A Sound Boost to Extreme Lasers

Technical set-up of a Brillouin diamond laser. (Source: Macquarie U.)

Researchers from Macquarie University have discovered a novel and practical approach to Brillouin lasers with output power already ten times higher than all other Brillouin lasers. Brillouin lasers rely on an effect in materials in which light and sound strongly interact to provide light amplification. To date, though, such lasers have been low power, and often seen as a curiosity. Indeed, in many systems the light-sound inter­action is seen as a nuisance because the effect spoils laser system per­formance when increasing power.

Diamond is a parti­cularly interes­ting material for this type of laser for two key reasons. Its high thermal conduc­tivity means it is possible to make miniature lasers that simul­taneously have high stability and high power. The speed of sound is also much higher compared with other materials. This gives the laser a secondary ability to directly synthesise fre­quencies in the hard-to-reach millimeter wave band. Now, the researchers show that the light-sound interaction is particularly strong in diamond, and have demons­trated the first bench-top Brillouin laser that uses diamond.

This result provides a highly practical approach to Brillouin lasers with a greatly increased range of perfor­mance. In contrast to earlier Brillouin lasers, the diamond version operated without having to confine the optical or sound waves in a waveguide to enhance the inter­action. This means Brillouin lasers can be more easily scaled in size and with much greater flexi­bility for controlling the laser properties as well as increasing power. Diamond provides a new way to begin to exploit the unique properties of Brillouin lasers. Only a very small amount of waste energy is deposited in the sound-carrying material. This leads to a host of features including beam gene­ration with ultra-pure and stable output frequency, the gene­ration of new frequencies, and poten­tially, lasers with excep­tionally high effi­ciency.

Macquarie University’s Rich Mildren says “This develop­ment provides a new pathway towards high power lasers that are extremely efficient and have exquisite frequency properties such as low-phase noise and narrow linewidth. These are properties needed for appli­cations that demand the highest standards of noise-free frequency properties, like ultra-sensitive detection of gravi­tational waves or mani­pulating large arrays of qubits in quantum computers.”

Another outcome is that the diamond can synthesise very pure frequencies beyond the microwave band. As a conse­quence of the very high speed of sound in diamond – a dashing 18 km/s – the frequency spacing between the input pump beam and the laser line is many times higher than in other materials. This property can be used to generate frequencies in the milli­meter wave band (30-300 GHz) using photo-mixing. Brillouin laser synthesis of these frequencies is important because there is an intrinsic mechanism that reduces the frequency noise to the levels needed by next-generation radar and wireless communi­cation systems. This has been a major challenge for electronics or other photonic-based generation schemes.

The work so far has quantified the strength of the light-sound inter­action in diamond, a funda­mental parameter for predicting future design and performance. It also demonstrated a practical device with over 10 W of power. Zhenxu Bai, lead PhD student on the project, says “We can now begin to think about the design of Brillouin lasers in a new way, rather than as a pheno­menon limited to small guided wave structures or as a detri­mental effect in fibre lasers.” The scientists are concentrating their future work on expanding the range of laser capability by demons­trating lasers with the higher levels of frequency purity and power needed to support future progress in quantum science, wireless communi­cations and sensing. (Source: Macquarie U.)

Reference: Z. Bai et al.: Diamond Brillouin laser in the visible, APL Phot. 5, 031301 (2020); DOI: 10.1063/1.5134907

Link: MQ Photonics Research Centre, Macquarie University, Sydney, Australia

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