Calibration is an important step in the construction of traditional spectrometers to ensure the accuracy of the obtained spectrum. Recent advancements in computational spectroscopy have also spurred the need for calibration with the aid of machine learning to enable the recovery of spectrums, but they generally require large datasets. In this paper, we present an arbitrary spectrum generation engine (ASGE) using a digital micromirror device (DMD) that can be configured to work in a broad wavelength range from visible to the near-infrared. The DMD allows for the independent modulation of spectral elements to output arbitrary spectrums and provide the large datasets required for training and calibration of computational spectrometers. The ASGE can also double as a normal spectrometer if a sampling accessory and a detector are included.
KEYWORDS: Spectroscopy, Spectral resolution, Digital micromirror devices, Near infrared, Infrared spectroscopy, Micromirrors, Matrices, Multiplexing, Signal to noise ratio, Printing
In this paper, we present a doubly-encoded single-pixel Hadamard transform spectrometer that has its spectral resolution decoupled from its throughput. The proof-of-concept is designed in the 1500 nm to 1600 nm near-infrared (NIR) wavelength range and uses a digital micromirror device (DMD) in conjunction with a fixed mask for encoding. The proposed system can easily be extended to other infrared (IR) wavelengths to achieve maximum throughput and multiplexing advantage.
In this paper, we present a single-pixel hyperspectral imager based on Hadamard transformat ion. The imager uses a micromirror array and a resonant scanning mirror to implement spatial and spectral encoding. For a proof of concept, the sensing wavelength of the imager is 450nm to 750nm, but it can easily be applied to the infrared wavelengths. It has high robustness and high frame rate compared with conventional single-pixel hyperspectral imagers. We also introduce a cascading method that can enhance the spatial resolution of the single-pixel hyperspectral imager. Some experimental results are presented in the paper to demonstrate the performance of our proposed system.
Novel concepts of on-chip Fourier transform spectrometers is proposed. The principal element in the spectrometer is semiconductor waveguide directional couplers. The optical path difference can be tuned by varying the coupling length or influencing the propagation mode of the directional coupler. Solutions of both these two methods are proposed, and the theories are verified by spectra recovering. They function well around 1.5 μm wavelength. Further enhancement can be achieved by cascading more stages of directional couplers or extending the coupling length. This design meets the requirement of small size, weight and power and may be useful in future on-chip spectroscopic sensors.
We report a miniaturized spectrometer based on Hadamard transformation. The compact spectrometer utilizes a single-pixel photodetector and thus can operate at any wavelengths at relatively low cost. The spectrometer utilizes a MEMS-driven moving mask to encode the light intensity at its imaging slit, and the encoded light is then collected by a single-pixel detector. The light spectrum is recovered through an inverse Hadamard transform after a complete set of encoding is measured and recorded. The spectrometer is experimentally demonstrated with a spectral resolution better than 3 nm covering the full visible spectrum from 400 nm to 700 nm, and is capable of measuring over 200 spectra per second.
Due to their advances in achieving large focal-length tuning ranges with compact structures, solid tunable lenses based on the Alvarez-Lohmann principle show a promising potential in various applications, especially modern miniature imaging systems. In this paper, we report miniature endoscopic systems integrated with solid tunable lenses for optical adjustable focusing or zooming. The solid tunable lenses are designed according to the improved Alvarez-Lohmann principle, where two independent extended polynomials are employed to govern the two freeform surfaces, respectively. Slim piezo benders aligned along the optical axis are utilized to drive the solid tunable lenses to move laterally. An image fiber bundle is used to transmit the images captured by the optical system to the external cameras. Results show that the endoscopic system is equipped with a capability of optical power tuning from about 135 diopters to about 205 diopters when there is a single solid tunable lens integrated in the system, which enables it to achieve adjustable focus for objects located at different positions. The integration of two solid tunable lenses and two fixed lenses further enables the endoscopic system to have the optical-zooming capability. A zoom ratio of 3x and a maximum full field of view as high as about 80 degrees are realized experimentally. The cross sectional diameter of the endoscopic probe is controlled below 4 mm. The captured images are clear and sharp. Such adjustable-focus or zoom endoscopic systems would be useful in future medical or industrial applications.
