This PDF file contains the front matter associated with SPIE Proceedings Volume 10925, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Traditionally optical design via computer optimization uses a numerical merit function to represent the optical performance of the simulated system. The conventional design approach is to maximize the nominal performance of the design, and then as a separate step, add fabrication tolerances to the nominal parameters so that upon manufacturing the resulting system still performs to specification. This paper will demonstrate an alternate approach. Because the angle rays make with respect to the normal on each surface are the primary drivers of optical aberrations and tolerance sensitivity, the method uses these ray angle as a fast, numerical approximation for the sensitivity to tolerance defects. This hybrid merit function thus includes the fabrication errors as part of the design process. The resulting design is effectively optimized for as-built, rather than nominal performance. Design examples will be provided which show that optimization using the hybrid merit function yields designs of different forms, which may have inferior nominal performance but superior as-built performance. The resulting alternate designs will be compared to conventional post-design tolerance analysis to demonstrate the reduction in tolerance sensitivity and superior resulting performance.

Calibration accuracy and long-term precision are key on-orbit performance metrics for Earth observing spaceborne sensors. The accuracy and consistency of environmental measurements across multiple instruments in low Earth and geostationary orbits are directly connected to the scientific understanding of complex systems, such as Earth’s weather and climate. It is common for instruments to carry on-board references for calibration at various wavelengths, but these are subject to degradation with time spent in-orbit, and also increase complexity, mass and power requirements.

ARCSTONE is a mission concept that provides a solution to the challenge of achieving and maintaining required instrument calibration accuracy on-orbit in the reflected solar wavelength range. As an orbiting spectrometer flying on a small satellite in low Earth orbit, ARCSTONE will provide lunar spectral reflectance with accuracy sufficient to establish the Moon as an SI-traceable absolute calibration standard for past, current, and future Earth weather, land imaging, and climate sensors in both low and geostationary Earth orbits.

The ARCSTONE instruments are required to provide spectral measurements in a thermal environment that varies by 40 °C or more depending on whether the instrument is in direct sunlight or shade. A Structural, Thermal, and Optical Performance (STOP) analysis is conducted to assess the robustness of these instruments in this thermal setting and to highlight areas for possible design improvement. The analysis is performed for transient thermal environments representing a thermal vacuum chamber (TVAC) test. Analysis was performed for both the ultraviolet – visible (UVVNIR) and infrared (SWIR) instruments, however, this paper will focus solely on the UVVNIR instrument. Additional considerations for the future flight units are presented, including modeling effects of preloads and sliding of lenses in their mounts on outcomes of the thermoelastic model. The ARCSTONE instrument design has been optimized based on the results of this analysis.

Modern interferometer-based optical-metrology technologies play an important role in many applications, and often consist of multi-disciplinary components. This reveals new ways of improving the performance or enriching the functionality of the system, while at the same time causing certain difficulties in system analysis and assembly. To overcome this, we present a physical-optics-based simulation approach. It starts from an electromagnetic representation of sources, which enables the modeling of coherence and polarization. We demonstrate how to integrate new components (from microscopy objectives to diffractive, micro- and nanostructures) in selected classical interferometric setups, and gauge whether the resulting system fulfills the desired functionality.

In this work, we discuss grid-based surface optimization in the OpticStudio lens design software. This new approach allows flexibility in designing freeform optical components that is not possible with traditional surface types. The system we focus on is a corrector plate with the rear surface specified by a variable grid sag surface. The goal of the corrector plate is to correct for wavefront error introduced by a Zernike phase surface. We optimize the sag values of the grid points to correct for this wavefront error and study the resulting behavior as the number of grid points are varied. We test grids from 5x5 to 25x25 and report the scaling of total computation time and improvement in wavefront error. We describe several findings, such as the importance of constraints on the grid variables and techniques that improve the calculation efficiency.

Time-gated image sensors with (sub-)nanosecond gating times have already found applications in multiple different domains such as 3D Time-of-Flight cameras, Fluorescence lifetime imaging (FLIM) and Tomography. Commercial timegated cameras are based on Image intensified CCDs (ICCD). The photomultiplier tubes used in these ICCDs have a limited quantum efficiency in visible and a fortiori in Near-Infrared (NIR). Furthermore, they are expensive, bulky, fragile and need high voltages to operate. We propose a time-gated camera based on the Current-Assisted Photonic Sampler (CAPS) which integrates the gating mechanism inside a silicon-based pixel without the need for photomultipliers. Due to particular pixel design, sub-nanosecond gating can be achieved while still attaining high quantum-efficiency even in NIR. A first proof-of-concept camera is demonstrated in this paper based on a 32x32-pixel CAPS array with specific timing circuitry to achieve precise and accurate high-resolution sensor gating. Quantitative results about the performance of the camera, such as gating speed and quantum efficiency will be presented and discussed. The cameras capabilities are demonstrated in two experimental setups. The first one: imaging a laser pulse traveling at the speed of light along the field of view. The second setup: making fluorescence lifetime images of two cuvettes containing fluorescent solutions with distinct lifetimes.

