Superimposed Fiber Bragg Gratings Research Articles

Triple-Wavelength Thulium-Doped Fiber Random Laser Based on Random Fiber Grating

We demonstrate a triple-wavelength thulium-doped fiber random laser using a 10 cm long random fiber grating to provide random distributed feedback and a superimposed fiber Bragg grating as the wavelength-selective mirror. The random fiber grating inscribed in single-mode fibers using a femtosecond laser provides strong random distributed feedback that avoids the use of long distance fibers and leads to a relatively low threshold power. Triple-wavelength random laser output at wavelengths of 1943.6, 1945.0 and 1946.3 nm was achieved with a relatively low threshold power of 2.01 W, a slope efficiency of 7.86% and a maximum output power of 151.8 mW when it was pumped using a 793 nm laser diode. The 3 dB linewidth was less than 0.1 nm and the optical signal-to-noise ratio was up to 45.6 dB. Good wavelength stability was achieved, which was attributed to the narrow band and stable reflection of the superimposed fiber Bragg grating. The time-domain characteristics of the laser output were also measured and analyzed, and some random self-pulsing caused by relaxation oscillations were observed.

Wavelength-switchable single-longitudinal-mode thulium-doped fiber laser at 2.05 µm using a superimposed fiber Bragg grating

Herein, we present a wavelength-switchable single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL), enabled by a three-channel superimposed fiber Bragg grating (SI-FBG), a compound-subring-cavity (CSC), and an unpumped TDF-based saturable absorber (SA). By adjusting a drop-in polarization controller (DI-PC), three single-wavelength operations can be obtained and switched flexibly. In each operation, the fiber laser exhibited a maximum optical signal-to-noise ratio (OSNR) of 62 dB as well as the wavelength and power fluctuations of ≤0.01 nm and ≤0.641 dB, respectively, in a 50-min time span. Furthermore, the SLM lasing characteristics of the proposed TDFL with the CSC and SA have been investigated in detail.

Fabrication of Multiple Superimposed Fiber Bragg Gratings for Multiple Parameter Sensing

Multiple superimposed fiber Bragg gratings (FBGs) are fabricated based on a non-uniform single-core As2Se3-Poly methyl methacrylate (PMMA) tapered fiber, by which superimposition of up to nine chirped FBGs is achieved. Multiple superimposed uniform FBGs are first inscribed on the nonuniform waveguide and multiple superimposed chirped FBGs are obtained by stretching the non-uniform waveguide. Simultaneous measurement of temperature and strain is realized based on two superimposed chirped FBGs at wavelengths of 1540 nm (FBG1) and 1550 nm (FBG2), respectively. The central wavelength shifts and bandwidth changes of the two superimposed chirped FBGs, when temperature and strain change, are measured, respectively. Four matrices are defined to predict the temperature and strain change, and the one defined by measuring the bandwidth change of FBG1 and the central wavelength shift of FBG2 gives the most accurate measurement with the sensitivities of 29.1 pm/°C for temperature measurement and 0.373 pm/μe for strain measurement by measuring the bandwidth change of FBG1, and 75.1pm/°C for temperature measurement and 0.245 pm/μe for strain measurement by measuring the central wavelength shift of FBG2, respectively. The resolutions for temperature and strain measurement are enhanced by more than a factor of 10, respectively, compared with those using the matrix defined by measuring the wavelength shifts of FBG1 and FBG2. A large strain change from 0 to 22000 μe is achieved due to the low stiffness of As2Se3-PMMA taper, which is far beyond the operating range of silica materials.

Fabrication of superimposed fiber Bragg gratings and applications in fiber laser oscillators

A novel dual-use superimposed fiber Bragg grating (FBG) consisting of a normal FBG operating at 1.55 μm and a chirped FBG (CFBG) operating at 1.08 μm is fabricated and applied in a fiber laser oscillator. The 1.08 μm CFBG is used as a reflector of the fiber oscillator and the FBG is used as an in-core temperature on-line monitor of the laser simultaneously. According to the temperature characteristics of the FBG, the real-time inner temperature can be obtained by detecting the shift of the reflection spectrum. Experimental results have proved the effectiveness of the proposed temperature monitor, which can give the temperature inside a fiber laser comparing to the normal surface measurement techniques such as infrared (IR) camera. This paper provides a simple method to detect the local in-core temperature of fiber lasers, which is useful for investigations of high-power fiber lasers system.

