Q13Engineering Physics
Question
Einstein's coefficients A and B relation.
Answer
The fundamental requirements for laser action are an Active Medium capable of stimulated emission, an external Pumping Source to provide energy, a Metastable State to temporarily trap atoms, and an Optical Resonator cavity to provide positive feedback and massive amplification of the coherent light.
The word LASER stands for Light Amplification by Stimulated Emission of Radiation. To successfully generate a highly intense, perfectly coherent, unidirectional laser beam, four critical, interrelated physical components and conditions must be meticulously engineered.
1. The Active Medium
The active medium is the actual physical material in which the laser action (stimulated emission) takes place. It contains a specific collection of atoms, molecules, or ions with a highly specific energy-level structure suitable for lasing. The composition of the active medium fundamentally determines the exact wavelength (color) of the resulting laser beam. Examples include a solid synthetic ruby crystal (Ruby laser), a mixture of Helium and Neon gases (He-Ne laser), or specially doped semiconductor materials (Diode lasers).
2. Pumping Source
A system left to itself in thermal equilibrium will always have more atoms residing in the low-energy ground state than in higher excited states. To achieve laser action, this natural order must be completely violently reversed—a state known as Population Inversion. The pumping source provides the massive external energy required to force ground-state atoms into higher energy levels. Pumping can be achieved via intense optical flashes (flashlamps in Ruby lasers), high-voltage electrical discharges (in gas lasers), or direct electric currents (in semiconductor lasers).
3. The Metastable State
Even with a powerful pumping source, achieving population inversion is impossible in a standard atom because excited states have incredibly short lifetimes (around seconds). The atoms decay back to the ground state instantly via spontaneous emission. Therefore, the active medium must possess a Metastable State. This is a special, long-lived quantum energy level where the electron is momentarily "trapped" for a significantly longer time (around seconds). This delay allows millions of pumped atoms to "bottle up" and accumulate in this state, successfully achieving the critical population inversion required to sustain an avalanche of stimulated emissions.
4. Optical Resonator (Resonant Cavity)
Stimulated emission alone produces coherent light, but it flies off in all directions and is relatively weak. To amplify it into a highly intense, parallel beam, the active medium must be enclosed within an Optical Resonator. This consists of two perfectly parallel mirrors placed at opposite ends of the medium. One mirror is reflective, and the other is partially transmissive (e.g., reflective, transmissive). As photons bounce relentlessly back and forth between these mirrors, they pass through the inverted medium millions of times, triggering exponentially more stimulated emissions on every pass. This provides massive positive feedback (amplification). The intense, highly directional laser beam finally escapes straight through the partially transmissive mirror.