This PDF file contains the front matter associated with SPIE Proceedings Volume 11161, including the title page, copyright information, table of contents, and author and conference committee lists.

This year marks the 25th anniversary of the first demonstration of quantum cascade lasers by Faist, et al. QCLs have proven to be unique sources of mid wave infrared (MWIR) and long wave infrared (LWIR) radiation, capable of operating in continuous wave mode at room temperature. QCLs are the only solid state sources for generating laser radiation at these wavelengths, converting electrical power directly into optical energy. The only other sources that meet the criterion of direct conversion of electrical power into optical power are the molecular gas lasers, such as carbon dioxide and carbon monoxide, which were first demonstrated fifty five years ago. Both of these systems have been commercially developed to produce high powers necessary for many industrial, defense and medical applications. However, both the CO₂ and the CO laser operate on multiple discrete infrared transitions of the respective molecules and thus are only discretely tunable over limited wavelength ranges, from ~9.0 μm to ~11.5 μm for the CO₂ laser (using normal and isotopic CO₂) and from ~5.0 μm to ~7.0 μm for the CO laser. On the other hand, QCLs are able to cover a spectral range from ~3.5 μm to terahertz, albeit, with much lower power. In spite of the lower powers available from QCLs, there are a very broad range of applications that have made QCLs attractive for continued research and development.

Incoherent beam combining of laser beams can increase the available power on target when the output power of a single laser source is limited, and it may also reduce turbulence effects by averaging of scintillations. We have investigated the optimization performance of different variations of the stochastic parallel gradient descent (SPGD) algorithm in a setup where five low power laser beams illuminate a cat’s eye retroreflector, and detectors next to the lasers are used to provide feedback for optimization. Angular adjustments of the laser beams are provided by displacement of fiber tips behind collimating lenses. This setup is representative of a dazzling application. Findings include that the optimization demands that there is some initial signal from all laser beams to provide rapid and dependable optimization, which means that the initial pointing errors cannot be much larger than the divergence of the individual beams. Parameter variations show that the sensitivity to settings is relatively low, often a factor two interval of parameter values give an acceptable performance.

Currently, the highest power quantum cascade lasers (QCLs) profit from (i) the efficient active region design, minimizing the energy fraction transferred into heat, (ii) relatively high number (40) of cascades, increasing the mode confinement factor, Γ, and (iii) narrow buried-heterostructure (BH) waveguides, which cool efficiently the active region due to the high lateral heat ow. This combination allows Watt-level continuous wave (cw) emission powers at room temperature. Moreover, narrow waveguides often support the fundamental TM₀₀ lateral mode, resulting into a nearly Gaussian laser beam. Power scaling is possible in this configuration only by laser length, which is limited by the needs for robust construction and transparent anti-reflection coatings. Broad-area (BA) QCLs with a small number of cascades can also deliver a Watt-level cw powers at room temperature, profiting from the enhanced vertical heat flow. Power can also be scaled for these lasers via the laser width, and is nearly unlimited. The usual drawback of the BA lasers is poor beam quality. In this paper we report on the first demonstration of the room temperature cw operation of the BA QCL at the fundamental TM₀₀ mode. The stable and reproducible transverse mode selection is achieved by the double-taper waveguide geometry. The laser emits at 4.6 μm and has demonstrated room-temperature cw power of 100 mW.

Optical countermeasures are widely used nowadays and quite often a laser is used as the optical source. Unfortunately such a laser beam can become severely distorted by optical turbulence when propagating through the atmosphere, resulting in effects such as beam spreading, beam wander, irradiance fluctuations, and loss of spatial coherence. These effects can be (partially) overcome using knowledge of the atmospheric conditions, as well as techniques to correct for amplitude and phase distortions. Our research focuses on the characterization of the atmospheric conditions, using adaptive optics, an in-house developed multi-aperture transmissometer, as well as a plenoptic sensor using phase distortion algorithms to compensate for effects caused by (strong) turbulence conditions.