Low-threshold wavelength-switchable photonic crystal fiber laser based on superimposed fiber Bragg gratings

• Quadruple superimposed FBGs are fabricated in a homemade photosensitive PCF. • A four-wavelength switchable PCF laser with stable output is demonstrated. • With optimized PCF and splicing, the laser has a low threshold power of ~0.85 mW. We report a wavelength-switchable photonic crystal fiber laser based on superimposed fiber Bragg gratings with a low threshold. Quadruple superimposed fiber Bragg gratings are fabricated in a photosensitive PCF and inserted between small core erbium-doped PCF and step-index fiber, which reduce the intracavity loss below 2 dB. Four wavelength switching stable outputs are obtained, and the lowest threshold power is only ∼0.85 mW with a slope efficiency of ∼13.3% correspondingly.

15 μm polarization-maintaining dual-wavelength single-frequency distributed Bragg reflection fiber laser with 28 GHz stable frequency difference

We demonstrate a polarization-maintaining (PM) dual-wavelength (DW) single-frequency Er-doped distributed Bragg reflection (DBR) fiber laser with 28GHz stable frequency difference. A homemade PM low-reflectivity superimposed fiber Bragg grating (SFBG) is employed as the output port of the DBR fiber laser. The SFBG has two reflection wavelengths located in the same grating region. The reflectivity of both DWs is around 85%. The achieved linear polarization extinction ratio is more than 20dB. The DWs of the laser output are located at 1552.2nm and 1552.43nm, respectively. The optical signal-to-noise ratio (SNR) is above 60dB. For each wavelength, only one longitudinal mode exists. The beat frequency of the two longitudinal modes is measured to be 28.4474GHz, with the SNR of more than 65dB and the linewidth less than 300Hz. During a 60-min-long measurement, the standard deviation of the frequency fluctuation is 58.592kHz.

Inscription process research and optimization for superimposed fiber Bragg gratings

Inscription process research and optimization for superimposed fiber Bragg gratings

Inscription of superimposed fiber Bragg gratings using a Talbot interferometer

In this paper, we present results on the inscription of superimposed Bragg gratings in various optical fibers, including specialized light guides produced in Russia, using an inscription setup based on the Talbot interferometer. A KrF excimer laser system of the master oscillator type—an Optosystems CL-7550 power amplifier—is used as the source of ultraviolet radiation for the experimental stand. The spectral characteristics of superimposing a different number of fiber Bragg gratings are demonstrated and the inscription of several Bragg gratings in the same region of the optical fiber is investigated.

Switchable 10 nm-spaced dual-wavelength SLM fiber laser with sub-kHz linewidth and high OSNR using a novel multiple-ring configuration

A switchable dual-wavelength (10 nm spacing) single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) that uses a multiple-ring configuration was demonstrated experimentally. A novel theoretically lossless triple-ring passive secondary cavity composed of three optical couplers was utilized to select the SLM from the dense main cavity longitudinal modes for the first time. Using two superimposed fiber Bragg gratings as one compact mode-restricting element and introducing the nonlinear polarization rotation effect to alleviate the strong mode competition for the fiber laser, a highly stable dual-wavelength mode-hop-free SLM operation with sub-kHz linewidths and optical signal to noise ratios (OSNRs) of >69 dB was achieved for both lasing wavelengths. By adjusting the polarization controllers, the dual-wavelength operation could be easily switched to single-wavelength lasing with a linewidth of <1 kHz and an OSNR of >72 dB. The proposed EDFL may find many important applications, such as the generation of very pure terahertz waves.

