Novel surface-relief diffraction grating

A surface-relief transmission diffraction grating assembly comprised of a substrate and, bonded to said substrate, a surface-relief transmission diffraction grating, wherein:

• (a) said substrate is optically homogeneous, consists essentially of material which has a coefficient of thermal expansion of from about 2×10−5 to about −1×10−6 per degree centigrade, has a refractive index of from about 1.4 to about 4.0, and has a transmittance of at least about 70 percent;
• (b) said surface-relief transmission diffraction grating is comprised of a grating forming layer and a surface-relief grating formed in said grating forming layer, wherein:
• 1. said surface-relief grating has a peak-to-trough height of from about 0.5 to about 5.0 microns, and
• 2. the grating aspect ratio of said peak-to-trough height of said surface-relief grating to said grating line spacing of said surface-relief grating is from about 0.8 to about 2.0;
• (c) when the incident optical beam to said surface-relief transmission diffraction grating assembly and the first order diffracted optical beam from said surface-relief transmission diffraction grating assembly have substantially equal angles with respect to the normal to the grating surface of said surface-relief transmission diffraction grating assembly, and
• 1. when the ratio of the wavelength of the incident optical beam to the grating line spacing is from about 0.8 to about 2.0, said surface-relief transmission diffraction grating assembly has a first diffraction efficiency of at least about 70 percent for the S polarized optical component of said incident optical beam, and
• 2. when the ratio of the wavelength of the incident optical beam to the grating line spacing is from about 0.8 to about 1.2, said surface-relief transmission diffraction grating assembly has a second diffraction efficiency of at least about 70 percent for the P polarized optical component of said incident optical beam and said second diffraction efficiency has a value that decreases from at least 70 percent when the ratio of the wavelength of the incident optical beam to grating line spacing is about 1.2 to a value of less than 10 percent when the ratio of the wavelength of the incident optical beam to grating line spacing is from about 1.43 to about 2.0;
• (d) after said surface-relief transmission diffraction grating assembly has been subjected to a temperature of 85 degrees centigrade and a relative humidity of 85 percent for 1,000 hours and then tested in accordance with the procedure specified in paragraph (c) to determine a third diffraction efficiency for the S polarized optical component and a fourth diffraction efficiency for the P polarized optical component, said third diffraction efficiency is less than 6 percent different than said first diffraction efficiency, said fourth diffraction efficiency is less than 6 percent different than said second diffraction efficiency;
• (e) said surface-relief transmission grating assembly is substantially optically clear;
• and
• (f) after said surface-relief transmission grating assembly has been subjected to a temperature of 85 degrees centigrade and a relative humidity of 85 percent for 1,000 hours, said surface-relief transmission grating assembly is still substantially optically clear.