For efficient visual inspection of moving targets, such as walls, surfaces of structures, roads, assembly lines, and so on in real-time, inspection systems must have a simple yet robust design capable of operating at high speed. However, high-speed motion degrades image quality due to motion blur and defocusing which are conventionally compensated separately. In this research, we propose a focus adjustable motion-blur compensation method using one deformable mirror capable of back-and-forth movement and curvature bending. The deformable mirror, installed between a target and a 2D camera, simultaneously tracks the moving target and adjusts the focus to achieve high-quality images in real-time without image processing. Through the experiment using a camera having VGA resolution and frame rate of 30Hz, a deformable mirror with a pupil size of 10mm and 43 actuators for deformation and tip/tilt, and a moving target, the results showed that our system improved motion blur and focusing at the same time with only one device. As future work, entire system can be packaged into inspection system.

In this work, we report the usage of multi-longitudinal mode laser as an input source to achieve a larger enhancement in the interaction length in cavity enhanced absorption spectroscopy. The MM laser source is constructed in the form of a ring laser using a long fiber coil, a directional coupler, a tunable filter with FWHM of 1 nm and a semiconductor optical amplifier pumped above the laser threshold. The ring has a length of about 1004 m with an FSR of 199.2 kHz. The gas cell is inserted into another ring cavity, which consists of two directional couplers and a gain medium pumped below threshold. The gain medium is used to compensate for the losses and boost the ring cavity finesse. The cavity ring has a length of about 6 m which gives an FSR of 33.3 MHz. The large ratio between the lengths of the two cavities (the MM laser cavity and the gas cell ring cavity) eliminates the need for a mode-locked technique. The acetylene gas cell is measured around 1535 nm. The interaction length is improved by a factor of about 37 compared to the direct absorption of the gas cell.

A focus error method photothermal microscope was designed for the simultaneous annealing and characterization of defects in thin film multilayer coatings for high power lasers. The technique relies in the detection of the thermal lens induced by the local absorption of a light power focused laser. A 10W CW laser at 1.06μm wavelength was used as a pump and a HeNe laser at 632.8nm as a probe. A 4 quadrant detector and specifically designed astigmatic optic is used to determine the defocusing of the transmitted probe beam at the modulation frequency of the pump. The instrument scans the surface and detects the evolution of the absorptance with time with sensitivity below 0.1ppm. The pump beam focus determines the spatial resolution of the instrument and the probe beam size, much larger than the pump, has to match the modulation frequency that yields a thermal diffusion distance of the order of the probe beam in one modulation period. The detailed design of the instrument will be presented showing the design parameters that should be considered for an adequate sensitivity. The sensitivity of the system is better than 0.1ppm and allows the realization of spatial sweeps and even measurements of the evolution of absorption as a function of time. These capabilities allow the location of defects and their characterization.

Photonic nanojet interferometry (PNI) permits three dimensional (3D) label-free and super-resolution surface characterization. PNI is based on coherence scanning interferometry (CSI), featuring Ångstrom level vertical resolution. Being an optical far-field technique, CSI is diffraction limited and according to the Abbe criteria, can laterally resolve, points that are separated by a few hundred nanometers. We overcame this limitation by using dielectric microspheres that generate photonic nanojets. Now sub 100 nm features can laterally be resolved while preserving the vertical resolution of the CSI system. The microsphere material could be polymer or glass with a diameter between 8 and 12 μm, which limits the field of view (FoV) of the PNI system to ~10 μm². Here we present a method to increase the FoV of a PNI based device by stitching a sequence of adjacent 3D images. We imaged a recordable Blu-ray Disc (BR-D) using a custom built Mirau type scanning white light interferometer with enhanced lateral resolution. Four 3D super-resolution images with constant 80% overlap, were stitched together using inhouse software. The resulting high fidelity image shows that 45% overlap and the above described procedure could be used to enlarge the FoV of label-free 3D super-resolution imaging systems.

Modern surfaces are often used as technical design elements. The high-quality appearance of these surfaces is crucial. Depending on the application, different requirements such as colour, reflectivity and resistance have to be met by the surfaces. Surface defects in highly transparent materials such as cover glasses, windows and displays affect long-term stability and distract the user. This paper shows the development of a camera-based measurement system for the investigation of structural defects in highly transparent materials. The requirements for a system like the one presented here are comparable to solution qualities of the human eye. The system is particularly suitable for the detection of surface imperfections with depths in the single µm range and widths below 50 μm as well as lengths in the mm range. At the same time, however, it can also be used to evaluate the gloss, scatter and contrast of the surface for rating the appearance quality of those. For this purpose, the setup was designed in which a sample in darkfield configuration is illuminated with an angled, multispectral LED array that can be individually switched sectorally and also uses the usual reflection approaches.