Electro-optical and infrared devices are vulnerable to the increasing threat of dazzle and damage from laser weapons, which will reduce the operational effectiveness of these surveillance devices critical to situational awareness. In this paper, we characterize the impact of laser radiation in-band to a mid-infrared thermal imager. In particular, we stress the importance of the detector integration time as a means to mitigate this threat. An Indium Antimonide focal-plane array of size 640 x 512 pixels was illuminated by a quantum cascade laser operating at a center wavelength of 4.58 micron. The power was increased up to a maximum of approximately 13 milliwatts. Video sequences were recorded for the imager operating at four pre-set integration times that were interlaced at a frame rate of 100 Hz. The extent of the laser dazzle is determined from the images and we assess the resultant impact on the scene contrast. Our results indicate that the spatial extent of the dazzle pattern is reduced as the integration time is also reduced, albeit at the probable detriment to background contrast in the image.

The use of complementary wavelengths bands in camera systems is a long known principle. The camera system’s spectral range is split in several spectral channels, where each channel possesses its own imaging sensor. Such an optical setup is used, for example, in high-quality 3-sensor colour cameras. A 3-sensor camera is less vulnerable to laser dazzle than a single-sensor camera. However, the separation of the individual channels is not sufficient high enough to suppress crosstalk and thus all three channels will suffer from laser dazzling. To solve that problem, we suggest two different optical designs in which the spectral separation of the channels is significantly increased. The first optical design is a 3-channel camera system, which was already presented earlier. The second design is a 2-channel camera system based on optical multiband elements, which delivers undisturbed colour images even under laser dazzle.

Based on our previous work1, we further investigate the laser-induced damage effects on digital micromirror devices (DMD) in comparison to different electro-optical imaging sensors such as complementary metal-oxide-semiconductors (CMOS) and charge-coupled devices (CCD). In our earlier work, we reported on damage thresholds obtained by pulsed laser radiation of nanosecond pulse width and by continuous-wave laser radiation utilizing irradiation times ranging from 250 milliseconds up to 10 seconds. The main objective of our current work is to fill the gap regarding the time scale of picosecond pulses. In the course of this research, we enhanced the experimental setup and we explicitly describe the achieved improvements in this work. Furthermore, we characterize the damage caused by laser pulse energies exceeding the laser-induced damage threshold (LIDT).

Their exceptionally large jam-to-signal ratios (J/S) make directed infrared countermeasure (DIRCM) systems the most efficient means for defeating heat seeking missiles. Although several studies have performed analyses on DIRCM systems, influence of optical turbulence on the effectiveness of jamming waveforms is usually ignored. However, due to turbulence originating from the exhaust plume of the air platform and the atmosphere, a DIRCM laser’s beam may be exposed to time-varying intensity variations which may drastically reduce the effective J/S at the seeker aperture compared to the one at the platform. Furthermore, previous studies have usually focused on the signal processing stages of seeker heads while neglecting the diffractive and aberrative properties of the missile optics. An analysis of the impact of turbulent air on the DIRCM effectiveness from a wave-optics point of view is required. In this paper, we first investigate the time-varying impact of several degrees of turbulence on laser beams modulated with a jamming pattern along the optical path from the air platform to the missile’s dome. In the turbulence model, the laser beam emerging from the platform with an arbitrary quality factor of M² transverses two different turbulent regimes when directed to the missile threat. First, it passes through the region of highly turbulent medium around a rotary-wing platform originating from the rotor downwash of the exhaust plume. Next, the beam travels much longer distances (on the order of few km’s) in the turbulent atmosphere until it reaches the missile. Beam propagation in both regions is simulated using the split-step method. Using the ZEMAX software and its wave-optics based Physical Optics Propagation (POP) package, we employ a generic model for the optical system of a first-generation spin-scan seeker and obtain the time-dependent intensity profiles of the laser beam at the focal point at various instants of the jamming pattern. Generic models for an uncooled lead-sulfide detector and the following signal processing stages have also been included in the model.

