Mode Coupling And Characteristics Of Long-period Fiber Gratings Inscribed In Few-mode Optical Fiber

Long-period fiber gratings(LPFGs)can be used to realize mode coupling as mode converters,which are important passive optical fiber components.The LPFG has the advantages of small size,low cost,simple fabrication,and low insertion loss and so on.With the flexibility of the fabrication technology of optical fiber and fiber devices,optical devices based on specialty fiber have very high research importance in the field of fiber communication system and optical fiber sensors.In this paper,we fabricated the LPFGs in two types of few-mode fibers(FMFs)using a CO₂ laser.The spectral,mode coupling characteristics,torsion,temperature,axial strain and bending characteristics of the LPFGs were studied experimentally.1.We fabricate all-fiber mode converters based on LPFGs inscribed in an FMF at 1.55?m waveband and 2?m waveband,respectively.The LPFGs have realized the mode coupling from LP₀₁ mode to high-order core modes(LP₁₁ mode,LP₂₁ mode,LP₀₂ mode,LP₃₁ mode,and LP₁₂ mode)with high coupling efficiencies at the 1.55?m waveband.The corresponding±1^st,±2^nd and±3^rd order orbital angular momentum(OAM)modes were all successfully generated by the LP₀₁-LP₁₁,LP₀₁-LP₂₁,LP₀₁-LP₃₁ mode converters at the corresponding resonance wavelength.At the 2?m waveband,the proposed LPFG realized the mode conversion from LP₀₁mode to three high-order fiber core modes(LP₁₁,LP₂₁ and LP₀₂ modes)with high conversion efficiencies,which is the first time of mode conversion of three high order core modes at 2?m waveband using all-fiber FMF-LPFG,to the best of our knowledge.The experimental results indicated the maximum torsion sensitivity of the LP₀₁-LP₁₁ mode converter at the wavelength of 2068.9 nm is-0.42 nm/(rad/m),which is 5.25 times higher than that of LPFG fabricated at 1.55?m waveband.The LPFG can be used as a high sensitivity torsion optical fiber sensor.2.The mode converters based on all-fiber FMF-LPFG with high diffraction order modes are proposed and fabricated.We made the simulation to study the transmission spectrum characteristics of the LPFGs with different diffraction orders.We experimentally fabricated the LPFGs with first,second,third,and fourth diffraction orders in the FMF by CO₂-laser,respectively.The same order OAM modes were generated by the LPFGs at the corresponding resonance wavelengths.3.We propose and fabricate the wavelength tunable high-order OAM mode converters based on the helical LPFGs and chiral LPFGs.The helical LPFGs with clockwise and anticlockwise helical refractive index modulation were fabricated for the mode conversion from LP₀₁ mode to high-order core modes(LP₁₁ mode and LP₂₁mode)with high conversion efficiencies.The±1^st order and±2^nd order OAM modes were directly excited by the helical LPFGs without any extra components for the introducing of a?/2 phase difference.The torsion sensitivities of the LP₀₁-LP₁₁ and LP₀₁-LP₂₁ mode converters based on anticlockwise helical LPFGs were-0.05nm/(rad/m)and-0.24 nm/(rad/m),respectively.The mode conversion from the LP₀₁mode to LP₁₁ mode,LP₂₁ mode and LP₃₁ mode were successfully realized by the fabricated chiral LPFGs with the second,eighth,and eleventh diffraction orders with the conversion efficiencies of 99.69%,99.89%and 99.94%,respectively.The-1^st,-2^nd,and-3^rd orders OAM modes were directly excited by the LP₀₁-²LP₁₁,LP₀₁-⁸LP₂₁and LP₀₁-¹¹LP₃₁ mode converters based on chiral LPFGs at the corresponding resonance wavelength.The torsion sensitivities of LP₀₁-²LP₁₁,LP₀₁-⁸LP₂₁ and LP₀₁-¹¹LP₃₁ chiral LPFGs mode converters were-0.243 nm/(rad/m),-0.016nm/(rad/m),and-0.120 nm/(rad/m),respectively.The proposed chiral LPFGs can be used as mode converters for the generation of OAM modes and high-sensitivity torsion sensors.4.The bandwidth tunable ultra-broadband mode converters based on LPFGs and helical LPFGs are fabricated and proposed.The LPFGs and helical LPFGs which fabricated using a CO₂ laser in the two-mode fiber(TMF)realized the mode conversion from LP₀₁ mode to LP₁₁ mode with a 10-d B bandwidth of 300.0 nm and297.0 nm,respectively.The±1^st order OAM modes were generated in a wide wavelength range.The helical LPFGs can excite the OAM mode directly without the extra polarization controller.The two proposed LPFGs can realize tunable bandwidth by adjusting the torsion and temperature of the gratings.The tunable range of 10-d B bandwidth for the LPFGs and helical LPFGs are 52 nm and 91 nm based on fiber rotation,respectively.The tunable range of 10-d B bandwidth for the LPFGs and helical LPFGs are 26 nm and 18 nm based on temperature controlling,respectively.Therefore,The LPFGs and helical LPFGs can be used as bandwidth tunable ultra-broadband mode converters in mode division multiplexing systems,and optical fiber sensor in optical fiber sensing applications based on FMFs.5.The ultra-broadband mode converters based on chirped LPFG are proposed and fabricated.The chirped LPFGs realized mode conversion from LP₀₁ mode to LP₁₁ mode with a 10-d B bandwidth of 409.1 nm,which covers completely the O+E+S+C bands.The±1^st order OAM modes were excited in a wide wavelength range by the chirped LPFGs.Besides,we proposed the tilted chirped LPFGs to optimize the bandwidth tunable range of ultra-broadband mode converters.The experimental results indicated that the 60°-titled chirped LPFG has a highest torsion sensitivity.The maximum tunable range of 10-d B bandwidth for the chirped LPFG with 60°-titled angle was 52 nm,which was twice higher than that of the non-tilted chirped LPFG.By adjusting the twist rate,the 10-d B bandwidth of the tilted chirped LPFGs could cover the O+E+S+C+L bands.6.A bandwidth tunable ultra-broadband mode converter based on chirped helical LPFG is fabricated experimentally.The proposed chirped helical LPFG can convert LP₀₁ mode to LP₁₁ mode with a 10-d B bandwidth and 15-d B bandwidth of480.0 nm and 379.0 nm,respectively.The wavelength range can cover O+E+S+C+L bands.The+1^st order OAM modes were directly generated over a wide wavelength range.In addition,the torsion,temperature and bending sensing characteristics of the chirped helical LPFG were experimentally studied.The 10-d B and 15-d B bandwidth can be tuned by fiber rotation.Therefore,the proposed chirped helical LPFG can be used as a bandwidth tunable ultra-broadband mode converter.