David L. K. Eng

Simple Fabrication and Processing of an All-Polymer Electrooptic Modulator

Emerging systems requiring large-scale manufacture and monolithic integration of photonic components have created demand for inexpensive and scalable processes for the production of conformal, low-drive voltage, and high bandwidth EO modulators. This paper discusses a device architecture for a phase modulator based on a recently developed organic EO material, IKD-1-50, using low-index, photocurable cladding layers on a Silicon platform. Theory and modeling for a TM waveguide and electrode configuration are presented, followed by the fabrication process and device characterization. The EO material serving as the core of the waveguide is poled using a poling and monitoring apparatus with procedures that were optimized for this material based on experimentation in simple slab-capacitor characterization devices. The challenges presented by the instability of OEOMs under common processing conditions have been addressed and a simple fabrication process has been developed using standard photolithography and reactive ion etching. The characterization methodology for phase modulators will be presented along with the results for modulators fabricated for this study. This study culminates in a VπL of roughly 3.3 V·cm which is comparable with the record demonstrations in the literature for a TM-mode inverted ridge-waveguide-based EO modulator.

Thin LiNbO 3 on insulator electro-optic modulator

This Letter presents a method for the fabrication and integration of a thin LiNbO3 substrate with a Si handle wafer. An inverted ridge structure guides a single optical mode in an electro-optic modulator fabricated on a mechanically thinned substrate. To define an optical waveguide, a ridge structure is first patterned on a 500 μm thick X-cut LiNbO3 wafer; then a low dielectric constant adhesive layer is used to bond the etched LiNbO3 to Si. The LiNbO3 is mechanically thinned to 4 μm, and planar electrodes are patterned. Experimental results demonstrating modulation with a V(π)L of 7.1 V-cm were shown, optical loss was low enough, and film quality high enough, to enable an interaction length of 0.8 cm.

All-polymer modulator for high frequency low drive voltage applications

Organic electro-optic material based optical modulators have been fervently pursued over the past two decades. The material properties of organic materials over crystalline electro-optic materials such as LiNbO3 have yielded devices with record low drive voltages and significant promise for high frequency operation that are ideal for implementation in many developing telecommunication technologies. This paper will discuss a TM electro-optic phase modulator based on a recently developed material IKD-1-50. A simple fabrication process that is compatible with wafer scale manufacturability using commercially available cladding materials, spin processing, standard photolithography, and dry etching will be presented. Non-centrosymmetric order is induced in the core material via a thermally enabled poling process that was developed based on work in simple slab waveguide material characterization devices, and optimized for polymer stack waveguide architectures. Basic phase modulators are characterized for half wave voltage and optical loss. In device r33 values are estimated from a combination of measured and simulated values. Additional work will be discussed including amplitude modulation and high frequency applications. The design for a Mach-Zehnder interferometer amplitude modulator that implements a multi mode interference cavity splitter will be presented along with plans for a microstrip transmission line traveling wave modulator.

Display of polarization information for passive millimeter-wave imagery

Abstract. A technique is described for displaying polarization information from passive millimeter-wave (mmW) sensors. This technique uses the hue of an image to display the polarization information and the lightness of an image to provide the unpolarized information. The fusion of both images is done in such a way that minimal information is lost from the unpolarized image while adding polarization information within a single image. The technique is applied to experimental imagery collected in a desert environment with two orthogonal linear polarization states of light and the results are discussed. Several objects such as footprints, ground textures, tire tracks, and shrubs display strong polarization features that are clearly visible with this technique, while materials with low polarization signatures such as metal are also clearly visible in the same image.

Broadband low-drive voltage polymer electro-optic modulator

An all-polymer high-frequency Mach-Zehnder modulator that can be fabricated using standard UV lithography is proposed. The optical waveguide structure consists of three polymer layers, two low-index, outer cladding layers and an organic-electro-optic material in a polymer host as the core. Lateral confinement is provided by a trench that is defined in the lower cladding layer, resulting in an inverted electro-optic polymer ridge waveguide. The inverted nature of this trench structure allows for a fabrication process in which the cladding layer is patterned, and the highly sensitive electrooptic material is simply spun on and cured. Microstrip transmission line electrodes patterned on the outer cladding, over the optical waveguides provide the modulation field. Similar devices using CLD1 or AJL8, as the electro-optic material have been numerically analyzed at up to 260GHz, and characterized at frequencies up to 40 GHz, but to date no electrooptic polymer device has been characterized at such high frequencies. A recently developed material, IKD-1-50, with electro-optic coefficients up to five times larger than CLD1 and AJL8 will be utilized as the core layer for the optical waveguide. The greater nonlinearity of these materials will yield a device with a lower Vπ. Additionally, high frequency characterization up to 300GHz will demonstrate the high bandwidth application possibilities of these new materials.

