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.
We report strain characterization of birefringent fiber Bragg gratings fabricated in hydrogen-free photosensitive Ge/B
codoped PS1250/1500 fiber by high-intensity femtosecond 264 nm pulses. These anisotropic FBGs demonstrate high
PDL value of 8 dB but, unlike the FBGs inscribed in PM fibers, show shape preservation under strain of [0, 700 &mgr;&Vegr;] by
interrogation based on broad-band source and optical spectrum analyzer.
We report on high power issues related to the reliability of fibre Bragg gratings inscribed with an infrared femtosecond laser using the point-by-point writing method. Conventionally, fibre Bragg gratings have usually been written in fibres using ultraviolet light, either holographically or using a phase mask. Since the coating is highly absorbing in the UV, this process normally requires that the protective polymer coating is stripped prior to inscription, with the fibre then being recoated. This results in a time consuming fabrication process that, unless great care is taken, can lead to fibre strength degradation, due to the presence of surface damage. The recent development of FBG inscription using NIR femtosecond lasers has eliminated the requirement for the stripping of the coating. At the same time the ability to write gratings point-by-point offers the potential for great flexibility in the grating design. There is, however, a requirement for reliability testing of these gratings, particularly for use in telecommunications systems where high powers are increasingly being used in long-haul transmission systems making use of Raman amplification. We report on a study of such gratings which has revealed the presence of broad spectrum power losses. When high powers are used, even at wavelengths far removed from the Bragg condition, these losses produce an increase in the fibre temperature due to absorption in the coating. We have monitored this temperature rise using the wavelength shift in the grating itself. At power levels of a few watts, various temperature increases were experienced ranging from a few degrees up to the point where the buffer completely melts off the fibre at the grating site. Further investigations are currently under way to study the optical loss mechanisms in order to optimise the inscription mechanism and minimise such losses.
We report on a low-coherence interferometric scheme for the measurement of the strain and temperature dependences of group delay and dispersion in short, index-guiding, 'endlessly-single-mode', photonic crystal fibre elements in the 840 nm and 1550 nm regions. Based on the measurements, we propose two schemes for simultaneous strain and temperature measurement using a single unmodified PCF element, without a requirement for any compensating components, and we project the measurement accuracies of these schemes.
We report a Fourier Transform Spectroscopy scheme for the simultaneous demodulation of a Bragg grating sensor array and the measurement of the intra-grating spectral structure of the individual gratings of the array, based on referencing to a transversely multimode laser. The performance for this reference, relative to that of a single-mode reference, is evaluated in the context of coherence effects, temperature sensing, and high-resolution intra-grating spectral measurement. The results indicate the potential for grating demodulation across the full spectrum of the E, S, C and L optical telecommunications bands.
We demonstrate a Fourier Transform Spectroscopic approach to achieve high-resolution measurement of the structural detail of the reflected spectra from an infrared FBG grating array, with the inherent measurement capability over a wavelength range of 900 nm. Measurements on all gratings in the array are achieved in a single scan of the OPD, from an interferogram captured on a single InGaAs photodiode. We demonstrate the approach for measurements on standard gratings and in the case of a grating subjected to a nonuniform measurand. The OPD is referenced from a visible HeNe beam propagated on the array downlead, and we demonstrate the robustness of the measurement in the presence of the complex transverse modal structure of that reference beam.
We propose and demonstrate an interferometric technique for the high precision measurement of displacement, based on Hilbert transform processing of the interferometric signal. The experimental arrangement is that of a laser illuminated Michelson interferometer in which a mirror, mounted on the target under test, is a component mirror of the interferometer. The optical configuration retains the simplicity of the fringe counting arrangement. Using standard sampling electronics, we capture the high coherence interferogram for the target motion to be analysed. We process the sampled interferogram, using Hubert transform techniques to generate the complex analytic signal of the real valued interferogram. The phase of the complex signal is extracted, unwrapped, and converted to displacement information corresponding to each sampling instant. We have achieved a X/100 for the individual displacement measurements, for analysis of a 1OOOtm travel of the target. In all cases, the displacement data is readily processed to measure velocity and acceleration. We report on both bulk optic and fibre optic versions of the interferometer. We have applied the technique to the characterisation of the motion of translation stages, with the objective of assessing their potential efficacy in temporally scanned interferometers. We report on our measurements of the consistency of scan rate and on the degree of 'out-of-plane' motion of the stages.
We report an interferometric scheme for comprehensive characterisation of the dispersion of birefringence in high birefringence fibres over a broad wavelength range, based on a single OPD scan. The approach is shown to support dispersion-insensitive measurements in a multiplexed serial array.
A low-coherence optical fiber interferometric technique for simultaneous measurement of geometric thickness and group refractive index of highly dispersive materials is described. The technique, immune to the dispersion-induced asymmetry of the low-coherence interferograms obtained, overcomes some of the drawbacks associated with recently reported low-coherence approaches to this simultaneous measurement. The technique uses 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. Dispersion- insensitive measurements of the group delay imbalances in the measurement interferometer are made using dispersive Fourier transform spectrometry (DFTS). Sample thickness and group refractive index are calculated from these group delays which are unambiguously determined from the optical frequency dependence of the measured phases of the interferograms. Thickness measurements accurate to within 1 micrometer and group index measurements accurate to within one part per thousand have been achieved for BK7 and fused-silica glass samples in the thickness range 2000 to 6000 micrometers.
Optical fibre interferometers have been widely used to measure strain. Most of the interferometers used are also sensitive to temperature. These two parameters can be distinguished by measuring phase for two optical modes or for two wavelengths. A linear transformation converts the two values of phase into independent values for strain and temperature , but this transformation is usually ill-conditioned, and so magnifies measurement errors.