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[[Image:Gaussian beam w40mm lambda30mm.png|thumb|right|Instantaneous intensity of a Gaussian beam.]] | |||
[[Image:Green laser pointer TEM00 profile.JPG|thumb|right|A 5 mW green laser pointer beam profile, showing the TEM<sub>00</sub> profile]] | |||
In [[optics]], a '''Gaussian beam''' is a [[Light beam|beam]] of [[electromagnetic radiation]] whose transverse [[electric field]] and [[irradiance|intensity]] (irradiance) distributions are well approximated by [[Gaussian function]]s. Many [[laser]]s emit beams that approximate a Gaussian profile, in which case the laser is said to be operating on the ''fundamental [[transverse mode]]'', or "TEM<sub>00</sub> mode" of the laser's [[optical resonator]]. When [[refraction|refracted]] by a [[Diffraction-limited system|diffraction-limited]] [[lens (optics)|lens]], a Gaussian beam is transformed into another Gaussian beam (characterized by a different set of parameters), which explains why it is a convenient, widespread [[mathematical models in physics|model]] in laser optics. | |||
The mathematical function that describes the Gaussian beam is a solution to the [[paraxial approximation|paraxial]] form of the [[Helmholtz equation#Paraxial approximation|Helmholtz equation]]. The solution, in the form of a Gaussian function, represents the [[complex number|complex]] amplitude of the beam's [[electric field]]. The electric field and [[magnetic field]] together propagate as an [[electromagnetic wave]]. A description of just one of the two fields is sufficient to describe the properties of the beam. | |||
The behavior of the field of a Gaussian beam as it propagates is described by a few parameters such as the spot size, the radius of curvature, and the Gouy phase.<ref name="svelto153">Svelto, pp. 153–5.</ref> | |||
<p> [ | Other solutions to the paraxial form of the Helmholtz equation exist. Solving the equation in Cartesian coordinates leads to a family of solutions known as the Hermite–Gaussian modes, while solving the equation in cylindrical coordinates leads to the Laguerre–Gaussian modes.<ref name="siegman642">Siegman, p. 642.</ref><ref name="goubau">probably first considered by Goubau and Schwering (1961).</ref> For both families, the lowest-order solution describes a Gaussian beam, while higher-order solutions describe higher-order transverse modes in an optical resonator. | ||
[[Image:Laser gaussian profile.svg|thumb|right|The top portion of the diagram shows the two-dimensional intensity profile of a Gaussian beam that is propagating out of the page. The blue curve, below, is a plot of the electric field amplitude as a function of distance from the center of the beam. The black curve is the corresponding intensity function.]] | |||
==Mathematical form== | |||
The Gaussian beam is a [[transverse electromagnetic mode|transverse electromagnetic (TEM) mode]].<ref name="svelto158">Svelto, p. 158.</ref> A mathematical expression for its complex electric field amplitude can be found by solving the paraxial Helmholtz equation, yielding<ref name="svelto153" /> | |||
:<math>E(r,z) = E_0 \frac{w_0}{w(z)} \exp \left( \frac{-r^2}{w(z)^2} -ikz -ik \frac{r^2}{2R(z)} +i \zeta(z) \right)\ , </math> | |||
where<ref name="svelto153" /> | |||
:<math>r</math> is the radial distance from the center axis of the beam, | |||
:<math>z</math> is the axial distance from the beam's narrowest point (the "waist"), | |||
:<math>i</math> is the [[imaginary unit]] (for which <math>i^2 = -1</math>), | |||
:<math> k = 2 \pi/\lambda</math> is the [[wave number]] (in [[radian]]s per meter), | |||
:<math>E_0 = |E(0,0)|</math>, | |||
:<math>w(z)</math> is the radius at which the field amplitude and intensity drop to 1/''e'' and 1/''e''<sup>2</sup> of their axial values, respectively, | |||
:<math>w_0 = w(0)</math> is the [[#Beam width or spot size|waist size]], | |||
:<math>R(z)</math> is the [[#Radius of curvature|radius of curvature]] of the beam's wavefronts, and | |||
:<math>\zeta(z)</math> is the [[#Gouy phase|Gouy phase shift]], an extra contribution to the phase that is seen in Gaussian beams. | |||
Additionally, the field has a time dependence factor <math>e^{i\omega t}</math> that has been suppressed in the above expression. | |||
The corresponding time-averaged [[intensity (physics)|intensity]] (or [[irradiance]]) distribution is | |||
< | :<math>I(r,z) = { |E(r,z)|^2 \over 2 \eta } = I_0 \left( \frac{w_0}{w(z)} \right)^2 \exp \left( \frac{-2r^2}{w^2(z)} \right)\ , </math> | ||
