Tilted Long Period Fiber Gratings Research Articles

Tilted long-period fiber grating strain sensor based on dual-peak resonance near PMTP

This paper puts forward a new kind of tilted long-period fiber grating (TLPFG) strain sensor's structure and its design methods based on high sensitivity of TLPFG sensors near the phase-matching turning point (PMTP). First, with the TLPFG phase-matching condition, the appropriate (2 m) cladding mode is selected and the grating period corresponding to PMTP under this mode is found. Further, the TLPFG transmission spectrum with strong attenuation peak is obtained by selecting the tilted angle and grating length. Finally, the dual-peak resonance characteristics and strain sensing characteristics of the TLPFG near PMTP is analyzed, it is found that dual peak wavelength spacing has a very high sensitivity and linearity to micro-strain. By selecting the appropriate grating structural parameters, the sensitivity could be effectively improved. The sensitivity of tilted long-period fiber grating strain sensor operating near PMTP is available to 525 pm/με, and the corresponding resolution can be available to 0.019με. Compared with the resolution (19.8με) of the reported tilted phase-shifted long-period fiber grating strain sensor, the resolving power is enhanced by three orders of magnitude. This new TLPFG strain sensor is expected to be applied in micro-strain monitoring and structural health monitoring.

Sensing Characteristics of Tilted Long Period Fiber Gratings Inscribed by Infrared Femtosecond Laser.

We propose a novel tilted long period fiber grating (TLPFG) design, inscribed using a line-by-line inscription technique and an infrared femtosecond (Fs) laser. The responses of this TLPFG to external refractive index, temperature, torsion, and strain were systematically investigated to determine its sensing characteristics. The external refractive index (RI) was measured to be −602.86 nm/RIU at an RI of ~1.432. The TLPFG was used to accurately measure temperatures up to 450 °C with a sensitivity of 103.8 pm/°C. The torsion and strain sensitivity of the device were 48.94 nm/(rad/mm) and −0.63 pm/µε, respectively. These results demonstrate that the proposed TLPFG could be used as sensors in a series of application fields including high temperatures and external environments.

Film sensor based on cascaded tilted long-period and tilted fiber Bragg grating

A film sensor based on a tilted long-period fiber grating (TLPFG) inserted before a tilted fiber Bragg grating (TFBG) is proposed. The sensor is described theoretically using the transfer matrix method. This structure has two reflected peaks in the reflection spectrum. One peak is for the selected recoupled cladding mode of azimuthal order l = 2 and the other one is for the coupled core mode. The tilt angles of the TLPFG and TFBG and the mode order of the l = 2 cladding mode mainly determine the reflected power of the recoupled-(l = 2) cladding-mode peak in the reflection spectrum. By analyzing the relation between the film parameters (film refractive index and film thickness) and reflection spectrum, the characteristics of the film sensor are studied. The results show that this film sensor has a high sensitivity to the film parameters and increases the sensitivity of the film refractive index by two orders of magnitude in comparison with the normal cascaded long-period fiber grating (LPFG) and the fiber Bragg grating (FBG). The resolutions of the refractive index and the thickness of the sensing film are predicted to be 10−6 and 10−3 nm.

Characteristics of dual-peak resonance in a tilted long-period fiber grating sensing film sensor

Based on the coupled-mode theory of tilted long-period fiber gratings (TLPFGs) and the dual-peak resonance theory, we build the structure of a TLPFG with the phenomenon of dual-peak resonance. The influences of grating tilt angle, grating structure parameters, surrounding refractive index (SRI), and film optical parameters on the dual-peak resonance are analyzed in the transmission spectra of this structure. As the results show, the characteristics of the dual-peak resonance in the (1m) cladding modes and the (2m) cladding modes are similar, and the sensitivity of the (2m) cladding modes to refractive index, as well as the grating tilt angle, grating structure parameters, and film optical parameters, is higher than that of the (1m) cladding modes in this structure. The research provides the theoretical foundation for design of a dual-peak resonance coated TLPFG sensor for SRI, gas, humidity, pH, bending curvature, and strain.

Core-cladding mode coupling of tilted long period fiber gratings in dual-mode fibers

We present a theoretical study on the mode coupling between core modes and cladding modes of the tilted long period fiber grating (TLPFG) in dual-mode fiber. The TLPFG can be applied as mode filters at several specific wavelengths. Using the full vector complex coupled mode theory, numerical simulation shows the maximum coupling coefficients between core modes (LP01 and LP11) and cladding modes can be found near the tilted angle of 86°. The transmission peaks of modes coupling are sensitive to the titled angle. Furthermore, several peak amplitude between the LP11 mode and same azimuthal order modes increase with the growth of the titled angle. With the band-stop characteristic of specific mode, the TLPFG written in dual-mode fiber could be exploited as mode filters.

Intensity-based magnetic field measurement employing tilted long-period fiber gratings

An intensity-interrogated fiber magnetic field sensor based on magnetic fluid (MF) and tilted long-period fiber grating (TLPFG) has been proposed. The sensor is constructed by a TLPFG immersed in the MF. The TLPFGs supported the coupling between the LP01 core mode and the forward-propagating cladding modes with different azimuthal order l = 0, 1, 2,…, which shows different spectral response behaviors with normal long-period fiber gratings. The experimental results and theoretical analysis show that resonance wavelengths of TLPFG present different characteristics by tuning the applied magnetic field intensity. The transmission loss of the TLPFG increases for some dips, while they reduce for the others with the increment of the applied magnetic field intensity. Taking advantage of the optical power difference for various wavelength regions, a magnetic field intensity sensitivity of 0.05 dB Oe−1 could be achieved for a magnetic intensity range from 75 Oe to 300 Oe. Due to its low cost and compactness, the proposed sensor is expected to find potential applications in the field of magnetic field measurement and photonic devices.

Theory analysis of mode coupling in tilted long period fiber grating based on the full vector complex coupled mode theory

The mode coupling in the tilted long period fiber grating (TLPG) is analyzed by using the full vector complex coupled mode theory (FV-CCMT). Compared with the non-tilted LPG, new resonance bands corresponding to the LP1m cladding modes are observed in the transmission spectrum of the TLPG. The coupling to LP1m cladding modes is the combination of the coupling to degenerate vector modes TE0m, TM0m and HE2m with different propagation constants. The maximum coupling between LP01 and LP1m modes occurs at a tilt angle of ∼87°. Numerical results show that the state of polarization (SOP) of the input light would induce the shift of the new resonance wavelength. Also, the RI sensitivity of the new resonance wavelength is much higher than that of the normal resonance wavelength corresponding to the LP0m cladding modes. Such TLPG would find application in chemical and biological sensing fields.