A Cooler Raman Laser

P ower dissipation, which results in heat generation in electronic chips, is the number one difficulty facing the semiconductor industry. The problem is so severe that it threatens to halt the huge development that has been enjoyed since the microprocessor industry’s inception — a doubling in processor performance roughly every 18 months, which was predicted by the celebrated Moore’s law. And this obstacle is not unique to microprocessors. The laser, another technological marvel that we rely on so heavily in today’s world, is plagued by the very same problem. Now, however, Nathalie Vermeulen and colleagues have proposed a way of reducing unwanted heat generation in an important class of laser, the Raman laser1. The essence of the dilemma sounds almost trivial, but its solution has eluded scientists for decades. The optical power produced by most lasers is only a small fraction of the power they consume, and the rest is wasted as heat. The problem is most severe in optically pumped lasers — the high-power workhorses of industrial material processing, medicine, directed-energy weapons and scientific research. This predicament is particularly difficult because it originates from fundamental physics rather than a technological imperfection. The source of heating is the so-called quantum defect: the output (Stokes) photons have lower energy than the input (pump) photons. The difference in energy unavoidably goes into heating the medium. This energy loss and associated temperature rise decreases the lasing efficiency and leads to other undesirable effects, such as deterioration of beam quality through a process known as thermal lensing. The problem is exacerbated by another inconvenient principle: as the temperature of the laser medium rises, its thermal conductivity drops. In other words, the hotter it becomes, the more heat it retains. This undesirable feedback can, in its worst form, damage or even destroy the lasing medium. To deal with this, high-power lasers must usually be equipped with elaborate cooling mechanisms, the cost, complexity and size of which can dwarf those of the laser itself. Vermeulen and colleagues1 have now proposed a solution to reduce heat generation in powerful optically pumped Raman lasers that generate wavelengths beyond the reach of other lasers. Their approach, an intrinsic heat mitigation mechanism, exploits an elegant nonlinear phenomenon called coherent anti-Stokes Raman scattering (CARS) to minimize the heat generated by the quantum-defect phenomenon. If it can be realized in practice, this approach may improve the performance, reliability and lifespan of Raman lasers. By reducing the need for thermal management, it also has the potential to facilitate device miniaturization. In Raman lasers, light amplification is based on stimulated Stokes Raman scattering (SSRS), a process in which atomic vibrations (phonons) in the medium mediate the transfer of power from the pump light at a frequency of ωp to the output (Stokes) beam with a lower frequency of ωs. This process, shown in Fig. 1a, leaves the medium in the excited Textbooks suggest that heating, caused by phonon emission, is an inevitable and intrinsic by-product of light generation in a Raman laser. Now a design has emerged that reduces the phonon emission and may lead to higher efficiency and smaller devices. LAseR design