Material scientists have developed computational modeling to predict the dynamic response of materials undergoing stress, but there is still a need to make precision measurements of surfaces undergoing shock compression. Miniature photonic Doppler velocimetry (PDV) probes have been developed to measure the velocity distribution from a moving surface traveling tens of kilometers per second. These probes use hundreds of optical fibers imaged by optical relays onto different regions of this moving surface. While previous work examined large surface areas, we have now developed a PDV microscope that can interrogate 37 different spots within a field of view of <1 mm, with a standoff distance of 17 mm, to analyze the motion differences across grain boundaries of the material undergoing dynamic stress. Each PDV fiber interrogates a 10 μm spot size on the moving surface. A separate imaging system using a coherent bundle records the location of the PDV spots relative to the grain boundaries prior to the dynamic event. Designing the mounting structures for the lenses, fibers, and coherent bundle was a challenge. To minimize back reflections, the fibers are index matched onto the first relay lens, which is made of fused silica. The PDV fibers are aligned normal to the moving surface. The imaging probe views the surface at an 18° angle. The coherent bundle is tilted 11° to its optical relay. All components are assembled into a single probe head assembly. The coherent bundle is removed from the probe head to be used for the next dynamic event. Alignment issues will also be discussed.
Photonic Doppler Velocimetry (PDV) has become widely and routinely used in many high-velocity experimental applications due to its improved ease of use, cost, experimental flexibility, data return, and robustness compared to earlier velocimetric methods. However, these earlier methods have advantages in applications with requirements beyond PDV’s current capabilities. Various classes of experiments at the National Ignition Facility (NIF) that are characterized by extremely high velocity or acceleration, or diagnostic requirements for high precision in timing and/or velocity, have historically seen a VISAR (velocity interferometer system for any surface) diagnostic employed due to such advantages. VISAR, however, requires specific, and sometimes challenging, experimental features, including planar geometry and normal incidence, high-reflectivity surface treatment, and a relatively large and inflexible diagnostic footprint. Therefore, the potential for implementing a PDV diagnostic at NIF has been evaluated by researchers from National Security Technologies, LLC and Lawrence Livermore National Laboratory. We present the results of this study, weigh the relative merits of the two methodologies with consideration of experimental phenomena and requirements, and discuss possible implementations and future directions.
A pulsed neutron source is used to interrogate a target, producing secondary gammas and neutrons. In order to make
good use of the relatively small number of gamma rays that emerge from the system after the neutron flash, our detector
system must be both efficient in converting gamma rays to a detectable electronic signal and reasonably large in volume.
Isotropic gamma rays are emitted from the target. These signals are converted to light within a large chamber of a liquid
scintillator. To provide adequate time-of-flight separation between the gamma and neutron signals, the liquid scintillator
is placed meters away from the target under interrogation. An acrylic PMMA (polymethyl methacrylate) light guide
directs the emission light from the chamber into a 5-inch-diameter photomultiplier tube. However, this PMMA light
guide produces a time delay for much of the light.
Illumination design programs count rays traced from the source to a receiver. By including the index of refraction of the
different materials that the rays pass through, the optical power at the receiver is calculated. An illumination design
program can be used to optimize the optical material geometries to maximize the ray count and/or the receiver power. A
macro was written to collect the optical path lengths of the rays and import them into a spreadsheet, where histograms of
the time histories of the rays are plotted. This method allows optimization on the time response of different optical
detector systems. One liquid scintillator chamber has been filled with a grid of reflective plates to improve its time
response. Cylindrical detector geometries are more efficient.
When a zoom lens views a tilted finite conjugate object, its image plane is both tilted and distorted depending on magnification. Our camera image plane moves with six degrees of freedom; only one moving doublet lens is required to change magnification. Two lens design models were analyzed. The first required the optical and mechanical axes to be collinear, resulting in a tilted stop. The second allowed the optical axis to be tilted from the lens mechanical axis with an untilted stop moving along the mechanical axis. Both designs produced useful zoom lenses with excellent resolution for a distorted image. For both lens designs, the stop is anchored to the moving doublet and its diameter is unchanged throughout magnification changes. This unusual outcome allows the light level at each camera pixel to remain constant, independent of magnification. As-built tolerance analysis is used to compare both optical models. The design application is for proton radiography. At the end of an accelerator, protons exit an aluminum vacuum window producing a shadowgraph image onto an LYSO (lutetium yttrium orthosilicate) scintillator. The 5″ square scintillator emission reflects off a pellicle and is collected by the zoom lenses located 24″ away. Four zoom lenses will view the same pellicle at different alpha and beta angles. Blue emission from the scintillator is viewed at an alpha angle of –14° or –23° and beta angles of ±9° or ±25°. The pellicle directs the light backwards to a zone where adequate shielding of the cameras can be achieved against radiation scattered from the aluminum window.
Optical designers assume a mathematically derived statistical distribution of the relevant design parameters for their Monte Carlo tolerancing simulations. However, there may be significant differences between the assumed distributions and the likely outcomes from manufacturing. Of particular interest for this study are the data analysis techniques and how they may be applied to optical and mechanical tolerance decisions. The effect of geometric factors and mechanical glass properties on lens manufacturability will be also be presented. Although the present work concerns lens grinding and polishing, some of the concepts and analysis techniques could also be applied to other processes such molding and single-point diamond turning.
High resolution, wide field-of-view and large depth-of-focus imaging systems are greatly desired and have received much attention from researchers who seek to extend the capabilities of cameras. Monocentric lenses are superior in performance over other wide field-of-view lenses with the drawback that they form a hemispheric image plane which is incompatible with current sensor technology. Fiber optic bundles can be used to relay the image the lens produces to the sensor's planar surface. This requires image processing to correct for artifacts inherent to fiber bundle image transfer. Using a prototype fiber coupled monocentric lens imager we capture single exposure focal swept images from which we seek to produce extended depth-of-focus images. Point spread functions (PSF) were measured in lab and found to be both angle and depth dependent. This spatial variance enforces the requirement that the inverse problem be treated as such. This synthesis of information allowed us to establish a framework upon which to mitigate fiber bundle artifacts and extend the depth-of-focus of the imaging system.
Cygnus is a high-energy radiographic x-ray source. The rod-pinch x-ray diode produces a point source measuring 1 mm
diameter. The target object is placed 1.5 m from the x-ray source, with a large LYSO scintillator at 2.4 m. Differentsized
objects are imploded within a containment vessel. A large pellicle deflects the scintillator light out of the x-ray
path into an 11-element zoom lens coupled to a CCD camera. The zoom lens and CCD must be as close as possible to
the scintillator to maximize light collection. A telecentric lens design minimizes image blur from a volume source. To
maximize the resolution of test objects of different sizes, the scintillator and zoom lens can be translated along the x-ray
axis. Zoom lens magnifications are changed when different-sized scintillators and recording cameras are used (50 or
62 mm square format). The LYSO scintillator measures 200 × 200 mm and is 5 mm thick. The scintillator produces blue
light peaking at 435 nm, so special lens materials are required. By swapping out one lens element and allowing all lenses
to move, the zoom lens can also use a CsI(Tl) scintillator that produces green light centered at 550 nm. All lenses are
coated with anti-reflective coating for both wavelength bands. Two sets of doublets, the stop, and the CCD camera move
during zoom operations. One doublet has XY compensation. The first three lenses use fused silica for radiation damage
control. The 60 lb of glass inside the 340 lb mechanical structure is oriented vertically.
Measurement of the concentration and size of suspended aerosol particles can be useful for a variety of applications.
Scanning nephelometers are frequently employed in such measurement; however, their methodology requires long
measurement times and places limitations on scattering angle. We have developed a non-scanning laser polar
nephelometer that allows for near-instantaneous measurement and greater angular range. This instrument uses both
refractive and reflective components to image light scattered from a volume of scattering particles onto an imaging
sensor, while allowing for introduction of a polarization analyzer. The scattering volume itself is imaged in the center of
the sensor, whereas the light scattered at various angles is imaged about the sensor. Angles from one hemisphere are
imaged, with an angular resolution of better than one degree. Preliminary data from suspended water droplets match
closely those quoted in the literature, with data collected from angles closer to forward- and back-scattered angles.
Design requirements; the optical design and implementation; and preliminary data and analysis are presented.
A new fisheye lens design is used as a miniature probe to measure the velocity distribution of an imploding surface
along many lines of sight. Laser light, directed and scattered back along each beam on the surface, is Doppler shifted by
the moving surface and collected into the launching fiber. The received light is mixed with reference laser light in each
optical fiber in a technique called photonic Doppler velocimetry, providing a continuous time record.
