Gunnar Ritt’s research while affiliated with Fraunhofer Institute of Optronics, System Technologies and Image Exploitation and other places

The optical limiting behavior of silver nanoparticles with different sizes and shapes is investigated and compared to the optical limiting performance of conventional carbon black suspension (CBS). The optical limiting behavior is characterized by means of nonlinear transmittance and scattered intensity measurements when submitted to nanosecond pulsed Nd:YAG lasers operating at the fundamental or the second harmonic wavelength. We found that the optical limiting effect is strongly particle size dependent and the best performance is achieved with the smaller particles. Moreover, it is shown that the surface plasmon resonance is not the main effect responsible for the nonlinear processes. A theoretical model based on the computation of the Mie scattering functions is exposed, and it is shown that the experimental results can be well explained from the calculations.

The optical limiting behaviour of silver nanoparticles with different sizes and shapes is investigated and compared to the optical limiting performances of conventional carbon black suspension. The optical limiting behaviour is characterized by means of nonlinear transmittance and scattered intensity measurements when submitted to a nanosecond pulsed Nd:YAG laser operating at the fundamental or the second harmonic wavelength. We found that the optical limiting effect is strongly particle size dependent, the best performance achieved with the smaller particles. Moreover, it is shown that the surface plasmon resonance is not the main effect responsible for the nonlinear processes. Especially, the particle size and its implication in the backscattering performance is outlined. A stronger backscattered radiation is observed for the 60 nm sized silver particles in comparison with the 580 nm large ones. Alternatively, a stronger scattering in the forward hemisphere is subsequent to nanoparticles whose sizes are significantly greater.

Electro-optical imaging sensors are widely distributed and used for many different tasks. Due to technical improvements, their pixel size has been steadily decreasing, resulting in a reduced saturation capacity. As a consequence, this progress makes them susceptible to intense point light sources. Developments in laser technology have led to very compact and powerful laser sources of any wavelength in the visible and near infrared spectral region, offered as laser pointers. The manifold of wavelengths makes it difficult to encounter sensor saturation over the complete operating waveband by conventional measures like absorption or interference filters. We present a concept for electro-optical sensors to suppress overexposure in the visible spectral region. The key element of the concept is a spatial light modulator in combination with wavelength multiplexing. This approach allows spectral filtering within a localized area in the field of view of the sensor. The system offers the possibility of automatic reduction of overexposure by monochromatic laser radiation.

The progress in laser technology leads to very compact but nevertheless powerful laser sources. In the visible and near infrared spectral region, lasers of any wavelength can be purchased. Especially continuous wave laser sources pose a serious threat to the human eye and electro-optical sensors due to their high proliferation and easy availability. The manifold of wavelengths cannot be encountered by conventional safety measures like absorption or interference filters. We present a protection concept for electro-optical sensors to suppress dazzling in the visible spectral region. The key element of the concept is the use of a digital micromirror device (DMD) in combination with wavelength multiplexing. This approach allows selective spectral filtering in defined regions of interest in the scene. The system offers the possibility of automatic attenuation of dazzling laser radiation. An anti-dazzle algorithm comprises the analysis of the laser wavelength and the subsequent activation of the appropriate micromirrors of the DMD.

The near- and off-resonant optical limiting properties of gold, silver and gold–silver alloy nanoparticles in methyl 2-methylprop-2-enoate for nanosecond laser pulses are presented. The nanoparticles are generated by picosecond pulsed laser ablation in liquid having hydrodynamic diameters from 26 to 30 nm. We use a Q-switched Nd:YAG laser working at a wavelength of 1064 or 532 nm, with a pulse width of 3 ns to characterize their behaviour by laser energy and fluence dependent transmittance measurements. To elucidate the contribution of nonlinear scattering to the optical limiting properties the scattered light energy at an angle of 90° is measured. The experimental results show that these nanoparticles have a strong nonlinear attenuation which can be attributed to intraband, interband and free carrier absorption and a thermal-induced scattering only at high input energies. Our results indicate in addition that the surface plasmon resonance does not contribute to the nonlinear processes at high input energies.

The rapid developments in the laser field through the last years led to very compact laser devices with high brightness. In the visual and near infrared spectral range practically each wavelength is now available. For optronical sensors, laser radiation states an increasing threat that cannot be encountered just by conventional safety measures like absorption or interference filters. We present a concept to protect imaging sensors against laser radiation of any wavelength. The system is based on the combination of a spatial light modulator and wavelength multiplexing and allows selective spectral filtering in a defined region of interest in the scene. Such a system offers the possibility to suppress annoying laser radiation without losing spatial information in the region of interest. Depending on the used imaging sensor, we discuss different ways to realize a control loop to activate the appropriate pixels of the spatial light modulator for the attenuation of the laser light.

The European Defence Agency (EDA) launched the Active Imaging (ACTIM) study to investigate the potential of active imaging, especially that of spectral laser imaging. The work included a literature survey, the identification of promising military applications, system analyses, a roadmap and recommendations. Passive multi- and hyper-spectral imaging allows discriminating between materials. But the measured radiance in the sensor is difficult to relate to spectral reflectance due to the dependence on e.g. solar angle, clouds, shadows... In turn, active spectral imaging offers a complete control of the illumination, thus eliminating these effects. In addition it allows observing details at long ranges, seeing through degraded atmospheric conditions, penetrating obscurants (foliage, camouflage...) or retrieving polarization information. When 3D, it is suited to producing numerical terrain models and to performing geometry-based identification. Hence fusing the knowledge of ladar and passive spectral imaging will result in new capabilities. We have identified three main application areas for active imaging, and for spectral active imaging in particular: (1) long range observation for identification, (2) mid-range mapping for reconnaissance, (3) shorter range perception for threat detection. We present the system analyses that have been performed for confirming the interests, limitations and requirements of spectral active imaging in these three prioritized applications.

We describe a new concept of an electro-optical sensor with the capability of simultaneous spatial and spectral filtering. It is based on a spatial light modulator, and in combination with the technique of wavelength multiplexing, it enables one to manipulate the spectral content of an indicated spot within the field of view of the sensor. This new concept allows the attenuation of monochromatic light of undetermined wavelengths in particular and is of worth for imaging vision systems to suppress unwanted detector overexposure.

Laser dazzling of electro-optical sensors states a serious problem. Especially when navigational tasks shall be fulfilled a disturbance of the sight is inacceptable. Classical protection measures like e.g. laser safety filters usually lack of the drawback of color distortion and a considerable loss in transmission. Therefore, new protection concepts should be aspired which are independent of the laser wavelength and do not affect the vision. In this paper we discuss a novel concept to protect electro-optical sensors against laser dazzling based on spatial light modulator technology and wavelength multiplexing. In particular, the method is suited as a protection measure against continuous wave laser sources.