JP2004328205A - Optical transmission device, optical transmission method, and optical communication system - Google Patents

Abstract

In optical space communication, a communication error is reduced by preventing light attenuation during rainfall.
The polarization state of linearly polarized light generated from a laser is controlled by a polarization rotation device. Light having a predetermined polarization state by the polarization rotation device 3 is input to the optical transmitter 4a, and the transmitted light is received by the optical receiver 6a via the spatial data communication path 5. The optical receiving unit 6a detects the optical receiving intensity, transmits data indicating the detected optical receiving intensity to the transmitting side, and receives the data by the optical receiving unit 6b. The reception output of the light receiving unit 6b is input to the polarization control device 8, and the polarization rotation device 3 is controlled so that the light reception intensity becomes higher.
[Selection] Figure 2

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmission device, an optical transmission method, an optical communication system, and an optical communication method applied to an optical free space communication system.
[0002]
[Prior art]
In the advanced information and communication society, it is expected that various multimedia information and communication devices and networks connecting them will penetrate into every corner of the society, and higher speed and larger capacity communication will be required. However, radio resources used for wireless communication are decreasing due to the rapid increase of mobile phones and the like. To alleviate such a situation, an optical space communication technology has been proposed. Optical space communication devices have already been used for indoor terminal communication or outdoor building communication.
[0003]
[Problems to be solved by the invention]
In the optical space communication, a transmission error occurs due to rain outside the propagation path. It is desired to solve the problem of deterioration of reliability.
[0004]
Therefore, an object of the present invention is to provide an optical transmission device and an optical transmission device capable of reducing the occurrence of communication errors at the time of rain by utilizing the fact that the atmospheric propagation characteristics of light during rain have strong polarization dependence. A method, an optical communication system and an optical communication method are provided.
[0005]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, an invention according to claim 1 is an optical transmission device for transmitting data through space, wherein linearly polarized light is generated by a laser light source, and the polarization state of the linearly polarized light can be controlled by optical means. And transmitting the light from the optical means to the spatial communication path as communication light, detecting the light receiving intensity on the receiving side, and performing feedback control on the optical means so as to increase the light receiving intensity.
[0006]
According to a sixth aspect of the present invention, in an optical transmission system for transmitting and receiving data through space, detection light having a plurality of polarization states and communication light of linear polarization are combined and transmitted to a spatial communication path, and the detection light and communication The light is received, the optimum polarization state is detected from the received detection light, and the communication light whose polarization state is optimally controlled by the polarization control signal is transmitted to the spatial communication path.
[0007]
In the present invention, focusing on the fact that the light reception intensity at the time of rain depends on the polarization state, feedback control or feedforward control of the polarization state of the communication light is performed so that the light reception intensity increases. Thereby, communication errors can be reduced.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a measurement example of the time variation of the average reception intensity during heavy rainfall, in which the normalized average reception intensity is plotted on the vertical axis and the time is plotted on the horizontal axis. Generally, the relationship between the average reception intensity and the bit error rate (BER) is an exponential function, and when the average reception intensity decreases, the BER rapidly increases. FIG. 1 shows a measurement at a fixed time interval using a pulse signal having a rate of 622 Mbps using a wavelength in the 1.55 μm band.
[0009]
In FIG. 1, black square marks indicate measured values of light N, white circle marks indicate measured values of polarized light P, and x marks indicate measured values of polarized light S. Here, P indicates the average reception intensity of linearly polarized light perpendicular to the ground, S indicates the average reception intensity of linearly polarized light parallel to the ground, and N indicates that the polarization state is not defined and the polarization state is not known. Shows the average reception strength of none. The polarizations P and S are controlled to pass through the same light propagation path.
[0010]
The average reception intensity increases as the light transmittance increases. As can be seen from FIG. 1, the graphs for the respective polarizations of P, S, and N are different from each other, and it can be seen that the light transmittance greatly differs depending on the polarization, particularly during heavy rainfall. Among them, it has been found that the transmission can be very increased at certain polarization states.
[0011]
The present invention intends to reduce the attenuation of light during rainfall by transmitting light while always maintaining a polarization state with a high transmittance based on such measurement results. Hereinafter, an optical spatial communication system according to an embodiment of the present invention will be described with reference to FIG.
