JP7804495B2 - Measurement method - Google Patents

Description

The present invention relates to a measurement method .

Measuring devices such as LiDAR (Light Detection and Ranging, Laser Imaging Detection and Ranging) are known that obtain distance information to surrounding objects by measuring light (see, for example, Patent Document 1). FMCW (Frequency Modulated Continuous Wave) measuring devices irradiate the object with frequency-modulated illumination light, and measure the distance to the object and its relative speed based on the beat frequency of the interference wave generated by interference between the light reflected from the object and a reference light.

Special Publication No. 2020-502503

In FMCW measurement devices, it is common to control the direct current of the laser light source to modulate the light so that the frequency increases or decreases linearly over time. However, due to response delays in the laser light source, the frequency of the light actually output from the laser light source may not increase or decrease linearly over time.

The present invention aims to achieve distance measurement using the FMCW method using a laser light source with a response delay.

One aspect of the present invention to achieve the above object is a measurement method that includes inputting a test current that changes linearly with time to a light source, outputting test light from the light source that is frequency-modulated in accordance with the test current, splitting the test light into first and second test lights, providing a predetermined time difference between the first and second test lights, causing the first and second test lights to interfere with each other to generate test interference light, causing a detection device to detect the test interference light and output a test beat signal, determining the beat frequency of the test beat signal per time, determining the change in the frequency of the test light based on the beat frequency of the test beat signal per time, outputting a correction current corrected based on the change in the frequency of the test light to the light source, outputting measurement light from the light source that is frequency-modulated in accordance with the correction current, irradiating an object with the measurement light and causing reflected light from the object to interfere with the measurement light to generate interference light, causing the detection device to detect the interference light and output a beat signal, and determining the distance to the object based on the beat frequency of the beat signal.

Furthermore, the problems disclosed in this application and the solutions thereto will be made clear in the detailed description and drawings.

According to the present invention, distance can be measured using the FMCW method with a laser light source that has a response delay.

FIG. 1 is an explanatory diagram of the overall configuration of a measurement device 1. 2A is a reference explanatory diagram showing the relationship between the input current input to the laser light source 13 when the input current is not corrected and the frequency analysis result (FFT analysis result) of the beat signal. FIG. 2B is an explanatory diagram outlining the measurement method of the present invention. FIG. 3 is an explanatory diagram of the correction data acquisition device 1'. FIG. 4 is a flow diagram of the measurement method of this embodiment. Fig. 5A is a graph of the test current, Fig. 5B is a graph showing the time change of the beat frequency of the test beat signal, and Fig. 5C is an explanatory diagram of how to determine the time change of the test light. FIG. 6 is an explanatory diagram of a method for measuring the beat frequency. FIG. 7 is an explanatory diagram showing the relationship between the test current and the frequency of the test light. 8A and 8B are diagrams illustrating a method for generating waveform data. FIG. 9 is an explanatory diagram of a modified measuring device 1. 10A and 10B are diagrams illustrating the case where light is modulated so that the frequency increases or decreases linearly with the passage of time. FIG. 11 is a graph showing the time variation of the frequencies of the measurement light and the reflected light when the object 90 is moving. 12A and 12B are diagrams illustrating the response delay of the laser light source 13. FIG.

The following describes embodiments of the present invention with reference to the drawings. Note that in the following description, identical or similar components may be designated by common reference numerals, and redundant explanations may be omitted.

<Overall structure>
FIG. 1 is an explanatory diagram of the overall configuration of a measurement device 1.

The measuring device 1 is a device that measures the distance to an object 90. The measuring device 1 functions as a so-called LiDAR (Light Detection and Ranging, Laser Imaging Detection and Ranging). The measuring device 1 measures the distance to the object 90 using the FMCW (Frequency Modulated Continuous Wave) method. That is, the measuring device 1 irradiates the object 90 with frequency-modulated measurement light (irradiation light), causes interference between the reflected light from the object 90 and the measurement light (reference light), and measures the distance to the object 90 based on the frequency (beat frequency) of the beat signal that is the detection result of the interference light. Note that the measuring device 1 can measure not only the distance to the object 90, but also the relative speed with respect to the object 90.

The measurement device 1 has a generating device 10, an optical device 20, a detecting device 30, and a signal processing device 40.

The generating device 10 is a device that generates frequency-modulated light (frequency-modulated light). The generating device 10 outputs the frequency-modulated light to the optical device 20. A portion of the light (measurement light) output from the generating device 10 becomes illumination light that is irradiated onto the object 90, and a portion becomes reference light that interferes with the reflected light. The generating device 10 has a current source 12, a laser light source 13, and a temperature controller 14.

The current source 12 generates a current signal for controlling the laser light source 13. As will be described later, the current source 12 has a correction unit 12A. The correction unit 12A corrects the current signal to be input to the laser light source 13 based on correction data. The correction unit 12A will be described later.
The laser light source 13 emits light whose frequency is modulated (frequency-modulated light). For example, the laser light source 13 is configured using a distributed feedback (DFB) laser element. The laser light source 13 generates laser light whose frequency corresponds to the current signal of the current source 12. The laser light generated is frequency-modulated in the range of 193.4024 to 193.4266 THz (λ=1549.903 to 1550.097 nm), for example. For example, the laser light source 13 generates laser light whose frequency gradually increases or decreases according to a triangular wave current signal (frequency-modulated light). The laser light source 13 outputs the laser light to the optical device 20.
The temperature regulator 14 regulates the temperature of the laser light source 13 (particularly the laser element) to a predetermined temperature. The temperature regulator 14 has, for example, a temperature sensor 14A and a thermoelectric element (e.g., a Peltier element), and measures the temperature of the laser light source 13 with the temperature sensor 14A and adjusts the laser light source 13 to a predetermined temperature by feedback-controlling the thermoelectric element based on the measurement result of the temperature sensor 14A.

