- spatial and temporal overlap of the two light fields
- phase coherence of the two light fields
- non-orthogonal polarization states
If those conditions are all fulfilled, 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 may 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 (rather than only two 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. So this is also a standing-wave pattern.
Interference effects also occur in multimode fibers. Figure 4 shows the simulated output intensity profiles of a multimode fiber when a monochromatic input beam is scanned across its input face.
Interference effects are often (and most easily) observed for monochromatic light, but monochromaticity is not strictly a precondition. An interferometer with a close to zero arm length difference can show interference even for broadband light. An example for that is the famous Michselson–Morley experiment, which was carried out with broadband light.
Importance of Interference Effects
- 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 essentially involved in the effect of spatial hole burning, e.g. in laser gain media.
- Interference is the basis of detecting beat notes in optical metrology.
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See also: interferometers, optics, optical phase, coherence, holography, beat note, The Photonics Spotlight 2007-09-27, The Photonics Spotlight 2009-05-22
and other articles in the categories general optics, physical foundations