We report for the first time successful inscription of high reflectivity Bragg grating in nanostructured core active fiber. Nanostructurization of the fiber core allows to separate the active and photosensitive areas and to distribute them all over the core. As a result unfavorable clustering between germanium and ytterbium particles is avoided. The distribution of discrete glass areas with feature size smaller than λ/5 results in effectively continuous refractive index profile of the fiber core. We present a single-mode fiber with built-in Bragg grating for laser application with the core composed of ytterbium and germanium doped silica rods. The core structure is arranged as a regular lattice of 1320 doped with ytterbium and 439 doped with germanium silica glass rods. The average germanium doping level within the core of only 1.1% mol allowed efficient inscription of Bragg grating. The nanostructured core was 8.6 μm and the internal cladding was 112 μm in diameter coated with low index polymer to achieve the double-clad structure. In the first proof-of-concept in the laser setup we achieved 35 % of slope efficiency in relation to launched power for the fiber length of 18 m. The output was single-mode with spectrum width below 1 nm. The maximum output power limited by pumping diode was 2.3 W. The nanostructurization opens new opportunities for development of fibers with a core composed of two or more types of glasses. It allows to control simultaneously the refractive index distribution, the active dopants distribution and photosensitivity distribution in the fiber core.
In this paper an application of fiber Bragg gratings (FBGs) based sensing system for stress measurement on the frame of the high mobility vehicle is reported. Demodulation of the strain value from the Bragg wavelength changes allows for fatigue strength calculation.
The magnetic resonance imaging (MRI) technique is a powerful diagnostic tool which is nowadays commonly used in many fields of medicine. In some cases, especially of the patients of intensive care units, it is highly recommended or even necessary to provide continuous monitoring of basic physiologic parameters, mainly the heart rate and the respiratory rate, during the MRI scan procedure. The presence of a strong magnetic field within the MRI chamber requires application of non-standard devices and solutions. The monitoring system needs to be immune to the strong magnetic field and simultaneously cannot negatively influence on the results of the scan. Therefore, application of optical sensing technologies could be potentially advantageous to fulfil these requirements. In this work we propose a novel optoelectronic measurement system, dedicated to monitoring of the patient during an MRI scan, immune to strong magnetic field and compatible with the MRI apparatus.
Fiber Bragg gratings (FBGs) are used as the sensing elements – the strain induced by the patient’s respiration and cardiac activity cause a change of the Bragg wavelength. These changes can be accurately measured and monitored in the time domain. The respiratory and heart rate can be extracted by further processing of the measured signal by dedicated software. The gratings are organized in a network to maximize the effective sensing area. Each of the FBGs has a different Bragg wavelength so that they can be connected in series.
The information from the sensors is read out using an interrogator based on an application specific photonic integrated circuit (ASPIC), designed and fabricated in an InP-based generic integration technology. The interrogator comprises a 36-channel arrayed waveguide grating wavelength demultiplexer, which outputs are connected to PIN photodiodes. Such a photonic circuit acts as a spectrometer and allows to reconstruct the reflection spectrum of many gratings simultaneously. An external superluminescent LED is used as the light source, however in the target configuration the source could be monolithically integrated with the interrogator. The Bragg gratings, the interrogator and the SLED are connected with each other using an optical circulator.
Initial tests of the monitoring system have been performed using a single fiber Bragg grating as the strain sensor and a commercially available optoelectronic interrogator. The fiber with an inscribed FBG was mounted using an epoxy glue on a PMMA board and deployed under the patient. Two signals can be distinguished out of the measured waves. The first one, with strong and slowly-varying peaks, reflects the respiration of the patient. The second signal, characterized by low-intensity and fast-varying peaks is a result of the cardiac activity. No influence of the magnetic field of the MRI instrument on the sensing system has been observed. The first results have confirmed both the correctness of the approach and the applicability of the system to monitoring the patient’s physical condition during MRI diagnosis.
This work was supported by the National Centre for Research and Development, project OPTO-SPARE, grant agreement PBS3/B9/41/2015.
