Mach-Zehnder optical modulators were fabricated using the CLD and FTC chromophores in polymer-on-silicon optical
waveguides. Up to 17 months of oven-ageing stability are reported for the poled polymer films. Modulators containing
an FTC-polyimide had the best over all aging performance. To model and extrapolate the ageing data, a relaxation
correlation function attributed to A. K. Jonscher was compared to the well-established stretched exponential correlation
function. Both models gave a good fit to the data. The Jonscher model predicted a slower relaxation rate in the out
years. Analysis showed that collecting data for a longer period relative to the relaxation time was more important for
generating useful predictions than the precision with which individual model parameters could be estimated. Thus from
a practical standpoint, time-temperature superposition must be assumed in order to generate meaningful predictions.
For this purpose, Arrhenius-type expressions were found to relate the model time constants to the ageing temperatures.
A method of chemical synthesis that allows for the facile attachment of a wide variety of chemical compounds, including highly active nonlinear optical chromophores, to polyimides has been developed recently at the Naval Air Warfare Center, Weapons Division. The synthesis of these compounds is presented, along with a discussion of their relevant physical and chemical properties, alone and in comparison to equivalent guest/host materials. Examples of attached chromophores include the well-known Disperse Red 1, along with high-activity chromophores of more recent interest such as FTC and CLD. The synthesis of structures that contain both attached chromophores and chemical functionalities that enable thermal cross-linking of the polyimides is also discussed.
Highly accurate, compact, and low cost inertial measurement units (IMUs) are needed for precision guidance in navigation systems. Active and passive polymer materials have been successfully used in fabricating two of the key guided-wave components, the phase modulator and the optical transceiver, for IMUs based on the interferometric fiber optic gyroscope (IFOG) technology. Advanced hybrid waveguide fabrication processes and novel optical integration techniques have been introduced. Backscatter compensated low loss phase modulators with low half-wave drive voltage (Vπ) have been fabricated with CLD- and FTC- type high performance electro-optic chromophores. A silicon-bench architecture has been used in fabricating high gain low noise transceivers with high optical power while maintaining the spectral quality and long lifetime. Gyro bias stability of less than 0.02 deg/hr has been demonstrated with these components. A review of the novel concepts introduced, fabrication and integration techniques developed and performance achieved are presented.
This is a brief overview of the technology of nonlinear optical polymers (NLOP) and their use in electro-optic (EO) modulators. This paper also covers preliminary results from the authors' laboratories on highly active CLD- and FTC-type chromophores in guest-host films of APC amorphous polycarbonate. Emphasis will be given to thermal stability and long-term EO modulator aging.
Techniques for the rapid evaluation of material properties of interest in the design of polymer integrated optical devices that have recently been developed in our laboratory are described. These include methods for determining optical loss and electrical resistivity. The use of the techniques is demonstrated with polyimide materials as an example. The level of precision that is reasonably attained from each technique is discussed, along with the relative merits of these techniques compared to other potential approaches to obtaining similar information.
New insights into the relationship between film processing conditions and properties for polyimide-based systems of interest in the fabrication of optical waveguides are presented. The critical role of the rate of solvent evaporation to ensuring high-quality films is explored, as are the effects of varying the casting solution concentration, spin casting, and baking conditions. In addition to film quality, the effect of the previously mentioned processing conditions on optical properties is also described.
Methods that successfully predict the refractive index at near-infrared wavelengths of negatively birefringent polymer films for optical waveguide applications are presented. The starting point for these methods is a correlation based on connectivity indexes originally developed by Bicerano for the refractive index of isotropic polymers at visible wavelengths. This correlation is applied to a set of polyimides at near infrared wavelengths with modifications in order to improve its predictive power. The polyimides were synthesized by condensation of monomers to form the precursor poly(amic acid)s followed by imidization in solution. Solutions of the polyimides were then spin coated onto glass substrates and baked to produce films of 2-3 microns in thickness with a variable negative birefringence. The refractive index profiles of these films near 1320 nm were then measured in both the TE- and TM- modes using a prism-coupling technique. The average refractive index of these films was then compared to the prediction generated by the model. The agreement between the predicted and observed values has been sufficient to enable the rapid development of materials for optical waveguides without the need for many rounds of trial-and-error investigation. These techniques facilitate the development of specialized polymers for optical waveguide applications.