The optical device 20 is a device that irradiates the object 90 with frequency-modulated light (measurement light) and causes interference between reflected light from the object 90 and reference light (measurement light). The optical device 20 uses a portion of the measurement light input from the generation device 10 as irradiation light to irradiate the object 90, and a portion of the measurement light input from the generation device 10 as reference light, and causes the reflected light from the object 90 to interfere with the reference light to generate interference light (interference wave). The optical device 20 outputs the interference light (interference wave) caused by interference between the reflected light and the reference light to the detection device 30. The optical device 20 has a branching device 21, a circulator 22, an optical system 23, an optical waveguide 24, and a coupler 25.
The splitter 21 splits the frequency-modulated light from the generation device 10. The splitter 21 is configured by, for example, an optical coupler. One of the split lights is output to the circulator 22 and becomes the irradiation light to be irradiated onto the object 90. The other of the split lights is output to the optical waveguide 24 and becomes the reference light to be interfered with the reflected wave.
The circulator 22 guides the light (irradiated light) from the branching device 21 to the optical system 23 , and also guides the light (reflected light) from the optical system 23 to the coupler 25 .
The optical system 23 irradiates light toward the object 90 and collects and outputs reflected light. The optical system 23 is composed of optical elements such as lenses, mirrors, and prisms. The optical system 23 has, for example, a light-projecting optical system that irradiates the irradiated light toward the object, and a light-receiving optical system that collects reflected light. The optical system 23 may also have a function of scanning the irradiated light. The optical system 23 outputs the collected reflected light to the circulator 22. The reflected light is input to the coupler 25 via the circulator 22.
The optical waveguide 24 forms an optical path of a predetermined length from the splitter 21 to the coupler 25. The optical waveguide 24 guides the reference light over a predetermined optical path length from the splitter 21 to the coupler 25. The optical waveguide 24 is formed of, for example, an optical fiber.
The coupler 25 combines the reflected light from the circulator 22 with the reference light from the optical waveguide 24. The coupler 25 is configured by, for example, an optical coupler. The coupler 25 functions as an interferometer that causes interference between the reflected light and the reference light, and generates interference light (interference wave) by causing interference between the reflected light and the reference light. The coupler 25 outputs the interference light to the detection device 30.

The detector 30 detects the interference light between the reflected light and the reference light and outputs a beat signal. The detector 30 includes a photoelectric converter 31 and an amplifier 32.
The photoelectric converter 31 outputs an electrical signal (current signal) corresponding to the intensity of the detected optical signal (here, interference light). The photoelectric converter 31 is, for example, a photodiode. The interference light detected by the photoelectric converter 31 is a wave whose amplitude changes periodically due to interference between reflected light and reference light, which have different frequencies.
The amplifier 32 converts the current signal of the photoelectric converter 31 into a voltage signal and outputs it. The amplifier 32 is configured, for example, by a transimpedance amplifier. The beat signal output from the amplifier 32 is a signal that indicates the difference in frequency between the reflected light and the reference light. The beat frequency of the beat signal corresponds to the frequency of the beat component of the interference light. The beat frequency of the beat signal also corresponds to the difference in frequency between the reflected light and the reference light.

The signal processing device 40 is a device that determines the distance to the object 90 based on the beat signal. The signal processing device 40 has an A/D converter, a calculation device, a storage device, etc. (not shown). The calculation device is composed of a calculation processing device such as a CPU, GPU, or MPU. The storage device is composed of a main storage device and an auxiliary storage device, and is a device that stores programs and data. The calculation device executes the programs stored in the storage device, thereby performing various processes for measuring the distance to the object 90. Figure 1 shows the various processes performed by the signal processing device 40 as functional blocks.

The signal processing device 40 has a signal acquisition unit 41, an analysis unit 42, and an output unit 43. The signal acquisition unit 41 acquires the beat signal from the detection device 30 as a digital signal. The signal acquisition unit 41 is configured, for example, by an A/D converter (such as an A/D conversion board). The analysis unit 42 calculates the distance to the object 90 based on the beat signal. The processing of the analysis unit 42 will be described later. The output unit 43 outputs the analysis results of the analysis unit 42 to the outside. For example, the output unit 43 outputs distance data indicating the distance to the object 90 and speed data indicating the relative speed of the object 90 to an external device, such as a vehicle ECU.

<Reference explanation 1>
First, a general FMCW measurement method will be described.

Figures 10A and 10B are explanatory diagrams of the case where light is modulated so that the frequency increases or decreases linearly over time. Figure 10A is a graph showing the temporal change in the frequency of the measurement light and the reflected light, with the horizontal axis representing time and the vertical axis representing frequency. Note that the period in which the frequency increases is sometimes called the gradual increase period, and the period in which the frequency decreases is sometimes called the gradual decrease period. Figure 10B is a graph showing the frequency analysis results of the beat signal (analysis results obtained by fast Fourier transform (FFT)), with the horizontal axis representing frequency and the vertical axis representing amplitude (intensity). First, we will explain the case where the object 90 is stationary (when the relative velocity with respect to the object 90 is zero).

In the figure, _fB indicates the frequency difference between the measurement light (illumination light, reference light) and the reflected light. Δt indicates the time it takes for the light to travel back and forth to the object 90. T indicates the gradual increase period or gradual decrease period (the modulation time for modulating the frequency). F indicates the modulation frequency width (the width of increase or decrease in frequency).

Here, since the frequency increases linearly with the passage of time, the slope of the graph is constant, and Δt is given by the following equation (1).
Δt=(T/F)・f _B ...(1)

Here, if the speed of light is c and the distance to the object 90 is R, it takes time Δt for light to travel to and from the object 90, so the distance R is given by the following equation (2).
2R = c Δt
R=(c・T/2F)・f _B ...(2)

In the above equation (2), the frequency _fB can be obtained by FFT analysis of the beat signal as shown in Fig. 10B. The speed of light c, modulation time T, and modulation frequency width F are known. Therefore, by performing FFT analysis of the beat signal to obtain the frequency _fB , the distance R to the target object 90 can be calculated.

Figure 11 is a graph showing the change in frequency of the measurement light and reflected light over time when the object 90 is moving. Note that the light is modulated so that the frequency increases or decreases linearly over time. As shown in the graph of reflected light, when the object 90 is moving, the frequency shifts due to the Doppler effect.

In the figure, f _dop indicates the amount of frequency shift due to the Doppler effect (Doppler shift frequency), f _up indicates the frequency difference between the measurement light (illumination light, reference light) and the reflected light during the gradual increase period, and f _dn indicates the frequency difference between the measurement light (illumination light, reference light) and the reflected light during the gradual decrease period.

