This PDF file contains the front matter associated with SPIE Proceedings Volume 11693, including the Title Page, Copyright Information, and Table of Contents.

The development of VR products has been exciting in the past six years. This talk provides a quick review on the progress that the industry made recently on the development and productization of VR optics and presents the current challenges.

We recently introduced a polariscopy method which can determine feature orientation below the diffraction limit. Measuring transmission at four linear polarisations (0,45,90,135°), information about the orientation of absorbers or patterns and features inside an object could be determined. This is applicable to transmitted/absorbed and scattered/reflected light as well, across the EM-spectrum. We investigate the feasibility of applying this technique to remote sensing satellites. Altimeters are able to determine many oceanographic parameters based on the surface height mapping (e.g., current directions, tidal waves etc). The nature of this measurement means the range and azimuth lateral resolution differs greatly, 100 km to 1 km scale. Synthetic aperture radar (SAR) data contains polarisation information and provides imaging of planetary surfaces. We aim to demonstrate the four polarisation technique in aerial imaging for recognition of feature alignment patterns which are beyond spatial resolution. Information from conventional intensity images (scalar) are augmented by the revealed orientation (vectorial) demonstrated for transmission/scattering in the visible spectrum. Visible as well as SAR imaging timetry can benefit from this augmented resolution.

Nano-Illumination Microscopy (NIM) is a technique that provides compact microscopes but at the present time only setups with limited Field-of-View (FOV) have been presented. Existing NIM setups reconstruct the image by measuring the light intensity that passes through the specimen when switching after one another the light emitting diodes (LEDs) on an array. The resolution of NIM is related to the LEDs pitch, while the FOV to the total area covered by the array. The first prototypes were demonstrated with 10 μm-pitch GaN-based 8x8 LED arrays giving rise to 80x80 μm² FOV. This work presents the first electronically-activated Scanning Transmission Optical Microscope (eSTOM) built with an Organic LED-on-silicon micro-display with 5 μm LEDs pitch, providing a FOV of 3.6 × 1.28 mm² . It is combined with a CMOS optical sensor with no other optical or mechanical components. We demonstrate how downscaling of the OLED array by means of optical lenses allows to further reduce the size of the light sources to explore the technique in more detail. Here we show steps towards the utility of NIM as a practical, low-cost and compact microscopy technique for biophotonics and many other applications.

This Conference Presentation, “Second-generation fluorescent nuclear track detector reader,” was recorded for the Photonics West 2021 Digital Forum.

The presented vision system integrates a focus tunable lens allows to perform fast autofocus and distance measurement at the same time. By deriving the best focus from the maximum position of a fitted distribution, it is not required to capture the image with the actual best focus during the autofocus sweep, resulting in high speed and robustness of the algorithm. In this work, we demonstrate that a focus tunable lens in conjunction with an autofocus algorithm can reliably measure distance to an arbitrary object in less than a second (depth from focus). The accuracy of the distance measurement is in line with the depth of field of the imaging system. No additional hardware is required apart from the imaging system comprising camera, objective lens and focus tunable lens. The fast and accurate focus and distance measurement enables and simplifies various applications ranging from robot vision to smart manufacturing control. The optics can be tailored to reach the desired precision and focus range, whereas there is generally a trade-off between the two.

A remote and heat-resistant Raman measurement probe has been developed for timegated PicoRaman utilizing a 532 nm pulsed laser and SPAD detector. The probe has 400 mm measuring distance with automated focus detection. The focus detection enables a fast auto-focusing to a varying sample surface distance – an obligatory requirement in many industrial applications including ore mineral scanning above conveyor belts in raw material sector or the inspection of metal product consistency in metal industry lines. In addition, the Raman probe has been designed to withstand thermal loads to be suitable for hot industrial process control.

LiDAR is an enabling sensor technology for autonomous vehicles, industrial automation, and robotics. However, large-scale adoption is hindered by cost and robustness. We have developed systems that address these challenges through a camera-like flash architecture, which eliminates moving parts and allows for simple optical and emitter designs that are robust and manufacturable at low cost. Large curved VCSEL arrays created using microtransfer printing illuminate the field of view, and 2D sensor arrays capture high resolution range data from the entire scene at once. Flash architecture with no moving parts enables robust LiDAR with lower cost to facilitate large-scale automation.

