Fundamentals of Laser Operation

The Gain Medium: The laser gain medium uses the pump energy to amplify, or increase the gain of the laser output. Gain media should have the ability to absorb the pump energy and store it in the form of excited electrons, and have at least one energy transition capable of producing laser emission at the desired wavelength. The gain medium can be a gas, liquid, solid, semiconductor, or free electrons. In most cases, the gain medium supplies the name of the laser, ie, carbon dioxide, argon, Alexandrite, Neodymium:Yttrium-Aluminum-Garnet (Nd:YAG), etc.

The Pump: Pumping refers to the method used to add energy to the gain medium. The pump source can be broadband or narrowband light supplied by a flashlamp, high voltage, DC voltage chemical, radio frequency, or even another laser. Typically:

The Resonator: The resonator is the optical path for reflecting photons in such a way that they resonate, or interfere constructively within the gain medium. Pump energy maintains the population inversion of excited electrons, and photons feeding back into the resonant path trigger more photons. One end of the resonator has a highly reflective (HR) mirror or other reflecting devices to feed photons back into the gain medium; the other end has a partially reflective mirror (output coupler, or OC) to allow laser photons to escape.

Energy States and Population Inversion: Quantum mechanics defines the allowable orbits ("orbitals") of electrons around an atomic nucleus. In the absence of added energy, these electrons are described as being in the ground state. When energy is added, electrons can jump to a higher allowable orbital, eventually returning to the original orbital and shedding a quantum of energy as a photon.This process of excitation and decay with spontaneous emission of a photon is called fluorescence, and continues as long as energy is added to the system.

If the gain medium is capable of storing energy in the form of excited electrons with a long lifetime before decaying back to the ground state, an increasing proportion of electrons will assume a higher energy state as the medium is pumped, until a population inversion occurs.

As an excited electron decays and emits a photon, the photon will interact with another excited electron and trigger the emission of an identical photon. These photons will repeat the process and trigger a cascade of identical photons. However, only photons traveling in the resonant optical path will oscillate and reinforce by constructive interference, and eventually escape through the output coupler as a laser beam. The others will surrender their energy back to the gain medium.

Energy transitions: A population inversion must be sustained for laser action to occur. Quantum considerations defines allowable energy transitions in a given gain medium, and there must be at least one downward transition capable of being triggered by stimulated emission. In the simplest, or 2 level system, an electron is excited to an upper energy state, and then emits a photon as it decays to the ground state. However, 2 level systems are impractical because they strongly absorb their own emitted photons, making maintenance of a population inversion difficult and operation in continuous mode problematic.

Most lasers operate as 3 or 4 level systems, in which an electron can be excited to level 3 or 4, decay one level, then emit as it decays to the ground state (in a 3 level system), or a lower energy level above the ground state (in a 4 level system) . Having more than a single transition prevents the emitted laser energy from being reabsorbed into the gain medium, lowers the amount of pump energy needed to maintain the population inversion and in some cases may allow a choice of wavelength, with proper selection of parameters in the resonator.



				Laser Fundamentals

All lasers exploit the principle of Stimulated Emission, in which energy transitions defined by quantum mechanics provide a source of photons. What makes Light Amplication by the Stimulation Emission of Radiation possible is a mechanism by which identical photons can be generated, captured, amplified and harnessed for use. All lasers have 3 essential components: A lasing or "*gain" medium* A source of energy to excite electrons in the gain medium to high energy states, referred to as "pump" energy An optical path which allows emitted photons to oscillate and interfere contructively as energy is added or "pumped" into the system, ie, a resonator
As energy is added to the system, electrons orbiting atoms in the gain medium are excited to higher energy states. As the energy states decay, photons identical in wavelength However, only those photons emitted exactly parallel to the axis of the resonator are reflected back into the gain medium by the HR (highly reflective) and OC (output coupler) mirrors, triggering a cascade of photons identical in direction, polarization and phase. The Gain Medium: The laser gain medium uses the pump energy to amplify, or increase the gain of the laser output. Gain media should have the ability to absorb the pump energy and store it in the form of excited electrons, and have at least one energy transition capable of producing laser emission at the desired wavelength. The gain medium can be a gas, liquid, solid, semiconductor, or free electrons. In most cases, the gain medium supplies the name of the laser, ie, carbon dioxide, argon, Alexandrite, Neodymium:Yttrium-Aluminum-Garnet (Nd:YAG), etc. The Pump: Pumping refers to the method used to add energy to the gain medium. The pump source can be broadband or narrowband light supplied by a flashlamp, high voltage, DC voltage chemical, radio frequency, or even another laser. Typically: Solid state "bulk" lasers (crystal or glass substrate doped with rare earth or transition metal ions) such as the ruby, alexandrite, Nd:YAG, etc) are optically pumped with flashlamps for higher power applications. Gas lasers are typically pumped with high voltage or RF. Semiconductor and diode lasers are pumped with DC current. Diode lasers powered by DC current can be used as a pump source for various gain media including solid state media (DPSS or Diode Pumped Solid State lasers), fiber lasers, and thin disk lasers. Chemical lasers are pumped by the energy of a chemical reaction (HF/DF, COIL) The Resonator: The resonator is the optical path for reflecting photons in such a way that they resonate, or interfere constructively within the gain medium. Pump energy maintains the population inversion of excited electrons, and photons feeding back into the resonant path trigger more photons. One end of the resonator has a highly reflective (HR) mirror or other reflecting devices to feed photons back into the gain medium; the other end has a partially reflective mirror (output coupler, or OC) to allow laser photons to escape. Energy States and Population Inversion: Quantum mechanics defines the allowable orbits ("orbitals") of electrons around an atomic nucleus. In the absence of added energy, these electrons are described as being in the ground state. When energy is added, electrons can jump to a higher allowable orbital, eventually returning to the original orbital and shedding a quantum of energy as a photon.This process of excitation and decay with spontaneous emission of a photon is called fluorescence, and continues as long as energy is added to the system. If the gain medium is capable of storing energy in the form of excited electrons with a long lifetime before decaying back to the ground state, an increasing proportion of electrons will assume a higher energy state as the medium is pumped, until a population inversion occurs. As an excited electron decays and emits a photon, the photon will interact with another excited electron and trigger the emission of an identical photon. These photons will repeat the process and trigger a cascade of identical photons. However, only photons traveling in the resonant optical path will oscillate and reinforce by constructive interference, and eventually escape through the output coupler as a laser beam. The others will surrender their energy back to the gain medium. Energy transitions: A population inversion must be sustained for laser action to occur. Quantum considerations defines allowable energy transitions in a given gain medium, and there must be at least one downward transition capable of being triggered by stimulated emission. In the simplest, or 2 level system, an electron is excited to an upper energy state, and then emits a photon as it decays to the ground state. However, 2 level systems are impractical because they strongly absorb their own emitted photons, making maintenance of a population inversion difficult and operation in continuous mode problematic. Most lasers operate as 3 or 4 level systems, in which an electron can be excited to level 3 or 4, decay one level, then emit as it decays to the ground state (in a 3 level system), or a lower energy level above the ground state (in a 4 level system) . Having more than a single transition prevents the emitted laser energy from being reabsorbed into the gain medium, lowers the amount of pump energy needed to maintain the population inversion and in some cases may allow a choice of wavelength, with proper selection of parameters in the resonator. Next: Laser Operation [home][disclaimer] Page last updated February 20 2008 © 2008 Albert Poet MD

Laser Fundamentals