We've put together some of the most frequently asked questions in the laser industry. Check out our list of laser glossary terms and FAQs.
Laser Glossary of Terms and FAQs
What is beam collimation?
Beam collimation is a measure of how parallel the light beams of a light source are to each other, with minimal divergence as they propagate. Collimation can be achieved in a number of ways - via specialised optics, fibres, or mirrors. The associated lens is called a collimator or converging lens. A collimator can be used to bundle the expanded beam from a laser source into a parallel or focused beam.
As the distance from the light source increases, the level of collimation increases. This effect is observed in the high level of collimation of star light.
What is beam diameter and beam waist?
The beam diameter is a measurement of the cross-section of a laser beam, indicating the area covered by the beam spot. The beam waist is the point where the beam diameter is at a minimum for its propagation, some distance from the aperture and related to the overall beam divergence.
If the beam is non-circular, where the beam profile does not have a constant radius in all directions, the position of the beam waist can vary depending on the orientation of the measurement - the measured beam diameter is only a minimum in one transverse direction. This effect is called astigmatism and it can be corrected, or indeed caused, by passing the beam through additional optics.
What is beam divergence?
The beam divergence is an angular measure of the change in beam diameter as it propagates from a source, some significant distance away from the beam waist. A measurement of beam divergence will show the rate of change in the actual beam size as it propagates from the source. A smaller the divergence value for a laser beam, the more collimated the output.
What is beam pointing?
Beam pointing refers to the stability in the direction of the output beam from a laser. Various thermal or mechanical effects can cause movement in the angular direction of the laser beam propagating from a source, which is quantified as a measurement of beam pointing stability over time and with respect to temperature change.
What is beam product parameter?
The measurements of beam divergence and beam waist can be combined into a single reference, known as the Beam Product Parameter (BPP). The BPP is calculated as the product of the beam divergence and the beam radius at the beam waist, and can be used as an indication of beam quality. The lower this BPP value, the higher the beam quality.
What is the coherence of a laser?
Coherence describes the extent to which a real electromagnetic wave, with statistically fluctuating amplitude and phase, matches an ideal sinusoidal wave, with a defined amplitude and phase. A distinction is made between spatial and temporal coherence - where light sources can be coherent, incoherent or partially coherent.
What is a continuous-wave laser?
A CW or continuous-wave laser emits constant and uninterrupted output power and intensity over time.
What is a diffraction-limited beam?
The beam quality K is the reciprocal of the diffraction index M^2 and therefore indicates the divergence angle of an ideal Gaussian beam, as compared to the divergence angle of a real laser beam with the same diameter at the beam waist.
The greater the beam quality, the smaller the minimum beam cross-section that can be achieved in the focal plane of a lens, which is a decisive factor for many applications.
Generally, the minimum focus radius is only limited by the diffraction - however, in practise, this is often limited by physical reasons. A good focusability of the radiation is particularly important if the laser is used in imaging applications, since a higher power density and better resolution can be achieved with a smaller cross section. A single frequency laser usually has a diffraction-limited beam.
Recently, researchers have succeeded in developing a method that makes it possible to go below the normal light microscopic resolution limit, formulated by Ernst Abbe. A STED microscope (STED = Stimulated Emission Depletion) is a special form of optical microscopy, the resolution of which is not diffraction-limited - allowing them to distinguish between structures that are much closer together. In 2014, Stefan Hell was awarded the Nobel Prize in Chemistry for his work on STED.
What is a diode-pumped solid-state (DPSS) laser?
The expression "diode-pumped solid-state laser" already reveals a lot about its structure - it is a laser with a solid laser medium (often a crystal) that is optically pumped with a laser diode. In order to stimulate the laser activity in the crystal, energy has to be supplied - this is called "pumping". In this case the crystal is optically pumped by a laser diode with high power, focused on the crystal. The excited crystal is then able to emit light, coherently focused into a laser beam.
What is ellipticity?
Laser ellipticity refers to the circularity, or roundness, of a laser beam profile. The beam shape dictates the size to which a laser beam can be focused. A small, focused spot size maintains a higher power-per-area distribution while a less circular, larger spot size is uneven in its power distribution. A beam with an elliptical shape is undesirable for high-precision applications that require a circularly symmetric Gaussian (TEM00) beam profile - such as Raman Spectroscopy, Flow Cytometry, and Quantum.
