Abstract:
An optical space communication apparatus for performing communication with an party apparatus by transmitting a light signal into a free space includes a signal selector for selecting and intercepting a beam including a light beam and background light from the party apparatus, a detector for detecting a beam via the signal selector, and a signal outputting device for comparing an output signal outputted from the detector when the signal selector intercepts the light beam from the party apparatus, with an output signal outputted from the detector when the signal selector does not intercept the light beam from the party apparatus, and for outputting a signal representing a difference between the output signals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical space communication apparatus which performs communication between two points remote from each other by propagating a beam of a light signal modulated by a transmitted signal, in the atmosphere. 
     2. Related Background Art 
     In general, communication making use of the light signal permits transmission of large-capacity information at high speed and, particularly, optical space communication using the free space as a transmission line has advantages of higher portability and easier installation of communication channel than wire communications using optical fibers and the like. Conventional communication apparatuses employ automatic tracking (autotracking) for controlling the angle of emission of the light beam so as to prevent the light signal from dropping off the apparatus, in order to improve the reliability of optical space communication. 
     FIG. 1 is a diagram showing the structure of an optical space communication apparatus having a tracking function in a conventional example, in which a lens  1  for transmission and reception is placed at a position opposite a party apparatus and in which a lens  2  and a movable mirror  3  are located on the optical path behind the lens  1 . A polarization beam splitter  4 , a lens  5 , and a light-emitting device  6  comprised of a semiconductor laser light source or the like are arranged in the direction of incidence of the movable mirror  3 . A beam splitting mirror  7 , a lens  8 , and a position detector  9  are arranged in the direction of reflection of the polarization beam splitter  4 , and a lens  10  and a light receiving element  11  are arranged in the direction of reflection of the beam splitting mirror  7 . 
     The output terminal of a multiplexer  12  is connected to the light-emitting device  6 , the output of a transmitted signal input terminal  13  is connected through an amplifier  14  to the multiplexer  12 , and the output of an oscillator  15  is also connected to the multiplexer  12 . Further, the output of the light receiving element  11  is connected through an amplifier  16  to a received signal output terminal  17 . 
     The output of position detector  9  is connected to a tracking control circuit  18  and the output of the tracking control circuit  18  is connected to the movable mirror  3  through two actuators  19 ,  20  for varying the angle of the movable mirror  3 . A collimation scope  21  for an inspector to verify collimation of axis by vision is provided nearly in parallel with the optical axis of the movable mirror  3 . 
     On the occasion of transmission, the transmitted signal is input via the transmitted signal input terminal  13 , amplified by the amplifier  14 , and further multiplexed with a signal from the oscillator  15  in the multiplexer  12 . Thereafter, the resultant signal is outputted to the light-emitting device  6 . The light-emitting device  6  intensity-modulates its emitting light according to the input signal to convert it into a light signal. The beam from the light-emitting device  6  is guided through the lens  5  to the polarization beam splitter  4 . Since this beam is polarized in parallel with the plane of the drawing, it is transmitted by the polarization beam splitter  4  as it is. Then the beam is reflected to the left by the movable mirror  3  and then passes through the lenses  2 ,  1  to be emitted toward the party apparatus. 
     On the occasion of reception, a light beam from the party apparatus is incident from the left into the lens  1 , passes through the lens  2  to be reflected downward by the movable mirror  3 , and then reaches the polarization beam splitter  4 . Since this beam is polarized normally to the plane of the drawing, it is reflected to the right by the bond face of the polarization beam splitter  4  to be split into two directions by the beam splitting mirror  7 . The beam reflected by the beam splitting mirror  7  passes through the lens  10  and then is received by the light receiving element  11  to be converted into an electric signal. Thereafter, the electric signal is amplified to an appropriate level by the amplifier  16  and the amplified signal is outputted from the received signal output terminal  17 . 
