14 is configured to store various programs to be executed by the first controller 13. The first storage unit 14 is configured to store a delay time 35 (see FIG. 7) which will be described later. The first storage unit 14 is a non-volatile storage device, such as, e.g., an HDD (Hard Disk Drive) and an SSD (Solid State Drive).

The absorptive green transmission filter 15 is configured to selectively transmit the light including the light of a wavelength band of the second light 31 and not including the light having a predetermined wavelength band of the first light 30. Further, the absorptive green transmission filter 15 is arranged at the pre-stage of the first light receiver 11. Specifically, the absorptive green transmission filter 15 is arranged at a position between the first light receiver 11 and the reflective green transmission filter 123. The details of the wavelength band that transmits through the absorptive green transmission filter 15 will be described later.

The first communication device 1 is configured to emit the first light 30 through the first window 16. Further, the first communication device 1 is configured to receive the second light 31 through the second window 17. The first window 16 and the second window 17 are formed of, for example, a glass plate or an acrylic plate.

The first housing 18 includes a first optical path forming member 18a, a first peripheral wall 18b, a first window 16, a first opening portion 18c closed by the first window 16, and a second opening portion 18d closed by the second window 17. The first peripheral wall 18b, the first window 16 for closing the first opening portion 18c, and the second window 17 for closing the second opening portion 18d constitute a closed first interior space 18e sealed in a water-tight manner.

As shown in FIG. 2, the first light emitter 10 and the green noise light removal filter 121 are held by the first optical path forming member 18a in the first housing 18.

The first optical path forming member 18a holds the first light emitter 10 and forms a first optical path which is an optical path of the first light 30. In this embodiment, the first optical path forming member 18a is configured to hold the green noise light removal filter 121 together with the first light emitter 10. The first optical path forming member 18a has a cylindrical configuration. Further, the first optical path forming member 18a is made of a metal material. The metal material is, for example, aluminum material. That is, the first optical path forming member 18a does not transmit light.

Further, the first light transmitting portion 18f is provided on a face of the first optical path forming member 18a facing the first window 16. The first light transmitting portion 18f is formed of, for example, a glass plate or an acrylic material. Therefore, the first light 30 emitted from the first light emitter 10 is emitted to the outside of the first communication device 1 via the first light transmitting portion 18f and the first window 16.

As shown in FIG. 2, the green noise light removal filter 121 is arranged at the post-stage of the first light emitter 10 in the first optical path forming member 18a. Specifically, the green noise light removal filter 121 is arranged at a position between the first light emitter 10 and the first window 16.

Further, as shown in FIG. 2, the first light receiver 11, the absorptive green transmission filter 15, and the reflective green transmission filter 123 are held by the second optical path forming member 122 in the first housing 18. The second optical path forming member 122 holds the first light receiver 11 and forms a second optical path that is an optical path for the second light 31. In this embodiment, the second optical path forming member 122 holds the absorptive green transmission filter 15 and the reflective green transmission filter 123 together with the first light receiver 11. The second optical path forming member 122 has a cylindrical configuration.

Further, the second light transmitting portion 122a is provided on a face of the second optical path forming member 122 facing the second window 17. The second light transmitting portion 122a is formed of, for example, a glass plate or an acrylic material. Therefore, the second light 31 incident on the first communication device 1 via the second window 17 is incident on the first light receiver 11 via the second light transmitting portion 122a.

As shown in FIG. 2, the absorptive green transmission filter 15 is arranged at the pre-stage of the first light receiver 11 in the second optical path forming member 122. Specifically, the absorptive green transmission filter 15 is arranged at a position between the first light receiver 11 and the reflective green transmission filter 123. Further, the fourth green light noise generation suppression member (reflective green transmission filter 123) is arranged at the pre-stage of the absorptive green transmission filter 15 in the second optical path forming member 122. Specifically, the reflective green transmission filter 123 is arranged at a position between the absorptive green transmission filter 15 and the second window 17.

(Second Optical Wireless Communication Device)

Next, the configuration of the second communication device 2 will be described with reference to FIG. 3.

As shown in FIG. 3, the second communication device 2 includes a second light emitter 20, a second light receiver 21, a second controller 22, a second storage unit 23, a third optical path forming member 24, a reflective blue transmission filter 25, a fourth optical path forming member 26, a third window 27, a fourth window 28, and a second housing 29.

The second communication device 2 is configured to emit signal light (second light 31) from the transmission unit (second light emitter 20) via the fourth light transmitting portion 26a and the fourth window 28 to transmit a light signal to a communication partner (first communication device 1) (see FIG. 1). Further, the second communication device 2 receives the signal light (first light 30) from the first communication device 1 by the receiving unit (second light receiver 21) via the third window 27 and the third light transmitting portion 24a and receives an optical signal (data). Further, the second controller 22 converts the data to be transmitted to the first communication device 1 into an electric signal and controls the emission of the second light 31 of the second light emitter 20. Further, the second controller 22 also converts the light-receiving signal of the second light receiver 21 into data by controlling the operation of the second light receiver 21.

The second light emitter 20 is configured to emit the second light 31 as signal light. The second light 31 is light of a second wavelength 41 (see FIG. 4) in a green wavelength band as a center wavelength. The second wavelength 41 is included in a wavelength band ranging from, for example, 532 nm to 518 nm. In this embodiment, the second wavelength 41 is, for example, 525 nm. Further, the second light 31 is light of a wavelength band having the second wavelength 41 as a center wavelength and not including red but including green. The second light emitter 20 includes a second laser light source 20a that generates the second light 31. The second laser light source 20a is, for example, a semiconductor laser light source.

The second light receiver 21 is configured to receive the first light 30 emitted from the first light emitter 10 (see FIG. 2). The second light receiver 21 includes, for example, a photomultiplier tube.

The second controller 22 is a computer, a processor, or a circuitry configured to include a CPU, a ROM, a RAM, and the like. The second controller 22 functions as a controller that controls each unit of the second communication device 2 by executing a predetermined control program stored in the second storage unit 23. The second controller 22 as hardware includes a second signal processing unit 22a as a functional block of software (program). The second controller 22 functions as the second signal processing unit 22a by executing a program stored in the second storage unit 23.

The second signal processing unit 22a is configured to change the first light 30 emitted from the first communication device 1 (see FIG. 2) and received by the second light receiver 21 into data.

The second storage unit 23 is configured to store various programs to be executed by the second controller 22. The second storage unit 23 is, for example, a non-volatile storage device, such as, e.g., an HDD or an SSD.