Progress towards dual vertical slot modulator for millimeter wave photonics

Dual vertical slot modulators leverage the field enhancement provided by the continuity of the normal electric flux density across a boundary between two dielectrics to increase modal confinement and overlap for the propagating optical and RF waves. This effect is achieved by aligning a conventional silicon-based optical slot waveguide with a titanium dioxide RF slot. The TiO2 has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. The dual slot design confines both the optical and RF modes to the same void between the silicon ribs of the optical slot waveguide. To obtain modulation of the optical signal, the void is filled with an organic electro optic material (OEOM), which offers a high optical non-linearity. The optical and RF refractive index of the OEOM is lower than silicon and can be deposited through spin processing. This design causes an extremely large mode overlap between the optical field and the RF field within the non-linear OEOM material which can result in a device with a low Vπ and a high operational bandwidth. We present work towards achieving various prototypes of the proposed device, and we discuss the fabrication challenges inherent to its design.

Measured comparison of contrast and crossover periods for passive millimeter-wave polarimetric imagery

Several targets are set-up outside and imaged by a passive millimeter-wave sensor over a 24 hour period. The sensor is capable of measuring two linear polarization states simultaneously and the contrasts of the targets are compared for the different polarizations. The choice of polarization is shown to have an impact on the contrast of different targets throughout the day. In an extreme case the contrast of a target experiences a crossover event and disappears for one polarization while it presents a strong contrast (9 K) with the other polarization. Experimental results are shown along with a simulation of the scene using a ray tracing program.

Dual slot modulator for millimeter wave photonics

Silicon slot waveguides leverage the field enhancement provided by the continuity of normal electric flux density across a dielectric boundary to confine an optical mode to a void between two proximal silicon strips. Silicon-organic hybrid slot modulators make use of this mode profile by infiltrating the slot region with a non-linear organic electro-optic material (OEOM) for modulation. The dual slot modulator takes this idea a step further by similarly confining a propagating RF mode to the same slot region to increase modal overlap for improved modulation efficiency. This effect is achieved by aligning a titanium dioxide RF slot along a conventional silicon slot waveguide. The TiO2 has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. As a result of the large modal overlap and high electro-optic activity of the OEOM this design can produce measured phase modulated VπL of less than 1.40 V•cm. Furthermore, as the modulator operates without the introduction of a doping scheme it can potentially realize high operational bandwidth and low loss. We present work towards achieving various working prototypes of the proposed device and progress towards high frequency operation.

Processing of organic electro-optic materials: solution-phase assisted reorientation of chromophores

Organic EO materials, sometimes called EO polymers, offer a variety of very promising properties that have improved at remarkable rates over the last decade, and will continue to improve. However, these materials rely on a “poling” process to afford EO activity, which is commonly cited as the bottleneck for the widespread implementation of organic EO material-containing devices. The Solution Phase-Assisted Reorientation of Chromophores (SPARC) is a process that utilizes the mobility of chromophores in the solution phase to afford acentric molecular order during deposition. The electric field can be generated by a corona discharge in a carefully-controlled gas environment. The absence of a poling director during conventional spin deposition forms centric pairs of chromophores which may compromise the efficacy of thermal poling. Direct spectroscopic evidence of linear dichroism in modern organic EO materials has estimated the poling-induced order of the chromophores to be 10-15% of its theoretical maximum, offering the potential for a manyfold enhancement in EO activity if poling is improved. SPARC is designed to overcome these limitations and also to allow the poling of polymeric hosts with temporal thermal (alignment) stabilities greater than the decomposition temperature of the guest chromophore. In this report evidence supporting the theory motivating the SPARC process and the resulting EO activities will be presented. Additionally, the results of trials towards a device demonstration of the SPARC process will be discussed.

Dual vertical slot modulator for millimeter wave photonics

Modern high frequency applications necessitate the utilization of the millimeter wave band. Slot waveguides have previously been used for electro optic modulators as the enhancement of the electric field strength in the slot creates a large overlap with the electro optic material. We present a design that utilizes the field enhancement provided by a slot waveguide geometry for both the optical field and the RF modulating field. The dual RF and optical slot configuration maximizes the overlap of the optical field and the modulating field in the electro optic material, creating the maximum amount of phase change per applied volt of modulating signal. This design presents unique fabrication challenges.