Patent ID: 8594142

Claim:
A wavelength-tunable light source comprising: a quantum cascade laser that emits light from respectively a first end and a second end; an optical system that collimates the light emitted from the first end; a first reflecting section on which the light collimated by the optical system is made incident, the first reflecting section diffracts a light at a particular wavelength in the incident light in a direction opposite to the incident direction, and returns the light at the particular wavelength to the first end; and a second reflecting section that partially reflects the light emitted from the second end and transmits the remaining light, the second reflecting section composes an external resonator with the first reflecting section, wherein the first reflecting section includes a plurality of diffractive gratings whose diffractive properties are different from each other and whose lattice plane directions are variable, and the light collimated by the optical system is made incident on one diffractive grating selected from the plurality of diffractive gratings, and the first reflecting section diffracts a light at a particular wavelength corresponding to the diffractive property and the lattice plane direction of the selected diffractive grating in the direction opposite to the incident direction, wherein the plurality of the respective diffractive gratings are blazed diffractive gratings whose lattice periods are equal to each other and whose groove depths are different from each other, wherein the first reflecting section is provided with diffractive gratings on a plurality of the respective side surfaces of a polygonal pillar shaped rotating member which is freely rotatable around a predetermined rotation central axis, and wherein the light collimated by the optical system is made incident on one diffractive grating selected from the plurality of diffractive gratings according to a rotational position of the rotating member, and a lattice plane direction of the selected diffractive grating is set according to a rotational position of the rotating member.