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17 Jun 2002
Polymers are finding their way into a whole host of components for optical communications, either on their own or in combination with conventional substrates. Mike Cowin looks at the advantages of this material.
From Opto & Laser Europe September 2001
Polymers are flexible materials. Not only do they exhibit mechanical flexibility, they also enable flexible production processes and their physical properties can easily be manipulated on a molecular level. All of these make polymers ideal for integration into optical components for applications such as wavelength-division multiplexing.
A number of start-ups are developing hybrid integration products that mix and match an array of different material-based technologies that are all built around a polymer photonic platform.
The photonics industry has found it hard to accept polymeric
materials, preferring to stick to what it knows best - silica. Recently, several start-up firms around the
world - especially in the US - have received funding to develop polymer-based components. However, the Heinrich Hertz Institute (HHI) in Berlin, Germany, dominates activities in
Europe. "Photonic devices
based on this material system will show more functionality than conventional ones. For example, you
can make low-power thermo-optic switches; electro-optic modulators with a greater bandwidth than
those based on lithium niobate; athermal devices, such as all-polymer-arrayed waveguides operating
without Peltier coolers; and active devices such as erbium-doped waveguide amplifiers." With polymers, the simple blending and copolymerization of suitably synthesized
monomers offers a superior index range that not only allows highly compact components but also
offers the optical designer greater freedom in building up polymer photonic structures amenable to
integration with a range of hybrid materials. However, it is the large thermo-optic coefficient and
low thermal conductivity of polymers that gives them real added value. Respectively, these are at least
30 times as much as, and one-tenth of the magnitude of that of silica. It is this thermal advantage that
sets polymers apart from the rest for thermally actuated components, giving devices such as
"thermo-optic switches with one-hundredth of the power consumption of that of equivalent silica-based
devices", according to Keil. Low-power switches are not the only product for which polymers
are suitable. The material's large thermal-index gradient can be used to make tunable filters with an
increased tuning range and variable optical attenuators with a greater dynamic range than conventional
components. This thermal advantage has led to a new class of hybrid devices developed at HHI
by Keil and his team. These are based on the integration of silica and polymer waveguides. The
vertically coupled switch array is based on the vertical coupling between a polymer waveguide and an
underlying silica waveguide, such that the low-power variation of the polymer's thermal index can be
combined with the low loss of silica. Terahertz Photonics aims to
bring out its first product by the end of 2002. "It will be a thermo-optically tunable filter that will use
glass, polymer and thin-film-coating technology," said Tooley. Meanwhile in the US, Rick
Tompane, CTO of Gemfire, explained his company's intention to build up a comprehensive
hybrid-platform technology: "We believe that polymers will represent an important element in a matrix
of materials, each suited to a particular task. A well integrated photonic circuit might consist of GaAs
lasers, InP detectors, lithium niobate gratings and garnet isolators all connected by polymer or glass
waveguides." So, while most traditional integrated photonic firms seem eager to be recognized
as one-material specialists, these new polymer companies are quite content to be seen cherry-picking
the best of the rest and then linking them all together with polymers. This pragmatic approach is
echoed by Louay Eldada, CTO and co-founder of US-based start-up Telephotonics. He said: "We see
polymers as being the basic platform. However, using polymers to do everything is unrealistic. We are
not blind supporters of polymers - we only use them when it makes sense. Our approach is to have
everything on a chip but to graft different materials to achieve functions in the ideal
material." This realistic approach also extends to making tough choices as to which material is
best for the job. Telephotonics intends to use garnate isolators but lithium niobate for electro-optic
modulation rather than electro-optic polymers. "Electro-optic polymers have not been proven to
be stable. It's a case where you have one good characteristic, such as a large electro-optic coefficient,
and basically you don't talk about the others. At 85°C and above you lose the electro-optic effect and
therefore the device," said Eldada. Another US company, Pacific Wave, is investigating these issues
and it expects to release its first commercial 40 GHz integrated-polymer modulator early next
year. When it comes to volume production, polymer photonics does seem to have
a head start on competing technologies. Using standard photolithographic equipment, the whole
waveguide production cycle is reduced to a simple three-step process using photosensitive
polymers. Unlike many material systems, polymers can be deposited onto most substrates with
waveguide and device formation achieved at low temperatures. Even on their own, polymers can
outperform silica. Keil and his team from HHI together with researchers from the Fraunhofer
Institute for Reliability and Microintegration, Germany, recently demonstrated an all-polymer athermal
arrayed-waveguide grating on a polymer substrate. The group believes that its all-polymer grating
shows a better performance than athermal silica-on-silicon approaches published up to now. So
perhaps it is time for the photonics industry to accept that polymers have advantages over conventional
materials. Whether used on their own or combined with other materials, the flexibility that polymers
offer in the manufacture of photonic integrated circuits is clear.
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