When is a laser a real laser?

Pulsed lasers are intense and coherent light sources, and the latest category is that of Free Electron Lasers, such as FERMI. First order coherence is a familiar phenomenon, and is manifested for example in diffraction phenomena. This represents the correlation between the amplitudes of a wave at different points in space (transverse coherence) or time (longitudinal coherence.) However, a high degree of first order coherence is not enough to define a laser, according to the Nobel laureate Roy Glauber, who stated that a laser can be defined as a source that is coherent in all orders. The higher order correlations are between intensity at different points in time and space. How are these correlations measured? For this one has to look at the statistics of the photons.
Glauber’s work was inspired by the famous Hanbury Brown and Twiss experiment, in which coincidences of photons (i.e. correlations) were measured of photons coming from distant stars. By varying the distance between two detectors, they were able to determine the degree of coherence of the star, and extract other information. This is the key to measuring the second order coherence of a light source: the intensity of light at different points is measured in coincidence, and statistical analysis is made. This experiment is considered by many as initiating the whole field of quantum optics. Now a team led by Ivan Vartaniants (DESY, Hamburg, and the National Research Nuclear University, Moscow) has performed a Hanbury Brown and Twiss experiment at FERMI. Instead of the two discrete photodetectors used originally, a CCD detector was used. Since all of the photons arrive in less than 100 fs, there is no need to use coincidence methods: the signal is naturally synchronised.
Measurements at other FELs, based on the Self Amplified Stimulated Emission process, have shown that although they are first order coherent, they are not coherent at higher orders. Thus they can be considered “chaotic” sources, like the stars measured by Hanbury Brown and Twiss. The measurements at FERMI showed that it is a different light source: it is not only first order coherent, as previously demonstrated, but also second order coherent, thus satisfying Glauber`s definition. This is an important result as it shows that the seed laser used at FERMI transfers all of the important properties of a laser, including the statistical and higher order properties, to the emitted light. As well, some quantum optical experiments require high order coherence, and so the way is open for this class of experiment.

 

Figure 1.  Difference between chaotic and coherent light sources. (a) photon correlation map for FERMI operated in seeded mode. (b) corresponding spectrum. (c) correlation map for FERMI operated in Self Amplified Stimulated Emission mode (the mode of operation of most Free Electron Lasers). (d) corresponding spectrum. Reprinted from O. Yu. Gorobtsov et al, Nature Communications 9 (2018) 4498. (Copyright Nature Publishing Group, reproduced with permission.)
 

 

 

This research was conducted by the following research team:

Oleg Yu. Gorobtsov,¹ Giuseppe Mercurio,² Flavio Capotondi,³ Petr Skopintsev,^1,4 Sergey Lazarev,^1,5 Ivan A. Zaluzhnyy,^1,6,10 Miltcho Danailov,³ Martina Dell`Angela,⁷ Michele Manfredda,³ Emanuele Pedersoli,³ Luca Giannessi,^3,8 Maya Kiskinova,³ Kevin C. Prince,^3,9 Wilfried Wurth,^1,2 and Ivan A. Vartanyants^1,6

 

¹ Deutsches Elektronen-Synchrotron DESY,  Hamburg, Germany
² Department of Physics, University of Hamburg and Center for Free Electron Laser Science, Hamburg, Germany
³ Elettra-Sincrotrone Trieste, Trieste, Italy
⁴ Present address: Laboratory for Biomolecular Research, Division of Biology and Chemistry, Paul Scherrer Institute, Villigen, Switzerland
⁵ National Research Tomsk Polytechnic University, Tomsk, Russia
⁶ National Research Nuclear University MEPhI, Moscow, Russia
⁷ CNR- IOM Istituto Officina dei Materiali, Trieste, Italy
⁸ ENEA C.R. Frascati, Rome, Italy
⁹ Molecular Model Discovery Laboratory, Department of Chemistry and Biotechnology, Swinburne University of Technology, Melbourne, Australia.
¹⁰ Present address: Department of Physics, University of California San Diego, La Jolla, USA.

Contact persons:

Ivan Vartaniants, e-mail: ivan.vartaniants@desy.de