HOE Tutorial

Diffractive Optics Family

What can you do with a wavefront?

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

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

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

Interferometric Testing With A Computer-Generated Hologram

Spectral Filters

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

Three Beam Optical Pickup For Compact Audio Disk Player

Optical Interconnections in Microelectronics

Fiber Optic Couplers

Wavefront Sampling of High Power Laser

Wavefront Transformation System

Faceted HOEs

Directional Diffusers

Solar Applications

Wavelength Multiplexing/Demultiplexing

The Basic Optical Processor

Anti-Reflective Structures

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

Holography

Advantages [img]holo1

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 [img]holo2

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 $phi$ 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 $lambda$ .

Single Element Dispersions Showing Hybrid Achromat Possibilities

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

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

Spin Coating Photoresist

Gelatin is similar but at lower speed.

Replication Methods

3 Step Conversion of Volume HOE to Surface Blazed HOE

Laboratory Optical Test Apparatus

Rotating Slit Scanners (Beam Scan)

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

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)

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

Hologram Exposure -- Single-Beam With Nonconformal Mirror

Introduced by Yuri M. Denisyuk in early 1960s.

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

Lloyd's Mirror

Gravity plateholder (after Abramson⁹ For NDT Apps

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

Full-Aperature Transfer Hologram

Rainbow Hologram (Benton, 1965)

Holographic Stereogram, after DeBitetto, 1968-69

35 mm Holocamera by David Rowley

Contact Printing (copying) Of Transmission Or Reflection Holograms

Secondary Holograms Formed By Scattered Light In A Construction Beam

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

Secondary Holograms Formed By Surface Reflections

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

Dual-beam construction method

Spurious (secondary) Holograms

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

Michaelson Interferometer, Table Check, Fringelocker Check

Last modified on 7/21/99
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