Kent B. Rochford

Low-coherence interferometric measurements of the dispersion of multiple fiber Bragg gratings

We show that the dispersion of multiple fiber Bragg gratings can be obtained from a single low-coherence interferometric measurement. The individual gratings can be identified either from the spatial separation of the interferometric signatures or from the unique wavelength-reflection bands of the gratings.

Fast and accurate low-coherence interferometric measurements of fiber Bragg grating dispersion and reflectance.

We demonstrate fast and accurate measurements of fiber Bragg grating dispersion and spectral reflectance using low-coherence interferometry. Both dispersion and spectral reflectance are obtained in less than 60 seconds, rendering the results immune to errors caused by temperature variations and instrumental drift. To examine the accuracy of the low-coherence technique, we compare the results with independent measurements and demonstrate an agreement better than 1.5 ps for dispersion and 25 pm for spectral reflectance wavelength.

Magneto-optic sensors based on iron garnets

The use of single crystal bulk and film iron garnets in optical sensors is reviewed. Magneto-optic sensitivity and its stability are important parameters that depend on a variety of factors, including optical design. Polarimetric and diffractive sensor technologies are summarized, and several recent demonstrations of magnetic field, current, and rotation sensing using garnets are described. Garnets also find application as important nonsensing components in sensor systems.

Accurate interferometric retardance measurements

A two-polarization Michelson interferometer with a low-retardance beam splitter and digital signal processing is used to measure the retardance of optical devices. Error analysis of the improved optical system and data processing shows that the measurement has an uncertainty of 0.039 degrees for measurements of nominally 90 degrees retarders. Retardance variations arising from coherent reflections in the retarder used for intercomparison add an uncertainty of from 0.005 degrees to 0.03 degrees , increasing the combined measurement uncertainty to as much as 0.049 degrees .

Faraday effect current sensor with improved sensitivity–bandwidth product

We report a new design for a Faraday effect current sensor based on yttrium iron garnet that has substantially greater bandwidth than previous designs and is much easier to fabricate. The measured sensitivity is 0.7°/A, with a −3-dB bandwidth of 500 MHz, which gives an improvement in sensitivity–bandwidth product of approximately 45. A noise-equivalent current of 840 nA/Hz1/2 was measured at 1.8 kHz by difference-over-sum processing. The use of turning prisms with phase-preserving coatings greatly simplifies construction, improves electrical isolation, and increases sensitivity through proximity effects.

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.

Demultiplexing of interferometrically interrogated fiber Bragg grating sensors using Hilbert transform processing

The peak reflectance wavelengths of gratings with reflectance maxima separated by less than 2 nm can be accurately determined through a demultiplexing method based on Hilbert transforms of interferograms. We demonstrate a wavelength demultiplexing of three fiber Bragg gratings (FBGs) with less than 4 pm crosstalk and repeatability and less than 19 pm uncertainty. We anticipate that a large number of gratings can be demultiplexed with a single broadband source and a single receiving interferometer, provided that the interferogram is sampled at accurate intervals slightly above the Nyquist rate.

Design and performance of a stable linear retarder

The National Institute of Standards and Technology (NIST) has developed a nominally quarter-wave linear retarder for wavelengths near 1.3 mum that is stable within +/-0.1 degrees retardance over a range of wavelength, input angle, temperature, and environmental variations. The device consists of two concatenated Fresnel rhombs made from a low stress-optic-coefficient glass that minimizes the residual birefringence from machining and packaging. Device machining, assembly, and antireflection coating tolerances are discussed, and the theoretical performance is compared with measurements. Humidity can modify retardance of the total-internal-reflection surfaces; we discuss packaging that mitigates this effect and provides an estimated 10-year lifetime for the device. Several measurement methods were intercompared to ensure that the device retardance can be measured with an uncertainty less than 0.1 degrees . Similar retarders will be certified by NIST and made available as Standard Reference Materials.

Demonstration of a programmable PMD source

A programmable polarization-mode-dispersion (PMD) source is demonstrated. The PMD source is calibrated and exhibits predictable, repeatable, and programmable performance. Such a source can be used to test for PMD impairments of optical links.

Polarization dependence of response functions in 3/spl times/3 Sagnac optical fiber current sensors

We show theoretically that the response functions of a lossless Sagnac optical fiber current sensor based on a 3/spl times/3 coupler fundamentally depend on the polarization state of light entering the coupler even for systems with no linear birefringence. For a lossless, zero-birefringence system the desired response functions, sinusoids separated by 120/spl deg/ phase shifts, are obtained only for circularly polarized input light. The response functions for linearly polarized and depolarized inputs are sinusoids separated by 180/spl deg/ and yield zero-slope small-signal responses; in addition, two outputs are degenerate, so the responses are similar to those observed in 2/spl times/2 systems. Thus, 3/spl times/3 Sagnac systems offer no advantage over 2/spl times/2 systems for linearly polarized input light. The predicted polarization dependence of 3/spl times/3 Sagnac response functions is experimentally confirmed. This result establishes the need for increased system complexity in 3/spl times/3 Sagnac current sensors since polarization control optics are required to provide the proper input polarization. >