HOE Tutorial

From HoloWiki - A Holography FAQ
Jump to: navigation, search

Contents

Diffractive Optics Family

Doptics.gif

What can you do with a wavefront?

Whatdo1.gif Whatdo2.gif
  • 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

enter

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

Suprhoe.gif

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

Abrcorfs.gif

Interferometric Testing With A Computer-Generated Hologram

Comgenho.gif

Spectral Filters

Spctfltr.gif

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

Colour.gif

Three Beam Optical Pickup For Compact Audio Disk Player

Cdplayer.gif

Optical Interconnections in Microelectronics

Opintmcr.gif

Fiber Optic Couplers

Fiberopc.gif

Wavefront Sampling of High Power Laser

Hipowlsr.gif

Wavefront Transformation System

Wavetran.gif

Faceted HOEs

Faceted.gif

Directional Diffusers

Dirdiff.gif

Solar Applications

Solarapp.gif

Wavelength Multiplexing/Demultiplexing

Wavelngt.gif

The Basic Optical Processor

Basoppro.gif

Anti-Reflective Structures

Antiref.gif
  • 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

Niteggls.gif

Holography

Advantages

Holo1.gif
  • 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

Holo2.gif
  • 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

Sngledis.gif

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

Dir.gif
  • 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

Photo.gif

Spin Coating Photoresist

Spincoat.gif

Replication Methods

Repmeth.gif

3 Step Conversion of Volume HOE to Surface Blazed HOE

3step.gif

Laboratory Optical Test Apparatus

Labtest.gif

Rotating Slit Scanners (Beam Scan)

Rotslit.gif
  • 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

Scattm.gif
  • 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)

Ronchi.gif
  • 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

Shutter.gif

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

Nncnmirr.gif

Introduced by Yuri M. Denisyuk in early 1960s.

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

Brewster.gif

Lloyd's Mirror

Loydsmir.gif

Gravity plateholder (after Abramson9 For NDT Apps

Gravity.gif

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

Filmhold.gif

Full-Aperature Transfer Hologram

Focusopt.gif

Rainbow Hologram (Benton, 1965)

Rainbowh.gif

Holographic Stereogram, after DeBitetto, 1968-69

Debiteto.gif

35 mm Holocamera by David Rowley

Holocam.gif

Contact Printing (copying) Of Transmission Or Reflection Holograms

Ctcprnt.gif

Secondary Holograms Formed By Scattered Light In A Construction Beam

Secndary.gif

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

Secondary Holograms Formed By Surface Reflections

Surfrefl.gif

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

Orevsd.gif

Spurious (secondary) Holograms

Spurius.gif
  • 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

Prvntsho.gif

Michaelson Interferometer, Table Check, Fringelocker Check

Micinter.gif

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