JP2010258598A - Optical space transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To always enable polarization separation of transmitting light and receiving light. <P>SOLUTION: The optical space transmission system 1 includes spatial optical communication devices 2 and 3 each having: a laser diode 22 for generating transmitting light A; a photodetector 20 for receiving light B; a polarization beam splitter 17 for reflecting either the transmitting light A at a predetermined polarization angle or the receiving light B at a polarization angle perpendicular to the polarization angle of the transmitting light A; a rotatable mirror 5 configured so as to reflect the transmitting light A to output it and reflect the receiving light B for incident; a photodetector 12 for position detection for detecting the incident position of the receiving light B; and a mirror driving control part 11 for rotatably driving the mirror 5 on the basis of the incident position of the receiving light B. The optical space transmission system 1 performs transmission and reception using the spatial optical communication devices 2 and 3 as a pair, wherein the elevation angle of the mirror 5 of the spatial optical communication device 2 is set opposite to the that of a mirror 7 of the spatial optical communication device 3. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical space transmission system.

  In an optical space communication device having a common optical axis and capable of two-way communication, for example, there is a polarization separation method as described in Patent Document 1 as a method for separating transmitted light and received light. In the optical space communication device described in Patent Document 1, by using a polarization beam splitter, the polarization angle of the laser light (transmitted light) transmitted from the laser diode is the polarization of the laser light (received light) received from the counterpart device. It is adjusted so as to be substantially orthogonal to the angle. Further, this apparatus performs tracking of the laser light by controlling the tracking mirror in two directions of azimuth and elevation based on the detection position of the received light on the position detection photodetector.

  Also, in such a polarization separation type optical space communication device, a λ / 2 wave plate is arranged in front of the transmission (reception) optical system, and the polarization state of the transmission light and the reception light is changed by rotating the wave plate. Patent Document 2 describes that transmission and reception light are completely separated by making them orthogonal.

JP-A-5-133716 Japanese Patent No. 2522379

  Here, in the optical space transmission system in which two optical communication devices described in Patent Document 1 are paired, when the optical communication device is installed in each of the fixed base station and the mobile station, the mobile station is moving. In addition, it is preferable that bidirectional communication can be performed between optical communication apparatuses.

  However, in the device described in Patent Document 1, when the tracking mirror is rotated in a wide range in the azimuth direction following the movement of the optical communication device on the mobile station side, the transmission light and the tracking mirror are rotated along with the rotation of the tracking mirror. Since the polarization angle of the received light changes in the opposite direction, it becomes impossible to maintain the polarization angle of the transmitted light and the polarization angle of the received light substantially orthogonal. As a result, the polarization separation of the transmitted / received light is not perfect, and there is a problem that the noise and the level of the transmitted / received light are lowered to affect the communication.

  Further, in the apparatus described in Patent Document 2, since it is necessary to control the rotation of the wave plate at any time, a mechanism for controlling the λ / 2 wave plate is required, and the apparatus becomes large.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an optical space transmission system capable of always separating the polarization of transmitted light and received light.

  As a result of intensive efforts, the present inventors have found that between the polarization angle of the transmitted light and the polarization angle of the received light, the orientation of each tracking mirror of the pair of spatial optical communication devices included in the optical spatial transmission system Even when the angles are rotated in the same direction, when the directions of the elevation angles of the optical mirrors with respect to the optical transmitter / receiver coincide with each other, it has been found that the polarization angle of the facing light has a relationship of rotating in the opposite direction in space. .

  That is, an optical space transmission system according to the present invention includes a light source that generates transmission light modulated by an information signal, a detector that receives the modulated reception light, and a predetermined polarization angle among transmission light generated from the light source. And a polarized beam splitter that reflects one of the received light received by the detector and the light having a polarization angle orthogonal to the polarization angle of the transmitted light and transmits the other, and exits from the polarized beam splitter A rotatable mirror configured to reflect and output the received transmitted light and reflect the received received light to be incident on the polarization beam splitter, and a position detector that detects a position where the received light is incident And first and second spatial light communication devices each having drive means for rotationally driving the mirror based on position information acquired from the signal detected by the position detector. The first spatial light communication device and the second spatial light communication device perform transmission / reception as a pair, the elevation angle of the mirror of the first spatial light communication device, and the elevation angle of the mirror of the second spatial light communication device Is configured to be reversed.

