Nonlinear dynamics in semiconductor lasers

Project description

The goal of the project is to develop a high-power broad area diode laser with a diffraction limited and single-frequency-mode output of multiple Watts. Such a diode laser (semiconductor laser) does not exist today. High-power diode lasers that are commercially available today suffer from very poor coherence properties, which ruin the focus ability of the laser beam essential in many applications.

The work within this project is to be founded on several years of research on feedback lasers in the Optics and Plasma Research Dept. (OPL) at Risų National Laboratory. Diode laser systems with off-axis external optical feedback have been developed to have excellent spatial and temporal coherence properties. The intention of the Ph.D.-project is to include the properties of the laser systems in a single laser diode. The significantly enhanced coherence properties of the output from the laser diode may lead to new applications within:

• biomedical optics
• pumping of active fiber lasers
• interferometric high power sensors
• coupling into single-mode fibers
• material processing
• new blue lasers based on frequency doubling

Possible biomedical applications of the laser diode include: high-resolution imaging in human tissue, ophthalmic photocoagulation, treatment of skin diseases, and ophthalmic fluorescence spectroscopy.

The improved coherence is due to nonlinear interactions between the laser light and the semiconductor material of the diode. The distribution of the intensity of light in the active region of the laser causes gratings, i.e. spatial patterns in the refractive index as well as in the gain of the material. However, the formation of the spatial gratings in a semiconductor laser is not well understood, and a theoretical investigation of the nonlinear processes in the laser cavity is needed. Within the project a numerical tool is to be constructed for the purpose of clarifying how gratings are formed and what their spatial locations, strengths, and orientations are, and at the same time simulate the propagation of light within the cavity. Such a laser simulator, in which the material properties, the optical field, and their effects upon each other are included simultaneously, is often termed self-consistent. The numerical tool must be versatile and general enough both to help disclosing previously unknown physical effects and to be well suited to simulate and test ideas for alterations in the design of the laser diode (such as tilted mirrors, UV-written gratings, and implanted apertures in the waveguide of the cavity.)

To gain true insight to the nonlinear processes in the diode lasers, experimental characterization is necessary in order to compare theory with measurements. This is to be done in the diode laser lab at the Optics and Plasma Research Department at Risų (OPL).

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