OPTICAL
Below is a sampling of the
variety of engineering projects that were worked on over the years.
It is by no means an inclusive list. Use the (
)
to navigate the text and pictures.
IR SYSTEM()
RADIOMETER()
32 BEAM BEAMSPLITTER()
CONVEX SEGMENTED APERTURE()
All coating and engineering products require test and evauation to verify that they will perform as expected. The test and evaluation section describes several examples of the special testing that has been required. This section by no means represents the extent of testing that was performed during the developments described on this web site.
TEST AND EVALUATION()
The system depicted below is a FLIR system capable of 0.25 C resolution. Packaging constraints resulted in several folds of the optical train. The system also included a derotation prism (not shown) located just after the dual fold mirrors and multielement lens cell. The derotation prism compensated for the image rotation caused by scan mirror at the front of the system. The system contained aspheric lenses, and a lens cell that provided passive thermal compensation. Alignment of this cell was done on an IR interferometer. The entrance window required "state of the art" multiple axis mills to produce a configuration with the required accuracy. Tests of the subassemblies and system required the design and development of substantial tooling to properly characterize the optical performance.
SEE PICTURE()
The photographs below illustrate a radiometer that was constructed to measure the in-situ properties of satellite materials before and after laser irradiation. The actual sample(~one inch square) was attached to a wheel that was located between mirror and the small block shown in the picture to the right. The sample was located at one focus of an elliptical mirror and an integrating sphere(the rectangular block) was located at the other focus. Light transferred from a monochromator was transferred through the small visible hole in the ellliptical mirror onto the sample. Changes caused by laser radiation could be determined by comparing the test sample to calibration samples on the wheel . The laser irradiation was performed in the same chamber by rotating the wheel so that the sample was moved to a portion of the chamber that had an IR window that would transmit the incident beam. A shield was placed between the irradiation and measurement sections of the chamber.
SEE PICTURE()
The 32 beam beamsplitter consists of 5 small parallel plates that are contacted onto a single bar. The plates are coated with a high reflector on the back side. One half of the front surface is coated with a 50% beamsplitter while the remainder of the front face is coated with an AR. An incident laser beam is injected at 45 degrees to the face of the plate onto the 50% beamsplitter coating. This beam is then 50% transmitted and 50% reflected. The portion that is transmitted is the reflected off the back face of the plate and back out the face on the AR coated portion. The result is that one incident beam is divided into two beams of approximately equal intensity. This occurs with all the beams at the face of each parallel plate with the result that for one beam in you get 32 beams out that have approximately equal intensity. The sketch below shows the configuration and the beam path after two of these mirrors.
SEE PICTURE()
The convex segmented aperture (CSA) is a device that was conceived and developed to convert the irregular intensity profile of a laser beam into a beam with a relatively uniform profile. The CSA concept was to arrange a number of smaller convex mirrors in a way that the reflected beams from each mirror would overlay on a common target area, as shown in (A) below. Computations of the resultant intensity profile, after adding the phased contribution from each of the mirrors in the CSA is shown in (B). Upon demonstration of the feasibility of the device a full up configuration that would handle the large laser power was designed and developed. This work was contracted to an appropriate engineering firm.
SEE PICTURES()
It is always neccessary to verify the optical or coating performance of a delivered product. The typical way to evaluate optical coatings is the use of ellipsometers, spectrophotometers, laser ratiometers, and other related equipment. I describe below several instances where the evaluation is not as commonplace.
In my early work with high
energy lasers it was important to determine the effects of high
energy laser fluences on various aerospace materials. The
striking photo()
shows what happened when a structural graphite epoxy composite
material was exposed to high energy CO2 laser radiation.
Testing materials in
this fashion always required lasers with smooth intensity profiles so
the results of the interaction could be modeled and understood.
A number of concepts to provide a smooth intensity from an irregular
laser input beam were evaluated. Shown in the picture()
is a concept that uses a rectangular cone to provide multiple
reflections for a laser input beam that is focused on the input aperture.
More typical of coating
evaluation is some measurement of the spectral performance of the
coating. Shown in the picture()
is an apparatus that consists of an environmental chamber
connected to a spectrophotometer. This gear was designed and
constructed to make transmission measurements over a range of
temperature and humidities to verify the stability of the coating in
these environments.
It was frequently necessary
to learn more about the microscopic nature of the films that were
deposited. Several different techniques were used to determine
film materials, film densities, film morphology, and film/surface
contaminants. Shown in the picture()
is an example of some analyses that were occasionally required.
The picture is an SEM photograph of a film consisting of many layers
and what happens when an impurity is injected into the film during
its deposition. Prior to the SEM photo the sample is prepared
by special polish and etch cycles to enhance the visibility of the layers.
In addition to examinations of this type electron diffraction analyses were used to determine the materials in the film. Rutherford scattering was also used to identify materials and their densities within each layer. Time of flight mass spectrometry was frequently used to determine the possible nature of surface contamination on both coated and uncoated parts.
Last but not least and
definitely not optical testing is a photo()
of the equipment that we designed and built to measure that low
temperature heat capacity of RbxWO3, a superconducting material.
It consists of concentric dewars and vacuum spaces that allow the
small sample container inside to have its temperature lowered from
room temperature to liquid helium temperatures(~ 1K). The
dewars are placed in the large electromagnet shown so that critical
field curves could be constructed from the changes in the
superconducting transition temperature induced by the magnetic field.