Microtechnology for compact, efficient high-power lasers

High power lasers are valuable tools for cutting and welding in industrial materials processing. Along with high power, an ideal laser would have a relatively short wavelength to provide a small spot size and would be both safe and convenient to operate. At present, however, no laser simultaneously meets all of these requirements. Commonly-used CO₂ lasers offer high power but a long wavelength; solid state lasers offer a shorter wavelength but limited power. The chemical oxygen iodine laser (COIL) offers both high power and a short wavelength, but its disadvantages (large often-inefficient hardware that is powered by chemical reactants rather than by electricity) have prevented its use in industry.

Mixing is a significant challenge for COIL lasers. Gaseous and liquid reactants must mix inside a chemical reactor called a singlet oxygen generator (SOG) to produce the pump species, singlet delta oxygen. This must then mix with the lasing species, iodine. Typical SOGs mix the reactants by bubbling the gas through the liquid, spraying jets of liquid through the gas, or rotating liquid-wetted surfaces through the gas.^1,2 Ideally the liquid would react uniformly. In practice, some liquid does not react, while other parts of the liquid react too much and form crystals that can clog the reactor. We examined a different approach to COIL that avoids many of these mixing challenges through fundamental physical scaling: we replaced the conventional hardware with high-performing arrays of microdevices in an approach that we call microCOIL.

Microdevices (also known as microelectromechanical systems, or MEMS) are fabricated using techniques derived from integrated circuit processing, but their capabilities can include combinations of electrical, mechanical, thermal, and fluidic functions. The benefits of MEMS can include smaller hardware or performance advantages that are enabled by favorable scaling. For microCOIL, we identified the parts of the system that could benefit from scaling to smaller sizes and designed, modeled, and—in the case of the SOG—experimentally demonstrated its performance.

We began by designing and modeling a MEMS-based COIL system.³ This included SOGs, supersonic mixing nozzles to mix the singlet delta oxygen with the lasing species, an optical cavity, and microscale steam ejectors to pump the flows through the system. The results showed that MEMS-based COIL offers several benefits that provide the potential for compact, efficient, high-performing systems. Enhanced mixing at the microscale and the high thermal conductivity of silicon enable some of the benefits. The ability to integrate MEMS components tightly offers lower losses between components for better overall system performance. Finally, small devices offer more benign failure modes than do larger devices. The largest single benefit came from the replacement of the macroscale SOG with an array of microchemical reactors. A schematic diagram of a microscale SOG is shown in Figure 1.

Figure 1. Schematic diagram of the microscale singlet oxygen generator (microSOG), shows the reactant distribution manifold, the array of packed bed channels where the reaction takes place, and the gas-liquid separator. (Figure courtesy Diana Park.)

To demonstrate the potential of the microCOIL approach, we built and tested a three-wafer silicon and pyrex microSOG (Figure 2) that mixes the gas phase and liquid reactants in an array of microstructured packed-bed reaction channels, separates the liquid byproducts from the exiting gas flow via an array of microscale capillary pores, and removes the excess heat of reaction through an array of integrated heat exchangers.⁴ The system works as shown in Figure 3; the microSOG produces and extracts singlet delta oxygen while maintaining a typical chip temperature below 0°C. The microSOG was also shown to work at ratios of liquid-to-gas-phase reactant very close to the theoretical limit for salt crystallization, which supports the claim of highly uniform mixing.

Figure 2. Photograph of the completed microSOG chip.

Figure 3. Photograph of the microSOG chip during operation. The flow channels glow red due to dimol emission when singlet delta oxygen is present.

The singlet delta oxygen concentration downstream of the SOG was determined by quantitatively-calibrated measurements of its spontaneous emission spectrum in collaboration with Physical Sciences Inc. The concentration was used to place a lower bound on the yield (the fraction of oxygen in the singlet delta state) and to determine the singlet delta oxygen flow rate. These results indicate a singlet delta yield of up to 78% at the outlet of the microSOG chip and a per-chip power in the flow at the chip outlet of greater than 1.3 W.

High performance MEMS devices offer significant benefits for more-compact efficient high-power chemical lasers. So far, we have shown that microSOGs can equal or exceed the performance of conventional macroscale SOGs. Our next steps are to create improved second generation microSOGs, to create additional elements of the MEMS-based chemical laser system to demonstrate lasing, and to expand to larger arrays of microcomponents for higher power levels.