Yes, laser light is coherent.
Understanding Laser Light Coherence
One of the defining characteristics of laser light is its coherence. Unlike light from sources like light bulbs or the sun, laser light exhibits a high degree of coherence, which contributes to its unique properties and applications.
According to the provided reference, "Because laser light is monochromatic and coherent, the distances with which lasers can be hazardous are quite long; typically miles long." This statement directly confirms that laser light possesses the property of coherence. The coherence, along with its monochromatic nature (single color), is a key factor in why laser beams can travel long distances and maintain their intensity and direction, making them potentially hazardous over significant ranges.
What Does Coherence Mean?
In simple terms, coherence refers to the predictable relationship between the phase of light waves at different points in space or time. There are primarily two types:
- Temporal Coherence: This describes how well a wave can interfere with itself at a later time. It relates to the spectral purity (how monochromatic) the light is. Since laser light is highly monochromatic (single wavelength/color), it is also highly temporally coherent.
- Spatial Coherence: This describes the correlation between the phase of light waves at different points across the beam's cross-section. A spatially coherent beam maintains a uniform phase front, allowing it to be focused to a very small spot or travel long distances without spreading significantly.
Laser light is typically highly both spatially and temporally coherent. This is achieved through the process of stimulated emission within the laser cavity, which forces the emitted photons to be in phase and travel in the same direction.
Properties Enabled by Coherence
The coherence of laser light gives it several distinct properties:
- Directionality: Laser beams spread very little over long distances, unlike conventional light sources which emit light in all directions.
- High Intensity: Since the light is concentrated into a narrow, directed beam, the intensity within the beam is much higher than that from an incoherent source of the same power.
- Ability to be Focused: Coherent light can be focused to an extremely small spot, enabling applications like cutting, drilling, and high-density data storage.
- Interference and Diffraction: Coherent light is essential for creating stable and high-contrast interference and diffraction patterns, which are fundamental to techniques like holography and interferometry.
These properties, stemming largely from its coherence (and monochromaticity), explain why lasers are used in a vast array of applications, from telecommunications and medical procedures to manufacturing and scientific research.