What is laser intensity?
The intensity of a laser is a measure of the power delivered per unit area - most often defined as optical intensity, or irradiance, given in W/m2. Here, these two terms have a slightly different meaning with respect to laser sources. Optical intensity is the power of an area perpendicular to the beam, whereas irradiance refers specifically to the power delivered to the area of a sample that is not necessarily perpendicular to the beam.
Optical intensity, or irradiance, should not be confused with radiance, or radiant intensity. Radiant intensity is a measure of the power delivered per solid angle - given as W/m2.sr - used in photometry and laser eye safety.
What is linewidth?
The linewidth of a light source is the frequency range of its spectral output. The term 'linewidth' is often used interchangably with 'bandwidth' - where 'linewidth' refers to the FWHM of a single frequency mode, usually specified in Hz, whereas 'bandwidth' refers to the total spectral range arising from a multimode source with several frequency lines comprising the output, usually specified in nm. The smallest possible spectral width of an atomic or molecular spectral line is called the natural linewidth. More generally, the bandwidth refers to the total spectral range that an optical element can process - related to the colloquial meaning of 'the total information that can be handled'.
What is the M2 of a laser?
M2, or beam propagation ratio, is a calculation of how close a beam profile is to a circularly symmetric Gaussian (TEM00) beam profile. In many M2 calculation methods, at least 5 measurements are taken at the beam waist to obtain the waist diameter, and a further 5 measurements are taken at least one Rayleigh length away from the waist to obtain the far-field measurements of beam divergence. A perfect Gaussian beam's M2 measurement is denoted as M2 = 1.
What is mode locking?
Mode locking is the creation of a constant phase relationship between neighboring resonator modes in order to generate short pulses in a laser.
What is laser modulation?
Modulation is a general term referring to a controlled change, often periodic, of one of the parameters of the laser output. The modulated parameter of the laser can include a number of different specifications, such as the power, phase, polarisation, and frequency. These parameters are controlled using a variety of different technologies, such as electro-optic, acousto-optic and mechanical solutions.
What is a monochromatic laser?
Monochromatic, is derived from 'mono' meaning one and 'chromatic' meaning colour. Monochromatic refers to light comprised of only one frequency or wavelength. In reality, all light sources have some associated bandwidth in their output and are not truly monochromatic - perfect monochrome is an ideal that cannot be realised. A laser emits a narrowly defined wavelength that corresponds to the transition between two energy levels, and so lasers are often referred to as quasi-monochromatic sources.
Polychromatic sources, such as incandescent bulbs, emit a broad range of wavelengths in their output.
What is a narrow linewidth?
The linewidth of a laser is the width of the frequency, or wavelength, interval that is covered by a particular spectral line. This is most often measured as the full width half maximum (FWHM) - the interval over the profile of the line under consideration where the spectral intensity is half the maximum value. This is vital for many applications, such as Raman spectroscopy, where a narrow linewidth is extremely important to ensure high resolution.
What is laser polarisation?
The polarisation of a laser source is a description of the direction in which the electric field component is oscillating as the laser beam propagates. This electric field is always at right angles to the direction of travel for the beam, though its orientation may be fixed and consistent, variable or random. Typical examples include linear polarisation, where the electric field vibrates in a fixed direction, or circular polarisation, where the direction of the electric field component rotates through 360 degrees periodically. The polarisation of light can be altered using optical devices such as wave plates, which have different refractive indexes for different polarisations.
What is laser power stability?
Laser power stability is a measurement of the output intensity of a laser source and how it can change with time, within a certain range. This can be measured in a variety of different ways, but in all cases characterises the maximum possible range of fluctuations in power over time found in the laser output.
This is distinct from laser power noise, which are small random fluctuations caused by inherent conditions, such as quantum instability, thermal effects, mechanical vibration, electrical noise and so on, often normalising to the average power. This can be measured by a number of mathematical models, such as RMS or peak to peak values, and is a statistical approximation, which can vary by measurement bandwidth, temperature, and time.
Generally, the power stability of a laser will give the range of power levels that the laser will emit over time, usually restricted by some feedback mechanism. The power noise will highlight the level of inherent fluctuation in power while operating at any given condition.
