Pair Of Fiber Bragg Gratings Research Articles

Short-length nanosecond pulsed 1.653-μm DBR fiber Raman laser oscillator fabricated by femtosecond laser direct-writing

We demonstrate a nanosecond pulsed 1.653-μm fiber Raman laser (FRL) oscillator in a short-length distributed Bragg reflector (DBR) cavity made by femtosecond laser direct-writing technique. The DBR FRL cavity is composed of commercial ultrahigh NA germanium-doped silica fiber with a short length of 10 m and a pair of fiber Bragg gratings (FBGs). The FBGs were directly inscribed on the ends of the Raman gain fiber, using femtosecond-laser point-by-point (PbP) writing method. A home-made 1.55-μm nanosecond fiber laser source was used as the pump of the FRL. 1.653-μm nanosecond pulsed laser oscillation with maximum average output power of 125 mW (corresponding to peak power of 6.6 W), 3 dB linewidth of ≤ 0.15 nm, and slope efficiency of 48 % was obtained under 1.55-μm pump with optimized repetition rate of 10 MHz. The optical signal-to-noise-ratio (OSNR) of the 1.653-μm narrow-linewidth Raman lasing is above 70 dB. The short-length DBR FRL output exhibits decently high degree of polarization (DOP) and high degree of linear polarization (DOLP) above 80 % in 3-hour period, benefitted from the birefringent FBGs made by femtosecond laser direct-writing method. It is expected that by further reducing the cavity length to < 1 m, the 1.653-μm DBR FRL oscillator made by femtosecond laser direct-writing method is promising as single-frequency narrow-linewidth, highly linearly polarized, high-OSNR laser sources for high-sensitivity CH4 gas detection.

Optimization of Dual-Wavelength linear cavity Erbium-Doped fiber laser based on pairs of fiber Bragg gratings

There are few reports in the field of linear cavity dual-wavelength fiber lasers about the laser output characteristics when 3-dB fiber loop mirror (FLM) and Fiber Bragg grating (FBG) are used as full reflective mirrors, respectively. Additionally, there is scarce research on further tuning of the FBG parameters (reflectivity and center wavelength interval) to study dual-wavelength fiber lasers. In this paper, we compare the performances of dual-wavelength lasing outputs between the 3-dB FLM-FBG cavity and the FBG-only cavity in a linear-cavity erbium-doped fiber laser (EDFL) based on the polarization hole burning effect. The results show that the optical signal-to-noise ratios (OSNRs) of the dual-wavelength lasing outputs with the FBG-only cavity are approximately 4 ∼ 5 dB higher than those with the 3-dB FLM-FBG cavity. Furthermore, the lasing output performances based on the FBG-only cavity are compared when the reflectivities of the output-mirror FBGs are the same or different. It is observed that the dual-wavelength laser with the same output reflectivity leads to higher OSNR, slope efficiency, and stability. Moreover, by varying the center wavelength interval of pairs of FBGs in the FBG-only cavity to values of 4 nm, 8 nm, and 12 nm respectively. It is shown that wider intervals result in higher OSNRs but lower laser slope efficiency. Finally, we demonstrate a dual- wavelength linear cavity EDFL operating at room temperature with two lasing wavelengths of 1550 nm and 1562 nm. The laser results in OSNRs around 50.62 dB and 50.93 dB; center wavelength variations of less than 0.030 nm and 0.035 nm; output power fluctuations of less than 0.061 mW and 0.059 mW, respectively.

220 W Raman fiber laser at 1.24 μm based on phosphosilicate fiber

A Raman fiber laser with more than 220 W output at 1.24 μm with phosphorus-doped single-mode fiber as gain medium is reported. The laser consists of a 30 m long piece of fiber and a pair of fiber Bragg gratings with different bandwidth, pumped with a high power 1064 nm Yb fiber laser. The optical conversion efficiency is 64% and the slope efficiency is 71.4%. The full width half-maximum linewidth at the maximum output is 5.6 nm. To the best of our knowledge, the reported power is an order magnitude higher than previous reports in phosphosilicate fiber Raman laser.

Multiplexed optical fiber cell temperature sensing system with high sensitivity and accuracy.

