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Laser applications, limits and safety

Connect coherence, linewidth, directionality and irradiance to laser applications while recognising diffraction, measurement limits and beam hazards.

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  • A-Level Laser Physics extension
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Lasers read barcodes, carry phone calls along optical fibres, measure distances and cut steel. Each job depends on a particular property of the beam, and the same concentrated beam that makes a laser useful can also damage the eye. Laser applications go beyond the 9478 syllabus.

Match the property to the task

An application explanation should name the relevant beam property and show why it helps. “Lasers are powerful” is rarely sufficient.

ApplicationUseful propertyPhysical reasonImportant limitation
interferometry and precision displacement sensingcoherence and narrow linewidthphase changes can produce stable fringe shiftsvibration, air-path changes and finite coherence still matter
fibre communicationdirectionality and controlled spectrumlight can be coupled into guided modes and modulated rapidlydispersion, attenuation and receiver noise limit performance
barcode or position scanninglow divergencea small spot can be maintained over the working distancediffraction and optical quality set the spot size
cutting or material processinghigh irradianceenergy is delivered to a small area quicklyabsorption, heat flow and plume formation affect the process
medical treatment and imagingselectable wavelength and controlled deliverytissue interaction depends strongly on wavelength and exposuresafe use requires trained control of power, time and beam path

Distance measurement

For a pulse that travels to a reflector and back in time t, the one-way distance is

d = ct/2.

The factor of two is essential. Timing resolution, atmospheric propagation, target shape and instrument calibration all contribute to uncertainty.

Worked example 1

Finding distance from round-trip time

Problem

A short laser pulse returns from a reflector after 80.0 ns. Estimate the reflector distance in air, taking the wave speed as 3.00 × 10⁸ m s⁻¹.
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  1. Convert the time

    Method

    t = 80.0 × 10⁻⁹ s.

    Reason

    The wave-speed value uses seconds.

    Working

    Answer

    80.0 ns = 80.0 × 10⁻⁹ s

  2. Recognise the path

    Method

    The measured time covers the outward and return journeys.

    Reason

    The pulse travels to the reflector and back to the detector.

    Working

    Answer

    ct = 2d

  3. Calculate one-way distance

    Method

    d = 12.0 m.

    Reason

    Divide the total path length by two.

    Working

    Answer

    d = ct/2 = ((3.00 × 10⁸)(80.0 × 10⁻⁹))/2 = 12.0 m

Diffraction cannot be removed

Even a well-collimated beam spreads. A larger beam diameter can reduce far-field diffraction angle, but tighter focusing increases angular spread. Laser directionality follows from source geometry and mode selection; it does not violate wave optics.

Safety is a system property

A narrow beam is not harmless. The eye can focus visible and near-infrared radiation onto a very small area of the retina. The risk depends on the wavelength, the power or pulse energy, the exposure time and access to the beam, not on how bright the beam looks. Never look into a laser beam, directly or through an optical instrument, and use only equipment and controls approved for the setting.

Useful controls include enclosing the beam path, preventing specular reflections, restricting access, using appropriate interlocks and following the device’s classification and institutional procedures. Protective eyewear is wavelength-specific and is not a substitute for engineering controls.

Common mistakes

  • Claiming laser light never spreads or is perfectly monochromatic. Every beam diffracts, and every laser line has a small spread of wavelengths.
  • Using coherence, intensity and directionality as interchangeable words. Coherence is a constant phase relation, intensity is power per unit area, and directionality is how little the beam spreads.
  • Explaining every application through high power. Many uses, such as barcode scanners and fibre links, depend on narrow bandwidth or a narrow beam, not power.
  • Forgetting the return path in time-of-flight calculations. The pulse travels to the target and back, so the distance is ct/2.
  • Giving generic safety advice. Identify the hazard: a direct or reflected beam entering the eye.

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Syllabus and review details

No official syllabus alignment is listed for this lesson.

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