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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 !