Rapid substance screening is a vital yet difficult task. Different materials are best analysed by different techniques, and therefore a single tool may not always be able to identify an unknown substance. This can be addressed by a multimodal approach, simultaneously combining several orthogonal techniques into a single tool. Here we demonstrate a proof-of-concept for a solution based on three complementing techniques - infrared spectroscopy, ultraviolet fluorescence spectroscopy and microscopic imaging - for rapid acquisition of a rich dataset well suited for biochemical samples classification, with dedicated signal processing extracting the descriptive features and identifying the nature of the material.

As the world pivots away from hydrocarbon to hydrogen energy sources, new detection methodologies will be required to maintain safety. A critical factor in the safe use of hydrogen energy sources is access to low-cost, high-performance stand-off detection technology which can readily and autonomously detect hydrogen leaks. The tried-and-trusted path of absorption spectroscopy cannot be utilized with hydrogen due to the absence of optical absorption features for hydrogen. In addition to this, the difficulty in performing range-resolved absorption measurements, precludes the use of backscatter-absorption techniques for hydrogen detection. However, the significant Raman scattering cross-section for hydrogen can be exploited as a route to detection. This approach mandates the use of time-correlated single photon techniques and so confers significant advantage over absorption techniques: specifically, revealing the nature and position of the target substance. We therefore exploit hydrogen's Raman-scattering cross-section, together with state-of-the-art UV excitation laser and single-photon detection technology to realize a practical handheld system permitting sub-percent level measurements within a 3m range with similar to 1second integration times. In this paper, we will outline the need for this detection methodology; the challenges associated with realizing practical systems based upon it; and demonstrate our recently developed hand-held hydrogen sensing device.

Good manufacturing practice for medicinal products is laid down in several guidelines and Directives of the European Commission. Those regulations imply, among other aspects, that medicinal product manufacturers have to ensure that the final products are fit for their intended use and do not place patients at risk due to the inadequate safety, quality, or efficacy. For the case of manufacturing of pharmaceutical blisters, the attainment of this quality objective often leads to the resourcing of qualified personnel for final visual verification of the blister pack content. The need for inline content verification of pharmaceutical blisters asks therefore for sensors that provide fast, noncontact, and accurate chemical information of each individual blister content. Here, we report on a quantum cascade laser (QCL)-based blister-verification sensor. The verification principle is substance chemical identification by means of backscattering mid-infrared (IR) spectroscopy. The light source is a palm-size wavelength-tunable mid-IR QCL with $\sim $ 1-kHz tuning speed. The blister content verification uses machine vision to obtain the required position information for each individual content and fast spatial scanning facilitated by a two-axis galvanometer scanner. Diffuse reflectance mid-IR spectra are acquired at each location, and their classification is conducted instantaneously. Different classifier approaches are evaluated and discussed including machine learning and standard cross correlation to Fourier-transform-IR (FTIR) data. Altogether, this sensor is capable of scanning a standard 12-pill blister pack in $\sim $ 0.3 s, whereas this scanning time is essentially related to the desired classification accuracy, but not to the spectral resolution, which is fixed. Using machine learning classification, 100% identification accuracy is demonstrated for 13 different medication types (i.e., with different chemical nature), whereas only 97.4% identification accuracy is achieved by standard cross correlation to FTIR data. The used pills have all similar size, shape, and color, so that classification by visual inspection is barely possible.

We explore the modulation bandwidth, data communication capabilities and nanosecond pulsed performance of Ultraviolet-C (UV-C) AlGaN LEDs with peak emission wavelengths ranging from 235-275 nm. Data rates from 0.5 to 2 Gbps were achieved with -3dB modulation bandwidths up to ~100MHz.

Here, we report on the development of a diode-pumped single-frequency transition-metal-doped crystalline lasers designed in a miniature Fabry-Perot-type resonator by utilizing a narrow-bandwidth volume Bragg grating output coupler. Namely, single-longitudinal-mode operation was achieved from Ti:sapphire and Alexandrite lasers with a maximum output power of 570 mW and 275 mW near 813 nm and 780 nm, respectively. The mode-hop-free laser frequency tunability of up to 30 GHz was achieved by the cavity temperature and length variation with the Ti:sapphire system. The laser linewidth was measured to be in the 180 kHz range when locked to an external reference cavity transmission peak.

