Volume 6, Issue 2 - Photonics in Security Systems / Guest Editors: Hans-Dieter Tholl, Heinz-Wilhelm Hübers

The idea behind hyperspectral imagers (HSI) is to generate an image with hundreds of contiguous narrow channels, the so-called spectral bands. As each material has a specific spectral signature, robust detection and classification of specific materials is now achievable. Spectra can be characterized by narrow features in their signatures that broadband and multispectral cameras cannot resolve. As a result of technical progress, new HSI with higher spatial resolution and better signal-to-noise ratios have been developed. Additionally, it is possible to buy small HSI that weigh less than 1 kg, which opens up new applications in surveillance and monitoring with unmanned aerial systems (UAS). Despite the capabilities of hyperspectral data evaluation, HSI is applied to surprisingly few tasks. This is a result of the sheer amount of recorded data that needs to be analyzed and the complex data pre-processing when the sensors are not used in a controlled environment. Also, extensive research is required to find the most efficient solution for a given task. The goal of this letter is to introduce and compare the different sensor techniques, discuss potential use for applications in civil security and give an outlook of future challenges.

We present here an open-path analyser, initially intended for security applications, specifically for the detection of gas plumes from illicit improvised explosive device (IED) manufacturing. Subsequently, the analysers were adapted for methane measurement and used to investigate its applicability for leak detection in different scenarios (e.g. unconventional gas extraction sites). Preliminary results showed consistent measurements of gas plumes in the open path.

Biological hazardous substances such as certain fungi and bacteria represent a high risk for the broad public if fallen into wrong hands. Incidents based on bio-agents are commonly considered to have unpredictable and complex consequences for first responders and people. The impact of such an event can be minimized by an early and fast detection of hazards. The presented approach is based on optical standoff detection applying laser-induced fluorescence (LIF) on bacteria. The LIF bio-detector has been designed for outdoor operation at standoff distances from 20 m up to more than 100 m. The detector acquires LIF spectral data for two different excitation wavelengths (280 and 355 nm) which can be used to classify suspicious samples. A correlation analysis and spectral classification by a decision tree is used to discriminate between the measured samples. In order to demonstrate the capabilities of the system, suspensions of the low-risk and non-pathogenic bacteria Bacillus thuringiensis , Bacillus atrophaeus , Bacillus subtilis , Brevibacillus brevis , Micrococcus luteus , Oligella urethralis , Paenibacillus polymyxa and Escherichia coli (K12) have been investigated with the system, resulting in a discrimination accuracy of about 90%.

In this work we present data analysis algorithms for detection of hazardous substances in hyperspectral observations acquired using active mid-infrared (MIR) backscattering spectroscopy. We present a novel background extraction algorithm based on the adaptive target generation process proposed by Ren and Chang called the adaptive background generation process (ABGP) that generates a robust and physically meaningful set of background spectra for operation of the well-known adaptive matched subspace detection (AMSD) algorithm. It is shown that the resulting AMSD-ABGP detection algorithm competes well with other widely used detection algorithms. The method is demonstrated in measurement data obtained by two fundamentally different active MIR hyperspectral data acquisition devices. A hyperspectral image sensor applicable in static scenes takes a wavelength sequential approach to hyperspectral data acquisition, whereas a rapid wavelength-scanning single-element detector variant of the same principle uses spatial scanning to generate the hyperspectral observation. It is shown that the measurement timescale of the latter is sufficient for the application of the data analysis algorithms even in dynamic scenarios.

A high peak-power rapidly tuneable laser system in the long-wave infrared is obtained using an external cavity quantum-cascade laser (EC-QCL) broadly tuneable from 8 to 10 μm and an optical parametric amplifier (OPA) based on quasi phase-matching in orientation-patterned gallium arsenide (OP-GaAs). To provide an efficient amplification, the nonlinear crystal is pumped by a pulsed fiber laser source. With a pump laser source tuneable around 2 μm, quasi phase-matching remains satisfied with a fixed grating period in the OP-GaAs crystal when the EC-QCL wavelength is swept from 8 to 10 μm. The OPA demonstrates parametric amplification from 8 to 10 μm and achieves output peak powers up to 140 W, with spectral linewidths below 3.5 cm −1 and a beam profile quality (M 2 ) below 3.4 in both horizontal and vertical directions.

