This study demonstrates that the cladding modes of a tilted superstructure fiber grating can be coupled back to the core mode to create induced channels by acousto-optic interactions when the acoustic wave traveling along the fiber axis vibrates the fiber. This phenomenon is based on the acoustic wave vector matching the difference between the core mode and the cladding mode wave vector. Acoustic power levels can be used to control the reflectivity and number of the induced wavelength channels. Moreover, the wavelength location of the induced channel can be tuned by varying the acoustic wave frequency. Thus, the proposed device may provide a switchable multiwavelength comb filter for applications in wavelength-division multiplexing systems, fiber lasers, or fiber sensors.
We demonstrated that a high-sensitivity fiber sensor based on a superstructure fiber grating (SFG) can simultaneously measure the pressure and temperature by encapsulating the grating in a polymer-half-filled metal cylinder, in which there are two openings on opposite sides of the wall filled with the polymer to sense the pressure. The mechanism of sensing pressure is to transfer the pressure into the axial extended-strain. According to the optical characteristics of an SFG composed of a fiber Bragg grating (FBG) and long period grating (LPG), the various pressure and temperature will cause the variation of the center-wavelength and reflection simultaneously. Thus, the sensor can be used for the measurement both of the pressure and temperature. The pressure sensitivity of 2.28×10<sup>-</sup><sup>2</sup>MPa<sup>-1</sup> and the temperature sensitivity both of 0.015nm/°C and -0.143dB/°C are obtained.
The induced cladding-mode performance in a tilted superstructure fiber grating (SFG) is first experimentally demonstrated. As a slanted SFG is heated to shift wide-band loss dips for obtaining the multi-narrow-band reflection moved at various positions of the loss dip, the strength and central wavelength of the induced cladding-mode depend on the loss depth and the positive/negative sides of the dip, respectively. The characteristics may provide the third physical-parameter measurements in a simultaneous multi-parameters fiber sensor.
An all-fiber pressure sensor based on a fiber Bragg grating with the pressure sensitivity of 2.2x10<sup>-2</sup> MPa<sup>-1</sup> has been demonstrated. The physical configuration includes a FBG encapsulated in a polymer-half-filled metal cylinder with its end bonded to the central of a round plate attached to the surface of polymer, and the Young’s modulus of the polymer is four orders lower than FBG. This cylinder has two opening on opposite side of the wall at the polymer part. Under the pressure environment, the polymer can be pressurized along one radial direction only, and responds an axial force acting on the round plate, producing an axial strain on FBG. With a nice linearity, this sensor should be applied potentially for the measurement of mediums pressure, liquid level and depth underwater.
The application of the acoustic-induced vibration on a fiber Bragg grating has been proposed as the function of controlling reflectivity levels and switching reflection wavelengths. Moreover, a switchable multi-wavelength optical filter is expected to develop for various applications in optics. Thus an acousto-optic interaction in a superstructure fiber grating (SFG) can provide a multi-wavelength reflective filter with the function of switching the operation wavelength. In this paper, we experimentally demonstrated that the channels of a blazed SFG could be increased or switched as the acoustic waves were launched into the fiber. When the acoustic wave is applied in the fiber and travels along the fiber axis, the cladding modes of a blazed SFG can couple back to the core mode by acousto-optic interaction in the fiber. The grating reflectivity and the number of the induced wavelength channels can be controlled by acoustic flexural amplitude. Thus, this device acts as a switchable multi-wavelength comb filter for the applications in a WDM system, or in fiber lasers or in fiber sensors.