5. Photon Absorption and Emission

6. Spontaneous and Stimulated Emission

7. Population Inversion For a laser you need a continuous number of available electrons for transition to a lower state emitting photons. In equilibrium most systems have a Boltzmann distribution of states and therefore the ground state always has the largest number of electrons. By electrical or optical pumping, carriers can be excited to higher states and the population of the states can be changed. For lasing, the upper radiative transition states must have a large population of electrons than the ground state i.e. population inversion. This also requires the lifetimes of each state to be correct to allow population inversion. (Most lasers, however, use four levels with a fast depopulation of the lower radiative transitions state but it is easier to illustrate population inversion using a three level laser system).

11. Three-Level Laser

18. IN MEDICAL

22. A laser beam fired into the heart can help people suffering from angina pectoris.

23. Lasers can be used to correct defects of the lens and cornea as well as repair tears and holes in the retina.

25. CUTTING

27. DRILLING Blind hole obtained by using an excimer laser beam on CFC Passing hole obtained by using an excimer laser beam on CFC

30. A long range laser rangefinder LRB20000 is capable of measuring distance up to 20 km; mounted on a tripod with an angular mount. The resulting system also provides azimuth and elevation measurements. A laser rangefinder is a device which uses a laser beam in order to determine the distance to a reflective object. it works on the principle of time of flight.

31. Revolver equipped with laser sight. Revolver equipped with laser sight. The laser has in most military applications been used as a tool to enhance the targeting of other weapon systems. For example, a laser sight is a small, usually visible-light laser placed on a handgun or rifle aligned to emit a beam parallel to the barrel. Most laser sights use a red laser diode Others use an infrared diode .

32. Airborne Laser (ABL)

34. This lidar scanner may be used to scan buildings, rock formations, etc., to produce a 3D model. The lidar can aim its laser beam in a wide range: its head rotates horizontally, a mirror flips vertically. The laser beam is used to measure the distance to the first object on its path.

36. COMMERCIAL USE Lasers used for visual effects during a musical performance

37. Laser printer A laser printer is a common type of computer printer that rapidly produces high quality text and graphics on plain paper. As with digital photocopiers and MFPs , laser printers employ a xerographic printing process but differ from analog photocopiers in that the image is produced by the direct scanning of a laser beam across the printer's photoreceptor HP LaserJet 4200 series printer

38. Laser engraving is the practice of using lasers to engrave or mark an object. The technique can be very technical and complex, and often a computer system is used to drive the movements of the laser head. Despite this complexity, very precise and clean engravings can be achieved at a high rate Laser marked electronic part