Broadband Spectrum of Short Laser Pulses

Real-time measurement of the start of an ultrashort pulse laser (Source: G. Herink, GAU)

Real-time measurement of the start of an ultrashort pulse laser (Source: G. Herink, GAU)

Extremely short, intense optical signals are essential in basic research, manu­facturing, and ophthal­mology. Emitted from a special breed of lasers, these ultrashort flashes of light are funda­mentally different from the output of a conventional, mono­chromatic laser. This flashed emission consists of a broadband spectrum – a rainbow – and the shorter the pulse, the more colors it contains. A colla­borative research effort at the University of Göttingen and the University of California, Los Angeles (UCLA) has now observed the emergence of these ultra­short laser pulses in real time, with a frame rate of 90 million snapshots per second.

The emission of a stable, pulsed laser is charac­terized by a periodic chain of light flashes. However, the start of this operation is irregular, developing from a tangle of random fluctua­tions. The fluctua­tions compete over many thousands of roundtrips as they reflect back and forth between the laser’s internal mirrors. This nuanced compe­tition makes the startup of the laser highly complex, with each transition to the steady state unique. In the quest to advance laser techno­logy, Georg Herink and Claus Ropers from Göttingen together with Daniel Solli and Bahram Jalali (UCLA) have harnessed the fastest type of spectro­meter available to examine this process.

This unique spectral measure­ment technique exposes the colors of each individual pulse despite the extreme conditions of the application. It is able to record the spectra of hundreds of thousands of conse­cutive pulses, each separated by mere billionths of a second. The technique makes use of the principle of chromatic dispersion: in a glass fiber, every color travels at a different speed, and so an ultra­short pulse spreads out as it travels. With a kilometer-long fiber, the colors of each pulse are separated in time, creating a signal that can be captured with special high-speed electronics. Using this technique, the scientists were able to observe the complete formation of the rainbow spectrum each time the laser was activated.

“Real-time spectros­copy fills a gap in laser diag­nostics. We get unique insights into laser systems and short-lived nonlinear effects in a split second,“ explained Herink. For example, the researchers found a previously unknown mechanism in which an interplay between two transient light fluc­tuations faci­litates the creation of a stable rainbow. Moreover, the team observed that it is not neces­sarily the most intense fluc­tuation in the initial random tangle that develops into the stable rainbow: the complex dynamics can suddenly favor a weaker distur­bance, ignoring stronger members that were seemingly the original favorites. The results may impact laser design and the study of nonlinear systems. (Source: GAU)

Reference: G. Herink et al.: Resolving the build-up of femtosecond mode-locking with single-shot spectroscopy at 90 MHz frame rate, Nat. Phot., online 14 March 2016, DOI: 10.1038/nphoton.2016.38

Links: Ultrafast Dynamics (C. Ropers), Faculty of Physics, Georg-August-University Göttingen, Germany • Jalali Lab, University of California, Los Angeles, USA

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