Download Veiling glare analysis is required to ensure high contrast lens

Ghost Reflections in Optical Systems
(Studying the unintended ray paths)
R. Edward English Jr.
3M Precision Optics, Inc.
April 8, 2008
Early consideration of ghost reflections in the
design process leads to practical system solutions
Requirements
Concept
s
Design
Build
Prototype
& Test
Analysis
Data
Understand?
Build
!
The atomic vapor laser isotope separation (AVLIS)
process selectively photoionizes U235 for
electrostatic collection and enrichment
Design a system to reliably and continuously inject
200 W copper vapor laser power into a 1 mm core
fused silica fiber
Parameter
Specification
Object size
51.3 mm
Object location
Front focal plane of parabolic reflector
with efl = 350 mm
Image size
0.9 mm
Numerical aperture
< 0.14
Image distortion
< 15%
Wavelength
578.2 nm (& 510.6 nm)
Beam quality (~NA in object space)
275 μrad (20x diffraction limited)
Entrance pupil
Telecentric (collimated laser beam)
The requirement for stable, long-term operation
helped determine the basic optical system
approach
• Underfill fiber in space and angle
– 0.9 mm image size on 1.0 mm fiber
– 0.14 NA light cone into 0.18 NA fiber
• Image a physical aperture onto the fiber
– Reduces sensitivity to damage
• Non-uniform illumination
– Insensitive to beam position
– Sensitive to beam direction
• More uniform illumination
– Sensitive to beam position
– Insensitive to beam direction
The laser beam first impinges on the off-axis
section of a parabola before encountering the relay
telescope and the optical fiber
Something happened during the initial test
Unintended light paths (and operational
techniques) must be considered
Shape of lens matters
Equi-convex vs. best form
Collimation matters
Mis-timed operation to allow
in-situ adjustment also
caused damage because
beam divergence is
substantial (ASE)
A design study explored the trade-off
between power density on the lenses and
length of the telescope
The system used the obscured region of the
annular laser to create multiple channels
1500 W laser split into green and
yellow by dichroic splitter
Different facet sizes for green and
yellow to meet dye chain pump
requirements
fiber
injectors
input beam
faceted mirror
dichroic mirror
parabolic mirror
Successful demonstration led to a fully
multiplexed, robust fiber distribution network
to pump a multiple dye laser system
The National Ignition Facility is the world’s largest
laser system, designed to heat a fusion capsule to
thermonuclear ignition
The 192 laser beams of NIF will generate (3ω)
Peak power of 500 TW
Pulse energy of 1.8 MJ
Pulse lengths from 3-20 ns
7500 large optics
(40 cm clear aperture)
Building is 704’ x 403’ x 85’
The optical path length through the main
laser to the target chamber is > 300 m
An afocal relay telescope is a standard
configuration for multi-pass laser cavities
• Cavity end mirrors are relayed (imaged) onto each other
• Amplifier slabs are oriented at Brewsters’ angle to
reduce Fresnel losses
• The spatial filter with pinhole restricts the angular spread
so as to “clean up” the beam and ensure a spatially
uniform irradiance profile
z1
f
Amplifier slabs
(at Brewsters’ angle)
z1+z2 = 2f ; total cavity length = 4f
f
Pinhole
z2
Spatial
filter
lens
Cavity
end
mirror
A Pockels cell allows the beam to make four passes
through the main laser amplifier before switching out
for a final pass through the power amplifier
1:1 Afocal telescope + two end
mirrors forms the main laser cavity
The National Ignition Facility is probably the
world’s largest, most complex optical system
Because the NIF laser beam energy can be 18kJ
at 1ω, a reflection of 0.1% creates an 18J
reflection (a respectable laser in its own right)
• The single-bounce backward
and double-bounce forward
focal lengths define the ghosts
for a collimated beam
n 1
f121 
 f  0.236  f
2n  1
f1212 
n 1
 f  0.134  f
3n  1
– Formulas shown are for
symmetric bi-convex lens
(zero thickness assumed)
– Exercise for the reader to
prove that the double-bounce
forward ghost does not
depend on lens shape
• The ghost foci for the diverging
beam can be similarly derived
z121 
n 1
 f  0.309  f
n
z1212 
n 1
 f  0.155  f
2n
The basic strategy is to not place components
near the single-bounce ghost foci because of the
substantial beam energy and high irradiance
The specific criteria stated that components should not be
placed where the ghost irradiance is 10% of the maximum
fluence of the main beam
z  0.289  f
The cavity spatial filter lens has a focal length of 11.75 m
Thus, the ghost stay-out length is ~3.4 m
The transport spatial filter lens was tilted to
reduce the ghost stay-out zone by ~2 m
• 30 m focal length would give a stay-out zone of 8.67 m
– This was reduced to 6.66 m by tilting the lens so that the ghost
reflections come to focus outside the beam
– The lens was fabricated as a bilaterally symmetric asphere to
compensate for the coma and astigmatism introduced
• This optical complication ...
