High spatial coherence of laser light arises from the existence of resonator modes, which define spatially correlated field patterns. Gaussian beams) with very high spatial coherence, and this is perhaps the most fundamental difference between laser light and radiation from other light sources. Lasers have the potential for generating beams (e.g. Coherence of Laser Light Spatial Coherence In that case, the temporal coherence is also somewhat degraded, while it would be completely preserved if only a stationary diffuser is used. Nevertheless, a real destruction of spatial coherence requires time-dependent wavefront distortions, as can be obtained with a rotating diffuser, for example. For most practical purposes, however, such a distorted beam will be spatially incoherent. It would then also in principle be possible to restore a simple beam shape by applying another optical element which compensates the complicated spatial distortions. If a laser beam with high spatial coherence is sent through an optical diffuser element (for example, a very inhomogeneous piece of glass) which totally scrambles the wavefront, the resulting distorted beam could in principle still be considered as spatially coherent, since the phase relationship between the electric fields at different points would still be fixed, as long as the wavefront distortions do not change with time. Figure 4: A laser beam with poor spatial coherence, but high temporal coherence. Such a beam can result from a single-frequency laser, when its output is sent through some optically inhomogeneous material. On the other hand, the beam is monochromatic, so that the spacing of the deformed wavefronts remains constant. The wavefronts are deformed, and this results in a high beam divergence and poor beam quality. Figure 3: A laser beam with high spatial coherence, but poor temporal coherence.įigure 4 shows a laser beam with reduced spatial coherence, but high temporal coherence. from the output of a supercontinuum source. Note that both the local amplitude and the spacing of the wavefronts vary to some extent. The wavefronts are formed as above, and the beam quality is still very high, but the amplitude and phase of the beam varies along the propagation direction. from the left side) with perfect spatial and temporal coherence.įigure 3 shows a beam with high spatial coherence, but poor temporal coherence. Figure 2: Electric field distribution around the focus of a Gaussian laser beam (coming e.g. For reference, Figure 2 shows a monochromatic Gaussian beam, exhibiting perfect spatial and temporal coherence. For example, the output of a single-frequency laser can exhibit a very high temporal coherence, as the electric field temporally evolves in a highly predictable fashion: it exhibits a clean sinusoidal oscillation over extended periods of time.įigures 2–4 further illustrate the difference between spatial and temporal coherence. Temporal coherence means a strong correlation between the electric fields at one location but different times.Spatial coherence is the essential prerequisite of the strong directionality of laser beams. For example, within a cross-section of a beam from a laser with diffraction-limited beam quality, the electric fields at different positions oscillate in a totally correlated way, even if the temporal structure is complicated by a superposition of different frequency components. Spatial coherence means a strong correlation (fixed phase relationship) between the electric fields at different locations across the beam profile.There are two very different aspects of coherence: For example, the general method of coherent beam combining relies on the mutual coherence of beams, whereas spectral (incoherent) beam combining does not Spatial Versus Temporal Coherence Figure 1: A prism is inserted into a spatially coherent laser beam, generating an interference pattern on the screen. In that case, “coherent” essentially means phase-sensitive. It is also common to call certain processes or techniques coherent or incoherent. There are various ways of quantifying the degree of coherence, as described below. Partial coherence means that there is some (although not perfect) correlation between phase values. A light field is called coherent when there is a fixed phase relationship between the electric field values at different locations or at different times. How to cite the article suggest additional literatureĬoherence is one of the most important concepts in optics and is strongly related to the ability of light to exhibit interference effects. More specific terms: phase coherence, temporal coherence, spatial coherence Definition: a fixed phase relationship between the electric field values at different locations or at different times
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