In the nuclear industry, there is a need for sensors that are resistant to both high temperatures and radiation. Fiber Bragg gratings inscribed into radiation resistant fibers are a potential solution to this as the femtosecond-infrared laser can inscribe Bragg gratings into fibers without a photosensitive core. In order for these gratings to be used for sensing, they need to be characterized to determine their temperature and radiation response and sensitivity. This paper characterized three commercially available fibers for use in high heat and high radiation environments. There are six fiber variants examined in this study. Three basic fiber designs are investigated: a germanium doped core, a germanium doped core with fluorine cladding, and a fluorine doped core and cladding. For each fiber design, normal and pre-irradiated versions are investigated. The fibers were tested for thermal response by heating them to 1000C and holding for 24 hours, and for radiation resistance by irradiating with gamma radiation. The germanium core doped fibers were more resistant to thermal effects but still had a wavelength shift during the 24-hour soak. The fluorine-doped fibers either had the gratings partially or completely erased during the thermal hold at 1000°C, but showed suitability for short term excursions to this temperature. The radiation data showed significant shifts in some cases, but there was not enough data to form a definitive conclusion. It appears that radiation introduces variability in the response of the Fiber Bragg Gratings (FBG).

New methods enabling the production of custom-tailored Gradient Index (GRIN) optical components brings the next challenge to the lens manufacturers. Simultaneously, for testing these optics, metrology has to evolve to accommodate new optics. In this paper, we describe how Experimental Ray Tracing (ERT) can be used to test GRIN optics produced using additive manufacturing. To evaluate this technique, we compare the results to those obtained using Phase Shifting Diffraction Interferometry (PSDI). The common way of lens manufacturers to verify their products is the measurement of the surface, e.g. using surface profilers or reflective interferometry. Determination of optical performance solely from surface topography includes the assumption of a completely homogeneous structure inside the lens. Since GRIN lenses introduce material inhomogeneity on purpose, these measurement techniques exceed their limits, as surface measurement techniques cannot see the material structure inside the lens. To overcome this problem, we propose the measurement of GRIN lenses using ERT. This reference free measurement technique measures the device under test in transmission. A narrow laser beam is introduced into the device under test (DUT) at a known position. By measuring the direction of the beam behind the DUT, its optical function at this position can be determined. Evaluating these local measurements to an optical powermap over the full aperture, details of the inside structure of the DUT can be seen. The results of the proposed measurement technique show good agreement with the results from measurements using PSDI. However, differences can be seen between the two techniques. Therefore, the results of both measurement techniques are evaluated and compared and the advantages and disadvantages of both techniques are presented.

Laser ablation of thin membranes for industrial applications such as Micro-Electrical-Meachanical Systems has high demands regarding the process stability, alignment, surface shape and surface roughness. In the production of laser structured membranes with thicknesses in the single-digit μm-range ablation monitoring is therefore desired at every stage of the manufacturing process. This works presents a shape-from-shading approach where the surface of microstructured circular steel membranes is illuminated by two different light sources in order to generate sufficient surface reflection data from which a three-dimensional depth profile is reconstructed. By rotational scanning of the sample under examination, data is gathered from different angles and wavelengths at the same time. The advantage of this novel approach is the gain in acquisition speed as the spectrally encoded angle information can be acquired within one camera frame. Data processing is performed on the R, G, and B channels of the recorded image in parallel. In experiments, steel membranes with thickness of 2:1 μm and ablated structures with approximately 1:5 μm depth were examined during the structuring process. In order to compare the results of the in-line metrology approach, the surface topography of the laser-ablated samples were characterized on a confocal laser microscope. A discussion of the implications of the results regarding the usability of the metrology approach in industrial use cases concludes the work.

The High-resolution Imaging Multiple-species Atmospheric Profiler (HiMAP) is an ultraviolet imaging spectro-polarimeter in development at the Jet Propulsion Laboratory for measuring O3 and NO2 concentrations in the troposphere from an airborne platform or satellite. In this paper: (1) the HiMAP design is illustrated and modeled using 3D polarization ray tracing calculus, (2) the dependency between the condition number of the systems polarization measurement matrix and properties of individual optical components is used as a method for tolerancing, and (3) the polarimeter capabilities of manufacturable thin film designs of polarizing and non-polarizing beam splitters is explored using numerical methods. The condition number of an optical system is calculated from a polarization ray tracing (PRT) matrix model of the polarimeter. Deviations of the condition number are calculated for non-ideal polarization elements and coatings to understand component and alignment tolerances.

In this work we present a novel wave front phase sensing technique developed by Wooptix. This new wave front phase sensor uses only standard imaging sensor, and does not need any specialized optical hardware to sample the optical field. In addition, the wave front phase recovery is zonal, thus, the obtained wave front phase map provides as much height data points, as pixels in the imaging sensor. We will develop the mathematical foundations of this instrument as well as theoretical and practical limits. Finally, we will expose the application of this sensor to silicon wafer metrology and comparisons against industry standard metrology instruments.

