JPH08307352A - Optical space transmission device - Google Patents

Abstract

(57) [Abstract] [Purpose] To receive signals accurately by keeping the light intensity on the light receiving element of the optical space transmission device constant. A liquid crystal device 8 is provided between the deflecting beam splitter and the lens 4, and the incident light L2 is condensed on the light receiving element 7 with its light intensity reduced. The liquid crystal of the liquid crystal device 8 is a ferroelectric liquid crystal having excellent high-speed response, and further,
Fine particles having a diameter of about 300 nm are added to the ferroelectric liquid crystal so that the contrast continuously changes depending on the applied voltage. The liquid crystal device 8 drives the liquid crystal device 8 via the liquid crystal control circuit 23 by the control signal of the liquid crystal device 8 being formed by the output of the light receiving element 7 and the reference voltage V1 and constantly outputs the optical power focused on the light receiving element 7. Keep it constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical space transmission device using a light emitting element as a signal light source, and more particularly to an optical space transmission device having a constant average light receiving intensity on the light receiving element.

[0002]

2. Description of the Related Art First of all, an optical space transmission device is a transmission side that modulates transmission information into a change in the intensity of light, emits the modulated light into the atmosphere toward the reception side, and the reception side uses the transmission side. By receiving the modulated light emitted from the device and demodulating it, desired information is transmitted through the atmospheric space.

That is, as shown in FIG. 6, optical space transmission performed between one optical space transmission apparatus 50A and the other optical space transmission apparatus 50B is transmitted from one optical space transmission apparatus 50A (or 50B). The light modulated by the signal is emitted through the lens 3, and the other optical space transmission device 50B (or 50A)
It is realized by receiving the similarly modulated light from (1) through (3) through the lens 3.

Next, an example of an optical system used in the optical space transmission device will be described with reference to FIG.

First, the transmission system comprises a light emitting element 6 made of a semiconductor laser, a lens 1, a deflecting beam splitter 5, a lens 2 and a lens 3. The light emitting element 6 is caused to emit light by the drive circuit 10 in response to a transmission signal,
The laser light L after being converted into parallel light by the lens 1 is once condensed by the lens 2 via the deflecting beam splitter 5, and then modulated from the lens 3 having a large diameter toward the partner device.
1 is emitted.

In the receiving system, the incident light L2 is condensed by the lens 3, collimated by the lens 2 and then collimated by the lens 4 through the deflecting beam splitter 5 onto the light receiving element 7. The light transmitted by the light receiving element 7 is converted into an electric signal, amplified and waveform-shaped by the light receiving circuit 11, and further demodulated by a circuit (not shown) following the latter stage to reproduce the transmission signal.

Although FIG. 6 and FIG. 7 show an optical space transmission device having a transmission and reception integrated structure, many devices (not shown) in which the transmission function and the reception function are separated from each other have hitherto been available. Have been proposed and put to practical use.

Now, the state of the space transmission line, for example, rain, fog,
If dust or the like is present in the transmission line, light transmission loss will occur, and the propagation direction of light will change due to fluctuations in the atmosphere, so it is not limited that it always enters the receiving side with a constant optical power. . Therefore, in order to compensate for this, transmission is performed with the optical power raised in advance. However, when the state of the spatial transmission path is good and the attenuation of the optical power is small, the light receiving element is saturated and it is difficult to properly reproduce the signal.

In addition, there is an optical space transmission device equipped with a device for rotating a deflection prism in the optical path in order to control the optical power. It dealt with slow changes, and the control range was not large enough.

[0010]

SUMMARY OF THE INVENTION Therefore, the object of the present invention is to control the light power incident on the light receiving element of the optical space transmission device so as to be substantially constant and to always receive light in an optimum state. is there.

[0011]

The present invention has been made in view of the above points, and a liquid crystal device whose light transmittance changes by voltage control is provided in front of a light receiving element, and the output of the light receiving element is provided. The light transmittance of the liquid crystal device is controlled according to the magnitude of the signal so that the incident light intensity on the light receiving element is substantially constant in the frequency component lower than the modulation frequency of the signal light.

The liquid crystal device is constructed by using a ferroelectric liquid crystal, and the drivable frequency is 1 KHz or more.