In this Letter, we report on the development of an ultra-compact single-frequency Ti:sapphire laser under direct diode pumping. Single-longitudinal-mode operation is realized from a miniature plane-parallel resonator using a volume Bragg grating as an output coupler. InGaN laser diodes operating at around 470 nm and 490 nm with a combined power of 6.7 W are used as an optical pump. A maximum output power of 700 mW is generated during single-frequency operation at 813.4 nm. A laser linewidth of 2.4 MHz is measured during free-running operation, which is reduced to about 180 kHz when the laser is locked to an external reference cavity.

Standoff scanning dual-comb spectroscopy of explosive materials is demonstrated with quantum cascade lasers at $\sim 8~\mu \text{m}$ . The proof-of-concept of the spectrometer, capable of the detection and identification of explosive materials at a distance of 3 m, has a detection limit of cyclotrimethylenetrinitramine (RDX) and pentaerythritol tetranitrate (PETN) on various surfaces of 5– $8~\mu \text{g}$ /cm2 in a scanning regime and 2– $3~\mu \text{g}$ /cm2 with stationary beam in reflection–absorption and backscattering modes.

A proof-of-concept of a scanning stand-off dual-comb spectrometer for explosives detection and identification at 3 m distance is demonstrated. Detection of two types of explosives: RDX and PETN on various surfaces was carried out in reflection-absorption and backscattering modes. A scanning area of 18 cm X 18 cm (400 pixels) was covered in ~2.5 sec. Identification method was based on Pearson’s correlation coefficients between the recorded reflection-absorption (backscatter reflection) spectra and transmission (reflection) FTIR of substances, with baseline subtraction using the asymmetric least square smoothing algorithm. Detection limits of the laboratory system of < 2 ug/cm2 were achieved.

The stand-off, range-resolved detection of hydrogen production rates is a valuable mechanism for the long-term condition monitoring of packages containing intermediate-level nuclear materials. To exploit this effect we have developed a long-range optical sensor system which uses Raman detection of hydrogen. Our need for operation over extended ranges (up to 100m) results in very low Raman signals. We therefore use time-correlated (with respect to the outgoing excitation laser pulse) and spectrally-resolved single-photon detection to ascertain molecular species, position and concentration as revealed by photon energy, arrival time and number, respectively.

The dual requirement for high spatial and substance specificity makes stand-off in-theatre biological detection of surface biological contaminants extremely challenging. We will describe a novel combined fluorescence multispectral imaging (MSI) and stand-off Raman approach which are united through their use of deep-UV (sub-250 nm excitation. This allows high-confidence location and classification of candidate contamination sites over the camera field of view, and subsequent resonance-Raman classification of these identified sites. Stand-off Raman is enabled through the use of a novel, extremely high-throughput Spatial Heterodyne spectrometer. The viability of this approach is confirmed through its use on application relevant biological simulant samples.

We report on an investigation addressing the challenge of the rapid detection of in-theatre surface chemical, biological and explosive (CBE) contaminants at a stand-off distance (<1m). The techniques we will describe are fundamentally underpinned by highly characteristic, molecule-specific Raman scattering. The implementation of Raman-at-range is problematic due to the extremely weak scattering cross-sections associated with this process, particularly when undertaken at the near-infrared wavelengths usually mandated by the need to suppress fluorescence. Excitation at shorter (near-UV) wavelengths can result in a two-order increase in scatter and this, combined with the extremely high throughput associated with Spatial Heterodyne Spectrometer (SHS) instrumentation, proves a viable route to Raman-at-range. We then implement time resolved spectral measurements on the ~100ps time scale to exploit the difference in generation timescale associated with Raman scatter and fluorescence generation; once so divorced the characteristics (both temporal and spectral) of the previously-troublesome fluorescent light can be embraced as an additional detection tool. We will show how SHS instrumentation, coupled with low-noise detector technology, can offer over four orders of magnitude improvement in spectral signal-to-noise level compared to conventional Czerny-Turner ‘slitted’ spectrometers using lower-cost linear CCD detectors. Finally, we show how a move to the deep-UV “Resonance-Raman” excitation region of sub- 250nm excitation leads both to enormous improvements in generated Raman signal, and spectral separation of the precious Raman from the troublesome fluorescence signal. We show the viability of this approach with biological spore simulant samples provided by DSTL.

