Contents Introduction
Items containing suspected dangerous substances, which are often hidden in everyday objects, such as luggage or boxes in public places, pose a significant security risk. They can contain explosives as well as explosive mechanisms that may be enriched with chemical, biological, radiological, nuclear, and explosive CBRNe elements. Conventional methods for assessing and neutralizing these threats often require direct physical contact, which is associated with risks. To mitigate these risks, remote handling technologies are favored that prioritize security while attempting to preserve forensic evidence. Many systems have already been developed to capture images of the shape and content of suspicious objects, such as X-ray-based technologies [1], [2], [3], [4], [5], [6], [7].
One of the most common techniques currently used to open suspicious items is manual opening by explosive ordnance disposal (EOD) technicians protected by bomb disposal suits [8], [9], [10]. Instead of opening suspicious objects manually by an EOD technician, mechanical devices are often used. Water jet systems are used for bomb disposal, where pressurized liquids are used to cut materials suitable for cutting large-caliber unexploded ordnance. The disadvantage of this technology can be the destruction of evidence inside the package or even the release of CBRNe substances acting on the object by mechanical forces [11], [12], [13], [14].
Laser processing methods for EOD applications have also been reported [15], [16]. These methods were developed to neutralize unexploded ordnance, but are not suitable for safely opening an improvised explosive device IED, especially for complex objects made of different materials to access their contents. In general, no laser cutting methods have been found in research that have extracted substances from laser processing. However, the device to develop is important for opening objects with an IED to prevent laser energy coupling in the next layer.
This study makes notable advancements in the field of explosive device neutralization and forensic evidence preservation through the development of a mobile laser cutting system (LCS) integrated with a novel sensor-based breakthrough detection technology. The research systematically establishes optimal laser parameters for cutting diverse materials, ensuring precise control to avoid unintended damage to underlying layers or ignition of explosive charges. Experimental validation demonstrates the system’s capability to achieve accurate breakthrough detection with a reliability exceeding 90%, utilizing acoustic and optical sensor data. Furthermore, the system is shown to preserve forensic integrity by enabling the detection of explosive residues as low as 10 ng following the cutting process. The LCS’s modular and portable design further enhances its applicability, allowing for safe, remote-controlled operation, thus providing a robust tool for EOD applications. These contributions collectively advance the state of the art in the safe handling and processing of hazardous materials under real-world conditions.
Laser machining is an advanced and highly precise material processing technique that utilizes focused laser beams to remove material through melting, vaporization, or ablation [17]. This method is favored in industries requiring intricate detailing and minimal thermal impact on the surrounding material. Pulsed lasers, such as Nd:YAG and CO2 lasers, are particularly advantageous due to their ability to deliver high peak power in short bursts, which reduces the heat-affected zone and minimizes thermal damage [17], [18], [19]. Due to these properties, pulsed lasers are used for micromachining, drilling, and cutting complex profiles in materials, such as ceramics, metals, and polymers. For example, in the laser drilling process described in [20], a pulsed laser is used to create high-precision boreholes, where the short pulse duration allows for controlled energy input and efficient material removal. Additionally, innovations, such as the use of a jet nozzle to remove molten material during the laser drilling process, further enhance the quality of machining by preventing recasting and ensuring clean cuts [21]. These advantages have led to the widespread adoption of pulsed laser machining in various high-precision applications, including aerospace and medical device manufacturing [21], [22], [23].
In laser machining, real-time process monitoring is essential for maintaining quality and precision, especially in applications where tolerances are tight and material integrity is crucial [24]. Sensory monitoring systems are integrated into laser machining setups to track key parameters, such as laser power, focus position, and material removal rates. For instance, the use of acoustic sensors to monitor the noise generated during laser ablation provides critical feedback that can be used to adjust the process in real time, ensuring consistent quality and preventing defects. In the study detailed in the investigation on acoustic monitoring, it was found that analyzing the acoustic signals produced during laser ablation can effectively differentiate between different material layers, such as distinguishing between necrotic and vital tissue in medical applications [21], [25]. Moreover, the integration of advanced sensor systems, as outlined in [22], allows for precise control over the laser machining process, improving safety and efficiency by enabling automated adjustments to the cutting parameters based on real-time feedback [26]. These developments underscore the importance of sensory monitoring in enhancing the capabilities of laser machining systems, ensuring high precision and safety across various industrial and medical applications [20], [24], [26].