Current methods for fabrication of optical components are complex, expensive and require specialized equipment, thus prohibiting rapid prototyping of optical components. We present a method allowing to rapidly shape curable liquids into a wide range of optical shapes without the need for any mechanical processing. After the desired shape is obtained, the liquid is cured to produce solid optical components with sub-nanometer surface quality. The method is inexpensive, does not require specialized equipment and can be implemented using a variety of curable liquids. In addition, the method is scale-invariant and can be used to produce optical components of any size.

Monolithic distributed feedback semiconductor lasers (1550 nm) for FMCW LiDAR applications have been designed, fabricated and tested. The strong optical frequency modulation distortion observed when a standard DFB laser is modulated with a triangular current waveform is significantly mitigated in our laser. A 100 kHz frequency modulation with amplitude of 0.9 GHz and nonlinear distortion of 0.3%, calculated as the standard deviation of the optical frequency after removal of a linear fit, was measured through an unbalanced fiber interferometer. This was achieved without electronic pre-distortion of the triangular waveform. The 60 kHz intrinsic linewidth of the laser was unaffected by the modulation. Two lasers were co-packaged in a 2.6 cm³ multi-layer ceramic package and coupled to fiber pigtails with micro-lenses. The pins of the ceramic package were soldered to a printed circuit board containing the current sources driving the lasers. This optical source was used in a two-channel LiDAR demonstrator built from off-the-shelf fiber optic components and a twodimensional gimbal scanning mirror. This demonstrator enabled detecting a target with 10 % Lambertian reflectivity up to a distance of >120 m and recording point clouds of different scenes. This shows that FMCW LiDAR in combination with highly coherent and linear DFB laser sources is a very promising technology for long range sensing. A version under development will include a silicon photonics chip for further integration and functionality including I/Q detection.

A pulse oximeter is an optical device that monitors tissue oxygenation levels. Traditionally, these devices estimate the oxygenation level by measuring the intensity of the transmitted light through the tissue and are embedded into everyday devices such as smartphones and smartwatches. However, these sensors require prior information and are susceptible to unwanted changes in the intensity, including ambient light, skin tone, and motion artefacts. Previous experiments have shown the potential of Time-of-Flight (ToF) techniques in measurements of tissue hemodynamics. Our proposed technology uses histograms of photon flight paths within the tissue to obtain tissue oxygenation, regardless of the changes in the intensity of the source. Our device is based on a 45ps time-to-digital converter (TDC) which is implemented in a Xilinx Zynq UltraScale+ field programmable gate array (FPGA), a CMOS Single Photon Avalanche Diode (SPAD) detector, and a low-cost compact laser source. All these components including the SPAD detector are manufactured using the latest commercially available technology, which leads to increased linearity, accuracy, and stability for ToF measurements. This proof-of-concept system is approximately 10cm×8cm×5cm in size, with a high potential for shrinkage through further system development and component integration. We demonstrate preliminary results of ToF pulse measurements and report the engineering details, trade-offs, and challenges of this design. We discuss the potential for mass adoption of ToF based pulse oximeters in everyday devices such as smartphones and wearables.

We report on a highly sensitive and selective optical sensor for detection of carbon monoxide (CO) in a sulfur hexafluoride (SF₆) gas matrix by using quartz-enhanced photoacoustic spectroscopy (QEPAS) technique. The sensor uses a mid-infrared quantum cascade laser with central wavelength at 4.61 μm as light source and a spectrophone consisting of a novel 8 kHz T-shaped quartz tuning fork with grooved prongs coupled with a pair of resonator tubes for photoacoustic detection. A minimum detection limit of 10 ppb at 10 s of signal integration time was achieved.

We report on the detection of the isotopologues ¹²CH₄ and ¹³CH₄ by employing a quartz-enhanced photoacoustic spectroscopy (QEPAS)-based sensor. By properly selecting the exciting light source and the working conditions, two absorption lines, having a negligible cross-section ratio temperature coefficient of -6.7‰/°C and a cross section ratio of ~ 0.06 for a natural abundance of each isotope, can be targeted. The QEPAS signal of the two isotopologues was acquired for mixtures in nitrogen of methane in natural abundance in a wide range of concentrations (0.02%-20%) showing a non-linear trend with high concentrations and a constant ratio comparable with the cross-section.

