An infrared camera system has been used to measure absorption in optical coatings and substrates. Laser
light is directed at the test sample and milliwatts of power are absorbed. The camera images the surface of
the sample and provides a direct measurement of the 8-12 micron radiation emitted. By considering the
effective emissivity of the sample and the ambient temperature, the surface temperature of the sample is
obtained. Through the use of an equivalent "reference" sample which is not heated by the laser,
background variations may be effectively eliminated. The application of standard calorimetric methods to
infrared imaging as well as the availability of improved sensors such as the microbolometer array has led to
our ability to resolve temperature excursions as low as 0.01°C with a S/N of 20 for typical samples.
The IR imaging method has been used to evaluate many optical coatings and window materials for the
Airborne Laser program. Because the method is noncontact, it has been used to directly measure
absorption on large optical surfaces. In some instances, defects have been observed and mapped using this
method. Variations in absorption which might be predicted from the coating design have been measured
directly. The IR imaging technique thus offers great flexibility and sensitivity comparable to precision
The space environment presents some unique problems for optics. Components must be designed to survive variations in temperature, exposure to ultraviolet radiation, particle radiation, atomic oxygen and contamination from the immediate environment. To determine the importance of these phenomena, a series of passive exposure experiments have been conducted which included, among others, the Long Duration Exposure Facility (LDEF, 1984-1990), the Passive Optical Sample Assembly (POSA, 1996-1997) and most recently, the Materials on the International Space Station Experiment (MISSE, 2001-2005). The MISSE program benefited greatly from past experience so that at the conclusion of this 4 year mission, samples which remained intact were in remarkable condition. This study will review data from different aspects of this experiment with emphasis on optical properties and performance.
High performance optical coatings are an enabling technology for many applications - navigation systems, telecom,
fusion, advanced measurement systems of many types as well as directed energy weapons. The results of recent testing
of superior optical coatings conducted at high flux levels have been presented. Failure of these coatings was rare.
However, induced damage was not expected from simple thermal models relating flux loading to induced temperatures.
Clearly, other mechanisms must play a role in the occurrence of laser damage. Contamination is an obvious
mechanism-both particulate and molecular. Less obvious are structural defects and the role of induced stresses. These
mechanisms are examined through simplified models and finite element analysis. The results of the models are
compared to experiment, for induced temperatures and observed stress levels. The role of each mechanism is described
and limiting performance is determined.
In this study, the environment inside an operational laser system was monitored over a period of three months using a surface acoustic wave sensor. The environment experienced by the sensor was subject to repeated vacuum pumpdown, nitrogen purge and chemical flow processes. The data collected during this period demonstrated the fact that this type of sensor is subject to both accumulation and desorption mechanisms. Surface conditions were clearly active and changing over time. By tailoring the sensor surface to be equivalent to that of the optical coatings in the system, it was believed that the sensor provided an excellent view of the condition of the surface of those optical coatings. Monitoring a system using a device of this type may, in the near term provide some knowledge of readiness. In the long term, this type of monitoring may assist in the selection of compatible materials and effective design for control of contamination.
High performance optical coatings are an enabling technology for many applications - navigation systems, telecom, fusion, advanced measurement systems of many types as well as directed energy weapons. The results of recent testing of superior optical coatings conducted at high flux levels will be presented. The diagnostics used in this type of nondestructive testing and the analysis of the data demonstrates the evolution of test methodology. Comparison of performance data under load to the predictions of thermal and optical models shows excellent agreement. These tests serve to anchor the models and validate the performance of the materials and coatings.
IR thermal imagin has been sued to study absorption in coated optical surface.s THis technique has demonstrated the ability to rapidly determine coating quality on thermally insulating substrates such as fused silica. The application of this technique to coatings deposited on thermal conductive substrates such as sapphire, silicon or copper is discussed. Data and the result of modeling are compared to show the limitations and potential of this technique for measurement and study of different classes of optical components.
The thermal response of a coated optical surface is an important consideration in the design of any high average power system. Finite element temperature distribution were calculated for both coating witness samples and calorimetry wafers and were compared to actual measured data under tightly controlled conditions. Coatings for ABL were deposited on various substrates including fused silica, ULE, Zerodur, and silicon. The witness samples were irradiate data high power levels at 1.315micrometers to evaluate laser damage thresholds and study absorption levels. Excellent agreement was obtained between temperature predictions and measured thermal response curves. When measured absorption values were not available, the code was used to predict coating absorption based on the measured temperature rise on the back surface. Using the finite element model, the damaging temperature rise can be predicted for a coating with known absorption based on run time, flux, and substrate material.
Coatings designed for use in the Airborne Laser (ABL) have stringent requirements for reflectance over several spectral bands in addition to extremely low absorption and high damage threshold at the 1315nm output of the chemical oxygen iodine laser. The complexity of these coatings leads to difficulty in design and fabrication particularly on curved optical surfaces with large apertures. A series of witness samples were fabricated to evaluate the state-of-the-art for this type of coating and provide appropriate design criteria for the ABL optical train. Damage testing at 1315nm under CW conditions was performed at the RADICL laser facility at Kirtland AFB. Limited optical characterization before and after the test was performed at the OCEL facility to evaluate the quality of the samples and to identify damage. The results of these test and characterization will be discussed.
The Kuiper Express is a mission to achieve the first reconnaissance of one of the primitive objects that reside in the Kuiper Belt. The objects in the Kuiper Belt are the remnants of the planetesimal swarm that formed the four giant planets of the outer Solar System. These objects, because they are far from the Sun, have not been processed by solar heating and are essentially in their primordial state. This makes them unique objects and their study will provide information on the composition of the solar nebula that cannot be extracted from a study of other objects in the Solar System. The Kuiper Express is a sciencecraft mission. A sciencecraft is an integrated unit that combines into a single system the essential elements (but no more) necessary to achieve the science objectives of the mission, including science instruments, electronics, telecommunications, power, and propulsion. The design of a sciencecraft begins with the definition of mission science objectives and cost constraint. An observational sequence and sensor subsystem are then designed. This sensor subsystem in turn becomes the design driver for the sciencecraft architecture and hardware subsystems needed to deliver the sensor to its target and return the science data to the earth. Throughout the design process, shared functionality, shared redundancy, and reduced cost are strongly emphasized. The Kuiper Express will be launched using a Delta vehicle and will use solar electric propulsion to add velocity and shape its trajectory in the inner Solar System, executing two earth gravity-assist flybys. It will also execute flybys of main belt asteroids, Mars, Uranus, and Neptune/Triton en route to its target in the Kuiper belt, where it will arrive about ten years after launch. It will use no nuclear power. The surface constituents and morphology of the objects visited will be measured and their atmospheres will be characterized. The cost of the detailed design, fabrication, and launch of the Kuiper Express is consistent with the $150M limit set by the NASA Discovery Program.