A non-destructive technique to measure an optical fiber's refractive index profile with sub-?m spatial resolution over a wavelength range spanning more than one octave (from 480 to 1040 nm) in a single measurement is described. Data showing the variation of refractive index with wavelength for several fiber types is presented.

Multi-Wavelength Optical Fiber Refractive Index Profiling by Spatially Resolved Fourier Transform Spectroscopy

The design and experimental demonstration of a snapshot hyperspectral imaging Fourier transform (SHIFT) spectrometer is presented. The sensor, which is based on a multiple-image FTS (MFTS), offers significant advantages over previous implementations using Michelson interferometers. Specifically, its use of birefringent interferometry creates a vibration insensitive and ultra-compact (15x15x10 mm(3)) common-path interferometer while offering rapid reconstruction rates through the graphics processing unit. The SHIFT spectrometer's theory and experimental prototype are described in detail. Included are reconstruction and spectral calibration procedures, followed by the spectrometer's validation using measurements of gas-discharge lamps. Lastly, outdoor measurements demonstrate the sensor's ability to resolve spectral signatures in typical outdoor lighting and environmental conditions.

Compact real-time birefringent imaging spectrometer.

We investigate in detail the process of CO2-laser writing of long-period fiber gratings (LPFGs) in unannealed and annealed boron-doped fiber samples by using repeated scanning of CO2-laser pulses. We find that the writing dynamics depends strongly on the CO2-laser energy density and the annealing temperature of the fiber. Our results from analyzing the writing dynamics reveal the relative importance of various physical effects arising from glass structure changes due to different writing conditions and thus provide a better understanding of the formation and the properties of CO2-laser written LPFGs.

Glass Structure Changes in CO $_{2}$ -Laser Writing of Long-Period Fiber Gratings in Boron-Doped Single-Mode Fibers

Review of RGB photoelasticity

We use the optical path difference OPD technique to quantify the organization of collagen fibers during skin repair of full- thickness burns following low-intensity polarized laser therapy with two different polarization incidence vectors. Three burns are cryogen- erated on the back of rats. Lesion L is irradiated using the electric field vector of the polarized laser radiation aligned in parallel with the rat's occipital-caudal direction. Lesion L is irradiated using the elec- tric field vector of the polarized laser radiation aligned perpendicu- larly to the aforementioned orientation. Lesion C is untreated. A healthy area labeled H is also evaluated. The tissue samples are col- lected and processed for polarized light microscopy. The overall find- ing is that the OPD for collagen fibers depends on the electric field vector of the incident polarized laser radiation. No significant differ- ences in OPDs are observed between L and H in the center, sides, and edges of the lesion. Lesions irradiated using the electric field vec- tor of the polarized laser radiation aligned in parallel with the rat's occipital-caudal direction show higher birefringence, indicating that collagen bundles in these lesions are more organized. © 2006 Society of

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Collagen Birefringence In Skin Repair In Response To Red Polarized-laser Therapy

The two-wave-plate compensator (TWC) technique is introduced for single-point retardation measurements. The TWC method uses a known wave plate together with a wave plate of unknown retardation and produces a linearly polarized output that allows a null of intensity to be detected. The TWC method is compared both theoretically and experimentally with the existing Brace–Kohler and Senarmont methods. The resolution of the TWC is shown to be 0.02 nm. TWC enables the measurement of a sample retardation with as little as 0.13% error and thus is more accurate than either the Brace–Kohler or the Senarmont method.

Two-wave-plate compensator method for single-point retardation measurements

An exact analysis of the working parameters of a modified Wollaston prism is presented. Parameters include the output splitting angle, the retardation, and the location of the plane of the interference fringes (plane of apparent splitting). Results are presented for the entire range of optical axis inclinations and wedge angles. Approximate expressions from the literature are evaluated. An angle of incidence that causes the plane of the interference fringes to be perpendicular to the axis of the optical system is found for each configuration analyzed. This is then applied to the design of modified Wollaston prisms for Nomarski differential interference contrast microscopy.

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Analysis and design of modified Wollaston prisms.

Residual stress profiles in optical fibers determined by the two-waveplate-compensator method

The two-wave-plate compensator (TWC) method is expanded for full-field retardation measurements by use of a polarization microscope. The sample image is projected onto a CCD camera connected to a computer, allowing the retardation to be measured at all pixels. The retardation accuracy of this implementation of the TWC is evaluated to be 0.06 nm. The method is applied to polarization-maintaining fibers and long-period fiber gratings. The measured retardation is in good agreement with the crossed-polarizer images of the fibers. The method achieves a spatial resolution of 0.45 µm and a retardation resolution of 0.07 nm. The full-field TWC method can thus be a useful tool for characterizing and monitoring the fabrication of optical devices.

Two-wave-plate compensator method for full-field retardation measurements.

Three algorithms for computing the refractive-index profile of azimuthally symmetric optical fibers via the inverse Abel transform are compared to determine their relative accuracies. Appropriate values of algorithm parameters are also determined. The direct differentiation algorithm, the iterative algorithm, and the Fourier algorithm are used to calculate the refractive-index profile from simulated measurements of the phase shift of light transmitted transversely through the fiber. The rms error in the calculated index profile is used to quantify the accuracy of each algorithm. The Fourier algorithm is typically the most accurate of the three.

Carole C. Montarou

Papers

Two-wave-plate compensator method for single-point retardation measurements

Analysis and design of modified Wollaston prisms.

Residual stress profiles in optical fibers determined by the two-waveplate-compensator method

Two-wave-plate compensator method for full-field retardation measurements.

Algorithm performance in the determination of the refractive-index profile of optical fibers