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In recent years, the demand for hyperspectral imaging devices has grown significantly, driven by their ability of capturing high-resolution spectral information. Among the several possible optical designs for acquiring hyperspectral images, there is a growing interest in interferometric spectral imaging systems based on division of aperture. These systems have the advantage of capturing snapshot acquisitions while maintaining a compact design. However, they require a careful calibration to operate properly. In this work, we present the interferometer response characterization algorithm (IRCA), a robust three-step procedure designed to characterize the transmittance response of multi-aperture imaging spectrometers based on the interferometry of Fabry-Perot. Additionally, we propose a formulation of the image formation model for such devices suitable to estimate the parameters of interest by considering the model under various regimes of finesse. The proposed algorithm processes the image output obtained from a set of monochromatic light sources and refines the results using nonlinear regression after an ad-hoc initialization. Through experimental analysis conducted on four different prototypes from the Image SPectrometer On Chip (ImSPOC) family, we validate the performance of our approach for characterization. The associated source code for this paper is available from Zenodo (http://dx.doi.org/10.5281/zenodo.7978514).

The stationary wave integrated Fourier transform spectrometer (SWIFTS) is based on the sampling of a stationary wave using nano-scattering centers on the surface of a channel waveguide. Single nano-scale scattering centers above the waveguide surface will radiate the sampled signal with wide angular distribution, which is not compatible with the buried detection area of infrared (IR) detectors, resulting in crosstalk between pixels. An implementation of multiple diffraction nano-grooves (antenna) for each sampling center is proposed as an alternative solution to improve directivity towards the detector pixel by narrowing the scattering angle of the extracted light. Its efficiency is demonstrated from both simulated and measured far-field radiative patterns exhibiting a promising method to be used for the future integrated IR-SWIFTS.

With the growing complexity of astronomical instruments devoted to interferometry, such as MATISSE (a 4 telescope beam combiner) or FIRST (a 9 sub-apertures beam combiner), and the rebirth of space projects such as LIFE (a mid-infrared interferometer), integrated optics devices can be an interesting and complementary approach for beam combination of a large number of apertures. Moreover, one of the approaches for beam combination is pairwise combination of the inputs (either from individual telescopes or from aperture masking on a single telescope), which scales as N(N-1)/2 for an N input system. Astrophotonics devices are attractive to reduce mass and system complexity, while achieving all the beam combination in a single chip, even for a high number of inputs. The aim of this work is to develop a compact photonic device for astronomical applications and demonstrate a proof-of-concept of a spectro-interferometer. In this paper ultrafast laser inscription is used to fabricate three arrayed waveguide gratings (AWGs) stacked vertically. This arrangement enables spectral dispersion and interferometry to be measured simultaneously. Individual AWGs were designed for operation at 633 nm, and demonstrated at 633nm and 830nm. A scan between 790 and 830nm was also achieved to study the wavelength behavior of the AWG. Using a segmented mirror, light at 633nm or 830nm was injected simultaneously into three AWGs layered 40 µm apart, showing analogous behavior for all three layers and no unexpected crosstalk. Finally the three outputs were vertically combined to obtain interference fringes, showing the feasibility of spectro-interferometry and opening the way for compact astrophotonic devices devoted to phase closure studies, used in astronomy to reduce the effect of atmospheric turbulence.

The next generation of extremely large telescopes (ELT), with diameters up to 39 meters, is planned to begin operation in the next decade and promises new challenges in the development of instruments since the instrument size increases in proportion to the telescope diameter D, and the cost as D 2 or faster. The growing field of astrophotonics (the use of photonic technologies in astronomy) could solve this problem by allowing mass production of fully integrated and robust instruments combining various optical functions, with the potential to reduce the size, complexity and cost of instruments. Astrophotonics allows for a broad range of new optical functions, with applications ranging from sky background filtering, high spatial and spectral resolution imaging and spectroscopy. In this paper, we want to provide astronomers with valuable keys to understand how photonics solutions can be implemented (or not) according to the foreseen applications. The paper introduces first key concepts linked to the characteristics of photonics technologies, placed in the framework of astronomy and spectroscopy. We then describe a series of merit criteria that help us determine the potential of a given micro-spectrograph technology for astronomy applications, and then take an inventory of the recent developments in integrated micro-spectrographs with potential for astronomy. We finally compare their performance, to finally draw a map of typical science requirements and pin the identified integrated technologies on it. We finally emphasize the necessary developments that must support micro-spectrograph in the coming years.

