With the recent progress in integrated silicon photonics technology and the recent development of efficient quantum cascade lasers (QCL), there is now a very good opportunity to investigate new gas sensors offering very high sensitivity, high selectivity (multi-gas sensing, atmosphere analysis) and low cost thanks to integration on planar Si substrates. Gas sensing generally requires a tunable source continuously covering the whole operational range of the QCL stack. This paper presents the design, fabrication and characterization of Array Waveguide Grating (AWG) devices aiming at the simultaneous detection of several gas using arrays of QCL sources. We have developed a new platform based on Ge cores surrounded by thick Si<sub>80</sub>Ge<sub>20</sub> layers. The index difference between the core and the cladding is around 0.5 on the 3-13 μm spectral range. The core has a typical cross section of 2.5 × 2.5 μm and is surrounded by 6 μm thick SiGe cladding layers. As test vehicle device, we designed a 35 inputs multiplexer working in the 9.5 μm operation range (1050-1250 cm<sup>-1</sup> ). The design was carried out to match the inputs of a Distributed Feed-Back-QCL array. Preliminary measurements on the waveguide showed losses in the 3.5 dB/cm range. AWG devices were fabricated and tested. They showed results in good agreement with the modeling. An almost flat transmission over a full 200 cm<sup>-1</sup> operational range was obtained, with a peak-to-valley modulation of -5dB.
In the framework of the preparation of the Darwin mission, Alcatel Space has been developing a Multi- Aperture Imaging Interferometer (MAI²) for ESA. The main purpose of this activity is to achieve a deep extinction rate, or "nulling", in the laboratory. The performance goal is a 106 nulling of a simulated star. A second objective is to image a planet-like source. The selected operating wavelength is centred around 1.55 microns, with a relative bandwidth of a few percent. The selected function conceptual designs can be updated to the Darwin spectral range (6-18μm).
With the recent progress in integrated silicon photonics technology and the recent development of efficient quantum cascade laser technology (QCL), there is now a very good opportunity to investigate new gas sensors offering both very high sensitivity, high selectivity (multi-gas sensing, atmosphere analysis) and low cost thanks to the integration on planar substrate. In this context, we have developed singlemode optical waveguides in the mid-infrared based on Silicon/Germanium alloy integrated on silicon. These waveguides, compatible with standard microelectronic technologies present very low loss in the 3300 – 1300 cm<sup>-1</sup> range. This paper presents the design, technological realization, and characterization of array waveguide grating devices specifically developed for the simultaneous detection of several gas using arrays of QCL sources. Gas sensing generally requires a tunable source continuously covering the whole operational range of the QCL stack. With this objective, specific design has been adopted to flatten the optical transfer function of the whole multiplexers. Samples devices around 2235cm<sup>-1</sup> were realized and tested and showed results in good agreement with the modeling, flat transmission over a full 100 cm<sup>-1</sup> operational range were obtained with a peak-to-valley modulation of -5dB were experimentally measured. These devices will be soon associated with QCL arrays in order to provide integrated, powerful, multi wavelength, laser sources in the 2235 cm<sup>-1</sup> region applicable to NO, CO, and CO<sup>2</sup> multi-gas sensor.
Gravity is one of the second-generation instruments of the Very Large Telescope Interferometer that operates in the near infrared range and that is designed for precision narrow-angle astrometry and interferometric imaging. With its infrared wavefront sensors, pupil stabilization, fringe tracker, and metrology, the instrument is tailored to provide a high sensitivity, imaging with 4-millisecond resolution, and astrometry with a 10μarcsec precision. It will probe physics close to the event horizon of the Galactic Centre black hole, and allow to study mass accretion and jets in young stellar objects and active galactic nuclei, planet formation in circumstellar discs, or detect and measure the masses of black holes in massive star clusters throughout the Milky Way. As the instrument required an outstanding level of precision and stability, integrated optics has been chosen to collect and combine the four VLTI beams in the K band. A dedicated integrated optics chip glued to a fiber array has been developed. Technology breakthroughs have been mandatory to fulfill all the specifications. This paper is focused on the interferometric beam combination system of Gravity. Once the combiner concept described, the paper details the developments that have been led, the integration and the performance of the assemblies.
