High order resonances between core mode and cladding supermodes in long period fiber gratings inscribed in photonic bandgap fibers

High diffraction order cladding modes of helical long-period gratings inscribed by CO2 laser

The helical long-period gratings (HLPGs) with resonance at high diffraction order are fabricated in single-mode fiber using a CO₂ laser. A series of HLPGs with different pitches are fabricated, and the phase-matching curves of the HLPGs with first and second diffraction orders are presented based on the experimental results. The temperature, surrounding refractive index (SRI), and torsion-sensing characteristics of the HLPGs with different diffraction orders have been investigated experimentally. The maximum torsion sensitivity of resonance at the second diffraction order is about 0.228 nm/(rad/m), which is twice as high as that of the first diffraction order cladding mode. The HLPG offers great potential to perform simultaneous multiparameter measurement due to the resonance dips at different diffraction orders having quite different sensitivities to temperature, SRI, and torsion.

Recent Progress in Fabrications and Applications of Heating-Induced Long Period Fiber Gratings

This paper presents a review of our work concerning the recent progress in fabrications and applications of heating-induced long period fiber gratings (LPFGs). Firstly, three kinds of heating fabrication techniques based on CO₂ laser, hydrogen–oxygen flame and arc discharge are demonstrated to fabricate LPFGs, i.e., standard LPFGs (SLPFGs) and helical LPFGs (HLPFGs), in different types of optical fibers such as conventional fibers, photonic crystal fibers, and photonic bandgap fibers. Secondly, the all-fiber orbital angular momentum (OAM) mode converters based on heating-induced SLPFGs and HLPFGs in different types of fibers are studied to increase the transmission capacity. Finally, the heating-induced SLPFGs and HLPFGs are investigated to develop various LPFG-based strain, pressure, torsion and biochemical sensors.

Higher-order diffraction of long-period microfiber gratings realized by arc discharge method

We report novel microfiber long period gratings (MF-LPGs) characterized by higher-order diffraction, which are fabricated using an arc discharge method. It is shown that an 11-period MF-LPG can exhibit an extremely high resonant dip (>30 dB) and a low transmission loss (<1.0 dB). A series of grating samples with elongated periods, from 400 μm to 1000 μm, and different diffraction orders have been fabricated and studied in contrast to the previously reported counterparts. The proposed structures have high reproducibility, stability, flexibility, and low production costs. Moreover, the resonant wavelength has a large refractive index (RI) sensitivity (up to ~3762.31 nm/RI-unit around RI = 1.383) and a very low temperature coefficient (~3.09 pm/°C at 1401.3 nm) for a structure with a diameter of 9.6 μm. The theoretical analysis shows good agreement with the experimental results. Our study should be useful for future applications of MF-LPGs in micro-scale in-fiber devices and sensors.

Supermode Bragg grating combined Mach-Zehnder interferometer for temperature-strain discrimination

We report on a compact sensor by integrating a Mach-Zehnder interference and a cladding Bragg grating in a same section of all-solid photonic bandgap fiber. Theoretical investigation reveals that the Bragg grating resonance stems from the coupling of counter-propagating cladding LP01-like supermodes and the Mach-Zehnder interference works between a LP01-like supermode and LP01 core mode. Compared with the interference fringe, such supermode grating dip responses to axial strain in a more sensitive and opposite-direction manner. Whereas, the interference fringe shows a higher temperature sensitivity than the supermode grating dip. By means of these different responses, this device finds a useful application in the discrimination of temperature and axial strain.

Mechanism and characteristics of long period fiber gratings in simplified hollow-core photonic crystal fibers

We demonstrate the fabrication of high-quality LPFGs in simplified hollow-core photonic crystal fibers, composed of a hollow hexagonal core and six crown-like air holes, using CO2-laser-irradiation method. Theoretical and experimental investigations indicate that the LPFGs are originated from the strong mode-coupling between the LP01 and LP11 core modes. And a dominant physical mechanism for the mode-coupling is experimentally confirmed to be the periodic microbends rather than the deformations of the cross-section or other common factors. In addition, the LPFGs are highly sensitive to strain and nearly insensitive to temperature, and are promising candidates for gas sensors and nonlinear optical devices.

Mode-beating-enabled stopband narrowing in all-solid photonic bandgap fiber and sensing applications

In this paper, core-cladding modal beating in a short piece of all-solid photonic bandgap fiber (AS-PBF) is observed in longitudinal propagation direction. It is demonstrated that at the stopband range of AS-PBF, the power could transfer back and forth between the fiber core and the first layer of high-index rods. Both experimental results and the theoretical analysis from transverse coupled mode theory confirm that the 3-dB width of the sharp stopband could be significantly narrowed by multicycles of such core-cladding modal couplings, which is of great benefit to the high-resolution sensing applications. Based on such a guiding regime, a high-temperature sensor head is also made and its response to temperature is tested to be of 59.9 pm/°C.