Magnifying time reveals the details of ultrafast physicsThe measurement of light pulses that last only a millionth of a millionth of a second requires new approaches from researchers. In his doctoral dissertation, Mikko Närhi explores new tools for capturing ultrafast pulses with unrivalled precision.
Ultrafast lasers are capable of producing high peak power pulses below picosecond durations on a regular basis. These densely packed energy packets consist of photons and are widely used in industrial applications and research. Their operating mechanism is based on nonlinear optics, where the electrons of the material can get so confused by the high energy density of the pulse that they start radiating light at entirely new wavelengths.
"This is quite exceptional in everyday situations. Imagine if the observed colour of a car driving by depended on whether you see it on the street or through a window," says Närhi.
Deep down, many natural phenomena are nonlinear when the perturbation to the physical system is strong enough. Nonlinearity can also cause practical issues, as it can make a system entirely unpredictable. In these situations, a tiny change in the initial condition can cause large differences at the end, which is also referred to as the butterfly effect in popular science. This can occur, for example, in optical fibers, when the pulse breaks up spontaneously into several subpulses in an entirely random manner and possibly causes higher peak powers at the output compared to the original pulse. These cases are referred to as optical rogue waves.
Understanding the phenomenon requires that observations are made on a single pulse basis for a large ensemble of measurements. In his dissertation, Mikko Närhi studies the break-up dynamics with a time lens, which is capable of enlarging these ultrafast pulses in time into a directly measurable time scale.
"What makes the topic extremely interesting is that the same mathematical equation governing laser pulse propagation in an optical fiber also governs the propagation of waves in deep seas. Once we discover something fundamental about the light pulses, such as the reason for the formation of optical rogue waves, we will automatically also gain insights into the physics of the waves. However, our experiments are much more straightforward to perform," Närhi describes.
These new measurement techniques allow the comparison of theoretical results with experiments and allow for optimized laser design and could provide a pathway for developing rogue wave warning systems for ships. In addition, Närhi has demonstrated the potential usefulness of the sensitivity of a nonlinear system to input conditions as an optical signal amplifier. The unprecedented sensitivity holds promise for measurement applications in future.
Public defence of a doctoral dissertation on Friday, 24 November 2017
The doctoral dissertation of MSc (Tech) Mikko Närhi in the field of physics titled ”Measurements of Noise-seeded Dynamics in Nonlinear Fiber Optics” will be publicly examined in the Faculty of Natural Sciences at Tampere University of Technology (TUT) in room RG202 in the Rakennustalo building (address: Korkeakoulunkatu 3, Tampere, Finland) at 12.00 on Friday, 24 November 2017. The opponent will be Professor Jérome Kasparian from the University of Geneva. Professor Goëry Genty from the Laboratory of Photonics at TUT will act as Chairman.
Mikko Närhi is 29 years old and comes from Tampere, Finland. He currently works as an R&D engineer at NKT Photonics in Denmark.
The dissertation is available online at http://urn.fi/URN:ISBN:978-952-15-4060-8
Further information: Mikko Närhi, tel. +358 40 7289219 or +45 5219 2903, email@example.com / firstname.lastname@example.org