The emerging dual-focus lenses are drawing increasing attention recently due to their wide applications in both academia and industries, including laser cutting systems, microscopy systems, and interferometer-based surface profilers. In this paper, a miniature electrically tunable rotary dual-focus lens is developed. Such a lens consists of two optical elements, each having an optical flat surface and one freeform surface. The two freeform surfaces are initialized with the governing equation Ar2θ (A is the constant to be determined, r and θ denote the radii and angles in the polar coordinate system) and then optimized by ray tracing technique with additional Zernike polynomial terms for aberration correction. The freeform surfaces are achieved by a single-point diamond turning technique and then a PDMS-based replication process is utilized to materialize the final lens elements. To drive the two coaxial elements to rotate independently, two MEMS thermal rotary actuators are developed and fabricated by a standard MUMPs process. The experimental results show that the MEMS thermal actuator provides a maximum rotation angle of about 8.2 degrees with an input DC voltage of 6.5 V, leading to a wide tuning range for both the two focal lengths of the lens. Specifically, one focal length can be tuned from about 30 mm to 20 mm while the other one can be adjusted from about 30 mm to 60 mm.
Photonic crystal split-beam nanocavities allow for ultra-sensitive optomechanical transductions but are degraded due to their relatively low optical quality factors. We report our recent work in designing a new type of one-dimensional photonic crystal split-beam nanocavity optimized for an ultra-high optical quality factor. The design is based on the combination of the deterministic method and hill-climbing algorithm. The latter is the simplest and most straightforward method of the local search algorithm, which provides the local maximum of the chosen quality factors. This split-beam nanocavity is made up of two mechanical uncoupled cantilever beams with Bragg mirrors patterned onto it and separated by a 75 nm air gap. Experimental results emphasize that the quality factor of the second order TE mode can be as high as 19,900. Additionally, one beam of the device is actuated in the lateral direction with the aid of a NEMS actuator and the quality factor maintains quite well even there’s a lateral offset up to 64 nm. We also apply Fano resonance to further increase the Q-factor by constructing two interfering channels. Before tuning, the original Q-factor is 60,000; it’s noteworthy that the topmost Q-factor reaches 67,000 throughout out-of-plane electrostatic force tuning. The dynamic mechanical modes of two devices is analyzed as well. Potentially promising applications, such as ultra-sensitive optomechanical torque sensor, local tuning of fano resonance, all-optical-reconfigurable filters etc, are foreseen.
Nowadays, nano-electro-mechanical systems (NEMS) actuators using electrostatic forces are facing the bottleneck of the
electromagnetic interference which greatly degrades their performances. On the contrary, the hybrid circuits driven by
optical gradient forces which are immune to the electromagnetic interference show prominent advantages in
communication, quantum computation, and other application systems. In this paper we propose an optical actuator
utilizing the optical gradient force generated by a hetero-structure photonic crystal cavity. This type of cavity has a
longitudinal air-slot and characteristics of ultrahigh quality factor (Q) and ultra-small mode volume (V) which is capable
of producing a much larger force compared with the waveguide-based structures. Due to the symmetry property,
attractive optical gradient force is generated. Additionally, the optomechanical coefficient (gom) of this cavity is two
orders of magnitude larger than that of the coupled nanobeam photonic crystal cavities. The 2D hetero-structure cavity,
comb drives, folded beam suspensions and the displacement sensor compose the whole device. The cavity serves as the
optical actuator whilst the butt-coupled waveguide acts as the displacement sensor which is theoretically proved to be
insensitive to the temperature variations. As known, the thermo-optic effect prevails especially in the cavity-based
structures. The butt-coupled waveguide can be used to decouple the thermal effect and the optoemchanical effect (OM)
with the aid of comb drives. The results demonstrate that the proposed optical gradient force actuator show great
potential in the future of all-optical reconfigurable circuits.
