JP7781366B2 - Distance measuring device - Google Patents

Description

This disclosure relates to a distance measurement device.

Under consideration is being given to the underwater use of a distance measuring device that scans a pulsed laser beam, detects the reflected light from the object being measured using a light-receiving element, and calculates the distance from the time between emission and detection and the speed of light.

JP 2011-013138 A JP 2010-169405 A International Publication No. 2013/084616

The purpose of this disclosure is to provide a distance measurement device that can extend the measurable distance range.

A distance measurement device according to one embodiment of the present invention includes a first laser light source that outputs a first pulsed light of a first wavelength; a second laser light source that outputs a second pulsed light of a second wavelength different from the first wavelength during a period when the first pulsed light is off; an optical system that combines the first pulsed light and the second pulsed light on the same optical axis and irradiates the object to be measured; a first light-receiving element that receives and photoelectrically converts first reflected light that is reflected by the object to be measured and is the combination of the first pulsed light and the second pulsed light; and a processing unit that receives a signal from the first light-receiving element and generates an electrical signal corresponding to the difference between the low level and high level of the energy of the first reflected light.

This disclosure makes it possible to provide a distance measurement device that can extend the measurable distance range.

1 is a schematic diagram of a distance measurement device according to an embodiment of the present invention. 1 is a schematic diagram of a distance measurement device according to an embodiment of the present invention. 1 is a schematic diagram of a distance measurement device according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a processing unit according to an embodiment of the present invention. 1 is a flowchart illustrating a distance measurement method using the distance measurement device according to an embodiment of the present invention. FIG. 4 is an output waveform diagram of a first pulsed light in the embodiment of the present invention. FIG. 4 is an output waveform diagram of a second pulsed light in the embodiment of the present invention. 6B is a waveform diagram of the energy of a first reflected light that is reflected by a measurement target located at a first distance (short distance) and input to a first light receiving element. FIG. 7B is a waveform diagram of the energy detected by the first light receiving element from the first reflected light shown in FIG. 7A. FIG. FIG. 7C is a waveform diagram of an AC voltage extracted from the detected energy shown in FIG. 7B. FIG. 10 is a waveform diagram of the energy of the first reflected light that is reflected from a measurement object located at a second distance (long distance) and input to the first light receiving element in the embodiment of the present invention. 8B is a waveform diagram of energy detected by a first light receiving element from the first reflected light shown in FIG. 8A. FIG. FIG. 8C is a waveform diagram of an AC voltage extracted from the detected energy shown in FIG. 8B. FIG. 4 is an output waveform diagram of a first pulsed light in the embodiment of the present invention. FIG. 4 is an output waveform diagram of a second pulsed light in the embodiment of the present invention. FIG. 10 is an output waveform diagram of a third pulsed light in the embodiment of the present invention. This is a waveform diagram of the energy of the first reflected light that is input to the first light receiving element after the combined light of the pulsed light shown in Figures 9A to 9C is reflected by a measurement object located at a first distance (short distance). 10B is a waveform diagram of energy detected by a first light receiving element from the first reflected light shown in FIG. 10A. FIG. FIG. 10C is a waveform diagram of an AC voltage extracted from the detected energy shown in FIG. 10B. This is a waveform diagram of the energy of the first reflected light that is input to the first light receiving element after the combined light of the pulsed light shown in Figures 9A to 9C is reflected by the measurement target located at the second distance (medium distance). 11B is a waveform diagram of the energy of the first reflected light shown in FIG. 11A detected by the first light receiving element. FIG. FIG. 11C is a waveform diagram of an AC voltage extracted from the detected energy shown in FIG. 11B. This is a waveform diagram of the energy of the first reflected light that is input to the first light receiving element after the combined light of the pulsed light shown in Figures 9A to 9C is reflected by a measurement object located at a third distance (long distance). 12B is a waveform diagram of the energy of the first reflected light shown in FIG. 12A detected by the first light receiving element. FIG. FIG. 12C is a waveform diagram of an AC voltage extracted from the detected energy shown in FIG. 12B.

The following describes the embodiments with reference to the drawings. In each drawing, the same components are designated by the same reference numerals. Note that the drawings are schematic or conceptual illustrations of the embodiments, and therefore the scale, spacing, and positional relationships of the components may not necessarily be the same as those in reality. In addition, some components may not be shown.

As shown in FIG. 1, the distance measurement device of this embodiment has a first laser light source 11, a second laser light source 12, an optical system 30, a first light receiving element 21, and a second light receiving element 22.

