1. Field of the Invention
The present invention relates to a reflective collection-type light receiving unit and a light receiving apparatus for spatial light communications using the reflective collection-type light receiving unit and more particularly to a reflective collection-type light receiving unit for spatial light communications for performing communications using visible light or infrared light radiated in a space and a light receiving apparatus for spatial light communications in which a plurality of reflective collection-type light receiving units are arranged in an array to realize communications higher in sensitivity and responsiveness.
2. Description of Related Art
Wireless communications by using radio waves as a communication medium have found many uses in various fields such as cellular phone networks, wireless LANs and short-distance wireless communications.
However, in wireless communications in which radio waves are used as a medium, it is impossible to increase the transmission power, with the influence of electromagnetic waves on the human body taken into account, where the transmitting and receiving are performed near people. Further, since the frequency range of radio waves used in wireless communications has already been allocated in many areas and actually used, it is impossible to use freely frequencies over a wide range. Still further, under special circumstances such as medical institutions, there are some restrictions, for example, the limited use of radio waves.
Thus, in recent years, there has been developed visible light communications in which visible light is used as a communication medium, and a visible light communication system has been proposed in Japanese Published Patent Application No. 2007-266794 A.
In this type of a conventional visible light communication system, ordinarily, a light receiving element equipped with an optical system including a collecting lens is used as a light receiving unit on the side of a receiver, and light projected from a projector on the side of a transmitter is collected by the optical system and focused on the light receiving element, thereby outputting a light receiving signal. In particular, in visible light communications movable in a relative position between the transmitter and the receiver, in order to increase a light collecting percentage of light, with consideration given to a case where light radiated from the projector of the transmitter is lower in the amount of light arriving at a light receiving unit of the receiver, the optical system including the collecting lens is required to be larger in size, the light receiving unit is made relatively larger in configuration, and in particular the use of the collecting lens increases the thickness dimension of the light receiving unit, thus posing a problem that it is difficult to downsize the optical system.
On the other hand, Japanese Published Patent Application No. H10-322276 A has disclosed a light receiving unit for spatial light communications in which a reflector lens is arranged on a light receiving path of a light receiving element to decrease the thickness dimension of the light receiving unit. However, in this reflective collection-type light receiving unit, a convex mirror and a concave mirror are arranged inside the reflector lens to constitute an optical system and a light receiving element is arranged behind the reflector lens, thereby posing another problem that the optical system is increased in size.
Further, the light receiving unit of this type of a conventional spatial light communication apparatus is constituted in such a manner that a plurality of reflective collection-type light receiving units are arranged in an array and a light receiving element of each of the reflective collection-type light receiving units is ordinarily mounted on a substrate of a light receiving circuit. In particular, where the plurality of light receiving elements are connected in parallel to the input side of an amplifying part of the light receiving circuit, a floating electrostatic capacitance generated between a pair of electrode leads of each light receiving element and the ground is multiplied only by the number of light receiving elements to be used and increased accordingly. There has been posed such a problem that an electrostatic capacitance generated on the electrode leads of the anode and the cathode of the light receiving element is increased to reduce a response speed of the light receiving element, thereby inhibiting spatial light communications at high speed.