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We have investigated the properties of fluorocarbons as coolants for a high-power laser diode bar mounted on a funryu-jet heat sink. The thermal conductivities of fluorocarbons are about ten times lower than that of water, but they are less corrosive to the funryu copper heat sink and their boiling points may be higher than that of water. Since the cooling efficiency of such a high power laser diode (LD) bar may be a result of the dynamic viscosity, the heat capacity, the thermal resistance, and the boiling temperature of the coolant, we have compared two types of “Fluorinert”, 3M's FC77 and FC43, and water as coolants. Wavelength distributions along a 1 cm bar, operated under a 52 A driving current, are measured to be ±1.1 nm for FC77, ±2.1 nm for FC43 and ±0.6 nm for water. From their thermal and material characteristics given by 3M, the lower viscosity of FC77 is important for our funryu heat sink, which has a high flow resistance. A thermal resistance of 0.7°C/W is achieved at a flow rate of 1.0 L/min for FC77. FC77 is adopted as the coolant for CW 30 W operation and has operated continuously for over 800 h.

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