What is the difference between power noise and optical noise?
Laser noise measurements describe fluctuations in different laser characteristics and takes the form of intensity (power) noise, and phase (optical) noise. Intensity, or power, noise is a measure of a laser's fluctuating output power over time - the noise itself a result of inherent conditions such as quantum instability, thermal effects, and mechanical vibration. Phase, or optical, noise is a measure of a laser's fluctuating phase (wavelength) over time and occurs through the stimulated emission/amplification of the gain media.
What is Rayleigh length?
The Rayleigh Length for a laser is the distance from the beam waist to where the beam spot has doubled in area. This assumes a circular beam spot, where higher M2 values reduce this length for any given laser. The Rayleigh Length is also related to the Confocal Parameter, b, which is double the Rayleigh length and determines the possible depth of focus.
What is a resonator mode?
A resonator mode is the natural electromagnetic oscillation of a resonator. A mode is the shape of the electromagnetic wave, which is defined by direction, frequency, and polarisation. In a laser resonator, a mode is uniquely designated by TEMplq or TEMmnq - where plq is the designation for circular mirrors and mnq is for rectangular mirrors. The index q is the longitudinal mode number and the indices p, l, m, and n are the transverse mode numbers.
What is a single frequency laser?
Single frequency lasers is a term used interchangeably with single longitudinal mode (SLM) lasers. These lasers operate with one longitudinal mode resonating in the cavity, or with one dominant mode and suppression of other bands.
What is a single mode laser?
Single mode laser is a term that is used interchangably between two separate parameters; single longitudinal mode lasers and single transverse mode lasers.
Transverse mode refers to the spatial distribution of intensity over the beam profile and is the most intuitive meaning of 'single mode', as the beam profile is visible and known, where TEM00 refers to the fundamental Gaussian distribution.
The longitudinal mode refers to the distinct frequencies that a laser operates on, perpendicular to the transverse mode, and dictates the spectral properties of the output. Single mode operation means only one frequency is present and is key for achieving the narrowest possible linewidth.
What is the difference between single mode and multi-mode lasers?
A longitudinal mode describes the electromagnetic natural oscillation state of a system in the longitudinal direction, i.e. in the direction of propagation. Light is reflected back and forth in a resonator. A standing wave is formed by the superposition of the two running directions. The longitudinal mode number q is the number of half waves in the resonator and follows from the resonator length L and wavelength λ. Typically, the resonator length is much larger than the wavelength, so the mode number q is very large, hence a multi-mode laser. The distance between two adjacent longitudinal modes (mode distance) is equal to the free spectral range of the resonator. Single frequency operation of a laser system can be enforced by using methods that prevent oscillations in several modes. Single frequency laser radiation with diffraction-limited beam quality is characterized by the ability to generate the highest spatial and spectral power densities.
What is a solid-state laser?
Solid-state lasers are optically-excited lasers, whose active medium consists of a crystalline or glass-like solid. Solid-state lasers are always optically pumped. They can be used in pulsed or continuous-wave operation.
What is spatial coherence?
Spatial coherence is a measure of how in phase a light wave is at any given location in time - measured in both transverse and longitudinal directions. A laser oscillating with a single transverse mode, like TEM00, is spatially coherent. As the coherence can be measured in two transverse dimensions a coherence area can be defined, which is dependent on the size of the light emitter and the distance of the light wave from the emitter - light waves will flatten as they move out from an emitting body, meaning starlight has a high level of spatially coherence.
What is a laser spectral range?
The electromagnetic spectrum consists of different frequency and wavelength ranges. They are not strictly defined and partially overlap. Wavelengths of 0.1 nm are usually referred to as X-rays, but if their origin is a nuclear decay, one speaks of gamma rays. The wavelength range of 400-700 nm is visible to the human eye and is therefore called the visible spectral range. The range of shorter wavelengths that adjoins it at the bottom is called the ultraviolet range. At the top there is a range with larger / longer wavelengths. This is the infrared range and is divided into near, medium and far infrared. The thermal radiation of bodies at normal temperature is, for example, in the infrared range. Above this lies the range of microwaves and radio waves.
