Phase-shifted Fiber Bragg Grating Research Articles

Pd–Ta alloy films hydrogen sensors based on partially coated π -phase-shifted FBG

A fiber-optic hydrogen sensor based on Pd–Ta alloy films partially coated π-phase-shifted fiber-Bragg grating (FBG) is proposed and experimentally demonstrated. Three types of Pd–Ta alloy films with different Ta content were prepared via co-sputtering technology. Material characterization shows the transformation of Pd–Ta alloy to amorphous phase with the increase of Ta content, and the phase transition between α-PdTaHx and β-PdTaHx is inhibited at 2 % hydrogen concentration. Hydrogen sensing experiments show that in-situ temperature compensation is possible with Pd/Ta films partially coated on π-phase-shifted FBG. Moreover, the sensor's hysteresis phenomenon was eliminated with an increase in Ta content due to the first-order phase transition. Pd–Ta alloy films hydrogen sensor with a Ta content of 27 % is capable of in-situ temperature compensation, fast response, good stability, and reliability.

Wavelength injection locking actively Q-switched random fiber laser based on random phase-shifted fiber Bragg grating and electro-optic modulator

Wavelength injection locking actively Q-switched random fiber laser based on random phase-shifted fiber Bragg grating and electro-optic modulator

Air Gap Fiber Bragg Grating for Simultaneous Strain and Temperature Measurement.

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.

Microwave Photonic Time–Frequency Analysis Based on Period-One Oscillation and Phase-Shifted Fiber Bragg Grating

Microwave Photonic Time–Frequency Analysis Based on Period-One Oscillation and Phase-Shifted Fiber Bragg Grating

Enhancing Gain Uniformity in WDM Networks Across C and L Bands Through Hybrid Optical Amplifier Integration

This study explores strategies for optimizing EDFA gain in the context of (WDM and DWDM systems. By reviewing recent literature, we investigate various approaches to enhance EDFA performance, considering factors such as input power, pump power, and fiber length. Our analysis covers optimization algorithms, gain flattening techniques, adaptive gain equalization methods, and hybrid amplifier configurations. Additionally, we examine studies addressing nonlinear effects and the allocation of pump power to maximize gain efficiency. Recent advancements, including machine learning-based approaches and novel gain equalization techniques using phase-shifted fiber Bragg gratings, are also discussed. The findings underscore the significance of tailored EDFA gain optimization methods for improving the overall performance and efficiency of WDM and DWDM systems in optical communication networks.

All-fiber laser self-mixing interferometry for signal enhancement with phase-shifted fiber Bragg grating

In this paper, phase-shifted fiber Bragg grating (PSFBG) is introduced in an all-fiber laser self-mixing interferometry (SMI) for signal enhancement. PSFBG is used as an optical filter to enhance the signal to noise ratio (SNR) of SMI signal. When the laser wavelength locates at the edge of the filter, the frequency modulation (FM) can be mapped to the easy-to-detect amplitude modulation (AM) to enhance the SMI signal. Simulation and experimental comparison of unenhanced SMI signal, FBG-enhanced SMI signal and PSFBG-enhanced SMI signal are presented. The enhancement effects of different reflectivity surfaces are comparatively analyzed and the maximum spatial test ranges of lower reflectivity surfaces are compared between unenhanced and PSFBG-enhanced. As well as applying the PSFBG-enhanced to the Doppler effect experiment. Due to the extremely narrow linewidth of PSFBG, the PSFBG-based SMI has the better performance in longer detection range and lower backscattering light. PSFBG-enhanced achieves an impressive enhancement of up to 800 times compared to unenhanced signals.

Acoustic Emission Testing at Cryogenic Temperatures Using Cantilevered Phase-Shifted Fiber Bragg Grating

Acoustic Emission Testing at Cryogenic Temperatures Using Cantilevered Phase-Shifted Fiber Bragg Grating

Phase-shifted fiber Bragg grating filters based on counter-propagating cladding modes coupling.

In this paper, we comprehensively analyze counter-propagating cladding mode assisted phase-shifted fiber Bragg gratings (FBGs) and propose an ultra-narrow bandwidth laser line filter based on such gratings. Full vector modal analysis has been used to obtain the mode guiding and its coupling characteristics. We show that the transmission spectrum of the counter-propagating cladding mode assisted FBGs can be tailored by incorporating single or multiple phase shifts along the grating length. Phase shifts open up narrowband transmission windows inside the stop band of the Bragg grating, and the transmitted wavelength can be altered by controlling the amount of phase shift. Unlike conventional FBGs, the grating discussed here has access to the evanescent field of the excited counter-propagating cladding mode, which opens up the possibility of refractive index based tuning of resonance wavelength. Further, the bandwidth of the proposed grating is two orders of magnitude smaller than that of the conventional LPGs. As an alternative application, we also show that with the right length ratio, the multiple phase shifts can also be used to create a very flattened single transmission peak inside the stop band. The effects of grating regulating factors such as the grating length, grating strength, location of the phase shift, and index apodization on the linewidth of the narrow central peak of π PS-FBG are also investigated. We present a clear physical explanation of various factors involved in the counter-propagating cladding mode coupling in such phase-shifted Bragg grating. Due to its extremely narrow transmission window, our study can find application in developing sensors for measuring parameters with greater accuracy. Such phase-shifted Bragg gratings can also be used to create an all-fiber demultiplexer for multichannel systems and fiber optic sensors as a particular application.

