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FOCUS TECHNOLOGY
‘BICSEL’ holds promise for faster computing and telecom links
R
esearchers at the University of California San Diego have demonstrated a new type of laser that has the potential to be more compact and energy efficient than the
standard vertical-cavity surface emitting lasers (VCSELs) used in many computing and optical networking links. Te new laser is based on an unconventional
physics phenomenon called ‘bound states in the continuum’ (BIC), which are resonant states. First proposed in quantum mechanics theory in 1929, only recently was it realised BICs are a general wave phenomenon that could also be applied to optics. In a previous study, the UC San Diego researchers demonstrated, at microwave frequencies, that BICs could be used to efficiently trap and store light. Now, they’re harnessing BICs to demonstrate new types of lasers at telecom wavelengths. ‘Te popular VCSEL may one day be replaced by
what we’re calling the “BICSEL” – bound state in the continuum surface-emitting laser – which could lead to smaller devices that consume less power,’ claimed Boubacar Kanté, electrical engineering professor at the UC San Diego Jacobs School of Engineering who led the research. Until now, ‘bound in continuum’ light states
have only been created in passive systems such as waveguides. In the latest research, Kanté and colleagues constructed a nanostructured material in which such bound states in a continuum are used to produce laser action at room temperature for the first time. Te team has filed a patent for the new type of light source. To make a laser, light is trapped between mirrors
or in a cavity. Tis confinement is vital as it allows the light to build in intensity so that stimulated
Left: BIC laser schematic: A high frequency laser beam (blue) powers the membrane to emit a laser beam at
telecom frequency (red); Right Researchers at UC San Diego demonstrate the first laser using bound states in the continuum. Left to right: Yeshaiahu Fainman, Boubacar Kanté, Ashok Kodigala and Babak Bahari
emission – lasing – occurs. BICs are resonant waves that defy conventional wisdom, by remaining localised even though they are in an open system where other frequencies can carry energy away. Te BIC laser in this work is constructed from a
thin semiconductor membrane made of indium, gallium, arsenic and phosphorus. Te membrane is formed from an array of cylinders, each about 500nm across, interconnected by a network of supporting bridges that provide mechanical stability. By powering the membrane with a high-frequency laser beam, researchers induced it to emit its own laser beam at around 1550nm. ‘Tis is a proof of concept demonstration that we can indeed achieve lasing action with BICs,’ Kanté said. Te team’s next step is to make BIC lasers
electrically powered by mounting the membrane on a conductive substrate. Currently, the device’s efficiency is about 20 per cent, though researchers believe it can reach much higher values. Te scaling of the BIC lasers is unique and can
enable much smaller devices, Kanté added. ‘What’s remarkable is we can get surface lasing to occur
with [nano-resonator] arrays as small as 8 × 8.’ In comparison, VCSELs and other types of surface- emitting lasers need larger (100 times) surface areas – and thus more power – to achieve lasing. BIC lasers could be designed to emit beams with
specially engineered shapes such as spiral, doughnut or bell curve – called vector beams – which could enable optical communication links that carry up to 10 times more information. Vector beams possess orbital angular momentum, a property that can be exploited as another degree of freedom in the design of optical links. Systems based on orbital angular momentum have been shown to scale to terabit capacities using mode- division multiplexing. Te lasers could be developed as high-power
lasers for industrial and defence applications, and for use in research labs to explore novel light– matter interaction effects. ‘Because they are unconventional, BIC lasers offer unique and unprecedented properties,’ Kanté concluded. Teir work was reported inNature: doi:10.1038/nature20799.l
Fine-tuning network design can save wavelength resources O
ptimising the design of networks that use distance-adaptive transmission can save
50 per cent of wavelength resources compared with fixed-rate transmission, according to research to be presented at the upcoming Optical Fiber Conference (OFC) in Los Angeles on 19–23 March. A researcher from Nokia Bell
Labs in Murray Hill has developed a mathematical model that could improve the flow of internet traffic generated by cloud computing by optimising the placement of data centres while adopting distance- adaptive transmission technology. In optical systems, capacity and
reach are antagonistic – increase one and you decrease the other. High-capacity, shorter-reach
channels also occupy more spectrum. Te latest coherent transmission systems offer distance-adaptive transmission – also referred to as flex-coherent or flex-grid transmission – to optimise total system capacity by adjusting the spacing between channels to avoid leaving unused spectrum. However, network design does not typically consider the distance
that data must travel, despite the fact that shorter distances can support higher rates. Yet as data traffic grows in volume, the industry has become increasingly aware of the limitations of this mode of transmission. Experts estimate the amount of data stored ‘in the cloud’, in remote data centres around the world, will quintuple in the next five years. ‘Te challenge for legacy systems
Issue 15 • Spring 2017 FIBRE SYSTEMS 11
Kanté group at UC San Diego
Kanté group at UC San Diego
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