Chengliang Zhu

The practical adoption of orbital angular momentum (OAM) in technologies ranging from high-capacity communications to quantum information science has been severely hindered by the lack of integrated devices capable of dynamic OAM manipulation across broad bandwidths. Existing solutions, particularly in optical fiber, are fundamentally constrained to narrowband operation by wavelength-specific phase-matching conditions. Here, we break this long-standing paradigm by demonstrating a fiber-optic vortex converter that achieves an ultra-broad bandwidth exceeding 2000 nm, spanning from the deep ultraviolet to the short-wave infrared. This unprecedented performance is enabled by a novel mechanism—equivalent continuously phase-modulated acousto-optic interaction in a customized graded-index few-mode fiber. This approach effectively decouples the OAM mode-conversion process from wavelength-dependent constraints. Our device simultaneously delivers dynamic tunability and high conversion efficiency, establishing a versatile platform that unlocks the full potential of OAM in ultrahigh-capacity communications, chiral photonics, ultrafast optics, and quantum information processing. The demonstrated >2000-nm bandwidth corresponds to the first-order azimuthal LP01 → LP11 mode-conversion process as directly verified by bar-transmission measurements. Subsequent high-purity OAM generation is achieved via a polarization-interference scheme and experimentally verified at 633 nm and across the 1480–1640 nm range as representative of the visible and near-infrared bands, with the phase-control principle supporting operation across the full bandwidth. The present demonstration is limited to the first-order azimuthal mode conversion; extension to higher-order modes would require cascaded conversion stages.

We demonstrate a phase-shifted superstructure fiber Bragg grating (PSSFBG) fabricated using a femtosecond laser-based point-by-point direct writing technique. The proposed PSSFBG simultaneously generates multiple pairs of equivalent phase-shifted resonance peaks that are mirror images of each other. The magnitude of the inserted phases can be precisely controlled due to a three-orders-of-magnitude reduction in fabrication precision requirements. Specifically, PSSFBGs with phase shifts ranging from 0 to 2π at fine intervals of 0.125π have been successfully implemented—a task challenging to achieve with high yield rates using previous methods. The proposed PSSFBG not only fulfills nearly all applications of ordinary phase-shifted FBGs but also enables specialized applications such as dual-wavelength switches, odd-even channel filters, and phase-demodulated FBG sensors. As an application example, we developed a differential-type high refractive index (RI) sensor based on the PSSFBG design, enhanced by micro-groove processing in the phase insertion region. This sensor demonstrates remarkable effectiveness in measuring RI values higher than those of the fiber itself, exhibiting exceptional linear sensitivity of 544 dB/RIU, which is competitive within the family of power-interrogated fiber-optic RI sensors. Furthermore, it features high precision and effectively mitigates the impact of power perturbations from the light source and optical path.

We demonstrate a submillimeter-length single-helix chiral grating embedded in a high-numerical-aperture single-mode fiber (HNA-SMF) for efficient generation of third-order orbital angular momentum (OAM), specifically the OAM_{±3, 12} mode. This design facilitates enhanced coupling of higher azimuthal modes due to significant perturbations arising from both the geometric effect of the thin-core offset under high-NA conditions and the elasto-optic effect induced by intense helical stress with a small twist pitch. As a result, we achieve an unprecedented device length of 0.88 mm with a resonance dip of −28 dB, corresponding to a high conversion efficiency of 99.8%—the shortest among all fiber grating-based OAM generators reported to date, to the best of our knowledge. It also exhibits excellent robustness against external disturbances, demonstrating low responsivities to torsion, strain, and temperature at 12 pm/(rad/m), 0.5 pm/με, and 24 pm/°C, respectively. These features make it promising for applications in OAM-based particle sensing and manipulation, particularly for chiral compounds.

We have theoretically and experimentally proposed and demonstrated an all-in-fiber-core mode-selective Michelson interferometer (MS-MI) sensor utilizing a single-ended helical graded-index few-mode fiber (SE-HGIFMF). The SE-HGIFMF effectively eliminates multi-mode interference, remains insensitive to external refractive index (RI) variations, and significantly enhances sensing sensitivity due to the minimal disparity in group RI between the two guided core modes. We investigated the torsion, strain, and temperature characteristics of the sensor, achieving sensitivities of 1.785 nm/(rad/m) for directional torsion, 0.0316 nm/µϵ for strain, and 0.624 nm/°C for temperature. These sensitivities are competitive within the fiber optic sensor family and represent the highest values reported for mode-selective MI and Mach-Zehnder interferometer (MZI) sensors to our knowledge. The proposed sensor not only demonstrates exceptional sensitivity but also offers directionality identification for torsion, ease of use, flexible installation, low loss, and high reliability, showcasing robustness against environmental fluctuations.

