Researchers at Columbia University and IBM's Thomas J. Watson Research Center built an optical parametric amplifier (OPA) on a single silicon chip.
Scanning electron microscope cross-section of a 700 x 425 nm silicon waveguide. The color map illustrates the Ey electric field component of the fundamental transverse-magnetic mode at 2.2 µm. Silicon dioxide and silicon oxynitride cladding layers sandwich the silicon waveguide core.
An optical amplifier dramatically boosts the power of mid-infrared light in a compact package. Researchers at Columbia University and IBM's Thomas J. Watson Research Center built an optical parametric amplifier (OPA) on a single silicon chip (Nature Photon. 4, 557). This technology could increase the use of 2- to 3-µm wavelengths for biochemical sensing, medical diagnostics and imaging, and free-space optical communications. It may also, eventually, be useful for telecommunications.
William Green's group at IBM demonstrated the silicon nanophotonic OPA with broadband gain as high as 25.4 dB, using a 2.2-µm pump. Their four-wave-mixing approach can boost signals over wavelengths from 2.05 to 2.250 µm to as much as 400 times their original strength. The gain is high enough to compensate all insertion losses, resulting in 13-dB net off-chip amplification, using only a 4-mm silicon chip.
IBM has been developing silicon photonics for on-chip optical interconnects, a method of bypassing the bottleneck of using electricity to transport signals between microelectronic chips. The silicon devices, made using established wafer-scale fabrication processes, could enable other applications in the mid-IR that need compact broadband sources or amplifiers.
The researchers believe that they can engineer higher-order waveguide dispersion to allow mid-infrared-pumped silicon optical parametric oscillators (OPOs) and OPAs to work for telecom-band optical signals. The current on-chip amplifiers do not work for these wavelengths (around 1.5 µm) due to parasitic two-photon absorption in silicon: Although silicon is transparent in the near-IR, at high intensities two photons work together to excite an electron into the conduction band, and these free carriers absorb light.
In the same issue of Nature Photonics, Sanja Zlatanovic and others at the University of California, San Diego, and the University of Central Florida reported another, lower-power OPO on a silicon chip, in the mid-IR (Nature Photon. 4, 561). While the IBM researchers pumped their device with a Ti:sapphire laser, Zlatanovic's group managed to generate light as long as 2.388 µm using compact light sources (a diode laser at 1.3 µm and a fiber laser, followed by an erbium-doped fiber amplifier, at 1.589 µm) developed for telecom applications.
Yvonne Carts-Powell is a freelance science writer who specializes in optics and photonics.