surgical procedures to cutting and welding metals, diode-pumped
solid-state lasers offer a wealth of uses. A Livermore team has
won an R&D 100 Award for a modular packaging technology for
the smallest, most powerful, and least expensive laser diode pumps
microchannel cooling system makes it possible. This technology,
called SiMM for silicon monolithic microchannel, relies on photolithography
and high-production etching techniques to produce thousands of miniscule,
30-micrometer-wide channels in silicon substrates. Water flowing
through these microchannels cools the laser diode bars that are
attached to the silicon, allowing the diodes to perform at higher
average power than previously possible.
On each SiMM, a tiny package
of 10 diode bars can be combined with as many microlenseswhich
collimate the laser lightto create a unit from which large
diode arrays can be built. The microlenses attached to each package,
developed and patented by Livermore scientists, give unsurpassed
optical brightness. Livermore holds three other patents as well
for new developments associated with SiMM.
To date, Livermore has fabricated arrays that put out power
of up to 45 kilowatts. Yet these powerful arrays measure just 10
by 18 centimeters.
|Members of the SiMM team are,
from left, Larain DiMercurio, Joe Satariano, Jacqueline Crawford,
Barry Freitas, Gary Loomis, Terri Delima-Hergert, Dave Van Lue,
Ray Beach, Kurt Cutter, and Everett Utterback. Missing from
the photo are Cathy Reinhardt and Jay Skidmore.
Cooling Is Key
Because laser diode bar arrays are semiconductor
devices, their performance suffers as their temperature increases,
notes physicist Ray Beach, who leads the team. Cooling is a challenging
problem because laser diodes generate high heat intensity, yet must
operate near room temperature. Efficient cooling is thus the cornerstone
of any technology that proposes to increase the power output of
Although laser diodes are
extremely efficient devices by ordinary laser standards, typically
converting nearly 50 percent of their electrical consumption into
light, the remaining 50 percent shows up as heat.
The use of silicon in the
cooling system is critical. Because photolithographic and etching
technologies are so well developed for silicon, arrays of precision
microchannels can be easily and inexpensively fabricated in this
material. Silicon makes it possible to place thousands of 30-micrometer-wide
microchannels close to the heat-producing laser diode bar arrays.
It also allows multiple bars to be located on a single substrate,
with an equal number of cylindrical microlenses, all attached in
a single fabrication step.
But why use silicon rather
than materials with higher thermal conductivities, such as copper?
In compact heat sink structures with flowing water, the best way
to control the overall temperature rise is to minimize the thickness
of the boundary layer where stagnant water meets flowing water.
It is in this boundary layer that the largest temperature rise occurs.
Because boundary-layer thickness scales relative to channel width
for the flow conditions in the SiMM package, the best material for
the cooling system is one that permits easy fabrication of narrow
channels. With copper, the channels would have to be wider. It turns
out that better thermal performance is gained by using a material
that permits tiny microchannel fabricationsiliconrather
than a material with higher thermal conductivity.
The laser diode bars can be precisely placed on the SiMM in V-shaped
grooves etched on the front surface of the package. These grooves
are generated with the same technology that creates the microchannels
in the back side of the silicon. Because the V-shaped grooves are
defined with a photolithographic process, the diode bars can be
located with micrometer precision relative to one another over the
entire SiMM package. Senior engineering associate Barry Freitas,
lead developer of the SIMM package, is responsible for this innovative
diode bar mounting technology.
The cylindrical microlenses
are located with the same micrometer precision in a ladderlike frame
of silicon runners. The microlenses are preloaded and glued into
the silicon runners to form a structure of 10 lenses. The entire
10-lens assembly is then attached to the SiMM package in a single
step. The microlens array serves to collimate the radiation of the
laser diode bars from its original 30-degree divergence angle down
to a beam with a divergence angle of only about 0.5 degrees. Finally,
a glass block seals off the base of the microchannels and serves
as a manifold for the cooling water as it flows in and out of the
At left is a 41-kilowatt laser
diode array constructed from 28 individual packages of silicon
monolithic microchannels (SiMMs). In a SiMM package below,
next to a quarter for scale, 10 diode bars placed in V-shaped
grooves are combined with 10 microlenses glued into silicon
Two other approaches compete
with the new SiMM cooling technology. The first one relies on stacking
single diode bar array packages, a process known as rack and stack.
The individual packages are fabricated of copper and use larger
macrochannels to flow cooling water. The other technology relies
on mounting the laser diode bars on thermally conductive bar mounts
and attaching them to a backplane cooler through which water flows.
The primary improvements
of the SiMM package over these technologies are in its integration
of high-performance heat removal within the high-density, multibar
package and the use of low-cost fabrication methods. First, the
thermal engineering of the SiMM package allows it to produce an
average exitance (irradiance in the emitted laser beam) that is
2.6 times greater than that of its nearest competitor. Second, the
use of a monolithic cooler is unique to SiMM. By attaching 10 individual
laser diode bars to each SiMM cooler, rather than one bar per cooler
as in the rack-and-stack method, the cost of the cooler package
is spread over 10 bars. Finally, the majority of the cost of diode-pumped
solid-state lasers is in the diode arrays that serve as their pump
excitation sources. These lasers thus benefit tremendously from
the low-cost fabrication methods in SiMM. SiMMs cost per watt
is less than one-third that of its nearest competitor.
innovative SiMM is already being incorporated into new military
defense systems. In the near future, a 1-megawatt version will become
a key element in the award-winning laser described on p. 8. That
laser, the most powerful solid-state laser system in the world,
is currently pumped using flash lamps but will soon incorporate
the smaller SiMM diode-pump array. An award winner meets an award
Key Words: R&D
100 Award, silicon monolithic microchannel (SiMM) laser diode array,
solid-state laser diodes.
For further information contact Ray Beach (925) 423-8986 (firstname.lastname@example.org).