A multiplexed fiber laser sensing system for cell temperature is proposed. To the best of the authors' knowledge, this is the first multilongitudinal mode (MLM) optical fiber laser sensor array designed for cell temperature sensing. A two-channel cell temperature sensing system with high sensitivity and real-time sensing capability is achieved. The temperature change of human hepatoellular carcinomas (HepG2) cells under the influence of exogenous chemical aflatoxin B1 (AFB1) can be monitored in real time. A fiber laser cavity consists of a pair of fiber Bragg gratings (FBGs) with matched central wavelengths and a piece of erbium-doped fiber (EDF). The static FBG is utilized for design of fiber laser cavity and laser modes selection. In comparison, the sensing FBG is used for cell temperature sensing. The sensing FBG has a length of 10mm and a diameter of . Beat frequency signals (BFS) are generated by MLM lasers after optical-to-electrical conversion at a photodetector. Frequency change of a BFS is closely related to the reflected wavelength change of the sensing FBG. Through frequency division multiplexing, two fiber laser cavities are designed in the sensing system for two-channel temperature sensing. Frequency shift of a BFS that represents temperature change of cells can be automatically recorded in seconds. A two-channel cell temperature sensing system is designed with high sensitivities of 101.62 and , respectively. The temperature change of HepG2 cells under the influence of exogenous chemical AFB1 is monitored in real time. The proposed system has the advantages of simple structure, high sensitivity, and two-channel sensing capability. Our study provides a simple and effective method to design a fiber laser sensor system without complex demodulation techniques and expensive optical components.

Femtosecond laser fabrication of large-core fiber Bragg gratings for high-power fiber oscillators

In this paper, a fs-laser phase mask inscription system based on a galvanometer scanning strategy is designed and set up for the fabrication of large-core fiber Bragg gratings (FBGs). Based on this setup, a homogeneous cross-sectional refractive index modulation can be achieved in the core of a large-mode-area fiber, and a pair of FBGs are fabricated in fibers with a core diameter of 30 µm. To investigate the performance of the fabricated FBGs, a high power all-fiber oscillator is built using a pure backward pumping structure. The FBGs work well, and the maximum output power of 7920 W is achieved with an optical–optical conversion efficiency of 77.3%. To the best of our knowledge, this is the highest power of all-fiber oscillators based on fs-written FBGs. This work provides a flexible, stable, and economic scanning strategy for large-core FBG inscription and exhibits excellent performance for high power fiber lasers.

Selection principle of seed power in high-power narrow linewidth fiber amplifier seeded by a FBGs-based fiber oscillator.

Here, we have experimentally demonstrated the selection principle of the seed power in a narrow linewidth fiber amplifier seeded by fiber oscillator based on a pair of fiber Bragg gratings. During the study on the selection of seed power, the spectral instability of the amplifier is found when a low power seed with bad temporal characteristics is amplified. This phenomenon is thoroughly analyzed from seed itself and the influence of the amplifier. Increasing the seed power or isolating the backward light of amplifier could effectively eliminate the spectral instability. Based on this point, we optimize the seed power and utilize a band pass filter circulator to isolate the backward light and filter the Raman noise. Finally, a 4.2 kW narrow linewidth output power is achieved with signal to noise ratio of 35 dB, which has exceeded the value under the highest output power reported in this type of narrow linewidth fiber amplifiers. This work provides a solution for high power and high signal to noise ratio narrow-linewidth fiber amplifiers seeded by FBGs-based fiber oscillator.

Anomaly detection of train wheels utilizing short-time Fourier transform and unsupervised learning algorithms

Anomaly detection of train wheels helps railway operators to find wheel defects and save cost by enabling condition-based maintenance. Existing approaches focus on applications on freight trains and use supervised data-driven methods which need substantial volume of fault data. This is often unavailable in passenger railways that normally provide reliable services. In this regard, this paper reports an unsupervised data-driven approach for anomaly detection of passenger train wheels. Particularly, short-time Fourier transform (STFT) is used to extract time–frequency features from the vibration signal collected with a pair of fiber Bragg grating (FBG) sensors during normal operating hours. Then, four common unsupervised learning algorithms, namely, non-negative matrix factorization (NMF), one-class support vector machine (OC-SVM), multilayer perceptron autoencoder (MLP-AE), and convolutional neural network autoencoder (CNN-AE), are used to derive five health indexes for monitoring the health condition of the train wheels. The proposed workflow is tested against a dataset complied with respect to the wheel turning record. Our results show that the health indexes obtained from the proposed workflow lead to improved correlation with the condition of train wheels compared with existing health metric. In addition, comparison of the four learning algorithms indicates that NMF and MLP-AE outperformed OC-SVM and CNN-AE.

