distance measuring device of FIG. 1 includes a modulation signal generator 1, an injection current source 2, a semiconductor laser 3, an interferometer 4, an optical detector 12, a beat frequency detection device 13, and a signal processing device 14. And prepare. In FIG. 1, a directly modulated semiconductor laser is shown as an example of a frequency-modulated laser. The interferometer 4 is mainly composed of a beam splitter 5a, an optical circulator 6, a beam splitter 5b, a reflector 7, and an object 11. As shown in FIG. 1, the output of the modulation signal generator 1 is input to the semiconductor laser 3 via the injection current source 2 to modulate the frequency of the output light. The output light of the semiconductor laser 3 is split into two by a beam splitter 5a, one of which is the reference light 8 and the other of which is the probe light 9. The probe light 9 is irradiated to the object 11 via the optical circulator 6. The reflected light from the object 11 is guided to the beam splitter 5b via the optical circulator 6, and the reflected light 10 and the reference light 8 are combined and received by the photodetector 12. Since there is a time difference between the frequency-modulated reference light 8 and the reflected light 10 according to the distance to the object 11, a frequency difference occurs. A beat signal corresponding to the frequency difference is generated at the output of the photodetector 12. The output of the photodetector 12 is input to the beat frequency detection device 13, the beat frequency is detected, and then the output is input to the arithmetic processing device 14. The arithmetic processing apparatus 14 records the relationship between the pre-calibrated distance and the average beat frequency using the semiconductor laser 3. The arithmetic processing apparatus 14 calculates the average value of the detected beat frequencies, and calculates the distance to the object 11 based on the relationship between the pre-calibrated distance and the average beat frequency.

The following will be described in detail using mathematical formulas. In FIG. 1, the electric field _ER (t) of the reference light 8 can be expressed by the following equation.

On the other hand, the electric field _ES (t) of the reflected light 10 can be expressed by the following equation.

Here, _AR and _AS are the amplitude of the electric field, ν ₀ is the center frequency, φ (t) is the instantaneous phase caused by the frequency modulation of the semiconductor laser 3, and τ _d is the round-trip time of light to the object 11. .. Assuming that the distance to the object 11 is L, the round-trip time τ _d of light can be expressed by τ _d = 2 L / c using the speed of light c. φ (t) is related to the optical frequency ν (t) of the semiconductor laser 3 by the following equation.

In the FMCW rider, it is common to use a triangular wave or a sawtooth wave as the frequency modulation signal, but as will be described later, if the relationship between the distance and the average beat frequency is calibrated in advance, these waveforms can be obtained. Not limited.

The beat signal V _B (t) output from the photodetector 12 can be expressed by the following equation.

Here, η is a constant determined by the sensitivity of the photodetector 12 and the like. The beat frequency detection device 13 detects the beat frequency f _B (t) represented by the following equation from the beat signal represented by the equation (Equation 6).

Next, in the arithmetic processing apparatus 14, the average beat frequencies f _B , avg (t ₁ , t ₂ ) expressed by the following equation are calculated from the beat frequency f _B (t) expressed by the equation (Equation 7). ..

The average beat frequency f _B , avg (t ₁ , t ₂ ) is obtained by integrating the beat frequency f _B (t) over a finite interval [t ₁ , t ₂ ] and then dividing by the time t ₂ − t ₁ in the finite interval. It is the value that was set. When the beat frequency is measured as N sample values f _B ( _tk ) ( _tk = 1, 2, ..., N), the average beat frequency is the sum of the N sample values based on the number of samples N. It becomes the value divided. The averaging interval [t ₁ , t ₂ ] is arbitrary, but when the motion velocity of the object 11 is obtained from the Doppler shift, a half cycle corresponding to the up and down chirps of the frequency modulation is desirable. If it is not necessary to obtain the motion velocity, it may be an integral multiple of the modulation period. The shorter the averaging interval, the shorter the measurement time.

The arithmetic processing apparatus 14 records or stores the relationship between the pre-calibrated distance L and the average beat frequencies f _B , avg (t ₁ , t ₂ ). Regarding the relationship between the two, it is possible to record the data of the average frequency with respect to the distance in the format of a table, but it is desirable to record it in a format that can be expressed analytically. For example, consider the case where the average beat frequencies f _B , avg (t ₁ , t ₂ ) are proportional to the distance L as shown in the following equation.