The main limitations of tunable diode laser absorption spectroscopy (TDLAS) sensors are represented by the high cost, limited detection bandwidth and low adaptability of photodetectors to work in harsh environments. In this work we present an extensive study on quartz tuning forks (QTFs) used as photodetectors, exploiting the opto-thermo-elastic energy conversion arising from the laser radiation-QTF interaction. The role of the strain field, accumulation time and working pressure of the quartz resonator in this Light-Induced Thermo-Elastic Spectroscopy (LITES) approach was then evaluated for a whole set of tuning forks. Once identified the most performant resonator, this QTF was implemented in a TDLAS setup and it was combined with laser diodes, interband- and quantum-cascade laser sources emitting from 1 μm to 10.5 μm and targeting different gas spacies. The detection limits achieved for the QTF were comparable or even lower down to one order of magnitude with respect to market-available photodetectors.

Quantum cascade lasers (QCLs) are optical sources exploiting radiative intersubband transitions within the conduction band of semiconductor heterostructures.¹ Mid-infrared QCLs have been thoroughly considered for applications such as spectroscopy,² free-space communications³ and countermeasure systems.⁴ Under self-optical feedback, QCLs have been proven to operate in several non-linear dynamic regimes,⁵ including low-frequency fluctuations and deterministic chaos, which are suitable for private communications taking advantage of both chaos masking and background stealth. However, the previous experiments focused on distributed feedback (DFB) quantum cascade lasers emitting at 5.7 µm, which is not an optimized wavelength for free-space applications. Indeed the atmosphere is characterized by two transparency windows between 3-4 µm and 8-12 µm, which are called bandpass L and bandpass N, respectively.⁶ Furthermore, the 5.7 µm lasers were studied at the chip level, which means that end users must own the dedicated mounts, connectors and mid-infrared optics in order to take advantage of these quantum cascade sources. This work extends our knowledge by exploring the non- linear dynamics of a packaged Fabry-Perot (FP) QCL emitting at 4 µm. The advantage of the FP configuration is an increased output-power compared to DFB sources, though the FP configuration is not well-known yet.⁷ Moreover, this laser comes in a handy environment with embedded focusing optics and high-heat load (HHL) packaging for plug-and-play operation. Consequently, the current findings pave the way for off-the-shelf private s at mid-infrared wavelength where high-power and compact turnkey systems are required.

3D-imaging is used in a wide range of applications such as robotics, computer interfaces, autonomous driving or even capturing the flight of birds. Current systems are often based on stereoscopy or structured light approaches, which impose limitations on standoff distance (range), and require textures in the scene or accurate projection patterns. Furthermore, there may be significant computational requirements for the generation of 3D maps. This work considers a system based on the alternative approach of time-of-flight. A state-of-the art single-photon avalanche diode (SPAD) image sensor is used in combination with pulsed, flood-type illumination. The sensor generates photon timing histograms in pixel, achieving a photon throughput of 100’s of Gigaphotons per second. This in turn enables the capture of 3D maps at frame rates >1kFPS, even in high ambient conditions and with minimal latency. We present initial results on processing data frames from the sensor (in the form of 64×32, 16-bin timing histograms, and 256×128 photon counts) using convolutional neural networks, with the view to localize and classify objects in the field of view, with low latency. In tests involving three different hand signs, with data frames acquired with millisecond exposures, a classification accuracy of >90% is obtained, with histogram-based classification consistently outperforming intensity based processing, despite the former’s relatively low lateral resolution. The total, GPU-assisted, processing time for detecting and classifying a sign is under 25 ms. We believe these results are relevant to robotics or self-driving cars, where fast perception, exceeding human reaction times is often desired.

