Search Results (33)

Fiber Bragg gratings (FBGs) inscribed by UV light and different femtosecond laser techniques (phase mask, point-by-point, and plane-by-plane) were exposed—in several irradiation cycles—to accumulated high doses of gamma rays (up to 124 MGy) and neutron fluence (8.7 × 10¹⁸/cm²) in a research-grade nuclear reactor. The FBG peak wavelengths were measured continuously in order to monitor radiation-induced shifts. Gratings inscribed on pure silica core fibers using near-IR femtosecond pulses through a phase mask showed the smallest shifts (<30 pm), indicating that these FBGs are suitable for temperature measurement even under extreme ionizing radiation. In contrast, the pointwise inscribed femtosecond gratings and a UV-inscribed grating showed maximal shifts of around 100 pm and 400 pm, respectively. Radiation-induced red shifts are believed to arise from gamma radiation damage, which may partially recover after irradiation is stopped. At the highest neutron exposures, grating peak blue shifts started to appear, apparently due to fiber compaction. Full article

This paper presents a new type of phase-shifted Fiber Bragg Grating (FBG): the sliced-FBG (SFBG). The fabrication process involves cutting a standard FBG inside its grating region. As a result, the last grating pitch is shorter than the others. The optical output signal consists of the overlap between the FBG reflection and the reflection at the fiber-cleaved tip. This new fiber optic device has been studied as a vibration sensor, allowing for the characterization of this sensor in the frequency range of 150 Hz to 70 kHz. How the phase shift in the FBG can be controlled by changing the length of the last pitch is also shown. This device can be used as a filter and a sensing element. As a sensing element, we will demonstrate its application as a vibration sensor that can be utilized in various applications, particularly in monitoring mechanical structures. Full article

We propose an air gap fiber Bragg grating (g-FBG) sensor that can measure strain and temperature simultaneously. The sensor is made by aligning two fiber Bragg gratings (FBGs), and an air gap exists between these two sub-gratings. This sensor’s architecture allows it to form a spectrum with phase-shifted fiber Bragg grating (PSFBG) spectroscopy and Fabry–Perot interference (FPI) spectroscopy. Since the sensitivity of PSFBG and FPI spectra is different for strain and temperature, it is possible to measure both strain and temperature by measuring one of the reflected dips of PSFBG and the interference dip of FPI. The experimental results show that the strain sensitivity is about 11.95 pm/με via the dip wavelength detection of FPI, and the temperature sensitivity is about 9.64 pm/°C via the dip wavelength detection of PSFBG. The g-FBG sensor demonstrates a resolution of approximately ±3.7 με within the strain range of 0 to 1000 με and about ±0.6 °C within the temperature range of 25 °C to 120 °C. The proposed g-FBG sensor, characterized by its simple structure, compact size, and cost-effectiveness, exhibits significant potential in the field of multi-parameter measurements. Full article

This paper describes design, theoretical analysis, and experimental evaluation of a π-Phase-Shifted Fiber Bragg Grating (π-PSFBG) inscribed in the standard telecom fiber for slow light generation. At first, the grating was designed for its use in the reflection mode with a central wavelength of 1552 nm and a pass band width of less than 100 pm. The impact of fabrication imperfections was experimentally investigated and compared to model predictions. The optical spectra obtained experimentally show that the spectral region used for slow light generation is narrower (less than 10 pm), thus allowing for too-low levels of slow light optical-output power. In the next step, the optimization of the grating design was conducted to account for fabrication errors, to improve the grating’s spectral behavior and its temporal performance, and to widen the spectral interval for slow light generation in the grating’s transmission mode. The targeted central wavelength was 1553 nm. The π-PSFBG was then commercially fabricated, and the achieved parameters were experimentally investigated. For the region of (1551–1554) nm, a 15-fold increase in the grating’s pass band width was achieved. We have shown that a pair of retarded optical pulses were generated. The measured group delay was found to be ~10.5 ps (compared to 19 ps predicted by the model). The π-PSFBG operating in its transmission mode has the potential to operate as tunable delay line for applications in RF photonics, ultra-fast signal processing, and optical communications, where tunable high precision delay lines are highly desirable. The π-PSFBG can be designed and used for the generation of variable group delays from tens to hundreds of ps, depending on application needs. Full article

Wind power is growing rapidly as a green and clean energy source. As the core part of a wind turbine, the blades are subjected to enormous stress in harsh environments over a long period of time and are therefore extremely susceptible to damage, while at the same time, they are costly, so it is important to monitor their damage in a timely manner. This paper is based on the detection of blade damage using acoustic emission signals, which can detect early minor damage and internal damage to the blades. Instead of conventional piezoelectric sensors, we use fiber optic gratings as sensing units, which have the advantage of small size and corrosion resistance. Furthermore, the sensitivity of the system is doubled by replacing the conventional FBG (fiber Bragg grating) with a π-phase-shifted FBG. For the noise problem existing in the system, this paper combines the traditional WPD (wavelet packet decomposition) denoising method with EMD (empirical mode decomposition) to achieve a better noise reduction effect. Finally, small wind turbine blades are used in the experiment and their acoustic emission signals with different damage are collected for feature analysis, which sets the stage for the subsequent detection of different damage degrees and types. Full article