We present a scheme for the realization of high performances, large tuning range, fully integrated and possibly low cost mid infrared laser source based on quantum cascade lasers and silicon based integrated optics. It is composed of a laser array and a laser combiner. We show that our metal grating approach gives many advantages for the fabrication yield of those laser arrays. We show the results of such a fabrication at 1350 cm<sup>-1</sup> with 60 cm<sup>-1</sup> tuning range. The silicon is a low cost option for the size consuming combiner. In the development of the SiGe platform, we present the loss measurement set up and we show losses below 1dB/cm at 4.5μm.
Gravity aims at enhancing infrared imaging at VLTI to significantly improve our understanding of the physical processes
related to gravitation and accretion within compact objects. With its fiber-fed integrated optics, infrared wavefront
sensors, fringe tracker, beam stabilization and a novel metrology concept, GRAVITY will push the sensitivity and
accuracy of astrometry and interferometric imaging far beyond what is offered today. Four telescopes will be combined
in dual feed in the K band providing precision astrometry of order 10 micro-arcseconds, and imaging with 4-
milliarcsecond resolution. The fringe tracker and the scientific instrument host an identical integrated optics beam
combiner made by silica-on-silicon etching technology that is put inside a cryogenic vessel and cooled down to 200K to reduce thermal background and increase sensitivity.
This paper gives the design of the integrated beam combiner and of its fibered array that allows feeding the combiner
with stellar light. Lab measurement of spectral throughput and interferometric performance for beam combiners made by Flame Hydrolysis Deposition and by Plasma-Enhanced Chemical Vapor Deposition (PECVD) are given. The procedure
to glue together the beam combiner and its fibered array is described as well as the tests to validate the performance and the ageing effects at low temperature. Finally the thermal analysis and the eigen-frequency study of the whole device are presented.
Gravity is a 2nd generation interferometric instrument for VLTI. It will combine 4 telescopes in dual feed in the K band
to study general relativity effects around the Galactic Center black hole. The concept of Gravity is based on two
equivalent beam combiner instruments: the scientific one fed by the science target (Sgr A*) and the fringe tracker fed by
a bright reference star (See Gillessen et al.<sup>1</sup>). Both beam combination instruments are based on silica on silicon
integrated optics (IO) component glued to fluoride glass fiber array. The beam combiners are implemented in a
cryogenic vessel cooled at 200°K and back-illuminated by a high power laser used for metrology (Bartko et al.<sup>2</sup>). This
paper is dedicated to the description of the development of the integrated beam combiner assembly.
The VLTI Spectro Imager (VSI) was proposed as a second-generation instrument of the Very Large Telescope Interferometer
providing the ESO community with spectrally-resolved, near-infrared images at angular resolutions
down to 1.1 milliarcsecond and spectral resolutions up to R = 12000. Targets as faint as K = 13 will be imaged
without requiring a brighter nearby reference object; fainter targets can be accessed if a suitable reference is
available. The unique combination of high-dynamic-range imaging at high angular resolution and high spectral
resolution enables a scientific program which serves a broad user community and at the same time provides the
opportunity for breakthroughs in many areas of astrophysics. The high level specifications of the instrument are
derived from a detailed science case based on the capability to obtain, for the first time, milliarcsecond-resolution
images of a wide range of targets including: probing the initial conditions for planet formation in the AU-scale
environments of young stars; imaging convective cells and other phenomena on the surfaces of stars; mapping
the chemical and physical environments of evolved stars, stellar remnants, and stellar winds; and disentangling the central regions of active galactic nuclei and supermassive black holes. VSI will provide these new capabilities
using technologies which have been extensively tested in the past and VSI requires little in terms of new
infrastructure on the VLTI. At the same time, VSI will be able to make maximum use of new infrastructure as it
becomes available; for example, by combining 4, 6 and eventually 8 telescopes, enabling rapid imaging through
the measurement of up to 28 visibilities in every wavelength channel within a few minutes. The current studies
are focused on a 4-telescope version with an upgrade to a 6-telescope one. The instrument contains its own
fringe tracker and tip-tilt control in order to reduce the constraints on the VLTI infrastructure and maximize
the scientific return.