  According to such an optical space transmission system, the elevation angle of the mirror of the first spatial optical communication device and the elevation angle of the mirror of the second spatial optical communication device are arranged in opposite directions. For this reason, the azimuth rotation directions of the mirrors of the first and second spatial light communication devices in the tracking operation are reversed, and the polarization angles of the transmission light and the reception light rotate in the same direction. As a result, the polarization angle of the transmission light and the polarization angle of the reception light can always be maintained substantially orthogonal, and the transmission light and the reception light can always be separated from each other.

  The optical space transmission system further includes signal processing means for correcting the position information of the received light acquired by the position detector according to the azimuth angle of the mirror, and the driving means is corrected by the signal processing means. It is preferable to rotate the mirror based on the positional information. As a result, the mirror can be driven to rotate after correcting the change in position information of the received light due to the azimuth angle of the mirror, so that the accuracy of the tracking operation of the received light by the mirror is improved.

  Similarly, an optical space transmission system detects the position of received light using an optical image rotation element arranged on the optical axis of the received light between the polarization beam splitter and the position detector according to the azimuth angle of the mirror. It is preferable to further have a correcting means for correcting the position incident on the device. As a result, the position detector can detect the incident position of the received light without being affected by the azimuth angle of the mirror, so that the accuracy of the tracking operation of the received light by the mirror is improved.

  Similarly, in the optical space transmission system, the mirror has a transmission part that transmits a part of the reception light, and the position detector is arranged to detect a part of the reception light transmitted through the transmission part of the mirror. At the same time, it is preferable that the driving means is disposed so as to be rotatable together with the mirror. As a result, the position detector can detect the incident position of the received light without being affected by the azimuth angle of the mirror, so that the accuracy of the tracking operation of the received light by the mirror is improved.

  According to the optical space transmission system of the present invention, transmission light and reception light can always be polarized and separated.

1 is a diagram showing an overall configuration of an optical space transmission system according to a first embodiment of the present invention. It is a figure which shows the structure of the space optical communication apparatus in FIG. It is the schematic which shows the change of the polarization angle of the transmitted light and the received light when the spatial optical communication apparatus installed in the mobile station moves from the apparatus upper direction. It is the figure which represented the detection position of the received light at the time of using the photodetector of many pixels, such as CCD and PSD, for a position detection photodetector with a rectangular coordinate. It is the figure which represented the detection position of the received light by a polar coordinate at the time of using the photodetector of many pixels, such as CCD and PSD, for the photodetector for position detection. It is the schematic which shows a mode that the beam spot was imaged on the 4-part dividing photodiode when a 4-part dividing photodiode is used for the photodetector for position detection, and the azimuth angle of a tracking mirror exists in an initial angle. It is the schematic which shows arrangement | positioning of the energy component of a beam spot when a quadrant photodiode is used for the photodetector for position detection, and the azimuth angle of a tracking mirror changes by angle (theta). It is a figure which shows the structure of the space optical communication apparatus in 2nd Embodiment. It is a figure which shows the structure from a beam splitter to the photodetector for position detection in detail. It is a figure which shows the structure of the spatial optical communication apparatus in 3rd Embodiment.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of an optical space transmission system according to the invention will be described with reference to the drawings. In the description, the same reference numerals are used for the same elements or elements having the same function, and redundant description is omitted. FIG. 1 is a diagram showing an overall configuration of an optical space transmission system 1 according to the first embodiment of the present invention.

  The optical space transmission system 1 performs bidirectional transmission / reception between a pair of space optical communication apparatuses 2 and 3. For example, one of the pair of space optical communication apparatuses 2 and 3 is installed in a fixed base station and the other is installed in a mobile station. In FIG. 1, a spatial optical communication device (first spatial optical communication device) 2 is installed on the fixed base station side, and a spatial optical communication device (second spatial optical communication device) 3 is installed on the mobile station side. The spatial optical communication device 2 includes an optical wireless module 4 in which an optical system for transmission / reception is arranged, and an azimuth angle so that the optical system of the optical wireless module 4 can track the light beam from the counterpart station. And a tracking mirror 5 that can be controlled in two directions of elevation. The spatial optical communication device 3 also includes an optical wireless module 6 and a tracking mirror 7 that have the same configuration as the spatial optical communication device 2.

