Development of fundamental buildings blocks needed for high spectral range integrated optics spectrometry: active phase modulation to increase sampling efficiency in Fourier transform spectrometers

This paper focuses on the development of an essential building block needed to achieve high spectral etendue in integrated optical spectrometers based on Fourier Transform methods: the active phase modulation of the fringes to sample. The long term objectives of this project are to achieve high-resolution spectrometry in a large spectral range, using compact spectrometers based on the SWIFTS (Standing Wave Fourier Transform Spectrometer) approach. The primary applications in astronomy will be precise measurement of atmospheric compositions of detected exoplanets as well as other celestial bodies, such as the detection and analysis of specific gases like carbon dioxide (CO2) and methane (CH4) that are linked to life. The proposed on-chip Fourier transform spectrometer (SWIFTS) approach offers several advantages, including high spectral resolution, compact size, and a robust design. However, the principle of sampling in a simple, passive SWIFTS, implies to extract the signal with the spatial frequency of the detector's pixel pitch. As the pixels' pitch is typically 10 m, the interferogram is strongly under-sampled, and the resulting spectral range without aliasing is small (typically tens of nm). The work presented in this paper is devoted to increasing the spectral range by temporal multiplexing, achieving on-chip phase modulation thanks to electro-optic properties of Lithium Niobate. By phase shifting the fringes under the sampling centers, we are able to reduce the effective distance between sampled values, therefore increasing the spectral etendue. After a brief introduction on the SWIFTS principle, we will focus on the electro-optic modulation of the fringes, and show preliminary results that validate the temporal multiplexing approach and discuss further improvements and the range of application of this active phase spectrometer.