A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.

ABSTRACT4This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e., the strong nonuniformity of the strain field), the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical fiber Bragg gratings with different gage lengths are embedded near the notch tip. The form of the spectra in transmission varies between gages that are at different distances from the notch tip under given loading conditions. This variation is shown to be due to the difference in the distribution of strain along the gage length. By using the strain field measured using electronic speckle pattern interferometry on the specimen surface and a discretized model of the grating, the spectra in transmission are then calculated analytically. For a known strain distribution, it is then shown that one can determine the magnitude of the applied force on the specimen. Thus, by considering the nonuniformity of the strain field, the optical fiber Bragg gage functions well as an embedded strain gage near the stress concentration.KEY WORDS--Optical fiber sensor, Bragg grating, embedded sensor, strain distribution, nondestructive evaluationThe use of optical fiber gages to measure temperature, strain, or even detect fracture in a material has great potential due to their relatively small size, sensitivity and immunity to electrical fietds.l Surface-mounted optical fibers, for example, have been used in a variety of strain gage applications. 2 Furthermore, due to their dimensions, optical fiber gages can be embedded unobtrusively into materials, particularly composites already containing fiber reinforcements. However, once an optical fiber is embedded, the interpretation of the gage response becomes more complex due to the effects of May 20, 1999. Final manuscript received: October 10, 2000 interface between the fiber and the material, as well as the multiple components of strain applied to the fiber. 3-6 The problem is further complicated when the strain field surrounding the gage is not sufficiently uniform with respect to the scale of the gage length.As an example, the optical fiber Bragg grating (OFBG) sensor permits the localized measurement of axial strain in an optical fiber. In comparison with a simple optical fiber displacement gage, the response of the OFBG sensor is only affected by the strain (or temperature) field at the location of the grating and not along other portions of the opticai fiber. This property makes the OFBG sensor especially useful for measuring localized phenomena such as the strain conditions near a region of fracture. 7The conventional treatment of the OFBG as a strain gage assumes that the reflected spectrum is a single distinct peak whose shift is linearly proportional to the applied strain. Naturally, this assumption is only valid if the gage le...

We present a digital holographic microscope that permits one to image polarization state. This technique results from the coupling of digital holographic microscopy and polarization digital holography. The interference between two orthogonally polarized reference waves and the wave transmitted by a microscopic sample, magnified by a microscope objective, is recorded on a CCD camera. The off-axis geometry permits one to reconstruct separately from this single hologram two wavefronts that are used to image the object-wave Jones vector. We applied this technique to image the birefringence of a bent fiber. To evaluate the precision of the phase-difference measurement, the birefringence induced by internal stress in an optical fiber is measured and compared to the birefringence profile captured by a standard method, which had been developed to obtain high-resolution birefringence profiles of optical fibers.

In this paper, the behaviour of fibre Bragg grating sensors subjected to transversal as well as axial strains is characterized, both in the case of low-birefringent and polarization-maintaining single-mode optical fibres. Two configurations are considered. Firstly, diametrical compression is studied and the results compared to those previously obtained in the literature. Secondly, the sensors are embedded in an epoxy specimen and their response monitored when the latter is subjected to biaxial loading. In both cases, the experimental results are compared to those obtained by means of finite-element simulations and an appropriate analytical description of the opto-mechanical response of polarization-maintaining fibres.

The tension on the core of single-mode fibers is strongly increased by the formation of a Bragg grating. This tension increase lowers the refractive index because of the photoelastic effect. On the other hand, the compaction of the core network results in an increased refractive index. The two contributions are evaluated from axial stress measurements, from the determined index modulation amplitude, and from the mean index change of the Bragg gratings. The total Bragg grating index modulation is smaller than the compaction-induced index modulation by 30%–35% because of the photoelastic effect.