Tomasz Osuch

Numerical model of tapered fiber Bragg gratings for comprehensive analysis and optimization of their sensing and strain-induced tunable dispersion properties

A versatile numerical model for spectral transmission/reflection, group delay characteristic analysis, and design of tapered fiber Bragg gratings (TFBGs) is presented. This approach ensures flexibility with defining both distribution of refractive index change of the gratings (including apodization) and shape of the taper profile. Additionally, sensing and tunable dispersion properties of the TFBGs were fully examined, considering strain-induced effects. The presented numerical approach, together with Pareto optimization, were also used to design the best tanh apodization profiles of the TFBG in terms of maximizing its spectral width with simultaneous minimization of the group delay oscillations. Experimental verification of the model confirms its correctness. The combination of model versatility and possibility to define the other objective functions of Pareto optimization creates a universal tool for TFBG analysis and design.

System for modification of exposure time in fiber Bragg gratings fabrication with using scanning phase mask method

In this paper there are described setup for modification of exposure time in fiber Bragg gratings fabrication with using scanning phase mask method. Using this laboratory stage we are able to write uniform, apodized and chirped gratings. Depending on exposure time and grating length it is possible to write gratings with various attenuation for Bragg wavelength. Also, it is possible to change Bragg wavelength, and obtain chirped gratings by linear and gradient post-processing respectively.

Simultaneous Measurement of Liquid Level and Temperature Using Tilted Fiber Bragg Grating

We present a tilted fiber Bragg grating (TFBG)-based fiber optic sensor for the simultaneous measurement of liquid level and temperature. Due to different responses of cladding modes and core mode in TFBG spectrum to liquid level and temperature changes, both values can be measured independently. The examined 3.5° TFBG exhibits linear liquid level, as well as temperature, responses and sensitivities of -0.456 ± 0.009 dB/mm and 11.4 ± 0.2 pm/°C, respectively. In addition, greater sensor applicability is proposed for liquids having lower refractive indexes using TFBG with greater tilt angle. The presented system is capable of being applied in chemical, food, and automotive industries as well as, thanks to its all-fiber structure, in potentially hazardous environments where the immunity to electromagnetic fields or/and electric isolation is required.

Fiber-Optic Strain Sensors Based on Linearly Chirped Tapered Fiber Bragg Gratings With Tailored Intrinsic Chirp

In this paper, spectrally tailored tapered chirped fiber Bragg gratings (TCFBGs) are considered for use as strain sensors. Both gratings were written in fused tapered optical fiber using linearly chirped fiber Bragg gratings in co-directional and counter-directional chirp configurations. Theoretical and numerical analysis as well as experimental verification of the influence of applied strain on spectral width of both TCFBG structures were carried out. The results show that TCFBGs exhibit monotonic strain response over the wide operating range of the applied force. Compared with standard tapered FBG written using uniform phase mask, in the case of co-directionally written TCFBG with substantially larger grating chirp the monotonic operating range can be easily broadened toward the higher strain values (even above the applied force at which the optical fiber breaks). In turn, the intrinsic chirp of the counter-directionally written TCFBG can be tailored in such a way as to ensure that the monotonicity of its strain response is always satisfied when tensile force is applied. This is due to the spectral broadening of the reflected spectrum when strain increases.

Fabrication of phase masks with variable diffraction efficiency using HEBS glass technology

A new fabrication method of apodized diffractive optical elements is proposed. It relies on using high energy beam sensitive glass as a halftone mask for variable diffraction efficiency phase masks generation in a resist layer. The presented technology is especially effective in mass production. Although fabrication of an amplitude mask is required, it is then repeatedly used in a single shot projection photolithography, which is much simpler and less laborious than the direct variable-dose pattern writing. Three prototypes of apodized phase masks were manufactured and characterized. The main advantages as well as limitations of the proposed technology are discussed.

Modeling of amplification and light generation in one-dimensional photonic crystal using a multiwavelength transfer matrix approach

We present an analysis of amplification and lasing in one-dimensional isotropic nonlinear photonic crystal (1D PC), which is based on a generalized (multiwavelength) transfer matrix method. This approach was used for modeling a Raman signal amplification in 1D PC and in an homogenous structure, showing advantages of a stratified medium. Moreover, the threshold operation of a 1D PC Raman laser is studied, assuming both strong as well as depleted pump. The normalized threshold gain characteristics for various end reflections and photonic crystal laser length were calculated.

Numerical analysis of double chirp effect in tapered and linearly chirped fiber Bragg gratings

In this paper, a theoretical analysis of recently developed tapered chirped fiber Bragg gratings (TCFBG) written in co-directional and counter-directional configurations is presented. In particular, the effects of the synthesis of chirps resulting from both a fused taper profile and a linearly chirped fringe pattern of the induced refractive index changes within the fiber core are extensively examined. For this purpose, a numerical model based on the transfer matrix method (TMM) and the coupled mode theory (CMT) was developed for such a grating. The impact of TCFBG parameters, such as grating length and steepness of the taper transition, as well as the effect of the fringe pattern chirp rate on the spectral properties of the resulting gratings, are presented. Results show that, by using the appropriate design process, TCFBGs with reduced or enhanced resulting chirp, and thus with widely tailored spectral responses, can be easily achieved. In turn, it reveals a great potential application of such structures. The presented numerical approach provides an excellent tool for TCFBG design.

Quasi-Uniform Fiber Bragg Gratings

A new simple and versatile method for inscribing fiber Bragg gratings with an arbitrarily chosen Bragg wavelength is presented. Owing to the optimization of the irradiation process and application of the phase mask with a properly matched chirp, the method offers the ability to adjust the FBG resonance wavelength adjustment within a range of tens of nanometer. However; the resulting grating has a chirp; its spectral properties closely resemble those of the uniform one, which explains why such gratings are called quasi-uniform fiber Bragg gratings (QUFBGs). Optimization of the inscription process involved a two parameter Pareto method. The research presented here was conducted for a 2.5 cm phase mask with a 0.35 nm/mm chirp and allowed for the modelling, optimization and inscription of QUFBGs of FWHM 0.8-0.9 nm, with a transmission minimum lower than -8 dB and Bragg wavelength adjustment range of 11.4 nm. It was estimated that if extended to a commercially available 15 cm phase mask, the method would allow a record-breaking 74.7 nm adjustment range to be obtained. Together with clear consistency of the numerical and experimental results with the design assumptions for QUFBGs inscription, this record value makes the proposed method competitive with more sophisticated and less reproducible interferometric techniques.

Modeling of fiber Bragg gratings written in tapered optical fibers

In this paper the numerical results of tapered fiber Bragg gratings modeling are shown. Presented model, based on coupled mode theory and transfer matrix method for tapered fiber Bragg gratings, gives highly accurate results for reflection width evaluation. Few simulations were made for three different configurations of tapered fiber Bragg structures, as well as comparison between theoretical and experimental results.

Temperature Independent Tapered Fiber Bragg Grating-Based Inclinometer

A new concept for an optical fiber inclinometer based on tapered fiber Bragg grating (TFBG) is presented. The sensor was characterized by the experimental measurement of the dependence of inclination angle on the spectral response in fluctuating temperature conditions. The research showed that the TFBG made it possible to simultaneously measure both the inclination angle and temperature, and to operate as a temperature insensitive 0°-90° tilt sensor, in a different interrogation regime. Inscription of the FBG in the tapered optical fiber section offered not only sensor miniaturization, but also the possibility of temperature sensing by the fiber section adjacent to the inclination area. Moreover, due to the reflective mode of the TFBG, the sensor offers excellent sensitivity due to the double-loss operation.