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Fiber Interface Thermal Management

Description:

OBJECTIVE: Develop an end of fiber holding and cooling system for high power (>1kW) fiber amplifiers. DESCRIPTION: The demands placed upon the last 10 cm of fiber used in high power Yb:silica fiber amplifiers can be strenuous. First the laser emission is accompanied by large amounts of residual 976nm pump that must be extracted and absorbed. Second the high energy density of the lasing emission can result in severe facet heating and even catastrophic failure. This is due to absorbing contaminants on the facet surface or scratches. And third, they must be accurately held in place for pointing stability. This latter issue is accentuated by temperature fluctuations as the laser is cycled from off to maximum power. And while the first two issues can be easily addressed, it is this pointing stability criterion that adds significant complexity to the problem. Traditional thermal management techniques have not been employed due to the small spatial scales involved. Novel approaches are being sought to address fiber holding and thermal management in the vicinity of optical fiber facets for amplifiers that develop over 1kW in output emission. The envisioned device is a fiber holding and cooling system on the last few centimeters (up to 10cm) of a large mode area fiber with, and also without, a fused silica end cap attached. The device is to rigidly hold and cool the fiber with minimal effect on the optical characteristics of output radiation due to mechanical or thermal stress. The heat load can be on the order of 100W. The holder cannot significantly alter the wavefront, polarization or mode structure of the optical radiation. The pointing stability should be less than 1 micro-radian and also not be compromised as the amplifier power (i.e. the heat load) is increased. Actuated tilt control of the device should be considered. The impact of jitter due to the cooling system is to be addressed. Future applications (Phase III) may involve close-packed arrays of fibers. Hence it should be easily adapted to a small packing size or multiple fibers. PHASE I: The successful effort will develop a proof of concept, which includes materials development, and packaging verified by finite element analysis modeling. PHASE II: A deliverable prototype for a single fiber will be constructed, validated, and delivered to AFRL/RDLA for evaluation. Concepts for holding multiple fiber amplifiers will be presented. PHASE III: In Phase III, the technology will be completed on a single multi-kW fiber amplifier. A prototype for multiple fibers (5-10) will be built. The technology will be transitioned to government/industry. REFERENCES: 1. Fan, Y., et al.,"Thermal effects in kilowatt all-fiber MOPA,"Optics Express, vol. 19(16), pp. 15162-15172, 2011. 2. Gwinn, J. P., and R. L. Webb,"Performance and testing of thermal interface materials,"Microelectronics Journal, L Vol. 34(3), pp 215-222, 2003.
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