Measurement of reduced backscattering noise in laser-driven fiber optic gyroscopes.

We report what we believe to be the first demonstration of a laser-driven fiber optic gyroscope (FOG) built with an air-core fiber. Its phase noise is measured to be 130 murad/ radicalHz. When the sensing fiber is replaced with a conventional fiber, this figure drops to 12 murad/ radicalHz. Comparison between these values suggests that the air-core fiber gyro is most likely not limited solely by backscattering noise but by reflections at the solid-core/air-core interface. By minimizing additional noise sources and reducing the air-core fiber loss to its theoretical limit (approximately 0.1 dB/km), we predict that the backscattering noise of the laser-driven air-core FOG will drop below the level of current FOGs. Compared with commercial FOGs, this FOG will exhibit a lower noise, improved thermal and mean-wavelength stability, and reduced magnetic-field sensitivity.

siRNA silencing of keratinocyte-specific GFP expression in a transgenic mouse skin model.

Small interfering RNAs (siRNAs) can be designed to specifically and potently target and silence a mutant allele, with little or no effect on the corresponding wild-type allele expression, presenting an opportunity for therapeutic intervention. Although several siRNAs have entered clinical trials, the development of siRNA therapeutics as a new drug class will require the development of improved delivery technologies. In this study, a reporter mouse model (transgenic click beetle luciferase/humanized monster green fluorescent protein) was developed to enable the study of siRNA delivery to skin; in this transgenic mouse, green fluorescent protein reporter gene expression is confined to the epidermis. Intradermal injection of siRNAs targeting the reporter gene resulted in marked reduction of green fluorescent protein expression in the localized treatment areas as measured by histology, real-time quantitative polymerase chain reaction and intravital imaging using a dual-axes confocal fluorescence microscope. These results indicate that this transgenic mouse skin model, coupled with in vivo imaging, will be useful for development of efficient and 'patient-friendly' siRNA delivery techniques and should facilitate the translation of siRNA-based therapeutics to the clinic for treatment of skin disorders.

Improving the mismatch between light and nanoscale objects with gold bowtie nanoantennas.

Metallic bowtie nanoantennas should provide optical fields that are confined to spatial scales far below the diffraction limit. To improve the mismatch between optical wavelengths and nanoscale objects, we have lithographically fabricated Au bowties with lengths approximately 75 nm and gaps of tens of nm. Using two-photon-excited photoluminescence of Au, the local intensity enhancement factor relative to that for the incident diffraction-limited beam has been experimentally determined for the first time. Enhancements >10(3) occur for 20 nm gap bowties, in good agreement with theoretical simulations.

Iterative processing of second-order optical nonlinearity depth profiles.

We show through numerical simulations and experimental data that a fast and simple iterative loop known as the Fienup algorithm can be used to process the measured Maker-fringe curve of a nonlinear sample to retrieve the sample's nonlinearity profile. This algorithm is extremely accurate for any profile that exhibits one or two dominant peaks, which covers a wide range of practical profiles, including any nonlinear film of crystalline or organic material (rectangular profiles) and poled silica, for which an excellent experimental demonstration is provided. This algorithm can also be applied to improve the accuracy of the nonlinearity profile obtained by an inverse Fourier transform technique.

Characterization of thermally poled germanosilicate thin films.

We report measurements of the nonlinearity profile of thermally poled low-loss germanosilicate films deposited on fused-silica substrates by PECVD, of interest as potential electro-optic devices. The profiles of films grown and poled under various conditions all exhibit a sharp peak ~0.5 microm beneath the anode surface, followed by a weaker pedestal of approximately constant amplitude down to a depth of 13-16 microm, without the sign reversal typical of poled undoped fused silica. These features suggest that during poling, the films significantly slow down the injection of positive ions into the structure. After local optimization, we demonstrate a record peak nonlinear coefficient of ~1.6 pm/V, approximately twice as strong as the highest reliable value reported in thermally poled fused silica glass, a significant improvement that was qualitatively expected from the presence of Ge.

Focusing in microlenses close to a wavelength in diameter.

Light focused from air into a spherical microlens is affected by diffraction at the lens surface as its diameter approaches the wavelength of light. Through an extension of Mie theory, we show that a converging wave that is incident upon a Si microlens with a diameter less than approximately 4lambda creates a spot as much as 25% smaller than predicted with vector diffraction theory. Si microlenses only a wavelength in diameter are shown to be virtually insensitive to variations in the maximum illumination angle, and changes in index of refraction are not found to cause the proportional changes in spot size that would be expected from vector diffraction theory.

Tunable mechanically induced long-period fiber gratings.

We report the experimental characterization of mechanically induced long-period fiber gratings (LPFG's) made by pressing a plate with periodic grooves against a short length of fiber. This filter, which is simple and inexpensive, exhibits transmission spectra and temperature stability similar to those of photoinduced LPFG's. It also offers the unique advantages of being tunable, erasable, and reconfigurable. Its polarization dependence also makes it useful as a polarizer, although it can also be eliminated with simple mechanical designs.

Absolute measurement of the second-order nonlinearity profile in poled silica.

Measurement of the thin nonlinearity profile of poled silica by the Maker fringe technique has been impossible because of total internal reflection (TIR) at the back surface of the sample. We demonstrate that this limitation can be removed by placing a prism against each face of the sample, thus avoiding TIR. This novel technique allows, for the first time to our knowledge, the nonlinearity profile of a thin film to be inferred by the Maker fringe technique. Applied to a silica sample thermally poled under standard conditions (275 degrees C and 5.3 kV for 30 min), it suggests a Gaussian profile with a 1/e width of 8 mum and a maximum d(33) of 0.34 pm/V.

