1.6. LASER PHYSICS
We have continued to investigate coherence and noise properties of laser light and to apply the lasers im spectrospic studies. Mainly we have utilized diode lasers in the red wavelength range. Of special interest have been the studies concerning nonlinear dynamics induced by external optical injection. Similarly to previous years starting from the year 1985 we have continued our collaboration with The Finnish Geodetic Institute. These investigations have concerned stabilized helium-neon lasers. A potentially consequential new collaboration between our group and The Department of Biosciences was started during the autumn. The aim of this work is to study laser induced processes to capture and to localize micro- and nanosize particles.
For our research activities we acknowledge financial support from the University of Helsinki, Svenska Kulturfonden and Svenska Tekniska Vetenskapsakademien.
Birger Ståhlberg and Åsa Lindberg
INVESTIGATION OF DIODE LASERS SUBJECT TO EXTERNAL OPTICAL INJECTION
Stefan Eriksson and Åsa Lindberg
The term external optical injection refers to the situation when light from one laser impinges on another laser. We have conducted studies of external optical injection in a master-slave configuration where the injected laser becomes controlled by the external laser light. In the experiment a polarization stabilized helium-neon laser is used as a highly coherent and stable master laser. The slave laser is a temperature controlled AIGaInP laser diode with a multiple-quantum well structure. By adjusting the temperature of the diode laser so that the master and slave laser frequencies become nearly degenerate, different nonlinear dynamical states can be excited in the slave laser. The frequency detuning between master and slave lasers and the field amplitude of the injected light are key parameters in this context. Existing theory prodicts a strong dependency on the linewidth-enhancement factor of the mapping of dynamical states in this parameter space. The linewidth-enhancement factor describes the strength of the coupling between the refractive index and the gain of a diode laser. The coupling is a property specific to the laser material. We have measured the linewidth-enhancement factor of the diode lasers used in the experiment.
CHARACTERIZATION AND APPLICATION OF EXTENDED CAVITY DIODE LASERS
Stefan Eriksson, Åsa Lindberg and Birger Ståhlberg
We have continued the work with extended cavity diode lasers. The fundamental linewidth and lineshape of a previously constructed laser was measured and the light from this laser was used to probe a closed transition in a neon discharge . This laser is frequency-tuneable in a wavelength region of 633-640 nm, by the use of optical injection from a grating cavity. A second laser source based on similar principles has been constructed. This allows a detailed study of laser frequency stability. In addition, the second laser can be used as a reference source in spectroscopy. When constructing the second laser, special emphasis was laid on the cavity design. The cavity can easily be modified for different experiments including the study of optical injection. The laser system employs previously developed servo electronics whose sensitivity to radio frequency fields is minimized. All mechanical parts of the laser system were skillfully manufactured at the laboratory workshop.
1. S. Eriksson, Å.M. Lindberg and B. Ståhlberg, Opt Las Technol 31 (1999) 473-477
MEASUREMENTS OF THE STABILITY OF THE POLARIZATION STABILIZED He-Ne LASERS INCORPORATED IN THE ABSOLUTE GRAVIMETERS OF THE FINNISH GEODETIC INSTITUTE
Birger Ståhlberg and Jaakko Mäkinen*
Polarization stabilized two-mode He-Ne lasers are employed as light sources in many gravimeters. We have continued to measure the stability of two such lasers incorporated in the gravimeters FG I and FG II. We have performed the measurements with a few months intervals during the year. The measurements have been carried out by heterodyning the polarization stabilized lasers against an iodine stabilized He-Ne meter laser. Similarly to the previous years the results show a few MHz differences between the measurements.
* The Finnish Geodetic Institute, Masala
CONSTRUCTION OF AN OPTICAL TWEEZER FOR MANIPULATION OF BIOLOGICAL OBJECTS
Roman Tuma*, Kaleva Salminen*, Dennis Bamford* and Birger Ståhlberg
Optical tweezers, or optical traps, exploit the fact that light exerts force on matter. Dielectric particles, such as uniform beads or bacterial cells, are attracted to and trapped near the waist of a laser beam that has been focused through a microscope objective. We have constructed an optical trap the main parts of which originate from a microscope from the Department of Biosciences. As a light source we have employed the laboratory large frame argon ion laser. We have performed preliminary experiments on micrometer sized beads.
* Department of Biosciences, University of Helsinki