The first laser light source 11 and the second laser light source 12 are, for example, semiconductor lasers (or laser diodes) that use a semiconductor as the medium that causes stimulated emission. Alternatively, the first laser light source 11 and the second laser light source 12 can also be solid-state lasers that use an insulating solid material as the medium.

The first laser light source 11 outputs a first pulsed light P1 having a first wavelength λ1. The second laser light source 12 outputs a second pulsed light P2 having a second wavelength λ2 that is different from the first wavelength λ1. For example, the first wavelength λ1 can be set in the range of 350 nm to 550 nm, and the second wavelength λ2 can be set in the range of 500 nm to 1600 nm.

As described below, the optical system 30 combines the first pulsed light P1 and the second pulsed light P2 on the same optical axis and irradiates the measurement object 100 with the combined light. The optical system 30 includes a dichroic mirror 31, a beam splitter 32, a transmissive/reflective member 33, and a condenser lens 34.

The first laser light source 11, dichroic mirror 31, beam splitter 32, and transmissive/reflective member 33 are arranged on the same optical axis. On the optical axis, the dichroic mirror 31 is arranged between the first laser light source 11 and the beam splitter 32, and the beam splitter 32 is arranged between the dichroic mirror 31 and the transmissive/reflective member 33.

The first pulsed light P1 and the second pulsed light P2 are combined on the same optical axis by the dichroic mirror 31. The first pulsed light P1 passes through the dichroic mirror 31. The second pulsed light P2 is reflected by the dichroic mirror 31 and combined with the first pulsed light P1 on the same optical axis as the first pulsed light P1 that passed through the dichroic mirror 31.

The first laser light source 11 and the second laser light source 12 can also be positioned in a position interchanged with the position shown in FIG. 1. In this case, the second pulsed light P2 from the second laser light source 12 passes through the dichroic mirror 31, and the first pulsed light P1 from the first laser light source 11 is reflected by the dichroic mirror 31.

The beam splitter 32 is disposed between the first laser light source 11 and the transmissive/reflective member 33, and between the second laser light source 12 and the transmissive/reflective member 33. The beam splitter 32 splits the combined light P of the first pulsed light P1 and the second pulsed light P2 into transmitted light directed toward the transmissive/reflective member 33 and second reflected light R2 directed toward the second light receiving element 22.

The transmissive-reflective member 33 has a hole 33a that transmits the combined light P that has passed through the beam splitter 32. The transmissive-reflective member 33 also has a mirror 33b that reflects the first reflected light R1 reflected by the measurement object 100 toward the first light receiving element 21.

The first light receiving element 21 receives the first reflected light R1 that is reflected by the measurement object 100 and passes through the mirror portion 33b of the transmissive reflector 33, and performs photoelectric conversion. The first light receiving element 21 is, for example, a photodiode. A condenser lens 34 is disposed between the first light receiving element 21 and the transmissive reflector 33.

The second light receiving element 22 receives the second reflected light R2, which is the combined light P reflected and split by the beam splitter 32, and performs photoelectric conversion. The second light receiving element 22 is, for example, a photodiode.

The combiner that combines the first pulsed light P1 and the second pulsed light P2 onto the same optical axis is not limited to the dichroic mirror 31; a combining beam splitter 35 shown in Figure 2 can also be used. The combining beam splitter 35 is a polarizing beam splitter that, for example, transmits P-polarized light and reflects S-polarized light.

Normally, the laser light output from a semiconductor laser is linearly polarized, and the polarization direction can be controlled by rotating the semiconductor laser. For example, with respect to the beam splitter 35, the first pulse light P1 controlled to a P-polarized state passes through the beam splitter 35, while the second pulse light P2 controlled to an S-polarized state is reflected by the beam splitter 35 and combined on the same optical axis with the first pulse light P1 that passed through the beam splitter 35.

Furthermore, the fiber coupler 36 shown in Figure 3 can also be used as a combiner that combines the first pulsed light P1 and the second pulsed light P2 onto the same optical axis.

Two optical fibers 37a and 37b are coupled in the fiber coupler 36. The first pulsed light P1 from the first laser light source 11 is incident on the first optical fiber 37a, and the second pulsed light P2 from the second laser light source 12 is incident on the second optical fiber 37b. The first pulsed light P1 propagated through the first optical fiber 37a and the second pulsed light P2 propagated through the second optical fiber 37b are combined on the same optical axis in the fiber coupler 36.

