A novel fiber-optic probe measures the velocity distribution of an imploding surface along many lines of sight. Reflected
light from each spot on the moving surface is Doppler shifted with a small portion of this light propagating backwards
through the launching fiber. The reflected light is mixed with a reference laser in a technique called photon Doppler
velocimetry, providing continuous time records.
Within the probe, a matrix array of 56 single-mode fibers sends light through an optical relay consisting of three types of
lenses. Seven sets of these relay lenses are grouped into a close-packed array allowing the interrogation of seven regions
of interest. A six-faceted prism with a hole drilled into its center directs the light beams to the different regions. Several
types of relay lens systems have been evaluated, including doublets and molded aspheric singlets. The optical design
minimizes beam diameters and also provides excellent imaging capabilities. One of the fiber matrix arrays can be
replaced by an imaging coherent bundle.
This close-packed array of seven relay systems provides up to 476 beam trajectories. The pyramid prism has its six
facets polished at two different angles that will vary the density of surface point coverage. Fibers in the matrix arrays are
angle polished at 8°to minimize back reflections. This causes the minimum beam waist to vary along different
trajectories. Precision metrology on the direction cosine trajectories is measured to satisfy environmental requirements
for vibration and temperature.
A velocimetry experiment has been designed to measure shock properties for small cylindrical metal targets
(8-mm-diameter by 2-mm thick). A target is accelerated by high explosives, caught, and retrieved for later inspection.
The target is expected to move at a velocity of 0.1 to 3 km/sec. The complete experiment canister is approximately
105 mm in diameter and 380 mm long. Optical velocimetry diagnostics include the Velocity Interferometer System for
Any Reflector (VISAR) and Photon Doppler Velocimetry (PDV). The packaging of the velocity diagnostics is not
allowed to interfere with the catchment or an X-ray imaging diagnostic. A single optical relay, using commercial lenses,
collects Doppler-shifted light for both VISAR and PDV. The use of fiber optics allows measurement of point velocities
on the target surface during accelerations occurring over 15 mm of travel. The VISAR operates at 532 nm and has
separate illumination fibers requiring alignment. The PDV diagnostic operates at 1550 nm, but is aligned and focused at
670 nm. The VISAR and PDV diagnostics are complementary measurements and they image spots in close proximity on
the target surface. Because the optical relay uses commercial glass, the axial positions of the optical fibers for PDV and
VISAR are offset to compensate for chromatic aberrations. The optomechanical design requires careful attention to fiber
management, mechanical assembly and disassembly, positioning of the foam catchment, and X-ray diagnostic field-of-view.
Calibration and alignment data are archived at each stage of the assembly sequence.
We have designed and used for several years now a ¼ inch O.D., 11.5 inch length optical probe for imaging light from a surface area inside a confined space. The design is based on a commercial SelFoc gradient index objective and relay rod combination with acceptance angle +-30 degrees. We have used our probe both for framing camera images and for imaging spots on a surface onto a fiber array for interferometry. Probe efficiency is 1x10-6 at an object distance of 10 centimeters, where, for imaging onto the array, the probe has a depth of field from 2 cm to infinity. If a spot size of 1 mm is acceptable, the object can be brought within a few mm. For interferometry, the probe images enough of the surface to require automation from the analysis software. Below we report our probe construction and performance calculations, and software automation and analysis improvements.
The total losses due to absorption and scatter from the best optical coatings can be made as low as <EQ 3 ppm, at the limit of sensitivity of present optical-absorption measurement techniques. We show by measurement and calculation that a dramatic increase in the sensitivity of absorption measurements is obtained by using a supercritical fluid, instead of an ordinary (non-critical) fluid, as the sensing fluid in a collinear photothermal-deflection apparatus. The noise floor in our surface-absorption measurements using supercritical xenon, Tc equals 16.7 degree(s)C, corresponds to an absorptance A equals Pabsorbed/Pincident equals 10-10 under illumination of 1 W. Bulk absorption measurements are similarly enhanced: the noise floor corresponds to an absorption coefficient of (alpha) equals 10-13 cm-1 for 1 W of illumination in a sample of length 1 cm. These levels are three orders of magnitude more sensitive than any previously reported. The enhancement is brought about by the divergence in the coefficient of thermal expansion of a fluid near the critical point. In attempting to use this sensitivity to measure the absorption in transmission of low-absorbing (<EQ few ppm) anti-reflection coatings, we found that the bare superpolished fused-silica and sapphire substrates absorb at A approximately 2 X 10-5. The low-level absorption at uncoated polished optical surfaces thus appears to be an important question.
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