Tilted Fiber Bragg Grating Research Articles

High Temperature Sustainable Tilted Fiber Bragg Gratings via Femtosecond Laser Plane-by-Plane Writing Technology

In this paper, high-quality tilted fiber Bragg gratings (TFBGs) inscribed by using femtosecond laser and the slit beam shaping technique were reported. By adjusting the included angle between the slit and the fiber, we realized a series of tilted gratings with tilt angles of 0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$°$</tex-math></inline-formula> (FBG), 5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$°$</tex-math></inline-formula> , 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$°$</tex-math></inline-formula> , 15 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$°$</tex-math></inline-formula> , and 20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$°$</tex-math></inline-formula> in a single-mode-fiber. For a 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$°$</tex-math></inline-formula> TFBG, the Bragg resonance was -1.90 dB at 1604.51 nm, and the maximum cladding-mode resonance reached 36.17 dB with an insertion loss of 1.44 dB due to the method of Plane-by-Plane (Pl-by-Pl) inscription. The effects of slit widths and different laser pulse energies on the Pl-by-Pl writing of TFBGs were also studied. The refractive index response sensitivity of the 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$°$</tex-math></inline-formula> TFBG was measured at 449.45 nm/RIU. Moreover, the TFBG written by this method was a type-II grating, which could withstand a high temperature of 1100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$°$</tex-math></inline-formula> C for a long time.

In situ tumor cells detection using nanotube-functionalized & microfluidic-controlling multiresonance optical fiber

In situ and highly sensitive monitoring of tumor cells provides a fundamental understanding of the valuation of cancer diagnosis. However, existing techniques are largely unable to rapidly capture and precisely quantify the tumor cells. To address this, we propose a novel approach based on a nanotube-functionalized & microfluidic-controlling multiresonance optical fiber sensor. The sensor consists of a tilted fiber Bragg grating (TFBG) inscribed in the fiber core and specially designed halloysite nanotubes (HNTs) coated over the fiber surface. In particular, the HNTs are arranged in an orderly manner along the fiber surface to form slit-like patterned layers for enhanced the interaction with tumor cells, resulting in more effective capture of tumor cells. Such a sensor provides a powerful light scattering sensing ability. Based on the spectral area interrogation method, normal cells and tumor cells can be unambiguously discriminated within a few minutes, providing high sensitivity (limit of detection of 10 cells/mL), and a linear response for a large cell concentration range (10 ∼ 105 cells/mL). Meanwhile, by integrating the fiber sensor with a well-designed microfluidic chip, rapid and precise measurement of different tumor cell samples with low consumption (sub-microliter volumes) can be achieved, which provides a potential tool for cancer diagnosis and treatment.

A passively Q-switched multi-wavelength erbium-doped fiber laser modulated by 12° tilted fiber Bragg grating

As a promising small-angle TFBG, the 12° tilted fiber Bragg grating (TFBG) has been successfully used in biochemical sensors, fiber optic anemometers, and continuously tunable photon fraction time differentiators. On the other hand, multi-wavelength Q-switched fiber lasers have widely potential and practical applications in various fields, such as laser processing, laser marking, medical diagnosis, military applications, and so on. In this paper, first, a 12° TFBG is prepared by femtosecond laser direct inscription method (FSL-DIM), then, a pulse multi-wavelength erbium-doped fiber laser (EDFL) passively Q-switched by the as-prepared 12° TFBG is exclusively and systematically demonstrated for the first time. When the pump power is raised from 100 mW to 500 mW, the repetition rate is boosted from 51.64 kHz to 149.4 kHz resulting in a repetition rate range about 97.76 kHz, while the pulse duration is compressed from 3.928 μs to 1.033 μs. The average output power monotonically increases up to the maximum value of 6.357 mW corresponding to the maximum pulse energy of 42.55 nJ. The signal-to-noise ratio (SNR) is measured to be 53 dB, which indicates a high stability. The experimental results are comparable to other Q-switched multi-wavelength EDFLs reported so far, and during the experiment, the 12° TFBG exhibits excellent stability and reliability. As far as we can explain, the multi-wavelength Q-switched operation obtained in this paper benefits from the filtering effect and polarization dependent loss (PDL) effect of the 12° TFBG. The work not only provides a reliable reference about the effective-SA performance of 12° TFBG, but also enriches the studies of passively Q-switched lasers based on different kinds of modulators.

