An ultra-compact particle size analyser using a CMOS image sensor and machine learning.

Light scattering is a fundamental property that can be exploited to create essential devices such as particle analysers. The most common particle size analyser relies on measuring the angle-dependent diffracted light from a sample illuminated by a laser beam. Compared to other non-light-based counterparts, such a laser diffraction scheme offers precision, but it does so at the expense of size, complexity and cost. In this paper, we introduce the concept of a new particle size analyser in a collimated beam configuration using a consumer electronic camera and machine learning. The key novelty is a small form factor angular spatial filter that allows for the collection of light scattered by the particles up to predefined discrete angles. The filter is combined with a light-emitting diode and a complementary metal-oxide-semiconductor image sensor array to acquire angularly resolved scattering images. From these images, a machine learning model predicts the volume median diameter of the particles. To validate the proposed device, glass beads with diameters ranging from 13 to 125 µm were measured in suspension at several concentrations. We were able to correct for multiple scattering effects and predict the particle size with mean absolute percentage errors of 5.09% and 2.5% for the cases without and with concentration as an input parameter, respectively. When only spherical particles were analysed, the former error was significantly reduced (0.72%). Given that it is compact (on the order of ten cm) and built with low-cost consumer electronics, the newly designed particle size analyser has significant potential for use outside a standard laboratory, for example, in online and in-line industrial process monitoring.

Fabry-Perot interferometers built by photonic crystal fiber pressurization during fusion splicing.

We report on a microscopic Fabry-Perot interferometer whose cavity is a bubble trapped inside an optical fiber. The microcavity is formed by pressuring a photonic crystal fiber (PCF) with large voids during fusion splicing with a conventional single-mode fiber. The technique allows achieving high repeatability and full control over the cavity size and shape. It was found that the size of the PCF voids contributes to control the cavity size independently of the pressure in the PCF. Our devices exhibit a record fringe contrast of 30 dB (visibility of 0.999) due to the ellipsoidal cavity whose surfaces compensate for the diffraction of the reflected beam. The strain sensitivity of the interferometers is higher when the cavities are ellipsoidal than when they are spherical.

Photonic crystal fiber sensor array based on modes overlapping.

An alternative method to build point and sensor array based on photonic crystal fibers (PCFs) is presented. A short length (in the 9-12 mm range) of properly selected index-guiding PCF is fusion spliced between conventional single mode fibers. By selective excitation and overlapping of specific modes in the PCF we make the transmission spectra of the sensors to exhibit a single and narrow notch. The notch position changes with external perturbation which allows sensing diverse parameters. The well-defined single notch, the extinction ratio exceeding 30 dB and the low overall insertion loss allow placing the sensors in series. This makes the implementation of sensor networks possible.

Embedded optical micro/nano-fibers for stable devices.

We present a technique to embed silica micro and nanofibers in low-index material (Teflon) using an inexpensive and straightforward fabrication process based on spin coating. The optical properties of the silica micro/nano-fibers have been investigated when they are bare or completely or partially embedded. Optical degradation occurs in bare fibers with diameters smaller than twice the wavelength of the guided light, thus making protection through embedding necessary. Our results also show that completely embedded fibers do not degrade over a long time, while partially embedded fibers can preserve the large evanescent waves without undergoing considerable degradation, which would be further reduced or even become negligible with functional overlayers. The results represent a step forward toward the development of durable and stable devices based on optical micro/nano fibers.

Thermally stabilized PCF-based sensor for temperature measurements up to 1000 degrees C.

We report on the development of a stable Photonic Crystal Fiber (PCF) based two-mode interferometric sensor for ultra-high temperature measurements (up to 1000 degrees C). The device consists of a stub of PCF spliced to standard optical fiber. In the splice regions, the voids of the PCF are fully collapsed, thus allowing the excitation and recombination of two core modes. The device spectrum exhibits sinusoidal interference pattern which shifts with temperature. We show that, despite being compact and robust, the proposed sensor head needs a quite long burn in (thermal annealing) to achieve an adequate and stable functionality level. The burn in process eliminates the residual stress in the fiber structure, which had been accumulated during the drawing phase, and changes the glass fictive temperature.

Photonic-crystal-fiber-enabled micro-Fabry-Perot interferometer.

We report on the fabrication of a monolithic fiber Fabry-Perot interferometer whose cavity is a microscopic air bubble. The latter is formed when splicing together a conventional single-mode fiber and an index-guiding photonic crystal fiber with the standard arc-discharge technique. Spherical microcavities with diameters ranging from 20 to 58 microm were fabricated with such a technique. The interferometers exhibited low thermal sensitivity (less than 1.0 pm/ degrees C), high mechanical strength, broad operation wavelength range, and fringe contrast in the 8-12 dB range. The applications of the interferometers for strain sensing (up to 5000 micro(epsilon) is demonstrated.

Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity.

We report an in-reflection photonic crystal fiber (PCF) interferometer which exhibits high sensitivity to different volatile organic compounds (VOCs), without the need of any permeable material. The interferometer is compact, robust, and consists of a stub of PCF spliced to standard optical fiber. In the splice the voids of the PCF are fully collapsed, thus allowing the excitation and recombination of two core modes. The device reflection spectrum exhibits sinusoidal interference pattern which shifts differently when the voids of the PCF are infiltrated with VOC molecules. The volume of voids responsible for the shift is less than 600 picoliters whereas the detectable levels are in the nanomole range.

An embedded optical nanowire loop resonator refractometric sensor.

A novel refractometric sensor based on an embedded optical nanowire loop resonator is presented. The device sensitivity has been studied in two typical configurations and its dependence on the nanowire diameter and coating thickness determined.

Ultra-low-loss optical fiber nanotapers.

Optical fiber tapers with a waist size larger than 1microm are commonplace in telecommunications and sensor applications. However the fabrication of low-loss optical fiber tapers with subwavelength diameters was previously thought to be impractical due to difficulties associated with control of the surface roughness and diameter uniformity. In this paper we show that very-long ultra-low-loss tapers can in fact be produced using a conventional fiber taper rig incorporating a simple burner configuration. For single-mode operation, the optical losses we achieve at 1.55microm are one order of magnitude lower than losses previously reported in the literature for tapers of a similar size. SEM images confirm excellent taper uniformity. We believe that these low-loss structures should pave the way to a whole range of fiber nanodevices.

Solid microstructured optical fiber.

An all-solid microstructured fiber based on two thermallymatched silicate glasses with a high index contrast has been fabricated for the first time. The microstructured cladding was shown to be essentially unchanged during fiber drawing. Fiber attenuation was measured as 5dB/m at 1.55m by the cutback method. High nonlinearity 230 W-1km-1 has been predicted and experimentally demonstrated in this fiber at 1.55m. In addition, modeling predicts that near-zero dispersion can be achieved between 1.5-1.6m in this class of high nonlinear fiber.

Effect of periodic background loss on grating spectra.

The effect of periodic loss on the performance of refractive-index gratings has been studied in detail. It is shown that the loss periodicity and relative phase strongly affects the symmetry of the reflection, transmission, and loss spectra. This asymmetry is explained successfully through consideration of the overlap between the standing-wave intensity distribution and the periodic loss pattern.