Yongqiang Shi

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.

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.

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.

Electro-optic polymer modulators with 0.8 V half-wave voltage

We report the fabrication and test results for polymeric electro-optic modulators with a half-wave voltage of 0.8 V and a half-wave voltage-interaction length product of 2.2 V cm. These modulators employ an optical push–pull architecture and are made from poly(methylmethacrylate) with a high molecular hyperpolarizability polyene bridge-type chromophore. An electro-optic coefficient of 58 pm/V was obtained at a 1318 nm wavelength. The guest–host polymer system exhibited a thermal stability to 75 °C and a relatively stable nonlinearity at ambient conditions. The experimental results have demonstrated not only the sub-1 V half-wave voltage electro-optic polymer modulator but also the potential of polymeric electro-optic materials for photonic applications.

Large photoinduced birefringence in an optically nonlinear polyester polymer

A nonlinear optical polyester with disperse red side groups exhibited a stable photoinduced birefringence, from 0.14 at 850 nm to 0.21 at 633 nm, after exposure to linearly polarized short‐wavelength visible or ultraviolet light. Thin‐film wave plates, birefringent diffraction gratings and waveguides were written in this polymer using photoexposure. Thin‐ film wave plates of <1 μm thickness showed ≳π/4 phase delay and good temporal stability. A 9:1 diffraction efficiency ratio of the two polarization eigenmodes was measured from thin birefringent gratings. This ratio is in good agreement with the theoretical prediction obtained using a simple three‐level model.

Fabrication and characterization of high-speed polyurethane-disperse red 19 integrated electrooptic modulators for analog system applications

The fabrication and characterization of polymeric electrooptic modulators, made of a thermally crosslinkable polyurethane with Disperse Red 19 side chains, are summarized in this paper. Straight channel and Mach-Zehnder modulators have been fabricated, packaged and tested for the fiber-optic analog transmission system applications. Device performances including halfwave voltage, insertion loss, on-off ratio, and modulation frequency responses were measured. Long-term halfwave voltage stability, dc-bias voltage stability, and optical power handling capability at 1.3-/spl mu/m wavelength were investigated. A carrier-to-noise ratio of 53 dB and 80-channel television transmission have been demonstrated using the packaged polymer modulators.

Optical heterodyne detection of 60 GHz electro‐optic modulation from polymer waveguide modulators

An optical heterodyne technique has been used to demonstrate electro‐optic modulation up to 60 GHz, the highest reported to date, in polymer waveguide modulators. The frequency response of the device from 40 to 60 GHz was obtained by measuring the frequency down converted modulation signal with a low frequency photodetector. No rolling‐off was identifiable from the measured device response, indicating no fundamental material limitations in this frequency range for the polymer material which we used. Such an optical heterdyne system can be a powerful tool for modulator characterization at millimeter wave frequencies because of its high sensitivity and low frequency demands on photoreceivers.

Traveling wave electro‐optic phase modulator using cross‐linked nonlinear optical polymer

We report the fabrication and characterization of a high frequency traveling wave electro‐optic phase modulator made from thermally cross‐linked polyurethane‐Disperse Red 19 polymers. The device has been fabricated using a three layer structure with an integrated 50 Ω microstrip line circuit. The straight channel optical waveguides were defined laterally by standard photolithography and oxygen reactive ion etching. The measured half‐wave voltage was in good agreement with that extrapolated from the second harmonic generation measurements. Optical modulation was observed directly on a spectrum analyzer up to 18 GHz. Long‐term evaluation over an extensive length of time found no observable loss of performance.

Push-pull poled polymer Mach-Zehnder modulators with a single microstrip line electrode

A push-pull structure has been realized for integrated Mach-Zehnder modulators based on a thermoset electrooptic polymer. The two modulator waveguide arms were poled in the opposite directions and covered by a single microstrip line electrode. This device structure can reduce the half-wave voltage by 50% without compromising wide-band frequency response. Efficient poling was achieved by using a compatible cladding material to lower the poling voltage, and by using a top cover piece and an inert gas to suppress air breakdown between the poling electrodes. Our fabricated devices exhibited the predicted 50% half-wave voltage reduction compared with non-push-pull devices fabricated on the same chip.

Thermosetting nonlinear optical polymer: Polyurethane with disperse red 19 side groups

A new thermosetting second‐order nonlinear optical polyurethane polymer with disperse red 19 pendant groups was studied by second‐harmonic generation and electro‐optic modulation methods. The polymer can be crosslinked by thermal curing during electric field poling. The electro‐optic coefficient r13 measured with a Mach–Zehnder interferometer ranged from 5 pm/V at 800 nm to 13 pm/V at 633 nm. The polymer crosslinked during poling showed no significant decay of the nonlinearity at room temperature for thousands of hours. A long‐term thermal stability of the dipole alignment at an elevated temperature, 90 °C, was demonstrated. The thermosetting polymer showed improved alignment stability under ultraviolet exposure.