William H. Steier

Polymer micro-ring filters and modulators

Micro-ring wavelength filters and resonant modulators using polymer materials at 1300 nm and 1550 nm are analyzed, designed, and demonstrated. The rings are integrated with vertically coupled input and output waveguides. The devices are fabricated using optical lithography. Filters with a finesse of 141 and free spectral range of 5 nm at 1300 nm and finesse of 117 with a free spectral range (FSR) of 8 nm at 1550 nm are demonstrated. Ring resonators with a Q as high as 1.3 /spl times/ 10/sup 5/ at 1300 nm are demonstrated. The filters can be temperature tuned at the rate of 14 GHz//spl deg/C. Resonant ring modulators, which use an electrooptic polymer, are demonstrated. The resonance wavelength voltage tunes at the rate of 0.82 GHz/V. The modulators have a bandwidth larger than 2 GHz. Using the resonant modulator, and open eye diagram at 1 Gb/s is demonstrated.

DEMONSTRATION OF 110 GHZ ELECTRO-OPTIC POLYMER MODULATORS

Electro-optic modulation up to 113 GHz has been demonstrated using traveling wave polymer modulators. The modulation signal was directly detected at 1.3 μm using a laser heterodyne system with an external-cavity tunable semiconductor laser. The device optical response variation, as a function of frequency over the whole W band, was within 3 dB. A well-matched coplanar probe was used to launch W band millimeter wave driving power into the microstrip line electrode on the device. Based upon these measurements, high speed electrodes with integrated millimeter wave transitions had been fabricated and tested.

From molecules to opto-chips: organic electro-optic materials

Recent advances in polymeric electro-optic materials and device fabrication techniques have significantly increased the potential for incorporation of these materials and devices into modern high bandwidth (fiber and wireless) telecommunication, information processing, and radar systems. Charge transfer π-electron chromophores characterized by molecular first hyperpolarizability (second order optical non-linearity) values approaching 3000×10 –30 esu have been synthesized. Elucidation of the role of intermolecular electrostatic interactions in inhibiting the efficient translation of molecular optical non-linearity to macroscopic electro-optic activity has permitted systematic modification of materials to achieve electro-optic coefficients approaching 100 pm V –1 . Improvements in the optical loss of polymeric materials at wavelengths of 1.3 and 1.55 µm have been effected. Mode matching of passive transmission and active electro-optic waveguides has been addressed, permitting a dramatic reduction in insertion loss. The putative ability of polymeric electro-optic materials to be efficiently integrated with very large scale integration semiconductor electronic circuitry and with passive optical circuitry has been demonstrated. Several devices of varying degrees of complexity have been fabricated and evaluated to operational frequencies as high as 150 GHz. The operational stability of polymeric devices is very competitive with devices fabricated from lithium niobate and gallium arsenide.

Waveguides: characteristic modes of hollow rectangular dielectric waveguides.

The field configurations and propagation constants of the normal modes of a hollow rectangular dielectric waveguide have been determined. In addition, the coupling coefficients of a Gaussian free-space mode into the normal modes of a square guide were calculated. The attenuation of each mode is found to be inversely proportional to the cube of the guide aperture 2alpha and proportional to the square of the free-space wavelength lambda. For a hollow dielectric square guide with 2alpha = 1 mm and lambda = 10.6 microm, an attenuation of 0.140 dB/m is predicted for SiO(2) and 0.032 dB/m for BeO. All modes are found to be hybrid modes, although they very closely approximate linearly polarized TEM modes.

The molecular and supramolecular engineering of polymeric electro-optic materials

Abstract A new class of electro-optic chromophores, of which 2-dicyanomethylen-3-cyano-4-{2-[ E -(4- N , N -di(2-acetoxyethyl)-amino)-phenylene-(3,4-dibutyl)thien-5]- E -vinyl}-5,5-dimethyl-2,5-dihydrofuran (denoted FTC) is the prototype, has been prepared, characterized, and used to fabricate electro-optic devices. The molecular hyperpolarizability and thermal stability of these chromophore molecules are exceptional. Strong intermolecular electrostatic interactions inhibit the efficient poling of these molecules. A statistical mechanical theoretical treatment is used to quantitatively predict the competition of poling, intermolecular electrostatic interactions, and thermal effects in defining achievable acentric order and hence macroscopic optical nonlinearity. Theory is used to predict the optimum chromophore structure and material composition (chromophore loading in a polymer matrix) for maximum electro-optic activity and minimum optical loss. Problems associated with lattice hardening to lock-in poling-induced order are discussed briefly.

