Ultra-short Fiber Bragg Grating Research Articles

Optical Vernier Effect in Fabry-Perot Interferometers Based on Ultra-Short Fiber Bragg Gratings

Optical Vernier Effect in Fabry-Perot Interferometers Based on Ultra-Short Fiber Bragg Gratings

Ultra-Short Fiber Bragg Grating Composed of Cascaded Microchannels in a Microprobe for Refractive Index Measurement

An ultra-short fiber Bragg grating (US-FBG) refractive index (RI) sensor is presented and demonstrated. The US-FBG is composed of a series of cascaded microchannels which carved by femtosecond (fs) laser in the microprobe. According to the simulation results, an US-FBG with length/period of 149.8/5.35 μm is fabricated and used to measure refractive index (RI) of liquids. The experimental results show that the sensitivity of the proposed sensor can reach 600.5622 nm/RIU with a linear response range of 1.3323 to 1.3787. Such a compact size, high sensitivity and the inherent high reliability and ease of modulation of the US-FBG is expected to be used in the field of biosensing related to RI.

Interrogation Method with Temperature Compensation Using Ultra-Short Fiber Bragg Gratings in Silica and Polymer Optical Fibers as Edge Filters.

The use of simpler and less bulky equipment, with a reliable performance and at relative low cost is increasingly important when assembling sensing configurations for a wide variety of applications. Based on this concept, this paper proposes a simple, efficient and relative low-cost fiber Bragg grating (FBG) interrogation solution using ultra-short FBGs (USFBGs) as edge filters. USFBGs with different lengths and reflection bandwidths were produced in silica optical fiber and in poly(methyl methacrylate) (PMMA) microstructured polymer optical fiber (mPOF), and by adjusting specific inscription parameters and the diffraction pattern, these gratings can present self-apodization and unique spectral characteristics suitable for filtering operations. In addition to being a cost-effective edge filter solution, USFBGs and standard uniform FBGs in silica fiber have similar thermal sensitivities, which results in a straightforward operation without complex equipment or calculations. This FBG interrogation configuration is also quite promising for dynamic measurements, and due to its multiplexing capabilities multiple USFBGs can be inscribed in the same optical fiber, allowing to incorporate several filters with identical or different spectral characteristics at specific wavelength regions in the same fiber, thus showing great potential to create and develop new sensing configurations.

High-Spatial-Resolution Strain and Vibration Dual-Parameter Sensor Based on Ultra-Short FBG

A method for simultaneous measurement of static strain and dynamic vibration with high spatial resolution based on millimeter sized ultra-short fiber Bragg grating (FBG) is proposed and experimented in this paper. Dual-parameter can be measured on the single unit of cascaded FBGs, which interrogate with optical coherence tomography method by a designed wavelength swept light source. In experiments, the sensing units were precisely located with mm degrees by inverse FFT of coherent signals and the phase difference calculation. And also the strain and vibration could be simultaneously measured in time and frequency domain: demodulating strain with low frequency component’s phase shift of coherent signals by extracting envelope and interpolation; demodulating vibration by finding peak positions of sidebands in FFT spectrum. Spatial resolution of positioning can reach 0.77 mm. Sensitivity of strain measurement reaches 0.811 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\times} 10^{-3} rad/\mu \varepsilon $</tex-math></inline-formula> under a wavelength sweep rate of 0.2 nm/ms, and forces of several milli newtons can be perceived. The measurement accuracy of kHz vibration frequency is over 98%. Moreover, by comparing simultaneous and respective measurement of dual-parameter, it can be seen that there is almost no crosstalk affectting the results. The results shows great application potential in the field of mechanical sensing, especially in bionic tactile sensing for its simultaneous dynamic and static measurement with accurate positioning performance.

High-Spatial-Resolution High-Temperature Sensor Based on Ultra-Short Fiber Bragg Gratings With Dual-Wavelength Differential Detection

Type II fiber Bragg gratings (FBGs) inscribed with femtosecond ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> s) laser possess significant potential for high temperature sensing. In this work, we propose and demonstrate a method for fabricating a parallel-integrated ultra-short type II FBG (PI-US-FBG) by using an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> s laser point-by-point technology. The PI-US-FBG, featuring by an ultra-short grating length of 80 µm, consists of six identical FBGs parallel-inscribed into a fiber core at different radial positions in the same cross section. The fabricated PI-US-FBG exhibits a broadband Gaussian-shape spectrum with a low reflectivity of ∼10%, an ultra-low out-of-band insertion loss of 0.01 dB, and a large full width at half maximum bandwidth of 9.4 nm. Moreover, this PI-US-FBG could be used as a high temperature sensor with a wide measurement range from 25 to 1000 °C, and an excellent linearity was achieved with a dual-wavelength differential detection. The temperature sensitivity could be increased from 0.00316 to 0.00945 (dB/ °C) by enlarging the wavelength spacing of the tunable laser. In addition, two cascaded PI-US-FBGs were used to precisely measure the temperature field distribution in a CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser spot with a very high spatial resolution of 100 µm. Hence, the proposed PI-US-FBGs could be used for a large-scale fiber sensor network, especially in future distributed temperature measurement with a high spatial resolution.

