OSA flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Snell's law

At a smooth interface, n₁ sinθ₁ = n₂ sinθ₂, where angles are measured from the surface normal. It governs refraction, and with n₁ > n₂ it predicts total internal reflection beyond the critical angle.

Critical angle

For light traveling from a higher-index medium n₁ into a lower-index medium n₂, the critical angle is θc = arcsin(n₂/n₁). Beyond it, no refracted ray exists and total internal reflection occurs.

Numerical aperture

NA = n sinθ_max, where θ_max is the half-angle of the accepted cone. It sets resolution and light-gathering in microscopes and the acceptance angle of step-index fibers.

Brewster angle

At θB = arctan(n₂/n₁), p-polarized light has zero reflection at a dielectric interface. The reflected beam is then fully s-polarized, a common way to produce polarized light.

Rayleigh range

The distance z_R = πw₀²/λ from the beam waist over which a Gaussian beam expands to √2 times its waist radius. It separates the near-field collimated region from the far-field divergence region.

Airy disk radius

The first dark ring of a diffraction-limited focused spot lies at radius 1.22λ·f/#, or 0.61λ/NA. It sets the diffraction-limited resolution scale for an imaging system.

Abbe sine condition

A lens satisfies h = f sinθ when coma is corrected; the marginal ray height relates to focal length and angle. Violating it produces coma even if spherical aberration is corrected.

Abbe number

vd = (nd − 1)/(nF − nC), where nF and nC are indices at the hydrogen F and C lines. A low value means high dispersion, useful for choosing achromat glass pairs.

Fused silica

An amorphous synthetic glass with nd near 1.459 and Abbe number near 68. It transmits from the deep ultraviolet to the near infrared and has low thermal expansion, making it a common broadband substrate.

Quarter-wave AR coating

A single layer of thickness λ/(4n) and index near √n_substrate gives destructive interference at the design wavelength. Reflection rises away from that wavelength, so broadband use requires multilayer stacks.

Grating equation

d(sinα ± sinβ) = mλ relates incidence angle α, diffraction angle β, groove spacing d, order m, and wavelength. The sign convention depends on whether the diffracted ray is on the same side.

Zero-order waveplate

A retarder with total retardance near one quarter or half wave, avoiding the strong wavelength and angle sensitivity of multi-order plates. It is preferred for broadband or wide-angle use.

Dielectric mirror stack

Alternating quarter-wave layers of high and low index produce high reflectance through constructive interference. Reflectance and bandwidth grow with the number of layer pairs; absorption is very low.

Aperture stop

The physical aperture that limits the on-axis ray bundle. Its images through preceding and following optics define the entrance and exit pupils, which set f/# and telecentricity.

Telecentric lens

A lens with the aperture stop at a focal point, so the chief ray is parallel to the axis in object or image space. This reduces perspective and magnification errors with defocus.

Petzval curvature

A field aberration in which the best-focus surface is curved rather than flat. It depends on element powers and indices, and is corrected by negative elements or a field flattener.

Achromatic doublet

A cemented pair combining a positive low-dispersion element with a negative high-dispersion element. It brings two wavelengths to a common focus, correcting primary axial color but not secondary spectrum.

Tolerance analysis

A study assigning manufacturing and alignment tolerances, identifying compensators, then predicting performance loss by sensitivity tables or Monte Carlo. It links drawings to expected image quality.

Laser threshold condition

Lasing begins when round-trip gain equals round-trip loss, including mirror transmission. For a linear cavity this is 2g₀L = 2αL + ln(1/(R₁R₂)).

Longitudinal mode spacing

For a cavity of optical length L, adjacent longitudinal modes are separated by Δν = c/(2L). The gain bandwidth then determines how many modes oscillate and the mode-locked pulse duration.

Q-switching

A technique that holds the cavity at high loss to store population inversion, then switches to low loss to release a short, high-energy pulse, typically nanoseconds.

ABCD matrix propagation

Ray transfer matrices multiply to describe an optical system, and the complex beam parameter transforms as q₂ = (Aq₁ + B)/(Cq₁ + D). This propagates Gaussian beams through lenses and spaces.

Phase matching

A nonlinear process is efficient only when photon momentum is conserved, so the wave-vector mismatch Δk is near zero. In second-harmonic generation this is achieved by angle, temperature, or periodic poling.

Nyquist sampling criterion

To resolve a spatial frequency, the detector must sample at more than twice that frequency. In imaging this means at least two pixels per resolvable element, otherwise aliasing occurs.

Quantum efficiency

The ratio of signal electrons generated to incident photons at a given wavelength. It drives detector sensitivity and, with read noise and dark current, sets achievable signal-to-noise ratio.

Lidar range equation

For an extended target, received power scales as P_t·ρ·A_r·η/(πR²). Because power falls as R², doubling range at fixed SNR requires roughly four times the transmitted power.

NETD

Noise-equivalent temperature difference is the scene temperature change giving a signal equal to the noise. It is the standard sensitivity metric for thermal infrared imagers.

Fizeau interferometer

A reference flat placed near the test surface produces fringes from the air gap between them. Analysis of fringe shape gives surface figure, but the reference error is included unless calibrated.

Surface figure versus finish

Figure refers to low-spatial-frequency shape deviation, reported as peak-to-valley or RMS waves. Finish refers to high-frequency roughness, measured by profilometry or white-light interferometry.

Slanted-edge MTF

A tilted knife edge is imaged and its edge spread function differentiated to give the line spread function; its Fourier magnitude yields MTF. The slant avoids aliasing of the sampled edge.

Optical density

OD = log₁₀(1/T), so OD 3 transmits 0.1 percent. Laser eyewear ratings state OD at a specific wavelength, and required OD follows from the maximum permissible exposure.

Laser Class 4

Under IEC 60825-1, Class 4 lasers exceed Class 3B limits and are hazardous to eyes and skin, including diffuse reflections. They can also ignite materials, so controls and a safety officer are required.