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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. [ABSTRACT FROM AUTHOR]

We present an optimization process to improve the rejection ratio in integrated beam combiners by locking the dark fringe and then monitoring its intensity. The method proposed here uses the electro-optic effect of lithium niobate in order to lock the dark fringe and to real-time balance the photometric flux by means of a twostage Mach--Zehnder interferometer waveguide. By applying a control voltage on the output Y-junction, we are able to lock the phase and stay in the dark fringe, while an independent second voltage is applied on the firststage intensity modulator, to finely balance the photometries. We have obtained a rejection ratio of 4600 (36.6 dB) at 3.39 µm in transverse electric polarization, corresponding to 99.98% fringe contrast, and shown that the system can compensate external phase perturbations (a piston variation of 100 nm) up to around 1 kHz. We also show the preliminary results of this process on wide-band modulation, where a contrast of 38% in 3.25- to 3.65-µm spectral range is obtained. These preliminary results on wide-band need to be optimized, in particular, for reducing scattered light of the device at the Y-junction. We expect this active method to be useful in high-contrast interferometry, in particular, for astronomical spatial projects actually under study. [ABSTRACT FROM AUTHOR]