Donal A. Flavin

Dispersion of birefringence and differential group delay in polarization-maintaining fiber

We present an interferometric technique for measurement of the dispersion of birefringence in polarization-maintaining fibers. The approach yields measurements over a broad spectral range from analysis of single interferograms obtained in a tandem inteferometer. The technique is demonstrated to measure first-, second-, and third-order dispersion of the differential propagation constant, corresponding to differential group delay (DGD) and its dispersion to second order; measurements are immune to asymmetry in the interferomgram that is being processed. The technique is further applied to measurement of the temperature dependence of DGD and its first-order dispersion.

Dispersion-insensitive measurement of thickness and group refractive index by low-coherence interferometry

We describe a low-coherence interferometric technique for simultaneous measurement of geometric thickness and group refractive index of highly dispersive samples. The technique is immune to the dispersion-induced asymmetry of the interferograms, thus overcoming limitations associated with some other low-coherence approaches to this simultaneous measurement. We use the experimental configuration of a tandem interferometer, with the samples to be characterized placed in an air gap in one arm of the measurement interferometer. Unambiguous, dispersion-insensitive measurements of critical group-delay imbalances in the measurement interferometer are determined from the optical frequency dependence of interferogram phases, by means of dispersive Fourier transform spectrometry. Sample thickness and group refractive index are calculated from these group delays. A thickness measurement precision of 0.2 microm and group index measurement accuracy of 5 parts in 10(5) across a wavelength range of 150 nm have been achieved for BK7 and fused-silica glass samples in the thickness range 2000 to 6000 microm.

COMBINED TEMPERATURE AND STRAIN MEASUREMENT WITH A DISPERSIVE OPTICAL FIBER FOURIER-TRANSFORM SPECTROMETER

We report simultaneous measurement of strain and temperature in single-mode optical fiber by broadband interferometry. A Mach-Zehnder interferometer, illuminated by a xenon-arc lamp, has a sensing element in one arm. Scanning an air path generates interferograms that are calibrated by a monochromatic reference interferogram. Values of group delay and dispersion, obtained from the phase of the fast Fourier transform of the sampled interferogram, give strain and temperature through a well-conditioned matrix transformation without phase ambiguity. We obtained measurement ranges and resolutions of 1500 +/- 12 microstrain and 25.0 +/- 0.4 K using a 0.8-m sensing element.

Interferometric interrogation of in-fiber Bragg grating sensors without mechanical path length scanning

We report the interrogation of a fiber Bragg grating (FBG) using an interferometer with a tilted mirror such that the optical path difference is a function of position on an array detector. Absolute measurements of mean resonant wavelength from the phase of the analytic signal of the spatial interferogram are determined, and a technique based on using a reference laser to compensate for performance degrading effects otherwise associated with spatially scanned interferometers is introduced. These measurements are not critically dependent on the accurate location of zero phase position. We have applied the technique to the absolute measurement of temperature-induced shifts in the grating resonant wavelength. A resolution of 0.025 nm for a spatially scanned optical path delay of only 200 /spl mu/m was achieved. The technique has the potential for higher resolutions and for multiplexing.

Statistical model of migration-assisted upconversion in a high-concentration erbium-doped fiber amplifier

We report a new model of a high-concentration erbium-doped fiber amplifier (EDFA) accounting for the statistical nature of the migration and up-conversion processes. By fitting experimental results, we conclude that the statistical model shows better applicability for the characterization of high-concentration EDFAs than the model accounting for the homogeneous upconversion and pair-induced quenching.

Short-scan interferometric interrogation and multiplexing of fibre Bragg grating sensors

We describe an interferometric technique for the demodulation of serial fibre Bragg grating sensor arrays, yielding absolute measurement of the individual grating mean wavelengths. The composite beam reflected from the array illuminates a scanning Michelson interferometer but, in contrast to spectral measurement by Fourier Transform Spectroscopy, our technique requires an OPD scan far shorter than the coherence length of the grating reflections. The technique is based on the high-resolution measurement of the phases of the complex analytic signals and of the relationship of these phases to interferometric delay. The analytic signals are derived via the Hilbert transform, allowing frequency domain filtering of individual signals within the signal processing. The technique has yielded a resolution of 0.007 nm for an OPD scan of 1.2 mm.

Demodulation of polarimetric optical fibre sensors by dispersive Fourier transform spectroscopy

We describe an interrogation technique for polarimetric optical fibre sensors, based on the measurement of the differential group delay between the eigenmodes of the birefringent sensing element by tandem interferometry using a low-coherence source. Measurement of the group delay has the advantage of avoiding the 2π rad ambiguity inherent in the phase measurement. This differential group delay is strongly dispersive, inhibiting measurement using central fringe identification. We measure the delay by dispersive Fourier transform spectroscopy, thus avoiding the errors introduced by dispersion. We demonstrate the application of this technique to temperature and strain measurement.

Statically scanned single and tandem low-coherence interferometers

Statically scanned single and tandem Michelson interferometer configurations are compared for the remote measurement of thermally induced group delay change in optically dispersive glass samples. A broadband tungsten filament bulb was used to illuminate the single interferometer, and a much narrower spectral bandwidth superluminescent diode (SLD) was used to illuminate the tandem interferometer. For a BK7 glass sample, measurements of thermally induced group delay changes were made with <0.5 fs root mean square error for optical path delay (OPD) scan lengths of only 260 µm when applied to low-coherence interferograms with signal-to-noise ratios as low as 6.5 dB. These results demonstrate the power of dispersive Fourier transform spectrometry (DFTS) applied to noisy, dispersion distorted low-coherence interferograms captured with non-mechanical, short path length scans. Further, following experimentally observed source bandwidth-induced measurement resolution limitations between the different illuminating sources, simulations were performed to examine this feature.

Non-Mechanically Scanned DFTS

Non-mechanically scanned dispersive Fourier transform spectrometry (DFTS) is reported for dispersion-insensitive measurements of thermally-induced change in dispersive group delay; optical path scan lengths of 260 microns yield 0.5fs resolution for a dispersive optical sample.

A statically scanned tandem interferometer

A static tandem Michelson interferometer configuration is reported for remote measurements of group delay change in a thermally modulated, optically dispersive BK7 glass sample. Using a superluminescent diode (SLD) to illuminate the interferometer, low-coherence measurement interferograms with signal-to-noise ratios as low as 16 dB were captured and subsequently processed using dispersive Fourier transform spectrometry (DFTS). Measurements of thermallyinduced delay change were made with < 2 fs root mean square error for optical path delay scans lengths of only 260 μm.