Recently, as a light source of an optical information processing technology, for example, research and development of a visible light laser such as green light laser and blue light laser are under way. As a type of visible light lasers, a wavelength conversion laser device that applies a wavelength conversion technique to make near infrared laser beams have a shorter wavelength has been known. Generally, in the wavelength conversion laser device, a wavelength conversion element made of a nonlinear optical material is provided inside or outside of an optical resonator of a semiconductor laser or a solid-state laser, and laser beams (fundamental waves) generated by the optical resonator are propagated to the wavelength conversion element, thereby outputting a second harmonic, which is wavelength-converted to half a wavelength (a double frequency) with respect to the fundamental waves.
At this time, an oscillation wavelength bandwidth of the optical resonator needs to be matched with a phase matching width of the wavelength conversion element. However, the phase matching width of the wavelength conversion element is generally very narrow, and an output of the wavelength conversion laser device fluctuates due to its external environment. Therefore, to achieve wavelength conversion by the wavelength conversion element in a highly efficient manner, a coherent light source has been proposed as a wavelength conversion laser device having a configuration such that a light source wavelength is fixed within a tolerance of the wavelength conversion element so that it is not much affected by a change of the external environment (see, for example, Patent Document 1). In this conventional coherent light source, fundamental waves from the laser medium are converted to a harmonic by the wavelength conversion element, and the fundamental waves reflected by a reflector are returned to the laser medium, thereby fixing the oscillation wavelength of the laser medium to the wavelength of return light so that the oscillation wavelength of the laser medium is automatically fixed to a phase matching wavelength of the wavelength conversion element.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-19603