Guided optics spectrometers can be essentially classified into two main families: based on Fourier transform or dispersion. In the first case, an interferogram generated inside an optical waveguide and containing the spectral information is sampled using spatially distributed nanodetectors. These scatter quasi-non-perturbingly light into the detector that is in contact with the waveguide, helping to reconstruct the stationary wave. A dedicated FFT processing is needed in order to recover the spectrum with high resolution but limited spectral range. Another way is to directly disperse the different wavelengths to different pixels, either introducing differential optical path in the same propagation plane (multiple Mach-Zehnder interferometers or Arrayed Waveguides Gratings), or using a periodic structure to perpendicularly extract the optical signal confined in a waveguide (photonic crystals or surface gratings), and by means of a relay optics, generate the spectrum on the Fourier plane of the lens, where the detector is placed. Following this second approach, we present a laser-fabricated high-resolution compact dispersive spectro-interferometer (R>2500, 30nm spectral range at λ = 1560nm), using four parallel waveguides that can provide up to three non-redundant interferometric combinations. The device is based on guided optics technology embedded in bulk optical glass. Ultrafast laser photoinscription with 3D laser index engineering in bulk chalcogenide Gallium Lanthanium Sulfide glass is utilized to fabricate large mode area waveguides in an evanescently-coupled hexagonal multicore array configuration, followed by subsequent realization of nanoscaled scattering centers via one dimensional nanovoids across the waveguide, written in a non-diffractive Bessel configuration. A simple relay optics, with limited optical aberrations, reimages the diffracted signal on the focal plane array, leading to a robust, easy to align instrument.

We study the optical coupling between a gold nanowire and a silver ion-exchanged waveguide, with special emphasis on the nanowire antenna radiation pattern. We measure the radiation patterns of waveguide-coupled gold nanowires with a height of 70 nm and width of 50 or 150 nm in the 450-700 nm spectral range for TE and TM polarizations. We perform a systematic theoretical study on the wavelength, polarization, nanowire size, and material dependences on the properties of the radiation pattern. We also give some elements concerning absorption and near-field. Experiments and calculations show localized plasmon resonance for the polarization orthogonal to the wire (far-field resonance at 580 nm for the smallest wire and 670 nm for the widest). It is shown that a great variety of radiation patterns can be obtained, together with a high sensitivity to a change of one parameter, particularly near-resonance.

Miniaturization is the main goal for system design in future cameras. This paper offers a novel method to scale down the optical system and to improve the image quality. As with the human retina, the detector array is spherically bent to fit the curved image surface; so the field curvature aberration is directly suppressed, leading to a better resolution and a simplified optical design. By thinning the substrate, the device is monolithically curved without modifying the fabrication process of the active pixels. Optical characterizations have been performed on planar and curved focal plane based cameras to illustrate the optical advantages of detector array curvature.

Design of miniature and light cameras requires an optical design breakthrough to achieve good optical performance. Solutions inspired by animals' eyes are the most promising. The curvature of the retina offers several advantages, such as uniform intensity and no field curvature, but this feature is not used. The work presented here is a solution to spherically bend monolithic IR detectors. Compared to state-of-the-art methods, a higher fill factor is obtained and the device fabrication process is not modified. We made an IR eye camera with a single lens and a curved IR bolometer. Images captured are well resolved and have good contrast, and the modulation transfer function shows better quality when comparing with planar systems.

The realization and the characterization of the leaky loop integrated Fourier transform spectrometer (LLIFTS) is described. The principle of the LLIFTS lies on a two-beam interferometer in planar design using a leaky loop waveguide structure. The interference pattern is measured at the edge of the component. The LLIFTS has been realized using the silver/sodium ion exchange on glass substrate technology, which is low cost and requires only a single lithography step. A mask has been designed considering a numerical model recently developed. Interference patterns have been measured in the wavelength range from 1500 to 1630 nm. Wavelength resolutions of 14 and 11 nm have been measured, respectively, on the 350 and the 500 microm radii leaky loop structures on a compact optical device.

A compact static Fourier transform spectrometer for integrated optics is proposed. It is based on a plane leaky loop structure combined with a plane waveguide. The interference pattern produced in the loop structure leaks outside of it and is guided in the plane waveguide to the photodetector array. This configuration allows one to control the shape of the field pattern at the end of the plane waveguide. A large fringe pattern with a high interference fringe contrast is obtained. A two-dimensional model based on an aperiodic Fourier modal method is used to modelize the coupling between the bent and the plane waveguides, completed with the Helmholtz-Kirchhoff propagation. This concept gives access to plan and compact spectrometers requiring only a single low-cost realization process step. The simulation has been done to realize a spectrometer in glass integrated optics (Deltalambda=6.1 nm at 1500 nm).