Assessment of Latent Heat Reservoirs for Thermal Management of QCW Laser Diodes
There is great interest in improving the thermal management of laser diodes intended for use as pumps in inertial confinement fusion systems. Laser diode power is currently constrained by heat dissipation in the diodes. Diodes typically dissipate a quantity of heat that is comparable to their optical power output. This heating of the diode junction causes a thermal rollover that prevents the output power from scaling linearly with current drive, and also results in reliability limits due to catastrophic failure at diode mirror facets. For the pulsed, quasi-continuous wave (QCW) operating mode employed for LIFE and certain DOD applications, {approx}5 kW/cm{sup 2} of heat must be removed on timescales of {approx}100{micro}s, which is determined by thermal paths located within {approx}200 {micro}m of the laser junction. For these reasons, QCW thermal management is extremely challenging. Reducing the diode junction temperature enables more efficient operation, reduced thermal chirp, and operation at higher output power without compromised reliability - which improves the diode costs as measured in $/W. We have proposed the use of latent heat reservoirs to improve thermal management of diodes used in pulsed, quasi-continuous wave (QCW) operation. Our basic concept involves placement of a reservoir of low-melting-point metal within a few hundred microns of the laser junction, as in Fig. 1-1. This metal's latent heat of fusion maintains a nearly constant temperature (like a cold plate) in the very near vicinity of the diode junction. This cold reservoir creates large thermal gradients, which in turn are anticipated to drive a large heat flow from the diode. In contrast, conventional QCW devices rely on thermal diffusion into a large solid mass which cannot be held at a fixed temperature, which significantly limits the thermal extraction. Our operational concept involves phase changes within the reservoir during every QCW pulse. During the early portion of the pulse, heating of the diode and its surrounding material initiates melting within the latent heat reservoir. This phase change results in a near-constant reservoir temperature that facilitates heat transfer. During the long ({approx}100 ms) time between QCW pulses, the reservoir metal resolidifies. A simple back-of-the-envelope calculation based on Gallium metal shows that a 50 {micro}m thick Gallium reservoir is sufficient to absorb all heat generated by a 350 {micro}s pulse at 5 kW/cm{sup 2}. While this calculation shows that a latent heat reservoir can provide sufficient capacity to handle the magnitude of heat generated, it does not address the transient change in the diode junction temperature, which depends on details the heat flow into and through the reservoir. For this reason, we undertook a set of numerical experiments to quantitatively assess the impact of latent heat reservoirs on junction temperature. This report documents the results of these simulations.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 973856
- Report Number(s):
- LLNL-TR-425903; TRN: US1002198
- Country of Publication:
- United States
- Language:
- English
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