JP7380382B2 - range finder - Google Patents

Description

The present invention relates to a range finder, for example, a range finder using laser light that performs phase difference detection by coherent detection.

Techniques used in range finders that measure the distance or displacement of an object include phase difference detection technology, TOF (Time of Flight) technology, and triangulation distance measurement technology (for example, see Non-Patent Document 1).

Among these, phase difference detection technology irradiates a measurement target with laser light that is amplitude-modulated at a predetermined fundamental frequency. By measuring the phase difference between the laser beam output from the range finder and the reflected light scattered from the object to be measured, the time from the output of the laser light from the range finder to the reception of the reflected light is determined. By multiplying that time by the speed of light, we find the absolute distance or displacement to the object.

https://www.keyence.co.jp/ss/products/process/levelsensor/type/laser.jsp

Since phase difference detection technology uses direct detection, the signal-to-noise ratio of scattered light is low. Therefore, it is sensitive to external disturbances and attenuation of reflected light due to the shape of the object to be measured, making stable measurement difficult. Therefore, a method using optical coherent detection that can ensure a high signal-to-noise ratio is desired.

However, in order to realize optical coherent detection, a very expensive narrow linewidth light source is required.

This invention has been made in view of the above-mentioned problems. An object of the present invention is to provide a range finder using optical coherent detection using an inexpensive light source.

In order to achieve the above-mentioned object, the range finder of the present invention splits a continuous light into two, one of which becomes a local light, and the other of which is amplitude-modulated with a sine wave of a predetermined frequency Δf to produce a probe light. an interference signal generation section that generates an interference signal by coherently detecting the scattered light from the probe light scattered by the object to be measured and the local light, and an interference signal generation section that samples the interference signal at a predetermined sampling frequency and converts it into a digital signal. It includes a digital signal generation section that converts the signal, and a signal processing section that extracts the phase difference between the scattered light and the local light from the digital signal and calculates the position or displacement of the object to be measured.

According to a preferred embodiment of the range finder of the present invention, the probe light generation section includes a continuous light source that generates continuous light, a splitter that splits the continuous light into two, and a splitter that splits the continuous light into two. A Mach-Zehnder intensity modulator is provided that generates a probe light by amplitude modulating the continuous light obtained by using a sine wave having a predetermined frequency Δf.

Further, according to a preferred embodiment of the range finder of the present invention, the signal processing section squares the digital signals into which the first interference signal and the second interference signal included in the interference signal are respectively converted. A squaring means, a second squaring means, an addition means for adding the calculation results of the first squaring means and the second squaring means, and extracting a frequency component with a frequency of 4πΔf from the calculation result of the addition means. FIR filter means, down-conversion means for down-converting the calculation result extracted by the FIR filter means, complex conjugation means for taking the complex conjugate of the calculation result of the down-conversion means, and phase unwrapping for the calculation result of the complex conjugation means. and a multiplication means that calculates the position or displacement of the object to be measured from the calculation result of the phase unwrapping means.

According to the range finder of the present invention, optical coherent detection can be performed using an inexpensive light source.

FIG. 1 is a schematic diagram for explaining a range finder of the present invention. FIG. 2 is a schematic diagram showing a frequency spectrum in a Mach-Zehnder intensity modulator. FIG. 3 is a diagram for explaining the operation of an interference signal generation section. FIG. 3 is a schematic diagram of a signal processing section. FIG. 3 is a schematic diagram showing a frequency spectrum corresponding to a calculation result of an adding means.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the shapes, sizes, and arrangement relationships of each component are merely shown schematically to the extent that the present invention can be understood. Further, although preferred configuration examples of the present invention will be described below, these are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes and modifications can be made that can achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

An embodiment of the range finder of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram for explaining the range finder of the present invention.

The range finder includes, for example, a probe light generation section 100, an optical circulator 200, a collimator lens 300, an interference signal generation section 400, a digital signal generation section 500, and a signal processing section 600.

The probe light generation unit 100 generates probe light by amplitude modulating continuous light with a sine wave having a predetermined frequency Δf. The probe light generation unit 100 includes, for example, a continuous light source 110, a demultiplexer 120, an oscillator 130, and a Mach-Zehnder intensity modulator 140.