This work addresses these challenges by developing a LCS capable of remotely opening IED-containing containers without destroying a potential explosive charge or similar. The following steps have to be carried out to make a cut on suspected objects. In the first step, an X-ray image of the suspected object (e.g., suitcase) is acquired to identify the dangerous load in the object. The LCS is then placed on the object and a test cut is made at a nonhazardous location to feed process data to the system preparing the aimed cutting. After this test cut and checking all the necessary parameters, the LCS is placed in the correct position and the outer material (here: suitcase) is cut. Once the material has been removed, the IED can be treated appropriately using the LCS (defusing or sampling) or other equipment. The required system must integrate advanced sensor technology that monitors and controls the cutting process in real time. This technology must ensure precise thermal management to minimize the risk of accidental ignition or detonation of the explosives inside. The laser system must be designed so that it can be mounted to robotic manipulators or modular positioning systems. This allows defusing teams to work from a safe distance and minimize the risk to personnel. This article presents the development of the sensory monitoring of laser cutting.
Experimental
A. Setup
At the beginning of the research work, an attempt was made to carry out initial cutting tests in order to generate specifications for the laser system. Initial tests were also carried out on enclosed and open samples containing explosives. Fig. 1 shows the setup used for the first series of measurements (in the following LS I).
Schematic setup of the laser system used for preliminary experiments: 1. laser, 2. parabolic mirror, 3. optical fiber, 4. spectrometer, 5. lens, 6. xy table (z manual), 7. sample chamber, 8. sample holder, 9. sample, 10. exhaust air, 11. physical sensors, 12. chemical sensors, 13. beam splitter with breakthrough diode, 14. power meter, and 15. solar cell.
The pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was initially used for the investigations into laser cutting and performance parameters. This laser emits nanosecond pulses at three different wavelengths (1064, 532, and 355nm) and offers the possibility of infinitely variable regulation of individual pulse energies. The Nd:YAG-laser has a maximum output power of 3W at 532nm and a pulse frequency of 2000 Hz. Each burst consists of exactly ten pulses with a single-pulse energy of 1.5 mJ, with each pulse having a duration of 10ns.
To refine and concentrate the optical power, a converging lens with a focal length of 250 mm is integrated into the system. Additionally, a polarization filter is used to precisely adjust the laser power. The power was monitored using a power meter, and power fluctuations during adjustment were around ± 1 mW. These meticulous adjustments ensured the precision and safety necessary for processing the relevant explosive materials.
The experimental series included investigations into the influence of laser fluence on the machinability and penetration duration when cutting various predefined, practically relevant materials. The cutting process was realized by repeatedly moving the sample being hit by the laser beam on a defined linear trajectory (Fig. 2).
Stationary laser system (LS I): a. xy-table, b. sample, c. focus point, d. laser beam, e. focusing lens, f. shutter, and g. laser system.
After determining the relevant parameters, a new LCS was used (in the following LS II). After merging it with the sensors and integrating it into the overall system. The structure looked as shown in Fig. 3.
Sketch of the final mobile LCS: 1. support and positioning system in the form of a mobile stand with swiveling spring arm device, 2. laser system, and 3. sensor tube.
This system allowed a flexible positioning of the laser beam via a galvanometer scanner as well as precise focus tracking and focusing to change the laser spot area using a motorized focus control. The system had a maximum pulse energy of 5 mJ for a wavelength of 515nm. Additionally, it was possible to use the wavelength of 1030nm after replacing wavelength-dependent optical elements.
