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An optical fiber current sensor based on helical long-period fiber grating (HLPG) inscribed in polarization-maintaining fiber (PMF) by CO 2 laser has been proposed. The PMF-HLPG is put into the solenoids with a direct current (DC) current source, and the value of applied current can be measured by monitoring the rotation angle of the input elliptical polarized light due to the changes of the magnetic field along the grating. The main factors that affect the rotation angle have been investigated experimentally. The experimental results show a good linear relationship between the rotation angle and the applied current, which indicate that the PMF-HLPG can be used as all fiber current sensor. The maximum sensitivity of the current measurement can be achieved by adjusting the parameters of the input elliptical polarized light. The current sensor based on the PMF-HLPG exhibits high sensitivity even for the detection of small electric current. • The PMF-HLPGs show a large optical activity. • The length of the PMF-HLPG is short. • The fabrication of PMF-HLPG is simpler than that of other optical fibers for current sensing. • The current sensitivity can be changed by adjusting related parameters. [ABSTRACT FROM AUTHOR]

A wideband true-time-delay (TTD) unit employing a novel fiber grating prism (FGP) for phased array beamforming is proposed in this paper. The FGP consists of a single-mode fiber delay line, a chirped grating delay line and three discrete fiber Bragg grating (FBG) delay lines. The first delay line is a length of single-mode fiber, which provides a fixed time delay for all wavelengths. The second delay line, which employs a chirped grating, is used to provide small time delays. The discrete FBG delay lines, which incorporate an array of 13 discrete FBGs, are used to provide large time delays. A 5×13 fiber-grating prism is constructed and experimented. The results show that the time delays produced by the fiber grating delay lines are independent of the microwave frequency and agree well with the calculated time delays. Based on the measured time delays, the radiation patterns of 5-element array antenna are calculated and analyzed. The beampointing direction of the antenna is independent of the microwave frequency and can be controlled by tuning the wavelength of the optical carrier. This 5×13 TTD unit can provide phased array beamforming at microwave frequencies up to 6 GHz. [Copyright &y& Elsevier]

We demonstrated a torsion sensor based on an in-fiber Mach-Zehnder interferometer (MZI), which consists of two identical cascaded long-period fiber gratings (CLPFGs) inscribed in two-mode fiber (TMF) by high-frequency CO 2 laser. The transmission and mode coupling performances, as well as the spectral properties have been proved theoretically and experimentally. The proposed TMF-CLPFGs could not only be used as an efficient multi-wavelength LP 01 -LP 11 mode converter, but also achieve the vector torsion measurements. As the twist induced, the proposed TMF-CLPFGs present vector torsion ability through the intensity of interference dips varying. The torsion sensitivity of TMF-CLPFGs with spacing length 1 cm can reach −1.07 dB/(rad/m) in counterclockwise direction and 0.9 dB/(rad/m) in clockwise direction. Moreover, the temperature sensitivity of TMF-CLPFGs was measured to be 0.055 dB/°C, which could be negligible to torsion measurement. Therefore, the proposed TMF-CLPFGs could be potential in few-mode fiber-based communication systems and sensing fields. [ABSTRACT FROM AUTHOR]

A high-sensitivity bending sensor based on dual-dip long period fiber gratings (LPFGs) in a single-mode fiber is proposed. The length of the gratings is very short only 7.2 mm. The asymmetric perturbation in fiber is generated during the fabrication, which makes the dual-dip LPFGs show different bending sensing characteristics at opposite directions. Meanwhile, the highest bending sensitivity of −35.91 nm/m−1 and −20.74 dB/m−1 can be obtained at bending directions of 0° and 180°, respectively. Furthermore, the torsion and temperature sensing characteristics are investigated experimentally. The torsion sensitivities follow sine function showing potential for directional sensing. The temperature sensitivities are 41.9 p.m./°C and 45.1 p.m./°C are close, which can eliminate the cross-sensitivity to temperature. • A high-sensitivity bending sensor based on dual-dip long period fiber gratings. • The directional bending is realized by asymmetric electric arc discharge during the fabrication. • The torsion sensitivities follow sine function showing potential for directional sensing. • Cross-sensitivity of temperature can be eliminated for the same sensitivity. [ABSTRACT FROM AUTHOR]

We demonstrate the fabrication of the interlaced tilted long-period fiber grating (ITLPFG) using focused CO 2 laser. Due to the asymmetric refractive index distribution, the grating has very weak sensitivity in the two opposite directions to reduce the influence on the other two bending directions with higher bending sensitivities of −9.16 and 5.14 nm/m−1, respectively. The temperature and refractive index sensing characteristics of the ITLPFGs are also investigated experimentally. The ITLPFGs could be adopted as temperature-insensitive directional bending sensor. [ABSTRACT FROM AUTHOR]