The distance measurement device of this embodiment further includes a processing unit 50 shown in FIG. 4. The processing unit 50 includes a first amplifier 51 electrically connected to the first light receiving element 21, an AC coupling capacitor 52 electrically connected to the first amplifier 51, a second amplifier 53 electrically connected to the AC coupling capacitor 52, and an AD converter 54 electrically connected to the second amplifier 53.

Next, a method for measuring the distance to the measurement target 100 using the distance measurement device of an embodiment of the present invention will be described with reference to the flowchart shown in Figure 5 and the waveform diagrams shown in Figures 6A to 8C in addition to the figures described above.

A distance measurement device according to an embodiment of the present invention can be installed in water while housed in a case, or mounted on a device that moves underwater, and can measure the distance to a measurement target 100 located outside the case.

<Step S1>
A first pulsed light P1 having a first wavelength λ1 is output from the first laser light source 11, and a second pulsed light P2 having a second wavelength λ2 is output from the second laser light source 12. The first pulsed light P1 and the second pulsed light P2 are not single pulses but continuous pulses.

Figure 6A is a diagram showing the output waveform of the first pulsed light P1, and Figure 6B is a diagram showing the output waveform of the second pulsed light P2. In Figures 6A and 6B, the horizontal axis represents time [seconds] and the vertical axis represents output [W].

The second laser light source 12 outputs the second pulsed light P2 during the period when the first pulsed light P1 is off. In other words, the phases of the first pulsed light P1 and the second pulsed light P2 are in an antiphase relationship.

When the distance measurement device of this embodiment is installed underwater, the attenuation rate in water of the second pulsed light P2 of the second wavelength λ2 is greater than the attenuation rate in water of the first pulsed light P1 of the first wavelength λ1. The second laser light source 12 outputs the second pulsed light P2 with a peak output lower than the peak output of the first pulsed light P1.

The first pulsed light P1 and the second pulsed light P2 are combined on the same optical axis by the dichroic mirror 31 shown in FIG. 1, the combining beam splitter 35 shown in FIG. 2, or the fiber coupler 36 shown in FIG. 3. The combined light P of the first pulsed light P1 and the second pulsed light P2 is a pulsed light in which the first pulsed light P1 and the second pulsed light P2 are superimposed, with the first pulsed light P1 and the second pulsed light P2 being in an antiphase relationship.

<Step S2>
The combined light P is split by the beam splitter 32 into transmitted light directed toward the transmissive/reflective member 33 and second reflected light R2 directed toward the second light receiving element 22 .

<Step S3>
The combined light P that has passed through the beam splitter 32 passes through the hole 33a of the transmissive/reflective member 33, and is then emitted to the outside of the case and irradiated onto the measurement object 100. The combined light P that has been irradiated onto the measurement object 100 is diffusely reflected by the measurement object 100.

The first reflected light R1 reflected by the measurement object 100 returns inside the case and is incident on the mirror portion 33b of the transmissive reflector 33. The first reflected light R1 incident on the mirror portion 33b is reflected by the mirror portion 33b, is focused by the focusing lens 34, and is incident on the first light receiving element 21. The first reflected light R1 is the combined light P of the first pulsed light P1 and the second pulsed light P2 reflected by the measurement object 100, and is a pulsed light.

<Step S4A>
The first light receiving element 21 receives the first reflected light R1 and outputs a current corresponding to the energy of the first reflected light R1 to the processing unit 50 shown in Fig. 4. Upon receiving a signal (current signal) from the first light receiving element 21, the processing unit 50 generates an electrical signal corresponding to the difference between the low level and high level of the energy of the first reflected light R1, which is pulsed light, through steps S5A to S8A described below.

<Step S5A>
4, the waveforms of the input and output signals of each element are shown in the balloons. The first amplifier 51 converts the current signal A1 from the first light receiving element 21 into a voltage signal V1 and amplifies it.

<Step S6A>
The AC coupling capacitor 52 attenuates (or blocks) the DC component from the voltage signal V1 output from the first amplifier 51, extracts the AC component, and generates the AC voltage V2. Fig. 4 shows, for example, the waveform of the AC voltage V2 consisting only of the AC component.

<Step S7A>
The second amplifier 53 generates a voltage V3 by adjusting the low level of the AC component of the AC voltage V2 to 0 V. The signals handled in the processes up to this point are analog signals.

