LASER TECHNOLOGY

Laser as family devices have found widespread use throughout the industrial and scientific environments. Laser devices are used in the operating room to repair eye tissue and in the machine shop to cut, drill, and weld. The information carried by optical fiber communication systems is usually put into the cable by a semiconductor laser. Continuous wave gas lasers are used in the optical laboratory to align, test, and characterize optical elements and devices. The word laser is an acronym for light amplification by stimulated emission of radiation. The central element gas laser is a gas-filled tube much like a gas lamp. It is within this tube that the stimulated emission and light amplification takes place. The low pressure gas filled tube is excited into emission by a high value DC voltage or by a radio frequency (RF) voltage source.

The DC excitation of the gas takes place through electrode extending into the gas just like electrode of gas lamp. A typical laboratory low power helium-neon laser will use  DC voltage of 7000 V to start the arc in the gas and have an operating voltage of 1800 V. Current flow will be about 5 mA. This type of laser will deliver 3 mW of radiated power for an electrical power input of 9 W. Radiation will emitted from the gas when electrons make energy transitions.

In RF laser, voltage sources cause excitation by capacitive coupling or inductive. In these laser types the atoms are excited by the rapidly changing RF electric or magnetic fields. RF excited lasers do not have electrodes embedded in the glass; rather energy is transferred through the glass by capacitive coupling or magnetic induction. Coils of wire about the glass tube or capacitive plates on provide the means of energy transfer. RF-excited laser typically use a power supply frequency in the range of 20 to 30 KHz. In general, the RF voltages used are much lower than the DC voltage used, but the RF current will be much higher than typical DC current.

The next key elements in a laser are the end mirrors. Reflecting mirrors are placed at each end of the excited gas tube. Gas laser can contain two or more gases. The applied electric field causes the atoms of one of these gases to become excited. Collisions of the excited gas with the other gas force it into excitation and emission. The emitted photons reflecting back and ford between the mirrors cause the amplification process of stimulated emission to take place. Stimulated emission takes place when a photon falls on an atom that is in an excited state. When this occurs, the atom immediately makes energy level transition. The transition causes the atom to emit a photon of the same wavelength and phase as the incident photon. The gases used in the laser tube, the excitation level achieved, the transmission and  the reflection  characteristic of the end mirror all affect the laser’s output.