A simple and novel distributed tilted fiber Bragg gratings (TFBGs) based transverse load sensing system using optical
frequency domain reflectometry (OFDR) is proposed and demonstrated for the first time. The transverse load compresses
an elastomer material around the fiber and induces a broadband loss in the spectrum of the TFBG. The additional loss is
directly measured by the return loss spectrum of Fresnel reflection points introduced in the Rayleigh backscattering
trace. With the wavelength sweeping characteristic of OFDR, we realized distributed sensing by interrogating each of the
TFBGs with different wavelengths and observed a minimum crosstalk effect between sensors.
The noise power spectrum of solid-state lasers - including fiber lasers - exhibits a characteristic peak at the relaxation
oscillation frequency. The tails associated with this peak extend to neighboring spectral ranges and may increase the
noise level above acceptable limits in applications using weak signals. One of the key factors to reduce the relative
intensity noise (RIN) amplitude is a low loss laser resonator. We describe a method to ultimately reduce the intensity
noise in single frequency phosphate fiber lasers by minimizing intra-cavity losses caused by fusion splices between
fibers made of different materials. Conventional fiber Bragg gratings written in silica fibers have been replaced with
gratings written in phosphate glass fibers. The quality of the intra-cavity fusion splice has been improved due to material
similarity. All-phosphate fiber laser devices have been built and tested utilizing the new gratings. The results show
relative intensity noise amplitudes that are very similar to those of conventionally fabricated devices. Challenges in the
grating writing process are currently preventing the new devices from surpassing their commercial counterparts in terms
of performance. However, this type of all phosphate glass fiber lasers may ultimately lead to a new generation of
commercial single frequency fiber lasers with improved intensity noise performance.
Previous work has demonstrated that the thermal stability of Fiber Bragg gratings can be influenced by pre- or postirradiation
of the grating with uniform (non-modulated) light, thereby changing the grating contrast (or modulation
index). We present new experimental results about the thermal stability of gratings where the contrast is determined by
the fiber-phase mask distance during the irradiation with excimer laser light. Due to the low spatial coherence of the
excimer laser light, the fringe contrast behind the mask drops from near 100% to less than 10% over 1 mm.
In this paper, we present the differential strain sensitivity characteristics of core and cladding modes in weakly TFBG. Both experiment and analysis results are presented, and they are well matched. The results show that there are three different strain sensitivity regions for cladding mode resonances: the short wavelength region, the ghost mode region and the nearly linear sensitivity change region between them. By monitoring the cladding modes with different strain sensitivities, and noting that the different cladding modes have similar temperature sensitivities, weakly TFBG are attractive candidates for more accurate temperature-independent strain sensors.
Fiber Bragg gratings with grating planes tilted at small angles relative to the fiber axis couple light both to backward propagating core modes and cladding modes. The resonant wavelengths for these mode couplings depend differentially on external perturbations. Using the core mode back reflection resonance as a reference wavelength, the relative shift of the cladding mode resonances can be used to selectively measure perturbations affecting the region outside the cladding independently of temperature. We have measured a relative wavelength shift lower than 0.4 pm/degree in conventional single mode fiber while the sensitivity to external changes in refractive index can be larger than 300 pm per % index change. Experimental results on the bending selective sensitivity (relative to uniform axial strain) are also reported.
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