In related art, a fluorescent light source device is known in which laser light is applied as excitation light to a phosphor to cause the phosphor to emit fluorescent light.
As illustrated in FIG. 16 of the accompanying drawings, one kind of such fluorescent light source devices includes a wavelength conversion member. In the wavelength conversion member, a fluorescent member 61 formed of a YAG sintered body (sintered compact) is disposed on an upper surface (front surface) of a substrate 62 formed of an AlN sintered body, with a barium sulfate layer 63 interposed therebetween (for example, see Patent Literature Document 1). A heat radiation member 65 having a heat radiation (dissipation) fin 65a is provided on a lower surface (back surface) of the substrate 62. In the fluorescent light source device, an upper surface of the fluorescent member 61 serves as an excitation light receiving surface of the wavelength conversion member and also serves as a fluorescent light emitting surface.
In such fluorescent light source device, however, it is difficult to emit fluorescent light, which is generated inside the fluorescent member 61, from the fluorescent light emitting surface with high efficiency. Thus, there is a problem that the fluorescent light is not used effectively. In addition, the fluorescent member 61 is heated by the fluorescent light that cannot exit from the fluorescent light emitting surface. This results in high temperature of the phosphor itself, and in turn causes temperature quenching. Accordingly, sufficient fluorescent light flux is not obtained.
To deal with such problem of the fluorescent light source device, there is known a technology that provides, on a surface configured to serve as the fluorescent light emitting surface of the wavelength conversion member, a periodic (cyclic) structure in which projections are arranged periodically, in order to improve emission efficiency of the fluorescent light from the fluorescent light emitting surface.
However, it is difficult to form a fine rugged (concave-convex) structure with a desired size on the fluorescent member itself because the fluorescent member is formed of a single-crystal or poly-crystal phosphor such as a YAG sintered body.
Accordingly, another technology is studied. Specifically, a periodic structure layer that is made from an easy-to-machine material such as a polymer material is stacked on a fluorescent member. The periodic structure layer includes projections arranged on the surface thereof. The periodic structure layer serves as the periodic structure. However, in particular, when the surface of a periodic structure layer is used as a fluorescent light emitting surface and also as an excitation light receiving surface, the shape of the periodic structure layer is deformed by heat of the laser light (i.e., excitation light), and emission efficiency of the fluorescent light from the fluorescent light emitting surface of the wavelength conversion member drops due to the deformation of the periodic structure layer.