Look holographic grating!

Holographic gratings generally refers to the principle of utilizing holography grating structure formed in a predetermined thickness, a relatively thick holographic grating is also called the volume holographic grating.

 

 

Recording holographic grating needs two coherent laser good, usually we put a bunch of them called the object beam, another beam called reference light. Was light and reference light form interference in the recording material to produce grating, called a hologram. Tilt the diffraction grating is a relatively simple law hologram. After formation of the grating, just use the reference light when recording with the same angle to illuminate the grating will diffract light effluent. And the thicker the higher the diffraction efficiency of the grating.

 

Holographic grating optical coupling when the need to satisfy the Bragg diffraction conditions, ie the angle of the incident light has very strict requirements, which led to an angle of the grating response only to a small range of angles of incident light, otherwise optical coupling will not occur, but directly through the grating. This severely limits the viewing angle. Therefore, the use of a lot of records HoloLens angles of inclination of the diffraction grating to increase the viewing angle. The more recording angle, meaning that the viewing angle increases. But not unlimited multi-angle recording continues, and if the difference between the two recording angles smaller than the angle selective Bragg diffraction conditions, it will cause crosstalk optical coupling process. And you want to get a smaller recording angle difference, the process will lead to a substantial increase in difficulty and the yield decreases, so the production costs increase. That is why Hololens full angle of 40 ° only reason to do so.