The present invention relates to an optical communication system capable of transmitting and receiving an optical signal by means of an optical fiber and relates, in particular, to an optical communication system applicable to domestic communication, communication between electronic equipment, LAN (Local Area Network) and so on using a plastic optical fiber as a transmission medium.
An optical communication system using an optical fiber has a transmission system on one end side of a signal transmission path of the optical fiber and has a reception system on the other end side of the transmission path. The transmission system includes a light source (light emitting element) of, for example, a light emitting diode, a semiconductor laser or the like and makes a signal light, which is obtained by making the light emitting source emit light under control, incident on the optical fiber. On the other hand, the reception system includes a light receiving element of, for example, a photodiode or the like, and the light receiving element receives the signal light emitted from the optical fiber and converts the signal into an electrical signal.
The performance of the optical communication system largely depends on the transmission efficiency of the signal light. Moreover, the transmission efficiency is determined mainly by the transmission efficiency of the optical fiber itself, the coupling efficiency of the light emitting source to the optical fiber and the coupling efficiency of the optical fiber to the light receiving element.
The reception systems of the conventional optical communication systems are roughly categorized into two types: a type that receives outgoing light from the optical fiber directly on the light receiving element and a type that receives the outgoing light by collecting the light via an optical system such as f a lens or the like arranged between the optical fiber and the light receiving element.
Such a scheme of optical coupling between the optical fiber and the light receiving element is widely used for a quartz fiber whose core diameter is on the order of micrometers. However, a problem occurs in the case of a plastic optical fiber whose core diameter is on the order of millimeters. The plastic optical fiber is the optical fiber that has recently attracted attention for home network and the like. The plastic optical fiber has a large fiber diameter of 0.5 to 2 mm and is easy to connect, whereas it has a problem that the coupling efficiency to the receiver is reduced because of a large aperture. Normally, the diameter of light reception of the light receiving element used for optical fiber communication is several hundreds of micrometers to one hundred micrometers, and accordingly, there is no problem in the case of an optical fiber of a small core diameter. However, in the case of, for example, a plastic optical fiber having an aperture of 1 mm, it is difficult to collect light to a size smaller than the size of the light source even when a lens or the like is used. Particularly, the diameter of light reception is required to be reduced in relation to capacity as the transmission rate is increased, and therefore, a reduction in the coupling efficiency, i.e., the reception efficiency occurs.
To solve the problem, an optical communication system, which has a structure for coupling the optical fiber with the light receiving element as shown in FIG. 25, is known. In this optical communication system, an optical guide 101, which has an optical guide path 102 enclosed by a highly reflective reflection surface 103, is placed between an optical fiber 104 and a light receiving element 105, and a signal light emitted from the optical fiber 104 is guided to the light receiving element 105 by the optical guide 101. With this arrangement, the optical fiber 104 is optically coupled with the light receiving element 105 with high efficiency, and even outgoing light from an optical fiber of a large core diameter such as the plastic optical fiber can efficiently be collected to a photodiode of a small aperture (refer to JP 10-221573 A, paragraph 0008 and FIGS. 1 and 3).
In the case of the structure as shown in FIG. 25, when the numerical aperture (NA) of the light emitted from the optical fiber is changed, and particularly enlarged, there is a drawback that the outgoing light 106 tends to return to the optical fiber 104 side as shown in FIG. 26, so that the coupling efficiency is reduced. Moreover, the structure has a manufacturing problem that the aspect ratio of the hole depth to the aperture is large, so that it is difficult to uniformly deposit a reflection coating.