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DigiLens Creates 'Liquid Gratings'

Light Reading
News Analysis
Light Reading
10/23/2000

Plenty of startups are trying to integrate wavelength multiplexing with other means of manipulating light on the same chip, but DigiLens Inc. has come up with a totally different way of doing it .

Like other component developers, it’s splitting light into different wavelengths using Bragg gratings. But unlike others in this field, it’s forming these Bragg gratings in liquid crystals rather than solid substrates such as silica and silicon. As the characteristics of the liquid crystal can be modified by applying an electric current, Digilens can split off a specific wavelength and then adjust its power or switch it in a single operation.

"We’re compressing functionality onto a waveguide that previously would have required multiple separate devices," says Jonathan Waldern, CEO of DigiLens. This means that Digilens’s components will be simpler to make and thus lower in cost, he adds.

Digilens calls its technology electrically-switchable Bragg grating (ESBG) or “S-Bug". Components are made by creating a row of ESBGs, each one handling a specific wavelength, on top of a single silica waveguide. The ESBGs themselves are formed from a mixture of polymer and liquid crystal, which have Bragg gratings (a series of stripes of different refractive index) created in them to reflect back specific wavelengths. This is done by exposing the polymer and liquid crystal mixture to ultraviolet light from intersecting laser beams, which form an interference pattern. The liquid crystal diffuses to areas of high light intensity, creating microdroplets.

In use, when a voltage is applied to this arrangement, the refractive index of the microdroplets is reduced, effectively erasing the grating and letting all the light through. With no applied voltage, the grating diffracts light at a specific wavelength out of the waveguide.

Digilens has been using this technology for making microdisplays for some time, and now it's moving into the telecom market.

It's starting by developing dynamic spectral equalizers (DSEs), devices that extend the range of dense wavelength-division multiplexing (DWDM) systems by ensuring that power levels are the same in every channel. It's aiming on shipping 6- and 10-channel models this quarter.

At least two other vendors already make DSEs. One of them, Corning Inc. (NYSE: GLW) uses liquid crystals to adjust power levels in the same way as Digilens. But it requires a string of other devices to split the light into different wavelengths and handle polarization issues (see What's Hot At The OFC). The other vendor, Ultraband Fiber Optics Inc., is basing its developments on acousto-optical tunable filters.

Digilens is also planning on making an electronic variable optical attenuator, a single channel version of its DSE, in the same time frame.

In the longer term, Digilens is planning to develop 1x1 and 2x2 optical switches suitable for restoration and protection functions, with a target shipment date in 2002.

The benefits of ESBGs become apparent in optical add-drop muxes (OADMs), which Digilens is also planning to develop. In today’s OADMs, the wavelengths must be broken out so that each one can be directed by a separate 2x2 switch. Finally, all the wavelengths must be recombined. Waldern claims that DigiLens could do all this in a single integrated component. For a six-channel device, this would reduce the component count by a factor of eight, and reduce the cost by about a third, he says.

DigiLens licenses the intellectual property rights for the ESBG from Science Applications International Corp. (SAIC), an independent R&D company which owns Telcordia Technologies Inc. (formerly Bellcore). SAIC is the biggest investor in DigiLens with a 10 percent stake.



Digilens recently raised a fourth round of $40 million, bringing total funding to date to $67 million (see Round 4 for DigiLens: $40M).

-- Pauline Rigby, senior editor, Light Reading http://www.lightreading.com

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