FIRST (Fibered Imager foR a Single Telescope instrument) is an instrument that enables high contrast imaging and spectroscopy, thanks to a unique combination of sparse aperture masking, spatial filtering by single-mode waveguides and cross-dispersion in the visible. In order to increase the instrument’s stability and sensitivity, we propose an active hybrid photonic beam combiner. The device consists on a 5T integrated optics beam combiner. The idea is to split the architecture in two parts: A first part, concerning input beam splitting and active phase modulation, requiring relatively simple optical circuits (Y junctions and straight waveguides) is obtained in an electro-optic crystal (Lithium Niobate). A second part, where the complex beam recombination of all the split inputs is achieved (for N inputs, N(N-1)/2 recombinations). This stage implies many waveguide crossings, bendings and lengthy waveguides. Therefore, a high transmission, high confining glass is used. In both cases, classical lithography and ion in-diffusion techniques are used to fabricate the waveguides. Both stages have been optimized in terms of mode matching and single mode spectral bandwidth. They have been assembled together and with input/output fibered V-grooves. The work presented here consists on the characterization of the hybrid 5T beam combiner on the optical bench simulator of the FIRST/SUBARU instrument that is developed at LESIA. We will present results in terms of transmission, polarization and active phase modulation, showing that with relative low voltages, active fringe scan is achieved directly on-chip, at frequencies only limited by the readout time of the camera.
This work aims to present a new miniature spectrometer in the mid InfraRed (L Band), using the SWIFTS (Stationary Wave Integrated Fourier Transform Spectrometer) technology. A stationary wave obtained by injecting light on both sides of a channel waveguide (Gabor configuration) is sampled using nano-scattering centres (grooves) on the surface of the waveguide. A single groove per scattering centre will radiate the sampled signal with wide angular distribution, which is not compatible with the buried detection area of infrared detectors, resulting in crosstalk between pixels. An implementation of multiple grooves (antenna) for each sampling centre is proposed as a solution to improve directivity towards the detector pixel by narrowing the scattering angle of the extracted light. Here, the results are obtained using a Lithium Niobate (LiNbO3) substrate, as its electro-optic properties allow for an active modulation of the phase, and the technology explored for its fabrication is Direct Laser Writing, that allows to have buried 3D waveguides and nanogrooves. In order to integrate the detector in the device, different configurations are explored so as to obtain a robust and high-resolution device useful mainly for astronomical applications such as spectro-interferometry.
KEYWORDS: Sensors, Antennas, Crystals, Waveguides, Terahertz radiation, Optical sensors, Electric field sensors, Plasmas, Optical microsystems, Near field
The measurement of microwave electric-field (E-field) exposure is an ever-evolving subject that has recently led the International Commission on Non-Ionizing Radiation Protection to change its recommendations. With frequencies increasing toward terahertz (THz), stimulated by 5G deployment, the measurement specifications reveal ever more demanding challenges in terms of bandwidth (BW) and miniaturization. We propose a focus on minimally invasive E-field sensors, which are crucial for the in situ and near-field characterization of E-fields both in harsh environments such as plasmas and in the vicinity of emitters. We browse the large varieties of measurement devices, among which the electro-optic (EO) probes stand out for their potential of high BW up to THz, minimal invasiveness, and ability of vector measurements. We describe and compare the three main categories of EO sensors, from bulk systems to nanoprobes. First, we show how bulk-sensors have evolved toward attractive fibered systems that are advantageously employed in plasmas, resonance magnetic imagings chambers or for radiation-pattern imaging up to THz frequencies. Then we describe how the integration of waveguides helps to gain robustness, lateral resolution, and sensitivity. The third part is dedicated to the ultra-miniaturization of components allowing ultimate steps toward electromagnetic invisibility. This review aims at pointing out the recent evolutions over the past 10 years, with a highlight on the specificities of each photonic architecture. It also shows the way to future multi-physics and multi-arrays smart sensing platforms.
FIRST (Fibered Imager foR a Single Telescope instrument) is an instrument that enables high contrast imaging and spectroscopy, thanks to a unique combination of sparse aperture masking, spatial filtering by single-mode waveguides and cross-dispersion in the visible. In order to increase the instrument’s stability and sensitivity, we proposed a new series of photonic beam combiners. The idea is to achieve phase modulation inside an optical chip and get rid of external delay lines, and improve the transmission by using novel techniques that will allow for beam combination in 3D, avoiding planar X-crossings and large bending radii observed in planar integrated optics instruments, between first and last inputs to combine, when the inputs separation is large (i.e. in 9 telescopes beam combiners). In a previous paper [4] we presented first prototypes of beam combiners for FIRST/SUBARU 9T. Planar 2D concepts were studied, but transmission was low due to the high number of crossings and the sharp bending angles needed to achieve beam combination within the length of the wafer. In this paper we will present our recent results on improved designs concerning: A) A hybrid Lithium-Niobate active beam splitter and phase modulator (9T, 1x8), coupled to a passive glass beam combiner (72x36, by pairs). B) A full passive device (5T splitter+beam combiner) and C) a narrow 5T splitter + phase modulator based on lithium niobate, to reduce the bending losses and optimize the overall transmission once coupled to the passive combiner. A comparative analysis of different performances will be presented.
