This work aims to present the first results towards a mid-infrared (L-band : 3.4 μm - 4.1 μm) high resolution integrated spectrometer, based on the Gabor approach of SWIFTS (Stationary Wave Integrated Fourier Transform Spectrometer). In this configuration, a stationary wave is obtained by injecting the light from the source on both sides of a channel waveguide. The centre of the coherent interferogram is obtained in the middle of the waveguide, allowing for full characterisation of the source, and all differential dispersion effects between the two optical paths. The stationary wave is sampled by nano-scattering centres (= nanogrooves) placed on top of the waveguide. They extract the interferogram, and the spectrum of the source is retrieved through a Fourier Transform. In the mid-infrared, the detection area of the detectors is buried : using only one groove per scattering centre leads to a diffracted signal too wide and causes pixel crosstalk. Several grooves per scattering centres have therefore been implemented, as this configuration creates a small diffraction grating, and reduces the angular divergence of the flux. In addition to avoiding crosstalk, this allows to extract more flux per scattering centre, thus increasing the signal to noise ratio. Our samples are made in Lithium Niobate (LiNbO3), an electro-optic crystal, using two technologies. First, Direct Laser Writing for both the waveguides and the nanogrooves, and secondly Titane diffusion for the waveguides and Focused Ion Beam for the nanogrooves. Because of the electrooptic properties of Lithium Niobate, applying an electric field ramp modulation will change the refractive index of the material, allowing to finely scan the fringe packet under the sampling centres, thus increasing the sampling rate of the interferogram by temporal multiplexing. We demonstrate that our waveguides are fully functional and have a correct transmission rate, and that our antennas are extracting the stationary wave as expected, in both technologies.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.