     On the other hand, the beam transmitted through the beam splitting mirror  7  is converged by the lens  8  to be received as a spot image S by the position detector  9 . The position detector  9  detects the position of the spot image S and outputs it as a position signal to the tracking control circuit  18 . The tracking control circuit  18  computes an angle of the light beam from the party apparatus with respect to the optical path of the present apparatus, based on this position signal, and makes driving signals for the actuators  19 ,  20 . The actuators  19 ,  20  adjust the angle of the movable mirror  3  so that the spot image S falls on the center of the position detector  9 . 
     With this adjustment, the position of the light-emitting device  6  is also adjusted, whereby the optical path of the emitted beam comes to agree with that of the incident beam. Therefore, the light beam is accurately emitted toward the party apparatus. If the apparatus is inclined during communication to shift the optical path of the received light to deviate the position of the spot image S on the position detector  9  from the center, then the movable mirror  3  will be moved immediately to successively correct the optical path of incidence of the light beam so as to receive the spot image S at the center of the position detector  9 , thereby preventing the incident light beam from dropping off the apparatus. 
     Here, the position detector  9  selectively detects only an ac pilot signal of a specific frequency in order to avoid influence of background light around the party apparatus, i.e., in order to prevent the position detection signal from being drawn to a strong background light so as to cause an error. This pilot signal is generated from the oscillator  15  and is multiplexed with the transmitted signal in the multiplexer  12  to be transmitted to the party apparatus. 
     FIG. 2 is a front elevation of a position detector  9  divided into four sections. The four segmental photodetecting elements  9   a  to  9   d  obtain the position of the spot image S by comparison of their outputs. FIG. 3 shows a two-dimensional light position detector  9   e,  in which a vertical position of the spot light S on this position detector  9   e  is detected by a difference between input voltages at the positive input terminal and at the negative input terminal of amplifier  22  and a horizontal position thereof by a difference between input voltages at the positive input terminal and at the negative input terminal of amplifier  23 . In either case of FIG.  2  and FIG. 3, the background light, in addition to the spot light S, is also incident on the position detector  9   a  to  9   e,  and components of frequencies other than that of the pilot signal are removed by an electric circuit such as a filter or a frequency selective amplifier at the rear end of the position detector and are thus not detected. 
     The tracking function stated above does not work unless the light beam from the party apparatus arrives at a level that can be received and unless the spot image S impinges on a part of the position detector  9 . In initial adjustment during installation of the apparatus, an operator fixes the movable mirror  3  at an initial position near the middle point and manually adjusts the direction of the apparatus while observing the party apparatus through the collimation scope  21 . 
     The optical space communication apparatus of the conventional example described above, however, has the following problems, because it uses the pilot signal in order to avoid the influence of the background light. 
     (1) The apparatus has to incorporate an oscillator  15  and a multiplexer  12  of the pilot signal and the receiving section has to include a circuit for extracting only the pilot signal. Therefore, the cost is high. 
     (2) Since the pilot signal is superimposed on the transmitted signal, signal interference such as intermodulation, occurs because of nonlinearity of the light-emitting device  6 , the light receiving element  11 , the amplifier  16 , and so on. This degrades the quality of the signal. 
     (3) In order to prevent the frequency band of the transmitted signal from overlapping with that of the pilot signal, the frequency band of the transmitted signal is limited. Further, since the position detector  9  normally has a small photoreceptive area, the initial adjustment needs to be conducted by observing the party apparatus through the collimation scope  21  in order to make the received light incident correctly. Since the positional relation needs to be adjusted with precision between the collimation scope  21  and the position detector  9 , there is the cost necessary for installation and adjustment of the collimation scope  21 . 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an optical space communication apparatus that eliminates the influence of background light without using a pilot signal, solving the above problems. 
