1. Field of the Invention
This invention relates to a transmitter-receiver for an optical signal for use for optical communication and a bidirectional optical amplifier for repeating and amplifying an optical signal.
2. Description of the Related Art
A bidirectional optical communication instrument in which transmission and reception are performed by means of a single optical fiber is conventionally known and disclosed, for example, in Hasegawa et al., "Waveguide Type Optical Separator-Coupler", Shimazu Review, Vol. 47, No. 3, pp.339-344, 1990. The bidirectional optical communication instrument of the type mentioned is shown in FIG. 6.
Referring to FIG. 6, in the conventional bidirectional optical communication instrument shown, an optical signal from a light emitting element 1a of an optical transmitter-receiver 4a is combined by an optical separator-coupler 13a and is sent to another optical transmitter-receiver 4b by way of an optical fiber 5. In the optical transmitter-receiver 4b, the optical signal is separated by an optical separator-coupler 13b and sent to a light receiving element 2b. On the contrary, a light signal from the optical transmitter-receiver 4b follows the route of a light emitting element 1b, the optical separator-coupler 13b, the optical fiber 5, the optical separator-coupler 13a and a light receiving element 2a. Bidirectional optical communication is performed in this manner.
Another bidirectional optical amplifier for repeating and amplification is also known and disclosed, for example, in C. Barnard et al., "Bidirectional Fiber Amplifiers" IEEE Photon Technol Lett , Vol 4, No 8, pp.911-913, 1992. The bidirectional optical amplifier just mentioned is shown in FIG. 7.
Referring to FIG. 7, in the bidirectional optical amplifier shown, each of a pair of Er doped fiber optical amplifiers 6a and 6b is pumped by pumping light from a light source 7. The bidirectional optical amplifier further includes a pair of band-pass filters 8a and 8b for separating pumping light and signal light from each other, a pair of directional couplers 9a and 9b each having a wavelength selection performance, and another directional coupler 10. The bidirectional optical amplifier further includes a pair of optical circulators 14a and 14b for suppressing returning light to prevent oscillations of the Er doped fiber optical amplifiers 6a and 6b.
In the bidirectional optical communication instrument shown in FIG. 6, light transmitted from the optical fiber 5 to the optical transmitter-receiver 4b is separated by the optical separator-coupler 13b and diverted to the light receiving element 2b. However, since the optical separator-coupler 13b serves as an equally branching path, the light is branched not only to the light receiving element 2b but also to the light emitting element 1b. This raises the problem of a loss of the reception signal, and the loss of approximately 3 dB is caused. Further, where the light emitting element 1b is a laser diode, there is another problem in that laser oscillations of it are disturbed by the reception signal branched to the light emitting element 1b side. Such phenomenon similarly applies to an optical signal from the optical transmitter-receiver 4b.
Meanwhile, the optical circulators 14a and 14b employed in the bidirectional optical communication instrument shown in FIG. 7 which employs the amplifiers 6a and 6b make use of a magneto-optical effect and are disadvantageous in that they are expensive in cost.