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Buried waveguides wire-up silicon

22 Jun 2005

Scientists use an ultrafast laser to create waveguides in crystalline silicon that could enable 3-D optical circuitry.

A laser processing technique that writes low-loss optical waveguides in crystalline silicon could one day enable 3-D optical circuits in a chip say its developers, Friedrich-Schiller-Universit├Ąt, Germany, and University of Toronto, Canada. By focusing mid-infrared femtosecond laser pulses at depths of up to 370 microns below the surface, scientists were able to induce refractive index changes and form buried waveguides. (OPTICS LETTERS 30 964)

Although such waveguides have been fabricated previously in glass, the team says that this is the first time they have been written in crystalline silicon. In the future, they could be used to link optoelectronics devices on a silicon chip.

"This bulk [silicon substrate] is a valuable and unexploited resource where low index contrast waveguides can be formed in multiple layers to create optical routing with negligible cross-talk," Amir Nejadmalayeri from The University of Toronto's Photonics Group told Optics.org. "With recent developments of the silicon Raman laser and silicon GHz optical modulators, our technology will facilitate the integration of CMOS electronics and basic photonic building blocks."

The set-up features a chirped-pulse-amplified Ti:sapphire laser that delivers 70 fs pulses with an energy of 600 µJ at 810 nm. Because silicon is opaque at this wavelength, the researchers use an optical parametric amplifier to shift the output to 2400 nm. By operating in the mid-infrared, the team ensures that the pulse energy is delivered to the laser's focal point with minimal energy loss to the surrounding material.

Taking a crystalline silicon wafer coated with a 20 µm thick silicon oxide (silica) layer, the group focussed its laser at depths of up to 370 µm from the silica-silicon interface. By moving the sample at a speed of 2 mm/min, the scientists managed to write buried optical waveguides. Tests with 1320 and 1550 nm sources show that the resulting waveguides have damping losses with an upper bound of 1.2dB/cm.

Interestingly, images of the silicon wafer's end facet revealed that the laser-inscribed waveguides appeared at distances of 5 to 20 µm below the silica-silicon interface irrespective of focal depth. Currently, the group is busy working to overcome this limitation and is open to scientific collaborations and commercial partnerships.

Author
James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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