| 摘要 |
PURPOSE:To permit improvement in signal receiving sensitivity by using a light source for monitoring, the wavelength of which is approximately equaled to the wavelength of carrier light and executing control on the reception side in such a manner that the plane of polarization of the light from the light source for monitoring and the plane of polarization of locally oscillated light are maintained in the same direction. CONSTITUTION:The light from at least one light source 23 for monitoring having the wavelength approximately equal to the wavelength of the carrier light is transmitted to a transmission path in such a manner that the plane of polarization thereof coincides with the plane of polarization of the carrier light 20 on the transmission side. The control is so executed on the reception side that the plane of polarization of the light fro the light source 23 and the plane of polarization of the locally oscillated 36 light are maintained in the same direction. Since the utilization of the light from the light source for monitoring for the control of the plane of polarization is thereby permitted, the need for using the energy of the signal light for the control is eliminated and the signal reception sensitivity is improved. |