We have developed a 32x24 pixel sensor array based on single-photon avalanche diodes (SPADs). Beside conventional 2- dimensional imaging, this sensor allows for precise timing of single-photon arrival times which can be exploited in a variety of technical and scientific approaches like 3D image acquisition, quantum imaging and quantum random number generation. Thus, such a sensor is eligible for many fields of application such as autonomous driving, remote and non-lineof- sight sensing, safety, robotics and more recently random number generation for statistical applications or data encryption. The novel sensor contains CMOS integrated backside illuminated SPADs which are connected to an underlying read-out IC by wafer-to-wafer bonding. Their single-photon sensitivity (quantum efficiency QE=60 % @ 580 nm) and high-speed performance (readout frequency 𝑓 = 25 kHz, temporal resolution 𝑡_TDC = 312.5 ps) make the sensor a promising choice for, e.g. quantum imaging with photon-pairs where a 2-dimensional spatial and temporal resolution are as crucial as a low noise level. SPADs also offer exciting opportunities for random number generation by using the randomness of photon generation paired with time-resolved detection and post-processing. Another potential application of the sensor is light detection and ranging for which we integrated the sensor into a demonstrator system for direct time-of-flight measurements. It is capable of coincidence detection using 4 SPADs in each pixel, which allows for background light suppression in outdoor situations. This combination of single-photon sensitivity, precise photon arrival timing and our recent developments in wafer-to-wafer bonding technology gives access to a new generation of optical sensors for a variety of applications.

Different consumer applications using 3D imaging are being introduced in the market. This has been essentially facilitated by the downsizing of semiconductor technologies. Different techniques with more or less advantages are developed with a major lack of efficiency under extreme light conditions. The FMCW 3D imaging technique may be one way to overpass this issue. We present a demonstration of a free space 3D imaging FMCW bench system that is envisioned to be miniaturized for a future integration in portable applications. Our first results show a very good immunity against ambient light. We demonstrate a 3D scene imaging range of 2 meters with only 1𝑚𝑊 optical projected-laser power.

Three dimensional (3D) sensing has recently been introduced in mobile devices for mass-market applications such as facial or gesture recognition. The two technologies widely used in smartphones, structured light and time of flight, are based on active imaging and enable operating in darkness. Nevertheless, ocular security constraints limit the accessible outdoor range in sunlight conditions. In contrast, immunity to ambient light is a natural advantage of Frequency Modulated Continuous Wave (FMCW) LIght Detection And Ranging (LIDAR), which is also an active technique, using the coherent nature of the laser source, through a reference arm to amplify the signal from the scene. However, the FMCW LIDAR potentially requires complex opto-electronic components for sequential scanning of the scene. Therefore, obtaining high resolution images at video rate becomes a real issue. In this paper, we investigate low power FMCW imaging (~1mW) with instantaneous active illumination of the scene and heterodyne detection with an image sensor, to combine the advantages of previous techniques with each other. We consider several configurations for the optical system, common to the scene and reference beams. Ideal criteria include uniform illumination of the image sensor by the reference arm, wavefront matching of the scene and reference beams for an optimal heterodyne yield at every pixel location, and minimization of optical loss. Moreover, the average speckle size, with regards to the pixel size and numerical aperture influences the statistical performance of the whole sensor. Finally, the optimal system choice is reinforced by experimental results on a bench setup built in the visible range.

Optical-time-domain-reflectometer (OTDR) suffers from the existence of dead-zones along a deployed fiber under test (FUT). Within a dead-zone, OTDR typically fails to provide any reliable diagnostic information. We here use a fewmode fiber (FMF) to completely cancel the OTDR dead-zone produced by the front facet reflection of the FUT. In particular, we launch the optical pulses in the form of the LP₀₁ mode into the FMF, and meanwhile we record the Rayleigh signal from the higher-order modes. The developed system successfully monitors the amplitude and frequency of a vibration event produced by a piezoelectric transducer (PZT) located within the dead-zone.

Optical fiber sensors based on multimode interference (MMI) have been widely used and developed into various applications. The sensing principle is mostly based on the induced wavelength shift of selected dips (or peaks) in the transmission spectrum. A simple structure to obtain MMI devices is a so-called single- mode—multimode—single-mode (SMS) fiber structure, which is composed of a short section of multimode fiber (MMF) fusion spliced between two single-mode fibers (SMFs). However, most of these MMI-based fiber sensors are related to circular core MMFs. Although the sensitivity is enhanced and the fabrication process is improved, some common problems still exist and need to be solved. To solve current challenges, such as linearity, crosstalk, and compactness, it is essential to find solutions like using a new structure or a new type of fiber. Recently, different new fibers, like hollow annular core fiber (HACF) and square no-core fiber, have been demonstrated to have advantages to overcome some of these limits. The applications of these fibers provide possibilities to study specifically shaped core multimode fibers. In this contribution, we propose a compact MMI-based fiber sensor for temperature measurement. Instead of the standard MMF used in the SMS structure, a square-core fiber (SCF) with a circular cladding is implemented as the sensing element. To the best knowledge of the authors, the SCF has not been investigated yet for sensing. Therefore, the sensing characteristics are studied experimentally. The proposed fiber sensor reaches a sensitivity of 45 pm/^◦C. It proves the sensing capability of SCF is promising, provides great potential for further works.