< | where <math>I_0 = I(0,0)</math> is the intensity at the center of the beam at its waist. The constant <math>\eta \,</math> is the [[Wave impedance|characteristic impedance]] of the medium in which the beam is propagating. For free space, <math> \eta = \eta_0 = \sqrt{\mu_0/\varepsilon_0} = 1/(\varepsilon_0 c) \approx 376.7 \ \Omega</math>. | ||
== Beam parameters == | |||
The geometry and behavior of a Gaussian beam are governed by a set of '''beam parameters''', which are defined in the following sections. | |||
< | ===Beam width or spot size===<!--Beam waist redirects here--> | ||
{{see also|Beam diameter}} | |||
[[Image:GaussianBeamWaist.svg|thumb|350px|right|Gaussian beam width ''w''(''z'') as a function of the axial distance ''z''. ''w''<sub>0</sub>: beam waist; ''b'': depth of focus; ''z''<sub>R</sub>: [[Rayleigh range]]; <math>\Theta</math>: total angular spread]] | |||
< | For a Gaussian beam propagating in free space, the spot size (radius) ''w''(''z'') will be at a minimum value ''w''<sub>0</sub> at one place along the beam axis, known as the ''beam waist''. For a beam of [[wavelength]] λ at a distance ''z'' along the beam from the beam waist, the variation of the spot size is given by<ref name="svelto153" /> | ||
< | :<math>w(z) = w_0 \, \sqrt{ 1+ {\left( \frac{z}{z_\mathrm{R}} \right)}^2 } \ . </math> | ||
where the origin of the z-axis is defined, without loss of generality, to coincide with the beam waist, and where<ref name="svelto153" /> | |||
:<math>z_\mathrm{R} = \frac{\pi w_0^2}{\lambda}</math> | |||
is called the [[Rayleigh range]]. | |||
===Rayleigh range and confocal parameter=== | |||
At a distance from the waist equal to the Rayleigh range ''z''<sub>R</sub>, the width ''w'' of the beam is<ref name="svelto153" /> | |||
:<math> w(\pm z_\mathrm{R}) = \sqrt{2} w_0.</math> | |||
The distance between these two points is called the ''confocal parameter'' or ''depth of focus'' of the beam: | |||
:<math>b = 2 z_\mathrm{R} = \frac{2 \pi w_0^2}{\lambda}\,.</math> | |||
=== Radius of curvature === | |||
''R''(''z'') is the ''[[Radius of curvature (optics)|radius of curvature]]'' of the wavefronts comprising the beam. Its value as a function of position is<ref name="svelto153" /> | |||
:<math>R(z) = z \left[{ 1+ {\left( \frac{z_\mathrm{R}}{z} \right)}^2 } \right] \ . </math> | |||
< | ===Beam divergence=== | ||
The parameter <math>w(z)</math> increases linearly with <math>z</math> for <math>z \gg z_\mathrm{R}</math>. This means that far from the waist, the beam is cone-shaped. The angle between the straight line <math>r=w(z)</math> and the central axis of the beam (<math>r=0</math>) is called the ''divergence'' of the beam. It is given by<ref name="svelto153" /> | |||
< | :<math>\theta \simeq \frac{\lambda}{\pi w_0} \qquad (\theta \mathrm{\ in\ radians}). </math> | ||
< | The total angular spread of the beam far from the waist is then given by | ||
:<math>\Theta = 2 \theta\ .</math> | |||
Because the divergence is inversely proportional to the spot size, a Gaussian beam that is focused to a small spot spreads out rapidly as it propagates away from that spot. To keep a laser beam very well [[Collimated light|collimated]], it must have a large diameter. This relationship between beam width and divergence is due to [[diffraction]]. Non-Gaussian beams also exhibit this effect, but a Gaussian beam is a special case where the product of width and divergence is the smallest possible. | |||
Since the gaussian beam model uses the paraxial approximation, it fails when wavefronts are tilted by more than about 30° from the direction of propagation.<ref>Siegman (1986) p. 630.</ref> From the above expression for divergence, this means the Gaussian beam model is valid only for beams with waists larger than about <math>2\lambda/\pi</math>. | |||
< | [[Laser beam quality]] is quantified by the [[beam parameter product]] (BPP). For a Gaussian beam, the BPP is the product of the beam's divergence and waist size <math>w_0</math>. The BPP of a real beam is obtained by measuring the beam's minimum diameter and far-field divergence, and taking their product. The ratio of the BPP of the real beam to that of an ideal Gaussian beam at the same wavelength is known as ''M''<sup>2</sup> ("[[M squared]]"). The ''M''<sup>2</sup> for a Gaussian beam is one. All real laser beams have ''M''<sup>2</sup> values greater than one, although very high quality beams can have values very close to one. | ||
