WO2012086283A1

WO2012086283A1 - Modulator, transmitter and communication system

Info

Publication number: WO2012086283A1
Application number: PCT/JP2011/072196
Authority: WO
Inventors: 顕次柴田; 功今岡; 上山　智
Original assignee: 株式会社豊田自動織機; 学校法人名城大学
Priority date: 2010-12-24
Filing date: 2011-09-28
Publication date: 2012-06-28
Also published as: JP2012137541A

Abstract

Disclosed are a modulator, a transmitter and a communication system which improve the total data transfer rate. An AWG (13) disperses white light (L) emitted from a white LED (11) into multiple spectral components (La-Ld) each having different wavelengths. The modulator (12) is provided with multiple modulation means (14a-14d). A modulation means (14a) generates a changing electric field in the material of a moth eye structure layer (142) by means of applying a voltage to a first electrode layer (141) and a second electrode layer (143), and changes the refractive index of said material to modulate the spectral component (La). The other modulation means (14b-14d) modulate the spectral components (Lb-Ld) in the same way. The modulation means (14a-14d) modulate the spectral components (La-Ld) on the basis of different signals.

Description

Modulator, transmitter and communication system

The present invention relates to a modulator, a transmitter, and a communication system.

In recent years, white LEDs using a combination of a short wavelength LED element made of a nitride semiconductor and a phosphor have been put into practical use, and the white LED is expected as a light source for future illumination. Unlike incandescent bulbs and fluorescent lamps, which are conventional illumination light sources, white LEDs can modulate light at a relatively high speed, and are expected to be applied to visible light communication utilizing this feature. Research projects aiming to build information communication networks in offices and homes such as the Internet through visible light communication of 1 to 10 Mbps from lighting devices are being actively promoted in Japan and overseas.

Also, as this type of visible light communication device, a device that drives a blue light excitation type white LED based on a drive current signal generated based on transmission data and outputs a visible light signal to a receiver is proposed. ing. In Patent Document 1, this visible light communication device generates a multi-tone drive current signal by adding a rising pulse at the rising edge of transmission data and adding a falling pulse at the falling edge of transmission data. It is described that a multi-tone drive means is provided.

JP 2010-103979 A

However, the current broadband communication using optical fibers and the like has reached 1000 Mbps, and compared with this, the transfer speed in the configuration using the light of the conventional LED or incandescent bulb is not sufficient.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a modulator, a transmitter, and a communication system capable of improving the total data transfer rate. In this specification, “total transfer rate” refers to the overall data transfer rate considering both the transfer rate by a single transmitter and the capacity to operate multiple transmitters in parallel. means.

In order to solve the above-described problems, a modulator according to the present invention includes a spectroscopic unit that splits light including a plurality of wavelengths into a plurality of lights having different wavelengths, and a plurality of modulation units. Each includes a structure including a material having concave or convex portions formed periodically on the surface, and an electric field adjusting unit that generates an electric field that changes in the material and changes the refractive index of the material, and includes a modulation unit. Each modulates one of the plurality of light by a change in refractive index.

According to such a configuration, the electric field adjusting unit changes the refractive index to change the light amount at the same speed as the change in the refractive index. In addition, the electric field adjustment unit of each modulation unit modulates different spectra in parallel.

In the plurality of modulation units, the period in which the concave portion or the convex portion is formed may be different.
In each of the plurality of modulation means, the period in which the concave portion or the convex portion is formed may be longer than the optical wavelength of the light modulated by the modulation means and smaller than the coherent length of the light modulated by the modulation means. .

In addition, a transmitter according to the present invention includes a light source that emits light including a plurality of wavelengths, and the modulator described above.
In addition, a communication system according to the present invention includes the above-described transmitter and a receiver having light detection means for detecting a plurality of lights.

The modulator, transmitter, and communication system of the present invention can improve the total data transfer rate by improving the data transfer rate and transfer capacity.

It is a schematic diagram which shows the outline of a structure of the communication system which concerns on this invention. It is a schematic diagram which shows the outline of a structure of the modulation | alteration means of FIG. It is explanatory drawing which shows the diffraction effect of the light in the interface of a different refractive index, (a) shows the state reflected in an interface, (b) shows the state which passes an interface. It is explanatory drawing which shows the diffraction effect of the light which injects into the moth-eye structure layer of FIG. It is a schematic diagram which shows the outline of a structure of the receiver of FIG. FIG. 10 is a diagram showing changes in signal contents in the second embodiment. It is a schematic diagram which shows the modification of the modulation | alteration means of FIG. It is a schematic diagram which shows the modification of the modulation | alteration means of FIG.

