Tunable Diffraction Grating

Abstract

The invention is directed to a tunable diffraction grating (1) with optically active elements (5) arranged at a distance (A) next to each other. The optically active elements (5) are displaceable relative to each other in a lateral direction (x, y) and mechanically interconnected to a layer (4), which is made out of a deformable material. A deformation of the layer (4) in a direction (z) in general perpendicular to the lateral direction (x, y) causes a change in the relative distance (A) of the optically active elements (5).

Description

FIELD OF THE INVENTION[0001]The invention lies in the field of diffraction gratings, especially tunable diffraction gratings with a significant spectral tuning range.BACKGROUND OF THE INVENTION[0002]A diffraction grating is an optical element that consists of a reflecting or trans-parent substrate whose surface contains fine, parallel grooves or rulings that are equally spaced. When light is incident on a diffraction grating, diffractive and mutual interference effects occur and light is reflected or transmitted in discrete directions called orders. Because of their dispersive properties, gratings are commonly used in monochromators and spectrometers. These devices were first manufactured by German physicist Joseph von Fraunhofer in 1821.[0003]Diffraction gratings have been a research topic for many decades. At the beginning, the design and the functional principle of the gratings were of major interest. In recent years, tunable diffraction gratings became popular, which allow to mo...

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Benefits of technology

[0019]The combination of diffraction gratings and piezoceramics is also known from the prior art, but experiments have shown that such devices have only a limited optical tuning range. This is mainly due to the limited strain (<0.2%) that can be achieved with piezoceramics. In a preferred embodiment the actuator and the grating are implemented in at least one deformable material. One advantage of such an implementation is a higher linear strain in grating period direction. This has the effect that the grating can achieve a large tuning range (about 150 times higher than any known implementations based on piezo-ceramic actuator materials). Further advantages of the present invention consist in that the manufacturing process is at a very low cost, simple and fast. In difference to previous systems which require several complicated micromachining processes (i.e. comb-drive driven tunable gratings), the production of a preferred embodiment of the invention requires relative inexpensive and commercially available materials such as PDMS (Polydimethylsiloxane, a widely used silicon-based organic polymer), Carbon Black or 3M VHB4910 Acrylic Elastomers and coating metals such as Au (Gold), Al (Aluminum) or ITO (Indium Tin Oxide), whereas this is only a selection of products. When relatively inflexible material is used e.g. as coating material, for electrodes, improved flexibility may be achieved by applying a special shape to the inflexible material such that at least in one direction an improved lateral flexibility is obtained. Good results are achieved by a wavy, concertina-fold like development which allows lateral deformation mainly due to bending. In a preferred embodiment the concertina-fold like development is a result of the grating itself. In a preferred embodiment a concertina-fold like layer has in an undeformed state a wavy cross-section with straight side walls which are interconnected by sharp edges or blends of a certain radius, or a sequence of interconnected semicircles.

[0023]A further advantage of the herein discussed invention consists in that it can be miniaturized which is important e.g. to make displays with small pixels. Diffraction gratings have good diffractive properties (wavelength separation) down to 10 lines per diffraction grating. This means that theoretically, a resolution of 10×10 μm2 can be achieved. A further advantage is that the mechanical impact is in plane (not as in rotated grating structures).

[0039]A tunable diffraction grating according to the present invention may be applied in many fields of technology such as for displays, light sources or in communication systems where light needs to be switched between different channels. The invention has the potential for many commercial applications because of simple and inexpensive production and the significant tuning range that can be achieved with the proposed device. One possibility is the use as beam expander, e.g. for virtual display devices similar to the application described in US pat. 2004109234. It can also be used as monochromatic light source. When white light is shone onto the grating, light is split up into its monochromatic components. This monochromatic light can be used in display devices. One advantage of such displays is that the displayed colors are not limited to the color gamut of state of the art display devices.

[0042]A first class of EAPs can be summarized as dielectric electroactive polymers (DEAP) wherein actuation is caused by electrostatic forces between electrodes. The electrodes are in general arranged opposite to each other on either side of a deformable polymeric material. When voltage is applied the opposite electrodes are attracted to each other and the in-between the electrodes arranged polymeric material is compressed. Due to poisson's ratio the compressed material expands in a perpendicular direction (lateral direction). This kind of electroactive polymers is characterized by a relatively large actuation voltage and comparable low electrical power consumption. To reduce hindering stresses it is advantageous that the electrodes are made deformable. A second class of electroactive polymers are ionic electroactive polymers. In this class deformation is caused by the displacement of ions inside the polymer. Only a relative low voltage is needed for actuation, but due to ionic flow a higher electrical power is needed.

Problems solved by technology

One major problem is the fact that the diffraction gratings are not continuously tunable without employing a complicated macroscopic rotation mechanism or expensive and complicated microscopic mechanical actuators.

However, from the prior art it is not known to tune a grating by using elastomer actuators, especially dielectric elastomer actuators.

Electrical tuning has significant advantages compared to methods know from prior art: Tuning by heating or cooling is very slow; tuning by external pressure is difficult to integrate in small devices.

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