The third optical path forming member 24 is configured to hold the second light receiver 21 and the reflective blue transmission filter 25 in the second housing 29. The third optical path forming member 24 has a cylindrical configuration. The third optical path forming member 24 is made of a metal material. The metal material is, for example, aluminum material. Therefore, the third optical path forming member 24 is configured not to transmit light.

The reflective blue transmission filter 25 is configured to selectively transmit the first light 30 among the light incident on the second light receiver 21. Further, as shown in FIG. 3, the reflective blue transmission filter 25 is arranged at a pre-stage of the second light receiver 21. Specifically, the reflective blue transmission filter 25 is arranged between the second light receiver 21 and the third window 27. The detailed configuration of the reflective blue transmission filter 25 will be described later.

The fourth optical path forming member 26 is configured to hold the second light emitter 20 in the second housing 29. The fourth optical path forming member 26 has a cylindrical configuration. Further, the fourth optical path forming member 26 is made of a metal material. Accordingly, the fourth optical path forming member 26 is configured not to transmit light.

The second communication device 2 is configured to receive the first light 30 via the third window 27. Further, the second communication device 2 is configured to emit the second light 31 through the fourth window 28. The third window 27 and the fourth window 28 are each formed by, for example, a glass plate or an acrylic plate.

Further, the third light transmitting portion 24a is provided on a face of the third optical path forming member 24 facing the third window 27. The third light transmitting portion 24a is formed of, for example, a glass plate or an acrylic material. Therefore, the first light 30 incident on the second communication device 2 via the third window 27 is incident on the second light receiver 21 via the third light transmitting portion 24a.

Further, the fourth optical path forming member 26 is provided with the fourth light transmitting portion 26a on a face facing the fourth window 28. The fourth light transmitting portion 26a is formed of, for example, a glass plate or an acrylic material. Therefore, the second light 31 emitted from the second light emitter 20 is emitted to the outside of the second communication device 2 via the fourth light transmitting portion 26a and the fourth window 28.

The second housing 29 has a second peripheral wall 29a, a third opening portion 29b closed by the third window 27, and a fourth opening portion 29c closed by the fourth window 28. The second peripheral wall 29a, the third window 27 closing the third opening portion 29b, and the fourth window 28 closing the fourth opening portion 29c constitute a closed second interior space 29d sealed in a water-tight manner.

In this embodiment, the underwater optical wireless communication system 100 (see FIG. 1) is configured to communicate between the first communication device 1 and the second communication device 2 by receiving the first light 30 emitted from the first communication device 1 (see FIG. 1) by the second communication device 2 and communicate between the second communication device 2 and the first communication device 1 by receiving the second light 31 emitted from the second communication device 2 by the first communication device 1.

In this embodiment, the moving body 80 (see FIG. 1) travels in the sea to inspect, for example, a structure laid on the seabed 90. The first communication device 1 is configured to transmit the inspection result acquired by a detector (not shown) provided in the moving body 80 to the second communication device 2 by the first light 30. Further, the second communication device 2 is configured to receive the inspection result transmitted from the first communication device 1 and transmit the received inspection result to a communication device provided on land or on a mother ship or the like. In a case where communication is performed between the first communication device 1 and the second communication device 2, the moving body 80 is moved to a communicable region to perform communication.

The first light 30 and the second light 31 are different in color from each other. Accordingly, the first communication device 1 and the second communication device 2 are configured such that multiple optical communication for performing bidirectional communication can be performed by simultaneously emitting the first light 30 and the second light 31.

(Absorptive Green Transmission Filter and Reflective Blue Transmission Filter)

The absorptive green transmission filter 15 (see FIG. 2) and the reflective blue transmission filter 25 (see FIG. 3) will now be described with reference to FIG. 4 and FIG. 5.

FIG. 4 is a graph 60 showing a predetermined wavelength band allowed to be passed by the absorptive green transmission filter 15 (see FIG. 2). In the graph 60, the vertical axis represents the transmittance, and the horizontal axis represents the wavelength.

The transmittance change curve 60a shown in FIG. 4 is a curve representing the relation between the wavelength of light and the transmittance in the absorptive green transmission filter 15 (see FIG. 2). As shown in the transmittance change curve 60a, in this embodiment, the absorptive green transmission filter 15 is configured to absorb light having a wavelength shorter than that of the second wavelength 41. That is, the absorptive green transmission filter 15 is a so-called absorptive long-pass filter that selectively transmits light including the light having a wavelength of the second light 31 (see FIG. 2) and not including the light of a predetermined wavelength band of the first light 30 (See FIG. 2). Note that the absorptive long-pass filter is a filter that absorbs light other than light having a predetermined wavelength region and converts the absorbed light into heat.

The absorptive green transmission filter 15 (see FIG. 2) has a first transmission band 200 including the second wavelength 41 and a first lower blocking region 201 that is shorter in wavelength than the first transmission band 200. In this embodiment, since the second light 31 is green light, the first transmission band 200 is set in the range capable of transmitting the light of the green wavelength band. Further, the second light 31 emitted from the second light emitter 20 is slightly shifted in wavelength due to the product error. Therefore, the lower limit of the first transmission band 200 is set on a shorter wavelength side than 500 nm by a predetermined wavelength.

Since the first communication device 1 (see FIG. 2) has the absorptive green transmission filter 15, the second light 31 (see FIG. 2) incident through the second window 17 (see FIG. 2) can be selectively transmitted and made incident on the first light receiver 11 (see FIG. 2).

FIG. 5 is a graph 61 showing the wavelength band through which the reflective blue transmission filter 25 (see FIG. 3) allows to transmit. In the graph 61, the vertical axis represents the transmittance, and the horizontal axis represents the wavelength.

The transmittance change curve 61a shown in FIG. 5 is a curve showing the relation between the wavelength of light and the transmittance in the reflective blue transmission filter 25 (see FIG. 3). As shown in the transmittance change curve 61a, in this embodiment, the reflective blue transmission filter 25 is configured to selectively transmit the light of a predetermined wavelength band including the first wavelength 40 and not include the second wavelength 41.

The reflective blue transmission filter 25 is, for example, a so-called reflective band-pass filter that selectively transmits the light of the wavelength band including the first wavelength 40 by reflecting the light of a wavelength other than the wavelength band. The reflective bandpass filter is a filter that reflects light of a wavelength in a wavelength band other than a predetermined wavelength band and transmits the light of a wavelength in a predetermined wavelength band without reflecting, thereby selectively transmitting the light of a wavelength in a predetermined wavelength band.

The reflective blue transmission filter 25 (see FIG. 3) includes a second transmission band 202 having a first wavelength 40 as a center wavelength, a second lower blocking region 203 on a shorter wavelength side than the second transmission band 202, and a second upper blocking region 204 on a longer wavelength side than the second transmission band 202. In this embodiment, since the first light 30 is blue light, the second transmission band 202 is configured within a range capable of transmitting the light of a blue wavelength band.

