Optical space communication apparatus

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 Field of the Invention The present invention relates to an optical space communication apparatus which performs communication between two points apart from each other by propagating a beam of a light signal modulated by a transmitted signal, in the atmosphere. 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 the wire communication using optical fibers and the like. The conventional communication apparatus employed 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, see for example EP 0 724 340. Fig. 1 is a diagram to show the structure of an optical space communication apparatus having the tracking function in a conventional example, in which a lens 1 for transmission and reception is placed at a position opposite to an 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 multiplexer 12 is connected to the light-emitting device 6, the output of transmitted signal input terminal 13 is connected through an amplifier 14 to the multiplexer 12, and the output of oscillator 15 is also connected to the multiplexer. 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, the 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 by 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 the strong background light 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 9a to 9d obtain the position of the spot image S by comparison of their outputs. Fig. 3 shows a two-dimensional light position detector 9e, in which a vertical position of the spot light S on this position detector 9e 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 to the position detector 9a to 9e, and the components of frequencies except for 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 in 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 the oscillator 15 and multiplexer 12 of the pilot signal and the receiving section has to include the circuit for extracting only the pilot signal. Therefore, the cost is high. (2) Since the pilot signal is superimposed on the transmitted signal, interference of signal 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 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 with 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. JP 07 154 326 discloses an optical transmission apparatus wherein the signal and background light are selected and detected in parallel. This leads to unstable results in case of drifting receivers. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical space communication apparatus that can eliminate the influence of the background light without using the pilot signal, solving the above problems. The above problems are overcome and the aforementioned object is achieved by the optical space communication apparatus according to claim 1 and the method of adjusting an angle of orientation of an optical system in an optical space communication system according to claim 10. The dependent claims relate to further developments. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram to show the structure of the optical space communication apparatus of the conventional example; Fig. 2 is a front elevation of an example of the position detector; Fig. 3 is a diagram to show the structure of another conventional example of the position detector; Fig. 4 is a diagram to show the structure of the first embodiment of the optical space communication apparatus according to the present invention; Fig. 5A, Fig. 5B, and Fig. 5C are graphs for explaining elimination of the influence of background light; and Fig. 6 is a diagram to show the structure of the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail by reference to the embodiments illustrated in Figs. 4 to 6. Fig. 4 is a diagram to show 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 the position opposite to the party apparatus and in which a lens 31 and a movable mirror 32 are placed on the optical path behind this lens 30. On the optical path in the direction of incidence of the movable mirror 32 there are a polarization beam splitter 33 which has a function to a light beam of infrared wavelength region such as a signal light to transmit the polarization component parallel to the plane of the drawing but to reflect the polarization component normal to the plane of the drawing by its combination face, a lens 34, and a light-emitting device 35 such as a semiconductor laser for emitting a light beam of infrared wavelength region, and the output of 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 liquid crystal control unit 47 is connected to the liquid crystal sheet 43. The liquid crystal sheet 43 and 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 outputted 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, the 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 outputted 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 to 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. 5A. The peak in the central part is the received signal light. When the voltage is applied to the liquid crystal sheet 43, the liquid crystal sheet 43 comes to have optical rotatory 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 the voltage to the liquid crystal sheet 43. Accordingly, only the background light is incident to the CCD imaging device 46 and the image signal is sent as a signal D2 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 to be sent as a signal D1 to the subtracter 51. The subtracter 51 executes subtraction between the signal D2 and the signal D1 by hardware by means of a logic circuit or by software by means of the CPU, and outputs the result. The signal of the line corresponding to Fig. 5A, in the signal D2 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. 5A 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. 5C. This signal is outputted as a signal D3 to the tracking control circuit 52. The tracking control circuit 52 checks the position of the peak of the signal D3 to detect the position of the signal light spot. Since the signal D3 is free of the 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 the error caused when the position detection result is drawn to the 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 to be outputted 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 to show the structure of the second embodiment, in which a position detector 56, similar to the four segmental elements 9a to 9d of Fig. 2 or to the two-dimensional light position detector 9e 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 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 the voltage is not applied to the liquid crystal sheet 43, the received signal light and background light both passes through the liquid crystal sheet 43 and polarizing plate 44 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 little, an extra memory like that in the first embodiment is not necessary. Next, the voltage is applied to the liquid crystal sheet 43 to intercept the received signal light, so that only the background light is incident to 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 needs to incorporate the collimation scope 58, but does not have to use the pilot signal. Therefore, the present embodiment is free of the degradation of signal due to interference of the pilot signal and of the limitation of the frequency band of signal. In addition to the combination of the liquid crystal sheet 44 and the polarizing plate 45 in the first and second embodiments, there are the following methods as the 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 is used instead of the liquid crystal sheet 43, and 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 is used without using the liquid crystal sheet 43, and 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 are embodied as the optical space communication apparatus having the 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 the autotracking. As described above, the optical space communication apparatus according to the present invention can perform the accurate angle detection at low cost without using the 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.

