Lasers

Author: the photonics expert Dr. Rüdiger Paschotta

Key questions:

• What special properties does laser light have?
• What types of devices are included under the term “laser”?
• What is the basic principle of operation of any laser?
• What enables lasers to form very directed beams with low divergence?
• What factors allow a laser to emit light within a very small bandwidth?
• What methods are used for pulse generation in lasers?
• How can laser light have extremely high optical intensities?
• What are the common types of lasers?
• What are tunable lasers and their benefits?
• Why are narrow-linewidth lasers significant?
• How do superluminescent sources differ from lasers?
• What is parametric amplification in optical parametric generators?
• What are the main safety concerns when working with lasers?
• What are some important applications of laser technology?
• Why are lasers relatively expensive to manufacture?
• What factors are vital for business success in the laser industry?

More questions…

“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 a 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.

Laser technology is at the core of the wider area of photonics, essentially because laser light has a number of very special properties:

• 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 [5]) 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”.

In laser technology, a wide range of optical components such as laser crystals, laser mirrors, polarizers, Faraday isolators and tunable optical filters are used; see the article on laser optics.

How a Laser Works

Basic Principle of Operation

The basic operation principle of lasers can be well understood by considering how light could be “stored” (maintained) over extended times:

First, we need to somehow spatially confine light such that it cannot escape our apparatus:

• The simplest approach would be using two plane mirrors, reflecting light between each other:

• However, the natural tendency of a light beam to diverge also needs to be counteracted. That can simply be done by using a concave curvature of at least one of the mirrors:

• Here, the spatial profile of the light beam can stay stable constant forever.
• The two remaining problems are (a) to initially get the light into the resonator and (b) to keep the light energy constant despite some unavoidable losses, e.g. by scattering of light on non-perfect mirror surfaces.
• We first solve problem (b) by inserting some light-amplifying medium into the resonator:

• If the light is sufficiently amplified in each path through that “gain medium”, any losses of light (e.g. at the mirrors) can be compensated, so that the optical power in the resonator stays constant.
• The gain medium then also solves problem (a): even if no light is present initially, the gain medium will emit some fluorescence light, some of which has the right direction of propagation to be captured in the resonator. Even if it is weak initially, during many resonator round-trips it can be amplified to a substantial power level.

• The mentioned amplification mechanism is based on stimulated emission of light, which can occur if light hits some electronically excited atoms or ions in the medium. (See the article on stimulated emission for more details.) Depending on the type of laser gain medium, very different mechanisms can be utilized to prepare that excitation which enables stimulated emission. In any case, some kind of energy supply must be provided, since light amplifications means transferring energy to the light. The energy supply may work through electricity, through light, or sometimes with other means such as a beam of fast electrons or chemical energy.

What is still missing for a useful laser is a way to generate a usable output laser beam. This is done by making one of the resonator mirrors only partially reflective, so that part of the circulating light power can escape. Of course, that will substantially increase the losses of the contained light; one will need accordingly stronger amplification in the laser gain medium to maintain laser operation. The higher the achieved gain of the laser gain medium, the more transmission of the output coupler mirror one can afford.

In conclusion, 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 laser gain medium (e.g. a laser crystal), which serves to amplify the light. Below you see an example: an optically pumped solid-state laser:

A laser cannot 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. Defects of a laser often raise the threshold to values which cannot be reached, so that laser operation becomes impossible.

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.

Note that many lasers are not operated continuously, but rather in a pulsed mode of operation, so that laser pulses are emitted. This is explained in a later section.

Directed Emission and 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 – very much in contrast to what happens in incandescent lamps, for example, where individual atoms emit in a completely uncoordinated manner.

The explained mechanism for coordinated emission is the physical basis for the potential of lasers to form very directed laser beams with low divergence, and for focusing light to very small spots. Quantitatively, this potential can be described via the spatial coherence of laser light.

The resulting amplitude and phase profile of a laser beam is largely determined by the properties of the laser resonator, usually much less by details of the laser gain medium. This is particularly so for lasers which operate on a single transverse mode, which usually leads to the emission of a close to Gaussian beam with nearly perfect beam quality, i.e., highest spatial coherence and optimum potential for focusing.

Emission Wavelengths, Temporal Coherence

Temporal coherence is a different issue, and it has completely different origins. It is related to the optical bandwidth of laser emission (although generally in a non-trivial way).

Most types of lasers can work only with a very restricted choice of emission wavelengths, if not with one emission wavelength only. However, different types can overall cover a very wide range of emission wavelengths from the far light to the ultraviolet light.

Some laser gain media can amplify light only in a narrow spectral range, while others have a large gain bandwidth. Even in the latter case, a laser often (particularly in continuous-wave operation) emits light only within a very small bandwidth, i.e., in a narrow range around a certain wavelength or optical frequency. This is because the conditions are often such that a net zero round-trip gain is possible only for that wavelength, while all other wavelengths exhibit a negative net round-trip gain. As a result, only light within that narrow bandwidth can “survive”, while light at any other wavelengths (even well inside the gain bandwidth) will “die out” without some number of resonator round trips. The emission bandwidth is often orders of magnitude smaller than the gain bandwidth – and particularly small when only a single resonator mode can oscillate (→ single-frequency operation).

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. Laser gain media like Ti:sapphire and Cr:ZnSe, having a large gain bandwidth, allow wavelength tuning over hundreds of nanometers.

The smaller the emission linewidth (i.e., the narrower the optical spectrum of emitted light), the higher is generally the degree of temporal coherence. A high temporal coherence means that the optical phase stays stable (predictable) over a relatively long time. 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). Even far shorter light pulses in the attosecond regime can be generated by sending intense femtosecond pulses into a gas, where high harmonic generation can occur.

Particularly when applying the technique of Q switching, and in conjunction with some pulsed laser amplifiers, a laser medium can accumulate some amount of energy over some “pumping” time in order to then release it within a much shorter time. This allows one to obtain an output peak power which is orders of magnitude higher than the applied pump power.

Particularly for femtosecond lasers, the emission bandwidth can be very large – even if the instantaneous frequency remains nearly constant during the pulse. That bandwidth is related to the properties of Fourier transforms and can also be explained with considerations on the excitation of oscillators exposed to the light field. In extreme cases, the bandwidth of ultrashort pulses can span about a full octave. Nevertheless, there can strong coherence between subsequently emitted pulses, which is related to the frequency comb structure of the optical spectrum.

Essential Properties of Laser Light

To summarize some already explained aspects, laser light can have very special properties in various respects, which are relevant for many laser applications:

• Its high spatial coherence allows for very directed propagation over large distances and also for tight focusing to small spots. However, not all lasers exhibit perfect beam quality.
• Its temporal coherence, related to its emission bandwidth, is extremely high at least in some cases. However, there are also laser sources with broadband emission and low temporal coherence.
• Laser light may be pulsed, often with short or even extremely short pulse durations, and often with very high peak powers.
• The tight concentration of light both spatially and temporally (in case of laser pulses) can lead to extremely high optical intensities.

Laser Noise

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 (containing a waveguide structure) 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.

Not very common are chemical and nuclear pumped lasers, free electron lasers and X-ray 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.

Safety Aspects

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.

Laser Applications

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.

More to Learn

Encyclopedia articles:

• laser light
• laser optics
• laser applications
• laser physics
• laser design