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Pulsed laser
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<h2 id="q-switching">Q-switching</h2> <p class="note">Main article: <a href="/facts/Q-switching/BNVh2d4N">Q-switching</a></p> <p>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. </p> <h2 id="mode-locking">Mode-locking</h2> <p class="note">Main article: <a href="/facts/Mode-locking/7p9MjBjT">Mode-locking</a></p> <p>A mode-locked laser is capable of emitting extremely short pulses on the order of tens of <a href="/facts/Picosecond/62W8DK3Q">picoseconds</a> down to less than 10 <a href="/facts/Femtoseconds/vaVXTVyR">femtoseconds</a>. These pulses will 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 <a href="/facts/Fourier_uncertainty_principle/XXhKJtc8">Fourier limit</a> (also known as energy-time <a href="/facts/Uncertainty_principle/UUSoSp4T">uncertainty</a>), a pulse of such short temporal length has a spectrum spread over a considerable bandwidth. Thus such a <a href="/facts/Gain_medium/Gmcharyh">gain medium</a> must have a gain bandwidth sufficiently broad to amplify those frequencies. An example of a suitable material is <a href="/facts/Titanium/53ArnDoy">titanium</a>-doped, artificially grown <a href="/facts/Sapphire/C11GJAmm">sapphire</a> (<a href="/facts/Ti-sapphire_laser/pUWECwSc">Ti:sapphire</a>) which has a very wide gain bandwidth and can thus produce pulses of only a few femtoseconds duration. </p><p>Such mode-locked lasers are a most versatile tool for researching processes occurring on extremely short time scales (known as femtosecond physics, <a href="/facts/Femtosecond_chemistry/6GHRJ2ns">femtosecond chemistry</a> and <a href="/facts/Ultrafast_science/V8joGdvF">ultrafast science</a>), for maximizing the effect of <a href="/facts/Nonlinear_optics/maerngXd">nonlinearity</a> in optical materials (e.g. in <a href="/facts/Second-harmonic_generation/NzWSrT3J">second-harmonic generation</a>, <a href="/facts/Parametric_down-conversion/yLRBgEkH">parametric down-conversion</a>, <a href="/facts/Optical_parametric_oscillator/K6RBDNjY">optical parametric oscillators</a> and the like) due to the large peak power, and in ablation applications.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> Again, because of the extremely short pulse duration, such a laser will produce pulses which achieve an extremely high peak power. </p> <h2 id="pulsed-pumping">Pulsed pumping</h2> <p>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 which 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 <a href="/facts/Capacitors/hyowqv7n">capacitors</a> which are then switched to discharge through <a href="/facts/Flashlamp/AMS4bfA5">flashlamps</a>, 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. </p> <h2 id="applications">Applications</h2> <p>Pulsed <a href="/facts/Nd%3aYAG_laser/n3Y2LHRc">Nd:YAG</a> and <a href="/facts/Er%3aYAG_laser/NWXt9jkJ">Er:YAG lasers</a> are used in laser <a href="/facts/Tattoo_removal/LqQcR2jY">tattoo removal</a> and <a href="/facts/Laser_range_finder/XlejNw9I">laser range finders</a> among other applications. </p><p>Pulsed lasers are also used in <a href="/facts/Soft_tissue/cHBwqLpP">soft-tissue</a> surgery. When a laser beam comes into contact with soft-tissue, one important factor is to not overheat surrounding tissue, so <a href="/facts/Necrosis/zCPsTeWx">necrosis</a> can be prevented.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> Laser pulses must be spaced out to allow for efficient tissue cooling (thermal relaxation time) between pulses.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Ultrashort_pulse_laser/d2BqT57e">Ultrashort pulse laser</a></li> <li><a href="/facts/Pulsed_laser_deposition/YaF3a2XG">Pulsed laser deposition</a></li> <li><a href="/facts/Laser_ablation/VmXJnyLU">Laser ablation</a></li> <li><a href="/facts/Laser_scalpel/LJsjmA6t">Laser scalpel</a></li> <li><a href="/facts/Soft-tissue_laser_surgery/LJsjmA6t">Soft-tissue laser surgery</a></li></ul> <h2 id="bibliography">Bibliography</h2> <ul><li>Siegman, Anthony E. (1986). <i>Lasers</i>, University Science Books. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-935702-11-3</li> <li>Svelto, Orazio (1998). <i>Principles of Lasers</i>, 4th ed. (trans. David Hanna), Springer. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-306-45748-2</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Silfvast, William T. (1996). Laser Fundamentals, Cambridge University Press. ISBN 0-521-55617-1 <a href="/wiki/William_T._Silfvast" target="_blank">/wiki/William_T._Silfvast</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Paschotta, Dr Rüdiger. "Q-switched lasers". www.rp-photonics.com. doi:10.61835/our. Retrieved 2024-05-01. <a href="https://www.rp-photonics.com/q_switched_lasers.html" target="_blank">https://www.rp-photonics.com/q_switched_lasers.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Choi, B.; Welch, A. J. (2001-01-01). "Analysis of thermal relaxation during laser irradiation of tissue". Lasers in Surgery and Medicine. 29 (4): 351–359. doi:10.1002/lsm.1128. ISSN 0196-8092. PMID 11746113. <a href="https://escholarship.org/uc/item/5ww5949f" target="_blank">https://escholarship.org/uc/item/5ww5949f</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Choi, B.; Welch, A. J. (2001-01-01). "Analysis of thermal relaxation during laser irradiation of tissue". Lasers in Surgery and Medicine. 29 (4): 351–359. doi:10.1002/lsm.1128. ISSN 0196-8092. PMID 11746113. <a href="https://escholarship.org/uc/item/5ww5949f" target="_blank">https://escholarship.org/uc/item/5ww5949f</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> </ol>