Fiber Bundle Optical Coupler with Circle-Distributed for Diffuse Light Collecting of Grain Sample

In this paper, a method to design fiber bundle optical coupler is introduced. The structure of coupler is parameterized, and it could be optimized by coupling factor. The geometrical characteristics of coupler are circle-distributed, discrete and
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   Fiber Bundle Optical Coupler with Circle-Distributed  for Diffuse Light Collecting of Grain Sample Feng.Zheng, Liying.Liu, Lingxi Zhu, Ye.Li, Kewei.Huan, Xiaoguang.Shi * , Faculty of Science of Changchun University of Science and Technology Changchun, 130022, China Corresponding author: Xiaoguang.Shi Guojun.Liu State Key Laboratory on High-Power Semiconductor Laser Changchun University of Science and Technology Changchun, 130022, China E-mail   Abstract- In this paper, a method to design fiber bundle optical coupler is introduced. The structure of coupler is parameterized, and it could be optimized by coupling factor. The geometrical characteristics of coupler are circle-distributed, discrete and multi-layer. It is good for diffuse light collecting of grain sample with a large-diameter of incident beam.   Keywords-optical coupler; circle-distributed; diffuse light collecting I.   I  NTRODUCTION In recent years, with the rapid development of Diffuse Optical Spectroscopy (DOS) in biology, agriculture, medicine and so on, collecting diffuse light plays a very important role in the non-invasive or non-destructive measurement [1-4] . The conventional way to collect diffuse light is integrating sphere collection. Spheroidal structure and inner diffuse reflection wall could collect diffuse light from each direction and make the diffuse light field uniform. For various samples and testing requirements, the key parameter of integrating sphere is the aperture-ball diameter ratio. The large-diameter of light beam or grain sample could make the integrating sphere not applicable. For natural sample, it is more difficult to test gain sample. It is a challenge to get stabilized spectrum data when keeping samples in natural state. In the process of diffuse light collecting, there are some unstable factors, such as grains state, power distribution in cross-section of incident beam, and sample loading. So, sophisticated structure of diffuse light collecting will enhance the stability of diffuse spectrum data [5] . II.   D IFFUSE L IGHT C OLLECTING    A.   Scattering Effect in Diffuse Light The spectral analysis method extracts sample’s information from the absorption-attenuation characteristic by light-sample interaction. There is evident linear correlation  between spectrum data and sample’s chemical or physical information when chemical composition of sample is simple. But for grain sample, both physical structure and chemical composition are rather complex, distribution of refractive index has a comparative wide range. Correspondingly, dielectric constant   of electromagnetic field becomes random variation in spatiotemporal domain. So amplitude and phase of optical field becomes random variation in spatiotemporal domain when the optical field propagates in biological tissue. As the case stands, there is no analytic solution to the Maxwell equations. Thus, there is only a statistical description. Undoubtedly, the information of grain sample given by diffuse spectroscopy is equalizing value in a statistical sense. So, structural design of diffuse light collecting should be fully considered in statistical perspective.  B.   Statistical Effect in Diffuse Light In diffuse light of grain sample, the scattering effect is far more than the absorption effect. Thus, the scattering effect takes a very important part in extracting sample information. The information about absorption-attenuation characteristic of light-sample interaction is carried by scattering process. Scattering process is a multi-scales action. It is composed of individual action in micro-scales and collective action in macro-scales. Generally speaking, information from different scales will be up-scaling by statistical average. The  precondition of up-scaling is that structures should be similar in different scales. For grain sample, self-similarity of structure has been broken in granular-scale. So the sample is divided into two-scales. In the granular-scale, abundant grains are piled up in the sample container. The structure in granular-scale forms a discontinuous medium in a relative slack state. And the topmost layer of sample forms a boundary layer. In the single grain, it is regarded as a quasi-continuous medium. And the coat layer of single grain is a boundary layer. So, the energy and information coupling and transferring in the two scales will have the scale effect. Figure 1a shows light incident upon a single grain. In this case, one part of energy will be reflected back, the other part of energy will enter into the internal of grain. In the internal of grain, energy of light will split into three portions. The first portion will transmit through the grain along the incident direction, the second portion will  be absorbed, the third portion will be scattered to all directions. Figure 1b shows light incident upon heaps of grains. After interacting with the single grain, the unabsorbed energy of light will interact with other circumjacent grains times after times. So, the grains’ distribution will have a strong impact on energy distribution of diffuse reflection. It means that chemistry composition distribution will be modulated by grains’ distribution in spatial domain. The random variation of grains’ distribution plays a very important role in the process of information up-scaling. Accordingly, to 2015 International Conference on Optoelectronics and Microelectronics (ICOM)978-1-4673-7462-0/15/$31.00 ©2015 IEEE273Jul. 16-18, 2015  reduce the effect of random variation of graone of the most considered factors. a) Incident light upon a b) Incident l single grain heaps of grainFigure 1. Incident light upon a single grain and The average of grain size (or clearanceimportant parameter of grains’ distribution.  