The frequency f _up can be obtained by FFT analysis of the beat signal during the gradual increase period. The frequency f _dn can be obtained by FFT analysis of the beat signal during the gradual decrease period. Note that the frequency analysis of the beat signal is performed separately for each of the gradual increase period and the gradual decrease period. Then, based on the frequency f _up and the frequency f _dn , the frequency f _B can be obtained as shown in the following equation (3), and the distance R can be obtained based on the above-mentioned equation (2).
_fB = ( _fup + _fdn )/2...(3)

Furthermore, based on the frequency f _up and the frequency f _dn , the Doppler shift frequency f _dop can be calculated as in the following equation (4), and the relative velocity V can be calculated based on the following equation (5).
f _dop = (f _up - f _dn )/2 (4)
V=(λ/2)・f _dop ...(5)
(λ is the wavelength of light)

<Reference explanation 2: Light source response delay>
12A and 12B are diagrams illustrating the response delay of the laser light source 13. Fig. 12A is a graph showing the time change of the input current input to the laser light source 13, with the horizontal axis representing time and the vertical axis representing current. Fig. 12B is a graph showing the time change of the frequency of the measurement light (irradiation light, reference light) and the reflected light, with the horizontal axis representing time and the vertical axis representing frequency.

In an actual laser light source 13, there is a response delay between a change in input current and a change in the frequency of the laser light. As a result, even if a triangular wave current signal is input to the laser light source 13 as shown in Fig. 12A, the frequency of the laser light output from the laser light source 13 does not increase or decrease linearly over time as shown in Fig. 12B. When the frequency changes nonlinearly in this way, the difference in frequency between the measurement light (irradiation light, reference light) and the reflected light does not remain constant (the frequencies _fB1 , _fB2 , and _fB3 shown in Fig. 12B are not constant).

FIG. 2A is a reference explanatory diagram showing the relationship between the input current input to the laser light source 13 and the frequency analysis results (FFT analysis results) of the beat signal when the input current is not corrected. The graph on the left side of the figure shows the time change of the input current to the laser light source 13. The graph on the right side of the figure shows the results of the frequency analysis (FFT analysis) of the beat signal. Due to the effect of the response delay of the laser light source 13 (see FIG. 12B ), as shown in the right diagram of FIG. 2A , even when frequency analysis (FFT analysis) is performed on the beat signal, it is difficult to obtain an intensity peak at a specific frequency. Therefore, when the current signal input to the laser light source 13 is a triangular wave, it is difficult to determine the beat frequency _fB by frequency analysis of the beat signal, and as a result, it is difficult to determine the distance R based on the above-mentioned equation (2).

Figure 2B is an overview of the measurement method used in this study. The graph on the left side of the figure shows the time change in the input current to the laser light source 13. In this study, a triangular wave current signal (see the left side of Figure 2A), in which the current increases or decreases linearly with time, is corrected to a current signal in which the current changes nonlinearly with time, and the corrected current signal (see the left side of Figure 2B; correction current) is input to the laser light source 13. The current signal (correction current) shown in the left side of Figure 2B is corrected based on the time change in the frequency of the frequency-modulated light when the current signal (see the left side of Figure 2A), in which the current changes linearly with time, is input to the laser light source 13. Due to the response delay of the laser light source 13, when the current signal (correction current) shown in the left side of Figure 2B is input to the laser light source 13, the frequency of the laser light output from the laser light source 13 increases or decreases linearly with time. As a result, as shown in the right side of Figure 2B, when frequency analysis (FFT analysis) is performed on the beat signal, an intensity peak appears at a specific frequency. This makes it possible to measure distance using the FMCW method with a laser light source 13 that has a response delay. This measurement method is explained in detail below.

<Measurement method>
FIG. 3 is an explanatory diagram of the correction data acquisition device 1'.
The correction data acquisition device 1' is a device that acquires correction data for correcting a current signal input to the laser light source 13. The correction data acquisition device 1' includes a generating device 10', a test optical device 20', a detecting device 30', and a signal processing device 40'. Some of the components of the correction data acquisition device 1' have a structure common to the components of the measurement device 1 described above. The correction data acquisition device 1' and the measurement device 1 may be provided as separate devices, or some of the components of the measurement device 1 may be used as the correction data acquisition device 1' (described later). Components common to the components of the measurement device 1 described above are assigned the same reference numerals, and descriptions thereof may be omitted.

Figure 4 is a flow diagram of the measurement method of this embodiment. S101 to S106 in the diagram are processes performed by the correction data acquisition device 1'. Note that S101 is a process performed by the generation device 10', S102 is a process performed by the test optical device 20', S103 is a process performed by the detection device 30', and S104 to S106 are processes performed by the signal processing device 40'. S107 is a process performed by the measurement device 1 (see Figure 1). However, some of the processes of S101 to S106 (e.g., S106) may be performed by the measurement device 1 (see Figure 1).

・S101
The generating device 10' inputs a triangular wave current signal (test current) to the laser light source 13', and outputs test light (frequency-modulated light) from the laser light source 13' (S101).

In S101, current source 12' generates a current signal (triangular wave current signal; test current) that linearly increases or decreases with time, and inputs the test current to laser light source 13'. Figure 5A is a graph of the test current output by current source 12' in S101. Figure 5A shows the test current during the gradual increase period. The test current changes linearly (increases in this case) with time, and the ratio of the change in current to the change in time (corresponding to the slope of the graph) is constant.

The laser light source 13' outputs light (test light) that is frequency-modulated in accordance with the test current (S101). As previously explained, the laser light source 13' has a response delay between when the input current changes and when the frequency of the laser light changes. As a result, the frequency of the test light output from the laser light source 13' changes nonlinearly with time (see Figure 12B).

・S102
The test optical device 20' splits the test light into a first test light and a second test light, provides a predetermined time difference between the first test light and the second test light, and causes the first test light and the second test light to interfere with each other to generate test interference light (S102).