At this stage, the adjustment of the laser pulse energy and the position of the focal plane was still done manually or with the help of preliminary operator software. The integration of a spectrometer to capture radiation in the UV and visible wavelength ranges was achieved by coupling it via an optical fiber with the optical setup and beam path of LS II. This setup allowed for the detection of reflected and emitted light from the processing zone. Fig. 4 shows the interior of the laser head, featuring a motorized power adjustment (1), motorized focus tracking (2), a laser switch/shutter (3), a fiber connection to the spectrometer (4), a camera with a filter for topography scanning and positioning aid (5), an f-theta lens (6), a galvanoscanner (7), and a laser diode (laser class 2) (8). Throughout the project, and in preparation for hardware integration, additional components were added or replaced to facilitate easier handling and control of LS II.
Interior of the laser system: 1. motorized power adjustment, 2. motorized focus tracking, 3. laser switch/shutter, 4. fiber connection to spectrometer, 5. camera + filter for topography scan and positioning aid, 6. f-theta lens, 7. galvanoscanner, and 8. laser diode (laser class 2).
1) Topography Scan
The new LCS was also equipped with a topography scan unit. This system operates by projecting a laser (safe for the human eye) onto the object’s surface, while the reflected light is captured. By analyzing the distortions in the projected pattern, the system calculates the precise 3-D geometry of the surface, creating a detailed topographical map. The focus point can then be adjusted automatically to follow the surface contour based on the acquired data.
2) Explosives
A specially designed sample holder was used for measurements with explosives. The explosive, in this test series triacetone triperoxide (TATP), was fixed in a cylindrical plastic container, which was placed inside a metal cylinder. The reason for selecting TATP was to test an explosive that was as sensitive as possible and hazardous (typical terrorist explosive) in order to identify critical parameters for processing. The sample holder was covered with a plastic plate (5) to simulate the packaging of an IED. The material to be processed was placed so that it was directly exposed to the laser beam (Fig. 5). The figure shows a schematic representation of the sample holder used for measurements on explosives, consisting of a bracket (1), a sleeve (2), the explosive (3), a PU hose (4), a polystyrene (PS) cover (5), and cardboard (6).
Schematic representation of the sample holder for measurements on explosives: 1. bracket, 2. sleeve, 3. explosive, 4. PU hose, 5. PS cover, and 6. cardboard.
B. Sensors
Sensory monitoring in laser processing comprises various optical and acoustic sensors. These have been integrated into the LSS to monitor and visualize the cutting process. By evaluating the sensor responses, online cutting monitoring is implemented to optimize the cutting process. The entire sensor system is mounted in a tube (Fig. 6). This tube shields the cutting machine from stray light and blocks interfering light from external sources. Furthermore, the tube enables the extraction of reaction gases. This protects the environment and allows for the analysis of the extracted gases. The extraction system, including the gas analysis setup, is shown in orange. At the top of the tube is the sensor ring where the sensors are mounted (Fig. 7). The sensors used are listed below.
Two microphones (4) for acoustic evaluation. During laser processing experiments, acoustic monitoring was conducted with two MEMS microphones (ELV MEMS1). The intensity levels allow conclusions to be drawn about the processing intensity. The microphones were sampled at 1000 Hz.
Broadband and UV photodiodes (2) for detecting emitted and scattered light. These diodes were purchased from Roithner LaserTechnik GmbH. Some were used for the UV range (330–445 nm), the laser wavelength (515nm), and the visible range. UV diodes are particularly useful for monitoring plasma generated during the processing of some materials. The diodes were sampled at 1000 Hz.
Metal oxide gas sensors (not shown) for evaluating changes in gas composition. Semiconductor gas sensors from UST Umweltsensortechnik GmbH (1000_series and 3000_series) were used to monitor environmental (reference) and process gases. The 1000_series is sensitive to hydrocarbons, hydrogen, and alcohols, while the 3000_series is sensitive only to hydrocarbons and alcohols. One set of sensors was mounted outside the tube to ensure no reaction gases escaped, while another set was placed in the exhaust stream to detect gases from the processed material.
A spectrometer (not discussed in this publication) from Ocean Optics (HR series) for analyzing emitted radiation in the UV/VIS spectral range (220–1100nm). Inline measurements were performed, and the backscattered light was decoupled using a beam splitter (Fig. 4). The spectrometer provides information directly from the processing spot.
