Diffractive Optical Elements (DOEs) are now widely used in technical optics because they are planar elements which can easily be mounted, packaged, because they can be made by means of high productivity batch planar technologies, and also because they can perform complex optical functions which refractive light processing techniques can not, or can hardly achieve by simple and low cost means.
One of the hurdles in diffractive optics technologies is the difficulty of obtaining high diffraction efficiency. One of the widely used solutions is to decompose the desired analogue groove profile in a number of discrete levels in a staircase form. This is a costly multilevel manufacturing process which is presently limited to elements of small angular aperture. Another solution is to rely upon a gray scale process technology capable of photolithographically and physically transferring the desired analogue profile with fidelity in one single technological step. This solution is not available yet as a manufacturing technology. Yet another solution is to replicate the mould of an analogue surface relief generated by means of highly resolving writing means such as an electron beam pattern generator. This technology is only suitable for manufacturing large volumes of identical elements and suffers from the thermal instability and ageing of the replicable material.
There is therefore the need for a technical solution providing high diffraction efficiency gratings, diffractive optical elements, holograms, even in the presence of more than one propagating diffraction order without having to resort to a multilevel technology to generate analogue groove profiles.
An optical diffraction device is described in U.S. Pat. No. 6,219,478 B1 which discloses a reflective diffraction grating where the diffraction event provides large, possibly 100% diffraction efficiency for the sole −1st propagating diffraction orders in the incidence medium in a direction which is not parallel to the incident wave. This document U.S. Pat. No. 6,219,478 B1 discloses the conditions for the incident beam field to accumulate into a leaky mode of a layer placed on top of the mirror. A leaky mode is a transverse field resonance leaking into the cover medium. A diffraction grating written on, or within the layer acts as a tap regulating the rate of field accumulation so that the accumulated field leaking into the incident medium damps or cancels out the reflection in the direction of the Fresnel reflection by destructive interference between the wave directly reflected from the top surface of the layer and the leaking wave which is accumulated in the leaky mode. Consequently, and provided the incidence configuration is not autocollimation, the optical energy has nowhere else to propagate but to be directed along the −1st diffraction order of the grating.
Although U.S. Pat. No. 6,219,478 B1 discloses the means to possibly achieve up to 100% diffraction efficiency in a grating even when the incidence angle θc (the incidence angle θc is defined from the normal to the general plane in which the grating extends) is relatively small, there are structures where the refractive index difference between the leaky mode propagating layer and the cover medium is too small, and/or there are incidence configurations where the incidence angle is too small, to provide sufficient field accumulation in the leaky mode to permit the cancellation of the reflection, therefore to give rise to 100% diffraction efficiency. In cases where the reflection can nevertheless be cancelled, deep grooves are required which implies that the leaky mode resonance is spectrally and angularly broad. According to U.S. Pat. No. 6,219,478 B1, it is under substantially grazing incidence and/or in the presence of a large index contrast between the leaky mode propagating layer and the cover medium that the reflection coefficient of the top boundary of the dielectric layer is large, i.e., that close to 100% diffraction efficiency can be achieved.
It would therefore be advantageous to achieve the cancellation of the reflection on the leaky mode propagating layer in all cases where the incident or the diffracted beam angle is small and in cases where the refractive index contrast between leaky mode propagating layer and cover is small (as for instance in the case of holograms and most visual diffractive structures which are often coated by a protective layer; when such a coating is applied on top of the leaky mode propagating layer, it is even impossible to obtain large incidence angle at the top surface of this layer since it would correspond to total reflection at the air-coating interface), and to achieve high, possibly 100% −1st order diffraction efficiency by means of a relatively weak corrugation or index modulation (besides, it is not always desired or possible to fabricate a deep corrugation or to cause a large index modulation in the layer propagating the leaky mode).
The documents of the scientific literature dealing with high diffraction efficiency of the −1st order of a reflection grating usually consider the diffraction configuration of the −1st order Littrow incidence where the diffracted beam is diffracted back in the direction of the incident beam. The Littrow incidence condition for the −1st order at vacuum wavelength λ from an incidence medium of refractive index nc on a periodic grating of period Λ is characterized by the Littrow angle θL such that sin(θL)=λ/(2Λnc). This incidence condition is also currently called the autocollimation configuration since the reflected −1st order diffracted wave propagates back parallel to the incident wave. The off-Littrow incidence configuration will hereafter refer to configurations where the angle of incidence θc essentially differs from θL, i.e., where the −1st order reflected diffracted wave is not parallel to the incident wave. Apart from a major functional difference, there is a fundamental difference between the autocollimation diffraction configuration and the off-Littrow configuration. Unlike in the off-Littrow diffraction configuration, the autocollimation configuration is known to always permit 100% diffraction efficiency (provided the layer average thickness is larger than a minimum thickness and provided the sole 0th and −1st orders can propagate) as calculated for instance in the case of high efficiency femtosecond pulse compression gratings in document M. D. Perry, R. D. Boyd, J. A. Britten, D. Decker, B. W. Shore, C. Shannon, E. Shults, “High-efficiency multilayer dielectric diffraction gratings”, Opt. Lett. 20 No 8, 940-942 (1995) and as analyzed by document H. Wei, L. Li, “All-dielectric reflection gratings: a study of the physical mechanism for achieving high efficiency”, Appl. Opt., Vol. 42 No 31, 6255-6260 (2003) which explains the mechanism of high efficiency and limits itself to the Littrow case. So does the document by V. A. Sychugov, B. A. Usievich, K. E. Zinov'ev, O. Parriaux, “Autocollimation diffraction gratings based on waveguides with leakage mode”, Quantum Electronics, 30(12) 1094-1098 (2000) which reports on the use of a semi-reflective structure on top of a dielectric layer with the objective of obtaining the achievable 100% diffraction efficiency in the −1st order Littrow configuration by means of a grating of a smaller corrugation depth. The semi-reflective structure consists of quarter wave layers at the wavelength, and at the incidence and diffraction direction of the autocollimation configuration.
The reason for such specific feature of the Littrow configuration is that the field of the diffracted wave in the mirrored corrugated structure is the same as that of the incident wave. For instance, the mirror ensuring the reflection of the incident wave reflects the diffracted wave identically. Similarly, in the structure dealt with by the above mentioned paper by V. A. Sychugov et al, the semi-reflective structure ensuring some degree of field concentration in the leaky mode inherently ensures the same degree of field concentration for the diffracted leaky mode. The autocollimation configuration is analogous to the reflection from a quarter wave multilayer mirror or from a fibre Bragg grating mirror with the specificity that the two waves participating in the −1st order reflection (the −1st order directed into the cover and the −1st order directed to the mirror, then reflected into the cover) have to interfere essentially constructively.
Because its symmetry, the autocollimation configuration implies first order coupling between the two counterpropagating leaky modes if the leaky mode propagation condition is satisfied. It is to be noted that the autocollimation configuration permits to obtain 100% diffraction efficiency even if the leaky mode propagation condition is not satisfied.
It is an object of the invention to provide an optical diffraction device having high and possibly 100% diffraction efficiency for the −1st diffraction order in a diffraction configuration where neither the incident beam nor the diffracted beam are grazing, where the diffracted beam is not parallel to the incident beam (i.e., outside the Littrow configuration), by means of a diffractive element or structure having relatively shallow depth or/and weak index modulation.