Type IIA Bragg Gratings Research Articles

Formation and Applications of the Secondary Fiber Bragg Grating.

Being one of the most proven fiber optic devices, the fiber Bragg grating has developed continually to extend its applications, particularly in extreme environments. Accompanying the growth of Type-IIa Bragg gratings in some active fibers, a new resonance appears at the shorter wavelength. This new type of grating was named “secondary Bragg grating” (SBG). This paper describes the formation and applications of the SBGs. The formation of the SBG is attributed to the intracore Talbot-type-fringes as a result of multi-order diffractions of the inscribing beams. The SBG presents a variety of interesting characteristics, including dip merge, high-temperature resistance, distinct temperature response, and the strong higher-order harmonic reflection. These features enable its promising applications in fiber lasers and fiber sensing technology.

Type IIa Bragg gratings formed in microfibers.

In this Letter, Type IIa Bragg gratings are inscribed into microfibers. The large germanium-doped core region of the multimode fiber provides the necessary photosensitivity to form a Type IIa grating when it is drawn down to the microscale. Reducing the diameter of the microfiber due to lower saturate modulation and the amplified tension-strain transformation effect can accelerate the formation of a Type IIa grating. This provides an efficient method for the fabrication of fiber gratings with 800°C temperature resistance.

Type IIA photosensitivity and formation of pores in optical fibers under intense ultraviolet irradiation

Formation of the type IIA Bragg gratings in germanosilicate optical fibers is studied. We report the observation of such a type of gratings in the standard single-mode fiber (Corning SMF-28) under different experimental conditions. A mechanism for the type IIA photosensitivity in optical fibers is proposed which is based on nucleation and evolution of pores from vacancy-type defects in fiber areas where a high level of mechanical stress is induced under intense ultraviolet (UV) light. Evolution of fiber core temperature under influence of a single 20 ns light pulse from a KrF excimer laser was measured and compared with theoretical calculations. It was shown that transient thermoinduced stress in the fiber core can achieve a level sufficient for effective nucleation of pores. A theory describing formation of pores in optical fibers has been developed and was used to estimate the pore nucleation rate, concentration, and other parameters of pore evolution for different levels of UV fluence and fiber core stress.

Formation of pores in the optical fiber exposed to intense pulsed UV radiation

A new method of forming refractive index gratings (type-IIA Bragg gratings) in the optical fiber by exposing it to intense UF laser radiation is suggested. Central in this method is the generation and propagation of micropores in the parts of the fiber where mechanical stresses are localized. It is shown that such parts are the center of the core and the core-cladding interface. The temperature of the radiation-heated germaniumdoped core is found experimentally and theoretically. Thermal stresses arising in the heated fiber are calculated. A theory of cracking and pore formation in the fiber exposed to intense laser pulses is worked out. The pore size distribution, pore formation and growth rate, and pore density versus time of laser action are estimated.

Characterization of thermal (in)stability and temperature-dependence of type-I and type-IIA Bragg gratings written in B–Ge co-doped fiber

The temperature-dependence and the thermal stability of UV-exposed type-I and type-IIA fiber Bragg gratings written in boron–germanium co-doped silica fiber were examined, in terms of their refractive-index modulations and the change in the Bragg wavelengths. The decay in the reflectivity of the gratings with time at different temperatures was studied and modelled according to a power-law and a master aging curve approach. The distribution of the activation energy, for these types of gratings, was evaluated to describe their thermal stability and then the comparison of their stabilities has been made. Temperature-induced reversible and irreversible changes in the grating characteristics were observed. The temperature-induced reversible spectral characteristics of type-I and type-IIA gratings were found to be different in nature. All these results were discussed within the framework of the current theoretical approaches for predicting the stability and operational lifetime of these types of gratings.

Optical power transversely scattered from fiber Bragg gratings: far-field measurements and modeling

Far-field properties of the optical power scattered from type IIA Bragg gratings photowritten within high-N.A. germanosilicate fibers have been studied under various experimental conditions. Experiments were carried out in order to measure the angular distribution of the diffracted beam intensity. The optical power detected in the far field was shown to be scattered around a cone centered on the fiber axis. The azimuthal distribution of scattered optical power was nonuniform. A theoretical model, which takes into account the LP01-mode coupling to radiation modes for wavelengths well below the Bragg wavelength, properly fits the features of the observed pattern. This theoretical model assumes the existence of a nonuniform UV-induced refractive index change across a plane perpendicular to the fiber axis.