Thermal Modulation of a Chaotic Fiber Laser by Using a Phase-Shifted Fiber Bragg Grating

Thermal Modulation of a Chaotic Fiber Laser by Using a Phase-Shifted Fiber Bragg Grating

Sub-kHz-linewidth laser generation by self-injection locked distributed feedback fiber laser

We presented an integrated all-fiber sub-kHz-linewidth distributed feedback fiber laser (DFB-FL) assisted by self- injection locking. A π phase-shifted fiber Bragg grating (π-FBG) was employed to achieve single longitudinal mode operation, which was UV-inscribed in Er/Yb co-doped fiber with super-Gaussian apodization profile. The experiment results have shown that the apodized π-FBG based laser cavity has got the higher slope efficiency and the narrower linewidth, compared with the normal grating, which are consistent with the simulation one very well. To further compress the linewidth, we have employed the self-injection locking mechanism, which the linewidth of output laser could reach to around 350 Hz with 10-m delay fiber. After integrated encapsulation, the relative intensity noise (RIN) peak of the DFB-FL was reduced to below −125 dB/Hz. The beat frequency fluctuation was less than 15 MHz within 1000 s and power fluctuation was less than 1% in 7 h.

High-Sensitivity Fiber-Optic Voltage Sensor Based on an Optoelectronic Oscillator Using a PZT-Stack and an Equivalent Phase-Shifted Fiber Bragg Grating

High-Sensitivity Fiber-Optic Voltage Sensor Based on an Optoelectronic Oscillator Using a PZT-Stack and an Equivalent Phase-Shifted Fiber Bragg Grating

Multi-directional strain measurement under thermomechanical loading using embedded phase-shifted fiber Bragg grating

This study presents a multi-directional strain measurement technique under thermomechanical loading, utilizing the sensitive birefringence of an embedded phase-shifted fiber Bragg grating. The spectral response of the phase-shifted fiber Bragg grating to temperature and multi-directional strains is mathematically elucidated and simulated using a modified transfer matrix method considering the polarization state. Strain decoupling and reconstruction algorithms are newly proposed based on thermomechanical analysis, which relate the spectral characteristics, strains in the sensor, and strains in the host material. After calibration of the temperature and multi-directional strain responses, a phase-shifted fiber Bragg grating is embedded into a polyetherimide specimen and subjected to thermomechanical loading, achieving the measurement of pressure as well as three-directional strains in the sensor and polyetherimide. The presented technique is demonstrated to be reliable for accurate multi-directional strain measurement under thermomechanical loading with minimal errors and shows potential for in-situ measurement of advanced structures.

Wavelength-Tunable Single-Longitudinal-Mode Narrow-Linewidth Thulium/Holmium Co-Doped Fiber Laser with Phase-Shifted Fiber Bragg Grating and Dual-Coupler-Ring Filter

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.

Study on Time-frequency Imaging of Ultrasonic Detection with Phase Shifted Fiber Bragg Grating Sensing

Abstract The influence of the wavelength difference between the laser source and the phase-shifted fiber Bragg grating (PS-FBG) on the intensity of the power demodulation system based on an adjustable laser source was studied experimentally, and the optimum of the output laser wavelength was determined. Then, the research on time-frequency imaging damage identification based on smooth pseudo-Wigner-Ville distribution was carried out. The Time of Flight of the acoustic wave signal was calculated and time compensation was made according to the Wigner-Ville distribution and the Lamb wave dispersion curve. The ultrasonic waves before and after damage were measured with spatially arranged PS-FBGs. The difference signals were processed in a window, and then the time-frequency energy of the normalized difference signal was imaged to assess the damage detection and location. Although the mode and group velocity of ultrasound measured by each fiber grating were different, the accurate location and identification of artificial damage in an aluminum alloy plate was realized by using only three PS-FBGs and a smooth Wigner time-frequency imaging method.