The liquid crystal device is constructed by using a ferroelectric liquid crystal containing fine particles whose diameter is distributed in the range of about 100 nm to about 500 nm, and the light transmittance is continuously changed according to the applied voltage. And solve the above problems.

[0014]

In the liquid crystal device, the light transmittance changes due to the voltage control according to the magnitude of the output signal of the light receiving element, and the average amount of light incident on the light receiving element becomes substantially constant. Therefore, the light is always received at the optimum level regardless of the state of the space transmission path.

[0015]

Embodiments of the present invention will be described with reference to FIGS. It should be noted that the configuration is different from the conventional example in that a liquid crystal device for controlling the amount of incident light is used in front of the light-receiving element. Therefore, the same configuration and operation as those in the conventional example are the same. The reference numeral is attached and the description thereof is omitted.

First, as shown in FIG. 1, the structure of the optical space transmission device of the present invention is such that a liquid crystal device 8 is provided in front of the light receiving element 7, and the incident light L2 is transmitted through the liquid crystal device 8 to receive the light receiving element. Focused on 7. In the present embodiment, the liquid crystal device 8 is set between the deflecting beam splitter 5 and the lens 4, but it may be provided between the lens 4 and the light receiving element 7. In the transmission / reception integrated optical system of FIG. 1, it is not preferable to provide the deflection beam splitter 5 on the emission side because the intensity of emitted light is changed.

Next, the control of the light transmission amount of the liquid crystal device 8, which is a feature of the present invention, will be described with reference to FIG.

The incident light L2 passes through the liquid crystal device 8 and is focused on the light receiving element 7 by the lens 4. A voltage corresponding to the received light intensity is generated at the connection point between the light receiving element 7 and the resistor R1 provided between the light receiving element 7 and GND, and the OP amplifier 20
After the impedance is converted by, the input impedance is input to the OP amplifier 21 and amplified according to the circuit characteristics formed by the resistor R2, the resistor R3, and the capacitor C1. After that, it is compared with the reference voltage V1 by the OP amplifier 22, and the compared voltage is amplified by the amplification factor determined by the resistors R4 and R5 and input to the liquid crystal control circuit 23. The liquid crystal control circuit 23 drives the liquid crystal device 8 based on this signal to control the light transmittance,
The amount of incident light reaching the light receiving element 7 is made constant.

Incidentally, it goes without saying that the structure of the control circuit is not limited to the above-mentioned one, and may be another circuit structure having the same function.

FIG. 3 shows the structure of the liquid crystal device 8 used in the present invention. A ferroelectric liquid crystal 30 containing fine particles 37, which will be described later, and both sides of the liquid crystal 30 with spacers 31 interposed therebetween.
A transparent substrate 35 having an alignment film 33 and a transparent electrode 34 formed thereon is adhered with an adhesive 32 to enclose the liquid crystal 30. Further, a deflection plate 36 is provided on the outer surface of the transparent substrate 35 on the opposite side to which light is incident.

The liquid crystal 30 is a ferroelectric liquid crystal having a drivable frequency of 1 KHz or more, a fast operation, and is suitable for use in the present invention.

The fine particles 37 in the liquid crystal described above are effective for continuously changing the amount of transmitted light, that is, the contrast of the light with respect to the applied voltage. See FIGS. 4 and 5. And explain in detail.

FIG. 4 is a diagram in which the contrasts of the ferroelectric liquid crystal to which fine particles are added and the liquid crystal to which no fine particle is added are measured with respect to the applied voltage. As can be seen from the figure, in the case where the fine particles are not added, the contrast sharply changes when the applied voltage is approximately 12V. On the other hand, in the case of adding fine particles, the contrast continuously changes in response to the applied voltage. From the figure, by adding fine particles to the ferroelectric liquid crystal, the contrast with respect to the applied voltage is extremely good. It shows continuity.

Next, the continuity of contrast depending on the size of the added fine particles will be described with reference to FIG.
FIG. 5 (a) shows the distribution of the diameters of the two types of particles to be added. In the A type, the center of the diameter distribution is about 70 nm, and the distribution is about 30 nm to 150 nm. Has a distribution center of approximately 300 nm and approximately 100 n
It is distributed from m to 500 nm.

