Heat spreaders are essential for cooling high-power laser diodes. With its high thermal conductivity, diamond has joined the ranks of other dielectrics and metal composites as a practical material for heat spreaders.
The demand for higher optical output from high-power pump lasers requires efficient thermal management because laser output drops drastically with increasing chip temperature. Thermal management of pump lasers critically affects laser wavelength, output power, threshold current, slope efficiency, and operating lifetime. For example, lasing wavelength shifts by about 0.3 nm/K for 980-nm pump lasers. The maximum optical power is limited by thermal rollover, while the power coupled into an optical fiber is limited by kink, which is caused by local heating in the active region of the laser chip. The threshold current of a pump laser increases while the slope efficiency decreases exponentially with the junction temperature. For these reasons, thermal management of pump lasers is crucial to device performance and extended long-term reliability.
Heat can be dissipated in two steps-first by spreading heat out, and then by sinking it away. A heat spreader is not unlike a water hose; it transmits heat (akin to dispensing water) and is described in terms of thermal conductivity. A heat sink is similar to a reservoir; it stores heat (akin to holding water) and is described in terms of heat capacity. A heat spreader does not retain heat, so it requires a combination of high thermal diffusivity and low heat capacity. In contrast, a heat sink needs high thermal diffusivity and high heat capacity.
Many materials are used in semiconductor and photonic packaging, each with its associated positive and negative characteristics. Metals have a high concentration of electrons and their thermal conductivity is dominated by electrons. For nonmetallic materials, in which electrons are not available for thermal conduction, thermal transport typically occurs through phonons (for example, in diamond). Many applications require both electrical isolation (dielectric-based heat spreaders) and thermal conduction. Dielectric heat spreaders are ideal for laser diodes and all semiconductor integrated circuits (see Table 1).
The diamond prepared by CSMH has ultra-high thermal conductivity, up to 2200W/(m.K). For laser applications, it provides high-quality diamond heat sinks, diamond wafers, diamond windows, diamond heterogeneous integrated composite substrates and other products.
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