Highly efficient optical gratings with reduced thickness requirements and impedance-matching layers

Abstract

An optical grating comprising a grating layer and two surface layers, the layers being arranged with the grating layer between the surface layers. The grating layer comprises a set of multiple, discrete, elongated first grating regions that comprise a first dielectric material and are arranged with intervening elongated second grating regions. The bulk refractive index of the dielectric material of the first grating regions is larger than the bulk refractive index of the second grating regions. The first surface layer comprises a first impedance matching layer, and the second surface layer comprises either (i) a second impedance matching layer or (ii) a reflective layer. Each said impedance matching layer is arranged to reduce reflection of an optical signal transmitted through the corresponding surface of the grating layer, relative to reflection of the optical signal in the absence of said impedance matching layer.

What is claimed is: 
     
         1 . An optical grating comprising:
 a grating layer comprising a set of multiple, discrete, elongated first grating regions that comprise a first dielectric material having a first grating bulk refractive index, the first grating regions being arranged with intervening elongated second grating regions having a second grating bulk refractive index that is lower than the first grating index;   a first surface layer on a first surface of the grating layer, the first surface layer comprising a first impedance matching layer; and   a second surface layer on a second surface of the grating layer, the second surface layer comprising either (i) a second impedance matching layer or (ii) a reflective layer, the grating layer being arranged between the first and second surface layers,   wherein each said impedance matching layer is arranged to reduce reflection of an optical signal transmitted through the corresponding surface of the grating layer, relative to reflection of the optical signal in the absence of said impedance matching layer.   
     
     
         2 . The optical grating of  claim 1  wherein the first dielectric material comprises silicon, doped silicon, silicon nitride, silicon oxynitride, titanium dioxide, cerium dioxide, aluminum oxide, tantalum pentoxide, aluminum oxynitride, beryllium oxide, bismuth oxide, chromium oxide, germanium, doped germanium, hafnium oxide, magnesium oxide, neodymium oxide, praseodymium oxide, scandium oxide, zinc selenide, zinc sulfide, or zirconium oxide. 
     
     
         3 . The optical grating of  claim 1  wherein the second grating regions comprise a second dielectric material. 
     
     
         4 . The optical grating of  claim 3  wherein the second dielectric material comprises silica, doped silica, borophosphate glass, borosilicate glass, soda lime glass, silicon nitride, silicon oxynitride, titanium dioxide, cerium dioxide, aluminum oxide, tantalum pentoxide, polymer, ambient atmosphere, air, or inert gas. 
     
     
         5 . The optical grating of  claim 1  wherein the second grating index is about equal to unity. 
     
     
         6 . The optical grating of  claim 1  wherein each said impedance matching layer comprises silicon nitride, silicon oxynitride, titanium dioxide, cerium dioxide, aluminum oxide, tantalum pentoxide, silica, doped silica, borophosphate glass, borosilicate glass, soda lime glass, polymer, beryllium oxide, calcium fluoride, cerium fluoride, cryolite, hafnium fluoride, lanthanum fluoride, strontium fluoride, or ytterbium fluoride. 
     
     
         7 . The optical grating of  claim 1  wherein at least one said impedance matching layer comprises distinct regions of differing dielectric materials that are sufficiently small that the impedance matching layer interacts with the optical signal as if it were a homogeneous layer. 
     
     
         8 . The optical grating of  claim 1  wherein a transverse cross section of each of the first and second grating regions is substantially rectangular or substantially trapezoidal. 
     
     
         9 . The optical grating of  claim 1  wherein the second surface layer comprises a reflective layer, and the optical grating comprises a reflection grating. 
     
     
         10 . The optical grating of  claim 9  wherein the reflective layer comprises a multilayer dielectric stack. 
     
     
         11 . The optical grating of  claim 9  wherein the reflective layer comprises a metal layer. 
     
     
         12 . The optical grating of  claim 9  wherein the reflective layer comprises gold, silver, or aluminum. 
     
     
         13 . The optical grating of  claim 1  wherein the second surface layer comprises a second impedance matching layer, and the optical grating comprises a transmission grating. 
     
     
         14 . The optical grating of  claim 1  wherein at least one said impedance matching layer comprises a substantially continuous layer on the first and second grating regions. 
     
     
         15 . The optical grating of  claim 14  wherein the substantially continuous impedance matching layer comprises a dielectric material having (i) a bulk refractive index between an average refractive index of the grating layer and a bulk refractive index of an adjacent medium on the corresponding side of the optical grating, and (ii) a thickness selected to yield a phase difference about equal to an odd multiple of it between optical signals reflected from the two surfaces of the impedance matching layer. 
     
