Planar waveguide amplifier and laser radar device

A planar waveguide amplifier comprising a planar waveguide including:

• a flat plate-like core to amplify signal light, with the core excited by pumping light;
• a first cladding to reflect light having propagated from the core back to the core, the first cladding being provided on a first principal face of the core;
• and
• a second cladding to reflect light having propagated from the core back to the core, the second cladding being provided on a second principal face of the core on an opposite side to the first principal face, wherein
• the signal light and the pumping light travel into the planar waveguide so that the signal light and the pumping light propagate inside the core in such a manner that optical paths of the signal light and the pumping light overlap each other,
• wherein a state in which the optical paths of the signal light and the pumping light overlap each other includes a state in which optical axes of the optical paths match each other and a state in which the optical axes are slightly shifted from each other such that a beam cross-section of the signal light and a beam cross-section of the pumping light overlap each other so as to include their optical axes,
• wherein the signal light and the pumping light travel into the planar waveguide so that the signal light and the pumping light propagate inside the core in directions opposite to each other along a zig-zag path in the plane of the planar waveguide amplifier, so that a portion through which the signal light and the pumping light do not pass is formed inside the core, and
• the core is an amplification medium that contains a rare-earth element serving as an active ion of a three-level system, and absorbs scattered light of the signal light and amplified spontaneous emission in a portion through which the pumping light does not pass.