JPWO2016088157A1 - Light receiving device for optical communication, optical communication module, and visible light communication system - Google Patents

Abstract

[Summary] [Problem] To provide a light receiving device for optical communication and the like capable of reducing adverse effects due to noise. [Solution] A light receiving device for optical communication according to the present invention is capable of transmitting and extracting an arbitrary light beam from incident light. It includes at least a light shielding part that shields light from the space between the space dividing part and the light receiving part.

Description

  The present invention relates to a light receiving device for optical communication, an optical communication module, and a visible light communication system, and in particular, light having good detection sensitivity so as to be able to be used by suppressing the influence of disturbance light and reducing received light signal noise even outdoors. The present invention relates to a light receiving device for communication, an optical communication module, and a visible light communication system.

With the spread of LED (Light Emitting Diode) lighting and the like, optical communication that performs optical communication using a light source such as an LED has been put into practical use.
In conventional spatial light communication represented by an infrared remote controller and an infrared LAN, an infrared LED or LD (Laser Diode) is used as a light source, and an infrared filter and a silicon photodiode are used as a light receiving unit.
In particular, in visible light communication, since visible light is used, there is a problem that sunlight or illumination light is mixed into a photodiode as a light receiving unit.
In addition, the received light power in optical communication is greatly attenuated in inverse proportion to the square of the distance to the light source, so that it has a large dynamic range characteristic to perform optical communication well at a distance of several tens of meters or more. There is a need for a light receiving device that has good detection sensitivity and that does not saturate the output even with a large amount of light.

Various methods for removing the influence of disturbance light, sunlight, illumination light and the like mixed in the light receiving device have been proposed. For example, the following methods are known.
(1) A method of using a filter corresponding to a specific light source frequency.
(2) A method in which the receiving device, the light receiving sensor, the light receiving lens, and the like are directed toward the light source.
(3) A method of narrowing the light receiving angle.
(4) A method of increasing detection sensitivity by operating an optical system such as a photoconductive fiber or a prism.

On the other hand, the applicant of the present invention has developed a method of using the space dividing device described in Patent Document 1 below as a method of selectively receiving only a light source to be communicated while minimizing the influence of these disturbance lights. .
This space division device is configured to capture only the light beam that is desired to be guided to the light receiver among a plurality of light sources in the space, and this device structure eliminates the influence of sunlight and disturbance light. The desired light source signal can be received.
This space dividing device has a structure in which an assembly of mirrors (DMD, Digital Micromirror Device) each capable of changing the reflection angle is provided in front of the light receiver to reflect and extract only a specific light source signal.

Japanese Patent No. 5265788

By using the space dividing device described in Patent Document 1, it has become possible to take out an arbitrary light beam with less influence of disturbance light. In this method, for example, among the following points, There is room for improvement in at least one of the points.
(1) In addition to a desired light beam, influences such as irregular reflection of a captured light beam and diffracted light may occur.
(2) The influence of leakage light or the like from an internal light source on the transmission side incorporated in the optical communication apparatus may occur.
(3) When using an optical communication device outdoors, the effects of natural phenomena (sunlight, rain, snow, wind, etc.) and light, temperature, humidity, etc. from the illumination light equipment near the optical communication device occur as noise. Cheap. In particular, since scattered light, reflected light, diffraction, and the like of sunlight cause a large influence as noise, it is necessary to minimize these influences.

  Therefore, at least one object of the present invention is to provide a light receiving device for optical communication, an optical communication module, and a visible light communication system that can reduce the adverse effects of noise.

The present invention, which has been made to solve the above-mentioned problems, includes a space dividing unit capable of transmitting an arbitrary ray from incident light and taking out the light, a light receiving unit that receives the light transmitted through the space dividing unit, and the space. Provided is a light receiving device for optical communication, comprising at least a light shielding portion that shields a space between a dividing portion and a light receiving portion.
In the present invention, the space dividing unit may be a liquid crystal in which a plurality of pixels are arranged on a plane and the transmittance can be controlled in units of pixels.
In the present invention, the light receiving section can be a light receiving sensor capable of detecting weak light with an input light quantity of 10 ⁻⁶ W or less.
Moreover, in the said invention, it can comprise so that the said space division part and a light-receiving part may be integrated by the said light-shielding part.

