HOE Tutorial

Contents

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

File:Car.gif

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

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

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

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

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 accumulating 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

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

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

File:Birefrin.gif

• 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