Pair Of Bragg Gratings Research Articles

All-fiber chirp tuning of ultrashort pulses via long chirped fiber Bragg grating pair

Chirp tuning of ultrashort pulses is crucial for nonlinear fiber amplification and nonlinear dynamics investigations. Here we demonstrate all-fiber chirp tuning via a chirped fiber Bragg grating (CFBG) pair. Two identically long CFBGs were placed reversely to cancel out most of their huge dispersion (∼40 ps/nm or ∼22.4 ps2 @1030 nm), while the controllable temperature gradient along them could be used for precise chirp tuning with a tuning range of ∼ps2, verified by dispersion measurement and ultrashort pulse broadening. This relatively large chirp tuning could be used in prechirp management in nonlinear fiber amplifiers, exemplified by the optical spectrum tailoring therein. In addition, we also show this precise chirp tuning capability could be very helpful for pulse temporal quality diagnosis, which is indispensable for seed pulse optimization. We believe this all-fiber chirp tuning technique would find wide applications in nonlinear amplification and ultrafast nonlinear dynamics investigations.

Co-located angularly offset fiber Bragg grating pair for temperature-compensated unambiguous 3D shape sensing.

A 10 mm-long three-dimensional shape sensor in a single-mode fiber is described and demonstrated experimentally. The sensor is based on a pair of fiber Bragg gratings inscribed at the same location along the fiber axis but offset along different radial directions away from the fiber center. Each offset grating generates cladding mode resonances over a ${\sim}{20}\;{\rm{nm}}$-wide spectral bandwidth, and the two gratings are also offset in period so that their transmission spectra are separated by 40 nm, and thus non-overlapping and fully distinguishable. Directional bending sensitivity results from the differential amplitude response of the cladding mode resonances from the two gratings, depending on the relative orientation of the bend with the azimuthal direction of the grating offsets. It is further demonstrated that both axial deformation and temperature have no influence on the shape measurement as they both only cause a global wavelength shift of the spectra without amplitude change. The experimental results demonstrate that the shape orientation of an object can be unambiguously determined for bend directions covering the full 360° range around the fiber axis with sensitivities of the order of ${{1}}\;{\rm{dB/}}{{\rm{m}}^{- 1}}$ and small curvatures between 0 and ${{1}}\;{{\rm{m}}^{- 1}}$.

Temperature-Insensitive Contact Force Sensing in Bi-Directional Catheter Using Fiber Bragg Grating Pair

Bi-directional catheters are commonly used in electrophysiology mapping and ablation procedures. We have designed and demonstrated a temperature-insensitive contact force sensor for bi-directional catheters with variable curve diameter during deflection. The compact force sensor is based on a fiber Bragg grating (FBG) pair, where both FBGs have the same center wavelength and are fixed at two opposite deflection arcs of the bi-directional catheter. When the catheter is deflected or when contact force is applied to the catheter, bifurcation of the FBG pair spectrum is observed. The amount of spectral bifurcation and the applied contact force has a linear relationship. Contact force measurement, therefore, is accomplished by measuring the change in the spectral bifurcation of the FBG pair, i.e., the change in the Bragg wavelength difference between the two FBGs. Since the approach relies solely on the amount of spectral bifurcation of the FBG pair instead of the Bragg wavelength shift of one single FBG, the proposed contact force sensor is temperature insensitive.

Experimental investigation of the thin fiber-optic hydrophone array based on fiber Bragg gratings

Abstract The paper presents the results of experimental investigations of the fiber optic hydrophone array consisting of six sensors, placed in one thin sensitive cable. Sensors were formed by pairs of Bragg gratings spaced 1.5 m apart and recorded in a birefringent optical fiber with the elliptical stressed coating. To form an extended sensor array the optical fiber was additionally covered with a silicone material RTV655 and protective coatings. Experimental investigations of the array showed that fiber-optic sensors pressure sensitivity increases as the acoustic frequency decreases at average value from −169.4 dB re rad/uPa at 495 Hz to −143.7 dB re rad/uPa at 40 Hz. The minimum detectable pressure was at average value from 53 mPa/√Hz at 495 Hz to 8.3 mPa/√Hz at 40 Hz. The obtained results might be used for developing and producing long thin hydroacoustic arrays for geophysical investigations and other hydroacoustic applications.