Two of the three instruments proposed to ESO for the second generation instrumentation of the VLTI would
use integrated optics for beam combination. Several design are studied, including co-axial and multi-axial
recombination. An extensive quantity of combiners are therefore under test in our laboratories. We will present
the various components, and the method used to validate and compare the different combiners. Finally, we will
discuss the performances and their implication for both VSI and Gravity VLTI instruments.
The VLTI Spectro Imager project aims to perform imaging with a temporal resolution of 1 night and with a maximum
angular resolution of 1 milliarcsecond, making best use of the Very Large Telescope Interferometer capabilities. To
fulfill the scientific goals (see Garcia et. al.), the system requirements are: a) combining 4 to 6 beams; b) working in
spectral bands J, H and K; c) spectral resolution from R= 100 to 12000; and d) internal fringe tracking on-axis, or off-axis
when associated to the PRIMA dual-beam facility.
The concept of VSI consists on 6 sub-systems: a common path distributing the light between the fringe tracker and the
scientific instrument, the fringe tracker ensuring the co-phasing of the array, the scientific instrument delivering the
interferometric observables and a calibration tool providing sources for internal alignment and interferometric
calibrations. The two remaining sub-systems are the control system and the observation support software dedicated to the
reduction of the interferometric data.
This paper presents the global concept of VSI science path including the common path, the scientific instrument and the
calibration tool. The scientific combination using a set of integrated optics multi-way beam combiners to provide high-stability
visibility and closure phase measurements are also described. Finally we will address the performance budget of
the global VSI instrument. The fringe tracker and scientific spectrograph will be shortly described.
This paper presents the development and tests in the thermal infrared of Integrated Optics (IO) technology in preparation of ESA's space mission <i>Darwin</i>. This mission aims to detect and characterize earth-like planets orbiting solar-type stars, using nulling interferometry in the spectral range 6 - 20 μm. Since typically 1:1e6 rejection of starlight is required, wavefront modal filtering is mandatory. Thus, mid-infrared single-mode IO is being developed in the framework of the ESA-funded "Integrated Optics for Darwin" project. Beyond its wavefront filtering capabilities, an IO component may support various optical functions, and is thus likely to ease instrumental design. This paper addresses the manufacturing process and the characterization tests results of newly developed IO devices. Investigated solutions are dielectric waveguides based on Chalcogenide glasses and Hollow Metallic Waveguides. In a first phase, the pre-selected technological solutions were validated and modal behavior of the manufactured devices was demonstrated, both through polarization and spectral analysis. Preliminary nulling ratios up to 5000 have been obtained with an IO modal filter in the 6 - 20 μm range. In a second phase of the project, the development of more complex IO functions was attempted. The methods used to validate the waveguide behavior and interferometric capabilities are also discussed. After achieving 1:1e5 polychromatic extinctions with similar solutions in the near IR, the presented results further underline the credibility of a mid-infrared IO concept for Darwin.
We present a new four-telescope integrated optics (IO) beam combiner in the near-infrared H band, and preliminary
photometric and interferometric measurements obtained in laboratory. The combiners tested and characterized
in our experiments are at the heart of the VSI/VITRUV instrument, whose goal is to combine four to
six telescopes of the VLTI. In this paper, we describe the combiners which incorporate phase-shifting devices
and their characterization through the analysis of polarization properties, instrumental visibilities and phases.
Our results were obtained with an eight-telescope laboratory interferometer, specially developed to simulate the
VLTI. These results demonstrate one more time that the integrated optics technology is particularly well suited
for interferometric combination of multiple beams, and therefore to achieve aperture synthesis imaging with the
VLTi Spectro-Imager (VSI) is a proposition for a second generation VLTI instrument which is aimed at providing
the ESO community with the capability of performing image synthesis at milli-arcsecond angular resolution. VSI
provides the VLTI with an instrument able to combine 4 telescopes in a baseline version and optionally up to
6 telescopes in the near-infrared spectral domain with moderate to high spectral resolution. The instrument
contains its own fringe tracker in order to relax the constraints onto the VLTI infrastructure. VSI will do
imaging at the milli-arcsecond scale with spectral resolution of: a) the close environments of young stars probing
the initial conditions for planet formation; b) the surfaces of stars; c) the environment of evolved stars, stellar
remnants and stellar winds, and d) the central region of active galactic nuclei and supermassive black holes. The
science cases allowed us to specify the astrophysical requirements of the instrument and to define the necessary
studies of the science group for phase A.