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.

Lasers are an unnatural occurrence, rendered almost impossible in nature due to the laws of thermodynamics. Thus, the presence of laser radiation is always accompanied by an intent for that laser such as sensing, targeting, range finding etc. Detection of laser radiation is therefore important as it may be a precursor to impending action. Laser warning receivers have been around for decades and have been aligned with the type of laser threat. In the last few years new threats have appeared in the form of low-cost diode lasers with dangerously high power levels (several Watts for a few hundred US dollars) and an ever expanding range of wavelengths. Protecting against such threats requires its detection, analysis and classification. In this paper we will discuss the types of technologies that have been used to detect lasers and the properties they can discern. We then focus on the developments in the detection of coherence properties and its ability to detect weak continuous wave (CW) laser sources.

The availability of low cost but relatively high power laser pointers (hundreds of mW) has led to misuse with potentially dangerous consequences, such as dazzling aircraft which has raised concerns about aircraft safety. A low cost laser detection system based on coherence detection has been developed and is able to detect weak, continuous laser sources even against bright background light. In this paper, we introduce the use of a cone mirror to extend the horizontal field of view of the detector (originally at 3°) to 360° to detect incoming beams from different directions. With the additional use of a camera in the system, we also determine the direction of the incoming laser beam. Finally, the sensitivity between the original system and the cone mirror system are compared: the new system showed promising results with a sensitivity below 100nW.

Detection of optical threats in the form of weapon sights and target acquisition systems is important and can be made using retro-detection or other techniques. It is also of great value if one can classify the optical systems so that proper counter action can be initiated. Several techniques to estimate the aperture size of the threatening aperture may be applied. In this paper a method based on image processing utilizing high resolution photographs will be presented. In order to investigate the potential to detect optical apertures, and also estimate the aperture sizes, some experiments were made using small black metal circular plates as well as real optics out to 2 km range. The targets were photographed using a commercial digital SLR camera with a telephoto lens. This was done from a laboratory about 20 meter above ground level with targets on the ground, which means that the turbulence was lower than it would have been closer to the ground. Detection and size estimation of optical apertures by image processing, has some potential which will be exemplified by the experimental results and some simulations.

The ability to detect optics is important for military surveillance, and allows early threat detection. Present-day optics detection systems are exploiting that focusing optics are retro-reflective. Hence, it is possible to discover threats, including riflescopes, electro-optical sensors, and magnifying optical assemblies used for weapon guidance, by illuminating them with a laser. However, the sensors have performance limitations and do not usually provide range information about the target. This work suggests a scanning optics detection system that uses a linear array of avalanche photo diodes (APD) with high sensitivity providing information about the target range and angular location. An experimental system using four pixels of a 16×1 linear APD array was constructed and tested against reference targets outdoors. The receiver assembly consisted of a micro-lens array, focusing optics, bandpass filter, and pre-amplifier circuit. The system also contained a pulsed NIR-laser, motorized pan-tilt stages for the scanning, and a calibrated scene camera to measure the background signal. It was possible to detect reference targets at over kilometre range while distinguishing the background, using dedicated signal analysis and noise reduction. The suggested scheme definitely benefit in long-range performance compared to similar techniques that use CCD/CMOS-sensors. The drawback using an APD array lies in reduced angular resolution and increased complexity of data acquisition electronics. In addition, the experimental results will be discussed in terms of a performance model, influence from turbulence effects and suggestions for future sensor improvements.

Characterization of potentially useful polarization signatures is discussed. We experimentally evaluate a liquid crystal retarder switch used for beamsteering as a function of angle and applied voltage, and compare to a CCR at 633 nm. Distinguishable results between the various measured Mueller matrix elements, angle of incidence, applied voltage and samples tested are shown, which indicates this method may be useful in discriminating between various return signals and background clutter.