[0012]
In FIG. 2, reference numeral 1 is an input terminal for transmission data. Although not shown, the transmission data is generated by processing such as error correction coding and modulation. Transmission data is added to a laser 2a such as a semiconductor laser, and linearly polarized light whose intensity is modulated according to the data is generated from the laser 2a. In order to modulate the intensity of the laser light according to the data, the laser light generated by the laser may be modulated by an optical modulator such as an electro-optic modulator or an acousto-optic modulator.
[0013]
The linearly polarized light generated from the laser 2a is incident on the polarization rotation device 3. The polarization rotation device 3 can rotate the polarized light by 180 °. In response to an electrical control signal from the polarization controller 8, it is possible to output linearly polarized light having any polarization state between P-polarized light and S-polarized light, for example, which produces the measurement results of FIG. Light having a predetermined polarization state by the polarization rotation device 3 is input to the optical transmission unit 4a. The optical transmission unit 4a has an optical configuration including a transmission telescope and the like, and communication light is transmitted to the spatial data communication path 5 by the optical transmission unit 4a.
[0014]
The communication light passing through the spatial data communication path 5 is received by the light receiving unit 6a. The light receiving unit 6a includes a receiving telescope, a photodiode (photodetector), and the like. Receiving light by the light receiving section 6a is taken to the converted output terminal 7 a into an electric signal (received data). Although not shown, the received data undergoes processing such as error correction and decoding using an error correction code.
[0015]
In order to perform bidirectional communication, transmission data input from the input terminal 1b is input to the signal processing unit 9, and the transmission data undergoes processing such as encoding using an error correction code and conversion of the transmission data into a format. Transmission data from the signal processing unit 9 is supplied to the laser 2b, and the intensity of the laser light is modulated according to the transmission data. The linearly polarized light generated by the laser 2b is supplied to the optical transmission unit 4b. Like the optical transmission unit 4a, the optical transmission unit 4b includes a polarization rotation device and a polarization control device in addition to an optical configuration including a transmission telescope and the like. The communication light is transmitted to the spatial data communication path 5 by the optical transmitter 4b.
[0016]
The light receiving unit 6a detects the light receiving intensity. For example, the optical reception intensity is detected at predetermined time intervals. Data indicating the detected light reception intensity is supplied to the signal processing unit 9. In the signal processing unit 9, data indicating the optical reception intensity is inserted into the transmission data. Note that control data for the polarization rotation device generated from the data indicating the light reception intensity may be supplied to the signal processing unit 9.
[0017]
Communication light passing through the spatial data communication path 5 is received by an optical receiver 6b provided on the transmission side. The light receiving unit 6b includes a receiving telescope, a photodiode (photodetector), and the like, like the light receiving unit 6a. The received light is converted into an electrical signal by the optical receiver 6b, and the received data is extracted to the output terminal 7b. Further, data indicating the light reception intensity separated from the reception data by the light receiving unit 6b is supplied to the polarization control device 8.
[0018]
The polarization control device 8 controls the polarization rotation device 3 based on data indicating the light reception intensity. For example, the optical receiving intensity is monitored based on the data from the optical receiving unit 6b. When the optical receiving intensity decreases by a predetermined amount, the transmitting side determines that there is rainfall and changes the polarization to 90. Start rotating at a speed of about ° / sec. In fine weather where the light reception intensity does not decrease by a predetermined amount, the polarization rotation device 3 is fixed.
[0019]
When the polarization rotation device 3 is rotating, the polarization rotation device 3 is fixed in a polarization state where the light reception intensity is the highest. After that, the polarization rotation device 3 changes the polarization angle continuously and stepwise within the range of 0 ° to ± 45 °, and finds the polarization control device 3 so as to find a peak at which the light reception intensity is maximum in that range. 8.