== | ===Gouy phase=== | ||
The ''longitudinal phase delay'' or ''Gouy phase'' of the beam is<ref name="svelto153" /> | |||
< | :<math>\zeta(z) = \arctan \left( \frac{z}{z_\mathrm{R}} \right) \ .</math> | ||
The Gouy phase indicates that as a Gaussian beam passes through a focus, it acquires an additional phase shift of π, in addition to the usual <math>e^{-ikz}</math> phase shift that would be expected from a plane wave.<ref name="svelto153" /> | |||
===Complex beam parameter=== | |||
{{main|Complex beam parameter}} | |||
Information about the spot size and radius of curvature of a Gaussian beam can be encoded in the complex beam parameter, <math>q(z)</math>:<ref name="siegman638">Siegman, pp. 638–40.</ref> | |||
< | :<math> q(z) = z + q_0 = z + iz_\mathrm{R} \ .</math> | ||
The reciprocal <math>1/q(z)</math> shows the relationship between <math>q(z)</math>, <math>w(z)</math>, and <math>R(z)</math> explicitly:<ref name="siegman638" /> | |||
:<math> { 1 \over q(z) } = { 1 \over z + iz_\mathrm{R} } = { z \over z^2 + z_\mathrm{R}^2 } - i { z_\mathrm{R} \over z^2 + z_\mathrm{R}^2 } = {1 \over R(z) } - i { \lambda \over \pi w^2(z) }.</math> | |||
The complex beam parameter plays a key role in the analysis of Gaussian beam propagation, and especially in the analysis of [[optical cavity|optical resonator cavities]] using [[ray transfer matrix analysis|ray transfer matrices]]. | |||
In terms of the complex beam parameter <math>{q}</math>, a Gaussian field with one transverse dimension is proportional to | |||
:<math> | |||
{u}(x,z) = \frac{1}{\sqrt{{q}_x(z)}} \exp\left(-i k \frac{x^2}{2 {q}_x(z)}\right). | |||
</math> | |||
In two dimensions one can write the potentially elliptical or astigmatic beam as the product | |||
:<math> | |||
{u}(x,y,z) = {u}(x,z)\, {u}(y,z), | |||
</math> | |||
<p | which for the common case of [[circular symmetry]] where <math>{q}_x = {q}_y = {q}</math> and <math>x^2 + y^2 = r^2</math> yields<ref>See Siegman (1986) p. 639. Eq. 29</ref> | ||
< | :<math> | ||
{u}(r,z) = \frac{1}{{q}(z)}\exp\left( -i k\frac{r^2}{2 {q}(z)}\right). | |||
</math> | |||
==Power and intensity== | |||
=== Power through an aperture === | |||
The [[power (physics)|power]] ''P'' passing through a circle of radius ''r'' in the transverse plane at position ''z'' is | |||
:<math> P(r,z) = P_0 \left[ 1 - e^{-2r^2 / w^2(z)} \right]\ ,</math> | |||
where | |||
< | :<math> P_0 = { 1 \over 2 } \pi I_0 w_0^2 </math> | ||
is the total power transmitted by the beam. | |||
< | For a circle of radius <math>r = w(z) \, </math>, the fraction of power transmitted through the circle is | ||
:<math>{ P(z) \over P_0 } = 1 - e^{-2} \approx 0.865\ .</math> | |||
Similarly, about 95 percent of the beam's power will flow through a circle of radius <math>r = 1.224\cdot w(z) \, </math>. | |||
< | === Peak and average intensity === | ||
The peak intensity at an axial distance <math>z</math> from the beam waist is calculated using [[L'Hôpital's rule]] as the limit of the enclosed power within a circle of radius <math>r</math>, divided by the area of the circle <math>\pi r^2</math>: | |||
< | :<math>I(0,z) =\lim_{r\to 0} \frac {P_0 \left[ 1 - e^{-2r^2 / w^2(z)} \right]} {\pi r^2} | ||
= \frac{P_0}{\pi} \lim_{r\to 0} \frac { \left[ -(-2)(2r) e^{-2r^2 / w^2(z)} \right]} {w^2(z)(2r)} | |||
= {2P_0 \over \pi w^2(z)}. </math> | |||
The peak intensity is thus exactly twice the ''average intensity'', obtained by dividing the total power by the area within the radius <math>w(z)</math>. | |||
< | ==Derivation== | ||
The Gaussian beam formalism begins with the [[electromagnetic wave equation|wave equation for an electromagnetic field]] in free space or in a homogeneous dielectric medium:<ref name="svelto148">Svelto, pp. 148–9.</ref> | |||
:<math> \nabla^2 U = \frac{1}{c^2} \frac{\partial^2 U}{\partial t^2},</math> | |||
where <math>U</math> may stand for any one of the six field components <math>E_x</math>, <math>E_y</math>, <math>E_z</math>, <math>B_x</math>, <math>B_y</math>, or <math>B_z</math>. The Gaussian beam formalism proceeds by writing down a solution of the form<ref name="svelto148" /> | |||
:<math> U(x,y,z,t) = u(x,y,z) e^{-i(kz-\omega t)},</math> | |||
where it is assumed that the beam is sufficiently [[collimated]] along the <math>z</math> axis that <math>\partial^2 u/\partial z^2</math> may be neglected. Substituting this solution into the wave equation above yields the [[Helmholtz equation#Paraxial approximation|paraxial approximation]] to the wave equation:<ref name="svelto148" /> | |||