In this paper we report the simulation of an achromatised LiNbO3 phase modulator with a range of 300nm centered around 670nm by using a two stage cascaded Mach-Zehnder controlled by independent electric field on each stage to overcome the chromatic dispersion brought by the Pockels effect. Development of a two stages electro-optic phase modulator test setup and first proofs of phase compensation between the stages will be presented.
Isotropic sillenite crystals such as Bismuth Silicon Oxide (BSO) present highly interesting opto-electronics properties including electro-optic effect and photorefractivity. BSO is also a highly suitable candidate for sensitive temperature-independent electric field sensors [1]. Then the production of low cost BSO-based optical-waveguides is becoming a major challenge. However, BSO high density (> 7 g.cm3) and non-standard dimensions are a hurdle for standard fabrication approaches such as ion diffusion or exchange and standard clean-room technologies.
Here we report for the first time the successful fabrication of low loss BSO ridge waveguides with high index contrast. The proposed technique is based on optical-grade dicing [2], which allows low cost and massive production of photonic devices in different types of material. Ridge waveguides are made in a 15-µm thick chemical mechanical polished thin layer of BSO bonded on a lithium-niobate wafer. Propagation losses, group velocity and modal birefringence of optical modes have been measured by Optical-Coherence-Tomography. The waveguides support both TE and TM guided modes at telecom wavelength (1.55 µm) and present propagation losses lower than 2 dB/cm. This approach promises to be powerful for shaping single crystal thin films even in exotic formats. We expect low loss optical-waveguide in BSO will pave the way toward compact and highly sensitive electric-field sensors, scintillators, LED and laser applications.
[1] I. Saniour et al, NMR in Biomedicine,31, (2018).
[2] N. Courjal et al, Journal of Physics D: Applied Physics, 305101,(2011).
The astronomical L band is particularly well suited for the hunt of low mass companions and the study of planet forming discs. In this paper, we present the concept of a spectro-interferometer with application to high precision interferometry, in projects such as Hi-5: a high-contrast thermal near-infrared imager for the VLTI. The interest of our system is that it allows, for the first time, spectro-interferometry in the mid-infrared (L-Band), in an integrated optic device, with a resolution of R=2000 in a 500μm long sampling zone. Fringe scan and photometry balancing are achieved on-chip, using an external applied voltage. This kind of devices has already been used for high contrast interferometry (36dB rejection ratio) and spectrometry, and first developments have been achieved in passive spectro-interferometers. This first demonstrator is a key milestone towards an interferometric nulling combiner dedicated to high contrast observations
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.
Lithium niobate (LiN bO3) microresonators have attracted much interest over the last decade, due to the electrooptical, acousto-optic and non-linear properties of the material, that can advantageously be employed in combination with thin resonances of optical microcavities for applications as varied as integrated gyrometers, spectrometers or dynamic filters. However the integration of micrometer scale cavities with an input/output waveguide is still a critical issue. Here we propose an innovative approach, allowing low insertion losses and easy pigtailing with SMF fibers. The approach consists in producing and optimizing separately a membrane-based LiNbO3 waveguide with Spot-Size Converters, and a thin microdisk. The two elements are dynamically assembled and fixed in a second step. Additionally to the proposed integrated microresonator, this approach opens the way to the production of 3D hybrid photonic systems.
Biomedical engineering (BME), electrophysiology, Electromagnetic Compatibility (EMC) or aerospace and defense fields demand compact electric field sensors with very small spatial resolution, low sensitivity and large bandwidth. We show that the electro-optical property of lithium niobate coupled with the tunability of photonic crystals can answer this request through Lab-on-Fiber technology.
First, band diagram calculations and Finite Difference Time Domain (FDTD) simulations analysis lead to the design of the most suitable two-dimensional photonic crystal geometry. We show that light normal incidence on rectangular array of air holes in free standing X-cut thin film lithium niobate produces a very sharp and E-field sensitive Fano resonance at a wavelength of 1550nm. Then, in order to concentrate the E-Field to be detected in the photonic crystal area (20μm*20μm*0.7μm) we design a thin metallic antenna, scaled down them in such a way that it does not produce any disturbances while increasing the sensitivity.
