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The cancellation of birefringence in the distributed Bragg reflector based on 90° rotation offset method is demonstrated. It is found that the birefringence, which causes the peak bifurcation has been eliminated and a single peak is produced at each resonance in the output spectrum. This modification is an economic solution for eliminating the birefringence of the optical fibre devices.

CO2 laser has been widely used in material processing, manufacturing, and medical and military applications since its invention in 1964. Typically for optical fiber components fabrication and treatment, CO2 laser has been reported intensively in stress and birefringence modification, long period grating fabrication, thermal regeneration, optical device fabrication, surface polishing, photosensitivity enhancement, and so on. The reason behind this is due to the advantages of high absorption, fast thermal response, low contamination, dynamic control, and focused heating area owned by the CO2 laser treatment technique. In this paper, a wide variety of CO2 laser applications in optical fiber components fabrication and treatment are reviewed in terms of procedure and principle. The advantages of using CO2 laser and the supremacy of the optical fiber components fabricated or treated by CO2 laser are discussed as well.

In this study the thermal degradation of gratings inscribed in three types of fiber namely, PS 1250/1500, SM 1500 and zero water peak single mode fiber is demonstrated. A comparative investigation is carried out on the aging characteristics of the gratings at three different temperature ramping rates of 3 °C/min, 6 °C/min and 9 °C/min. During the thermal annealing treatment, a significant enhancement in the grating reflectivity is observed for PS 1250/1500 fiber from ∼1.2 eV until 1.4 eV which indicates a thermal induced reversible effect. Higher temperature ramping rates lead to a higher regeneration temperature. In addition, the investigation also reflects that regardless of the temperature ramping rate the thermal decay behavior of a specific fiber can be successfully characterized when represented in a demarcation energy domain. Moreover, this technique can be accommodated when predicting the thermal decay characteristics of a specific fiber.

In this study, the thermal degradation of a fiber Bragg grating written on hydrogen loaded photosensitive Germanium/Boron co-doped fiber with 193 nm ArF excimer laser is investigated. A noteworthy increase in the grating reflectivity during annealing at 425 °C is observed indicating a thermal induced reversible effect deviating from the general thermal decay behavior. Both stepwise and continuous annealing procedures indicate a similar trend in the accelerated aging characteristics when analyzed with respect to demarcation energy (Ed). In the characterization of the lifetime of the treated FBG, a thermal sustainability of thousand years with less than 2% decay resulting in a grating reflectivity of 98% is observed at 300 °C for a grating as short as 10 mm. The ease of characterization and obtaining detailed information through the analysis of transmission spectra is also considered as an advantage in this study. These thermal enduring FBG sensors based on thermal annealing over a temperature range of 200–400 °C lay the foundation for a new direction of research in the investigation of durable temperature sensors.

The thermal regeneration of fiber Bragg gratings inscribed on hydrogenated gallosilicate optical fiber, fabricated using a modified chemical vapor deposition (MCVD) technique followed by a solution doping process, is demonstrated for the first time. A type-I seed grating is thermally regenerated and the evolution of the grating reflectivity in a temperature range between 25 °C and 720 °C is investigated. Temperature responses of 15.2 pm/°C and 15.0 pm/°C are obtained during heating and cooling processes of the regenerated grating (RG), respectively, when performing thermal calibration over a temperature range of 25 °C–750 °C and vice versa. Gallosilicate-based RGs are potential candidates as high-temperature resistant gratings in rare-earth-doped fiber, since they provide high photosensitivity and thermal sustainability with the involvement of a single dopant with a concentration as low as 5 wt%.

This work demonstrated for the first time, the thermal regeneration of two and four modes graded index fiber Bragg gratings using high temperature tube furnace. During the regeneration process, the seed grating is erased and a new grating with lower index contrast is formed. The thermal calibration shows that the temperature sensitivity of regenerated grating is slightly higher for fiber with larger core. On the other hand, the regeneration temperature is lower for fiber with smaller core. The temperature sustainability up to 900 °C is observed for the produced regenerated gratings in few mode fibers.

In this paper, we have demonstrated birefringence modification in regenerated grating in polarization maintaining fiber (RGPMF) by using a CO 2 laser annealing technique. After conducting an isothermal annealing procedure followed by a slow cooling process, the birefringence of the RGPMF has been increased. This phenomenon can be explained by the changes in the thermal expansion coefficient and glass transition temperature of the stress applying part at different cooling rates. This process was reversible by reannealing with a subsequent fast cooling process. In our observation, the birefringence exponentially increases with a decreasing cooling rate. The highest and lowest records in the birefringence throughout the annealing process are $4.72 \times 10^{-4}$ and $3.46 \times 10^{-4}$ , respectively (a difference of $1.26 \times 10^{-4}$ ). This finding is useful for the study of the birefringence modification, measurement range, sensitivity, and accuracy of PMF or PMF-related devices.

