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.
Two precision mirror gimbals were designed using slit diaphragm flexures to provide two-axis precision mirror alignment in space-limited applications. Both gimbals are currently in use in diagnostics at the National Ignition Facility: one design in the Gamma Reaction History (GRH) diagnostic and the other in the Neutron Imaging System (NIS) diagnostic. The GRH gimbal has an adjustment sensitivity of 0.1 mrad about both axes and a total adjustment capability of ±6°; the NIS gimbal has an adjustment sensitivity of 0.8 μrad about both axes and a total adjustment range of ±3°. Both slit diaphragm flexures were electro-discharge machined out of high-strength titanium and utilize stainless steel stiffeners. The stiffener-flexure design results in adjustment axes with excellent orthogonality and centering with respect to the mirror in a single stage; a typical two-axis gimbal flexure requires two stages. Finite element analyses are presented for both flexure designs, and a design optimization of the GRH flexure is discussed.
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 (NIF) has a need for measuring gamma radiation as part of a nuclear diagnostic program.
A new gamma-detection diagnostic uses 90° off-axis parabolic mirrors to relay Cherenkov light from a volume of
pressurized gas. This nonimaging optical system has the high-speed detector placed at a stop position with the
Cherenkov light delayed until after the prompt gammas have passed through the detector. Because of the wavelength
range (250 to 700 nm), the optical element surface finish was a key design constraint. A cluster of four channels (each
set to a different gas pressure) will collect the time histories for different energy ranges of gammas.
The National Ignition Facility will begin testing DT fuel capsules yielding greater than 10<sup>13</sup> neutrons during 2010.
Neutron imaging is an important diagnostic for understanding capsule behavior. Neutrons are imaged at a scintillator
after passing through a pinhole. The pixelated, 160-mm square scintillator is made up of 1/4 mm diameter rods 50 mm
long. Shielding and distance (28 m) are used to preserve the recording diagnostic hardware. Neutron imaging is light
starved. We designed a large nine-element collecting lens to relay as much scintillator light as reasonable onto a 75 mm
gated microchannel plate (MCP) intensifier. The image from the intensifier's phosphor passes through a fiber taper onto
a CCD camera for digital storage. Alignment of the pinhole and tilting of the scintillator is performed before the relay
lens and MCP can be aligned. Careful tilting of the scintillator is done so that each neutron only passes through one rod
(no crosstalk allowed). The 3.2 ns decay time scintillator emits light in the deep blue, requiring special glass materials.
The glass within the lens housing weighs 26 lbs, with the largest element being 7.7 inches in diameter. The distance
between the scintillator and the MCP is only 27 inches. The scintillator emits light with 0.56 NA and the lens collects
light at 0.15 NA. Thus, the MCP collects only 7% of the available light. Baffling the stray light is a major concern in the
design of the optics. Glass cost considerations, tolerancing, and alignment of this lens system will be discussed.
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.
The design and assembly of a nine-element lens that achieves >2000 lp/mm resolution at a 355-nm wavelength
(ultraviolet) has been completed. By adding a doublet to this lens system, operation at a 532-nm wavelength (green) with
>1100 lp/mm resolution is achieved. This lens is used with high-power laser light to record holograms of fast-moving
ejecta particles from a shocked metal surface located inside a test package. Part of the lens and the entire test package are
under vacuum with a 1-cm air gap separation. Holograms have been recorded with both doubled and tripled Nd:YAG
laser light. The UV operation is very sensitive to the package window's tilt. If this window is tilted by more than 0.1
degrees, the green operation performs with better resolution than that of the UV operation. The setup and alignment are
performed with green light, but the dynamic recording can be done with either UV light or green light. A resolution plate
can be temporarily placed inside the test package so that a television microscope located beyond the hologram position
can archive images of resolution patterns that prove that the calibration wires, interference filter, holographic plate, and
relay lenses are in their correct positions. Part of this lens is under vacuum, at the point where the laser illumination
passes through a focus. Alignment and tolerancing of this high-resolution lens are presented. Resolution variation across
the 12-mm field of view and throughout the 5-mm depth of field is discussed for both wavelengths.
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.
Shock waves passing through a metal sample can produce ejecta particulates at a metal-vacuum interface. Holography
records particle size distributions by using a high-power, short-pulse laser to freeze particle motion. The sizes of the
ejecta particles are recorded using an in-line Fraunhofer holography technique. Because the holographic plate would be
destroyed in an energetic environment, a high-resolution lens has been designed to relay the scattered and unscattered
light to a safe environment where the interference fringes are recorded on film. These interference fringes allow particles
to be reconstructed within a 12-mm-diameter, 5-mm-thick volume. To achieve resolution down to 0.5 μm, both a high-resolution
optical relay lens and ultraviolet laser (UV) light were implemented. The design and assembly of a nine-element
lens that achieves >2000 lp/mm resolution and operates at f/0.89 will be described. To set up this lens system, a
doublet lens is temporarily attached that enables operation with 532-nm laser light and 1100 lp/mm resolution. Thus, the
setup and alignment are performed with green light, but the dynamic recording is done with UV light. During setup, the
532-nm beam provides enough focus shift to accommodate the placement of a resolution target outside the ejecta
volume; this resolution target does not interfere with the calibrated wires and pegs surrounding the ejecta volume. A
television microscope archives images of resolution patterns that prove that the calibration wires, interference filter,
holographic plate, and relay lenses are in their correct positions. Part of this lens is under vacuum, at the point where the
laser illumination passes through a focus. Alignment and tolerancing of this high-resolution lens will be presented, and
resolution variation through the 5-mm depth of field will be discussed.