BLOCKS of glass may soon replace the electrical switchgear that handles
fibre-optic signals in telephone exchanges. Researchers at Harvard University
say they can burn 鈥渨aveguides鈥� into blocks of glass, forming the equivalent of
wires to carry light around optical circuits.
Optical fibres can carry many telephone conversations and other streams of
data at once by encoding each stream on different wavelengths of light. To
redirect the data at an exchange, these data streams have to be split up and
processed. Because there aren鈥檛 any circuits that can do this optically, the
signals must be converted into electrical pulses, processed electronically, and
then converted back into light.
Now Chris Schaffer and Eric Mazur at Harvard believe they have come a step
closer to making an entirely optical circuit by creating waveguides inside a
block of glass. They released their findings at a meeting of the Materials
Research Society in San Francisco earlier this week.
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鈥淚f you use a very, very short laser pulse and you focus it tightly in the
glass, it deposits energy there, heats the material and melts it,鈥� says
Schaffer. Each pulse from the laser melts a sphere of glass around 10
micrometres in diameter, he says.
To burn a waveguide into the glass, you simply scan the laser across it,
melting a column within the block. The glass at the edge of the column cools
first, forming low-density glass with a low refractive index. This increases the
pressure on the molten glass in the centre of the column, which then cools to
form a denser glass with a higher refractive index. This high-density glass
forms a waveguide for optical signals. 鈥淎s you move the laser across the
surface, you can make a waveguide that runs through the glass beneath the
surface,鈥� says Schaffer.
鈥淲e end up with waveguides around 8 micrometres in diameter, which is ideal
for telecommunications,鈥� says Schaffer. The researchers were able to write
waveguides into the glass up to 1 centimetre beneath the surface by using a tiny
lens to change the focal point of the laser.
They also found they could make junctions between waveguides, and thus create
complete optical circuits within the glass block. To test the junctions, the
researchers created three waveguides that crossed over at their midpoints. 鈥淲hen
you launch light down one waveguide, it leaks into the other two at the
intersection and you get three outputs,鈥� says Schaffer. The team now plans to
build a device that splits different wavelengths of light, separating out the
data streams.
鈥淭he holy grail is to be able to switch light into different channels, and
this is a step towards that,鈥� says Karen Ness of the Institute of Photonics at
the University of Strathclyde. 鈥淭he way things are going, we鈥檒l be dealing with
96 or more channels,鈥� she adds. 鈥淚t could be easier to lay down these structures
within a block.鈥�
