Laser Cavity Simulator
Interactive visualization of how a laser works through stimulated emission and optical feedback. Explore the relationship between pump rate, mirror reflectivity, and laser threshold.
- Pump Rate: Controls how much energy is supplied to excite atoms (0% to 100%)
- Output Mirror Reflectivity: Sets how much light is reflected back into the cavity (50% to 100%)
- Cavity Length: Distance between the two mirrors (5 to 30 cm); longer cavities build up more slowly
- Internal Losses: Energy lost per round trip due to absorption and scattering (0% to 30%)
- Threshold: When gain exceeds losses, laser action begins—watch for the intensity to build up!
- Use preset buttons to see different operating regimes
- Toggle view options to focus on energy levels or intensity buildup
📚 Physics Background
💡 How a Laser Works
LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser produces coherent, monochromatic light through a combination of two key processes: stimulated emission (the gain mechanism) and optical feedback (provided by the cavity mirrors).
Unlike ordinary light sources that emit photons randomly in all directions, a laser produces photons that are all in phase, traveling in the same direction, with the same wavelength.
⚛️ Energy Levels and Population Inversion
Atoms in the gain medium exist in discrete energy states. For laser action to occur, we need population inversion—more atoms in the excited state than in the ground state.
N2 > N1 (Population Inversion Condition)
Where:
- N2: Number of atoms in the excited (upper) energy level
- N1: Number of atoms in the lower energy level
This is achieved by pumping—supplying energy (optical, electrical, or chemical) to excite atoms from the ground state to higher energy levels.
✨ Stimulated Emission
When a photon with the right energy encounters an excited atom, it can trigger stimulated emission: the atom drops to a lower energy state and emits a second photon that is:
- Identical in wavelength (same color/frequency)
- Identical in phase (waves aligned)
- Traveling in the same direction
Ephoton = E2 - E1 = hν
This process creates optical gain—each photon can produce more photons, leading to light amplification.
🪞 The Optical Cavity (Resonator)
The gain medium is placed between two mirrors that form an optical cavity. Photons bounce back and forth, passing through the gain medium multiple times and triggering more stimulated emission with each pass.
- Back Mirror: Highly reflective (99%+), keeps most light in the cavity
- Output Mirror: Partially transmitting (95-99%), allows the laser beam to exit
- Cavity Length (L): Determines resonant frequencies (longitudinal modes)
Round-trip time = 2L/c
The cavity provides feedback—photons make multiple passes, building up the light intensity exponentially when above threshold.
🎯 Laser Threshold
Lasing occurs only when gain exceeds losses. The threshold condition is:
G ≥ Ltotal = Lmirror + Linternal
Where:
- G: Round-trip gain from stimulated emission
- Lmirror: Losses from imperfect mirror reflectivity
- Linternal: Internal losses (absorption, scattering)
⬇️ Below Threshold
Losses exceed gain. The light intensity decays with each round trip. Only spontaneous emission occurs—the output is dim and incoherent, similar to a LED.
⚡ At Threshold
Gain exactly balances losses. Intensity remains constant but low. This is the critical pump power where laser action just begins.
🔥 Above Threshold
Gain exceeds losses. Intensity grows exponentially until gain saturation limits the output. The laser produces bright, coherent light with characteristic laser properties.
📈 Intensity Build-up
Above threshold, the intracavity intensity grows according to:
I(n) = I0 × (G × R1 × R2 × e-αL)n
Where:
- I(n): Intensity after n round trips
- G: Single-pass gain
- R1, R2: Mirror reflectivities
- α: Internal loss coefficient
- L: Cavity length
The intensity grows until gain saturation occurs—the gain medium cannot provide more amplification, and the laser reaches steady-state operation.
🔬 Applications of Lasers
Lasers are used in countless applications due to their unique properties:
- Communications: Fiber optic networks transmit data as laser pulses
- Medicine: Laser surgery, eye correction (LASIK), dermatology
- Manufacturing: Cutting, welding, and engraving materials
- Science: Spectroscopy, interferometry, atomic cooling
- Consumer Electronics: CD/DVD/Blu-ray players, laser printers, barcode scanners
- Defense: Targeting, range finding, directed energy weapons