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21 Mar 2006
Researchers in the US have come up with a microfluidic waveguide that relies on the thermal gradient across its core and cladding liquids to channel light.
Liquid-based, optical waveguides that can be reconfigured in real-time are being developed by a team at Harvard University, US. The device is said to be much less sensitive to manufacturing flaws thanks to the optically smooth liquid interface between core and cladding fluids. (Appl. Phys. Lett. 88 061112)
"One advantage of a liquid waveguide is that the optical and physical properties of the core and cladding can be changed simply by introducing different fluids," Sindy Tang told Optics.Org. "Varying the thermal gradient across the two components provides another parameter to control the waveguide's output."
In the Harvard device, the core and cladding liquids are identical (water), but introduced into the channel at different temperatures to provide a contrast in refractive index. Using the same liquid for both components of the waveguide means that the fluids are easy to reclaim and re-use.
The core liquid was supplied directly at room temperature (21 degC), whereas the cladding fluid was preheated and injected at various temperatures from 30 to 80 degC. Fabricated from a polymer called PDMS, the waveguide's microfluidic channel measures 5 mm long and has a flow area of 400 x 125 µm.
Light from a laser diode (635 nm) or quartz halogen lamp was coupled into the waveguide using a singlemode optical fiber and imaged with a CCD to assess transmission. The scientists found that the efficiency of light confinement in the core liquid improved with an increase in total flow rate from 6 to 60 ml/hr. Higher flow rates help to reduce thermal diffusion across the channel's width and preserve the difference in refractive index.
The Harvard group feels that the waveguide's principles can be extended to other optical structures such as gradient index lenses, optical splitters or switches and is currently working on a range of designs.
Author
James Tyrrell is News Editor on Optics.org and Opto & Laser Europe magazine.
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