World Scietific Research Joural (WSRJ) ISSN: 2472-3703 www.wsr-j.org Research o Fresel Les Optical Receivig Atea i Idoor Visible Light Commuicatio Zhihua Du College of Electroics Egieerig, Chogqig Uiversity of Posts ad Telecommuicatios, Chogqig 400065, Chia 727025145@qq.com Abstract: I order to collect more optical sigals at the receivig ed of the visible light commuicatio (VLC), a Fresel les is desiged, ad at the same time, a hemispherical les is added at the receivig ed i order to icrease the field of view agle ad the receivig optical gai. The results show that the field of view reaches 40 ad the optical gai is 4.6. Keywords: VLC, the field of view, optical gai. 1. INTRODUCTION At preset, the research o the optical receivig atea of the idoor VLC system has attracted much attetio. The additio of a optical receivig atea at the receivig frot ed ca collect more sigals ad improve the commuicatio quality of the system. The optical receivig ateas maily iclude plao-covex leses, hemispherical leses, compoud parabolic cocetrators, ad Fresel leses. Amog them, the Fresel les is a part of the material that does ot chage the optical path i the covetioal flat les, so as to reduce the atteuatio of light i the les. Compared with ordiary leses, Fresel leses have the advatages of high light trasmittace, small size, light weight, ad ecoomical applicatio. Fresel leses have a good applicatio prospect as a refractive optical receivig atea i the field of optical commuicatios. Oe research built a VLC system. The system uses ultra-bright white light-emittig diodes, low-cost commercial photodiodes ad Fresel leses, ad LED drivers cosist of power MOSFETs ad MOSFET drivers. I order to icrease the system badwidth, a Fresel les was added i frot of the PIN photodiode to icrease the sigal-to-oise ratio by 40 db [1]. For the problem that idoor VLC could ot satisfy both high gai ad large field of view, oe research desiged a atea combiig a Fresel les ad a compoud surface cocetrator. The simulatio results show that the field of view reaches 40. Usig Matlab, the received power distributio of the desiged optical receivig atea is aalyzed. The results show that the 78
average power is icreased by 7dBm compared with the case where o optical atea is added at the receivig ed [2]. Therefore, this paper desigs a Fresel les as the optical receivig atea of the VLC system to collect light eergy i free space ad improve the commuicatio quality of the system. 2. FRESNEL LENS DESIGN The Fresel les cosists of a series of cocetric circles that look like a circle of thread, also kow as a threaded les [3-6]. I the study of Augusti Fresel, the refractio of the icidet light o the les surface maily comes from the cotiuous surface, ad the icidet light does ot chage through the uiform les material o the surface. Therefore, Augusti Fresel believes that the part of the les that does ot affect the optical path are itercepted ad the remaiig part are traslated to a plae. Fresel les [7, 8] is formed by usig this method. Each prismatic groove i the Fresel les correspods to a idepedet refractive surface, ad the icidet light coverges to a commo focal poit through these idepedet refractive surfaces. The mai structural parameters of the Fresel les are: rig-to-bad spacig d, umber of prisms, workig side agle, ad material refractive idex. Performace p ai () parameters are: les diameter D, focal legth f. The Fresel les desiged i this paper is a plaar poit focusig structure as show i Figure 1. I Fig.1, the icidet ray is icidet from the left side of the les ad is focused o the right side of the les F, which focuses o the agle () i betwee the cetral ray of the prism ad the cetral axis of the les. The basic parameters of the Fresel les ca be derived from the optical refractio law ad the plae geometry. The specific calculatio is as show i (1) From the sie formula: ( i 1/ 2) d ta ( i) f Available from the les structure: From the iverse trigoometric fuctio: The plae geometry we ca see from Fig. 1: The law of refractio of light shows: 2pd i 1/ 2 ( i) arcta 2 pf / D (1) D (2) (3) () i (4) ( i) ( i) (5) si (6) si Substitutig equatios (3), (4), ad (5) ito equatio (6) is simplified: 79
si ( i) ( i) arcta cos ( i) (7) Figure 1. Fresel les prism structure Fresel les material is optical plastic PMMA, caliber D 40mm, F 1, les rig umber. The focusig performace of the desiged Fresel les is simulated. Figure 2 p 20 shows the simulated ray tracig of the Fresel les. Figure 2. Fresel les ray tracig Figure 3. Distributio of irradiace at the receivig surface of the Fresel les 80