[0020]
In the case of two-way communication, the optical receiving unit 6b detects the optical receiving intensity, and inserts the detected optical receiving intensity data or control data into the transmission data in the signal processing unit. Alternatively, a configuration is possible in which control data is separated in the optical receiving unit 6a and supplied to the polarization controller in the optical transmitting unit 4b. Actually, in the case of two-way communication, since the space through which communication light in each direction passes is substantially the same, the polarization rotation in the light transmission unit 4b is performed based on the result of detecting the light reception intensity only in the light reception unit 6a. It is adapted to control the device. The polarization control device in the light transmission unit 4b controls the polarization rotation device in the same manner as the polarization control device 8 controls the polarization rotation device 3 described above. By this feedback control, the polarization state of the transmission laser light transmitted from the optical transmitter 4b to the optical receiver 6b is controlled, and the communication error at the time of rainfall is reduced.
[0021]
Next, another embodiment of the present invention will be described with reference to FIG. In another embodiment, feed-forward control is performed in contrast to the above-described embodiment in which feedback control is performed. Also, in consideration of the fact that the light passes in each direction in the two-way communication and the space is almost the same, the polarization dependence is measured only from the output of the light receiving unit 17b.
[0022]
In FIG. 3, reference numeral 11a denotes an input terminal for transmission data. Transmission data is applied to a laser 12a such as a semiconductor laser, and the transmission for linearly polarized light whose intensity is modulated according to the data from the laser 12a. Laser light is generated. In order to modulate the intensity of the laser light according to the data, an optical modulator such as an electro-optic modulator or an acousto-optic modulator may be used.
[0023]
The transmission laser beam for communication from the laser 12a is incident on the light combining unit 13. The light combining section 13 combines the lights having many polarization states generated by the detection light generating section 14. The detection light generating unit 14 is a light emitting diode, white light, or laser light in which a part of the transmission laser light is switched to various polarization states within about 1 millisecond or less. The light combining unit 13 generates communication light in which detection lights having various polarization states are combined. The communication light is transmitted to the spatial data communication path 16 by the optical transmission unit 15a.
[0024]
The communication light passing through the spatial data communication path 16 is received by the light receiving unit 17b. The received light is converted into an electric signal (received data) by the light receiving unit 17a and extracted to the output terminal 18b. The communication light received by the light receiving unit 17b is incident on the polarization dependency measuring unit 19. The polarization dependence measuring unit 19 measures the polarization dependence of the received light intensity, and detects the polarization state with the highest transmittance (the strongest reception intensity).
[0025]
An example of a method for measuring the polarization dependence of received light will be described. A part of the received light is separated by a beam splitter and is incident on a polarizing plate. The polarizing plate is rotated, and the relationship between the polarization state and the intensity of the received light is measured. In this case, since the received light is intensity-modulated according to the signal, a process for excluding the signal light is performed. For example, since the signal light is generated by the laser 12a, the signal light can be identified and excluded from the characteristics that the polarization state of the signal light is known in advance and that the signal light has the sharpest largest peak.
[0026]
The polarization dependence measuring unit 19 measures the relationship between each polarization state and the light reception intensity, and generates a polarization control signal indicating the optimal polarization state. This polarization control signal is supplied to the optical transmission unit 15b and the signal processing unit 20. The optical transmitter 15b receives linearly polarized light from the laser 12b. The polarization control signal from the polarization dependent measurement unit 19 is supplied to the polarization rotation device provided in the light transmission unit 15b, and the polarization state is optimally controlled by the polarization rotation device. Communication light whose polarization state is optimally controlled is transmitted to the spatial data communication path 16.
[0027]
The transmission data from the input terminal 11b is supplied to the signal processing unit 20, and the polarization control signal from the polarization dependence measurement unit 19 is inserted into the transmission data. The intensity of the laser beam generated by the laser 12b is modulated by the transmission data into which the polarization control signal has been inserted. In the optical transmission unit 15b, the communication light whose polarization state is controlled as described above is transmitted to the spatial data communication path 16. The optical receiver 17a extracts the received data to the output terminal 18a and separates the polarization control signal. The separated polarization control signal is supplied to a polarization rotation device in the optical transmission unit 15a, and the polarization state is controlled to an optimum one. In another embodiment of the present invention, the polarization state is optimally controlled by an open loop using the measurement result of the polarization dependence measurement unit 19.