Decoupling bulk and surface characteristics with a bare tilted fiber Bragg grating.

The tilted fiber Bragg grating (TFBG) with dense comb-like resonances offers a promising fiber-optic sensing platform but could suffer from cross sensitivity dependent on bulk and surface environment. In this work, the decoupling of bulk and surface characteristics (indicated by bulk refractive index (RI) and surface-localized binding film) from each other is attained theoretically with a bare TFBG sensor. This is realized with the proposed decoupling approach based on differential spectral responses of cut-off mode resonance and mode dispersion represented as wavelength interval between P- and S-polarized resonances of the TFBG to the bulk RI and surface film thickness. The results demonstrate that with this method the sensing performance for decoupling bulk RI and surface film thickness is comparative to the cases in which either the bulk or surface environment of the TFBG sensor changes, with the bulk and surface sensitivities over 540 nm/RIU and 12 pm/nm, respectively.

Ge-Sb-Se-Te -coated tilted fiber Bragg gratings sensor for the refractive index measurement of transformer oils

In this work, a tilted fiber Bragg gratings coated with a 130 nm thick Ge-Sb-Se-Te (GSST) layer, a high refractive index soft glass had been fabricated and demonstrated for the refractometric test of transformer oil. The findings indicated that the introduction of the high refractive index coating had enhanced the polarization property and extended the refractive index detection range of the tilted fiber Bragg gratings beyond the cut-off point of the cladding modes. In the experiment with transformer oils, the Ge-Sb-Se-Te -coated tilted fiber Bragg gratings has shown a good linear sensitivity of 9.952 dB/RIU for the refractive index range of 1.472–1.484 RIU.

Spectral characteristics and interrogation of a fiber-optic ball resonator biosensor modulated by a tilted fiber Bragg grating

In this work, we describe the design and analyze the characteristic of a fiber-optic biosensing device formed by a fiber-tip ball resonator (BR) assisted by an in-line tilted fiber Bragg grating (TFBG). The BR-TFBG device acts as miniature fiber-tip biosensor, characterized by a broad reflection spectrum remodulated by the cladding modes of the grating that enhance the amplitude sensitivity. We demonstrate an interrogation method that improves the sensitivity on the cladding mode comb, showing a sensitivity up to 274.0 dB/RIU in amplitude by using polarization-based analysis, and 154.7 nm/RIU for wavelength shift by using a cross-correlation method. Then, we demonstrate the biosensing application of the BR-TFBG for the detection of human epidermal growth factor receptor-2 (HER2) in buffer, within 0.5 – 128.0 ng/ml concentration.

Ultra-sensitive and specific detection of pathogenic nucleic acids using composite-excited hyperfine plasma spectroscopy combs sensitized by Au nanoarrays functionalized with 2D Ta2C-MXene

Accurate and rapid screening techniques on a population scale are crucial for preventing and managing epidemics like COVID-19. The standard gold test for nucleic acids in pathogenic infections is primarily the reverse transcription polymerase chain reaction (RT-PCR). However, this method is not suitable for widespread screening due to its reliance on large-scale equipment and time-consuming extraction and amplification processes. Here, we developed a collaborative system that combines high-load hybridization probes targeting N and OFR1a with Au NPs@Ta2C-M modified gold-coated tilted fiber Bragg grating (TFBG) sensors to enable direct nucleic acid detection. Multiple activation sites of SARS-CoV-2 were saturable modified on the surface of a homogeneous arrayed AuNPs@Ta2C-M/Au structure based on a segmental modification approach. The combination of hybrid probe synergy and composite polarisation response in the excitation structure results in highly specific hybridization analysis and excellent signal transduction of trace target sequences. The system demonstrates excellent trace specificity, with a limit of detection of 0.2 pg/mL, and achieves a rapid response time of 1.5 min for clinical samples without amplification. The results showed high agreement with the RT-PCR test (Kappa index = 1). And the gradient-based detection of 10-in-1 mixed samples exhibits high-intensity interference immunity and excellent trace identification. Therefore, the proposed synergistic detection platform has a good tendency to curb the global spread of epidemics such as COVID-19.