Embodiment 1 FIG.
FIG. 1 is a schematic diagram showing an outline of a configuration of a communication system according to the present invention. The communication system according to the present invention includes a transmitter 10 and a receiver 20. A DC power supply 30 is connected to the transmitter 10, and the DC power supply 30 supplies power to the transmitter 10.
The transmitter 10 includes a white LED 11 as a light source. The white LED 11 emits, for example, white light L as light including a plurality of wavelengths. The transmitter 10 also includes a modulator 12. The modulator 12 modulates the light emitted from the white LED 11 based on an input signal (described later with reference to FIGS. 2 to 4).

The modulator 12 includes an AWG (arrayed waveguide grating) 13 as spectroscopic means. The AWG 13 splits the white light L emitted from the white LED 11 into a plurality of lights having different wavelengths. In the example of FIG. 1, the light is split into four light beams, La to Ld. In this example, it is assumed that the spectral wavelength is the shortest in the spectral La, and becomes longer in the order of the spectral Lb, Lc, and Ld. In the present embodiment, the number of spectra is four for convenience of explanation, but it is theoretically possible to split up to 400.

Further, the modulator 12 includes a plurality of modulation means 14. Each of the modulation means 14 modulates this corresponding to any one of the spectra La to Ld. In FIG. 1, the modulators 14a to 14d are in order from those corresponding to the short wavelength spectrum. The light beams La to Ld modulated by the modulation means 14a to 14d are referred to as signal lights La 'to Ld', respectively.

FIG. 2 is a schematic diagram showing an outline of the configuration of the modulation means 14a. The modulation means 14a includes a first electrode layer 141, a moth-eye structure layer 142 as a structure, and a second electrode layer 143. These layers are laminated in order of the second electrode layer 143, the moth-eye structure layer 142, and the first electrode layer 141 from the side on which the spectral La is incident toward the side on which the signal light La 'is emitted.

Also, a signal input power source 145 is connected to the modulation means 14a. One pole of the signal input power source 145 is connected to the first electrode layer 141, and the other pole is connected to the second electrode layer 143. The signal input power source 145 controls the voltage applied between the first electrode layer 141 and the second electrode layer 143 based on a signal to be transmitted.

The first electrode layer 141 is an n ⁺ -AlGaN layer having an AlN molar fraction of 0.2 or less. The first electrode layer 141 may be made of other materials as long as the sheet resistance is smaller than that of the moth-eye structure layer 142.

The moth-eye structure layer 142 is formed on the first electrode layer 141, and convex portions 144 protruding to the second electrode layer 143 side are periodically formed. In the present embodiment, each convex portion 144 is formed in alignment with the intersection of the virtual triangular lattice at a predetermined cycle so that the center is the position of the apex of the regular triangle in plan view. The period of each convex part 144 is larger than the optical wavelength of the spectral La and smaller than the coherent length of the spectral La. Here, the period refers to the distance between the peak positions of the depths of the adjacent convex portions 144. The optical wavelength means a value obtained by dividing the actual wavelength by the refractive index. Furthermore, the coherent length corresponds to a distance until the periodic vibrations of the waves cancel each other and the coherence disappears due to the difference in the individual wavelengths of the photon group having a predetermined spectral width. The coherent length lc is approximately lc = (λ ² / Δλ), where λ is the wavelength of light and Δλ is the half width of the light. Here, the period of each convex part 144 is preferably larger than twice the optical wavelength of the spectrum La. Moreover, it is preferable that the period of each convex part 144 is below half of the coherent length of spectroscopy La.

In the present embodiment, the period of each convex portion 144 is 500 nm. If the wavelength of the spectral La is 450 nm, for example, the refractive index of the group III nitride semiconductor layer is 2.4, so the optical wavelength is 187.5 nm. If the half width of the spectrum La is 63 nm, the coherent length is 3214 nm. That is, the period of each convex part 144 is larger than twice the optical wavelength of the spectrum La and is equal to or less than half of the coherent length.

Further, in the present embodiment, each convex portion 144 is formed in a conical shape whose diameter is reduced toward the tip. Specifically, each convex portion 144 has a base end portion with a diameter of 200 nm and a depth of 250 nm. The tip of each convex portion 144 may have a sharp shape (for example, FIG. 3) or may have a non-pointed shape (for example, a flat shape as in FIG. 4). Further, the surface of the moth-eye structure layer 142 is flat except for the convex portions 144.

