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Pound–Drever–Hall technique
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<p>The Pound–Drever–Hall (PDH) technique is a widely used and powerful approach for stabilizing the frequency of <a href="/facts/Light/jeofpMn4">light</a> emitted by a <a href="/facts/Laser/jk6PTvvT">laser</a> by means of <a href="/facts/Feedback/XnuWH3zp">locking</a> to a stable cavity. The PDH technique has a broad range of applications including <a href="/facts/LIGO/jX6jyJzM">interferometric gravitational wave detectors</a>, <a href="/facts/Saturated_absorption_spectroscopy/FfIq3Qks">atomic physics</a>, and <a href="/facts/Atomic_clock/MF8nsXDR">time measurement standards</a>, many of which also use related techniques such as <a href="/facts/Frequency_modulation/qIExDdtv">frequency modulation</a> spectroscopy. Named after <a href="/facts/R._V._Pound/NKhat84D">R. V. Pound</a>, <a href="/facts/Ronald_Drever/ow1DINvH">Ronald Drever</a>, and <a href="/facts/John_L._Hall/fufDdncd">John L. Hall</a>, the PDH technique was described in 1983 by Drever, Hall and others working at the <a href="/facts/University_of_Glasgow/D0XAYMtq">University of Glasgow</a> and the U. S. <a href="/facts/National_Bureau_of_Standards/gr9Fnh85">National Bureau of Standards</a>. This optical technique has many similarities to an older frequency-modulation technique developed by Pound for microwave cavities. </p><p>Since a wide range of conditions contribute to determine the <a href="/facts/Laser_linewidth/A86VJ6dL">linewidth</a> produced by a laser, the PDH technique provides a means to <a href="/facts/Feedback/XnuWH3zp">control</a> and decrease the laser's linewidth, provided an <a href="/facts/Optical_cavity/BITPG3ga">optical cavity</a> that is more stable than the laser source. Alternatively, if a stable laser is available, the PDH technique can be used to stabilize and/or measure the instabilities in an optical cavity length. The PDH technique responds to the frequency of laser emission independently of intensity, which is significant because many other methods that control laser frequency, such as a side-of-fringe lock are also affected by intensity instabilities. </p>