Recovery of a highly reflective Bragg grating in DPDS-doped polymer optical fiber by thermal annealing.

We report fiber Bragg grating manufacturing in poly(methyl methacrylate) (PMMA)-based polymer optical fibers (POFs) with a diphenyl disulfide (DPDS)-doped core by means of a 266 nm pulsed laser and the phase mask technique. Gratings were inscribed with different pulse energies ranging from 2.2 mJ to 2.7 mJ. For the latter, the grating reflectivity reached 91% upon 18-pulse illumination. Though the as-fabricated gratings decayed, they were recovered by post-annealing at 80°C for 1 day, after which they showed an even higher reflectivity of up to 98%. This methodology for the fabrication of highly reflective gratings could be applied for the production of high-quality tilted fiber Bragg gratings (TFBGs) in POFs for biochemical applications.

Femtosecond laser fabrication of chirped and tilted fiber Bragg gratings for stimulated Raman scattering suppression in kilowatt-level fiber lasers.

Chirped and tilted fiber Bragg gratings (CTFBGs) are important all-fiber filtering components in high-power fiber lasers for stimulated Raman scattering (SRS) suppression. The fabrication of CTFBGs in large-mode-area double-cladding fibers (LMA-DCFs) by femtosecond (fs) laser is reported for the first time to the best of our knowledge. The chirped and tilted grating structure is obtained by scanning the fiber obliquely and moving the fs-laser beam relative to the chirped phase mask at the same time. By this method, the CTFBGs with different chirp rates, grating lengths, and tilted angles are fabricated, and the maximum rejection depth and bandwidth are ∼25 dB and ∼12 nm, respectively. To test the performance of the fabricated CTFBGs, one is inserted between the seed laser and the amplifier stage of a 2.7 kW fiber amplifier, and an SRS suppression ratio of ∼4 dB is achieved with no reduction in laser efficiency and degradation in beam quality. This work provides a highly fast and flexible method to fabricate large-core CTFBGs, which is of great significance to the development of high-power fiber laser systems.

TFBG Side Modes and Fresnel Reflection-Based Sensing System for Solution Concentration Measurement

Solution concentration (SC) measurement is of great significance in various fields. Various optical fiber SC sensors have been developed accordingly, in which the Fresnel refraction-based optical fiber sensor has the advantages of avoiding damage to sensing units, reusability, and low maintenance cost. In this study, a tilted fiber Bragg grating (TFBG) side modes and Fresnel reflection-based sensing system for SC measurement is proposed. Theoretically and experimentally, the nonuniformity of the grating period causing the forming of irregular TFBG side mode fringes is simulated and proved, and the feasibility of the dip intensity of the TFBG side mode resonance to quantitatively characterize the SC is demonstrated. As a result, a maximum concentration sensitivity of −0.471 dB/% at the glycerol SC (GSC) of 95.83% is achieved by increasing outpower of the light source and using an optical circulator. In addition, a method for simultaneous measurement of solution temperature and refractive index (RI) by using one side mode resonance signal is proposed and proved, which can significantly reduce the response time and improve the accuracy of concentration measurement.

Femtosecond laser line-by-line tilted Bragg grating inscription in single-mode step-index TOPAS/ZEONEX polymer optical fiber.

