Improvement and Commissioning of a Novel Technology for the Measurement of Laser Beam Profiles Royal Observatory, Edinburgh Science and Technology Facilities Council Innovations Club and Photonics Knowledge Transfer Network
Wavefront Metrology Simon Hall
National Physical Laboratory
Outline
• NPL intro • Why is metrology required? • Project to produce an adaptive eye
simulator • Components of adaptive optics systems • NPL artefacts for characterisation of
wavefront sensors • Aberration generator • Laser beam propagation measurement
NPL operation
• The laboratory is owned by the UK Government (through the Department for Innovation, Universities and Skills) and operated under contract by NPL Management Ltd, a wholly owned subsidiary of Serco Group plc
NPL operation
NPL is a world leading centre for the development and application of highly accurate measurement techniques
As the UK’s national standards laboratory, NPL underpins the national measurement system, ensuring consistency and traceability of measurements throughout the UK
Basic Adaptive Optic System Schematic
Wavefront sensor Wavefront Modulator
Light Path
Traceability & Linearity measurement have a role in ensuring interoperability of components
Adaptive Optics applied to physical emulation of Human Eye • Product development of laser and intense light
source consumer devices requires insight into how the light propagates within the eye under all eye accommodation conditions (e.g. Development of laser projection TV; Superbright LED optical sources; Intense Pulsed Light sources for medical applications)
• NPL is producing an eye analogue with a lens that can scan through the range of accommodation available to the human eye.
The human eye (front view)
Entrance Pupil (2 mm to 7 mm)
Iris Diaphragm
Conjunctiva (UV Conjunctivitis)
Ultraviolet light tends to be harmful to the front elements of the eye
Many laser injuries are to the rear of the eye (retina)
The human eye (side schematic)
Lens
Cornea
Fovea
Macula
• Refractive power of the eye – Pupil acts as aperture stop – Can focus light to 20 micron spot size – Image centered on fovea
• Region of sharpest visual acuity • Highest density of rods and cones
Imaging properties
Blind spot & Optic Nerve
this region)
Accommodation Distance
• Closest distance of sharpest focus – ≈ 100 to 200 mm for normal human vision – ‘Reasonably foreseeable’ viewing distance of source? – Less than 100 mm image of source will be blurred
• Unless viewing aids are used – Microscope, eye loupe
IEC 60825-1 International
Radiation is transmitted and focused by lens
300 nm to 1400 nm
Ideal Laser Beam Propagation
Calculation of spot size on retina
d01 beam waist diameter of input beam L1 waist to lens distance Zr1 Rayleigh length of input beam fe focal length of lens L2 lens to transformed waist distance
Zr1 Rayleigh length of output beam d02 beam waist diameter of output beam dr beam diameter on retina Lr transformed waist to retina distance
Adaptive Eye Simulator Liquid Lens CCD
Liquid Lens Schematic Constructed of two different drops of liquid, one electrically conducting, one insulating.
Drops sit on top of each other in conic ring of metal.
When electrical charge applied to metal ring conductive liquid changes its electrowetting characteristics
Liquid lens no voltage applied
Water
Glass Liquid lens
no voltage applied
Constructed of two different drops of liquid, one electrically conducting, one insulating.
Drops sit on top of each other in conic ring of metal.
When electrical charge applied to metal ring conductive liquid changes its electrowetting characteristics
Ray Diagram for liquid lens with no applied voltage
Liquid Lens No voltage applied: Ray Diagram
Liquid lens voltage applied
SSL Light Engine ‘Quarterlite’
• LED Safety during manufacture? – assembly and testing of SSL engine
• Cool White HB-LED source: – Direct viewing of source indicated as
being ‘Low risk’
International Standards used for Optical Radiation Safety
IEC 60825-1:2007 Ed. 2.0 IEC 62471:2006 Ed. 1.0
Apparent Source Size
Retinal Thermal Damage
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
Sp ec
Essential Optical Hazard Parameter “Angular Subtense” can be measured using adaptive optics
α = angular subtense as used for laser hazard measurement in the international standard IEC 60825-1 and the CIE S 009:E/2002 Photobiological Safety of Lamps and Lamp Systems’ now also adopted as IEC 62471
Apparent Source Size
reflector reflected ray
chipsubstrate
Movie of HB LED viewed through a changing focal length lens
LED Beam Propagation
Medical diagnostics using optical probing of the eye
Abberations induced in the input wavefront are detected as they reflect back from structures in the eye. This can provide valuable measurements of whole body concentrations of certain analytes eg Glucose.
