Topic：Novel ultrafast fiber laser sources utilizing fiber nonlinearities
Date：Nov.20, 11am
Place：Rm 938, OECE Building
Lecturer：Cao Qian
Invited by：Zhan Qiwen
Abstract：Optical frequency comb has become a powerful tool in frequency metrology and other scientific applications such as spectroscopy, atomic optical clock and standard synchronization. Frequency comb in the mid-infrared (mid-IR) wavelength (2-20 μm) is of particular interest, as a large number of gas molecules exhibit strong absorption in this range due to the molecular vibrational transitions. Mid-IR frequency comb thus opens up possibilities of detecting and distinguishing gas molecules. Moreover, the atmosphere possesses two transparent windows (3-5 μm and 8-13 μm) in the mid-IR range, which enables the applications of atmospheric remote sensing and long-distance synchronization.
Due to lack of direct active medium in mid-IR, especially in the long wavelength region (> 5 μm), the development of mid-IR sources relies upon the optical parametric processes such as optical parametric amplifier (OPA), optical parametric oscillator (OPO) and difference frequency generation (DFG). Among them, DFG has the most straight forward implementation and the widest range of wavelength tunability.
The build of a DFG-based mid-IR laser requires two synchronized inputs (the pump and the signal) being frequency mixed inside a nonlinear crystal. The difference between their center frequencies later becomes the center frequency of the output DFG in mid-IR. In the laboratory, the signal pulse can be derived from the pump pulse by fiber nonlinearities such as self-phase modulation (SPM) and intra-pulse Raman scattering effect. This approach leads to the automatic cancelation of the carrier envelope offset (ceo) phase on the output DFG pulse. After stabilizing the repetition rate of the laser system, the resulting mid-IR source forms a mid-IR frequency comb.
In this talk, I firstly present the numerical study on two wavelength shifting techniques based upon soliton self-frequency shift (SSFS) and self-phase-modulation enabled spectral selection (SESS). The spectral and temporal evolution of the pulse propagation for both methods is demonstrated. I discuss in detail their optical noise transfer characteristic and make a comparison for them.
In the second part of the talk, I show the energy scalability of general DFG process. Finally, I show two realizations of DFG systems. One of them generates DFG at 1 μm using SSFS and the other generates DFG in the mid-IR range using SESS.
Lecturer Resume：Cao Qian received his B.Sc degree from Department of Optical Science and Engineering in Fudan University in 2012 and his M.Sc degree from Electro-Optics Program in University of Dayton in 2014. He is currently pursuing his Ph.D. degree in physics from Universität Hamburg. He is also a student in the International Max Planck Research School (IMPRS). His research interest includes fiber lasers, spatio-temporal wave packets and other researches in ultrafast optics. He is scheduled to get his PhD degree after his defense of his thesis in 2019 Spring.
