Reliability of Long Pulsewidth High Power Laser Diode Arrays
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Reliability of Long Pulsewidth High Power Laser Diode Arrays Byron L. Meadowsa, Farzin Amzajerdiana, Bruce W. Barnesa, Nathaniel R. Bakerb, Rene P. Baggottc, Upendra N. Singha, Michael J. Kavayaa a NASA Langley Research Center, MS 468, Hampton, Virginia 23681-2199 b Lockheed Martin Engineering and Science Company, Langley Research Center, MS 468, Hampton, Virginia, 23681 c Science and Technology Corporation, Langley Research Center, MS 468, Hampton, Virginia, 23681
ABSTRACT Space-borne laser remote sensing systems typically rely on conductively cooled, diodepumped solid-state lasers as their transmitter source. Since space-borne instruments incur high developmental and launch costs and are inaccessible for maintenance, their reliability is of great importance. Therefore, it is crucial to address the reliability of high power laser pump arrays, which essentially dictate the reliability and lifetime of the laser systems. The most common solid-state lasers used for remote sensing applications are Neodymium-based, 1-micron lasers and Thulium/Holmium based 2-micron lasers. 2-micron lasers require a pump wavelength of around 10 to 20 nm shorter compared with 1-micron lasers, and require pump pulse durations 5 to 10 times longer. This work focuses on the long pulsewidth laser diode arrays (LDAs) operating at a central wavelength of 792 nm used for optically pumping 2-micron solid-state laser materials. Such LDAs are required to operate at relatively high pulse energies with pulse durations on the order of one millisecond. However, such relatively long pulse durations cause the laser diode active region to experience high peak temperatures and drastic thermal cycling. This extreme localized heating and thermal cycling of the active regions are considered the primary contributing factors for both gradual and catastrophic degradation of LDAs, thus limiting their reliability and lifetime. One method for mitigating this damage is to incorporate materials that can improve thermo-mechanical properties by increasing the rate of heat dissipation and reducing internal stresses due to differences in thermal expansion and thus increasing lifetime. This paper explains the need for long pulsewidth operation, how this affects reliability and lifetime and presents some results from characterization and life testing of these devices.
INTRODUCTION AND BACKGROUND 2-micron laser remote sensing systems are typically designed around Thulium/Holmium solid state lasers that are optically pumped by high power, quasi-CW, 2D laser diode arrays (LDAs). Absorption dynamics and pump manifold lifetimes of Tm/Ho necessitate that pump diodes be operated at a central wavelength of 792 nm, a spectral width of 3-5 nm, for pulse durations of at least 1 millisecond and preferably 3 milliseconds or longer [1-4]. Figure 1 shows the absorption cross-section for Tm/Ho for sigma and pi polarizations, which makes evident the need for LDAs at this fairly distinctive wavelength.
Absorption Coeficient
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