As shown in FIG. 3, the test optical device 20' has a splitter 21', a coupler 25', a first optical waveguide 26', and a second optical waveguide 27'. The splitter 21' splits the test light (frequency-modulated light) from the generation device 10'. One of the split lights is output to the first optical waveguide 26' and becomes the first test light. The other of the split lights is output to the second optical waveguide 27' and becomes the second test light. The first optical waveguide 26' guides the first test light from the splitter 21' to the coupler 25' over a predetermined optical path length. The second optical waveguide 27' guides the second test light from the splitter 21' to the coupler 25' over an optical path length longer than that of the first optical waveguide 26'. The difference in optical path length between the first optical waveguide 26' and the second optical waveguide 27' provides a predetermined time difference (Δt) between the first test light and the second test light. Here, because the second optical waveguide 27' has a longer optical path length than the first optical waveguide 26', the second test light arrives at the coupler 25' with a predetermined time difference (Δt) delay from the first test light. The coupler 25' combines the first test light and the second test light. The coupler 25' functions as an interferometer that causes interference between the reflected light and the reference light, and generates interference light caused by interference between the first test light and the second test light. In the following description, the interference light caused by interference between the first test light and the second test light is referred to as test interference light.

The first optical waveguide 26' and the second optical waveguide 27' are both made of optical fiber, and the first test light and the second test light are guided from the splitter 21' to the coupler 25' by optical waveguides made of optical fiber. This has the advantage that noise is less likely to be mixed into the first test light and the second test light, and also into the test interference light (in contrast, in the measurement device 1 of Figure 1, noise is more likely to be mixed into the interference light because it is taken in from outside the measurement device 1). Furthermore, by configuring the first optical waveguide 26' and the second optical waveguide 27' with a predetermined optical path length, it becomes easy to impart a predetermined time difference (a known time difference) between the first test light and the second test light (in contrast, if reflected light is used as the second test light, it becomes more difficult to impart a predetermined time difference between the first test light and the second test light).

・S103
The detecting device 30' detects the test interference light and outputs a beat signal of the test interference light (S103). In the following description, the beat signal of the test interference light may be referred to as a test beat signal. The detecting device 30' has the same configuration as the detecting device 30 of the measuring device 1.

S104
In S104, the analysis unit 42' measures the beat frequency of the test beat signal (test beat frequency) at predetermined time intervals (S104). Fig. 6 is an explanatory diagram of a method for measuring the beat frequency. The graph in the figure shows the test beat signal, with the horizontal axis representing time and the vertical axis representing voltage.

The analysis unit 42' first determines the time (peak time) at which the test beat signal reaches its peak. The black circles on the graph in the figure indicate the peaks (extreme values; local maximum or minimum values) of the test beat signal. For example, the analysis unit 42' sequentially measures the times at which the sign of the voltage slope of the test beat signal (the differential value of the beat signal) changes as peak times. Here, the analysis unit 42' measures the peak time at which the test beat signal reaches its maximum value (the time at which the sign of the voltage slope changes from positive to negative) and the peak time at which the test beat signal reaches its minimum value (the time at which the sign of the voltage slope changes from negative to positive). However, the analysis unit 42' may measure only the peak time of the test beat signal's maximum value or only the peak time of the test beat signal's minimum value. Instead of measuring the peak time of each peak of the test beat signal, the analysis unit 42' may detect the peak time, for example, at a predetermined cycle or at a predetermined time interval. The analysis unit 42' will measure multiple peak times during the gradual increase period or during the gradual increase period.

Next, the analysis unit 42' calculates the beat frequency of the test beat signal (test beat frequency) based on the interval between the two peak times. For example, the analysis unit 42' calculates the beat frequency at a given time (timing) based on the interval between the peak time of a certain maximum value of the test beat signal and the peak time of the next maximum value (the time of one cycle of the beat signal). The analysis unit 42' may also calculate the beat frequency at a given time based on the interval between the peak time of a certain minimum value of the test beat signal and the peak time of the next minimum value (the time of one cycle of the test beat signal). The analysis unit 42' may also calculate the beat frequency at a given time based on the interval between the peak time of a certain maximum value (or minimum value) of the test beat signal and the peak time of the next minimum value (the time of half a cycle of the test beat signal). Alternatively, the analysis unit 42' may calculate the beat frequency at a given time based on the interval between the peak time of a certain maximum value (or minimum value) of the test beat signal and the peak time of a maximum value (or minimum value) several cycles later. The analysis unit 42' measures the beat frequency at each of multiple times (multiple timings) during the gradual increase period or gradual decrease period.

In the example shown in FIG. 6, the analysis unit 42' alternately calculates the beat frequency based on the peak times of two maximum values and the beat frequency based on the peak times of two minimum values every half period. This allows the beat frequency to be calculated for every half period of the test beat signal, making it possible to calculate the beat frequency with high time resolution. This also makes it possible to calculate the beat frequency at a specified time (see the black circles in FIG. 5B) with high accuracy.

5B is a graph showing the change over time in the beat frequency of the test beat signal. The horizontal axis represents time, and the vertical axis represents the beat frequency _fB . In the following description, the beat frequency of the test beat signal at a predetermined time (instant) t _i may be referred to as f _B (t _i ). The times t _i (i=0, 1, 2, 3, ...) represent the time (timing) of each predetermined time. Time t ₀ is a reference time (for example, the start time of the gradual increase period), and the predetermined times t _i are determined in the order t ₁ , t ₂ , t _{3 ...} from the reference time at each predetermined time.

The black circles on the graph in the figure indicate the beat frequencies determined by the analysis unit 42'. As shown in the figure, the analysis unit 42' determines the beat frequencies _fB ( _t1 ), _fB ( _t2 ), _fB ( _t3 ), ... of the test beat signal at each predetermined time Δt. Note that due to the effect of a response delay of the laser light source 13, the beat frequency of the test beat signal does not become a constant value.
The predetermined time Δt shown in Fig. 5B corresponds to the predetermined time difference (Δt) given between the first test light and the second test light due to the difference in optical path length between the first optical waveguide 26' and the second optical waveguide 27' shown in Fig. 3. In other words, the analyzer 42' obtains the beat frequency of the test beat signal for each predetermined time difference (Δt) given between the first test light and the second test light.

・S105
Next, the analysis unit 42' determines the change in the test light over time based on the beat frequency (test beat frequency) of the test beat signal at each predetermined time (S105).

5C is an explanatory diagram of how to determine the change over time of the test light. The horizontal axis represents time, and the vertical axis represents frequency f. In the following description, the frequency of the test light at time t _i may be referred to as f(t _i ). The solid line graph in the figure represents the change over time of the frequency of the test light (first test light). The dotted line graph in the figure represents the change over time of the frequency of the second test light.

