Polymer optical waveguides on polymer substrates

An optical waveguide is provided. The optical waveguide includes a polymer substrate and a lower cladding disposed on the substrate. The lower cladding is a first perhalogenated polymer. The optical waveguide also includes a core disposed on at least a portion of the lower cladding. A method of manufacturing the optical waveguide is also provided.

FIELD OF THE INVENTION

The present invention relates to polymer optical waveguides disposed on polymer substrates, and more specifically, to perfluorinated polymer optical waveguides.

BACKGROUND OF THE INVENTION

Optical waveguides can be formed in polymers by using a core polymer and a cladding polymer with the core polymer refractive index slightly higher than that of the cladding polymer in the near infrared region of the third optical telecommunication wavelength window (around 1550 nm). In order to form useful optical waveguide devices such as integrated splitters, couplers, arrayed waveguide gratings, and optical waveguide amplifiers, it is essential to have stable and low loss optical waveguides. The optical loss, or attenuation of an optical waveguide, originates primarily from two sources: 1) optical absorption by the core and cladding material and 2) optical signal scattering from the waveguide.

A general approach to making polymer optical waveguides is to dispose an undercladding polymer film layer on a silicon substrate and then a polymer core film layer on top of the undercladding layer. The polymer core layer film subsequently undergoes lithography and etching processes from which a rectangular cross-section channel is formed An overcladding polymer film layer is then disposed on top of the waveguide core and the exposed undercladding film layer.

It has been found that, during the processes of forming the undercladding, core and overcladding layers, such as spin coating and subsequent drying of solvents, temperature variations usually occur throughout the polymer waveguide layers. Such temperature variations cause polymer shrinkage or expansion in accordance with thermal expansion coefficients (CTE) of the polymer materials, which typically run between approximately 50 to 300 parts per million (ppm) per degree Celsius, depending on the particular polymer. Generally simultaneously, the waveguide substrate undergoes similar shrinkage or expansion as the temperature changes. However, in contrast to the CTE for polymers, the CTE for silicon is approximately 4.2 ppm per degree Celsius. The mismatch of CTE between the silicon substrate and the polymer waveguide claddings and core can cause polymer film cracking and stress build-up in the polymer layers. These effects will increase the polymer waveguide attenuation, preventing practical waveguide device application of polymer waveguides. This tendency can be further quantified through the following expression:
σf=Ef(CTEf−CTEs)(Tproc−Tamb)  Equation 1
where:σfis the stress in the film;Efis the elastic modulus of the film;CTEfis the CTE of the polymer film;CTEsis the CTE of the substrate;Tprocis the processing temperature; andTambis the ambient temperature.

It is believed that polymers have been used as a substrate as well as the waveguide disposed on the substrate. Keil et al. have disclosed fluoroacrylate-type polymers such as pentafluorostyrene, trifluoroethylmethacrylate, and glycidylmethacrylate disposed on a polymer substrate. However, these fluoroacrylate-type polymers contain numerous CH bonds. Polymers with CH bonds typically have high absorption in the infrared region where the optical communication signals reside, at approximately 1.5 μm. This absorption causes optical communication signal loss. To alleviate the signal loss problem, CF bonds are used to substitute the CH bonds in the polymer. Perfluorinated polymers have no CH bonds, resulting in extremely low absorption loss around the 1.5 μm infrared communication wavelength.

It is desirable to have a low loss optical waveguide in which the coefficient of thermal expansion of both the substrate and the polymer layers disposed oil the substrate are such that the polymer layers do not crack or develop high stress on the substrate.

BRIEF SUMMARY OF THE INVENTION

Briefly, the present invention provides an optical waveguide. The optical waveguide comprises a polymer substrate and a lower cladding disposed on the substrate. The lower cladding is a first perhalogenated polymer. The optical waveguide also comprises a core disposed on at least a portion of the lower cladding.

Additionally, the present invention provides a method of manufacturing an optical waveguide. The method comprises providing a polymer substrate; depositing a first perhalogenated polymer onto the substrate; depositing a first polymer onto the first perhalogenated polymer; and depositing a second polymer onto the first polymer.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings, like numerals indicate like elements throughout. U.S. Pat. No. 6,603,917 B2, filed on even date, which is owned by the assignee of the present invention, is incorporated herein by reference in its entirety. As used herein, the term “element” is defined to mean ions, atoms, isotopes, and species of atoms of the Periodic Table.