The LN membrane with the antenna is fabricated by standard clean room processes and Focused Ion Beam (FIB) is used to mill the photonic crystal. Then, by means of a flexible/bendable transparent membrane, we were able to align and to attach the photonic crystal onto a ferrule ending polarization maintained optical fiber.
Optical characterizations show that the Fano resonance is easily modulated (wavelength shifted) by the surrounding E-field. The novel non-intrusive E-field sensor shows linearity, low sensitivity, large bandwidth (up to 100GHz) and a very small spatial resolution (≈20μm). To the best of our knowledge, this spatial resolution has never been achieved in E-field optical sensing before.
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.
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.
We present an optimization process to improve the rejection ratio in integrated beam combiners by locking the dark fringe and then monitoring its intensity. The method proposed here uses the electro-optic effect of lithium niobate in order to lock the dark fringe and to real-time balance the photometric flux by means of a two-stage Mach–Zehnder interferometer waveguide. By applying a control voltage on the output Y-junction, we are able to lock the phase and stay in the dark fringe, while an independent second voltage is applied on the first-stage intensity modulator, to finely balance the photometries. We have obtained a rejection ratio of 4600 (36.6 dB) at 3.39 μm in transverse electric polarization, corresponding to 99.98% fringe contrast, and shown that the system can compensate external phase perturbations (a piston variation of 100 nm) up to around 1 kHz. We also show the preliminary results of this process on wide-band modulation, where a contrast of 38% in 3.25- to 3.65-μm spectral range is obtained. These preliminary results on wide-band need to be optimized, in particular, for reducing scattered light of the device at the Y-junction. We expect this active method to be useful in high-contrast interferometry, in particular, for astronomical spatial projects actually under study.
We present easy-to-implement technologies to produce LiNbO3 PhCs in confined optical waveguides. Ti-indiffusion or
Annealed Proton Exchange (APE) are combined with optical grade dicing to fabricate ridge waveguides with
propagation losses that can be lower than 0.2 dB/cm. Firstly we show how a PhC inscribed in a confined ridge
waveguide can be exploited as a temperature sensor with an unexpectedly high 8 nm/°C temperature sensitivity. LiNbO3
PhCs with high aspect ratio are also demonstrated. The performance is achieved by properly tilting the ridge before
patterning its walls by Focused Ion Beam (FIB). A eight micrometer long 1D-PhC on a Ti:LiNbO3 ridge waveguide has
been fabricated and its reflectivity has been evaluated using an optical coherence tomography (OCT) system: it is
measured to be 53 % for the TM wave and 47 % for the TE wave. The period can be optimized in order to increase the
reflection of the 1D-PhC up to 80 %. These developments open the way to the dense integration of compact dynamic
devices such as modulators, spectral filters or electric field sensors.
We present our work on integrated optic beam combiners devoted to mid-infrared applications in the field of stellar
interferometry, in particular for nulling interferometry, where high rejection ratios are needed. The main results obtained are the single-mode behavior of the waveguides at a central wavelength λ=3.39μm, in both TE and TM polarization, a high rejection ratio on the modulated signal (best value of 30dB) and low dispersion in wide-band configuration. In a second time, in order to improve the electro-optic response of the modulators, we have realized a photonic crystal inside one of the arms of the Y-junction. Theoretical results predict simultaneous TE and TM group index enhancement, which should give better electro-optic response, however, our preliminary experimental results do not show significant difference with the initial combiners. Perspectives on the future work will be presented.
We present three different techniques for single-mode waveguide realization in Lithium Niobate at the 3.39μm
atmospheric transmission band, named L-band. These methods include Titanium diffusion, Ion Beam Implantation and
Photo-inscription. After describing the fabrication process and waveguide characterization, we will present an integrated
interferometer based on the Young's double slit experiment. From the recorded interferogram we recover information
about the source, namely, its peak emission lines.
We report two novel kinds of LiNbO3 electro-optic modulators. The first one is oriented toward long haul high bit rate telecommunication systems. An original single-ended structure with a poled section and phase reversal electrodes is proposed to prevent the intensity modulation from chirp, without sacrifice on the driving voltage. We also show that improvements can be performed with the use of several poled sections. To remain attracting, LiNbO3 modulators should also exhibit a lower size. The second configuration described here is a new generation of LiNbO3 modulators based on photonic crystals, with a micrometric active length. We theoretically show that the optimal photonic structures for an efficient electro-optical tuning are based on a triangular array of holes integrated on a X-cut substrate. The first optical characterizations confirm the theoretical predictions, and exhibit a -12dB extinction ratio in the transmission response.
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