Real-time Lissajous imaging with a low-voltage 2-axis MEMS scanner based on electrothermal actuation.

Laser scanning based on Micro-Electro-Mechanical Systems (MEMS) scanners has become very attractive for biomedical endoscopic imaging, such as confocal microscopy or Optical Coherence Tomography (OCT). These scanners are required to be fast to achieve real-time image reconstruction while working at low actuation voltage to comply with medical standards. In this context, we report a 2-axis Micro-Electro-Mechanical Systems (MEMS) electrothermal micro-scannercapable of imaging large fields of view at high frame rates, e.g. from 10 to 80 frames per second. For this purpose, Lissajous scan parameters are chosen to provide the optimal image quality within the scanner capabilities and the sampling rate limit, resulting from the limited A-scan rate of typical swept-sources used for OCT. Images of 233 px × 203 px and 53 px × 53 px at 10 fps and 61 fps, respectively, are experimentally obtained and demonstrate the potential of this micro-scannerfor high definition and high frame rate endoscopic Lissajous imaging.

Mitigation of Temperature-Induced Light-Shift Effects in Miniaturized Atomic Clocks.

The demonstration of miniature atomic clocks (MACs) based on coherent population trapping (CPT) with improved mid- and long-term frequency stability benefits from the implementation of additional stabilization loops to reduce temperature-induced light-shift effects. In this article, we report and highlight the individual and combined benefits of such servo loops on the frequency stability of a CPT-based MAC. The first loop stabilizes the actual temperature of the vertical-cavity surface-emitting laser (VCSEL) chip using a compensation method in which the reading of external temperature variations is derived from the atomic vapor output signal. The second loop maintains the total microwave power absorbed by the laser to a value that maximizes the optical absorption and significantly reduces the laser power dependence of the clock frequency. Experimental tests are performed onto a miniaturized CPT-clock physics package using a chip-VCSEL tuned on the Cs D1 line ( λ = 895 nm). The VCSEL temperature compensation technique improves, by a factor of 4, the Allan deviation of the clock at 104 s. The simultaneous operation of both servo loops improves, by a factor of 7, the clock fractional frequency stability at 104 s. The clock demonstrates a fractional frequency stability of 7.5×10 -11 at 1 s and better than 2×10-11 at 1 day.

Microfabrication of axicons by glass blowing at a wafer-level.

This Letter reports on the generation of glass-based axicons realized at the wafer level by means of microfabrication. The technique is based on micro glass blowing allowing parallel fabrication of numerous components at a time. Blowing is achieved due to cavities containing a gas that expands when the wafer stack is introduced in a furnace. Such cavities, generated in a silicon wafer and sealed by a bonded glass wafer, act as pistons pushing locally the other side of the glass wafer where the micro-optical component profile emerges. After cavities' removal by polishing, it is shown that such a component produces nondiffracting Bessel beams.

Technological Platform for Vertical Multi-Wafer Integration of Microscanners and Micro-Optical Components.

We describe an original integration technological platform for the miniaturization of micromachined on-chip optical microscopes, such as the laser scanning confocal microscope. The platform employs the multi-wafer vertical integration approach, combined with integrated glass-based micro-optics as well as micro-electro-mechanical systems (MEMS) components, where the assembly uses the heterogeneous bonding and interconnecting technologies. Various heterogeneous components are disposed in vertically stacked building blocks (glass microlens, MEMS actuator, beamsplitter, etc.) in a minimum space. The platform offers the integrity and potential of MEMS microactuators integrated with micro-optics, providing miniaturized and low cost solutions to create micromachined on-chip optical microscopes.

An Optical Diffuse Reflectance Model for the Characterization of a Si Wafer with an Evaporated SiO2 Layer.

Thin films are a type of coating that have a very wide spectrum of applications. They may be used as single layers or composed in multilayer stacks, which significantly extend their applications. One of the most commonly used material for thin films is silicon dioxide, SiO2. Although there are other tools that can be used to measure the thickness of SiO2 films, these tools are very complex and sophisticated. In this article, we propose the use of an exponential two-layer light-material interaction model, throughout its diffuse reflectance spectra, as an alternative for the measurement of the thickness of evaporated SiO2 on Si wafers. The proposed model is evaluated experimentally by means of a 980-nm-thick SiO2 layer evaporated on a Si wafer. The results show that the proposed model has a strong correlation with the thickness measurements obtained using commercial equipment.

Analytical Expressions of the Dark Resonance Parameters in a Vacuum Vapor Cell.

This paper reports a dedicated theoretical and experimental study on the properties (signal amplitude and linewidth) of coherent population trapping resonances detected in vacuum vapor cells. Results are presented for conventional single-lambda schemes of atomic energy levels but also for double-lambda schemes, now widely used in various applications including atomic clocks and magnetometers. Approximate compact analytical expressions, valid for a wide range of light-wave intensities, i.e., beyond the low intensities or pump-probe regime, have been obtained. Analytical results are found to be in excellent agreement with exact numerical solutions based on the optical Bloch equations. Experimental results, obtained in a Cs vapor microfabricated cell, are reported and found to be in correct agreement with theoretical expressions.