Here, γ represents the rate of change of the average beat frequency with respect to the distance. If the value of γ is known, the distance L'can be calculated from the measured average beat frequency using the following equation.

In this way, since the distance can be calculated using only a single parameter γ, the time required for the arithmetic processing can be shortened. The interval [t ₁ , t ₂ ] for calculating the average beat frequency must be set to be the same as the calibration performed in advance.

The relative error ε between the distance L'calculated by the equation (Equation 10) and the actual distance L can be expressed by the following equation.

Hereinafter, the results of theoretically calculating the relationship between the average beat frequencies f _B , avg (t ₁ , t ₂ ) and the distance L will be described using a specific waveform. First, consider an ideal triangular wave as the optical frequency ν (t). The ideal triangle wave can be expressed by the Fourier series expansion of the following equation.

Here, _Δν is the chirp bandwidth and fm is the modulation frequency. As shown by Eq. (Equation 12), an ideal triangular wave can be developed by a fundamental wave and odd-order harmonics.

FIG. 2 is a diagram showing the results of theoretical calculations for an ideal triangular wave. FIG. 2A shows the optical frequency (broken line) of the equation (Equation 12) and the beat frequency (solid line) calculated by substituting the equation (Equation 12) into the equation (Equation 7). The modulation frequency is fm = 5 kHz, and the chirp bandwidth is _Δν = 15 GHz. L represents the distance to the object, and the beat frequency is the calculation result for the distances of 5, 50, 500, and 5000 m.

As shown by the beat frequency in FIG. 2A, the beat frequency becomes a constant value in the linear chirp region of the triangular wave. On the other hand, at the apex of the triangular wave, the up chirp and the down chirp are switched, so that the beat frequency changes like a spike. In the FMCW rider, a region having a constant beat frequency is extracted, spectrum analysis is performed, and a distance is calculated. As the distance increases, the region where the beat frequency is constant decreases, and the data used for spectrum analysis decreases.

FIG. 2B shows the distance between the average beat frequency (solid line) calculated using Eq. (Equation 8) and the relative error (single point chain line) calculated using Eq. (Equation 11) for an ideal triangular wave. It is plotted as a function. The dotted line is the beat frequency f _FMCW expressed by the equation (Equation 1). The modulation frequency is fm = 5 kHz, the chirp bandwidth is Δν = 15 GHz, and the integration range in the equation (Equation 8) is [ _0,200 μs].

The average beat frequency in FIG. 2B increases in proportion to the distance and is almost the same as the beat frequency f _FMCW of the equation (Equation 1), but tends to be saturated in the region of the distance of 1 km or more. This is because the beat frequency f _FMCW considers only the region where the beat frequency in FIG. 2 (a) is constant, whereas the average beat frequency is calculated including spike-like changes. ..

In the region where the distance is 1 km or less, the average beat frequency in FIG. 2B can be related to the distance by the following equation.

The beat frequency f _FMCW of the equation (Equation 1) increases at a rate of 1.000 MHz / m with respect to the distance, while the average beat frequency of FIG. 2 (b) increases at a rate of 0.9978 MHz / m with respect to the distance. It changes with. The reason why the rate of change is slightly small is that the spike-like beat frequency change is included.

The relative error in FIG. 2B tends to increase with a distance, and the measurement accuracy deteriorates sharply at a distance of 1 km or more. Considering practical measurement accuracy (about 5%) and laser coherence, it is considered that the method of calculating the distance from the frequency change rate γ can be applied to the measurement of the distance of 1 km or less.

Next, consider a triangular wave having a nonlinear chirp as the optical frequency ν (t). A triangular wave with a non-linear chirp can be represented by the Fourier series expansion of the following equation.

The Fourier series expansion of an ideal triangular wave consists of a fundamental wave and odd-order harmonics, but when it has a non-linear charp, as shown by Eq. (Equation 14), even-order harmonics due to non-linearity appear, and at the same time, The values of the amplitudes C _k and D _k of the Fourier coefficient and the phases ψ _k and θ _k change.

