James H. Bechtel

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.

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.

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.

Double-end crosslinked electro-optic polymer modulators with high optical power handling capability

Integrated Mach–Zehnder and straight channel electro-optic modulators have been fabricated with a double-end crosslinked polymer containing amino-sulfone chromophores. The optical power handling capability of these modulators was tested at 1.32 μm wavelength and at input optical power levels compatible with commercial analog transmitters. At a cw peak intensity of 0.9 MW/cm2 inside the waveguide, the double-end crosslinked polymer waveguide modulators exhibited no observable increase in optical loss or degradation of nonlinearity during the experiment period. The poled polymer showed a long-term thermal stability of the electro-optic coefficient at 100 °C and photochemical stability at 633 nm wavelength.

Long-term stable direct current bias operation in electro-optic polymer modulators with an electrically compatible multilayer structure

The drift of the direct current (dc) quadrature bias voltage was monitored in electro-optic polymer Mach–Zehnder modulators for over 600 h. Traceable bias voltage was observed in polyurethane-Disperse Red 19 modulators. Electric field relaxation analysis showed that steady state dc and alternating current (ac) field distributions on polymer layers depended on the dielectric properties of the materials. The long-term bias stability of our modulators was originated from the low conductivity of the electro-optic polymer guiding layer. A concept of material electrical compatibility is introduced as a materials selection guideline for the fabrication of polymer modulators with the minimum bias drift.

High-isolation photonic microwave mixer/link for wideband signal processing and transmission

We have proposed, analyzed, and demonstrated a high-isolation photonic microwave mixer using an integrated, dual-stage balanced-bridge Mach-Zehnder modulator. The proposed balanced photonic microwave mixer provides not only high isolation between radio frequency (RF) signal and local oscillator (LO) ports, but also excellent isolation between intermediate frequency (IF) and RF/LO ports without any filters. In this paper, the structure, principle, and operation settings are discussed in detail. Experimental results showed >60 dB electrical isolation between RF and LO ports and >50 dB isolation between IF and RF/LO ports in a demonstrative photonic microwave mixer. This device can extend the bandwidth of a modulator and have wide applications in RF photonic links where RF signal conversion and processing are required.

Photoinduced molecular alignment relaxation in poled electro‐optic polymer thin films

The photoinduced nonlinearity decay in poled nonlinear optical polymer, polyurethane/Disperse Red 19, films was studied at wavelengths from 543 to 1320 nm. Electro‐optic gratings and waveguide modulators were used in measuring and monitoring material’s nonlinearity. No photoinduced nonlinearity decay was detected at near infrared wavelengths. However, the photoinduced relaxation rate increased approximately 5 orders of magnitude from 670 to 543 nm, as the excitation wavelength approaches π‐π* transition of the chromophores. The photoinduced chromophore reorientation was identified as the dominant relaxation mechanism.

Fabrication of polymer waveguide tapers to minimize insertion loss

Polymer based electro-optic (EO) modulators and other integrated optic devices have the potential to provide low cost and lightweight alternative for high-speed digital as well as analog RF links. To be truly competitive with existing technologies such as LiNbO3, EO polymer modulators must also meet the criteria of low loss. There are two major causes of loss in EO modulators: waveguide loss (including material loss, scattering, etc.), and fiber- to-waveguide coupling (butting) loss. Various techniques can be utilized to minimize these coupling losses, however, to maintain low cost of component, we resort to the simplest possible approach which is easy to manufacture. Pigtails using standard single mode fiber produce coupling loss on the order of 3 to 5 dB/connection. In order to improve mode size matching yet maintain low drive voltage we incorporate waveguide and fibers tapers. Waveguide tapers resulted to butting losses as low as 1.5 dB/connection, whereas fiber tapers resulted to 2.5 dB/connection butting losses. Combining both techniques together, it was possible to produce 1.3 dB/connection butting loss, however, tapered waveguide devices were less sensitive to alignment tolerance than tapered fiber devices, and therefore less sensitive to environmental conditions.

Analysis of poling-induced polymer waveguide losses in push-pull Mach-Zehnder modulators

Poling induced losses of split-ground plane, push-pull polymeric electro-optic modulators have been investigated. Two sources of loss are found: loss due to the presence of oxygen and loss due to deforming the waveguide structure by large poling fields. Deformation is the most severe at the edges of the electrodes, where the electric field amplitude is largest. Experiments were done by poling waveguides with different architectures and poling in air and in an inert atmosphere. There is an apparent rapid increase in poling induced loss (to the 4th power) with poling voltage due to the presence of oxygen (up to 6.5 dB/cm for poling field of 170 V/micrometers ), whereas loss due to deformation increases linearly with poling voltage (up to 2.5 dB/cm). Oxygen-induced loss can be minimized by poling in inert atmosphere, while deformation induced loss can be minimized by optimizing device architecture.

Low-driving-voltage electro-optic polymer modulators for advanced photonic applications

The rapid development of ultra-wide bandwidth fiber optical communications networks has challenged circuit designers to obtain ever-increasing link gain-bandwidth products with new electro-optic modulators. As the link bandwidth increases, high power modulator drivers become very costly and have low efficiency. To overcome the limited gain-bandwidth product, it is important to reduce the required driving voltage or the so-called halfwave voltage of current electro-optic modulators. In this paper, we report results on new polymeric electro-optic modulators with a halfwave voltage of 0.8V and a halfwave voltage-interaction length product of 2.2V-cm. The low driving voltage allows electro-optic modulators to be driven directly by high-speed logic circuits without an amplifier. The driving electronics and the system cost can be significantly reduced when these modulators are implemented. Here, low halfwave voltage modulators are based on recent developments in materials and modulator fabrication technologies. The incorporation of a new second-order nonlinear optical chromophore CLD-1 in a poly(methylmethacrylate) matrix has a demonstrated electrooptic coefficient approximately 60 pm/V at 1318 nm wavelength. Using this material system and an optical push- pull modulator design, sub-1 volt Mach Zehnder modulators and temporal stability of these modulators will be reported.