Light emitting diodes work on the same basic principle as regular diodes. A diode consists of two halves of silicon. Each half of the diode has been doped with a different type of atom. Doping the silicon means that a small portion of the silicon atoms in the crystal lattice have been replaced with a different type. One half of the diode is doped with boron atoms and the other half has been doped with phosphorus atoms. Phosphorus has one more electron in the valence shell than silicon. The extra electron remains free when a phosphorus atom replaces a silicon atom in the crystal structure. This half of the diode has a surplus of electrons and is called n-type referring to the negative charge. The other half of the diode is doped with boron, an atom that has one less valence electron than silicon. A hole is left when boron replaces a silicon atom. A hole has a positive charge and is free to move around the crystal. The Boron doped half of the diode is called p-type for the positive holes. When the n-type and p-type halves are brought together a depletion region is formed in the center. Holes and electrons cannot cross this region unless a sufficient voltage is applied across the diode.
The semiconductor material of a light emitting diode varies from a traditional diode. A red LED can be made out of materials such as AlGaAs or GaAsP. A photon is emitted when an electron in the conducting band recombines with a hole in the valence band. The wavelength of the emitted light is dependent on the difference in energy between the valance band and conducting band called the band gap energy. A photon is emitted in a diode when the two halves of the diode have a direct band gap. A band gap is called a direct gap when the crystal momentum in the two halves of the diode are the same.
Optical phase conjugation is a nonlinear effect that causes a beam of light to exactly reverse its direction after entering a nonlinear medium called a phase conjugate mirror. Phase conjugate mirrors use a process called four-wave mixing. When three frequencies are sent into a nonlinear medium, a fourth frequency will be formed. In order to produce a conjugate beam, two pumping beams are sent anti-parallel through a nonlinear medium so that their wave vectors add to zero. A third probe beam is sent in and a conjugate beam will be formed so that the wave vectors of the conjugate and the probe beam add to zero. One use of phase conjugation is removing aberrations caused by various distorting elements.
The Faraday effect is an interaction between light and a magnetic field in a dielectric medium. Michael Faraday noticed that the polarization of light rotates when passing through a medium that has an axial magnetic field applied to it. The angle of rotation is proportional to the strength of the magnetic field and the path length of the light through the medium. The formula is θ = BVD. Where B is the magnetic field, D is the path length and V is the Verdet constant for specific material. The Verdet constant has a strong dependence on the wavelength of light, increasing as the wavelength decrease. The verdet constant also has a weaker dependence on the temperature and density of the medium.
Linear polarized light can be thought of as a vector that oscillates between a positive and negative magnitude in a single plane. The frequency of this oscillation is the same as the frequency of the light. Linear polarized light can also be represented by the vector sum of two vectors with constant magnitude rotating in opposite directions around the axis of propagation. When a magnitic field is applied to certain mediums the material becomes circularly birefringent. That is, the index of refraction for left hand circularly polarized light will differ from right hand circularly polarized light. Therefore, when linear polarized light travels through a medium, in the presence of a magnetic field, the phase of the two circularly polarized components will be shifted. This results in a rotation of the linear polarization.
A useful application of the Faraday Effect prevents a reflected laser beam from reentering the laser. The device that accomplishes this is called a Faraday isolator. An isolator consists of a medium with an applied axial magnetic field in between two polarizers. The first polarizer will polarize the light vertically and the second polarizer will be rotated by 45 degrees. The system will be setup to rotate the vertically polarized light 45 degrees and allow all of the light to pass through the second polarizer. Linear polarized light that is reflected reverses the handedness of the circularly polarized components. That is, circularly polarized light in the clockwise direction will be polarized in the counterclockwise direction and vice versa. The polarization of reflected light propagating through the isolator in the negative direction will be rotated an additional 45 degrees in the same direction as before, because of the reversal of the circularly polarized components. Thus, when the light reaches the vertical polarizer, it will be horizontally polarized. Therefore, all of the light will be extinguished and none of it will enter the laser.