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ECE 5616Curtis
Testing
• Imaging Systems• Aberrations and Resolution
• Interferometric testing• General light measurements• Components• Wavefront Sensor – Shack Hartmann
ECE 5616Curtis
Basic Imaging
ECE 5616Curtis
Nodal Slide – find principle planes
If not at Nodal point, lens rotation results in image motion
Joseph M. Geary, “Introduction to Optical Testing”, SPIE
ECE 5616Curtis
Measuring lens f/#
Joseph M. Geary, “Introduction to Optical Testing”, SPIE
ECE 5616Curtis
Measure FFL/ BFL and EFL
Use autocollimator to get collimation correction – measure BFL/FFL
Lens equation can be used to find EFLIf PP are not accurately known iterations of these measurements will converge
Warren Smith, “Modern Optical Engineering”, SPIE
ECE 5616Curtis
EFL by Magnification
Use know collimator lens with Point Source and then measure image size with lens to be tested.
Used with hybred diffractive and refractive systems and fully reflective systems since the PP are commonly outside the optical package itself and thus a nodal slide would run out of travel
ECE 5616Curtis
Relative Intensity Falloff
Integrating sphere with detector
Lens transmission is measured as a function of chief ray angle
ECE 5616Curtis
Aberrations and Resolution
ECE 5616Curtis
Field Curvature
Measure image location along the chief ray as a function of field angle
Find COLC in z
ECE 5616Curtis
DistortionLateral displacement in x
ECE 5616Curtis
Resolution
∑−=i
iii VHAAAWAR 1
US Air Force sixth root of 2 target. Series of vertical and Horizontal bars with each set of bars smaller than the predecessor by sixth root of 2. A line pair is on black bar and one white bar. The reciprocal of this width is lp/mm.If H in focus and V not then astigmatism is present. Area weighted resolution is
ECE 5616Curtis
MTF and OTFMTF curve
ECE 5616Curtis
MTF via Knife Edge Scan
Fourier Transform of LSF is MTF through origin
Freqre
spon
se
Joseph M. Geary, “Introduction to Optical Testing”, SPIE
ECE 5616Curtis
Interferometer
Use reference surface of extremely high quality. Fringes reveal wavefront errors which are converted into Seidel aberrations or Strehl ratios or …. Need to make certain the measurement is taken over correct area.
ECE 5616Curtis
Using a Null Lens
Mounting is critical. Bad mounting can warp the component.
ECE 5616Curtis
Zygo Readout of HDS Disk
ECE 5616Curtis
Ronchi Test
ECE 5616Curtis
General light measurements
ECE 5616Curtis
Measuring temporal coherenceMichelson interferometer
Fringe Visibility
Joseph M. Geary, “Introduction to Optical Testing”, SPIE
ECE 5616Curtis
Measuring Spatial Coherence
Joseph M. Geary, “Introduction to Optical Testing”, SPIE
Young’s experiment
ECE 5616Curtis
Ellipsometer
Intensity ratio (take square of min and max gives semi major and semi minor axis of polarization ellipse. With and without quarter wave plate can determine handedness of wave (otherwise two solutions give same result).
ECE 5616Curtis
Ellipsometer
Perfect V polarization V polarization after reflection off of metal mirror at an angle
ECE 5616Curtis
Collimation
ECE 5616Curtis
Beam pointing stability(PSD)
ECE 5616Curtis
Long Term StabilityLaser Table/Setup
Determine roughly OPL difference of needed experiment. Set OPL of interferometer to this difference and make interferometer as small as possible and cover with box. Aperture should be smaller than a fringe and just a couple of fringes visible. Output of detector to strip chart recorder and leave on ~20 hours and record output. Jumps are mode-hops, gradual changes are thermal or other drift.
Once laser is stable enough, then make the interferometer as large as the load bearing part of optical table (inside legs) keep OPL difference the same. Repeat experiment. Drifts can be thermal or bad mounts, low freqs are noise through table legs (elevators, etc), high freq acoustic
ECE 5616Curtis
Components
ECE 5616Curtis
InterferenceExample Newton’s Rings
Wikipedia
The radius of the Nth Newton's bright ring is given by
rn = ((N-1/2)λR)1/2
where N is the bright ring number, R is the radius of curvature of the lens the light is passing through, and λ is the wavelength of the light passing through the glass.
ECE 5616Curtis
Pulfrich Refractometer
Accurate to 5 decimal places
ECE 5616Curtis
Surface RoughnessProfilometer
ECE 5616Curtis
Scattering
Red laser
Blue laser
spatialfilter
spatialfilter
PMPM
PMPM
photo
diodephoto
diode
chopper
shutterND filter
sample
M
M
M
FM W
M - mirrorFM - flipper mirrorW - wedgePM - power meter
ECE 5616Curtis
Scatter• Scatter power must be calibrated vs. the solid angle subsumed by
the detector. The solid angle is calculated from the diameter of the iris (I7) and the distance from the sample to I7. The solid angle is defined as
where d is the iris diameter, and r is the distance from the sample to the iris.
• The scatter function, BSDF, is defined as
where Psc is the scattered power, and Pin is the incident power on the sample. θ is the angle between the monitor photodiode and the normal of the sample, reflecting a geometric factor of the laser beam shape ‘viewed’ by the photodiode.
2
2
2r
d
s
⎟⎠⎞
⎜⎝⎛
=Ωπ
θcos1*
in
s
sc
P
P
BSDFΩ
=
ECE 5616Curtis
Scatter from 2 different plastic substrates
0.0E+00
5.0E-03
1.0E-02
1.5E-02
2.0E-02
2.5E-02
0 10 20 30 40 50 60 70 80 90 100
Scatter Angle (degrees)
BSD
F
Substrate 1
Substrate 2
ECE 5616Curtis
Absorption/transmission
Need two spectrometers to cover entire region of light.
ECE 5616Curtis
Spectrometer• Most spectrometer consist of three parts:
– sample– A light source covering the range of interest (infrared, UV etc..)– A monochromator to discriminate wavelengths– A detector to measure the transmitted intensity through the sample
ECE 5616Curtis
Wavefront Sensor
Shack Hartmann
ECE 5616Curtis
Shack Hartmann Wavefront SensorShack-Hartmann wavefront sensor advantages:• no moving parts • The incident radiation does not have to be coherent. Can function over wide range of wavelengths• Measures of both the irradiance and phase distributions of the light. • single CCD image captures the information, reduces sensitivity to vibration and pulsed sources may be analyzed and aligned. • Measures large range of aberrations (particularly larger than what interferometers can). • Yields the PSF, MTF, Zernike wavefront decomposition (related to Siedels), beam quality parameter' and other analysis parameters can also be performed..
Disadvantages – not as accurate as interferometer, limited resolution (need to fill sensor for good readings), limited dynamic range (can saturate)
ECE 5616Curtis
Shack Hartmann Wavefront Sensor
The lens array creates a pattern of focal spots on the CCD. The CCD image is broken into subapertures (called Areas Of Interest or AOIs) of N x N pixels (corresponding to the region behind each lens) in which the spot position is determined: The algorithm usually used to determine the position of the focal spots on theCCD is called the first moment or centroiding. The size of the irradiance distribution on the CCD can be characterized by the second moment of the distribution. From this the wavefront can be determined once tilt and overall beam centering is aligned out.
ECE 5616Curtis
Shack Hartmann Wavefront Sensor
F/5 lens, rotated -15°, large F/5 lens, rotated -15°, Evenastigmatism and defocus terms subtracted