Figure 3 shows the illumiace distributio of the fresel les o the receivig surface. The 10 2 maximum irradiace at the receivig surface is 2.13 10 W/m, the average irradiace is 1.64 10 W/m, ad the les codesig efficiecy reaches 90%. The ifluece of the receivig field of view o the cocetratig performace of the desiged Fresel les is aalyzed. The icidet agles of the parallel rays are chaged i order to simulate the plae irradiace simulatio of the Fresel les at the receivig agles of 0.5, 1, ad 1.5, respectively, as show i Figure 4. Figure 4. Distributio of irradiace at the receivig surface of the Fresel les Accordig to the simulatio results i Fig. 4, whe the agle of icidet light icidet o the Fresel les is withi 0.5, the maximum irradiace at the receivig surface is the average irradiace is 9 2 2.13 10 W/m, 1.64 10 W/m, ad the les codesig efficiecy reaches 90%. The radius of the spot is 0.6mm; whe the agle of icidece of icidet light o the Fresel les is 81
9 2 withi 1, the maximum irradiace at the receivig surface is 1.07 10 W/m, the average irradiace is 1.58 10 W/m, ad the les codesig efficiecy reaches 86%. The radius of the spot is 1.2mm. Whe the agle of icidece of icidet light o the Fresel les is withi 1.5, the maximum irradiace at the receivig surface is 7.92 10 W/m, the average irradiace is 1.35 10 W/m, ad the les codesig efficiecy reaches 74%. The spot radius of the spot has icreased; therefore, whe the agle of icidece of icidet light o the Fresel les icreases, the optical efficiecy of the Fresel les decreases, ad the area of the focused spot icreases. Diffusio spots i the image poit become larger. 3. THE FIELD OF VIEW OPTIMIZATION The study shows that the optical gai of the hemispherical optical atea is costat i the large field agle, which is suitable for the light collectio i the wide FOV. Therefore, combied with these two kids of optical atea characteristics, we optimize the fresel atea. The light ray is collected by Fresel les, ad focused by the hemispherical les o the photoelectric detector. we set the hemispherical les which the radius of is 5mm, the refractive idex is 1.517. the distace betwee the Fresel les ad hemispherical les is set to 40mm (Fresel les focus). Via Tracepro, The light from the radomly distributed poit light source is used to simulate the light. Whe the divergece half agle of radom light source is α, the agle of icidet light is evely distributed betwee 0~α. The optical gai is defied the ratio of the amout of light rays are received by the same detector with ad without the optical atea at the same agle α. The distributio of the received irradiace of the system o the receivig surface is obtaied at the FOV of 40. The amout of rays collected by o-optical atea the fresel les receivig atea, ad fresel ad hemispherical receivig atea which are 40, 50 ad 186, respectively as show i Fig. 6 Figure 5. Power received by: (a)o-optical atea (b) sigle Fresel (c) Fresel ad hemispherical atea 82
Table 1. The gai of the differet optical atea at differet FOV 40 Optical receivig atea Oly fresel les Fresel ad hemispherical les gai 1.25 4.6 The result show that the gai of the Fresel ad hemispherical receivig atea is 4.6, whe the Fresel les is 1.25. By compariso, the Fresel ad hemispherical les optical atea meets the required field of view, which has a high optical gai as show i table 1 4. CONCLUSION This paper desig a Fresel les optical receivig atea to collect light eergy i free space ad improve the quality of the VLC system. The results show that the viewig agle reaches 40 ad the optical gai is 4.6. REFERENCES [1] Kim M S, Soh K R. Performace ivestigatio of Visible Light Commuicatio usig super bright white LED ad Fresel les [J]. Joural of the Korea Society of Marie Egieerig, 2015, 39 (1): 63-67. [2] Yi-lu ZHANG, La BAI, Mig-guag GAO, et al. Desig of a 2-cascade idoor visible light commuicatio optical receivig atea [J]. Acta Phys. Si., 2015, 64 (16): 238-243. [3] Leutz R, Suzuki A, Akisawa A, et al. Desig of a oimagig Fresel les for solar cocetrators1 [J]. Solar eergy, 1999, 65 (6): 379-387. [4] Lorezo E, Luque A. Fresel les aalysis for solar eergy applicatios [J]. Applied Optics, 1981, 20 (17): 2941-2945. [5] Hua Xu, Xiagig Li, Guo Zhou. Desig of large pitch equal thickess Fresel les based o Zemax software [J]. Joural of Shaghai Uiversity of Techology, 2007, 29 (1): 99-102. [6] Guaghui Yag, Mig Wei, Big Che, et al. Desig ad research of equal-thickess flat Fresel les [J]. Joural of Applied Optics, 2013, 34 (6): 898-902. [7] Wag T, Sekercioglu Y, Armstrog J. Aalysis of a optical wireless receiver usig a Hemispherical les with applicatio i MIMO Visible Light Commuicatios[J]. Joural of Lightwave Techology, 2013, 31 (11): 1744-1754. [8] Xie W, Dai Y, Wag R, et al. Cocetrated solar eergy applicatios usig Fresel leses: a review [J]. Reewable & Sustaiable Eergy Reviews, 2011, 15 (6): 2588-2606. 83