The basic principle on which optical source work is absorption, spontaneous emission and stimulated emission.

When light is incident on a semiconductor material, some photons get absorbed in the material which then transfer their energy to the electrons in the ground state and cause them to migrate to the excited state. This phenomenon is called absorption.

The transition of an electron from the excited state to the ground state can happen as a result of the natural tendency of the electron without the action of any external agent. The radiation produced as a result of such transitions is called as spontaneous radiation.

Stimulated emission of radiation is the process whereby photons are used to generate other photons that have exact phase and wavelength as that of parent photon. Thus stimulated emission is caused by external stimuli.

The emitted photon is given by:

Eph=E2−E1=hν

where h is the Planck’s constant.

and ν is angular frequency

Part B: LED Structure

There are five major types of LED structure and although only two have found extensive use in optical fiber combination. These are the surface emitter, the edge emitter, and the super luminescent LED respectively. The other two structures, the planar and dome LED.

Surface Emitter LED (SLED)

• Working:

i. Surface emitter LED (SLED) operates at 850nm wavelength. SLED is a five layered double hetrojunction on device consisting of a GaAs and GaAlAs layers.

ii. The design of SLED was based on massive electron injection into a thin active layer for recombination of electron and holes and enhanced focus of emitted light into the optical fiber.

iii. The plane of the active light emitting region is oriented perpendicularly to the axis of the fiber. From the substrate of the device, a well is etched. Fibers are connected in the well to accept the emitted light.

iv. The circular active area in practical surface emitters in normally 50μm in diameter and up to 2.5μm thick SLED has a low thermal impedance in the active region which allows high current densities and give high radiance emission into the optical fiber.

v. The isotropic pattern from a SLED is Lamberitian pattern. In this pattern source is equally bright when viewed from any direction, but the power diminishes as cosθ.

i.e P=Pocosθ Where θ is the angle between viewing direction and normal of the surface and Po is the value of power P at θ=0°