CMN Optics Developments

Diffraction Gratings with Tailored Spectral Response

SEM image of a grating structure with two different kinds of sub-wavelength entities, i.e. pillars and voids.
© Fraunhofer IOF

SEM image of a grating structure with two different kinds of sub-wavelength entities, i.e. pillars and voids.

Hyperspectral imaging is a technique of combining imaging and spectroscopy to survey a scene or target, and extract detailed information. The scene’s spectral properties are obtained at hundreds to thousands of narrow wavelength bands within an entire spectral field of view, which typically spans over one octave or more. The collected and processed data serve the function of characterizing and identifying chemical compositions, materials, or liquids, which can then be referenced to a precise location in a scene.

The correct balancing of the optical sensitivity throughout the entire spectral bandwidth is one of the major challenges of designing hyperspectral instruments and it comprises all parts of the system – from the detector and optical components and, of course, the dispersive element(s). Using gratings as dispersive elements is most often the method of choice as they allow compact and lightweight devices.

Two grating patterns with different spectral characteristics. First grating (blue) provides an almost constant spectral response and second grating (cyan) has an efficiency peak at a wavelength of 700 nm but a larger gradient at the spectral limits.
© Fraunhofer IOF

Two grating patterns with different spectral characteristics. First grating (blue) provides an almost constant spectral response and second grating (cyan) has an efficiency peak at a wavelength of 700 nm but a larger gradient at the spectral limits.

Here at Fraunhofer IOF, we have established a technology process that allows the design and manufacturing of diffraction gratings with purposefully tailored spectral response, high wavefront accuracy, and thermal stability. The tailoring of the spectral response can be used to flatten the grating’s spectral sensitivity (broad and uniform) or to emphasize particular subsets of the spectrum.

In order to do so, the grating structure (within a single period) is composed of several tenth binary entities with lateral sizes in the sub-wavelength domain. The patterning process is performed by electron beam lithography such that the particular sizing and placement of all individual entities can be controlled to a very large extent.

 

Authors: Frank Burmeister, Thomas Käsebier, Uwe Zeitner, Thomas Flügel-Paul