Further, the first light 30 emitted from the first light emitter 10 (see FIG. 2) is slightly shifted in wavelength due to the product error. Therefore, the lower limit of the second transmission band 202 is set to a wavelength on a shorter wavelength side than 440 nm by a predetermined wavelength. Further, the upper limit of the second transmission band 202 is set to a wavelength on a longer wavelength side than 450 nm by a predetermined wavelength.

Since the second communication device 2 (see FIG. 3) has a reflective blue transmission filter 25 (see FIG. 3), it is possible to selectively transmit the first light 30 (see FIG. 3) incident through the third window 27 (see FIG. 3) to make the first light 30 to be incident on the the second light receiver 21 (see FIG. 3).

(Green Noise Light)

Here, the inventors of the present invention have intensively studied and found the fact that the green light received by the first communication device 1 (see FIG. 2) includes green light (green noise light 50a) other than the second light 31 (see FIG. 3) which is green light emitted by the second communication device 2 (see FIG. 3).

Specifically, as shown in FIG. 6A to FIG. 6D, the inventors of the present invention have found the fact that green noise light 50a is included as green light other than the second light 31 (see FIG. 3). The green noise light 50a includes, for example, first green noise light 51a to fourth green noise light 51d.

As shown in FIG. 6A, the first green noise light 51a is a green Raman scattered light caused by the Raman scatter of the first light 30 (see FIG. 2) in water. Like the first light 30, when blue light is emitted in water, green light (first green noise light 51a) in which the wavelength band is shifted toward a long wavelength side is generated by Raman scatter. When the first green noise light 51a occurs, the first green noise light 51a transmits through the absorptive green transmission filter 15 (see FIG. 2). Therefore, the first green noise light 51a reduces the S/N ratio of the second light 31 (see FIG. 2).

As shown in FIG. 6B, the light emitted from the first light emitter 10 includes the first light 30 which is blue light and green light. That is, the second green noise light 51b is green light emitted from the first light emitter 10 together with the first light 30 (see FIG. 2). After the second green noise light 51b is emitted from the first light emitter 10 (see FIG. 2) together with the first light 30, it may be reflected or scattered in the first communication device 1 (see FIG. 2) or in water or the like, thereby entering the first light receiver 11 (see FIG. 2). In this case, the second green noise light 51b reduces the S/N ratio of the second light 31 (see FIG. 2).

As shown in FIG. 6C, in a case where the optical path forming member or the like is formed of a resin member 3, when the first light 30 which is blue light is emitted to the resin member 3, green fluorescence is generated. That is, the third green noise light 51c is green fluorescence emitted from the resin member 3 when the resin member 3 is irradiated with the first light 30 (see FIG. 2). When the third green noise light 51c is incident on the first light receiver 11 (see FIG. 2), the third green noise light 51c reduces the S/N ratio of the second light 31 (see FIG. 2).

As shown in FIG. 6D, when the first light 30 which is blue light is emitted to the absorptive green transmission filter 15, green fluorescence is generated. That is, the fourth green noise light 51d is green fluorescence emitted from the absorptive green transmission filter 15 when the first light 30 is emitted to the absorptive green transmission filter 15. The first light receiver 11 (see FIG. 2) is provided at the post-stage of the absorptive green transmission filter 15. Therefore, the fourth green noise light 51d generated when the first light 30 is irradiated to the absorptive green transmission filter 15 enters the first light receiver 11. When the fourth green noise light 51d enters the first light receiver 11, the fourth green noise light 51d reduces the S/N ratio of the second light 31.

Therefore, in this embodiment, the deterioration of the communication accuracy of the second light 31 due to the first green noise light 51a to the fourth green noise light 51d by the green light noise signal removal circuit 120 (see FIG. 2) is suppressed by the green noise light removal filter 120 (see FIG. 2), the second optical path forming member 122 (see FIG. 2), and the reflective green transmission filter 123 (see FIG. 2).

(Removal of Green Light Noise Signal Due to First Green Noise Light)

With reference to FIG. 7 to FIG. 10, the configuration in which the green light noise signal removal circuit 120 (see FIG. 7) removes the green light noise signal 50b caused by the first green noise light 51a (see FIG. 7) will be described.

The first green noise light 51a (see FIG. 7) is green light caused by Raman scattering in water of the first light 30 (see FIG. 2) emitted by the first light emitter 10 (see FIG. 2). Since the green light noise signal removal circuit 120 can acquire the timing at which the first light emitter 10 emits the light from the first controller 13 (see FIG. 2), it is possible to acquire the timing at which the first green noise light 51a occurs.

Thus, in this embodiment, as shown in FIG. 7 and FIG. 8, the green light noise signal removal circuit 120 is configured to remove the green light noise signal 50b based on the first green noise light 51a (see FIG. 7) included in the light-receiving signal 33 (See FIG. 10) in the first light receiver 11 (see FIG. 2), based on the driving signal 32 when the first light emitter 10 (See FIG. 2) emits the first light 30 (see FIG. 2).

The processing of removing the green light noise signal 50b by the green light noise signal removal circuit 120 includes processing of acquiring the delay time 35 and the waveform information 36 on the green light noise signal at the time of calibration by the green light noise signal removal circuit 120, and processing of generating a compensation signal 34 (see FIG. 8) based on the waveform information 36 on the delay time 35 and the waveform information 36 on the green light noise signal when the first light 30 (see FIG. 2) is emitted during the receipt of the second light 31 and removing the green light noise signal 50b by the generated compensation signal 34.

(Acquisition of Compensation Signal and Delay Time)

First, with reference to FIG. 7, the configuration in which the green light noise signal removal circuit 120 acquires the delay time 35 and the waveform information 36 on the green light noise signal will be described.

The green light noise signal removal circuit 120 acquires a compensation signal 34 (see FIG. 8) at the time of calibration in a state in which the first light receiver 11 is not receiving the second light 31 (see FIG. 2). The compensation signal 34 is an electric signal of the light received by the first light receiver 11 when the first light emitter 10 (see FIG. 2) emits the first light 30 (see FIG. 2). Further, the green light noise signal removal circuit 120 acquires the delay time 35 from the time when the first light 30 is emitted to the time when the first green noise light 51a is detected during the calibration.

As shown in FIG. 7, the green light noise signal removal circuit 120 includes an AD conversion unit 120a, a delay time acquisition unit 120b, and a green light noise information acquisition unit 120c.