A high-quality flat wave front is usually used to calibrate the Shach-Hartmann wave-front sensors. The article discusses the possibility of calibrating sensors with spherical wave fronts. Special attention is paid to the consideration of calibration in standard laboratory conditions. The mathematical apparatus and the scheme of the experiment are considered.

Fabrication of thin-film structures sets high demands on quality, precision and reliability of the manufacturing process. Appropriate thin-film characterization should deliver nanometer-accurate film thickness and 3D topographical resolution, as well as the ability to characterize mm-sized surface areas in an in-line manner. This work presents a dispersion-encoded low-coherence interferometer in a Mach-Zehnder configuration which is operated in a dual-channel mode. The primary channel utilizes a dispersive element to provide a controlled phase variation of the interference signal in the spectral domain. This phase variation is traced and used as measure for film parameters. The signal detection is performed by an imaging spectrometer to allow the scan-free data acquisition in one lateral domain. The second channel utilizes the back-reflected light from the sample's substrate material. This enables the in-system evaluation of substrate parameters to improve the accuracy of the measurement. The experimental setup was established and evaluated on industrial-grade indium-tin-oxide coated PET-foil substrates. From the gathered data it could be shown that a thickness resolution of the film thickness is in the order of 5 nm and can be achieved with a lateral spatial resolution of 4 μm. The advantage over other approaches is that signal processing is fast and spatially resolved data is gathered in a scan-free approach.

Manufacturing of multi-element optical lenses requires compliance with design specifications within strict tolerances. Commercially available metrology systems allow verification of surface curvatures, center thicknesses, and overall centration. However, there is no single system that can perform all these measurements, which results in a complex and costly verification process. Additionally, currently available systems do not provide information on relative orientation between different elements within one lens assembly. Additional manufacturing errors may potentially arise when relying on third-party glass suppliers because glass types of individual elements within a compound lens cannot be verified. Here, we describe a method for full characterization of individual elements of compound optical lenses using reflectance confocal microscopy (RCM), low-coherence interferometry (LCI), and computational ray-tracing. RCM provides information about focal plane shifts within an optical elements and LCI is used to measure the corresponding optical thickness at different wavelengths. LCI was also used to measure radius of curvature and center thickness. Computational ray-tracing models were performed to a) estimate individual glass materials, and b) correct for optical distortions for accurate estimation of internal geometry. A database of glass properties of commercially available glass types was built using publicly available information, and glass estimation was performed by exhaustive optimization against the measured parameters for each glass type. We validated our method on commercially available singlet, doublet, and compound lenses. We believe this method may have applications in the lens manufacturing pipeline by providing an integrated-system for verification and quality assurance of compound lenses.

An experimental study of the variation of quality factor (Q-factor) of mm-size whispering-gallery mode (WGM) resonators manufactured with fluoride crystals as a function of surface roughness is proposed. Q-factors of the order of 1 billion are measured at 1550 nm. The experimental procedure needs repeated polishing steps, after which the surface roughness is measured by quantitative phase imaging, based on a white-light phase-shifting interferometry approach, while the Q-factors are determined using the cavity-ring-down method. This process allows us to reach an explicit curve linking the Q-factor of the disk-resonator to the surface roughness of the rim. The variations of Q-factor as a function of surface roughness is universal, in the sense that it is globally independent of the bulk material under consideration. We used a white-light interferometer to investigate the dependency of Q-factors considering three different difluoride crystals as bulk materials; in all cases, we have found that a billion Q-factors at 1550 nm are achieved when the rms surface roughness has a nanometer order of magnitude. We have also compared our experimental data with theoretical estimations. This comparison enabled us to highlight a mismatch, which can be explained by the many physical constraints imposed by the mechanical grinding and polishing protocol. We expect that our work will contribute to a better understanding of the Q-factor limitations for mm-size WGM resonators, which are finding applications in a broad range of areas.

FocalSpec chromatic line confocal sensor technology is presented. Performance of the technology is described in terms of z-precision and accuracy. We report on the latest technical developments that improve the measuring speed by 6x. With these developments we are able to deliver sub-micron precise 3D point clouds with speeds up to 24 Million points per second. Potential applications are discussed.

A compact economical system for broadband multichannel spectroscopy with exceptionally high SNR, high sensitivity and high throughput is introduced. This system using the state-of-the-art optics and CCD is capable of combining more than 10 channels in one enclosure, any channel fully configurable to each experiment needs. The SNR value, Limit of detection (LOD) and limit of quantitation (LOQ) are identified as well-known criteria for sensitivity measure. These values were found to be superior comparing to the reported values for the leading benchtop fluorimeters. This system was tested measuring very low concentration of different samples simultaneously and demonstrated excellent performance.

