Compact Bidimensional Sampling Spectrometer – CoBiSS

Fourier Transform Spectrometer (FTS) is the tool of choice for visible and IR spectral analysis. However, the FTS technique suffers from a lack of compactness and/or robustness due the presence of scanning parts. Furthermore, standard technologies make available cumbersome devices particularly in the near and middle infrared. For more than five years, UTT and UJF are involved in the development of new compact Fourier Transform Spectrometers based on direct near-field detection of standing waves on both integrated (SWIFTS: Patent WO2007017588) and free space (CoBiSS: Patent WO2009127794) optical technologies. These technologies pave the way for parallel spectral analysis in a minimal volume without moving parts (compact and robust) and offer probably the most efficient integration of the spectrometric function.
In terms of miniaturization and among the diversity of other existing technologies, the CoBiss (Compact Bidimensional Sampling Spectrometer) technology seems to offer the best trade-off between the size of the spectrometer and the performances (spectral resolution, spectral bandwidth) it provides due to a specific sampling of the interferogram. CoBiss takes advantage of micro/nano-fabrication and near field optical technologies to build-up a bi-dimensional spatial sampling of an evanescent interferogram localised on a facet of a prism. The sampling is achieved using a 2D array of optical nanoprobes which is placed in the evanescent part of the stationary wave in order to extract (scatters) a fraction of the evanescent field. By tilting the nanoprobe array with respect to interference fringe lines, with a well defined angle, the sampling frequency is no more limited by the detector pixel pitch which is not the case in alternative devices using 2D array detectors in FTS. Experimental studies have been carried out in the frame of OSEO program for innovation (2009) and showed a wide spectral bandwidth in the visible range, down to 380nm (limited by the nanoprobes diameter 80nm) with spectral resolution of 1.6 nm around 780nm. The spectral resolution was mainly limited by the maximum size (180µm) of the 2D array of nanoprobes. It has to be stressed that, CoBiss has no intrinsic limitation in resolution which depends only on the length of the probed interferogram. LNIO and LTM are both involved in large scale (>1×1mm2) 2D nano-structured surface issues. Several techniques are investigated: electronic lithography, holography (LNIO) and nanoimprint (LTM).
Nanoimprint is best suited for industrial applications and will be therefore investigated more deeply in this program. Another major task is to be able to design and scale CoBiss for different specific applications. Simulation tools (FDTD, RCWA, Green 3D) will be developed for a best understanding of the interaction mechanisms between the evanescent light and the scattering nanoprobes used to sample the interferogram (cross section, radiation diagram). A far field interferometric microscope will be set up to characterize the scattering cross section and the radiation diagram for different sizes, shapes and materials of nanoprobes. The experimental results will be used to assess our simulation tools. A last aspect of the project is dealing with data analysis when rebuilding the spectrum by Fourier transformation. We have shown in previous studies that the obtained FT spectrum depends drastically on the relative alignment between the 2D array of nanoprobes and the fringe lines. Smart algorithms will be developed at LM2S to take into account a residual misalignment in the sampling procedure. In the mean time, alternatives to Fourier transformation will be investigated.
We have formed an interdisciplinary consortium consisting of three major national research laboratories (LNIO, LTM, LM2S) gathering near field microscopy knowledge, optical instrumentation, nanofabrication know-how, Nanoimprint processing, data analysis and image processing competencies.