The state-of-the-art spectroscopy instruments are designed to collect as much light as possible, especially for astronomy applications. In this context, a blazed sub-wavelength disperser (called a metasurface) is a promising alternative to the widely used sawtooth blazed gratings with a metal coating. In order to find the optimal opto-geometric characteristics of such a device, topology optimization based on Finite Element modelling of Maxwell’s equations is used. This paper deals with the application of this powerful optimization process, using different materials and introducing the manufacturability constraints. The reflection averaged on the [400,1500]nm wavelength range can reach 80% with a broadband optimization on silica. It is 28% higher than that of the sawtooth blazed grating (which reaches 52%) in absolute terms, and 54% in relative terms. First samples of metasurface gratings have been manufactured.
This paper presents an overview of technologies of MEMS Cs microcells for CPT-based atomic clocks, obtained in FEMTO-ST Institute over the last decade. We discuss the challenges in microfabrication of miniature cells: the different configurations of cells, two methods for filling alkali vapor cells with Cs from a dispenser pill and from the dispensing paste, and a study on the permeability of our microcells to the buffer gas limiting the sealing performances. Finally, we report on the aging tests and resulting short-term and long-term clock stability measurements.
The early diagnosis of cancer is essential since it can be treated more effectively when detected earlier. Visual inspection followed by histological examination is, still today, the gold standard for clinicians. However, a large number of unnecessary surgical procedures are still performed. New diagnostics aids are emerging including the recent techniques of optical coherence tomography (OCT) which permits non-invasive 3D optical biopsies of biological tissues, improving patient’s quality of life. Nevertheless, the existing bulk or fiber optics systems are expensive, only affordable at the hospital and thus, not sufficiently used by physicians or cancer’s specialists as an early diagnosis tool. We developed an endoscopic microsystems based on Mirau interferometry and applied for swept source OCT imaging applied for gastroenterology. The architecture is based on a miniature spectrally tuned a single-channel Mirau interferometer integrated with an electro-thermal MEMS microscanner scanning the sample area.
The specific conical shape of the axicon makes them well suited for the generation of so-called Bessel beams. We present here a technological micro-glass blowing platform developed for miniaturization of micro-optical components. This employs the glass-blowing process, silicon micromachining and heterogeneous bonding. Here, we focus on the unconventional fabrication methods of micro-optical components in borosilicate glass. As an example, an original concept of a micro-machined micro-axicons for generation of Bessel beams is demonstrated and experimentally validated.
In this paper, we present the construction and preliminary experimental results of a MOEMS fiber-based integrated probe for endoscopic optical imaging of stomach tissue using a Swept-Source Optical Coherence Tomography (SSOCT). The probe consists of a Mirau micro-interferometer, combined with a GRIN lens collimator and a micromirror scanner. We describe the building blocks of the probe, especially the monolithically integrated Mirau mirointerferometer, fabricated by wafer-level vertical stacking and anodic bonding of Si/glass components, and the electrothermal 2-axis MEMS microscanner allowing large swept angles (up to ±22°) at high frequencies (> kHz) for low driving voltages (<20 V). The results of probe characterization, performed in a designed SS-OCT system, have confirmed proper operation of the probe. The B-scan images were obtained for central wavelength of λc = 840 nm, swept range of Dλ = 60 nm and A-scan frequency of fA= 110 kHz. The axial resolution of the probe is equal to 5.2 μm (determined by applied swept source), whereas the lateral resolution, measured by use of USAF test pattern, is 9.8 μm.