  Further, the present invention provides an optical communication module comprising at least a condensing lens and a light receiving device for optical communication described above, wherein the light shielding unit further shields a space between the condensing lens and the space dividing unit. Can also be provided.

  The present invention can also provide a visible light communication system including at least the light receiving device for optical communication or the optical communication module.

According to the present invention, at least one of the effects described below can be obtained.
(1) It is possible to provide a light receiving device or the like that is not affected by disturbance light, internal scattered light, or the like. Therefore, visible light communication with an improved communication distance is possible without being affected by sunlight or natural phenomena even outdoors.
(2) Since the influence of disturbance light, internal scattered light, etc. is small, a highly sensitive light receiving sensor can be used. Therefore, visible light communication with improved light receiving sensitivity is possible.

The schematic block diagram which shows an example of the visible light receiver of a prior art. The figure which shows the relationship between the electric current of a photodiode, and an output. The figure which shows the relationship between the output electric power of MPPC, and incident light quantity. The figure which shows the utilization range of each light receiving element. Schematic which shows the structure of the visible light communication system which concerns on this invention. The relationship diagram of incident light and noise concerning a prior art. The relationship diagram of incident light and noise concerning the present invention. The figure which shows the received signal waveform in this invention. The constellation figure of the QAM signal in this invention. The figure which shows the 1st structural example of the optical communication module which concerns on this invention. The figure which shows the 2nd structural example of the optical communication module which concerns on this invention.

[1] Conventional technology.
First, the prior art that led to the idea of the present invention will be described with reference to FIGS. 1A to 1D.
This “prior art” refers to an invention devised and studied by the applicant prior to the idea of the present invention, and is different from the prior art (known art).

[2] Overall configuration.
FIG. 1A shows a schematic diagram of a receiving device for visible light communication according to the prior art.
The receiving device has a structure in which the light beam collected through the objective condenser lens a reflects only a specific light source signal to the light receiving device d by the spatial division device c using DMD through the polarizing filter b. ing.

[3] Light receiver.
The light receiver d is provided with a light receiving sensor that receives a light source signal.
As the light receiving sensor, a photoelectric effect type is more suitable than a thermal effect type, and a photomultiplier tube (photomultiplier), a photoconductive cell, a Si cell using Si, Ge as a base, a photodiode such as a Ge cell (Si-PD). , Ge-PD), a phototransistor, or the like can be used.

[4] Type of photodiode.
As the photodiode (PD) used for the light receiving sensor, pn-PD, pin-PD, Si-PD (cooling), Si-APD (Avalanche Photo Diode), and the like can be considered.
On the other hand, as a light receiving sensor that can be used for visible light communication,
(I) excellent sensitivity in the visible light band (wavelength of about 380 to 780 nm);
(Ii) no saturation due to incident light including sunlight (large dynamic range);
(Iii) excellent response characteristics;
Etc. are required.
In consideration of outdoor use, it is limited to those that can withstand a wide range of temperature and humidity changes depending on the natural environment.
Furthermore, since visible light communication based on the present invention uses DC to about 250 MHz in frequency characteristics and is often used with modulation as a communication system, it is excellent in frequency characteristics with a junction capacitance as small as possible as a photodiode characteristic. Things are needed.
Therefore, pn-PD, pin-PD, and APD due to the photovoltaic effect can be considered as satisfying these conditions.

[4.1] When adopting pin-PD as a light receiver.
FIGS. 1B (a) and 1 (b) are diagrams showing the relationship between the output voltage and current of the pin-PD.
1B, the resistance of the PD when the light strikes the PD is reduced by a quantity, V = E in the dark, to detect maximum light during at V = E-RI _L, and the output voltage due to a change in light intensity.
When the sensitivity of this PD is increased, noise in the vicinity of darkness increases, and there are many problems in using it for visible light communication outdoors due to various problems such as the need to cool the element due to the influence of temperature and the like.