We present a brief review of recent scientific and technical advances at the Infrared Optical Telescope Array (IOTA). IOTA is a long-baseline interferometer located atop Mount Hopkins, Arizona. Recent work has emphasized the use of the three-telescope interferometer completed in 2002. We report on results obtained on a range of scientific targets, including AGB stars, Herbig AeBe Stars, binary stars, and the recent outburst of the recurrent nova RS Oph. We report the completion of a new spectrometer which allows visibility measurements at several high spectral resolution channels simultaneously. Finally, it is our sad duty to report that IOTA will be closed this year.
Stellar interferometry is an old technique (first successful measurement of Titan diameter by Michelson in 1890), which have recently been dramatically improved by the implementation of integrated optical devices. The technique, consisting in combining coherently several beams coming from distinct telescopes, allows to reconstruct images with a very high angular resolution, typically 10 times better than the diffraction limit of the biggest telescopes on Earth and about 20 times better than the Hubble space telescope. During the last few years, LETI, in collaboration with IMEP (Institut de MicroElectronique et Photonique) and LAOG (Laboratoire d'Astrophysique de l'Observatoire de Grenoble) has developed several components for stellar interferometry, either using its well-established silica on silicon technology, or developing a new silicon technology for mid-infrared metallic hollow waveguides adapted for the ESADARWIN mission. This paper will present the latest developments made by LETI in this field, describing the silicon technologies involved, the realized devices as well as their behaviour on laboratory set-ups or on the sky.
This paper reports the design, realisation, and characterisation of singlemode hollow conductive waveguides for stellar interferometry. These waveguides are developed in the frame of technological developments for the ESA DARWIN mission, which aims at direct detection of exoplanets and biomarkers on them (proof of life) using nulling interferometry in the 6-20 μm spectral range. The use of singlemode waveguides is mandatory in order to meet DARWIN required performance by achieving a modal filtering better than 10<sup>-6</sup>. While there is ongoing developments of infrared dielectric fibers or integrated waveguides, both using chalcogenide glasses or silver halide compounds, this paper presents the first realisation and characterisation of singlemode hollow conductive waveguides in the DARWIN spectral range, by means of standard microelectronic and wafer bonding technologies.
Optical chemical sensors and biosensors are attracting research interest in applications such as environmental monitoring and biomedical diagnostics. Structured Integrated Optical Waveguide is one solution to reduce the reader's cost and size. The principle is the capture of fluorescence emitted by Qdots at the surface of a rib waveguide, which collects then guides it at the end-face of the chip to be detected. However, fluorescence coupling into a waveguide is
still not easy to predict as it depends on fluorophore's environment and dipole's orientation and location. We report here the validation of a simple theory concerning optimization of optical waveguide's thickness considering a fluorophore's position. Optimisation of coupling power between a dipole and a guided mode can be simplified by the optimisation of the guided mode's intensity ratio integrated in the 5 nm region over the guide's core surface (where QDots are supposed to settle) divided by the whole guided intensity. A model has been developed from the work of Marcuse<sup>1</sup>: coupled power is proportional to the square of the electrical field of the guided wave. As a result, this model gives an optimal core's thickness and efficiency of coupling depends on polarisation. Moreover, FDTD simulations do complete this study. Three thicknesses have been therefore experimentally deposited: 100 nm, 125 nm and 150 nm. To conclude, experimentation corresponds to the model. A new, sensitive and potentially low cost portable transducer for the analysis of all kinds of biomolecular affinity systems has been developed and validated.
