We report on a new experimental technique for monitoring laser-induced shock waves and thermal waves above the
sample surface called total internal reflection based photothermal or photoacoustic deflection (TIR based PTD/PAD
deflection). It is based on the changes in transmissivity of a prism which is operated near the condition of total internal
reflection for a HeNe laser beam propagating parallel to the sample surface at a small distance. The HeNe laser beam is
probing photoacoustic or photothermal waves originating from a sample surface due to interaction with a pulsed
Nd:YAG laser beam. The method is compared with standard online detection techniques like scatter probe monitoring
and plasma detection, and found to be a very sensitive and practical tool. It also showed its suitability for selectively
monitoring several surfaces (e. g. front and rear surface) of optical components, and attributing the damage starting
point. Therefore, the method might be used for monitoring of surface damage on laser crystals or valuable components.
Keywords: photothermal deflection, photoacoustic deflection, laser damage, total internal reflection.
Discussed is a novel method of manufacturing an Angularly Sensitive Micro-Sensor (ASMS). The process
employed utilizes excimer laser ablation to write out the microlens on the curved surface of the master lens. This
master lens element is manufactured with fused optical fibers, such that if the registration is maintained, the light
from each microlens goes via the fiber to a specific pixel in a focal plane array (FPA). Such a system allows for a
field of view greatly in excess of 180 degrees. If local imaging is required for specific tasks the fiber can send the
angularly localized image to a pixel set. Image fusing may then be required.
Infrared and ultraviolet versions can be manufactured. A more general application allows for a multi-spectral
sensor. After one ASMS is constructed, then an inverse mask (mould) can be created and the monolithic sphere,
retaining its registration, is covered in liquid plastic and placed into the mould and the exact replica is re-created.
The advantage is low cost and rapid manufacture of the ASMS.
The paper focuses on this sensor as a Task-Oriented Optical Processing (TOP) system; where the processing is
performed primarily by the optics leaving a greatly reduced requirement for an electronic processor. This is a
critical issue for micro, insect sized platforms where the weight budget is devoted to the energy and propulsive
systems. An important aspect of this approach is that the sensor samples amplitude and angular space rather than
amplitude and position space as conventional sensors currently do. This makes the ASMS processing paradigm
completely different from conventional image processing. For example using several fiber/pixel elements to
comprise a UV polarimeter allows for simple storage and processing of vector elements for simple navigation. The
home position may be treated as "Look up table" reference matrix (RM). That base table can be modified to account
for the passage of time (and hence change in solar position from the UV polarimeter, as appropriate). A second
"real time" travel matrix (TRM) is then created. Eventually, a target matrix (TAM) would also be created. Simply
driving changes in the TRM towards the RM would be used for navigating the return trip back to home base. When
the difference between the two matrices goes to a null matrix the platform would be home.
Discussed will be the development of a model for both a line laser diode as well as an area array consisting of line laser diode elements. The need for this work comes from the difficult task of using a single strip laser diode with dimensions such as 1 centimeter in the y direction and 1 micron in the x direction while exhibiting substantially different divergences in each axis as well. The intention is to create a fairly uniform optical pump beam in both the x and the y axis through as much of the gain media as possible. The modeled, virtual diode, is telecentric only in the long or “slow” axis where the divergence is generally less than the diode's “fast” axis. The virtual diode model is used as a source in a lens design software program to predict performance of diodes and their accompanying optics as pump sources for micro-lasers. Corroboration between the model and actual systems have been experimentally verified and will be presented. The single, linear laser diode source is used as the basis for a laser diode array model is also presented.
The Microwave-to-Optical Transformation (MOT) method, reported previously, makes use of an optical thin film filter design took known as the Optical Admittance Diagram, the characteristic matrix and the quarter wave rule. Stripline elements are re-configured and modeled as sequential quarter wave optically thin film layers. The previous work was qualitative and mainly focused on the intuitively useful analysis that the MOT method offers the microwave engineer. A more rigorous formalism shall be presented in this paper which allows the computation of E- Field, both amplitude and phase. In addition, new graphical techniques that provide the designer further insight will accompany the E-field analysis. This is a novel extension of the previous work. Microwave and optical computing circuits and components that span a vast wavelength range may be adaptable to this technique depending on their specific utilization. This paper will first develop an extension of the MOT method for electric field strength characterization. Secondly, this technique will be applied to the Wilkinson Power Divider used in the previous work. Use of this already MOT-characterized stripline component will validate the utility of this technique by also analyzing the electric field through the same simple microwave circuit. Finally, this technique will be applied to the design of a co-planar waveguide transition component used for launching power to a Field Emitter Array.
The single shot laser induced damage onset and morphology of ultrathin silicon wafers is investigated using a Q-switched, single longitudinal and transverse mode Nd:YAG laser operating at 1.06 micrometers . The wafers had a thickness of 2.5 - 33 micrometers with identical front and back surface polish and <100> orientation. Comparisons are made for simultaneous front and back surface damage at the lowest level detectable surface modification. The morphology of laser damage on front and back surface due to the application of multiple shots on one site (N/1) at higher fluence values was also monitored. A scatter probe system consisting of a laser beam analyzer and a CCD array was used. It allowed the in-situ observation of the development of damage on the samples.
The unique aspect of this work is that the defects found in an AR optical coating represent a degenerate ensemble that is uniformly distributed throughout the optical coating at high spatial densities. This ensures that various stages of damage may be observed for each irradiation and that the defect source can be identified.