Q-switched lasers operating at wavelengths around 2 µm have many applications including materials processing and LIDAR. However, the low gain of the quasi-three-level gain media available at 2 µm can lead to problems with pulse-to-pulse fluctuations in their output, known as jitter. Here we present a methodology for characterising the level of jitter in a Q-switched laser and apply it to a Tm:YAP system. We also look at the causes of jitter and evaluate some methods of reducing it. The methodology developed here will aid in the development and characterisation of Q-switched lasers at any wavelength.

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.

We present a spectroscopic technique based upon optical phase-fluctuation spectroscopy for very high levels of sensitivity and specificity with application for detecting the presence of concealed explosives by detection in the vapor phase. The approach enables recent advances in deep-infrared QCL spectroscopic sources to be utilised without the need for cooled detectors and gives multi-pass Herriott-type cell performance from a highly compact form factor. The system has been evaluated in the mid-infrared using a continuous-wave optical parametric oscillator as a spectroscopic excitation source, and Ethane as a sample molecule for detection. With this setup we have demonstrated the specificity of the device by being able to resolve characteristic spectral lines of the molecule of interest against other contaminants in the sample with similar spectral response, and a noise-equivalent sensitivity of 15ppb. Sensitivity is currently limited by ambient mechanical noise and routes to minimize this are considered.

Here we report our recent achievements towards a compact, portable, handheld device for contactless real-time detection and identification of explosives and hazardous substances via reflectance spectroscopy in the 7.5 mu m - 10 mu m spectral region. The mid-IR spectroscopic measurement principle relies on selective illumination of the target using broadly tunable external cavity quantum cascade lasers (EC-QCLs). A resonant micro-opto-electro-mechanical systems (MOEMS) grating enables fast wavelength tuning in the external cavity, allowing the full spectral scan to be completed in < 1 ms. The diffusely backscattered light's intensity dependence on illumination wavelength provides spectroscopic information to identify threat compounds via our spectral database, containing a large number of materials relevant in a security context. We present a handheld portable, albeit tethered, device capable of real-time identification of hazardous substances at a range of 1 m. We will outline future improvements to increase the system's usability, such as integrated computing power, automated focusing to that allow use over a range of detection distances and spatial scanning for background subtraction.

Abstract. We report on mid-IR spectroscopic measurements performed with rapidly tunable external cavity quantum cascade lasers (EC-QCLs). Fast wavelength tuning in the external cavity is realized by a microoptoelectromechanical systems (MOEMS) grating oscillating at a resonance frequency of about 1 kHz with a deflection amplitude of up to 10 deg. The entire spectral range of the broadband QCL can therefore be covered in just 500  μs, paving the way for real-time spectroscopy in the mid-IR region. In addition to its use in spectroscopic measurements conducted in backscattering and transmission geometry, the MOEMS-based laser source is characterized regarding pulse intensity noise, wavelength reproducibility, and spectral resolution.

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.

Laser absorption spectroscopy utilizes a tunable infrared source, providing the necessary selectivity, to detect the characteristic fingerprint spectral absorption of an abundant gas. In a simple embodiment such as single-pass absorption, sensitivity is limited as attenuation becomes minuscule for trace level concentrations; a problem exacerbated in the mid-infrared region due to significant detector noise. Sensitivity can be improved by increasing interaction between the optical field and molecular ensemble with methods such as a multiple-pass Herriot cell or resonant cavity ring-down spectroscopy but these techniques have a substantial overhead in instrumentation. An alternative approach to this problem is Phase Fluctuation Optical Heterodyne (PFLOH) spectroscopy. Here, interferometric effects are used to detect the minute heating of the sample gas when incident laser light of the appropriate wavelength is absorbed. More specifically, by placing the absorption chamber within one arm of a Mach-Zehnder interferometer, heat-induced changes in the optical path length can be detected with great sensitivity through the resulting fringe modulation. A secondary benefit is that although excitation occurs in the infrared, its effects can be detected using visible lasers and silicon detectors, thereby obviating the need for cooled, infrared detectors. We will present our results used to detect ethane using absorption in the 3.33-3.37 mu m region. The Mach-Zehnder interferometer used a Helium Neon laser for the probe laser, and a broadly tunable Optical Parametric Oscillator (OPO) for spectroscopic excitation. We have demonstrated detection levels at parts per billion with further sensitivity possible by implementing several identified improvements.

Measuring the linewidth of single-frequency lasers is challenging, and great care must be taken to understand the capabilities and limitations of different measurement techniques to obtain consistent results. A detailed comparative study will be presented.