First, the analysis unit 42' calculates the frequency of the test light at each predetermined time interval. Here, the analysis unit 42' calculates the frequency f(t _i ) of the test light at each time t _i corresponding to a black circle on the graph in the figure.
As already explained, due to the difference in optical path length between the first optical waveguide 26' and the second optical waveguide 27' shown in Fig. 3, a predetermined time difference (Δt) is provided between the solid line graph and the dotted line graph in the figure. Also, as already explained, in S104, the analysis unit 42' calculates the beat frequencies f _B (t ₁ ), f _B (t ₂ ), f _B (t ₃ ), ... of the test beat signal for each predetermined time Δt. As shown in Fig. 5C, the frequency f(t _i ) of the test light at a certain time t _i corresponds to the sum of the frequency f(t _i-1 ) of the test light at a time t _i-1 , a predetermined time earlier, and the beat frequency f _B (t _i ) (f(t _i ) = f(t _i-1 ) + f _B (t _i )). In this way, the analysis unit 42' determines the frequency f(t _i ) of the test light for each predetermined time based on the beat frequency f _B (t _i ) of the test beat signal for each predetermined time determined in S104.

In order to find the frequency f(t _i ) of the test light at a certain time t _i , it is necessary to determine the frequency f(t ₀ ) of the test light at the reference time t _0. However, since the time change in the frequency of the test light (corresponding to the slope of the solid line graph in FIG. 5C ) is ultimately utilized, the frequency f(t ₀ ) of the test light at the reference time t ₀ can be set to any value.

Next, the analysis unit 42' determines the time change in the frequency of the test light based on the frequency f(t _i ) of the test light for each predetermined time. Here, the analysis unit 42' determines the time change in the frequency of the test light corresponding to the solid line graph in FIG. 5C based on the frequency f(t _i ) of the test light for each predetermined time indicated by the black dots on the graph in FIG. 5C. For example, the analysis unit 42' can determine the time change in the frequency of the test light corresponding to the solid line graph in FIG. 5C by performing an interpolation process on the frequency f(t _i ) of the test light for each predetermined time indicated by the black dots on the graph in FIG. 5C. This determines the time change in the frequency of the test light corresponding to each time. Note that the time change in frequency is the ratio of the amount of change in frequency to the amount of change in time, and corresponds to the slope of the solid line graph in FIG. 5C. In the following description, the value indicating the time change in the frequency of the test light (the slope of the solid line graph in FIG. 5C) may be referred to as α'.

Figure 7 is an explanatory diagram showing the relationship between the test current and the frequency of the test light. The horizontal axis in the diagram represents time. The vertical axis on the right side of the diagram represents current, and the vertical axis on the left side of the diagram represents frequency. The dotted line graph in Figure 7 shows the time change of the current signal (test current) input to the laser light source 13' in S101 (see the right vertical axis), as well as the time change of the frequency of the ideal frequency-modulated light (the target frequency of the measurement light) (see the left vertical axis). The solid line graph in Figure 7 shows the time change of the frequency of the test light (see the left vertical axis; also see Figures 5C and 12B). In other words, the solid line graph in Figure 7 shows the time change of the frequency of the test light calculated by the analysis unit 42' in S105. In the dotted line graph in Figure 7, the current or frequency increases linearly over time. In the solid line graph in Figure 7, the frequency increases nonlinearly over time due to the effect of the response delay of the laser light source. Thus, the time change of the frequency of the test light deviates from the time change of the ideal frequency (the time change of the target frequency of the measurement light).

・S106
Next, the analysis unit 42' generates waveform data based on the time change in the frequency of the test light. The waveform data is data (current profile data) that indicates the waveform of the current (correction current) input to the laser light source 13, and is data for generating the correction current shown in the left diagram of FIG. 2B, for example. A method for generating the waveform data (current profile data) will be described below.

The analysis unit 42' divides the gradual increase period (or gradual decrease period) into a number of unit times (Δt _j ) and calculates a correction current for each unit time. The correction current for each unit time represents a portion of the waveform data (waveform data fragment). The analysis unit 42' then connects the correction currents (waveform data fragments) calculated for each unit time in chronological order to generate waveform data representing the waveform of the correction current. Here, the generation of the correction currents (waveform data fragments) at times t _A and t _B in Figure 7 will be described.

8A and 8B are explanatory diagrams of a method for generating waveform data. In other words, Fig. 8A and Fig. 8B are diagrams showing a method for correcting a current input to the laser light source 13. Fig. 8A shows a method for generating a correction current (waveform data fragment) at time _tA in Fig. 7. Fig. 8B shows a method for generating a correction current (waveform data fragment) at time _tB in Fig. 7.

The dotted line in the upper diagrams of Figures 8A and 8B indicates the time change in the frequency of the ideal frequency-modulated light (time change in the target frequency of the measurement light), and the solid line indicates the time change in the frequency of the test light. The horizontal axis in the upper diagrams of Figures 8A and 8B indicates time, and the vertical axis indicates frequency. α in the diagrams indicates the slope of the dotted line, and indicates the value of the time change in the frequency of the ideal frequency-modulated light (time change in the target frequency of the measurement light) (the time width in the upper diagrams of Figures 8A and 8B is 1). Note that the slope α is a known value and is constant regardless of time. α _A ' in the diagrams indicates the slope of the solid line at time t _A in Figure 7. α _B ' indicates the slope of the solid line at time t _B in Figure 7. Note that in the following description, the slope of the solid line at any time in Figure 7 may be simply referred to as α' without a subscript. The slope α' is a value corresponding to the time change in the frequency of the test light (the ratio of the change in the frequency of the test light to the change in time), and is the value calculated in S106 described above. As shown in the upper diagrams of Figures 8A and 8B, the slope α' at _{time tA} is smaller than the slope α (α > _α '), and the slope α _' at time _tB is larger than the slope α (α < _α ').