     
         16 . The optical grating of  claim 1  wherein at least one said impedance matching layer comprises a set of multiple, discrete, elongated regions each positioned on a corresponding one of the first grating regions. 
     
     
         17 . The optical grating of  claim 16  wherein the regions of the impedance matching layer comprise a dielectric material having (i) a bulk refractive index between the first grating index and a bulk refractive index of an adjacent medium on the corresponding side of the optical grating, and (ii) a thickness selected to yield a phase difference about equal to an odd multiple of it between optical signals reflected from the two surfaces of the impedance matching layer. 
     
     
         18 . The optical grating of  claim 1  wherein:
 the first and second grating regions are characterized locally by (i) a local grating spacing along the grating layer in a local direction substantially perpendicular to the elongated grating regions, (ii) a local grating duty cycle, and (iii) a local grating thickness along a direction normal to the grating layer; 
 for at least a first localized area of the grating layer, the first and second grating regions are arranged to exhibit a first specified grating spacing to diffract at a first specified diffracted angle an optical signal at a specified wavelength within an operational wavelength range when said optical signal is incident on the first localized area of the grating layer at a first specified incidence angle; and 
 for at least the first localized area of the grating layer, the first and second grating regions are arranged so that first and second optical modes at the specified wavelength propagating in a direction normal to the grating layer (i) propagate with respective first and second modal indices, and (ii) accrue a phase difference about equal to an odd multiple of π upon either (a) propagating through the grating layer once or (b) being reflected to propagate through the grating layer twice. 
 
     
     
         19 . The optical grating of  claim 18  wherein, for at least the first localized area of the grating layer, an efficiency for diffraction of the optical signal, incident at the first incidence angle and diffracted at the first diffracted angle, is greater than about 90% over the operational wavelength range, and the operational wavelength range is between about 1525 nm and about 1565 nm. 
     
     
         20 . The optical grating of  claim 18  wherein, for at least the first localized area of the grating layer, the first and second grating regions are arranged to exhibit a first specified grating duty cycle to yield an efficiency for diffraction of the optical signal, incident at the first incidence angle and diffracted at the first diffracted angle, that varies with polarization of the optical signal only within a specified operationally acceptable range. 
     
     
         21 . The optical grating of  claim 20  wherein, for at least the first localized area of the grating layer, the efficiency for diffraction of the optical signal varies with polarization by less than ±0.25 dB over the operational wavelength range, and the operational wavelength range is between about 1525 nm and about 1565 nm. 
     
     
         22 . The optical grating of  claim 1  wherein the grating layer and the first and second surface layers are supported by at least one substrate. 
     
     
         23 . The optical grating of  claim 1  wherein the grating layer and the first and second layers are sandwiched between two substrates. 
     
     
         24 . The optical grating of  claim 1  wherein the grating layer is substantially flat. 
     
     
         25 . The optical grating of  claim 1  wherein the first grating regions comprise a set of substantially straight, substantially parallel linear regions. 
     
     
         26 . The optical grating of  claim 1  wherein the first grating regions comprise a set of curvilinear regions. 
     
     
         27 . The optical grating of  claim 1  wherein the first and second grating regions are arranged according to an interference pattern derived from computed interference in the grating layer between a simulated design input optical signal and a simulated design output optical signal. 
     
     
         28 . The optical grating of  claim 1  wherein the grating layer is substantially flat, and the sets of first and second grating regions are arranged so that respective wavefronts of an incident optical signal and a portion of that signal diffracted by the optical grating exhibit differing convergence, divergence, or collimation properties. 
     
     
         29 . A method comprising:
 forming a grating layer comprising a set of multiple, discrete, elongated first grating regions that comprise a first dielectric material having a first grating bulk refractive index, the first grating regions being arranged with intervening elongated second grating regions having a second grating bulk refractive index that is lower than the first grating index;   forming a first surface layer on a first surface of the grating layer, the first surface layer comprising a first impedance matching layer;   forming a second surface layer on a second surface of the grating layer, the second surface layer comprising either (i) a second impedance matching layer or (ii) a reflective layer; and   arranging each said impedance matching layer to reduce reflection of an optical signal transmitted through the corresponding surface of the grating layer, relative to reflection of the optical signal in the absence of said impedance matching layer.   
     
     
         30 . The method of  claim 29  wherein the second grating regions comprise a second dielectric material. 
     
     
         31 . The method of  claim 29  wherein the second grating index is about equal to unity. 
     
     
         32 . The method of  claim 29  wherein forming the grating layer comprises etching the first dielectric material so that a transverse cross section of each of the first and second grating regions is substantially rectangular or substantially trapezoidal. 
     
     
         33 . The method of  claim 32  wherein at least one said impedance matching layer is etched along with the first dielectric material. 
     