This paper describes the design and electro-mechanical characterizations of a three-axial micro piezoresistive force sensor fabricated by microelectromechanical systems (MEMS) technologies. This is the first three-axial MEMS micro force sensor applied to the study of Micro Aerial Vehicle (MAV) aerodynamics. A standard dry etching fabrication process using Silicon On Insulator (SOI) wafer is employed to fabricate the multi-axis sensors. Conventional cross-beam structure is employed. There are eight piezoresistors on the beams, and each of the silicon strain gauge size is 15 μm in width, and between 400 and 500 μm in length. The Finite Element Method (FEM) analysis for confirming the piezoresistors attachment locations is performed. The miniaturized force sensor (11×11 mm2) is attached at the wing base of a micro flapping wing system (MAV, 70×30 mm2 ) by a short pillar. The sensor is designed to detect the dynamic drag force and lift force generated by a single wing under a moderate flapping frequency (5~10Hz) simultaneously. The characterizations are experimentally investigated. The sensor should be stiff enough to withstand the high inertial force (200 millinewton) and also has high resolution to detect the minimal force correctly. Measurements show that the resolution is on the order of a millinewton. High linearity and low hysteresis under normal forces and tangential forces are demonstrated by applying forces from 0 to 0.1 N. The micro flapping wing mechanism and the assembly of wing and sensor are also discussed in this paper.
In this paper, mechanical resonant frequencies of a nanoelectromechanical systems (NEMS) device are tuned by light, in
which optical spring effect plays a role. A NEMS spring mechanism with multi-degree of freedom (DOF) is adopted
here. Double-coupled nanobeam photonic crystal cavities (PCCs) are utilized to pump the optical spring effect. One of
the PCCs is fixed, while the other is driven by the NEMS mechanism. We investigate the shift of the mechanism’s firstorder
in-plane translational (I1) resonance mode tuned by the incident laser wavelengths across the fourth-order even
(TEe,4) and odd (TEo,4) modes of the coupled cavities. It shows a nonlinear relationship between the I1 mechanical
frequency and the laser wavelength. We also investigate the frequency of the third-order torsional (T3) mode versus the
wavelengths across the same cavity TEe,4 mode and it shows a weaker torsional optical spring effect.
KEYWORDS: Nanoelectromechanical systems, Transparency, Photonic crystals, Analog electronics, Electron beam lithography, Electrodes, Scanning electron microscopy, Waveguides, Radio propagation, Optical fibers
In this paper, the analog to electromagnetically induced transparency (EIT) in the double-coupled one-dimensional photonic crystal cavities are proposed and experimentally observed. This EIT-like effect is due to the interference of two resonance modes and the leaky propagation mode. A nanoelectromechanical systems (NEMS) comb drive is used to align the two resonant wavelengths up, which is first used in the studies of the EIT-like effect. The fabrication of the device bases on the standard semiconductor process. Finally, the evolution of the EIT-like transmission spectrum with the applied voltages is shown in the last part of this paper.
In this paper, we demonstrate a configuration of optical force actuator based on coupled one-dimensional photonic crystal cavities (1D PCCs). A NEMS structure, which consists of 3 cascaded folded-beam-springs and an electrostatic comb drive, is integrated into the device to finely tune the gap between cavities so that the relation between the cavities’ resonance shift and their gap changes can be precisely and straightforwardly characterized. Resonance modes of the cavities are utilized to drive the spring structures, which can generate much larger optical forces than waveguide modes due to their high quality factors. The even resonance mode produces an attractive force, while the odd mode produces a repulsive force. In addition, there is the thermo-optic effect accompanying with the optical forces. Here, a decoupling method is also introduced by calibrating the relations of resonance shift versus gap change with the help of the NEMS and resonance shift versus temperature variation in advance. The experimental results show that one cavity is pulled to (pushed away from) the other cavity by 37.1 nm (11.4 nm) for the optomechanical actuator proposed here. This kind of optical actuator has the potential applications of all-optical circuits in future communication and sensing systems.
In this paper, we demonstrate a novel split-ladder photonic crystal cavity, which can be tuned across a very large wavelength range by an on-chip integrated nano-electromechanical actuator. The nano-electromechanical actuator is used to control the cavity’s central gap. When the cavity gap is widened, the resonance wavelength is blue-shifted correspondingly. The dimensions of the proposed split-ladder cavity are designed by 3D Finite-difference time-domain (FDTD) calculations. We fabricated and tested the devices. Experimental results show that the optomechanical coupling coefficient of the second order resonance mode is about 80 GHz/nm. In addition, there is a relatively linear relationship between the resonance shift and the gap change. The maximum measured resonance wavelength shift is about 17 nm, corresponding to a cavity central gap increase of 26 nm.
