Enhanced photoacoustic spectroscopy sensitivity through intra-cavity OPO excitation

We report an optical molecular gas sensor exhibiting high levels of selectivity and sensitivity. The outstanding sensitivity demonstrated by our technology is rooted in a novel combination of photoacoustic spectroscopy (PAS) operated within the cavity of a continuous-wave, intra-cavity Optical Parametric Oscillator (OPO). We exploit the very high circulating field present within the resonant down-converted cavity as the excitation source of the photoacoustic effect, conferring orders-of-magnitude improvement in optical excitation power. Additionally, the wide selectivity of the system arises from the inherent broad tunability and narrow optical linewidth of an OPO. Here we report the use of this technology for the detection of ammonia (NH3) as a simulant target molecule. A 3-D printed miniature PAS cell with microelectromechanical systems based (MEMS) microphone is used for the gas detection. The resonance frequency of the cell was measured at 17.9 kHz with a Q-factor of 9. The down-converted signal wave resonating within its optical cavity was tuned to 6605.6cm-1 (corresponding to a strong local NH3 absorption line) through a combination of phase matching and intra-cavity etalon control. The laser was amplitude modulated at the resonance frequency of the PAS cell, producing an average optical excitation power of ~10W in the signal arm of the OPO, to induce the photoacoustic effect for only 4W of primary diode pump power. In this work we show detection limit at the level of single parts-per-billion (ppb). Additionally, we will discuss how this technology could be readily refined to potentially demonstrate a sensitivity of tens parts-per-quadrillion

Classification for hyperspectral imaging

Hyperspectral Imaging is a method of collecting and processing the information across pre-defined electromagnetic spectrum. These measurements make it possible to derive a continuous spectrum for each pixel of the image. After necessary adjustments these image spectra can be compared with database of reflectance spectra in order to recognise tested materials. This project is conducted in cooperation between Fraunhofer Centre for Applied Photonics and Heriot-Watt Industrial Doctorate Centre in Photonics and Optics Technologies in partnership with University of Strathclyde. Fraunhofer Institute is known of world-class photonics solutions and this project aims in enhancement of one of their Hyperspectral Imaging systems with signal processing techniques. Set of classification procedures would be applied for the output of imaging spectrometer with the intention of spatial and spectral classification of objects captured by the spectrometer. Spatial classification is based on Support Vector Machine (SVM) classifier. Use of texture features of the objects is considered as a base for labelling of detected items. Spectral classification is based on Partial Least Squares (PLS) method. With database of calibration reflectance spectra, method this can be used for prediction of “end members” concentration and therefore identification of the objects captured on the hyperspectral image.

Remote oil spill detection and monitoring on ice-covered waters

The spillage of oil in Polar Regions is particularly serious due to the threat to the environment and the difficulties in detecting and tracking the full extent of the oil seepage beneath the sea ice. Development of fast and reliable sensing techniques is highly desirable. In this paper hyperspectral imaging is proposed as a potential tool to detect the presence of oil beneath the sea ice. A feasibility study project was initiated to explore the detectability of the oil under ice layer. Some preliminary results obtained during this project are discussed

Widely-tunable mid-infrared ring cavity pump-enhanced OPO and application in photo-thermal interferometric trace ethane detection

Funding: Innovate UK (133076); Engineering and Physical Sciences Research Council (EP//L01596X/1, EP/M01326X/1, EP/M024385/1); European Research Council (ERC-2018-STG 803665).The development of a broadly and accurately tunable single-frequency mid-infrared laser source and its application to a sensitive laser absorption detection method are described. Photo-thermal interferometric spectroscopy is employed as a phase-sensitive method to detect the minute refractive index change caused by the heating of a gas under laser radiation. A separate probe beam allows for the spectrally-interesting mid-infrared region to be examined whilst utilizing low cost, high detectivity photodetectors in the visible/near-infrared region. We also describe the implementation of a Sagnac interferometer to minimize the effects of environmental perturbation and provide inherent passive stability. A continuous-wave ring-cavity pump-enhanced OPO has been developed to provide excitation light from 3–4 µm at 140 mW with the ability to mode-hop tune continuously over 90 cm−1 in 0.07 cm−1 steps. Complementary use of both detection apparatus and excitation source has allowed for presence of ethane to be detected down to 200 parts per billion.developed to provide excitation light from 3–4 µm at 140 mW with the ability to mode-hop tune continuously over 90 cm−1 in 0.07 cm−1 steps. Complementary use of both detection apparatus and excitation source has allowed for presence of ethane to be detected down to 200 parts per billion.Publisher PDFPeer reviewe

Mid-infrared photonic crystal waveguides in SOI

We demonstrate the design, fabrication and characterization of mid-infrared photonic crystal waveguides on a silicon-on-insulator platform, showing guided modes in the wavelength regime between 2.9 and 3.9 μm.</p

Mid-infrared photonic crystal waveguides in silicon

We demonstrate the design, fabrication and characterization of mid-infrared photonic crystal waveguides on a silicon-on-insulator platform, showing guided modes in the wavelength regime between 2.9 and 3.9 µm. The characterization is performed with a proprietary intra-cavity Optical Parametric Oscillator in a free space optical setup and with a fibre coupled setup using a commercial Quantum Cascade Laser. We discuss the use of an integrated Mach-Zehnder interferometer for dispersion measurements and report a measured group velocity of up to a value of ng = 12, and determine the propagation loss to be 20 dB/cm.<br/