Optical transmitter including rare earth-doped fiber

Claim 1

An optical transmitter comprising: a signal light source for outputting signal light having a first predetermined plane of polarization; a pumping light source for outputting pumping light having a second predetermined plane of polarization; a first Faraday rotator for rotating 45 degrees the plane of polarization of the signal light from said signal light source, said first Faraday rotator rotating a plane of polarization of a reflected and returning beam transmitting therethrough backwardly so that the plane of polarization of the reflected and returning beam output from said first Faraday rotator becomes perpendicular to the first predetermined plane of polarization; a second Faraday rotator for rotating 45 degrees the plane of polarization of the pumping light from said pumping light source, said first and second Faraday rotators being arranged such that the plane of polarization of the signal light output from said first Faraday rotator and the plane of polarization of the pumping light output from said second Faraday rotator are perpendicular to each other, said second Faraday rotator rotating a plane of polarization of a reflected and returning beam transmitting therethrough backwardly so that the plane of polarization of the reflected and returning beam output from said second Faraday rotator becomes perpendicular to the second predetermined plane of polarization; a polarization coupler for polarization-coupling the signal light from said first Faraday rotator and the pumping light from said second Faraday rotator and outputting the coupled light, said polarization coupler including a polarization beam splitting film transmitting both the signal light from said first Faraday rotator and a first component of the reflected and returning beam having a plane of polarization inclined at 45 degrees relative to the first predetermined plane of polarization and reflecting both the pumping light from said second Faraday rotator and a second component of the reflected and returning beam having a plane of polarization inclined at 45 degrees relative to the second predetermined plane of polarization, said polarization beam splitting film thus multiplexing the signal light and the pumping light and polarization splitting the reflected and returning beam; and a rare earth-doped fiber into which the signal light and the pumping light from said polarization coupler are introduced.