In this Letter, we demonstrate 8°-tilted fiber Bragg grating (TFBG) inscription in single-mode step-index TOPAS/ZEONEX polymer optical fibers (POFs) using a 520 nm femtosecond laser and the line-by-line (LbL) writing technique. As a result of the tilt angle and the fiber refractive index, a large spectral range of cladding mode resonances covering 147 nm is obtained. The evolution of the transmitted spectrum is analyzed as a function of the surrounding refractive index (SRI) in a large range from 1.30 to 1.50. The cutoff cladding mode shows a refractive index sensitivity of 507 nm/RIU (refractive index unit). For single-resonance tracking near the cutoff mode, the sensitivity is at least 6 nm/RIU, depending on the exact wavelength position of the cladding modes. The main originality of our work is that it produces, for the first time, to the best of our knowledge, a TFBG in POF that operates in the refractive index range of aqueous solutions. The sensing capability for a large range of refractive index values is also relevant for (bio)chemical sensing in different media.

A Wavelet Derivative Spectrum Length Method of TFBG Sensor Demodulation.

Fibre optic sensors using tilted fibre Bragg grating (TFBG) have high sensitivity for refractive index measurements. In order to achieve good metrological parameters of the measurement, an appropriate method of spectrum demodulation must be used. The method proposed in the article is an improvement of the spectral length algorithm. The spectral length parameter is treated as the sum of the derivative filter responses. In the original version, the first difference of spectrum elements was used, while this article proposes to use the wavelet transform to calculate the numerical derivative approximation. The advantage of this solution is an easy way to select the level of smoothing filtration by changing the scale parameter. The derivation is appropriate even for a relatively low signal-to-noise level. The approximation of the spectral length by the derivative calculated using the wavelet transform eliminates the high-frequency noise of the optical signal. The absolute value of determined spectral derivatives after significant smoothing can be used to estimate the wavelength of the decay of modes. After analyzing experimental data and performing calculations, it turns out that this is a linear method with better resolution than the contour length algorithm.

Immunosensing for Early Detection of Rheumatoid Arthritis Biomarkers: Anti-Cyclic Citrullinated Peptide Antibodies Based on Tilted-Fiber Bragg Grating Biosensor

Rheumatoid arthritis (RA) is regarded as a chronic, immune-mediated disease that leads to the damage of various types of immune cells and signal networks, followed by inappropriate tissue repair and organ damage. RA is primarily manifested in the joints, but also manifests in the lungs and the vascular system. This study developed a method for the in vitro detection of RA through cyclic citrullinated peptide (CCP) antibodies and antigens. The diameter of a tilted-fiber Bragg grating (TFBG) biosensor was etched to 50 μm and then bonded with CCP antigens and antibodies. The small variations in the external refractive index and the optical fiber cladding were measured. The results indicated that the self-assembled layer of the TFBG biosensor was capable of detecting pre- and post-immune CCP antigen and CCP peptide concentrations within four minutes. A minimum CCP concentration of 1 ng/mL was detected with this method. This method is characterized by the sensor's specificity, ability to detect CCP reactions, user-friendliness, and lack of requirement for professional analytical skills, as the detections are carried out by simply loading and releasing the test samples onto the platform. This study provides a novel approach to medical immunosensing analysis and detection. Although the results for the detection of different concentrations of CCP antigen are not yet clear, it was possible to prove the concept that the biosensor is feasible even if the measurement is not easy and accurate at this stage. Further study and improvement are required.

Plasmonic biosensing with tilted fiber Bragg gratings interrogated using a 512-pixel spectrometer.

Plasmonic tilted fiber Bragg gratings (TFBGs) are very efficient for fast, accurate, and minimally invasive biosensing. Their transmitted amplitude spectrum is a dense comb of narrowband cladding mode resonances (full width at half maximum < 1 nm) that is usually demodulated using highly resolved (wavelength resolution < 10 pm) devices. This work demonstrates the possibility of using a coarsely resolved spectrometer (166 pm) to read out the amplitude spectrum of a gold-coated TFBG. A refined analysis of the spectral content has allowed us to develop signal processing that provides a refractometric sensitivity of 2656 nm/RIU. This is a fivefold improvement compared to previously reported read-out techniques. Biosensing has then been successfully implemented with gold-coated TFBGs implemented in reflection mode for the detection of insulin, with specific antibodies grafted on the gold surface. Our experimental work is a first step toward the industrialization of the FBG technology, as it opens the door to fast parallel biosensing, profiting from the multiple sensing channels (up to 64) of the interrogator and its high processing speed (repetition rate up to 3 kHz).