FIG. 3 is a diagram showing the result of theoretical calculation for a triangular wave having a non-linear chirp. FIG. 3A shows the optical frequency (broken line) of the equation (Equation 14) and the beat frequency (solid line) calculated by substituting the equation (Equation 14) into the equation (Equation 7). The modulation frequency is fm = 5 kHz, and the chirp bandwidth is _Δν = 15 GHz. L represents the distance to the object, and the beat frequency is the calculation result for the distances of 5, 50, 500, and 5000 m.

As shown by the beat frequency in FIG. 3A, the beat frequency changes like a spike near the apex of the triangular wave. On the other hand, in the other regions, the beat frequency changes slowly under the influence of the nonlinear chirp. Further, due to the existence of the even-order harmonics in the equation (Equation 14), the frequency change in the down chirp region becomes larger than that in the up chirp region. Due to the influence of non-linear chirps, it is difficult to accurately calculate the distance from the beat frequency. For example, when the distance to the object 11 is 5 m, the beat frequency f _FMCW of the equation (Equation 1) is 5.00 MHz, whereas the beat frequency changes in the range of 1.05 MHz to 6.77 MHz. This frequency change corresponds to a distance of 1.05 m to 6.77 m.

FIG. 3B shows the distance between the average beat frequency (solid line) calculated using Eq. (Equation 8) and the relative error (single point chain line) calculated using Eq. (Equation 11) for a triangular wave having a non-linear chirp. It is plotted as a function of. The dotted line is the beat frequency f _FMCW expressed by the equation (Equation 1). The modulation frequency is fm = 5 kHz, the chirp bandwidth is Δν = 15 GHz, and the integration range in the equation (Equation 8) is [ _0,200 μs].

As in the case of the ideal triangular wave, the average beat frequency in FIG. 3 (b) increases in proportion to the distance and is almost the same as the beat frequency f _FMCW in the equation (Equation 1), but in a region of 1 km or more in the distance. Then, it tends to be saturated.

In the region where the distance is 1 km or less, the average beat frequency in FIG. 3B can be related to the distance by the following equation.

The beat frequency f _FMCW of the equation (Equation 1) increases at a rate of 1.000 MHz / m with respect to the distance. On the other hand, the average beat frequency in FIG. 3B changes at a rate of 1.0018 MHz / m with respect to the distance. The reason why the rate of change is slightly large even though the spike-shaped beat frequency change is included is that the beat frequency change due to the non-linear chirp is large.

The relative error in FIG. 3B tends to increase with a distance, and the measurement accuracy deteriorates sharply at a distance of 1 km or more. The relative error in the region with a distance of 1 km or less is about ½ of that in the case of an ideal triangular wave. This is because the decrease in the average beat frequency due to the spike-like frequency change is canceled by the increase due to the nonlinear chirp. Even when a triangular wave having a non-linear chirp is used, it is considered that the method of calculating the distance from the frequency change rate γ can be applied to the distance measurement of 1 km or less.

In the distance calculation by spectrum analysis, which is a conventional technique, a signal in a region where the beat frequency is a constant value is extracted, so it is necessary to determine the data region to be used according to the distance. Therefore, there is a problem that the amount of data decreases as the distance increases and the frequency resolution decreases. On the other hand, when the distance is calculated from the average beat frequency, all the data is used regardless of the distance, so that it is not necessary to determine the available data area and there is no problem of deterioration of the frequency resolution.

In the examples of FIGS. 2 and 3, a case where a triangular wave is used as a modulation signal has been described, but if the distance can be uniquely calculated from the relationship between the pre-calibrated distance and the average beat frequency, other waveforms can be used. The same can be performed when a modulated signal is used.

(First Embodiment)
The optical distance measuring device of the first embodiment is a case where a device for calculating the instantaneous phase of a beat signal and obtaining the beat frequency from the time derivative of the instantaneous phase is used as the beat frequency detecting device. The following will be described with reference to 4 to 8.

FIG. 4 is a diagram showing a process performed by the beat frequency detection device 13. The beat signal input to the beat frequency detection device 13 is divided into two and input to the mixer 17a and the mixer 17b, respectively. The output of the local oscillator 15 having a frequency of f _C as the local oscillator signal of the mixer 17a and the output of the local oscillator 15 as the local oscillator signal of the mixer 17b are given a phase shift of π / 2 by the phase shifter 16, respectively. input. The output of the mixer 17a and the output of the mixer 17b are input to the low-pass filter 18a and the low-pass filter 18b, respectively, the high-frequency component is removed, and the difference frequency component between the beat signal and the local signal is output. The common mode component 19 (I (t)) of the beat signal represented by the following equation is output to the output of the low-pass filter 18a.

