Research And Fabrication Of Long Period Fiber Grating Refraction Index Sensor

Long Period Fiber Grating-LPFG is a kind of new grating of period from dozens to hundreds microns. It is mostly concerned these years because of its little insertion loss, low back-reflection, simple preparation and relatively low cost. LPFG is highly sensitive to temperature, stress, bending, cross loading and environmental refractive index. Therefore it has a very high applied value in the sensor region. However, LPFG has lower sensitivity and bad measuring selectivity problems, comparing with Surface Plasmon Resonance-SPR. Therefore, it is of great significance to promote the sensitivity and selectivity of LPFG by a large margin in the practical process.LPFG is a transmission grating, in which the transmission peaks come from the coupling of basic mode and cladding mode in the same direction. Hence the change of cladding directly influences the transmission spectrum of LPFG. The concentration and species of chemicals in the circumstances can directly result in the change of effective refractive index. The transmitting traits of grating spectrums are varied consequently. Hence LPFG is sensitive to refractive index. In this text, refractive index sensor is made which has high sensitivity and selectivity to gas and solution, in relevance to the sensitivity to refractive index. Theoretical analysis is also made here. Concrete work is as follows:1. With the three-layered model of optical fiber and Helmholtz equation, I deduced the characteristic equation of core guide mode and cladding mode. After that, figure out the effective refractive index with this equation. Meanwhile, analyze the spectrum characteristic of LPFG, and obtain the relative parameters. These figures are prepared for making sensors which correspond to LPFG refractive index. Introduce the function and significance of Electrostatic l-b-l Self-Assembly, as well as means and foundation of the modification of cladding of fiber grating.2. Corning SMF-28 grating is used in the text, whose parameters RI of core n1=1.4681, RI of cladding n2= 1. 4628, radius of core a1= 4. 15, radius of cladding a2= 62. 5, respectively. Prepare the fiber grating of 500um with the traditional method of ultraviolet read-in. KrF excimer laser was used in the experiment. The sensitized fiber is exposed by the UV laser under an amplitude mask, during which an ANDO infrared spectrometric analyzer is used to monitor the transmission spectrum of the LPFG. This means is structurally simple and easy to control. We can make LPFG of little inserting loss and narrow bandwidth.3. Erode the LPFG and make sure of reaching more sensitive regions under the condition of good mechanical strength. According to the analysis above, the cladding radius of LPFG could influence the sensitivity of the refractive index. Therefore fiber grating soaking in 40% HF solution was used to decrease the cladding radius to more sensitive extent, aiming at promoting the sensitivity of LPFG.4. According to the relationship between the transmission spectrum of LPFG and surrounding refractive index in Chapter 2, I studied the modification of LPFG through Electrostatic l-b-l Self-Assembly, in order to the refractive index sensitivity of LPFG. First of all, assemble the polyelectrolyte of PAH-PSS onto the cladding of the fiber grating. The fiber grating is most sensitive to water when it comes to 105th layer, and reach 3nm/layer, which is one order of magnitude larger than the values in relative literatures, 0.5nm/layer. This reduces the difficulty of LPFG sensor's signals, to a large extent. Second, considering that excess layers could possibly bring about long experimental period and more errors, PDDA-PSS was used instead of PAH-PSS. It was found that the most sensitive point was reached at 80th layer and the wavelength movement was over 10nm/layer. This indicates that polyelectrolytes of higher thickness can help to get the most sensitive point at fewer layers. The reason is that the thickness of the film influences a lot the refractive index of the combination of the film and the media. Polyelectrolytes of a thicker film influence the effective refractive index more markedly. Hence, it is easier to make the environmental refractive index close to that of LPFG, and get the most sensitive point.5. In the assembling process, LPFG was used to detect five media and analyze the rules of the most sensitive points of them. It was found that, the most sensitive points came at different number of layers and it had a common rule: in the PAH-PSS assembling process, isopropanol, alcohol, methanol, water and air give rise to the sensitive regions successively. The reason for the phenomenon is that isopropanol, which has the refractive index of a relatively high refractive index reacting together with PAH-PSS film is more close to the effective refractive index of fiber cladding. The refractive index of air is rather small, which means the environmental refractive index is small. Hence more layers need to be assembled to make the environmental refractive index more close that of fiber cladding. The sequence of LPFG to the sensitive region of the five media, which is the result of loose structure of PDDA-PSS as well as the tendency to swell.This article is just the preliminary study of preparation of high-sensitivity and high-selectivity LPFG refractive index through self-assembly means. To make further promotion of the sensitivity and selectivity of LPFG to medium under test through this means, systematical study of physical and chemical properties of various polyelectrolytes as well as their coaction with gas and solution. It needs some further investigation and exploration to make its application in the daily life come true.