As the continuous light source 110, any suitable conventionally known laser light source can be used. Continuous light source 110 generates continuous light S1. Continuous light S1 generated by continuous light source 110 is sent to demultiplexer 120. The electric field y(t) of the continuous light S1 is given by the following equation (1).

Here, p is the average intensity of the continuous light S1, _f0 is the frequency of the continuous light S1, and θ(t) is the phase noise in the continuous light source 110. Note that the initial phase is omitted because it is not essential.

The demultiplexer 120 branches the continuous light S1 into two. One of the continuous lights S2 branched into two by the demultiplexer 120 is sent to the interference signal generation section 400 as local light. The electric field y _LO (t) of the local light S2 is given by the following equation (2). Here, p _LO is the average intensity of the local light S2.

The other continuous light S3 branched into two by the demultiplexer 120 is sent to the Mach-Zehnder intensity modulator 140.

A sine wave electrical signal of a predetermined frequency Δf generated by the oscillator 130 is input to the Mach-Zehnder intensity modulator 140 . The Mach-Zehnder intensity modulator 140 amplitude-modulates the input continuous light S3 with a sine wave having a predetermined frequency Δf to generate probe light S4.

With reference to FIG. 2, the frequency spectra of the continuous light S3 input to the Mach-Zehnder intensity modulator 140 and the probe light S4 output from the Mach-Zehnder intensity modulator 140 will be described. FIG. 2 is a schematic diagram showing a frequency spectrum in a Mach-Zehnder intensity modulator. 2(A) shows the frequency spectrum of the continuous light S3 input to the Mach-Zehnder type intensity modulator 140, and FIG. 2(B) shows the frequency spectrum of the probe light S4 outputted from the Mach-Zehnder type intensity modulator 140. Showing the spectrum. As shown in FIG. 2A, the frequency spectrum of the continuous light S3 includes the frequency f ₀ of the continuous light generated by the continuous light source 110 as a component. Further _, as shown in FIG. 2B, the frequency spectrum of the probe light S4 is the sum ( _f0 +Δf) and the difference (f ₀ −Δf) as components.

The output y ₄ (t) of the probe light S4 is given by the following equation (3).

The probe light S4 is output from the probe light generation section 100, passes through the optical circulator 200 and the collimator lens 300 as a light emitting section, and is irradiated onto the measurement target 1000. Note that although an example using the optical circulator 200 and the collimator lens 300 will be described here, this is just an example and is not limited thereto.

Scattered light S5 from measurement object 1000 returns along the same path as the probe light, passes through collimator lens 300 and optical circulator 200, and is sent to interference signal generation section 400. The electric field y _S (t) of the scattered light S5 is given by the following equation (4).

Here, _pS is the average intensity of the scattered light S5 when received by the interference signal generation unit 400, c is the speed of light, and n is the average refractive index of the optical path of the probe light S4 and the scattered light S5. In addition, d, x(t) and v(t) are the initial distance from the range finder to the measurement target 1000, the displacement of the measurement target 1000 from the initial distance, and the speed of the measurement target 1000, respectively. be. Here, by using the approximation c>>v(t), the following equation (5) can be obtained from the above equation (4).

The interference signal generation section 400 coherently detects the local light S2 generated by the probe light generation section 100 and the scattered light S5 from the measurement object 1000 to generate an interference signal. The interference signal generation section 400 includes, for example, an optical hybrid coupler 410 and a balanced detector 420. Further, the interference signal generation section 400 includes an amplifier 430 and a bandpass filter 440 as necessary.

The operation of interference signal generation section 400 will be explained with reference to FIG. 3. FIG. 3 is a diagram for explaining the operation of the interference signal generation section 400. In FIG. 3, an optical hybrid coupler 410 and a balanced detector 420 are shown.

