Plasmonic metamaterials are artificial structures whose optical response can be tailored to achieve several effects by playing with the geometrical parameters of the components. In this talk, we discuss how to apply the metamaterial design rules to develop band-stop linear filters and nonlinear filters, operating as intensity limiters. In both regimes, the filters share some common qualities: their optical response does not change for a broad range of incidence angles, at least up to 30 degrees, and is only weakly dependent on the polarisation of the incident light. These properties make these ultrathin filters useful in open field applications. The metamaterial is based on an array of gold nanotubes (i.e., a cylindrical gold shell with a dielectric core) embedded in a dielectric matrix. In the linear regime, the metamaterial displays an absorption resonance independent of the polarisation and the angle of incidence of the light, which can be tuned throughout the visible spectral range by changing the geometrical parameters of the array. In the nonlinear regime - based on free-electron Au nonlinearity and tested with ns-long pulses at 532 nm - the metamaterial limits the output peak fluence, keeping it constant across several order of magnitudes of the incoming fluence. The proposed metamaterial approach can be useful for designing optical spectral filters and intensity limiters over broad range of wavelengths.
A low-cost camera-based method of detecting continuous-wave (cw) lasers has been developed at Defence Science and Technology Laboratory. The detector uses a simple optical modification to a standard color camera combined with image processing techniques to distinguish lasers from other illumination sources and measure the wavelength, direction, and irradiance of the laser light. Such a detector has applications in collecting information on aircraft laser dazzle incidents, providing the evidence required to inform on aircrew laser exposure events and to assess if engagements are eye safe. A prototype has been developed using entirely commercially available off-the-shelf (COTS) components, costing ≈£600, and assessed in laboratory conditions with the capability of measuring laser wavelengths to ±5 nm and irradiances to ±10 % . A realistic handheld laser engagement scenario, using a range of relevant wavelengths and irradiances, was simulated during the Moonraker trial where the prototype was capable of measuring laser wavelengths to an accuracy of ±10 nm and peak irradiances to ±25 % . All laser engagements were detected over a total data collection period of 9 h with zero false alarms. Comparisons were made with a COTS laser detector, which showed an equivalent performance. This technology offers a low-cost approach to cw laser detection, which is capable of extracting a range of parameters while maintaining a relatively wide field of view and angular resolution.
In October 2018, NATO SET-249 performed a common trial at WTD 52, Oberjettenberg, Germany, to study laser dazzle effects in an airborne scenario. The facility is equipped with a cable car and is ideal for slanted path experiments from the base station to the cable car where the sensors were mounted. NATO SET-249’s background is laser threat evaluation and the evaluation of the impact of laser eye dazzle on the visual performance of humans. This work gives an overview on the various measurements performed here: 1. Assessment of dazzle effects originating from light scattering at an aircraft canopy by comparing the images of two cameras: one outside and one inside the canopy. The general findings showed that the canopy, which had been used previously on an aircraft, substantially affected the dazzle pattern in the camera within the canopy as compared to the camera outside. 2. Sensor dazzling: Laser dazzling of complementary metal-oxide-semiconductor (CMOS) cameras in the visible domain and, in addition, laser dazzling of a camera equipped with a fisheye lens, which is commonly present in micro-unmanned aerial vehicles, is demonstrated. The dazzled area in the camera field of view (FoV) grows with increasing laser irradiance, and dazzling is effective at irradiance levels around a few μW/cm². 3. An overview on realistic handheld laser engagement scenarios to test the capabilities of a DSTL-developed Laser Event Recorder (LER) is provided. This technology is able to detect continuous wave (CW) and pulsed lasers, and extract their wavelengths, irradiances, Pulse Repetition Frequency (PRF) and directionality. Applications for this LER include collecting information on aircraft laser exposure events, giving information to assess if engagements are eye safe. 4. Measurements performed on various Fraunhofer IOSB developed sensor systems hardened against laser dazzle: The hardening measure of these systems is based either on the use of spatial light modulators or on the implementation of the principle of complementary wavelength bands. The field trial offered the possibility to generate data of the hardened systems under real life conditions.
A novel, low-cost, camera-based method of detecting Continuous Wave (CW) lasers has been developed at DSTL. The detector uses a simple optical modification to a standard colour camera combined with image processing techniques to distinguish lasers from other illumination sources, as well as measuring the wavelength, direction and irradiance of the laser light. Such a detector has applications in collecting information on aircraft laser dazzle incidents: providing the evidence required to inform on aircrew laser exposure events and to assess if engagements are eye safe. A prototype has been developed using entirely Commercially available Off-The-Shelf (COTS) components, costing ≈£600, and assessed in the laboratory conditions, with the capability of measuring laser wavelengths to ±5nm and irradiances to ±10%. A realistic hand-held laser engagement scenario, using a range of relevant wavelengths and irradiances, was simulated during the Moonraker trial where the prototype was capable of measuring laser wavelengths to an accuracy of ±10nm, and peak irradiances to ±25%. Comparisons were made with a COTS laser detector, and showed an equivalent performance. This technology offers a low cost approach to CW laser detection, which is capable of extracting a range of parameters, whilst maintaining a relatively wide Field of View (FOV) and angular resolution.
The laser-induced damage thresholds of commercial off-the-shelf visible and shortwave infrared cameras are measured under laboratory and outdoor conditions for infrared laser exposure times varying from microseconds to continuous wave. The damage threshold results are compared with a simple thermal model, which shows strong correlation with the experimental data, allowing the model to be used to predict camera damage thresholds across a range of exposure durations and wavelengths.
Applications involving the outdoor use of pulsed lasers systems can be affected by atmospheric turbulence and scintillation. In particular, deterministic prediction of the risk of injury or damage due to pulsed laser radiation can be difficult due to uncertainty over the focal plane fluence of radiation that has traversed through a turbulent medium. In this study, focussed beam profiles of nanosecond laser pulses are recorded for visible laser pulses that have traversed 1400m paths through turbulent atmospheres. Beam profiles are also taken under laboratory conditions. These pulses are characterised in terms of their peak focal plane fluence, total collected energy and Strehl ratio. Measured pulses are then compared statistically to pulse profiles generated by a two-dimensional phase screen propagation model based on the Von Karman power spectrum distribution. The model takes into account the refractive index structure constant (𝐶𝑛2), the wavelength, the path geometry and macroscopic beam steering. Analysis shows good correlation between the measured and simulated data, inferring that the Von Karman phase screen model can be used to predict focal plane fluence distributions for outdoor applications.