Multilayer interference optical mirror coatings are traditionally fluence-limited by nodular inclusions. Planarization of these defects modifies the geometrically and interference-induced light intensification to increase the laser resistance of mirror coatings. Previous studies using engineered defects on the substrate or buried in the middle of the coating stack have focused only on understanding the improvement in laser resistance. However, real coating defects are distributed throughout the coating. To better understand differences between the critical fluence-limiting defects of both planarized and non-planarized mirror coatings, laser damage pit depths were determined as a function of laser fluence.
Multilayer mirrors are fluence-limited by nodular defects. Such defects originate from the deposition source, inadequate
cleaning, transport, pump down, heating, shedding from rotating hardware, etc. These overcoated inclusions behave as
micro-lenses resulting in light intensification within the multilayer structure. To minimize the impact of these defects, a
planarization process has been developed to reduce geometric-induced light intensification. By exploiting the angledependent
etching rate of materials, a deposit-and-etch process reduces nodular defect height and diameter. Planarized
defects demonstrate a greater than 20x increase in laser resistance at a wavelength of 1064 nm and pulse length of 10 ns.
Process parameters were explored such as planarization efficiency of the coating materials, discrete versus continuous
etching, thick planarization layers for substrate defects, and etching throughout the multilayer to planarize coating
A system of customized spatial light modulators has been installed onto the front end of the laser system at the National
Ignition Facility (NIF). The devices are capable of shaping the beam profile at a low-fluence relay plane upstream of the
amplifier chain. Their primary function is to introduce "blocker" obscurations at programmed locations within the beam
profile. These obscurations are positioned to shadow small, isolated flaws on downstream optical components that might
otherwise limit the system operating energy. The modulators were designed to enable a drop-in retrofit of each of the 48
existing Pre Amplifier Modules (PAMs) without compromising their original performance specifications. This was
accomplished by use of transmissive Optically Addressable Light Valves (OALV) based on a Bismuth Silicon Oxide
photoconductive layer in series with a twisted nematic liquid crystal (LC) layer. These Programmable Spatial Shaper
packages in combination with a flaw inspection system and optic registration strategy have provided a robust approach
for extending the operational lifetime of high fluence laser optics on NIF.
Substrate scratches can limit the laser resistance of multilayer mirror coatings on high-peak-power laser systems. To
date, the mechanism by which substrate surface defects affect the performance of coating layers under high power
laser irradiation is not well defined. In this study, we combine experimental approaches with theoretical simulations
to delineate the correlation between laser damage resistance of coating layers and the physical properties of the
substrate surface defects including scratches. A focused ion beam technique is used to reveal the morphological
evolution of coating layers on surface scratches. Preliminary results show that coating layers initially follow the
trench morphology on the substrate surface, and as the thickness increases, gradually overcoat voids and planarize
the surface. Simulations of the electrical-field distribution of the defective layers using the finite-difference timedomain
(FDTD) method show that field intensification exists mostly near the top surface region of the coating near
convex focusing structures. The light intensification could be responsible for the reduced damage threshold.
Damage testing under 1064 nm, 3 ns laser irradiation over coating layers on substrates with designed scratches show
that damage probability and threshold of the multilayer depend on substrate scratch density and width. Our
preliminary results show that damage occurs on the region of the coating where substrate scratches reside and
etching of the substrate before coating does not seem to improve the laser damage resistance.
Customized spatial light modulators have been designed and fabricated for use as precision beam shaping devices in
fusion class laser systems. By inserting this device in a low-fluence relay plane upstream of the amplifier chain,
"blocker" obscurations can be programmed into the beam profile to shadow small isolated flaws on downstream optical
components that might otherwise limit the system operating energy. In this two stage system, 1920 × 1080 bitmap
images are first imprinted on incoherent, 470 nm address beams via pixelated liquid crystal on silicon (LCoS)
modulators. To realize defined masking functions with smooth apodized shapes and no pixelization artifacts, address
beam images are projected onto custom fabricated
optically-addressable light valves. Each valve consists of a large,
single pixel liquid cell in series with a photoconductive Bismuth silicon Oxide (BSO) crystal. The BSO crystal enables
bright and dark regions of the address image to locally control the voltage supplied to the liquid crystal layer which in
turn modulates the amplitude of the coherent beams at 1053 nm. Valves as large as 24 mm × 36 mm have been
fabricated with low wavefront distortion (<0.5 waves) and antireflection coatings for high transmission (>90%) and
etalon suppression to avoid spectral and temporal ripple. This device in combination with a flaw inspection system and
optic registration strategy represents a new approach for extending the operational lifetime of high fluence laser optics.
Replacing growing damage sites with benign, laser damage resistant features in multilayer dielectric films may enable
large mirrors to be operated at significantly higher fluences. Laser damage resistant features have been created in high
reflecting coatings on glass substrates using femtosecond laser machining. These prototype features have been damage
tested to over 40 J/cm2 (1064nm, 3ns pulselength) and have been shown not to damage upon repeated irradiation at
40J/cm2. Further work to optimize feature shape and laser machining parameters is ongoing.