Electrooptic films based on a new class of porphyrin chromophores with exceptionally large molecular first hyperpolarizabilities are begin developed. These chromophores feature donor and acceptor groups linked to the porphyrin macrocycle at the 5- and 15-meso positions through intervening ethynyl moieties. We report herein preliminary result that describe the ambient temperature assembly of ordered, electrooptic films using the Langmuir-Blodgett-Kuhn method. The facile formation of such films was mae possible by the preparation of amphiphilic chromophores containing hydrophobic aliphatic substituents to enhance the polar alignment at the air-water interface. Second harmonic generation and UV-Vis absorption were used to characterize the films as a function of the number of layers deposited. Polar films containing up to 10 layers were made by simple Y-type homolayer deposition, and up to six layers by the Y- type heterolayer method in which the porphyrin layers were interleaved with poly(t-butyl methacrylate). The polar order in these films relaxed exponentially. Ionic structures are recommended for improved stability.
Acentric films of stilbazolium-substituted polyepichlorohydrin (SPECH) and poly(sodium 4- styrenesulfonate) (PSS) were formed using alternating polyelectrolyte deposition (APD) on hydrophobic glass substrates. APD is a layer-by-layer technique for the formation of polymer films by alternately immersing a substrate in aqueous solutions of a polyanion and a polycation. APD provides precise control of the overall film thickness that through automated processing may exceed a thousand layers. The peak maximum UV-visible absorbance in transmission through the SPECH/PSS films was linear as a function of the number of bilayers. Second harmonic generation (SHG) was used as a tool to indicate acentric order to polarizable side-chain chromophores within the APD films. The SHG exhibited the expected quadratic intensity increase with film thickness after 30 bilayers had been deposited. During slow temperature ramping of the SPECH/PSS APD films on hydrophobic glass, in situ SHG measurements revealed that 90 percent of the polar order is retained at temperature well over 120 degrees C. The additional minima in the Maker fringe data, created by interference of the second harmonic waves generated at the two sides of a double-coated substrate, were found to go to zero, which is indicative of high quality films. The observed high thermal stability of the polar order makes the APD films attractive for device applications.
This is a preliminary report on alternating polyelectrolyte deposition (APD) with the goal of making electro-optic films at room temperature. By means of the APD method, films were built up layer-by-layer at room temperature by a dipping process. APD requires two complementary polymers, one being negatively charged and the other being positively charged. The polymer in solution is attracted to the solid substrate by coulombic forces to form the next layer on the substrate. For the first time, second-order nonlinear optical (NLO) polymer films were prepared by APD in which both polymers were NLO-active. The cationic polymer contained a side-chain stilbazole chromophore, and the anionic polymer contained a main-chain cinnamoyl chromophore. The intensity of the second harmonic signal generated in the film increased quadratically with each layer for the first twenty layers.
Alternating polyelectrolyte deposition (APD) in aqueous solutions may be used to process nonlinear optical polymers (NLOPs) into noncentrosymmetric ordered films at ambient temperature. Second-order NLOP films were prepared by alternately dipping a substrate into aqueous solutions of a polycation and a polyanion. Polyepichlorohydrin substituted with stilbazolium side-chain chromophore was used as the cationic NLOP. The inactive polyanion was polystyrene sulfonate. Uniform layer to layer deposition is observed as evidenced by a linear increase of UV-Visible absorbance and quadratic increase of second harmonic generated light intensity as a function of film thickness. Films have been uniformly deposited up to 24 bilayers. Films have been further characterized by contact angle measurements, interferometry, and polarized light microscopy. Work is in progress to deposit thicker films of the same quality and to quantify NLO figures of merit.
Electro-optic (EO) polymer films for use in optical modulators and switches continue to show promise for lowering costs, increasing bandwidth, and allowing high levels of monolithic integration. Now more emphasis must be placed on designing EO polymer films with low optical loss while maintaining adequate EO coefficient and thermal stability. Recently demonstrated, highly rigid and polarizable polymers have quite high thermal stability, but their optical clarity has suffered. The higher optical loss in films of these highly rigid polymers is an unsolved problem at this point. Furthermore, electricfield- poled films of these rigid polymers often have lower EO coefficients, which may be due to higher electrical conductivity at poling temperature that prevents the build-up of an adequate electric field across the film. Crosslinking of moderately rigid, fluorinecontaining films may provide the best compromise of thermal stability, optical clarity and EO coefficient.
