Characteristics Investigation And Fabrication Of Novel Structural Long Period Fiber Grating

Fiber gratings which could directly modulate the fiber modes on the centimeter, millimeter, and even micrometer scale, and possess the advantages of light weight, small size, immunity to electromagnetic interference, and so on, have been widely used in the field of optical communications and optical sensors, and maybe play an very important role in the new generation IT. Among them, the structural long period fiber gratings (SLPFGs) with strong modulation, good thermal stability, more compact size and other unique characteristics, are more conducive to integration and miniaturization of optical devices, and have become one of the development trends of fiber gratings.Basing on this reality demands as mentioned above, this article proposed and experimentally demonstrated a new SLPFG. To study its sensing properties, the class structural LPFG were applied to bending, twisting, strain, temperature, refractive index, and other mechanical or environmental parameters measurement by observing the evolution of the grating transmission spectrum. And then, some structural LPFG-based sensors, such as he bend-insensitive high temperature sensor, directional twist sensor, simultaneous measurement of strain and temperature sensor, and micro-displacement sensor with high sensitivity were proposed and experimentally achieved. The main contents and research results are as follows:1The "micro-core-offset"(MCO) were firstly introduced into LPFG as the grating modulation. Due to abrupt changes of the fiber geometry, the MCO-SLPFG possess strong grating modulations, and thus, the grating in short fiber size but a deeper loss peak could be achieved. In the experiment, a MCO-SLPFG with the offset value of3.5μm and two pitches could be fabricated with a peak loss over20dB and ultra short grating length of1.09mm. Due to the asymmetry of the offset structure, MCO-SLPFG has a strong asymmetric mode coupling effect. For the offset value of2.5μm, grating could coupling the core fundamental mode LP01to LP14mode, this asymmetrical modulation structure and mode coupling characteristics can be applied to directional sensing measurements.2The "waist-enlarged bitapers"(WFB) is firstly introduced into the LPFG as the grating modulations. WFB induced fiber radius increases, causing the fiber mode field mismatch, thereby forming a light field intensity modulation. By adjusting the overlap value, the geometrical parameters of enlarged region such as, the radius and taper-length will be changed, and then its ability to modulate the light field will be different. When the overlap value is60μm, a WFB-SLPFG could be formed by only five grating period and displayed a peak loss over30dB. Fiber core mode can be coupled to different orders mode under different overlap value of WFB-SLPFG s. When the overlap values are60μm and70μm, WFB-SLPFG s achieved the mode coupling from LP01to LP14, and LP01to LP15, respectively. This feature indicates the WFB-SLPFG s could modulate the fiber modes more flexible and convenient, and the applications in the mode conversion-related fields.3A novel bend-insensitive high temperature sensor is proposed and experimentally achieved based on the geometrical structure of MCO-SLPFG s. Because of the asymmetrical structure, the bending sensitivity of MCO-SLPFG s is bending direction-dependent. At the most sensitive direction, the sensitivity is0.1877nm/m-1, which a one order lower than that of conventional LPFGs; and at the the most insensitive direction the sensitivity is approximately0.0097nm/m-1, which is three order lower than that of traditional LPFGs and can be considered as bend-insensitive. The irreversible fiber geometry changes could survive at high temperatures, and thus could be used to measure the high temperature changes. By selecting the appropriate fiber orientation, MCO-SLPFG s could be applied to bend insensitive high temperature sensor.4Based on the resonant wavelength and amplitude changes of one peak of the WFB-SLPFG s, the simultaneous measurements of strain and temperature sensor is proposed and experimentally demonstrated. The periodically changes of the fiber radius induced by WFB result in WFB-SLPFG s with a high strain sensitivity of1.75nm/mε in the strain range of0-2.7mε, which is3-6times higher than that of the SLPFG fabricated by arc discharge method and CO2laser radiation. To solve the problem of cross sensitivity between strain and temperature, a two parameters measurement sensor was introduced, which indicated a practical significance of the WFB-SLPFG.5A compact all-fiber micro-displacement sensor with high sensitivity was achieved based on a WFB-SLPFG. The very short dimension guarantees a compact device based on WFB-SLPFG, and the wide broadband is conducive to the formation of interference fringes. An all-fiber displacement sensor based on WFB-SLPFG and fiber tip structure is presented in this article. The maximum wavelength shift sensitivity is measured to be934pm/μm which is four times as that of the combined structure of LPFG and an air chamber. The maximum amplitude variation sensitivity is approximately to be-1.97dB/μm, which is the highest value currently reported. The displacement sensor can be used for high-precision positioning required in the industry and scientific fields.