“Laser” (rarely written as l.a.s.e.r.) is an acronym for “Light Amplification by Stimulated Emission of Radiation”, coined in 1957 by the laser pioneer Gordon Gould. Although this original meaning denotes an principle of operation (exploiting stimulated emission from excited atoms or ions), the term is now mostly used for devices generating light based on the laser principle. More specifically, one usually means laser oscillators, but sometimes also includes devices with laser amplifiers, called master oscillator power amplifier (MOPA). An even wider interpretation includes nonlinear devices like optical parametric oscillators and Raman lasers, which also produce laser-like light beams and are usually pumped with a laser, but are strictly speaking not lasers themselves.
- It is usually emitted as a well directed laser beam which due to its high spatial coherence can propagate over long lengths without much divergence (often limited only by diffraction) and can be focused to very small spots, where a high intensity is achieved.
- It often has a very narrow optical bandwidth (high temporal coherence), whereas e.g. most lamps emit light with a very broad optical spectrum. However, there are also broadband lasers, particularly among ultrafast lasers.
- Laser light may be emitted continuously, or alternatively in the form of short or ultrashort pulses, with pulse durations from microseconds down to a few femtoseconds. The temporal concentration of pulse energy – in addition to the potential of strong spatial confinement in a beam focus – allows for even far higher intensities to be generated. Particularly extreme intensity values are used in high-intensity physics.
These properties, which make laser light very interesting for a range of applications, are to a large extent the consequences of the very high degree of spatial and/or temporal coherence of laser radiation. The articles on laser light and laser applications give more details.
The first laser was a pulsed lamp-pumped ruby laser (a kind of solid-state laser), demonstrated by Theodore Maiman in 1960 [2, 3]. In the same year, the first gas laser (a helium–neon laser ) and the first laser diode were made. Before this experimental work, Arthur Schawlow, Charles Hard Townes, Nikolay Basov and Alexander Prokhorov had published ground-breaking theoretical work on the operation principles of lasers, and a microwave amplifier and oscillator (maser) had been developed by Townes' group in 1953. The term “optical maser” (MASER = microwave amplification by stimulated amplification of radiation) was initially used, but later replaced with “laser”.
How a Laser Works
A laser oscillator usually comprises an optical resonator (laser resonator, laser cavity) in which light can circulate (e.g. between two mirrors), and within this resonator a gain medium (e.g. a laser crystal), which serves to amplify the light. Without the gain medium, the circulating light would become weaker and weaker in each resonator round trip because it experiences some losses, e.g. upon reflection at mirrors. However, the gain medium can amplify the circulating light, thus compensating the losses if the gain is high enough. The gain medium requires some external supply of energy – it needs to be “pumped”, e.g. by injecting light (optical pumping) or an electric current (electrical pumping → semiconductor lasers). The principle of laser amplification is stimulated emission.
A laser can not operate if the gain is smaller than the resonator losses; the device is then below the so-called laser threshold and only emits some weak luminescence light. Significant power output is achieved only for pump powers above the laser threshold, where the gain can reach (or temporarily exceed) the level of the resonator losses.
If the gain is larger than the losses, the power of the light in the laser resonator rises very rapidly, starting e.g. with low levels of light from fluorescence. Note that the resonator round-trip time is usually very small (e.g. a few nanoseconds, for compact laser types even much less), so that even a small net round-trip gain implies rapid exponential growth of the intracavity power. As high laser powers saturate the gain by extracting energy from the gain medium, the laser power will in the steady state reach a level so that the saturated gain just equals the resonator losses (→ gain clamping). Before reaching this steady state, a laser often undergoes relaxation oscillations (just one aspect of laser dynamics). The threshold pump power is the pump power where the small-signal gain is just sufficient for lasing.
Some fraction of the light power circulating in the resonator is usually transmitted by a partially transparent mirror, the so-called output coupler mirror. The resulting beam constitutes the useful output of the laser. The transmission of the output coupler mirror can be optimized for maximum output power (see also: slope efficiency). In most cases, one has only a single output coupler.