This paper presents a method for writing a Fabry-Perot (F-P) cavity in the tapered fiber. General idea for writing the F-P
cavity in tapered fiber is based on utilization of chirped tapered fiber Bragg gratings (CTFBG) in counter-directional
configuration. In such approach, the chirp resulting from tapered fiber and linearly increasing periods of the grating, cancel
each other out. Presented numerical results consists of qualitative model, that provides the general design approach, as
well as quantitative analysis, that allows for more detailed studies on presented structure. For this purpose Coupled Mode
Theory (CMT) together with Transfer Matrix Method (TMM) were utilized.
In this paper a new concept of human breath detection by means of fiber Bragg grating (FBG) based temperature sensor
is reported. In presented approach, temperature of exhaled air is directly measured by FBG, and its Bragg wavelength
changes allow for determine breathing rate. Experimental results of comparison the FBG sensor with electronic one,
show significant advantages of our solution, especially when faster and/or plate breathing is detected. Another advantage
of proposed fiber Bragg grating based breathing rate sensor is its ability to work in the high electromagnetic field
environment, for example during the magnetic resonance imaging.
A concept, technology and applications of novel linearly chirped Bragg gratings written in fused tapered optical fiber are presented. The novelty of proposed structure is based on 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. Depending on the orientation of the optical fiber taper relative to the variable-pitch phase mask during UV exposure, tapered and linearly chirped fiber Bragg gratings (TCFBG) with resultant co-directional or counter-directional chirps can achieved and thus their spectral responses can be widely tailored. Moreover the detailed numerical analysis of spectral shaping in TCFBGs is presented. Strain sensing applications of TCFBGs written in both co-direction and counter-directional chirp configurations are proposed and their advantages over typical tapered (uniform) FBG based sensors are highlighted.
This article presents implementation of the Simulated Annealing (SA) algorithm for tapered fiber Bragg gratings (TFBGs) design. Particularly, together with well-known Coupled Mode Theory (CMT) and Transfer Matrix Method (TMM) the algorithm optimizes the group delay response of TFBG, by simultaneous shaping of both apodization function and tapered fiber transition profile. Prior to the optimization process, numerical model for TFBG design has been validated. Preliminary results reveal great potential of the SA-based approach and with proper definition of the design criteria may be even applied for optimization of the spectral properties of TFBGs.
In this paper, the concept and experimental characterization of tilted fiber Bragg grating (TFBG) based sensor for temperature and liquid level measurement are presented. It is shown that, when liquid level increases the peak amplitudes of cladding modes linearly decreases (in dB). In turn, changes in temperature causes a shift of the TFBG transmission spectrum, which can be accurately measured by monitoring the Bragg wavelength corresponding to the liquid level independent core mode. The main advantages of proposed sensor are simple design as well as linear responses to liquid level and temperature.
In this paper, extensive study on the apodization of fiber Bragg gratings is presented. Particularly, not only influence of non-true and true apodization for gratings with uniform period is analyzed, but also the influence of both of these techniques impact on spectral properties of chirped gratings. Both numerical and experimental results are provided. For true apodized uniform gratings, impact on spectral response of true apodization is evaluated for three apodization windows, i.e. cos2, Gaussian and triangle.
In this paper fiber ring lasers (FRL) as interrogation units for distributed fiber Bragg grating (FBG) based sensor networks are studied. In particular, two configurations of the fiber laser with erbium-doped fiber amplifier (EDFA) and semiconductor optical amplifier (SOA) as gain medium were analyzed. In the case of EDFA-based fiber interrogation systems, CW as well as active-mode locking operation were taken into account. The influence of spectral overlapping of FBGs spectra on detection capabilities of examined FRLs are presented. Experimental results show that the SOA-based fiber laser interrogation unit can operate as a multi-parametric sensing system. In turn, using an actively mode-locked fiber ring laser with an EDFA, an electronically switchable FBG based sensing system can be realized.