Characterization of commercially available vertical-cavity surface-emitting lasers tuned on Cs D1 line at 894.6 nm for miniature atomic clocks.

We report on the metrological characterization of novel commercially available 894.6 nm vertical-cavity surface-emitting lasers (VCSELs), dedicated to Cs D1 line spectroscopy experiments. The thermal behavior of the VCSELs is reported, highlighting the existence of a minimum threshold current and maximum output power in the 55°C-60°C range. The laser relative intensity noise, measured to be -108 dB/Hz at 10 Hz Fourier frequency f for a laser power of 25 µW, is reduced with increased power. The VCSELs frequency noise is 108 Hz2/Hz at f=100 Hz. The spectral linewidth of the VCSELs is about 30 MHz. VCSELs injection current can be directly modulated at 4.596 GHz with microwave power in the range of -10 to +0 dBm to generate optical sidebands. A VCSEL was used in a microcell-based Cs atomic clock based on coherent population trapping. A preliminary clock short-term fractional frequency stability of 8×10-11τ-1/2 up to about 100 s is reported, demonstrating the suitability of these VCSELs for miniature atomic clock applications.

Miniature Schwarzschild objective as a micro-optical component free of main aberrations: concept, design, and first realization with silicon-glass micromachining.

This paper presents the conception of a new micro-optical component fabricated within the wafer-level approach: a micromachined reflective objective, the so-called micro-Schwarzschild objective, characterized by superior optical performances than widespread microlenses. The system, made of two vertically integrated mirrors, works in transmission similarly as microlenses. While the specific geometric configuration of the two-mirrors allows elimination of most common optical aberrations, the reflective architecture provides inherent achromaticity. This paper presents in detail the optical design and analyzes fabrication tolerances. It also describes a fabrication flow chart based on silicon micromachining done at the wafer level that could allow production of thousands of such micro-optical devices within a single fabrication run. The realized prototype employs the two-step KOH etching process to generate the micromirror pairs followed by glass reflow for the secondary mirror generation and selective metallic deposition. Despite an insufficient mirror quality attributed to this specific silicon etching technique and highlighted by the reflective configuration, the objective fabrication in terms of alignment, bonding, and coating is shown as feasible.

Wafer-level fabrication of multi-element glass lenses: lens doublet with improved optical performances.

This Letter reports on the fabrication of glass lens doublets arranged in arrays and realized at wafer level by means of micro-fabrication. The technique is based on the accurate vertical assembly of separately fabricated glass lens arrays. Since each one of these arrays is obtained by glass melting in silicon cavities, silicon is employed as a spacer in order to build a well-aligned and robust optical module. It is shown that optical performance achieved by the lens doublet is better than for a single lens of equivalent numerical aperture, thanks to lower optical aberrations. The technique has good potential to match the optical requirements of miniature imaging systems.

Simple method based on intensity measurements for characterization of aberrations from micro-optical components.

We report a simple method, based on intensity measurements, for the characterization of the wavefront and aberrations produced by micro-optical focusing elements. This method employs the setup presented earlier in [Opt. Express 22, 13202 (2014)] for measurements of the 3D point spread function, on which a basic phase-retrieval algorithm is applied. This combination allows for retrieval of the wavefront generated by the micro-optical element and, in addition, quantification of the optical aberrations through the wavefront decomposition with Zernike polynomials. The optical setup requires only an in-motion imaging system. The technique, adapted for the optimization of micro-optical component fabrication, is demonstrated by characterizing a planoconvex microlens.

Micro-optical design of a three-dimensional microlens scanner for vertically integrated micro-opto-electro-mechanical systems.

This paper presents the optical design of a miniature 3D scanning system, which is fully compatible with the vertical integration technology of micro-opto-electro-mechanical systems (MOEMS). The constraints related to this integration strategy are considered, resulting in a simple three-element micro-optical setup based on an afocal scanning microlens doublet and a focusing microlens, which is tolerant to axial position inaccuracy. The 3D scanning is achieved by axial and lateral displacement of microlenses of the scanning doublet, realized by micro-electro-mechanical systems microactuators (the transmission scanning approach). Optical scanning performance of the system is determined analytically by use of the extended ray transfer matrix method, leading to two different optical configurations, relying either on a ball lens or plano-convex microlenses. The presented system is aimed to be a core component of miniature MOEMS-based optical devices, which require a 3D optical scanning function, e.g., miniature imaging systems (confocal or optical coherence microscopes) or optical tweezers.

Impact of mirror spider legs on imaging quality in Mirau micro-interferometry.

We report the impact on imaging quality of mirror suspensions, referred to as spider legs, used to support the reference mirror in a Mirau micro-interferometer that requires the vertical alignment of lens, mirror, and beamsplitter. Because the light goes from the microlens to the beamsplitter through the mirror plane, the spider legs are a source of diffraction. This impact is studied as a function of different parameters of the spider legs design. Imaging criteria, such as the resolution as well as the symmetry of the imaging system, are determined using the point spread function and the modulation transfer function of the pupil. These imaging criteria are used to determine the optimum radius of curvature, thickness, and number of legs of the spider structure. We show that 3 curved legs give performances, with specific radius of curvature and thickness, similar to a suspension-free mirror.