Referring toFIGS. 1 and 2, an optical waveguide assembly100is comprised of a polymer substrate10with a polymer optical waveguide20disposed on the substrate10. The waveguide20is comprised of a lower cladding22, a core24disposed on at least a portion of the lower cladding22, and an upper cladding26disposed on the core24and a remaining portion of the lower cladding22. Preferably, the lower cladding22, the core24, and the upper cladding26are all polymers, and more preferably, all perhalogenated polymers, and most preferably, perfluoropolymers.

Preferably, the substrate10is from the group consisting of polycarbonate, acrylic, polymethyl methacrylate, cellulosic, thermoplastic elastomer, ethylene butyl acrylate, ethylene vinyl alcohol, ethylene tetrafluoroethylene, fluorinated ethylene propylene, polyetherimide. polyethersulfone, polyetheretherketone, polyperfluoroalkoxyethylene, nylon, polybenzimidazole, polyester, polyethylene, polynorbornene, polyimide, polystyrene, polysulfone, polyvinyl chloride, polyvinylidene fluoride, ABS polymers, polyacrylonitrile butadiene styrene, acetal copolymer, poly[2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene] (sold under the trademark TEFLON® AF), poly[2,3-(perfluoroalkenyl)perfluorotetrahydrofuran] (sold under the trademark CYTOP®), poly[2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole-co-tetrafluoroethylene] (sold under the trademark HYFLON®), and any other thermoplastic polymers; and thermoset polymers, such is diallyl phthalate, epoxy, furan, phenolic, thermoset polyester, polyurethane, and vinyl ester. However, those skilled in the art will recognize that a blend of at least two of the polymers listed above, or other polymers, can be used. It is also preferred that the substrate10has a CTE of approximately between 50 and 300 parts per million per degree Celsius. Preferably, the substrate10is generally circular and is approximately between 7.5 and 15 centimeters (3 and 6 inches) in diameter.

Preferably, the lower cladding22is a halogenated polymer, more preferably a fluoropolymer, and most preferably, a perfluoropolymer including a perfluoropolymer from the group consisting of TEFLON® AF, CYTOP®, and HYFLON®, although those skilled in the art will recognize that other polymers or polymer blends can be used for the lower cladding22. It is also preferred that the lower cladding22has a CTE of approximately between 50 and 300 parts per million per degree Celsius. It is also preferred that the CTE of the substrate10and the CTE of the lower cladding22differ by less than approximately 40%. Such a difference greatly reduces thermal expansion differences between the substrate10and the lower cladding22, minimizing the likelihood of developing cracks and stress in the lower cladding22during further manufacture of the waveguide assembly100.

The core24is preferably a polymer, more preferably a halogenated polymer, and most preferably a perfluoropolymer. More preferably, for optical amplifier applications, the core24is constructed from a perfluoropolymer containing at least one rare earth element from the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Examples of preferred perfluoropolymers are disclosed in U.S. patent application Ser. Nos. 09/507,582, filed Feb. 18, 2000; 09/722,821, filed Nov. 28, 2000; and 09/722, 282, filed Nov. 28, 2000, and 60/314,902, filed Aug. 24, 2001, which are all owned by the assignee of the present invention and are all incorporated herein by reference in their entireties. However, those skilled in the art will recognize that other polymers containing at least one rare earth element can also be used. Further, the core24can be a blend of polymers including at least a first polymer containing one of the rare earth elements disclosed above and at least a second polymer, such as the polymer used as the lower cladding22.

The upper cladding26is preferably a polymer, more preferably a halogenated polymer, and most preferably a perfluoropolymer. More preferably, the upper cladding26is the same polymer or polymer blend as the lower cladding22. However, those skilled in the art will recognize that the upper cladding26and the lower cladding22need not necessarily be the same polymer, although it is preferred that the upper cladding26have the same, or nearly the same, refractive index nclas the lower cladding22.

Preferably, the lower cladding22and the upper cladding26have a common refractive index ncland the core24has a refractive index ncothat differs from the refractive index nclby a small enough amount such that the waveguide assembly100propagates a signal light λSin a single mode. For the case where the cladding layers22,26are homogeneous, with a single refractive index ncl, the relationship between dimensions of the core24and Δn (nco−ncl) is well-captured by the dimensionless V parameter, defined by:V=2⁢πλ⁢a⁢Δ⁢⁢nEquation⁢⁢2
where λ is the wavelength, preferably in nanometers, of light to be transmitted through the core24and α is the width of the core24, also preferably in nanometers. The V parameter must be less than 2.5 in order to achieve the single-mode condition. When Δn is large, α must be kept small to achieve V<2.5. Such a requirement may result in low optical efficiency coupling to an optical fiber, resulting in undesired signal loss. For a V of 2.5, with Δn of approximately 0.04, at a wavelength λ of 1550 nanometers, α is approximately 3000 nanometers, or 3 microns.

