Optical waveguides – Having particular optical characteristic modifying chemical... – Of waveguide cladding
Reexamination Certificate
1999-10-28
2002-05-14
Bovernick, Rodney (Department: 2874)
Optical waveguides
Having particular optical characteristic modifying chemical...
Of waveguide cladding
C385S141000, C385S144000, C528S353000
Reexamination Certificate
active
06389215
ABSTRACT:
MICROFICHE APPENDIX
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to polyimide compositions useful in optical waveguide structures and subsystems. More particularly, the invention relates to a polyimide film and process for making the film that are used as cladding or core in polymeric optical waveguides. Still more particularly, the polyimide cladding or core of the present invention provides high optical transparency, low birefringence, resistance to solvents, strong adhesion to solid substrates, and a high thermal stability in passive optical films.
2. Description of the Related Art
Electronic information transfer systems distribute data through electrical signals. Optical information transfer systems distribute data at high rates through photonic signals. Optical systems use optical waveguides to carry the data. Passive optical waveguides connect optical information processing devices. A glass optical fiber is a passive optical waveguide, for example. A ribbon of polymer on a glass substrate can also be a passive waveguide. Passive waveguides are used to split, combine, couple and route optical signals. These optical waveguides typically comprise a transparent core and a transparent cladding surrounding the core, with the cladding material generally having a lower index of refraction than the core material.
Desirable characteristic for optical waveguides include high optical transparency, low birefringence, resistance to solvents, strong adhesion to solid substrates, and a high thermal stability. Low optical transmission loss, including low optical absorbance and scattering, allow the optical waveguide to transfer data efficiently over large distances.
Birefringence is a measure of the difference in index of refraction between the two orthogonal polarizations that normally exist in the waveguide. Birefringence may result, in part, from the chemical structure of the polymer used in the waveguide cladding or core. For example, a linear (para substituted) chemical structure in the polymer backbone may result in orientation of the polymer chains in the plane of the film causing increased birefringence. Large birefringence is generally detrimental to waveguide performance, causing the two polarizations to have different properties, such as mode size and propagation velocity. For example, a mode size mismatch between the waveguide and the optical fiber reduces the efficiency of coupling. A difference in propagation velocity leads to dispersion of the optical signals and places an upper bound on the rate at which data may be effectively transmitted through the device. Birefringence is measured as a continuous scale with 0.0 being the ideal. Birefringence is a measure of the optical anisotropy of the waveguide. It also is normally desired to minimize the difference in birefringence between the core and cladding materials in planer waveguides. Polymer chains in flat waveguide configurations are often oriented in the plane of the film and this generally leads to negative birefringence in the film. High birefringence is normally undesirable for optical waveguides and may contribute to high optical scattering losses and dispersion. Polyimides are of interest because of their high thermal stability. Accordingly, polyimides having relatively low birefringence are especially useful in planar waveguide applications. Since polyimides generally have a stiff backbone, low birefringence is difficult to achieve.
Optical waveguides should possess high thermal stability, which permits the waveguide to endure the electronics packaging and assembly processes used in manufacturing. The optical waveguide material should also allow etching (wet or dry) of a rib in the core or of a channel in the cladding having smooth sidewalls. Additionally, it is desirable to overcoat multiple polyimide layers without degradation of the previous layers from the solvent used in the added overcoat layers.
Several patents have disclosed polyimide structures in waveguide applications. U.S. Pat. No. 5,649,045 discloses polyimide structures having increased birefringence though the use of BPDA (biphenyl dianhydride). U.S. Pat. No. 5,317,082 discloses photodefinable photosensitive copolyimides and waveguide structures thereof having a 6FDA/BTDA dianhydride component, an aromatic diamine component having bulky methyl groups ortho to the amine, and a fluorinated co-diamine component to reduce birefringence. U.S. Pat. Nos. 5,598,501 and 5,572,619 disclose electron-beam irradiation of the film to define the core region of the waveguide. U.S. Pat. No. 5,344,916 discloses polyimide and copolyimide films having negative birefringence for use in liquid crystal displays, and a method for increasing the negative birefringence by increasing the amount of PMDA (pyromellitidianhydride) in the dianhydride component. None of these patents discloses the advantages of the substantially meta-linked polyimide composition for low birefringence of the present invention.
SUMMARY OF THE INVENTION
The present invention provides an optical waveguide subsystem comprising at least one cladding in contact with at least one core, wherein the cladding has a refractive index less than the refractive index of the core, and, at least one of the cladding or the core comprises a crosslinked polyimide that is substantially meta-linked and has an absolute birefringence of from about 0.01 to about zero.
The present invention also provides a process for fabricating an optical waveguide subsystem comprising the steps of (a) forming a polyimide from about 1 mole percent to about 15 mole percent of a first diamine, wherein the first diamine possesses a crosslinkable site, from about 35 mole percent to about 49 mole percent of at least one additional diamine, wherein the combined first and additional diamines possess a substantial amount of meta-linking configuration and the combined mole percent of the first and additional diamines equals approximately 50 mole percent, and approximately 50 mole percent of a dianhydride; (b) forming a solution of from about 20 weight percent to about 50 weight percent of the formed polyimide with from about 50 weight percent to about 80 weight percent of a solvent, and optionally from about 0.5 to about 10 weight percent of a crosslinking agent; (c) filtering the formed solution through a submicron filter; (d) spin-coating the filtered solution at from about 200 rpm to about 8000 rpm to form a film on a substrate; and, (e) baking the film at a final temperature which is the higher of from about 5° C. to about 30° C. above the glass transition temperature of the polyimide or approximately about 220° C. for time period of from about 30 minutes to about 240 minutes. The polyimide may optionally be photocrosslinked.
The present invention provides crosslinked polyimide structures that are substantially meta-linked in optical waveguides for low birefringence and controllable refractive index properties. Meta-linked refers to the bond placement along the backbone of the polyimide, described below.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides optical waveguide subsystems and a process for making them. The optical waveguide subsystem comprises at least one cladding in contact with at least one core. Either the cladding or core, or both, comprises a crosslinked polyimide that is substantially meta-linked along the backbone. The cladding has a refractive index less than the refractive index of the core. The polyimide cladding, core or preferably both, has a low birefringence of from about 0.01 to about zero. With the cladding in contact with the core, additional cladding and/or core components may be included in the present invention as needed for a given purpose of the optical waveguide device.
The present invention comprises a cladding or core component that comprises a substantially meta-linked polyimide. The polyimide of the present invention is formed from the combination of diamines and dianhydride compounds. In a preferred embodiment, the crosslinked polyim
Chafin Andrew P.
Lindsay Geoffrey A.
Yee Rena
Zarras Peter
Bovernick Rodney
Pak Sung
Serventi Anthony J.
The United States of America as represented by the Secretary of
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