     An optical space communication apparatus according to the present invention for accomplishing the above object is an optical space communication apparatus for performing communication with a party apparatus by transmitting a light signal into free space, where the optical space communication apparatus and the party apparatus are spaced apart at two positions opposite each other. The optical space communication apparatus comprises signal selecting means for selecting and intercepting a beam including a light beam and background light from the party apparatus, detecting means for detecting a beam via the signal selecting means, and signal outputting means for comparing an output signal output from the detecting means when the signal selecting means intercepts the light beam from the party apparatus, with an output signal output from the detecting means when the signal selecting means does not intercept a light beam from the party apparatus, and for outputting a signal representing a difference between the output signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the structure of a conventional optical space communication apparatus; 
     FIG. 2 is a front elevation of an example of the position detector; 
     FIG. 3 is a diagram showing the structure of another conventional position detector; 
     FIG. 4 is a diagram showing the structure of the first embodiment of the optical space communication apparatus according to the present invention; 
     FIGS. 5A,  5 B and  5 C are graphs for explaining elimination of the influence of background 
     FIG. 6 is a diagram showing the structure of a second embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail with reference to the embodiments illustrated in FIGS. 4 to  6 . 
     FIG. 4 is a diagram showing the structure of the first embodiment of the optical space communication apparatus according to the present invention, in which a lens  30  for transmission and reception is located at a position opposite a party apparatus (not shown) and in which a lens  31  and a movable mirror  32  are placed on the optical path behind the lens  30 . On the optical path in the direction of incidence of the movable mirror  32  there is a polarization beam splitter  33  which functions as a polarization beam splitter for a light beam in the infrared wavelength region, such as a signal light. The polarization beamsplitter  33  transmits the polarization component parallel to the plane of the drawing but reflects the polarization component normal to the plane of the drawing by its combination face. The optical path further includes a lens  34 , and a light-emitting device  35 , such as a semiconductor laser for emitting a light beam of infrared wavelength region. The output of a transmitted signal input terminal  36  is connected through an amplifier  37  to the light-emitting device  35 . 
     A light splitting mirror  38  is placed in the direction of reflection of the polarization beam splitter  33 . On the optical path in the direction of reflection of the beam splitting mirror  38  there are a lens  39  and a light receiving element  40 , such as an avalanche photodiode or a pin diode, and the output of the light receiving element  40  is connected through an amplifier  41  to a received signal output terminal  42 . 
     On the optical path in the direction of transmission of the beam splitting mirror  38  there are arranged, in order, a liquid crystal sheet  43 , a polarizing plate  44  for transmitting polarized light normal to the plane of the drawing, a lens  45 , and a CCD imaging device  46  serving as a position detector and as a finder for collimation. The output of the liquid crystal control unit  47  is connected to the liquid crystal sheet  43 . The liquid crystal sheet  43  and the polarizing plate  44  may be positioned between the lens  45  and the CCD imaging device  46 . 
     The output of the CCD imaging device  46  is successively coupled to a CCD control circuit  48 , an A/D converter  49 , an image memory  50 , a subtracter  51 , and a tracking control circuit  52 . The output of the CCD control circuit  48  is also connected to a finder monitor terminal  53  and the output of the A/D converter  49  is also connected directly to the subtracter  51 . The output of the tracking control circuit  52  is connected through two actuators  54 ,  55  to the movable mirror  32  and is also connected to the liquid crystal control unit,  47  and to the image memory  50 . 
     On the occasion of transmission, the transmitted signal is input via the transmitted signal input terminal  36  and the transmitted signal is amplified by the amplifier  37  to be output to the light-emitting device  35 . The transmitted signal is converted into a light signal in the light-emitting device  35 , and the light signal is guided through the lens  34  to the polarization beam splitter  33 . Since this beam is polarized in parallel to the plane of the drawing, the beam is transmitted by the polarization beam splitter  33  as it is, and it is then reflected to the left by the movable mirror  32  to be emitted through the lenses  31 ,  30  toward the party apparatus. 
     On the occasion of reception, a light beam from the party apparatus is incident into the lens  30  from the left, passes through the lens  31  to be reflected downward by the movable mirror  32 , and then reaches the polarization beam splitter  33 . Since this beam is polarized normally to the plane of the drawing, the beam is reflected to the right by the bond surface of the polarization beam splitter  33  and is then split into two directions by the beam splitting mirror  38 . The beam reflected by the beam splitting mirror  38  is guided through the lens  39  and then is received by the light receiving element  40  to be converted into an electric signal. Thereafter, the electric signal is amplified to an appropriate level by the amplifier  41  and the amplified signal is output from the received signal output terminal  42 . 