To detect earth-like planets orbiting around solar-type stars in the mid-infrared spectral range, a typical rejection ratio of 10<sup>6</sup> of the stellar flux must be achieved. Space missions like Darwin/TPF aim at achieving such contrasts using nulling interferometry between 4 μm and 20 μm. The instrumental constraints on beam combination, spatial filtering, intensity and phase mismatches must then be accurately considered. This paper presents the first characterization results of mid-infrared waveguides for integrated optics (IO) developed in the frame of an ESA contract. Taking into account the scientific achievements already obtained with IO components in the near infrared range, results demonstrate that these technologies can also be used for future nulling devices as an alternative to bulk optics instrumentation in the mid-infrared spectral range. Good waveguiding behaviour has been obtained on dielectric waveguides based on Chalcogenide or Zinc Selenide glasses and Hollow Metallic Waveguides. The single-mode behavior, spatial filtering and polarization control capabilities of the hollow metallic channel waveguides have been also demonstrated. This paper focuses on the methods used to validate the waveguide behaviour and the first laboratory results obtained with the different technologies used in the mid-infrared.
During the last years, LETI has been developing integrated optics components for stellar interferometry imaging using its proprietary silica on silicon technology. Recent astrophysical results obtained with the three-telescope combiner IONIC3 on IOTA confirm the great interest of this kind of integrated devices in terms of overall performance and stability. In this paper, after a brief explanation of the stellar interferometry imaging technique, silica on silicon integrated optical components for stellar interferometry are presented.
This paper reports the design, fabrication and opto-mechanical characterization of a deformable mirror to correct spherical aberration in future optical data storage standard (Blu-Ray Disc). The integrated mirror is realized in standard semiconductor technology to produce very low cost devices. The device is based on the electrostatic actuation of a 10 &mu;m thick silicon membrane (4 mm diameter) obtained from a 4' SOI wafer glued with polymer paste over concentric or hexagonal electrodes obtained from another 4' silicon wafer. Optical wavefront measurements compared with theoretical calculations demonstrate that an applied voltage of only 40 V on the three concentric electrodes allow to perfectly correct aberrations. Moreover we showed that the shape of the optical deformation induced by the mirror can precisely be controlled by the design of electrodes and applied voltages. A Peak to Valley optical deformation up to 4 &mu;m can be achieved with an applied voltage of only 70 V. Finally dynamic measurement showed that the device is able to work at a frequency of 100 Hz that is higher that needed for foreseen application.
We are working towards imaging the surfaces and circumstellar envelopes of Mira stars in the near-infrared, using the IOTA interferometer and the IONIC integrated-optics 3-beam combiner. In order to study atmospheric structures of these stars, we installed 3 narrow-band filters that subdivide H-band into 3 roughly equal-width sub-bands - a central one for continuum, and 2 adjacent ones to sample Mira star's (mostly water) absorption-bands. We present here our characterization of the IOTA 3-Telescope interferometer for closure-phase measurements with broad and narrow-band filters in the H atmospheric window. This includes characterizing the stability, chromaticity, and polarization effects of the present IOTA optics with the IONIC beam-combiner, and characterizing the accuracy of our closure phase measurements.
During the last years, LETI has developed integrated optics components for stellar interferometry using silica on silicon technology. Recent astrophysical results obtained with the three-telescope combiner IONIC3 on IOTA confirm the interest of this kind of integrated devices in terms of overall performance and stability. New combiners based on symmetric three-waveguide couplers have been theoretically studied, manufactured, and tested in laboratory in collaboration with LAOG over the H-band. Simulation et experimental preliminary results are presented and compared to asymmetric two-waveguide couplers. Applications to three- and four-telescope combination are discussed.
The science objectives of VITRUV is to investigate the morphology of compact astrophysical objects in optical wavelengths like the environment of AGN, star forming regions, stellar surfaces. This instrument will take full advantage of the VLTI site with 4 very large telescopes and 4 auxiliary telescopes. The instrument concept is to built aperture synthesis images like the millimeter-wave radiointerferometer of the IRAM Plateau de Bure. VITRUV coupled to the VLTI will have similar and even better resolution than ALMA. The astrophysical specifications although not yet finalized will be a temporal resolution of the order of 1 day, spectral resolution from 100 to 30,000, image dynamic from 100 to 1,000, a field of view of 1 arcsec for an initial wavlength coverage from 1 to 2.5 microns that could be extended from 0.5 to 5 microns. The technology that is contemplated at this stage is integrated optics.
