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The beam diameter or beam width of an electromagnetic beam is the diameter along any specified line that is perpendicular to the beam axis and intersects it. Since beams typically do not have sharp edges, the diameter can be defined in many different ways. Five definitions of the beam width are in common use: D4σ, 10/90 or 20/80 knife-edge, 1/e2, FWHM, and D86. The beam width can be measured in units of length at a particular plane perpendicular to the beam axis, but it can also refer to the angular width, which is the angle subtended by the beam at the source. The angular width is also called the beam divergence.

“It comes back, I take that round-trip time, divide by two, multiply it times the speed of light, and great I’ve got the distance,” said Jaeger.

For a Gaussian beam, the relationship between the 1/e2 width and the full width at half maximum is 2 w = 2   F W H M ln ⁡ 2 = 1.699 × F W H M {\displaystyle 2w={\frac {{\sqrt {2}}\ \mathrm {FWHM} }{\sqrt {\ln 2}}}=1.699\times \mathrm {FWHM} } , where 2 w {\displaystyle 2w} is the full width of the beam at 1/e2.[2]

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“But it is so complex, that with as much light as you send out and getting just that single photon back, it would be like going to a beach that’s say 1,000 kilometers long, 30 kilometers or so wide, and 30-40 feet deep, and someone telling you exactly what grain of sand you need to pick out of that whole pile.”

The D4σ width of a beam in the horizontal or vertical direction is 4 times σ, where σ is the standard deviation of the horizontal or vertical marginal distribution respectively. Mathematically, the D4σ beam width in the x dimension for the beam profile I ( x , y ) {\displaystyle I(x,y)} is expressed as[3]

When a beam is measured with a laser beam profiler, the wings of the beam profile influence the D4σ value more than the center of the profile, since the wings are weighted by the square of its distance, x2, from the center of the beam. If the beam does not fill more than a third of the beam profiler's sensor area, then there will be a significant number of pixels at the edges of the sensor that register a small baseline value (the background value). If the baseline value is large or if it is not subtracted out of the image, then the computed D4σ value will be larger than the actual value because the baseline value near the edges of the sensor are weighted in the D4σ integral by x2. Therefore, baseline subtraction is necessary for accurate D4σ measurements. The baseline is easily measured by recording the average value for each pixel when the sensor is not illuminated. The D4σ width, unlike the FWHM and 1/e2 widths, is meaningful for multimodal marginal distributions — that is, beam profiles with multiple peaks — but requires careful subtraction of the baseline for accurate results. The D4σ is the ISO international standard definition for beam width.

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International standard ISO 11146-1:2005 specifies methods for measuring beam widths (diameters), divergence angles and beam propagation ratios of laser beams (if the beam is stigmatic) and for general astigmatic beams ISO 11146-2 is applicable.[6][7] The D4σ beam width is the ISO standard definition and the measurement of the M2 beam quality parameter requires the measurement of the D4σ widths.[6][7][8]

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An American flag wasn’t the only thing astronauts left behind on the moon 50 years ago. One of the experiments taken into space on Apollo 11 was called the Lunar Laser Retro-Reflector, and it still works even today (though perhaps not as well as it did), just like it did in a 2010 episode of “The Big Bang Theory.”

The simplest way to define the width of a beam is to choose two diametrically opposite points at which the irradiance is a specified fraction of the beam's peak irradiance, and take the distance between them as a measure of the beam's width. An obvious choice for this fraction is ⁠1/2⁠ (−3 dB), in which case the diameter obtained is the full width of the beam at half its maximum intensity (FWHM). This is also called the half-power beam width (HPBW).

The angle between the maximum peak of radiated power and the first null (no power radiated in this direction) is called the Rayleigh beamwidth.

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“The idea behind it was we needed to accurately measure the distance between the earth and the moon, see if that distance varies, map out its orbit,” explained Todd Jaeger, a lunar laser expert who used to work at NASA, and is now with Heraeus Conamic, a German technology company that made the fused silica reflector mirrors used in the experiment.

The 1/e2 width is equal to the distance between the two points on the marginal distribution that are 1/e2 = 0.135 times the maximum value. In many cases, it makes more sense to take the distance between points where the intensity falls to 1/e2 = 0.135 times the maximum value. If there are more than two points that are 1/e2 times the maximum value, then the two points closest to the maximum are chosen. The 1/e2 width is important in the mathematics of Gaussian beams, in which the intensity profile is described by I ( r ) = I 0 ( w 0 w ) 2 exp ( − 2 r 2 w 2 ) {\displaystyle I(r)=I_{0}\left({\frac {w_{0}}{w}}\right)^{2}\exp \!\left(\!-2{\frac {r^{2}}{w^{2}}}\right)} .

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Originally those reflectors were designed to last about a decade, but they still work today, and with the right equipment you can still pull off the experiment, like they suggested in that episode of “The Big Bang Theory.”

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The main drawback of the knife-edge technique is that the measured value is displayed only on the scanning direction, minimizing the amount of relevant beam information. To overcome this drawback, an innovative technology offered commercially allows multiple directions beam scanning to create an image like beam representation.[4]

“Now, the actual signal coming back from [the reflector] has gone down to about 10% of what it was, but luckily lasers have gotten 10-to-100 times more powerful,” Jaeger said. “So we’ve made up for that degradation from moon dust and micrometeorite impact, etc.”

