| Along with the research into the procedure of light-guiding, there come out a series of new integrated optical devices. Waveguide has become a large family in newly designed sensors.When guided modes are excited with a prism-waveguide coupler, a series of so-called m-lines can be observed in the reflected light. Measuring the angles of the reflection of the light beam trapped in the waveguide can help to determine parameters of the waveguide structure, such as the refractive indexes of different layers. This allows the measuring of refractive indexes of liquid samples come into reality. So I design a refractive index sensor in this paper based on the m-lines technique.1. Relevant deduction and design of the sensor structureThe field strength of the evanescent wave decays exponentially,δxiis the penetration depth from waveguide layer into lower refractive index medium. Eigenvalue equation of the waveguide above can be described as: When is different from , for TE mode, eigenvalue equation can be varied into the form below: From this equation, we deduced n4 can be calculated as: For TE mode,to make sure excited modes with the highest order N exist in the waveguide, the thickness of waveguide hshould be above hc(u Nt) :Based on prism-waveguide coupling technique and the deduction above, I designed the structure of the waveguide: Prism-waveguide coupling sensor for measuring refractive index2. Realization of the waveguide structure in laboratoryFabricate waveguide layer by coating two thin layers of special chemicals on the bottom of the prism:Considering the confines and relationships between different parameters in the waveguide structure, Al 2O3( n1=1.58)is chosen to form the waveguide layer, and ( =1.38)is chosen to form the middle layer instead of the air gap in the former quartz plate idea. Coating two layers of chemicals on the prism bottom3. Design of the chamber to hold liquid samples Have designed two different types of chamber as below: This type of chamber has a simple structure which makes the handling and setting of it very easily. It is also easily to be cleaned .It is suitable to fetch liquid samples with unchanged refractive indexes. Matching of the three main parts: holder, chamber and coated prism4. Experiment systemThe experiment system is composed with: m-lines measuring system(laser diode, collimator ,θ2θprecise angle measuring rotary table,optical intensity detector,computer), prism with coated waveguide on it, chamber, CCD, graphing control software. Optical routine 1.laser diode 2.polarizer3.λ4shift plate 4.semi- transparent plate 5.pin-hole 6. optical intensity detector 7.θ2θrotary table 8. prism with coated waveguide 5. Experiments of glycerine solution samples1) Calibrating of waveguide thickness to each mode angle Prepare three liquid samples with different percentages of glycerine: 50%,75% and 99%.Graphed the probing curve and measured the mode angles of each samples under 20℃. Take the mode angles into relevant calculation which have been programmed with software,we finally got refractive indexes of all the three samples.Due to the defections in the coating technique, so thick a layer can't been made very plane and even. The roughness of the layer has caused the impreciseness of the calculated refractive indexes ( ). So it is necessary to calibrate the waveguide thickness to each mode angle.Precise sample refractive indexes are essential to the calibration of the waveguide thickness . Use Abbe-meter we got a relatively precise refractive indexes of each samples, then took these numbers into relevant calculation, waveguide thickness to each mode angle was found out like this.Take the mode angles into relevant calculation again,we finally got much more precise refractive indexes( N4)of all the three samples. Calibrated waveguide thickness to each mode angle: 2) Graphing"refractive index-percentage"curve of the glycerine solution samples Prepare another 4 glycerine solution samples with different percentage: 60%,70%,80% and 90%. With a program of ORIGIN software,"refractive index-percentage"curve was drawn out based on the refractive indexes( )of the glycerine solution N4 Then with the same program of ORIGIN software, we found out an empirical equation that can describe the relationship between refractive index N4and the glycerine percentage: Y=0.041988 X2+7.64114X-10.3584 (X= refractive index,Y= glycerine percentage)With this empirical equation, we can determine the glycerine percentage of a random liquid sample by measuring its refractive index.3) Example experimentPrepare another glycerine solution sample with a percentage of 65%. After measuring the sample two times and dealing with the data, was found out to be 1.4295 in both times. Take into the empirical equation, we find out the result to be 0.6504(65.04%),very close to the real percentage65%.This result had proved the empirical equation is well simulated on one hand and can work well on the other hand. We can use this sensor to determine the glycerine percentage of a random liquid sample.6.Experiment of honey sampleWe can use this sensor to determine the sugar share(sugar percentage) of a honey sample. Sugar share is an important character to differ good quality honey from bad one.Sugar share is usually measured by a very simple device named Baume-meter. The reading of Baume-meter is gave out in"°Bé", a very special unit. But it cann't give out a very precise data because of it's simple and rough structure.Use a bottle of honey, which was bought from a supermarket and has a mark of 41.5°Béon its label, as a sample. Determine its sugar share with both the prism-waveguide coupling sensor and Baume-meter, we can get similar results.With a program of ORIGIN software,"refractive index-sugar share"curve of this honey was drawn based on the data given out by the"national standard of honey's sugar share". Then with the same program of ORIGIN, we found out a empirical equation that can describe the relationship between refractive index and the glycerine percentage: Y=-7.14234X 2+25.0735X-20.705 (X= refractive index,Y= sugar share)Measure the honey 2 times, in the first time, is 1.48561, in the second time, is 1.48569. According to the data given out by the"national standard of honey's sugar share", honey of 41.5°Béshould have a refractiveWe did another test of this honey with a Baume-meter, it also gave a result of 41.5°Bé.From the above, we can declare this bottle honey to be of good quality. And the prism-waveguide coupling sensor can be applied into uses such as determining honey's sugar share or parameters of other liquid samples. |
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