Eric D. Moore

Long period grating resonances in photonic bandgap fiber

We demonstrate the formation of stress-induced long period gratings (LPGs) in fluid-filled photonic bandgap fiber (PBGF). Based on our experimental results, simulations, and theoretical understanding of LPGs, we identify coupling to a guided LP(11)-like mode of the core and lossy LP1x-like modes of cladding microstructure for a single grating period. The periodic modal properties of PBGFs allow for coupling to the same mode at multiple wavelengths without a dispersion turning point. Simulations identify inherent differences in the modal structure of even and odd bands.

Tuning properties of long period gratings in photonic bandgap fibers.

We investigate the thermal tuning properties of long period gratings (LPGs) in a fluid-filled photonic bandgap fiber (PBGF). The combination of strong, resonant waveguide dispersion, characteristic of all PBGF modes, and the large thermo-optic coefficients of fluids yields highly tunable grating resonances. We measure grating resonances in three transmission bands with large tuning coefficients of up to -1.58 nm/degrees C, which match numerical results. We derive an analytic model for the PBGF LPG tuning coefficient to show how it depends on both the shift of the transmission bands and the dispersion of the coupled modes.

Correction of sampling errors due to laser tuning rate fluctuations in swept-wavelength interferometry

The frequency-sampling method is widely used to accommodate nonlinear laser tuning in swept-wavelength interferometric techniques such as optical frequency domain reflectometry (OFDR) and swept-wavelength optical coherence tomography (OCT). In this paper we analyze the frequency-sampling method and identify two sources of sampling errors. One source of error is the limit of an underlying approximation for long interferometer path mismatches and fast laser tuning rates. A second source of error is transmission delays in data acquisition hardware. We show that the measurement system can be configured such that the two error sources cancel to second order. We present experimental verification of sampling error correction using a general swept-wavelength interferometer with a significantly nonlinear laser sweep.

Phase-sensitive swept-source interferometry for absolute ranging with application to measurements of group refractive index and thickness

Interferometric range measurements using a wavelength-tunable source form the basis of several measurement techniques, including optical frequency domain reflectometry (OFDR), swept-source optical coherence tomography (SS-OCT), and frequency-modulated continuous wave (FMCW) lidar. We present a phase-sensitive and self-referenced approach to swept-source interferometry that yields absolute range measurements with axial precision three orders of magnitude better than the transform-limited axial resolution of the system. As an example application, we implement the proposed method for a simultaneous measurement of group refractive index and thickness of an optical glass sample.

Direct ice sensing and localized closed-loop heating for active de-icing of wind turbine blades

Experimental results of closed-loop de-icing are presented using distributed optical ice sensors, temperature sensors, and resistive heaters on a stationary turbine blade part at a fixed pitch angle inside a custom icing chamber. Optical frequency domain reflectometry (OFDR) is used for direct detection of ice on the blade. Distributed temperature sensors mounted at the leading edge of the blade are used for input to the closed-loop controller. Each resistive heater is surrounded by optical ice sensors which are used to inform a heater on/off decision. Scaling up, the experiments show that using combined OFDR with temperature sensing and distributed PID control uses a total power expenditure of less than 0.5% of the rated power under light/medium icing conditions; de-icing could yield a larger percentage of power improvement and a longer turbine up-time in cold regions. The power consumption for this localized heating is only about 10% of uniformly heating the blade. Furthermore, de-icing performance of high-intensity pulsed actuation versus continuous low intensity actuation is investigated. The results show that using high intensity pulse amplitude modulation (PAM) actuation achieves better de-icing performance than continuous PID control.

Three-dimensional waveguide arrays via projection lithography into a moving photopolymer

We demonstrate a projection lithography method that induces optical index changes in a flexible polymer cable that is continuously translated through the image plane. We demonstrate that a static spot pattern generates a grid of waveguides along the cable length via a continuous extrusion process. Rotations or scaling of the optical spot array can fabricate image inverters or magnifying face plates in a single process step. The resulting polymer devices have applications in optical backplanes, endoscopes for medical applications and lightweight imaging systems.

Index Contrast Measurement Using Scanning Optical Frequency Domain Reflectometry

We demonstrate direct measurements of local refractive index contrast using a scanning optical frequency domain reflectometer. Measurement results for step index fiber, gradient index fiber, and a volume holographic grating are presented.

Permanent Holographic Waveguide Arrays

Permanent two-dimensional optical waveguide arrays are demonstrated by exposing diffusion-mediated photopolymer with a multiple-beam interference pattern. A fiber-based phase control system ensures a stable interference pattern during exposure.

Microbend long period gratings in fluid-filled photonic bandgap fiber

We demonstrate the formation of long period gratings in fluid-filled photonic bandgap fiber (PBGF). The unique modal properties of PBGFs allow for coupling to LP11-like modes at multiple wavelengths. We obtain good agreement with simulations.

Tunable long period gratings in photonic bandgap fiber

Fluid filled photonic bandgap fibers (PBGFS) incorporate materials with a large thermo-optic coefficient. the modes of these fibers also have very strong waveguide dispersion. we exploit these two properties of PBGFS to demonstrate highly tunable long period gratings.