:<math>\frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} = 2ik \frac{\partial u}{\partial z}.</math> | |||
Solving this differential equation yields an infinite set of functions, of which the Gaussian beam is the lowest-order solution or ''[[transverse mode|mode]]''. | |||
==Higher-order modes== | |||
{{see also|Transverse mode}} | |||
Gaussian beams are just one possible solution to the paraxial wave equation. Various other sets of [[orthogonal]] solutions are used for modelling laser beams. In the general case, if a complete [[Basis (linear algebra)|basis set]] of solutions is chosen, any real laser beam can be described as a superposition of solutions from this set. The design of the laser determines which basis set of solutions is most useful. In some cases the output of a laser may closely approximate a single higher-order mode. Hermite-Gaussian modes are particularly common, since many laser systems have Cartesian reflection symmetry in the plane perpendicular to the beam's propagation direction. | |||
=== Hermite-Gaussian modes === <!--Hermite-Gaussian mode redirects here--> | |||
[[Image:Hermite-gaussian.png|thumb|250px|right|Twelve Hermite-Gaussian modes]] | |||
Hermite-Gaussian modes are a convenient description for the output of lasers whose cavity design is not radially symmetric, but rather has a distinction between horizontal and vertical. In terms of the previously defined complex <math>q</math> parameter, the amplitude distribution in the <math>x</math>-plane is proportional to | |||
:<math> | |||
{u}_n(x,z) = \left(\frac{2}{\pi}\right)^{1/4} \left(\frac{1}{2^n n! w_0}\right)^{1/2} \left( \frac{{q}_0}{{q}(z)}\right)^{1/2} \left[\frac{{q}_0}{{q}_0^\ast} \frac{{q}^\ast(z)}{{q}(z)}\right]^{n/2} H_n\left(\frac{\sqrt{2}x}{w(z)}\right) \exp\left[-i \frac{k x^2}{2 {q}(z)}\right] | |||
</math> | |||
where the function <math>H_n(x)</math> is the [[Hermite polynomial]] of order <math>n</math> (physicists' form, i.e. <math>H_1(x)=2x\,</math>), and the asterisk indicates [[complex conjugation]]. For the case <math>n=0</math> the equation yields a Gaussian transverse distribution. | |||
For two-dimensional [[rectangular coordinates]] one constructs a function <math>{u}_{mn}(x,y,z)=u_m(x,z) u_n(y,z)</math>, where <math>u_n(y,z)</math> has the same form as <math>u_m(x,z)</math>. Mathematically this property is due to the [[separation of variables]] applied to the [[paraxial Helmholtz equation]] for [[Cartesian coordinate system|Cartesian coordinates]].<ref>Siegman (1986), p645, eq. 54</ref> | |||
< | Hermite-Gaussian modes are typically designated "TEM<sub>''mn''</sub>", where ''m'' and ''n'' are the polynomial indices in the x and y directions. A Gaussian beam is thus TEM<sub>00</sub>. | ||
=== Laguerre-Gaussian modes === <!--Several terms redirect here.--> | |||
[[Image:LG-wiki.jpg|thumb|right|upright=1.5|The intensity profiles of twelve Laguerre-Gaussian modes]] | |||
If the problem is cylindrically symmetric, the natural solutions of the paraxial wave equation are Laguerre-Gaussian modes.<ref name="goubau">probably first considered by Goubau and Schwering (1961).</ref> They are written in [[cylindrical coordinates]] using [[Laguerre polynomial]]s | |||
:<math>{u}(r,\phi,z)=\frac{C^{LG}_{lp}}{w(z)}\left(\frac{r \sqrt{2}}{w(z)}\right)^{|l|}\exp\left(-\frac{r^2}{w^2(z)}\right)L_p^{|l|} \left(\frac{2r^2}{w^2(z)}\right) | |||
\exp\left( i k \frac{r^2}{2 R(z)}\right)\exp(i l \phi)\exp\left[i(2p+|l|+1)\zeta(z)\right], | |||
</math> | |||
where <math>L_p^l</math> are the generalized Laguerre polynomials, the radial index <math>p\ge 0</math> and the azimuthal index is <math>l</math>. <math>C^{LG}_{lp}</math> is an appropriate normalization constant; <math>w(z)</math>, <math>R(z)</math> and <math>\zeta(z)</math> are beam parameters defined [[#Beam parameters|above]]. | |||
=== Ince-Gaussian modes === | |||
In [[elliptic coordinates]], one can write the higher-order modes using [[Ince polynomial]]s. The even and odd Ince-Gaussian modes are given by <ref>Bandres and Gutierrez-Vega (2004)</ref> | |||
<p | :<math> | ||
u_\varepsilon \left( \xi ,\eta ,z\right) = \frac{w_{0}}{w\left( | |||
z\right) }\mathrm{C}_{p}^{m}\left( i\xi ,\varepsilon \right) \mathrm{C} | |||
_{p}^{m}\left( \eta ,\varepsilon \right) \exp \left[ -ik\frac{r^{2}}{ | |||
2q\left( z\right) }-\left( p+1\right) \psi _{GS}\left( z\right) \right] , | |||
</math> | |||