Simulation of a high-speed superimposed fiber Bragg gratings interrogation system.

This paper presents a simulation of high-speed nonuniform random sampling in a superimposed fiber Bragg gratings (SFBGs) interrogation system. The simulated Gauss SFBGs are used to generate a nonuniform sensing pulse train during each scanning cycle. Six different conditions that can cause nonuniform sampling are simulated, and a random sine-wave driving method to improve the driving speed is proposed. An 11.8kHz dynamic strain is measured by generating an additive nonuniform randomly distributed 12kHz optical sensing pulse train from a mean 2kHz sinusoidal periodically changing scanning frequency and three SFBGs. Four conditions that can improve the sampling results are also simulated.

Analysis of all fiber acousto-optic tunable filter based on superimposed fiber Bragg gratings

In this paper, an all fiber acousto-optic tunable filter based on superimposed fiber Bragg gratings (SFBG-AOTF) is demonstrated and studied. Compared to the normal fiber Bragg gratings based all fiber acousto-optic tunable filter, SFBG-AOTF can modulate the two optical resonant wavelengths of the gratings synchronously. The spectrum of SFBG-AOTF at various acoustic frequencies and under acoustically induced strains, has been analyzed theoretically. Based on simulation results, one can find that each order of the secondary reflection peak is symmetrical with respect to the two primary reflection peaks with SFBG as the center, and the resonant wavelength spacing between the primary reflection peak and the secondary reflection peak which is modulated by the former, is proportional to the acoustic frequency. But the resonant wavelength between the same order secondary reflection peaks which are modulated by two different primary reflection peaks, is independent of the acoustic frequency. The acoustically induced strains mainly affect the variation of the reflectivities of the primary and secondary reflection peaks. In the experiment, the spectra of SFBG-AOTF with acoustic frequencies of 390 and 710 kHz, are measured. The variation trend of the experimental results accords well with the simulated one.

Switchable and tunable dual-wavelength single-longitudinal-mode erbium-doped fiber laser with special subring-cavity and superimposed fiber Bragg gratings

A switchable and tunable dual-wavelength single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) has been proposed and demonstrated. Two superimposed fiber Bragg gratings (SI-FBG) have been used as one original wavelength selection component. A special subring-cavity was employed to select the SLM. By properly adjusting a polarization controller, single- and dual-wavelength switchable operations with high wavelength and power stability were realized experimentally. Regardless of which work mode, the signal to noise ratio was higher than 60 dB and the polarization extinction ratio was higher than 20 dB. The wavelength-spacing was 9.96 nm, indicating that it can be used to generate continuous-wave terahertz waves. The linewidth of each lasing wavelength measured by the delayed self-heterodyne method was approximately 1.25 kHz and 1 kHz, respectively. By stretching the SI-FBG, the EDFL had a wavelength-tunable range of 5.96 nm in both the single- and dual-wavelength operations.

Fast Interrogation Method Based on Two Superimposed Fiber Bragg Gratings

A fast interrogation method based on two Superimposed Fiber Bragg Gratings (SI-FBGs) is researched. The two SI-FBGs under100Hz dynamic sinusoidal strain with linear triple driving voltage and 500Hz FFP-TF scanning frequency with 20nm wavelength range is simulated based on transfer matrix. A four times increase in the interrogation speed of dynamic strain, by generating a 2kHz optical sensing pulse train from a 500Hz scanning frequency, is demonstrated experimentally. The simulation and experimental results show that the interrogation speed can be increased by times with more SI-FBGs used.

A stable single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser with superimposed FBG and an in-line two-taper MZI filter

A simple approach to fabricate a stable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber ring laser with one superimposed fiber Bragg grating (FBG) is proposed and demonstrated. The superimposed FBG is fabricated by the dual-exposure method for wavelength selection. A home-made in-line two-taper Mach–Zehnder interferometer is incorporated to balance the intra-cavity loss for the wavelengths. The SLM operation is guaranteed by a feedback fiber loop and saturable absorber. A stable dual-wavelength SLM fiber ring laser with a wavelength spacing of approximately 8.53 nm is experimentally realized. The optical signal-to-noise ratio of the laser output is over 50 dB and the linewidth of each wavelength measured by the delayed self-heterodyne method is 9.67 kHz and 9.07 kHz, respectively. The proposed scheme offers the possibility for difference frequency generation of terahertz radiation.

