Optical multiplexer

An optical multiplexer that multiplexes a plurality of laser beams having different wavelengths and emits a multiplexed laser light, the optical multiplexer comprising:

• a plurality of light sources that emit a plurality of laser beams having different wavelengths;
• a collimator that collimates the plurality of laser beams emitted from the plurality of light sources;
• and
• a diffraction grating that diffracts the plurality of laser beams collimated by the collimator and emits the plurality of laser beams along a same optical path, the diffraction grating being of a transmission-type,
• wherein the plurality of light sources are linearly arranged on an incident side focal plane of the collimator,
• a grating surface of the diffraction grating is disposed on an emission side focal plane of the collimator,
• a direction along which the plurality of light sources are arranged and a direction along which grooves of the diffraction grating extend are orthogonal to each other, and
• a distance D between two adjacent light sources among the plurality of light sources is set to satisfy D=f×(λ1−λ2)/p, where λ1, λ2 (λ1>λ2) are respectively wavelengths of laser beams emitted from the two adjacent light sources, f is a focal length of the collimator, and p is a pitch of the diffraction grating.