Spatial Coherence of Laser Radiation
A high degree of spatial coherence of the laser radiation can be achieved, essentially because the light emission is triggered (stimulated) by the intracavity radiation (i.e., the light circulating in the laser resonator) itself, rather than occurring spontaneously in an uncoordinated fashion. In the stimulated emission process, the laser-active ions are made to emit light in the direction of already existing light, and also with the same optical phase. In effect, the circulating laser light serves to strongly coordinate the emission of many atoms or ions. The resulting amplitude and phase profile of the laser beam is largely determined by the properties of the laser resonator, not usually by the laser gain medium.
As explained above, the spatial coherence is the physical basis for the potential of forming very directed laser beams with low divergence, and for focusing light to very small spots.
Temporal coherence is a different issue, and it has completely different origins. Some laser gain media can emit light only in a narrow spectral range. However, even if that is not the case, a laser often (particularly in continuous-wave operation) emits light only at a precisely defined wavelength or frequency because the conditions are such that a net zero round-trip gain is possible only for that wavelength, and other wavelengths exhibit a negative net round-trip gain. A laser may be tuned to the exact wanted wavelength (within the emission region of the gain medium) e.g. by using a tunable intracavity bandpass filter, such as a Lyot filter. Again, the mechanism of stimulated emission is of crucial importance: the laser-active ions can be made to emit at exactly the optical frequency of already existing light. The smaller the emission linewidth (i.e., the narrower the optical spectrum of emitted light), the higher the degree of temporal coherence.
In extreme cases, the linewidth of a laser can be forced to values below 1 Hz (with certain means of laser stabilization). This is many orders of magnitude below the mean frequency (hundreds of terahertz). Optical clocks involve such highly stabilized lasers.
Interestingly, even ultrashort pulses can exhibit a very high degree of temporal coherence, in that case involving coherence between subsequent pulses in a regular pulse train. This is related to the formation of a frequency comb as the optical spectrum. While the optical spectrum can then overall be very wide, each comb line can be extremely narrow and well defined in frequency.
Generation of Light Pulses
Some lasers are operated in a continuous fashion, whereas others generate pulses, which can be particularly intense. There are various (very different) methods for pulse generation with lasers, allowing the generation of pulses with durations of microseconds, nanoseconds, picoseconds, or even down a few femtoseconds (→ ultrashort pulses from mode-locked lasers). Often, a laser medium can accumulate some amount of energy over some “pumping” time in order to then release it within a much shorter time.
The optical bandwidth (or linewidth) of a continuously operating laser may be very small when only a single resonator mode can oscillate (→ single-frequency operation). In other cases, particularly for mode-locked lasers, the bandwidth can be very large – in extreme cases, it can span about a full octave. The center frequency of the laser radiation is typically near the frequency of maximum gain, but if the resonator losses are made frequency-dependent, the laser wavelength can be tuned within the range where sufficient gain is available. Some broadband gain media such as Ti:sapphire and Cr:ZnSe allow wavelength tuning over hundreds of nanometers.
Due to various influences, the output of lasers always contains some noise in properties such as the output power or phase. For pulsed lasers, additional quantities can come into play, for example the timing jitter. For more details, see the article on laser noise.
Types of Lasers
Laser technology is a rather diverse field, utilizing a wide range of very different kinds of laser gain media, optical elements and techniques. Common types of lasers are:
- Semiconductor lasers (mostly laser diodes), electrically (or sometimes optically) pumped, efficiently generating very high output powers (but typically with poor beam quality), or low powers with good spatial properties (e.g. for application in CD and DVD players), or pulses (e.g. for telecom applications) with very high pulse repetition rates. Special types include quantum cascade lasers (for mid-infrared light) and surface-emitting semiconductor lasers (VCSELs, VECSELs and PCSELs). Some of those are also suitable for pulse generation with high powers.
- Solid-state lasers based on ion-doped crystals or glasses (doped insulator lasers), pumped with discharge lamps or laser diodes, generating high output powers, or lower powers with very high beam quality, spectral purity and/or stability (e.g. for measurement purposes), or ultrashort pulses with picosecond or femtosecond durations. Common gain media are Nd:YAG, Nd:YVO4, Nd:YLF, Nd:glass, Yb:YAG, Yb:glass, Ti:sapphire, Cr:YAG and Cr:LiSAF. A special type of ion-doped glass lasers are:
- Fiber lasers, based on optical glass fibers which are doped with some laser-active ions in the fiber core. Fiber lasers can achieve extremely high output powers (up to kilowatts) with high beam quality, allow for widely wavelength-tunable operation, narrow linewidth operation, etc.