An infrared camera (1) to assess heat development in the cutting zone. Heat monitoring is critical due to the material’s thermal conductivity. The IR camera (optris Xi series, 8000–14000nm, 50fps) monitored the temperature development along the cutting path. The data focused on the hottest point, corresponding to the machining location, enabling real-time process monitoring.
Interior view of the tube with a view of the sensor ring, the mentioned sensors are selected as examples: 1. IR camera, 2. UV-diode, 3. f-theta lens of the LCS, and 4. microphone.
Results
The experiments and results obtained are presented below. At this point, it is important to consider possible LCSs and sensors as well as the necessary interfaces for realization in a mobile LCS, taking into account the current state of science and technology as well as laser safety regulations.
A. Objective
The overall aim of the work is to optimize and control the laser cutting process with suitable parameters and to equip the LCS with relevant sensor technology in such a way that the cutting of different materials and stopping of the cutting process at a suitable point (avoiding initiation of the explosive charge) is achieved.
This section focuses on presenting the experimental results aimed at determining suitable cutting parameters for effectively cutting relevant materials and objects. Additionally, it addresses the identification of appropriate sensors and process monitoring methods for detecting the cutting process. The selection of relevant materials and containers was carried out in collaboration with the end user.
B. Determination of the Laser Parameters for the Laser System
The aim of these experiments was to determine the required performance characteristics of an LCS concerning laser parameters, beam guidance, cutting system, process gas guidance, and material extraction for processing explosives. These determinations were made through investigations using LS I.
It was observed that many materials could already be processed with the existing laser system, although limitations were reached, particularly with poorly absorbing materials and thicker metals. Experiments to determine material-specific plasma thresholds identified the laser fluence at which plasma emissions could be detected at the process site. Exceeding this threshold resulted in initial material removal, indicating the commencement of the cutting process. It was found that for processing relevant materials in an adequate time, especially those poorly absorbing ones, higher fluences around 40J/cm2 were necessary (Fig. 8). This value was considered the minimum necessary fluence, which, however, did not achieve sufficient material removal for the given scenario.
Measurements with explosives indicated that the safe laser parameters for drilling processes in packaging containing explosives, as determined in previous projects. The LAGEF project could not be transferred to laser cutting processes due to insufficient fluence, which leads to a too long procedure [27]. TATP was used as the energetic material in the experiments with explosives.
Explosives specifically mixed with highly light-absorbing particles posed an additional challenge. This necessitated defining a limited time window for the laser cutting process, within which the process should be terminated after penetrating the material thickness, without affecting the next material layer.
1) Requirements of the System
The results of the measurements led to the following requirements and properties of the new laser system for the LCS (LS II). The laser system should be a diode-pumped solid-state laser with pulse durations in the nanosecond range (3–10ns). The wavelength should be in the green spectral range (515–532nm) with an optional switchable mode to the NIR range (1030–1064nm), achievable, for example, via a folding mirror in the system. The individual pulse energy should be at least 5–10 mJ in the green spectral range and 10–20 mJ in the NIR range, with continuous attenuation possible down to a minimum of 0.05 mJ. The attenuation and radiation emission (on/off) must be controllable via a corresponding interface. The repetition rate must also be variably controllable, with a minimum of 500 Hz and a preference for 2 kHz. The beam quality must have an M2 value of less than 3. The fluence required for processing a variety of materials should be greater than 80J/cm2. The laser system should be operable with a standard 230-VAC connection, while being designed in a compact way and capable of being integrated in the whole system.
C. Development of the Laser System
Essential for the future use of the LCS is the monitoring of the cutting process through online evaluation of sensor data, enabling feedback on the process progress. Initial investigations on a test stand using an existing laser system examined various sensor principles (optical, spectroscopic, acoustic, and chemical) to monitor relevant materials, including those with explosives.
These studies provided insights into evaluating reflected laser radiation, emitted radiation during processing, sound development, and resulting reaction products. The aim was to determine their suitability and develop potential evaluation algorithms for intelligent control and automation of the cutting process. This enabled the acquisition of information regarding material classification, material breakthroughs, cutting progress, and material changes, which in turn allows for the selection and adjustment of power parameters and selective processing.
1) System Tests—Laser Cuts in Test Surfaces and Objects:
This section provides information on the actual cutting of various test materials.