On the other hand, the orthogonal component 20 (Q (t)) of the beat signal represented by the following equation is output to the output of the low-pass filter 18b.

Φ (t) in the equations (Equation 16) and (Equation 17) is expressed by the following equation. Φ (t) is the instantaneous phase of the beat signal.

In the arithmetic processing unit 21, the beat frequency f _B (t) is obtained from the common mode component I (t) and the orthogonal component Q (t). (Equation 19) and (Equation 20) will be specifically described.

First, in the arithmetic processing unit 21, the common mode component I (t) obtained as described above, the orthogonal component Q (t), and the following equation are used to calculate Φ (t) of the equation (Equation 17). ..

In a semiconductor laser in which the injection current is modulated, not only the frequency but also the light intensity is modulated at the same time. The beat signal also includes a component caused by light intensity modulation, which can be removed by the process of equation (Equation 19).

Next, in order to detect a phase change exceeding ± π, the arithmetic processing unit 21 performs unwrapping processing on the phase obtained by the equation (Equation 19). The unwrapping process is a process of adding a phase that is an integral multiple of 2π to a discontinuous phase change to make a continuous phase change. By time-differentiating the unwrapped phase Φ (t) and adding the frequency f _C of the local oscillator 15, the beat frequency f _B (t) represented by the following equation can be obtained.

FIG. 5 is a diagram illustrating an optical distance measuring device according to the first embodiment and a device for calibrating the relationship between the distance and the average beat frequency. The optical distance measuring device of FIG. 5 includes a modulation signal generator 1, an injection current source 2, a semiconductor laser 3, an optical system corresponding to an interferometer, a balanced optical detector 24, and a vector signal analyzer 26. And the arithmetic processing device 14. The optical system corresponding to the interferometer 4 in FIG. 1 is configured by using an optical fiber, and includes optical directional couplers 22a and 22b, a polarization controller 23, and an optical circulator 6. The output of the modulation signal generator 1 is input to the semiconductor laser 3 via the injection current source 2 to modulate the frequency of the output light. The output light of the semiconductor laser 3 is divided into two by the optical directional coupler 22a, and one of them is guided to the optical directional coupler 22b via the polarization controller 23 as reference light. The other is input as probe light to the optical delay line 25 via the optical circulator 6. The light subjected to Fresnel reflection at the output end face of the optical delay line 25 is guided to the optical directional coupler 22b via the optical circulator 6, combined with the reference light, and then received by the balanced photodetector 24. Output as a beat signal. The beat signal output from the balanced photodetector 24 is input to the vector signal analyzer 26 to convert it into a digital signal (resolution 12 bits) and calculate the beat frequency f _B (t). Here, after the beat signal is converted into a digital signal, the beat frequency f _B (t) is detected by software processing, but the same processing can also be performed using an analog electronic circuit. The calculation of the average beat frequency based on the equation (Equation 8) is executed by offline processing using the arithmetic processing device 14.

Specific Example 1 is shown. As the output of the modulation signal generator 1, a triangular wave having a frequency of 5 kHz is used, the modulation current amplitude of the semiconductor laser 3 is 60 mA _pp , and the chirp bandwidth of the output light is 15 GHz. The output light of the semiconductor laser 3 has a non-linear chirp, and the optical frequency can be expressed by the Fourier series of the equation (Equation 14). The chap bandwidth of 15 GHz corresponds to the resolution of equation (Equation 2) of 10 cm, but in the optical distance measuring device shown in FIG. 5, since there is only one reflection point, the distance can be measured with a higher resolution. can.