The AD conversion unit 120a is configured to convert the analog signal of the first green noise light 51a received by the first light receiver 11 into a green light noise signal 50b which is a digital signal. The AD conversion unit 120a includes, for example, an AD converting circuit. The AD conversion unit 120a outputs the converted green light noise signal 50b to the delay time acquisition unit 120b. Further, the AD conversion unit 120a outputs the converted green light noise signal 50b to the green light noise information acquisition unit 120c.

The delay time acquisition unit 120b and the green light noise information acquisition unit 120c are functional blocks that function when the green light noise signal removal circuit 120 executes programs stored in the first storage unit 14.

The delay time acquisition unit 120b is configured to acquire the delay time 35 based on the driving signal 32 and the green light noise signal 50b. Specifically, the delay time acquisition unit 120b acquires the driving signal 32 for driving the first light emitter 10 (see FIG. 2) from the first controller 13. The delay time acquisition unit 120b acquires the green light noise signal 50b from the AD conversion unit 120a. The delay time acquisition unit 120b acquires, as the delay time 35, the difference between the rise time t1 (see FIG. 9) of the driving signal 32 and the rise time t2 (see FIG. 9) of the green light noise signal 50b. Then, the delay time acquisition unit 120b outputs the acquired delay time 35 to the first storage unit 14.

Further, the green light noise information acquisition unit 120c acquires the waveform information 36 on the green light noise signal from the green light noise signal 50b acquired from the AD conversion unit 120a. The waveform information 36 on the green light noise signal includes, for example, the waveform and the amplitude of the green light noise signal 50b. The green light noise information acquisition unit 120c outputs the acquired waveform information 36 on the green light noise signal to the first storage unit 14.

The first storage unit 14 stores the delay time 35 input from the delay time acquisition unit 120b. Further, the first storage unit 14 stores the waveform information 36 on the green light noise signal input from the green light noise information acquisition unit 120c.

In this embodiment, the first communication device 1 (see FIG. 1) is provided in the moving body 80 (see FIG. 1) that travels in water and is movable in water. For this reason, the compensation signal 34 may vary in magnitude or the like depending on the environments of the moving body 80. Therefore, in this embodiment, the green light noise signal removal circuit 120 is configured to generate (update) the compensation signal 34 at predetermined time intervals.

(Removal of Green Light Noise Signal)

Next, the configuration in which the green light noise signal removal circuit 120 removes the green light noise signal 50b (see FIG. 7) will be described with reference to FIG. 8.

In this embodiment, the green light noise signal removal circuit 120 acquires the duration required for the compensation signal 34 to be input after the driving signal 32 is output. The green light noise signal removal circuit 120 is configured to remove the green light noise signal 50b by the delayed compensation signal 34 for the time required for the compensation signal 34 to be input. The time required for the compensation signal 34 to be input is the delay time 35 acquired at the time of calibration and stored in the first storage unit 14.

Specifically, as shown in FIG. 8, the green light noise signal removal circuit 120 is further provided with a compensation signal generation unit 120d, a DA conversion unit 120e, and a first green noise light removal unit 120f. The compensation signal generation unit 120d is a functional block that functions when the green light noise signal removal circuit 120 executes a program stored in the first storage unit 14. The DA conversion unit 120e is configured to convert the compensation signal 34, which is a digital signal, into an compensation waveform 34a, which is an analog signal. The DA conversion unit 120e includes, for example, a DA converting circuit. Further, the first green noise light removal unit 120f includes, for example, a differential amplifier.

When the first controller 13 outputs the driving signal 32, the driving signal 32 is input to the first light emitter 10 (see FIG. 2) and the compensation signal generation unit 120d. When the driving signal 32 is input, the compensation signal generation unit 120d acquires the delay time 35 and the waveform information 36 on the green light noise signal from the first storage unit 14. Then, the compensation signal generation unit 120d generates the compensation signal 34 based on the waveform information 36 on the green light noise signal. The compensation signal generation unit 120d delays the compensation signal 34 by the delay time 35 after the driving signal 32 is input and outputs the compensation signal 34 to the DA conversion unit 120e.

Note that the compensation signal generation unit 120d generates and outputs the compensation signal 34 only when the driving signal 32 is in an on state. When the driving signal 32 is in an off state, the compensation signal generation unit 120d does not generate and output the compensation signal 34.

The DA conversion unit 120e converts the compensation signal 34, which is a digital signal input from the compensation signal generation unit 120d, into a compensation waveform 34a, which is an analog signal. Then, the DA conversion unit 120e outputs the converted compensation waveform 34a to the first green noise light removal unit 120f.

To the first green noise light removal unit 120f, the light-receiving signal 33a is input from the first light receiver 11. The light-receiving signal 33a is an electric signal of the second light 31, including the first green noise light 51a (see FIG. 7). Further, the compensation waveform 34a is input from the DA conversion unit 120e to the first green noise light removal unit 120f. The first green noise light removal unit 120f acquires the second light 31 by removing the first green noise light 51a from the light-receiving signal 33a. Specifically, the first green noise light removal unit 120f acquires the light-receiving signal 33, which is an electric signal of the second light 31 from which the first green noise light 51a has been removed, by subtracting the compensation waveform 34a from the light-receiving signal 33a. The first green noise light removal unit 120f outputs the acquired light-receiving signal 33 to the first signal processing unit 13a.

FIG. 9 is a graph 62 showing the waveforms of the driving signal 32 and the green light noise signal 50b. In the graph 62, the horizontal axis represents time.

As shown in the graph 62, there is a difference between the time t1 at which the driving signal 32 rises and the time t2 at which the green light noise signal 50b rises. The difference between the time t1 and the time t2 is a difference between the time at which the driving signal 32 rises, the first light 30 (see FIG. 2) emitted from the first light emitter 10 (see FIG. 2) is Raman-scattered into the first green noise light 51a (see FIG. 6 A) and received by the first light receiver 11 (see FIG. 2) and the time at which the first green noise light 51a is converted into an electric signal.

Note that since the first green noise light 51a occurs instantaneously after the first light 30 is emitted, the difference between the time t1 and the time t2 is mainly the time required for the first green noise light 51a is converted into an electric signal. The difference between the time t1 and the time t2 is the delay time 35 (see FIG. 7) as described above.

Further, as shown in the graph 62, there also occurs a difference between the time t3 at which the driving signal 32 falls and the time t4 at which the green light noise signal 50b falls. This difference is a time interval similar to the delay time 35. Thus, as shown in graph 62, in a case where the first communication device 1 (see FIG. 2) communicates by the first light 30, it repeats that the driving signal 32 rises and then the green light noise signal 50b rises after the delay time 35 has elapsed, and the driving signal 32 falls and then the green light noise signal 50b falls after the delay time 35 has elapsed.