The moth-eye structure layer 142 as the structure is preferably made of a material having a large Pockels effect. Specifically, it is preferable to have piezoelectricity, and more preferably an isotropic crystal. The moth-eye structure layer 142 can be formed of, for example, SiO ₂ , AlN, LiNbO ₃ , ZnO, ITO, or the like, and is formed of AlN in this embodiment. The shape of each convex portion 144 can be a cone, a polygonal pyramid, or the like. In the present embodiment, a light diffraction effect can be obtained by the convex portions 144 arranged periodically.

The second electrode layer 143 is formed along the surface of the moth-eye structure layer 142. In the present embodiment, the second electrode layer 143 is composed of an n ⁺ -AIGaN layer having an AlN molar fraction of 0.2 or less. The second electrode layer 143 may be made of other materials as long as the sheet resistance is smaller than that of the moth-eye structure layer 142.

Here, as shown in FIG. 3A, when light is reflected at the interface of the moth-eye structure layer 142 from the Bragg diffraction condition, the condition that the reflection angle θ _ref should satisfy with respect to the incident angle θ _in is ,
d · n1 · (sin θ _in −sin θ _ref ) = m · λ (1)
It is. Here, n1 is the refractive index of the medium on the incident side, λ is the wavelength of the incident light, and m is an integer. In the present embodiment, n1 is the refractive index of air.

On the other hand, as shown in FIG. 3B, from the Bragg diffraction condition, when light is transmitted at the interface of the moth-eye structure layer 142, the condition that the transmission angle θ _out should satisfy with respect to the incident angle θ _in is:
d · (n1 · sin θ _in −n2 · sin θ _out ) = m ′ · λ (2)
It is. Here, n2 is the refractive index of the medium on the exit side, and m ′ is an integer. In the present embodiment, n2 is the refractive index of the moth-eye structure layer 142. That is, when the refractive index of the moth-eye structure layer 142 changes, the light transmission condition changes.

In order for the reflection angle θ _ref and the transmission angle θ _out that satisfy the diffraction conditions of the above equations (1) and (2) to exist, the period of each convex portion 144 is the optical wavelength inside the element (λ / n 1 ) Or (λ / n2). The generally known moth-eye structure has a period set smaller than (λ / n1) or (λ / n2), and there is no diffracted light. And the period of each convex part 144 must be smaller than the coherent length which light can maintain the property as a wave, and it is preferable to set it as the half or less of coherent length. By setting it to half or less of the coherent length, the intensity of reflected light and transmitted light by diffraction can be secured.

As shown in FIG. 4, light incident on the moth-eye structure layer 142 at an incident angle θ _in is reflected at a reflection angle θ _ref that satisfies the above equation (1), and at a transmission angle θ _out that satisfies the above equation (2). To Penetrate. Here, when the incident angle θ _in is greater than the total reflection critical angle, the intensity of the reflected light is high.
3 and 4, θ _out > θ _in , but the magnitude relationship varies depending on the refractive indexes n1 and n2, as is apparent from the above equation (2).

When a voltage is applied to the first electrode layer 141 and the second electrode layer 143 to generate an electric field in the moth-eye structure layer 142, the refractive index of the material of the moth-eye structure layer 142 changes. In this embodiment, since the moth-eye structure layer 142 has piezoelectricity, the refractive index can be changed efficiently using the Pockels effect. When the refractive index of the moth-eye structure layer 142 changes, n2 in the above equation (2) changes and the diffraction transmission condition changes. Thereby, the light quantity of the signal light La ′ can be changed by changing the potential of the first electrode layer 141.
That is, the first electrode layer 141 functions as a refractive index modulation electrode. In addition, the first electrode layer 141 and the second electrode layer 143 function as an electric field adjustment unit that changes the refractive index of the material of the moth-eye structure layer 142.

Therefore, the light can be modulated in accordance with the electric field generated in the moth-eye structure layer 142, and the modulation speed can be greatly improved as compared with the conventional method in which the on / off of light emission by the white LED 11 is controlled and modulated. it can.

In addition, since the moth-eye structure layer 142 is sandwiched between a pair of layers having a smaller sheet resistance than the moth-eye structure layer 142, an electric field can be reliably generated in the moth-eye structure layer 142.