been grinded to fine particle in the earlyspectroscopy. Statistically, it increases the in the interacting space. But, this methinstability from operating process, such anon-uniform of grain sizes. Thereby, enlargspace is a more effective method to fluctuation by random variation of grains’ intensity decreases as depth increasing. It different information in different sample laeffect, the most information is in the boincreasing the area of incident plane Distribution of incident light and structure ouseful factors to be considered for large-di beam. III.   F IBER BUNDLE OPTICAL COUPL  A.   Structure Model of Circle-Distributed Col  Figure 2a shows the schematic didistributed collecting. 1 and 2 are the fiber caxises of fiber   5 and 6 are envelopes of f end faces of fiber   9 and 10 are the tips ocentral axis of circle-distributed fiber bundl12 is the body of circle-distributed fibcoupler; 13 is the grain sample; 14 is the light; 15 is the incident light; 16 is the diffurectangular coordinate system could be estafigure 2b. a) Schematic diagram of circle-distributed ins’ distribution is ight upon heaps of grain size) is the most o, the sample has stage of diffuse quantity of grains d will introduce   s water losing or ng the interacting reduce statistical distribution. Light eans that there is ers. For interface ndary layer. So, is recommended. collector become meter of incident ER MODEL   lecting gram of circle-ores  3 and 4 are iber   7 and 8 are f fiber   11 is the e optical coupler; r bundle optical hole for incident se light. Then, the lished showed in collecting  b) Establishing the rectangular cFigure 2. Schematic diagram and par collectin  B.   Cone Model of Fiber Collec  Numerical Aperture (NA parameter of fiber for collectisuitable for quantitative desclight. Figure 3 shows the cocollecting light. Figure 3. Cone model of a s The generatrix of cone P-TPR=h, the radius of bottom Tline segment of axis is PQ=b, and plane MN, the half-angle MQN truncated by sample platwo ends of long axis and MN               Let z replace by h-z and transformation,              Let x replace by  Let z replace by  The conical equation is obt                The cross section truncateconical equation of the cross se        ordinate system in optical coupler ameterization of the circle-distributed structure ting Light ) is an important geometric ng light. The cone equation is ribing the effect of collecting e model of a single fiber for ngle fiber for collecting light  N is PN= l, the height of axis is RN is RN=RT=r, the truncated the angle   is between axis PR of cone is  . The cross section e is an ellipse, M and N is the d. The conical equation is: (1)   replace by x+r, by translation      (2)         ined                     (3)    by plane z=0 is an ellipse, the ction is:       (4)   2015 International Conference on Optoelectronics and Microelectronics (ICOM)978-1-4673-7462-0/15/$31.00 ©2015 IEEE274Jul. 16-18, 2015  C.   Collecting Capability Model of Optical C  Aiming to spatial distributed characterista diffuse light collecting system is designecollecting method. According to non-imaginthe collecting capability of distributed ener Etendue. This concept could be used toelements and beam itself. As for beam, Etegeometric properties of beam propagaelements, Etendue represents the capability of the optical elements. Etendue is an important concept in notheories, and it is used to describe geom beam which has certain aperture angle and Etendue is a two-dimension optical invarianthe converging and diverging between becharacteristic parameters of geometrical ocross section. The definition of Etendue is equals the area of the entrance pupil times tsource subtends as seen from the pupil. Tfollow:        Where   is included angle between nor and central axis of solid angle d  . Etendue is actually the integration of th beam and the spatial solid angle of beam. m2  sr, then it is totally a geometrical quanrefer to optical energy or intensity. The Eten                                       Where n is air refractive index(equreceiving panel, d   is receiving solid adiameter of fiber core,  1 is acceptance included angle between normal line of fiber sample.                                      Fiber bundle optical coupler with circldivide the total Etendue into several indepsub-Etendues. Every independent fiber has tof the same ring. That means the sample points are used to instead of continuous ring.IV.   D ESIGN AND IMPLEMENT  A.    