G. Ulliac

First Results in Near and Mid IR Lithium Niobate-Based Integrated Optics Interferometer Based on SWIFTS-Lippmann Concept

High-resolution spectrometers are nowadays achievable in compact devices using integrated optics. The approach developed here consists in obtaining a static interferogram by means of a Fresnel reflection at the waveguide output (Lippmann interference between forward and backward beams) and then sample the fringes by periodically etching the waveguide with transverse nanogrooves, that will collect a negligible part of the flux. We present the first SWIFTS-Lippmann interferometer in the near and mid-infrared, thanks to high form factor grooves obtained by focused ion beam in lithium niobate, which opens the way to electrooptic modulation of the interferogram and thus, sampling on-chip, without any moving part. Possible applications are high-resolution spectroscopy and accurate measurement of effective refractive index of a waveguide. A measurement of the effective group refractive index of the guided mode is presented.

Wavelength-Selective Ti:LiNbO

This paper described a wavelength-selective couplers dedicated to drop functionality (spectral-selective couplers dedicated to wavelength demultiplexing). It is consist of two Y junctions: an input two drop channels and an output. The Y-branches distributors of optical power were carried out by a specific geometry of Ti:LiNbO3 waveguides. Each Y junction is composed by two identical bends jointed at one with a weak inclination angle to decrease losses. On each arm of the Y-branches, a Bragg grating (BG) is etched by Focused Ion Beam (FIB) in order to extract a specific wavelength band. A two-channel configuration is adopted which demonstrate a drop functionality of two spectral bands centered at 1123 nm and 1527 nm. The complementary spectrum is avalaible at the coupler output.

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The context of this work is the development of integrated optic beam combiners devoted to high contrast interferometry, in particular for exoplanet spectral characterization and future spatial missions, where the use of compact and light optical beam combiners ensures robustness and stability of the interferometric signal. Thus, the development of materials allowing light confinement in both polarizations, together with a good transparency from the visible to the mid-IR and able to achieve electro-optic modulation, in order to finely tune the relative phase of the interacting fields, is knowing a rapid development. Lithium Niobate is an electro-optical material allowing index, and thus optical phase modification, by application of an external electric field. It is also well known for waveguide realization in the visible, near and midinfrared. Here we present results on near and mid-infrared beam combiners achieving different optical functions: a) three telescope AC beam combiner, devoted to phase closure studies; b) Phase locking and fringe scanning using double Mach-Zehnder concept. Optimization of the fringe contrast by real time on-chip phase and photometry balance and c) High Resolution Spectrometers in channel waveguides.

Multiple Y-Branch Coupler Based on Focused Ion Beam Milled Bragg Reflectors

In this paper we have studied effect of depth etching on the Bragg gratings (BGs) realized by Focused Ions Beam. This technique has the advantage to induce a direct waveguide structuring without intermediate media, comparing to traditional methods. A reflectivity of 96% within a window centred at 1550 nm is obtained. The effect of the depth etching on the transmittance and the bandwidth at half maximum is demonstrated.

Lithium Niobate active beam combiners: results of on-chip fringe locking, fringe scanning and high contrast integrated optics interferometry and spectrometry

Integrated optics spectrometers can be essentially classified into two main families: based on Fourier transform or dispersed modes. In the first case, an interferogram generated inside an optical waveguide is sampled using nanodetectors, these scatter light into the detector that is in contact with the waveguide. A dedicated FFT processing is needed in order to recover the spectrum with high resolution but limited spectral range. Another way is to extract the optical signal confined in a waveguide using a surface grating and directly obtain the spectrum by means of a relay optics that generates the spectrum on the Fourier plane of the lens, where the detector is placed. Following this second approach, we present a high-resolution compact dispersive spectrometer (δλ =1.5nm at λ=1050nm) based on guided optics technology. The propagating signal is dispersed out of a waveguide thanks to a surface grating that lays along it. Focused Ion Beam technique is used to etch nano-grooves that act as individual scattering centers and constitute the surface grating along the waveguide. The waveguide is realized using X-cut, Ypropagating Lithium Niobate substrate, where the effective index for TE and TM guided modes is different. This results in a strong angular separation of TE and TM diffracted modes, allowing simultaneous detection of spectra for both polarizations. 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.