An array of single-mode optical fibers sends laser light through the fisheye lens. The lens consists of an index-matching
positive element, two positive doublet groups, and two negative singlet elements. The optical design minimizes beam
diameters, physical size, and back reflections for excellent signal collection. The fiber array projected through the
fisheye lens provides many measurement points of surface coverage over a hemisphere with very little crosstalk. The
probe measures surface movement with only a small encroachment into the center of the cavity.
The fiber array is coupled to the index-matching element using index-matching gel. The array is bonded and sealed into
a blast tube for ease of assembly and focusing. This configuration also allows the fiber array to be flat polished at a
common object plane. In areas where increased measurement point density is desired, the fibers can be close packed. To
further increase surface density coverage, smaller-diameter cladding optical fibers may be used.
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.
The National Ignition Facility and the Omega Laser Facility both have a need for measuring prompt gamma radiation as
part of a nuclear diagnostic program. A new gamma-detection diagnostic using off-axis-parabolic mirrors has been built.
Some new techniques were used in the design, construction, and tolerancing of this gamma ray diagnostic. Because of
the wavelength requirement (250 to 700 nm), the optical element surface finishes were a key design consideration. The
optical enclosure had to satisfy pressure safety concerns and shielding against electromagnetic interference induced by
gammas and neutrons. Structural finite element analysis was needed to meet rigorous optical and safety requirements.
The optomechanical design is presented. Alignment issues are also discussed.
A diagnostic instrument has been developed for the acquisition of high-speed time-resolved images at the Dual-Axis
Radiographic Hydrodynamic Test (DARHT) Facility at Los Alamos National Laboratory. The instrument was developed
in order to create time histories of the electron beam. Four discrete optical subsystems view Cerenkov light generated at
an x-ray target inside of a vacuum envelope. Each system employs cylindrical optics to image light in one direction and
collapse light in the orthogonal direction. Each of the four systems images and collapses in unique axes, thereby capturing
unique information. Light along the imaging axis is relayed via optical fiber to streak cameras. A computer is used
to reconstruct the original image from the four optically collapsed images. Due to DARHT's adverse environment, the
instrument can be operated remotely to adjust optical parameters and contains a subsystem for remote calibration. The
instrument was deployed and calibrated, and has been used to capture and reconstruct images. Matters of alignment,
calibration, control, resolution, and adverse conditions will be discussed.
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.
An instrument has been developed to acquire time-resolved tomographic data from the electron beam at the DARHT [Dual-Axis Radiographic Hydrodynamic Test] facility at Los Alamos National Laboratory. The instrument contains four optical lines of sight that view a single tilted object. The lens design optically integrates along one optical axis for each line of sight. These images are relayed via fiber optic arrays to streak cameras, and the recorded streaks are used to reconstruct the original two-dimensional data. Installation of this instrument into the facility requires automation of both the optomechanical adjustments and calibration of the instrument in a constrained space. Additional design considerations include compound tilts on the object and image planes.
It is well known that the attenuation length of radiation in any dense material increases with radiation energy. We propose a novel method of measuring x-ray and gamma spectra based on this principle. The multispectral x-ray and gamma spectrometer concept employs a scintillating material and optical camera system coupled via optical fibers. The optical fibers are placed sequentially at increasing depth with respect to the radiation path along the length of the scintillating material. Light generated by the interaction of radiation with the scintillating material is transported to the camera for recording and subsequent analysis. The proposed system will be used to determine the spectrum of incident radiation by deconvolving the radiation spectrum from the optical intensity (as a function of depth) of the recorded signals.
We have developed a method for simple and highly sensitive detection of multivalent proteins using an optical waveguide sensor. The optical biosensor is based on optically tagged glycolipid receptors imbedded within a fluid phospholipid bilayer membrane formed on the surface of a planar optical waveguide. The binding of multivalent toxin initiates a fluorescence resonance energy transfer resulting in a distinctive spectral signature that is monitored by measuring emitted luminescence above the waveguide surface. The sensor methodology is highly sensitive and specific, and requires no additional reagents or washing steps. Demonstration of the utility of protein-receptor recognition using planar optical waveguides is shown here by the detection of cholera toxin.