FIG. 10A is a graph 63 showing the light-receiving signal 33a of the second light 31a (see FIG. 8) including the first green noise light 51a (see FIG. 7). FIG. 10B is a graph 64 of the compensation signal 34. FIG. 10C is a graph 65 showing the light-receiving signal 33 of the second light 31 (see FIG. 8) after removing the first green noise light 51a. Note that the vertical axis of each of the graphs 63 to 65 represents signal strength, and the horizontal axis represents time.

As shown in the graph 63, the light-receiving signal 33a of the second light 31a (see FIG. 8) including the first green noise light 51a (see FIG. 7) includes the signal component 31b of the second light 31 (see FIG. 8) and the green light noise signal 50b. Note that in the graph 63, for simplicity, the signal components 31b of the second light 31 and the green light noise signal 50b are hatched differently from each other.

By subtracting the compensation signal 34 shown in the graph 64 from the light-receiving signal 33a shown in the graph 63, the light-receiving signal 33 from which the green light noise signal 50b has been removed can be acquired as shown in the graph 65.

(Removal of Second Green Noise Light)

Next, with reference to FIG. 11, the configuration in which the green noise light removal filter 121 removes the second green noise light 51b will be described.

The green noise light removal filter 121 has optical properties similar to those of the reflective blue transmission filter 25 (see FIG. 3) and is configured to selectively transmit light of a blue wavelength band without transmitting light of a green wavelength band. That is, the green noise light removal filter 121 is configured to selectively transmit the first light 30 without transmitting the second green noise light 51b. Therefore, as shown in FIG. 11, only the first light 30, out of the first light 30 emitted from the first light emitter 10 and the second green noise light 51b which is green light included in the first light 30, is emitted to the outside of the first communication device 1 (see FIG. 2) through the first window 16 (see FIG. 2).

(Suppression of Generation of Third Green Noise Light)

Next, with reference to FIG. 12, the configuration in which the third green light noise generation suppression unit (second optical path forming member 122) suppresses the generation of the third green noise light 51c will be described. The third green light noise generation suppression unit (second optical path forming member 122) is configured to suppress the generation of the third green noise light 51c due to the first light 30 emitted to the second optical path forming member 122. Specifically, the second optical path forming member 122 is formed of a metal material. The metal material is, for example, aluminum material. Thus, the second optical path forming member 122 does not transmit light.

Here, a metal material, such as, e.g., an aluminum material, is less likely to generate green fluorescence even when irradiated with blue light. Therefore, as shown in FIG. 12, even when the second optical path forming member 122, which is a metal material, such as, e.g., aluminum material, is irradiated with the first light 30, which is blue light, generation of the third green noise light 51c, which is green fluorescence, is suppressed. Note that in FIG. 12, the third green noise light 51c is represented by a broken line in which the generation of third green noise light 51c is suppressed.

(Suppression of Generation of Fourth Green Noise Light)

Next, the configuration for suppressing the occurrence of the fourth green noise light 51d (see FIG. 6 D) will be described with reference to FIG. 13 and FIG. 14. The fourth green light noise generation suppression member is configured to suppress the generation of the fourth green noise light 51d. The fourth green light noise generation suppression member is the reflective green transmission filter 123 that suppresses the generation of the fourth green noise light 51d.

As shown in FIG. 13, by providing the reflective green transmission filter 123 at the pre-stage of the absorptive green transmission filter 15, even if the first light 30 and the second light 31 is incident on the absorptive green transmission filter 15 via the second window 17 (see FIG. 2) and the second light transmitting portion 122a (see FIG. 2), the first light 30 is removed by the reflective green transmission filter 123. Therefore, it is possible to prevent the first light 30 from being emitted to the absorptive green transmission filter 15. Therefore, it is possible to suppress that the first light 30 is emitted to the absorptive green transmission filter 15 to generate the fourth green noise light 51d (see FIG. 6) from the absorptive green transmission filter 15. Note that FIG. 13 shows a configuration example in which one reflective green transmission filter 123 is provided at the pre-stage of the absorptive green transmission filter 15.

FIG. 14 is a graph 66 showing a predetermined wavelength band through which the reflective green transmission filter 123 (see FIG. 13) allows to transmit. In the graph 66, the vertical axis is transmittance and the horizontal axis is wavelength.

The transmittance change curve 66a shown in FIG. 14 is a curve representing the relation between the wavelength of the light and the transmittance in the reflective green transmission filter 123. As shown in the transmittance change curve 66a, in this embodiment, the fourth green light noise generation suppression member (reflective green transmission filter 123 (see FIG. 13)) is configured to suppress the generation of the fourth green noise light 51d (see FIG. 6) due to the first light 30 (see FIG. 2) emitted to the absorptive green transmission filter 15 (see FIG. 2).

Specifically, the reflective green transmission filter 123 selectively transmits the light of a predetermined wavelength not including the wavelength band of the first light 30 by reflecting the light of the wavelength band of the first light 30. The reflective green transmission filter 123 is, for example, a so-called reflective band-pass filter that selectively transmits the light of a wavelength band including the second wavelength 41 by reflecting the light of a wavelength other than the wavelength band including the second wavelength. Accordingly, the first light 30, which is the light of a wavelength band including the first wavelength 40, is reflected by the reflective green transmission filter 123.

The reflective green transmission filter 123 includes a third transmission band 205 having a second wavelength 41 as a center wavelength, a third lower blocking area 206 on the shorter wavelength side than the third transmission band 205, and a third upper blocking area 207 on the longer wavelength side than the third transmission band 205.

In this embodiment, since the second light 31 is green light, the third transmission band 205 is configured within a range capable of transmitting a green wavelength band. Further, the second light 31 (see FIG. 13) emitted from the second light emitter 20 (see FIG. 3) is slightly shifted in wavelength due to the product error. Therefore, the lower limit of the third transmission band 205 is set to a wavelength on the shorter wavelength side than 518 nm by a predetermined wavelength. Further, the upper limit of the third transmission band 205 is set to a wavelength on the longer wave side than 532 nm by a predetermined wavelength.

(Reflective Green Transmission Filter Holding Member)

As shown in FIG. 15A, in this embodiment, the first communication device 1 includes a pair of reflective green transmission filter holding members 124 for holding the reflective green transmission filter 123 by sandwiching the reflective green transmission filter 123. The pair of reflective green transmission filter holding members 124 includes one side holding member 124a and the other side holding member 124b.