Second generation VLTI instruments will be able to use of the array full imaging capability with up to 8 telescopes. Such an instrument will allow astronomers to measure 28 visibilities and 21 independent closure-phases at the same time, providing therefore rapidly imaging abilities with a spatial resolution of one milliarcsecond in the near infrared range.
The VITRUV project is a proposition to achieve the VLTI interferometric combination thanks to single-mode planar optics (the so-called integrated optics, IO). IO technologies allow to design integrated combiners with remarkable stability and self allignement properties. In addition, modal filtering associated with photometric calibration will lead to accurate visibility and closure-phase measurements. In this paper we present a detailed analysis of beam combination concepts that takes into account several constraints: throughput, signal to noise ratio, interferometric efficiency, integrated optics circuit design constrains and astrophysical requirements for imaging mode.
The Multi-Aperture Imaging Interferometer (MAI<sup>2</sup>), which Alcatel Space has been developing for ESA for deep nulling demonstration in preparation of the Darwin project, is based on an innovative layout, where both beam combination and modal wave-front filtering functions are achieved by means of an Integrated Optics (IO) component. Two different components, based on different designs and technologies, have been developed and characterised by LAOG with detailed design and manufacturing performed by IMEP/GeeO/LETI. SAGEIS-CSO (optical path control) and Alcatel Space have developed the other breadboard functions. The MAI² interferometer achieved stable Darwin-class nulling (10-5) of a simulated star in monochromatic light, and with a relative bandwidth of several percent (10-4). Operation in non-polarised light, with unchanged nulling performances, was also demonstrated. Preliminary characterisation of the relationship between nulling and bandwidth is also provided.
The design and characterization of very compact passive devices based on Silicon On Insulator (SOI) are investigated. Our devices have been realised on 8 inch wafer at CEA LETI using microelectronics technology for the demonstration of an optical clock distribution. We have studied strip waveguide of 0.3μm width and 0.38 μm in height, Y junctions, MMI splitters and two kinds of microbends with constant or progressive radius of curvature.
The IRSI/DARWIN spatial interferometer of the European Space Agency (ESA) is aimed at detecting extrasolar planets. The high difference in flux emission between the star and the planet is tackled by using nulling interferometry as a coronographic method. By star light extinction, one can retrieve the planet signal, and thus have access to high resolution imaging by interferometric measurements. Critical technological solutions are to be developed in order to reach the high level performances of such instruments. This is the scope of the Multi-Aperture Imaging Interferometer (MAI<sup>2</sup>) breadboard developed by Alcatel Space in an ESA contract. The goal of this laboratory experiment, based on integrated optics (IO) beam combination, is to obtain stable rejection of a star signal at a level of 10<sup>6</sup>.
Several scientific topics linked to the observation of extended structures around astrophysical sources (dust torus around AGN, disks around young stars, envelopes around AGBs) require imaging capability with milli-arcsecond spatial resolution. The current VLTI instruments, AMBER and MIDI, will provide in the coming months the
required high angular resolution, yet without actual imaging. As a rule of thumb, the image quality accessible with an optical interferometer is directly related to the number of telescopes used simultaneously: the more the apertures, the better and the faster the reconstruction of the image. We propose an instrument concept to
achieve interferometric combination of N telescopes (4 ≤ <i>N</i> ≤ 8) thanks to planar optics technology: 4 x 8-m telescopes in the short term and/or 8 x 1.8-m telescopes in the long term. The foreseen image reconstruction quality in the visible and/or in the near infrared will be equivalent to the one achieved with millimeter radio interferometers. Achievable spatial resolution will be better than the one foreseen with ALMA. This instrument would be able to acquire routinely 1 mas resolution images. A 13 to 20 magnitude sensitivity in spectral ranges from 0.6 to 2.5 <i>μm</i> is expected depending on the choice of the phase referencing guide source. High dynamic range, even on faint objects, is achievable thanks to the high accuracy provided by integrated optics
for visibility amplitude and phase measurements. Based on recent validations of integrated optics presented here an imaging instrument concept can be proposed. The results obtained using the VLTI facilities give a demonstration of the potential of the proposed technique.
