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 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 on the position detector 9a to 9e, 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.
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.