ANTI-REFLECTIVE COATINGS ON OPTICAL WAVEGUIDES
An anti-reflective waveguide assembly comprising a waveguide substrate having a first index of refraction, a plurality of diffractive optical elements disposed upon a first surface of the waveguide and an anti-reflective coating disposed upon a second surface of the waveguide. The anti-reflective coating preferably increases absorption of light through a surface to which it is applied into the waveguide so that at least 97 percent of the light is transmitted. The anti- reflective coating is composed of four layers of material having different indices of refraction that the first index of refraction and an imaginary refractive index less than 1×10 −3 but preferably less than 5×10 −4 .
1 . A method of propagating light, comprising:
propagating the light through an anti-reflective waveguide, comprising:
a planar waveguide substrate having a first index of refraction;
a plurality of diffractive optical elements disposed upon a first surface of the waveguide; and
an anti-reflective coating disposed upon a second surface of the waveguide.
2 . The method of claim 1 , wherein the waveguide is planar and the light propagates by total internal reflection between the plurality of diffractive optical elements and the anti-reflective coating in a substantially first direction, and outcouple light in a second direction substantially orthogonal to the first direction.
3 . The method of claim 2 , wherein the light propagating by total internal reflection comprises an s polarization component and a p polarization component.
4 . The method of claim 3 , wherein the anti-reflective coating reduces phase retardation between the two components such that an angle of incidence of the s component is substantially similar to that of the p component through the waveguide.
5 . The method of claim 4 , wherein the anti-reflective coating reduces reflection from and increases transmission of light through the second surface into the waveguide.
6 . The method of claim 5 , wherein at least 97 percent of the light is transmitted through the second surface.
7 . The method of claim 3 , wherein the waveguide substrate is glass and the anti-reflective coating comprises a layer of MgF 2 .
8 . The method of claim 7 , wherein the layer of MgF 2 has a thickness between 75 and 125 nm.
9 . The method of claim 7 , wherein the anti-reflective coating comprises a layer of SiO 2 .
10 . The method of claim 8 , wherein the layer of MgF 2 is disposed immediately adjacent to the second surface.
11 . The method of claim 10 , wherein a layer of SiO 2 is disposed upon the layer of MgF 2 .
12 . The method of claim 11 , wherein a cumulative index of refraction of the anti-reflective coating has an imaginary refractive index component value less than 5×10 −4 .
13 . The method of claim 11 , wherein a cumulative index of refraction of the anti-reflective coating has an imaginary refractive index component value between 5×10 −4 and 1×10 −3 .
14 . The method of claim 3 , wherein the anti-reflective coating comprised less than eight layers alternating between a first material and a second material.
15 . The method of claim 14 , wherein the anti-reflective coating consists of four layers.
16 . The method of claim 14 , wherein the first material has comparatively higher index of refraction than the second material.
17 . The method of claim 14 , wherein the first material is TiO 2 .
18 . The method of claim 14 , wherein each layer of TiO 2 has an index of refraction greater than 2.
19 . The method of claim 14 , wherein the second material is SiO 2 .
20 . The method of claim 19 , wherein each layer of SiO 2 has an index of refraction between 1.45 and 1.58.
21 . The method of claim 20 , wherein a cumulative index of refraction of the anti-reflective coating has an imaginary refractive index component value less than 5×10 −4 .
22 . The method of claim 20 , wherein a cumulative index of refraction of the anti-reflective coating has an imaginary refractive index component value between 5×10 −4 and 1×10 −3 .
23 . The method of claim 1 , wherein a cumulative index of refraction of the anti-reflective coating has an imaginary refractive index component value less than 5×10 −4 .