  In particular, in the present embodiment, the positional relationship between the optical wireless module 4 and the tracking mirror 5 of the spatial optical communication device 2 and the positional relationship between the optical wireless module 6 and the tracking mirror 7 of the spatial optical communication device 3 are mutually up and down. They are arranged to be reversed. That is, as shown in FIG. 1, when the spatial optical communication device 2 on the fixed base station side arranges the tracking mirror 5 above the optical wireless module 4, the spatial optical communication device 3 on the mobile station side A tracking mirror 7 is disposed below the module 6. The spatial light communication devices 2 and 3 have a common optical axis between the tracking mirrors 5 and 7, and the elevation angles of the tracking mirrors 5 and 7 are opposite so that bidirectional communication can be performed. It is comprised so that it may become.

  FIG. 2 is a diagram illustrating a configuration of the spatial light communication device 2. As shown in FIG. 2, in the present embodiment, the azimuth angle of the tracking mirror 5 refers to a rotation angle around the axis of the optical wireless module 4, and the elevation angle is orthogonal to the axis of the optical wireless module 4. This is the angle between the surface and the tracking mirror 5. The tracking mirror 5 is adjusted by the mirror drive control unit (drive means) 11 so that the optical system of the optical wireless module 4 can track the light beam from the partner station (spatial optical communication device 3). Two directions of elevation can be automatically adjusted. In the tracking operation of the tracking mirror 5, first, information on the detection position of the light beam from the counterpart station, which is obtained from the signal detected by the position detection photodetector (position detector) 12, is detected by the angle detection unit 13. The signal processing unit (signal processing means) 14 performs a correction calculation based on information on the azimuth angle of the tracking mirror 5 acquired from the signal detected by the mirror, and the mirror drive control unit uses the position information calculated by the correction. 11 is performed by driving and controlling the tracking mirror 5. Details of the signal processing in the signal processing unit 14 will be described later.

  The optical system as a transmission function of the optical wireless module 4 includes a light projecting / receiving optical lens 15, a beam expander unit 16, a polarization beam splitter 17, and an optical lens 23 that collects light from a laser diode (light source) 22. The The laser diode 22 generates light that is modulated and polarized by a transmission signal from a transmission / reception circuit (not shown) (polarized light in the vertical direction in the example of FIG. 1). The light beam condensed by the optical lens 23 and made almost parallel is reflected by the polarization beam splitter 17, converted into a large aperture beam by the beam expander unit 16, and passes through the light projecting / receiving optical lens 15 to follow the tracking mirror 5. And transmitted to the spatial optical communication device 3 of the counterpart station as transmission light A.

  The optical system as a reception function of the optical wireless module 4 includes a light projecting / receiving optical lens 15, a beam expander unit 16, a polarization beam splitter 17, a beam splitter 18, and an optical lens 19. A laser beam having a polarization angle different from that of the transmission light A by about 90 degrees is received as the reception light B from the spatial light communication device 3 of the partner station that is installed facing. The received light B reflected by the tracking mirror 5, passing through the light projecting / receiving optical lens 15 and becoming a small aperture beam at the beam expander unit 16 is transmitted through the polarization beam splitter 17 and the beam splitter 18, and is transmitted by the optical lens 19. The light is collected on a photodetector (detector) 20. The received signal from the photodetector 20 is sent to a transmission / reception circuit (not shown). A part of the received light B is reflected by the beam splitter 18 and condensed on the position detection photodetector 12 by the optical lens 21. The position detection photodetector 12 includes, for example, a sensor such as a CCD, a PSD, or a four-division photodiode, and the position signal of the collected reception light B is sent to the mirror drive control unit 11 via the signal processing unit 14. , The tracking operation of the tracking mirror 5 is performed so that the optical axis of the light projecting / receiving optical lens 15 coincides with the optical path center of the received light B.