- Gas lasers (e.g. helium–neon lasers, CO2 lasers, argon ion lasers and excimer lasers), based on gases which are typically excited with electrical discharges. Frequently used gases include CO2, argon, krypton, and gas mixtures such as helium–neon. Common excimers are ArF, KrF, XeF, and F2. As far as gas molecules are involved in the laser process, such lasers are also called molecular lasers.
Specifically Designed Lasers
Some types of lasers can be specifically be design to achieve high performance in a certain sense. Some examples:
- High-power lasers are optimized for a particularly high output power. Only few laser architectures are suitable for the concept of power scaling of lasers, while many others can at least to some extent be optimized for higher powers (with methods different from scaling).
- Tunable lasers are constructed such that their emission wavelength can be tuned over some region.
- Narrow-linewidth lasers are optimized for a narrow emission bandwidth. Usually, they are single-frequency lasers.
Laser Sources in a Wider Sense
There are some light sources which are not strictly lasers, but are nevertheless often called laser sources:
- In some cases, the term is used for amplifying devices emitting light without an input (excluding seeded amplifiers). An example are X-ray lasers, which are usually superluminescent sources, based on spontaneous emission followed by single-pass amplification. There is then no laser resonator.
- A similar situation occurs for optical parametric generators, where the amplification, however, is not based on stimulated emission; it is parametric amplification based on optical nonlinearities.
- Raman lasers utilize amplification based on stimulated Raman scattering.
Light from such devices can have laser-like properties, such as strongly directional emission, high spatial and temporal coherence and a narrow optical bandwidth.
In other cases, the term laser sources is justified by the fact that the source contains a laser, among other components. This is the case for combinations of lasers and amplifiers (→ master oscillator power amplifier), and also for sources based on nonlinear frequency conversion of laser radiation, e.g. with frequency doublers or optical parametric oscillators.
The work with lasers can raise serious safety issues. Some of those are directly related to the laser light, in particular to the high optical intensities achievable, but there are also various other hazards related to laser sources.
See the article on laser safety for details.
There is an enormously wide range of applications for a great variety of different laser devices. They are largely based on various special properties of laser light, many of which cannot be achieved with any other kind of light sources. Particularly important application areas are laser material processing, optical data transmission and storage and optical metrology. See the article on laser applications for an overview.
Still, many potential laser applications cannot be practically realized so far because lasers are relatively expensive to make – or more precisely because they are so far mostly made with relatively expensive methods. Most lasers are fabricated in relatively small volumes and with a limited degree of automation. Another aspect is that lasers are relatively sensitive in various respects, for example concerning the precise alignment of optical components, mechanical vibrations and dust particles. Therefore, there is ongoing research and development for finding more cost-effective and robust solutions.
For business success, it is often vital not just to develop lasers with high performance and low cost, but also to identify the best suited applications, or develop lasers which are best suited for particular applications. Also, the knowledge of the application details can be very important. For example, in laser material processing it is vital to know the exact requirements in terms of laser wavelength, beam quality, pulse energy, pulse duration etc. for optimum processing results.
HÜBNER PHOTONICS offers a full range of high performance lasers including single and multi-line Cobolt lasers, tunable C-WAVE lasers, C-FLEX laser combiners. All our lasers are manufactured in a clean room environment, by skilled staff and with the highest of quality.
At FYLA we develop ultrafast fiber lasers with pulse durations in the range of picoseconds and femtoseconds. Our lasers are used in a lot of applications, from microscopy (single-molecule fluorescence, OCT, FRET, TIRF, etc.) up to optical characterization, providing a greater level of robustness, higher lifetimes, and a cost-effective solution.