where <math>\xi</math> and <math>\eta</math> are the radial and angular elliptic coordinates | |||
defined by | |||
:<math> | |||
x = \sqrt{\varepsilon /2}w\left( z\right) \cosh \xi \cos \eta , | |||
</math> | |||
:<math> | |||
y = \sqrt{\varepsilon /2}w\left( z\right) \sinh \xi \sin \eta. | |||
</math> | |||
<math>{C}_{p}^{m}\left( \eta ,\epsilon \right)</math> are the even Ince | |||
polynomials of order <math>p</math> and degree <math>m</math>, <math>\varepsilon</math> is the ellipticity parameter, and <math>\psi _{GS}\left( z\right) =\arctan \left( z/z_\mathrm{R}\right)</math> | |||
is the Gouy phase. The Hermite-Gaussian and Laguerre-Gaussian modes are a special case of the Ince-Gaussian modes for <math>\varepsilon=\infty</math> and <math>\varepsilon=0</math> respectively. | |||
=== Hypergeometric-Gaussian modes === | |||
There is another important class of paraxial wave modes in [[polar coordinates]] in which the complex amplitude is proportional to a [[confluent hypergeometric function]]. | |||
These modes have a [[Mathematical singularity|singular]] phase profile and are eigenfunctions of the [[photon orbital angular momentum]]. The intensity profile is characterized by a single brilliant ring with a [[Mathematical singularity|singularity]] at its center, where the field amplitude vanishes.<ref>Karimi et. al (2007)</ref> | |||
<p | :<math> | ||
u_{pm}(\rho,\theta;\zeta)= | |||
\sqrt{\frac{2^{p+|m|+1}}{\pi\Gamma(p+|m|+1)}} \frac{\Gamma(1+|m|+\frac{p}{2})}{\Gamma(|m|+1)} | |||
\,\,i^{|m|+1}\zeta^{\frac{p}{2}}(\zeta+i)^{-(1+|m|+\frac{p}{2})}\rho^{|m|}e^{-\frac{i\rho^2}{(\zeta+i)}}e^{im\phi}{}_{1}F_{1}\left(-\frac{p}{2}, |m|+1;\frac{r^2}{\zeta(\zeta+i)}\right), | |||
</math> | |||
<p> | where <math> m </math> is integer, <math> p\ge-|m| </math> is real valued, <math> \Gamma(x) </math> is the gamma function and <math> {}_{1}F_{1}(a,b;x) </math> is a confluent hypergeometric | ||
function. | |||
Some subfamilies of hypergeometric-Gaussian (HyGG) modes can be listed as the modified Bessel-Gaussian modes, the modified exponential Gaussian modes, and the modified Laguerre–Gaussian modes. | |||
The set of hypergeometric-Gaussian modes is overcomplete and is not an orthogonal set of modes. In spite of its complicated field profile, HyGG modes have a very simple profile at the pupil plane: | |||
< | :<math> | ||
u(\rho,\phi,0) \propto \rho^{p+|m|}e^{-\rho^2+im\phi}. | |||
</math> | |||
< | See [[Optical vortex]], which explains that the outcoming wave from a pitch-fork hologram is a sub-family of HyGG modes. The HyGG profile while beam propagates along <math>\zeta</math> has a dramatic change and it is not a stable mode below the [[Rayleigh range]]. | ||
==See also== | |||
* [[Bessel beam]] | |||
* [[Tophat beam]] | |||
* [[Laser beam profiler]] | |||
==Notes== | |||
<references/> | |||
==References== | |||
*{{cite book | title = Fundamentals of Photonics | author = Saleh, Bahaa E. A. and Teich, Malvin Carl | publisher = John Wiley & Sons | location = New York | year = 1991 | isbn= 0-471-83965-5 }} Chapter 3, "Beam Optics," pp. 80–107. | |||
*{{cite book | title = Optical Coherence and Quantum Optics | author = Mandel, Leonard and Wolf, Emil | publisher = Cambridge University Press | location = Cambridge | year = 1995 | isbn= 0-521-41711-2 }} Chapter 5, "Optical Beams," pp. 267. | |||
*{{cite book | first = Anthony E.|last=Siegman|year=1986|title=Lasers|publisher=University Science Books|isbn= 0-935702-11-3}} Chapter 16. | |||
*{{cite book | first = Orazio | last = Svelto | title = Principles of Lasers | edition=5th | year=2010 }} | |||
*{{cite book | first =Amnon | last =Yariv | year =1989 | title = Quantum Electronics| edition =3rd | publisher =Wiley | isbn =0-471-60997-8}} | |||
*{{cite arxiv | |||
| author=F. Pampaloni and J. Enderlein | |||
| title=Gaussian, Hermite-Gaussian, and Laguerre-Gaussian beams: A primer | |||
| journal= | |||
| year = 2004 | |||
| eprint = physics/0410021 | |||
| class=physics.optics | |||
}} | |||
*{{cite journal | |||
| author= G. Goubau and F. Schwering | |||
| title=On the guided propagation of electromagnetic wave beams | |||
| journal= IRE Trans. | |||
| volume = 9 | |||
| year = 1961 | |||
| pages = 248-256 | |||
| doi= 10.1109/TAP.1961.1144999 | |||
| MR = 0134166 }} | |||
*{{cite journal | |||
| author= Miguel A. Bandres and Julio C. Gutierrez-Vega | |||
| title=Ince Gaussian beams | |||
| journal= Opt. Lett. | |||
| pages = 144–146 | |||
| publisher = OSA | |||
| volume = 29 | |||