Fiber Bragg Grating Sensors for Performance Evaluation of Fast Magnetic Resonance Thermometry on Synthetic Phantom.

The increasing recognition of minimally invasive thermal treatment of tumors motivate the development of accurate thermometry approaches for guaranteeing the therapeutic efficacy and safety. Magnetic Resonance Thermometry Imaging (MRTI) is nowadays considered the gold-standard in thermometry for tumor thermal therapy, and assessment of its performances is required for clinical applications. This study evaluates the accuracy of fast MRTI on a synthetic phantom, using dense ultra-short Fiber Bragg Grating (FBG) array, as a reference. Fast MRTI is achieved with a multi-slice gradient-echo echo-planar imaging (GRE-EPI) sequence, allowing monitoring the temperature increase induced with a 980 nm laser source. The temperature distributions measured with 1 mm-spatial resolution with both FBGs and MRTI were compared. The root mean squared error (RMSE) value obtained by comparing temperature profiles showed a maximum error of 1.2 °C. The Bland-Altman analysis revealed a mean of difference of 0.1 °C and limits of agreement 1.5/−1.3 °C. FBG sensors allowed to extensively assess the performances of the GRE-EPI sequence, in addition to the information on the MRTI precision estimated by considering the signal-to-noise ratio of the images (0.4 °C). Overall, the results obtained for the GRE-EPI fully satisfy the accuracy (~2 °C) required for proper temperature monitoring during thermal therapies.

Characterization of 100 Micron Long Fiber Bragg Gratings and Applications in Ultrafast Shock Wave Experiments

Ultrashort Fiber Bragg Gratings (FBGs) may be very useful whenever highly localized pressure measurements are desired. Obviously, very short FBGs will exhibit lower, as well as spectrally broader, reflections; however, these deficiencies may be outweighed by the advantage of the potential for highly localized measurement. In addition, with such short gratings, their accurate localization along the fiber is of great importance. In this paper, commercial 100 μ m FBGs are characterized, and their location along the fiber is accurately determined by two methods: Spectral and white-light interferometry. We also demonstrate the advantage of ultrashort gratings in a shock-wave experiment; when a solid target is hit by a projectile, the resultant pressure waves may change rapidly in both location and time. In this paper, the suitability of a 100 μ m grating to this scenario is demonstrated, revealing features in good agreement with theory—in particular, an approximately 0.7 mm long constant pressure region. Although these pressure waves may be monitored by ordinary (∼6–8 mm long) FBGs, due to the high-pressure gradients involved, the FBG response will be very complicated and data retrieval may be ambiguous. Thus the advantage of ultrashort FBGs is demonstrated.

Microstructured Optical Fiber Based Distributed Sensor for In Vivo Pressure Detection

Microstructured optical fiber based distributed pressure sensor is proposed and demonstrated for high resolution manometry (HRM), which adopts the hybrid wavelength and frequency division multiplexing architecture, and hyperelastic packaging. The microstructure acting as a sensing element is composed of two identical and closely spaced ultrashort fiber Bragg gratings with the reflectivity of only 1%. In theory, 243 microstructures can be multiplexed along one single fiber with an interval of 10 mm beneficial from the compact structure, hybrid encoding feature, and low insertion loss. Homogeneous elastic packaging is explored to protect the fiber and greatly enhance the lateral pressure sensitivity of the sensor. Moreover, a mechanical model is established through the finite element method to analyze the sensitizing effect of the packaging. A prototype system is built and experimentally demonstrated the distributed pressure measurement with the spatial resolution of 10 mm and the pressure sensitivity up to 2.2 Nm/mPa. Due to the advantage of high spatial resolution, high multiplexing capacity, and high pressure sensitivity, the proposed sensor is suitable for HRM in medical application.

Micro-Cavity Array With High Accuracy for Fully Distributed Optical Fiber Sensing

A micro-cavity array (MCA) fiber based on a dense ultra-short fiber Bragg grating (FBG) array is proposed for high-accuracy fully distributed sensing. In the MCA fiber, each pair of adjacent FBGs forms a micro-cavity, which acts as the sensing element. The high finesse interference spectra of the micro-cavities facilitate high demodulation accuracy. Large-scale multiplexing of 8557 FBGs results in 8556 micro-cavities in a 7.18-m MCA fiber. Combined with optical frequency-domain reflectometry, fully distributed temperature sensing with a spatial resolution less than 1 mm and measurement precision of 0.157 °C is demonstrated.

Simultaneous distributed static and dynamic sensing based on ultra-short fiber Bragg gratings.