Removal of laser-induced damage sites provides a possible mitigation pathway to improve damage resistance of coated
multilayer dielectric mirrors. In an effort to determine the optimal mitigation geometry which will not generate
secondary damage precursors, the electric field distribution within the coating layers for a variety of mitigation shapes
under different irradiation angles has been estimated using the finite difference time domain (FDTD) method. The
coating consists of twenty-four alternating layers of hafnia and silica with a quarter-wave reflector design. A conical
geometrical shape with different cone angles is investigated in the present study. Beam incident angles range from 0° to
60° at 5° increments. We find that light intensification (square of electric field, |E|2) within the multilayers depends
strongly on the beam incident direction and the cone angle. By comparing the field intensification for each cone angle
under all angles of incidence, we find that a 30° conical pit generates the least field intensification within the multilayer
film. Our results suggest that conical pits with shallow cone angles (≤ 30°) can be used as potential optimal mitigation
The presence of defects in optical materials can lead to bulk damage or downstream modulation and subsequent surface
damage in high fluence laser systems. An inclusion detection system has been developed by the National Ignition
Facility Optics Metrology Group. The system detects small inclusions in optical materials with increased sensitivity and
speed over previous methods. The system has detected all known inclusions and defects and has detected previously
undetected defects smaller than 5 microns.
The Mercury laser uses ytterbium-doped strontium fluorapatite (Yb:S-FAP) crystals as the gain medium with a nominal
clear aperture of 4 x 6 cm. Recent damage test data have indicated the existence of bulk precursors in Yb:S-FAP that
initiate damage starting at approximately 10 J/cm2 at 9 ns under 1064 nm irradiation. In this paper, we report on
preliminary results on bulk damage studies on Yb:S-FAP crystals.
An automated laser damage test system has been developed by the National Ignition Facility small optics metrology
group. The Small Optics Laser Damage (SOLD) system measures the fluence at which laser damage occurs in optical
coatings and substrates following the requirements of MEL01-013-OD. Irradiation of the sample is by a 1064nm, 8ns
pulse with a 1mm 1/e2 diameter. The test protocol requires raster scanning of a 1cm2 area at increasing fluence levels.
Real-time high-resolution imaging of the surface during raster scanning enables automated detection and sizing of
defects to 10 microns. Improved imaging resolves actual size of damage events while the automated damage detection
removes the subjectivity of the human operator in thresholding damage events. In addition, a map is created enabling
additional functions such as excluding damage sites on future scans and to returning to the damage site for growth
The laser damage test for qualifying a coating run of anti-reflection coated optics consists of scanning a pulsed 1064 nm laser to illuminate approximately 2400 sites over a 1 cm x 1 cm area on a test sample. Scans are repeated at 3 J/cm2 increments until the fluence specification for the optic is reached. In the past, initiation of 1 or more damage sites was classified as a failed coating run, requiring the production optics in the corresponding coating lot be reworked and recoated. Recent laser damage growth tests of 300 repetitive pulses performed on numerous damage sites revealed that all were stable up to 20 J/cm2. Therefore the acceptance criteria has been modified to allow a moderate number of damage sites, as long as they are smaller than the allowed dig size and are stable (do not grow). Consequently many coating runs that previously would have been rejected are now accepted, resulting in higher yield, lower cost, and improved delivery schedule. The new test also provides assurance that initiated damage sites are stable during long term operation.
Mueller matrix imaging polarimetry was used to study stress in a series of a high numerical aperture molded glass lenses. An interesting radially symmetric retardance was found which resembled the polarization aberration induced by coatings. Upon investigation the source of the polarization aberration is traced to a remarkably symmetric radial stress birefringence in the glass believed to arise during fire-polishing of the surfaces. While annealing the lenses relieves much of the stress birefringence, reducing the retardance of the lenses by a factor of five, the lenses remained unusable for critical polarization applications.
A high speed Mueller matrix imaging polarimeter is presented. The instrument enables measurement of the full Mueller matrix in transmission, reflection, or retro-reflection. The Mueller matrix provides a complete description of the polarization transforming properties of the sample. The retardance, diattenuation, polarizance, and depolarization are all characterized by the polarimeter. The polarimeter is able to measure the polarization properties of samples ranging from sub-millimeter optical components to large optics. The imaging capabilities can be modified to measure the polarization properties across the surface of the sample or as a function of the angle through the sample. The dual rotating retarder polarimeter provides up to sixty-two full Mueller matrix images per second. Instrument details, measurement techniques, example data, and applications are presented.
Reflective Liquid Crystal on Silicon (LCoS) panels are widely used as light valves for projection displays. LCoS panels and the associated beam splitters, retardance films, and dichroic beam splitters display significant variations in polarization properties over the area, angle of incidence and spectral bandwidth of the projector. This paper surveys these polarization aberrations and describes a high speed Mueller Matrix Imaging Polarimeter (MMIP) for the characterization of these polarization aberrations. The characterization of projection systems and components by the MMIP enables advanced modeling and compensation of polarization aberrations.