Metrologie

Question 1

What is a polarimeter?

Answer 1

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))

Question 2

What’s a Shack-Hartmann Wavefront Sensor?

Answer 2

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.

Question 3

What is ellipsometry?

Answer 3

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

Question 4

How would you do temperature sensing?

Answer 4

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

Question 5

What is M squared?

Answer 5

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.

Question 6

What is the coherence needed for OCT sources?

Answer 6

Low temporal coherence, high spatial coherence. (High spatial coherence : very straight, collimated light)

Question 7

When would you want low temporal coherence?

Answer 7

For OCT, imaging (projector screens).

Question 8

When would you want high temporal coherence?

Answer 8

Holography, sensors, general laser applications (application: coherent beam combining)

Question 9

What’s the Rayleigh Range?

Answer 9

Distance along propagation direction of beam from the waist to an area with 2x cross sectional area. This value determines depth of focus.

Question 10

What is the confocal parameter?

Answer 10

2x Rayleigh Range. This is the range over which the beam is in focus.

Question 11

Axial resolution/coherence length formula for OCT?

Answer 11

2 * log(2) * \lambda^2 / (pi * delta lambda)

Question 12

Lateral resolution formula?

Answer 12

Simple lens formula; delta x = 4lambdaf / pi*d

Question 13

Difference between superluminescent diode and laser diode?

Answer 13

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.

Question 14

What is finesse?

Answer 14

How narrow resonances are in relation to the frequency distance (Free spectral range / FWHM bandwidth). High finesse = sharp resonances

Question 15

How does a lock-in amplifier work?

Answer 15

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.

Question 16

Negative vs positive photoresist?

Answer 16

Negative: parts exposed to UV are crosslinked, ress dissolves away
Positive: parts exposed to UV degrade, anything masked is preserved

Question 17

Advantage of VCSELs?

Answer 17

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.

Question 18

What’s stimulated emission?

Answer 18

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.

Question 19

How does a laser work?

Answer 19

Induce stimulated emission with a population inversion (4-level state; slow laser transition with fast radiationless transitions)

Question 20

Benefits/drawbacks of using a laser for ellipsometry?

Answer 20

Laser = single-wavelength ellipsometry. Something like a HeNe. Benefits: high power, high resolution. Downside: one set of psi and delta value per measurement.

Question 21

Benefits/drawbacks of broadband source for ellipsometry?

Answer 21

You can get complex refractive index tensor; much more information simultaneously.

Question 22

What is shot noise?

Answer 22

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.

Question 23

How do you measure shot noise?

Answer 23

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.

Question 24

What is intensity noise, where does it come from, and how do you measure it?

Answer 24

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.

Question 25

What’s read noise, how do you measure it?

Answer 25

Read noise is the noise generated by electronics when the charge from the pixels is transferred to the camera; it’s applied uniformly across the CCD. To measure, take two bias (0ms exposure), then subtract one from the other and take the standard deviation on a pixel per pixel basis.

Question 26

What is telecentricity and how do you achieve it?

Answer 26

Telecentricity means that the entrance (or exit pupil) is at infinity, so any an object placed at varying distances from a lens will not change physical size, just the focus. You can achieve it by putting an aperture stop in the back focal plane (for object-space telecentric) and front focal plane (for lens-space telecentric)

Question 27

What are chief and marginal rays?

Answer 27

The chief ray begins at the edge of the object and goes through the center of the entrance pupil, exit pupil, and stop (and thus defines the size of object and image and pupil locations)
The marginal ray begins on-axis at object plane, encounters edge of pupils and stops. This defines the location of the object and image and sizes of the pupils.

Question 28

What is etendue?

Answer 28

How “spread out” the light is in area and angle

Question 29

Why is the sky blue?

Answer 29

As white light passes thru the atmosphere, it interacts with the air. Because scattering increases with decreasing wavelength, blue light scatters more than red light and thus it’s visible in the sky.

Question 30

What’s the difference between an LED and LD?

Answer 30

LDs are cleaved at the side to generate the highly reflective laser cavity.

Question 31

Compare and contrast CMOS and CCD cameras?

Answer 31

CCD: analog device, more expensive, more power consumption, slower, less noise, higher sensitivity, smaller
CMOS: digital, active electronics, cheaper, higher framerates, more sensitive to IR, more efficient

Question 32

What are typical NA values for microscope objectives?

Answer 32

Low end: 0.1, high end: 0.95 (for a dry objective)
1.5 or higher: immersion objectives

Question 33

What is an infinity-corrected objective lens?

Answer 33

The standard type of objective lens. Light coming from the object gets turned into parallel beams before going into the tube lens, allowing you to adjust the distance between the distance between the eyepiece and the objective lens (otherwise you need a fixed tube distance). You can also add filters (polarizers, waveplates, etc) in between the eyepiece and specimen as a result.

Question 34

Tradeoff between depth of field and NA in OCT?

Answer 34

Higher the NA, lower the depth of field.

Question 35

What is an f-number?

Answer 35

Ratio of the focal length of a system to the diameter of its entrance pupil (clear aperture).

Related to the amount of illumination given a specific exposure time of a capture image; the greater the f-number, the darker the images (compensate with extra exposure time).

Higher f-number means higher depth of field, lower f-number used for nature photographs for stronger bokeh (background blur)

Question 36

What are dispersion equations for grating?

Answer 36

Angular dispersion (change in diffraction angle per wavelength): m / d*cos(beta), where N is groove density, m is order of light, and beta is diffraction angle.

Reciprocal linear dispersion (change in wavelength per slit) is d* cos(theta) / m*f where f is focal length.