Even if a lot of applications are today focused on optical communications, other industrial fields can also take benefit from the advantages offered by MOEMS solutions. Many Optical MEMS are based on electro-mechanical devices providing optical function, for example moving micro-mirrors for optical switches, attenuators or scanners. But LETI and a few other labs have also developed an original MOEMS technology by combining on the same substrate integrated optical planar waveguides with micro-mechanical structures. To reach this aim, a specific 'silica micro-machining' technology has been developed. Based on the moving waveguide technology, LETI have fabricated a micro-vibration sensor for the surveillance of rotating machines in electrical generators and optical switches for optical network protection and reconfiguration. Other devices are also based on silica micro-machining technology since they use moving silica optical structures, for example micro-lenses for micro-scanners. After general ideas on definitions and Optical MEMS applications, the paper continues with a short overview of the state of the art of Optical MEMS based on moving waveguides. Then it is more focused on silica on silicon technology. Specific technological problems are presented and the discussion is illustrated by the work carried out by LETI in this area.
Proc. SPIE. 4035, Laser Radar Technology and Applications V
KEYWORDS: Signal to noise ratio, Electronics, Avalanche photodetectors, Sensors, Laser range finders, Interference (communication), Receivers, Distance measurement, Analog electronics, Signal detection
In this paper, a digital time of flight laser range finder is presented. This measurement method has been simulated and experimentally evaluated using an eye-safe (class IIIA) laser range finder based on a passively Q-switched microchip laser as emitter and an avalanche photodiode as receiver. This new concept of measurement overcomes instability of classical time of flight laser range finder versus the reflectivity of the target, the atmospheric transmittance and the photoelectric noise. Classical laser range finder uses analog techniques to measure the time elapsed between the start and stop laser pulses: start time and stop time are defined by analog thresholds, therefore accuracy is greatly affected by the SNR in the receiver and associated electronics. This SNR is also dependent on emitter and receiver geometric configuration. Therefore the power seen by the receiver as a function of the distance measurement has been estimated theoretically and observed in experiments in order to optimize SNR over a chosen range. A new technique to overcome this lack of accuracy has been implemented using a fast digitizer with an acquisition rate of 10 GS/s and digital signal processing algorithms. Accuracy of about +/- 5 mm for a single shot without averaging and +/- 0.5 mm for averaging 20 shots have been demonstrated from 10 meters to 250 meters using only one photodetector for the start and stop pulse. Such a system will have an interest in the area of 3D vision laser ranging where there is a need of high accuracy measurements.
We have developed an integrated system for laser beam deflection, using a monolithic silica on silicon structure including microlenses and an electromechanical actuator. The scanner shows a maximum deflection angle of +/- 5 degree(s) for a static Voltage of about 130 V or continuous scanning of a field of 20 degree(s) for a 45 V signal at the resonance frequency. The device is fabricated by a surface micromachining process of a silica layer on silicon substrate with no subsequent assembling steps involved.
In this paper a theoretical and an experimental study of a sensor which uses a self-mixing interference effect inside a microchip laser are reported. The so-called FMCW approach has been implemented by using a intracavity electrooptic modulator which provides a total 10 GHz frequency-shift of the optical wave emitted by a Nd<SUP>3+</SUP>:YAG-LiTaO3 microchip laser. We demonstrated that the resonant behavior of the microchip laser provides a substantial amplification of the return wave and that such a phenomenon can be used to perform a highly sensitive detection for absolute distance (from 1 to 20 m +/- 0.1%) and velocity (from 0.1 to 25 m.s<SUP>-1</SUP> +/- 0.1%) measurements of a remote scattering target.
A 1:2 Micro-Opto-Mechanical switch has been achieved using the combination of 2 technologies: `Integrated Optics' and `Micromachining on Silicon'. The commutation is obtained by means of the mechanical deflection of a cantilever beam driven by an electrostatic force. The first devices show promising characteristics.