Laser amplification comes from stimulated emission. This process exponentially increases photons sharing same quantum state.
Light field’s intensity and density are directly proportional to photons per unit volume. So laser amplification is basically an coherent light amplification process.
To realize this amplification, critical prerequisite is presence of enough microscopic particles on an excited state. This raises a question: how many particles must get excited for coherent light amplification? Under normal circumstances, matter tends to absorb light instead of amplifying it.
At thermal equilibrium, rates of photon absorption and emission by microscopic particles equal.
When absorption dominates, medium shows net light absorption. Conversely, when emission exceeds absorption, medium amplifies incident light.
Achieving light amplification needs an Population Inversion—a state where density of particles on an excited energy state surpasses that on a lower energy state. This inversion is necessary physical condition for laser action.
Process of exciting particles to higher energy states is called Pumping.
To establish population inversion, we need effective pumping mechanism to lift enough particles into excited state.
However, simply pumping particles isn’t enough if excited state has very short lifetime. For particles quickly return to ground state, preventing accumulation.
Therefore, second essential condition for population inversion is using an gain medium with Metastable State—an excited state with relatively long lifetime. This lets particles accumulate sufficiently on excited state to maintain inversion.
Most lasers work on at least one Three-Level system. Because two-level systems can’t sustain population inversion. Due to equal probabilities of upward and downward transitions stimulated by same radiation.
In three-level system, pumping excites particles from ground state to higher energy level (level 3). It rapidly decays non-radiatively to metastable intermediate state (level 2).
Laser transition then happens from this metastable state down to ground state (level 1). This enables population inversion between levels 2 and 1.
Ruby Laser is classic example of three-level solid-state laser. Its Gain Medium is synthetic ruby crystal (Cr³⁺:Al₂O₃).
Optical pumping, typically by xenon flashlamp, excites chromium ions to high-energy state. It quickly relaxes to metastable state with lifetime on order of milliseconds. This metastable state enables population inversion relative to ground state.
Laser emits coherent red light at wavelength of approximately 694.3 nm, known as R1 line.
Ruby laser’s design includes Optical Resonator formed by mirrors at ends of ruby rod. It facilitates stimulated emission process and coherent light amplification.
| Condition | Description |
|---|---|
| Population Inversion | More particles on excited state than on lower state to enable net stimulated emission |
| Effective Pumping | Mechanism (optical, electrical, etc.) to excite particles into higher energy states |
| Metastable State | Excited state with sufficiently long lifetime to accumulate particles and maintain inversion |
| Suitable Gain Medium | Material with appropriate energy level structure (e.g., ruby crystal) |
| Optical Resonator | Reflective cavity to sustain and amplify coherent light |
These conditions together ensure stimulated emission dominates over absorption and spontaneous emission. They let laser produce coherent, amplified light beam.
This explanation combines fundamental principles of laser amplification and population inversion. It highlights role of metastable states and pumping, with ruby laser as prototypical example.
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