Multiparameter Single Sensor for Space Silicone Adhesive Monitoring Under High-Vacuum Ultraviolet Exposure

Tilted fiber Bragg grating (TFBG) sensors were demonstrated to simultaneously measure the material thermomechanical and refractometric state in which they are embedded. In this work, for the first time, TFBGs are investigated for three-parameter monitoring of space-qualified NuSil® CV16-2500 silicone operating during high-vacuum ultraviolet (UV) exposure. The first part of the work is focused on the ultraviolet effect on the TFBG spectrum when the sensor is 1) directly exposed to the radiation, 2) covered by a thin cover glass, and with a Kapton layer on top. Successively, the silicone is used as an adhesive in a sandwich structure in which the TFBGs are embedded and exposed under high vacuum to various UV/vacuum UV intensity radiations and durations. The sensors’ spectra were acquired and demodulated to detect the silicone strain–temperature–refractive index variations and correlate the silicone refractometric changes with the equivalent exposure solar hours. The second part of the paper is on silicone degradation state evaluation using the same sensor but during a direct exposure of the adhesive to the radiation. This allowed the UV effects on the silicone to be enhanced but needed a method to compensate for the damaging effect of UV radiation on the TFBG spectrum.

Probe type TFBG-excited SPR fiber sensor for simultaneous measurement of multiple ocean parameters assisted by CFBG.

The tilted fiber Bragg grating(TFBG), chirped fiber Bragg grating(CFBG), Vernier effect and metal surface plasmon resonance(SPR) effect are effectively combined to form a probe type fiber sensor for simultaneous measurement of seawater salinity, temperature and depth(STD). The SPR effect excited by the TFBG is achieved by covering a gold layer around the TFBG, which is used to measure the refractive index (RI) of seawater. The core mode of TFBG is used to detect the change of seawater temperature and the measurement of TFBG reflection spectrum is realized by inscribing a CFBG after the TFBG, which makes the sensor have a probe type design and more beneficial to practical applications. The fusion of quartz micro-spheres on the end face of the sensing fiber and the parallel connection of an Fabry Perot(F-P) interference cavity enables the use of Vernier effect to detect the depth of the ocean. Femtosecond laser line-by-line method is used to the inscribing of TFBG, which allows the grating parameters to be changed flexibly depending on the desired spectrum. The experimental results show that the temperature sensitivity is 10.82pm/°C, the salinity sensitivity is 0.122nm/g/Kg, the depth sensitivity is 116.85 pm/m and the depth can be tested to 1000 m or even deeper.

A 3.7-kW Oscillating-Amplifying Integrated Fiber Laser Featuring a Compact Oval-Shaped Cylinder Package.

Combining the advantages of high efficiency, environmental robustness, and anti-reflection behavior, oscillating-amplifying integrated fiber lasers have become popular for use in high-power laser structures in industrial applications, wherein the size of the laser source matters. Here, an oscillating-amplifying integrated fiber laser in an oval-shaped cylinder package has been proposed and demonstrated, the footprint for which only occupies an area of 0.024 m2 apart from the pump diode, which is much smaller than in traditional planar fiber laser packages. Numerical simulations have been carried out, which have revealed that an oval-shaped cylinder package can effectively suppress the high-order mode in large mode area fiber setups and thereby benefit the integration of fusion points and the unpackaged elements at the same time. Over 3.7 kW of transverse mode instability (TMI)-free output power has been obtained, with a slope efficiency higher than 80%. With a custom-made chirped and tilted fiber Bragg grating (CTFBG), the Raman suppression ratio is improved to reach 38 dB at peak output power. The oval-shaped design has been verified to assist with the realization of TMI suppression and improve the integration of high-power fiber lasers. To the best of our knowledge, this fiber laser has among the smallest footprints of the various fiber sources at such high-power operating levels.

