Aside from the laser source itself, optics are one of the most important aspects of a laser system. They are used to focus, manipulate and shape laser light into beams with parameters tailored for a wide range of applications.
They are therefore essential in all aspects of laser usage, from the cutting, welding, marking and additive manufacturing tasks of laser-based manufacturing, to the high-energy physics experiments of research institutes and the various laser treatments available in medicine.
Laser optics are available in a staggering number of varieties, so much so that the catalogue of our sponsor for this article, Edmund Optics, is no less than 170 pages long – boasting a countless array of lenses, prisms, mirrors, filters, polarisers, beam expanders, beam splitters, windows and couplers, made from (and coated in) an equally wide range of exotic materials.
In this article we provide an introductory view of the many optical building blocks used in laser systems.
Mirrors
Laser mirrors ensure that a beam is directed correctly, featuring tight surface qualities in order to minimise light scatter. They are often used for beam steering or folding, interferometry, and for constructing laser resonators.
Laser mirrors are often made from materials such as copper, nickel, borosilicate glass, and fused silica. To increase reflectivity, they’re usually coated with a thin layer of metal such as aluminium, gold, rhodium, or silver. The type of coating depends on the wavelength range of the laser and the desired reflectivity. For example, gold is often used in the near-IR to far-IR range, aluminium is effective in the visible and near-infrared range, and rhodium-coated nickel is favoured for its resistance to thermal and physical damage.
However, these coatings are delicate – they can easily scratch or degrade when exposed to the elements. To prevent this, additional coatings are often applied to laser mirrors. Dielectric coatings, for instance, provide higher reflection than metallic coatings and are optimised for common laser wavelengths, while laser line mirror coatings can help prevent laser damage and ensure a long lifetime.
Lenses
While mirrors direct the collimated light from a laser beam, optical lenses focus it. When multiple lenses are combined in a beam shaping system, they can be used to form spots, lines, rings and other shapes of varying size. Like those found in a camera, laser lenses can often be adjusted to produce ‘close ups’ or ‘wide angles’ of light for tasks such as laser illumination, scanning, welding, cutting, and other types of material processing.
These tasks require a variety of lens types in a range of wavelengths, such as spherical, aspherical, plano-convex, cylindrical, conical, and line-generating lenses. These lenses have different focal distances for different applications. The smaller the beam diameter, the higher the energy density of the laser beam, which results in a more aggressive beam. Like laser mirrors, laser lenses are also coated with numerous exotic materials to ensure they perform their intended task with the highest performance and greatest reliability and longevity.
Prisms
Perhaps most recognisable from the cover of a Pink Floyd album, prisms have been around for centuries. Indeed, it was Sir Isaac Newton who discovered that glass prisms could be used to steer, direct, and disperse light – something he demonstrated by splitting a beam of sunlight into the seven colours of the rainbow.
Prisms are commonly used for beam steering in laser applications (Image: Shutterstock/Borkin Vadim)
Lasers, however, are usually only one colour. So instead of being split like in Newton’s experiment, laser beams tend to bend when shined through a prism. For this reason, laser prisms are often designed to internally reflect a laser beam off of multiple surfaces in order to redirect the beam path, making them useful for beam steering or beam manipulation applications.
As with laser mirrors and lenses, laser prisms come in a wide range of forms, including: right-angle, dispersion, penta, image rotation, retroreflection, wedge, and more. All these prisms can be found in various scientific and industrial settings for tasks such as laser tracking, alignment, and range finding, as well as atmospheric monitoring.
Filters
Laser filters are used to essentially ‘clean up’ laser systems by selectively transmitting or blocking specific wavelengths of light, depending on which are required for the application at hand. By screening out interference such as background radiation, ambient light, and noise, filters can prevent damage to optical components caused by changes in power levels or temperature. More specifically, laser line and shortpass edge filters improve the signal quality at the laser source, while longpass edge and laser rejection filters tackle unwanted noise at a detector. Notch and dichroic filters are also commonly deployed in laser systems.
Similar to the lasers themselves, laser filters are deployed across a wide range of applications, including material processing, spectroscopy (measuring chemical and physical properties of samples), medical and life sciences (surgical procedures, diagnostics, and therapeutics), and optical communications (efficient data transfer over optical fibres). They are also used to improve the contrast and resolution of images and protect sensitive optical systems in the defence industry, while in astronomy they are used in telescopes to collect the right types of light required to capture stars, galaxies, and other celestial objects in perfect clarity.
Beam splitters and expanders
Beam splitters and expanders are used to change the radius of collimated laser beams. These components often constitute a series of mirrors, prisms, lenses, and filters.
Beam splitters split a light beam into two or more beams, usually by wavelength or polarity. They can also work in reverse to combine multiple light beams into one. There are various types of beamsplitters: dichroic beamsplitters reflect some wavelengths while transmitting others; polarising beam splitters split light beams based on polarity while non-polarising splitters do so independently of the polarity; and lateral beam splitters divide an incident beam into two parallel beams.
Beam expanders, on the other hand, increase the diameter of a light beam to reduce laser power density and prevent damage to optical components. They can maintain beam collimation in long-path systems and are useful for remote sensing, interferometry, laser scanning and more. Like their light-splitting counterpart, beam expanders can also be used in reverse to reduce the radius of the beam.
