optical transmission as we know it today needed two enabling technologies, an efficient way of generating photons and a low-loss medium to carry that energy. Albert Einstein had theorised about the principles of stimulated emission in 1917. Stimulated emission is when an already excited atom is bombarded by yet another photon, causing it to release that photon along with the photon that previously excited it. The emitted photon and bombarding photon emerge in phase and the same direction. In order to produce a steady source of radiation many atoms or molecules have to be pumped to a higher energy state. A laser uses an arrangement of mirrors within a cavity that reflect light waves SO that they reinforce one another. An excitable gas or liquid or solid in the cavity determines the wavelength of the resulting beam. The process of pumping introduces energy into the cavity that excites the atoms causing a population inversion. A population inversion is a state in which there are more excited atoms than grounded atoms. This is stimulated emission. The released photons oscillate backwards and forwards 9 p://www-03.ibm.com/press/us/en/pressrelease/34726.wss Physical Layer Connectivity between the mirrors in the cavity causing other atoms to release more photons. One of the mirrors allows some of the released photons to escape the cavity to create the beam. This is either launched into free space (free-space optics, free-space photonics, optical wireless) or into fibre. Charles Townes working at the Bell Laboratories during the Second World War and at Columbia University with Arthur Schawlow produced a device that used ammonia gas and microwave radiation that became known as the MASER, microwave amplification by stimu- lated emission of radiation. The device was used to amplify radio signals and as a detector for space research. Gordon Gould, one of Townes students at Columbia, was probably the first person to construct an optical laser in 1958, Ali Javan produced the first gas laser using helium neon in 1960, Robert Ha