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Innovative Passivation to Increase the Power at Which Laser Diode Fails


OBJECTIVE: Improve the reliability/lifetime and increase power and performance of high power laser diodes (LD). DESCRIPTION: There is a compelling need for substantially increasing the power and brightness of LD optical-pumps in the 9xx nm spectral range for scaling single-mode narrow-line fiber lasers to high power for DoD high energy laser (HEL) applications. The power and brightness of state-of-the-art LDs are severely limited by catastrophic optical-damage (COD) at the front facet. COD severely limits the power/bar that could be attained and hence a larger number of LD bars are required for a given LD pump power. The larger number of bars increases system complexity and decreases efficiency of the high power laser system. In addition it results in an increase in size, weight and cost of the laser system. The focus of this SBIR is to significantly improve the reliability of high-power semiconductor LDs so they can be reliably operated at 6-7X higher power density per bar than the present state-of-the-art. Specifically, state-of-the-art 980nm, 20 percent fill-factor, 10mm wide bars operate at approximately 70W. Achieving this goal of 400-500W/bar may directly impact DARPA"s high-power fiber lasers such as Revolution in Fiber Lasers RIFL by increasing the specific power of laser diodes pumps from the present 1kW/kg to 6-7kW/kg. Since LD pumps contribute about 50% of the cost and weight of the high power laser system, increasing the specific power (kW/kg) will have a significant impact on the size of the high energy laser system. In addition, the cost of the laser diode pumps is inversely proportional to the power/bar and increase of 6-7x in power that could be obtained from a bar decreases the cost by a similar factor. The weight and cost of LD pumps is estimated to be approximately 50% of the laser system so decreasing them by 6X will decrease the all-important weight and cost of the HEL by 40%. This technology will also provide similar benefits to the HEL solid-state lasers. PHASE I: Determine the technical feasibility of the growth of a single-crystal passivation layer on the (110) facet of a 9xx laser diode formed at low temperature and in ultra-high vacuum. Current passivation techniques are either amorphous, resulting in significant residual surface state density within the bandgap, or require high temperature growth which degrades the Ohmic contacts. Low temperature growth (<= 400 degrees C) is therefore required to ensure compatibility with existing laser diode processing and ultra-high vacuum (<1e-9 Torr) is required to prevent oxidation of the cleaved surface. The passivation layer should fully passivate the facet and prevent the defects. It should also prevent absorption of the laser line. Phase I deliverables will include a demonstration of lattice matched high band-gap crystal growth on the cleaved end of the GaAs laser diode. PHASE II: Develop, demonstrate and validate reliable operation (500hr) at 500W of a 10mm-wide, 980nm laser-diode bar with fill-factor =20% with innovative passivation demonstrated in Phase I. PHASE III: High-power LDs have a large $3 billion market that is growing at 20 percent annually. The technology developed in this SBIR may be a valuable asset for this market as it should significantly decrease the all-important cost or dollars/watt by 6X. The passivation technology developed under this SBIR may have an impact on DoD HEL systems that use LD pumps.
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