[0028]
The present invention is not limited to the above-described embodiment of the present invention, and various modifications and applications are possible without departing from the gist of the present invention. For example, the present invention is not limited to the optical communication system described above, and can be applied to a measurement system.
[0029]
【The invention's effect】
According to the present invention, in an optical communication or measurement system using an outdoor space as a transmission path, the polarization state of light passing through the spatial transmission path is controlled so that the light reception intensity increases during rainfall. According to the present invention, light can be transmitted while maintaining a polarization state with a high transmittance. Since the communication error increases exponentially with the decrease in the optical reception intensity, the communication error can be reduced by avoiding the decrease in the optical reception intensity.
[Brief description of the drawings]
FIG. 1 is a graph of a measurement example showing that the average reception intensity at the time of rain varies depending on polarization.
FIG. 2 is a block diagram of a communication system according to an embodiment of the present invention.
FIG. 3 is a block diagram of a communication system according to another embodiment of the present invention.
[Explanation of symbols]
2 ... Laser, 3 ... Polarization rotating device, 4a, 4b ... Optical transmitter, 5 ... Spatial data communication path, 6a, 6b ... Optical receiver, 8 ... Polarization controller

Claims (7)

In an optical transmission device that transmits data through space,
A laser light source that generates linearly polarized light,
Optical means for receiving linearly polarized light from the laser light source and controlling the polarization state of the linearly polarized light,
Transmitting means for transmitting light from the optical means to the spatial communication path as communication light,
Receiving means for receiving a signal of optical reception intensity from the receiving side,
An optical transmission device comprising: a control unit that controls the optical unit so that the optical reception intensity is higher. In claim 1,
The optical transmission device, wherein the optical means starts the control operation when the optical reception intensity decreases by a predetermined amount. In an optical transmission method for transmitting data through space,
Generates linearly polarized light,
Transmitting the light whose polarization state of the linearly polarized light is controlled to the spatial communication path as communication light,
Receiving the signal of the optical reception intensity from the receiving side,
An optical transmission method for controlling the polarization state so that the optical reception intensity is higher. In an optical communication system for transmitting and receiving data through space,
The transmitting device is
A laser light source that generates linearly polarized light,
Optical means for receiving linearly polarized light from the laser light source and controlling the polarization state of the linearly polarized light,
Transmitting means for transmitting light from the optical means to the spatial communication path as communication light,
Receiving means for receiving a signal of optical reception intensity from the receiving side,
As the light receiving intensity is higher, the control means for controlling the optical means,
The receiving device
Receiving means for receiving the communication light,
An optical communication system comprising: a transmission unit that detects an optical reception intensity and transmits a signal of the optical reception intensity to the transmission device. In an optical communication method for transmitting and receiving data through space,
Generates linearly polarized light,
Transmitting the light whose polarization state of the linearly polarized light is controlled to the spatial communication path as communication light,
Receiving the communication light,
Detecting the light reception intensity of the received light, transmitting a signal of the light reception intensity,
Receiving the signal of the optical reception intensity from the receiving side,
An optical communication method for controlling the polarization state so that the optical reception intensity is higher. In an optical transmission system that transmits and receives data through space,
A laser light source that generates linearly polarized light,
A light combining unit that combines detection light having a plurality of polarization states with the linearly polarized light,
First transmitting means for transmitting the light from the light combining section as communication light to a spatial communication path;
First receiving means for receiving the detection light and the communication light;
Polarization dependency measuring means for detecting an optimum polarization state from the detection light received by the first receiving means,
A second transmission unit that transmits communication light whose polarization state is optimally controlled by the polarization control signal from the polarization dependence measurement unit to the spatial communication path;
An optical communication system comprising: a second receiving unit that receives the communication light. In an optical communication method for transmitting and receiving data through space,
Generates linearly polarized light,
Combining the detection light having a plurality of polarization states with the linearly polarized light,
Sending out the combined light as a communication light to a spatial communication path,
Receiving the communication light,
Detecting the optimum polarization state from the received communication light,
The communication light whose polarization state is optimally controlled by the polarization control signal indicating the optimal polarization state is transmitted to the spatial communication path,
An optical communication method for receiving the communication light.

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