Coupling characteristic analysis and refractive index sensing based on in-line Michelson interferometer

The detection of ammonia over a wide range from parts per millions (PPM) to 1000’s ppm in a single sensor is of great importance for industrial applications. We have been exploring Vapochromic Coordination Polymers (VCP) specifically Zn[Au(CN)₂]₂, that was developed to achieve fluorescence when exposed to NH₃. Upon high concentration ammonia exposure, the fluorescent peak under near-UV stimulation undergoes a spectral shift from 470nm to 530nm, while the intensity increases by 3~4X. However, at ammonia concentrations < 100 ppm, there is almost no peak wavelength spectral shift or intensity change and only subtle fluorescent spectrum alterations. Using a 405nm laser diode excitation source provides a narrow (4nm) stimulation easily separated from the emission peak. The emission is focused on a USB portable spectrometer (430 to 650 nm). First we create a method that gives unique values over the range <1000 ppm by dividing the spectrum into 20 nm bins, and integrate the emission in each bin, relative to that of 0 ppm exposure (Sum of Integrated Emissions). The key point in this analysis is to note that the way the spectrum changes in each wavelength bin varies at different ammonia ppm exposures. SIE gives excellent sensitivity between 0-50 ppm and <300 ppm, but the 100-300 ppm region has low accuracy. There we change the metric to the Spectral Region Subtraction (SRS) by separating the spectrum into (A) 430-516 nm and (B) from 516 -650 nm, integrate the spectrum and subtract A from B, giving a rapid change within 100-300 ppm.

A compact broadband atmospheric gas spectrometer has been developed in the framework of the EU-H2020 FLAIR project. The system is composed of a mid-IR 2-4um broadband supercontinuum source, a temperature controlled 10- meter-long multipass-cell for light-gas interaction, a diffraction grating, and an uncooled PbSe-on-CMOS matrix detector recording absorption spectra. The detection limit has been measured at sub-ppm level on methane under laboratory conditions. We also present 2 successful field measurement campaigns aboard airborne platforms: a hot-air airship for controlled methane release experiments, and a helicopter tracking ship exhaust fumes abroad the coastline of Denmark, with special emphasis on methane detection.

Autonomous driving enable more use of in-vehicle interior lighting effects using advanced interior materials for illumination. Targeting premium visual ergonomics requires additional know how of today’s and future materials regarding light scattering. Within the presented work, we improved a goniophotometer towards a 2 standard deviation of < 0.08 %. This is in the range of a suitable limit so we successfully measured light scattering in terms of BSDF and/or BTDF of wood, transparent plastic (including 3D printed), and leather. These materials scatter light differently, so the results enable the base for optical simulation tools to design special visual effects.

A new photon counting X-ray imager with a 4-sided buttable structure is proposed. The imager consists of a stacked structure of a semiconductor detector such as Cadmium Telluride detector and a Si-based Read-Out Integrated Circuit (ROIC). The imager can be arranged in two dimensions with small gaps of less than 100 μm, because input/output pads of the ROIC are located on the back using through silicon via technology. In addition, daisy-chaining between imagers extends the number of tilings without increasing the number of external connections. This allows to connect small imagers to achieve a larger imaging area.

In this paper the possibility offered by a Vis-SWIR spectroscopy-based analysis is described, carried out directly in the field, to recognize post harvested olive fruit attacked by olive fruit fly (i.e. Bactrocera oleae). To reach this goal, chemometric techniques were used, that is: Principal Component Analysis (PCA) for exploratory data analysis and Partial Least Square – Discriminant Analysis (PLS-DA) for classification of attacked and un-attacked olive fruits. Itrana cultivar, at different degree of ripeness, coming from three different locations, was investigated. An ASD FieldSpec 4® Standard- Res field portable spectroradiometer working in the range 350-2500 nm was utilized. The developed classification model and the achieved results showed a promising ability to recognize attacked olive fruits, reaching Sensitivity and Specificity values in prediction of 0.939 and 0.698, respectively.