SHIPS TODAY: AeroDIODE offers fiber-coupled laser diodes between 520 nm and 1650 nm as stock items or associated with a CW laser diode driver or pulsed laser diode driver. They are compatible with our high speed nanosecond pulsed drivers or high power CW drivers with air coolingfor the multimode high power laser diode versions. The single mode laser diodes (either Fabry–Pérot laser diode or DFB laser diode) can reach high power in nanosecond pulse regime up to 500 mW. Most turn-key diode & driver solutions are optimized for single-shot to CW performances with pulse durations down to 1 ns. The laser diode precision pulses are generated internally by an on-board pulse generator, or on demand from an external TTL signal. Many multimode versions are available with CW emission up to 300 W in a 200-µm core multimode fiber or up to 250 W in a 135-µm core fiber or 160 W in a 105 µm core fiber.
See also our tutorial on fiber-coupled laser diodes.
NKT Photonics is at the forefront of optical fiber and laser technology. Expect exceptional performance when you want to push forward and move the definition of what’s possible. We offer a wide range of lasers spanning from pulsed diode lasers and single-frequency fiber lasers over ultrafast fiber lasers and femtosecond lasers to supercontinuum white light lasers. Whatever your laser needs, we have a system for you!
RPMC Lasers offers the widest selection of solid-state lasers in North America. From ≈ 1500 standard products to full customization capabilities, we are sure to have what you need: pulsed and CW sources ranging in wavelength from the UV through the LWIR regimes. Pulsed lasers include DPSS lasers, fiber lasers, microlasers/microchip Lasers, ultrafast lasers, and more. Additionally, CW laser modules including single-mode and multimode DPSS laser and laser diode modules are available in both fiber-coupled and free space configurations, as well as gas and fiber lasers, line modules, and many laser diode types, including superluminescent laser diodes, multi-wavelength lasers, and quantum cascade laser diodes. Let RPMC help you find the right laser today!
The Stuttgart Instruments Alpha is an ultrafast and fully wavelength-tunable frequency conversion system in an ultra-compact and completely passively stable system based on revolutionary parametric oscillator design which guarantees outstanding stability, reproducibility and shot-noise limited performance.
The revolutionary design of Stuttgart Instruments Alpha, characterized by outstanding low noise and passive long-term stability, is based on the fiber-feedback optical parametric oscillator (FFOPO) technology and results in outstanding performance and high flexibility at the same time.
The Alpha covers a gap-free rapid tunable spectral range from 700 nm to 20 µm wavelengths, while maintaining high output power up to the Watt-level with femto- or picosecond pulses at several MHz pulse repetition rates. It provides multiple simultaneously tunable outputs with a selectable bandwidth from a few to 100 cm-1. Shot-noise limited performance above 300 kHz, passive spectral stability (< 0.02% rms) and wavelength-independent stable beam pointing (< 30 µrad) enable excellent sensitivity. In addition, each Alpha is equipped with a user-friendly ethernet and Wi-Fi interface and a matching graphical user interface (GUI) as well as easy to access API interfaces for e.g. LabView, Python, C++.
Typically, the Alpha is pumped by an ultra-low-noise Primus pump laser, which provides more than 8 W average output power at 1040 nm wavelength and 450 fs pulse duration at 42 MHz repetition rate. In addition, the Alpha can be operated with other pump lasers around 1 µm wavelength and enough power.
Due to our modular platform, the Alpha can be adapted and optimized for various applications and is particularly suited for spectroscopic applications requiring a robust and reliable tunable radiation with low noise.
Alpes Lasers offers a wide range of lasers with wavelengths ranging from 4 to 14 μm and powers up to several watts. This includes FP, DFB, THz, frequency comb and external cavity lasers in the mid-IR. Additionally, Alpes offers uniquely fast and widely tuneable lasers with our ET and XT product line.
GWU-Lasertechnik has more than 30 years of experience in lasers and nonlinear optics. We are the pioneer of commercial BBO OPO technology. Our widely tunable laser sources cover the spectral range from the deep-UV at <190 nm to the IR at >2700 nm. We offer pulsed solutions for nano-, pico- and femtosecond pulses with best performance and highest reliability.
Frankfurt Laser Company offers the widest wavelength range for laser diodes on the world market from 370 nm to 12 µm, single mode & multimode, broad area, DFB and DBR, fiber Bragg grating stabilized, quantum cascade, VCSELs, superluminescent diodes and mid-IR light emitting diodes. We offer wavelength selection and custom packaging; please contact us to discuss your requirements.