A first example of calibration is shown. First, as the optical delay line 25 in FIG. 5, the frequency change rate was calibrated using a manual variable optical delay line capable of precisely setting the optical path length. The minimum scale of the variable optical delay line is 0.08 mm. In the calibration, the optical path length was changed in units of 8.0 mm, the beat frequency was measured, and the average value over one cycle of modulation was calculated. FIG. 6A is a diagram showing a beat frequency when the variable optical delay line is set to 0 and 80.0 mm. On the scale of FIG. 6 (a), the two are almost overlapped. FIG. 6B is an enlarged view of a part of FIG. 6A (the part surrounded by the dotted line), and when enlarged, there is a slight difference between 0 mm (solid line) and 80 mm (dotted line, grayed out). You can see that.

FIG. 7 is a diagram showing the relationship between the set value of the variable optical delay line and the average beat frequency. The black circles represent the average beat frequency, and the white circles represent the error between the distance calculated from the average beat frequency using the equation (Equation 10) and the set value of the variable optical delay line. The average beat frequency of 5.60 MHz when the delay of the variable optical delay line 25 is 0 corresponds to the optical path difference of the optical fiber interferometer itself. The average beat frequency increases in proportion to the delay setting. The rate of change of the average beat frequency with respect to the distance is 1.023 MHz / m, which is almost the same as the theoretical value. The calculated distance error is within ± 1 mm, indicating that the distance can be measured accurately from the average beat frequency.

Next, a second example of calibration is shown. Using an optical fiber patch cord as the optical delay line 25, the optical path length was changed from 0 to 10 m in 1 m units, and the frequency change rate was calibrated. Patch cords with lengths of 1, 2, 3, 5, and 10 m were used in combination. The patch cord is manufactured up to +5 cm longer than the specifications. FIG. 8A is a diagram showing a change in the beat frequency when the optical fiber length is changed. It can be seen that the beat frequency increases corresponding to the optical fiber length.

FIG. 8B is a diagram showing the relationship between the optical fiber length and the average beat frequency. The fiber length on the horizontal axis is a patch cord specification and differs from the actual fiber length. The frequency of 2.36 MHz when the fiber length is 0 corresponds to the optical path difference of the optical fiber interferometer itself. The rate of change of the average beat frequency with respect to the distance calculated in consideration of the refractive index (1.467) of the optical fiber is 1.0095 MHz / m, which is slightly larger than the theoretical value and the value calibrated by the variable optical delay line. .. The reason is that the optical fiber patch cord is manufactured longer than the specification. The result of FIG. 8B shows the proportional relationship between the optical fiber length and the average beat frequency, and shows that the distance can be calculated from the average beat frequency even for a change in the optical path length in units of 1 m. There is.

(Second embodiment)
The optical distance measuring device of the second embodiment is a case where a device for obtaining the beat frequency from the time of the zero cross point of the beat signal is used as the beat frequency detecting device, and the following is referred to with reference to FIGS. 9 to 13. explain.

FIG. 9 is a diagram illustrating a beat frequency detection device used in the device of the present embodiment and its operation. FIG. 9A is a schematic diagram of the circuit of the beat frequency detection device. In FIG. 9A, the beat frequency detection device 13 is composed of an AD converter 27 and an arithmetic processing unit 21, and a modulation signal is also input to these from the output of the modulation signal generator 1. The beat signal output from the photodetector 12 of FIG. 1 is input to the AD converter 27. FIG. 9B is a diagram showing a waveform of a beat signal output from the photodetector 12 of FIG. The beat signal from which the DC component has been removed has a plurality of zero cross points t ₁ , t ₂ , ..., T _N , t _{N + 1} , .... The beat frequency at t _N at the time can be approximately expressed by the following equation.

Therefore, the beat frequency can be obtained by detecting the time of the zero cross point included in the beat signal. The sampling of the AD converter 27 is synchronized with the output of the modulation signal generator 1, but is not synchronized with the beat signal output from the photodetector 12. Therefore, the sampling point of the AD converter 27 does not always coincide with the zero cross point.

FIG. 9 (c) is an enlarged view of a part (a portion surrounded by a dotted line) of FIG. 9 (b), and is a diagram for explaining a method of obtaining a zero cross point from the sample value of the AD converter 27. The time of a zero cross point is represented by t _N , and the time and voltage of two sample points close to the time t _N are represented by (t _Na , V _Na ) and (t _Nb , _{VN b} ) (t _Nb> t _Na ). ). If there is a zero cross point between the two sampling points, the voltage product V _Na · V _Nb becomes negative. Therefore, the position of the zero cross point can be specified by calculating the product of the voltages of two adjacent points for all the sample values. The time of the zero cross point can be obtained by interpolating the sample value by the following equation for t _N.

