A Nd:GSAG laser operated at the 4F3/2
→4I9/2 transition was tuned by a FPI-etalon achieving a tuning range of
1.5 nm with a center wavelength at 942.7 nm. Three water vapor absorption wavelengths with different absorption
strength as suitable for a water vapor LIDAR are within this tuning range and lasing could be achieved at all
three wavelength. Q-switched pulse energies up to 26mJ were obtained as required for long range detection.
An actively Q-switched Nd:GSAG laser with 942nm wavelength was frequency doubled in a critically type-I
phase-matched LBO. Maximum pulse energy of 8mJ with 300ns pulse duration at 471 nm was obtained with 19mJ
incident radiation at 10Hz. The corresponding conversion efficiency was 42%. The frequency doubling of a focused
Gaussian beam involves spatially dependent phase mismatching due to beam divergence. It decreases the conversion
efficiency and deteriorates the beam quality. According to the theoretical calculation, elliptical focusing was used to
improve the second harmonic beam quality and slightly increase the conversion efficiency.
The wavelength-dependent Jones matrix representation of a twisted-nematic liquid crystal (TN-LC) cell contains
four independent parameters. The absolute values of these parameters and two mutual sign relationships can
be determined from comparatively simple transmission measurements of the TN-LC cells sandwiched between
two rotatable polarizers. The physical parameters of the cell (twist angle α, director orientation ψ, birefringence
β) can be retrieved if the Jones matrix is known for more than one wavelength. We have measured the Jones
matrices of the TN-LC cells of a translucent Sony LCX-016 microdisplay for six wavelengths ranging from 488nm
to 1064nm and determined the physical parameters of the cell. We have also measured the Jones matrices for
one wavelength for a number of applied voltages. These experimental results show that it is not sufficiently exact
to calculate the Jones matrix from the known physical parameters of the cell assuming a voltage-dependent
birefringence only. We attribute the deviations from the theoretical model to edge effects which are not taken
into account. The direct experimental determination of the Jones matrix components is therefore preferable and
permits a more accurate simulation of the TN-LC microdisplay in experimental configurations involving other
polarization-dependent optical components.
Water vapour absorption wavelengths have been directly generated by diode pumped Nd:YGG crystals emitting at 935 nm and with Nd:GSAG crystals emitting at 942 nm in cw and pulsed operation. In addition the 1064 nm fundamental wavelength from Nd:YAG pump lasers with pulse lengths of 10 or 20 ns was shifted using Stimulated Raman Scattering (SRS) or Ti:Sapphire (TiSa) lasers. The potential of Nd:GSAG, Nd:YGG, SRS and TiSa laser systems is compared for future incorporation into a satellite based Lidar system. High output energies are possible by recent advances of fiber coupled diode sources allowing pulsed longitudinal pumping of Q-switched solid state lasers.
For weather forecast, especially for civil protection from high-impact weather events, measuring the three-dimensional
distribution of water vapour by DIAL techniques is a fundamental concern. Especially for development and evaluation of
atmospheric models, knowledge of water vapour distribution is important. Suitable wavelengths for a water vapour
DIAL are e.g. around 943 nm. This region can be reached with well established technologies such as the optical
parametric oscillator (OPO) and the Ti:Sapphire laser. But these systems suffer from low efficiency and complex set-up.
In contrast the Nd:GSAG laser presented here can be directly pumped with 808 nm laser diodes. This supports the
realisation of an efficient and compact laser system. Different oscillator and amplifier setups working at 943 nm were
realised. An output energy of >17 mJ in a 100 ns pulse with 10 Hz repetition rate was demonstrated. In a MOPA system
a double pass gain of 1.5 and an output energy of >18 mJ was achieved. The Nd:GSAG laser oscillator was successfully
injection seeded with DFB laser diode from FBH-Berlin. Also the gain cross section in a Nd:GSAG laser crystal from
941-944 nm was measured. The FWHM of the homogeneous line is 2 nm with a peak stimulated emission cross section
of 4.0•10-20 cm2 at 942.7 nm.
The three-dimensional measurement of the global water vapor distribution in the atmosphere considerably improves the reliability of the weather forecast and climate modeling. A spaceborne Differential Absorption Lidar (DIAL) is able to per-form this task by use of suitable absorption lines of the broad absorption spectrum of water vapor. Because no interference with the absorption of other molecules exists, the range of 935/936 nm, 942/943 nm are the most preferred wavelength ranges for a water vapor DIAL. The challenge is to develop a dedicated efficient high power laser source emitting at these wavelengths. The comparison between frequency converters based on stimulated Raman scattering (SRS) and Ti:Sapphire and the directly generated Mixed Garnet laser shows the favorable properties of each concept and helps to evaluate the most suitable concept. Development of Raman frequency converters for high pulse energies concentrates on linear resonator de-signs and seeding using the Raman material as a direct amplifier based on Raman four-wave-mixing. In addition a seeded and frequency stabilized pulsed Ti:Sapphire laser system with output pulses up to 22 mJ injection-seeded at the water vapor absorption line at 935.684 nm with a spectral purity up to 99.9 % has been developed. Direct generation of the wavelengths 935/936 nm and 942/943 nm required for water vapor detection is possible with diode-pumped, Nd-doped YGG- and GSAG-crystals. First experiments resulted in pulse energies of 18 mJ in Q-switched and 86 mJ in free-running operation at 942 nm wavelength.
