ARLINGTON, Va.—The power and performance of lasers are continually increasing to meet the expanding requirements of diverse future technologies. Their capacity for handling high power is critical to a host of applications requiring beam strength of a kilowatt and above—such as power beaming, directed energy, management of space debris, space propulsion, clean energy generation (laser fusion), artificial “guide-stars” for astronomy, and other uses yet to be discovered.
Sponsored by the Office of Naval Research (ONR) Global and the Asian Office of Aerospace Research and Development (AOARD), scientists from the Department of Physics and Astronomy of Macquarie University in Australia—led by Professor Rich Mildren—are studying the optical and laser properties of diamond. This material possesses extreme thermal properties, including a thermal conductivity that is 100 times that of most optical materials.
Dr. Simin Feng, science director ONR Global Tokyo, said, “This research will enrich our understanding of the rapid heat-dissipation mechanism in diamond—and improve future designs of high-power diamond laser with high quality of output beam.
“In this project the researchers discovered a new method to generate random states (quantum random polarization states),” she added. “This discovery has potential application for quantum random number generation, which is one of the critical ingredients for quantum information technology.”
Diamond sits well apart from well-known gemstone materials such as ruby, sapphire and garnet, which are robust and high-heat-handling laser hosts. Diamond has a problem, however, that its lattice structure is too dense to allow the introduction of suitable active laser ions.”
Professor Mildren states, “Having demonstrated efficient diamond lasers previously (greater than 50 percent), and at powers up to 1.2 kilowatts for short bursts, our main objective now is to increase laser power and understand its performance limits in terms of power and coherence.
“The stimulated scattering light gain mechanism is perhaps more challenging to harness than their more widely studied stimulated emission counterparts,” he continued, “but offers a large variety of possibilities—including lasers with very high power density and high beam quality, large wavelength range and narrow linewidth.”
Main challenges and opportunities
The main challenge for diamond lasers is derived from the fact that the transfer of input power to the generated laser beam occurs in a fundamentally different way compared to conventional lasers. This has the attraction that there is good scope to discover new phenomena. At the same time, however, the systems optimize differently and theoretical models specific to this system need to be developed to guide design as we extend the performance range.
The project from Macquarie University is currently focusing on determining the main factors that will ultimately limit power and beam quality, and influence the output spectrum. They have recently discovered that the stimulated scattering readily occurs in diamond via a second process: Brillouin scattering, in addition to the more usual Raman scattering, which brings a range of new possibilities for extending the range of laser capability.
Hence, the studies are opening a range of new directions. According to Professor Mildren, the focus is currently on teasing-out the unexpected and extreme properties of diamond, whether it is in high-power or in quantum science.
With respect to increasing power, Mildren said, “We have recently found that thermal lensing starts to impact upon performance at power levels of a kilowatt. Hence, we are looking at ways to increase power beyond that level by either designing around the thermal lens or mitigating its strength by adjusting material properties.”
“We’ve also found that the diamond lasers are extremely good at producing high-purity laser frequencies with simultaneous high power,” he continued. “With diamond able to lase at almost any wavelength, this is interesting for applications that require highly coherent sources such as cold atom physics and laser guidestar beacons.”
Mildren added, “In the area of quantum science, we have recently shown a diamond laser configuration that directly produces truly random pulses with their polarization direction (through the randomness of a quantum statistical seed). This may have interesting applications in areas such as quantum key distribution and quantum simulation.”
Dr. Feng concluded, “This is exactly the beauty of basic research. With a small amount of seeding funding and creative minds, we can get several spin-off projects for a wide range of applications. In this example, we are funding a high-power laser project for directed-energy application, but we are getting three extra projects for three different applications in quantum technology, electronic warfare, and space technology.
ONR Global sponsors scientific efforts outside of the U.S., working with scientists and partners worldwide to discover and advance naval capabilities.