Lasers for Safety and Security

Safety and security is one of the key areas of attention for the European Commission. To show how laser science and technology can contribute to a safer world, in this focus section we give some examples of recent safety and security related initiatives within Laserlab-Europe. In 2011, in a transnational access project conducted at LaserLaB Amsterdam, Spanish forensic experts were for the first time able to detect the presence of DNT, a material found in many explosive materials, through layers of non-transparent plastics.  In 2014, CLF spinout Cobalt Light Systems received the MacRobert Award, the UK’s most prestigious engineering prize, for their application of Spatially Offset Raman Spectroscopy (SORS) in an airport security scanner that allows airports to remove the existing hand-luggage liquid ban. The technique identifies explosive threat materials inside containers in seconds without opening them. More recently, IOE-MUT (Warsaw, Poland) created a multifunctional lidar system for stand-off detection of biological clouds at distances up to several kilometres, which can be used as an early warning device for biological warfare.
More generally, several partners of Laserlab-Europe have developed laser-based detection techniques that can be used to distinguish between toxic and non-toxic varieties of the same molecule, for example, and to measure pollution levels that can be considered a threat to our health and as such a safety risk as well. Finally, we should also mention the many laser techniques applied to biomedical cases that have been highlighted in Laserlab Forum over the years. Those, in a way, can also be seen as concerning issues of safety, especially when these techniques reduce the risk of dying of life-threatening diseases like cancer.

Detection of hidden objects by X- ray imaging using a laser-generated electron beam

Left: A diagram of the set-up of the array of
test objects. Right:
Example of an X-ray
backscatter image of the object array shown
left. An array of objects including 38 mm thick
aluminium and
0.14 m thick insulation foam
are shown. For more details see:
R. Deas et al., J. X-ray Science and Technology 23,
791-7, 2015.

For detection of objects through barriers such as items smuggled in a container crate or buried landmines, standard X-ray detection techniques are inadequate. In collaboration with the UK’s Defence Science and Technology Laboratory, scientists from Laserlab-Europe partner the Central Laser Facility (CLF) have developed and demonstrated a new approach to probe for hidden objects or items surrounded by sand or soil.

X-ray backscatter imaging is currently used in a range of technologies, from portal security, where it is used to scan airline passengers, vehicles and containers, to industrial inspection, studying the internal structure of low density materials, and applications requiring single sided imaging. Currently, the application of this technique to the detection of landmines is limited due to the surrounding sand or soil strongly attenuating the 10s to 100s of keV X-rays required for backscatter imaging.

In collaboration with the UK’s Defence Science and Technology Laboratory the CLF have developed and demonstrated a new approach using a high energy 140 MeV short-pulse (< 100 fs) electron beam, generated by laser-driven acceleration, to probe the sample. High en-ergy electrons are able to penetrate to greater depths in a sample; these electrons will then produce X-rays via bremsstrahlung emission, which then backscatter and travel back through the sample before being detected. The backscattered X-ray pulses coming from deeper within the sample will take longer to reach the detectors, therefore a depth profile can be formed. Scanning across the sample allows one to generate a full 3D like image.

An experiment carried out using the Gemini laser system generated the electron beam by focusing the laser pulse in a supersonic gas jet. A variety of detector and scintillator configurations were used to measure the backscattered X-ray pulses coming from various depths within the sample, with the main challenge being the capability of the detectors to resolve pulses that hit the detector, only billionths of a second apart. Despite this ex- treme challenge, an X-ray backscatter image of an array of different density and atomic number items was demonstrated and is the first time a backscatter image with depth information has been acquired using a laser-driven electron beam to generate X-ray emission in the imaging target itself.

Although this research is in its very early stages, it is hoped that it will ultimately lead to a deployable system that can be used to help detect buried or hidden objects such as landmines or contraband.
David Neely (CLF)

Laser lightning protection

French Laserlab-Europe partner LOA will lead a new FET-OPEN programme called Laser Lightning Rod, aimed at developing a new type of lightning protection. The goal of Laser Lightning Rod is to investigate and develop a new type of lightning protection based on the use of upward lightning discharges initiated by a high-repetition-rate, multi-terawatt laser.

The feasibility of the novel technique is based on recent research providing new insights into the mechanism responsible for the guiding of electrical discharges by laser filaments, as well as on cutting-edge high-power laser technology, and the availability of the uniquely suitable Säntis lightning measurement station in Northeastern Switzerland, located at an altitude of 2500 metres. Because of the optical Kerr effect, a terawatt ultra- short laser pulse propagating in air will self-organise into thin light channels called filaments. This process results in long-range propagation of a pulse with multi GW/cm 2 peak intensity. Due to ionisation, a plasma track and a low-density channel are left in the wake of the pulse.

Such long-lived low density channels form a preferential path for lightning precursors, as has been demonstrated in laboratory experiments where guiding of electric discharges has been obtained over distances of 4 metres. Using a powerful kHz laser in conjunction with a new type of focusing system should allow the formation of a long and permanent low-density channel able to initiate upward lightning discharges in real conditions.

