Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-01-22
2001-04-17
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S130000, C385S131000
Reexamination Certificate
active
06219478
ABSTRACT:
The present invention concerns a light wave diffraction device. In particular, the invention concerns a device of this type formed by a layer, in particular a dielectric layer, in which light waves can propagate, and which has a diffraction grating arranged on one face thereof.
Devices of this type have been described recently in scientific publications, in particular in the article “Diffraction gratings with high optical strength for laser resonant cavities”, Quantum Electronics, 24 (3), pages 233 to 235, 1994. This document describes only one use for a device of the aforementioned type under the particular Littrow condition, i.e. with a “−1” order diffracted wave propagating along the line of the incident wave but in the opposite direction. In these conditions, an approximate analytical formulation was established for the diffraction efficiency (about 85%) according to the “−1” order. Within the scope of this research, it was shown that in these particular conditions, high levels of efficiency could be obtained for angles of incidence of around 45°. According to this document, when the angle of incidence &thgr; (defined from the direction normal to the diffraction grating) increases, there is efficiency of approximately 60% (&thgr;=68°). For still larger angles of incidence, for example for &thgr;=85°, it is mentioned that calculations have shown that it is possible to expect levels of efficiency of up to 50%.
First, it will be noted that the aforementioned document only provides teaching for a use of the aforementioned type of diffraction device under the particular Littrow condition. Those skilled in the art are thus not provided with any teaching concerning the situation in which the “−1” order diffracted light propagates along a different direction to the direction of incidence.
Secondly, on the basis of the aforementioned document, those skilled in the art cannot expect very high diffraction efficiency values, when the angle of incidence is large, for example greater than 70°. For angles of incidence of approximately 80°, one does not expect to obtain an efficiency greater than 50% with the teaching of the prior art. Thus, with a diffraction efficiency not exceeding 50%, numerous applications have no future with the proposed diffraction device.
A lack of teaching is thus noted for diffraction devices whose first order diffracted wave direction is different to the direction of incidence although this situation is favourable to numerous applications. Next, those skilled in the art who would modify the spatial period of the diffraction grating so as to obtain such a situation for the incident wave diffraction can expect a relatively low diffraction efficiency, i.e. less than or equal to 50%, when the angle of incidence defines a relatively large angle, i.e. close to 90° with respect to the direction perpendicular to the diffraction grating plane.
Thirdly, the preconceived ideas of those skilled in the art are confirmed hen a first conventional analysis is performed on the rays reflected, refracted or diffracted by a diffraction structure of the type currently under consideration. Indeed, during a conventional analysis of the different light rays present, those skilled in the art will note that with an angle of incidence of approximately 45°, the intensity of the reflected wave is initially of an order comparable with the intensity of the wave transmitted in the dielectric layer and with the intensity of the wave diffracted in said dielectric layer in accordance with the “−1” order diffraction order. Thus, for angles of incidence of approximately 45°, those skilled in the art expect to be able to compensate the wave reflected by destructive interference with the transmitted wave re-exiting the dielectric layer and also with the contribution of the “−1” order diffracted wave in the layer and re-exiting the latter also by diffraction in the direction of the reflected wave. Conversely, when the angle of incidence increases and becomes relatively large, i.e. it approaches 90° and in particular in the situation of a grazing incidence, the intensity of the reflected wave, relative to the intensity of the incident wave, is very significant and close to 100%. Within the scope of such an analysis, which those skilled in the art generally perform in order to orient their research, a priori confirmation is received of the fact that the diffraction efficiency for a low incidence, in particular grazing incidence, is limited.
Despite the situation described hereinbefore, which prevails in the field of optical diffraction devices prior to the present invention, and the preconceived idea of those skilled in the art of designing such high efficiency arrangements for a grazing incident wave or a grazing “−1” order diffracted wave, inventors have directed their research in this direction by seeking to determine to what extent the dielectric layer, at a surface of which the diffraction grating is arranged or in which it is recorded, can be used to resolve or can contribute to resolving the problem of cancelling the high intensity of reflected light. Within the scope of such research, they have ended up using the dielectric layer as a leaky waveguide propagating a leaky mode. They have thus demonstrated that by correctly determining the characteristic parameters of the diffraction device which forms the subject of the present invention, resonance can be generated in the dielectric layer and a high energy wave can thus be propagated in this layer in accordance with a corresponding leaky mode. As a result of the excitation of resonance in the dielectric layer by an incident wave of a given wavelength and large angle of incidence, it is possible to accumulate sufficient energy in this layer for the amplitude of a wave exiting the layer in the direction of reflection to have an amplitude comparable to the amplitude of the initially reflected wave and a phase shift of substantially &pgr; allowing destructive interference. This allows the intensity of the reflected light to be reduced. The adjustment of the two amplitudes to equality is performed by the diffraction grating which thus allows cancellation of the reflected wave. In order to do this, a specific depth of the diffraction grating is determined. Thus, the luminous intensity reflected by the diffraction device propagates essentially along the diffraction directions. By correctly selecting the spatial period of the diffraction grating, it is possible to limit diffraction to the “−1” order outside this diffraction device. Thus, a very high diffraction efficiency is obtained, which can theoretically be equal to 100%.
The invention concerns a light wave diffraction device formed of a layer in which light waves can propagate and a diffraction grating or equivalent diffraction means arranged on one face of or within said layer. This device is characterised in that the mean height of said layer substantially satisfies the resonance condition for a leaky mode in said layer for at least one incident wave having a wavelength &lgr; and a given angle of incidence defined relative to the direction perpendicular to the grating or the equivalent diffraction means, and in that this grating or these equivalent diffraction means has or have a spatial period whose value is determined to diffract said incident wave essentially along the first negative order of diffraction and at an angle of diffraction &egr;
c
having a different value to that of said angle of incidence.
As a result of the features of the device of the invention it is possible to obtain very high diffraction efficiency values outside the situation corresponding to the Littrow condition, in particular at a large angle of incidence. The solution of the present invention does not a priori allow a high diffraction efficiency to be obtained in the Littrow condition given that there is a symmetry in the dielectric layer between the refracted wave and the “−1” order diffracted wave. Thus, the conditions of the present invention assuring res
Parriaux Olivier M.
Sychugov Vladimir A.
Tishchenko Alexandre V.
Griffin & Szipl, P.C.
Kang Juliana K.
Lee John D.
Parriaux Olivier M.
LandOfFree
Light wave diffraction device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Light wave diffraction device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Light wave diffraction device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2463497