Volume 86, Issue 3 - Mechatronische Lasersysteme / Gastherausgeber: Jürgen Czarske

Zusammenfassung Thermische Effekte limitieren die Leistungsskalierbarkeit in Hochleistungsscheibenlasern und können durch konstruktive Maßnahmen nur bedingt reduziert werden. Ein neuartiger Ansatz zur Kompensation der thermisch induzierten Effekte wird durch den resonatorinternen Einsatz hochleistungstauglicher adaptiver Spiegel ermöglicht. In diesem Beitrag werden hierfür zwei mögliche Technologien vorgestellt und charakterisiert: Während pneumatisch aktuierte Flächenlastspiegel eine gezielte Kompensation bekannter Wellenfrontstörungen erlauben, bieten optisch adressierte deformierbare Spiegel die Möglichkeit nahezu arbiträre Deformationen zu realisieren. Jene Bauform nutzt thermoelastische Effekte infolge der lokalen Erwärmung des Spiegels um gewünschte Deformationen zu erzielen.

Zusammenfassung Breitbandig durchstimmbare Infrarotstrahlung im mittleren Wellenlängenbereich von etwa 3 bis 10 µm bzw. 1000 bis 3200 Wellenzahlen kann effizient zur Spektroskopie gasförmiger, flüssiger oder fester Stoffe und Gemische verwendet werden. In diesem Beitrag wird ein neuartiges quasi-statisches mikro-opto-elektro-mechanisches (MOEMS) Gitter zur spektralen Durchstimmung einer Infrarotlichtquelle auf Basis eines Quantenkaskadenlasers (QCL) vorgestellt. Das Konzept vereint die Vorteile breitbandiger Quellen mit denen kohärenter Laserquellen in einem miniaturisierten Aufbau. Das quasi-statische Antriebsprinzip erlaubt im Gegensatz zur resonanten Bewegung das Scannen mit vorgegebenen Trajektorien und einstellbarer Geschwindigkeiten und damit eine höhere spektrale Auflösung. Zur Stabilisierung der Bewegung wird MOEMS-Bauelement mittels integrierter Positionssensoren geregelt. Dieser Beitrag stellt die dynamischen Eigenschaften eines MOEMS-Mikrospiegels mit integriertem Beugungsgitter, sowie die damit gewonnenen experimentellen Ergebnisse beim Durchstimmen eines QCL im Bereich von 4150 bis 4600 nm Wellenlänge vor.

Endoscopes enable optical keyhole access in many applications for instance in biomedicine. In general, coherent fiber bundles (CFB) are used in conjunction with rigid lens systems which determine a fixed image plane. However, the lens system limits the minimum diameter of the endoscope typically to several millimeters. Additionally, only pixelated two-dimensional amplitude patterns can be transferred due to phase scrambling between adjacent cores. These limitations can be overcome by digital optical elements. Thus, in principle thinner, lensless, holographic endoscopes with a three-dimensional adjustable focus for imaging and illumination can be realized. So far, several techniques based on single mode CFB and multi mode fibers (MMF) have been presented. However, these techniques require access to both sides of the fiber, in order to calibrate the bending and temperature sensitive phase distortion, which is not possible in a real application. We present the feasibility of an in-situ calibration and compensation of a CFB with single sided access. A lensless endoscope with a diameter of only 500 µm, a spatial resolution around 1 µm and video rate capability is realized.

This article presents a large aperture micro scanning mirror (MSM) array especially developed for the panoramic 3D-ToF camera Fovea-3D. The Fovea-3D system uses a fiber amplified pulsed laser ToF technique at λ=1550nm\lambda =1550\hspace{0.1667em}\text{nm} with 1 MVoxel distance measuring rate. It targets for real time 3D imaging with a panoramic optical field of view (FOV) of 360°×60°360^\circ \times 60^\circ (horizontal × vertical) combined with a large distance measurement range of 100 m and a video-like frame rate of 10 Hz. For fast vertical scan axis a MEMS scanner module with large receiver aperture was especially developed. It increases the scanning rate to 3200 Hz which is four times faster in comparison to state-of-the-art fast macroscopic polygon scanning systems used for LIDAR systems. To guarantee at the same time a large reception aperture of Deff=23mm{D_{\mathit{eff}}}=23\hspace{0.1667em}\text{mm}, large FOV of 60°60^\circ and high vertical scanning rate of 3200 Hz, a hybrid assembled MSM array was developed consisting of 22 reception mirrors and a separate emitting mirror for laser scanning of the target. For Fovea3D hybrid assembly of frequency selected scanner elements was chosen instead of a monolithic MEMS scanner array to guaranty a high yield of MEMS fabrication. All MSM are driven in parametric resonance to enable a fully synchronized operation of all individual MEMS scanner elements. Therefore, piezo-resistive position sensors are integrated on each MEMS chip for position feedback of driving control. The paper discusses details of the MEMS system integration including the synchronized operation of multiple MEMS scanning elements.

Optical measurement systems are an important part of the portfolio of 3D topography sensors. By precise, contactless and rapid measurements these sensors constitute an alternative to tactile instruments. In this contribution the principle of a laser interferometric distance sensor is presented, which in combination with lateral scan axes acts as a topography sensor and also as distance sensor for the compensation of vibrations in a coherence scanning Linnik interferometer. An advantage of this distance sensor is its high acquisition rate of height values, which in case of working as a topography sensor enables high scan velocities as it is demonstrated at a chirp standard measured with a scan velocity of 80 mm/s. This is much higher than the scan velocity of tactile instruments, which are typically limited up to 1 mm/s. In addition, the compensation of vibration disturbances demonstrates the capability of the fast distance measurement. In contrast to other existing high-speed point sensors the relevant components are mass products. This keeps the costs of the sensor setup in a limited range. Furthermore, the sensor shows potential of much higher measurement rates than 116 kHz provided by the sensor used here.

In-situ measurements of the deformation and of the structural dynamical behavior of moving composite structures, such as rotors made of glass fiber reinforced polymers (GFRP), are necessary in order to validate newly developed simulation models. Local methods like strain gauges and fiber Bragg gratings lack spatial resolution, while contactless optical methods like image correlation or speckle interferometry suffer from noise effects in the presence of fast rigid body movements. A novel compact sensor – based on the diffraction grating method – is introduced for spatially and temporally resolved strain measurement. The use of a line camera allows the measurement of vibrations up to several tens of kHz. With a scanning movement, strain fields at submillimeter resolution can be recorded. The use of two diffraction orders and an objective lens reduces cross sensitivities to rigid body movements on the strain measurement by two to three orders of magnitude. A validation on a GFRP probe was conducted in a quasi-static tensile test with an optical extensometer up to 14500 µ ϵ . Furthermore, a strain measurement on a moving rotor at surface speeds up to 75 m/s was performed and the results were compared with those of strain gauges as a gold standard. The statistical standard deviation was around 10 µ ϵ and independent of the rotational speed.