  The configuration of the spatial light communication device 3 is such that the polarization angle of the light (the above-described received light B) generated from the laser diode 22 as the light source is substantially orthogonal to that of the spatial light communication device 2 (in the example of FIG. Except for the point that is polarized in the direction), it is the same as that of the spatial optical communication apparatus 2 shown in FIG. In the spatial optical communication device 3, the arrangement of the optical wireless module 6 and the tracking mirror 7 is opposite to the arrangement of the optical wireless module 4 and the tracking mirror 5 of the spatial optical communication device 2.

  In the optical space transmission system 1 configured as described above, when the space optical communication device 3 installed in the mobile station is moving in the direction of the arrow 30 shown in FIG. The angle varies as shown in FIG. FIG. 3 is a schematic diagram showing changes in the polarization angles of the transmission light A and the reception light B when the spatial optical communication device 3 installed in the mobile station moves, from above the device.

  As shown in FIG. 3, when the spatial optical communication device 3 moves to the left, the tracking mirror 5 of the spatial optical communication device 2 counterclockwise so as to track the azimuth angle in the moving direction of the spatial optical communication device 3. Rotate. At this time, since the azimuth angle of the optical wireless module 4 does not change, the polarization angle of the transmission light A output from the spatial optical communication device 2 rotates clockwise from the vertical direction according to the rotation angle of the tracking mirror 5.

  On the other hand, in the spatial light communication device 3, the tracking mirror 7 rotates counterclockwise so as to track the azimuth angle in the direction of the spatial light communication device 2, as with the tracking mirror 5. 7 is in the opposite direction to that of the tracking mirror 5, the polarization angle of the transmitted light (received light B) output from the spatial light communication device 3 is also rotated in the opposite direction of the transmitted light A. That is, the received light B rotates counterclockwise from the horizontal direction when viewed from the spatial light communication device 3 side.

  Accordingly, the transmission light A and the reception light B have their polarization angles rotated in the same direction in space, as indicated by reference numeral 31 in FIG. For example, when a train is considered as a mobile station and the traveling direction of the mobile station is almost constant with respect to the fixed base station, such as when a fixed base station is installed at a station, the polarization rotation angles of the transmitted and received light coincide with each other. .

  According to such an optical space transmission system 1, the elevation angle of the tracking mirror 5 of the spatial light communication device 2 and the elevation angle of the tracking mirror 7 of the spatial light communication device 3 are arranged in opposite directions. For this reason, the azimuth rotation direction of the tracking mirrors 5 and 7 in the tracking operation is reversed, and the polarization angles of the transmission light A and the reception light B rotate in the same direction. As a result, the polarization angle of the transmission light A and the polarization angle of the reception light B can always be maintained substantially orthogonal, and the transmission light A and the reception light B can always be separated from each other.

  Next, the signal processing in the signal processing unit 14 will be described with reference to FIGS. In the present embodiment, the change in the azimuth angle of the tracking mirrors 5 and 7 is equivalent to the rotation of the image formed on the position detection photodetector 12 in the optical wireless modules 4 and 6. That is, the direction of the deviation amount from the tracking target indicated by the position of the detected beam spot changes depending on the azimuth angle of the tracking mirrors 5 and 7 at that time. Therefore, it is desirable that the tracking control can always be performed in the correct direction regardless of the change in the azimuth direction of the tracking mirrors 5 and 7. The signal processing unit 14 has a function of correcting the positional information deviation of the received light acquired by the position detection photodetector 12 for this purpose.

  The signal processing in the signal processing unit 14 is performed in a specific manner depending on whether (a) a multi-pixel photodetector such as a CCD or PSD is used for the position detection photodetector 12, or (b) a quadrant photodiode is used. Processing is different.

  First, with reference to FIGS. 4 and 5, processing when a multi-pixel photodetector such as a CCD or PSD is used as the position detection photodetector 12 will be described.

  In a multi-pixel photodetector such as a CCD or an optical position detector such as a PSD, the spot position can be determined as a position on orthogonal coordinates as shown in FIG.