Design and development of an economical temperature compensated bidirectional fiber Bragg grating flow sensor

A target-type cantilever based temperature compensated fiber Bragg grating (FBG) flow sensor is designed and developed with economical demodulation and data acquisition schemes. FBG is glued on the surface of a thin elastic stainless steel cantilever in pre-strained condition. The cantilever is mounted inside a flow channel by a special purpose brass holder with a height and direction adjustable threaded stem. The sensor is interrogated by an optical spectrum analyzer (OSA) and indigenously developed twin FBGs and chirped FBG—FBG intensity demodulation schemes. Response of the sensor is investigated in the flow rate range of 0–10 l min−1 in both forward and reverse flow directions. In spectral demodulation, the flow sensor offered a linear response in the full investigated range with a sensitivity of 19.2 pm/(L/min). However, the twin FBGs demodulation scheme presented a linear behaviour in the range of 4–10 l min−1 with sensitivities of 1.22 ± 0.03 μW/(L/min) and 29.89 ± 0.55 mV/(L/min) detected by the high speed power meter and photodetector, respectively. The novel CFBG-FBG intensity demodulator offered a linear response throughout its tested range and can offer a flow rate detection up to 265 l min−1 at the present sensitivity of the sensor with the condition that the cantilever-FBG system must be strong enough to withstand the higher flow rates. The temperature of the fluid is monitored by an FBG temperature sensing probe connected to the flow channel using intensity demodulation scheme by another pair of twin FBGs. Temperature compensation of the flow sensor is performed by arithmetic operations over the reflection intensities due to temperature probe to the results of the flow rates using a cost-effective data acquisition scheme.

Monolithic silica fiber laser operating at 585 nm.

We report, to the best of our knowledge, the first monolithic silica fiber laser operating in the visible. The laser cavity is based on a dysprosium-doped aluminosilicate fiber bounded by a pair of fiber Bragg gratings operating at 585 nm. The yellow laser signal reaches a record output power of 147 mW. Although the pump irradiation causes photodarkening, significant reduction of the photoinduced absorption losses is demonstrated via a photobleaching process with visible light.

A Simultaneous Distinguishing Measurement Method of Carbon Dioxide Gas Concentration and Air Humidity Based on Double Fiber Bragg Gratings

Due to the advantages of small size, high sensitivity and strong anti-interference ability, optical fiber sensors are widely used in gas detection. Meanwhile, concentration and humidity detection based on optical fiber sensing are paramount technologies in the field of gas sensing. However, how to solve the issue of cross sensitivity of concentration and humidity of optical fiber gas sensors and realize synchronous detection of two parameters are current common challenges. In this study, taking carbon dioxide as an example, a detection method based on double fiber Bragg grating (FBG) string is proposed to measure carbon dioxide concentration and air humidity simultaneously. Here, a pair of FBGs are welded in series on an optical fiber after being filled with special materials. Using the control variable method, the performance parameters of the sensor can be determined through calculation. The experimental results demonstrate that the gas concentration sensitivity is 0.5952 pm/ppm in the range of 0 ppm~2250 ppm. In the range of 25%~65%, the air humidity sensitivity is 28.62 pm/RH. Moreover, it is less affected by temperature and has satisfactory repeatability and reproducibility. The sensor realizes high sensitivity and low crosstalk measurement of concentration and humidity and can be applied to the simultaneous detection of carbon dioxide concentration and air humidity in specific confined space.

Fiber oscillator of 5 kW using fiber Bragg gratings inscribed by a visible femtosecond laser

We fabricate a pair of fiber Bragg gratings (FBGs) by a visible femtosecond laser phase mask scanning technique on passive large-mode-area double-cladding fibers for multi-kilowatt fiber oscillators. The bandwidth of high-reflection (HR) and low-reflection (LR) FBG is ∼1.6 nm and 0.3 nm, respectively. The reflection of the HR-FBG is higher than 99%, and that of the LR-FBG is about 10%. A bidirectional pumped all-fiber oscillator is constructed using this pair of FBGs, a record output power of 5027 W located in the signal core is achieved with a slope efficiency of ∼82.1%, and the beam quality factor M2 is measured to be ∼1.6 at the maximum power. The FBGs are simply fixed on a water cooling plate without a special package, and the thermal efficiency of the HR-FBG and the LR-FBG is 2.76°C/kW and 1°C/kW, respectively. Our research provides an effective solution for robust high-power all-fiber laser oscillators.

