We demonstrated the fabrication of bandwidth tunable ultra-broadband mode converters based on CO2-laser inscribed long-period fiber gratings (LPFGs) and helical long-period gratings (HLPGs) in a two-mode fiber (TMF). The simulation and experimental results show that there is a dual-resonance coupling from LP01 to LP11 core mode at the dispersion turning point. The mode converters based on the TMF-LPFG and TMF-HLPG provide a 10-dB bandwidth of ∼300 nm and ∼297 nm, respectively, which covers O + E+S + C band. The 1st order orbital angular momentum (OAM) mode based on TMF-LPFG was generated by adjusting the polarization controllers (PCs), while the 1st order OAM mode can be generated directly by the TMF-HLPG. When the twist rate is varied from -36 rad/m ∼ 36 rad/m, the tunable range of the 10-dB bandwidth is ∼52 nm and ∼91 nm for the LPFG and HLPG mode converters, respectively. The ultra-broadband mode converter can be adopted as a bandwidth tunable mode converter, which can be applied in ultra-broadband mode-division-multiplexing transmission systems and optical fiber sensing systems based on few-mode fibers.

All-fiber bandwidth tunable ultra-broadband mode converters based on long-period fiber gratings and helical long-period gratings.

We propose a fiber structure based on a single-mode-multimode-single-mode one. A theoretical analysis model based on leaky mode expansion along with a new concept of leaky mode interference is presented to facilitate and enlighten the analysis for the leaky structure. Then we focus on transmission characteristics of the structure in response to surrounding refractive indices (RI) that are higher than that of the silica fiber. In our simulation, output intensity of the structure increases monotonically for RI from 1.46 to 1.55, and changes over 2.0 dB for a change of 0.01 in RI range from 1.46 to 1.48, which is well verified by the following experiments.

Guided-mode-leaky-mode-guided-mode fiber structure and its application to high refractive index sensing

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.

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

In this study, we propose and investigate a novel grating-assisted surface plasmon resonance (SPR) platform based on a silver coated long period fiber grating having a photosensitive cladding (C-LPFG). We show that the SPR mode is transited from the higher EH mode with an effective refractive index (ERI) close to that of the surrounding refractive index (SRI) and is highly sensitive to a change in the SRI. Compared with a conventional SPR sensor, a much higher sensitivity is obtained for the novel C-LPFG-based SPR sensor. The sensitivity can be further improved by reducing the fiber diameter. The numerical results show that the highest local sensitivity, which can be as high as ∼4900  nm/RIU, and the corresponding resolution of ∼2.04×10(-6)  RIU are achieved for the reduced silver coated C-LPFG sensor. Moreover, the Q-factor and the figure of merit exhibit better characteristics than those of the conventional LPFG-based SPR sensor. Our findings provide insight into the C-LPFG-based SPR platform being a potentially important area to explore for biochemical sensing.

Highly sensitive surface plasmon resonance sensor utilizing a long period grating with photosensitive cladding.

In this paper, we have briefly review the developing history and recent advances made with regard to helical long-period fiber gratings (HLPGs) in three aspects, i.e., the mode-coupling theories, the fabrication techniques, and the applications. It is shown that, due to the intrinsic helicity characteristics, which are especially suitable to control the loss, polarization, and orbit-angular-momentum (OAM) states of the light in optical fiber, HLPGs have recently attracted great research interest and have found various applications, such as the mode-converters, the torsion sensors, the band-rejection filters, wave plates, linear- and circular-light polarizers, and OAM mode generators, etc. It is believed that HLPGs and the HLPGs-based devices would find further applications to not only the fields of optical sensors and optical communication, but also other fields such as ultrahigh precision measurement, quantum optics, and biochemistry, etc.