Effect of depth etching on Bragg reflectors realized by focused ion beam in Ti:LiNbO3 waveguide

In this paper, we propose a new concept of acoustic resonator based on a waveguide structure. In the telecommunication market, composants used are indeed generally bases on Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) devices. However, those systems present technological limits as short-circuits between the electrodes of the interdigital transducers (for SAW device) or the precise control of the piezoelectric material thick resonator (for BAW device). We suggest a new concept based on a periodically poled transducer (PPT) in a ferroelectric substrate (LiNbO3 or LiTaO3), embedded between two guiding substrates in order to create an acoustic waveguide. Periodically poled transducers have been investigated recently as an alternative to classical inter-digital transducers for the excitation and detection of guided acoustic waves. A resonator operating at 131MHz has been successfully fabricated and used in order to stabilize an oscillator at this frequency. However, the developed resonator presents a significant thermal sensitivity. The following experiments have consisted in studying a Si/thinned PPT layer/Si structure in order to reduce the thermal sensitivity.

High resolution TE&TM near infrared compact spectrometer based on waveguide grating structures

In this paper, we present new results on the development of piezoelectric transducers based on periodically poled ferroelectric domains on lithium niobate. The fabrication of test devices operating in the range 17-665 MHz has been achieved on 3 500 mum thick wafers. These devices have been bonded on silicon and lapped down to a few tens of microns. Guided elliptic as well as partially guided longitudinal modes are excited, yielding phase velocity of about 3800 and 7000 m.s-1 respectively. The experimental responses are compared to the theoretical results. Finally, the temperature coefficients of frequency are measured and simulated by introducing temperature variations in our FEA/BEM code showing a good agreement between both results.

New acoustic resonator based on periodically poled transducer in lithium niobate or tantalate

The paper addresses two microwave photonics functionalities based on device potentialities. The first one is related to the specific use of a mode locked (multi-section) laser diode in the analog domain where different modulation functions can be achieved. The second one deals with the specific design of a photodetector aiming to separately detect multiple modulated input beams and to group the resulting photocurrents into a single one. This leads to build an optoelectronic add function that can be used, for example, in microwave photonics signal processing applications such as optical beamforming networks for phased array antennas.

Characterization of guided modes excited by periodically poled transducers on Si

Integrated optic devices are nowadays achieving extremely high performances in the field of astronomical interferometry, as shown by the PIONIER and GRAVITY instruments. Progress remains to be made in order to increase the number of apertures/beams/channels to be combined (up to 9) and eventually ensure on-chip phase modulation (for fringe temporal scanning). We present a novel generation of beam combiners, based on the hybridization of two integrated optic devices: (i) one producing glass waveguides, that can ensure very sharp bend radius, high confinement and low propagation losses, with (ii) a lithium niobate device providing phase modulators and channel waveguides that can achieve on-chip, fast (<100kHz) phase modulation. The aim of this work is to compare three different concepts for the new generation FIRST/SUBARU 9T instrument, in terms of transmission, birefringence, half-wave voltage modulation and spectral range.

Novel device concepts for microwave photonics functionalities

We present our work on achromatic beam combiners to achieve wide band interferometry, using electro-optic devices. We also present our developments in hybrid integrated optics interferometers coupling passive devices but allowing sharp bending radius waveguides and active devices, allowing phase modulation in lithium niobate channel waveguides. These systems have been developed for visible applications, in the context of LITHIUM cubesat project and FIRST 9 channel beam combiner for FIRST/SUBARU telescope.