In this paper we show preliminary experimental studies on accelerated aging-tests of tapered fiber Bragg gratings (TFBG). Examined gratings were fabricated on tapered and then hydrogenated optical fibers using uniform phase mask and scanning technique. In particular, the impact of isothermal annealing of TFBGs on their spectral and dispersion characteristics was analyzed. Experimental results show that although reflectance levels of TFBGs significantly reduce due to the aging process, slopes of the group delay characteristics remain practically unchanged. New knowledge on spectral and dispersion characteristics of TFBGs written in hydrogen loaded optical fibers is important in possible applications, where long-term stability and reliability is required.
In this work, numerical model of tapered fiber Bragg grating (TFBG) based on transfer matrix method (TMM) and coupled mode theory (CMT) is presented. It describes the behavior of TFBG when the axial strain is applied. In particular, the influence of various tensile forces on dispersion characteristics of tapered fiber Bragg gratings as fiber optic components with variable group delay were examined. Besides the numerical results, measured group delay characteristics are presented as well and compared with theoretical ones.
In this paper the comparative study of spectral transmission characteristics of tilted Bragg gratings made using uniform
phase mask and chirped phase mask are presented. Results are focused on low tilt angles with special interest on
cladding modes behavior. The spectral characteristic shape for simultaneously tilted and chirped Bragg gratings point out
its potential applications as amplitude discriminator in fiber sensing applications and gain flattening element for EDFA.
This paper describes possible use of Sagnac interferometer, based on the polarization maintaining fiber, as an optical microphone. Operation of investigated setup is based on the refractive index changes induced by acoustic wave and fiber strain. In the conducted experiment, Sagnac interferometer working as an optical microphone was able to achieve 6 dB bandwidth up to 4.5 kHz. Possibilities for linear operation was examined as well. Experimental results are preceded by a mathematical analysis of the operating principles.
In this paper automated system for simultaneous measurement of spectral response and dispersion characteristics of optical fibers and passive fiber optic components (such as fiber Bragg gratings) is presented and validated. Using this setup transmitted and reflected spectra can be measured. The system is validated by comparison relative group delay characteristics of dispersion shifted fiber (DSF) obtained using presented setup as well as commercially available equipment. Moreover, the spectral reflection characteristics of linearly chirped fiber Bragg grating obtained using both presented system and optical spectrum analyzer are compared and discussed.
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.
This paper presents design of the linear binary phase diffractive axicon with radially varying diffraction efficiency. The designed element was manufactured using the method based on the application of the High Energy Beam Sensitive (HEBS) glasses. The parameters of the produced axicon were measured showing close resemblance to the Bessel beam. The obtained results lead to the conclusion that the linear diffractive axicon with radially varying diffraction efficiency represents an asymptotic version of the Bessel beam and can be successfully used as a single element for producing focal segments of constant width and constant axial intensity.
We present simulations of averaged intensity of light behind apodized phase masks. Two types of apodization profile were assumed: Gaussian and tanh. In reality, because of limitations of electron-beam exposure system used for phase mask fabrication, we simulated phase masks with eight values of step height. For comparison, the averaged intensity distributions behind ideal phase masks with variable intensity were also calculated. Simulations and description of
intensity distribution perturbations due to phase jumps in real apodized phase masks were performed.
In this paper we present results of research on fiber Bragg gratings (FBGs) fabricated on hydrogen-loaded fibers using scanning phase mask method. The influence of temperature on Bragg wavelength, reflectivity and FWHM for various exposure time and hydrogenation time was investigated. We also have carried out aging tests of fiber Bragg gratings using climatic chamber. Results of these tests can be useful for optimization of FBGs fabrication. We have estimated of fiber Bragg gratings usability as wavelength standards for calibration of fiber optics devices especially optical spectrum analyzers (OSA).
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.
The key to efficient systems based on fiber bragg gratings is a flexible and accurate inscription of these components with demanded parameters. This paper presents basic information about fiber Bragg gratings and offers a flexible method of writing long gratings at any wavelength with the same phase mask.
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