Advances on Mode-Coupling Theories, Fabrication Techniques, and Applications of the Helical Long-Period Fiber Gratings: A Review

To facilitate the analysis of radiation mode couplings, quasi leaky mode approximations were utilized in coupled-mode analysis. The key to effectively and accurately apply this approach is how to well approximate radiation modes by the quasi leaky modes in an equivalent closed waveguide model. In this paper, the principle, applicability and accuracy of the approximations are demonstrated, and the detailed implementation is also suggested by applying a unified coupled-mode analysis to fiber gratings. First of all, based on a thorough study on the characteristics of the complex modes, for the first time, quasi leaky modes are classified into guided-mode-like inner-cladding and radiation-mode-like outer-cladding leaky modes so as to explicitly establish equivalence relationships between the discrete leaky modes and the continuous radiation modes. With this new insight, the whole analysis process especially for some of the practically tricky issues such as the criteria for developing the proper equivalent waveguide model and the subsequent mode expansion basis are better understood and easier to be dealt with for different problems where radiation modes come into play. Moreover, as essential preconditions to extend the conventional coupled mode analysis to the present unified one, the couplings between the guided core mode and a leaky mode are studied in a systematic and consistent manner. An intuitive and then a deep understanding on the roles of complex modes on mode coupling and power exchanging are thus gained for further simulations. Lastly, the transmission spectra of fiber gratings with different surrounding indices are simulated. The simulated results agree well with those obtained theoretically and experimentally in the literatures, which strongly validate the principle of quasi leaky mode approximations and its implementation on the unified coupled-mode analysis expounded in this paper.

New insight into quasi leaky mode approximations for unified coupled-mode analysis

Based on the dual-resonance principle around the dispersion turning point (DTP), for the first time, an ultra-broadband orbital angular momentum (OAM) mode converter formed by the helical long-period fiber grating (HLPG) is proposed. The converter used for delivering the OAM operation with 3-dB bandwidth of 287 nm, which is about 7 times of general OAM converters and has only one mode. Furthermore, by chirping the HLPG working around DTP, a flat-top broadband OAM mode converter with bandwidth of ∼182 nm@3 dB is conveniently achieved. The flatness of spectrum can be increased by apodizing and optimizing the length of the chirped HLPG. Subsequently, we significantly developed a flat-top broadband rejection filter with >30 dB bandwidth of a high level of ∼100 nm@1 dB by double-cascading the HLPG. It is shown that the performances of the OAM mode converter and the flat-top broadband rejection filter can be remarkably improved by accomplishing the DTP in the mode phase-matching for the HLPG.

Ultra-broadband conversion of OAM mode near the dispersion turning point in helical fiber gratings

A novel method to control the parameters of a chiral fiber grating structure is proposed. Mode couplings are controlled in real time during the twisting fabrication process. This chiral grating structure can satisfy the phase-matching condition for generating high-quality orbital angular momentum (OAM) beams, with an order mode of conversion efficiency over 99.9%. Both theoretical analysis and experimental results of this OAM mode conversion have been investigated, with good agreement. The results demonstrate a dual-OAM beam converter with a charge of ±1 for the right- and left-handed CLPGs, respectively. The high-quality OAM beam generated in this twisted single-mode fiber process may find excellent applications in optical communications.

Excitation of high-quality orbital angular momentum vortex beams in an adiabatically helical-twisted single-mode fiber.

In this paper, for the first time, we propose a large-broadband orbital angular momentum (OAM) mode converter based on helical long period fiber grating (HLPG) working at turning point (TP). Owing to the combination of dual-resonance peaks at TP, an OAM mode converter with 3-dB bandwidth of ~287 nm is readily obtained, and a high coupling efficiency of ~100% is achieved. We believe that this OAM mode converter will play a key role in fiber communication and other fields.

Li Yang

Papers

New insight into quasi leaky mode approximations for unified coupled-mode analysis

Ultra-broadband conversion of OAM mode near the dispersion turning point in helical fiber gratings

Excitation of high-quality orbital angular momentum vortex beams in an adiabatically helical-twisted single-mode fiber.

The study of large-broadband OAM mode converter based on helical long period fiber grating