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Measurements of the 1/e2 width only depend on three points on the marginal distribution, unlike D4σ and knife-edge widths that depend on the integral of the marginal distribution. 1/e2 width measurements are noisier than D4σ width measurements. For multimodal marginal distributions (a beam profile with multiple peaks), the 1/e2 width usually does not yield a meaningful value and can grossly underestimate the inherent width of the beam. For multimodal distributions, the D4σ width is a better choice. For an ideal single-mode Gaussian beam, the D4σ, D86 and 1/e2 width measurements would give the same value.

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The definition given before holds for stigmatic (circular symmetric) beams only. For astigmatic beams, however, a more rigorous definition of the beam width has to be used:[5]

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This definition also incorporates information about x–y correlation ⟨ x y ⟩ {\displaystyle \langle xy\rangle } , but for circular symmetric beams, both definitions are the same.

When Buzz Aldrin left the moon lander, he laid out the reflector module on the surface that enabled scientists here on Earth to shoot a laser at the moon, and have that light reflected right back them.

Before the advent of the CCD beam profiler, the beam width was estimated using the knife-edge technique: slice a laser beam with a razor and measure the power of the clipped beam as a function of the razor position. The measured curve is the integral of the marginal distribution, and starts at the total beam power and decreases monotonically to zero power. The width of the beam is defined as the distance between the points of the measured curve that are 10% and 90% (or 20% and 80%) of the maximum value. If the baseline value is small or subtracted out, the knife-edge beam width always corresponds to 60%, in the case of 20/80, or 80%, in the case of 10/90, of the total beam power no matter what the beam profile. On the other hand, the D4σ, 1/e2, and FWHM widths encompass fractions of power that are beam-shape dependent. Therefore, the 10/90 or 20/80 knife-edge width is a useful metric when the user wishes to be sure that the width encompasses a fixed fraction of total beam power. Most CCD beam profiler's software can compute the knife-edge width numerically.

The American National Standard Z136.1-2007 for Safe Use of Lasers (p. 6) defines the beam diameter as the distance between diametrically opposed points in that cross-section of a beam where the power per unit area is 1/e (0.368) times that of the peak power per unit area. This is the beam diameter definition that is used for computing the maximum permissible exposure to a laser beam. In addition, the Federal Aviation Administration also uses the 1/e definition for laser safety calculations in FAA Order JO 7400.2, Para. 29-1-5d.[1]

The other definitions provide complementary information to the D4σ. The D4σ and knife-edge widths are sensitive to the baseline value, whereas the 1/e2 and FWHM widths are not. The fraction of total beam power encompassed by the beam width depends on which definition is used.

The D86 width is defined as the diameter of the circle that is centered at the centroid of the beam profile and contains 86% of the beam power. The solution for D86 is found by computing the area of increasingly larger circles around the centroid until the area contains 0.86 of the total power. Unlike the previous beam width definitions, the D86 width is not derived from marginal distributions. The percentage of 86, rather than 50, 80, or 90, is chosen because a circular Gaussian beam profile integrated down to 1/e2 of its peak value contains 86% of its total power. The D86 width is often used in applications that are concerned with knowing exactly how much power is in a given area. For example, applications of high-energy laser weapons and lidars require precise knowledge of how much transmitted power actually illuminates the target.

Using this general definition, also the beam azimuthal angle ϕ {\displaystyle \phi } can be expressed. It is the angle between the beam directions of minimal and maximal elongations, known as principal axes, and the laboratory system, being the x {\displaystyle x} and y {\displaystyle y} axes of the detector and given by

Beam diameter usually refers to a beam of circular cross section, but not necessarily so. A beam may, for example, have an elliptical cross section, in which case the orientation of the beam diameter must be specified, for example with respect to the major or minor axis of the elliptical cross section. The term "beam width" may be preferred in applications where the beam does not have circular symmetry.

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The success of the project ultimately helped lay the foundation decades later for GPS and other satellite-based technology that we’re so dependent on today.

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By mechanically moving the knife edge across the beam, the amount of energy impinging the detector area is determined by the obstruction. The profile is then measured from the knife-edge velocity and its relation to the detector's energy reading. Unlike other systems, a unique scanning technique uses several different oriented knife-edges to sweep across the beam. By using tomographic reconstruction, mathematical processes reconstruct the laser beam size in different orientations to an image similar to the one produced by CCD cameras. The main advantage of this scanning method is that it is free from pixel size limitations (as in CCD cameras) and allows beam reconstructions with wavelengths not usable with existing CCD technology. Reconstruction is possible for beams in deep UV to far IR.

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“You do need to have a high-powered laser, which is going to set you back about six figures, you have to have the right detector, which is 10 grand or more, the right computers, and of course a $10-to-14 million telescope, which aren’t just laying around,” Jaeger said. “You could rent one I suppose.”

“We, of course, had to have the same sort of algorithms, calculations worked in, to figure out how to get these into a sustainable orbit around the earth,” Jaeger said. “They had to work this in to the algorithms and calculations that the processors do on these satellites, otherwise the accuracy would have drifted out in minutes. In fact, where we’re used to them being accurate to a few feet for a civilian system, they’d drift off in accuracy by five miles after a day.”

Beam diameter is usually used to characterize electromagnetic beams in the optical regime, and occasionally in the microwave regime, that is, cases in which the aperture from which the beam emerges is very large with respect to the wavelength.