Laser light routing in an elongated micromachined vapor cell with diffraction gratings for atomic clock applications.

This paper reports on an original architecture of microfabricated alkali vapor cell designed for miniature atomic clocks. The cell combines diffraction gratings with anisotropically etched single-crystalline silicon sidewalls to route a normally-incident beam in a cavity oriented along the substrate plane. Gratings have been specifically designed to diffract circularly polarized light in the first order, the latter having an angle of diffraction matching the (111) sidewalls orientation. Then, the length of the cavity where light interacts with alkali atoms can be extended. We demonstrate that a longer cell allows to reduce the beam diameter, while preserving the clock performances. As the cavity depth and the beam diameter are reduced, collimation can be performed in a tighter space. This solution relaxes the constraints on the device packaging and is suitable for wafer-level assembly. Several cells have been fabricated and characterized in a clock setup using coherent population trapping spectroscopy. The measured signals exhibit null power linewidths down to 2.23 kHz and high transmission contrasts up to 17%. A high contrast-to-linewidth ratio is found at a linewidth of 4.17 kHz and a contrast of 5.2% in a 7-mm-long cell despite a beam diameter reduced to 600 µm.

Dense arrays of millimeter-sized glass lenses fabricated at wafer-level.

This paper presents the study of a fabrication technique of lenses arrays based on the reflow of glass inside cylindrical silicon cavities. Lenses whose sizes are out of the microfabrication standards are considered. In particular, the case of high fill factor arrays is discussed in detail since the proximity between lenses generates undesired effects. These effects, not experienced when lenses are sufficiently separated so that they can be considered as single items, are corrected by properly designing the silicon cavities. Complete topographic as well as optical characterizations are reported. The compatibility of materials with Micro-Opto-Electromechanical Systems (MOEMS) integration processes makes this technology attractive for the miniaturization of inspection systems, especially those devoted to imaging.

A simple method for quality evaluation of micro-optical components based on 3D IPSF measurement.

This paper presents a simple method based on the measurement of the 3D intensity point spread function for the quality evaluation of high numerical aperture micro-optical components. The different slices of the focal volume are imaged thanks to a microscope objective and a standard camera. Depending on the optical architecture, it allows characterizing both transmissive and reflective components, for which either the imaging part or the component itself are moved along the optical axis, respectively. This method can be used to measure focal length, Strehl ratio, resolution and overall wavefront RMS and to estimate optical aberrations. The measurement setup and its implementation are detailed and its advantages are demonstrated with micro-ball lenses and micro-mirrors. This intuitive method is adapted for optimization of micro-optical components fabrication processes, especially because heavy equipments and/or data analysis are not required.

A high-performance frequency stability compact CPT clock based on a Cs-Ne microcell.

This paper reports on a compact table-top Cs clock based on coherent population trapping (CPT) with advanced frequency stability performance. The heart of the clock is a single buffer gas Cs-Ne microfabricated cell. Using a distributed feedback (DFB) laser resonant with the Cs D1 line, the contrast of the CPT signal is found to be maximized around 80°C, a value for which the temperature dependence of the Cs clock frequency is canceled. Advanced techniques are implemented to actively stabilize the clock operation on a zero-light-shift point. The clock frequency stability is measured to be 3.8 × 10(-11) at 1 s and well below 10(-11) until 50,000 s. These results demonstrate the possibility to develop high-performance chip-scale atomic clocks using vapor cells containing a single buffer gas.

Fabrication of spherical microlenses by a combination of isotropic wet etching of silicon and molding techniques.

We report a novel process technology of hemispherical shaped microlenses, using isotropic wet etching of silicon in an acid solution to produce the microlenses molds. Governed by process parameters such as temperature and etchant concentration, the isotropic wet etching is controlled to minimize various defects that appear during the molding creation. From the molds, microlenses are fabricated in polymer by conventional replication techniques such as hot embossing and UV-molding. The characterization of molds and measurements of replicated microlenses demonstrate high smoothness of the surfaces, excellent repeatability of mold fabrication and good optical properties. Using the proposed method, a wide range of lens geometries and lens arrays can be achieved.

Feedback-induced voltage change of a Vertical-Cavity Surface-Emitting Laser as an active detection system for miniature optical scanning probe microscopes.

We propose a novel detection technique for scanning probe microscopy based on the measuring of the feedback-induced voltage change of 780-nm VCSEL operating at constant current in far-field regime when we modulate mechanically the length of a coupled-cavity generating the feedback conditions. The voltage change of the VCSEL is produced by light back reflected from the sample to the laser cavity. Two-dimensional image probing is successfully demonstrated with high temporal resolution, offering a viable solution for miniature parallel scanning probe optical microscopes, such as confocal microscope, where the use of a photodetector is avoided. This approach opens the possibility to perform imaging tasks in a low cost and hand-held miniature device with much improved effective-space.