All Integrated Lithium Niobate Standing Wave Fourier Transform Electro-Optic Spectrometer
Résumé
In a standard stationary wave Fourier-transform spectrometer, a static interferogram obtained in a single-mode channel waveguide is sampled by scattering centers that are regularly spaced on top of a channel waveguide. Each scattering center is placed right below a pixel, so that the sampling step is limited by the pixel pitch to avoid crosstalk. For micron-size pixels, this implies under-sampling of the fringes, and therefore, a reduced spectral etendue. In this paper, we propose to overcome this subsampling limitation by using the electro-optical properties of lithium niobate. An integrated optic inversed Y-junction finished by a mirror is used to inject in the single waveguide output two waves characterized by an optical phase difference (OPD). The mirror reflection induces a static interferogram with an OPD null at the mirror (interference of each incident wave with its own reflection) and a dynamic interferogram coming from the interference between the one wave from one arm of the Y-junction and the wave from the other arm, after being reflected by the mirror. The dynamic feature is observed by the integration of an electro-optic phase modulator set on the two arms of the reversed Y-junction inputs. Using moderate control voltages (<100 V), a dynamic wideband fringe wave packet is shifted on several fringe periods under the sampling centers fabricated on the surface of the single waveguide output. We show that this solution can compensate the subsampling related to their large spacing and rebuild a complete interferogram, sufficiently sampled with a small acquisition time, thanks to the electro-optical performances of lithium niobate modulators. Moreover, we show that an equivalent long distance fringe scan can be obtained by superimposing the different fragments of the wideband spectrum seen by each individual sampling center for the same voltage scan. This paper opens the way to high resolution, large spectral etendue compact Fourier-transform spectrometers, where external optical path delay scan can be replaced by the internal phase-modulation using electro-optic properties of lithium niobate or similar materials.