<Step S8A>
The AD converter 54 converts the voltage V3 into digital data.

After step S2, the second reflected light R2 split by the beam splitter 32 is also subjected to the same signal processing in steps S4B to S8B as in steps S4A to S8A.

<Step S4B>
In step S4B, the second light receiving element 22 receives the second reflected light R2 that does not pass through the measurement object 100 and outputs a current corresponding to the energy of the second reflected light R2 to the processing unit 50. The second reflected light R2 becomes reference light (pulsed light) for calculating the distance to the measurement object 100 using the first reflected light R1 reflected by the measurement object 100.

<Steps S5B to S8B>
Thereafter, the same processes as steps S5A to S8A are executed as steps S5B to S8B in the processing unit 50. As a result, in step S8B, an electrical signal (digital data) corresponding to the difference between the low level and the high level of the energy of the reference light received by the second light receiving element 22 is generated.

<Step S9>
The distance to the object 100 is calculated from the time difference between the time (reference time) when the second light receiving element 22 receives the second reflected light R2 (reference light) and the time when the first light receiving element 21 receives the first reflected light R1.

When using laser light to measure distance underwater, for example, attenuation of the laser light in water can easily result in large differences in the energy of the light reflected from the object being measured when measuring close and long distances, making it difficult to accommodate the input range of the electrical circuit for the entire energy range of the reflected light. Generally, the range of signal energy input to the electrical circuit can be adjusted to fit within the input range of the electrical circuit by varying the energy of the laser light or the gain of the light-receiving element depending on the situation. However, because high-precision distance measurements require measuring very short periods of time, there is a problem in that measurement errors can easily increase due to distortion of the signal waveform caused by varying the energy of the laser light or the gain of the light-receiving element.

According to an embodiment of the present invention, the processing unit 50 utilizes the difference between the low and high levels of the first reflected light R1, rather than the total energy of the first reflected light R1.

Figure 7A is an output waveform diagram of the first reflected light R1 that is reflected by a measurement target located at a first distance from the distance measurement device and input to the first light receiving element 21. In Figure 7A, the horizontal axis represents time [seconds] and the vertical axis represents output [W]. As mentioned above, the first reflected light R1 is a combined light obtained by combining the first pulsed light (solid line) shown in Figure 6A and the second pulsed light (dashed line) shown in Figure 6B on the same optical axis.

The first light receiving element 21 detects the first reflected light R1 shown in Figure 7A as the energy shown in Figure 7B, without distinguishing between the first pulsed light and the second pulsed light. In Figure 7B, the horizontal axis represents time [seconds] and the vertical axis represents energy [W].

The first pulsed light and the second pulsed light are in an antiphase relationship. The peak output of the second pulsed light is lower than the peak output of the first pulsed light. The attenuation rate of laser light in water depends on the wavelength of the laser light. The wavelengths of the first pulsed light and the second pulsed light are different. Due to this difference in wavelength, in this embodiment, the attenuation rate of the second pulsed light in water is greater than the attenuation rate of the first pulsed light in water. Under these conditions, a difference between the high and low levels of the detected energy of the first light receiving element 21 shown in Figure 7B is likely to occur.

Figure 8A is a waveform diagram of the energy of the first reflected light R1 that is reflected from the distance measurement device by a measurement target located at a second distance longer than the first distance and input to the first light receiving element 21. In Figure 8A, the horizontal axis represents time [seconds] and the vertical axis represents energy [W].

Because the second distance is longer than the first distance, the time it takes for the first reflected light R1 reflected by the measurement object located at the second distance to be detected by the first light receiving element 21 is longer than the time it takes for the first reflected light R1 reflected by the measurement object located at the first distance to be detected by the first light receiving element 21.

When measuring the second distance, the first light receiving element 21 also detects the first reflected light R1 shown in FIG. 8A as the energy shown in FIG. 8B, without distinguishing between the first pulsed light and the second pulsed light. In FIG. 8B, the horizontal axis represents time [seconds] and the vertical axis represents energy [W]. Due to the above conditions, when measuring the second distance, differences are likely to occur between the high and low levels of the detected energy of the first light receiving element 21 shown in FIG. 8B.

Here, because the second distance is longer than the first distance, the first reflected light R1 shown in Figure 8A is attenuated more than the first reflected light R1 shown in Figure 7A and inputs to the first light receiving element 21. Figure 8A shows an example in which the second pulsed light is attenuated so that the energy level of the second pulsed light is almost zero.

