Laser Operation and Components
- The process of light stimulated emission is fundamental to laser operation.
- Laser light is produced by an active medium, or gain medium inside the laser optical cavity. The active medium is a collection of atoms, or molecules that can undergo stimulated emission. The active medium can be in a gaseous, liquid or solid form.
- For lasing to take place, the active medium must be pumped into an excited state capable of undergoing stimulated emission. The energy required for excitation is often supplied by an electric current or an intense light source, such as a flashlamp.
- To induce stimulated emission, the laser cavity must provide a means to reflect, or feedback emitted light into the gain medium.
- A laser must have an output coupler to allow a portion of the laser light to leave the optical cavity.
Laser Optical Cavity
Sketch showing the main components of a laser optical (or resonator) cavity. The optical cavity is formed by a pair of mirrors that surround the gain medium and enable feedback of light into the medium. The output coupler is a partially reflective mirror that allows a portion of the laser radiation to leave the cavity. The gain medium is excited by an external source (not shown), such as a flash lamp, electric current or another laser. The light trapped between the mirrors forms standing wave structures called modes. Although beyond the scope of this discussion, the reader interested in cavity modes can consult References 7-10 and the “Laser Radiation Properties” section.
Stimulated Emission 7-10, 12, 13
- Stimulated emission occurs when a photon of light induces an atom or molecule to lose energy by producing a second photon. The second photon has the same phase, frequency, direction of travel and polarization state as the stimulating photon.
- Since from one photon a second identical photon is produced, stimulated emission leads to light amplification.
- Stimulated emission can be understood from an energy level diagram within the context of the competing optical processes of stimulated absorption and spontaneous emission.
- For stimulated emission to take place, a population inversion must be created in the laser gain medium.
- For more on stimulated emission, see energy level diagrams and subsequent sections.
Energy Level Diagrams
- An energy level diagram displays states of an atom, molecule or material as levels ordered vertically according to energy.
- The states contain contributions from several sources, as appropriate for the matter considered. Sources include the orbital and spin angular momentum of electrons, vibrations of nuclei, molecular rotations, and spin contributions from nuclei.
- The lowest energy level is called the ground state.
- Absorption and emission of energy occurs when matter undergoes transitions between states.
Energy level diagram showing states of a sodium atom. Each state is labeled by a term symbol and includes effects of electron orbital and spin angular momentum.
- Term symbols are a shorthand for describing the angular momentum and coupling interactions among electrons in atoms and molecules.
- As a starting point for understanding a term symbol, write the electron configuration for the state considered. For Na, the electron configuration of the ground state is: 1s22s22p63s1
- The central letter describes the total orbital angular momentum. Only the valence electrons need to be considered. For Na, there is one valence electron, and it occupies an s-orbital. The angular momentum quantum number for an s-orbital is l = 0. The total orbital angular momentum for ground state Na is L = l = 0. Symbols are assigned to the values of L as follows: L = 0 (S), L = 1 (P), L = 2 (D), etc.
- The left superscript reflects the coupling of valence electron spin angular momentum and gives the degeneracy of spin states. For Na, s = 1/2 for the valence electron; therefore, the total spin, S = 1/2 and the degeneracy = (2 S + 1) = 2.
- The right subscript reflects the coupling between spin and orbital angular momentum. For ground state Na, J = L + S = 1/2.
- For a detailed discussion of term symbols, see Ref 11.
Absorption and Emission Processes and Transitions Between Energy States
- Stimulated absorption (a) occurs when light, or a photon of light (hν), excites matter to a higher energy (or excited) state.
- Spontaneous emission (b) is a process whereby energy is spontaneously released from matter as light.
- Molecules typically transition to vibrationally excited levels within the excited electronic state.
- Following excitation, the vibrational energy is quickly released by non-radiative pathways (c).
- In molecules, spontaneous emission known as fluorescence (b) occurs by transition from the lowest level in the excited electronic state, to upper vibrational levels of the lower electronic state.
Energy level diagram for a typical dye molecule. The vibrational levels of each electronic state, labeled by S0 and S1, are included.
Stimulated Emission - Details 7-10, 12, 13
- Laser radiation is produced when energy in atoms or molecules is released as stimulated emission (c).
- Stimulated emission requires a population inversion in the laser gain medium.
- A population inversion occurs when the number of atoms or molecules in an excited state exceeds the number in lower levels (usually the ground state).
- To create the population inversion, the gain medium must transition to a metastable state, which is long lived relative to spontaneous emission.
- The three-level diagram (below) shows excitation followed by non-radiative (nr) decay (b) to 2E states. The 2E states are long lived, because the transition to 4A2 requires a change in the electron spin state.
- A photon of the same energy as the 2E → 4A2 transition can stimulate the emission of a second photon (c), leading to light amplification, or lasing.
Three-level energy diagram. Simplified diagram showing transitions for Cr3+ in a ruby laser.
Three and Four Level Lasers 7-10, 12, 13
- Three-level lasers require intense pumping to maintain the population inversion, because the lasing transition re-populates the ground state.
- Lasers based on transitions between four energy levels (see below), can be more efficiently pumped, because the lower level of the lasing transition is not the ground state.
- Only four-level lasers provide continuous output. HeNe and Nd:YAG are common four-level lasers.
- A population inversion is necessary for lasing, because without one, the photon inducing stimulated emission would instead have a greater probability of undergoing absorption in the gain medium.
- For more in depth information about laser transitions and population inversion, Refs 7-10, 12 (pg 96) and 13 can be consulted.
Four-level energy diagram. Simplified diagram showing transitions for Nd3+ in a Nd:YAG laser.