In this paper the dynamic processing of interferometric fringe patterns obtained by real-time optical measurement methods like holographic interferometry is shown. A hologram of the tested component is superimposed with the hologram of the stressed component. The achieved fringe patterns vary according to the degree of stress applied. To evaluate these varying fringe patterns in real time, dynamic filtering is required. A hybrid opto-electronic sytem with a digital image processing and optical correlation module based on liquid-crystal spatial light modulators gives us the possibility to use dynamic filters and input images. In order to process interferometric fringes the adaptive wavelet transformation is applied.
In this paper the dynamic processing of interferometric fringe patterns obtained by real-time optical measurement methods like holographic interferometry is shown. A hologram of the tested component is superimposed with the hologram of the stressed component. The achieved fringe patterns vary according to the degree of stress applied. To evaluate these varying fringe patterns in real time, dynamic filtering is required. A hybrid opto-electronic system with a digital image processing and optical correlation module based on liquid-crystal spatial light modulators gives us the possibility to use dynamic filters and input images. In order to process interferometric fringes the adaptive wavelet transformation is applied.
We will show two methods of dynamic filtering. Firstly a static filter is used to process varying fringe patterns. With this method changes of features in the fringe patterns can be observed correlating to changes of stress applied on the tested component. Another application of dynamic filtering uses a static input image and dynamic filters. This method is used for the classification of interferometric fringe patterns. A set of different wavelet filters is applied to the input image using the ability of the spatial light modulator to display images in video frame rates. Comparing the wavelet filters and the output images it is possible to assign the fringe patterns to a fault class.
A new spatial light modulator system based on a high-resolution LCOS micro-display is presented. The parameters like pixel size and number as well as the fill factor and especially the phase modulation properties open up many application in coherent optical applications. The light efficiency of this system can compete with other static optical solutions. A major application field diffractive optics will be illustrated and performance and perspectives of this system will be discussed. Investigations were done mostly on the addressing of the 2.3 Mega pixel device. The effect of ghost images and cross talk due to the addressing scheme will be discussed concerning the influence of the optical performance. General optical measurements like complex modulation is presented, particularly focused on the phase modulation properties. Furthermore, possibilities of adapting the modulation by the use of "Gamma-correction" curves are shown. Various experiments which illustrate different application fields the modulator system are presented.
The detection and classification of faults is a major task for optical nondestructive testing in industrial quality control. Interferometric fringes, obtained by real-time optical measurement methods, contain a large amount of image data with information about possible defect features. This mass of data must be reduced for further evaluation. One possible way is the filtering of these images applying the adaptive wavelet transform. The wavelet transform has been proved to be a capable tool in the detection of structures with definite spatial resolution. In this paper it is shown the extraction and classification of disturbances in interferometric fringe patterns, the application of several wavelet functions with different parameters for the detection of faults, and the combination of wavelet filters for fault classification. Furthermore the implementation of complex valued wavelet filters and correlation filters is shown. We will present an algorithm to classify interferometric fringe patterns. In order to achieve real-time processing a hybrid opto-electronic system with a digital image processing and an optical correlation module is favored. The calculated wavelet filters are implemented into the optical correlator system that is based on liquid-crystal spatial light modulators. So, all discussed items were verified experimentally in the optical setup.
The application of two dimensional addressable high resolution displays as spatial light modulators for the reconstruction of coherent masks is investigated. Based on their amplitude and/or phase modulating properties they are implementable as dynamic diffractive element and thus they are qualified for the efficient reconstruction of digital holograms. Parameters are deduced for a geometrically exact reconstruction of an object-wave field for its use as coherent mask in an interferometric measuring system. Results for different spatial light modulators are presented.
A 2D dynamic phase-modulating system (DPS) addressed by a digital signal at video frame rate can realize several optical transmission functions in the field of refractive as well as diffractive optics. Critical parameters are the space-bandwidth product, the phase shift and the light efficiency of the system. We demonstrate result of our current research activities on a compact system based on the SVGA spatial light modulator and a 635 nm laser diode. Points of discussion are limitations of pixilated diffractive optical systems, which cause boundaries for the application in the field of beam shaping and beam splitting. Furthermore, light efficiency and compactness of the systems architecture are main tasks. Examples are shown, which deal with the current state of beam shaping possibilities and which show the potential in the field of rapid-prototyping for optical transmission functions.
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