| year = 2004 | |||
| url = http://www.opticsinfobase.org/abstract.cfm?URI=ol-29-2-144 | |||
| doi= 10.1364/OL.29.000144 | |||
| pmid= 14743992 | |||
| issue= 2 | |||
|bibcode = 2004OptL...29..144B }} | |||
*{{cite journal | |||
| author= E. Karimi, G. Zito, B. Piccirillo, L. Marrucci, and E. Santamato | |||
| title= Hypergeometric-Gaussian beams | |||
| journal= Opt. Lett. | |||
| pages = 3053–3055 | |||
| publisher = OSA | |||
| volume = 32 | |||
| year = 2007 | |||
| url = http://www.opticsinfobase.org/abstract.cfm?URI=ol-32-21-3053 | |||
| doi= 10.1364/OL.32.003053 | |||
| pmid= 17975594 | |||
| issue= 21 | |||
|bibcode = 2007OptL...32.3053K |arxiv = 0712.0782 }} | |||
* [http://www.cvimellesgriot.com/Products/Documents/TechnicalGuide/Gaussian-Beam-Optics.pdf Gaussian Beam Propagation] - CVI Melles Griot Technical Guide | |||
* [http://www.newport.com/Gaussian-Beam-Optics/144899/1033/content.aspx Gaussian Beam Optics Tutorial, Newport] | |||
{{DEFAULTSORT:Gaussian Beam}} | |||
[[Category:Laser science]] | |||
[[Category:Electromagnetic radiation]] | |||
Revision as of 09:59, 1 December 2013

In optics, a Gaussian beam is a beam of electromagnetic radiation whose transverse electric field and intensity (irradiance) distributions are well approximated by Gaussian functions. Many lasers emit beams that approximate a Gaussian profile, in which case the laser is said to be operating on the fundamental transverse mode, or "TEM00 mode" of the laser's optical resonator. When refracted by a diffraction-limited lens, a Gaussian beam is transformed into another Gaussian beam (characterized by a different set of parameters), which explains why it is a convenient, widespread model in laser optics.
The mathematical function that describes the Gaussian beam is a solution to the paraxial form of the Helmholtz equation. The solution, in the form of a Gaussian function, represents the complex amplitude of the beam's electric field. The electric field and magnetic field together propagate as an electromagnetic wave. A description of just one of the two fields is sufficient to describe the properties of the beam.
The behavior of the field of a Gaussian beam as it propagates is described by a few parameters such as the spot size, the radius of curvature, and the Gouy phase.[1]
Other solutions to the paraxial form of the Helmholtz equation exist. Solving the equation in Cartesian coordinates leads to a family of solutions known as the Hermite–Gaussian modes, while solving the equation in cylindrical coordinates leads to the Laguerre–Gaussian modes.[2][3] For both families, the lowest-order solution describes a Gaussian beam, while higher-order solutions describe higher-order transverse modes in an optical resonator.
Mathematical form
The Gaussian beam is a transverse electromagnetic (TEM) mode.[4] A mathematical expression for its complex electric field amplitude can be found by solving the paraxial Helmholtz equation, yielding[1]
where[1]
- is the radial distance from the center axis of the beam,
- is the axial distance from the beam's narrowest point (the "waist"),
- is the imaginary unit (for which ),
- is the wave number (in radians per meter),
- ,
- is the radius at which the field amplitude and intensity drop to 1/e and 1/e2 of their axial values, respectively,
- is the waist size,
- is the radius of curvature of the beam's wavefronts, and
- is the Gouy phase shift, an extra contribution to the phase that is seen in Gaussian beams.
Additionally, the field has a time dependence factor that has been suppressed in the above expression.
The corresponding time-averaged intensity (or irradiance) distribution is
where is the intensity at the center of the beam at its waist. The constant is the characteristic impedance of the medium in which the beam is propagating. For free space, .
Beam parameters
The geometry and behavior of a Gaussian beam are governed by a set of beam parameters, which are defined in the following sections.
Beam width or spot size
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For a Gaussian beam propagating in free space, the spot size (radius) w(z) will be at a minimum value w0 at one place along the beam axis, known as the beam waist. For a beam of wavelength λ at a distance z along the beam from the beam waist, the variation of the spot size is given by[1]
where the origin of the z-axis is defined, without loss of generality, to coincide with the beam waist, and where[1]
is called the Rayleigh range.
