Optical waveguides – Planar optical waveguide
Reexamination Certificate
2001-05-01
2002-11-26
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S131000
Reexamination Certificate
active
06487354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical waveguides, or more particularly to lithographically formed single-mode optical waveguides employing organic polymeric materials. The waveguide structure has a low propagation loss.
2. Technical Background
In optical communication systems, messages are transmitted by carrier waves at optical frequencies that are generated by such sources as lasers and light-emitting diodes. There is interest in such optical communication systems because they offer several advantages over conventional communication systems. They have a greatly increased number of channels of communication as well as the ability to transmit messages at much higher speeds than electronic systems using copper wires. This invention is concerned with the formation of light-transmissive optical waveguide devices. The operation of an optical waveguide is based on the fact that when a medium which is transparent to light is surrounded or otherwise bounded by another medium having a lower refractive index, light introduced along the inner medium's axis is highly reflected at the boundary with the surrounding medium, thus producing a guiding effect.
It is possible to produce polymeric optical waveguides and other optical interconnect devices which transport optical signals in optical circuitry or optical fiber networks. One method used to form an optical device involves the application of standard photolithographic processes. Photopolymers are of particular interest for optical interconnect applications because they can be patterned by photolithographic techniques which are well known in the art. Photopolymers also offer opportunity for simpler, more cost effective manufacturing processes. Lithographic processes are used to define a pattern in a light-sensitive, photopolymer containing layer deposited on a substrate. Among the many known photopolymers, acrylate materials have been widely used as waveguide materials because of their optical clarity, low birefringence and the ready availability of a wide range of monomers.
Planar polymer waveguides typically comprise layers of low loss optical materials of precise indices of refraction. Both step index and gradient index waveguide structures are known in the art. For planar polymer and glass waveguides, in particular, step index structures are most easily achieved through successive coating of materials with differing indices of refraction. Typically, the core has a refractive index which is 0.5% to 2% higher than the clad. The magnitude of this refractive index difference (&Dgr;n) is set to optimize the performance of the planar waveguides or to match light modes when the transition is made from the planar device to an optical fiber. In reality, most planar waveguide structures have a configuration where the underclad is applied first, followed by application and definition of the core layer, and followed finally by application of the overclad. Due to the height of the core, the overclad typically has a bump on it that can be quite large. This can occur in polymer waveguides in which polymers must be spin cast from a solvent solution due to their high molecular weight and viscosity. It can also occur in silica waveguides in which chemical vapor deposition of the overclad applies a uniform layer on top of the rib. In addition, reactive-ion etching of polymer or glass waveguide cores can result in high propagation losses due to scattering of light caused by rough sidewalls.
Waveguides can be made using photopolymerizable optical materials which can be coated and cured on a substrate. Typically, the materials include mixtures of monomeric and oligomeric components which are blended to provide the correct index of refraction. Mixtures are blended to provide a &Dgr;n between core and clad, of typically 0.5 to 2 percent. In the photolithography of these curing mixtures, typically a guiding region having an index gradient instead of a step index can be formed in the underclad layer. Also, a region can form at the side and the top of the core in which an index gradient is found instead of a step index. The formation of the gradient index in the region surrounding the core is due to migration of dissimilar chemical components, particularly a monomer component moving from the core layer into the cladding layers. In the region directly under the core, the monomer component can further react during the formation of the core forming an unwanted guiding region within the undercladding layer. When the lower clad region is of about the same thickness as the core, a guiding layer can be formed that penetrates the full thickness of the clad. In extreme cases it can be as intensely guiding as the core itself and allows light to reach the substrate surface. Since the substrates of this invention may be absorbing at optical wavelengths of importance to telecommunications, any portion of the propagating light that reaches the substrate is subject to absorption. Absorption of light by the substrate leads to a severe undesirable polarization-dependent loss of optical power from the propagating signal.
Other attempts have been made in the art to resolve these issues. One potential solution is using a thick undercladding layer to isolate the core from the substrate to prevent this undesirable result. Eliminating the problem to the desired degree, however, requires the use of an impracticably thick undercladding. Another solution includes using a buffer region with an index which is 2% or more lower than the core, wherein the buffer region is below the underclad. Even if monomer diffusion occurs deeply through the underclad and slightly into the buffer, the guiding in the buffer will be greatly suppressed, eliminating most light absorption by the silicon. However, the underclad can still guide light and multimode waveguides with residual polarization effects can still result.
One method of lithographically forming optical elements uses an acrylic photoactive composition which is capable of forming a waveguide material upon polymerization. However, this utilizes polymers with as high a glass transition temperature as possible in order to provide for the greatest operating temperatures. Another method involves the production of waveguides using light polymerizable compositions such as acrylics having a Tg of at least 100° C. The foregoing waveguides suffer from undesirably high optical loss.
SUMMARY OF THE INVENTION
The invention provides a single-mode optical waveguide fabricated on a substrate wherein the substrate defines a surface. The single-mode optical waveguide comprises a polymeric buffer layer on the surface of the substrate, wherein the buffer layer defining a surface and having an index of refraction n
b
. A thin, polymeric undercladding layer is on the surface of the buffer layer, wherein the undercladding layer defining a surface and having an index of refraction layer n
u
. A pattern of a light-transmissive single-mode polymeric core is on the surface of the undercladding layer, wherein the core defines a top surface and sidewalls and wherein the core has an index of refraction n
c
. A polymeric overcladding layer is on the top surface of the core and on the sidewalls of the core and on a portion of the undercladding layer and having an index of refraction n
o
. The undercladding layer has a thickness of from about 10 percent to about 50 percent of a thickness of the core. The core index of refraction n
c
is greater than the index of refraction of the overcladding layer n
o
and also greater than the index of refraction of the undercladding layer n
u
. In the waveguide, &Dgr;n=n
c
−n
o
, and the difference between n
c
and the index of refraction of the buffer n
b
is at least about 1.5 times &Dgr;n, and the value of &Dgr;n is such that it produces a single-mode waveguide at optical communication wavelengths.
The invention also provides a method for forming a single-mode optical waveguide on a surface of a substrate. The method comprises the steps of depositing a polymeric
Battell Kevin
Ferm Paul M.
Shacklette Lawrence W.
Corning Incorporated
Knauss Scott
Roberts & Mercanti LLP
Sanghavi Hemang
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