  Next, if the coordinates represented by the orthogonal coordinates are converted into polar coordinates, they are represented as shown in FIG. Here, it is assumed that the rotation center axis of the tracking mirrors 5 and 7 and the control target coincide with the origin of the coordinates, and the vertical axis assumed on the position detection photodetector 12 is a straight line connecting the origin and the coordinate points. Let the angle be θ.

At this time, if the amount of azimuth change from the initial angle of the tracking mirrors 5 and 7 is γ, the tracking mirrors 5 and 7 After the azimuth angle changes, the coordinates obtained from the position detection photodetector 12 are converted into polar coordinates and γ is added to the angle component. That is, as a calculation flow, (1) obtaining orthogonal coordinates [x, y] from the position detection photodetector 12 (2) converting the orthogonal coordinates [x, y] into polar coordinates [ρsinθ, ρcosθ].
(3) Add the mirror azimuth angle γ obtained from the angle detector 13 to the angle component. (4) Convert polar coordinates [ρsin (θ + γ), ρcos (θ + γ)] to Cartesian coordinates [X, Y]. (5) The tracking mirrors 5 and 7 are controlled based on the coordinate deviation determined in (4) above.

  Next, with reference to FIGS. 6 and 7, processing when a quadrant photodiode is used for the position detection photodetector 12 will be described.

  FIG. 6 shows a state in which the beam spot is imaged on the quadrant photodiode. Here, the size of the beam spot is enlarged for easy understanding, and the center is made to coincide with the center of the detector.

  When the tracking mirror is at the initial angle, the energy components of the beam spots detected by the respective pixels a, b, c, and d are A, B, C, and D, respectively. Each energy component A, B, C, D is equal to a control signal used to control the tracking mirrors 5, 7.

  At this time, when the azimuth angle of the tracking mirrors 5 and 7 is changed by the angle θ by the tracking operation, the arrangement of the energy components is as shown in FIG.

When the control signals A, B, C, and D are obtained with reference to FIG.
(1) For 0 ° ≦ θ <90 °, a substitution variable γ is assumed to simplify the calculation.
γ = θ

(2) When 90 ° ≦ θ <180 °, γ = θ−90

(3) When 180 ° ≦ θ <270 °, γ = θ−180

と表せる。 (4) When 270 ° ≦ θ <360 °, γ = θ−270

It can be expressed.

  By obtaining a control signal from the components A, B, C, and D thus obtained, the amount of change (deviation) in the positional information of the received light due to the azimuth angle of the tracking mirrors 5 and 7 is corrected, and then the tracking mirror Therefore, the accuracy of the tracking operation of the received light by the tracking mirror can be improved.

  Next, a second embodiment of the present invention will be described with reference to FIGS. FIG. 8 is a diagram illustrating a configuration of the spatial light communication device 2a according to the second embodiment. The spatial optical communication device 2a of the present embodiment includes an optical image rotation element 24a such as an image rotator prism between the beam splitter 18 and the position detection photodetector 12 in that the signal processing unit 14 is not provided. It differs from the spatial light communication apparatus 2 of the first embodiment in that the correction means 24 used is provided.

  FIG. 9 is a diagram showing in detail the configuration from the beam splitter 18 to the position detection photodetector 12 in FIG. As shown in FIG. 9, in the present embodiment, the correction means 24 is arranged on the optical axis before the position detection photodetector 12, and is matched with the rotation angle of the tracking mirror 5 detected by the angle detection unit 13. Thus, the position where the received light enters the position detection photodetector 12 is optically corrected by rotating the image rotator prism (dub prism) 24a of the correction means 24 around the optical axis direction. However, since the rotation angle of the image by the image rotator prism 24a is twice the rotation angle of the prism 24a, it is necessary to control the rotation angle at half of the change in the azimuth angle of the tracking mirror 5.

  With such a configuration, the position detection photodetector 12 can detect the incident position of the received light without being affected by the azimuth angle of the tracking mirror 5, so that the accuracy of the tracking operation of the received light by the tracking mirror is improved. It becomes possible to improve.

