Group of Lange and Ackemann
Nonlinear optics and quantum optics
Institute of Applied Physics
WWU Münster

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Optical pattern
formation
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Vertical-cavity
surface-emitting
lasers (VCSELs)
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Nonlinear beam
propagation
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Optical pattern formation

Introduction
The lab
The team
Publications
Shortcuts to projects in optical pattern formation:
Polarization Patterns
Hexagons, flowers and dynamical phenomena in a quasi-scalar situation
Formation of superlattices and quasiperiodic optical patterns
Dynamics of patterns and spatio-temporal structures
Intrinsic dynamics: spirals and target patterns
Drift Instability/ Locking
Localized Structures

Introduction

What is the reason that an initially homogeneous system evolves spontaneously into a modulated, "structured" state? This intriguing question arises in many sciences, in nature as well as in the laboratory. Spontaneous self-organization phenomena in space and time are also ubiquitous in optical systems in which intense laser beams interact with a nonlinear medium, i.e. a medium in which the optical properties (refractive index or absorption coefficient) depend on the intensity of the incident light. The interplay of spatial coupling by diffraction and nonlinearity is responsible for the pattern formation. It is highly fascinating that the properties of structures in such different regions of science like hydrodynamics, chemical reactions, gas discharges and optics possess remarkably universal aspects, and optics is beginning to promote the knowledge on dissipative patterns by demonstrating phenomena not known before. Therefore the investigation of optical patterns is an important topic of interdisciplinary research using equipment of technical relevance and might on the other hand form the basis for future all-optical data processing.
 

Nonlinear effects occur in many media and in many different configurations. Concerning the medium, we focus on experiments using sodium vapor. The nonlinearity is due to optical pumping. Beside technical advantages (high optical quality, easy variation of parameters over a broad range, high resonant nonlinearity) the benefit of using an atomic vapor is that the equations governing the light-matter interaction can be derived directly from quantum mechanics via the density matrix approach. Regarding the investigated configurations, investigations have been done in most of the situations in which spatial optical structures are known to occur. These are
nonlinear beam propagation
driven cavities filled with a nonlinear medium
single-mirror feedback systems
The present focus is on the single-mirror feedback system since it is conceptually very simple.
 


The Lab

A view to the lab
Here you can see some random pictures from the optical table within the lab on which our experiments were performed.
Picture 1: Shows the dye laser in the background. To the left you see the stabilizing unit for the laser frequency.
Picture 2: To the left you see the mirror mount that brings the laser beam to the constant height of 5 cm above the table. On the right hand of that a part of the beam is coupled into an optical fibre (orange coated) in order to measure the optical frequency with it.
Picture 3: Shows the last part of the way of the beam, where the beam quality is enhanced by spatial filtering. After that the beam is lifted again to the height of the vapor cell which is surrounded by 3-dimensional Helmholtz-coils.
Main Equipment
  • argon ion laser 15 W (Spectra Physics 2030-15)
  • frequency stabilized ring dye laser (Spectra Physics 308D)
  • intensified CCD-camera Proxitronic HF4 S 5N (gate duration downto 5 ns)
  • trigger logic for slow-motion pictures of repetive fast phenomena with a time resolution of 100 ns (video-sampling method)
  • CCD-camera Pulnix TM-765 (glassless, selected for laser beam analysis)
  • data acquisition and image processing system based on LabView (National Instruments)
  • wavemeter (accuracy 500 MHz absolute, 200 MHz relative)
  • digital oscilloscope LeCroy 9400 (175 MHz)

Detailed Experimental Setup
The setup can be schematically divided into four parts:
  • the laser system itself,
  • the  preparation of the laser beam for the experiment,
  • the sodium cell, and
  • the detection system.


The purpose of the beam preparation section is to provide a well controlled input beam for the experiment. This includes spatial filtering to ensure a smooth and rotationally symmetric beam profile, the control of the polarization state and means for power stabilization and ramping. The most important part of the detection system are CCD-cameras which record two-dimensional images of the intensity distribution in real space inside the vapor (near field) and - in the back focal plane of a lens - simultaneously its Fourier spectrum (far field). This allows a complete characterization of the field.

The next figure shows the complete setup as it was mounted for the investigation of the patterns with circularly polarized excitation (click here for references).



(click on the image to get a better view)

L lens, BS beam splitter,
FP Fabry-Perot interferometer,
IU, OU fiber coupling units,
M mirror, EOM electro-optical modulator,
lambda/4 , lambda/2 retardation plates,
D detector (photodiode),
CCD charge-coupled device camera,
FTIR variable beam splitter based on frustrated total internal reflection,
FM feedback mirror (R=0.915), B system of three orthogonal pairs of Helmholtz coils.


 


The Team

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AG Lange & Ackemann
Institut für Angewandte Physik · Universität Münster Corrensstr. 2/4 · 48149 Münster