Don C. Abeysinghe

Agile wide-angle beam steering with electrowetting microprisms

A novel basis for beam steering with electrowetting microprisms (EMPs) is reported. EMPs utilize electrowetting modulation of liquid contact angle in order to mimic the refractive behavior for various classical prism geometries. Continuous beam steering through an angle of 14 degrees (+/-7 degrees ) has been demonstrated with a liquid index of n=1.359. Experimental results are well-matched to theoretical behavior up to the point of electrowetting contact-angle saturation. Projections show that use of higher index liquids (n~1.6) will result in steering through ~30 degrees (+/-15 degrees ). Fundamental factors defining achievable deflection range, and issues for Ladar use, are reviewed. This approach is capable of good switching speed (~ms), polarization independent operation, modulation of beam field-of-view (lensing), and high steering efficiency that is independent of deflection angle.

A novel MEMS pressure sensor fabricated on an optical fiber

We describe the fabrication, and initial testing of a novel optically interrogated, microelectromechanical system (MEMS) pressure sensor in which the entire MEMS structure is fabricated directly on an optical fiber A new micromachining process for use on a flat fiber end face that includes photolithographic patterning, wet etching of a cavity, and anodic bonding of a silicon diaphragm is utilized. We have employed both 200- and 400-/spl mu/m-diameter multimode optical fibers. A pressure sensor fabricated on an optical fiber has been tested displaying an approximately linear response to static pressure (0-80 psi). This sensor is expected to find application in situations where small size is advantageous and where dense arrays may be useful.

Novel MEMS pressure and temperature sensors fabricated on optical fibers

We present the fabrication and initial testing of novel optically interrogated pressure and temperature sensors fabricated directly on optical fibers using microelectromechanical systems (MEMS) technology. A new micromachining process for use on a flat fiber end face that includes photolithographic patterning, wet etching of a cavity and anodic bonding of a silicon diaphragm is utilized. Two prototype pressure sensors, fabricated on 400 μm diameter multimode fibers, have been tested displaying an approximately linear response to static pressure (14–80 psi). A prototype temperature sensor, fabricated by anodically bonding an ultra-thin crystalline silicon onto a fiber end face, has been tested in the range 25–300 °C. A minimum detectable temperature variation of 6 °C is observed. Since these sensors are significantly miniaturized, they will find application in situations where small size is advantageous and where dense arrays may be useful.

Generating cylindrical vector beams with subwavelength concentric metallic gratings fabricated on optical fibers

We report the generation of cylindrical vector beams using a concentric metallic grating fabricated on optical fibers with a period smaller than the wavelength of the incident light. Similar to the wiregrid linear polarizer, such a subwavelength metallic annular structure strongly reflects azimuthal polarization and allows radial polarization to transmit through. Due to the polarization selectivity of the concentric metallic grating, a cylindrical vector beam is obtained when a circularly polarized light is launched into the fiber. Such a device is suitable for the end mirror coupler in an all-fiber laser design to produce radially polarized modes.

Wavelength multiplexing of microelectromechanical system pressure and temperature sensors using fiber Bragg gratings and arrayed waveguide gratings

Wavelength multiplexing of optically interrogated microelectromechanical system (MEMS) pressure and temperature sensors using both fiber Bragg gratings (FBGs) and arrayed waveguide gratings (AWGs) is demonstrated. The multiplexed sensor system using FBGs has the potential to multiplex about 70 sensors, depending on the FBG bandwidth and the one using AWGs has the potential to multiplex about eight sensors, depending on the number of AWG channels and the bandwidth of each channel. A dual-wavelength method incorporating a tunable laser is used to interrogate either the applied pressure or temperature experienced by the sensor, while a three-wavelength method can be used to simultaneously interrogate pressure and temperature. The Fabry-Perot cavity-based pressure sensors and temperature sensors are designed and fabricated using MEMS techniques. Experimental results, including response as a function of pressure or temperature, are characterized by good agreement between experimental and theoretical results. There is no observable crosstalk between the multiplexed sensors.

