Diffraction element

Abstract

A diffraction element having concave/convex-like diffraction gratings in its two surfaces from which at least two separated light beams can be taken in the same direction without changing largely the propagating direction of diffracted light even if the temperature of the operating environment changes. A diffraction grating having a concave/convex shape in cross-section is formed in the incoming-side surface of the transparent substrate and two diffraction gratings of concave/convex shape in cross-section are formed in the outgoing-side surface wherein the grating pitch of the first one is made equal to the grating pitch of one of the second ones. In addition, a reflection type diffraction element exhibiting a good wavelength dependence of diffraction efficiency without being dependent largely on the direction of polarization of an incoming light is provided. A pseudo sawtooth-like diffraction grating is formed in either surface of the transparent substrate, a reflective film is formed on a diffraction grating portion, and an antireflective film is formed on the opposite surface.

1 . A diffraction element comprising a transparent substrate and diffraction gratings formed in both surfaces of the transparent substrate and having a concave/convex shape in cross-section;
 wherein an incoming-side surface, into which an external light is incident, in both surfaces of the transparent substrate is provided with an incoming-side diffraction grating in its central region, and two outgoing-side diffraction gratings are formed in an outgoing-side surface which is opposite to said incoming-side surface; and   wherein the two outgoing-side diffraction gratings are formed on light paths of the external light diffracted by said incoming-side diffraction grating, the concave/convex shape in cross-section of each of the two outgoing-side diffraction gratings comprises a sawtooth-like diffraction grating or a pseudo sawtooth-like diffraction grating whose shape is approximated by stairs, and a grating pitch of the incoming-side diffraction grating is substantially equal to a grating pitch of at least one of the two outgoing-side diffraction gratings.   
   
   
       2 . The diffraction element according to  claim 1 , wherein the concave/convex shape in cross-section of the incoming-side diffraction grating comprises a sawtooth-like diffraction grating or a pseudo sawtooth-like diffraction whose shape is approximated by stairs. 
   
   
       3 . The diffraction element according the  claim 1 , wherein the diffraction gratings are formed directly in the surfaces of the transparent substrate. 
   
   
       4 . The diffraction element according to  claim 1 , wherein the diffraction gratings are formed in an inorganic film formed on the surfaces of the transparent substrate. 
   
   
       5 . The diffraction element according the  claim 1 , wherein the external light is incident obliquely with respect to a normal line of a surface of the substrate with the incoming-side diffraction grating formed therein; a grating pitch of one of the two outgoing-side diffraction gratings is substantially equal to the grating pitch of the incoming-side diffraction grating; and a pitch of the other outgoing-side diffraction gratings wider than the grating pitch of the incoming-side diffraction grating.