Distributed static and dynamic sensing is demonstrated with an ultra-short fiber Bragg grating (USFBG) array. The USFBGs serve as the sensors and reflection mirrors at the same time. Distributed static sensing is performed by demodulating the strain-induced or temperature-induced wavelength shift of each USFBG. Dynamic vibration sensing is realized based on phase variation between two adjacent USFBG reflected pulses. Static temperature and dynamic vibration are applied to the sensing ultra-short FBG array simultaneously. The experiments show that the temperature measurement from 30 °C to 80 °C is achieved successfully. And dynamic sensing of nε scale vibration and 12.5 kHz acoustic wave are demonstrated at a sampling rate of 50 kHz.

Interrogation of weak Bragg grating sensors based on dual-wavelength differential detection.

It is shown that for weak Bragg gratings the logarithmic ratio of reflected intensities at any two wavelengths within the spectrum follows a linear relationship with the Bragg wavelength shift, with a slope proportional to their wavelength spacing. This finding is exploited to develop a flexible, efficient, and cheap interrogation solution of weak fiber Bragg grating (FBGs), especially ultra-short FBGs, in distributed sensing based on dual-wavelength differential detection. The concept is experimentally studied in both single and distributed sensing systems with ultra-short FBG sensors. The work may form the basis of new and promising FBG interrogation techniques based on detecting discrete rather than continuous spectra.

Ultra-short FBG based distributed sensing using shifted optical Gaussian filters and microwave-network analysis.

Ultrashort fiber Bragg gratings (US-FBGs) have significant potential as weak grating sensors for distributed sensing, but the exploitation have been limited by their inherent broad spectra that are undesirable for most traditional wavelength measurements. To address this, we have recently introduced a new interrogation concept using shifted optical Gaussian filters (SOGF) which is well suitable for US-FBG measurements. Here, we apply it to demonstrate, for the first time, an US-FBG-based self-referencing distributed optical sensing technique, with the advantages of adjustable sensitivity and range, high-speed and wide-range (potentially >14000 με) intensity-based detection, and resistance to disturbance by nonuniform parameter distribution. The entire system is essentially based on a microwave network, which incorporates the SOGF with a fiber delay-line between the two arms. Differential detections of the cascaded US-FBGs are performed individually in the network time-domain response which can be obtained by analyzing its complex frequency response. Experimental results are presented and discussed using eight cascaded US-FBGs. A comprehensive numerical analysis is also conducted to assess the system performance, which shows that the use of US-FBGs instead of conventional weak FBGs could significantly improve the power budget and capacity of the distributed sensing system while maintaining the crosstalk level and intensity decay rate, providing a promising route for future sensing applications.

Interrogation of Ultrashort Bragg Grating Sensors Using Shifted Optical Gaussian Filters

Ultrashort fiber Bragg gratings (FBGs) possess significant potential as weak sensing units for distributed measurements, the exploitation of which, however, have been usually limited by their inherent wide spectra which is undesirable for most traditional wavelength measurements. In this letter, we propose to use shifted optical Gaussian filters to interrogate such wide spectrum gratings. However, instead of acting like Gaussian edge filters as previously done, to take advantage of spectral feature of ultrashort gratings, here they have a role which is more like shifted matched filters. The measurement inherits the important features of a common shifted Gaussian filter interrogation technique, namely its high flexibility, natural insensitivity to intensity variations, promising future for multichannel interrogation, and a potentially wider measuring range relative to common intensity-based approaches. We believe that this simple concept for ultrashort Bragg gratings should offer the strong foundation for their future applications in distributed measurements. Interrogation of a strain-tuned ultrashort grating was accomplished, with a wide sensitivity tuning range from 2.8 to 10.4 dB/nm achieved.

Acoustic emission source linear localization based on an ultra-short FBGs sensing system

An acoustic emission (AE) linear location system was proposed, which employed fiber Bragg gratings (FBGs) as AE sensors. It was demonstrated that the FBG wavelength could be modulated as the static case when the grating length was much shorter than the AE wavelength. In addition, an improved AE location method based on the Gabor wavelet transform (WT) and threshold analysis was represented. The method was testified through AE linear location experiments based on a tunable narrow-band laser interrogation system using ultra-short FBG sensors as AE sensors. Results of the experiments showed that 86% of the linear location errors were less than 10 mm.

Mode Locking and Electrical Tuning of a Hybrid Laser Source Using a Connectorized Ultra-Short Fiber Bragg Grating

Electrical tuning and mode locking of an external-cavity semiconductor laser with an ultrashort (0.4 mm) fiber-Bragg-grating (FBG) reflector are studied theoretically and experimentally. Evolution of electrical tuning of longitudinal modes in a laser with broad-bandwidth FBG is analyzed theoretically. Tuning over an 11.4-nm range was experimentally possible by mechanically changing the connectorized FBG reflector in the external cavity of a diode laser package. With a 40%-reflection FBG, the external cavity laser was mode locked and generated at < 50-ps optical pulses at (6.3 plusmn 0.3) GHz within a broad frequency range of 600 MHz. The authors have simulated the intensity-modulation response of this laser and have shown that the broad frequency range mode locking of these lasers is determined by a chirp-related resonance rather than an external cavity resonance. The theoretical analysis agrees with the experimental results