Investigation into Fiber Optic Seismic Sensor incorporating Fiber Bragg Grating Array

This paper presents a theoretical investigation on a type of Fiber Optic Sensing system used for seismic monitoring which incorporates an array of Fiber Bragg Gratings (FBGs) in the sensing medium. An interferometry array contains a group of interferometry sub-arrays, every sub-array including a plurality of interferometry. Each interferometer in a sub-array is realized with a particular pair of fiber Bragg gratings and a sensing length of optical fiber located between the respective pair of fiber Bragg gratings. The fiber Bragg gratings in every respective couple of fiber Bragg gratings have a similar characteristic wavelength that is altered from the characteristic wavelength of every other couple of fiber Bragg gratings in the sub-array. The sub-arrays are interconnected to minimize the common-wavelength crosstalk among the sensors in the overall interferometry array. Our work focuses on investigating the selection of the optimum wavelength for use in the system. The study was performed using state-of-the-art simulation techniques for strain and temperature changes in the FBG sensor array. The cumulative results obtained can be used to help designers produce an optimized performance for similar fiber optic hydrophones in the future. It has been shown that the wavelength 1300nm region gives the best performance in this type of fiber optic hydrophone system.

In-place fiber-optic inclinometer based on a vertical cantilever beam and dual FBGs

Inclinometers are commonly used in boreholes to monitor the profile of land/soil displacement, which is vital in geotechnical engineering. A new in-place fiber-optic inclinometer (FOI) is developed based on a vertical cantilever beam and dual fiber Bragg gratings (FBGs). This FOI is characterized by its robust design, bidirectional large range, multiplexing capability, and push-pull configuration with temperature insensitivity. A pair of pre-stretched FBGs with different reflection wavelengths are respectively glued on the opposite surfaces of a vertical cantilever beam with tip mass to form a push-pull structure, which is further encapsulated by a solid cylinder made of aluminum alloy. Calibration tests of the proposed inclinometer are performed in the laboratory to examine its measurement performance, revealing a measurement sensitivity of ∼ 0.1 nm/°, resolution of ∼ 0.01°, and a maximum bidirectional measurement range of ± 30°. The numerically calculated sensitivity based on actual parameters agrees well with the experimental result. Furthermore, three inclinometer units are connected by spacers to conduct an assembled inclinometer test in the laboratory, four typical profile patterns are validated with a sensing error of <6.73 %.

Triple-wavelength laser from a femtosecond laser directly-written fiber cavity for microwave generation.

We report an all-fiber triple-wavelength single-frequency distributed Bragg reflector (DBR) laser for 1-10 GHz microwave (MW) generation. The DBR cavity contains a non-polarization-maintaining Er fiber and a pair of fiber Bragg gratings (FBGs) made using a femtosecond laser line-by-line (LbL) direct-writing method. Such a configuration combining a short cavity and radial asymmetry leads to frequency locking and phase controlling of multi-wavelength fiber lasers. A 1.59-µm triple-wavelength laser with high coherence, spectrum purity, and polarization purity has been demonstrated; 3-6 GHz triple-frequency MW was generated.

High-temperature linearly polarized single-frequency fiber lasers based on a non-polarization-maintaining FBG preparation through a femtosecond laser

A high-temperature-resistant linearly polarized single-frequency distributed Bragg reflector fiber laser is demonstrated by using a femtosecond laser and directly fabricating a pair of fiber Bragg gratings (FBGs) into an erbium-doped fiber (EDF). A high-reflection FBG with high birefringence prepared by femtosecond laser overexposure is used as a polarization selector. The integrated resonator cavity is 0.82 cm to ensure single-frequency operation. After annealing treatment, the laser can work stably at 600°C, and no mode hopping happens at different temperatures. By using the residual pump light and a suitable EDF to amplify the laser, a narrow linewidth laser with an output power of 26.3 mW, a degree of polarization reaching 0.984, and a linewidth less than 4 kHz is obtained.

High-quality fiber Bragg grating inscribed in ZBLAN fiber using femtosecond laser point-by-point technology.

