Lingyun Xiong

Directional Bend Sensor Based on Re-Grown Tilted Fiber Bragg Grating

A novel fiber optic bend sensor is implemented by using a re-grown tilted fiber Bragg grating (TFBG) written in a small core single mode fiber with UV overexposure. The spectrum of the re-grown TFBG contrasts with that of normal TFBG by exhibiting large differences in the amplitude between neighboring symmetric (LP0m) and asymmetric (LP1m) cladding mode resonances, moreover each asymmetric cladding mode resonance splits into two peaks (corresponding to two orthogonal polarization states). The differential response of the three individual resonances of such group provides quantitative information about the magnitude and directions of bends in the TFBG. Numerical simulations indicate that the changes in the cladding-mode profiles in a bent fiber are responsible for this behavior through their impact on coupling coefficients. A bend sensitivity of 0.4 dB. m (for the 18th order group of cladding modes) is experimentally demonstrated within a range of 0-10.6 m- 1.

Differential sensitivity characteristics of tilted fiber Bragg grating 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.

Differential strain sensitivity of higher order cladding modes in weakly tilted fiber Bragg gratings

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.

Polarization-Resolved Near- and Far-Field Radiation from Near-Infrared Tilted Fiber Bragg Gratings

The radiation pattern of near-infrared light from 10° tilted fiber Bragg gratings is measured as a function of position along the grating, azimuthal direction, and input light polarization. An anomalous spatial periodicity in the radiation pattern is found and assigned to the presence of imperfect zero-order nulling and on the uncertainty in the exact alignment between the optical fiber and the phase mask used to form the grating. Furthermore, the intensity of the radiated light is correlated with the transmission spectrum of the tilted grating, showing narrowband spectral regions of increased and decreased radiation.

Effect of writing beam spatial coherence on fiber Bragg grating modulation contrast and thermal stability

We present a method to fabricate fiber Bragg gratings with adjustable refractive index contrast by using the standard phase mask technique. A theoretical analysis of the diffracted field from the phase mask is performed by considering the effect of the spatial coherence of the incident UV beam. The numerical results show that the grating index contrast decreases as the separation between the fiber and the phase mask increases. Strong gratings with various index contrasts have been inscribed in hydrogen-loaded single mode fibers at different writing distances, and the measured index contrast values are in good agreement with the simulation results. Furthermore, thermal decay tests on the gratings demonstrate that the thermal stability of the grating reflectivity is improved for those gratings fabricated at larger separations between the fiber and the phase mask. These results suggest a one-step process to fabricate gratings with an enhanced thermal stability.

Deep UV-induced near-infrared photodarkening of Er/Yb-doped and undoped phosphate fibers.

Photodarkening (PD) of Er/Yb-doped and undoped phosphate fibers caused by pulsed 193 nm irradiation from an ArF excimer laser to form Bragg grating mirrors is investigated. Doped and undoped phosphate fibers exhibit the same level of significant UV-induced PD loss, which is associated with the formation of a color center band at 467 nm. The UV-induced absorption extends into the NIR and creates a loss on the order of 1 dB/cm across the C-band. Photo-bleaching by a high-power supercontinuum source, and thermal-bleaching processes are performed on photodarkened samples. Both bleaching processes are found effective to fully erase the UV-induced PD loss.

Low-noise single frequency all phosphate fiber laser

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.

Photosensitivity and thermal stability of UV-induced fiber Bragg gratings in phosphate glass fibers

The photosensitivity of highly Er/Yb doped and undoped phosphate glass fibers is characterized under irradiation with intense pulsed 193 nm light from an ArF excimer laser through a phase mask. The ultraviolet photosensitivity of the active fibers is shown to be roughly half that of the passive fibers. We also demonstrate that the strong growth of the fiber Bragg grating reflectivity observed upon heating at temperatures between 100 – 250 °C is directly related to the UV irradiation time, but not to the size of the index modulation of the seed grating or even to the fiber type (Er/Yb doped or undoped). The conditions to reliably obtain final index modulations amplitudes between 5 and 10 × 10−5 are given.

Thermal stability of Excimer laser-written Fiber Bragg Gratings as a function of fiber/phase mask distance

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.