Due to the fact that different cutting geometries and sizes may be required, the standard test cut (PS, 4W, 40 mm/s) was converted and cut to other and larger cutting geometries. The values for larger diameters were calculated using the test cuts of 2-cm circular geometries with standard parameters. The first breakthroughs occurred in the test cut after 60 s and the cut sample fell off after 70 s. This results in breakthrough times of 150 s for a 5-cm circle and the average should occur after 175 s.
Fig. 9 presents an example of sensor data during the laser cutting of suitcase material (LS II). In this case, only microphone data is shown and the solar cell data as a reference. The plot on the left shows the microphone and solar cell data of a 2-cm cut of PS (one cycle: 1.57 s) and the plot on the right shows a 5-cm circular cut of the same material at an offset position (one cycle: 3.93 s). The two microphones capture signal curves that represent the laser cutting process and the material’s circular cut-out over time. The signal curve from microphone 2 reveals three phases: 1) initial laser entry into the surface; 2) the cutting phase within the material; and 3) the final penetration phase. The solar cell, functioning as the reference sensor, detects laser light passing through the material. Although all other sensors produce similar patterns, they vary in their temporal resolutions.
Microphone and solar cell data of exemplary 2 and 5 cm cuts of PS: Mic1: microphone is placed inside the sensor tube, Mic2: microphone is placed outside the sensor tube, and the solar cell is located below the sample to be cut.
In the left-hand plot, the solar cell data show that the first breakthrough through the material occurs after about 75 s (increase in the solar cell signal). In addition, the actual cut through (material is cut by the interaction with the laser beam, the laser beam hits the solar cell permanently) of the material takes place after 91 s, which can be read from the bending of the microphone signal in combination with the solar cell data. Assuming a linearly transferable cut, the events of the cut on the larger circle should occur 188 s (gray line) and 228 s (black line). However, the data shows that the first breakthrough is around 210 s and the cut through is at almost 250 s.
Because repeating experiments showed similar results, investigations were carried out with regard to laser power and laser beam profile. The beam profile image showed the following profile at different z-axis shifts (0 and 2.5 cm) (Fig. 10).
Beam profile measurement of the laser beam of the LCS: shift in focus: left: 0 cm and right: 2.5 cm.
The images show an oval-shaped profile with a further widening when the focal position is shifted, mainly caused by the profile shape in combination with the selected cutting parameters. The laser spot geometry caused one axis to irradiate more than the other. As a pulsed laser system was used, the individual pulses overlapped more frequently on one side, which resulted in a stronger treatment of individual areas (Fig. 11).
Schematic representation of the overlapping of individual laser pulses: the green circle represents the scan trajectory and the blue shapes the laser pulses.
As already mentioned, sensors, such as photodiodes, microphones, and others, were used to develop and control the cutting process. The following illustration (Fig. 12) shows the sensor data development of an exemplary cut. Plotted are recorded data using photodiodes and solar cells (reference sensor) against time, around 25 s corresponding to one scan. The first breakthroughs can be seen after 30 s (gray line; 2nd scan). It can be observed that after an increase in breakthroughs, the material is cut after about 50 s (black line). The cut sample falls off after approx. 78 s (green line). The sensor data of the diodes DPFOC, DP2, and DP3 show an increase contrary to the periodicity of the previous course (DPFOC and the DP series are abbreviations for photodiodes). The drop in the intensity of the solar cell (between the red dotted lines) can be explained by the fact that at this point the laser beam hits again on the sample, which has already fallen off but is in the beam path (under the clamped cut sample; above the solar cell). This interrupts the contact of the laser beam with the solar cell in this area.
Sensor data development (photodiodes and solar cell) based on an exemplary section: gray line: first breakthroughs, black line: cut through, and green line: material falls off.
The IR camera and the spectrometer were used intensively in the development of early breakthrough detection. The spectrum shown in Fig. 13 is from a laser cutting test on black PS. The sample was cut with a laser power of 5W and the spectra were integrated over 100 ms. It can be clearly seen that the light backscattered from the sample has a relatively high intensity. The recorded intensity at 515nm originates from the laser beam and is not completely filtered out. Approximately 5–10nm is missing to the right and left of the laser signal. This is due to the fact that a 515-nm notch filter was used to prevent too much light from entering the spectrometer directly.
