LED lasers, also known as laser diodes, function by generating light through electrical current injection into a p-n junction, ultimately producing laser light via stimulated emission. They aren't technically LEDs in the traditional sense but share similarities in construction.
Here's a breakdown of the process:
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P-N Junction Formation: A laser diode is created by joining a p-type semiconductor material (with an abundance of holes) and an n-type semiconductor material (with an abundance of electrons). This creates a p-n junction.
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Forward Bias: When a voltage is applied across the diode in the forward direction (positive to p-side, negative to n-side), electrons from the n-side and holes from the p-side are pushed towards the junction.
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Recombination and Photon Emission: At the junction, electrons and holes recombine. This recombination process releases energy in the form of photons (light particles).
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Optical Cavity: A key feature differentiating laser diodes from standard LEDs is the presence of an optical cavity. This cavity is formed by highly reflective mirrors on either end of the active region (the region where recombination occurs). These mirrors reflect the photons back and forth through the active region.
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Stimulated Emission: As photons bounce between the mirrors, they interact with other excited electrons (electrons that are ready to recombine). This interaction triggers stimulated emission. Stimulated emission occurs when an incoming photon causes an excited electron to recombine and emit a new photon. Crucially, this new photon is an exact copy of the original photon – same wavelength, phase, and direction. This leads to amplification of the light.
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Laser Output: One of the mirrors is partially reflective, allowing a portion of the amplified light to escape as a coherent laser beam.
Key Differences Between Laser Diodes and LEDs
| Feature | LED | Laser Diode |
|---|---|---|
| Emission Type | Spontaneous Emission | Stimulated Emission |
| Coherence | Incoherent | Coherent |
| Directionality | Omnidirectional (wide beam angle) | Highly Directional (narrow beam angle) |
| Output Power | Lower | Higher |
| Optical Cavity | Absent | Present |
In summary, while both LEDs and laser diodes use electrical current to generate light at a p-n junction, the laser diode utilizes an optical cavity and stimulated emission to produce a highly directional, coherent, and higher-powered laser beam, whereas LEDs produce light via spontaneous emission resulting in incoherent, omnidirectional light.
