Photonics Flashcards
What is the simple way of calculating irradiance?
power/area
What is the refractive index?
The ratio of the speed of the light in a vacuum to the speed of light in a material.
Equation for the velocity of EM waves?
What is the variation of the refractive index with frequency?
Depending on the frequency of the incident EM wave, the polarisation induced is caused due to different mechanisms.
At low frequencies, molecules with built-in dipoles may rotate, producing large variations in polarization at the applied frequency.
At higher frequencies, polarization occurs mainly due to the displacement of electrons.
At optical frequencies, it is mainly due to the displacement of electrons.
Topic 2: The equation that relates Ξ΅r to frequency?
Topic 2: Clausius-Mossotti equation
Topic 2: What is the dispersion?
The variation of refractive index with wavelength is known as dispersion.
In other words, light at different wavelengths travels at different velocities.
It is the reasons why prisms break up white into its constituent colours.
Topic 2 - What is group velocity?
Group velocity, vg, is the speed at which energy can be propagated.
This is because any real or information-carrying signal will not be truly monochromatic, so different frequency components in the signal will travel at different velocities.
In a vacuum, group velocity would equal phase velocity.
Topic 1: Optical field described by the E-Field alone?
The equation πΈπ₯(π§, π‘) = πΈ0 cos( ππ‘ - π½π§ + π0) describes a general monochromatic traveling wave which is propagating in the z-direction.
A wavefront is defined by the phase term, ππ‘ - π½π§ + π0, being constant.
As t increases, z must also increase in order to keep the phase term zero, and thus the maxima of the electric field moves in the positive z-direction with time.
Topic 1: Phase Velocity?
The phase velocity is the velocity of a point of constant phase on a wave.
Phase velocity is the velocity of any one frequency component.
You can imagine the phase velocity as the internal oscillations moving forward as the whole wave moves forward.
Topic 2: What is the group refractive index given as?
Topic 2: What do you need to remember about the wavelength term in the propagation constant?
Ξ»0 is the wavelength in the medium
Topic 2: What can be concluded about the group refractive index and the phase refractive index?
Less than a 2% difference between the two.
Phase refractive index decreases monotonically with wavelength, the group index has a minimum at roughly Ξ»0 = 1300 nm
Topic 2: What is the optical path length?
The optical path length is equal to n**L.
The optical path length is the distance the light would have to travel in a vacuum to experience the same phase change as it does in the material.
Optical path length can also be defined by the following integral:
Topic 2: Why will light be attenuated as it passes through a material?
The light will be attenuated due to:
Absorption - loss of power of the EM wave due to the conversion of the light to other forms of energy, for example, lattice vibrations or excitation of the electrons from the valence band to the conduction band.
Scattering - some power of the light wave will be sent into directions other than the main propagation direction. This can be due to reflections at microscopic regions of different refractive index.
In either case, power will be seen to decay exponentially, with a characteristic loss coefficient.
Topic 2: What are Fresnelβs Equations?
(Donβt need to remember these! But you should understand how to use them: power reflectance at normal incidence, TIR, Brewster angle, etc.)
Topic 2: What is the Power Reflectance given as?
Topic 2: What is the power transmission defined as?
