Shelley M. Etzel

Verdet constant dispersion in annealed optical fiber current sensors

The Verdet constant in annealed optical fiber current sensors has been measured at wavelengths from 636 to 1320 nm. The measurements are fitted to two models, one classical and the other an expansion of the classical model that includes a nonlinear term. These measurements and models are compared to previous measurements made in optical fiber and bulk SiO/sub 2/. Our measurements have an average accuracy of /spl plusmn/0.6% and an average measurement uncertainty of /spl plusmn/0.5% over the 636 to 1320 nm range.

Fundamental limits in fiber Bragg grating peak wavelength measurements

We discuss the fundamental limits of fiber Bragg grating (FBG) wavelength metrology. High-accuracy wavelength measurements are critical for FBG strain sensors because a wavelength measurement uncertainty as small as 1 pm leads to an uncertainty of nearly 1 microstrain. We compare the measurement uncertainties for several common wavelength measurement systems, including tunable laser, optical spectrum analyzer (OSA), and interferometric. We show that when using an OSA it is difficult to achieve a measurement uncertainty better than 10 pm, and if the OSA is not accurately calibrated to a known wavelength reference, then the wavelength measurement uncertainty can be as large as 1 nm. We describe the uncertainties involved in determining peak and/or centroid wavelength from a measured data set. We also discuss calibration references for FBG sensor interrogation units. Wavelength references that are based on molecular absorption lines are often an excellent choice for FBG sensor calibration. However, some interrogation units require a wavelength reference unit based on narrow reflection lines rather than absorption lines. We investigated the application of athermally packaged FBGs as wavelength references, but we found that their wavelengths will drift with time and can undergo large jumps. We concluded that it is difficult to achieve stability better than 4 pm/year in athermally packaged FBGs.

Optical fiber current sensors in high electric field environments

This paper analyzes the response of optical fiber current sensors that are subjected to high electric fields, such as fields encountered in gas-insulated systems. This paper shows that through the electrooptic (EO) Kerr effect, these fields can cause harmonic distortion of the measured ac current waveform. This harmonic distortion was confirmed experimentally. Also, this paper shows that it is possible to simultaneously measure both current and voltage waveforms and the phase between them using this effect. To minimize the electrooptic Kerr effect, optical fiber current sensors must be screened from high-electric fields.

Wavelength and temperature performance of polarization-transforming fiber.

We have theoretically and experimentally investigated an optical fiber with circular polarization modes on one end and linear polarization modes on the other end. We call this fiber a polarization-transforming fiber because the local modes, or polarization states they represent, are converted from linear to circular, and visa versa, in the fiber. We have developed and implemented a postdraw process for making polarization-transforming fiber samples 30 mm long with losses less than 1 dB and a polarization-mode conversion from circular to linear greater than 20 dB. Also, we have modeled and measured the dependence on wavelength and temperature of polarization-transforming fiber samples. The measured normalized wavelength dependence of a sample fiber 30 mm long was approximately 1.4 x 10(-4) nm(-1), and the measured normalized temperature dependence was approximately 6 x 10(-4) degrees C(-1). These values are better in some cases than values for conventional high-birefringent fiber quarter-wave plates.

Key metrology considerations for fiber Bragg grating sensors

We discuss the current status of fiber Bragg grating (FBG) sensor metrology. High-accuracy wavelength measurements are critical for FBG strain sensors, because wavelength measurement uncertainties even as small as 1 pm lead to an uncertainty of nearly 1 microstrain. We administered an FBG wavelength measurements round robin in which twelve participants measured the spectral reflectance of four FBGs; we found that the measured peak wavelengths of a single FBG varied by as much as 35 pm. We have determined that this variation results from two factors: the uncertainty of the measurement systems and the wavelength instability of the athermally packaged FBGs. We found that the wavelengths of athermally packaged gratings will drift with time and can undergo large jumps and that it is difficult to achieve stability better than 4 pm/year. We discuss the measurement uncertainties for several common wavelength measurement systems, including tunable laser, optical spectrum analyzer (OSA), and interferometric measurements. We show that when using an OSA, as many of the round-robin participants did, it is difficult to achieve a measurement uncertainty better than 10 pm, and if the OSA is not accurately calibrated to a known wavelength reference; then wavelength measurement uncertainty can be as large as 1 nm. Wavelength references that are based on molecular absorption lines are ideal for calibrating OSAs, but they are not optimal for calibrating FBG interrogation units where a reference based on reflection lines rather than absorption lines is often preferred. We discuss other possible candidates for wavelength references for FBG sensor interrogation units.

