David A. Wargowski

LOSS MECHANISMS IN POLYIMIDE WAVEGUIDES

Waveguide losses in thin film polyimides using waveguide loss spectroscopy and photothermal deflection spectroscopy as a function of cure cycle and structure were studied. Fluorinated sidegroups on the polyimide backbone lead to decreases in birefringence and absorption. The primary waveguide loss mechanism is absorption, not scattering. Waveguide losses as low as 0.4 dB/cm at 800 nm have been measured. Losses as low as 0.3 dB/cm at 1300 nm can be inferred from the photothermal deflection spectroscopy.

Crosslinked polyimide electro-optic materials

We report studies of the optical and electro‐optic properties of guest–host polymeric nonlinear optical materials based on aromatic, fluorinated, fully imidized, organic soluble, thermally, and photochemically crosslinkable, guest–host polyimides. We have introduced temperature stable nonlinear optical chromophores into these polyimides and studied optical losses, electric field poling, electro‐optic properties, and orientational stability. We measured electro‐optic coefficients of 5.5 and 12.0 pm/V for ((2,6‐Bis(2‐(3‐(9‐(ethyl)carbazolyl))ethenyl)4H‐pyran‐4‐ylidene)propanedinitrile (4‐(Dicyanomethylene)‐2‐methyl‐6‐(p ‐dimethylaminostyryl)‐4H‐pyran) DCM‐doped guest–host systems at 800 nm using a poling field of 1.3 MV/cm. Poling induced nonlinearities in single‐layer films were in agreement with the oriented gas model, but were lower in three‐layer films due to voltage division across the layers.

Fabrication of low loss polyimide optical waveguides using thin-film multichip module process technology

Polyimides containing fluorine are attractive for integrated optics since the introduction of fluorine increases the optical transparency in the visible and near infrared spectrum. Polyimides are easily integrated with silicon or GaAs optical devices, can be spin-coated on a number of substrates and are resistant to chemical etchants, solvents and package assembly temperatures. Photosensitive polyimides are being widely used as dielectrics in multichip module fabrication because they offer a combination of good electrical and mechanical properties and low cost processing relative to non-photosensitive polyimides. In this paper, a simple, wet chemical patterning process for the fabrication of low loss waveguides using photosensitive polyimides is described. Optically transparent, fluorinated polyimides are modified by copolymerizing a low concentration of photocrosslinking groups into the backbone. The polyimides can then be patterned into channel, ribs, or Y branches by UV exposure through a photomask followed by wet chemical development. Sidewall smoothness and sidewall profiles can be controlled by varying exposure and development conditions. >

Optical polyimides for single-mode waveguides

The synthesis and optical characterization of fluorinated polyimide systems with potential use in passive waveguides and electro-optic devices is reported. The effect of fluorination on optical properties such as refractive index, birefringence, and near-infrared absorbance is reviewed in terms of optical performance requirements. Synthetic methods of tuning the refractive index in order to achieve appropriate core/cladding differentials is discussed. The relation between processing parameters and refractive index for several polyimide structures also is reported. We describe the microlithographic fabrication of a multilayer polyimide rib- type waveguide that is suitable for single mode guiding. The waveguide is fabricated using photosensitive polyimide systems via negative resist imaging. A comparison of wall profiles and resolution limits afforded by the wet-chemical patterning techniques is presented. Results on channel guide coupling, propagation, and loss are described, as well as progress in producing active guides.

Polyimide-based electro-optic materials

The properties of new, high temperature optical materials based on dye-doped Ultradel 9000D polyimides are presented. Ultradel 9000D is a soluble, pre-imidized, fluorinated polymer with properties optimized for integrated optical applications. When thermally or photochemically cross-linked, it has a Tg approaching 400

Characterization of thermally stable dye-doped polyimide based electrooptic materials

DEGC and retains excellent optical transparency as measured by both waveguide loss spectroscopy (WLS) and photothermal deflection spectroscopy (PDS). The agreement between WLS and PDS data indicates that losses in polyimides are due to absorption, not scattering. Two thermally stable, donor-acceptor oxazole-based dyes were designed, synthesized, and doped into the polyimide at concentrations up to 25 percent by weight. The Tg of the doped polymers decreased from the neat polymer, but remained above 300

Characterization of crosslinked electro-optic polyimides

DEGC. The effects of doping on the dielectric constant, refractive index, and coefficient of thermal expansion of the polyimide are presented.

Cross-linked polyimides for integrated optics

Polymeric electrooptic materials have the potential to replace electronic switches in applications which require minimization of heat dissipation while maintaining high switching speeds. Polyimide matrices incorporating electrooptic dyes are promising materials for such applications due to their low cost and compatibility with existing processing environments. Preparation and characterization of novel dye-doped polyimide films for electrooptics is described. Thermal stabilities of donor-acceptor 2,5-diaryl oxazoles were evaluated by differential scanning calorimetry. Absorptive losses in thin films of Ultradel 9000D{reg_sign} doped with donor-acceptor oxazoles were measured by photothermal deflection spectroscopy. Absorptive losses at high doping levels may be explainable by dye-dye aggregation or dye degradation during the curing process. Lower doping levels, however, show losses of {le} 3.0 dB/cm at 830 nm and {le} 2.4 dB/cm at 1,320 nm.

Photodefinable optical waveguides

The electro-optical properties of UltradelR 9000D polyimides doped with DCM and DADC, a bis(carbazole) analog of DCM with improved thermal stability, are reported. Cure temperatures were restricted to 240 degree(s)C or less to minimize potential thermal degradation of these dyes. Low poling fields of 30 V/micrometers were used in these experiments and yielded r13 coefficients in the 0.1 - 0.8 pm/V range. Photothermal deflection measurements of dye-doped Ultradel 9000D samples showed low optical absorption losses in systems cured at 175 degree(s)C, but losses exceeded 20 dB/cm in samples cured at 300 degree(s)C.

Polymide optical waveguide structures

We have investigated a promising class of polyimide materials for both passive and active electro-optic devices, namely crosslinkable polyimides. These fluorinated polyimides are soluble in the imidized form and are both thermally and photo-crosslinkable leading to easy processability into waveguide structures and the possibility of stable electro-optic properties. We have fabricated channel and slab waveguides and investigated the mechanism of optical propagation loss using photothermal deflection spectroscopy and waveguide loss spectroscopy, and found the losses to arise from residual absorption due to the formation of charge transfer states. The absorption is inhibited by fluorination leading to propagation losses as low as 0.3 dB/cm in the near infrared. Because of the ability to photocrosslink, channel waveguides are fabricated using a simple wet-etch process. Channel waveguides so formed are observed to have no excess loss over slab structures. Solubility followed by thermal cross-linking allows the formation of multilayer structures. We have produced electro-optic polymers by doping with the nonlinear optical chromophores, DCM and DADC; and a process of concurrent poling and thermal crosslinking. Multilayer structures have been investigated and poling fields optimized in the active layer by doping the cladding with an anti-static agent. The high glass-transition temperature and cross-linking leads to very stable electro-optic properties. We are currently building electro-optic modulators based on these materials. Progress and results in this area also are reported.