Interference is an optical effect which can occur when two or more light beams are superimposed. More precisely, for interference to occur, several conditions have to be met:
- spatial and temporal overlap of the two light fields
- phase coherence of the two light fields
- non-orthogonal polarization states
In this case, the resulting total light field does not have an optical intensity which equals the sum of the intensities of the superimposed beams. Instead, its complex amplitude is the sum of the amplitudes of the superimposed beams. For example, the amplitudes of two equally intense light beams can have opposite signs at some location, so that they can cancel each other (destructive interference). On the other hand, with equal signs (equal phases) of both contributions (constructive interference), the total intensity can be four times that of the single beams. Nevertheless, the total energy is conserved in any case. For example, if two light beams of equal intensity, frequency and polarization are superimposed on a screen with some angle between the beams, an interference pattern occurs which consists of bright and dark stripes (see Figure 1). It is called a standing wave pattern, since the minima and maxima of the total optical intensity can stay at their positions, although the optical waves are moving with high velocity.
Figure 2 illustrates the superposition of two circular waves with the same frequency but different source points. It shows a snapshot, i.e. the field distribution at one particular moment in time. As time progresses, the spatial patterns move away from the point sources.
By averaging the optical intensity corresponding to this pattern over one oscillating period, the interference pattern in Figure 3 is obtained.
Interference effects also occur in multimode fibers. Figure 4 shows the simulated output intensity profiles of a multimode fiber when an input beam is scanned across its input face.
Importance of Interference Effects
The phenomenon of interference is of great importance in optics in general, and also in laser physics. Some examples:
- Interference governs the operation of interferometers, which are used in many variations for a wide range of applications, and is the basis of holography.
- Interference is essential for the formation of resonator modes, and thus for many physical details of laser operation.
- Interference is essentially involved in the effect of spatial hole burning.
- Interference is the basis of detecting beat notes in optical metrology.
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