Question

What distinguishes the light emitted by a LASER diode from an LED?

• A. LED light is monochromatic
• B. LASER light is coherent
• C. LED light is directional
• D. LASER light is non-directional

Hint:

LASER light is produced by stimulated emission, making it highly coherent (synchronized phase), monochromatic, and directional, whereas LED light is produced by random spontaneous emission.

Answer 1

The Correct Option is B

Concept: Both Light Emitting Diodes (LEDs) and Light Amplification by Stimulated Emission of Radiation (LASER) diodes are optoelectronic semiconductor devices that emit light via carrier recombination across a forward-biased p-n junction. However, their underlying light emission mechanisms are fundamentally different:
• LED Emission: An LED relies on spontaneous emission, where excited electrons drop randomly back to the valence band, releasing photons with random phase relationships, directions, and a broad spectral width.
• LASER Emission: A LASER diode relies on stimulated emission occurring inside an optical resonant cavity (such as a Fabry-Perot cavity). An incoming photon stimulates an excited electron to drop down, releasing a second photon that is identical in phase, frequency, polarization, and direction.

Step 1: Comparing coherence, monochromaticity, and directionality.
Let us analyze the distinct properties of LASER emission:
• Coherence: Because of stimulated emission, all generated photons are locked in a fixed phase relationship over both time and space. This makes LASER light highly coherent, whereas LED light is completely incoherent.
• Monochromaticity: A LASER has an extremely narrow spectral linewidth, making it highly monochromatic compared to an LED.
• Directionality: LASER light forms a tightly focused, highly directional beam, whereas an LED radiates light widely in all directions.

Step 2: Matching with the correct option statement.
Reviewing the given choices: Option (A) is incorrect because LASER light is far more monochromatic than LED light; Option (C) and (D) are incorrect because LASER light is directional, not the LED. Option (B) correctly identifies that LASER light is coherent, which is its primary distinguishing characteristic. This matches Option (B).