[4.2] When APD is adopted for the light receiver.
APD with high sensitivity and internal multiplication function in the device is much more sensitive than pin-PD and can detect faint light, but it requires high voltage back electromotive force (several tens of volts to 100 volts). There is a problem that the multiplication factor greatly changes depending on the temperature characteristics.

[4.3] When MPCC is adopted for the optical receiver.
There is an optical semiconductor element called MPPC (Multi-Pixel Photon Counter) or Si-PM (Silicon Photomultiplier) as a multi-pixel APD.
This element is used as a photon counter as an alternative to PMT (Photo Multiplier Tube). For this reason, it is said that the sensitivity to light is extremely high, and the number of incident photons can be measured to 102 or less (about 6 light stars).
However, since this element is intended for photon counters, it is suitable for measuring ultra-weak light (number of photons), but as a receiver for visible light communication, the element generates heat due to incidence of a large amount of light. In general, it is not considered to be a suitable element due to the characteristic that the multiplication factor decreases and becomes saturated.
The upper limit incident light amount of the dynamic range in which the possible linearity of this element can be obtained is about 10 ⁻¹⁵ to 10 ⁻⁷ W even for an element corresponding to a relatively high incident light amount. The incident light amount-output current characteristic of this element is as shown in FIG. 1C.

That is, the upper limit of the incident light amount in the MPPC element described above is saturated at about 10 ⁻⁷ W. For this reason, when visible light communication is performed outdoors, a light source signal cannot be detected due to the influence of slight noise light such as the influence of sunlight, disturbance light, other scattered light, or internal light.

Of course, a method of increasing the light source signal intensity with an optical system such as a laser beam or a lens is also conceivable. However, in visible light communication, visible light near λ = 470 nm, which is close to blue, is often used, and the sensitivity of the MPPC element is high. Has a peak at λ = 400 to 500 nm, and is considered suitable for use outside the saturation region as a receiver for visible light communication.
In addition, in order to secure a communication distance as long as possible, it is necessary to accurately capture weak light.

[4.4] Viewpoint from characteristics of incident light region.
FIG. 1D shows an incident light region covered by a commercially available photodiode used for a photo counter or the like.
Si-PD covers about 10 ⁻⁶ to 10 ⁻¹⁴ W, Si-APD covers about 10 ⁻⁶ to 10 ⁻¹⁶ W, and MPPC covers about 10 ⁻⁸ to 10 ⁻¹⁸ W. doing.
That is, when Si-PD or Si-APD is used as the light receiving sensor, it is necessary to limit the use to 10 ⁻⁶ W or less, and when MPPC is used as the light receiving sensor, it is necessary to use it at 10 ⁻⁸ W or less. is there.
In other words, the upper limit incident light amount for practical use of these light receivers is 10 ⁻⁶ W or less, and this upper limit incident light amount corresponds to the brightness at full moon under room lighting, and must be kept below this level. It will not be available.

In order to use these elements for visible light communication, a method of greatly reducing the amount of incident light is conceivable.
However, if an attempt is made to use the portion that does not reach the saturation region by reducing the total amount of light incident on the visible light receiver, the incident signal ratio between the light source signal and noise light such as sunlight disturbance light or internal disturbance light (so-called S / N ratio) decreases, and it is considered that the light source signal cannot be extracted.

Therefore, in the present invention, the ratio of noise light and incident signal is greatly improved, and an ultra-weak light detection sensor element such as Si-PD, Si-APD, or MPPC, which is a photon counter element usually used in the ultra-weak light region, is provided. We propose a system configuration that can be used as a highly sensitive light receiver.
Hereinafter, embodiments of the present invention will be described with reference to FIG.