Quasi-epitaxial growth of organic multiple quantum well structures by organic molecular beam deposition

Multiple quantum well structures consisting of alternating layers of two crystalline organic semiconductors, namely, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) and 3,4,7,8 naphthalenetetracarboxylic dianhydride (NTCDA), have been grown by organic molecular beam deposition. The individual layer thicknesses in the multilayer samples were varied from 10 to 200 A. X‐ray diffraction and birefringence data show that there is a strong structural ordering in all layers, as well as across large spatial distances along the sample surface. Thus, the growth is ‘‘quasi‐epitaxial’’ even though the PTCDA and NTCDA crystal structures are incommensurate. From the optical absorption spectra, it was found that the lowest energy PTCDA singlet exciton line shifts to higher energy with decreasing layer thickness. Comparison of these results with a quantum mechanical model based on exciton confinement in the PTCDA layers is proposed to describe the energy shift.

Recent advances in electrooptic polymer modulators incorporating highly nonlinear chromophore

Based on a nonlinear optical polymer with a highly nonlinear chromophore (CLD) dispersed in an amorphous polycarbonate (APC), we have developed electrooptic (EO) polymer modulators operating at 1550-nm wavelength with low loss and good thermal stability. By incorporating polymer insulation layer, push-pull poling was successfully performed without film damages. We also demonstrated that the propagation loss of the EO polymer waveguide could be reduced as low as 1.2 dB/cm at 1550 nm when the large core waveguide structure was incorporated. The long-term reliabilities of the EO polymer modulator made of CLD/APC polymer were investigated. When the modulator was hermetically sealed in an inert gas, the V/sub /spl pi// change of a Mach-Zehnder modulator was negligible over 30 d of operation with 20-mW exposure to the waveguide input. In the thermal stability measurement, 25% V/sub /spl pi// increase was observed from the sample heated to 60/spl deg/C over 40 d, though the sample left at room temperature showed no decay of nonlinearity.

Three-dimensional integrated optics using polymers

Some of the key components are demonstrated to make three-dimensional (3-D) optical integrated circuits possible using polymers. Fabrication techniques of shadow reactive ion etching, shadow photolithography, and gray-level photolithography to produce complex 3-D integrated optic structures are demonstrated. Vertical waveguide bends exhibit excess losses of <0.3 dB, and vertical power splitters possess predictable output splitting ratios between multiple core levels with excess losses of <0.5 dB. Vertical polarization splitters exhibit power extinction ratios of 15 dB between the output core layers. A 1/spl times/4 vertical-horizontal power splitter is also demonstrated. Additionally, these techniques are used to integrate different polymer materials into the same optical circuit while easily solving the mode mismatch problem. To show the technique, a polymer electrooptic modulator is vertically integrated with a low-loss waveguide.

Photorefractivity at 1.5 μm in CdTe:V

We have for the first time demonstrated two‐beam coupling energy transfer at a wavelength of 1.5 μm. Beam coupling gain coefficients of 0.6 cm−1 have been obtained in vanadium ‐doped CdTe with only 5 mW/cm2 incident intensity. These gain coefficients exceed typical gain coefficients in GaAs at 1.06 μm wavelength by 50%. In preliminary measurements using the moving grating technique, we have measured a gain coefficient of 2.4 cm−1. Through adjustment of the doping level, CdTe:V can be used as a sensitive photorefractive material through the 0.9–1.5 μm spectral range.

Large stable photoinduced refractive index change in a nonlinear optical polyester polymer with disperse red side groups

The photoinduced refractive index change of polyester with disperse red side groups was studied at several different wavelengths. This material exhibits a large photoinduced index change (0.3 at 633 nm) and in addition when poled has a sizable second‐order nonlinear optical effect. This index change was monitored over 1000 h and long‐term stability was demonstrated. Based on these results, a method using only photomasks to define channel waveguides and other patterns in such polymers was suggested and experimentally demonstrated. This photoinduced index change may have wide applications in integrated optical systems when these or other similar polymers are involved.