Spectrum of black PS during a cutting test: the gray arrow shows the wavelength of the laser, laser power: 5 W, and integration time: 100 ms.
Fig. 14 shows an example of data acquisition with an IR camera. This is also a circular section of 2 cm on PS. The highest temperature in the measurement field (hotspot temperature) was plotted and the data from the solar cell was used as a reference sensor (the blue line displays the moving average of the data). The data show that the temperature in the material rises with increasing processing until it first stagnates. The hotspot temperature begins to scatter from around 20 s onward. This can be explained by the higher energy input and the higher sum after a certain time in the material. Another reason could be the further penetration into the material and the uneven distribution of energy. The first breakthroughs through the material are difficult to recognize with the IR camera alone. In the range of 70 to 78 s, the material buckles (the hotspot temperature drops). After approx. 78 s, the material collapses. The increase in the hotspot temperature is also due to the laser beam hitting the collapsed material again.
Viewing the hottest pixel in the IR cam’s field of view and correlating solar cell data.
Fig. 15 shows a stacked plot of the sensor data from a cutting test on real suitcase material. A laser power of 4W and a cutting speed of 40 mm/s was used. This figure shows the sensor responses of the two microphones, the hotspot temperature, the integral of the recorded spectra over the measurement time, the solar cell voltage, and the intensity of the wavelength of 515nm (indicator for the laser power). At this point, an attempt was made to use a threshold value (red line in the figure on the microphone data) to identify the termination point of the cutting process before processing of the materials/objects behind can take place. It can therefore be recommended that the cutting process should be terminated if the measured voltage falls below the specified value. As shown in the figure, the relevant microphone level falls below after around 65 s. A detailed analysis of the data progression shows that the first breakthroughs occur from this point onward (area marked in green in the figure). In the further course, the material is cut through (yellow area in the figure). The laser power was also measured in order to be able to trace and guarantee power-specific events and a uniform progression.
Cutting test on a plastic case: diameter of the circle: 2 cm, traversing speed of the laser: 40 mm/s, and laser power: 4W.
With the aim of developing a complete breakthrough detection system, a LabView program was written for live monitoring of the cutting process. The program provides a visual representation of the cutting process based on the sensor responses. In this way, the cutting process can be monitored. Fig. 16 shows an example of the program output. In the software-generated illustration, the breakthroughs are visible as green points on a circular trajectory. Nearly identical breakthroughs can be seen on the real sample (right-hand side—light behind sample demonstrates cut through parts) in comparison to the breakthrough detection program output. The breakthrough detection measures the heating and the processing intensity of the points on the cutting trajectory. On the one hand, the data from the spectrometer is used, which is well suited for breakthrough detection due to its high spatial resolution. In addition, the microphone data of the inner microphone is used and the spatial resolution is calculated using the spectrometer. This data can be used to calculate the extent to which a point has been affected on the trajectory. Furthermore, if one of the signals breaks away at a certain point, it can be determined whether the material is no longer being processed, as the laser does not hit the material in the focus.
Software-generated view of a cutting process (left) and a real cutting sample (right).
Various materials, such as plastic sheets, metal plates, glass, and an aluminum, can contaminated with trace amounts of explosive residue were processed, along with combinations of these materials. The topography scan was tested on angled plastic samples to evaluate its functionality. The majority of tested materials were successfully cut using the LCS, with the cut material being removable. Organic and nontransparent materials, such as black PS, were particularly effectively cut in a controlled manner (sensor-monitored with laser breakthrough detection). Thinner aluminum plates were also cuttable after prolonged, consistent laser exposure. However, cutting thicker metals (
Cut samples with respective breakthrough times: from top to bottom: leather case, plastic case, and textile bag with zip fastener.
