laser speckle (original) (raw)
Definition: light patterns resulting from the reflection of coherent light at rough surfaces
Alternative term: speckle noise
general opticsRelated: coherencecoherence lengthlinewidthsuperluminescent sourcesThe Plague of a Narrow Emission Linewidth
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What is Laser Speckle?
The term speckle refers to a random granular pattern which can be observed e.g. when a highly coherent light beam (e.g. from a laser) is diffusely reflected at a surface with a complicated (rough) structure, such as a piece of paper, white paint, a display screen, or a metallic surface. This phenomenon results from the interference of many different reflected portions of the incident beam with random relative optical phases. Even minor changes in the conditions, such as changes of the illuminated spot or the direction of the incident laser beam, can change the detailed shape of a speckle pattern.
It can be shown that under most conditions laser speckle exhibits so-called Rayleigh statistics. This means that the probability of finding a certain intensity at a certain location in a speckle pattern decays exponentially with increasing intensity: the probability density is p(I) = \frac{{\exp ( - I/\left\langle I \right\rangle )}}{{\left\langle I \right\rangle }}$$
where ($\langle I \rangle$) is the average intensity.
Speckle patterns can severely degrade the image quality of laser projection displays, or disturb position measurements with laser pointers. Speckle can often be avoided, e.g. by using a light source with sufficiently large optical bandwidth (short coherence length) such as a superluminescent source. (How much bandwidth is required for that purpose depends on the situation.) For projection displays, one may also work with an optimized screen. However, there are also applications of speckle, e.g. in speckle interferometry.
The complicated intensity patterns observed at the ends of multimode fibers are also sometimes called speckle. They result from the interference of many propagation modes, where the relative phases depend on the detailed launching conditions.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is laser speckle?
Laser speckle is a random, granular intensity pattern observed when a coherent light beam, such as from a laser, is reflected from a rough surface. It arises from the interference of many reflected light portions having random relative optical phases.
Why is laser speckle often a problem?
Speckle patterns can be problematic because they can severely degrade the image quality in applications like laser projection displays or disturb precise position measurements made with laser pointers.
How can laser speckle be reduced or avoided?
Speckle can often be avoided by using a light source with lower coherence (i.e., a larger optical bandwidth), such as a superluminescent source. For projection displays, using an optimized screen can also help.
Are there any useful applications of speckle?
Yes, speckle is not always a nuisance. For example, it is utilized in speckle interferometry for high-precision measurements.
What is meant by Rayleigh statistics of speckle?
Under most conditions, the intensity distribution in a speckle pattern follows Rayleigh statistics. This means the probability of finding a high intensity at any given point decreases exponentially with the intensity level.
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Bibliography
| [1] | R. V. Langmuir, “Scattering of laser light”, Appl. Phys. Lett. 2, 29 (1963); doi:10.1063/1.1753756 |
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| [2] | J. W. Goodman, “Some fundamental properties of speckle”, J. Opt. Soc. Am. 66 (11), 1145 (1976); doi:10.1364/JOSA.66.001145 |
| [3] | K. Creath, “Phase-shifting speckle interferometry”, Appl. Opt. 24 (18), 3053 (1985); doi:10.1364/AO.24.003053 |
| [4] | B. Shapiro, “Large intensity fluctuations for wave propagation in random media”, Phys. Rev. Lett. 57, 2168 (1986); doi:10.1103/PhysRevLett.57.2168 |
| [5] | Justin Dove and Jeffrey H. Shapiro, “Speckled speckled speckle”, Opt. Express 28 (15), 22105 (2020); doi:10.1364/OE.398226 |
| [6] | J. C. Dainty, Laser Speckle and Related Phenomena, Springer, Berlin (1975) |
(Suggest additional literature!)
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