Operation of Hartmann-Shack Wavefront Sensor
f
CCDLenslet Array
•The incoming wavefront is divided up and focussed onto a CCD by the microlens array
Flat Wavefront
•An area of interest is found for each spot within which the centroid can be calculated
Operation of Hartmann-Shack Wavefront Sensor
Tilted Wavefront
CCDLenslet Array
•The wavefront slope over each lenslet can be measured from the deviation of the centroid from a reference •The wavefront is then reconstructed by integrating the wavefront slope measurements
Traceable Wavefront Measurements
Distance / mm
Va ria
tio n
of m
ea su
re d
w av
ef ro
Traceable Wavefront Measurements
-2
-1
0
1
2
3
4
5
Introduced tilt / mr
Traceable Wavefront Measurements
Hartmann-Shack Peak-Valley Measurement / waves
Zygo Peak-Valley Measurement / waves
0.35 ± 0.03 0.37 ± 0.02
Aberration Generator
• Previous NPL work has produced artefacts which produce known types and amounts of aberration (i.e. Spherical Radius of Curvature; Tilt; Spherical Aberration; Astigmatism)
• To properly characterise wavefront sensors a continuum of calibrated aberrations is required to enable linearity, hysteresis, as well as traceability at fixed points.
• Proposed aberration generators are deformable mirrors which are characterised against the primary NPL interferometer.
MMDM mirror
• 40 mm diameter • 59 electrodes •Maximum deflection of the mirror centre 15μm
PDM mirror
of the mirror centre 9μm
Characterisation of Deformable Mirrors
•Traceable interferometric measurements of actuator influence functions
•Measurement of actuator hysteresis
Characterisation of Deformable Mirrors
•Traceable interferometric measurements of actuator influence functions
•Measurement of actuator hysteresis
in front of the carriage
Measuring a Beam
Move the carriage and virtual CCD through the beam – measuring diameter either side of the waist.
Plot Hyperbola onto sampled beam diameters
0 0.1 0.2 0.3 0.4 0.5 0
5 .10 4
Iteration Number
W id
th (m
Iteration Number
W id
th (m
Iteration Number
W id
th (m
Iteration Number
W id
th (m
Iteration Number
W id
th (m
Iteration Number
W id
th (m
Iteration Number
W id
th (m
General Astigmatism
Ellipticity of the beam rotates as the beam propagates (twist).
ISO 11146-1 Methodology Measurement Sequence • Set CCD at known distance from Laser aperture • Measure irradiance profile • Calculate converging second moment (CSM) • Set CCD to new distance • Repeat measurements and CSM calculation • Fit hyperbola to CSM values • Find beam waist size (hyperbola minimum) and
location from Laser aperture • Calculate M2
ISO 11146-1 Beam parameter measurement equipment
Laser source
Novel beam propagation measurement device incorporates a Crossed Fresnel grating
cross-distorted diffraction phase grating (IMP® grating) to obtain
intensity profiles simultaneously from nine planes
around the beam waist.
Schematic of IMP grating operation relating planes of laser beam to images on camera
CCD
AWS-100 Device
Using a Variable M2 source to compare the ISO 11146-1 method and a grating Wavefront Sensor
VCSEL Focusing lens CCD
sensor on z-axis carriage
NPL 11146-1 measurement apparatus
previous work which provided the first traceable metrology available for Wavefront Sensors and Laser Beam Propagation Measurement.
• Commercialisation of adaptive optics systems will require traceability to ensure interoperability of individual components and to allow manufacture of generic systems rather than the current bespoke products
• The project aims to address problems with optical hazard measurement of optical sources and to provide a basic physical eye model that can be used to examine medical and commercial interactions of light with the human eye.
Thank you for your attention
Optical Technologies and Scientific Computing Division
I’d like to acknowledge my colleagues Steven Knox, Mike Shaw and Richard Stevens for their assistance with this
presentation
Laser Applications of Adaptive Optics  Royal Observatory, Edinburgh Science and Technology Facilities Council           Inn
Outline
Adaptive Optics applied to physical emulation of Human Eye
The human eye (front view)
The human eye (side schematic)
Imaging properties
Ideal Laser Beam Propagation
Adaptive Eye Simulator
Liquid Lens Schematic
Liquid Lens No voltage applied: Ray Diagram
Liquid Lens With voltage applied: Ray Diagram
SSL Light Engine ‘Quarterlite’
Apparent Source Size
Essential Optical Hazard Parameter “Angular Subtense” can be measured using adaptive optics
Apparent Source Size
What is the apparent source of an LED?
Movie of HB LED viewed through a changing focal length lens
LED Beam Propagation
Operation of Hartmann-Shack Wavefront Sensor
Operation of Hartmann-Shack Wavefront Sensor
Traceable Wavefront Measurements
Traceable Wavefront Measurements
Traceable Wavefront Measurements
Traceable Wavefront Measurements
Converging Second Moment: Real Measurement
General Astigmatism
ISO 11146-1 Beam parameter measurement equipment
Novel beam propagation measurement device incorporates a Crossed Fresnel grating
Schematic of IMP grating operation relating planes of laser beam to images on camera
Using a Variable M2 source to compare the ISO 11146-1 method and a grating Wavefront Sensor
Summary