Most light enabled sensing and imaging techniques have depth resolutions that are larger than a few micrometers. An exception to this is the single wavelength laser coherent interferometry, known for its sensitivity and resolution lambda/1000. However, due to 2pi periodicity of the cosine function, the unambiguous range of single wavelength interferometry systems is limited to lambda/2. Researchers in the past have performed interferometry with multiple wavelengths and detected phase on a single photodetector. The beat signal of the multiple wavelengths has been used to extend the range, but this technique worsens the resolution by the same factor. To alleviate this range-resolution tradeoff, our system uses multiple wavelengths for the interference, but wavelength demultiplexes the interferograms before detecting them on separate photodetectors. The resolution is preserved to that of a single wavelength interferometer, while the range is extended to the lowest common multiple of the multiple wavelengths. In our experiment, we use two wavelengths, 1525 nm and 1550 nm, and employ techniques of simultaneous phase interferometry to extract and unwrap the phase. We are able to measure discrete positions of a piezo electric stage up to an unambiguous distance of 94.55µm, with nanometer precision. Furthermore, by incorporating 4F lens systems with our technique, we demonstrate the capabilities of measuring samples with high precision. Our system is instantaneous, computationally cheap, utilizes inexpensive components, and has the highest dynamic range of 1e5.

The photo-elastic stress metrology is well known measurement technique widely used in mechanical engineering applications since at least 1940 [1]. Its use in semiconductor manufacturing has been limited since the direct measurements of the stress in silicon are complicated by relatively low values of stress-optic coefficient and need of use on NIR array detectors. The advent of flexible electronics and wide spread of use of PI films as passivation layer give opportunity to take advantage of very strong stress induced birefringence effect in PI for practical application. Here we present practical tool enabling measurement of stress in PI films with resolution down to 1 MPa. The optical system comprises of (light emitting device) LED panel (light source), polarization components, color filters and camera. Due to the birefringence caused by the stress, the sample changes the light into the elliptically polarized light. To analyze the elliptically polarized light and to eliminate the ambiguity when unwrapping the phase, we employed three (550 nm, 589 nm, and 632 nm) placed directly in front of the camera. We demonstrate performance of this system for flat panel displays having dimensions up 185 cm x 150 cm (G6). Discuss throughput and repeatability of this metrology. We also discuss scalability of this metrology. [1] K. Ramesh, “Digital Photoelasticity Advanced Techniques and Applications,” Springer, 2000.

Micro-reflectance (μ-R) spectroscopy is a powerful technique for investigating the micro-scaled surfaces and interfaces, such as semiconductors, metals, etc. We discuss and compare the μ-R spectroscopy on specular (Si wafer) and scattering (MoS₂ flake) surfaces using various objectives with different NA. μ-R is calculated by the ratio of sample to reference spectra, it follows the sequence of NA on scattering surface, which is proven by larger NA showing better performance for scattered irradiance due to its wider collection angle. Micro-reflectance difference (μ-RD) of each wavelength is further calculated, and it’s 30 times larger on scattering MoS2 flake surface than specular Si wafer surface.

We present an overview of current opto-electronic terahertz platforms designed for industrial applications. We discuss current and future market perspectives with respect to competing technologies and killer applications. “Make-or-break” features for industrial use are cost and volume reduction alongside with increased robustness and measurement speed. These market challenges are discussed for different technologies, and one representative industrial application is shown for each technology.

Because of its high range and resolution, light detection and ranging (LIDAR) is a significant technology for numerous applications, such as autonomous vehicles, robotics, aerial or terrestrial mapping, and atmospheric research. Current lidar market is mainly occupied by conventional pulsed time of flight lidars. However, recently emerging companies are utilizing frequency modulated continuous wave lidars for improved and robust range resolution, dynamic range, sensitivity and simultaneous velocity measurement. Here, we propose and demonstrate multi-tone modulated continuous wave (MTCW) lidar system made of a CW laser with multiple fixed RF tones for a high precision range finding and velocimetry. In the proposed approach, the interference of the scattered light with the reference is detected by a PIN photodiode to extract the modulation information. Since, the acquired light is traveled all the way to the target and back to the beam splitter, it carries the range and velocity information about the target as phase and frequency shift, respectively, on the RF modulation tones. We use 1550nm light source and multiple RF tone modulations ranging from 50 MHz to 6 GHz to demonstrate proof of principle for range finding. We also provide sine fitting algorithms on the measured RF tones to extract the range and velocity information in a single shot RF measurement. We show that the precision and range information are scaled by the selection of RF tones. By an engineered selection of RF tones and a laser source, the measurement precision can be increased without compromising the range.

Optical frequency-modulated continuous-wave (FMCW) interferometry using a swept laser source allows for highly precise distance measurements. The FMCW technique has the distinct capability to perform both diffuse surface and sub-surface ranging with micron-level accuracy over meters of depth. Similar to swept-source optical coherence tomography, this technique can also produce detailed images of internal structures over large volumes. Recent advances such as active sweep linearization and laser phase-noise suppression techniques have greatly improved the ranging depth and accuracy, allowing for high-resolution 3D imaging of complex objects. Understanding how these techniques can affect the FMCW signal is key in determining system performance limits. The application of an opto-electronic phaselocked loop (OPLL) can greatly reduce both the laser phase noise and deterministic sweep errors. The OPLL uses optical feedback to suppress sweep nonlinearities while also controlling drifts that can be extremely detrimental to the accuracy of the interferometric beat signal. In this work, an analysis of various error sources encountered in activelylinearized FMCW ranging systems is presented.