In the visible range there are many frequently used laser wavelengths such as 640 nm (red), 594 nm (orange), 561 nm (yellow), 532 nm (green), 488 nm (blue) or 442 nm (blue-violet). In the UV range it is 349 nm or 266 nm, in the infrared 813 nm or 1064 nm, all of which are no longer visible to the human eye.
What is spectral stability?
Spectral, or wavelength stability of an optical source (laser) is a measure of the maximum deviation of the peak wavelength from its mean value over time. High spectral stability refers to a more stable wavelength - useful in applications requiring exact wavelength outputs, such as atomic clocks and optical tweezing.
What is a TEM00 beam?
Laser radiation is characterized by various properties such as average power, beam diameter, beam divergence and color, wavelength or frequency. The Gaussian beam (TEM00) is the fundamental mode of a laser and is ideal for many laser technology applications because of its minimal divergence. In practice, however, many lasers often deviate from this ideal case. The basic mode TEM00 has a Gaussian intensity distribution I (x, y) with the beam radius w and the beam diameter d = 2w.
What is temporal coherence?
Temporal coherence is the total length of time that a propagating real wave will match with the theoretical ideal sinusoidal wave - the coherence time. This means that the electromagnetic field is developing in a predictable way - the wavefronts are equally spaced with one frequency - and so it is possible to measure the electric field component at a given point in time and to know what it will look like in the future. The temporal coherence can be measured experimentally with a Michelson interferometer. As the speed of light is fixed, the coherence time of a light source will give rise to an associated coherence length. The coherence time can be calculated as the reciprocal of the bandwidth - for instance, sunlight has very low temporal coherence, due to the width of the solar spectrum.
What is a clean-up filter?
Clean-up filters are a type of bandpass optical filter. They provide the high transmission and high rejection required to isolate narrow spectral ranges for a wide variety of applications. These bandpass filters typically achieve a transmission of > 90% at their specified laser wavelength and a high level of blocking of the other wavelength ranges. Steep edges are used to eliminate optical noise from non-lasing (plasma) lines and spontaneous emission, in order to optimize the signal-to-noise ratio. Clean-up filters are used in laser-based fluorescence instruments, in Raman spectroscopy, as well as in analytical and medical laser systems. These filters are available for all wavelengths of gas and solid-state lasers.
What is a laser cavity?
The laser cavity, resonator, or optical cavity, houses many of the optical components that form the laser - including mirrors, lenses, and gain media. Optical radiation circulates within the cavity, bouncing between the mirrors and the gain media to form the resulting laser beam. The cavity may also include optical elements that facilitate laser characteristics such as wavelength tuning and mode locking.
What is fibre coupling?
Many applications, such as spectroscopy, require complex light beam geometries to perform the imaging methods necessary for analysis. Special fibre optic solutions can be used for this - where the light is brought to the measurement location via a flexible fibre of a certain length. It is possible to use both single-mode and multi-mode fibres, with different core diameters allowing for higher coupling efficiency.
What is the gain medium of a laser?
Gain medium, or active laser medium, amplifies a laser beam through stimulated emission with the intent of creating population inversion. Medium refers to the matter used, with each media resulting in different laser properties (such as wavelength, efficiency, output power). Gain refers to the amount of light amplification achieved. Gain media can be in the form of crystals, gases, liquid laser dyes, and direct band gap semiconductors.
What is a laser GUI?
A GUI (graphical user interface), in the context of UniKLasers, gives customers control over the laser management software through a straightforward and intuitive interface. The UniKLasers GUI lets users power on the laser via a PC and view certain laser characteristics in real-time - such as wavelength and output power.
What is a heatsink?
A heatsink is a component that transfers energy supplied to it into an adjacent medium - this could be a solid, gas, or liquid. In technical applications, this cooling device is used to dissipate excess heat in order to prevent the connected components from overheating.
In the laser field, a heatsink also serves to stabilise the laser output power and spectral stability. For example, the emitted wavelengths from diode lasers change depending on their temperature coefficient. This effect can be significant, moving by several nm. This has a negative effect on pump absorption, especially in the case of diode-pumped solid-state lasers, as some laser crystals have a narrow pump absorption bandwidth, which can lead to unstable laser performance.