As mentioned above, an AC component is extracted from the energy detected by the first light-receiving element 21 in the AC coupling capacitor 52 of the processing unit 50. Figure 7C is a waveform diagram of the AC component (AC voltage) extracted from the detected energy shown in Figure 7B, and Figure 8C is a waveform diagram of the AC component (AC voltage) extracted from the detected energy shown in Figure 8B. In Figures 7C and 8C, the horizontal axis represents time [seconds] and the vertical axis represents voltage [V].

As shown in Figures 7B and 8B, even if there is a large difference in the detected energy of the first light receiving element 21 depending on whether the distance to the measurement object is short (first distance) or long (second distance), signal processing in the processing unit 50 eliminates large differences in the level of the AC component (voltage level) whether the distance to the measurement object is short (first distance) or long (second distance), as shown in Figures 7C and 8C, making it possible to process signals within the input range of the electrical circuit. As a result, according to embodiments of the present invention, the distance range that can be measured with high accuracy can be extended.

It is sufficient that there is a difference between the high and low levels of the detected energy of the first light receiving element 21 shown in Figures 7B and 8B. If the attenuation rate of the second pulsed light in water is greater than the attenuation rate of the first pulsed light in water, and the peak output of the second pulsed light is made higher than the peak output of the first pulsed light, the second pulsed light may be attenuated, and the first reflected light R1 may be input to the first light receiving element 21 in a state where the high level of the second pulsed light is the same as the high level of the first pulsed light. In this case, there is no difference between the high and low levels of the detected energy of the first light receiving element 21 shown in Figures 7B and 8B.

Therefore, if the attenuation rate of the second pulsed light in water is greater than the attenuation rate of the first pulsed light in water, it is preferable to set the peak output of the second pulsed light lower than the peak output of the first pulsed light. Conversely, if the attenuation rate of the first pulsed light in water is greater than the attenuation rate of the second pulsed light in water, it is preferable to set the peak output of the first pulsed light lower than the peak output of the second pulsed light.

Even if the peak output of the first pulsed light and the peak output of the second pulsed light are the same, a difference may occur between the high and low levels of the detected energy of the first light receiving element 21 shown in Figures 7B and 8B depending on the relationship between the attenuation rate of the first pulsed light in water and the attenuation rate of the second pulsed light in water.

It is also possible to calculate the distance to the measurement object 100 without disposing the beam splitter 32 and second light receiving element 22, from the time difference between the emission timing (emission time) of the pulsed light from the first laser light source 11 and the second laser light source 12 and the time when the first reflected light R1 is received by the first light receiving element 21.

In addition, to obtain an accurate time difference between the reference time and the time when the first light receiving element 21 receives the first reflected light R1, it is preferable to use the time when the light is received by the second light receiving element 22 as in the above embodiment, rather than using the emission time of the laser light sources 11 and 12 themselves as the reference time.

The distance measurement device may further include a third laser light source. The third laser light source outputs third pulsed light having a third wavelength λ3 that is different from the first wavelength λ1 of the first pulsed light and the second wavelength λ2 of the second pulsed light. For example, the second wavelength λ2 is longer than the first wavelength λ1, and the third wavelength λ3 is longer than the second wavelength λ2.

Figure 9A is a diagram of the output waveform of the first pulsed light, Figure 9B is a diagram of the output waveform of the second pulsed light, and Figure 9C is a diagram of the output waveform of the third pulsed light. In Figures 9A to 9C, the horizontal axis represents time [seconds] and the vertical axis represents output [W].

The second laser light source 12 outputs the second pulsed light during the period when the first pulsed light is off, and the third laser light source outputs the third pulsed light during the period when the first pulsed light is off. In other words, the phases of the second pulsed light and the third pulsed light are opposite in phase to the phase of the first pulsed light.

When the distance measurement device is placed underwater, the attenuation rate of the third pulsed light in water is greater than the attenuation rate of the first pulsed light from the first laser light source 11, and is different from the attenuation rate of the second pulsed light from the second laser light source 12. For example, the attenuation rate of the second pulsed light in water is greater than the attenuation rate of the first pulsed light in water, and the attenuation rate of the third pulsed light in water is greater than the attenuation rate of the second pulsed light in water. The peak output of the second pulsed light is lower than the peak output of the first pulsed light, and the peak output of the third pulsed light is lower than the peak output of the second pulsed light.