We demonstrate for the first time, to the best of our knowledge, the fabrication of a high-quality fiber Bragg grating (FBG) in ZBLAN fiber by using an efficient femtosecond laser point-by-point technology. Two types of FBG, e.g., high coupling coefficient and narrow bandwidth grating, are successfully obtained. The coupling coefficient is strongly dependent on the grating order and pulse energy. A second-order FBG with an ultrahigh coupling coefficient of 325 m-1 and reflectivity of 97.8% is inscribed in the ZBLAN fiber. A pair of FBGs with a narrow FWHM of 0.30 and 0.09 nm are also demonstrated.

A sensitivity difference approach to overcome temperature influence on a fiber optic force sensor with a pair of FBGs

A temperature-insensitive fiber optic sensor is devised to measure the contact force at the tool tip of transluminal surgery. The force sensor is made with a pair of fiber Bragg gratings (FBGs), which are multiplexed on a single mode fiber and sit in the state of tension inside two tubes, a nitinol tube and a stainless steel tube. Their material stiffness difference leads to the sensitivity difference to force, which is analyzed by the finite element method and is validated by the sensor’s calibration. A decoupled model based on sensitivity difference is used to compute the force from the wavelength shifts of the two FBGs, overcoming the influence of temperature change during force measurement. Comparative experiments have been done to assess the model, and we find that the force computed by the compensation model is much more accurate than the non-compensation model. The devised sensor can be used to accurately measure force under varying temperature.

Liquid Level Sensor with Two FBGs Embedded in a PDMS Diaphragm: Analysis of the Linearity and Sensitivity.

This paper presents a fiber optic, liquid level sensor system based on a pair of fiber Bragg gratings (FBGs), embedded in a circular silicone (PDMS—polydimethylsiloxane) rubber diaphragm. The measurement principles of this sensor, whose diaphragm structure is about 2.2 mm thick with 45 mm in diameter, are introduced. To analyze the linearity and sensitivity of the sensor, the diaphragm was subjected to compression tests as well as to liquid level loading and unloading. The force and liquid level increase tests showed that inserting two FBGs (0.99453 for force and 0.99163 for liquid level) in the diaphragm resulted in a system with greater linearity than that with individual FBGs. This occurred where FBG1 showed 0.97684 for force and 0.98848 for liquid level and FBG2 presented 0.89461 for force and 0.93408 for liquid level. However, the compression and water level decrease tests showed that the system (R2 = 0.97142) had greater linearity with FBG2 (0.94123) and lower linearity with FBG1 (0.98271). Temperature characterization was also performed, and we found that sensitivity to FBG1 temperature variation was 11.73 pm/°C and for FGB2 it was 10.29 pm/°C. Temperature sensitivity was improved for both FBGs when compared with uncoated FBGs with typical values of 9.75 pm/°C. Therefore, the proposed FBG-based sensor system is capable of simultaneous measurement of force and temperature in a compact diaphragm-embedded system.

Fabrication of Fiber Bragg Gratings by Visible Femtosecond Laser for Multi-kW Fiber Oscillator

In this paper, we fabricate a pair of fiber Bragg gratings (FBGs) in large mode area double cladding fiber (LMA-DCF, core/inner cladding diameter: 20/400 μm) by visible (515 nm) femtosecond laser and phase mask for the first time to the best of our knowledge. Then this pair of FBGs are utilized to construct a high power fiber oscillator operated near 1080 nm. The maximum output laser power is more than 3.2 kW with a slope efficiency of ∼77.9%, and the beam quality factor M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is about 1.28. The FBGs are simply fixed on water-cooling plate without any special package, and the temperature of the FBGs is about 40 °C at the maximum output power. Our research work confirms the reliability of visible femtosecond laser in fabrication of FBGs intended to be applied in high power fiber oscillators.

An optical fiber sensor for the simultaneous measurement of pressure and position based on a pair of fiber Bragg gratings

In this study, an optical fiber sensor for the simultaneous measurement of pressure and position based on a pair of fiber Bragg gratings (FBG) in a clamped beam is proposed. The FBG pair are pasted on a one-meter-long beam, and the pasted position is at 20 cm and 80 cm. The pressure and the position information applied at any point of the beam could be obtained through the wavelength shift of the FBG pair. The experimental results show that in the pressure range of 0 to 40 N, the maximum pressure sensitivity of the sensor is 54 pm/N. In the position range of 20 to 80 cm, the minimum error of position detection is 0.38 cm. The proposed sensor has the advantages of good stability, high sensitivity, simple manufacturing process, and low cost.