Equivalence of ruled, holographic, and lamellar gratings in constant deviation mountings

Blazed gratings with both controllable blaze angle and anti-blaze angle fabricated by using a twice oblique ion beam etching method

A twice oblique ion beam etching method is proposed to fabricate triangular blazed gratings that have controllable both blaze angle and anti-blaze angle. The anti-blaze angle is controlled by first obliquely etching the photoresist mask to obtain an asymmetrical trapezoidal grating, one sidewall of which then evolves into the anti-blaze facet in the second etch step. The blaze angle is controlled by obliquely etching the asymmetrical trapezoidal grating to obtain a triangular blazed grating. We show the key process steps of the method by fabricating a blazed grating with the blaze angle of 20.9° and the anti-blaze angle of 89.0°. The method is verified not only to increase the anti-blaze angle to near 90°, but also have a good tolerance against over-etching.

Design of dense transmission diffraction gratings for high efficiency

We propose a design method for dense surface-relief diffraction gratings with high efficiency in transmission mode. Closed-form analytical relations between diffraction efficiency, polarization, and grating parameters are derived and verified in the resonance domain of diffraction under general three-dimensional angles of incidence traditionally termed conical mounting. A powerful tool for rigorous design of computer-generated holograms and diffractive optical elements with spectroscopic scale periods is now enabled.

Optimization of nonbinary slanted surface-relief gratings as high-efficiency broadband couplers for light guides

We propose and investigate the use of slanted surface-relief gratings with nonbinary profiles as high-efficiency broadband couplers for light guides. First, a Chandezon-method-based rigorous numerical formulation is presented for modeling the slanted gratings with overhanging profiles. Then, two typical types of slanted grating couplers--a sinusoidal one and a trapezoidal one--are studied and optimized numerically, both exhibiting a high coupling efficiency of over 50% over the full band of white LED under the normal illumination of unpolarized light. Reasonable structural parameters with nice tolerance have been obtained for the optimized designs. It is found that the performance of the couplers depends little on the grating profile shape, but primarily on the grating period and the slant angle of the ridge. The underlying mechanism is analyzed by the equivalence rules of gratings, which provide useful guidelines for the design and fabrication of the couplers. Preliminary investigation has been performed on the fabrication and replication of the slanted overhanging grating couplers, which shows the feasibility of fabrication with mature microfabrication techniques and the perspective for mass production.

Measurement of duty cycles of photoresist grating masks made on top of multilayer dielectric stacks

We present a novel and nondestructive method for measuring the duty cycles (ratio of ridge width to period) of submicrometer rectangular photoresist gratings made on top of multilayer dielectric stacks. The method exploits the fact that the effective index of the leaky mode that has a strong evanescent tail in the cladding changes with the duty cycle of the grating situated at the interface between the top dielectric layer and the cladding. By comparing measured coupling angles of the leaky mode with a theoretical or experimentally calibrated relationship between coupling angles and duty cycle, one can determine the duty cycle of the grating. This method is applicable even when the grating period is less than the measurement wavelength. It is simple because it does not require any power measurement. Most importantly, it is virtually independent of groove depth. The physical principle of the method and the results of experimental verification are presented.

Effective grating theory for resonance domain surface-relief diffraction gratings

An effective grating model, which generalizes effective-medium theory to the case of resonance domain surface-relief gratings, is presented. In addition to the zero order, it takes into account the first diffraction order, which obeys the Bragg condition. Modeling the surface-relief grating as an effective grating with two diffraction orders provides closed-form analytical relationships between efficiency and grating parameters. The aspect ratio, the grating period, and the required incidence angle that would lead to high diffraction efficiencies are predicted for TE and TM polarization and verified by rigorous numerical calculations.

High-efficiency metallic diffraction gratings for laser applications

The design and fabrication of large-area, high-efficiency metallic gratings for use in high-power laser systems is described. The gratings exhibit a diffraction efficiency in excess of 95% in the m = -1 order (Littrow mount) and have a high threshold for laser damage. Computations and experimental measurements are presented that illustrate the effect of grating shape and polarization on efficiency. A simple theory for optical damage to metallic diffraction gratings is developed and compared with experimental measurements of the laser-damage threshold over the pulse range from 400 fs to >1 ns.

Holographic Fourier diffraction gratings with a high diffraction efficiency optimized for optical communication systems

Novel diffraction gratings in a near-infrared region that is specially optimized for optical communication systems are studied and demonstrated by using a simple holographic exposure process. These gratings have a unique groove profile. The gratings of 770 lines/mm have a very high efficiency of 95% at a wavelength of 1.31 microm and more than 85% at a wavelength of 1.52 microm for nonpolarized light in the Littrow mounting. They also show a very low dependence (less than 2%) of the efficiency on the polarization.

Extended Rayleigh-Fano theory and its application to grating polarization

A simple method of calculating the degree of polarization of light diffracted by a grating having shallow grooves and finite conductivity has been developed by extending the Rayleigh-Fano theory. Its validity is examined by comparing the computed degree of polarization with the experimental data obtained in the visible and vacuum ultraviolet for the +/- 1st and -2nd orders of Al- and Au-coated blazed gratings and holographic gratings used in the specific mountings. The calculations are in good agreement with the observed polarization behavior of the gratings, verifying the usefulness of the present method.

Asymmetrical trapezoidal grating efficiency

The diffraction efficiency behavior of reflection gratings with an asymmetrical trapezoidal groove profile has been studied. Theoretical calculations predict that at certain grating parameters superior efficiency compared to the conventional ruled gratings can be achieved.