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Thermal runaway in germanium laser windows.

|January 23, 2010

Summary

This study measured the absorption of germanium at 10.6 micrometers, finding free carrier effects dominate. Germanium windows showed critical power densities for thermal runaway, with sub-ambient cooling yielding the highest stability.

Area of Science:

Materials Science
Optics
Solid State Physics

Background:

Germanium (Ge) is a crucial material for infrared optics, particularly for CO(2) lasers operating at 10.6 micrometers.
Understanding its optical absorption and thermal properties is vital for high-power laser applications.

Purpose of the Study:

To measure the absorption coefficient of germanium across a range of wavenumbers and temperatures.
To investigate the thermal runaway characteristics of germanium windows in a 10.6 micrometer CO(2) laser system.

Main Methods:

Absorption coefficient measurements of 50-Omega cm germanium from 400-2000 cm(-1) at temperatures ranging from 300 K to 450 K.
Studying thermal runaway by varying power density and employing different cooling mechanisms for germanium windows.

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Main Results:

Lattice absorption in germanium is negligible above 900 cm(-1).
At 10.6 micrometers, absorption is primarily due to free carrier effects.
A critical power density (P(c)) was identified for thermal runaway, with the highest value of 88 W cm(-2) achieved through sub-ambient cooling.

Conclusions:

Free carrier absorption is the dominant mechanism in germanium at 10.6 micrometers.
Effective cooling is essential to prevent thermal runaway in germanium laser windows.
Sub-ambient cooling significantly enhances the critical power density, improving operational stability.