Reduction of scattering loss of silicon slot waveguides by RCA smoothing

Because of stronger optical confinement density, silicon slot waveguides tend to have higher scattering loss than normal ridge waveguides with same sidewall roughness. A wet chemical process is found to be highly effective in reducing the surface roughness and scattering loss. A reduction in scattering loss by 10.2 dB/cm for TE and 8.5 dB/cm for TM polarizations has been achieved.

Hybrid spiral plasmonic lens: towards an efficient miniature circular polarization analyzer

A hybrid spiral plasmonic lens that consists of alternating spiral slot and spiral triangular sub-aperture array can differentiate circular polarization of different handedness and enable a miniature circular polarization analyzer design with high efficiency. The improved performance compared to pure spiral slot lens comes from the fact that the hybrid lens is capable of focusing both the radial and the azimuthal polarization components of a circular polarization, doubling the coupling efficiency. In this paper, the spin-dependent plasmonic focusing properties of a spatially arranged triangular sub-aperture array and a hybrid spiral plasmonic lens are demonstrated using a collection mode near field scanning optical microscope. The coupling efficiency could be further improved through optimizing the geometry of the hybrid lens.

Generation of vectorial optical fields with slot-antenna-based metasurface.

A transmission-type metasurface composed of carefully designed rectangular slot antennas for the generation of vectorial optical fields is proposed and demonstrated. Acting as local linear polarizers, these slot antennas enable the spatial modulation of optical fields in amplitude, phase, and polarization for the cross-polarized component of the scattered field. As an illustration, a metasurface capable of forming a radially polarized scattered field with specific vectorial beam patterns with appropriate excitation at normal incidence is designed, fabricated, and tested. The radially polarized scattered field is designed to be further tightly focused by a high numerical aperture objective lens in order to obtain a uniform longitudinally polarized optical needle field along the propagation direction. Characterization experiments demonstrate that its overall extinction ratio satisfies the amplitude modulation requirement, and a corresponding π phase modulation is realized as proposed.

Achieving Higher Modulation Efficiency in Electrooptic Polymer Modulator With Slotted Silicon Waveguide

Silicon slot waveguide based Mach-Zehnder interferometric modulators with electrooptic polymers in the slot have the advantage of low half-wave voltage-length product (Vπ *L). Several key aspects of this unconventional electrooptic polymer modulator design to optimize the modulator performance are studied in this work. Both computer simulation and experiments have been conducted to understand the relationship between modulator performance such as modulation efficiency, optical loss and the waveguide design parameters. Techniques to achieve efficient poling of electrooptic polymers in the silicon slot waveguide have been developed. The doping of the silicon to enhance conductivity for efficient poling and the trade-off between conductivity and optical loss are experimentally investigated. Surface passivation of silicon nanophotonic structures has been found to be effective in improving poling efficiency. By applying these techniques to a silicon slot waveguide Mach-Zehnder modulator, a low Vπ*L of 0.52 V ·cm has been achieved. Finally travelling wave electrode designs have been evaluated and the results show that the bandwidth is mainly limited by the attenuation of the radio frequency signal in the electrode and a standard coplanar waveguide electrode design is able to reach 20 GHz in modulators of silicon slot waveguide embedded in electrooptic polymer.

Microelectromechanical system pressure sensor integrated onto optical fiber by anodic bonding.

Optical microelectromechanical system pressure sensors based on the principle of Fabry-Perot interferometry have been developed and fabricated using the technique of silicon-to-silicon anodic bonding. The pressure sensor is then integrated onto an optical fiber by a novel technique of anodic bonding without use of any adhesives. In this anodic bonding technique we use ultrathin silicon of thickness 10 microm to bond the optical fiber to the sensor head. The ultrathin silicon plays the role of a stress-reducing layer, which helps the bonding of an optical fiber to silicon having conventional wafer thickness. The pressure-sensing membrane is formed by 8 microm thick ultrathin silicon acting as a membrane, thus eliminating the need for bulk silicon etching. The pressure sensor integrated onto an optical fiber is tested for static response, and experimental results indicate degradation in the fringe visibility of the Fabry-Perot interferometer. This effect was mainly due to divergent light rays from the fiber degrading the fringe visibility. This effect is demonstrated in brief by an analytical model.