2 to 4 show only the configuration of the modulation means 14a, the configuration of the modulation means 14b to 14b is the same. However, as described above, since the modulation means 14a to 14d modulate the spectra La to Ld having different wavelengths, the dimensions depending on the wavelength in the configuration are different. A person skilled in the art can design other modulation means 14b to 14d by making necessary changes in accordance with the wavelength of light to be modulated based on the description of the configuration of the modulation means 14a. For example, the period at which the convex portion of the moth-eye structure layer is formed in the modulating unit 14b is not 500 nm as in the modulating unit 14a, but is determined to be an optimal value based on the optical wavelength, coherent length, etc. of the spectral Lb. For example, as schematically shown in FIG. 1, the period at which the convex portions of the moth-eye structure layer are formed differs from one modulation means to another, the modulation means 14a is the smallest, and thereafter increases in the order of the modulation means 14b, 14c, 14d.
In this way, the modulation means 14a to 14d modulate one of the plurality of spectral lines La to Lb by changing the refractive index of the material of the moth-eye structure layer included therein.

The modulation means 14a to 14d modulate the corresponding spectrum based on different signals. That is, different information can be given to each wavelength of light. Since the modulation means 14a to 14d can operate simultaneously, the spectrums La to Ld can be modulated and transmitted in parallel based on the four types of signals, and the signal transfer capacity can be increased.

FIG. 5 is a schematic diagram showing an outline of the configuration of the receiver 20. The receiver 20 includes a plurality of optical filters 21 and a plurality of light detection means 22. The optical filter 21 and the light detection means 22 are provided corresponding to the signal lights La ′ to Ld ′. For example, the optical filter 21a transmits light having a wavelength corresponding to the signal light La ', and the light detection unit 22a detects light having a wavelength corresponding to the signal light La'.

The light detection means 22a to 22d can be configured by a phototransistor, a photodiode, a photo IC, or the like having an operation speed that can correspond to the modulation speed of the modulation means 14. For example, in a CCD camera, an optical filter corresponding to a different wavelength may be provided for each light receiving element (or each group of light receiving elements). The optical filters 21a to 21d may be arranged in the same chip as the light detection means 22a to 22d, or may be arranged outside the chip.

In this way, the receiver 20 separates and detects the signal lights La ′ to Ld ′ having different wavelengths based on the wavelengths, so that even if the signal lights La ′ to Ld ′ are spatially overlapped, they are detected. Can be received separately. That is, even when the distance between the transmitter 10 and the receiver 20 is large and the modulation means 14 cannot be individually recognized with the resolution (spatial resolution) of the receiver 20, the signal lights La ′ to Ld ′ are spectrally reflected. Can be received separately.

As described above, according to the transmitter 10 and the receiver 20 according to the present embodiment, high-speed modulation can be performed using the electric field generated in the moth-eye structure layer 142, so that the data transfer rate is improved. Can do. In addition, since a signal is modulated and transmitted in parallel using a plurality of wavelengths, the data transfer capacity can be improved. Therefore, the total transfer rate in visible light communication can be improved by improving the transfer rate and transfer capacity.

Moreover, since the white LED 11 is used as a light source, the light source for indoor illumination can be diverted or shared for information transfer.

Embodiment 2. FIG.
In the second embodiment, a part of the signal light La ′ to Ld ′ is a dummy in the first embodiment, and the signal light corresponding to the actual signal is changed with time. Such an embodiment is useful, for example, in cryptographic communications.

FIG. 6 shows changes in signal contents in the communication system according to the second embodiment.
In period 1, modulation means 14a and 14b perform modulation based on an actual signal to be transmitted, and modulation means 14c and 14d perform modulation based on a dummy signal not related to the actual signal. That is, the signal lights La ′ and Lb ′ represent actual signals, and the signal lights Lc ′ and Ld ′ represent dummy signals. In FIG. 6, real signals are represented by solid lines, and dummy signals are represented by broken lines.

Next, it is assumed that time 1 has elapsed and period 1 has ended. When the period is switched (in this example, period 2 is started), a part or all of the modulation means using the real signal and the modulation means using the dummy signal are switched. In the example of the period 2 in FIG. 6, unlike the period 1, the signal lights La ′ and Ld ′ represent actual signals, and the signal lights Lb ′ and Lc ′ represent dummy signals.
Similarly, when the period 2 ends, in the subsequent period 3, the signal lights Lb ′ and Lc ′ represent actual signals, and the signal lights La ′ and Ld ′ represent dummy signals.

Although not shown, in the second embodiment, information indicating which of the wavelengths represents a real signal and which of the wavelengths represents a dummy signal is obtained from the receiving device (for example, a receiver or a demodulator) by any method. Communicated or stored in advance. Therefore, on the receiving side, only those representing the actual signal among the wavelengths can be selected, and the original signal can be restored using these.