     On the other hand, since the beam transmitted by the beam splitting mirror  38  is polarized normally to the plane of the drawing, in a state in which a voltage is not applied to the liquid crystal sheet  43  by a timing signal T from the tracking control circuit  52 , the beam passes through the liquid crystal sheet  43  and polarizing plate  44  with little attenuation, then passes through the lens  45 , and enters the CCD imaging device  46 . The background light also passes on the same optical path to enter the CCD imaging device  46 , but it is attenuated to half intensity during transmission through the polarizing plate  44 , because the background light includes all the polarization components and the polarization beam splitter  33  functions as a typical beam splitter to the light beam of infrared wavelength region such as the background light. The received signal light and background light incident on the CCD imaging device  46  is converted into a video signal by the CCD control circuit  48  and the video signal is converted into a digital signal by the A/D converter  49 . Video information of one field or one frame is written into the image memory  50 . One line along the time axis near the central part of the image information thus written is, for example, as illustrated in FIG.  5 A. The peak in the central part is the received signal light. 
     When voltage is applied to the liquid crystal sheet  43 , the liquid crystal sheet  43  develops optical rotary power, so that the direction of polarization of the light transmitted thereby rotates 90°. As a result, most of the received signal light does not pass through the polarizing plate  44 , whereas the background light is also attenuated to half intensity. Therefore, there is no change in the intensity of the background light between before and after the application of voltage to the liquid crystal sheet  43 . Accordingly, only the background light is incident on the CCD imaging device  46  and the image signal is sent as a signal D 2  via the CCD control circuit  48  and A/D converter  49  to the subtracter  51 . On the other hand, the image signal written in the image memory  50  is read at the same timing so as to be sent as a signal D 1  to the subtractor  51 . The subtracter  51  executes subtraction between the signal D 2  and the signal D 1  using hardware, e.g., by means of a logic circuit, or using software, e.g., by means of the CPU, and outputs the result. 
     The signal of the line corresponding to FIG. 5A, in the signal D 2  includes the background light and signal light, while a signal of a line corresponding to FIG. 5B, includes almost the same background light but no signal light. Since there is little difference between capture times of the signal of FIG.  5 A and the signal of FIG. 5B, there is little change of the background light during the period between the capture times. The subtraction between them eliminates the components of the background light and extracts only the signal light to obtain the signal as illustrated in FIG.  5 C. This signal is output as a signal D 3  to the tracking control circuit  52 . The tracking control circuit  52  checks the position of the peak of the signal D 3  to detect the position of the signal light spot. Since the signal D 3  is free of noise due to the components of the background light, the peak position can be detected readily and with accuracy. This method is also free of any error caused when the position detection result is drawn to a strong background light. 
     Control of timing for transmission/non-transmission of the signal light by the signal selecting means and timing of reading/writing from or in the image memory  50  is carried out by the timing signal T from the tracking control circuit  52 . The tracking control circuit  52  generates driving signals for the actuators  54 ,  55 , based on the position signal detected, thereby adjusting the angle of the movable mirror  32  in the same manner as in the conventional example. 
     The video signal from the CCD control circuit  48  is branched and output to the finder monitor terminal  53 . A television monitor can be connected to the finder monitor terminal  53  to observe the picture picked up by the CCD imaging device  46 . This obviates the need for the collimation scope and permits observation from a remote place by use of an extension cable. 