Design According to parametric description of of fiber coupler with circle-distributed coul parameter set    , and the specific parameare expressed by              . Thefiber coupler with circle-distributed a                          upler ic of diffuse light, d with distributed g optical theories, y is expressed by describe optical due expresses the ion; for optical f receiving beams n-imaging optical tric properties of cross section. The t, and it describes m and beam and tics related with that: the Etendue he solid angle the he equation is as (5)   al line of area dA e area crossed by o the unit of it is ity and it doesn’t ue at point P1 is:      (6)     ls to 1), dA is ngle, d is core angle,  1 is the end and surface of (7)        (8)   e-distributed is to endent distributed e same parameter with N discrete TION  model, the design  be expressed by ters of each layer , the two layers e expressed as [6]. One design case in diff system of wheat is given bestructure support body of fiber of incident light lens in this sy bundle is adopted. When the  N1=9 and N2=10. The paramin figure 4, and         Figure 4. Structure sup The complete structure diawith circle-distributed is giveand  are standard fiber portcombiner,  and   are SMA9  is locknut,  is location hol  is structure support body an Figure 5. Circle-distributed Based on cone model, reccircle-distributed fiber bundle and two layers are shown in fi Figure 6. Received solid angle simuloptical coupler with  B.   Operating Distance of Optic According to figure 5, fibcircle-distributed is used to pse reflection light collecting low. Seen from figure 4, the coupler is shown. The diameter tem is 48mm, and 1 to 19 fiber 1+N2=19, with equal division, ter P is determined by structure   . ort body of fiber coupler ram of two layers fiber coupler in figure 5. Among them,  s SMA905,  is optical fiber 05 adapters,  is lock washer, e for circle-distributed coupler,  is distributed fibers. fiber bundle optical coupler eived solid angle simulation of  ptical coupler with single layer ure 6. ation of circle-distributed fiber bundle single and two layers al Coupler er bundle optical coupler with t bowl structure support body 2015 International Conference on Optoelectronics and Microelectronics (ICOM)978-1-4673-7462-0/15/$31.00 ©2015 IEEE275Jul. 16-18, 2015  cover the surface of measured sample. The of fiber coupler is defined as the distansurface of coupler and surface of meassimulation effect in figure 6 shows that the section on the surface of sample will chandistance changes. So in practical applicationdistance need be determined to get expecteIn figure 7 shows the simulation effect of r sample in different distances between scoupler. The operating distance is from d=1step distance is 1mm. Figure 7. Simulation of received section on sample i between sample and fiber couple According to the principle of optical r the light source is arranged on the receivdistribution could be formed on the sampsimulation results could be verified by expshows the experimental photographs of resample surface in different distance betweecoupler. These results are basically the sameffect. There is additional optical power inf 8. The brighter part in the picture means the  be collected more and the larger area of thmeans that the grain sample could be covthese two factors between the area of the ligthe intensity of light, it can be considered toperating distance ce between front red sample. The effective received e when operating , proper operating collecting effect. ceived section on ample and fiber 1mm to d=42mm, different distances eversibility, when ed end, the light le surface. Thus, eriment. Figure 8 ceived section on sample and fiber as the simulation rmation in Figure diffuse light could light distribution red more. Weigh t distribution and at the appropriate range of operating distance of fiber coupler is from d=25mm Figure 8. Experimental results of redistances between sa V.   CO By using the flexible distributed coupling technolcoupler with circle-distributed for the diffuse light collectdistributed structure makes receiving structure increase.  power distribution, the collecti be adjusted and optimized, anthe collection efficiency of suitable for grain sample w beam. It is a useful supplesphere diffuse collection. R  EFE[1]   Bing Yu etc., “Diffuse reflectanca smart fiber-optic probe”, Bio689 [2]   Bing Yu etc., “Instrumenspectroscopy”,  J. Biomed. Opt  .2[3]   S.F.Bish, etc., “Development spectroscopy probe for measurinOpt. 2011,16(12):120505. [4]   M. B. van der Mark and A. DesjHigh Collection Efficiency," i  Photonic Applications , OSA Tec[5]   Feng.Zheng etc. , “Restriction anCondition in NIR Diffuse Spectroscopy and Spectral Anal[6]   Feng.Zheng etc., “The ParameteOptic Coupler Based on Circle-The 3rd Chinese Conference on the two-layer circle-distributed to d=30mm. ceived section on sample in different  ple and fiber coupler CLUSIONS   light guide technology and gy, the fiber bundle optical has been designed and applied ing of grain sample. Circle-the adjustable parameters of ccording to the incident light g features and capability could it is advantageous to improve diffuse light. Especially, it is ith large-diameter of incident ent for replacing integrating ENCES   e spectroscopy of epithelial tissue with edical Optics Express,2014,5(3): 675- independent diffuse reflectance 011,16(1): 011010 of a noncontact diffuse optical g tissue optical properties,” J. Biomed. rdins, "Diffuse Spectroscopy with Very CLEO:2011 - Laser Applications to nical Digest (CD), paper ATuB4. d Parameter Description of Geometrical Reflectance Spectrum Measuring”, sis, 2010,30  11):193-194. ization Method to Design Fiber Bundle istributed for NIR Diffuse Collecting”, ear Infrared Spectroscopy,2010. 2015 International Conference on Optoelectronics and Microelectronics (ICOM)978-1-4673-7462-0/15/$31.00 ©2015 IEEE276Jul. 16-18, 2015
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