We demonstrate the inscription of long-period gratings (LPGs) in a fluorine-doped double-cladding fiber (DCF) with a low refractive index trench. The grating has a single resonance dip in the wide wavelength range from 1200 nm to 2400 nm. The transmission spectra and cladding mode evolution of the DCF-LPG during the cladding etching process are investigated theoretically and experimentally. The mode order of the cladding modes decreases with one order of every 20 μ m diameter etching, while the resonance wavelength of the DCF-LPG barely shifts when the fiber diameter decreases gradually. Only LP 11 mode can be excited in the etched DCF-LPG when the fiber diameter is less than 40 μ m. The characteristics of the DCF-LPGs are investigated experimentally. The torsion sensitivity was measured to be −0.154 nm/(rad/m) and −0.271 nm/(rad/m) at 1. 55 μ m and 2. 0 μ m wavebands, respectively, which are one order of magnitude higher than that of the conventional LPG. The proposed DCF-LPG has potential application as a tunable band-rejection filter in broad wavebands. • Only one resonance dip of the DCF-LPG can be observed in the wavelength range from 1200 nm to 2400 nm. • The mode order of the cladding modes decrease with diameter etching, while the resonance wavelength barely shifts. • The twist sensitivity of the DCF-LPGs at 1. 55 μ m and 2 μ m wavebands was one order of magnitude higher than that of the conventional LPG. [ABSTRACT FROM AUTHOR]

The thermal stability of uniform fiber Bragg gratings (FBGs) inscribed in single mode double cladding fiber (DCF), standard single mode fiber (SMF-28) and dispersion shifted fiber (DSF) by 193 nm excimer laser was experimentally investigated. When the temperature rises from 30 °C to 800 °C, the grating contrast of FBGs with the saturated refractive index modulation drops slowly, and the corresponding 3 dB bandwidth gradually decrease After annealing at 400 °C for 2000 h, it is found that the resonance wavelength shift of FBGs mainly occur during the first 200 h. It is also observed that the FBG inscribed in DCF requires a longer time to stabilize wavelength than the FBG written in SMF-28, about 1500 h, which suggests that long-term pre-annealing is required to ensure accurate wavelength measurement. After 2000 h of annealing treatment, the FBGs can still have excellent sensing performance in the temperature range of 0–400 °C. • The thermal stability of FBGs fabricated in three types of optical fibers by 193 nm excimer laser are investigated. • The characteristics of the transmission contrast and bandwidth of FBGs during the annealing process are analyzed. • The wavelength shift rate of FBGs are investigated after the annealing at 400 °C for 2000 h. [ABSTRACT FROM AUTHOR]

We propose a fiber torsion sensor based on double-helix long-period grating (DH-LPG) written in a tapered polarization-maintaining fiber (PMF). A series of DH-LPGs were fabricated in the tapered PMFs with waist diameters of 85 to 67 μ m by a CO 2 -laser heating and twisting system. The torsion, temperature and surrounding refractive index sensing characteristics were investigated experimentally. Thanks to the special structures combined DH-LPGs with tapered PMFs, the proposed sensor can realize directional torsion measurement. The maximum torsion sensitivity of the proposed grating could be up to -2.28 nm/(rad/m) when the tapered waist diameter was 67 μ m. The DH-LPGs have potential application in field of all-fiber high sensitivity directional torsion sensing. • Novel and repeatable fabrication methods for double-helix long-period gratings. • Detailed analysis in the sensing characteristics with various waist diameters. • Available for vector torsion measurements. • High torsion sensitivities and wide measurable twist rate ranges. [ABSTRACT FROM AUTHOR]

A wavelength tunable erbium-doped fiber ring laser with flattened output power spectrum over a broadband is proposed and demonstrated. The power flattening is achieved using a high-birefringence fiber loop mirror, in which a number of high-birefringence fiber sections and polarization controllers are used to get a reflection spectrum that can compensate for the output power spectrum. The wavelength tuning is realized by compressing or stretching the FBGs in the laser cavity. A 1×3 switchable fiber Bragg grating array is incorporated into the fiber ring to get a wideband tuning range of 38 nm, from 1527 to 1565 nm. Within this range, the output power uniformity is controlled within ±0.8 dB. The total output power is about 4 dBm, the 3-dB linewidth is 0.01 nm, and the side mode suppression ratio is more than 48 dB. [Copyright &y& Elsevier]

In this work, a terahertz (THz) fiber directional coupler with a symmetric dual-suspended core structure has been proposed for realizing low loss and polarization-insensitive coupling length in the THz frequency region. The fiber core of the directional coupler consists of two round solid dielectric rods supported by two intersecting thin dielectric layers. Numerical simulations performed using the full-vector finite element method show that a short polarization-independent coupling length can be obtained by adjusting the geometric parameters of the fiber. Moreover, a low-loss THz fiber directional coupling performance can also be achieved at a central frequency of 1.00-THz and approximately 0.16 THz of bandwidth. For example, a coupler with a length of 0.421 cm has a transmission loss of 0.30 dB and 0.26 dB for the x- and y-polarization modes, respectively. • The fiber core consists of two dielectric rods supported by dielectric layers. • The length of the coupler can be tuned to shorter lengths. • The coupling length is made polarization-independent. • The loss of the device is reduced effectively. [ABSTRACT FROM AUTHOR]