HOE Tutorial

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Contents

Diffractive Optics Family

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What can you do with a wavefront?

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  • Diffractive elements can be single-order or multi-order
  • Patterning resolution x Area (SBWP) is a measure of absolute design freedom
  • Phase encoding techniques provide the effective design freedom
  • Very large SBWP can be made by combining holographic recording with computed DOEs

Diffractive Optical Element Basic Functions

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Application Examples

  • Beam-combiners for display systems
  • Laser scanners
  • Low noise and high performance diffraction gratings
  • Asphere testing elements
  • Spectral notch filters
  • Holographic laser optical heads
  • Optical interconnections in microelectronics
  • Wavefront sampling
  • Wavefront transformation-diffusers
  • Solar concentrations
  • Wavelength multiplexers/demultiplexers
  • Various unique laser optical elements

HUD with combiner laminated into the windshield for Volkswagon


Multi Order Super HOE Scanner

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Potential Advantages Of Holographic Disk Scanners

  • Simpler optical arrangement
  • Larger tolerances for wobble
  • Less air turbulence
  • Each facet can have a different focal length
  • Lower production cost per unit
  • Scan angle is independent of the number of facets

Aberration-Corrected HOE Grating For Spectrometer

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Interferometric Testing With A Computer-Generated Hologram

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Spectral Filters

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Advantages of Holographic Spectral Filters

  • Easy fabrication of large filters
  • High efficiency
  • Parallel layering is not a constraint
  • Free from extranious passbands

Colour Combination/Colour Separation

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Three Beam Optical Pickup For Compact Audio Disk Player

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Optical Interconnections in Microelectronics

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Fiber Optic Couplers

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Wavefront Sampling of High Power Laser

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Wavefront Transformation System

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Faceted HOEs

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Directional Diffusers

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Solar Applications

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Wavelength Multiplexing/Demultiplexing

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The Basic Optical Processor

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Anti-Reflective Structures

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  • As the grating period gets smaller, the diffraction angles increase
  • Ultimately, gratings have only zero order transmitted and zero order reflected
  • Tailoring duty cycle and etch depth one can control the power in these two remaining orders
  • This is the same as an impedance match in electricity and magnetism

Concept For Holographic Night Goggles

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Holography

Advantages

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  • Images to a point with no aberrations
  • Aberration control possible
  • Highly dispersive
  • Optical power on a flat surface
  • Off-axis geometry
  • Can be transmissive or reflective
  • Narrowband response
  • Can be replicated

Disadvantages

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  • High dispersion
  • Large aberration away from construction conditions
  • Efficient over small wavelength band
  • Limited design flexibility
  • Difficulty with control of holographic emulsions

Advantages of HOE Diffraction Gratings (HOEDGs)

Property Classical Gratings HOEDGs
Efficiencies 60 to 99% 50 to 90% (surface relief only)
Efficiency at blaze is lower but the efficiency curve is flatter
Ghosts At best 10-5 (usually 10-2) of parent line No ghosts at all
Scattered light At best 10-5 to 10-6 at 5Å of laser line in visible At best 10-6 to 10-8 at 5Å of laser line in visible
Size In general standard sizes are limited to 8x8" Up to Φ 17", but can be larger
Number of grooves Maximum 3600 lines/mm (There are rare exceptions.)
Scattered light increases drastically with density
Up to 6000 lines/mm
No increases of scatter with groove density
Optical power No Yes
Volume HOEs can diffract 99% @ Bragg angle and center λ.

Single Element Dispersions Showing Hybrid Achromat Possibilities

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Comparisons of Fabrication Methods Of Diffractive Optics


Diamond Turning Direct-Write Holographic or Photo-Lithography Embossing Injection Molding
Practical production volumes 100 ~ 102 100 ~ 102 100 ~ 105 103 ~ 105 103 ~ 107
Initial tooling costs low-moderate low moderate-high low-moderate high
Precision low-moderate good-excellent excellent moderate moderate
Materials metals, plastics glasses, semiconductors glasses, semiconductors plastics plastics
Volume production costs high high low-moderate low low

Direct Laser Writing

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  • Spot sizes ~1 - 5um
  • Tightly Focuses, modulated He-Cd or Argon-ion laser scanned across photresists surface
  • Up to 256 phase levels
  • Serial Process
  • Difficult to accurately transfer structure into substrate
  • Direct ablation of polyimide layer on substrate using an excimer laser is also possible
  • Pattern can be transferred to a VHOE by processing in a 4f optical processor.

