Chengjiu Wu

Electro-optic determination of the nonlinear-optical properties of a covalently functionalized Disperse Red 1 copolymer

We report the development of a methodology for the comprehensive characterization of the linear- and nonlinear-optical properties of a model copolymer, Disperse Red 1, covalently functionalized to a methyl methacrylate backbone. From refractive-index and electro-optic measurements, we have evaluated both fundamental macroscopic and microscopic nonlinear-optical properties. We have measured the refractive indices of the copolymer as a function of chromophore concentration, wavelength, and poling field. For the linear-optical properties we find that the wavelength dependence is well described by a single-oscillator Sellmeier equation and that the poling-field dependence is well described by a simple theory related to the order parameter. From the observed refractive-index anisotropy as a function of poling field, we have obtained an effective dipole moment for the Disperse Red chromophore. We have measured the linear electro-optic effect in these systems as a function of poling temperature, film thickness, poling-field strength, wavelength, and concentration. We have found that the electro-optic coefficients may be modeled by an independent-response two-level dispersion model, from which an independent determination of the effective dipole moment, nonresonant second-order susceptibility χ(2), and nonresonant molecular hyperpolarizability β have been obtained.

Advanced polymer systems for optoelectronic integrated circuit applications

An advanced versatile low-cost polymeric waveguide technology is proposed for optoelectronic integrated circuit applications. We have developed high-performance organic polymeric materials that can be readily made into both multimode and single-mode optical waveguide structures of controlled numerical aperture (NA) and geometry. These materials are formed from highly crosslinked acrylate monomers with specific linkages that determine properties such as flexibility, toughness, loss, and stability against yellowing and humidity. These monomers are intermiscible, providing for precise adjustment of the refractive index from 1.30 to 1.60. Waveguides are formed photolithographically, with the liquid monomer mixture polymerizing upon illumination in the UV via either mask exposure or laser direct-writing. A wide range of rigid and flexible substrates can be used, including glass, quartz, oxidized silicon, glass-filled epoxy printed circuit board substrate, and flexible polyimide film. We discuss the use of these materials on chips and on multi-chip modules (MCMs), specifically in transceivers where we adaptively produced waveguides on vertical-cavity surface-emitting lasers (VCSELs) embedded in transmitter MCMs and on high- speed photodetector chips in receiver MCMs. Light coupling from and to chips is achieved by cutting 45 degree mirrors using excimer laser ablation. The fabrication of our polymeric structures directly on the modules provides for stability, ruggedness, and hermeticity in packaging.

Femtosecond response of electro‐optic poled polymers

We investigate the ultrafast electro‐optic response and sensitivity of a poled side chain polymer film via the electro‐optic sampling technique. A 760 fs rise‐time electrical transient is observed corresponding to a bandwidth of 460 GHz. We believe this to be device limited and not due to limitations in the speed of response of the polymer.

Electrooptic characterization of organic media

The authors have developed an electrooptic characterization apparatus based on an AC-modulated Senarmont compensator. The apparatus is particularly suitable for studying the electrooptic response of thin polymer films. The system is capable of measuring the Pockels effect at a variety of wavelengths with a minimum phase shift sensitivity of 1 mu rad. Sample-to-sample variations in the coefficient measurements are less than 15%. The authors have made a systematic evaluation of thin, poled electrooptic polymer films on coplanar electrodes to demonstrate the usefulness of the apparatus. Corrections for the fringing effects of the applied electric field are described. The reproducibility of the electrooptic data through an examination of the polymer thickness dependence has been verified. The authors have also measured the poling field dependence of the electrooptic coefficient and obtained a best-fit molecular dipole value in excellent agreement with the reported value obtained from EFISH and solvatochromism measurements. >

