Laser: Light Amplification by Stimulated Emission. The energy of an incoming photon, E = hυ, stimulates the emission process by inducing an electron to pass to a lower energy level. This process allows obtaining photon amplification: One incoming photon results in two outgoing photons that have the same direction, wavelength and phase.
LEDs presented in previous chapter are based in the spontaneous emission mechanism while LDs (laser diodes) are based on the stimulated emission principle.
In order to achieve an amplification of the light by stimulated emission, the probability of a photon emission must be above that of absorption for the spectral range concerned. When the stimulated emission is dominant, the light is amplified, and laser occurs. Stimulated emission is the dominant mechanism when the probability of finding an electron in CB is greater than the probability of finding an electron in the VB. This happens in presence of a population inversion. The population inversion is achieved when the difference between Fermi energy of electrons, EFN, and Fermi energy of holes, EFN, is bigger than the bandgap, Eg. In order to separate these Fermi energy levels is necessary to pump energy in form of electrical current into the semiconductor. Then, by pumping the laser, when a threshold current is injected, the semiconductor is shifted into a state of population inversion.
Optical cavities, such as the FP (Fabry-Perot) or dielectric mirrors DBRs (distributed Bragg reflectors), containing a laser between two reflecting surfaces are used as optical resonators. In steady state, there are stationary EM (electromagnetic) oscillations in the optical cavity. These oscillations reflect on the reflecting surfaces of the optical cavity. The optical cavity axes perpendicular to the current flow. At each reflection, the wave is partially transmitted through the reflective facets. Laser oscillation begins when the amount of amplification becomes equal to the total amount lost through the sides of the resonator, scattering along propagation in the medium and through absorption by the crystal.
There are two main types of LDs: Edge Emitting and surface emitting LDs. Edge emitting LDs have wide and astigmatic emission, while surface emitting LDs present narrower beam emission.
VCSELs (vertical cavity surface emitting lasers) are lasers with a very short active region that have the optical cavity axis along the direction of current flow.
In these lasers, light emission occurs in a direction perpendicular to the active region. CVSELs allow data communications up to 10 Gbs-1.
An EOM (electro-optic modulator) is a device which can be used for controlling the power, phase or polarization of a laser beam by means of an electrical control signal.
LDs are the most common type of lasers and are used in a wide range of applications. These devices have small size; relatively low price and a long lifetime making them a good component for multiple applications that include, but are not limited to, optical communications, barcode readers, laser pointers, CD/DVD/Blu-ray Disc reading and recording, laser scanning and printing or directional lighting sources.