Rayleigh range and confocal parameter
At a distance from the waist equal to the Rayleigh range zR, the width w of the beam is[1]
The distance between these two points is called the confocal parameter or depth of focus of the beam:
Radius of curvature
R(z) is the radius of curvature of the wavefronts comprising the beam. Its value as a function of position is[1]
Beam divergence
The parameter increases linearly with for . This means that far from the waist, the beam is cone-shaped. The angle between the straight line and the central axis of the beam () is called the divergence of the beam. It is given by[1]
The total angular spread of the beam far from the waist is then given by
Because the divergence is inversely proportional to the spot size, a Gaussian beam that is focused to a small spot spreads out rapidly as it propagates away from that spot. To keep a laser beam very well collimated, it must have a large diameter. This relationship between beam width and divergence is due to diffraction. Non-Gaussian beams also exhibit this effect, but a Gaussian beam is a special case where the product of width and divergence is the smallest possible.
Since the gaussian beam model uses the paraxial approximation, it fails when wavefronts are tilted by more than about 30° from the direction of propagation.[5] From the above expression for divergence, this means the Gaussian beam model is valid only for beams with waists larger than about .
Laser beam quality is quantified by the beam parameter product (BPP). For a Gaussian beam, the BPP is the product of the beam's divergence and waist size . The BPP of a real beam is obtained by measuring the beam's minimum diameter and far-field divergence, and taking their product. The ratio of the BPP of the real beam to that of an ideal Gaussian beam at the same wavelength is known as M2 ("M squared"). The M2 for a Gaussian beam is one. All real laser beams have M2 values greater than one, although very high quality beams can have values very close to one.
Gouy phase
The longitudinal phase delay or Gouy phase of the beam is[1]
The Gouy phase indicates that as a Gaussian beam passes through a focus, it acquires an additional phase shift of π, in addition to the usual phase shift that would be expected from a plane wave.[1]
Complex beam parameter
Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. Information about the spot size and radius of curvature of a Gaussian beam can be encoded in the complex beam parameter, :[6]
The reciprocal shows the relationship between , , and explicitly:[6]
The complex beam parameter plays a key role in the analysis of Gaussian beam propagation, and especially in the analysis of optical resonator cavities using ray transfer matrices.
In terms of the complex beam parameter , a Gaussian field with one transverse dimension is proportional to
In two dimensions one can write the potentially elliptical or astigmatic beam as the product
which for the common case of circular symmetry where and yields[7]
Power and intensity
Power through an aperture
The power P passing through a circle of radius r in the transverse plane at position z is
where
is the total power transmitted by the beam.
For a circle of radius , the fraction of power transmitted through the circle is
Similarly, about 95 percent of the beam's power will flow through a circle of radius .
Peak and average intensity
The peak intensity at an axial distance from the beam waist is calculated using L'Hôpital's rule as the limit of the enclosed power within a circle of radius , divided by the area of the circle :
The peak intensity is thus exactly twice the average intensity, obtained by dividing the total power by the area within the radius .
Derivation
The Gaussian beam formalism begins with the wave equation for an electromagnetic field in free space or in a homogeneous dielectric medium:[8]
where may stand for any one of the six field components , , , , , or . The Gaussian beam formalism proceeds by writing down a solution of the form[8]
where it is assumed that the beam is sufficiently collimated along the axis that may be neglected. Substituting this solution into the wave equation above yields the paraxial approximation to the wave equation:[8]
Solving this differential equation yields an infinite set of functions, of which the Gaussian beam is the lowest-order solution or mode.
Higher-order modes
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We will not only get you a property at a rock-backside price but also in an space that you've got longed for. You simply must chill out back after giving us the accountability. We will assure you 100% satisfaction. Since we now have been working in the Singapore actual property market for a very long time, we know the place you may get the best property at the right price. You will also be extremely benefited by choosing us, as we may even let you know about the precise time to invest in the Singapore actual property market.
The Hexacube is offering new ec launch singapore business property for sale Singapore investors want to contemplate. Residents of the realm will likely appreciate that they'll customize the business area that they wish to purchase as properly. This venture represents one of the crucial expansive buildings offered in Singapore up to now. Many investors will possible want to try how they will customise the property that they do determine to buy by means of here. This location has offered folks the prospect that they should understand extra about how this course of can work as well.
Singapore has been beckoning to traders ever since the value of properties in Singapore started sky rocketing just a few years again. Many businesses have their places of work in Singapore and prefer to own their own workplace area within the country once they decide to have a everlasting office. Rentals in Singapore in the corporate sector can make sense for some time until a business has discovered a agency footing. Finding Commercial Property Singapore takes a variety of time and effort but might be very rewarding in the long term.
is changing into a rising pattern among Singaporeans as the standard of living is increasing over time and more Singaporeans have abundance of capital to invest on properties. Investing in the personal properties in Singapore I would like to applaud you for arising with such a book which covers the secrets and techniques and tips of among the profitable Singapore property buyers. I believe many novice investors will profit quite a bit from studying and making use of some of the tips shared by the gurus." – Woo Chee Hoe Special bonus for consumers of Secrets of Singapore Property Gurus Actually, I can't consider one other resource on the market that teaches you all the points above about Singapore property at such a low value. Can you? Condominium For Sale (D09) – Yong An Park For Lease
In 12 months 2013, c ommercial retails, shoebox residences and mass market properties continued to be the celebrities of the property market. Models are snapped up in report time and at document breaking prices. Builders are having fun with overwhelming demand and patrons need more. We feel that these segments of the property market are booming is a repercussion of the property cooling measures no.6 and no. 7. With additional buyer's stamp responsibility imposed on residential properties, buyers change their focus to commercial and industrial properties. I imagine every property purchasers need their property funding to understand in value.
