Patent ID: 7292613

Claim:
An optical device designed for operating at a predetermined operating wavelength λ op <λ, where λ is a predetermined design wavelength, the device comprising: a) two quarter-wave stacks, wherein the quarter-wave stacks comprise alternating layers of a low-index material having refractive index n l and a high-index material having refractive index n h , where n l <n h , wherein the alternating layers have alternating quarter-wave thicknesses λ/4n l and λ/4n h , and b) a spacer layer sandwiched between the two quarter-wave stacks, wherein the spacer layer has a thickness of approximately λ/2n l and comprises: i) two spacer layers of low-index material having refractive index n l and ii) a subspacer layer of high-index material having refractive index n h and a thickness less than λ/4n h , wherein the subspacer layer is sandwiched between the two spacer layers of low-index material.