Selective transverse mode operation of an all-fiber laser with a mode-selective fiber Bragg grating pair.

We propose a novel approach for achieving selective transverse mode operation of few-mode all-fiber lasers. Lasing in a specific transverse mode is enabled by employing a pair of few-mode fiber Bragg gratings as an efficient transverse mode selector. As a proof-of-principle, we have implemented a ytterbium-doped all-fiber laser operating in the second-order (LP11) transverse mode. The achieved output power is 81 mW, and the slope efficiency is as high as 54%. This simple and compact all-fiber laser is also capable of delivering cylindrical vector beams through appropriate polarization control. Furthermore, the approach has great scalability and can be applied to other working modes and spectral regimes.

Large anomalous-dispersion mode-locked fiber laser based on a chirped fiber Bragg grating pair

A carbon-nanotube mode-locked erbium fiber laser with large net anomalous dispersion is presented. A chirped fiber Bragg grating (CFBG) pair is incorporated to increase the net-cavity anomalous dispersion and soliton splitting threshold. Self-started mode-locked laser produces stable pulses with repetition rate of 9.26 MHz. Laser spectrum is centered at ~1560 nm with 3 dB bandwidth of 0.43 nm. The typical output pulse energy and duration is 0.21 nJ and 8.05 ps, respectively.

High Intrinsic Sensitivity Etched Polymer Fiber Bragg Grating Pair for Simultaneous Strain and Temperature Measurements

A sensing configuration for simultaneous measurement of strain and temperature with enhanced intrinsic sensitivity based on a fiber Bragg grating (FBG) pair with one grating inscribed in the etched and the other in unetched polymer fiber region is demonstrated. A poly (methyl methacrylate) based single-mode polymer fiber is etched to different diameters, and it is observed that etching can lead to change in the material properties of the fiber, such as Young’s modulus and thermal expansion coefficient, which can play a vital role in improving its intrinsic sensing capabilities. Thus, exploiting the different strain and temperature sensitivities exhibited by etched and unetched polymer FBGs, strain and temperature can be simultaneously measured with very high sensitivity. Experimental results show that rms deviations of ± 8.42 $\mu \epsilon $ and ± 0.39 °C for strain and temperature, respectively, in a real simultaneous measurement. The effect of individual thermal and strain sensitivity coefficients on measurement accuracy is also analyzed.

A Highly Sensitive Temperature Sensor Based on Power Changes of Fiber Bragg Grating Pairs

A highly sensitive temperature sensor based on power change of double fiber Bragg gratings (FBGs) structure is proposed in this letter. It consists of two identical FBGs (FBG1, FBG2), in which the Bragg wavelength of FBG1 is modulated by the temperature and its reflecting light is injected into FBG2. Thus, the power difference of two peaks that appear in the transmission spectrum of FBG2 can be modulated by temperature around FBG1. By shifting the Bragg wavelength of FBG2, the sensor can achieve the segmental measurement of temperature. In the experiment, dynamic range of 25 °C-135 °C and sensitivity of 1.35 ± 0.02 dBm/°C have been achieved.

