Many military applications require reliable laser diode operation across a wide range of elevated temperatures. Size, Weight and Power (SWaP) restrictions often limit cooling options and necessitate that high-power emission at the desired pump wavelength is maintained across wide temperature ranges. A family of QCW diode arrays has been developed for operation in these harsh environments. The arrays may be constructed with either multi-wavelength diode bars or wavelength locked with Volume Bragg Gratings (VBGs) to optimize absorption across a wide range of temperatures. The arrays have been designed to withstand the mechanical shock and vibration requirements common to military environments.
This paper includes a comparison of the multi-wavelength arrays and VBG-locked arrays at high temperatures. Both sets of arrays were characterized across a broad temperature range and exposed to MIL-STD shock and vibration testing. VBG locked arrays are shown to provide >90% locking across a 15° operating range whereas multi-wavelength arrays allowed power absorption to be maintained across incredibly wide ranges (e.g. -40° to 70°C). Life test results from arrays operated at 80oC, 250A, 60Hz for over 600 million pulses are also presented. These arrays demonstrate excellent high-temperature reliability over a pulse count well in excess of the requirement of many military applications (e.g. range finders / target designators).
A low-SWaP (Size, Weight and Power) diode array has been developed for a high-power fiber-coupled application. High efficiency (~65%) diodes enable high optical powers while minimizing thermal losses. A large amount of waste heat is still generated and must be extracted. Custom ceramic microchannel-coolers (MCCs) are used to dissipate the waste heat. The custom ceramic MCC was designed to accommodate long cavity length diodes and micro-lenses. The coolers provide similar thermal performance as copper MCCs however they are not susceptible to erosion and can be cooled with standard filtered water. <p> </p>The custom ceramic micro-channel cooled array was designed to be a form/fit replacement for an existing copperbased solution. Each array consisted of three-vertically stacked MCCs with 4 mm CL, 976 nm diodes and beamshaping micro-optics. The erosion and corrosion resistance of ceramic array is intended to mitigate the risk of copperbased MCC corrosion failures. Elimination of the water delivery requirements (pH, resistivity and dissolved oxygen control) further reduces the system SWaP while maintaining reliability. <p> </p>The arrays were fabricated and fully characterized. This work discusses the advantages of the ceramic MCC technology and describes the design parameters that were tailored for the fiber-coupled application. Additional configuration options (form/fit, micro-lensing, alternate coolants, etc.) and on-going design improvements are also discussed.
We report on the development of a novel, ultra-low thermal resistance active heat sink (AHS) for thermal management of high-power laser diodes (HPLD) and other electronic and photonic components. AHS uses a liquid metal coolant flowing at high speed in a miniature closed and sealed loop. The liquid metal coolant receives waste heat from an HPLD at high flux and transfers it at much reduced flux to environment, primary coolant fluid, heat pipe, or structure. Liquid metal flow is maintained electromagnetically without any moving parts. Velocity of liquid metal flow can be controlled electronically, thus allowing for temperature control of HPLD wavelength. This feature also enables operation at a stable wavelength over a broad range of ambient conditions. Results from testing an HPLD cooled by AHS are presented.
Northrop Grumman Cutting Edge Optronics has developed large kilowatt class lensed laser diode arrays with subnanometer
spectral width using Volume Bragg Grating (VBG) reflectors. Using these CW arrays with 100W bars at
885nm, excellent absorption in Nd:YAG is achieved, with lower thermal aberration than can be attained with 808nm
pumps. The additional cost of the VBG reflectors and their alignment is partially offset by the much broader wavelength
tolerance that is allowed in the unlocked array enhancing bar yield. Furthermore, the center wavelength of the arrays
exhibit lower temperature sensitivity allowing the arrays to be operated over a wider current or temperature range than
arrays without wavelength control. While there is an efficiency penalty associated with the addition of VBGs of 5-8%, it
is more than compensated for by enhanced absorption, especially when used with narrowband absorption lines, such as
885nm in Nd:YAG. An overview of the design and manufacturing issues for arrays that are wavelength-locked with
VBGs is presented along with the effect of post-construction hard UV exposure.
Northrop Grumman Cutting Edge Optronics (NGCEO) has developed a laser diode array package with minimal bar-tobar
spacing. These High Density Stack (HDS) packages allow for a power density increase on the order of ~ 2.5x when
compared to industry-standard arrays. Power densities as high as 15 kW/cm<sup>2</sup> can be achieved when operated at 200
This work provides a detailed description of the duty factor, pulse width and power limitations of high density arrays.
The absence of the interposing heatsinks requires that all of the heat generated by the interior bars must travel through
the adjacent bars to the electrical contacts. This results in limitations to the allowable operating envelope of the HDS
arrays. Thermal effects such as wavelength shifts across large HDS arrays are discussed.