Preferably, the waveguide assembly100is adapted to amplify light for use in an optical amplifier, although those skilled in the art will recognize that the waveguide assembly100can be an optical splitter, an optical combiner, or other optical components that can be constructed from a waveguide. For such non-amplification uses, the core24need not contain the rare earth element as described above.

To manufacture the waveguide assembly100, the substrate10is first prepared. The surface of the substrate10is cleaned to remove any adhesive residue which may be present on the surface of the substrate10. Typically, a substrate is cast or injection molded, providing a relatively smooth surface on which it can be difficult to deposit a perfluoropolymer, owing to the non-adhesive characteristics of perfluoropolymers in general. After cleaning, the substrate10is prepared to provide better adhesion of the lower cladding22to the surface of the substrate10. The substrate10can be prepared by roughening the surface or by changing the chemical properties of the surface to better retain the perfluoropolymer comprising the lower cladding layer22. The preferred roughening method is to perform reactive ion etching (RIE) using argon. The argon physically deforms the surface of the substrate10, generating a desired roughness of approximately 50 to 100 nanometers in depth. The preferred method that changes the chemical properties of the surface of the substrate10is to perform RIE using oxygen. The oxygen combines with the polymer comprising the surface of the substrate10, causing a chemical reaction on the surface of the substrate10and oxygenating the surface of the substrate10. The oxygenation of the substrate10allows the molecules of the perfluoropolymer comprising the lower cladding22to bond with the substrate10. Although RIE with argon and oxygen is disclosed, those skilled in the art will recognize that other methods can be used to prepare the substrate10. Alternatively, the substrate10can be prepared by applying a fluorinated coupling agent, such as a fluorosilane, to the substrate10.

The lower cladding22is then deposited onto the substrate10. For a lower cladding22constructed from HYFLON® solid HYFLON® is dissolved in a solvent, perfluoro(2-butyltetrahydrofuran), which is sold under the trademark FC-75, as well as perfluoroalkylamine, which is sold under the trademark FC-40. Other potential solvents are a perfluolinated polyether, such as that sold tinder the trademark H GALDEN® series HT170, or a hydrofluoropolyether, such as that sold under the trademarks H GALDEN® series ZT180 and ZT130. For a lower cladding22constructed from other polymers, each polymer is dissolved in a suitable solvent to form a polymer solution. The polymer solution is then spin-coated onto the substrate10using known spin-coating techniques. The substrate10and the lower cladding22are then heated to evaporate the solvent from the solution.

Preferably, the lower cladding22is spincoated in layers, such that a first layer22ais applied to the substrate10and annealed to evaporate the solvent, a second layer22bis applied to the first layer22aand annealed, and a third layer22cis applied to the second layer22band annealed. Preferably, after all of the layers22a,22b,22care applied, the lower cladding22has achieved a height of between 8 and 12 micrometers. Although the application of three layers22a,22b,22care described, those skilled in the art will recognize that more or less than three layers22a,22b,22ccan be used.

Since adhesion of the lower cladding22to the substrate10can be poor due to the non-adhesiveness of the perfluoropolymer used as the lower cladding22, adhesion testing was performed to determine the adhesion of the lower cladding22to the substrate10. The testing was performed with several substrates; first, with a substrate that had not been cleaned or prepared prior to applying the lower cladding22, second, with a substrate that had been cleaned but not prepared prior to applying the lower cladding22, and third, with a substrate that had been both cleaned and prepared prior to applying the lower cladding22. For the prepared substrate, the preparation consisted of performing RIE with oxygen. The test consisted of taking a cross hatch cutter with four cutting edges and cutting through the lower cladding22and just into each substrate10, forming a first series of cuts. The cross hatch cutter was then taken at right angles to the first series of cuts to make a second series of cuts, forming a lattice pattern. The lower cladding22was brushed lightly to remove any loose flakes of lower cladding22. Tape with an adhesive strength of approximately 12.3 Newtons/25 mm (45.0 oz/in) was applied to the lattice pattern and removed. The lower cladding22on each substrate10was then viewed under a microscope.

