MICROSTRUCTURED FIBER OPTICS: Polymer-fiber cores are doped with QDs and silica nanoparticles

Researchers at the University of Sydney (Sydney, Australia), CeramiSphere, the University of New South Wales, and the Australian Nuclear Science and Technology Organization (all in New South Wales, Australia) have developed a method to incorporate active dopants—including materials incompatible with the polymer matrix such as quantum dots (QDs) and silica nanoparticles—into the cores of microstructured polymer optical fibers (mPOFs).¹ The breakthrough combines the low-temperature, low-cost manufacturing attributes of mPOF with the many capabilities enabled by QDs and silica nanoparticles.

Previous attempts to incorporate these particles into mPOF typically involved applying QDs or nanoparticles in solution to the cladding holes of the fiber; however, light-guiding interactions were minimal and usually limited to evanescent-field effects. The new process instead embeds dopant particles in the mPOF core, achieving a homogeneous, controlled, and fixed spatial distribution that maximizes the interaction of the nanoparticles with the guided light.

Drawing, drawing again

The fiber-fabrication process begins with an 11-mm-diameter polymethylmethacrylate (PMMA) intermediate preform with a central air hole surrounded by six air holes (see figure). The particle-doped material is formed into rods of approximately 5 mm diameter sleeved with another PMMA tube and drawn down to a diameter of 2.5 mm. This 2.5 mm rod is then placed in the core air hole of the intermediate mPOF preform and further drawn to the desired final fiber diameter.

To create the silica nanoparticles, sol-gel and emulsion technology are combined. A silicon alkoxide solution is hydrolyzed and then polymerized, which results in the encapsulation of rhodamine isothiocyanate (RITC). The RITC dye-doped silica-nanoparticle solution (with particle diameters on the order of 50 nm) is mixed with PMMA solution in a 0.12-weight-percent ratio of RITC to PMMA and then evaporated and ground to powder to eliminate solvents. The powder is then fused under vacuum into the 5 mm rod used in the final fabrication process.

The QD-doped core rods were synthesized in a similar process, but began with commercially available “Hops Yellow” QDs purchased from Evident Technologies (Troy, NY) that were combined with PMMA in a 0.017-weight-percent QD mixture.

The doped-core rod was inserted into the center hole of an intermediate preform and then drawn to a final fiber diameter of 400 µm and a core diameter of 130 µm. Spectral analysis of the resultant fibers produced results consistent with the optical characteristics of the dopant materials.

This new fabrication process enables such future applications as the insertion of rare-earth materials for the amplification of optical fibers, the creation of in-fiber single-photon sources for quantum communications, and magneto-optically active fibers for use in optical switching and optical isolator devices.

“Now we’re able to simultaneously tailor the microstructure pattern and the material composition of mPOFs to extend fiber properties beyond what was previously possible,” says researcher Helmut Yu. “We are currently developing a number of exciting applications that will take advantage of these properties.”

REFERENCE

1. H.C.Y. Yu et al., Optics Express15, 16, 9989 (Aug. 6, 2007).