FIG. 5 (b) is a graph showing the contrast with respect to the applied voltage when the A type fine particles are added, and the applied voltage changes at about 16V to 22V. On the other hand, FIG. 5C is a diagram showing the contrast with respect to the applied voltage when the B type fine particles are added, and the applied voltage continuously changes at about 11V to 25V. From this, it is understood that the type B of the fine particles to be added has a wider control voltage than the type A and is preferable in the control of the contrast.

Therefore, as described above, the liquid crystal of the liquid crystal device 8 is a ferroelectric liquid crystal capable of high-speed operation, and the diameter distribution center is approximately 300 nm in order to obtain continuous contrast.
It is preferable to add fine particles distributed in a range of about 100 nm to 500 nm.

[0027]

Therefore, according to the present invention, the light transmittance of the liquid crystal device is changed by the voltage control according to the magnitude of the output signal of the light receiving element, and the average amount of light incident on the light receiving element becomes substantially constant. , Regardless of the state of the spatial transmission line, it is possible to always receive light at an optimum level and reproduce the signal.

The amount of light received is greatly influenced by the length of the spatial transmission path, and particularly in short-range communication using microwaves and millimeter waves, the receiving state is adjusted by removing the antenna from the receiving device or adding an attenuator. However, the optical space transmission device including the liquid crystal device can be used as it is regardless of the length of the space transmission path.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical free space transmission apparatus according to the present invention.

FIG. 2 is a configuration diagram of a control system for controlling the light reception intensity of the optical free space transmission apparatus according to the present invention.

FIG. 3 is a diagram showing a liquid crystal device used in the optical free space transmission apparatus of the present invention.

FIG. 4 shows the relationship between applied voltage of liquid crystal and contrast.
It is a figure which shows about the case where a liquid crystal contains a fine particle, and does not contain it.

FIG. 5 is a diagram showing the relationship between the diameter size of fine particles added to the liquid crystal and the contrast, wherein (a) shows the distribution of A type and B type sizes of fine particles, and (b) shows A.
The relationship between the applied voltage and the contrast when the type is used, and (C) shows the relationship between the applied voltage and the contrast when the type B is used.

FIG. 6 is a diagram for explaining a transmission state of the optical free space transmission apparatus.

FIG. 7 is a diagram showing a configuration of a conventional optical space transmission device.

[Explanation of symbols]

 1 to 4 lens 5 polarization beam splitter 6 light emitting element 7 light receiving element 8 liquid crystal device 10 driving circuit 11 light receiving circuit 20 to 22 OP amplifier 23 liquid crystal control circuit 31 spacer 32 adhesive 33 alignment film 34 transparent electrode 35 transparent substrate 36 deflector 37 Fine particles

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Claims (8)

[Claims] 1. A light emitting element as a signal light source, transmitting means for transmitting light by modulating the light of the light emitting element according to a transmission signal and emitting the light to the outside, and modulated light incident from the outside. In an optical space transmission device including a light receiving element for receiving light and a receiving means for demodulating a signal received by the light receiving element to obtain a transmission signal, a liquid crystal device in which light transmittance changes by control by voltage An optical space transmission device arranged in front of the light receiving element. 2. The light transmittance of the liquid crystal device is controlled according to the magnitude of the output signal of the light receiving element, and the intensity of light incident on the light receiving element is lower than the modulation frequency of the signal light. The optical space transmission device according to claim 1, wherein the components are configured to be substantially constant. 3. The drivable frequency of the liquid crystal device is 1 KH.
The optical space transmission device according to claim 1, wherein the optical space transmission device has z or more. 4. The optical space transmission device according to claim 1, wherein the liquid crystal device is configured by using a ferroelectric liquid crystal. 5. The light transmittance of the liquid crystal device is configured to continuously change according to an applied voltage,
The optical space transmission device according to claim 1. 6. The optical space transmission device according to claim 1, wherein the liquid crystal device is configured by using a ferroelectric liquid crystal containing fine particles. 7. The optical space transmission device according to claim 1, wherein the diameters of the fine particles included in the liquid crystal device are distributed in a range of 100 nm to 500 nm. 8. The optical space transmission device according to claim 1, wherein the polarization plane of the light incident on the liquid crystal device coincides with the polarization plane of the light from the other device.