Etienne Le Coarer

Wavelength-scale stationary-wave integrated Fourier-transform spectrometry

Spectrometry is a general physical-analysis approach for investigating light-matter interactions. However, the complex designs of existing spectrometers render them resistant to simplification and miniaturization, both of which are vital for applications in micro- and nanotechnology and which are now undergoing intensive research. Stationary-wave integrated Fourier-transform spectrometry (SWIFTS)-an approach based on direct intensity detection of a standing wave resulting from either reflection (as in the principle of colour photography by Gabriel Lippmann) or counterpropagative interference phenomenon-is expected to be able to overcome this drawback. Here, we present a SWIFTS-based spectrometer relying on an original optical near-field detection method in which optical nanoprobes are used to sample directly the evanescent standing wave in the waveguide. Combined with integrated optics, we report a way of reducing the volume of the spectrometer to a few hundreds of cubic wavelengths. This is the first attempt, using SWIFTS, to produce a very small integrated one-dimensional spectrometer suitable for applications where microspectrometers are essential.

Curved focal plane detector array for wide field cameras

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 a compact static Fourier transform spectrometer in integrated optics based on a leaky loop structure.

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).

Realization of the compact static Fourier transform spectrometer LLIFTS in glass integrated optics.

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.

Toward a revival of stellar intensity interferometry

Building on technological developments over the last 35 years, intensity interferometry now appears a feasible option by which to achieve diffraction-limited imaging over a square-kilometer synthetic aperture. Upcoming Atmospheric Cherenkov Telescope projects will consist of up to 100 telescopes, each with ~100m2 of light gathering area, and distributed over ~1km2. These large facilities will offer thousands of baselines from 50m to more than 1km and an unprecedented (u,v) plane coverage. The revival of interest in Intensity Interferometry has recently led to the formation of a IAU working group. Here we report on various ongoing efforts towards implementing modern Stellar Intensity Interferometry.

Fabry–Perot optical fiber strainmeter with an embeddable, low-power interrogation system

Detection of earthquakes and tidal variations via measurement of strain in the Earth’s crust requires compact and robust instrumentation with low power usage that can be deployed in the field. Here we demonstrate a stationary-wave integrated Fourier transform spectrometer (SWIFTS) and measure the variations induced by ground strain on an optical fiber Bragg grating sensor using two short (17±2  mm) Fabry–Perot (FP) cavities, one for the sensor, and one for temperature compensation. The SWIFTS delivers spatial interferograms that are then Fourier transformed to deduce the deformation from a cross-spectral analysis of the FP spectra. The full system is tested in field conditions to record crustal earth strain signals and successfully detect the earth tide and an earthquake signal. With this low-coherency interferometry technique, this system offers an excellent compromise between the resolution needed and the cost of a fully autonomous field instrument.

Full integrated beam combiner instrument based on SWIFTS concept

We present how photonics associated with new arising detection technologies is able to provide fully integrated instrument for interferometric beam combination. The feasibility and operation of on-chip beam combiners have been demonstrated now according to various combination schemes. More recently we have proposed a novel detection principle that allows to directly sample and extract the spectral information of the incoming optical signal together with the flux level measurement. The so-called SWIFTS concept that stands for Stationary-Wave Integrated Fourier Transform Spectrometer, is able to provide the full spectral and spatial information recorded simultaneously thanks to a motionless detecting device. Throughout the newly available detection principle that could be considered for SWIFTS implementation, some technologies are even able to provide photo-counting operation that may bring a significant extension of the astrophysical domain of investigation of interferometry. The proposed new concept is applicable either to a fringe tracker instrument with fast and sensitive capabilities, or to a dispersive instrument with high spectral resolution capabilities.

High-performance high-speed spectrum analysis of laser sources with SWIFTS technology

The ZOOM Spectra spectrometer is the first fully integrated system to benefit from the disruptive SWIFTS technology, providing a high-resolution high-rate solution for the characterization of lasers. It allows for the first time a dynamic real-time view of their behavior. The instrument is an alliance of integrated guided optics, groundbreaking nanotechnologies, microelectronics and advanced software. The device has been designed to be a rapid solution for checking the tuning of a laser, the existence of hopping modes and the correct suppression of a side mode. Their performances are particularly valuable for analysis of custom sources such as Distributed Feedback (DFB) lasers, Vertical Cavity Surface Emitting Lasers (VCSELs), External Cavity Lasers (ECL), or Optical Parametric Oscillator (OPO) sources.

Compact high-resolution micro-spectrometer on chip: spectral calibration and first spectrum

Compact and hand-held spectrometers may be very interesting for the measurement of spectral signatures of chemicals or objects. To achieve this goal, ONERA and IPAG have developed a new on chip Fourier Transform Spectrometer operating in the visible spectral range with a high spectral resolution (near 2 cm-1), named visible HR SPOC (visible High Resolution Spectrometer On Chip). It is directly inspired from the MICROSPOC infrared spectrometer, studied at ONERA in the past years. This spectrometer is made of a stair-step two-wave interferometer directly glued on a CMOS detector making it a very compact prototype. After calibrating the optical path difference, measurements of experimental spectra are presented.

Defining requirements and identifying relevant technologies in astrophotonics

Astrophotonics offers a solution to some of the problems of building instruments for the next generation of telescopes through the use of photonic devices to miniaturise and simplify instruments. It has already proved its worth in interferometry over the last decade and is now being applied to nightsky background suppression. Astrophotonics offers a radically different approach to highly-multiplexed spectroscopy to the benefit of galaxy surveys such as are required to determine the evolution of the cosmic equation of state. The Astrophotonica Europa partnership funded by the EU via OPTICON is undertaking a wide-ranging survey of the technological opportunities and their applicability to high-priority astrophysical goals of the next generation of observatories. Here we summarise some of the conclusions.