Further, the pair of reflective green transmission filter holding members 124 each has a U-shape in cross-section. Specifically, the one side holding member 124a has a protrusion 125a protruding toward the other side holding member 124b. Therefore, the one side holding member 124a has a U-shape in cross-section when viewed from the side surface 123b of the reflective green transmission filter 123. Note that FIG. 15A and FIG. 15B are diagrams assuming that the first light 30 (see FIG. 2) is emitted from the other side holding member 124b.

The other side holding member 124b has a protrusion 125b protruding toward the one side holding member 124a. Therefore, the other side holding member 124b has a U-shape in cross-section when viewed from the side surface 123b of the reflective green transmission filter 123.

The pair of reflective green transmission filter holding members 124 is configured to be capable of forming a region 70 that overlap each other when viewed from the side surface 123a (see FIG. 15A) of the reflective green transmission filter 123.

Specifically, as shown in FIG. 15B, in the case of holding the reflective green transmission filter 123 by the pair of reflective green transmission filter holding members 124, in a state in which the reflective green transmission filter 123 is sandwiched by the one side holding member 124a and the other side holding member 124b, the one side holding member 124a and the other side holding member 124b are fixed. At this time, the protrusion 125a of the one side holding member 124a and the protrusion 125b of the other side holding member 124b overlap each other when viewed from the side surface 123a of the reflective green transmission filter 123 to form the region 70.

The region 70 is a region not irradiated with the first light 30 (see FIG. 2) by the one side holding member 124a and the other side holding member 124b. Therefore, for example, by applying an adhesive 126 to the region 70, even in a case where the one side holding member 124a and the other side holding member 124b are fixed, it is possible to prevent the adhesive 126 from being irradiated with the first light 30. Therefore, it is possible to suppress the generation of the green fluorescence due to the fact that the adhesive 126 is irradiated with the first light 30.

The region 70 is superimposed on each other when viewed from the side surface 123a (see FIG. 15A) of the reflective green transmission filter 123. Therefore, even if the first light 30 is multiple-scattered in the reflective green transmission filter holding member 124, the first light 30 only reaches the inner surface of the protrusion 125a. Therefore, in this embodiment, the reflective green transmission filter holding member 124 can suppress the adhesive 126 from being irradiated with the first light 30.

(Calibration Processing)

Next, with reference to FIG. 16, the processing that the green light noise signal removal circuit 120 (see FIG. 7) acquires the delay time 35 (see FIG. 7) will be described.

In Step 101, the first controller 13 (see FIG. 7) emits the first light 30 (see FIG. 2) by outputting the driving signal 32 (see FIG. 7) to the first light emitter 10 (see FIG. 2).

In Step 102, the delay time acquisition unit 120b (see FIG. 7) acquires the driving signal 32 from the first controller 13. Note that the driving signal 32 is output also to the delay time acquisition unit 120b when it is output from the first controller 13 to the first light emitter 10. Accordingly, the delay time acquisition unit 120b may acquire the timing at which the first light 30 is emitted from the first light emitter 10.

In Step 103, the delay time acquisition unit 120b acquires the green light noise signal 50b.

At Step 104, the delay time acquisition unit 120b acquires the delay time 35 (see FIG. 7). Specifically, the delay time acquisition unit 120b acquires the delay time 35 based on the driving signal 32 acquired in Step 102 and the green light noise signal 50b acquired in Step 103.

In Step 105, the first storage unit 14 stores the delay time 35.

In Step 106, the green light noise information acquisition unit 120c (see FIG. 7) acquires the waveform information 36 on the green light noise signal.

In Step 107, the first storage unit 14 stores the waveform information 36 on the green light noise signal. Thereafter, the processing ends.

Note that either the processing in Step 102 to Step 104 or the processing in Step 106 and Step 107 may be performed first. The processing in Step 102 to Step 104 and the processing in Step 106 and Step 107 may be executed in parallel.

(Green Light Noise Signal Removal Processing)

Next, with reference to FIG. 17, the processing in which the green light noise signal removal circuit 120 (see FIG. 8) removes the green light noise signal 50b (see FIG. 8) will be described. Note that the processing shown in FIG. 17 is initiated when the compensation signal generation unit 120d (see FIG. 8) acquires the first controller 13 (see FIG. 8) from the driving signal 32 (see FIG. 8).

In Step 110, the compensation signal generation unit 120d determines whether the driving signal 32 (see FIG. 8) has been output. When the driving signal 32 is output, the processing proceeds to Step 111. When the driving signal 32 has not been output, the processing ends.

In Step 111, the compensation signal generation unit 120d acquires the delay time 35 (see FIG. 8) and the waveform information 36 (see FIG. 8) on the green light noise signal from the first storage unit 14 (see FIG. 8).

In Step 112, compensation signal generation unit 120d generates a compensation signal 34 (see FIG. 8).

In Step 113, the compensation signal generation unit 120d acquires the driving signal 32 and then outputs the compensation signal 34 in a state in which it is delayed by the delay time 35.

In Step 114, the first green noise light removal unit 120f (see FIG. 8) acquires the light-receiving signal 33 (see FIG. 8) of the second light 31 by subtracting the compensation signal 34 from the light-receiving signal 33a (see FIG. 8) of the second light 31 (FIG. 3) including the first green noise light 51a (see FIG. 7). That is, the first green noise light removal unit 120f removes the green light noise signal 50b (see FIG. 7) from the second light 31a including the first green noise light 51a.

In Step 115, the compensation signal generation unit 120d determines whether the driving signal 32 has been stopped. When the driving signal 32 has not been stopped, the processing proceeds to Step 114. When the driving signal 32 is stopped, the process proceeds to Step 116.

In Step 116, the compensation signal generation unit 120d stops the output of the compensation signal 34. Note that when the driving signal 32 is stopped, the first light 30 (see FIG. 2) is stopped. However, even when the light emission of first light 30 is stopped, the first green noise light 51a received by the first light receiver 11 (see FIG. 2) is converted into the green light noise signal 50b. Therefore, even when the driving signal 32 is stopped, the green light noise signal 50b is acquired after the delay by 35 minutes of the delay time. Therefore, the green light noise signal removal circuit 120 continuously processes the removal of the green light noise signal 50b for 35 minutes of the delay time even after the driving signal 32 is stopped. Thereafter, the processing ends.

Effects of this Embodiment

In this embodiment, the following effects can be obtained.

In this embodiment, as described above, the underwater optical wireless communication device (first communication device 1) is an underwater optical wireless communication device for performing optical wireless communication in water. It is provided with the first light emitter 10 configured to emit the first light 30 of the first wavelength 40 included in the blue wavelength band as a center wavelength, the first light receiver 11 configured to receive the second light 31 of the second wavelength 41 included in the green wavelength band as the center wavelength, and a noise suppression unit configured to suppress the noise caused by the green light generated due to the first light 30.