Different kinds of heatsinks are used, depending on the design and performance of the laser, and the application to which it will be applied. For example, passive cooling through an attached heatsink using air cooling can cause vibration and turbulence in the output beam. Since applications such as holography cannot allow for this, water-cooled systems can be used instead.
What is laser pumping?
Laser pumping is the application of energy to a gain medium that results in population inversion. To achieve population inversion, pumping excites atoms and moves a majority of them from a lower energy level, to a higher energy level. Lasers can be pumped using light, electrical, gas, chemical, and microwave sources.
What is a Michelson interferometer?
The Michelson interferometer splits a light beam into two separate beams using a semi-transparent beam splitter. One beam strikes a fixed mirror and the other beam strikes an adjustable mirror. The two beams then overlap behind the semi-transparent beam splitter and cause an interference fringe system. The two mirrors have different and variable distances from the beam splitter, so that the partial beams are delayed according to the different transit times. The delay can be so great that interference no longer occurs. With a Michelson interferometer it is possible to determine the coherence time and length of lasers, or the change in the refractive index of liquids or gases. A Michelson interferometer can also be used to measure gravitational waves.
What is an optical filter?
Optical filters are glass or plastic devices used to influence light depending on its wavelength - causing it to be transmitted, absorbed, or reflected. The incident radiation may also be affected by the light's polarisation state or the direction of incidence. Filters are generally categorised as absorption filters and interference (dichroic) filters.
The best-known absorption filters are used in photography. For scientific applications, absorption filters are selected based on how they influence light.
- 'Shortpass filters' - for transmitting wavelengths below a cut-off to be transmitted while preventing longer wavelengths
- 'Longpass filters' - for transmitting wavelengths above the cut-off
- 'Bandpass filters' - for transmitting wavelengths within a particular spectral range
- 'Neutral density filters' - for uniformly absorbing a range of wavelengths to reduce the tranmission overall
Interference filters are optical coatings, or thin films, using the principle of deconstructive interference and reflection, as opposed to absorption, to remove specified wavelengths.
What is a Volume Bragg Grating (VBG)?
We have to differentiate between surface and volume gratings. Starting from a surface grating, if you stack transparent surface gratings on top of each other, the refractive index changes through the entire volume to a point at which there is only one diffraction order. This represents the transition from a surface to a volume grating behavior. Volume Bragg gratings (VBG) are gratings that take up the volume of a medium.
The term goes back to Sir William Bragg, who in 1915 used the diffraction of light that propagates through a crystal to determine the lattice structure of this crystal. He observed that the light is strongly diffracted by the crystal when certain conditions are met. This state is called the "Bragg state" or "resonance state".
There are basically two types of VBGs. On the one hand the Transmission Bragg grating (TBG), in which the incident light, if it fulfills the Bragg condition, is not transmitted but also diffracted, and secondly the Reflection Bragg grating (RBG), which behaves like a mirror to incident light, which corresponds to the Bragg condition. Volume Bragg gratings are used in many applications. These include holography, measurement technology and the stabilization of lasers.
The Brillouin Effect is the inelastic scattering of photons caused by their parametric interaction with thermal phonons, as found in Raman spectroscopy. This is a result of the interaction of light with phonons vibrating within the acoustic range (sound waves). These dynamic thermal fluctuations cause changes in the dielectric constant. The refractive index of a carrier material, produces a weak inelastic scattering effect as a photon passes through. This inelastic interaction causes a change in frequency within the incident light, proportional to the relative velocity of the phonon. This results in an energy change, or Stokes shift, several orders of magnitude smaller than the Raman shift due to the comparative speeds of sound and light.
A gravimeter, or gravity meter, is an ultra-precise accelerometer that measures constant downward acceleration of gravity and allows for the detection of gravitational variations and the calculation of local gravitational field strength. The acceleration caused by gravity on an object varies - dependent on factors such as local topography, subterranean density, or latitude. By ultra-precise monitoring of the acceleration due to gravity on a calibrated mass, correcting for all other effects, it is possible to infer, or map, the surrounding density of mass. In this way, gravimeters allow for the underground mapping of existing infrastructure or the monitoring of oil, water, and volcanic magma.