AFS offers various femtosecond laser platforms based on ytterbium or thulium-doped fiber amplifiers, OPCPA/OPA and XUV extensions covering all wavelength ranges for compact systems up to complex multi-functional beamlines.
TOPTICA's products provide an ultra-broad laser wavelength coverage: 190 nm – 0.1 THz (corresponding to 3 mm). They enable a big variety of demanding applications in quantum optics, spectroscopy, biophotonics, microscopy, test & measurement, as well as materials inspection. The unique wavelength range is based on three major product categories:
MegaWatt Lasers Inc. spezializes on lamp-pumped pulsed lasers for a wide range of applications. Our pump chamber designs provide unsurpassed performance, reliability and leak integrity to meet the demanding needs of your laser system design. Our lasers produce multi-joule nanosecond pulses at eye-safe wavelength, e.g. based on Er:YAG or CTH:YAG rods. We also offer Nd:YAG and alexandrite lasers.
We maintain an inventory of standard pump chambers for immediate delivery, while modular design allows for cost effective custom solutions.
Optogama offers various types of lasers: eye-safe 1,54 μm diode-pumped passively Q-switched and CW erbium glass lasers as well as passively Q-switched Nd:YAG lasers.
Eye-safe 1,54 μm wavelength nanosecond lasers of the “KAUKAS“ (1, 2, 3) series have a unique compact design and are available in OEM models for dedicated applications. These lasers deliver up to 3 mJ energy per pulse with pulse duration <10–15 ns, TEM00 beam profile and repetition rate of up to 5 Hz.
„KAUKAS HR“ laser models have adjustable repetition rate feature and deliver more than 30 µJ energy per pulse with a repetition rate of up to 1 kHz available on request.
“KAUKAS CW” diode-pumped, solid-state laser models deliver up to 400 mW of continuous-wave power at 6 wavelength in the 1,5–1.6 μm spectral range.
Nd:YAG passively Q-switched lasers of the "WAVEGUARD" series with sub-nanosecond pulse widths and a peak power of several tens of kilowatts are very compact. Additional harmonics modules for 532 nm, 355 nm, or 266 nm wavelengths are available on request for all models.
Radiantis manufacturers broadly tunable laser systems based on Optical Parametric Oscillators (OPOs). Femtosecond, picosecond and continuous-wave (CW) lasers are available that cover the visible and IR spectral regimes. The laser systems include both a pump laser and the OPO in the same enclosure.
Customised devices for specific wavelengths or pulse duration can be developed.
Bright Solutions offers various types of lasers:
Lumibird manufactures a wide range of lasers thanks to its expertise in three key technologies: pulsed solid-state lasers (nanosecond range), CW and pulsed fiber lasers and fiber amplifiers, and laser diodes. Various application areas are addressed, in industry (manufacturing, lidar sensors), science (laboratories and universities), medical (ophthalmology) and defense.
Class 5 Photonics delivers ultrafast, high-power laser technology at outstanding performance to advance demanding applications from bio-imaging to ultrafast material science and attosecond science. Our robust optical parametric chirped pulse amplifiers (OPCPA) provide high-power, tunable femtosecond pulses and user-friendly operation.
QPC Lasers manufactures fiber-coupled diode laser modules with the highest powers and brightness in the industry at wavelengths ranging from 780 to 2000 nm.
Products range from sub-watt single-mode PM fiber coupled diodes for LIDAR and communications to multi-mode fiber-coupled modules with outputs in the hundreds of watts for medical, materials processing and pumping applications. Optional features include Brightlock monolithically spectrally stabilized diodes for unmatched linewidth and spectral control.
LumIR offers mid-IR fiber lasers, based on fluorine glass fibers, with up to 10 W output power and emission wavelength between 2.79 μm and 3.3 μm. They are ideal for medical, material processing and sensing applications.
AdValue Photonics has developed a number of industrial fiber laser products, operating in different wavelengths regions and pulse duration regimes, or in continuous-wave mode. They are suitable for a range of industrial applications, including laser cutting, drilling and ablation.
See also our overview on fiber laser products!