     
         34 . The method of  claim 32  wherein the first impedance matching layer is formed on the grating layer after etching the first dielectric material. 
     
     
         35 . The method of  claim 29  wherein the second surface layer comprises a reflective layer, and the optical grating comprises a reflection grating. 
     
     
         36 . The method of  claim 29  wherein the second surface layer comprises a second impedance matching layer, and the optical grating comprises a transmission grating. 
     
     
         37 . The method of  claim 29  wherein at least one said impedance matching layer comprises a substantially continuous layer on the first and second grating regions. 
     
     
         38 . The method of  claim 29  wherein at least one said impedance matching layer comprises a set of multiple, discrete, elongated regions each positioned on a corresponding one of the first grating regions. 
     
     
         39 . The method of  claim 29  wherein:
 the first and second grating regions are characterized locally by (i) a local grating spacing along the grating layer in a local direction substantially perpendicular to the elongated grating regions, (ii) a local grating duty cycle, and (iii) a local grating thickness along a direction normal to the grating layer; 
 for at least a first localized area of the grating layer, the first and second grating regions are arranged to exhibit a first specified grating spacing to diffract at a first specified diffracted angle an optical signal at a specified wavelength within an operational wavelength range when said optical signal is incident on the first localized area of the grating layer at a first specified incidence angle; and 
 for at least the first localized area of the grating layer, the first and second grating regions are arranged so that first and second optical modes at the specified wavelength propagating in a direction normal to the grating layer (i) propagate with respective first and second modal indices, and (ii) accrue a phase difference about equal to an odd multiple of π upon either (a) propagating through the grating layer once or (b) being reflected to propagate through the grating layer twice. 
 
     
     
         40 . The method of  claim 39  wherein, for at least the first localized area of the grating layer, the first and second grating regions are arranged to exhibit a first specified grating duty cycle to yield an efficiency for diffraction of the optical signal, incident at the first incidence angle and diffracted at the first diffracted angle, that varies with polarization of the optical signal only within a specified operationally acceptable range. 
     
     
         41 . The method of  claim 29  wherein the grating layer and the first and second surface layers are formed on at least one substrate. 
     
     
         42 . The method of  claim 29  wherein the grating layer and the first and second layers are sandwiched between two substrates. 
     
     
         43 . The method of  claim 42  wherein the grating layer is formed on a first substrate, at least one said impedance matching layer is formed on a discrete second substrate, and the first and second substrates are assembled to position said impedance matching layer against the grating layer. 
     
     
         44 . The method of  claim 42  wherein the grating layer is formed on a first substrate, the reflective layer is formed on a discrete second substrate, and the first and second substrates are assembled to position the reflective layer against the grating layer. 
     
     
         45 . The method of  claim 29  further comprising arranging the first and second grating regions are arranged according to an interference pattern derived from computed interference in the grating layer between a simulated design input optical signal and a simulated design output optical signal. 
     
     
         46 . The method of  claim 29  wherein the grating layer is substantially flat, further comprising arranging the sets of first and second grating regions so that respective wavefronts of an incident optical signal and a portion of that signal diffracted by the optical grating exhibit differing convergence, divergence, or collimation properties. 
     
     
         47 . An optical grating comprising a set of multiple, discrete, elongated first grating regions that comprise a first dielectric material having a first bulk refractive index, the first regions being arranged with intervening elongated second grating regions having a second bulk refractive index that is lower than the first index, the first and second regions defining a grating layer, wherein:
 the first and second grating regions are characterized locally by (i) a local grating spacing along the grating layer in a local direction substantially perpendicular to the elongated grating regions, (ii) a local grating duty cycle, and (iii) a local grating thickness along a direction normal to the grating layer;   for at least a first localized area of the grating layer, the first and second grating regions are arranged to exhibit a first specified grating spacing to diffract at a first specified diffracted angle an optical signal at a specified wavelength within an operational wavelength range when said optical signal is incident on the first localized area of the grating layer at a first specified incidence angle;   for at least the first localized area of the grating layer, the first and second grating regions are arranged so that first and second optical modes at the specified wavelength propagating in a direction normal to the grating layer (i) propagate with respective first and second modal indices, and (ii) accrue a phase difference substantially equal to an odd multiple of π upon either (a) propagating through the grating layer once or (b) being reflected to propagate through the grating layer twice; and   for at least the first localized area of the grating layer, the first and second grating regions are arranged to exhibit a first specified grating duty cycle to yield an efficiency for diffraction of the optical signal, incident at the first incidence angle and diffracted at the first diffracted angle, that varies with polarization of the optical signal only within a specified operationally acceptable range.