Spectroscopic phase-dispersion optical coherence tomography measurements of scattering phantoms

We demonstrate a novel technique to determine the size of Mie scatterers with high sensitivity. Our technique is based on spectral domain optical coherence tomography measurements of the dispersion that is induced by the scattering process. We use both Mie scattering predictions and dispersion measurements of phantoms to show that the scattering dispersion is very sensitive to small changes in the size and/or refractive index of the scatterer. We also show the light scattered from a single sphere is, in some cases, non-minimum phase, and therefore the phase of the scattered light is independent of the intensity. Phase dispersion measurements may have application to distinguishing the size and refractive index of scattering particles in biological tissue samples.

Analysis of photoconductive gain as it applies to single-photon detection

We detail a mathematical framework for photoconductive gain applied to the detection of single photons. Because photoconductive gain is derived from the ability to measure current change for an extended period, its magnitude is reduced as detection speed is increased. We theoretically show that high-speed detection is still possible as long as the noise spectrum of the device is 1/f in nature. Using signal analysis techniques, we develop tools to apply to device noise spectra to determine the performance of single-photon detectors that utilize photoconductive gain. We show that there is no speed penalty when one considers the signal-to-noise ratio for the fundamental 1/f noise typical of high electron mobility transistors. We outline a technique for quickly characterizing a detector’s sensitivity and speed through purely electrical measurements of the device’s noise spectra. Consequently, the performance of the detector can be determined and optimized without conducting optical measurements. Finally, we e...

Temperature dependence of the single-photon sensitivity of a quantum dot, optically gated, field-effect transistor

We present a systematic study of the temperature dependence of the electrical noise in a quantum dot, optically gated, field-effect transistor (QDOGFET) and detail how the noise influences the sensitivity of these novel single-photon detectors. Previous studies have shown that when cooled to 4 K, QDOGFETs exhibit single-photon sensitivity and photon-number-resolving capabilities; however, there has been no systematic study of how operating temperature affects their performance. Here, we measure the noise spectra of a device for a range of sample temperatures between 7 K and 60 K. We use the noise data to determine the signal-to-noise ratio of the optical responses of the devices for various temperatures and detection rates. Our analysis indicates that QDOGFETs can operate over a broad range of temperatures, where increased operating temperature can be traded for decreased sensitivity.

Dispersion of the temperature dependence of the retardance in SiO 2 and MgF 2

We have directly measured the retardance versus temperature for single-crystal quartz (SiO(2)) and magnesium fluoride (MgF(2)) at wavelengths of 633, 788, 1318, and 1539 nm and over a temperature range of 24-80 degrees C. To our knowledge, the temperature dependence of retardance for these two materials has not been directly measured. We compared our direct measurements of the normalized temperature derivative of the retardance gamma with derived values from previously reported indirect measurements and found our results to be in agreement and our measurement uncertainties to be typically a factor of 4 smaller. Our overall mean value for gamma(SiO(2)) is -1.23 x 10(-4) with a combined standard uncertainty of 0.02 x 10(-4) and little wavelength dependence over the 633-1539-nm range. Our overall mean value for gamma(MgF(2)) is -5.37 x 10(-5) with a combined standard uncertainty of 0.17 x 10(-5) and with a small wavelength dependence over the 633-1539-nm range.

Stability of fiber Bragg grating wavelength calibration references

We discuss the key considerations necessary to create stable fiber Bragg grating wavelength references. We describe two high-accuracy measurements to characterize the gratings, and we demonstrate gratings with wavelength stability better than 1 pm over 75 days.