Beam expanders often come in the form of optical telescopes that have two lenses or curved mirrors. Keplerian and Galilean telescopes are the two most common types of beam expanders. A Keplerian telescope has two focusing lenses separated by their combined focal lengths, with a beam waist between them. This means that changing the focal length of the lenses changes the beam radius. A Galilean telescope, which is a little more compact, uses a focusing lens and a defocusing lens, with the distance between them equal to the sum of their focal lengths, where one is negative.
Beam shapers
Laser beam shapers are used to collimate laser beams, transform beam profiles, convert beam shapes and more. They are required as, typically, laser sources emit beams with a non-uniform intensity that are not ideal for most applications, and many applications require a beam shape that laser sources cannot naturally produce, such as a line or ring.
Some of the many types of beam shapers include: flat top beam shapers, used to convert Gaussian beams into flat top beams with a uniform intensity distribution; cylinder lenses, used to convert an oval or differently shaped profile to a circular one; slow axis collimators and fast axis collimators, used to collimate the light of laser bars and laser diodes, respectively; microlens arrays, for homogenising a variety of light emitters such as line-narrowed excimer lasers and high-power LEDs; diffractive optical elements, which transform laser beams with a nearly-Gaussian profile into a defined 2D shape with uniform intensity distribution at the focal point of a lens; and multi-focus objectives, which can focus laser light to multiple foci along the optical axis, increasing the effective depth of focus (useful in high-speed cutting applications).
Crystals
Laser crystals are solid-state materials used as the active medium (or ‘gain media’) in laser devices. They’re designed to amplify light and generate a coherent beam via the process of stimulated emission. These crystals are highly transparent to prevent scattering and light absorption and must be extremely durable to withstand the thermal effects of a high-power laser. Crystals are often doped with impurities such as rare earth or transition metal ions to alter their properties. The density of this doping, along with the shape of the crystal, influences the characteristics of the laser beam, including its wavelength, power stability and beam coherence. The crystals are known as “host crystals”, with common types including garnets, vanadates, fluorides, sapphires, and chalcogenides.
Optics are used to focus, manipulate and shape laser light into beams with parameters tailored for a wide range of applications (Image: Shutterstock/Mike_shots)
Neodymium-doped yttrium aluminium garnet (Nd:YAG) crystals, for example, are commonly used in high-power Q-switched lasers emitting light in the infrared range, while neodymium-doped vanadate (Nd:YVO4) crystals are suited to mode-locked infrared lasers with high pulse repetition rate; titanium-doped sapphire crystals have particularly broad bandwidths and are often used in tuneable and ultrashort pulse lasers; and chalcogenide crystals have broad absorption and emission bands, meaning they can be used to emit a range of different beam types such as continuous-wave, gain-switched, Q-switched, and mode-locked.
Isolators
When a light beam is passed through an optical fibre it can reflect back on the laser source and damage it. Optical isolators prevent this by only allowing light to only be transmitted in one direction. Types of optical isolators include polarised, composite, and magnetic. Polarised and composite isolators use polarisation to control light transmission, while magnetic isolators use the Faraday effect to control the polarisation of light.
Faraday isolators have three main components: an input and output polariser, and a Faraday rotator. The Faraday rotator rotates the polarisation of light passing through it by 45 degrees, and the input and output polarisers ensure that light can only pass through in one direction. The optical isolator operates in two modes – forward mode and backward mode. In forward mode, light enters the input polariser, becomes linearly polarised, passes through the Faraday rotator, and then exits through the output polariser. In backward mode, light enters the output polariser, is rotated by the Faraday rotator, and then reflected by the input polariser.
Isolators can often be found in laboratory, industrial, and corporate environments being used to improve signal levels and stability in optical communication systems, amplifiers, and laser diodes.
Ultrafast optics
Ultrafast optics, as the name suggests, are designed specifically for ultrafast lasers. These lasers are characterised by their ultrashort pulse durations (picoseconds, femtoseconds and even attoseconds) and are used across a wide range of applications. For example, they can be used to micromachine materials with extreme precision, or observe physical processes that occur at small time scales, such as photosynthesis, human vision, protein folding, and molecular vibrations.
Ultrafast lasers are also deployed across medical applications, nonlinear imaging and microscopy, spectroscopy, communications, defence, and more. Ultrafast optics encompass mirrors, lenses, prisms, filters, crystals, gratings, windows, beam expanders, splitters and shapers, all of which are designed to withstand the ultrashort pulse durations, broad wavelength ranges, and extremely high peak powers of ultrafast lasers.
Windows
Laser windows provide a high degree of transmission of specified wavelengths, used in both laser safety and laser applications. For laser safety, windows are configured to protect users from laser radiation while still allowing them to observe the beam and application as a whole. Laser windows are made from a wide array of polymers and glass, available in various shades and colours depending on the wavelength and power of laser light being blocked/transmitted.
In laser applications, similar to a filter, laser windows can offer exceptional transmission of desired wavelengths while effectively reflecting unwanted wavelengths. They can be used to isolate a laser beam, and are available in high-power versions for when particularly high damage thresholds are required.