A laser beam is often described as a parallel beam, but its diameter generally changes as it propagates. This spreading is called beam divergence. Understanding its definition helps when selecting a laser, designing optics, or estimating the spot size at a target.<\/p>\n
A beam with a finite transverse size cannot remain perfectly parallel indefinitely. Diffraction produces an angular spread even in an ideal optical system, so divergence is not necessarily a defect. A narrower beam generally has a larger diffraction-limited divergence, while increasing the beam diameter can reduce that spread.<\/p>\n
In practical systems, the beam also depends on the laser source, optical alignment, lens quality, and operating conditions. A larger beam may require larger optics and more careful alignment, so minimizing divergence is only one part of the design.<\/p>\n
A Gaussian beam has an approximately bell-shaped intensity distribution. Its smallest radius occurs at the beam waist. Away from the waist, the beam radius increases; sufficiently far from it, that increase is approximately linear with distance.<\/p>\n
An ideal Gaussian beam defines a diffraction-limited reference. The beam propagation factor M\u00b2 describes how a real beam differs from that reference, with an ideal value of 1. Beam quality, wavelength, and waist size all influence propagation. The smallest possible divergence cannot be specified from wavelength alone.<\/p>\n
The half-angle is measured between the beam axis and one side of the expanding beam. The full-angle is the angle between both sides and is twice the half-angle. Specifications often use milliradians (mrad), so checking which angle is quoted is essential.<\/p>\n
For example, in the far field and at small angles, a full-angle divergence of 1 mrad corresponds to approximately 1 mm of increase in beam diameter over 1 m of propagation. If 1 mrad is a half-angle instead, the diameter increase is approximately 2 mm over the same distance.<\/p>\n
A beam does not always have a sharp edge. Diameter therefore needs a defined measurement convention. For a Gaussian intensity profile, the 1\/e\u00b2 diameter is measured where intensity has fallen to about 13.5% of its peak. Full width at half maximum (FWHM) uses a different threshold and gives a different width.<\/p>\n
Some beams are elliptical, with different divergence along two perpendicular axes. Diode-laser specifications may identify fast and slow axes. Compare values only when the angle definition, beam-width convention, axes, and measurement conditions are compatible.<\/p>\n
One practical method measures beam diameters D\u2081 and D\u2082 at two positions separated by a distance L. In the far field, where beam diameter changes approximately linearly, the small-angle estimate of full-angle divergence is:<\/p>\n
\u0398 \u2248 (D\u2082 \u2212 D\u2081) \/ L<\/strong><\/p>\n Use consistent units. The result is in radians; multiply by 1,000 for milliradians. The corresponding half-angle is \u03b8 = \u0398 \/ 2. This expression uses diameters, not radii, and assumes the beam is expanding between the measurement positions.<\/p>\n Measurements close to the waist can give misleading results because the beam envelope is curved there. For more reliable characterization, take measurements at multiple positions and fit an appropriate propagation model. Keep the same width definition and measure both axes when the beam is elliptical.<\/p>\n Check the wavelength, beam diameter, waist location where relevant, M\u00b2, angle convention, and whether divergence is specified for the bare emitter or after collimating optics. A diode emitter and a complete collimated laser module can have very different angular spreads.<\/p>\n All else being equal, shorter wavelengths and larger waist sizes support smaller diffraction-limited divergence. Real performance still depends on beam quality and optics, so confirm the actual specification rather than inferring it from a single parameter.<\/p>\n Spatial coherence is related to the phase relationship across a beam and is relevant to its propagation. Temporal coherence concerns phase relationships over time and is associated with spectral properties and coherence length. A long coherence length does not, by itself, establish a small divergence angle.<\/p>\n Select the required properties according to the application. Interference measurements, beam delivery, and illumination may place different demands on coherence, beam shape, stability, and divergence.<\/p>\n A collimating lens is used to reduce the angular spread from a source. Its position relative to the source is important: an incorrectly positioned lens can leave the beam converging or diverging. A beam expander increases beam diameter and can reduce divergence, provided the beam and optical system support the intended performance.<\/p>\n Optical quality, available aperture, alignment, and mounting stability all matter. Avoid clipping the beam, and distinguish changes in beam width from changes in pointing direction. A beam can remain narrow while its central axis moves.<\/p>\n Focusing introduces a trade-off between spot size and the axial range over which that spot remains small. Focal length, numerical aperture, incoming beam diameter, and beam quality influence the result. A shorter focal length can help produce a small spot, but may reduce working clearance and tighten alignment tolerances.<\/p>\n Design around the required working distance and usable focus range, not only the smallest calculated spot. The lens aperture should accommodate the beam without unintended clipping.<\/p>\n A diode laser may have an elliptical beam, different divergence in the two axes, and astigmatism. Suitable collimating or beam-shaping optics may be needed. Do not assume that a low-divergence specification for a finished module also applies to its bare diode.<\/p>\n The mechanical axis of a housing is not always identical to the optical axis. A small pointing error can produce a large displacement at a distant target. Temperature can change wavelength, output, beam profile, and alignment, while mounting stress, heat flow, and cable forces can also affect stability.<\/p>\n Where appropriate, observing the spot while rotating a mounted source can help identify an offset between mechanical and optical axes. Such checks should be interpreted together with the mounting geometry and performed using suitable laser-safety measures.<\/p>\n He\u2013Ne lasers often provide a relatively circular, stable beam, while many semiconductor lasers require attention to axis-dependent beam properties. However, performance varies substantially between products and optical configurations. Compare actual specifications for the complete system rather than assigning one universal divergence value to a laser type.<\/p>\n As a beam spreads, its illuminated area increases and its intensity distribution changes. In measurement, this can affect spatial resolution, the light returned from a surface, and signal-to-noise ratio. Target reflectance, surface texture, scattering, and the collection optics must be considered alongside beam diameter.<\/p>\n In processing, spot size and energy density influence the resulting feature width and the usable focus range. A design that performs well at one target distance may behave differently when the target moves. Evaluate the beam under the distances, temperatures, and surface conditions expected in operation.<\/p>\n Beam divergence is an inherent part of finite-beam propagation. To compare specifications correctly, check half-angle versus full-angle, the beam-width convention, measurement position, and optical configuration. Combining an appropriate beam diameter and optics with stable mechanical and thermal design helps achieve the required performance over the intended working distance.<\/p>\n","protected":false},"excerpt":{"rendered":" A laser beam is often described as a parallel beam, but its diameter generally changes as it propagates. This <\/p>\n","protected":false},"author":2,"featured_media":5360,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_locale":"en_US","_original_post":"https:\/\/www.katsura-opto.com\/?p=4754","footnotes":""},"categories":[18],"tags":[],"class_list":["post-6746","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-column","en-US"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/posts\/6746","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/comments?post=6746"}],"version-history":[{"count":1,"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/posts\/6746\/revisions"}],"predecessor-version":[{"id":6750,"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/posts\/6746\/revisions\/6750"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/media\/5360"}],"wp:attachment":[{"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/media?parent=6746"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/categories?post=6746"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.katsura-opto.com\/wp-json\/wp\/v2\/tags?post=6746"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Reading a specification sheet<\/h3>\n
Divergence and coherence are different properties<\/h2>\n
Controlling a beam with optics<\/h2>\n
Collimators and beam expanders<\/h3>\n
Focusing and working distance<\/h3>\n
Practical considerations for diode lasers<\/h2>\n
Comparing laser types<\/h3>\n
Effects in measurement and processing<\/h2>\n
Summary<\/h2>\n