A dual-wavelength tunable laser with superimposed fiber Bragg gratings

We report a dual-wavelength tunable fiber laser. The cavity is formed by two superimposed fiber Bragg gratings (FBGs) and a temperature tunable high-birefringence fiber optical loop mirror (FOLM). FBGs with wavelengths of 1548.5 and 1538.5 nm were printed in the same section of a fiber using two different masks. The superimposed FBGs were placed on a mechanical mount that allows stretch or compression of the FBGs. As a result of the FBG strain both lines are shifted simultaneously. Dual-wavelength generation requires a fine adjustment of the cavity loss for both wavelengths.

Additive non-uniform random sampling in superimposed fiber Bragg grating strain gauge

This paper demonstrates an additive non-uniform random sampling and interrogation method for dynamic and/or static strain gauge using a reflection spectrum from two superimposed fiber Bragg gratings (FBGs). The superimposed FBGs are designed to generate non-equidistant space of a sensing pulse train in the time domain during dynamic strain gauge. By combining centroid finding with smooth filtering methods, both the interrogation speed and accuracy are improved. A 1.9 kHz dynamic strain is measured by generating an additive non-uniform randomly distributed 2 kHz optical sensing pulse train from a mean 500 Hz triangular periodically changing scanning frequency.

基于随机非均匀采样的重叠双光栅动态应变传感实验研究

基于随机非均匀采样的重叠双光栅动态应变传感实验研究

Dynamic and static strain gauge using superimposed fiber Bragg gratings

This paper demonstrates a simple and fast interrogation method for the dynamic and/or static strain gauge using a reflection spectrum from two superimposed fiber Bragg gratings (FBGs). The superimposed FBGs are designed to decrease nonequidistant space of generated a sensing pulse train in a time domain during dynamic strain gauge. By combining centroid finding with smooth filtering methods, both the interrogation speed and accuracy are improved. A four times increase in the interrogation speed of dynamic strain, by generating a 2 kHz optical sensing pulse train from a 500 Hz scanning frequency, is demonstrated experimentally. The interrogation uncertainty and total harmonic distortion characterization of superimposed FBGs are tested and less than 4 pm standard deviation is obtained.

Phase-controlled superimposed FBGs and their applications in spectral-phase en/decoding

A novel kind of superimposed fiber Bragg gratings (SI-FBGs) named SI-sampled FBGs (SI-SFBGs) is proposed to control the phase relationship among SI sub-gratings by modulating the sampling periods. The realization of such phase-controlled SI-SFBGs just needs a single uniform phase mask and sub-micrometer precision moving stage. The success of phase-controll is expected to encourage SI-SFBGs' applications in more sophisticated fields. As a demonstration, their applications in spectral-phase en/decoding are testified by both simulation and experiment. The spectral-phase encoded (SPE) encoders with the longest code-length that FBG-based SPE encoders can achieve, i.e., 64-frequency bins, are experimentally fabricated for the first time. The results show the advantages accompanying the SI-SFBGs-based SPE encoders compared with the traditional methods.

Free-spectral range tunable Fabry–Perot filter with superimposed fiber Bragg gratings

A novel free-spectral range (FSR) tunable Fabry–Perot filter (FPF) with two superimposed fiber Bragg gratings (SFBGs) is proposed and experimentally demonstrated. The SFBGs, initially uniform in period with slightly different Bragg wavelengths, were glued onto the lateral surface of a specially-designed cantilever beam and linearly chirped by beam bending-induced strain gradient. As a result, their reflection bands overlap and a grating-based FPF with a tunable FSR is formed. A continuously tunable FSR of 0.1–0.5 nm has been achieved in the experiment.