A 16-Element Multiplexed Heterodyning Fiber Grating Laser Sensor Array

In this paper, we demonstrate the implementation of a 16-element array of heterodyning fiber grating laser sensors cascaded in a single optical fiber. Each laser consists of a Bragg grating pair inscribed in a short section of Er-doped fiber. The laser offers a RF-domain beat signal between orthogonally polarization modes and can act as a photonic sensor for the measurement of external perturbations, by monitoring the beat frequency. The laser sensors are wavelength multiplexed by inscribing fiber gratings with different pitches, and frequency multiplexed in RF domain by controlling the intracavity birefringence via CO2 laser side irradiation. The pump power depletion as a result of mode mismatch between the active and passive fibers is a main limitation factor for the multiplexing scale. We can multiplex ten laser sensors at most with a single pump laser with an output power of 130 mW and six more sensors can be multiplexed with an additional pump laser at the other end. The measurement ability of an individual laser sensor can be affected when multiplexed in array, in terms of abrupt frequency jumps and the raised noise level. As a result, the force resolution at 1 kHz is changed from 3 to 6 nN/ $\sqrt {{\rm Hz}}$ , and the minimal detectable static force is changed from 68 to 120 μN. This is attributed to the off-resonance grating couplings and the reflections at the splicing points. This effect can be weakened by selecting mode-matching fibers and optimizing grating profiles, or simply using additional optical isolators.

A Fabry-Perot acousto-optical cavity for the terahertz band

A periodic structure that is a pair of Bragg gratings produced by a decaying acoustic wave is considered. The problem of light diffraction by such structures is solved. Changes of the cavity transmission function are found with allowance for the attenuation of the propagating acoustic wave. Analytical expressions for the light wave transmission and reflection functions are derived, and it is also shown that narrow reflection (or transmission) lines are formed along with standard transmission windows. In contrast to cavities formed by dielectric mirrors, such a cavity supports a mode with a modulated Q factor. In particular, it is stated that a Fabry-Perot cavity can be applied in the terahertz wavelength band.

High-Power and Narrow-Linewidth Er, Yb Fiber Laser Locked by a Volume Bragg Grating-Pair

A high-power and narrow-linewidth Er, Yb co-doped fiber laser is demonstrated by using two serially-paired volume Bragg gratings. A maximum output power of 19.4 W at 1545.3 nm with a full-width at half-maximum linewidth of is obtained for 65.3 W of launched pump power, corresponding to a slope efficiency of 30.1% with respect to launched pump power.

Synchronous tunable wavelength spacing dual-wavelength SOA fiber ring laser using Fiber Bragg grating pair in a hybrid tuning package

A Dual-Wavelength Semiconductor Optical Amplifier (DW-SOA) based fiber ring laser with synchronous wavelength tunability is proposed and experimentally demonstrated. The SOA gain medium strongly suppresses mode competition, thus allowing stable dual-wavelength laser oscillation. The wavelength spacing of the two lasers can be tuned synchronously using a modified hybrid-tuning package incorporating a pair of Fiber Bragg Gratings (FBGs). The DW-SOA demonstrates a laser output with a wavelength spacing of between 0.10 and 8.30 nm (wavelength shift inequality of 0.08 to 0.75 nm). The relationship between the applied strain and wavelength shift of the two tuning modes is also analyzed.

High frequency fiber laser emission generated by pump spiking

A stable and short pulse train of ∼100 MHz repetition frequency was obtained from an erbium doped fiber laser excited by a “continuous” semiconductor laser and by using a linear cavity defined by a Bragg grating pair. The operation frequency of the fiber laser was greater (∼5–15 times) than the cavity round-trip frequency. Emission properties obtained from the erbium doped fiber laser were correlated with those taken from the pump laser, which presented a particular optical noise (very short pulses) added to the continuous emission. From the temporal and radio-frequency analysis of both systems, we conclude that the pump emission characteristics are the responsible of the fiber laser pulsed behaviour.

Ultra-high-sensitivity two-dimensional bend sensor

A multicore fibre Fabry-Perot-based strain sensor interrogated with tandem interferometry for bend measurement is described. Curvature in two dimensions is obtained by measuring the difference in strain between three co-located low finesse Fabry-Perot interferometers formed in each core of the fibre by pairs of Bragg gratings. This sensor provides a responsivity enhancement of up to 30 times that of a previously reported fibre Bragg grating based sensor. Strain resolutions of 0.6 nɛ/Hz1/2 above 1 Hz are demonstrated, which corresponds to a curvature resolution of ∼0.012 km−1/Hz1/2.

