Published 5 July 2012 by Alexander Bastidas Fry

Hall and Hänsch on Lasers

Some fifty years ago the first lasers were developed. They were seen as a curious diversion of physics by some, but since that time they have proved to be a success story of science theory and application. The original nascent concepts of how light behaved were discovered by James Clerk Maxwell just before the 20th century then later Einstein’s work with the atomic coefficients of emission, absorption, and stimulated emission laid the groundwork for creating the laser. The nature of the laser is evident in the fact that laser is actually an acronym for light amplification for stimulated emission or radiation.

By the nature of scientific discovery it is impossible to predict what research will be fruitful or what practical applications there will be. The laser is perhaps the most fantastic example of theoretical science gone practical. The laser has been completely integrated into modern society with ubiquitous technology applications. Yesterday in lectures by Hall and Hänsch I heard about the course of scientific research with lasers and their applications. Hall described research that has driven lasers to eclipse tangible weights and measures as the standards of international science measurement. For example in 1983 the meter was redefined at the length light travels in a vacuum during the time interval of 1/299,792,458th of a second. Hall described how the precision of lasers for measurement seems to approach a limit because measuring something that is precise in frequency by measuring something precise in time are normally conjugate – observation of one property interferes with observation of the other property.

The 2005 Nobel Prize in physics was awarded to John Hall, Roy Glauber, and Theodor Hänsch for their fundamental contributions to optical frequency combs. A frequency comb is a spectrum of light which is split up into a very fine series of pulses at equally spaced intervals of frequency. By shifting to thinking in Fourier space, where signals are decomposed into their constituent frequencies, light waves composed of many sharp pulses of light have allowed some incredible advancements.

Light is repeatedly reflected back and forth within the mirrored cavity of a laser and here the wave nature of light is very important. There will exist many unique discrete wavelengths, eigen modes, of light in the cavity which will (such as in the case of phase or mode locked lasers) interfere with each other in constructive and destructive modes.

The eigen modes of laser inside a cavity reinforce at characteristic frequencies. Source NIST

The waves of light superimpose on each other and end up emitting a train of evenly spaced pulses of light at range frequencies. This wave superposition of light is analogous to a much more common situation as Hänsch showed. Pendulums have a frequency of oscillation which depends on their length so if several different length pendulums are all started at the time time they will fall in and out of phase at regular intervals. Take a look:


Mode locked lasers and optical combs are practical in many areas. Including but not limited to nuclear fusion, biological imaging of chemical reactions, precision astronomical spectroscopy, and even fundamental physics. Hänsch at then end of his talk mentioned the fascinating prospecting of uncovering discrepancies between matter and antimatter. Particle accelerators, such as the LHC, create and capture antiparticles which have now been held stable long enough to be combined into anti-hydrogen. If there exist any differences at all between the available electron energy levels in hydrogen and anti-hydrogen then this could be a clue as to the fundamental asymmetry in our universe of matter over antimatter. 

Alexander Bastidas Fry