This work describes a novel system to control the stability of a 1583 nm telecommunications laser diode via measurement of junction voltage. This electronic technique dispenses with the optical components used in conventional wavelength locking schemes and shifts wavelength control to system level electronic instrumentation. The approach employs real-time measurement of diode series resistance (R_s), which is used to compensate the measured forward voltage (V_f) and recover the junction voltage (V_j) of the laser. Control of V_j provides wavelength control without introducing a significant error when the package temperature varies. This was implemented by measuring Rs as the dynamic resistance, δV/δI, by modulating the injection current. Recent work has reduced the modulation amplitude and noise in the electronics. Using a frequency deviation of 1 GHz, we achieved a centre wavelength variation of ± 2 pm over a package temperature variation of 20-55 °C. This gives a wavelength/ temperature coefficient of 0.03 pm/°C, which is an improvement on 0.34 pm/ °C, as typically achieved for optical locking systems. The system has been further developed using board-level components within a compact demonstrator unit. Work is on-going to further enhance this performance over a package temperature variation of 0-70°C.

A hybrid sensor based on microstructured hollow core fiber is proposed for the simultaneous measurement of strain and temperature. The fiber, consisting of four silica capillaries with wall thickness of ~1 μm and a cladding with a thickness of ~26 μm, is spliced between two sections of single mode fiber. Using a low arc discharge power to splice the two fibers, a Fabry-Perot interferometer is formed. In this situation, light travels in the hollow core and the behavior of a twowave interferometer is observed. However, when the power of the arc discharge is increased, the structure near the splice area changes, generating new interferometric paths and giving rise to a different spectral response. In this work, sensors with a single degenerated area are analyzed. In such case, both Fabry-Perot and Michelson interferometers are created and different sensitivities to strain and temperature are obtained. The different spectral frequencies are analyzed, enabling the discrimination between the two parameters. For a sensor with a length of ~385 μm, strain sensitivities of 2.46 pm/με and -0.52 pm/με are obtained for the Fabry-Perot and for the Michelson interferometer, respectively. Regarding temperature, a sensitivity of 1.81 pm/°C was attained for the former, whereas for the last the sensitivity was of 42.23 pm/°C. Keywords: Hyb

NASA has described the need for rugged, compact spectrometers for deep ultraviolet (DUV) analysis of atmospheric properties during the entry, descent, and landing (EDL) phase of the mission profile for planetary landing craft. The EDL phase presents a brief opportunity to gather useful altitude profiles of atmospheric components and pressures. However, EDL is a high-risk phase and presents severe challenges to instrument design, including large temperature changes over short periods and payload restrictions. Therefore, the preferred features of an EDL spectrometer are compact design, stable performance across wide temperatures, DUV sensitivity, and simple temperature management systems. In this paper, a compact EDL spectrometer is described, which includes the system-level optical design and analysis of the novel silicon-carbide integrated circuits.

Light scattering-type sensors have been utilized in order to measure the amount of particulate matter in the atmosphere in variety of locations. Optically detection characteristics of a light scattering-type particle sensor with a double-side mirror reflector structure which can detect efficiently the weakly scattered light intensity from a particle are studied in simulation. Dependence properties of an irradiated laser power, a particle size and a spatial particle distribution are revealed by the simulation with regard to a minimum particle size of 0.3 μm in diameter. The calculated results with the simulation model corresponds reasonably well with the measured results.

This paper proposes a new vector Brillouin optical time-domain analysis optical fiber sensor with large dynamic range and high signal-to-noise ratio that combines distributed Raman amplification with optical pulse coding. The optimized Raman pumping configurations are numerically simulated by solving the coupled differential equations of the hybrid Brillouin-Raman process, and experimentally investigated with respect to the Brillouin pump pulse. A vector network analyzer is adopted to extract both the amplitude and phase spectrograms of the Brillouin interaction in a distributed fashion which effectively lessens the impact of the Raman relative intensity noise transfer problem and achieve high accuracy measurement over a long sensing distance. Advanced pulse coding is further introduced to increase the sensing range under high spatial resolution. Initial experimental results of phase and amplitude from a custom built BOTDA system is presented. Compared to typically tens of kilometers measurement distance of conventional Brillouin optical time-domain analysis techniques, the proposed optical fiber Brillouin sensor has the potential to greatly enhances sensing range up to one hundred kilometers or greater, providing distributed temperature and strain monitoring of high spatial resolution and high sensing resolution in structures such as oil and natural gas pipelines.