Photoresist Processes For Lithography

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Spin Coating Photoresist

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Replication Methods

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3 Step Conversion of Volume HOE to Surface Blazed HOE

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Laboratory Optical Test Apparatus

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Rotating Slit Scanners (Beam Scan)

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  • Narrow, rotating slit is scanned through pattern
  • Measure irradiance profiles with ~micron lateral precision
  • Slit widths down to 1 um
  • Scan areas over 10 mm are possible
  • Measurement of both near and far-field diffraction patterns
  • Both 1-D and 2-D scans can be performed

Scatterometer

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  • Measures irradiance patterns from DOE's by scanning a detector and pinhole
  • Scanning and data acquisition is computer controlled (LabView™ software)
  • Precision depends on pinhole size and step-size of motorized stage
  • Slow process
  • Can be difficult to align scan axis

Ronchi Rule -- Gaussian Spot Sise Measurement. (Lee Dickson)

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  • do = 1/e2 spot
  • w = bar width
  • K = pmin/pmax
  • do/w = 2.2K + 1

Side view of ruling in beam

An Electromagnetic Shutter From A D'Arsenual

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Shutter is silent and can easily be configured to close after accumulating a preset energy per unit area.

Hologram Exposure -- Single-Beam With Nonconformal Mirror

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Introduced by Yuri M. Denisyuk in early 1960s.

Single Beam Frame Using All Second Surface Mirror Without Ghosts (from Saxby)

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Lloyd's Mirror

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Gravity plateholder (after Abramson9 For NDT Apps

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Film Holder With Xylene Well (after Benton, 1960s)

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Full-Aperature Transfer Hologram

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Rainbow Hologram (Benton, 1965)

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Holographic Stereogram, after DeBitetto, 1968-69

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35 mm Holocamera by David Rowley

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Contact Printing (copying) Of Transmission Or Reflection Holograms

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Secondary Holograms Formed By Scattered Light In A Construction Beam

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Any stray or scattered light can combine with a construction beam to form secondary transmission and reflections holograms

Secondary Holograms Formed By Surface Reflections

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The reflection portion of construction wave 1 combines with construction wave 2 to form a Transmission hologram

Prevention Of Secondary Holograms Formed By Surface Reflections

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Spurious (secondary) Holograms

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  • Desired hologram:
    • Reflection hologram AB
  • Spurious holograms:
    • Reflection hologram AA1
    • Reflection hologram BB1
    • Reflection hologram A1B1
    • Transmission hologram AB1
    • Transmission hologram A1B

Prevention of Secondary Holograms

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Michaelson Interferometer, Table Check, Fringelocker Check

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Unique Characteristics Of HOEs

  • Perfect imaging between two points for a single wavelength
  • Useful in unusual (I.E., not in-line) geometries
  • Shape independent (I.E., flat surfaces can have optical power)
  • Extremely dispersive (effice v-number of -3.45)
  • Angle selection
  • Wavelength selective
  • Multiple functions
  • Multiple elements in the same aperture
  • Compact and light weight
  • Relatively inexpensive - low cost "photographic" replication

Requirements on Construction Optical System

  • Hight quality optical elements
  • Minimize multiple reflections between surfaces of construction optics and hologram substrate
  • Scattered light should be prevented from falling on hologram plate
  • Mechanical and thermal stability during exposure
  • Proper coherence length
  • Polarization of two recording beams should be maintained properly
  • Active fringe stabilization system for long exposures

Form Birefringence

  • Subwavelength gratings behave somewhat like biaxial crystals
  • As the period gets small relative to the wavelength, we can calculate an equivalent dielectric constant or index of refraction (n)


Last modified on 7/21/99