Jitendra Kumar

Enhanced Temperature

This letter reports a first time study on enhanced temperature (~800°C) stability of type-IIa as compared with type-I fiber Bragg gratings (FBGs) written by high repetition rate (5.5 kHz) 255 nm UV beam. Both the FBGs were written in high photosensitive Ge doped (~10 mole%) single mode fiber without hydrogen loading. Both the FBGs survived 800°C over 9 h, however, time rate of fall of type-IIa FBG reflectivity was ~3 times lower as compared with type-I. The high temperature stability of 255 nm written FBGs is attributed to enhanced core-cladding stress release in the strong seed grating written by highly stabilized interference fringes and high cumulative fluence of 255 nm, 5.5 kHz UV pulses.

({\sim}{800}^{\circ}{\rm C})

This paper presents an experiment and analysis on the factors affecting nonlinear evolution of Bragg wavelength with change in temperature in typical bare and embedded fiber Bragg grating-based (FBG) temperature sensors. The purpose of the study was to find the constants in the function required to evaluate temperature from Bragg wavelength shift. The temperature sensitivity of bare FBGs was found to increase with temperature elevation, and is different for FBGs written in different fiber types. The average temperature sensitivity increased by about 20% when the bare FBG temperature was elevated from 25°C to 525°C. The average temperature sensitivity of the embedded FBG sensor, investigated in the temperature range of 30°C-90°C, was a factor of 2-3 times larger than for bare FBG, depending on its fastened length with the substrate. Analytically, it is shown that the nonuniform behavior of temperature sensitivity in bare FBGs is the result of both the thermal expansion effect of the fiber and the temperature derivatives of the effective refractive index. The strain transfer and temperature coefficients of thermal expansion of the substrate affect the nonuniform behavior of temperature sensitivity in embedded FBG sensors.

Stability of Type-IIa FBG Written by 255 nm Beam

Abstract. An experimental study on hydrofluoric acid (HF)-based clad etching and chemical sensing characteristics of fiber Bragg gratings (FBG) of different reflectivity fabricated under different UV (255 nm) exposure times is presented. Two FBGs of reflectivity 11% and 93% were inscribed by phase mask–based exposition of the photosensitive fibers by a 5.5 kHz repetition rate of 255 nm UV pulses for 15 s and 10 min, respectively. These two FBGs, employed in an HF-based clad etching experiment, revealed a much higher etching rate of 2.03  μm/min for the grating of reflectivity 11% as compared to 1.69  μm/min for the grating of reflectivity 93%. The performance of these etched FBGs were also studied for refractive index sensing of the chemicals ethanol and ethylene glycol under different fiber etching times, hence of different residual cladding diameter. It was observed that the same refractive index sensitivity for both the chemicals could be achieved under smaller etching time, i.e., larger residual cladding diameter, for the FBG with lower reflectivity. This differentiating behavior of FBGs under etching and sensing may be linked to the different degree of densification in fused silica fiber cladding under different UV fluence exposures.

Study of the nonuniform behavior of temperature sensitivity in bare and embedded fiber Bragg gratings: experimental results and analysis.

This paper presents studies on the pulsed optogalvanic effect and isotope-selective excitation of Yb 555.648 nm (0 cm−1 → 17 992.007 cm−1) and 581.067 nm (17 992.007 cm−1 → 35 196.98 cm−1) transitions, in a Yb/Ne hollow cathode lamp. The Yb atoms were excited by narrow linewidth (500–1000 MHz) Rh110 and Rh6G dye based pulsed lasers. Optogalvanic signal inversion for ground state transition at 555.648 nm was observed beyond a hollow cathode discharge current of 8.5 mA, in contrast to normal optogalvanic signal at 581.067 nm up to maximum current of 14 mA. The isotope-selective excitation studies of Yb were carried out by recording Doppler limited optogalvanic signals as a function of dye laser wavelength. For the 581.067 nm transition, three even isotopes, 172Yb, 174Yb, and 176Yb, and one odd isotope, 171Yb, were clearly resolved. These data were compared with selective isotope excitation by 10 MHz linewidth continuous-wave dye laser. For 555.648 nm transition, isotopes were not clearly resolved, although isotope peaks of low modulation were observed.

Study on hydrofluoric acid-based clad etching and chemical sensing characteristics of fiber Bragg gratings of different reflectivity fabricated under different UV exposure times

Abstract. A numerical analysis on the refractive index modulation in first and second order of type I fiber Bragg gratings (FBGs) written by prism interferometer fringes is presented. The analysis of FBG written by biprism interferometers has been carried out to optimize the writing position and by Lloyd prism interferometer to optimize FBG length with respect to ultraviolet (UV) beam parameter. It is analytically shown that in the biprism fringe depth, the fiber positions of maximum reflectivity in first and second orders are different and both are less than the distance of maximum beam overlap. The refractive index modulation of FBGs written by Lloyd prism varies along the FBG length. The evolution and saturation of the FBGs written by biprism and Llyod prism are different due to difference in magnitude and/or profile of the UV fringes contrast in the FBG writing plane.

Studies on the Optogalvanic Effect and Isotope-Selective Excitation of Ytterbium in a Hollow Cathode Discharge Lamp Using a Pulsed Dye Laser

This paper presents studies on the long term thermal stability of regenerated fiber Bragg gratings (FBGs) at 1100 oC. The seed type-I FBGs of almost similar reflectivity (∼ 35 dB) were inscribed in hydrogen loaded fused silica fibers using high repetition rate (5.5 kHz) 255 nm UV radiation. These FBGs were subjected to multi-step annealing process upto 1100 oC spanning over 60 hours for regeneration and stabilisation of regenerated grating. The residual reflectivity of 40% and 10% was obtained for Ge doping of 3 mol% and 18 mol% respectively, which was stable even after ∼10 hours annealing at 1100 oC.

Analysis of ultraviolet fringes contrast on first and second order Fiber Bragg gratings written by prism interferometers

We report the maximum reflectivity (12 dB, 93.7 %) so far observed for FBGs written by biprism interferometer, in single mode photosensitive fiber without hydrogen loading.

Studies on the stability of regenerated fiber Bragg gratings at 1100 °C

The effect of Ge doping concentration on refractive index rollover fluence and thermal annealing characteristics of type IIa FBG are studied. The temperature sustainability was higher for grating written at higher refractive index rollover fluence.

Optimization of FBG writing by biprism interferometer

Abstract. This paper presents studies on the role of Ge-doping concentration (6 to 18 mol. %) in the refractive index rollover fluence and thermal annealing characteristics of type IIa fiber Bragg gratings (FBGs). A 255 nm UV beam of low-pulse energy density (∼2.2  mJ/cm2), nanosecond (∼30  ns) duration, and high-repetition rate (∼5.5  kHz) was used for FBG inscription. It is observed that the UV fluence needed for refractive index rollover was higher for fiber having low Ge-doping (∼6  mol.%). The temperature sustainability of these gratings has been studied in a multistep thermal annealing process up to 800°C. It was observed that the higher the total UV fluence required for refractive index rollover, the higher the temperature sustainability of a type IIa grating. The temperature rise of the fiber for a single UV pulse and at the maximum cumulative UV fluence was estimated for different Ge-doping concentrations. The thermal stability of the grating is linked to the amount of Ge-doping concentration of the fiber. These observations may be due to the fact that a high cumulative fluence resulted in a larger stress relaxation, leading to enhanced FBG temperature stability.