Metrologie Flashcards
What is a polarimeter?
Used for measuring polarization (measures the angle by which polarization is rotated when passes through an optically active medium)
First: polarize light, then send it through a medium, then rotate the second linear polarizer until the transmission power is at its minimum; use Malus’ law.
Example polarizer: Nicol prism (calcite crystals which are naturally birefringent (refractive index depends on direction; 1.6 vs 1.4))
What’s a Shack-Hartmann Wavefront Sensor?
Used for measuring phase aberrations in eyes or telescopes, or laser beams.
Measures wavefronts and field amplitudes
System: array of lenses of identical focal length, each focused onto a photon sensor placed at the geometric focal plane. Traces individual rays through the optical system of a large telescope.
What is ellipsometry?
Used to measure dielectric properties (complex refractive index/permittivity), thin film thickness (indirectly), and other material properties.
Gives complex reflectance ratio (rho), parametrized by amplitude (psi) and phase difference (delta); $$\rho = \frac{r_p}{r_s} = \tan \Psi \cdot exp{i*\Delta}$$ , r_p and r_s are p-polarized and s-polarized reflectances
Angle used in ellipsometry is close to Brewster angle to maximize ratio.
Technique is fairly insensitive to scatter and fluctuations, does not require reference beam
How would you do temperature sensing?
Fiber Bragg Gratings: temperature affects peak reflectivity wavelength (need to know thermal expansion coefficient and refractive index difference)
Raman scattering (ratio of stokes to anti-stokes scattering), Brillouin scattering
What is M squared?
Beam quality factor. If it’s 1 for a diffraction limited beam, you know it’s gaussian.
Half angle beam divergence = \theta = M^2 \frac{\lambda}{\pi w_0}
Can be found by measuring beam size at multiple locations along the propagation direction.
Tells you to what degree a beam can be focused for a given divergence angle.
What is the coherence needed for OCT sources?
Low temporal coherence, high spatial coherence. (High spatial coherence : very straight, collimated light)
When would you want low temporal coherence?
For OCT, imaging (projector screens).
When would you want high temporal coherence?
Holography, sensors, general laser applications (application: coherent beam combining)
What’s the Rayleigh Range?
Distance along propagation direction of beam from the waist to an area with 2x cross sectional area. This value determines depth of focus.
What is the confocal parameter?
2x Rayleigh Range. This is the range over which the beam is in focus.
Axial resolution/coherence length formula for OCT?
2 * log(2) * \lambda^2 / (pi * delta lambda)
Lateral resolution formula?
Simple lens formula; delta x = 4lambdaf / pi*d
Difference between superluminescent diode and laser diode?
SLDs are designed to prevent reflections that would induce lasing (resonance). Typically in LDs you’d cleave both ends to create a fabry perot etalon, while in SLDs you have an AR coating. They also use angled waveguides in SLDs; weak spontaneous emission with highly fluctuating phase and spectrum gets amplified at whatever stage it’s at in the output.
What is finesse?
How narrow resonances are in relation to the frequency distance (Free spectral range / FWHM bandwidth). High finesse = sharp resonances
How does a lock-in amplifier work?
Mixes two signals together, FTs to get components at 0Hz and 2f (you choose f w/ chopper or something similar), DC is separated from 2f using low-ass filter.
Negative vs positive photoresist?
Negative: parts exposed to UV are crosslinked, ress dissolves away
Positive: parts exposed to UV degrade, anything masked is preserved
Advantage of VCSELs?
Beam comes out the top, not the side (easily integratable into photonic circuits), low threshold current, single mode)
The longer the cavity length, the narrower the bandwidth.
What’s stimulated emission?
The process by which an incoming photon of specific frequency interacts with an excited state electron, causing it to drop to a lower level and emit a second photon that is totally coherent with the first.
How does a laser work?
Induce stimulated emission with a population inversion (4-level state; slow laser transition with fast radiationless transitions)
Benefits/drawbacks of using a laser for ellipsometry?
Laser = single-wavelength ellipsometry. Something like a HeNe. Benefits: high power, high resolution. Downside: one set of psi and delta value per measurement.
Benefits/drawbacks of broadband source for ellipsometry?
You can get complex refractive index tensor; much more information simultaneously.
What is shot noise?
A quantum noise. The fundamental minimum of optical intensity noise; related to discreteness of photons and electrons. Sometimes contributed to by detector, but generally a characteristic of the light field itself.
How do you measure shot noise?
Use of a photomultiplier or avalanche photodiode, where you can detect individual photons; it’s simply the fluctuation in the number of photons counted vs expected.
What is intensity noise, where does it come from, and how do you measure it?
Fluctuations in laser power over time. Comes from laser gain, resonator losses, and various external factors like thermal fluctuations. Measure intensity (or power) with a fast photodetector and evaluate the noise spectrum with an electronic spectrum analyzer.