With this, the noise suppression unit suppresses the noise caused by the green light generated due to the first light 30, and therefore, it is possible to prevent the S/N ratio of the second light 31 from decreasing. Consequently, it is possible to suppress the decrease in communication accuracy due to the green light noise caused by the green light other than the second light 31 which is the green light received from the communication partner (second communication device 2).

Further, in this embodiment, as described above, the underwater optical wireless communication system 100 is an underwater optical wireless communication system that performs optical wireless communication in water. It is provided with the first optical wireless communication device (first communication device 1) configured to emit the first light 30 of the first wavelength 40 included in the blue wavelength band as a center wavelength and receive the second light 31 of the second wavelength 41 included in the green wavelength band as a center wavelength and the second underwater optical communication device (second communication device 2) configured to emit the second light 31 and receive the first light 30. The first underwater optical wireless communication device (first communication device 1) includes the first light emitter 10 configured to emit the first light 30, the first light receiver 11 configured to receive the second light 31, and the noise suppression unit configured to suppress the noise caused by the green light generated due to the first light 30.

With this configuration, like the underwater optical wireless communication device (first communication device 1), it is possible to provide the underwater optical wireless communication system 100 capable of suppressing the decrease in communication accuracy caused by the green light due to the green light other than the second light 31 which is green light received from the communication partner (second communication device 2).

Further, in the above-described embodiment, the following further effects can be obtained by the following configuration.

That is, in this embodiment, as described above, the noise suppression unit includes at least one of the green light noise removal unit 12a for removing the green light noise which is noise caused by the green light generated due to the first light 30 and the green light noise generation suppression unit 12b for suppressing the generation of the green light noise.

Thus, for example, in a case where the noise suppression unit includes the green light noise removal unit 12a, even when the green light noise is generated due to the light received by the first light receiver 11, the green light noise is removed by the green light noise removal unit 12a, and therefore, it is possible to prevent the communication accuracy of the second light 31 from decreasing due to the green light noise. Further, for example, in a case where the noise suppression unit includes the green light noise generation suppression unit 12b, the generation of the green light noise is suppressed by the green light noise generation suppression unit 12b, and therefore, the quantity of the green light noise can be reduced. Consequently, an increase in the ratio of the green light noise to the second light 31 can be suppressed.

Further, in this embodiment, as described above, the green light noise includes at least one of the green noise light 50a which is the green light generated due to the first light 30 and the green light noise signal 50b in which the green light noise is converted to an electric signal by the first light receiver 11. The green light noise removal unit 12a includes at least one of the green light noise signal removal circuit 120 for electrically removing the green light noise signal 50b and the green noise light removal filter 121 for optically removing the green noise light 50a.

With this configuration, for example, in a case where the green light noise includes the green light noise signal 50b, and the green light noise removal unit 12a includes the green light noise signal removal circuit 120, even in a case where the light received by the first light receiver 11 includes the green noise light 50a, the green light noise signal 50b is removed by the green light noise signal removal circuit 120. Therefore, it is possible to suppress the decrease in the communication accuracy of the second light 31 due to the green light noise signal 50b.

Further, for example, in a case where the green light noise includes the green noise light 50a, and the green light noise removal unit 12a includes the green noise light removal filter 121, it is possible to prevent the green noise light 50a from entering the first light receiver 11. Consequently, the quantity of the green light noise caused by the green noise light 50a can be reduced, and therefore, an increase in the ratio of the green light noise to the second light 31 can be suppressed.

Further, in this embodiment, as described above, the green noise light 50a is at least one of the first green noise light 51a which is green Raman-scattered light generated by Raman scattering in water and the second green noise light 51b which is green light included in the first light 30. With this configuration, it is possible to provide an underwater optical wireless communication device (first communication device 1) capable of suppressing a decrease in the communication accuracy of the second light 31 due to at least one of the first green noise light 51a and the second green noise light 51b causing green light noise.

Further, in this embodiment, as described above, the green light noise signal removal circuit 120 is configured to remove the green light noise signal 50b based on the first green noise light 51a included in the light-receiving signal 33 in the first light receiver 11, based on the driving signal 32 when the first light emitter 10 emits the first light 30.

With this configuration, even in a case where the green light noise signal 50b is included in the light-receiving signal 33, the green light noise signal 50b is removed by the green light noise signal removal circuit. Therefore, even in a case where the first green noise light 51a enters the first light receiver 11, it is possible to suppress a decrease in the communication accuracy of the second light 31 due to the green light noise signal 50b based on the first green noise light 51a.

Further, in this embodiment, as described above, the green light noise signal removal circuit 120 is configured to, at the time of calibration in a state in which the first light receiver 11 is not receiving the second light 31, acquire the compensation signal 34 which is an electric signal of light received the first light receiver 11 when the first light emitter 10 emits the first light 30 and the time (delayed time 35) required from the time when the driving signal 32 is output to the time when the compensation signal 34 is input, and remove the green light noise signal 50b by the delayed compensation signal 34 by the time required for the compensation signal 34 is input.

With this configuration, it is possible to easily remove the green light noise signal 50b by the compensation signal 34 acquired at the time of calibration and the delay time 35 when the first communication device 1 and the second communication device 2 communicate with each other.

Further, as described above, in this embodiment, it is further provided with the first optical path forming member 18a for holding the first light emitter 10 and forming the first optical path which is an optical path for the first light 30. The green noise light removal filter 121 is arranged at the post-stage of the first light emitter 10 in the first optical path forming member 18a and is configured to selectively transmit the first light 30 without transmitting the second the green noise light 51b.

With this configuration, it is possible to suppress the second green noise light 51b from being emitted from the first optical path forming member 18a. Therefore, it is possible to prevent the second green noise light 51b from being emitted (or scattered) in the water or in the first communication device 1 and being emitted onto the second light receiver 21. As a consequence, the quantity of green light noise caused by the second green noise light 51b can be reduced, and therefore, an increase in the ratio of the green light to the second light 31 can be suppressed.

Further, in this embodiment, as described above, it is further provided with the absorptive green transmission filter 15 arranged at the pre-stage of the first light receiver 11 and configured to selectively transmit the light including the light of the wavelength band of the second light 31 and not including the light of a predetermined wavelength band of the first light 30 by absorbing the light of the wavelength band of the first light, and the second optical path forming member 122 configured to hold the first light receiver 11 and form the second light path which is an optical path for the second light 31. The green noise light 50a is at least one of the third green noise light 51c which is green fluorescence emitted from the resin member 3 when the first light 30 is emitted to the resin member 3 and the fourth green noise light 51d which is green fluorescence emitted from the resin member 3 when the first light 30 is emitted to the absorptive green transmission filter 15.