     FIG. 6 is a diagram showing the structure of a second embodiment, in which a position detector  56 , similar to the four segmental elements  9   a  to  9   d  of FIG. 2 or to the two-dimensional light position detector  9   e  of FIG. 3 in the conventional example, is used as the position detector  9 . The output of the position detector  56  is connected to the tracking control circuit  57 , similar to that in the conventional example, and the present embodiment does not use the CCD imaging device  46 , CCD control circuit  48 , A/D converter  49 , image memory  50 , subtracter  51 , and tracking control circuit  52  of the first embodiment. The apparatus of the present embodiment is equipped with a collimation scope  58 , similar to the conventional example, and the present embodiment does not use the pilot signal oscillator  15  and the multiplexer  12  of the conventional example. The other structure is similar to that of the first embodiment and the same members are denoted by the same symbols. 
     First, in the state in which voltage is not applied to the liquid crystal sheet  43 , the received signal light and background light both pass through the liquid crystal sheet  43 , the polarizing plate  44  and the lens  45  to enter the position detector  56 . At this time, the position detector  56  outputs a position detection signal that represents a position corresponding to the center of gravity of intensity distribution of light which is superposition of the received signal light and the background light. 
     The tracking control circuit  57  temporarily stores this signal in the memory of the CPU. Since the amount of information to be stored is small, an extra memory like that in the first embodiment is not necessary. Next, voltage is applied to the liquid crystal sheet  43  to intercept the received signal light, so that only the background light is incident on the position detector  56 . The output from the position detector  56  at this time represents the position of the center of gravity of intensity distribution of the background light. The tracking control circuit  57  subtracts the position signal resulting from only the background light, obtained this time, from the position signal resulting from the superposition of the received signal light and background light, stored previously, thereby obtaining a position signal of the component of only the received signal light with the background light components being canceled out. This permits accurate position detection of only the received signal light, independent of the state of the background light. 
     The present embodiment incorporates a collimation scope  58 , and therefore does not use a pilot signal. Therefore, the present embodiment is free of any degradation of signal due to interference of the pilot signal and of any limitation of the frequency band of the signal. 
     In addition to the combination of the liquid crystal sheet  44  and the polarizing plate  45  in the first and second embodiments, the following method may be provided as signal selecting means for transmitting/intercepting only the received signal light at desired timing without affecting the intensity of the background light incident to the position detector  56 . 
     (1) A half wave plate may be used instead of the liquid crystal sheet  43 , where the half wave plate rotates the plane of polarization of emerging light by an angle equal to double the angle between the direction of polarization of incident light and the crystallographic axis. Therefore, switching between transmission and interception of the signal light can be effected by rotating the half wave plate 45° about the optical axis of the optical system (lens  45 ). 
     (2) Only the polarizing plate  44  may be used without using the liquid crystal sheet  43 , where the polarizing plate  44  is arranged to be capable of rotating about the optical axis of the optical system (lens  45 ). The switching between transmission and interception of the signal light can be achieved by rotating the polarizing plate 90°. 
     (3) The signal light is normally light in the infrared wavelength region of semiconductor laser light or the like, whereas the background light is mostly light in the visible wavelength region. Accordingly, instead of the combination of the liquid crystal sheet  43  with the polarizing plate  44 , a wavelength selective optical filter that transmits visible light but cuts infrared light may be interposed on the received light path, whereby the switching from transmission to interception can be achieved only of the signal light. As an example, the switching between transmission and interception of signal can be achieved by using a disk glass plate. a semicircular part of which is processed as a wavelength transmitting filter and by rotating the disk glass plate about the optical axis thereof. 
     The above embodiments illustrate an optical space communication apparatus having an autotracking function, but the present invention can also be applied to an angle detecting method of apparatus adapted to perform adjustment of angle by hand without the autotracking function. In this case, the adjustment work is carried out manually by adjusting the angle while observing an indication of a meter or a display of numbers or the like of the output from the position detector  46 ,  56 , and so on, whereby accurate angle adjustment can be achieved without being affected by the background light, as in the case of autotracking. 
     As described above, the optical space communication apparatus according to the present invention can perform accurate angle detection at low cost without using a pilot signal for detection of angle and without being affected by the background light, and the apparatus can perform optical communication with high accuracy, because the transmitted/received signal is not affected by the pilot signal.