Ultra-low-loss polymeric waveguides for optical interconnection

AlliedSignal scientists have developed new polymeric materials for ultra-low-loss optical interconnection, particularly for the key wavelengths of 0.83, 1.3, and 1.55 microns. Developments of these materials has required a thorough understanding of fundamental principles of optical absorption due to both vibrational and electronic resonant absorptions. We have thus created materials with measure losses at 830 nm which are in the range of 0.02 dB/cm/ At longer wavelengths, the losses can be higher due to the vibrational absorption within the polymer. However through careful selection of chemical structure, polymeric materials with intrinsic loss below 0.08 dB/cm have been demonstrated at 1.55 micron wavelength. For wavelengths longer than 830 nm, single-mode and multimode waveguides with losses equal to the intrinsic loses have been fabricated.

Loss measurements in electro-optic polymer waveguides

We report loss measurements in polymer-bound Disperse Red I slab and photodelineated channel waveguides. Losses resulting from electronic charge-transfer and vibrational carbon- hydrogen stretch overtone absorptions, trans to cis isomerization, exposure to visible or ultraviolet (UV) light and changes in dye pendant group number density are investigated. A waveguide absorption spectrometer is described which can measured waveguide losses (alpha) ((lambda) ) from 600 - 1800 nm. Absorption losses are compared to the wavelength dependent electro-optic coefficient r33((lambda) ) and a figure-of-merit r33((lambda) )/(alpha) ((lambda) ) is determined for the material.

Polymeric materials for guided wave devices

Organic polymeric materials offer great promise for the creation ofoptical guided-wave structures for use with silicon or gallium arsenide semiconductor devices. We have developed a number of new polymeric materials for which the refractive index may be photochemically controlled. These materials are ifiustrated by solid solutions of novel nitrone compounds in polymer hosts such as PMMA. We have demonstrated the creation of planar guided-wave structures in these materials both with direct laser writing and with traditional photolithographic techniques. We have also developed polymeric materials which are electro-optically active and which provide for the photochemical delineation ofguided-wave structures. We have utilized these materials to create electro-optic devices such as optical modulators.

Laser-written polymeric optical waveguides for integrated optical device applications

We describe a laser-writing technique for fabricating polymeric optical waveguides by photo- crosslinking acrylate monomer materials. We report loss measurements as low as 0.01 dB/cm in straight laser-written photopolymer waveguides and describe changes in waveguide losses at temperatures up to 250 degree(s)C. Loss measurements are also summarized for 90 degree(s) bends and laser-written 8 X 8 star couplers.

Polymer optical interconnection technology: toward WDM applications

We have developed organic polymeric materials that can be readily made into both multimode and single-mode optical waveguide structures of controlled numerical aperture and geometry, making them excellent candidates for WDM applications. Waveguides are formed lithographically, with the liquid monomer mixture polymerizing upon illumination in the UV via either mask exposure or laser writing. Our waveguides are low loss (0.03 dB/cm at 840 nm multimode) as well as temperature resistant (up to 10 years at 120 degree(s)C), enabling use in a variety of applications. Single-mode structures such as directional couplers have been made via laser writing. We further discuss the fundamental optical properties of these polymers and as they relate to WDM applications. As an example, we discuss an inorganic multimode WDM sensor that has been developed for aerospace applications and its integration with multimode polymer waveguides.

Polymeric Electro-Optic Materials and Devices: Meeting the Challenges of Practical Applications

Over the past five years, a new class of electro-optic polymeric materials has evolved which provides for the first time the capability to fabricate simple and inexpensive electro-optic devices on a variety of substrates. More importantly, these materials possess optical dielectric constants (or refractive indexes) comparable to radio-frequency dielectric constants allowing for fabrication of devices in which the electric field and the optical field propagate at the same velocity. Finally, the low dielectric constant of these materials relative to inorganic ionic crystals provides for operation of devices at much higher efficiency. Although the above facts have been clear for some time, the practical applications of these materials cannot be realized until materials can be created which satisfy a host of practical requirements and until device architectures and fabrication techniques appropriate for these materials can be developed. We will describe here research directed toward both of these ends.