Abstract: Inertia-less optical scanners are an essential building block for many systems, including remote sensing, spectroscopy, and optronics. Although many solutions provide efficient scanning devices in the visible to near-infrared spectral range today, none of these devices offers good performances in longer wavelengths like in the mid-IR range. The new rationale that is described in this paper is to take advantage of existing and well-proven steering techniques in the near IR and to reach mid-IR by frequency conversion in a specifically designed Fourier transform optical parametric oscillator.

Adaptive holographic interferometry is a promising method for high-sensitivity phase-modulation measurements in the presence of slow perturbations from the environment. This technique is based on the use of a nonlinear recombining medium. We report the realization of an adaptive holographic interferometer relying on an optically addressed liquid crystal spatial light modulator operating at 1.55 μm. The beam-coupling process that occurs in a GaAs-liquid crystal device, allows obtaining a phase-modulation sensitivity of 200 μrad/sqrt (Hz) at 1 kHz. The interferometer behaves as an optical high-pass filter, with a cutoff frequency of approximately 10 Hz, thus, filtering slow-phase disturbances, such as due to temperature variations or low-frequency fluctuations, and keeping the detection linear without the need of heterodyne or active stabilization. Moreover, owing to the basic principle of holography, this technique can be used with complex wave fronts such as the speckled field reflected by a highly scattering surface or the optical field at the output of a multimode optical fiber. We demonstrate both theoretically and experimentally that using a multimode optical fiber as a sensing element, rather than a single-mode fiber, allows improving the interferometer phase sensitivity. Finally, we present a phase-optical time domain reflectometry (OTDR) optical fiber sensor using the adaptive holographic interferometer.

Ego localization is an important prerequisite for several scientific, commercial, and statutory tasks. Only by knowing one’s own position, can guidance be provided, inspections be executed, and autonomous vehicles be operated. Localization becomes challenging if satellite-based navigation systems are not available, or data quality is not sufficient. To overcome this problem, a team of the German Aerospace Center (DLR) developed a multi-sensor system based on the human head and its navigation sensors – the eyes and the vestibular system. This system is called integrated positioning system (IPS) and contains a stereo camera and an inertial measurement unit for determining an ego pose in six degrees of freedom in a local coordinate system. IPS is able to operate in real time and can be applied for indoor and outdoor scenarios without any external reference or prior knowledge. In this paper, the system and its key hardware and software components are introduced. The main issues during the development of such complex multi-sensor measurement systems are identified and discussed, and the performance of this technology is demonstrated. The developer team started from scratch and transfers this technology into a commercial product right now. The paper finishes with an outlook.

Chalcogenide glasses are emerging as important enabling materials for low-cost infrared imaging by virtue of their transparency in the key short-wave infrared (SWIR) to long-wave infrared (LWIR) bands and the ability to be mass produced and molded into near-net shape lenses. In this paper, we introduce a new family of chalcogenide glasses, which offer visible as well as infrared transmission and improved thermal and mechanical properties. These glasses are based on Ga 2 S 3 -La 2 S 3 with added Ga 2 Se 3 -up to complete the substitution of Ga 2 S 3 for Ga 2 Se 3 . The samples are prepared via the melt-quench method in an argon-purged atmosphere. All the studied compositions showed a lower glass transition temperature and an extended transmission window. Particularly, the LWIR transmission was extended from about 9 μm for gallium lanthanum sulfide (Ga-La-S) glass to about 15 μm for Se-added Ga-La-S retaining visible transmission from around 463 nm. The thermal and mechanical properties were investigated to prove the suitability of these novel materials for the production of optical components such as visible to LWIR lenses. Their suitability for drawing into optical fibers is also discussed.

Abstract: Optical analysis of integrated circuits (IC) is a powerful tool for analyzing security functions that are implemented in an IC. We present a photon emission microscope for picosecond imaging of hot carrier luminescence in ICs in the near-infrared spectral range from 900 to 1700 nm. It allows for a semi-invasive signal tracking in fully operational ICs on the gate or transistor level with a timing precision of approximately 6 ps. The capabilities of the microscope are demonstrated by imaging the operation of two ICs made by 180 and 60 nm process technology.