Q-switched all-fiber laser based on magnetostriction modulation of a Bragg grating

We report an actively Q-switched all-fiber laser based on magnetostriction modulation of a Bragg grating. The laser employs a pair of Bragg gratings as reflective mirrors, one of which is bonded to a magnetostrictive element. Lengthening of the magnetostrictive element when a magnetic field is applied shifts the Bragg wavelength of the grating, allowing control of the Q-factor of the cavity and, thus, performing active Q-switching. The magnetostrictive modulator is small, compact and requires less than 300 mW electrical drive power. Using erbium-doped fiber and a maximum pump power of 120 mW, Q-switch pulses of more than 1 W peak power were obtained, with a pulse repetition rate that can be continuously varied from 1 Hz to 125 kHz.

Superimposed Bragg gratings in high-birefringence fibre optics: three-parameter simultaneous measurements

We used a pair of Bragg gratings written in high birefringence fibre optics to measure, simultaneously, longitudinal and transverse strain and temperature. The Bragg gratings are superimposed in the same position of the fibre optic, so as to behave as a punctual sensor. The sensitivity of the spectral response of the device to longitudinal strain, transverse strain and temperature are all characterized, and the results of its application as a three-parameter sensor are also presented.

Mode recoupling in a novel Bragg grating pair.

A new type of mode recoupling in a Bragg grating pair, in which one of the gratings is written in the fiber cladding, is proposed to overcome limitations of concatenated long-period and fiber Bragg gratings reported earlier [Opt. Lett. 27, 1214 (2002)]. Its reflection spectrum is similar to that of the previously reported concatenated grating structure; however, the reflectivity can be much larger than the previous limitation of 50%. Furthermore, avoidance of the loss induced by a long-period grating makes such a Bragg grating pair more attractive for practical applications.

Monitoring fiber Bragg grating pair interferometer sensor with a modulated diode laser

A new technique of monitoring a fiber Bragg grating pair interferometer (FBGPI) sensor with a modulated diode laser is demonstrated. The sensing element includes a pair of fiber Bragg grating (FBG) sections separated with a short fiber cavity, so that the reflectance spectrum is a superposition of a gradually changing reflectance envelope over a sinusoidal modulation. Modulating the drive current of a diode laser produces laser chirping to scan over ∼2.6 fringes of the FBGPI reflectance spectrum. The spectral position of the sensor is precisely determined from the phase of the scanned interference pattern. It is an absolute measurement because the fringe order numbers of the interference pattern can be determined according to the average reflectance changes. Temperature measurements are performed over the range from 24 to 367 °C with a resolution of 0.004 °C.

Adaptive dispersion slope equalizer using a nonlinearly chirped fiber Bragg grating pair with a novel dispersion detection technique

We realized the first adaptive-dispersion equalizer that equalizes dispersion over a wide wavelength range (6 nm) in the zero-dispersion wavelength region of a dispersion-shifted fiber (DSF). The equalizer is based on a pair of nonlinearly chirped fiber Bragg gratings, which are designed to equalize exactly the dispersion profile of a DSF. The dispersion changes were tracked using a technique that employs opposite dispersion fibers to identify the sign of the dispersion change. Unlike previous approaches, no additional sources or changes in the source wavelength are required. We demonstrate the adaptive equalization of the dispersion changes in an 83-km DSF, induced by temperature changes between -10/spl deg/C and 60/spl deg/C.

A wavelength-tunable dispersion equalizer using a nonlinearly chirped fiber Bragg grating pair mounted on multilayer piezoelectric transducers

We realized a wavelength-tunable dispersion equalizer using a pair of nonlinearly chirped fiber Bragg gratings mounted on multilayer piezoelectric transducers. The operation region can be continuously tuned up to 7 nm by applying 50 V. This device enables us to equalize the second- and third-order dispersion of dispersion-shifted fibers with different zero dispersion wavelengths over 6 mn. This performance shows that our equalizer can be employed for adaptive equalization.