It has been shown that the Admittance Diagram along with the Quarterwave Rule can be used in the design and characterization of optical thin film coatings. However, this same tool may be utilized in the design and characterization of some microwave components as well. A simple design example of a Wilkinson power divider is presented to illustrate the utility of this optical technique for microwave circuit design and analysis.
It has been shown previously that an optical analysis based on a ray trace can prove to be beneficial in the design of binary type phase holograms, because the optical path of each ray must be taken into account. Binary elements converted directly from thick, classical designs to Fresnel/binary equivalent without a redesign will probably have reduced performance. Furthermore, the phase prescription developed by sophisticated lens design programs may not be manufacturable. A simple design example is presented in order to illustrate natural and manufacturing constraints.
Unusual behavior has been reported for D20/H20-dosed CaF2 optical thin films. This behavior includes resistance to laser-induced damage at 2.7-µm radiation, improved damage morphology, and reduced optical absorption in the water band for even 1-120-dosed CaF2 optical thin films. Analysis of the films strongly suggests that the modification of film properties is not the result of inclusion of the D20/1420 into the film matrix or chemical contamination. Rather, the act of dosing has modified the structural properties of the films, thus producing the effects observed.
Laser damage to optical surfaces, particularly coated surfaces, is typically initiated by material defects that couple strongly to the laser radiation. Knowledge of damage-related defect characteristics is therefore essential to optical material development efforts and to quality control. Such characteristics include individual defect failure levels, defect densities, and content of optically absorbing impurities. Defect failure distributions containing defect density information can be inferred from properly designed laser damage measurements. The data also permit distinctions among different damage mechanisms, including non-defectrelated mechanisms involving the substrate. Inclusions containing volatile, optically absorbing impurities can be detected by mass analysis of laser-desorbed vapors. These methods are described, and examples of results are presented.
Scanning electron microscopy (SEM) was used to identify four distinct laser damage mor-phologies in ZnSe/ThF4 multilayer mirrors. There were three types of defect-initiated damage morphologies. Oblong-shaped damage sites oriented perpendicular to the electric field vector of the laser were associated with particulates on or near the surface of the ZnSe/ThF4 multilayers. Circular-shaped damage sites were initiated by particulates embed-ded beneath the top ZnSe layer. Selective laser damage at pinholes was identified as the third defectinitiated damage morphology. In addition to defect-initiated damage, stress-related damage was indicated by cracks near or within laser damage craters and erosion sites. Selective laser damage at nodular growth defects in SiH/Si02 multilayers was also observed using SEM. Samples with different numbers of nodules were prepared in-house using RF-diode, reactive sputtering. The low-defect mirror had the highest laser damage onset, and the mirror with the highest number of nodules had the lowest laser damage onset.
Laser damage to optical surfaces, particularly coated surfaces, is typically initiated by material defects which couple strongly to the laser radiation. Knowledge of damage-related defect characteristics is therefore essential to optical material development efforts and to quality control. Such characteristics include individual defect failure levels, defect densities, and content of optically absorbing impurities. Defect failure distributions containing defect density information can be inferred from properly designed laser damage measurements. The data also permit distinction between different damage mechanisms, including nondefect-related mechanisms involving the substrate. Inclusions containing volatile, optically absorbing impurities can be detected by mass analysis of laser-desorbed vapors. These methods will be described, and examples of results will be presented.
We report the results of an effort to correlate visible small-spot laser damage with changes in the transmittance of optical components at 10.6 pm. The test source used was a pulsed CO2 TEA laser, operating in the lowest order spatial mode and producing plane-polarized, gain-switched pulses of 0.1 is nominal length.
A process for producing calibration markers directly on the photoconductive surface of video camera tubes has been developed. This process includes the use of either a pulsed Nd:YAG laser, operating at 1.06 μm with a 9.5-ns pulse width (full width at half maximum), or a continuous helium-neon laser. The Nd:YAG laser was constrained to operate in the TEMoo spatial mode by intercavity aperturing. The use of this technology has produced an increase of up to 50 times the accuracy of geometric measurement. This is accomplished by a decrease in geometric distortion and an increase in geometric scaling. The process by which these laser-formed video calibrations are made will be discussed.
This paper relates a technique for increasing the optical strength of single-crystal NaCl. The 1.06-μm pulse laser-induced damage threshold was increased by a factor of 4.6 by a temperature annealing process. The starting material for this work was single-crystal "laser grade" NaCl acquired from the Harshaw Chemical Company. The bulk laser-induced damage threshold of the crystal was measured prior to and after the heat treatment using a Nd:YAG laser operated at 1.06 pm in the TEM00 spatial mode with a pulse width of 9 nsec, full width at half maximum. After the laser-induced damage threshold of the untreated crystal was meas-ured, it was mounted in a quartz tube and placed in an oven for the heat treatment. The quartz tube was continuously flushed with dry nitrogen throughout the heat treatment. The NaC1 sample was slowly heated to 794°C (approximately 7°C below its melting temperature). The sample was maintained at this temperature for a short time and was then removed from the oven to allow rapid cooling. The specimen required repolishing after the heat treatment because of surface sublimation which occurred at temperatures near melting. After repolishing, the bulk laser-induced breakdown intensity was remeasured and found to be 4.6 times greater than the value measured for the untreated crystal.
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