Each laser is has three basic elements, it needs to generate laser light:
On this page we will discuss the various pump or energy sources of lasers:
The “pump” is another major component of a laser, along with the already mentioned lasing (or amplification) medium and the resonating cavity. As already described above, laser light will only develop, if a population inversion of ground-state atoms to “excited-state” atoms can be achieved. In order to achieve this population inversion, energy in a suitable form needs to be supplied to the system. Different types of energy delivery systems are available, and each is unique to an individual type of laser. Excitation by light has been discussed above and is referred to as optical pumping. Excitation by electricity is another option and is usually found in gas and semiconductor media. Finally, the energy derived by chemical reactions is also used as an energy source.
Optical Pumping
Optical pumping is a process, in which light energy is used to “raise” electrons from lower energy level to a higher energy level in an atom. In the construction of a laser this is used to achieve a population inversion in the lasing medium. The concept of optical pumping has been discovered approximately ten years prior to the construction of the first laser, by Alfred Kastler. Suitable sources of light for this pumping process are high intensity flash lights, or even other lasers. Light itself will emit photons in a variety of wavelengths. Only those wavelengths, which correspond in energy to exactly the energy difference between two orbits in the lasing medium atoms will contribute towards the pumping action. Therefore, lasing media which have a larger number of energy levels or orbitals will increase the likelihood for absorption of the exact energy quanta necessary to lift (pump) certain electrons into their elevated and more energetic orbits. Because flash lights emit light at such a variety of wavelength, most of the flash light energy is actually wasted as heat in the lasing medium. Utilizing a laser as a pumping source is another alternative. Since the pump laser’s spectrum is very narrow, the energy transfer into the lasing medium is much more efficient, because more electrons can be “pumped” into higher orbits with the same amount of exposure time.
Electric Pumping
Electric glow discharge is one form of electric pumping. This system typically involves a gas discharge tube filled with an inert gas and two electric terminals on either side. In a DC setup there is usually an anode on one side and a cathode on the opposite side. As Michael Faraday already observed in the 1830s, a glow in the gas can be produced when a potential difference is applied between the two electrodes. Electrons originating at the cathode are accelerated, collide, transfer their energy and eventually slow down at the anode end and get transferred to the outside circuit. Since a great portion of the kinetic energy of the electrons is transferred to their individual collision partners, this energy can then be used as a “pump” to elevate electrons and populate the upper energy levels of the individual atoms. Often times a “booster gas” is added to the mix, which offers a metastable energy level that can store the excited electrons better. The metastable level of the booster gas needs to have approximately the same energy as the upper levels of the lasing gas.
Electric current is often used as a pumping source in semiconductor lasers, such as diode lasers. In this system, an electric current drives the electrons from one side of the diode (the N-doped side) to the other side of the diode, which is in demand for electrons (the P-doped side). This process is often also referred to as “charge injection” and diode lasers are therefore often referred to as injection lasers. The lasing action occurs at the junction of the N-doped and P-doped interface. Once an electron from the N-doped side “falls” into a “hole” of the electron deficient P-doped side of the semiconductor, a photon is spontaneously emitted. This spontaneous emission of photons is driven by an electric charge laid across the P-N junction of a semiconductor (such as a diode).
Chemical Reaction Pumping
Chemical lasers are pumped by the excess energy of a chemical reaction. Common examples of chemical lasers are hydrogen-flouride lasers, deuterium-flouride lasers or oxygen-iodine lasers. In this system to chemicals such as hydrogen and fluoride are combined in a lasing chamber (or flow chamber) where a certain amount of reaction heat is produced in the formation of hydrogen flouride. This allows for the excitation of the HF atoms into the third level, with rapid decay to the second level and simultaneous emission of photons. Chemical lasers can achieve continuous wave output in the megawatt range. Chemical lasers are therefore mostly used in industry for cutting and drilling or in the military as directed-energy weapons, however they have also found impressive application in medicine. The Excimer laser for eye surgeries, for instance derives its pumping energy from a chemical reaction.
