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Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.1
Coherence
Coherence is usually described in terms of the laser light’s waves, not the photons (see figure 12).
● Laser cavities are designed so that the stimulated light photons bounce off the mirrors back through the laser medium. These reflected photons then stimulate the emission of more photons.
● If viewed as waves, then we can see that the waves ‘line up’ – they are in phase, or, coherent. This means that the waves peaks all match up, while the waves troughs also match up. The waves are all travelling in the same direction, phase and velocity.
● This cannot occur naturally with light. Light from bulbs or the sun is messily incoherent – the waves are flying in all directions, with different wavelengths, completely out of phase. That is ‘normal’ light!
Figure 12: Coherent light waves
An alternative description of coherence is seen in figure 13:
(a)
Figure 13: Coherence versus incoherence
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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(b)