Phase-sensitivity measurement of a 10-sensor array with erbium-doped fiber amplifier telemetry.

We report what are believed to be the first measurements of the phase sensitivity of a fiber sensor array using multiple low-gain remotely pumped amplifiers with an interferometric sensor inserted. The measured phase sensitivities for individual rungs average 5.7 microrad(rms)/ radicalHz and exhibit no dependence on rung number, in agreement with predictions.

325-nm Interference Microscope.

We first discuss an interference microscope's 3D response in terms of Richards and Wolf's vector theory. We then report the results we obtained with a 325-nm interference microscope using an ultraviolet transparent beam splitter, short-working-distance Mirau interferometer. The microscope performs at near-ideal definitions with a measured FWHM of the intensity spot at 0.14 mum and a FWHM of the depth envelope intensity at 0.25 mum. Feasibility of a shorter wavelength system operating at 248 nm is demonstrated.

High-density optical recording using a solid immersion lens.

A solid immersion lens attached to a conventional objective increases the effective numerical aperture (NA(eff)) of an optical pickup and yields an areal recording density proportional to (NA(eff))(2). One version of this device, with an effective (NA(eff)) of 1.7, should be capable of very high density storage but would probably need a sealed system. Another simple configuration enables the use of this method for optical data storage in an unsealed environment and extends the spatial cutoff frequency 1.5 times. Experiments with these devices are compared with the full vector field theory of this type of imaging system.

Nitrogen-rich silicon nitride thin films for deep-ultraviolet Mirau interferometry.

We report experimental results on UV-transparent low-pressure chemical-vapor-deposition nitride thin films. We show that, by using nitrogen-rich rather than conventionally silicon-rich thin-film membranes, we are able to obtain more than 95% UV transparency below 250 nm, while keeping the stress of the membrane manageable and below standard nitride's giga-Pascal stress. Using these results, we were able to microfabricate a UV Mirau interferometer for correlation microscopy.

Micromachined scanning confocal optical microscope.

We have constructed a miniature confocal optical microscope for monochromatic imaging that uses single-mode fiber illumination and a two-phase off-axis zone plate objective lens. The scanning mechanism consists of two micromachined silicon torsional scanning mirrors with orthogonal axes of rotation. The objective lens is made of fused silica and has a N.A. of 0.24 at lambda = 0.6328 microm, with a 1.0-mm working distance. The device is side looking, with die dimensions of 1.2 mm x 2.5 mm x 6.5 mm. We have measured 1.0-microm resolution over a 100-microm field of view.

Electro-optic phase modulation in a silica channel waveguide.

We report what is to our knowledge the first demonstration of electro-optic phase modulation in a fused-silica channel waveguide. The nonlinearity is induced through elevated temperature poling of an electron-beamirradiated waveguide. A phase shift of 32 mrad was measured at lambda = 633 nm for a device interaction length of 4.8 mm and an applied electric field of 7.3 V/microm.

Optical pattern recognition measurements of trench arrays with submicrometer dimensions.

We describe two pattern recognition algorithms for measuring half-micrometer focus-exposure dense photoresist line structures by using optical microscopes, such as a confocal scanning microscope and an interference microscope. The first approach depends on a data-clustering technique that allows us to measure resist lines down to 0.3 µm. The second method is an improved calibration procedure that utilizes multiple regression and moments of a cloud plot. Linearity between optical and scanning electron microscopy measurements is achieved down to 0.3 µm for nested focus-exposure resist structures with a standard deviation of 10 nm.

Electro-optic focusing system for a high-density optical disk.

We have developed an electro-optic cell as a fine-adjustment focusing system for an optical disk. The cell is fabricated from transparent lead lanthanum zirconate titanate ceramic and has concentric-patterned indium tin oxide electrodes. The electro-optic cell can generate a relative phase shift in the pupil function. We have confirmed, theoretically and experimentally, that the phase shift induces a change in focal position that is as much as twice the focal depth without degrading the region near the main lobe of the point-spread function.

Three-dimensional image realization in interference microscopy.

We describe an efficient algorithm based on the Hilbert transform for reconstructing cross-sectional or three-dimensional images from the input images acquired by an interference microscope. First the design of this filter is presented, and cross-sectional images of an integrated circuit constructed with this algorithm are demonstrated. It is shown that this Hilbert transform algorithm can be easily implemented with a low-cost frame grabber so that the computation time required for image reconstruction is drastically reduced.

Phase measurements using the Mirau correlation microscope.

A new algorithm for extracting amplitude and phase information from the data collected by the Mirau correlation microscope is presented. The accuracy of the phase measurements is first determined by measuring phase steps and vertical resolutions in the nanometer range. Experimental results with metal gratings show good agreement with Fourier theory. An inverse filter is then used to improve the resolution of the system. The filtered image has sharper edges and better phase contrast.

Correlation microscope.

We have constructed a correlation microscope based on the Mirau interferometer configuration using a thin silicon nitride film beam splitter, and we have developed a method to extract the amplitude and phase information of the reflected signal from a sample located at the microscope object plane. An imaging theory for the interference microscope has been derived, which predicts accurately both the transverse response at a sharp edge and the range response to a perfect plane reflector.