Systems for imaging require to employ high quality optical components in order to dispose of optical aberrations and thus reach sufficient resolution. However, well-known methods to get rid of optical aberrations, such as aspherical profiles or diffractive corrections are not easy to apply to micro-optics. In particular, some of these methods rely on polymers which cannot be associated when such lenses are to be used in integrated devices requiring high temperature process for their further assembly and separation. Among the different approaches, the most common is the lens splitting that consists in dividing the focusing power between two or more optical components. In here, we propose to take advantage of a wafer-level technique, devoted to the generation of glass lenses, which involves thermal reflow in silicon cavities to generate lens doublets. After the convex lens sides are generated, grinding and polishing of both stack sides allow, on the first hand, to form the planar lens backside and, on the other hand, to open the silicon cavity. Nevertheless, silicon frames are then kept and thinned down to form well-controlled and auto-aligned spacers between the lenses. Subsequent accurate vertical assembly of the glass lens arrays is performed by anodic bonding. The latter ensures a high level of alignment both laterally and axially since no additional material is required. Thanks to polishing, the generated lens doublets are then as thin as several hundreds of microns and compatible with micro-opto-electro-systems (MOEMS) technologies since they are only made of glass and silicon. The generated optical module is then robust and provide improved optical performances. Indeed, theoretically, two stacked lenses with similar features and spherical profiles can be almost diffraction limited whereas a single lens characterized by the same numerical aperture than the doublet presents five times higher wavefront error. To demonstrate such assumption, we fabricated glass lens doublets and compared them to single lenses of equivalent focusing power. For similar illumination, the optical aberrations are significantly reduced.
Some of the critical limitations for widespread use in medical applications of optical devices, such as confocal or optical coherence tomography (OCT) systems, are related to their cost and large size. Indeed, although quite efficient systems are available on the market, e.g. in dermatology, they equip only a few hospitals and hence, are far from being used as an early detection tool, for instance in screening of patients for early detection of cancers. In this framework, the VIAMOS project aims at proposing a concept of miniaturized, batch-fabricated and lower-cost, OCT system dedicated to non-invasive skin inspection. In order to image a large skin area, the system is based on a full-field approach. Moreover, since it relies on micro-fabricated devices whose fields of view are limited, 16 small interferometers are arranged in a dense array to perform multi-channel simultaneous imaging. Gaps between each channel are then filled by scanning of the system followed by stitching. This approach allows imaging a large area without the need of large optics. It also avoids the use of very fast and often expensive laser sources, since instead of a single point detector, almost 250 thousands pixels are used simultaneously. The architecture is then based on an array of Mirau interferometers which are interesting for their vertical arrangement compatible with vertical assembly at the wafer-level. Each array is consequently a local part of a stack of seven wafers. This stack includes a glass lens doublet, an out-of-plane actuated micro-mirror for phase shifting, a spacer and a planar beam-splitter. Consequently, different materials, such as silicon and glass, are bonded together and well-aligned thanks to lithographic-based fabrication processes.
This paper proposes a method for the characterization of focusing micro-optical components such as microlens. Based on the measurement of the focal volume generated by the micro-element, the wavefront map reconstruction as well as the optical aberrations can be estimated. To record the slices of the focal volume, this technique requires a simple optical arrangement which consists of a microscope objective and a camera. Then, an iterative phase retrieval algorithm is applied on each recorded intensity slice. This approach is less sensitive to the environmental variations than interferometry and is less expensive than wavefront sampling sensors although it leads to similar results than interferometry. As an example, ball microlens with 596μm diameter and 0.56 numerical aperture, has been characterized and comparison with more conventional technique demonstrates the good performances of the proposed phase retrieval method.
In this work, vertical integration of miniaturized array-type Mirau interferometers at wafer level by using multi-stack anodic bonding is presented. Mirau interferometer is suitable for MEMS metrology and for medical imaging according to its vertical-, lateral- resolutions and working distances. Miniaturized Mirau interferometer can be a promising candidate as a key component of an optical coherence tomography (OCT) system. The miniaturized array-type interferometer consists of a microlens doublet, a Si-based MEMS Z scanner, a spacer for focus-adjustment and a beam splitter. Therefore, bonding technologies which are suitable for heterogeneous substrates are of high interest and necessary for the integration of MEMS/MOEMS devices. Multi-stack anodic bonding, which meets the optical and mechanical requirements of the MOEMS device, is adopted to integrate the array-type interferometers. First, the spacer and the beam splitter are bonded, followed by bonding of the MEMS Z scanner. In the meanwhile, two microlenses, which are composed of Si and glass wafers, are anodically bonded to form a microlens doublet. Then, the microlens doublet is aligned and bonded with the scanner/spacer/beam splitter stack. The bonded array-type interferometer is a 7- wafer stack and the thickness is approximately 5mm. To separate such a thick wafer stack with various substrates, 2-step laser cutting is used to dice the bonded stack into Mirau chips. To simplify fabrication process of each component, electrical connections are created at the last step by mounting a Mirau chip onto a flip chip PCB instead of through wafer vias. Stability of Au/Ti films on the MEMS Z scanner after anodic bonding, laser cutting and flip chip bonding are discussed as well.
