OPTICAL FIBERS: High-speed electronics are built into fiber

A technique to deposit precisely doped semiconductor materials into microstructured optical fibers (MOFs) is coming of age, demonstrated by gigahertz-bandwidth photodetectors built directly into a fiber. The work is the product of a long-standing collaboration between Penn State University (University Park, PA) and the University of Southampton (Southampton, England), who first reported basic electronic functionality inside MOFs in 2006.¹

The effort as a whole is designed to sidestep the practical difficulties in integrating the optical fibers with semiconductor chips, as Pier Sazio of Southampton explains. “The optical fiber is just normally seen as a passive photon-transport mechanism,” he says. “Your fiber would come onto a chip and you’d do the electronic conversion on-chip and you have to go back out onto the fiber.” For many applications that’s a difficult process—matching up round fibers to flat semiconductor optoelectronics that might be 100 times smaller, adding not only complexity but expense.

Capillary electronics

The MOFs consist of a central core surrounded by a hexagonal array of empty capillaries along the fibers’ length. The team’s technique of putting the electronics directly into the fiber has seen significant advances toward a long-imagined future in which light is generated, modulated, processed, and detected within only fibers. Their approach makes use of the MOFs’ capillaries as the space in which to build up exactly the functionality required. They use high-pressure chemical-vapor deposition to build up semiconductor devices layer by layer within the capillaries; dopants and materials are mixed with carrier gases and forced through the capillaries at pressures as high as 35 MPa.

In recent years, the team has determined how to adjust the recipe to not only get precisely controlled films and doping levels, but also to create sharp junctions between different structures along the fiber’s length. For the new work, they’ve put the technique to work by depositing platinum and doped silicon and germanium into MOFs. The result? “What we’ve done here is integrated semiconductor functionality inside as a high-speed photodiode,” Sazio says.

Using focused-ion-beam technology, they were able to insert electrodes to pull out the electrical signal that the semiconductor detector generated. They note that electrical contacts could be made using existing fiber side-polishing or femtosecond-laser machining techniques, so that contacts can be made not only at the fibers’ end faces but anywhere along their length. To test the diode’s response, a commercial supercontinuum source was filtered to produce 10 ps pulses at telecommunications wavelengths. The diodes exhibited a roughly 60 ps rise time and 100 ps fall time, for an estimated bandwidth of 3 GHz.²

But for the purposes of moving the technique out of the lab, it’s not just about building semiconductor structures into the fibers—it’s about doing it cheaply, without the usual infrastructure around semiconductor manufacturing. “We circumvented having to use a multimillion-dollar lithographic cleanroom to develop this thing,” Sazio explains.

Modulators next

As an advance in process development, the group’s work bodes well for other types of devices as well, and Sazio and his colleagues have their sights set on modulators next. The team has already succeeded in depositing high-quality crystalline direct-bandgap materials like zinc selenide into MOFs, opening the possibilities not only for electro-optic modulation but also for emission and wavelength conversion.

“It just shows how the materials improvements have mapped onto device functionality,” Sazio says. “The fact that we can integrate this sort of active device inside suggests that we can move this further along in terms of many other things you might want to put inside the fiber.” The Southampton and Penn State researchers are already discussing ways to commercialize the technology.

REFERENCES
1. P.J. A. Sazio et al., Sci., 311, 1583 (2006).
2. R. He et al., Nat. Photon., doi:10.1038/nphoton.2011.35.