New thermally stable, spin-castable, electro-optic (EO) polymers designed for high frequency optical modulators are reported (the third generation accordion polymers). The softening temperature (the glass transition temperature) is about 240 degrees Celsius, and the upper limit on short term thermal baking stability is about 320 degrees Celsius. The refractive index at 1.3 microns is about 1.73 and fairly birefringent. The second-order nonlinear optic coefficient, d33, of a second generation accordion polymer containing essentially the same chromophore, measured by second- harmonic generation at 1.06 microns, is 120 pm/V (resonance enhanced by the 495 nm absorption). Measurement of the electro-optic coefficient, r33, is in progress. The added thermal stability in these polymers is due to the all- aryl amine electron donor. The molecular topology of the polymer backbone makes it possible for over 85 weight percent of the bulk material to be comprised of EO-active chromophore. The chromophores are configured in a head-to- head mainchain topology. The films are completely amorphous (no microcrystalline scattering sites).
Progress on second-order nonlinear optical polymers for high speed optical signal modulation is reported. New accordion polymers containing head-to-head backbone chromophores extended with a styryl-thienyl-vinyl linkage are described. The maximum absorption of the electronic transition (ground state yields excited state) for these polymers occurs at a wavelength of 494 nm. A corona-poled film of the ortho-xylyl bridged accordion polymer has a resonance-enhanced second- order nonlinear optical coefficient of 120 pm/V measured at 1.06 microns, and a glass transition temperature of about 187 degree(s)C. Along with linear polymer, polar cyclic dimers were formed during the polymerization. An extraction process was developed to remove the cyclic dimers from the polymer.
One might say this paper is about something old, something new, something borrowed, something blue -- finding chromophoric brides to be wed to polymeric husbands. Several dyes have been synthesized that are red, magenta, or blue in color and have large ground-state dipole moments. Dyes synthesized in our laboratory were characterized by differential scanning calorimetry and UV-VIS spectroscopy. First molecular hyperpolarizabilities ((beta) o) of the dyes were calculated using MOPAC (V6). Some of the dyes were dissolved in poly(methyl methacrylate), corona poled above the glass transition temperature, and cooled to room temperature to freeze in the polar order. Second-order nonlinear optical properties of these guest-host films were compared by optical frequency doubling measurements.
The synthesis and preliminary characterization of new polypeptide-bound chromophores are reported. These polymers were made for the purpose of assembling (beta) -sheets at an air- water surface, and processing them into nonlinear optical films by Langmuir-Blodgett (LB) deposition. These films should be amenable to integration on semiconductor wafers, e.g., as active optical waveguides for phase shifting.
An amorphous, nonlinear optical polymer, which has a glass transition temperature of 208 degree(s)C, was prepared. It is soluble in common organic solvents, and can be processed by conventional photolithographic processes. Corona-poled thin films of this polymer exhibit second-order nonlinear optical properties. This polymer is comprised of mainchain cinnamamide chromophores in the syndioregic configuration. The rigid U-shaped links which bridge the chromophores in this polymer were designed to geometrically stabilize polar, folded conformations in the backbone.
New nonlinear optical polymers (NLOP) having potential utility in waveguides for the modulation and switching of optical signals are reported. A new class of chromophoric polymers which assume a folded, polar conformation of the backbone have been prepared. The polymers have a syndioregic arrangement of chromophores within the backbone (i.e., a head-to-head, tail-to-tail configuration). Polymers were synthesized by the polymerization of difunctional, precoupled pairs of chromophores and difunctional, bridging groups. Glassy, noncentrosymmetric films were prepared by electric field poling and by Langmuir-Blodgett (LB) deposition. Characterization of multilayer LB films by null ellipsometry to determine the anisotropic refractive parameters was performed at different angles of incidence and at a wavelength of 1.0 (mu).
Head-to-tail mainchain chromophoric polymers have been of interest because of their high chromophore densities [about 30 X 1021 chromophores/cm3 in the case of poly(4-N-ethylene-N-ethylamino)-(alpha) -cyanocinnamate], their proposed tendency to align in an electric field more readily than unconnected chromophores, and the proposed enhancement of the macroscopic hyperpolarizability coefficients. These proposed properties have yet to be proven for the solid state. In fact, results on mainchain nonlinear optical polymers indicate that a much lower degree of alignment resulted than would be expected for unconnected dipoles. It is possible that the head-to-tail configuration of mainchain polymers has too high of an energy barrier to rotation and alignment to allow proper alignment of the dipoles. Therefore, a new class of mainchain polymers, namely head-to-head polymers connected with various flexible spacers (which may allow the dipoles to align) was developed. The synthesis and characterization of these new materials is described. The nonlinear optical properties of several of this new class of mainchain nonlinear optical polymers were compared to the properties of the head-to-tail mainchain polymers.