Optical device and photodetection system

An optical scan device comprising:

• a first mirror having a first reflecting surface and extending along a first direction;
• a second mirror having a second reflecting surface that faces the first reflecting surface and extending along the first direction;
• and
• an optical waveguide layer, located between the first mirror and the second mirror, that causes light to propagate along the first direction,
• wherein
• a transmittance of the first mirror is higher than a transmittance of the second mirror,
• a reflection spectrum of at least either the first mirror or the second mirror with respect to light arriving from a direction normal to the reflecting surface includes, in a wavelength region in which a reflectance is higher than or equal to 90%, a local maximum point PLM and first and second points of inflection P1 and P2 located closer to a long-wavelength side than the local maximum point, and
• light emitted via the first mirror is polarized to do a scan on a space.