– Reduced building length and saved money
• 6 ft * bldg width (400 ft) * $2000/sf = $4.8 mm
• Incremental lens fabrication and beam dump cost was <$1.5 mm
– And eliminated energetic pencil beams
Pencil beams are formed from back
reflections (e.g., from lens surface) that are
clipped by spatial filter pinholes
• Lens reflections diverge toward the pinhole, which
transmits a beam the diameter of the pinhole
– CSF pinholes are ± 100-200 μrad (2.4-4.8 mm dia)
– Pencil beams remain small through the system
• Pencil beams are low energy initially
– Amplification can raise them to damage fluence levels
Output after PA ~ 15kJ
SF3 pencil beam ratio 5.9x10-5
Reflectivity of 1%
Two cavity pass (MA) + booster (PA)
(50x gain)  ~0.4 J
Beam area ~ π(0.25 cm)2 ~ 0.2 cm2
Fluence ~ 2 J/cm2
For NIF it is extremely important to anticipate and
mitigate a large number of unintended ray paths
• Testing on a single beam line in the 1990’s was essential
for validating these analyses
– Beamlet was used to study these effects as well as many other
laser physics effects
– Testing and modeling ensure understanding
• The final optics assembly (i.e., where frequency
conversion occurs and the beam is focused onto the
target) is an extremely sensitive location of ghost
reflections
– Several optical elements in close proximity
– High power operation “turns on” non-linear self-focusing effects
that limit the total system performance
– By design, the laser output power is limited by optical damage
threshold in the final optics
• 1.8 MJ / (35cm)2 / 192 beams * 1.5 modulation ~ 12 J/cm2
Projection lenses for television are moderately
fast, relatively wide field-of-view with an emphasis
on color correction and contrast
• The major design requirements include
–
–
–
–
–
–
–
Moderate to fast speed (f/2.4)
Magnification of ~70x
Low distortion (<1%)
High contrast (ANSI > 150:1, or more)
Color corrected (longitudinal and lateral)
Wide field-of-view (HFOV of 45° or more, if offset)
Resolution (resolve pixels, ~50 lpm, with MTFs above 80% onaxis and 60% in the corner)
– Fixed conjugates (no zoom required, focus during assembly)
– Low cost (<$50)
• The system configuration wants a relatively compact,
perhaps folded, lens
Most lens forms have a meniscus front
element, which is often a plastic asphere
USP 5,870,228
USP 6,144,503
USP 5,042,929
USP 5,760,965
 Compute marginal rays from boundary points A & B (in
Light
from the
display device will fall upon
space
of interest)
to target.
edges
ofAngle
optics
andofon
housing
 Veiling
Glare
 Slope
the the
steepest
of these. ... this
the
can
cause
veiling
glare
problems
 Veiling
Glare
Line applies
to entire
space of interest.
Tar Lx (steepest)
B
A
Tar Rx-1
Area seen by screen
screen space
target
5
Area illuminated by stray light
A veiling glare analysis permits definitive control of
stray light by sizing of elements, edge shape, and
housing design
Seen by screen,
but not illuminated
Illuminated, but
not seen by screen
Unintended ray paths can be frustrated in the design process,
avoiding last-minute “fixes” like paint and baffles
Unintended ray paths are evaluated with nonsequential analysis and studied in more detail with
traditional lens design software
• A reflection from an element in the rear group can form
an blurred image on the imager, which is then projected
onto the screen
15:54:44
– Hardware solutions include
• Better AR coatings
• Insertion of a quarter-wave plate
• Redesign of lens
ghost reflection
Understanding the origin of potentially offending
ghost reflections allowed control of them during the
design process
• If marginal ray angles at all surfaces exceed 2°, then
contrast reduction due to ghost reflections is minimal
• A reduced element count lens, with marginal ray angles
>5° on all glass surfaces gave us a lens with ANSI
contrast >350:1
Early consideration of ghost reflections in the
design process leads to practical system solutions
Requirements
Concepts
Design
Prototype
& Test
• Don’t wait too long to test something
– Sometimes testing is quicker than analysis
and provides key insights to guide the analysis
Analysis
Data
• But do the analysis, too!
Understand?
– Make sure your test truly tests the critical
requirements and assumptions
– Be relentless in understanding the results and having a model that
accurately explains the results
Build
• Learn how to think ahead and anticipate problems
• Hypothesis testing (through analysis and experiment) is
a vital part of successful optical engineering
!