An electrostatically-driven microelectromechanical systems (MEMS) based pyramidal-polygonal micro-scanner is developed for circumferential-scanning endoscopic probes. In this paper, an endoscopic optical coherence tomography (OCT) probe is proposed, which utilizes multiple parallel incident light beams to drastically reduce the required mechanical rotation angle to achieve near 360-degree circumferential scanning. Manual assembly is employed to construct this micro scanner, which combines a silicon micromachined MEMS actuator with four sets of electrostatic comb-drives and a polymer-based pyramidal-polygonal micro-reflector. The pyramidal-polygonal micro-reflector is developed using a combination of high-precision diamond turning and soft lithography molding technologies. The measured average surface roughness and root-mean-square roughness of the diamond turned mold are 7.02 nm and 9.02 nm respectively, and those of the PDMS casted polygon micro-reflector with an Au coating are 48.95 nm and 61.90 nm respectively. Since near-infrared light sources with wavelengths ranging from 930 nm to 1550 nm are typically utilized for OCT applications, the surface roughness of the polygon reflector is better than λ/10 of the operating wavelength indicating that its quality meets the requirement of the OCT bio-imaging applications. An overall optical scan angle of near-360 degrees is demonstrated experimentally at a resonant frequency of 180 Hz with 80 Vpp ac driving voltage. The proposed MEMS scanner offers an alternative solution for circumferential scanning in endoscopic OCT probes owing to its advantages of compact design, fast scanning speed and low operational power consumption.
We have devised an air-suspended nano-optomechanical structure that is capable of precisely detecting in-plane motion up to a precision of subnanometer level. To achieve the detection, we utilized a basic silicon photonic building block (i.e., a nanowire waveguide directional coupler) in conjunction with nanoelectromechanical systems (NEMS). We numerically optimized the design and experimentally demonstrated a displacement sensitivity of 8.83×10−5 V/nm−1 with a low noise-level resolution of 0.172 nm/√Hz in a 1 Hz bandwidth centered at 950 Hz. As the waveguide coupler design does not segmentize nanowire waveguides, we eliminated the undesirable insertion losses and coupling losses irrelevant to the measurand. Furthermore the design is simple, ultracompact, and can be easily integrated with on-chip photonic systems, which may be beneficial for applications that require a compact displacement sensor with high accuracy and precision.
Miniaturized low-power, high-speed scanners are tremendously useful in a variety of applications. Besides MEMS
micromirrors, the in-plane vibratory grating scanner is a development in this area which possesses several unique
features. The in-plane scanning mechanism minimizes the dynamic non-rigid-body out-of-plane deformation of the
mirror surface, allowing for higher-resolution displays. The dispersive element permits splitting the incoming beam into
its constituents for analysis. Coupling a grating platform to an in-plane moving structure is also useful for real-time
motion measurement which would otherwise be difficult to pick-up. The past developments till the current design are
explored in this paper. Possible alternative applications besides image display, for example spectral imaging and realtime
motion sensing are also described.
In this paper, a novel liquid lens design is presented, in which a diffractive surface with an aspherical phase contour,
combined with the spherical-like refractive surface, is adopted to improve the inherent chromatic and spherical
aberration performance when compared to conventional pure refractive-type configurations. Single-point diamond
turning together with soft lithography is used to realize this structure. Both simulation and test measurement results agree
well with each other and demonstrate significantly improved chromatic and spherical aberration within the tunable range
of the lens.
KEYWORDS: Near field, Waveguides, Silicon, Near field scanning optical microscopy, Photonic crystals, Optical design, Resonators, Control systems, Nanophotonics, Atomic force microscopy
Research interest for silicon nanophotonics is a topic of heavy interest currently due to the requirements for high density
communications of integrated devices with small footprints in the semiconductor industry. Silicon photonic crystals
(PhC) are nanoscale subwavelength periodic structures that possess the capability to induce strong interaction between
light and matter. PhC nanocavities utilizes the photonic bandgap effect to trap certain frequencies of light within a small
confined region for a diverse range of applications such as enhancement and suppression of spontaneous emission,
efficient and compact lasers, add/drop multiplexers, optical filters and sensing etc. In this paper, we describe a
mechanically-perturbative near-field probe with a special design shape to achieve low-loss and precise resonance control
of PhC nanocavities. One-dimensional (1D) PhC are chosen for our study due to the ease of integrating with low-loss
SOI waveguide technology and easy integration with nanomechanical structures. Sub-micron microelectromechanical
systems (MEMS/NEMS) technology is introduced as an ideal integration platform with such near-field probe designs
due to its capabilities to accurately control fine displacements without the need of bulky equipment such as atomic force
microscopy (AFM), scanning near field microscope (SNOM) or highly sensitive piezo-controlled micromanipulator
stages. We propose that such near-field probe designs are capable of achieving large resonance spectral shift of up to few
nm with high re-configurability, highly accurate actuation displacements, low power consumption, and portability. In
this work, we propose an approach utilizing numerical methods to study and characterize the electromagnetic interaction
between PhC nanocavities and nanomechanically displaced near-field nano-probes.
