Volume 3, Issue 2 - Active Imaging / Guest Editors: Martin Laurenzis and Hans Dieter Tholl

In a series of two contributions, decisive business-related aspects of the current process status to transfer research results on diffractive optical elements (DOEs) into commercial solutions are discussed. In part I, the focus was on the patent landscape. Here, in part II, market estimations concerning DOEs for selected applications are presented, comprising classical spectroscopic gratings, security features on banknotes, DOEs for high-end applications, e.g., for the semiconductor manufacturing market and diffractive intra-ocular lenses. The derived market sizes are referred to the optical elements, itself, rather than to the enabled instruments. The estimated market volumes are mainly addressed to scientifically and technologically oriented optical engineers to serve as a rough classification of the commercial dimensions of DOEs in the different market segments and do not claim to be exhaustive.

There is an increasing demand for wavelength agile laser sources covering the mid-infrared (MIR, 3.5–12 µm) wavelength range, among others in active imaging. The MIR range comprises a particularly interesting part of the electromagnetic spectrum for active hyperspectral imaging applications, due to the fact that the characteristic ‘fingerprint’ absorption spectra of many chemical compounds lie in that range. Conventional semiconductor diode laser technology runs out of steam at such long wavelengths. For many applications, MIR coherent light sources based on solid state lasers in combination with optical parametric oscillators are too complex and thus bulky and expensive. In contrast, quantum cascade lasers (QCLs) constitute a class of very compact and robust semiconductor-based lasers, which are able to cover the mentioned wavelength range using the same semiconductor material system. In this tutorial, a brief review will be given on the state-of-the-art of QCL technology. Special emphasis will be addressed on QCL variants with well-defined spectral properties and spectral tunability. As an example for the use of wavelength agile QCL for active hyperspectral imaging, stand-off detection of explosives based on imaging backscattering laser spectroscopy will be discussed.

In this paper, we shall present the active gated imaging system (AGIS) in relation to the automotive field. AGIS is based on a fast-gated camera and pulsed illuminator, synchronized in the time domain to record images of a certain range of interest. A dedicated gated CMOS imager sensor and near infra-red (NIR) pulsed laser illuminator, is presented in this paper to provide active gated technology. In recent years, we have developed these key components and learned the system parameters, which are most beneficial to nighttime (in all weather conditions) driving in terms of field of view, illumination profile, resolution, and processing power. We shall present our approach of a camera-based advanced driver assistance systems (ADAS) named BrightEye™, which makes use of the AGIS technology in the automotive field.

Active imaging and mapping using lasers as illumination sources have been of increasing interest during the last decades. Applications range from defense and security, remote sensing, medicine, robotics, and others. So far, these laser systems have mostly been based on a fix wavelength laser. Recent advances in lasers enable emission of tunable, multiline, or broadband emission, which together with the development of array detectors will extend the capabilities of active imaging and mapping. This paper will review some of the recent work on active imaging mainly for defense and security and remote sensing applications. A short survey of basic lidar relations and present fix wavelength laser systems is followed by a review of the benefits of adding the spectral dimension to active and/or passive electro-optical systems.

Laser diode stacks are applied for the realization of compact and efficient laser illumination devices for active imaging (e.g., laser gated viewing). An efficient use of illumination power and sensor arrays has to adapt the laser illumination to the sensor’s field of view (FOV) with a perfectly matched and homogeneous illumination field. In the past decades, ISL has studied different methods for beam shaping and homogenization of laser diode stacks. In this publication, the authors give a review of the development of different beam-shaping techniques for auto-stack, minibar stack in a specific configuration and standard laser diode stacks. The presented methods are based on the application of wedge-type waveguides, fast axis and slow axis collimation, and the rearrangement of the laser beams by polarization overlapping and virtual restacking. All presented methods have very high transmission efficiencies (>80%) and lead to a homogeneous illumination matched to the sensor’s field of view.

Time-correlated single-photon counting (TCSPC) range profiling and imaging provide high resolution laser radar data applicable in several optical remote sensing applications at short and long distances. Excellent range resolution, below centimetres, can be obtained and information about remote objects can be extracted from TCSPC range profiles. The present study describes a TCSPC range profiling system with subcentimetre range resolution applied for remote sensing of objects at short and longer ranges. Experimental results from interrogation of geometrical shapes, reflectance tomographic imaging and range profiling at longer distance in daylight conditions are presented.

We have developed an optical stimulator system for vision-based sensors. The stimulator is an efficient tool for stimulating a camera during on-ground testing with scenes representative of spacecraft flights. Such scenes include starry sky, planetary objects, and other spacecraft. The optical stimulator is used as a test bench to simulate high-precision navigation by different types of camera systems that are used onboard spacecraft, planetary rovers, and for spacecraft rendezvous and proximity maneuvers. Careful hardware design and preoperational calibration of the stimulator result in high precision and long-term stability. The system can be continuously used over several days. By facilitating a full camera including optics in the loop, the stimulator enables the more realistic simulation of flight maneuvers based on navigation cameras than pure computer simulations or camera stimulations without the involvement of the actual optics.