LOA will collaborate with Swiss institutions of higher education Université de Genève, École polytechnique fédérale de Lausanne (EPFL), and Haute Ecole Spécilisée de Suisse occidentale (Hesso), as well as laser company Trumpf Scientific Lasers, and aircraft manufacturer Airbus GI. The Laser Lightning Rod programme has a budget of 3.9 million euros.
Aurélien Houard (LOA)

In the Laser Lightning Rod project, upward lightning flashes will be initiated by a high-repetition-rate,
multi-terawatt laser, directing
lightning away from vulnerable objects.

Laser speed gun velocimeter with integrated camera

’Rapid Laser’ handheld speed gun prototype

Since Poland joined the EU, the country has seen a rapid increase in the number of modern, double belt highways. This, together with increasing demand among drivers for indisputable prove of their traffic offense, led to an urgent need for new methods of measuring speed of the vehicles. Accordingly, the Institute of Optoelectronics (IOE) of the Military University of Technology (MUT) in Warsaw developed a handheld laser speed gun velocimeter for law enforcement on public roads.

Widely used Doppler radar devices are becoming obsolete because they do not allow precise beam aiming. The answer to that problem is a pulsed laser diode. Combined with the right optics it can generate a narrow light beam with milliradian divergence, sufficient for accurate targeting. As a result of joint work of IOE MUT and the ZURAD company from Ostrów Mazowiecka, a prototype named ‘Rapid Laser’ was built based on lidar technology. IOE took the role of R&D department and ZURAD – the future manufac- turer of the device – contributed its experience in the law enforcement market.

The presented device is based on a 905 nm semiconductor pulsed laser diode. The energy of the emitted pulse is 0.5 mJ, an upper limit for class 1 laser products operating within a pulse width range of one to one hundred nanoseconds. The device emits a series of pulses in order to increase the accuracy of the measurement. Receiving and transmitting optics are based on a single aspheric lens which creates a 3 mrad divergence of the transmitted beam and a 4 mrad field of view for the receiver.

The device is meant to measure the speed of vehicles traveling within a range of 1250 metres. It can also measure the distance to a vehicle, record photos and videos, and is immune to laser jammers and other laser devices aiming for the same vehicle. It has features like a ‘distance gate’ when measurement results are marked and classified only when the vehicle is located in the specified range away from the device.

Aiming unambiguity is achieved by displaying the targeting point in the live video image. The device has two displays: a high-resolution touch display serving as data output and user interface, and a near-eye display meant to ease aiming. Performance of the device was proven in field trials against laser jammers available on the Polish market, and against crosstalk between various laser devices.
Michal Muzal (IOE MUT) 

Terahertz imaging of polyethylene bullet protection plates

To improve ballistic protection plates, detailed study of the material after impact is essential. At the Institute of Optoelectronics (IOE) of the Military University of Technology (MUT, Warsaw), laser-generated terahertz radiation is used to image the effect of the impact of a bullet on polyethylene fibre plates.

Polyethylene composites, which are manufactured as plates with a thickness of approximately 10-20 mm, are used as material for the ballistic protection of vehicles, helmets and bulletproof vests. They consist of a number of about 50 mm thick layers of fibres made of ultra-high-molecular-weight polyethylene (UHMWPE). The fibres are arranged in successive layers perpendicular to each other. During interaction of a projectile with the structure, a chamber is created and the composite will delaminate. Knowing the location, size and thickness of these delaminations is essential to determine the quality of the material and further research. Terahertz radiation in the range of 0.1-3 THz perfectly passes through the polyethylene structures and facilitates their accurate analysis and threedimensional visualisation.

To measure the sample, we used a time domain sys- tem (TDS) operating in the reflection configuration. In the TDS setup, a femtosecond laser and two photoconductive antennas are used to generate and detect a short pulse of electromagnetic radiation lasting about 1 ps. A THz pulse propagating in a multilayer structure encounters the interfaces between the layers with different refractive index and is partially reflected. The remaining part propagates further in the structure. As a result, one obtains a series of pulses delayed relative to each other by twice the propagation time in the individual layers. Scanning the sample point by point allows reconstruction of the internal structure and the position of delaminations.

Vertical and horizontal cross sections of the structure present the inlet channel of the projectile, the centrally located chamber, radially spreading delaminations, and other features. By means of signal processing of ambiguous THz waveforms reflected from the interior of the sample, we can unambiguously determine the distribution of delamination and size of the chamber.
Norbert Palka (IOE MUT)

THz B-scan of the sample in reflection along axes: PeO (left part) and GeO (right part). The scan was performed
from the front side of the sample. The image is saturated to emphasise delaminations.
Reproduced from N. Palka et al., Composites Part B 92 (2016) 315-325

Remote LIBS for assessing the safety of high-voltage outdoor insulators


Laser-induced plasma emission is collected through a telescope,
placed at a distance of almost 10 metres away from the target
insulator. An optical fibre is aligned at the focus of the telescope
and transmits the plasma light into the spectrometer. The inset
shows two typical spectra obtained upon irradiation of stock and
old (field) insulators (Nd:YAG laser; λ=1064 nm, τ = 10 ns).
courtesy of IESL-FORTH and HEDNO SA.

