DIRECTIONAL RADIATION BY ASYMMETRICAL
The present invention concerns the directional blaz- 5 ing of waves and, more particularly, discriminating between optical surface waves that are incident on a dielectric grating.
There have recently been developed a wide variety of components and systems utilizing surface waves, e.g. 10 delay lines, amplifiers, correlators and signal compressors. Thin-film structures currently in use in integrated optics are one example and consist typically of either multilayered or periodic grating configurations of the leaky-wave type. Attempts to enhance surface-waveto-beam couplers have resulted in using either a slanted boundary in the substrate region, which is inconvenient, or a special value for the grating thickness which may unfavorably affect the value of the leakage param- 2Q eter. The present invention is directed toward a novel grating structure which will improve the operation of these and other beam couplers and has potential application to other thin film devices.
Accordingly, it is an object of the present invention 25 to provide asymmetric dielectric gratings having strong spatial discrimination.
Anothr object of this invention is to provide dielectric gratings which make it possible to discriminate between optical surface waves incident thereon. 30
A further object of this invention is to provide dielectric gratings which distinguish between energy coming from opposite directions by deflecting respective energies into the regions above or below the grating.
Other objects, advantages and novel features of the 35 invention will become apparent from the following detailed description thereof when considered in conjunction with the accompanying drawings in which like numerals represent like parts throughout and wherein:
FIG. 1 is a schematic diagram illustrating the conven- 40 tional conversion of a surface wave into leaky-wave beams;
FIG. 2 is a sectional view of one embodiment of a dielectric grating of the present invention having an asymmetric profile; 45
FIG. 3 is a sectional schematic drawing of an alternate embodiment of the invention having greater flexibility of structure than the embodiment of FIG. 2;
FIG. 4 is a sectional view of a further dielectric grating of the invention; and 50
FIG. 5 presents a graphical comparison of the variation of leakage and efficiency in relation to lateral shifting of the top periodic layer of the embodiment of FIG. 3.
Blazing is the property whereby a grating can con- 55 centrate most of the energy from an incident wave into a non-specular diffraction order. The present invention, in general, is directed to a method of and means for producing a novel blazing effect which occurs when a surface wave, rather than a beam, is incident along a 60 dielectric grating of the thin-film variety. By properly shaping its periodic profile, the thin-film dielectric grating can concentrate the power from an incident surface wave into a beam that appears selectively either above or below the grating. The waves are guided by an 65 asymmetric grating profile which exhibits strong directional characteristics and can be used to selectively beam the energy into either one of two regions.
where 17 = 0, ±1, ±2,... ,j = a,s designates the upper, e.g. air, or lower, e.g. substrate, regions, respectively, ej is the relative permittivity of the jth medium and A:0 = 2tt/\ is the plane-wave propagation factor in vacuum. Here d is the periodicity length and p0 is the fundamental wavenumber of the leaky wave, which is usually very closely equal to the wavenumber fim of the incident surface wave. By properly choosing /30, d and \, it is possible to satisfy equation (1) so that only a single value of 7), i.e. either rf= —1 or +1, yields a real angle 8yj 0). A single beam is then radiated in each of the two unbounded regions above and below the grating as shown in FIG. 1.
FIG. 2 illustrates one embodiment of the invention in which the grating provides means for discriminating between the regions above and below the wave-carrying thin film. Such discrimination in this embodiment is accomplished by a serrated dielectric grating 25 into which a pair of opposing beams 26 and 27 may be individually or simultaneously directed. Grating 25 is given an asymmetric profile which in this embodiment is in the form of a plurality of non-symmetrical peaks 30 which have the effect of asymmetrically dividing beam 26 into a stronger beam 31 diverted into a dielectric substrate 32 on which grating 25 is mounted and a weaker beam 35 into the region above the grating. Incident light from the opposing direction, however, is divided by the grating into a stronger beam 36 into the region above the grating and a weaker beam 37 into substrate 32.
The foregoing division of beams 26 and 27 into unequal components is accounted for in the following manner. In the light incident from the left, i.e. beam 26 along the +x axis, the electric field in an air region above £ ating 25 can be written as: