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Properties of laser light

Distinguish laser coherence, narrow spectral width, low divergence and high irradiance, including diffraction and intensity calculations.

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  • A-Level Laser Physics extension
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A laser pointer makes a small, bright spot on a wall across the room, and its light can produce steady interference fringes. Four properties explain this: a narrow range of wavelengths, coherence, low divergence and high irradiance. None of them is perfect, and each comes from how the laser works. Laser beam properties go beyond the 9478 syllabus.

Directionality, spectral width and coherence of laser lightA parallel-ray beam, narrow spectral peak and aligned wave traces show three distinct properties of laser output.Low divergencenearly parallel raysNarrow spectrumsmall but non-zero linewidthHigh coherencestable phase relationshipBrightness alone does not establish any of these three properties.
Directionality, spectral width and coherence of laser lightA parallel-ray beam, narrow spectral peak and aligned wave traces show three distinct properties of laser output.Low divergencenearly parallel raysNarrow spectrumsmall but non-zero linewidthHigh coherencestable phase relationshipBrightness alone does not establish any of these three properties.
A laser beam combines low divergence, a narrow spectrum and strong phase correlation. These properties are related but not identical, and no real laser has exactly zero linewidth or zero divergence.

Narrow spectral width

Laser output is often described as monochromatic, meaning concentrated in a narrow frequency or wavelength interval. A real laser has finite linewidth and may support more than one longitudinal mode.

Coherence

Temporal coherence describes predictable phase over a time delay; spatial coherence describes phase correlation across the beam. Coherence enables stable interference, but it is not the same as brightness.

Low divergence

A well-designed laser produces a nearly collimated beam. It still diffracts: for an aperture or beam diameter D, a characteristic small angle is of order

θ∼λ/D.

The numerical factor depends on the aperture and beam profile.

High irradiance

Power concentrated into a small cross-sectional area gives large irradiance,

I = P/A.

High irradiance is possible but not required by the definition of a laser; low-power laser sources also exist.

Worked example 1

Estimating focused-beam irradiance

Problem

A 5.0 mW beam is spread uniformly over a circular spot of radius 0.40 mm. Estimate its irradiance.
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  1. Convert to SI

    Method

    P = 5.0 × 10⁻³ W and r = 0.40 × 10⁻³ m.

    Reason

    Irradiance in W m⁻² requires watts and metres.

    Working

    Answer

    5.0 mW = 5.0 × 10⁻³ W; 0.40 mm = 0.40 × 10⁻³ m

  2. Find the spot area

    Method

    A = 5.03 × 10⁻⁷ m².

    Reason

    The stated spot is circular and the given 0.40 mm is its radius.

    Working

    Answer

    A = π r² = π(0.40 × 10⁻³)²

  3. Calculate irradiance

    Method

    I = 9.9 × 10³ W m⁻², about 9.9 kW m⁻².

    Reason

    Uniform irradiance is power divided by illuminated area.

    Working

    Answer

    I = (5.0 × 10⁻³)/(5.03 × 10⁻⁷) = 9.9 × 10³ W m⁻²

  • Stimulated emission correlates newly emitted photons with the field already present.
  • Resonator feedback favours propagation near the cavity axis and supports selected frequencies.
  • A finite aperture imposes diffraction, so directionality is never perfect.
  • Focusing reduces spot area but increases the range of propagation angles; this is a diffraction trade-off.

Common mistakes

  • Treating “monochromatic” as exactly one wavelength. A laser line is very narrow, but it still has a small spread.
  • Defining coherence as “same frequency”. Equal frequencies can still have an unstable phase relation.
  • Claiming laser light does not diffract. A laser beam spreads less than ordinary light, but every beam diffracts at its aperture.
  • Using power as intensity. Intensity is power divided by the area the beam falls on.

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Syllabus and review details

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