The input signal light S _S input to the optical hybrid coupler 410 is split into two branches, one of which is sent to a first directional coupler 411 and the other to a second directional coupler 412 . In addition, the local light S _LO input to the optical hybrid coupler 410 is split into two branches, one of which is sent to the first directional coupler 411, and the other is subjected to a 90° phase change by a 90° phase shifter 413. Thereafter, it is sent to the second directional coupler 412. The first directional coupler 411 outputs E ₁ and E ₂ as two outputs of the optical hybrid coupler 410 . Further, the second directional coupler 412 outputs E ₃ and E ₄ as the two outputs of the optical hybrid coupler 410 .

If the average intensity, angular frequency and phase of the input signal light are p _S , ω _S and θ _S, respectively, and the average intensity, angular frequency and phase of the local light are p _LO , ω _LO and θ _LO , respectively, then the input signal The electric field E _S (t) of light and the electric field E _LO (t) of local light are given by the following equation (6).

At this time, the four output lights E ₁ to E ₄ from the optical hybrid coupler 410 are each given by the following equation (7).

Further, the output of the balanced detector 420 is given by the following equation (8).

Here, I _I (t) and I _Q (t) are currents output from the balance detector 420, respectively. The current I output from the balanced detector 420 is proportional to the square of the electric field E input to the balanced detector 420. R is this proportionality constant and is called conversion efficiency. Note that although not shown in the figure, a resistor with a load resistance r (Ω) is usually connected to the latter stage of the balance detector 420, and finally the voltage signals V _I (t) and V _Q (t ) is output.

The local light S2 given by the above equation (2) and the scattered light S5 given by the above equation (5) are respectively input to the interference signal generation section 400 described with reference to FIG. When calculating the above equations (7) and (8) for _{the LO} and input signal light _SS , the interference signal output from the interference signal generation section 400 is given by the following equations (9) and (10). It will be done.

As shown in equations (9) and (10) above, the scattered light is amplified by interference with the local light, resulting in so-called coherent detection. The output of the interference signal generation section 400 is sent to the digital signal generation section 500.

The digital signal generation section 500 is configured using, for example, an analog-to-digital converter (ADC). Digital signal generation section 500 samples the interference signal at a predetermined sampling frequency to generate a digital signal. Corresponding to the sampling in the digital signal generation section 500, the time t is assumed to be a discrete time t _i (i is an integer). The digital signal generated by the digital signal generation section 500 is sent to the signal processing section 600.

The signal processing section 600 will be explained with reference to FIG. FIG. 4 is a schematic diagram of the signal processing section 600.

The signal processing unit 600 includes a first square means 611, a second square means 612, an addition means 620, a finite impulse response (FIR) filter means 630, a down conversion means 640, a complex conjugate means 650, It is configured to include phase unwrapping means 660 and multiplication means 670. The signal processing section 600 is a section that performs digital signal processing, and can have any suitable configuration that implements each means. For example, the signal processing unit 600 can be configured with an FPGA (Field-Programmable Gate Array). Further, a configuration may be adopted in which each means is realized by a CPU (Central Processing Unit) executing a program. The signal processing unit 600 extracts the phase difference between the scattered light and the local light from the digital signal, and calculates the distance from the range finder to the object to be measured or the displacement of the object to be measured.

The first squaring means 611 squares _VI and sends the calculation result to the adding means 620. Further, the second squaring means 612 squares _VQ and sends the calculation result to the addition means 620. Adding means 620 adds the output results of first squaring means 611 and second squaring means 612. The calculation result of the addition means 620 is given by the following equation (11). Further, when the following equation (11) is expressed in a complex number signal representation, the following equation (12) is obtained.

FIG. 5 is a diagram showing a frequency spectrum corresponding to the above equation (12), which is the calculation result of the adding means 620. In FIG. 5, the component (I) with frequency 0 corresponds to the first term of the above equation (12), the component (II) with the frequency 4πΔf corresponds to the second term of the above equation (12), and the frequency - Component (III) of 4πΔf corresponds to the third term of the above equation (12).

The calculation result of addition means 620 is sent to FIR filter means 630. The FIR filter means 630 is a bandpass filter with complex coefficients. The FIR filter means 630 extracts the second term of the above equation (12).

The calculation result extracted by the FIR filter means 630 is sent to the down conversion means 640. The down conversion means 640 down converts the calculation result of the FIR filter means 630 by exp(-Δft _i j). The calculation result of the down conversion means 640 is sent to the complex conjugate means 650.

