Engineers from Harvard University (Cambridge, MA) have developed a new class of Raman laser small enough to operate on a photonic chip.1 The optical component uses a nanoscale racetrack-shaped diamond resonator to convert one frequency of laser light to an entirely different range of wavelengths, opening up new possibilities for broadband data communications and other applications.
According to the research team, this demonstration marks diamond as only the second material besides silicon to show Raman lasing in a completely integrated photonic chip.
Current on-chip lasers used in telecommunications operate at a narrow range of wavelengths around 1.55 µm. Though efficient, this limits the amount of data that can be transmitted through optical fibers. The ability to transmit and manipulate light across a broader range of wavelengths would help to alleviate some of the growing bottlenecks in telecommunications.
An efficient means of achieving this change of color or wavelength of a laser is through stimulated Raman scattering. If enough optical energy is pumped into a material, a small fraction of the input light loses energy to atomic vibrations and is shifted to a specific lower frequency. This results in amplification for the lower frequency shifted wave, which when combined with an optical resonator can yield a Raman laser. Such Raman lasers are well-known in optics and have applications in medical devices, chemical sensing, and telecommunications.
Giant color shifts
Though Raman lasing has been achieved in silicon, this material is not transparent across a wide range of colors, limiting its use to a few specific applications. Diamonds, on the other hand, are transparent across the ultraviolet, visible, and infrared parts of the electromagnetic spectrum. Diamond can also use Raman scattering to provide giant color shifts across the entire spectrum. However, Raman lasers in diamond are traditionally made from bulky plates in macroscopic cavities. They also require careful alignment of components and comparatively high energy to operate. These factors have limited the ways they can be integrated into chip-based technologies.
To make the diamond microresonators, the researchers used standard nanofabrication techniques. A diamond thin film was bonded to the surface of a silicon chip and the desired pattern written via electron beam lithography and etched onto the surface using an oxygen plasma.
"The manufacturing process is quite simple and enables us to produce resonators of multiple shapes and sizes that could be easily integrated into existing optoelectronic technologies," said Pawel Latawiec, Lončar Laboratory, Harvard University and co-author of the paper.
The Raman laser reported in the paper works at wavelengths near 2 µm, which has been identified for next generation optical telecommunication networks.
The researchers also report that since diamonds are transparent across almost the entire optical spectrum, the operating principle they demonstrate can be readily translated to other wavelength ranges simply by using different pump lasers.
Source: OSA
REFERENCE:
1. Pawel Latawiec et al., Optica (2015); doi: http://dx.doi.org/10.1364/OPTICA.2.000924.