Optical Supercontinuum from a Feedback System

• Applicant: Professor Dr. Fedor Mitschke
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2008 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 86545597

• Final Report Year: 2015

Final Report Abstract

Optical supercontinuum is a term describing light with broad spectrum and correspondingly short coherence time, but good spatial coherence. This renders supercontinuum much more useful than similarly broadband light from thermal sources like tungsten filament bulbs etc., and lets it find use in several applications. Typically, supercontinuum is generated with laser pulses in micro-structured optical fibers. The generation process serves as a testbed for a most complex interaction of several linear and nonlinear processes; this study aimed to clarify some issues. In the experiments we used an additive pulse mode-locked Nd:YAG laser which is a most efficient means of obtaining gain-bandwidth-limited pulses of ca. 10 ps duration and 10 kW peak power. Its light pulses were launched into a length of micro-structured fiber, with or without feedback of the output to the fiber input. Any intense pulse of light, launched into a single-mode fiber, is subject to both linear distortion and nonlinear distortions. Among the latter, self and cross phase modulation, four wave mixing etc. are consequences of third-order susceptibility. Raman scattering produces a substantial frequency shift. Relevant length scales for various processes were identified; their discussion shows why the spectrum develops rapidly in a first stage, then becomes nearly stationary once it is fully developed. The fiber’s dispersion curve allows phase matching at certain frequencies between pulses and radiation; new frequency components are generated this way. Their positions could be verified quantitatively. A thermodynamic treatment was tested and found to be generally valid, but not to possess much predictive power. Modulational instability renders the input pulse into what looks like a periodic series of pulses. Only recently has a comprehensive description been given in terms of the so-called Akhmediev breather. We could show that it is a misconception to think of the pulses in the train as solitons as was suggested by several authors: While the Akhmediev breather does contain solitons, it is erroneous to identify them with the individual pulses. We could also show that, in contrast to the integrable case as described by the Nonlinear Schrödinger equation, here an exchange of energy can take place from a weak radiative part into a much stronger soliton, a process which may contribute to rogue wave formation. We had expected that feeding back some of the light from the fiber end to its input would create interesting dynamics. We had also hoped that the threshold for supercontinuum generation could be lowered, opening a way to design a more power-efficient and thus economic supercontinuum source. It turns out that while these expectations are borne out in principle, the technical advantages may not be sufficient to affect commercial designs. The feedback arrangement does support what has been called soliton crystals before, a group of pulses in a fixed temporal array. Regimes of existence and parameters like temporal separations between pulses were discussed. Similarly, the setup can produce THz oscillations which were studied with a view to possible applications. However, the frequencies are dominated by the fiber’s dispersion curve which makes it quite difficult to provide frequency tunability. This study has led to a much more detailed understanding of the processes involved in the generation of optical continuum.

Publications

• "From optical rogue waves to optical transistors“. Nonlinear Phenomena in Complex Systems 16, 24 (2012)
  A. Demircan, Sh. Amiranashvili, C. Brée, F. Mitschke, G. Steinmeyer
  
• "Possibility of an Akhmediev breather decaying into solitons“. Physical Review A 82, 033808 (2012)
  Ch. Mahnke, F. Mitschke
  
• "Recent Insights about Solitons in Optical Fibers“. Nonlinear Phenomena in Complex Systems 15, 369 (2012)
  F. Mitschke, A. Hause, Ch. Mahnke, P. Rohrmann
  
• "Rogue events in the group velocity horizon“. Scientific Reports 2:850 (2012)
  A. Demircan, Sh. Amiranashvili, C. Brée, Ch. Mahnke, F. Mitschke, G. Steinmeyer
  
• "Rogue wave formation by accelerated solitons at an optical event horizon“. Appl. Phys. B 115, 343 (2014)
  A. Demircan, Sh. Amiranashvili, C. Brée, Ch. Mahnke, F. Mitschke, G. Steinmeyer
  
• "Ultrashort Light Pulses Generated from Modulation Instability: Background Removal and Soliton Content“. Appl. Phys. B 116, 15 (2014)
  Ch. Mahnke, F. Mitschke