Optical device and virtual image display device

A virtual image display device comprising:

• a light source to emit a pencil of rays;
• a collimating optical system to form the pencil from the light source into a parallel pencil;
• a scanning optical system to form, by horizontal and vertical scan, the parallel pencil into parallel pencil groups traveling in different directions;
• an optical waveguide to guide, by internal total reflection, parallel pencil groups meeting a condition of internal total reflection inside the optical waveguide;
• a first reflection volume hologram grating to diffract and reflect the parallel pencil groups incident upon the optical waveguide from the scanning optical system and traveling in different directions as they are so as to meet the condition of internal total reflection inside the optical waveguide;
• and
• a second reflection volume hologram grating to project the parallel pencil groups guided by internal total reflection inside the optical waveguide as they are from the optical waveguide by diffraction and reflection thereof so as to depart from the condition of internal total reflection inside the optical waveguide,
• wherein,
• some parallel pencils of the parallel pencil groups guided through the optical waveguide being totally reflected a different number of times for a period from external incidence upon the optical waveguide until outgoing from the optical waveguide,
• the second reflection volume hologram grating reflects and diffracts some parallel pencils of the incident parallel pencil groups a plurality of times to project the parallel pencils from the optical waveguide at each diffraction and reflections,
• the second reflecting volume hologram grating has, near the first reflection volume hologram grating, a first area having recorded therein a plurality of interference fringes having a second slant angle smaller than a first slant angle that is a slant angle of an interference fringe for a maximum diffraction efficiency of the parallel pencil groups diffracted and reflected perpendicularly to the second reflection volume hologram grating in order to diffract the incident parallel pencil groups with a maximum efficiency, and
• the second reflecting volume hologram grating has, far from the first reflection volume hologram grating, a second area having recorded therein a plurality of interference fringes having a third slant angle larger than the first slant angle in order to diffract the incident parallel pencil groups with a maximum efficiency.