<1> Overall Configuration FIG. 2 shows an example of a visible light communication system according to the present invention.
The visible light communication system according to the present invention includes a light receiving device A for optical communication, a condenser lens 50, and a dichroic mirror (or prism) 60.
The optical signal from the light source X collected by the condenser lens 50 reaches the light receiving device A for optical communication through the dichroic mirror (or prism) 60 and the like along with disturbance light such as sunlight.
When the dichroic mirror 60 is also used as a transmission / reception device, the dichroic mirror 60 is used to send out transmission light source light. As a light receiving device, the dichroic mirror 60 is used to narrow down the entire light amount and monitor incident light.
Details of the light receiving device for optical communication will be described below.

<2> A light receiving device for optical communication.
The light receiving device A for optical communication includes a space dividing unit 10 capable of transmitting an arbitrary light beam from incident light, a light receiving unit 20 that receives the light beam transmitted through the space dividing unit 10, and the space dividing unit 10 A light shielding unit 30 that shields light from a space between the light receiving unit 20 and at least a light shielding unit 30;

<3> Space Dividing Unit The space dividing unit 10 is an apparatus for extracting an arbitrary ray from a plurality of incident rays. The arbitrary light beam includes both a single light beam and a plurality of light beams.
The space dividing unit 10 is composed of liquid crystal arranged in a plane with respect to the optical axis.
The liquid crystal is formed by arranging a plurality of pixels (pixels) on a plane, and the transmittance can be controlled in units of the pixels. More specifically, it has a function of transmitting or limiting light transmission for each pixel by a control signal from the X axis / Y axis of the liquid crystal.
Therefore, only an arbitrary light beam (light signal) from the light source X is allowed to pass, and other light beams are not allowed to pass as noise, and serves as a space division filter or a shutter configured for each pixel.

  As the space dividing unit 10, not only a low-temperature and high-temperature polysilicon liquid crystal but also a transmissive ceramic PZT (Piezoelectric Zirconate Titanate) element can be used as long as the space dividing filter has a shutter characteristic for each pixel. .

Note that the space dividing unit 10 does not always transmit light of only one pixel, but may also pass a plurality of pixels or pixels in the vicinity of one pixel.
That is, it can be configured to be able to simultaneously receive signals from a plurality of light sources due to the difference in wavelength of the light sources. When signals having a plurality of wavelengths are received, the wavelengths are separated by a spectroscopic prism or the like before the light receiving unit and received by the plurality of light receiving units. Further, when the incident light source light extends over a plurality of pixels, it is configured to pass pixels in the vicinity of one pixel.

<4> A light receiving unit.
The light receiving unit 20 is a member for receiving a light source signal.
The light receiving unit 20 includes a light receiving sensor, and includes pn-PD, pin-PD, PMT (photomultiplier), Si-PD (cooling), Si-APD, MPCC, etc., and particularly Si-PD, Si-APD or It would be beneficial if an ultra-weak light detection sensor element such as MPPC could be used.

<5> A light shielding portion.
The light shielding unit 30 is a member that shields at least the space between the space dividing unit 10 and the light receiving unit 20.
In this embodiment, the light shielding unit 30 has a structure in which a dark portion is formed by sealing around the space between the space dividing unit 10 and the light receiving unit 20.
Accordingly, the inside of the light shielding portion 30 is a dark space 40 and is in a state in which diffuse reflection and diffraction of light inside as well as intrusion sealing of external light noise is suppressed as much as possible.
The inner surface of the light shielding unit 30 is preferably black or provided with a light irregular reflection preventing agent in order to prevent irregular reflection of light.

<6> Function / action.
When DMD is used for the space dividing device c described in the prior art, the optical axis is controlled by a mirror, and the reflected light is separated by the angle to receive a signal from a predetermined light source. The influence of irregularly reflected light, diffracted light, or the like occurs before the received optical signal reaches the light receiving unit.
However, in the present invention, since the space between the space dividing unit 10 and the light receiving unit 20 is made a dark part by the light blocking unit 30, noise light is received before reaching the light receiving unit 20 after taking in the light beam (signal light). There is nothing to be generated, and it becomes possible to take in signal light with an extremely improved S / N ratio.