With this, it is possible to provide an underwater optical wireless communication device (first communication device 1) capable of suppressing an increase in the ratio of green light noise to the second light 31 due to at least one of the third green noise light 51c and the fourth green noise light 51d causing green light noise.

Further, in this embodiment, as described above, the green light noise generation suppression unit 12b includes the third green light noise generation suppression unit that suppresses the generation of the third green noise light 51c due to the first light 30 emitted to the second optical path forming member 122.

Here, in a case where the second optical path forming member 122 does not include the third green light noise generation suppression unit, such as formed by a resin member 3, it emits the third green noise light 51c that is green fluorescence when being irradiated with the first light 30. Therefore, the third green noise light 51c emitted from the resin material to the first light receiver 11 becomes noise, which increases the ratio of the green light noise to the second light 31.

Therefore, as described above, since the second optical path forming member 122 includes the third green noise light suppressing unit, even when the second optical path forming member 122 is irradiated with the first light 30, it is possible to suppress the generation of the third green noise light 51c from the second optical path forming member 122. Consequently, it is possible to suppress the generation of the third green noise light 51c that becomes noise from the second optical path forming member 122, and therefore, it is possible to suppress an increase in the green light noise to the second light 31.

Further, in this embodiment, as described above, the third green light noise generation suppression unit is the second optical path forming member 122 formed of a metal material. Here, a metal material is less likely to generate green fluorescence even when irradiated with the first light 30 that is blue light. Therefore, as described above, by forming the second optical path forming member 122 by a metal material, it is possible to easily suppress the generation of fluorescence from the second optical path forming member 122 even when the second optical path forming member 122 is irradiated with the first light 30.

In this embodiment, as described above, the green light noise generation suppression unit 12b includes the fourth green light noise generation suppression member that suppresses the generation of the fourth green noise light 51d due to the first light 30 emitted to the absorptive green transmission filter 15. With this, since fourth the green noise light generation suppressing member suppresses the generation of the fourth green noise light 51d from the absorptive green transmission filter 15, it is possible to suppress an increase in the ratio of the green light noise to the second light 31 due to the fourth green noise light 51d.

Further, in this embodiment, as described above, the absorptive green transmission filter 15 is arranged at the pre-stage of the first light receiver 11 and is configured to absorb the light of a wavelength shorter than that of the second wavelength 41 in the second optical path forming member 122. The fourth green light noise generation suppression member is the reflective green transmission filter 123 arranged at the pre-stage of the absorptive green transmission filter 15 in the second optical path forming member 122 and configured to selectively transmit the light of a predetermined wavelength band not including the wavelength band of the first light 30 by reflecting the light of the wavelength band of the first light 30.

As a result, since the reflective green transmission filter 123 is arranged at the pre-stage of the absorptive green transmission filter 15, it is possible to prevent the absorptive green transmission filter 15 from being irradiated with the light of the wavelength band of the first light 30. Consequently, it is possible to easily suppress the generation of the fourth green noise light 51d from the absorptive green transmission filter 15.

Further, as described above, in this embodiment, it is further provided with a pair of reflective green transmission filter holding members 124 each formed in a U-shape in cross-section and configured to hold the reflective green transmission filter 123 by sandwiching the reflective green transmission filter 123. The pair of reflective green transmission filter holding members 124 is each configured such that the region 70 overlapping each other when viewed from the side surface 123a of the reflective green transmission filter 123 can be formed.

Thus, for example, even in the case of fixing the pair of reflective green transmission filter holding members 124 with the adhesive 126 applied to the region 70 when holding the reflective green transmission filter 123 by the pair of reflective green transmission filter holding members 124, it is possible to prevent the adhesive 126 from being irradiated with the first light 30. Consequently, it is possible to prevent the generation of green fluorescence due to the adhesive 126 for fixing the pair of reflective green transmission filter holding members 124.

Modified Embodiments

It should be understood that the embodiments disclosed here are examples in all respects and are not restrictive. The scope of the present invention is shown by the scope of the claims rather than the descriptions of the embodiments described above, and includes all changes (modifications) within the meaning of equivalent and the scope of claims.

For example, in the above-described embodiment, an example is shown in which the noise suppression unit includes both the green light noise removal unit 12a and the green light noise generation suppression unit 12b, but the present invention is not limited thereto. For example, it is enough that the noise suppression unit includes either the green light noise removal unit 12a or the green light noise generation suppression unit 12b. However, in a case where the noise suppression unit is configured to include only one of the green light noise removal unit 12a and the green light noise generation suppression unit 12b, the communication accuracy of the second light 31 is reduced due to the green light noise as compared with the case in which the noise suppression unit includes both the green light noise removal unit 12a and the green light noise generation suppression unit 12b. Accordingly, the noise suppression unit preferably includes both the green light noise removal unit 12a and the green light noise generation suppression unit 12b.

Further, in the above-described embodiment, an example is shown in which the green light noise includes both the green noise light 50a and the green light noise signal 50b, and the green light noise removal unit 12a is provided with both the green light noise signal removal circuit 120 and the green noise light removal filter 121, but the present invention is not limited thereto. The green light noise removal unit 12a may not be provided with both the green light noise signal removal circuit 120 and the green noise light removal filter 121. When the green light noise is only the green noise light 50a, it is enough that the green light noise removal unit 12a is provided with the green noise light removal filter 121. Further, in a case where the green light noise includes the green light noise signal 50b, it is enough that the green light noise removal unit 12a is provided with the green light noise signal removal circuit 120.

Further, in the above-described embodiment, an example is shown in which the green noise light 50a includes both the first green noise light 51a and the second green noise light 51b, but the present invention is not limited thereto. As the green noise light 50a, only the first green noise light 51a may be included. Also, as the green noise light 50a, only the second green noise light 51b may be included. In a case where the green noise light 50a includes only the first green noise light 51a, the green light noise removal unit 12a may include the green light noise signal removal circuit 120. In a case where the green noise light 50a includes only the second green noise light 51b, it is enough that the green light noise removal unit 12a includes the green noise light removal filter 121.

In the above-described embodiment, an example is shown in which the green light noise signal removal circuit 120 removes the green light noise signal 50b from the light-receiving signal 33, based on the driving signal 32, but the present invention is not limited thereto. As long as the green light noise signal 50b can be removed from the light-receiving signal 33, the green light noise signal removal circuit 120 may be configured in any manner.

Further, in the above-described embodiment, an example is shown in which the green noise light