Induced Fiber Grating Research Articles

A proposal for distributed humidity sensor based on the induced LPFG in a periodic polymer coated fiber structure

In this paper, a distributed relative humidity (RH) or moisture sensor is proposed. The proposed structure is a single mode telecommunication optical fiber coated by humidity sensitive and insensitive polymers periodically. The polymer coated fiber is surrounded by a high Young’s modulus material such as stainless steel mesh. The swelling of the coated Humidity Sensitive Polymers (HSPs) as a result of moisture absorption induces fiber gratings in the single mode fiber. Depending on the coating period relative to the light wavelength, the induced fiber grating can be Fiber Bragg Grating (FBG) or Long Period Fiber Grating (LPFG). The light reflected by the induced FBG or losses due to the coupled energy to the cladding modes in the induced LPFG can be employed to describe the sensor operation. Since long period length (LPFG) manufacturing is much simpler than short period length (FBG) sensors, this investigation is concentrated on the long period length sensors.

Generation of Cylindrical Vector Beams and Optical Vortex in a Solid-Core Ring Fiber Based on an Acoustically Induced Fiber Grating

We present a method for achieving the cylindrical vector beams (CVBs) and optical vortex generation in a solid-core ring fiber (SCRF) by using an acoustically induced fiber grating (AIFG). Theoretical analysis shows that the SCRF can introduce a refractive index difference of ~10-5 between the first group of adjacent high-order degenerate vector modes. In the experiment, the transmission spectra of AIFG constructed with the SCRF depicts three separate resonance peaks with 3-dB bandwidth and resonance wavelength interval of 0.16 and 2.5 nm, respectively, revealing that the high-order degenerate vector modes have been effectively separated. On the basis of the separation of the degenerate vector mode, the CVBs and the ±1-order optical vortex are further experimentally generated at three separate wavelengths using the AIFG.

Generation of cylindrical vector beams and optical vortex by two acoustically induced fiber gratings with orthogonal vibration directions.

Mode coupling from the fundamental vector mode (HE11x) to the cylindrical vector beams (CVBs) and optical vortex beams (OVBs) of a few-mode fiber excited by two acoustic flexural waves with orthogonal perturbations is achieved by using a composite acoustic transducer. The HE11x mode is converted to TM01 and TE01 modes, which have radial and azimuthal polarizations, by using the lowest-order acoustic flexural modes of F11x and F11y, respectively. Furthermore, HE11x mode can also be converted to the ± 1-order OVBs of HE21even±iHE21odd through the combined acoustic modes of F11x±iF11y. This technique provides a useful way of generating CVBs and OVBs in optical fiber with conveniently electrically-controlled mode conversion characteristics.

Generation of femtosecond optical vortex pulse in fiber based on an acoustically induced fiber grating.

We proposed a method for generation of a femtosecond optical vortex pulse in a two-mode fiber based on an acoustically induced fiber grating (AIFG) driven by a radio frequency source. Theoretical analysis and experimental results demonstrated that the left- and right-handed circular polarization fundamental modes of the femtosecond optical pulse could be converted to the linearly polarized ±1-order optical vortex modes through the AIFG with the mode conversion efficiency of ∼95%. The off-axial interference experiment and the polarization angle-dependent intensity examination were performed to verify the topological charge and the polarization state of the femtosecond optical vortex, respectively.

High-order optical vortex generation in a few-mode fiber via cascaded acoustically driven vector mode conversion.

We propose a method to generate the high-order optical vortex in a few-mode fiber via cascaded acoustically driven vector mode conversion. Theoretical analysis showed that the vector mode conversion induced by the acoustically induced fiber grating (AIFG) could occur between two HE modes with adjacent azimuthal numbers. In the experiment conducted at 532 nm, two AIFGs were simultaneously induced in the same segment of the fiber by a radio frequency source containing two different frequency components. One AIFG was used to convert the left- and right-handed circular polarization fundamental modes to the ±1-order vortex modes, which were then further converted to the ±2-order vortex modes by the other AIFG. The topological charges of the vortex modes were verified using both coaxial and off-axial interference methods, showing typical signature patterns of spiral forms and forklike fringes, respectively.

Prestressed Fiber Bragg Grating With High Temperature Stability

A prestressed fiber Bragg grating with high temperature stability has been successfully fabricated by use of 800 nm femtosecond laser pulse irradiation through high-temperature annealing, followed by a rapid cooling treatment. The reflectivity and the resonant wavelength of the grating are maintained constants for more than 26 h in an isothermal measurement up to 1200°C, showing significant improvement over conventional femtosecond laser pulse induced fiber gratings. Longitudinal tension tests demonstrate that the prestressed fiber exhibits enhanced mechanical robustness compared with the residual stress relaxed fiber. This paper reveals that residual stress plays an important role and essentially provides a new solution to enhance the thermal stability of fiber gratings at elevated temperatures and offering high reliability.