An overview of recent HDS design and manufacturing improvements is also presented. These improvements result in
reliable operation at higher power densities and increased duty factors. A comparison of the effect of bar geometry on
HDS performance is provided. Test data from arrays featuring these improvements based on both full 1 cm wide diode
bars as well as 3 mm wide mini-bars is also presented.
We report on the development of a novel active heat sink for high-power laser diodes offering unparalleled
capacity in high-heat flux handling and temperature control. The heat sink employs convective heat transfer
by a liquid metal flowing at high speed inside a miniature sealed flow loop. Liquid metal flow in the loop is
maintained electromagnetically without any moving parts. Thermal conductance of the heat sink is
electronically adjustable, allowing for precise control of diode temperature and the laser light wavelength.
This paper presents the principles and challenges of liquid metal cooling, and data from testing at high heat
flux and high heat loads.
Northrop Grumman Cutting Edge Optronics (NGCEO) has recently developed high-power laser diode arrays specifically
for long-life operation in quasi-CW applications. These arrays feature a new epitaxial wafer design that utilizes a large
optical cavity and are packaged using AuSn solder and CTE-matched heat sinks.
This work focuses on life test matrix of multiple epitaxial structures, multiple wavelengths, and multiple drive currents.
Particular emphasis is given to the 80x and 88x wavelength bands running at 100-300 Watts per bar. Reliable operating
points are identified for various applications including range finding (product lifetimes less than 1 billion shots) and
industrial machining (product lifetimes greater than 20 billion shots). In addition to life test data, a summary of
performance data for each epitaxial structure and each bar design is also presented.
One of the primary challenges of the Laser Inertial Fusion Engine (LIFE) project is the cost and availability of the laser
diode arrays needed to pump the solid-state laser gain media in the system. Current projections indicate that the arrays
need to be available for approximately one cent per Watt of output power, which is one to two orders of magnitude
cheaper than currently available.
This work focuses on potential manufacturing approaches to meet the projected specifications of the LIFE project.
Special attention will be paid to requirements related to power density (25 kW/cm<sup>2</sup>), bar pitch (150 - 400 microns),
output wavelength (87x), and fast-axis divergence (+/- 4 degrees). A summary of the supply limitations and cost
ramifications of each requirement is presented. Also discussed are potential supply chain limitations that are anticipated
as a result of the immense size of the LIFE project.
Northrop Grumman Cutting Edge Optronics has developed a family of arrays for high-power QCW operation. These
arrays are built using CTE-matched heat sinks and hard solder in order to maximize the reliability of the devices.
A summary of a recent life test is presented in order to quantify the reliability of QCW arrays and associated laser gain
modules. A statistical analysis of the raw lifetime data is presented in order to quantify the data in such a way that is
useful for laser system designers.
The life tests demonstrate the high level of reliability of these arrays in a number of operating regimes. For single-bar
arrays, a MTTF of 19.8 billion shots is predicted. For four-bar samples, a MTTF of 14.6 billion shots is predicted. In
addition, data representing a large pump source is analyzed and shown to have an expected lifetime of 13.5 billion shots.
This corresponds to an expected operational lifetime of greater than ten thousand hours at repetition rates less than 370
A family of laser diode arrays has been developed for QCW operation in adverse environmental conditions. The arrays
contain expansion-matched heatsinks, hard solder, and are built using a process that minimizes the packaging-induced
strain on the laser diode bars. The arrays are rated for operation at 200 Watts/bar under normal operating conditions.
This work contains test results for these arrays when run under a variety of harsh operating conditions. The conditions
were chosen to mimic those required by many military and aerospace laser programs.
Life test results are presented over a range of operating temperatures common to military specifications (-40 °C to + 70
°C) at a power level of approximately 215 Watts/bar. The arrays experienced no measurable degradation over the course
of the life test. Operation at the temperature extremes did not introduce any additional detectable failure mechanisms.
Also presented are results of characterization and reliability tests conducted at cryogenic temperatures. Diode arrays
have been subjected to repeated cycles in rapid succession between room temperature and 77 K with temperature ramp
rates up to 100 K/minute. Pre- and post- thermal cycle P-I-V data are compared. The results demonstrate the suitability
of these arrays for operation at cryogenic temperatures.
A next-generation microchannel cooler has been developed for packaging laser diode arrays that eliminates many of the
problems associated with typical copper-based cooling designs. The coolers are built on well-established Low-Temperature Cofired Ceramic technology and provide excellent thermal performance. They do not require the use of
This work highlights the strengths of the new cooler technology. The results of a long-term, high-flow-rate test which
demonstrates the excellent erosion resistance of these coolers are presented. Three devices have been tested for 2500
hours at a flow rate of 0.25 GPM and show minimal signs of erosion. This data is compared to a similar test conducted
with copper coolers.
Several design parameters are also addressed for the ceramic coolers. The available form and fit characteristics are
addressed, as is the custom-configurable nature of the devices.