Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2001-11-28
2002-10-29
Ullah, Akm E. (Department: 2874)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
Reexamination Certificate
active
06473551
ABSTRACT:
The present invention generally relates to thin film optical waveguides and methods for forming thin film optical waveguides.
BACKGROUND OF THE INVENTION
Thin film optical waveguides are increasingly used to provide advanced functions in optical communications networks such as, for example, splitting, wavelength multiplexing and demultiplexing, optical add/drop multiplexing, variable attenuation, modulation, switching and amplification. Thin film optical waveguides provide reduced size and higher reproducibility than conventional fiber-based solutions.
Polymers of high optical quality may be used to form thin film waveguides using low temperature lithographic processes. Polymeric thin film waveguides can be used to make splitters, wavelength multiplexers and demultiplexers, optical add/drop muxes, variable attenuators and switches, when coupled with thermo-optic actuation. Various polymers have also been developed to address modulation and amplification functions.
Typically, thin film polymer waveguides comprise multiple adjacent thin films formed by, for example, spin coating. In the production of thin film optical waveguides, it is desirable to build successive layers of material with slightly different optical properties, such as refractive indices, in order to tailor the optical properties of the particular optical waveguide. For example, one can make a thin film optical waveguide, or other structures that confine and channel light, from certain polymers by forming polymers having different compositions on top of one another. The refractive index of a guiding, or core, layer must be higher than the refractive index of buffer and cladding layers so that the optical modes can be conducted into the core layer by internal reflection.
A conventional method for forming a thin film polymeric waveguide is to select different co-polymers within a particular polymer/co-polymer system for adjacent thin film layers. That is, for each layer, one selects polymers/co-polymers with different refractive indices. However, in conventional thin film polymeric waveguides, the various polymers/co-polymers have similar composition and thus they also have similar solubility. For fabrication, the polymer that will eventually form the thin film layer is dissolved in a suitable solvent before spin coating. The polymer/solvent solution is spin coated onto a substrate and then subsequently dried, for example, on a hot plate or in an oven, to remove the solvent, leaving the thin film layer behind. The drying process, however, typically causes stress fields in the resultant polymer thin film layer. In addition, when the next layer of polymer is applied, the underlying dried polymeric thin film can experience significant additional stress from solvent swelling.
Solvent swelling readily occurs if the polymers in the two adjacent polymer thin film layers are substantially soluble in the same solvent. This effect is aggravated as additional layers having substantially the same solubility in the solvent are added. These stresses cause optical scattering, resulting in performance degradation. Solvent swelling also causes crazing, cracking, and de-lamination of the entire polymeric waveguide structure, resulting in greatly reduced reliability, and in some cases, component failure.
In addition to stress caused by solvent swelling, selection of different co-polymers within a particular polymer/co-polymer system for adjacent thin film layers may cause a dissolution zone where polymers from the upper layer and the lower layer are present. In this case, a loss of a distinct boundary occurs between the adjacent polymer layers. In conventional spin casting processes, there is little or no control of the make-up and uniformity of this dissolution layer. Uncontrolled dissolution of the polymers of adjacent layers can form localized pockets with high concentrations of the upper layer polymer and low concentrations of the lower layer polymer. If the amount of dissolution is extensive and occurs in an uncontrolled manner, it may promote scattering of light out of the waveguide.
One method of reducing the effects of solvent swelling to minimize stress is to deposit a very thin barrier layer between the lower and upper polymer layers. This method, however, requires additional processing steps to form the barrier layer to prevent ingress of the upper layer solvent into the lower polymer layer. Another method of stress reduction is to apply drying regimes in an attempt to produce a fully relaxed polymer layer that would not retract when exposed to the solvent used to form the next polymeric layer. These drying regimes, however, include long drying cycles, for example, and may require baking the substrate at high temperatures after each successive film deposition for two to three days in order to fully relax the polymer layer.
Thus, there is a need to overcome these and other problems of the prior art and to provide thin film optical waveguides with improved performance and reliability. The present invention, as illustrated in the following description, is directed to solving one or more of the problems set forth above.
SUMMARY OF THE INVENTION
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, in one exemplary embodiment, the invention relates to an optical element comprising a first polymeric layer and a second polymeric layer adjacent to the first polymeric layer and a method for making the optical waveguide are disclosed. The first polymeric layer has a refractive index of n
1
and solubility S
1
in a first solvent. The second polymeric layer has a refractive index of n
2
and solubility S
2
in the first solvent. In the optical waveguide, n
1
≠n
2
and S
1
/S
2
is at least 5.
Another exemplary embodiment of the invention relates to an optical waveguide having a first polymeric layer, a second polymeric layer adjacent to the first polymeric layer, and a third polymeric layer adjacent to the second polymeric layer. The first polymeric layer has a refractive index of n
1
, a solubility of S
1
in a first solvent, and a solubility of S
1′
in a second solvent. The second polymeric layer has a refractive index of n
2
, a solubility of S
2
in the first solvent, a solubility of S
2′
in a second solvent. Further, n
1
≠n
2
and S
1
/S
2
is at least 5. The third polymeric layer has a refractive index n
3
, a solubility of S
3′
in a second solvent. In addition, n
2
≠n
3
and S
2′
/S
3′
is at least 5.
Yet another exemplary embodiment of the invention is an optical waveguide having a first polymeric buffer layer, a first polymeric sublayer adjacent to the first polymeric buffer layer, a polymeric core layer adjacent to the first polymeric sublayer, a second sublayer adjacent the core layer, and a second buffer layer adjacent the second sublayer. The first polymeric buffer layer has a refractive index of n
1
, a solubility of S
1
in a first solvent, and a solubility of S
1′
in a second solvent. The first polymeric sublayer has a refractive index of n
2
, a solubility of S
2
in the first solvent, a solubility of S
2′
in a second solvent and the ratio of S
1
/S
2
is at least 5. The polymeric core layer has a refractive index of n
3
and a solubility of S
3′
in the second solvent, wherein n
3
>n
2
, n
3
>n
1
, and S
2′
/S
3′
is at least 5.
One exemplary embodiment of the invention, is an optical waveguide formed by providing a first polymeric layer, wherein the first polymeric layer has a refractive index of n
1
. Also, a second polymeric layer having refractive index of n
2
adjacent to the first polymeric layer is formed by providing a solution over the first polymeric layer. The solution comprises a first solvent that dissolves about one percent or less of the first polymeric layer. In this embodiment, the value of n
1
does not equal the value of n
2
.
Another exemplary embodiment of the invention relates to a method for making an optical waveguide comprising depositing a first pol
Norwood Robert A.
Teng Chia-Chi
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Photon-X, Inc.
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