A wavelength-tunable single-longitudinal-mode (SLM) narrow-linewidth thulium/holmium co-doped fiber laser (THDFL) was developed in this study. The lasing wavelength was determined by combining a phase-shifted fiber Bragg grating (PS-FBG) and a uniform FBG (UFBG). SLM oscillation was achieved by incorporating a dual-coupler ring filter with the PS-FBG. At a pump power of 2.0 W, the THDFL exhibited excellent SLM lasing performance with a stable optical spectrum. It operated at an output wavelength of ~2050 nm with an optical signal-to-noise ratio of >81 dB, an output power fluctuation of 0.15 dB, a linewidth of 8.468 kHz, a relative intensity noise of ≤−140.32 dB/Hz@≥5 MHz, a slope efficiency of 2.15%, and a threshold power of 436 mW. The lasing wavelength tunability was validated experimentally by stretching the PS-FBG and UFBG simultaneously. The proposed THDFL had significant potential for application in many fields, including free-space optical communication, LiDAR, and high-precision spectral measurement. Full article

Fiber Bragg grating (FBG) relative humidity (RH) sensors are fabricated in commercially available polyimide (PI)-coated optical fibers with diameters of 50 and 125 μm. Infrared (800 nm) femtosecond pulse duration laser pulses and a phase mask are used to inscribe Type-I and Type-II FBGs directly through the protective polyimide coatings of both 50 and 125 μm diameter fibers without typical fiber processing such as hydrogen loading, cryogenic storage, stripping, recoating or annealing. The devices are then evaluated for their performance as humidity sensors. At telecom wavelengths, the 50 μm diameter fiber devices with a 10 μm thick PI coating had a wavelength shift of the Bragg resonance at a constant temperature of 2.7 pm/%RH, whereas the 125 μm diameter fiber devices with a 17 μm thick PI coating had a wavelength shift of 1.8 pm/%RH. The humidity sensors in the 50 µm diameter fiber demonstrated a more rapid response time to small changes in humidity and a weaker hysteresis when compared to the 125 µm diameter fiber devices. No modification to the PI coatings was observed during fabrication. No difference in RH sensitivity was observed for Type-I devices when compared with Type-II devices with the same fiber. The applicability of this approach for fabricating distributed RH sensing arrays with hundreds of sensing elements on a single fiber is discussed. Full article

A five fiber Bragg grating (FBG) array is inscribed at the same spot with a single uniform phase-mask (PM). The inscription setup consists of a near-infrared femtosecond laser, a PM, a defocusing spherical lens and a cylindrical focusing lens. The tunability of the center Bragg wavelength is achieved by a defocusing lens, and by translating the PM, which results in a different magnification of the PM. A first FBG is inscribed, followed by four cascading FBGs, which are inscribed exactly at the same spot only after the translation of the PM. The transmission and reflection spectra of this array are measured, showing a second-order Bragg wavelength at ~1.56 µm with a transmission dip of ~−8 dB. The spectral wavelength shift between each consecutive FBG is ~2.9 nm, and the total wavelength shift is ~11.7 nm. The reflection spectrum of the third-order Bragg wavelength is measured at ~1.04 µm, showing a wavelength separation of ~1.97 nm between neighboring FBGs, and the total spectral span between the first FBG and the last one is ~8 nm. Finally, the wavelength sensitivity to strain and temperature is measured. Full article

In this work, it is demonstrated numerically that an asymmetric Moiré fiber grating operated in reflection can provide the required spectral response to implement an all-optical fractional differentiator. In our case, the accumulated phase shift is not associated with a point phase shift, as when working with fiber Bragg gratings and long-period gratings with punctual defects, but is distributed all over the grating. The proposed device is supported by numerical simulations, and a dimensionless deviation factor is calculated to make quantitative analysis feasible. The performance of the proposed device is analyzed using numerical simulations by computing the fractional time derivatives of the complex field of an arbitrary transform-limited Gaussian pulse. A comparison with the performance given by theoretical differentiation is also presented. Full article

Despite major progress in the design of power transformers, the Achilles’ heel remains the insulation system, which is affected by various parameters including moisture, heat, and vibrations. These important machines require extreme reliability to guarantee electricity distribution to end users. In this contribution, a fiber optic sensor (FOS), consisting of a Fabry–Perot cavity made up of two identical fiber Bragg gratings (FBGs), is proposed, to monitor the temperature and vibration of power transformer windings. A phase shifted gratings recoated sensor, with multilayers of polyimide films, is used to monitor the moisture content in oil. The feasibility is investigated using an experimental laboratory transformer model, especially fabricated for this application. The moisture contents are well correlated with those measured by a Karl Fisher titrator, while the values of temperature compare well with those recorded from thermocouples. It is also shown that the sensors can be used to concurrently detect vibration, as assessed by sensitivity to the loading current. The possibility of dynamically measuring humidity, vibrations, and temperatures right next to the winding, appears to be a new insight that was previously unavailable. This approach, with its triple ability, can help to reduce the required number of sensors and therefore simplify the wiring layout. Full article