In this paper, we present construction, fabrication and characterization of an electrostatic MOEMS vertical microscanner for generation of an optical phase shift in array-type interferometric microsystems. The microscanner employs asymmetric comb-drives for a vertical displacement of a large 4x4 array of reference micromirrors and for in-situ position sensing. The device is designed to be fully compatible with Mirau configuration and with vertical integration strategy. This enables further integration of the device within an "active" multi-channel Mirau micro-interferometer and implementation of the phase shifting interferometry (PSI) technique for imaging applications. The combination of micro-interferometer and PSI is particularly interesting in the swept-source optical coherence tomography, since it allows not only strong size reduction of a system but also improvement of its performance (sensitivity, removal of the image artefacts). The technology of device is based on double-side DRIE of SOI wafer and vapor HF releasing of the suspended platform. In the static mode, the device provides vertical displacement of micromirrors up to 2.8μm (0 - 40V), whereas at resonance (fo=500 Hz), it reaches 0.7 μm for only 1VDC+1VAC. In both operation modes, the measured displacement is much more than required for PSI implementation (352nm peak-to-peak). The presented device is a key component of array-type Mirau micro-interferometer that enables the construction of portable, low-cost interferometric systems, e.g. for in vivo medical diagnostics.
In recent years, optical coherence tomography (OCT) became gained importance in medical disciplines like ophthalmology, due to its noninvasive optical imaging technique with micrometer resolution and short measurement time. It enables e. g. the measurement and visualization of the depth structure of the retina. In other medical disciplines like dermatology, histopathological analysis is still the gold standard for skin cancer diagnosis. The EU-funded project VIAMOS (Vertically Integrated Array-type Mirau-based OCT System) proposes a new type of OCT system combined with micro-technologies to provide a hand-held, low-cost and miniaturized OCT system. The concept is a combination of full-field and full-range swept-source OCT (SS-OCT) detection in a multi-channel sensor based on a micro-optical Mirau-interferometer array, which is fabricated by means of wafer fabrication. This paper presents the study of an experimental proof-of-concept OCT system as a one-channel sensor with bulk optics. This sensor is a Linnik-interferometer type with similar optical parameters as the Mirau-interferometer array. A commercial wavelength tunable light source with a center wavelength at 845nm and 50nm spectral bandwidth is used with a camera for parallel OCT A-Scan detection. In addition, the reference microscope objective lens of the Linnik-interferometer is mounted on a piezo-actuated phase-shifter. Phase-shifting interferometry (PSI) techniques are applied for resolving the conjugate complex artifact and consequently contribute to an increase of image quality and depth range. A suppression ratio of the complex conjugate term of 36 dB is shown and a system sensitivity greater than 96 dB could be measured.
The EU-funded project VIAMOS1 proposes an optical coherence tomography system (OCT) for skin cancer detection, which combines full-field and full-range swept-source OCT in a multi-channel sensor for parallel detection. One of the project objectives is the development of new fabrication technologies for micro-optics, which makes it compatible to Micro-Opto-Electromechanical System technology (MOEMS). The basic system concept is a wafer-based Mirau interferometer array with an actuated reference mirror, which enables phase shifted interferogram detection and therefore reconstruction of the complex phase information, resulting in a higher measurement range with reduced image artifacts. This paper presents an experimental one-channel on-bench OCT system with bulk optics, which serves as a proof-of-concept setup for the final VIAMOS micro-system. It is based on a Linnik interferometer with a wavelength tuning light source and a camera for parallel A-Scan detection. Phase shifting interferometry techniques (PSI) are used for the suppression of the complex conjugate artifact, whose suppression reaches 36 dB. The sensitivity of the system is constant over the full-field with a mean value of 97 dB. OCT images are presented of a thin membrane microlens and a biological tissue (onion) as a preliminary demonstration.
