N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 5 - SONET, WDM, DWDM and B-ISDN Technologies                                                             Slide 14 of 76 _________________                                                                                  __________

                                                        Light Sources

                                            Light Emitting Diodes (LED)

                                            Laser Diodes or Semiconductor Lasers (LD)

_________________                                                                                  __________

Optical transmitters convert electrical input signals into modulated light for transmission over the optical fiber.

Light Emitting Diodes - LEAD illuminates light by the movement of electrons in a semiconductor material. A diode is a semiconductor device with a varying ability to conduct electricity. Most semiconductors are made of a poor conductor like Silicon. In an Elements Periodic Table, pure Silicon bonds perfectly with its neighbouring elements.

Silicon is doped with neighbouring elements to create a semiconductor with free electrons (N-Type) or with holes (P-Type) where electrons can go. A diode comprises of a section of N-Type material bonded with P-Type material. P-Type conductor is created by adding Phosphorous or Arsenic to Silicon. N-Type conductor is created by adding Boron or Gallium to Silicon.

How does LED work?

When no voltage is applied to the diode, electrons from the N-Type semiconductor fill holes in P-Type semiconductor between the layers. A depletion zone is created in the middle of the diode.

The depletion zone disappears when electrons move from N-Type area to P-Type area and holes from P-Type area to N-Type area. Free electrons falling into empty holes release energy in the form of photons.

Reverse polarity attachment to the diode creates the depletion zone. The interaction between electrons and holes generates light.

Laser Diodes - There are many different type of lasers. The laser material can be a solid, gas, liquid or semiconductor. These lasers are small and use low power. In an atom electrons spin around the nucleus in orbits ( See Silicon electron structure). Electrons can be exited to go to the higher orbit. The level of excitation depends on the amount of energy that is applied to the atom via heat, light or electricity. Once the electron moves to a higher-energy orbit, it wants to return to the original state. When it does, it releases its energy as a photon - a particle of light. Diodes are used to emit light wavelength.