The work is dedicated to a comparative analysis of the following methods for fiber Bragg grating (FBG) spectral response modeling. The Layer Sweep (LS) method, which is similar to the common layer peeling algorithm, is based on the reflectance and transmittance determination for the plane waves propagating through layered structures, which results in the solution of a system of linear equations for the transmittance and reflectance of each layer using the sweep method. Another considered method is based on the determination of transfer matrices (TM) for the FBG as a whole. Firstly, a homogeneous FBG was modeled using both methods, and the resulting reflectance spectra were compared to the one obtained via a specialized commercial software package. Secondly, modeling results of a π-phase-shifted FBG were presented and discussed. For both FBG models, the influence of the partition interval of the LS method on the simulated spectrum was studied. Based on the analysis of the simulation data, additional required modeling conditions for phase-shifted FBGs were established, which enhanced the modeling performance of the LS method. Full article

A tunable microwave-pulse-generation scheme is proposed and demonstrated by employing an actively mode-locked optoelectronic oscillator (OEO) based on a microwave photonic filter (MPF). The MPF mainly consists of a phase-shifted fiber Bragg grating (PS-FBG) and a phase modulator. The microwave pulse trains with variable repetition rates are achieved by injecting an external signal, of which the frequencies are equal to an integer multiple of the free spectrum range (FSR) of the OEO. The multi-mode oscillation mechanism is discussed in detail theoretically and experimentally. A microwave pulse train with a central frequency of 9.25 GHz and repetition rate of 1.68 MHz is demonstrated by setting the injecting signal frequency to be the same with the FSR of the OEO. A tunable center frequency of the microwave pulses from 5.47 GHz to 18.91 GHz can be easily generated by tuning the laser frequency benefit from adopting the MPF. Furthermore, the microwave pulses with different pulse periods of 297.62 ns, 198.69 ns, and 148.81 ns are also realized by harmonic mode-locking. The proposed tunable microwave-pulse-generation method has potential applications in the pulse Doppler radar and communications. Full article

In this paper, we investigate various designs of distributed Raman amplifier (DRA) to extend amplification bandwidth in mid-link optical phase conjugation (OPC) systems and compare bands 191–197 THz and 192–198 THz giving a total bandwidth of 6 THz using a single wavelength pump. We demonstrate the use of highly reflective fiber Bragg grating (FBG) to minimize gain variation across a WDM grid by optimizing forward and backward pump powers as well as the wavelength of FBGs for original and conjugated channels. Finally, we also simulate OSNR and Kerr nonlinear reduction as a product of signals asymmetry and nonlinear phase shift (NPS) for all channels. Full article

Partial discharge (PD) ultrasonic detection is an early sign of the insolation defects of power transformers. The early diagnosis of PD requires the high sensitivity and reliability of ultrasonic sensing systems. For this purpose, a reformative PD ultrasonic sensing system based on phase-shifted FBG (PS-FBG) was demonstrated. By using PS-FBG as the ultrasonic sensing unit, the ultrasonic sensing system improved the response to the ultrasonic signal and overcame the electromagnetic noise. To compensate for the influence of temperature change on the ultrasonic sensing system, an automatic wavelength scanning demodulating method was carried out. The wavelength spanning strategy was optimized based on the principle of cross-correlation, in order to quicken the spanning. A PD detection test in the transformer oil was conducted, and the result shows that PS-FBG was 17.5 times more sensitive than PZT. Because of the better ultrasonic response, the proposed system was able to achieve the early diagnosis of insolation faults in a power transformer. Full article

This paper presented the force and displacement analyses of a diaphragm-embedded fiber Bragg grating (FBG) sensor. In the first step, a numerical analysis (via finite element method) was performed considering linear elastic materials, where there is a linear variation on the strain in the optical fiber for both displacement and force (or pressure). In the second step, the experimental analysis was performed using two approaches: (i) controlling the displacement applied in the diaphragm-embedded FBG (while the force is also measured). (ii) Controlling the force applied in the sensor (also with the measurement of the displacement). Results showed reflected optical power variations and wavelength shift following the application of displacement and force. The sensitivities of both wavelength shift and optical power were different (and non-proportional) when displacement and force were compared. However, a higher correlation, determination coefficient (R²) of 0.998, was obtained in the analysis of the wavelength shift as a function of the displacement, which indicated that the strain transmission in the optical fiber is directly related to the strain in the diaphragm, whereas the force has an indirect relation with the strain and depends on the material features. Then, the possibility of simultaneous estimation of force and displacement was investigated, where the linear relation of both parameters (displacement and force) with the wavelength shift and the optical power were obtained in a limited range of displacement and force. In this range, root mean squared errors of 0.37 N and 0.05 mm were obtained for force and displacement, respectively. In addition, the force variation with a step displacement input also shows the possibility of using the proposed FBG device for the characterization of the materials’ viscoelastic features such as phase delay, creep, and stress relaxation, which can be employed for in situ characterization of different viscoelastic materials. Full article