Gaussian beams are just one possible solution to the paraxial wave equation. Various other sets of orthogonal solutions are used for modelling laser beams. In the general case, if a complete basis set of solutions is chosen, any real laser beam can be described as a superposition of solutions from this set. The design of the laser determines which basis set of solutions is most useful. In some cases the output of a laser may closely approximate a single higher-order mode. Hermite-Gaussian modes are particularly common, since many laser systems have Cartesian reflection symmetry in the plane perpendicular to the beam's propagation direction.
Hermite-Gaussian modes

Hermite-Gaussian modes are a convenient description for the output of lasers whose cavity design is not radially symmetric, but rather has a distinction between horizontal and vertical. In terms of the previously defined complex parameter, the amplitude distribution in the -plane is proportional to
where the function is the Hermite polynomial of order (physicists' form, i.e. ), and the asterisk indicates complex conjugation. For the case the equation yields a Gaussian transverse distribution.
For two-dimensional rectangular coordinates one constructs a function , where has the same form as . Mathematically this property is due to the separation of variables applied to the paraxial Helmholtz equation for Cartesian coordinates.[9]
Hermite-Gaussian modes are typically designated "TEMmn", where m and n are the polynomial indices in the x and y directions. A Gaussian beam is thus TEM00.
Laguerre-Gaussian modes

If the problem is cylindrically symmetric, the natural solutions of the paraxial wave equation are Laguerre-Gaussian modes.[3] They are written in cylindrical coordinates using Laguerre polynomials
where are the generalized Laguerre polynomials, the radial index and the azimuthal index is . is an appropriate normalization constant; , and are beam parameters defined above.
Ince-Gaussian modes
In elliptic coordinates, one can write the higher-order modes using Ince polynomials. The even and odd Ince-Gaussian modes are given by [10]
where and are the radial and angular elliptic coordinates defined by
are the even Ince polynomials of order and degree , is the ellipticity parameter, and is the Gouy phase. The Hermite-Gaussian and Laguerre-Gaussian modes are a special case of the Ince-Gaussian modes for and respectively.
Hypergeometric-Gaussian modes
There is another important class of paraxial wave modes in polar coordinates in which the complex amplitude is proportional to a confluent hypergeometric function.
These modes have a singular phase profile and are eigenfunctions of the photon orbital angular momentum. The intensity profile is characterized by a single brilliant ring with a singularity at its center, where the field amplitude vanishes.[11]
where is integer, is real valued, is the gamma function and is a confluent hypergeometric function.
Some subfamilies of hypergeometric-Gaussian (HyGG) modes can be listed as the modified Bessel-Gaussian modes, the modified exponential Gaussian modes, and the modified Laguerre–Gaussian modes.
The set of hypergeometric-Gaussian modes is overcomplete and is not an orthogonal set of modes. In spite of its complicated field profile, HyGG modes have a very simple profile at the pupil plane:
See Optical vortex, which explains that the outcoming wave from a pitch-fork hologram is a sub-family of HyGG modes. The HyGG profile while beam propagates along has a dramatic change and it is not a stable mode below the Rayleigh range.
See also
Notes
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Svelto, pp. 153–5.
- ↑ Siegman, p. 642.
- ↑ 3.0 3.1 probably first considered by Goubau and Schwering (1961).
- ↑ Svelto, p. 158.
- ↑ Siegman (1986) p. 630.
- ↑ 6.0 6.1 Siegman, pp. 638–40.
- ↑ See Siegman (1986) p. 639. Eq. 29
- ↑ 8.0 8.1 8.2 Svelto, pp. 148–9.
- ↑ Siegman (1986), p645, eq. 54
- ↑ Bandres and Gutierrez-Vega (2004)
- ↑ Karimi et. al (2007)
References
- 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 Chapter 3, "Beam Optics," pp. 80–107.
- 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 Chapter 5, "Optical Beams," pp. 267.
- 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 Chapter 16.
- 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
- 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.
My blog: http://www.primaboinca.com/view_profile.php?userid=5889534
- Template:Cite arxiv
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The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more
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In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang
Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules
Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.
A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running
The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more
There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang - One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting
In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang
Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules
Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.
A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running
The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more
There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang - Gaussian Beam Propagation - CVI Melles Griot Technical Guide
- Gaussian Beam Optics Tutorial, Newport