This study reports on the demonstration and characterisation of a short cavity fibre Fabry-Perot resonator (FPR) for the detection of ultrasonic waves propagating through a solid medium over a frequency range of 1 kHz–10 MHz. The proposed sensing device has some outstanding features in terms of simple fabrication technique, frequency response and high sensitivity over a wide frequency range. The response of the surface-mounted FPR sensor showed good agreement with that of a commercially available piezoelectric transducer during ultrasonic excitation.

In this letter, a pair of regenerated gratings with Bragg wavelengths of 1547 and 1304 nm in a single piece of single-mode fiber has been fabricated for simultaneous monitoring of temperature and strain in ultrahigh temperature environment. A $3\times 3$ matrix is used to characterize the cross sensitivity of the proposed sensor in which the strain sensitivity varies with temperature change. The proposed structure exhibits the deviation of 28.3 $\mu \varepsilon $ and 4.1 °C at the ranges of 0–1000 $\mu \varepsilon $ and 25 °C–900 °C, respectively.

In this paper, we demonstrated an optical notch filter constructed from a periodic microbent fiber Bragg grating attained by using two copper-wire-wound slabs. In the reflection spectrum, sideband peaks are created as a result of mechanically induced modulation on the grating period, and a higher number of peaks are observed when greater modulation is applied. The peak wavelength spacing depends upon the period of microbending, which can be varied by changing the diameter of the winding wire or the angle of placement of fiber Bragg grating placed in between the two wound slabs. Moreover, the trend of Bragg transmission loss (BTL) for the transmission spectrum changes with the changing of modulation period and amplitude. The proposed technique is very stable, and it can be used as an optical Gaussian notch filter.

Grating inscription in a Ga-doped silica core fiber (~5 wt. % Ga) has been demonstrated using ArF (193 nm) and KrF (248 nm) excimer lasers. In a comparative study with germanosilicate fiber with similar Ge concentration, a Ga-doped silica core fiber shows greater photosensitivity to an ArF excimer laser due to the higher absorbance in the region of 190-195 nm. In addition, the photosensitivity of a Ga-doped silica core fiber has been greatly enhanced with hydrogenation. Ga-doped fibers are potential photosensitive fibers for fiber Bragg grating production with an ArF excimer laser.

In this work, we have demonstrated for the first time grating regeneration in hydrogenated fibers by direct CO(2) laser annealing. During the annealing process, the center wavelength redshifts as the intensity of the focused CO(2) laser on the grating is elevated. The reflectivity of the grating begins to decay as the temperature induced in the grating approaches the erasure temperature. The grating is completely erased and regenerated afterwards. The observed spectral results have provided the proof of occurrence of dehydroxylation and stress relaxation in the fiber core during the annealing process. Regenerated gratings with low loss, good temperature sensitivities and sustainability have been successfully developed by this technique.

Grating regeneration using CO₂ laser has been demonstrated. Bragg wavelength redshifts as the irradiated laser power is elevated. The grating reflectivity begins to decay as the induced temperature is approaching the erasure temperature. The grating is completely erased and regenerated afterward.

In this work, we have demonstrated thermal stress relaxation in regenerated fiber Bragg gratings (RFBGs) by using direct CO₂-laser annealing technique. After the isothermal annealing and slow cooling process, the Bragg wavelength of the RFBG has been red-shifted. This modification is reversible by re-annealing and rapid cooling. It is repeatable with different cooling process in the subsequent annealing treatments. This phenomenon can be attributed to the thermal stress modification in the fiber core by means of manipulation of glass transition temperature with different cooling rates. This finding in this investigation is important for accurate temperature measurement of RFBG in dynamic environment.

In this study, a technique for measuring the grating visibility of the fiber Bragg grating (FBG) based on bent-spectral analysis is proposed. From varying ac and dc coupling coefficients at different bending radii, the grating visibility is estimated with the aid of a simple mathematical model. The investigation begins with the estimation of the grating visibility from the transmission spectra of the FBG during the inscription process. After that, the FBGs are subjected to a bending test with reducing radii, and again the transmission spectra are recorded. It is shown that the estimated grating visibility is in agreement with the result determined from the earlier inscription process.

This Letter presents a simple mathematical model developed from coupled-mode theory to describe the relationship between Bragg transmission loss (BTL), grating length, coupling coefficients, and bending loss in a bent fiber Bragg grating. In our investigation, the finding indicates that the decrement of BTL can be attributed to the increasing bending loss and degradation of both dc and ac coupling coefficients as the bending radius decreases. Besides, the center wavelength shifts as a result of coupling coefficients degradation. The validity of the proposed model is supported by experimental result.