State of the art laser-based refractive surgery instruments require precise optical beam steering and focusing capabilities to actively control an operational laser beam during eye surgery. As part of an industrial collaboration, Celera Motion and SwissOptic AG worked on a novel design approach to realize a compact, actively controlled lens moving mechanism for beam focusing. This system incorporates a closed loop voice coil driven air bearing that integrates a focusing lens of 10mm in diameter in a low module footprint. The application driven design approach, modularity, and scalability of the solution will be presented together with results of the practical system evaluation. Thereby, the achieved optical alignment accuracy of the moving lens of only a few microns along a moving range of up to 80mm in combination with a lateral movement bandwidth up to 100 Hz indicate best agreement with the design requirements for the driving unit. Since the developed driving mechanism offers high system performance for additional applications, the authors will give further application examples and show scalability approaches for various lens diameters and moving ranges for customized z-scan units in optical systems. As part of future developments, design modifications for applications with higher bandwidth requirements and reduced module footprint with lower weight will be discussed.

The continually increasing sensitivity required for advancement of far-infrared astronomy dictates that the next generation of space-based observatories must employ cryogenically cooled telescopes and instruments. Cryogenic operation of interferometers such as those proposed for future space missions poses particular challenges, including the need for robust low power dissipation cryogenic position metrology. Instrumentation must be cooled to <4 K to avoid a noise contribution from self-emission and often contain moving components whose position must be measured precisely at cryogenic temperatures. In 2018, we reported on the development of a three-phase fiber-fed laser homodyne interferometer for optical position metrology that achieved a displacement uncertainty of 2.3 nm RMS at 4 K. In that design, one arm of the interferometer had an additional 2 m of optical fiber to carry the probe signal to the 4 K work space. Subsequently, a 2 m, armored, differential fiber pair was developed to balance the lengths of the probe and reference interferometric beams that were subject to thermal gradients. Although this led to an improved dynamic performance in the measurement of an oscillating target, low velocity performance was limited by 1/f noise in the photodetector circuit. Building on that work, we present the design and review the performance of a new frequency-modulated laser interferometer system we have developed that improves upon the three-phase system by eliminating the need for a differential fiber pair in cryogenic applications and achieves 29 nm RMS uncertainty for mechanical displacement velocities from 0 to ~4 mm/s.

In the process industries, pressure measurement using bourdon tube is basic measurement technique. In the present work, we proposed a photonic based pressure measurement and transmitting technique of pressure which is capable to transmit the measured flow rate data through optical fiber with the help of Lithium Niobate (LiNbO₃) based Mach-Zehnder Interferometer (MZI). A modified bourdon tube based transducer developed using hall probe sensor. Transducer converts the pressure to the 1-5 Volt and 4-20 mA which is not suitable for inflammable regions of the process plant. For this purpose a measured pressure transmit to the remote location in optical domain. The required mathematical derivation and the principle of operation of the transmitter are shown in the paper.

Performing optical imaging of relatively small or weak targets in complicated background, as a primary approach used to explore the nature, is limited in performance by atmosphere-induced optical aberrations. Methods that characterize wavefront aberration have improved our understanding of how wavefront distortions degrade image quality. However, these approaches are unable to show the light-field and polarization information except for the wavefront aberration. The integrated imaging chip or microsystem that can acquire more dimensional information is crucial for researchers to see more clearly into the scene. To address this question, an electrically tunable focused plenoptic camera that is used to visualize objects directly in a snapshot with multi-dimension perception such as wavefront, polarization, and light-field information, is proposed and demonstrated. This prototyped camera, which is composed of a twist nematic liquid-crystal microlens array (TN-LCMLA) and a common CMOS sensor array, can directly sense two orthogonally polarized lightfield images via switching the working stated of TN-LCMLA. This prototype records not only intensity and polarization information but also the direction and position of incident beams, so as to indicate that the localized wavefront slope of incident beams can be extracted directly. In this study, the theoretical analysis, reconstruction algorithm and experiment results show that a high performance and functional detection are obtained by this novel configuration. Then, with the proposed algorithm, the prototyped camera has a potential to be used in adaptive imaging application and many other optical systems.

The potassium LIDAR at Arecibo Observatory utilizes the Light Age alexandrite high power laser which requires a well synchronized system and steady trigger repetition rate to achieve correct height determination and to extend the lifetime of the equipment. The system includes an optical chopper that prevents the detector from saturating before the altitude of interest is reached and a sequence of delayed pulses that execute the laser trigger which are generated by an external pulse/delay generator. The accuracy of the optical chopper limits the accuracy of the laser repetition rate as well as other equipment in the synchronized system. This work describes the implementation of a new microcontroller based single instrument optical chopper and laser trigger controller to improve stability and functionality. By programming a unifying USB user interface, the new capability of monitoring the system and manipulating relevant variables was achieved. This includes changing the repetition rate, moving the optical chopper edge to block out different altitudes, tuning PID constants, and more. The new system centralizes control, increasing ease of operation and allowing more flexible and efficient use. Furthermore, a laser only mode for testing has been implemented to send out a laser trigger sequence to the rest of the system without the need of an optical chopper. The new implementation has reduced steady state frequency jitter of the laser trigger by 60% and startup time by 77%.