The dotted lines in the lower diagrams of Figures 8A and 8B represent waveform data before correction (waveform data indicating a triangular wave current signal or a test current). The solid lines represent waveform data after correction (waveform data indicating a correction current). The horizontal axis in the lower diagrams of Figures 8A and 8B represents time, and the vertical axis represents current. The width of the lower diagrams of Figures 8A and 8B represents unit time _Δtj , which corresponds to the time required to update the current (current value) input to the laser light source 13. (The time width in the lower diagrams of Figures 8A and 8B is not necessarily 1.) In the following description, the current indicated by the dotted line will be referred to as I, and the current indicated by the solid line will be referred to as I'. Currents I( _jA ) and I'( _jA ) in the diagrams represent current values at timing _jA , which corresponds to time _tA in Figure 7. Currents I( _jB ) and I'( _jB ) represent current values at timing _jB , which corresponds to time _tB in Figure 7. The lower diagrams of FIGS. 8A and 8B show how the current indicated by the waveform data changes from I(j-1) to I(j) in unit time Δt _j between timing j-1 and the next timing j.

In uncorrected waveform data (waveform data representing a triangular wave current signal or test current), as shown by the dotted lines in the lower diagrams of Figures 8A and 8B, the current I(j) at a given time j is the current I(j-1) at the immediately preceding time j-1 increased by a predetermined amount of change β (I(j) = I(j-1) + β). Note that the amount of change β is a constant value (during the gradual increase period when the frequency is increased, the amount of change β is positive, and during the gradual decrease period when the frequency is decreased, the amount of change β is negative). Then, by repeatedly outputting the current I(j) increased by the amount of change β from the immediately preceding current I(j-1) every unit of time (in other words, by connecting the waveform data pieces in chronological order), waveform data representing a current (triangular wave current signal) that changes linearly over time is derived.

In this embodiment, the change amount β is corrected based on the change over time (slope α') in the frequency of the test light. Here, the corrected change amount β' is calculated by multiplying the pre-correction change amount β by the inverse of the ratio of the slope α' to the ideal slope α (α/α') (β' = β × α/α'). Therefore, the correction current I'(j) at a given timing j is the correction current I'(j-1) at the immediately preceding timing j-1, increased by a predetermined change amount β' (I'(j) = I'(j-1) + β').

For example, at timing _jA corresponding to time _tA , the corrected change amount _βA ' is the value obtained by multiplying the pre-correction change amount β by α/ _αA ' ( _βA ' = β x α/ _αA '). Because the slope _αA ' at time _tA is smaller than the slope α (α > αA _' ), the corrected change amount _βA ' is larger than the pre-correction change amount β. The current I'( _jA ) at timing _jA corresponding to time _tA is the current I'( _jA -1) at the immediately preceding timing _jA -1 increased by the corrected change amount _βA '(I'( _jA ) = I'( _jA -1) + _βA '). The time change in the corrected current I' at time _tA (corresponding to the slope of the solid line in the lower diagram of Figure 8A; _βA '/ _Δtj ) is larger than the time change in the current I before correction (corresponding to the slope of the dotted line in the lower diagram of Figure 8A; β/Δtj ₎ .
Furthermore, at timing _jB corresponding to time _tB , the corrected change amount _βB ' is the value obtained by multiplying the pre-correction change amount β by α/ _αB ' ( _βB ' = β x α/ _αB '). Because the slope _αB ' at time _tB is greater than the slope α (α < _αB '), the corrected change amount _βB ' is smaller than the pre-correction change amount β. The current I'( _jB ) at timing _jB corresponding to time _tB is the current I'( _jB -1) at the immediately preceding timing _jB -1 increased by the corrected change amount _βB '(I'( _jB ) = I'( _jB -1) + _βB '). The time change in the corrected current I' at time _tB (corresponding to the slope of the solid line in the lower diagram of Figure 8B; _βB '/ _Δtj ) is smaller than the time change in the current I before correction (corresponding to the slope of the dotted line in the lower diagram of Figure 8B; β/ _Δtj ).

In this way, if the time change (slope α') of the frequency of the test light at a certain time is smaller than the ideal time change (slope α), the current indicated by the waveform data is corrected so that the time change (β'/Δt _j ) of the current at that time is increased. Conversely, if the time change (slope α') of the frequency of the test light at a certain time is larger than the ideal time change (slope α), the current indicated by the waveform data is corrected so that the time change (β'/Δt _j ) of the current at that time is decreased. In this way, a correction current (waveform data piece) is generated for each unit time so that the time change of the frequency of the frequency-modulated light output from the laser light source 13 at that time approaches the ideal time change.

As described above, the analysis unit 42' divides the gradual increase period (or gradual decrease period) into a number of unit times _Δtj and calculates the correction current I'(j) corresponding to each unit time _Δtj . The analysis unit 42' generates waveform data representing the waveform of the correction current by connecting the correction currents (waveform data fragments) calculated for each unit time in chronological order. Note that since the slope α' varies over time, the slope of the correction current of each waveform data fragment (the slope of the solid line in the lower diagrams of Figures 8A and 8B) varies for each unit time. Therefore, the waveform data obtained by connecting the waveform data fragments becomes data representing the correction current I' that changes nonlinearly over time. Specifically, the processing of S106 generates waveform data such as that shown in the left diagram of Figure 2B.

After the analysis unit 42' generates the waveform data, the output unit 43' outputs the waveform data to the correction unit 12A of the measurement device 1 (see Figure 1), and causes the correction unit 12A to store the waveform data (correction data) (S106).

・S107
The correction unit 12A of the measurement device 1 (see FIG. 1) causes the current source 12 to output a current signal (correction current) corresponding to the waveform data (current profile data). In other words, the current source 12 generates a current signal (correction current) according to the waveform data (current profile data) stored in the correction unit 12A and inputs it to the laser light source 13 (S107). The correction current input to the laser light source 13 has a change amount β' per unit time _Δtj that varies over time. As a result, as shown in the left diagram of FIG. 2B, the correction current changes nonlinearly over time.

The laser light source 13 outputs measurement light (frequency-modulated light) with a frequency corresponding to the input correction current (S107). As described above, the laser light source 13 has a response delay between when the input current changes and when the frequency of the laser light changes. However, the current is corrected for each unit time _Δtj so that the time change in the frequency of the measurement light output from the laser light source 13 approaches the time change in the target frequency. For example, at time _tA , the time change in the input current to the laser light source 13 is corrected to be larger, thereby canceling out the effect of the response delay of the laser light source 13 and causing the time change in the frequency of the measurement light output from the laser light source 13 to approach the time change in the target frequency. Also, at time _tB , the time change in the input current to the laser light source 13 is corrected to be smaller, thereby canceling out the effect of the response delay of the laser light source 13 and causing the time change in the frequency of the measurement light output from the laser light source 13 to approach the time change in the target frequency. Therefore, the laser light source 13 can output measurement light whose frequency increases and decreases linearly over time.