Sacher Lasertechnik offers a wide range of diode lasers for scientific and industrial applications, covering a wide range of emission wavelengths and output powers. Apart from DFB and DBR lasers, Fabry–Pérot and tapered amplifier diodes, we offer quantum cascade lasers and terahertz emitters.
CNI offers a wide range of lasers, including
- laser diode modules, e.g. narrow-linewidth DFB lasers, picosecond lasers, alignment lasers and others
- diode-pumped solid-state lasers with high stability, low noise, high output power or pulse energy, Q-switched and mode-locked picosecond versions, etc.
- fiber lasers with SM or MM fiber output, pulse width <200 ps, tunable versions with 1–250 ns pulse width, modulation rates up to 1 MHz
and various others. We specialize in designing and manufacturing custom-made and OEM lasers to suit our clients' particular needs. In fact, 75% of the lasers manufactured involve some type of custom work.
Integrated Optics offers a perfect solution for scientific industry and researcher that requires fast and simple integration with smart functionality. MatchBox CW lasers come in compact, comfortable size, with up to 150 configurations that feature various wavelengths, output types, power levels and fiber-options. Many customers are happy to take advantage of convenience that MatchBox lasers provide in integrations process. MatchBox lasers can also combine up to 4 different diode wavelengths in one single housing that still has the same record- small footprint of 30 × 50 × 18 mm.
|A. L. Schawlow and C. H. Townes, “Infrared and optical masers”, Phys. Rev. 112 (6), 1940 (1958); https://doi.org/10.1103/PhysRev.112.1940 (ground-breaking work; also contains the famous Schawlow–Townes equation)
|T. H. Maiman, “Stimulated optical radiation in ruby”, Nature 187, 493 (1960); https://doi.org/10.1038/187493a0 (first experimental demonstration of a laser); https://doi.org/10.1038/187493a0
|T. H. Maiman, “Optical maser action in ruby”, Br. Commun. Electron. 7, 674 (1960)
|P. P. Sorokin and M. J. Stevenson, “Stimulated infrared emission from trivalent uranium”, Phys. Rev. Lett. 5 (12), 557 (1960); https://doi.org/10.1103/PhysRevLett.5.557 (the first four-level laser)
|A. Javan, W. R. Bennett Jr., and D. R. Herriott, “Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture”, Phys. Rev. Lett. 6 (3), 106 (1961); https://doi.org/10.1103/PhysRevLett.6.106
|T. Tomiyasu, “Laser bibliography”, IEEE J. Quantum Electron. 1 (3), 133 (1965); https://doi.org/10.1109/JQE.1965.1072194
|G. Smith, “The early laser years at Hughes Aircraft Company”, IEEE J. Quantum Electron. 20 (6), 577 (1984); https://doi.org/10.1109/JQE.1984.1072445
|R. E. Slusher, “Laser technology”, Rev. Mod. Phys. 71, S471 (1999); https://doi.org/10.1103/RevModPhys.71.S471
|J. M. Gill, “Lasers: a 40-year perspective”, IEEE J. Quantum Electron. 6 (6), 1111 (2000); https://doi.org/10.1109/2944.902159
|"Bright idea: the first lasers", American Institute of Physics (2010)
|J. Hecht, “Short history of laser development”, Opt. Eng. 49, 091002 (2010); https://doi.org/10.1364/AO.49.000F99
|A. E. Siegman, Lasers, University Science Books, Mill Valley, CA (1986)
|O. Svelto, Principles of Lasers, Plenum Press, New York (1998)
|F. Träger (ed.), Handbook of Lasers and Optics, Springer, Berlin (2007)
|R. Paschotta, Field Guide to Lasers, SPIE Press, Bellingham, WA (2007)
See also: laser light, laser optics, laser applications, laser physics, laser design, power scaling of lasers, laser resonators, laser crystals, laser gain media, diode-pumped lasers, lamp-pumped lasers, solid-state lasers, fiber lasers, waveguide lasers, upconversion lasers, semiconductor lasers, gas lasers, molecular lasers, X-ray lasers, mode-locked lasers, Q-switched lasers, visible lasers, laser threshold, slope efficiency, laser noise, linewidth, coherence, wavelength tuning, laser safety, laser specifications, scientific lasers
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics AG. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!
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