We describe a technological platform developed for miniaturization of optical imaging instruments, such as laser scanning
confocal microscopes or Optical Coherence Tomography devices. The platform employs multi-wafer vertical integration
approach, combined with integrated glass-based micro-optics and heterogeneous bonding and interconnecting technologies.
In this paper we focus on the unconventional fabrication methods of monolithic micro-optical structures and components in
borosilicate glass (e.g. micro beamsplitters, refractive microlenses) for optical beam shaping and routing. In addition, we
present hybrid laser-assisted integration of glass ball microlenses on the silicon MEMS actuators for transmissive beam
scanning as well as methods of electrical signals distribution through thick glass substrates, based on HF etched via holes.
The paper presents the multi-wafer bonding technology as well as the integration of electrical connection to the zscanner
wafer of the micromachined array-type Mirau interferometer. A Mirau interferometer, which is a key-component
of optical coherence tomography (OCT) microsystem, consists of a microlens doublet, a MOEMS Z-scanner, a focusadjustment
spacer and a beam splitter plate.
For the integration of this MOEMS device heterogeneous bonding of Si, glass and SOI wafers is necessary. Previously,
most of the existing methods for multilayer wafer bonding require annealing at high temperature, i.e., 1100°C. To be
compatible with MEMS devices, bonding of different material stacks at temperatures lower than 400°C has also been
investigated. However, if more components are involved, it becomes less effective due to the alignment accuracy or
degradation of surface quality of the not-bonded side after each bonding operation.
The proposed technology focuses on 3D integration of heterogeneous building blocks, where the assembly process is
compatible with the materials of each wafer stack and with position accuracy which fits optical requirement. A
demonstrator with up to 5 wafers bonded lower than 400°C is presented and bond interfaces are evaluated.
To avoid the complexity of through wafer vias, a design which creates electrical connections along vertical direction by
mounting a wafer stack on a flip chip PCB is proposed. The approach, which adopts vertically-stacked wafers along with
electrical connection functionality, provides not only a space-effective integration of MOEMS device but also a design
where the Mirau stack can be further integrated with other components of the OCT microsystem easily.
Scientific articles focusing on fabrication of micro-components often evaluate their optical performances by techniques such as scanning electron microscopy or surface topography only. However, deriving the optical characteristics from the shape of the optical element requires using propagation algorithms. In this paper, we present a simple and intuitive method, based on the measurement of the intensity point spread function generated by the micro-component. The setup is less expensive than common systems and does not require heavy equipments, since it requires only a microscope objective, a CMOS camera and a displacement stage. This direct characterization method consists in scanning axially and recording sequentially the focal volume. Our system, in transmissive configuration, consists in the investigation of the focus generated by the microlens, allowing measuring the axial and lateral resolutions, estimating the Strehl ratio and calculating the numerical aperture of the microlens. The optical system can also be used in reflective configuration in order to characterize micro-reflective components such as molds. The fixed imaging configuration allows rapid estimation of quality and repeatability of fabricated micro-optical elements.
In this paper, we adapt a technique employed for glass microlenses fabrication in order to obtain matrices of millimeter size lenses for inspection applications. The use of microfabrication processes and Micro-Electro-Mechanical Systems (MEMS) compatible materials allow the integration of lenses larger than usual in microsystems. Since the presented lenses can have 2 mm in diameter or more, some aspects apparently irrelevant when diameters are lower than 500 μm must be reviewed and taken into account. Indeed, when the lenses are in the millimeter range, problems such as size nonuniformities within a matrix and asymmetric shapes of each lens are dependent on parameters as mask design, depth of the silicon cavities and enclosed vacuum control after anodic bonding, glass reflow temperature and even the position of the lenses on the substrate. Issues related to the fabrication flow-chart are addressed in this paper and solutions are proposed. First results are shown to prove the pertinence of this technique to fabricate MEMS-compatible millimetersized lenses to be integrated in miniature inspection systems. We also discuss some of the paths to follow that could help improving the performances.