As already described, the optical device 20 irradiates the object 90 with frequency-modulated light (measurement light), causes reflected light from the object 90 to interfere with the reference light (measurement light), and generates interference light. The detection device 30 detects the interference light between the reflected light and the reference light and outputs a beat signal. The analysis unit 42 of the signal processing device 40 performs frequency analysis (fast Fourier transform (FFT)) on the beat signal. As shown in the right diagram of FIG. 2B , when frequency analysis (FFT analysis) is performed on the beat signal, an intensity peak appears at a specific frequency, allowing the analysis unit 42 to determine the beat frequency f _B of the beat signal. The analysis unit 42 calculates the distance R to the object 90 using the above-mentioned equations (2) and (3) based on the beat frequency f _B of the beat signal (S107). As described above, according to this embodiment, distance measurement using the FMCW method is possible even when a laser light source 13 with a response delay is used.

However, since the characteristics of the laser light source 13 (particularly the laser element) change with temperature, the response delay of the laser light source 13 also changes with temperature. Therefore, the response delay of the laser light source 13 changes depending on the temperature.
Therefore, it is desirable that the correction data acquisition device 1' performs the above-mentioned processes of S101 to S106 under different temperature environments, acquires correction data (e.g., waveform data) for each of a plurality of temperatures, and stores the correction data associated with the temperature in the correction unit 12A. Then, when performing the above-mentioned process of S107, the correction unit 12A preferably generates a current signal (correction current) based on the correction data corresponding to the temperature detected by the temperature sensor and inputs it to the laser light source 13. This can improve the accuracy of distance measurement.

In this case, the correction unit 12A may use the temperature detected by the temperature sensor 14A provided in the temperature regulator 14. This allows the temperature sensor 14A to be used for both purposes. However, the correction unit 12A may also use the temperature detected by a temperature sensor other than the temperature sensor 14A of the temperature regulator 14 to correct the current signal (correction current) based on correction data corresponding to the temperature.

<Modification 1>
9 is an explanatory diagram of a modified example of the measuring device 1. In this modified example, some of the components of the measuring device 1 are used as a correction data acquisition device 1'.

The optical device 20 of the modified measuring device 1 includes a test optical device 20' and optical switches 28A and 28B. The optical switch 28A switches the connection destination of the laser light source 13 between the splitter 21 and the splitter 21'. The input port of the optical switch 28A is connected to the laser light source 13, one of the two output ports of the optical switch 28A is connected to the splitter 21, and the other output port is connected to the splitter 21'. The optical switch 28B switches the connection destination of the detection device 30 between the coupler 25 and the coupler 25'. One of the two input ports of the optical switch 28B is connected to the coupler 25, the other input port is connected to the coupler 25', and the output port is connected to the detection device 30.

In Figure 4, the process of S101 is performed by the generation device 10 of the measurement device 1, the process of S102 is performed by the test optical device 20', the process of S103 is performed by the detection device 30 of the measurement device 1, and the processes of S104 to S106 are performed by the signal processing device 40 of the measurement device 1. When performing the processes of S101 to S103, the optical switch 28A connects the laser light source 13 to the splitter 21', and the optical switch 28B connects the coupler 25' to the detection device 30. This allows the test light to be split into first and second test lights, a predetermined time difference to be imparted between the first and second test lights, and the first and second test lights to interfere with each other to generate test interference light. In addition, the test interference light can be detected by the detection device 30 to generate a beat signal of the test interference light (test beat signal). When processing S107, optical switch 28A connects the laser light source 13 to the splitter 21, and optical switch 28B connects the coupler 25 to the detection device 30.

<Modification 2>
In the above description, the analysis unit 42′ (or the analysis unit 42) determines the time change in the frequency of the test light based on the beat frequency of the test beat signal at predetermined time intervals (S105), generates waveform data indicating the waveform of the correction current based on the time change in the frequency of the test light (S106; see the lower diagrams of FIGS. 8A and 8B ), and stores the waveform data as correction data in the correction unit 12A. However, the correction data is not limited to waveform data (current profile data) indicating the waveform of the correction current. For example, data indicating the time change (slope α′) of the frequency of the test light may be stored in the correction unit 12A as correction data, or the reciprocal (1/α′) of the time change in the frequency of the test light may be stored in the correction unit 12A as correction data. In this case, the correction unit 12A generates waveform data indicating the waveform of the correction current based on the correction data (see the lower diagrams of FIGS. 8A and 8B ), and the current source 12 outputs a current signal (correction current) corresponding to the waveform data determined by the correction unit 12A. In this way, part of the processing of S101 to S106 (in this case, S106) may be performed by the measurement device 1 (for example, the correction unit 12A). In this case, the correction unit 12A needs to include not only a memory for storing the correction data but also an arithmetic processing device (for example, a CPU, an MPU, etc.) for generating waveform data. Even in this way, the laser light source 13 can output measurement light whose frequency changes linearly with the passage of time.

In the above explanation, waveform data indicating the waveform of the correction current is obtained in advance, and the current source 12 outputs the correction current in accordance with the waveform data. However, the correction current may be generated without obtaining the waveform data in advance. Specifically, the current source 12 may output the correction current while calculating the correction current I' as shown in the lower diagrams of Figures 8A and 8B. However, outputting the correction current while calculating the correction current I' requires high-speed calculations. In contrast, obtaining waveform data in advance and having the current source 12 output the correction current in accordance with the waveform data is advantageous because high-speed calculations are not required.