The dichroic mirror 31 shown in FIG. 1, the beam combining beam splitter 35 shown in FIG. 2, or the fiber coupler 36 shown in FIG. 3 combines the first pulse light and at least either the second pulse light or the third pulse light on the same optical axis. In the above embodiment, an example was shown in which the first pulse light and the second pulse light were combined, but the first pulse light and the third pulse light can also be combined. Furthermore, the first pulse light, the second pulse light, and the third pulse light can also be combined.

As in the above embodiment, the combined light of the first pulsed light, second pulsed light, and third pulsed light is irradiated onto the measurement object 100 and is diffusely reflected by the measurement object 100. This first reflected light R1 is photoelectrically converted by the first light receiving element 21, and the photoelectrically converted electrical signal is subjected to signal processing in the processing unit 50 similar to that in the above embodiment.

Figure 10A is a waveform diagram of the energy of the first reflected light R1 that is reflected by a measurement target located at a first distance (short distance) from the distance measurement device and input to the first light receiving element 21. In Figure 10A, the horizontal axis represents time [seconds] and the vertical axis represents energy [W]. Here, the first reflected light R1 is a combined light obtained by combining the first pulsed light (solid line) shown in Figure 9A, the second pulsed light (dashed line) shown in Figure 9B, and the third pulsed light (dash-dotted line) shown in Figure 9C on the same optical axis.

The first light receiving element 21 detects the first reflected light R1 shown in Figure 10A as the energy shown in Figure 10B, without distinguishing between the first pulsed light, the second pulsed light, and the third pulsed light. In Figure 10B, the horizontal axis represents time [seconds] and the vertical axis represents energy [W].

Figure 11A is a waveform diagram of the energy of the first reflected light R1 reflected from a distance measurement device by a measurement target located at a second distance (medium distance) longer than the first distance, and input to the first light receiving element 21. In Figure 11A, the horizontal axis represents time [seconds], and the vertical axis represents energy [W]. Because the second distance is longer than the first distance, the time it takes for the first reflected light R1 reflected by a measurement target located at the second distance to be detected by the first light receiving element 21 is longer than the time it takes for the first reflected light R1 reflected by a measurement target located at the first distance to be detected by the first light receiving element 21.

When measuring the second distance, the first light receiving element 21 also detects the first reflected light R1 shown in FIG. 11A as the energy shown in FIG. 11B, without distinguishing between the first pulse light, the second pulse light, and the third pulse light. In FIG. 11B, the horizontal axis represents time [seconds] and the vertical axis represents energy [W]. Because the second distance is longer than the first distance, the first reflected light R1 shown in FIG. 11A is more attenuated than the first reflected light R1 shown in FIG. 10A and enters the first light receiving element 21.

Figure 12A is a waveform diagram of the energy of the first reflected light R1 reflected from a distance measurement device by a measurement target located at a third distance (long distance) longer than the second distance, and input to the first light receiving element 21. In Figure 12A, the horizontal axis represents time [seconds], and the vertical axis represents energy [W]. Because the third distance is longer than the second distance, the time it takes for the first reflected light R1 reflected by the measurement target located at the third distance to be detected by the first light receiving element 21 is longer than the time it takes for the first reflected light R1 reflected by the measurement target located at the second distance to be detected by the first light receiving element 21.

When measuring the third distance, the first light receiving element 21 also detects the first reflected light R1 shown in FIG. 12A as the energy shown in FIG. 12B, without distinguishing between the first pulsed light, the second pulsed light, and the third pulsed light. In FIG. 12B, the horizontal axis represents time [seconds] and the vertical axis represents energy [W]. Because the third distance is longer than the second distance, the first reflected light R1 shown in FIG. 12A is more attenuated than the first reflected light R1 shown in FIG. 11A and enters the first light receiving element 21. FIG. 12A shows an example in which the second pulsed light and the third pulsed light are attenuated, and the energy levels of the second pulsed light and the third pulsed light become almost zero.

As described above, an AC component is extracted from the energy detected by the first light-receiving element 21 in the AC coupling capacitor 52 of the processing unit 50. Figure 10C is a waveform diagram of the AC component (AC voltage) extracted from the detected energy shown in Figure 10B, Figure 11C is a waveform diagram of the AC component (AC voltage) extracted from the detected energy shown in Figure 11B, and Figure 12C is a waveform diagram of the AC component (AC voltage) extracted from the detected energy shown in Figure 12B. In Figures 10C, 11C, and 12C, the horizontal axis represents time [seconds] and the vertical axis represents voltage [V].