The presented paper shows the concept and optical design of an array-type Mirau-based OCT system for early diagnosis of skin cancer. The basic concept of the sensor is a full-field, full-range optical coherence tomography (OCT) sensor. The micro-optical interferometer array in Mirau configuration is a key element of the system allowing parallel imaging of multiple field of views (FOV). The optical design focuses on the imaging performance of a single channel of the interferometer array and the illumination design of the array. In addition a straylight analysis of this array sensor is given.
KEYWORDS: 3D metrology, Optical components, Objectives, Imaging systems, Point spread functions, Microscopes, Cameras, Microlens, CMOS cameras, High dynamic range imaging
High-resolution miniature imaging systems require high quality micro-optical elements. Therefore, it is essential to characterize their optical performances in order to optimize their fabrication. Usually, basic evaluation of micro-optical elements quality is based on the measurement of their topography since their optical properties are largely defined by their shape. However, optical characteristics have to be derived from the measured geometry. An alternative method is the direct measurement of their optical properties. Unlike topography measurement, it allows characterization of high numerical aperture components. Moreover, it can be applied to single elements but also to optical systems composed of several micro-optical components. In this work, we propose a simple method based on the measurement of the 3D intensity point spread function (IPSF). IPSF is defined by the 3D shape of the focal spot generated by the micro-element. The direct characterization of focusing response through the measurement of IPSF allows very precise estimation of micro-structures quality. The considered method consists in imaging different slices of the focal volume generated by the focusing component. It allows, depending on the configuration, characterizing both transmissive and reflective micro-optical components.
We present the preliminary design, construction and technology of a microoptical, millimeter-size 3-D microlens scanner,
which is a key-component for a number of optical on-chip microscopes with emphasis on the architecture of confocal
microscope. The construction of the device relies on the vertical integration of micromachined building blocks: top glass lid,
silicon electrostatic comb-drive X-Y and Z microactuators with integrated scanning microlenses, ceramic LTCC spacer, and
bottom lid with focusing microlens. All components are connected on the wafer level only by sequential anodic bonding. The
technology of through wafer vias is applied to create electrical connections through a stack of wafers. More generally, the
presented bonding/connection technologies are also of a great importance for the development of various silicon-based
devices based on vertical integration scheme. This approach offers a space-effective integration of complex MOEMS devices
and an effective integration of various heterogeneous technologies.
We present the development of an array-type micromachined Mirau interferometers, operating in the regime of low
coherence interferometry (LCI) and adapted for massively parallel inspection of MEMS. The system is a combination of
free-space microoptical technologies and silicon micromachining, based on the vertical assembly of two glass wafers.
The probing wafer is carrying an array of refractive microlenses, diffractive gratings to correct chromatic and spherical
aberrations and reference micro-mirrors. The semitransparent beam splitter plate is based on the deposition of a dielectric
multilayer, sandwiched between two glass wafers. The interferometer matrix is the key element of a novel inspection
system aimed to perform parallel inspection of MEMS. The fabricated demonstrator, including 5x5 channels, allows
consequently decreasing the measurement time by a factor of 25. In the following, the details of fabrication processes of
the micro-optical components and their assembly are described. The feasibility of the LCI is demonstrated for the
measurement of a wafer of MEMS sensors.
This paper reports a batch-fabrication technique based on micromachining of silicon molds to create, after replication,
arrays of microlenses characterized by high fill factors. The technique for single microlens generation (compatible with
various types of replication or integration so that microlenses made of plastics or glass can be generated) was reported
previously and showed its potential in terms of range of shapes and cost. However, subtleties of chemical etching makes
more difficult the generation of high fill factor matrices when microlenses size overcomes several tenth of microns.