  Next, a third embodiment of the present invention will be described with reference to FIG. FIG. 10 is a diagram illustrating a configuration of the spatial light communication device 2b according to the third embodiment. The spatial optical communication device 2b of the present embodiment does not include the signal processing unit 14 and the angle detection unit 13, and the optical lens 21 and the position detection photodetector 12 are not included in the optical wireless module 4 but in the tracking mirror 5. It differs from the spatial optical communication apparatus 2 of the first embodiment in that it is provided.

  As shown in FIG. 10, in this embodiment, a part of the tracking mirror 5 is provided with a transmission part 5a that is an opening or a partial transmission mirror, and a part of the received light is configured to pass through the transmission part 5a. The Then, the optical lens 21 and the position detection light detector 12 are arranged on the back side of the tracking mirror 5 so as to detect the received light transmitted through the transmission part 5a. Thereby, since the position detection photodetector 12 can detect the incident position of the received light without being affected by the azimuth angle of the tracking mirror 5, the accuracy of the received light tracking operation by the tracking mirror can be improved. Is possible.

  The optical space transmission system according to the present invention has been described with reference to the preferred embodiment, but the present invention is not limited to the above embodiment. For example, in the above embodiment, the polarizing beam splitter 12 reflects the transmission light and transmits the reception light. However, conversely, the transmission light may be transmitted and the reception light may be reflected. In this case, the arrangement of the transmission function and the reception function of the optical wireless module 4 shown in FIG. 2 is also reversed.

  In the above embodiment, the positional relationship between the optical wireless modules 4 and 6 and the tracking mirrors 5 and 7 is the vertical direction. However, as long as the relative positional relationship between the two is maintained, the other direction may be used. .

  DESCRIPTION OF SYMBOLS 1 ... Optical space transmission system, 2, 2a, 2b ... Spatial optical communication apparatus (1st spatial optical communication apparatus), 3 ... Spatial optical communication apparatus (2nd spatial optical communication apparatus), 4, 6 ... Optical wireless module 5, 7 ... Tracking mirror, 5a ... Transmission unit, 11 ... Mirror drive control unit (drive means), 12 ... Position detection photodetector (position detector), 13 ... Angle detection unit, 14 ... Signal processing unit (Signal processing means), 17 ... Polarizing beam splitter, 20 ... Photo detector (detector), 22 ... Laser diode (light source), 24 ... Correction means, 24a ... Image rotator prism (optical image rotating element), A ... Transmission Light, B ... Received light.

Claims (4)

A light source that generates transmission light modulated by an information signal;
A detector for receiving the modulated received light;
Reflects either one of the transmitted light generated from the light source having a predetermined polarization angle and one of the received light received by the detector and having a polarization angle orthogonal to the polarization angle of the transmitted light. A polarizing beam splitter that transmits the other,
A rotatable mirror configured to reflect and output transmission light emitted from the polarization beam splitter, and to reflect input reception light and enter the polarization beam splitter;
A position detector for detecting a position where the received light is incident;
A first and a second spatial light communication device each having driving means for rotationally driving the mirror based on position information of the received light acquired from a signal detected by the position detector;
The first spatial optical communication device and the second spatial optical communication device perform transmission and reception as a pair,
An optical space transmission system configured such that an elevation angle of the mirror of the first spatial light communication device is opposite to an elevation angle of the mirror of the second spatial light communication device. Signal processing means for correcting the position information of the received light acquired by the position detector according to the azimuth angle of the mirror;
2. The optical space transmission system according to claim 1, wherein the driving unit rotationally drives the mirror based on the position information corrected and calculated by the signal processing unit.   The received light is incident on the position detector by using an optical image rotation element arranged on the optical axis of the received light between the polarization beam splitter and the position detector according to the azimuth angle of the mirror. The optical space transmission system according to claim 1, further comprising correction means for correcting the position to be corrected. The mirror has a transmission part that transmits a part of the received light,
2. The position detector is disposed so as to detect a part of received light transmitted through a transmission part of the mirror, and is disposed rotatably with the mirror by the driving means. The optical space transmission system described in 1.

Images