A highly sensitive torsion sensor based on side-hole fiber (SHF) Sagnac interferometer is proposed and experimentally demonstrated in this paper. The theoretical analyses on sensing performances of the proposed interferometric sensor are in good accordance with their experimental measurement counterparts. Due to photo-elastic effect, the birefringence of SHF in the loop would vary with the torsional rate, giving rise to linear wavelength shift and sinusoidal power variation responses to torsion applied on the SHF. Based on the wavelength and intensity interrogations of transmission dips, the torsional sensitivities reach up to about 2.2 nm/(rad/m) and ±10 dB/(rad/m), respectively. Opposite wavelength shift responses for clockwise and counterclockwise torsion states equip the proposed sensor with good twisting direction distinguishability. Besides, the experimental results indicate that our proposed sensor has a low axial strain cross sensitivity of 4.3 pm/ $\mu \varepsilon $ . The side-hole-fiber-based interferometric torsion sensor possesses such desirable merits as high sensitivity, low axial strain cross sensitivity, good torsional direction distinguishability, and good mechanical robustness, ensuring its viability for practical use on torsion sensing occasions.

Highly Sensitive Torsion Sensor Based on Side-Hole-Fiber Sagnac Interferometer

A novel DC-sampling method allowing generation of a multichannel helical long-period fiber grating (HLPG) has been proposed. Unlike the previous phase-only sampling method where phases of the continuous sampling function must be discretely adopted and inserted into the local pitches of the grating (thus high precision setups for controlling all the individual pitches of the grating are essential), the proposed method can greatly facilitate the fabrication of the multichannel HLPG by using the CO2 laser writing technique. As one typical example, the DC-sampled 3-channel HLPG has been demonstrated both theoretically and experimentally. The orbital angular momentum (OAM) modes inherently existed in this kind of HLPGs have also been verified. This is the first report for generation of a multi-channel (wavelength) OAM mode by using only one HLPG.

DC-Sampled Helical Fiber Grating and its Application to Multi-Channel OAM Generator

Instead of inscribing the discrete phases into each local period of the seed HLPG, an equivalent approach to realize the phase-only sampled multichannel helical long-period fiber grating (HLPG) has been demonstrated both theoretically and experimentally which is realized by superposing the so-called sampling HLPGs with the seed HLPG in a same region of the fiber. The proposed approach can greatly facilitate the fabrication processes of the multichannel HLPG by using the direct CO2 laser writing technique, meanwhile it enables to provide much more design and fabrication flexibilities for high channel-count HLPG than the previous methods. As examples, a 3-channel and a 5-channel HLPG have been successfully demonstrated. Moreover, the orbital angular momentum (OAM) modes inherently existed at all channels of the fabricated HLPG have also been verified, which highlights that the proposed HLPGs could find application to multi-wavelength OAM mode converter as well.

The Superimposed Multi-Channel Helical Long-Period Fiber Grating and Its Application to Multi-Channel OAM Mode Generator

A curvature sensor based on single mode-no-core-twisted single mode fiber structure

In this study, a novel and efficient method enabling fabrication of a multichannel helical long-period fiber grating (HLPG) with almost the same channel spacing and the same rejection depth has been proposed and demonstrated for the first time both theoretically and experimentally, which is realized based on utilization of the so-called phase-only sampling technique. Unlike the previous amplitude-type sampling method where either a strong rectangular-type or a Sinc-like index-modulation distribution is generally demanded, the proposed method requires the minimum as well as a uniform index-modulation to the designed multichannel gratings, which considerably facilitates the fabrication process and makes the multichannel HLPGs to be fabricated in even a conventional single-mode fiber (SMF) by using the CO2 laser writing technique. As examples, a 3-channel and a 9-channel HLPGs have been successfully demonstrated numerically and experimentally. The experimental results agree with the designed ones, which represents the first realization of a multichannel HLPG, to the best of our knowledge. It is believed that the proposed multichannel HLPG could find potential applications to multi-wavelength orbit-angular-momentum (OAM) mode converter as well as the wavelength division multiplexing (WDM) sensors.

Optimal design and fabrication of multichannel helical long-period fiber gratings based on phase-only sampling method

We propose and demonstrate a novel type of sampled Bragg gratings by combining a helically twisted fiber and a Bragg grating. A comb-like spectrum with a series of harmonic narrow resonances is observed, and the influence of geometrical parameters on the resonances is studied. As a special application, the intrinsic nature of the device that contains the Bragg grating and helical fiber spectral responses permits the temperature to be detected from the former, whereas the mechanical torsion is extracted from the latter, suggesting a potential for the simultaneous measurement of these two parameters. The proposed configuration features simplification, easy fabrication, high flexibility, stability, and low cost, and therefore has good prospects for sensor applications, as well as other applications, such as multi-channel filters, distributed Bragg reflectors, etc.

Sampled Bragg gratings formed in helically twisted fibers and their potential application for the simultaneous measurement of mechanical torsion and temperature

Au/SiNCA-based SERS analysis coupled with machine learning for the early-stage diagnosis of cisplatin-induced liver injury.

We investigate the sensing characteristics for a sampled Bragg grating formed in a helical single-mode fiber. This sensor features structural simplicity, high integration, flexibility, and low cost and thus has prospects in practical applications.

Gaoyang Chen

Papers

Sampled Bragg gratings formed in helically twisted fibers and their potential application for the simultaneous measurement of mechanical torsion and temperature

Au/SiNCA-based SERS analysis coupled with machine learning for the early-stage diagnosis of cisplatin-induced liver injury.

Sensing Characteristics of Sampled Bragg Gratings Produced in Helical Fibers

Multiplex signal amplification strategy-based early-stage diagnosis of Parkinson's disease on a SERS-enabled LoC system.