In the arithmetic processing unit 21 of FIG. 9A, the time of the zero cross point is obtained from the sample value of the AD converter 27, and the beat frequency f _B (t _N ) is calculated using the equation (Equation 21). The arithmetic processing unit 21 of FIG. 9A can also be used as the arithmetic processing apparatus 14 of FIG.

FIG. 10 is a diagram illustrating an optical distance measuring device according to the present embodiment and a device for calibrating the relationship between the distance and the average beat frequency. The apparatus from the modulation signal generator 1 to the balanced photodetector 24 is the same as the apparatus of FIG. The beat signal output from the balanced photodetector 24 is input to the digital oscilloscope 28 (resolution 8 bits) and converted into a digital signal. Detection of zero cross point by Eq. (Equation 22), calculation of beat frequency f _B (t _N ) by Eq. (Equation 21), and average beat frequency f _B , avg (t ₁ , t ₂ ) by Eq. (Equation 8) Is calculated by offline processing using the arithmetic processing device 14. The arithmetic processing apparatus 14 of FIG. 10 also has a function of the arithmetic processing unit 21 of FIG. 9A.

Specific Example 2 is shown. As the output of the modulation signal generator 1, a triangular wave having a frequency of 5 kHz is used, the modulation current amplitude of the semiconductor laser 3 is 60 mA _pp , and the chirp bandwidth of the output light is 15 GHz. The output light of the semiconductor laser 3 has a non-linear chirp, and the optical frequency can be expressed by the Fourier series of the equation (Equation 14).

A third example of calibration is shown. First, as the optical delay line 25 in FIG. 10, the frequency change rate was calibrated using a manual variable optical delay line capable of precisely setting the optical path length. FIG. 11A is a diagram showing a waveform of a beat signal when the variable optical delay line is set to 0 mm. In the first half of the modulation cycle, the injection current of the semiconductor laser increases, so that the envelope of the beat signal increases and down chirp occurs. In the latter half of the modulation cycle, the injection current decreases, so the process is the reverse of the first half. 11 (b) is a diagram showing a waveform of a beat frequency obtained from the waveform of FIG. 11 (a). The waveform is the same as that of FIG. 6A obtained by the first embodiment, but the signal-to-noise ratio is smaller.

FIG. 12 is a diagram showing the relationship between the set value of the variable optical delay line and the average beat frequency. The black circles represent the average beat frequency, and the white circles represent the error between the distance calculated from the average beat frequency using the equation (Equation 10) and the set value of the variable optical delay line. The average beat frequency of 5.87 MHz when the delay of the variable optical delay line 25 is 0 corresponds to the optical path difference of the optical fiber interferometer itself. The average beat frequency increases in proportion to the delay setting. The rate of change of the average beat frequency with respect to the distance is 1.0323 MHz / m, which is slightly larger than the theoretical value and the value obtained by the first embodiment. Further, the error of the calculated distance is 6.4 mm at the maximum, which is larger than that of the first embodiment. Compared with the first embodiment, the value of the average beat frequency and the cause of the large error are the resolution (8 bits) of the digital oscilloscope 28 used as the AD converter.

Next, a fourth example of calibration is shown. Using an optical fiber patch cord as the optical delay line 25, the optical path length was changed from 0 to 10 m in 1 m units, and the frequency change rate was calibrated. FIG. 13A is a diagram showing waveforms of beat frequencies for optical fiber lengths of 0, 2 m, 4 m, 6 m, 8 m, and 10 m. It can be seen that the beat frequency increases corresponding to the optical fiber length.

FIG. 13B is a diagram showing the relationship between the optical fiber length and the average beat frequency. The frequency 2.43 MHz when the fiber length is 0 corresponds to the optical path difference of the optical fiber interferometer itself. The rate of change of the average beat frequency with respect to the distance calculated in consideration of the refractive index (1.467) of the optical fiber is 1.0651 MHz / m, which is slightly larger than the theoretical value and the value obtained by the first embodiment. big. The cause is the resolution (8 bits) of the digital oscilloscope 28. Therefore, the rate of change can be improved by optimizing the resolution. FIG. 13B shows the proportional relationship between the optical fiber length and the average beat frequency, and shows that the distance can be calculated from the average beat frequency even for a change in the optical path length in units of 1 m.

