Research On The Fabrication And Characteristics Of The Laser Written Helical Long-Period Fiber Gratings

Fiber grating is an important optical passive device, which is one of the most valuable optical components in the optical communications, fiber sensors and optical information processing systems. Long-period fiber gratings （LPFGs） can couple light from the core mode into the cladding modes, which is wildly used as high sensitive fiber sensors and passive devices for optical communications, such as optical fiber amplifier, filters, lasers, and mode converters. In recent years, with the development of big data technology, the spatial division multiplexing （SDM） and mode-division multiplexing （MDM） have been developed to expand the ever-increasing transmission capacity of optical communication networks. New requirements are raised for fiber gratings, and the development of new fiber gratings techniques and the novel components based on the specially designed fiber gratings could find promising application in the next generation optical communication system and optical sensors.In this thesis, we investigated experimentally the fabrication of the helical long-period fiber gratings （HLPG）. Two different kinds of HLPGs were fabricated by CO2 laser, which were the structure type HLPG （S-HLPG） and the index type HLPG（1-HLPG）. We investigated the coupling and sensing characteristics of the fabricated HLPGs, and explored their application in optical sensors and optical communications.Our researches provide a new method to fabricate LPFGs with different characteristics, which also would be helpful for the further application of HLPGs. The main research work and results are as follows.（1） We successfully fabricated two different kinds of HLPGs in the single-mode fiber （SMF） with CO2 laser. For the first time, we demonstrated the fabrication of the S-HLPG with simple one-time writing technique, which has less dependence on the high-grade precision machinery equipment. The stability of HLPG fabrication was greatly improved.（2） We investigated the coupling and sensing characteristics of the fabricated HLPGs, and found that the S-HLPG was sensitive to the bending, strain, and twist. In experiments, the bending sensitivity of S-HLPG was 14.69 nm/m-1, which was twice as that of conventional LPFGs. The sensitivities of the grating to the twist and axial strain were 0.21 nm/rad/m and 1.6 pm/με, respectively, which were one order magnitude higher than those of conventional LPFGs. Therefore, the S-HLPG can be used as a high sensitivity twist or strain sensor. The fabricated I-HLPG also had the higher twist and axial strain sensitivities than those of conventional LPFGs. We observed the pattern of I-HLPG and found that the grating coupled core mode into the asymmetric cladding mode of LPlncl, which verified to be consistent with the theoretical analysis.（3） To increase the twist sensitivity and reduce the temperature cross sensitivity,we proposed a high sensitivity twist sensor based on the HLPG inscribed in the two-mode fiber （TMF）. The twist sensor had three new features: firstly, it was insensitive to temperature with a sensitivity of 23.9 pm/℃, which was one third of that of the conventional LPFGs. Secondly, it had a high twist sensitivity of 0.47 nm/rad/m, which was twice higher than that of SMF-HLPG, and one order magnitude higher than that of conventional LPFGs. Finally, the sensor could detect the twist rate and twist direction simultaneously. All of these make the TMF-HLPG a promising prospect for the application of the twist monitor.（4） We proposed two kinds of mode converters based on the SMF-HLPG-TMF structure and TMF-HLPG. For the SMF-HLPG, the core mode can be coupled into the cladding modes of Llncl. Due to the difference of the fiber core diameters, the part of the LPlncl will travel into the core of TMF and propagate along the fiber as LP₁₁^co mode. Therefore, the SMF-HLPG can excite LP₁₁^co at the different resonance wavelength, and can be used as multi-wavelength mode converter. For the TMF-LPFG, the grating can couple the fundamental core mode of LP₀₁^co into the higher order core mode of LP₁₁^co. Given its high twist sensitivity, TMF-HLPG can be used as a wavelength tunable mode converter by twisting the grating. The helical index modulation of the HLPG leads to a π/2 phase shift in the coupling, and the orbital angular momentum （OAM） beams can be directly generated from the grating.Therefore, the HLPG could be used as OAM mode converter, and will have a promising prospect for the application in optical communications, especially in MDM.