Thus, in this paper, we describe the analysis of the chemical etching process and the corresponding adaptation of the
mask design to achieve 100% fill factors arrays of microlenses. The process to fabricate arrays of microlens, with
hexagonal footprints and element sizing from 30 to 270 microns and having NA from 0.2 to 0.4, is described. The
hexagonal footprint shape of the elements in the arrays leads to 100% geometrical fill factor of fabricated structures.
Isotropic etching used for the molds fabrication preserves the spherical profile of the resulting microlenses.
A variant of the Fox & Li method performing intra-cavity laser beam shaping for resonators containing an arbitrary number of amplitude and phase diffractive optics is presented. As an illustration, the problem of forcing a laser to oscillate on a single high-order transverse mode has been considered. In particular, from numerical simulation, we deduce a simple model for generating such modes with a pi-phase plate inserted into a plano-concave cavity. This model
has been tested experimentally within an active cavity with a diode-pumped Nd:YVO4 laser and an excellent agreement with numerical predictions has been found: a phase aperture located quite close to the concave mirror, and whose normalized radius κ is so that 2κ2 corresponds to a zero of the desired p-order Laguerre polynomial, was sufficient to generate single cylindrical TEMp0 modes (p = 1, 2, 3) as long as its radius is correctly chosen. The laser based on the optimized features was perfectly stable, whatever the order of the generated mode.
We consider the promising properties of very simple Diffractive Optical Elements (DOE) for reshaping the intensity profile of a
laser beam. The first type of DOE that we have considered is a phase aperture which consists in a transparent plate with a circular
relief introducing a π phase shift in the central region of the incident beam. The phase aperture is able to convert a Gaussian beam
into a super-Gaussian, a ring-shaped or a doughnut profile. The second DOE that has been considered is a phase step which is
able to transform a TEM00 beam into a TEM10 or TEM01 beam depending on its orientation. These beams have been used to
generate radial polarization.
Whereas microelectronic lithography is heading to the 32 nm node and discussing immersion and double-patterning
strategies, there is much which can be done with the 45 nm node in microoptics for white light processing. For instance,
one of the most demanding applications in terms of achievable period is the LCD lossless polarizer, which can transmit
the TM polarization and reflect the TE polarization evenly all through the visible spectrum - provided that a 1D metal
grid of 100 nm period can be fabricated. The manufacture of such polarizing panels cannot resort to the step & repeat
cameras of microelectronics since the substrates are too large, too thin, too wavy and full of contaminants. There is
therefore a need for specific fabrication techniques. It is one of these techniques that a subgroup of partners belonging to
two of the Networks of Excellence of the European Community, NEMO and ePIXnet, have decided to explore together.
We demonstrate that insertion of a simple diffractive optical element in the cavity of a self-Q-switched Cr3+ : LiSAF laser can speed up its dynamics so that the laser output pulse is shortened and its energy is increased.
An experiment has been performed to analyse a nonlinear lensing effect in a flashlamp pumped Cr3+:LiSAF laser which has been found to be adiverging lensing effect. The latter is due to the refrctive index change which is assumed to be proportional to the excited ion population. The corresponding constant of proportionality has been measured from the time variation of the laser pulses far-field divergence.
We present the promising properties of a very simple binary Diffractive Optical element which is a phase slit. The latter is made up of two plates of glass in which a phase step has been produced with ion-etching equipment. The two phase steps are set parallel and are able to slide each on the other so that the width of the phase shifting zone is continuously adjustable. The adjustable phase slit has been used for 1-D laser beam tailoring in which the key parameters that govern the shape of the far-field intensity distribution is the phase step height and the ratio of incident beam and slit widths. The phase slit is able to convert a Gaussian beam into a super-Gaussian or a hollow profile having a central depression whose depth can be continuously adjusted.
Beam propagation factor (usually referred to as M2 factor) and brightness of high-order symmetrical Laguerre Gauss (LG) beams that diffract upon a hard-aperture are investigated. We demonstrate that a circular aperture is able to decrease the M2 factor of a high-order symmetrical LG beam and also to enhance its brightness although induced losses.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.