As shown in Figures 10B, 11B, and 12B, even if there is a large difference in the detected energy of the first light receiving element 21 depending on whether the distance to the measurement object is short (first distance), medium (second distance), or long (third distance), signal processing in the processing unit 50 eliminates large differences in the level of the AC component (voltage level), as shown in Figures 10C, 11C, and 12C, making it possible to process signals within the input range of the electrical circuit. This makes it possible to extend the distance range that can be measured with high accuracy.

In the distance measurement described above, when a combined light of the first pulse light, second pulse light, and third pulse light is used, it is easier to generate a difference between the high and low levels in the detected energy of the first light receiving element 21 shown in Figures 10B, 11B, and 12B compared to when a combined light of the first pulse light and second pulse light is used.

Embodiments of the present invention have been described above with reference to specific examples. However, the present invention is not limited to these specific examples. All forms that a person skilled in the art can implement by appropriately modifying the design based on the above-described embodiments of the present invention fall within the scope of the present invention, as long as they encompass the gist of the present invention. In addition, a person skilled in the art may come up with various modifications and alterations within the scope of the concept of the present invention, and these modifications and alterations also fall within the scope of the present invention.

11...First laser light source, 12...Second laser light source, 30...Optical system, 21...First light receiving element, 22...Second light receiving element, 31...Dichroic mirror, 32...Beam splitter, 33...Transmissive/reflective member, 35...Combining beam splitter, 36...Fiber coupler, 50...Processing unit, 100...Measurement object, P1...First pulsed light, P2...Second pulsed light, P...Combined light, R1...First reflected light, R2...Second reflected light

Claims (10)

A distance measurement device that measures a distance to a measurement object underwater,
a first laser light source that outputs a first pulsed light having a first wavelength;
a second laser light source that outputs a second pulsed light having a second wavelength different from the first wavelength during a period in which the first pulsed light is off;
an optical system that combines the first pulsed light and the second pulsed light on the same optical axis and irradiates the measurement object with the combined light;
a first light receiving element that receives first reflected light that is reflected by the object to be measured and is formed by combining the first pulsed light and the second pulsed light, and performs photoelectric conversion on the first reflected light;
a processing unit that receives a signal from the first light receiving element and generates an electrical signal corresponding to a difference between a low level and a high level of energy of the first reflected light;
Equipped with
an attenuation rate of the second pulsed light in the water is greater than an attenuation rate of the first pulsed light in the water,
A distance measurement device in which the second laser light source outputs the second pulsed light having a peak output lower than the peak output of the first laser light source . The distance measurement device of claim 1, wherein the electrical signal is an AC voltage. the first wavelength is equal to or greater than 350 nm and equal to or less than 550 nm,
3. The distance measurement device according to claim 1 , wherein the second wavelength is not less than 500 nm and not more than 1600 nm. 4. The distance measurement device according to claim 1, further comprising a transmissive reflecting member that reflects the first reflected light toward the first light receiving element. A second light receiving element;
a beam splitter disposed between the first laser light source and the transmission/reflection member and between the second laser light source and the transmission/reflection member, the beam splitter splitting the first pulsed light and the second pulsed light into a transmitted light directed toward the transmission/reflection member and a second reflected light directed toward the second light receiving element;
The distance measurement device according to claim 4 , further comprising: The distance measurement device according to any one of claims 1 to 5, further comprising a third laser light source that outputs third pulsed light, the attenuation rate of which in water is greater than that of the first pulsed light and which is different from the second pulsed light, when the distance measurement device is installed underwater. 7. The distance measurement device according to claim 6 , wherein the optical system includes a dichroic mirror that combines the first pulsed light with at least one of the second pulsed light and the third pulsed light. 7. The distance measurement device according to claim 6 , wherein the optical system includes a fiber coupler that combines the first pulsed light with at least one of the second pulsed light and the third pulsed light. 7. The distance measurement device according to claim 6 , wherein the optical system includes a beam splitter for combining the first pulsed light with at least one of the second pulsed light and the third pulsed light. The processing unit
converting a current signal from the first light receiving element into a voltage signal;
Attenuating a DC component from the voltage signal and extracting an AC component;
generating a voltage in which the low level of the AC component is adjusted to 0 V;
10. The distance measurement device according to claim 1, wherein the voltage is converted into digital data.