The quality of an image captured by the human eye is typically better than that obtained relative to artificial images created by cameras or telescopes. This is because humans have curved retinas. In contrast, conventional imaging cameras have flat sensors that are not well matched to the curved focal surfaces of a camera lens or telescope objective. Thus, the image cannot be at the same focus across the entire sensor field of view. It is hypothesized that as the surface of the sensor approaches the curvature of the camera lens or telescope, the image quality increases. To test this, a commercially available ray tracing software was used. The curvatures were varied from flat (0 mm) to 12 mm. As the curvature reached 9 mm, the Petzval curvature, the quality of the captured images from the camera significantly improved. However, as the curvature increased beyond 9 mm, the quality of the artificial image decreased. In addition, a simulation of a classical Cassegrain telescope was also made. For the telescope, the curvatures were varied from 0 mm to 500 mm. As the curvature approached the telescope’s focal surface curvature of 350 mm, the distortion decreased. In addition to the optical simulations, two images were generated: one with a camera and the other by a reconstruction process. The latter was reconstructed by using the central part of images taken along that curve to create an image. A comparison of these images demonstrates the superior image produced with the latter method. Devices such as cameras and telescopes with curved focal plane array detectors produce images with higher quality than those produced using devices equipped with flat focal plane array detectors.

Several studies has reported that indoor air quality affects concentration and memory. For this reason, air quality control services has been required indoors, such as offices, schools and homes. For effective air quality control, not only conventional temperature and humidity adjustment, but also removal and addition of gas (odor) are important. To control gas (odor) concentration with high precision, gas sensors are required. In this report, we propose a compact photoacoustic optical gas sensor utilizing resonant cell with frustum hole for indoor air quality control.

The first aim of the development of compact spectrometers was to bring the lab measurements to the field or into the process line. This has been accomplished in many industrial applications – optical characterization, pharmaceutics, biotechnology, chemistry etc. – by maintaining the required performance level. It was at the time when miniature spectrometers emerged (size <10000 cm³ in the study). A few years later, the same spectrometers also opened up new applications where spectroscopy had not been used before: precision farming, recycling, process control, etc. The range of potential applications became vast. The market started to be split into a wide variety of niche adoptions, each having its own requirements (performance, costs, design, operating conditions, …). This forced the development of products specific to each segment. At the same time, micro spectrometers were first presented (size between 10000 cm³ and 100 cm³), offering a similar performance to miniature devices but in a handheld, portable design. They enabled the launch of new systems for professional users. At the moment, it is still research and industrial optical characterization that possess the biggest shares within the compact spectrometer market. However, the better knowledge of end-users needs results in improving medium series applications (agriculture, environment) and also in developing further solutions for professionals (hair analyzers, textile identifiers, cannabis testers etc.). A turning point for the market is coming. It is the arrival of global leaders, both in the role of manufacturers (AMS Technologies, Osram, Viavi Solutions) and end users (Huawei, Samsung, Bosch, Henkel). Big players will drive the market towards consumer and biomedical applications (image enhancement, personal monitoring etc.). This is possible due to the emergence of chip size spectral sensors (<1 cm³).

The correct phase unwrapping in the presence of noise is a difficult problem and it has been of great interest since the last decades as observed in the literature. This is due to the spatial dependency that exists in traditional 2-D phase-unwrapping algorithms and, as a consequence, spreads errors in the reconstructed phase map. To solve this, algorithms known as temporal phase unwrapping (TPU) have been proposed; here the intensity of a fringe image sequence changes as a function of time, and therefore the elements of the phase-map are independent of each other. Thus, the 3-D measurement vision system by projection of fringe images presented here consists in capturing a sequence of images with sinusoidal fringes deformed by the height of the object. Then, the phase is processed by using a TPU technique based the one-dimensional Continuous Wavelet Transform (CWT). It should be noted that the use of CWT in one dimension is to analyze the intensity variations of the temporal sampled phase images, and the importance of this is in the detection of the frequency from which the phase is obtained as a linear function by means of an unwrapping in one dimension. We present simulated experiments and some real applications in digital archaeology and zoological morphometrics that validate the proposal.