A compact and low power control of photonic crystal nanocavity resonance was devised, simulated, and experimentally validated utilizing a hybrid integration of a microelectromechanical systems driven nanoprobe. The experimental results demonstrated a reversible resonance tuning up to 5.4 nm with minimal Q-factor degradation.
We address a genetic algorithm (Gas) to achieve optimization design for diffractive optical elements (DOEs) for the laser beam shaping. A laser beam shaping system is investigated using genetic algorithm, in which an incident Gaussian profile laser beam is converted into a zero-order Bessel beam. This algorithm exploits the global nature of the genetic algorithms. High-quality DOEs can be achieved by use of the optimization procedure we proposed.
The electromechanical side-instability and the stable travel range of comb-drive actuators are investigated. The stable travel range depends on the finger gap spacing, the initial finger overlap, and the spring stiffness ratio of the compliant suspension. Proper design of the suspension structure is the most effective way to stabilize the actuator and therefore to achieve a large deflection. In this paper, an improved suspension design, so called tilted folded-beam suspension, is proposed. The expressions for the spring constants of the proposed suspension both in and perpendicular to the stroke direction are given. Using such suspension, the stability of the comb-drive actuator is improved and the stable travel range is enhanced. Comb drive actuators with various tilted folded-beam suspensions were fabricated using the standard surface micromachining technology and their operational performances were characterized. The experimental results are in good agreement with the theoretical predictions.
The concept of nondiffracting beams was first introduced by Durnin. The beam spot of nondiffracting beam undergoes diffraction-free spreading over a long propagating distance. Therefore, nondiffracting beams could have potential applications in precision alignment, optical interconnections, and power transport. In this paper, hybrid genetic algorithms that combine genetic algorithms (GAs) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive optical elements (DOE's) for the generation of nondiffracting beams. In the hybrid genetic algorithms, an offspring obtained by genetic operators, such as crossover and mutation, is not included in the next generation directly but used as a seed for the sequent local search. The local search method searches the neighborhood of each offspring, and selects a better point, which is included in the next generation. In such a manner, the efficient exploitation of local information is provided by the incorporated local search procedure and the reliable locating of the global minimum is provided by the use of mechanisms of nature selection. The proposed hybrid methods exploit the global nature of the GAs as well as the local improvement capabilities of the gradient-based local search techniques, and will perform a more improved search while comparing with both of the single ones. The incorporated local search technique we used here is the Davidon-Fletcher-Powell (DFP) method, which is well known for its good convergence property. Numerical results demonstrate that the designed DOE's can successfully produce both zero-order and high-order nondiffracting beams.
In this paper, we developed a nonsilicon surface- micromachining technique that uses a thick photoresist film and a spattered copper layer as two sacrificial layers and uses the electroplated ferronickel (FeNi) as the structure material. The proposed nonsilicon micromachining process is simpler, with relatively low temperature, and more flexible for various materials. By using such technique, silica or glass can be used as an optical material. Several out-of- plane multilevel diffractive optical elements (DOE's) including gratings, phase Fresnel lenses and some other optical components supported by FeNi microstructures are successfully constructed on the silicon wafer. Those DOE's are fabricated by several steps both of photolithography and reactive ion etching on the spattered silica layer. This technology offers a new approach to fabricate high quality phase micro-optical elements for free-space integrated micro-optics and other applications.
In this paper, a novel optimization algorithm, termed genetic local search algorithm (GLSA), that combines a genetic algorithm (GA) with a local search technique is proposed to design DOE's. This hybrid algorithm performs an improved, more goal-oriented search compare to a purely GA. A 1:17 cross pattern fan out grating and a uniform focal plane intensity profile generator are designed to demonstrate the algorithm we proposed. Numerical results proved that the proposed algorithm are highly robust and efficient. High-quality DOE's are achieved by using the algorithms we proposed.
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