Table I, below, provides results of the adhesion tests, which show that preparing the substrate10prior to depositing the lower cladding layer20virtually eliminated flaking of the lower cladding22from the surface of the substrate10.

TABLE ISubstrateISO ValueUncleaned and unprepared5Cleaned and unprepared5Cleaned and prepared0

An ISO value of 5 generally corresponds to any degree of flaking that exceeds 65% of the affected area after cross-cutting and an ISO value of 0 generally corresponds to the edges of the cross-cuts being completely smooth and none of the squares of the lattice being detached.

After the lower cladding22has dried, the rare earth containing core24is deposited onto the lower cladding22, preferably using the same technique as described above to deposit the lower cladding22onto the substrate10. However, instead of depositing several sub-layers of the core24onto the lower cladding22, preferably, only one layer of the core24is deposited onto the lower cladding22. Preferably, the core24is soluble in a solvent in which the lower cladding22is not soluble so that the solvent does not penetrate the lower cladding22and disturb the lower cladding22. For a core24constructed from CYTOP®, solid CYTOP® is dissolved in a solvent, such as perfluorotrialkylamine, which is sold under the trademark CT-SOLV 180™, or any other solvent that readily dissolves CYTOP®, forming a CYTOP® solution. Alternatively, CYTOP® can be commercially obtained already in solution. For a waveguide100that will be used as an optical amplifier, the rare earth containing perfluoropolymer is then blended with the CYTOP® solution and the combined rare earth containing perfluoropolymer/CYTOP® solution is applied over the lower cladding22. After the core24is dried, a preferred thickness of the core24and lower cladding22is approximately between 12 and 16 microns.

Next, the core24is etched to provide a desired core shape. Preferably, the etching is performed by RIE, which is well known in the art. However, those skilled in the art will also recognize that other methods of etching the core24may also be used. WhileFIG. 1discloses a generally straight core24, those skilled in the art will recognize that other shapes can be used, such as the curved waveguide shape disclosed in U.S. patent application Ser. No. 09/877,871, filed Jun. 8, 2001, which is owned by the assignee of the present invention and which is incorporated herein by reference in its entirety. Further, whileFIG. 2discloses a generally rectangular cross section for the core24, those skilled in the art will recognize that the cross section of the core24can be other shapes as well Preferably, the core24is not etched completely down to the lower cladding22, but a core layer24aof core material is retained for manufacturing purposes as will be described later herein.

Next, the upper cladding26is deposited onto the core24, the core layer24a,and any remaining portion of the lower cladding22not covered by the core24or the core layer24a.Preferably, similar to the lower cladding22, the upper cladding26is spincoated in layers, such that a first layer26ais applied to the core24and a remaining portion of the lower cladding layer22not covered by the core24and annealed to evaporate the solvent, a second layer26bis applied to the first layer26aand annealed, and a third layer26cis applied to the second layer26band annealed. Preferably, the upper cladding26is soluble in a solvent in which the core24and core layer24aare not soluble so that the solvent does not penetrate the core24and the core layer24aand disturb the core24or the core layer24a.The core layer24aprovides a barrier between the upper cladding26and the lower cladding22, so that, since the upper cladding26and the lower cladding22are preferably the same material, the solvent in which the upper cladding26is applied does not penetrate to the lower cladding22. Preferably, after all of the layers26a,26b,26care applied, the entire waveguide100has achieved a height of between 15 and up to approximately 50 micrometers. Although the application of three layers26a,26b,26care described, those skilled in the art will recognize that more or less than three layers26a,26b,26ccan be used. Alternatively, the upper cladding26can be a different material fiom the lower cladding22, but with approximately the same refractive index as the lower cladding22, for example, a photocuring fluorinated acrylate or a thermoset.

As can be seen inFIG. 2, the layers26a,26b,26care not necessarily flat, but contour around the core24with decreasing curvature for each successive layer26b,26c.Although the last layer26cis shown with a generally flat top surface, those skilled in the art will recognize that the top surface of the last layer26cneed not necessarily be flat. Those skilled in the art will also recognize that single layer claddings with high degrees of flatness or planarization can be achieved by either spincoating or casting processes.

After forming the waveguide100, the waveguide100is cut to a desired size and shape, preferably by dicing. As shown inFIG. 1, a desired shape is generally rectangular, although those skilled in the art will recognize that the waveguide100can be cut to other shapes as well.