Insulators are crucial for the reliable performance and safety of high-voltage (HV) systems. In the project POLYDIAGNO, scientists from IESL-FORTH (Crete), together with Greek partners, developed a laser diagnostics system which can be used to remotely assess the condition of outdoor high-voltage insulators, thereby reducing the risk of electrocution or fire.

Polymer-based composite HV outdoor insulators have been introduced in HV power systems technology more than thirty years ago, as an alternative to conventional insulators, made of porcelain or glass, due to their excellent long-term performance and reduced installation and maintenance costs. They consist of a fibre-reinforced epoxy rod, which is covered with an elastomeric housing. In particular, silicone rubber insulators are commonly used in overhead transmission lines of most electric power distribution stations due to their unique properties, such as low weight, high heat resistance, chemical stability and long-term hydrophobicity.

However, these insulators are subject to ageing and/ or degradation processes, which are often attributed to prolonged exposure to unfavourable environmental conditions and/or the occurrence of electric discharges. Contamination of salts, dust, moisture and biological growth on the insulator’s surface contribute to the formation of conductive layers, which serve as ideal paths for leakage current flow, which, under certain conditions, may cause a complete flashover.

In order to avoid such complications and secure the high efficiency of an electrical energy system, it is important to develop suitable non-destructive diagnostic techniques, which would enable the remote and real time evaluation of the insulator’s performance, without detaching them from the network. To this end, Laser-Induced Breakdown Spectroscopy (LIBS) has been found to be promising as a field-deployable technique for the efficient and reliable assessment of the operational state of HV outdoor insulators in service.

Remote LIBS measurements were performed successfully on site, at the TALOS High Voltage Test Station in Heraklion, Crete, a unique facility dedicated to the research of outdoor insulator systems operated by HEDNO (the Hellenic Electricity Distribution Network Operator).

In brief, focusing a high-intensity pulsed laser beam on the insulator’s surface, from a distance of over 5 metres, results in plasma formation. Plasma light is collected via a telescope, which is approximately 10 metres away, and is transferred into the spectrometer for analysis and recording of the spectrum, which provides information on the elemental composition of the silicone rubber housing.

Correlation of Si, C and CN emission intensity ratios obtained from field insulators with reference intensity ratios, corresponding to unused (stock) insulators, shows that their values differ systematically reflecting the extent of chemical modifications, induced to the polymeric housing of the insulators, as a result of ageing and/or structural deterioration. Therefore, using the emission peak intensity ratios as spectral indicators was found to be satisfactory for remotely evaluating the operational quality of silicone rub- ber insulators.

Based on the value of the percentual difference of the spectral indicators between field and stock insulators, a straightforward classification of the operational quality of the insulators can be made. Likewise, by use of proper spectral parameters, also extracted from LIBS spectra, it has become possible to differentiate different types of synthetic insulators as well as to detect environmental deposits on their surface.
Costas Kalpouzos and Demetrios Anglos (IESL-FORTH)

Detection of concealed explosives

Time-resolved Raman spectroscopy setup; a two layer sample is
excited with a 3-ps frequency-doubled Ti:sapphire laser at 460 nm;
Raman photons are detected in a backscatter geometry.
Photo: Maria Lopez-Lopez

Using a technique called time-resolved Raman spectroscopy, researchers at LaserLaB Amsterdam have been able to detect the presence of DNT (dinitrotoluene), a material found in many explosive materials, through layers of nontransparent plastics.

Detection and identification of explosives and their associated compounds in different environments is a problem of critical interest for security and forensic diagnostics. Many techniques have been investigated for this purpose, but the majority are not ideal for explosives detection in that they are invasive or require lengthy sample preparation. Raman spectroscopy is ideal for the rapid detection of potentially hazardous substances because it is non-invasive and provides a 'molecular fingerprint' that facilitates chemical identification. Applicability of conventional Raman spectroscopy for investigation of packed compounds is limited, however, as light from the surface layers tends to obscure Raman photons coming from inside the package. In time-resolved Raman spectroscopy, short laser pulses are sent into the object of interest. By carefully choosing the time window in which the returning photons are detected, one can discriminate between photons coming from the surface (the vast majority) and the photons emanating from inside. Thus, using a delay of several hundred picoseconds, only the retarded photons are detected – providing chemical information on the content of the package. In a project made possible through the Laserlab-Europe Access Programme, the LaserLaB Amsterdam team, in collaboration with Spanish forensic experts from the University of Acalá, has been able to detect the chemical compound DNT through layers of different diffusely scattering white plastic materials of various thicknesses, including typical non-transparent packaging containers. The association of DNT’s with some hazardous and explosive materials makes them of particular relevance for security. For example, they are used in the munition industry as a modifier for smokeless powders, as a plasticizer in propellants, and are products of degradation and impurities in the synthesis of the explosive TNT. Another application area of time-resolved Raman spectroscopy that the LaserLaB Amsterdam team is currently working on is non-invasive disease diagnosis through skin. Reference: I.E. Iping Petterson et al., Analytical Chemistry 2011 83 (22), 8517-8523 (2011)