To automate this process, topography measurement and focus plane tracking are necessary. In this procedure, the surface of the sample to be processed is illuminated with a laser diode before the actual cutting attempt, while a camera captures the reflected laser radiation. The surface information is then computed using software developed by a project partner and subsequently combined with the predetermined cutting trajectory. Consequently, the laser focus was adjusted to the sample’s topography during the cutting process, automatically fine-tuning the focus. An example of this functionality is shown in Fig. 18.
Scan of the surface of a curved plastic sample: curved sample (top left), scanned sample (top right), projected circular cut on the sample (bottom left), and processed sample (bottom right).
Furthermore, experiments were conducted to detect explosives on surfaces post-laser cutting. The surface of the sample (plastic, aluminum) to be cut was contaminated with a defined amount of explosive (1,3,5-Trinitroperhydro-1,3,5-triazine—RDX). The RDX contained in the solvent was dripped onto the object to be cut and waited until the solvent had evaporated. For this purpose, the sample containing the explosive was clamped in the beam path of the laser beam. The cut was performed with the known parameters. After completing the cutting process, a sampling strip for a mobile ion mobility spectrometer (IMS) was used to wipe the object’s surface, and the detectability of the explosive was assessed. The IMS was able to detect explosive amounts greater than 10ng.
Summary and Conclusion
This work aimed to develop and test a mobile LCS with sensor-based control for use on explosive objects. The project spanned several phases, each addressing specific components and goals necessary for the realization of this system. In the initial phase, system specifications and conditions were defined. This involved comprehensive state-of-the-art analyses and the identification of system requirements, possible laser and cutting systems, necessary sensors, overall system architecture, and adherence to laser safety regulations. Subsequently, the project focused on determining the laser parameters and acquiring and installing the system. Various laser parameters were tested using a frequency-converted Nd:YAG laser to establish the performance characteristics required for cutting relevant materials, including materials with explosives. These tests determined essential attributes, such as laser fluence, wavelength, pulse energy, and repetition rates. Based on these findings, a tailored laser system was specified. The development of subsystems was the next crucial step, which included sensing and cutting monitoring. Key developments included creating a sensor-equipped tube for monitoring and gas extraction, which was essential for capturing and analyzing cutting process data in real time. The integration of these subsystems with the LCS was a significant milestone, ensuring comprehensive control and safety. System tests and optimization were conducted to refine the demonstrator’s performance. These tests involved cutting various materials, including plastics, metals, and composites, to evaluate the system’s effectiveness. While initial tests showed promising results, further optimization was necessary. The system demonstrated significant potential for the safe and effective cutting of explosive containing objects. The final phase focused on testing and evaluating the system’s performance under real-world conditions. The investigations provided valuable insights and established a solid foundation for future advancements in this field. Further efforts are ongoing to secure funding for continued development and real-world testing to realize the system’s full potential.
The cuttability of various materials using a nanosecond-pulsed laser system at a wavelength of 515 nm depends on the energy coupling into the material. Good results were achieved with most plastics, wood, cardboard, thin glass, and metal. The system also effectively cuts cables or avoids cutting based on sensor responses, which is valuable for bomb disposal operations. However, thicker metal sheets, translucent plastics, and white glass showed poor cuttability, with cutting progress stagnating at deeper layers.
However, the reliability of breakthrough detection was not fully assessed. A thorough evaluation, especially in simulated applications during final testing, was not conducted. The tests carried out show the possibilities of sensory cut depth monitoring and offer a field in the fight against IEDs. For the application on real objects, a larger number of tests are required for which fine-tuning must be carried out. Research into miniaturization and adaptation/optimization of the system is continuing. For example, the use of ultrashort pulse (USP) lasers could open up further dimensions in cutting, although these are currently still too large. The cuttability of the materials has already been tested with the USP laser and delivered promising results, so that further research needs to be carried out into the cuttability of other materials (suitable cutting times) and the appropriate sensor technology.
Acknowledgement
Acknowledgment
The authors would like to acknowledge their project partners at Landeskriminalamt Nordrhein-Westfalen and ELP GmbH European Logistic Partners and the associated partners Bundeskriminalamt Wiesbaden and Bundespolizei Entschärfungsdienst Düsseldorf for their invaluable contributions to this work. The laser system was provided by Laser Zentrum Hannover e.V.