Complex conjugate means 650 takes the complex conjugate of the operation result of down conversion means 630. The calculation result of the complex conjugate means 650 is given by the following equation (13).

The calculation result of the complex conjugate means 650 is sent to the phase unwrap means 660. The phase unwrapping means 660 performs phase unwrapping on the calculation result of the complex conjugate means 650 given by the above equation (13). As a result, the calculation result of the phase unwrapping means 660 is given by the following equation (14).

The calculation result of phase unwrapping means 660 is sent to multiplication means 670. The multiplication means 670 multiplies or divides the calculation result of the phase unwrapping means 660, given by the above formula (14), by Δf, c, 8π, and n, thereby calculating the range of the object to be measured from the range finder. Distance d+x(t _i ), displacement x(t _i ), or velocity v(t) can be calculated.

Here, as a premise for performing phase unwrapping in the phase unwrapping means 660, the following equation (15) is required.

As long as the condition given by Equation (15) above is satisfied, displacement can be measured in the sampling time period, and by integrating them, the net displacement of the object can be measured.

Further, as long as the condition given by the following equation (16) is satisfied, the distance meter of the present invention can be used as an absolute distance meter by determining the distance d+x(t _i ).

Here, the above equation (14) does not include a component related to the phase noise θ(t) in the continuous light source 110. That is, measurement results can be obtained independent of the phase noise θ(t) of the continuous light source 110.

Conventionally, in order to realize optical coherent detection, a very expensive narrow linewidth light source has been required as the continuous light source 110. This has been a major barrier to realizing optical coherence. However, according to the range finder of the present invention described above, measurement can be performed independent of the phase noise of the continuous light source 110, so an expensive narrow linewidth light source is not required. Therefore, it is possible to realize a range finder using optical coherent detection using a light source that is cheaper than conventional range finders.

100 Probe light generation section 110 Continuous light source 120 Demultiplexer 130 Mach-Zehnder optical intensity modulator 140 Oscillator 200 Optical circulator 300 Collimator lens 400 Interference signal generation section 500 Digital signal generation section 600 Signal processing section 611, 612 Square means 620 addition means 630 FIR filter means 640 down conversion means 650 complex conjugate means 660 phase unwrapping means 670 multiplication means

Claims (2)

a probe light generation unit that splits the continuous light into two and uses one as a local light and amplitude-modulates the other with a sine wave of a predetermined frequency Δf as a probe light;
an interference signal generation unit that generates an interference signal by coherently detecting scattered light from the probe light scattered by the measurement object and the local light;
a digital signal generation unit that samples the interference signal at a predetermined sampling frequency and converts it into a digital signal;
a signal processing unit that extracts a phase difference between the scattered light and the local light from the digital signal and calculates the position or displacement of the measurement target;
Equipped with
The signal processing section includes:
a first squaring means and a second squaring means for respectively squaring digital signals obtained by converting the first interference signal and the second interference signal included in the interference signal;
Adding means for adding the calculation results of the first squaring means and the second squaring means to obtain the calculation result shown in the following formula (I);
FIR filter means for extracting a frequency component with a frequency of 4πΔf from the calculation result of the addition means shown in the following formula (I);
down-conversion means for down-converting the calculation result extracted by the FIR filter means;
a complex conjugate means for taking the complex conjugate of the calculation result of the down conversion means and obtaining the calculation result shown in the following formula (II);
A phase unwrapping means for performing phase unwrapping on the calculation result of the complex conjugate means shown in the following equation (II) to obtain the calculation result shown in the following equation (III);
a multiplication means for calculating the position or displacement of the measurement object from the calculation result of the phase unwrapping means shown in the following formula (III);
A rangefinder characterized by comprising:

The probe light generation section includes:
a continuous light source that generates the continuous light;
a splitter that branches the continuous light into two;
2. The probe according to claim 1, further comprising a Mach-Zehnder intensity modulator that generates the probe light by amplitude modulating one of the continuous lights split into two by the splitter with a sine wave of a predetermined frequency Δf. The rangefinder mentioned.

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