===Summary===
The above measurement method includes the steps of inputting a test current that changes linearly with time as shown in FIG. 5A to the laser light source 13′ (the laser light source 13 in the modified example), and outputting test light that has been frequency-modulated in accordance with the test current from the laser light source 13′ (S101), branching the test light into first test light and second test light, providing a predetermined time difference Δt between the first test light and the second test light, and causing the first test light and the second test light to interfere with each other to generate test interference light (S102), causing the detection device 30′ (the detection device 30 in the modified example) to detect the test interference light and output a test beat signal (S103), determining the beat frequency of the test beat signal with time (S104; see FIG. 5B), determining the change in frequency of the test light with time based on the beat frequency of the test beat signal with time (S105), and outputting a correction current corrected based on the change in frequency of the test light with time to the laser light source 13 (S106, S107). By outputting the thus corrected correction current to the laser light source 13, the measurement light (frequency-modulated light) output by the laser light source 13 in accordance with the correction current changes linearly with time. This makes it possible to measure distance by the FMCW method using a laser light source 13 with a response delay. That is, it is possible to irradiate the object 90 with the measurement light output from the laser light source 13 in accordance with the correction current, cause the reflected light from the object 90 to interfere with the measurement light to generate interference light, cause the detection device 30 to detect the interference light and output a beat signal, and determine the distance to the object 90 based on the beat frequency of the beat signal (the frequency of the peak shown in FIG. 2B ).

In addition, in the above measurement method, in the process of S104, the beat frequency of the test beat signal is determined for each predetermined time difference Δt between the first test light and the second test light (see Figure 5B). This makes it possible to determine the change in the frequency of the test light over time in the process of S105 based on the test beat frequency for each predetermined time (see Figure 5C).

Furthermore, in the above measurement method, as shown in Figure 6, the peak time at which the beat signal reaches its peak is determined, and the beat frequency is calculated based on the interval between peak times. This makes it possible to determine the change in beat frequency over time at a given time. However, the method for measuring beat frequency is not limited to this. For example, the time at which the beat signal reaches a given voltage (e.g., 0 V) can be determined, and the beat frequency can be calculated based on the interval at which the beat signal reaches a given voltage (e.g., the center voltage (average voltage) of the beat signal).

Furthermore, as shown in Figure 6, the above measurement method determines the peak time when the beat signal reaches its maximum value and the peak time when the beat signal reaches its minimum value, and then calculates the beat frequency for each half period of the beat signal based on the interval between the peak times. This makes it possible to calculate the beat frequency with high time resolution.

In addition, in the above measurement method, as shown in the lower diagrams of Figures 8A and 8B, the correction current I' is generated based on the ratio (α'/α) of the time change in the frequency of the test light (slope α') to the time change in the target frequency of the measurement light (slope α). This offsets the effects of the response delay of the laser light source 13 and corrects the current input to the laser light source 13 so that the time change in the frequency of the measurement light output from the laser light source 13 approaches the time change in the target frequency. Note that in the above explanation, the correction current I'(j) is generated based on the amount of change β' (= β × α/α') calculated by multiplying the inverse (α/α') of the ratio of the time change in the frequency of the test light to the time change in the target frequency of the measurement light by the amount of change β. However, the method for generating the correction current is not limited to this. For example, the amount of change β' (= β × C × α/α') may be calculated by multiplying the inverse (α/α') of the ratio of the time change in the frequency of the test light to the time change in the target frequency of the measurement light by a predetermined coefficient C.

The measurement device 1 includes a laser light source 13, an optical device 20, a detection device 30, a signal processing device 40, and a current source 12. The laser light source 13 outputs light having a frequency corresponding to an input current and frequency-modulated light corresponding to a current signal whose current varies over time. However, there is a response delay between a change in the input current and a change in the frequency of the laser light. Therefore, when a current signal whose current varies linearly over time is input to the laser light source 13, the beat frequency _fB of the beat signal varies over time (see FIG. 12B). As a result, it is difficult to determine the beat frequency _fB by frequency analysis of the beat signal (see the right diagram in FIG. 2A), which makes it difficult to measure distance using the FMCW method. Therefore, the current source 12 of this embodiment inputs to the laser light source 13 a correction current (current signal; see FIG. 2B) corrected based on the time change in the light frequency (see the solid line in FIG. 5C) when a current signal whose current varies linearly over time (see FIG. 5A) is input to the laser light source 13. As a result, the measurement light (frequency modulated light) output from the laser light source 13 in accordance with the correction current changes linearly with time, making it possible to measure distances using the FMCW method.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments and includes various modifications. Furthermore, the above embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those that include all of the described configurations. Furthermore, some of the configurations of the above embodiments can be added to, deleted from, or replaced with other configurations.

1 measuring device, 10 generating device,
12 current source, 13 laser light source,
14 temperature controller, 14A temperature sensor,
20 Optical device, 20' Test optical device,
21 branching device, 22 circulator, 23 optical system,
24 optical waveguide, 25 coupler,
26' first optical waveguide, 27' second optical waveguide,
28A, 28B optical switches,
30 Detector, 31 Photoelectric converter, 32 Amplifier,
40 signal processing device, 41 signal acquisition unit,
42 analysis unit, 43 output unit,
90 Object

Claims (5)

applying a test current to the light source that varies linearly with time;
outputting test light from the light source, the test light being frequency modulated in accordance with the test current;
splitting the test light into first test light and second test light, providing a predetermined time difference between the first test light and the second test light, and causing the first test light and the second test light to interfere with each other to generate test interference light;
causing a detection device to detect the test interference light and output a test beat signal;
determining a beat frequency of the test beat signal over time;
determining a time change in the frequency of the test light based on the beat frequency of the test beat signal for each time;
outputting a correction current to the light source that is corrected based on the time change in the frequency of the test light;
outputting measurement light from a light source, the measurement light being frequency-modulated in accordance with the correction current;
Irradiating the measurement light onto an object and causing reflected light from the object to interfere with the measurement light to generate interference light;
causing the detection device to detect the interference light and output a beat signal; and
A measurement method for determining the distance to the object based on the beat frequency of the beat signal. 2. The measurement method according to claim 1,
determining the beat frequency of the test beat signal for each of the predetermined time differences given between the first test light and the second test light;
Measurement method. The measurement method according to claim 2,
determining a peak time at which the test beat signal reaches a peak;
determining the beat frequency of the test beat signal based on the interval between the peak times;
Measurement method. The measurement method according to claim 3,
a peak time when the test beat signal reaches a maximum value and a peak time when the test beat signal reaches a minimum value are respectively determined;
determining the beat frequency for each half period of the test beat signal based on the interval between the peak times;
Measurement method. The measurement method according to any one of claims 1 to 4,
The measurement method further comprises generating the correction current based on a ratio of a time change in the frequency of the test light to a time change in the target frequency of the measurement light.