<7> Comparison of characteristics.
With reference to FIGS. 3A and 3B, the distribution of disturbance light such as sunlight, noise light due to the internal disturbance light, and the incident light quantity of the signal light will be described.
FIG. 3A shows a light receiving unit (MPPC) that can obtain the distribution of ambient light such as sunlight, noise light caused by internal light, and incident light quantity of signal light when using a conventional space division device c (DMD). It is the figure which showed the input-output characteristic of ().
In a receiving device using a conventional space division device c (DMD), noise light such as internal disturbance light is in the 10 ⁻⁸ to 10 ⁻⁶ W region, and there is a region where the influence of the noise light substantially overlaps with the signal light. Therefore, it is necessary to improve the S / N ratio between the signal light and the noise by another means (for example, increasing the intensity of the signal light).

3B shows the measurement results of the intensity distribution of the noise light and the signal light when the optical communication receiver A according to the present invention is used under the same environmental conditions of the signal light and the noise light in FIG. 3A. It is the figure shown on the input-output characteristic.
As shown in FIG. 3B, according to the optical communication receiver A according to the present invention, even if the entire incident light is narrowed down, the difference between the influence of sunlight and the absolute value of the light source signal is increased, and the light source signal and the disturbance light / An incident signal ratio (SN ratio) with noise light such as internal disturbance light is greatly increased.
This makes it possible to set the amount of light source signal incident light to 10 ⁻⁸ W or less, which is less than or equal to the MPPC use upper limit sensitivity.

<8> Application examples of a light receiving device for optical communication.
Next, an application example of the present invention to visible light communication will be described.
INDUSTRIAL APPLICABILITY The present invention can provide an apparatus / system that is not easily affected by sunlight or disturbance light such as outdoors in visible light communication.
In an example of visible light communication, when using LED illumination as a visible light communication system, the transmission side superimposes subcarriers using the LED illumination optical signal as a carrier, and the transmission signal is subjected to intensity modulation, frequency modulation, phase modulation, etc. Transmit using the modulation method. Communication systems that use LED lighting are not as coherent (phase) condensing as communication systems that use laser light. Compared with laser light, the light intensity is weaker and blurred. Therefore, in the light receiving device, the condensing efficiency can be increased by using a lens, and the communication distance can be improved by using a more sensitive light receiving unit.

In the embodiment of the present invention, visible light communication by QAM (Quadra Amplitude Modulator) quadrature amplitude modulation is performed using LED illumination as a transmission device, and a central signal light source at 10 ⁻¹² W can be captured by an ultrasensitive light receiving unit. It was.

  FIG. 4 shows received signal waveforms obtained by performing transmission / reception using the optical communication receiver A according to the present invention for visible light communication. The transmitted signal is an oscillograph of a received signal waveform when the signal is transmitted by QAM modulation using a light emitting element by an LED and using λ (wavelength) close to blue = 470 nm.

As shown in FIG. 4, light source signal light is received by being superimposed on noise light caused by diffused reflection of sunlight or disturbance light caused by ambient light.
In this state, the amount of incident light as a photon counter is detected at the photon level, so the number of photons incident on a light receiving cell of a light receiving unit such as Si-APD is superposed and measured.
In a visible light communication system, it is not necessary to measure the number of photons, and it is sufficient if a subcarrier signal modulated by light source intensity modulation or QAM modulation can be received accurately. It is possible with existing technology to cut the received optical signal at the threshold level and extract only the light source signal light.

FIG. 5 is a synchroscope diagram of a spatial diagram (constellation diagram) of the obtained QAM signal when visible light communication is performed by 64QAM using the present invention.
This figure shows a state in which the light source signal is reliably received using the visible light communication device according to the present invention.