In the first and second embodiments, the case where a triangular wave is used as the modulation signal has been described above. As described above, other modulated waveforms can be used as long as the relationship between the distance and the average beat frequency is known.

Further, the calibration of the first and second embodiments is performed under modulation conditions corresponding to a maximum measurement distance of 10 m and a resolution of 1 cm, assuming an environment recognition sensor mounted on an autonomous robot. When used as an in-vehicle sensor, specifications with a maximum measurement distance of 200 m and a resolution of 10 cm are required, but the chirp band and modulation frequency can be optimized and implemented in the same manner.

Further, in the first and second embodiments, the case where the semiconductor laser is used as the light source has been described, but any laser having a frequency modulation function can be used in the same manner.

The examples shown in the above embodiments and the like are described for the sake of easy understanding of the invention, and are not limited to this embodiment.

Since the optical distance measuring device and method of the present invention do not require an additional device for controlling the frequency modulation of the laser, they are industrially useful as a small, highly accurate and low-cost FMCW rider system. The possibility of using it for consumer devices, including its use as an environment recognition sensor for automobiles and autonomous robots, is great.

1 Modulation signal generator 2 Injection current source 3 Semiconductor laser 4 Interferometer 5a, 5b Beam splitter 6 Optical circulator 7 Reflector 8 Reference light 9 Probe light 10 Reflected light 11 Object 12 Optical detector 13 Beat frequency detector 14 Arithmetic processing Device 15 Local oscillator 16 Phase shifter 17a, 17b Mixer 18a, 18b Low-pass filter 19 In-phase component 20 Orthogonal component 21 Arithmetic processing unit 22a, 22b Optical directional coupler 23 Polarization controller 24 Balanced optical detector 25 Optical delay line 26 Vector Signal analyzer 27 AD converter 28 Digital oscilloscope

Claims (4)

With a frequency-modulated laser,
With a photodetector
The output light of the laser is divided into two parts, one of which is the reference light and the other of which is the probe light. The interferometer incident on the optical detector and
A beat frequency detection device that detects a beat frequency that is the frequency of a beat signal generated from the photodetector and that changes continuously with time.
The beat frequency detected by the beat frequency detection device is integrated over an arbitrary finite interval including an interval that is an integral multiple of the modulation period , and then divided by the time of the finite interval to obtain an average value, and the average value is calculated. , An arithmetic processing device that calculates the distance to the object in light of the relationship between the pre-calibrated distance and the average value, which indicates the rate of change of the pre-calculated average value with respect to the distance. , Prepare ,
The beat frequency detection device obtains the in-phase component I (t) and the orthogonal component Q (t) of the beat signal, and instantaneously obtains the beat signal from the inverse positive tangent tan ^-1 {Q (t) / I (t)}. An optical distance measuring device , characterized in that the phase is calculated and the beat frequency is obtained from the time derivative of the instantaneous phase . The optical distance measuring device according to claim 1 , wherein the frequency modulation waveform of the laser is a triangular wave. The output light of the frequency-modulated laser is divided into two, one is the reference light and the other is the probe light.
By irradiating the object with the probe light, the reflected light from the object and the reference light are combined and incident on the photodetector.
The frequency of the beat signal generated from the photodetector, which is the frequency of the beat signal that continuously changes with time, is detected.
The detected beat frequency was integrated over an arbitrary finite interval including an interval of an integral multiple of the modulation period , and then divided by the time of the finite interval to obtain an average value, and the average value was calculated in advance. The distance to the object is calculated in light of the relationship between the pre-calibrated distance and the average value, which indicates the rate of change of the average value with respect to the distance.
To detect the frequency of the beat signal, the in-phase component I (t) and the orthogonal component Q (t) of the beat signal are obtained, and the beat signal is obtained from the inverse positive tangent tan ^-1 {Q (t) / I (t)}. An optical distance measuring method, characterized in that the instantaneous phase of the above is calculated and the beat frequency is obtained from the time derivative of the instantaneous phase . The optical distance measuring method according to claim 3 , wherein the frequency modulation waveform of the laser is a triangular wave.

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