Fine-polishing techniques, such as chemical mechanical polishing treatments, are important techniques in glass substrate manufacturing. However, these techniques may cause microcracks under the surface of glass substrates because they use mechanical friction. We propose a Non-contact thermal Stress-Induced LightScattering Method (N-SILSM) using a heating device for inspecting surfaces to detect polishing-induced microcracks. The N-SILSM can detect microcracks in a product under a fine-polished surface. It is a technique for exposing microcracks by exploiting the change in light-scattering from microcrack tips due to temperature variation-induced stress. Additionally, optical properties change due to temperature variations. However, at manufacturing sites, it is ideal that inspection systems be able to distinguish between microcracks and tiny particles. In this report, we carry out the selective detection of microcracks and tiny particles using a N-SILSM with temperature variation. Experimental results showed that the amount of change in the lightscattering intensity alters the cubic function regardless of the size of the microcracks, and also confirmed that tiny particles show very little change in light-scattering intensity. In addition, the possibility of microcrack size estimation was suggested from the magnitude of the change in light-scattering intensity. From the above results, it has been shown that microcracks and tiny particles can be identified and measured by a N-SILSM utilizing temperature change, and that microcrack size estimation can be based on the change in light-scattering intensity. Thus, it has been suggested that N-SILSM is a useful inspection technique for distinguishing between microcracks and tiny particles.

Dual-comb spectroscopy (DCS) is a powerful tool for gas spectroscopy due to high resolution, high accuracy, broadband spectral coverage, and rapid data acquisition, based on optical frequency comb (OFC) traceable to a frequency standard. In DCS, after a temporal waveform of interferogram is acquired in time domain, the corresponding mode-resolved OFC spectrum is obtained by fast Fourier transform (FFT) calculation of the acquired interferogram. However, FFT calculation of huge-sized temporal data spends significantly longer time than the acquisition time of interferogram, making it difficult to response the transient signal change. In this article, we demonstrate frequency-domain DCS by a combination of DCS with lock-in detection (LID), namely LID-DCS. LID-DCS directly extracts an arbitrary OFC mode from a vast number of OFC modes without the need for FFT calculation by the synchronous detection at a LID reference frequency while maintaining high resolution and high accuracy. Usefulness of LID-DCS is demonstrated in rapid monitoring of transient signal change and spectroscopy of hydrogen cyanide gas by comparing with usual DCS.

Spectroscopic investigations of material samples on the microscopic and nano-scales, incorporating different modalities such as Raman, photoluminescence, Transmission- Absorption-Reflection(TAR), and transient spectroscopy, are growing increasingly popular to collect quantitative and functional information of materials. Typical samples can range from invitro and in-vivo tissue samples, living cells, quantum structures (dots, wires), material surfaces, microfluidics and nano-crystals. Enhancing traditional microscopy systems by coupling a spectrometer to them offers the added dimension of higher density multichannel spectral information. This information can unlock deeper chemical, dynamic and conformational information on various materials. Different approaches are taken in configuring such systems from the commercially available fully integrated ‘black box’ tailored for specific types of application and/or spectral modalities, to the in-house purpose-built modular systems where the end-users build and integrate their own system from various individual components. There are advantages and limitations with either approach. This article outlines various aspects and implications for taking the modular approach to constructing one’s microspectroscopy setup. In particular, examples are illustrated where Andor Technology’s dedicated, highly configurable interfaces for microspectroscopy utilize various optical and optomechanical components to facilitate flexible direct and indirect coupling of the spectrometer to the microscope. Software options are discussed for control of the system and data collection/processing to allow for multitrack and hyperspectral imaging, as well as fast chemical mapping. Sensitivity is a key technical challenge in most scenarios when dealing with such small sample volumes; an outline of where the latest CCD, EMCCD, and sCMOS technology offers benefits in sensitivity are also discussed.

Selecting a camera can be a difficult due to the different technologies available and the range of different camera models. Technical specifications of different camera technologies vary, sometimes by several orders of magnitude, or sometimes by a seemingly small amount, and it is not clear how these differences affect camera performance for a specific application. The key to selecting the most suitable camera is by combining several key specifications with the needs of the specific application. The important parameters that influence how well suited a camera will be for a specific application can be identified as: sensitivity, speed, field of view and in some cases, low dark noise. A further subset of factors such as: dynamic range, shuttering modes, connectivity or vibration can also be used to determine suitability. This information should simplify the decision tree allowing for flexibility that may be needed in multi-user, multiapplication/ technique environment spanning often radically different light regimes and sample sensitivities. Recently cameras based on back-illuminated sCMOS technology have become available which offer further improvements in sensitivity. This provides a further option for consideration to the existing sCMOS and EMCCD cameras available. Considering this new camera technology, we will characterize the new sCMOS camera model within the wider context of the camera technologies and other models available.