As described above, in the optical communication system using the light receiving unit for optical communication according to the present invention, the difference between noise light such as disturbance light due to sunlight or ambient light and internal light of the system optical system and the light source signal light can be greatly increased. Therefore, it is possible to use a photon counter measuring element such as Si-APD or MPPC, which is an ultra-weak light detecting element having an incident light quantity characteristic of 10 ⁻⁶ W or less, which is considered to be difficult to apply, in the light receiving unit 20.
These weak light detection elements can be used as more sensitive elements by arranging them in an array or by arranging a plurality of them, and in the visible light communication device, the communication distance is greatly increased. It becomes possible.

  Further, in the present invention, since the S / N ratio between the signal light and the noise light is greatly improved, it has been proposed that the high-sensitivity detection sensor for ultra-weak light can be used as the light receiving unit 20. Not only high-sensitivity MPPC but also photomultiplier tubes (photomultipliers), photoconductive cells, Si cells based on Si and Ge, photodiodes such as Ge cells (Si-PD, Ge-PD, Si-APD), Even if it is not a high-sensitivity detection sensor for ultra-weak light such as a phototransistor, an even better visible light communication distance can be secured and a light receiving device with a simple configuration can be provided.

Next, an embodiment of an optical communication module including a light receiving device for optical communication will be described.
The optical communication module according to the present invention can be configured to include at least the light receiving device A for optical communication and the condenser lens 50 described above.
At this time, the light shielding unit 30 can further shield the space between the condenser lens 50 and the space dividing unit 10.
Hereinafter, each module example will be described.

<1> Module example (part 1).
FIG. 6A shows an example in which the condenser lens 50, the polarizing filter 70 according to the present invention, and the dichroic prism (half mirror) 80 are integrated into a module.
In FIG. 6A, the received light source signal light is guided to the space dividing unit 10 by the condenser lens 50, only the signal light is extracted by the space dividing unit 10 of the present invention, and the light source signal is extracted by the light receiving unit 20. Since this integrated module passes through the dark part until the incident light taken in by the condenser lens 50 reaches the light receiving part 20, the S / N ratio can be more reliably ensured.

<2> Module example (part 2).
FIG. 6B shows another example of the module.
In the module configuration, the polarizing filter 70 and the dichroic prism 80 described above do not necessarily have to be taken into the module.
For the polarizing filter 70, it is possible to adopt a form in which the polarizing filter 70 is disposed in front of the condenser lens 50 outside the module (on the light source side) or a form that is omitted.
The dichroic prism (half mirror) 80 is necessary for simultaneous transmission / reception and separation of incident signal light of a plurality of channels. However, the dichroic prism (half mirror) 80 is required not to be used as a transmission device or to separate signals of a plurality of channels. If not, it can be omitted or placed outside the module.

a Objective condenser lens b Polarizing filter c Spatial dividing device d Light receiver A Optical communication light receiving device 10 Spatial dividing portion 20 Light receiving portion 30 Light shielding portion 40 Dark space 50 Condensing lens 60 Dichroic mirror 70 Polarizing filter 80 Dichroic prism X Light source

Claims (6)

A space dividing unit capable of transmitting an arbitrary ray from incident light and extracting it;
A light receiving unit that receives light transmitted through the space dividing unit;
A light shielding portion that shields light from the space between the space dividing portion and the light receiving portion;
A light receiving device for optical communication. 2. The light receiving device for optical communication according to claim 1, wherein the space dividing unit is a liquid crystal in which a plurality of pixels are arranged on a plane and the transmittance can be controlled in units of pixels. The light receiving device for optical communication according to claim 1, wherein the light receiving unit is a light receiving sensor capable of detecting weak light having an input light amount of 10 ⁻⁶ W or less. 4. The light receiving device for optical communication according to claim 1, wherein the space dividing unit and the light receiving unit are integrated by the light blocking unit. 5. A condenser lens;
A light receiving device for optical communication according to any one of claims 1 to 4,
The optical communication module, wherein the light shielding unit further shields a space between the condenser lens and the space dividing unit. A visible light communication system comprising at least the light receiving device for optical communication according to any one of claims 1 to 4 or the optical communication module according to claim 5.