1.  Field of the Invention
The present invention relates to an optical space transmission apparatus which communicates from a communication apparatus installed at one point with communication apparatuses installed at a plurality of points using an optical signal which propagates through the air. 
2.  Description of the Related Art
Using FIG.  6, an overview of a conventional optical space communication system will be explained.  Here, FIG.  6 is a schematic view of the conventional optical space communication system (Japanese Patent Laid-Open No.  2000-224112 (European Patent No.  1054520B1: European equivalent to the Japanese Patent)) and as shown in this schematic view, the optical space communication system is constructed by an optical space transmission apparatus 60 and remote apparatuses 61a to 61c.  
An optical signal radiated from a light source 62 of the optical space transmission apparatus 60 is changed to a substantially parallel light beam 64 which is spread a little when passing through an optical system 63, this light beam 64 is reflected on a movable mirror 65 and transmitted to the respective remote apparatuses 61a to 61c.  
Furthermore, the movable mirror 65 is driven according to a preset sequence and angle, and when a transmission to the remote apparatus 61a is completed, then the movable mirror 65 directs the light beam to the remote apparatus 61b, and when a transmission to the remote apparatus 61b is completed, then the movable mirror 65 directs the light beam to the remote apparatus 61c and transmits the light beam to the remote apparatus 61c.  In this way, by scanning the remote apparatuses sequentially and performing communications, communications are established between a communication apparatus installed at one point and communication apparatuses installed at a plurality of points. 
This scanning is performed at a high speed and the users of the respective remote apparatuses need not be aware of a waiting time when receiving signals.  Moreover, an optical signal is free of restrictions on the frequency band as in the case of radio waves, and can thereby transmit information at a high speed and communicate a sufficient volume of information even through intermittent transmissions using sequential scanning. 
Moreover, in the above described Japanese Patent, it is proposed that a center apparatus scans respective remote apparatuses using a mirror, is provided with a light source and an optical detector to perform bidirectional communications with the remote apparatuses. 
Furthermore, for reasons of safety of eyes, to prevent degradation of the communication quality of optical radio waves generated due to attenuation of light beams caused by weather conditions such as rain and snow under conditions in which the output levels of light beams are limited, it is necessary to narrow the diameters of light beams and correctly direct the light beams to the remote apparatuses.  As a method of realizing this, in the Japanese Patent Laid-Open No.  2000-224112, it is proposed about a function in which five photodiodes are arrayed and the directions of light beams based on the outputs of the respective photodiodes are corrected. 
Moreover, a method of calculating the directions of the remote apparatuses by projecting the light beams sent from the remote apparatuses onto an optical position detection element is also widely known as a method of correcting the directions of light beams. 
However, for the method of changing the angle of the mirror according to a preset sequence and angle and sequentially scanning all the remote apparatuses, communication channels are also established with remote apparatuses not requiring communications.  For this reason, when many remote apparatuses are scanned, the mirror is also driven to irradiate light beams to remote apparatuses not requiring communications, wasting the time to decide that communications are not necessary, unable to allocate sufficient communication times to remote apparatuses requiring communications.  Furthermore, the method of using an array of five photodiodes to correctly direct light beams to remote apparatuses and finding exact directions of the remote apparatuses based on the outputs of the respective photodiodes and the optical position detection elements, etc.,  is adopted, but when the power of a remote apparatus is OFF, the center apparatus cannot identify the remote apparatus and wastes time until the center apparatus searches the remote apparatus and decides consequently that a communication therewith is not possible, unable to realize efficient optical space communications between one point and multi points. 
Moreover, even when the power of the remote apparatus is ON, a processing time for correctly directing light beams to remote apparatuses not requiring communications is wasted, unable to realize efficient optical space communications between one point and multi points.