A reference-free dual-comb spectroscopy calibrated by passive devices

Dual-comb spectroscopy has enabled new approaches for optical precision measurements. Although Doppler-limited resolution can be achieved over long-time scales across a large bandwidth, the development of dual-comb spectroscopy is hindered by strict demands for light source stability. Typically, expensive and complex self-reference systems are required to lock the carrier-envelope offset frequency (fceo) of the laser. Additionally, simply locking the repetition frequency (frep) to a radio frequency reference source still results in residual relative timing jitter between light sources. Here we extracted the relative fceo fluctuation between the frep-locked lasers from the high-precision passive notch filtering characteristics of the phase-shifted fiber Bragg grating and then eliminated it through online phase calibration. By introducing a passive broadband Fabry–Perot cavity with excellent thermal wavelength stability, we subsequently corrected residual relative timing jitter with online wavelength calibration, and the standard deviation of the relative wavelength drift was reduced to less than 0.4 pm within the full operating range. The spectral profile can also be extracted and removed by the Fabry–Perot cavity through intensity calibration. By calibrating these three dimensions, we built a reference-free post-calibration dual-comb spectroscopy and used this powerful tool to measure the Fabry–Perot cavity resonance peaks, the notch filtering narrow band of phase-shifted fiber Bragg gratings, and the absorption characteristics of hydrogen cyanide gas. The system achieves a spectral resolution of 0.8 pm over a bandwidth of more than 100 nm. This low-cost and convenient scheme provides new ideas for the application of dual-comb spectroscopy systems.

Five-wavelength-switchable single-longitudinal-mode erbium-doped fiber laser using CDRC filter and cascaded PS-FBG

Wavelength-switchable fiber lasers play an important role in optical fiber communication systems. Although phase-shifted fiber Bragg grating (PS-FBG) has a narrow 3 dB bandwidth, it has rarely been reported as a multi-channel optical filter. Therefore, a five wavelength-switchable single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) was implemented and analyzed, where the cascaded PS-FBG with a 3 dB bandwidth of each channel narrower than 0.024 nm was used as a five-channel filter. The design, fabrication, and characterization of cascaded PS-FBG were implemented and analyzed in detail. By tuning the micro-displacement platform, five-single wavelength switchable SLM outputs with a high optical signal-to-noise ratio (OSNR) were generated and the outputs were monitored over 60 min. The measured maximum wavelength drift and the peak power fluctuation were 0.02 nm and 1.73 dB. The relaxation oscillation peaks were lower than −100 dB/Hz and the minimum linewidth at an integration time of 0.001 s was 1.3102 kHz. To the best of our knowledge, this is the first time that the PS-FBG is used as a multi-channel filter to realize multiple SLM single-wavelength outputs.

Wavelength injection locking actively Q-switched random fiber laser based on random phase-shifted fiber Bragg grating and EOM

Wavelength injection locking actively Q-switched random fiber laser based on random phase-shifted fiber Bragg grating and EOM

A passively Q-switched random fiber laser with cascaded doped fiber segments and random phase-shift Bragg grating

In this paper, stable pulse train generated from an all-optical passively Q-switched random fiber laser (RFL) with cascaded doped fiber segments and random phase-shift Bragg grating is demonstrated for the first time. A 25 mm random phase-shifted fiber Bragg grating (RPS-FBG) is used to provide random feedback. Two cascaded doped fiber segments, Erbium-doped fiber (EDF) and Erbium-Ytterbium co-doped fiber (EYDF), are used as saturable absorber when the fibers are incompletely pumped. The characteristics of the RFL under different power of two pump lasers are investigated. The doped fiber has different nonlinearities under different pump power combinations, which results in passively Q-switched pulse with repetition rate from 5.1 kHz to 29.3 kHz. The RFL not only has low-threshold but also exhibits stable random pulse with repetition rate and pulse width controlled by the combination of two pump powers.

Automatic Power Control for a Frequency Tunable Optoelectronic Oscillator

Automatic power control for a frequency tunable optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The OEO is implemented by a tunable microwave photonic filter using a phase-shifted fiber Bragg grating (PS-FBG) for frequency-tuning. Taking advantage of the automatic power control (APC), the problem of large power fluctuation in the OEO output signal during the frequency tuning or interfered by the external disturbance can be significantly improved and the functionality of equalization is realized. By adjusting the DC reference voltage of the automatic power control circuit, the tuning of RF signal power is conveniently achieved. To the best of the authors’ knowledge, this is the first time that the output power regulation of OEO has been implemented. For verification, the low-cost OEO at 2–7 GHz is established. Experiment results show that the output power fluctuation of less than 3 dB and a phase noise of −113 dBc/Hz@10 kHz are achieved.

Static and ultrasonic structural health monitoring of full-size aerospace multi-function capsule using FBG strain arrays and PSFBG acoustic emission sensors

In this study, we developed an integrated static and ultrasonic structural health monitoring (SHM), that was achieved using fiber Bragg grating (FBG) arrays and phase-shifted FBG (PSFBG) sensors, to monitor a full-size aerospace multi-function capsule under varying load. The FBG and PSFBG were bonded using direct and remote adhesive methods, respectively, to ensure the performance of the integrated SHM. The static strain measured by FBG accurately followed the loading curve during the initial 15 min of the test, and subsequently exhibited an irregular trend due to damage. Acoustic emission signals induced by damage were first detected by PSFBG at 15 min 11 sec. Following the detectable irregular strain and AE signals, the failure of the capsule manifested as a wrinkling ultimately occurred at 18 min. The FBG arrays and PSFBG sensors performed well for the integrated SHM and demonstrated that the newly proposed method can alert the unhealthy status of a large-scale aerospace structure before catastrophic failure.