Diode laser modules based on laser-machined, multi-layer ceramic substrates with integrated water cooling and micro-optics
Abstract
This thesis presents a study on the use of low temperature co-fired ceramic (LTCC)
material as a new platform for the packaging of multiple broad area single emitter diode
lasers. This will address the recent trend in the laser industry of combining multiple
laser diodes in a common package to reach the beam brightness and power required for
pumping fibre lasers and for direct-diode industrial applications, such as welding,
cutting, and etching. Packages based on multiple single emitters offer advantages over
those derived from monolithic diode bars such as higher brightness, negligible thermal
crosstalk between neighbouring emitters and protection against cascading failed
emitters. In addition, insulated sub-mounted laser diodes based on telecommunication
standards are preferred to diode bars and stacks because of the degree of assembly
automation, and improved lifetime. At present, lasers are packaged on Cu or CuW
platforms, whose high thermal conductivities allow an efficient passive cooling.
However, as the number of emitters per package increases and improvements in the
laser technology enable higher output power, the passive cooling will become
insufficient. To overcome this problem, a LTCC platform capable of actively removing
the heat generated by the lasers through impingement jet cooling was developed. It was
provided with an internal water manifold capable to impinge water at 0.15 lmin-1 flow
rate on the back surface of each laser with a variation of less than 2 °C in the
temperature between the diodes. The thermal impedance of 2.7°C/W obtained allows
the LTCC structure to cool the latest commercial broad area single emitter diode lasers
which deliver up to 13 W of optical power. Commonly, the emitters are placed in a
“staircase” formation to stack the emitters in the fast-axis, maintaining the brightness of
the diode lasers. However, due to technical difficulties of machining the LTCC structure
with a staircase-shaped face, a novel out-plane beam shaping method was proposed to
obtain an elegant and compact free space combination of the laser beam on board using
inexpensive optics. A compact arrangement was obtained using aligned folding mirrors,
which stacked the beams on top of each other in the fast direction with the minimum
dead space.