Flow cytometry - where 'flow' describes cells lining up in single file to facilitate analysis, 'cyto' means cells, and 'metry' refers to measurement. Flow cytometry is a method for the quantitative determination of cellular properties through analysing light incident on a focussed stream of fluid containing a heterogenous population of biological samples. Modern systems are equipped with several laser lines, covering a broad range of possible analysis - such as size, shape, health, surface properties, proteins, and by-products of cells.
Holography - where 'holo' in this case means whole and complete, and 'graphy' is the study of. Holography is the science of generating a 3D (whole) image by recording the interference patterns between two light beams; one reference beam and one object incidence beam. High levels of coherence in these light beams are required to resolve the interferometric detail of the image, making single frequency lasers a key component in this application. Most common applications use red, green, and blue lasers to re-create visible images - for example, holography is used to facilitate high resolution augmented reality. Holography can also be used outwith the visible spectrum in areas such as increasing the volume and security of data storage.
Interferometry - where 'interfero' refers to intereference, and 'metry' is measurement. Interferometry refers to a wide range of measurement techniques that rely on the superimposition of two coherent light paths, most often split from a single source, to form an interference pattern. The requirement for high coherence between the light paths and the need to resolve small feature details, requires the use of laser sources.
The basic principle of every laser spectroscopic method is the irradiation of electromagnetic waves in the form of a laser beam into a medium to be examined and the detection of the interaction between the wave and the particles in the medium. The linewidth of the laser radiation is decisive for the precision of the spectroscopic measurement. The lower this is, the more precisely the atomic or molecular transition to be spectroscoped can be measured. Depending on the structure of the resonator, a free-running laser has typical linewidths of several gigahertz. By means of frequency-selective elements that are introduced into the resonator, the line width can be reduced to a few megahertz or less, which is necessary for spectroscopic precision experiments. In order to achieve the highest spectral resolutions, single-frequency lasers with extremely small linewidths are used. Laser spectroscopy is used for many analytical techniques such as Raman spectroscopy, flow cytometry, and fluorescence.
Optical clocks, or optical atomic clocks, measure time as determined by the frequency of light emitted or absorbed when an atom changes energy states. These ultra-precise time measurement devices replace current standards of atomic clocks, based on microwave frequencies, and instead use light to measure atomic oscillations. This can be achieved with the use of frequency combs - providing a link between the microwave and optical frequency ranges used. Optical clocks can be used for setting time standards, GPS, and satellite applications.
In quantum sensors, precise manipulation of atoms by means of light interaction facilitates highly precise measurements of different parameters. This can be carried out with cold or quantum degenerate gases - and applied in areas such as improving the measurement accuracy of time (atomic clocks), and allowing for 3D mapping of dense materials (gravimeters) - where such sensors would enable the monitoring of oil, water, and volcanic magma.
What is Raman scattering?
In Rayleigh scattering, when a laser is incident on a sample, most of the photons will be scattered elastically and will not be subject to any energy change. Raman scattering events are significantly less frequent - around 1 in a million incident photons. A change in frequency (or Stokes shift) can be observed and a range of information about the sample can be determined. The energy for this wavelength shift comes from a change in the energy state of molecular bond(s). This is distinct from an interaction where the photon is absorbed by an atom and then re-emitted at a different wavelength - as seen in fluorescence spectroscopy. The wavelength shift in the Raman scattered light corresponds directly to the current energy states of the molecular bonds in the sample. As these are influenced not just by the atoms involved in those bonds but also by the total crystal structure and the strain the system is under, it is possible to interpret useful information from the Raman spectrum that can be difficult to obtain by other means.
What is Rayleigh scattering?
Rayleigh Scattering is the elastic movement of photons, caused by their interaction with electrons bound to molecules or atoms - if light is scattered onto particles that are very small compared to the wavelength of the light, the light is equally scattered forwards and backwards. This interaction does not cause any energy to be transferred between the photon and the electron, meaning that the photon emerges at the same frequency though with an altered trajectory.
Two scattering mechanisms can be distinguished, based on the relative size of the matter with which the photon is interacting - the electrons of the molecules and atoms cause Rayleigh scattering, whereas particles larger than the wavelength cause Mie scattering.
Blue light is scattered more than the red in the visible spectrum by these mechanisms, which leads to the blue colouring of the sky. At sunset, the sky appears to have a red hue because the blue portion is largely deflected from the direction of the solar radiation as a result of this scattering.