High power VCSEL arrays have experienced rapid growth and development. In this paper, we review the unique properties of VCSELs and present the progress that is making them very attractive for high power laser applications.
INTRODUCTIONOver the past five to ten years, high-power vertical-cavity surface emitting laser (VCSEL) arrays have experienced rapid growth and development. Traditionally known for low-power applications such as short reach datacom links, recent work has shown VCSELs to be capable of high efficiencies and very high output powers. Coupled with that are the unique properties of VCSELs, such as circularly symmetric output beam, narrow spectral linewidth, and a low-cost fabrication model; high-power VCSEL arrays are becoming increasingly attractive for high-power laser applications such as solid-state pumping, welding, illumination, and printing, to name a few.The leading diode laser technology currently supplied to the marketplace is the edge-emitting laser (EEL), which boasts very high conversion efficiencies (60% -70%), and can be stacked to achieve kW-class power levels. While current VCSEL array efficiencies are typically <50%, other properties lend to potential improved system level performance and lower system level costs. This paper reviews the benefits of VCSELs and presents the current state of the art performance from VCSEL arrays.
There is interest in the use of carbon nanotubes (CNTs) to create a field emission cathode. The primary benefit of this cathode is that a gas flow is not required to create electrons, which is of critical importance for space missions that must minimize propellant mass. A field emission cathode consisting of CNT arrays is fabricated by the Georgia Tech Research Institute and the Georgia Tech High-Power Electric Propulsion Laboratory. The cathode is tested at pressures below 3 x 10-5 Torr. The average output current density is measured over a cathode voltage range of 250-800 V relative to the gate for several CNT cathodes. The highest stable emission current density is 0. 6 mA/cm 2. The emission current density is measured throughout a 50-hour life test at a cathode voltage of 550 V which demonstrated an overall constant emission current density of 0.51 mA/cm 2 with an emission current to input power ratio of 1.7 mA/W.
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