Thus, according to the communication system according to the second embodiment, the frequency of the signal light representing the actual signal varies with time. For this reason, even if the signal lights La ′ to Ld ′ are intercepted or eavesdropped, it is difficult to restore the original signal if any of them is concealed as to whether it is a real signal or which is a dummy signal. . Therefore, the confidentiality of communication can be improved. In FIG. 6, the number of modulation means is four, but 400 modulation means can actually be provided as described above, and a highly confidential communication method can be realized.

In the second embodiment described above, the switching method between the real signal and the dummy signal can be appropriately designed using a well-known encryption communication protocol. In Embodiment 2, there are two modulation means corresponding to the actual signal in each period, but this may be a different number, or there may be a period in which all the modulation means transmit the actual signal. There may be a period in which all modulation means transmit dummy signals.

In the first and second embodiments, the moth-eye structure layer 142 is formed of AlN. For example, as shown in FIG. 7, n-type GaN layers 142b and AlN layers 142a are alternately stacked. May be formed. In this case, since the electric field generated in each AlN layer 142a becomes strong, the refractive index change of the moth-eye structure layer 142 can be increased. For the modulation of light, it is preferable to change the refractive index of the moth-eye structure layer 142 using the Pockels effect. However, although the amount of change in the refractive index is smaller than that of the Pockels effect, the refractive index is changed using the Kerr effect. You can also.

In Embodiments 1 and 2, as shown in FIG. 2, the convex portion 144 of the moth-eye structure layer 142 is convex toward the incident side, but this may be formed in the opposite direction. An example of such a configuration is shown in FIG. The modulation means 24 in FIG. 8 includes a first electrode layer 241, a moth-eye structure layer 242 as a structure, and a second electrode layer 243. These layers are laminated in the order of the first electrode layer 241, the moth-eye structure layer 242, and the second electrode layer 243 from the side on which the spectrum L2 is incident to the side on which the signal light L2 'is emitted. Here, the convex portion 244 of the moth-eye structure layer 242 is formed in a direction that is convex toward the side from which the signal light L2 'is emitted. Further, a signal input power source 245 is connected to the modulation means 24.

In the first and second embodiments, the moth-eye structure layer 142 is periodically formed with the convex portions 144, but the concave portions may be periodically formed. Further, for example, the convex portion or the concave portion may be formed in a prismatic shape and aligned with intersections of a virtual square lattice at a predetermined period. Furthermore, the convex portion or the concave portion may be a polygonal pyramid shape that is reduced in diameter toward the tip, such as a triangular pyramid shape or a quadrangular pyramid shape, and it is needless to say that a specific detailed structure and the like can be appropriately changed. .

In Embodiments 1 and 2, the modulation means 14a to 14d perform modulation based on different information, but a plurality of modulation means may perform modulation based on the same information. In this case, the redundancy of transmitted information can be improved and transmission errors can be suppressed.

In the first and second embodiments, the white LED 11 is used as the light source, but other configurations may be used as long as the light source emits light including a plurality of wavelengths. For example, a monochromatic LED having a spectral width that can be dispersed may be used. An incandescent bulb or a fluorescent lamp may be used. These may be diverted or used as a light source for indoor lighting.

In the first and second embodiments, the medium that meshes with the convex portion 144 of the moth-eye structure layer 142 is air, but this may not be air, and a layer of another medium may be provided. For example, you may seal the LED chip which comprises white LED in a dome shape with an epoxy resin. In this case, the refractive index n1 of the incident side medium in FIG. 3 is the refractive index of the epoxy resin.

Claims

A spectroscopic means for splitting light including a plurality of wavelengths into a plurality of lights having different wavelengths;
A plurality of modulation means,
Each of the modulation means includes
A structure including a material in which concave portions or convex portions are periodically formed on the surface;
An electric field adjusting unit that generates an electric field that changes in the material and changes a refractive index of the material, and
Each of the modulation means modulates one of the plurality of lights by changing the refractive index.
The modulator according to claim 1, wherein in the plurality of modulation means, the period in which the concave portion or the convex portion is formed is different.
The period in which the concave portion or the convex portion is formed in each of the plurality of modulation means is larger than an optical wavelength of light modulated by the modulation means and smaller than a coherent length of light modulated by the modulation means. Modulator.
A light source that emits the light comprising a plurality of wavelengths;
A transmitter comprising the modulator according to any one of claims 1 to 3.
A transmitter according to claim 4;
A communication system comprising: a receiver having light detecting means for detecting the plurality of lights.