Claims: An optical space communication apparatus for performing communication with a party apparatus by transmitting a light signal into free space, said optical space communication apparatus and said party apparatus being spaced at two positions apart opposite to each other, said optical space communication apparatus comprising: detecting means (45, 46, 56) for detecting a beam; an optical system (30, 31, 32) for transmitting a light signal when performing communication with the party apparatus; and control means (54) for adjusting an angle of orientation of the optical system, characterized by further comprising: signal selecting means (43, 44) for selecting and intercepting incoming light including a light beam from the party apparatus and background light on an optical path to the detecting means; signal outputting means (49, 50, 51, 52, 57) for comparing an output signal (D2) outputted from said detecting means when said signal selecting means intercepts the light beam from said party apparatus, with an-output signal (D1) outputted from said detecting means when said signal selecting means does not intercept the light beam from said party apparatus, and for outputting a signal (D3) representing a difference between said output signals; and said control means adjust the angle of orientation of the optical system, based on the signal from said signal outputting means. The optical space communication apparatus according to claim 1, wherein said detecting means (46) is a CCD imaging device. The optical space communication apparatus according to claim 1, wherein said detecting means (56) is a four-divisional photodetector. The optical space communication apparatus according to claim 1, wherein said detecting means (56) is a two-dimensional light position detector. The optical space communication apparatus according to claim 1, wherein said signal selecting means (43, 44) comprises a liquid crystal sheet (43) and a polarizing plate (44). The optical space communication apparatus according to any one of claims 1 to 4, wherein said detecting means (45, 46) comprises a further optical system (45), and wherein said signal selecting means comprises a half wave plate capable of rotating about a rotational axis along the optical axis of said further optical system, and a polarizing plate (44). The optical space communication apparatus according to any one of claims 1 to 4, wherein said detecting means (45, 56) comprises a further optical system (45), and wherein said signal selecting means comprises a polarizing plate (44) capable of rotating about a rotational axis along the optical axis of said further optical system. The optical space communication apparatus according to any one of claims 1 to 7, wherein said signal selecting means comprise a wavelength selective optical filter capable of being changed over from an interposed state to a non-interposed state on a received light path, and vice versa. The optical space communication apparatus according to any one of claims 1 to 8, wherein said optical system comprises a movable mirror (32). A method for adjusting an angle of orientation of an optical system (30, 31, 32) in an optical space communication system for performing communication with an party apparatus by transmitting a light signal into a free space, said optical space communication system and said party apparatus being spaced at two positions apart opposite to each other, said method comprising: a step of selecting and intercepting a beam including a light beam from the party apparatus and background light by signal selecting means (43,44); a step of detecting a beam via said signal selecting means, by detecting means (45, 46, 56); a step of comparing an output signal outputted from said detecting means when said signal selecting means intercepts the light beam from said party apparatus, with an output signal output from said detecting means when said signal selecting means does not intercept the light beam from said party apparatus, and outputting a signal representing a difference between said output signals from signal outputting means; and a step system of adjusting the angle of orientation of the optical system, based on the signal from said signal outputting means.

Inventors: SAKANAKA TETSUO, SAKANAKA, TETSUO, Sakanaka, Tetsuo, c/o Canon K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1998
Main classification: H04B10
IPC codes: H04B10