Focusing lenses

01

Pick the focal length: spot size versus depth of focus

RP Photonics defines the focal length of a thin focusing lens as the distance from the lens to the point where an incident collimated beam is focused, and notes that focal length depends on wavelength, causing chromatic aberration. [1] For a collimated Gaussian beam of radius w0 hitting a lens of focal length f, the focused waist radius is wf = λf/(πw0) — proportional to focal length and wavelength and inversely proportional to input beam radius — assuming the focus is much smaller than the input beam and the paraxial approximation holds. [1] Around a waist of radius w0 the beam stays near its minimum size over the Rayleigh length zR = πw0²/λ on each side, the confocal length being twice that; beams with non-ideal beam quality (M² above 1) have a larger beam parameter product than the ideal λ/π. [2]

Editorial calculation: assuming an ideal CO2 beam (10.6 µm) of 5 mm radius at the lens, a 63.5 mm (2.5 in) lens gives a focus radius of about 43 µm with a Rayleigh length of about 0.54 mm, so roughly 1.1 mm depth of focus; a 38.1 mm (1.5 in) lens gives about 26 µm radius but only about 0.4 mm depth of focus. The same 63.5 mm geometry at 1.07 µm (fiber laser) gives about 4.3 µm radius. Real beams with M² above 1 focus larger in proportion. [1] [2] [3]

Editorial note: short focal lengths suit thin material and fine engraving; longer focal lengths keep the beam narrow over thicker material and give more working distance. Confirm the choice with the head or machine maker.

Give the supplier the laser type and wavelength, laser power, the head or machine model, lens diameter and edge thickness, focal length, lens shape (plano-convex or meniscus), material and grade (e.g. CVD or PVD ZnSe for CO2), anti-reflection coating at the wavelength, surface quality and flatness, damage threshold data, mount or washer, and quantity.

02

Match material, wavelength and size to the head

Lens listings show how material and size are specified. Startnow, a laser-parts supplier, lists CO2 laser focusing lenses in zinc selenide (ZnSe), offered as CVD ZnSe and PVD ZnSe grades and as branded II-VI lenses, in diameters of 12, 18, 19.05, 20, 22 and 25.4 mm and focal lengths of 38.1, 50.8, 63.5, 76.2 and 190.5 mm — the longest for high-power mixed cutting — together with PTFE washers to seat the lens, and separate focusing and collimating lenses for fiber laser machines. [3] Sub-Lattice Optical lists F-theta lenses by wavelength — 10.6 µm CO2, 1030–1080 nm fiber, 355 nm UV and 266 nm — and Bessel-beam cutting heads for glass. [4]

Editorial note: a lens is designed and coated for one wavelength band; a CO2 lens will not work on a fiber laser or the reverse, and replacement lenses must match the head's diameter, thickness and focal length so the focus position and nozzle distance stay correct.

03

Compare what suppliers offer

Suppliers linked to this page list laser focusing lenses; each card links to the page reviewed.

Listings include CO2 ZnSe focusing lenses in 12–25.4 mm diameters and 38.1–190.5 mm focal lengths, fiber-laser focusing and collimating lenses, and focusing optics in Bessel glass-cutting heads; ask each for material grade, coating reflectance, surface specification and test data.

04

Trace what sets lens performance

This is a functional reference built from the sources cited; it is not one maker's design.

05

Fill the specification fields with values for your project

A focusing lens quotation needs values for your project. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the laser, head and lens size together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Laser: [CO2 10.6 µm; fiber ~1 µm; power]
Head: [maker; model]
Lens: [diameter; thickness; focal length; shape]
Material: [ZnSe CVD/PVD; other]
Coating: [AR at wavelength]
Quality: [surface; damage threshold]
Quantity: [pcs]

See manufacturer records Send an enquiry through CNFX