Accurate compensation and prediction of the temperature cross-sensitivity of tilted FBG cladding mode resonances.

The temperature dependence of core mode resonance has been thoroughly studied in fiber Bragg gratings (FBGs), but it is not the case for cladding mode resonances in multi-resonance gratings such as tilted FBGs (TFBGs). In this work, the temperature sensitivity of ultraviolet written TFBGs in SMF-28 fibers is assessed, demonstrating in the first, to the best of our knowledge, place that a single gauge factor K T=6.25⋅10-6±0.02⋅10-6 ∘ C -1 can be employed to characterize the response to temperature of the resonances over the full spectrum in the 10°-50°C range. Then, a simulation model is obtained, enabling to predict TFBG spectra in the 10°-50°C range with high accuracy. This requires a calibration of the core index and dispersion of the TFBG measured in air at 25°C, and determination of the glass refractive index thermo-optic coefficient (d n/d T=8.46⋅10-6±0.1⋅10-6 ∘ C -1, common to both core and cladding glasses), leading to a mean error on the wavelength position of resonances between 1 and 3pm. This mean error can be further reduced (less than 1pm) by considering a linear dependence with temperature of d n/d T. Therefore, this model will enable to completely remove the temperature-induced shifts of all resonances in TFBG sensing applications and measure with great accuracy the variables of interest by using the scaled averages of groups of resonances instead of (less accurate) individual shifts.

A low-frequency acoustic sensing scheme based on cladding mode of large-angle tilted fiber Bragg grating

A novel low frequency acoustic sensor based on the cladding mode of large-angle tilted fiber Bragg grating (TFBG) is proposed and verified in this work. It mainly uses the characteristic that the coupling mode of the core and cladding mode in TFBG is easy to change when the TFBG experiences micro-bend, which will finally causes a dramatic drift in the spectrum. By combining a large-angle TFBG with the designed polyethylene terephthalate (PET) transducer diaphragm and cavity structure, an effective low-frequency acoustic sensing system is obtained in this work. Under the action of applied acoustic wave, the transducer membrane will have periodic vibrations, which will makes the fixed TFBG dynamically bend, directly leading to a wavelength shift of the cladding mode spectrum. The experimental results show that the sensing system can achieve high-sensitivity acoustic detection in a frequency range of 45–220 Hz, and a maximum acoustic pressure sensitivity of 115.88 mV/Pa at 54 Hz. Moreover, the minimum detection sound pressure can achieve 539.2 μPa/Hz&lt;sup&gt;1/2&lt;/sup&gt;@54 Hz. Therefore, the sensor has the advantages of high sensitivity, good repeatability, simple structure, easy processing, etc. It has a great development prospect in the low-frequency acoustic detection related application fields.

Mitigation of stimulated Raman scattering in a high-power fiber master oscillator power amplifier laser based on a dual-structure fiber grating

Abstract With the increasing power of fiber lasers, single chirped and tilted fiber Bragg gratings (CTFBGs) cannot completely mitigate continuously enhanced system-excited stimulated Raman scattering (SRS). Although improving the loss rate of a single CTFBG or cascading multiple CTFBGs can provide better suppression of the stronger SRS, excessive insertion loss may cause significant attenuation of the output power. Confronting the challenge, we firstly present an SRS mitigation method based on a dual-structure fiber grating in this paper. The dual-structure fiber grating comprises a CTFBG and a fiber Bragg grating structure, which were designed and fabricated on a passive 25/400 double-clad fiber. To evaluate the performance of the grating, a 3 kW fiber master oscillator power amplifier laser is established. The experimental results demonstrate that the SRS mitigation rate of the grating is greater than 30 dB (99.9%), whereas the insertion loss is only approximately 3%, thus allowing for minimal deterioration of the output power. This solves the contradiction between high suppression rate and high insertion loss faced by CTFBGs, which in turn makes dual-structure fiber gratings particularly suitable for mitigating SRS in 3–5 kW high-power fiber lasers.