A novel, low-cost, camera-based method of detecting a single nanosecond laser pulse and kHz modulated continuous wave and pulsed lasers has been developed at DSTL. The detector uses a simple optical modification to a standard rolling shutter colour camera combined with image processing techniques to distinguish lasers from other illumination sources and extract a lasers wavelength and pulse repetition frequency. In addition the detector is also capable of detecting a single nanosecond laser pulse at any given time. Such a detector has applications in free-space optical communications, as a low cost broadband method of extracting information from multiple sources, and as a detector of laser range finders. A low cost prototype (≈£600) has been developed using entirely off-the-shelf components and assessed in laboratory conditions, with the ability to measure laser wavelengths to ±5nm and pulse repetition frequencies to within ±5%. In the laboratory the prototype was also able to detect each of the 1000 pulses generated by a 10Hz 10ns 532nm pulsed laser, as well as 100 pulses sent at random intervals, highlighting its capability to detect a ns pulse at any given time. The prototype was taken to the Moonraker NATO SET-249 field trial, where it was able to measure the pulse repetition frequency of a modulated continuous wave laser source to within ±5% at a distance of 660m. This novel technology offers a low cost method of detecting lasers and extracting their pulse repetition frequencies, with a wide field of view and high spatial resolution.

Jamming code development set-ups are generally employed for evaluating the jamming code effectiveness of Directed Infrared Countermeasure (DIRCM) Systems on seeker heads in laboratory conditions. In these set-ups, usually, the output beam of a mid-infrared (mid-IR) laser having similar properties with the DIRCM laser is expanded and collimated before directing it to the seeker head so that a beam with an almost-homogeneous intensity distribution is obtained at the seeker aperture. This simulates what would be expected in a real long distance engagement scenario where an infrared heat seeking missile is attacked by a countermeasure laser and the laser beam diverges to create an almost-homogenous intensity profile at the seeker aperture provided that the effect of atmospheric turbulence is neglected. Large aperture off-axis parabolic mirrors are often employed for the purpose of expanding and collimating the laser beam in these set-ups. However, instead, it is also possible to employ refractive beam shaping optical systems that are much smaller in size compared to these large collimating mirrors in an effort to reduce the laser power loss, required space, and cost for obtaining such uniform laser beam profiles. In this paper, we propose a simple design method for a Galilean beam shaping optical system that transforms a laser beam with a Gaussian intensity distribution into flattop, Super Gaussian, or Fermi-Dirac intensity distributions, hence facilitates obtaining an almost-homogenous intensity distribution for the purpose of future use in a jamming code development setup to be operating in mid-IR band. Similar to Galilean or Keplerian type conventional beam shaping optical systems, the method depends on using two aspheric lenses whose active and opposing surfaces shape the beam by refraction. The first surface redistribute the beam intensity distribution and the second surface collimates the beam. However, contrary to these conventional methods, we employ a simple numerical algorithm to generate the surface equations of the two aspheric lenses. Using our method, we present examples of optical system designs for chosen wavelengths in the mid-IR band. We use ZEMAX’s Physical Optics Propagation package to verify that our design method works.

At present, water spray cooling is widely used in the fields of fire prevention and cooling as an economical and convenient method. In recent years, surface ships have been using water curtains for surface cooling and infrared stealth, which is becoming more and more common. The LECHLER nozzle used in the infrared stealth field of ships adopts the built-in type, featuring a mounting surface flush with the shell plate, which is suitable for ship stealth. It has a water pressure of 0.5Mpa and a water volume of 4m³/h, and the water spray covers a diameter of 7m. However, through observing the inside, the author found that there is a total pressure loss existing in many structures in the nozzle, which generally results in a loss of flow energy inside the nozzle, thereby reducing the spray coverage of the nozzle. In this paper, the structural optimization research of the nozzle is carried out, and the dual control conditions of inlet water pressure and water supply are proposed. The pressure loss and the coverage of water spray are analyzed by numerical simulation. The calculation shows that the total pressure loss is reduced from the original 47.5% to 10% with the optimized design. Under the premise of controlling the inlet water pressure and flow, the spray coverage is increased by 86.6%.