Taming the Thermal Behavior of Solid-State Military Lasers
Laser diodes and laser diode arrays (LDAs) are widely used throughout military systems, for sighting and range finding, but also at high power levels as offensive and defensive weapons. In companion with much higher-power chemical lasers, these high-power optical sources can generate kilowatts of optical power (as arrays) for use in neutralizing incoming missiles, rockets, and armaments and for long-distance offensive strikes. But laser diodes are also still inefficient, and a great deal of the energy supplied to a laser diode or an LDA is converted into heat, which must be safely dissipated to ensure a long operating lifetime for the solid-state devices.
Fortunately, effective thermal management of high-power laser diodes and LDAs can be achieved through a proper understanding of the thermal mechanisms of the materials surrounding a laser diode, such as its circuit substrate and packaging materials. By choosing materials with high thermal conductivity to surround a high-power optical source, a good deal of the heat that it generates can be safely dissipated into the surrounding hardware and environment.
For any laser diode or LDA, the thermal load is a function of the “on” time of the diode, which is typically pulsed with narrow pulse widths and at high pulse repetition frequencies (PRFs). Materials with high thermal conductivity, such as copper and tungsten, are used as building materials for laser diode packaging, in order to provide a form of “thermal channel” away from the active device that is generating the heat. Laser diodes are notoriously inefficient. Their efficiency is 50%, meaning that one-half of the power that is supplied to a laser diode results in optical power, but the other one-half of the input power generates heat, which must be dissipated.
The Need for TECs
In telecommunications applications, where laser diodes may be in service for 20 years or more, thermoelectric coolers (TECs) are routinely designed and used in those applications to provide an active form of cooling as part of an optical telecommunications link. In such applications, where lasers provide signals for transmission over optical cables, often with different-wavelength laser diodes side by side and feeding the same optical cable, the thermal characteristics of each laser diode (such as drift) must be tightly controlled. Thus, the need for TECs.
Although the requirements are no less critical in military applications, the type of thermal control needed is somewhat different, where larger concentrations of generated heat from kilowatt lasers must be efficiently removed to preserve the operating characteristics and reliability of the heat-generating laser diode or LDA. Such heat removal depends not only on the thermal characteristics of the heat-removing materials but often also on the three-dimensional (3D) structures into which those materials are formed. The effectiveness of those structures can contribute a great deal to the efficiency of the heat removal.
For a material's basic thermal characteristics, different materials can be compared in terms of their thermal conductivity, which is how well they conduct heat. It is measured in watts (W) of power per meter (m) of material per degree Kelvin (K) or W/m-K. They can also be compared by the inverse of thermal conductivity, or thermal resistance, which is how well a material resists the flow of heat through it, or acts as an insulator against heat. As noted earlier, copper is a good thermal conductor, with high thermal conductivity of about 400 W/m-K. Another good thermal conductor, aluminum, has thermal conductivity of about 235 W/m-K while silver, an outstanding thermal conductor, is at 429 W/m-K.
Because of the rigorous demands placed on equipment in military applications, any form of heat-dispensing packaging used to surround highpower laser diodes or LDAs must also provide mechanical protection for them, and so a material with high mechanical strength is preferred if it can also provide good thermal conductivity. A comparison of copper and aluminum illustrates the typical tradeoffs in developing cooling structures and packages for high-power laser diodes: aluminum provides strength but sacrifices some of the thermal conductivity of copper.
By combining 3D structural design with thermally conductive materials, structures with cavities for air or some form of cooling fluid have been developed to more efficiently flow heat away from an active device, such as a power transistor or a high-power laser diode. Microchannel coolers (MCCs) are cooling structures formed of materials with low thermal resistance, such as copper bonded to different types of ceramic substrates, with internal chambers or cavities to accommodate the flow of a cooling fluid, such as deionized (DI) water. By increasing the flow rate of the cooling fluid through the cavity, the effective thermal resistance of the cooling structure is reduced and its cooling effectiveness is removed. The capability to achieve MCCs using materials with high thermal conductivity in miniature footprints is particularly important in support of the trend in higher-power laser diodes and LDAs for smaller-sized packages for military and aerospace applications.
MCCs for high-power laser diodes are typically composed of some combination of copper and ceramic substrate. The cooling structures are generally designed with multiple layers of ceramic and thermally conductive metal such as copper or aluminum, with different types of ceramic substrates lending different mechanical properties to the overall structure. Common ceramic substrates for MCCs include alumina (Al2O3) and aluminum nitride (AlN), with each exhibiting different mechanical properties in terms of strength and durability under thermal stresses.
The bond between the copper and the ceramic substrate is very important for the long-term reliability of MCCs used in any laser-diode cooling application because of the thermal stresses experienced during the on-off cycling of the high-power laser diodes and LDAs. The mechanical structure of an MCC (Figure 1) may be quite intricate, with multiple metal and ceramic layers, in order to form a large number of potential thermal pathways and fluid cavities to draw heat away from the laser diodes. Two proven processes are used to attach thermally conductive copper films to ceramic substrates of different types. In the direct bond copper (DBC) approach, pure copper is melted at high temperature and diffused onto the ceramic substrate. These attachment processes take place at extremely high temperatures, well in excess of +1000°C.
Due to the high densities of heat generated by such small devices as laser diodes, multiple cooling structures such as MCCs are often required in circuits for laser-based weapons systems to prevent thermal hotspots and build-up within the circuits and system. Not only can the reliability and the performance of the laser diodes be degraded by the excessive heat, but RF, analog, and digital circuitry in close-enough proximity to unchecked heat sources can also be affected, often requiring arrays of MCCs to complement the arrays of laser diodes mounted on a PCB.
Employing Multiple MCCs for Cooling
When multiple MCCs are used with laser diodes or other high-power active devices for cooling, they can be arranged in serial or parallel configurations, with or without cooling fluids. In series, cooling occurs from the inlet to the outlet of an MCC, with the thermal energy of each laser diode adding from beginning to end of the series of devices. In parallel, which often is a simpler layout to achieve, heat flows crosswise to the inlet and outlet of the MCC. Both configurations provide effective cooling with the choice of configuration largely dependent upon the circuit layout and placement of the laser diodes or LDAs.
Modern liquid-cooled materials such as curamik® CoolPerformance and curamik CoolPerformance Plus (Figure 2) from Rogers Corp. provide the capabilities to dissipate large amounts of heat in small areas, as generated by high-power laser diodes and LDAs. MCCs formed with these materials employ multiple layers of copper and ceramic materials, using 3D micro-channel, water-cooled structures to transfer the heat away from the active devices. While MCCs based on these materials may not be capable of handling the enormous thermal outputs of the chemical lasers in kilowatt-output weapons systems, they can readily dissipate thermal energy to about 200W in lower-power circuits using high-power diodes and LDAs, greatly extending the expected operating lifetimes of those devices and helping to minimize the thermal effects of those high-power optical devices on surrounding circuits.
This article was written by Manfred Goetz, Product Marketing Manager, Rogers Power Electronics Solutions (Eschenbach, Germany). For more information, visit here .