Editing Laser (section)

== Modes of operation ==
{{more citations needed|section|date=February 2023}}
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A laser can be classified as operating in either continuous or pulsed mode, depending on whether the power output is essentially continuous over time or whether its output takes the form of pulses of light on one or another time scale. Of course, even a laser whose output is normally continuous can be intentionally turned on and off at some rate to create pulses of light. When the modulation rate is on time scales much slower than the [[Q factor#Optical systems|cavity lifetime]] and the period over which energy can be stored in the lasing medium or pumping mechanism, then it is still classified as a "modulated" or "pulsed" continuous wave laser. Most laser diodes used in communication systems fall into that category.

=== Continuous-wave operation ===
{{^| [[Continuous wave laser]] and [[Continuous-wave laser]] redirect here}}Some applications of lasers depend on a beam whose output power is constant over time. Such a laser is known as a ''[[continuous-wave]]'' (''CW'') laser. Many types of lasers can be made to operate in continuous-wave mode to satisfy such an application. Many of these lasers lase in several longitudinal modes at the same time, and beats between the slightly different optical frequencies of those oscillations will produce amplitude variations on time scales shorter than the round-trip time (the reciprocal of the [[Free spectral range#Fabry–Pérot interferometer|frequency spacing]] between modes), typically a few nanoseconds or less. In most cases, these lasers are still termed "continuous-wave" as their output power is steady when averaged over longer periods, with the very high-frequency power variations having little or no impact on the intended application. (However, the term is not applied to [[Mode locking|mode-locked]] lasers, where the ''intention'' is to create very short pulses at the rate of the round-trip time.)

For continuous-wave operation, the population inversion of the gain medium needs to be continually replenished by a steady pump source. In some lasing media, this is impossible. In some other lasers, it would require pumping the laser at a very high continuous power level, which would be impractical, or destroying the laser by producing excessive heat. Such lasers cannot be run in CW mode.

=== Pulsed operation ===
{{Main|Pulsed laser}}

The pulsed operation of lasers refers to any laser not classified as a continuous wave so that the optical power appears in pulses of some duration at some repetition rate. This encompasses a wide range of technologies addressing many different motivations. Some lasers are pulsed simply because they cannot be run in [[#Continuous-wave operation|continuous]] mode.

In other cases, the application requires the production of pulses having as large an energy as possible. Since the pulse energy is equal to the average power divided by the repetition rate, this goal can sometimes be satisfied by lowering the rate of pulses so that more energy can be built up between pulses. In [[laser ablation]], for example, a small volume of material at the surface of a workpiece can be evaporated if it is heated in a very short time, while supplying the energy gradually would allow for the heat to be absorbed into the bulk of the piece, never attaining a sufficiently high temperature at a particular point.

Other applications rely on the peak pulse power (rather than the energy in the pulse), especially to obtain [[nonlinear optical]] effects. For a given pulse energy, this requires creating pulses of the shortest possible duration utilizing techniques such as [[Q-switching]].

The optical bandwidth of a pulse cannot be narrower than the reciprocal of the pulse width. In the case of extremely short pulses, that implies lasing over a considerable bandwidth, quite contrary to the very narrow bandwidths typical of CW lasers. The lasing medium in some ''dye lasers'' and ''vibronic solid-state lasers'' produces optical gain over a wide bandwidth, making a laser possible that can thus generate pulses of light as short as a few [[femtoseconds]] (10<sup>−15</sup> s).

==== Q-switching ====
{{Main|Q-switching}}

In a Q-switched laser, the population inversion is allowed to build up by introducing loss inside the resonator which exceeds the gain of the medium; this can also be described as a reduction of the quality factor or 'Q' of the cavity. Then, after the pump energy stored in the laser medium has approached the maximum possible level, the introduced loss mechanism (often an electro- or acousto-optical element) is rapidly removed (or that occurs by itself in a passive device), allowing lasing to begin which rapidly obtains the stored energy in the gain medium. This results in a short pulse incorporating that energy, and thus a high peak power.

==== Mode locking ====
{{Main|Mode locking}}

A mode-locked laser is capable of emitting extremely short pulses on the order of tens of [[picosecond]]s down to less than 10&nbsp;[[femtoseconds]]. These pulses repeat at the round-trip time, that is, the time that it takes light to complete one round trip between the mirrors comprising the resonator. Due to the [[Fourier uncertainty principle|Fourier limit]] (also known as energy–time [[Uncertainty principle|uncertainty]]), a pulse of such short temporal length has a spectrum spread over a considerable bandwidth. Thus such a gain medium must have a gain bandwidth sufficiently broad to amplify those frequencies. An example of a suitable material is [[titanium]]-doped, artificially grown [[sapphire]] ([[Ti-sapphire laser|Ti:sapphire]]), which has a very wide gain bandwidth and can thus produce pulses of only a few femtoseconds duration.

Such mode-locked lasers are a most versatile tool for researching processes occurring on extremely short time scales (known as femtosecond physics, [[femtosecond chemistry]] and [[ultrafast science]]), for maximizing the effect of [[nonlinear optics|nonlinearity]] in optical materials (e.g. in [[second-harmonic generation]], [[parametric down-conversion]], [[optical parametric oscillator]]s and the like). Unlike the giant pulse of a Q-switched laser, consecutive pulses from a mode-locked laser are phase-coherent; that is, the pulses (and not just their [[Envelope (waves)|envelopes]]) are identical and perfectly periodic. For this reason, and the extremely large peak powers attained by such short pulses, such lasers are invaluable in certain areas of research.

==== Pulsed pumping ====
Another method of achieving pulsed laser operation is to pump the laser material with a source that is itself pulsed, either through electronic charging in the case of flash lamps, or another laser that is already pulsed. Pulsed pumping was historically used with dye lasers where the inverted population lifetime of a dye molecule was so short that a high-energy, fast pump was needed. The way to overcome this problem was to charge up large [[capacitors]] which are then switched to discharge through flashlamps, producing an intense flash. Pulsed pumping is also required for three-level lasers in which the lower energy level rapidly becomes highly populated, preventing further lasing until those atoms relax to the ground state. These lasers, such as the excimer laser and the copper vapor laser, can never be operated in CW mode.