Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2001-01-31
2003-04-15
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
C385S008000, C385S130000, C385S131000
Reexamination Certificate
active
06549685
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electro-optic modulators and the materials and methods used to fabricate such modulators.
BACKGROUND OF INVENTION
Over the past thirty years, significant time and effort has been dedicated to the study of various materials such as, LiNbO
3
, III-V semiconductors, and organic polymers, to determine their respective characteristics for the fabrication and performance as electro-optic (EO) devices (e.g., modulators, waveguides, switches, emitters, detectors, and the like). When compared to inorganic crystal-based modulators, organic polymers provide significant advantages including higher speed and lower half-wave voltage (i.e., V
&pgr;
). That is due, at least in part, to the fact that organic polymers have lower dielectric constants than their inorganic counterparts, which permits optical signals (i.e., waves) and microwaves to propagate within a device constructed from such organic materials with nearly equal phase velocities. Moreover, organic polymers exhibit large molecular hyperpolarizabilities and the possibility of exceptionally high electro-optic coefficients (r
33
), which are also important for reducing V
&pgr;
. For example, organic modulators have been demonstrated at operating frequencies as high as 113 GHz and a V
&pgr;
of approximately 0.8V. However, such modulators have used electric field poled polymers as the electro-optic active layer. Such materials are, in general, characterized by loss-inducing chromophore aggregation effects, charge injection-induced degradation, and low degrees of polar orientation that can be achieved by the electric field poling process. In addition, it is difficult to achieve a sufficiently high glass transition temperature value, T
g
, to ensure a stable polar orientation over prolonged periods of time at realistic use temperatures for poled polymers. Unfortunately, known electro-optic modulators that use poled polymers require electric field poling to provide polar orientation, typically requiring a poling voltage of between 500 V and 1000 V, which increases the electrode size needed to accommodate that voltage. Moreover, polar alignment is likely to be lost over time, as the device experiences thermal cycling and exposure to elevated temperatures (both during processing and operation). All of the above-described disadvantages of known polymer-based electro-optic devices increase the design and fabrication complexity of the devices and, consequently, the cost.
Polar organic materials can exhibit far higher electro-optic coefficients (r
33
) and lower dielectric constants (&egr;) than conventional inorganic EO materials, suggesting the possibility of inexpensive organic modulators with digital-level operating voltages (i.e., low) and large bandwidths (e.g., compare the figure-of-merit, n
3
r
33
/&egr; for SAS (self-assembled superlattices)=20-140 pm/V, versus 10-40 pm/V for poled-polymers and 8.7 pm/V for LiNbO
3
). Practical organic materials must meet many criteria. For example, continuous on-chip device operating temperatures can range from 80-100° C., on average, and may reach as high as 250° C. during processing-induced excursions. Practical organic materials thus require noncentrosymmetric microstructures with long-term stability. As mentioned above, sophisticated poled-polymer modulators are rapidly approaching or even exceeding the performance of inorganic devices in terms of bandwidth (i.e., speed) and operating voltage. Further computationally aided development of efficient, robust and simple organic chromophores exhibiting large first (&bgr;) and second hyperpolarizabilities is expected to dramatically improve device performance. It is conceivable that applicable low-drive-voltage EO modulators based on ultra-high glass transition temperature polymers (T
g
≧300° C.), to prevent thermal randomization of chromophore orientation, might someday become a reality. Nevertheless, there is a great need for the innovative design of EO organic materials and modulators which do not require poling. For instance, polar chromophore alignment is generally induced near T
g
values using high electric fields (e.g., 10
6
V/cm), making incorporation of intrinsically acentric (i.e., not requiring poling) organic films into device structures a desirable alternative. One challenge is clear: can utilization of such materials reduce device design complexity? Minimizing processing steps would doubtless lead to smaller components and simplify large-scale fabrication.
The rapidly expanding use of optical telecommunications and public networking requires high speed optical-to-electrical data conversion, which in turns drives a vast need for advanced, extremely efficient electro-optic devices such as modulators, waveguides, switches, emitters, and detectors. Integration of “soft materials” in the form of organic thin films into such electro-optic device offers new device properties, enhanced performance, and eventually lower production costs.
SUMMARY OF THE INVENTION
The present invention utilizes layer-by-layer molecular self-assembled (SA) templated formation of intrinsically polar arrays of high-&bgr; chromophores grown directly on silicon or related substrates. That process does not require electric-field poling, poling electrodes, or electrically matched buffer layers and, therefore, enables ready integration into semiconductor electronics and into the latest all-organic microphotonic and nanophotonic circuits. Furthermore, programmed polar microstructures in accordance with the present invention often exhibit excellent chemical, photochemical, and thermal stability, and are compatible with soft lithography (including templated growth on patterned substrates). The very large chromophore densities (N
max
of approximately molecule/10
21
molecules per cm
−3
) and high degrees of net polar orientation result in superior EO responses (r
33
ranging from approximately 40 to 200 pm/V). Chemical modifications of the molecular building blocks, such as those described herein, allow systematic optimization of SA techniques, material characteristics, and eventually, device quality.
The present invention is directed to a new electro-optic (EO) phase modulator constructed from a combination of a low-loss passive polymer waveguide and a self-assembled chromophore superlattice (SAS) with an intrinsic polar structure. In contrast to typical polymer-based modulators, the present invention utilizes a siloxane SA methodology that enables the acentric alignment of constituent chromophores during film growth without the need for post-deposition electric-field poling. The present invention thus significantly reduces the size and complexity of electrodes required to fabricate electro-optic devices because the electrodes no longer need to accommodate the high poling voltages required to align molecules during formation of an electro-optic device, as is the case with prior art devices. The present invention further provides electro-optic devices that are thermally stable and less susceptible to variation in structure (e.g., polar alignment) and optical characteristics over time.
The use of SiO
2
, Cytop™ (a commercially available fluorinated polymer) and Cyclotene™ 3022-35 (a commercially available polybisbenzocyclobutane) glassy polymers in accordance with the present invention results in a straightforward device formation process that is compatible with the thermally and photochemically robust SAS materials. Other transparent polymers with appropriate refractive indices may also be used in place of Cytop™ and Cyclotene™ 3022-35. Thus, nanoscale control of the film architecture results in greatly simplified macroscopic device fabrication. The present invention thus provides a SAS-based electro-optic modulator demonstrating excellent electro-optic response properties and a &pgr; phase shift.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indi
Ho Seng-Tiong
Lu Wei-Kang
Marks Tobin J.
van der Boom Milko E.
Zhao Yi-Guang
Northwestern University
Palmer Phan T. H.
Reinhart Boerner Van Deuren s.c.
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