A novel amplitude-sampled long-period fiber grating with deep tapering (AS-LPFG-DT) is proposed and experimentally demonstrated, functioning as a multichannel LPFG that efficiently generates multiple parallel resonance dips with nearly identical channel spacing. Compared to previous methods, our approach achieves ultralow duty cycle and few numbers of sampling period due to the unparalleled coupling efficiency resulting from both refractive-index perturbation and geometric perturbation in the deep and symmetrical tapering structure. This significantly enhances effective channel count and covered bandwidth. Our configuration offers simplicity, ease of fabrication, flexibility, and high efficiency, making it highly promising for applications such as all-fiber multi-wavelength mode converters and wavelength division multiplexing sensors. As examples, we have successfully implemented 5-, 7-, and 15-channel AS-LPFGs-DT. Furthermore, we have demonstrated a strain and temperature sensor based on our proposed AS-LPFG-DT design that exhibits competitive sensing sensitivities while eliminating cross-sensitivity effects between these two parameters.

We propose and demonstrate a hybrid few-mode fiber configuration (HFMFC) that enables ultrahigh-channel-count orbital angular momentum (OAM) mode conversion. The HFMFC consists of periodically twisted graded-index few-mode fiber segments and a step-index few-mode fiber segment. Our proposed HFMFC-based multichannel OAM mode converter (OAM-MC) offers an exceptionally high channel count in a wide bandwidth, with customizable channel spacing down to 50 GHz (∼0.4 nm), achieved through optimization of the structural parameters of the HFMFC. By employing this methodology, we have successfully demonstrated 10, 32, 117, and 233 channel OAM mode conversions covering the entire C + L band, representing the highest performance among all reported fiber-based multichannel OAM-MCs to date, for the first time to the best of our knowledge. The suggested ultrahigh-channel-count OAM-MC exhibits promising potential for applications in various fields such as OAM fiber communication, OAM holography, OAM information processing, and OAM metrology.

A strong-coupling helical fiber needle (HFN) is proposed and demonstrated for the realization of bandwidth-enhanced broadband optical vortex beam (OVB) generation. The HFN is based on a single mode fiber and operates at the dispersion-turning-point (DTP) of the lowest radial order of the cladding mode (i.e., LP₁₁) but with a remarkably high mode coupling efficiency. By utilizing this novel, to the best of our knowledge, HFN, successful generation of the first-order OVB with an impressive bandwidth up to 556 nm at −10 dB and a center wavelength of ∼1570 nm has been achieved. This represents the broadest bandwidth demonstrated among all fiber grating-based OVB generators to date. The proposed HFN-based OVB generator exhibits a relatively compact size, ultra-wide bandwidth, and customizable center wavelength, making it highly promising for applications in optical vortex-based endoscopic imaging as well as particle detection and manipulation.

A new method enabling to provide an on-demand flat-top wideband orbital angular momentum (OAM) mode converter is proposed and experimentally demonstrated, which is based on utilization of a cladding-etched helical long-period fiber grating (CEHLPG). By appropriately selecting the grating period and precisely controlling the diameter of the CEHLPG in-situ, both the radial order and central wavelength of the flat-top band for the generated OAM mode can be flexibly tailored according to specific requirements. As typical examples, the first azimuthal order OAM modes with a flat-top bandwidth of 95 nm at -20 dB, a central operating wavelength of ∼1500 nm, and the radial-orders of 9, 8, 5, and 2, respectively, have been demonstrated consecutively. The proposed method provides an excellent flexibility and robustness in controlling both the radial order and the central wavelength of the resulting flat-top wideband OAM mode conversion, which may support a variety of practical optical vortex applications.

We demonstrated a cross-sensitivity-free torsion and strain sensor, which was achieved by using the two concatenated dispersion-turning-point (DTP)-customizable helical long-period fiber gratings (DTP-C-HLPGs). Each DTP-C-HLPG operated at the resonance wavelengths near its DTP, which could be customized by applying the vertical fiber-etching technique. Thus, the radial order of the coupled cladding mode and resonance wavelengths could be flexibly adjusted as needed by changing the grating diameter. As an example, an ultrahigh sensitivity up to 2.731 nm/(rad/m) in torsion and a high sensitivity up to 0.0179 nm/με in strain was successfully achieved. Furthermore, the cross-sensitivity effect between torsion and strain could be completely eliminated by using our proposed device.

In this study, the underlying mechanism for mode coupling and propagation in a strongly-coupled few-period long-period fiber grating (LPG) is firstly revealed both theoretically and experimentally, which is based on the idea that an ultra-short strongly-coupled LPG could be equivalently regarded as cascaded and phase-matched Mach-Zehnder interferometers (MZIs) but with phase-matched two arms. The proposed mechanism and the corresponding analysis method can also be applied to those conventional LPGs (with a distributed weak-perturbation through the whole length of the grating itself), which opens a new way for design and thereafter fabrication of any other kinds of the LPGs such as the LPG-based mode-converters, the multichannel-, the phase-shift- and the high-order LPGs, etc.