<p>A light source is provided that realizes a single spectral linewidth having a half value width of 1MHz or less and that is not influenced by the ambient temperature. A light source includes first laser (71) generating first laser light, second laser (12) generating second laser light, and nonlinear optical crystal (13) wherein the first laser light and the second laser light are injected into the nonlinear optical crystal to generate coherent light by the generation of a difference frequency or a sum frequency. The second laser (12) is a wavelength-tunable light source that includes therein a diffraction grating and that can sweep the wavelength of the second laser light. The first laser (71) is composed of semiconductor laser and a fiber grating that has a reflection bandwidth narrower than a resonance wavelength spacing determined by the laser chip length of the semiconductor laser.</p>
