Characteristic Analysis of Diffraction from the Restricted Output End Surface of Single Mode Planar Waveguide LI Lian—huang ,GUO Fu—yuan ,WANG Li—xiang ,HU Lin-shun (1.Fujian Provincial Key Laboratory of Photonic Technology,Institute of Laser&Optoelectronic Technology,Fujian Normal University,Fuzhou 350007,CHN;2.Department of Information Science&Elcetronic Engineering,Zhejiang University,Hangzhou 310027,CHN) Abstract:As for single mode symmetric step refractive index structure dielectric planar optical waveguide’ when its output end is restricted,the diffractive field distribution is derived.It cart be expressed as the convolution between the Dirac function and the Sinc and Lorenz functions. It is helpful to deepen the recognition of the restricted diffraction.The characteristic of the diffractive field is discussed.The variational curve of the full ang1e width at half maximum intensity of central bright fringe versus the half width of slit is Dresented by numerical calculation,and the fitting curve with the exponential function is close to it・ Key words:diffraction;slit;central bright fringe;full angle width CLC n哪ber:TN25 Document code:A Article ID:1007--0206(2007)01—0O83一O6 1 IntrOducti0n Since the initial stage of 1990s,the research on multiplexer in optical communication has been mainlY concentrating on the integrated opticsE .The fabrication of many kinds of integrated optics devices,such as arrayed waveguide grating multiplexer and concave etched grating multiplexerL2。5 ,is based on planar oDtica1 waveguide technology, and its basic operation principle is the unequal amplitude multi-beam interference of the diffractive field from the end facet of planar optical waveguide.Therefore,the study on characteristics of the diffractive field of planar optical waveguide has an important signit n 。Ior developing and improving integrated optics devices.The characteristics of diffraction from end facet ot p1anar optical waveguide have been reportedE 墙]. Meanwhile,many kinds of restricted diffraction,such as Gaussian beamE9一l 3l,have been developed。However,the investigation on the diffraction from the restricted output end surface of planar optical waveguide has not been found. The calculable formula of diffractive field from the restricted output end surface of dielectric planar optical waveguide is derived in this paper. At the same time,characteristics of the diffractive field and full ang1e width are discussed in detail.The classical full angle width at half maximum intensity of central bright fringe is a relative1y important sign for diffraction effect.The relationship of the full angle width versus the half width of slit is introduced by numerical calculation.These conclusions would enhance our recognition of the essence of the restricted diffraction. 2 Calculable Formula of the Restricted Diffraction Field The schematic diagram 0f diffraction from the restricted output end surface of the single mode symme ri step refractive index structure dielectric planar optical waveguide is shown in Fig・1. ReOeived date:2006--07--20;revised date:2006--08--26 F0叫dati∞item:Natural science Foundati。n。f Fujian Provinca(A054000 1);The Program。f the Science and Techn0l。gY Department of Fujian Province(K04022) 维普资讯 http://www.cqvip.com
84  ̄miconductor Fhotonic ̄and Technology 2OO7 The diffraction screen∑1 clings to the output end su rIace of planar optical waveguideWe assume that .the thickness of it could be neglected and its c.enter IS In accordance with the waveguide centerThe half width of slit in the diffraction screen∑1 is dThe .。 cular arc 2 is the observation surface of diffractive field that regards 0 as the centre and regards R as the radiusThe viewpoint P locates at .Fig.1. Schematic diagram of the restricted diffracti0n 0f planar optical waveguide he circular arc “d P 2.L is the distance between P ・Engaged on th R yleigh’s Criteri。n,it can be simplified as L=R—zsin S。the Rayleigh—S。mmerfield diffracti。n scalar ntegra¨ormu1a of electromagnetic wave㈨can be simplifiedThe amplitude distribution of diffraction ..shown in Fig・1It can be expressed as L一[(R—zsin ) +.722 cos2 ] /2(P)can be described approximately as[83 (P)=cosOexp(_/kR)r.+ ̄…, ̄(p。)exp(一次zsinO)d e. (1) where (P。)is the amplitude in the slit and 0 is the inclination ang1According to the definition of waveguide parameters and the description of normalized field distributi6n of f““d mental mode of waveguide㈣the amplitude ( )in the slit can be described as foilows,: If the half width of slit is sma11er than or equa1 to the half thickness of core 1ayer,i.e. ≤口,there is )一c。s( )rect(刍) If the half width of slit is bigger than the half thickness of core 1ayer,(2) i.e. >口,the field distribution of core layer(1 l≤ 乜)is exDressed as )一c。s( ) The field distribution of cladding layer(I z I>口)is exDressed as (3) co ∽exp卜 ( 一 )]rect(刍) the normaIlzed evanescent wave parameterc4, 。口is the half where U—n[(志。 1) 一J9。] and W=a[J9 一(志。 2) ] /2 are the normalized standing wave parameter and ,respectively,ko=2 7c/ is the wave number in vacuum, is the propagat。on constant of waveguide, is the wavelength of electromagnetic wave in vacuumthickness of core layer, 1 and 2 are the refractive index of core layer and cladding 1. ayer of the waveguide, respectively,and d is the half width of slitFor the convenience of analysis and without discount of validity,the exp[-i( 一cU£)]item in the above formu1a is omitted. From Eqs.(1)— (4),the amplitude distribution of the diffractive field from the restricted outDut end surface of dielectric planar optical waveguide (P)can be expressed as (P)一cosOe xp(ikR)S(0) V/,4』、 。(5) where S( )is the spatial frequency spectrum of diffractive field,and it is expressed as the c0nv0lution between functions as foilows! If the half width of slit is smalier than or equa1 to the half thickness of core 1ayer,i.e. ≤口,there is (6) sc 一[8(sin 一等)+ (sin +等)] sinc 2d ̄in0 If the half width of slit is bigger than the half thickness of core 1ayeri.e. :>口,there is ,s㈣ (s啪~等)+ (s-n0+意u )] nc( )+ 维普资讯 http://www.cqvip.com
Vol・13 No・1 LI L H,et al・Chara.Analysis of Dif.from the ResOutput End Surface of Single Mode Planar Waveguide 85 .[2 nc( )-2as ( )]⑧cosc队xp(w ×[ 带 where⑧refers to the operator of convolution integra1. 3 Characteristics of Diffractive Field by Numerical Calculation (7) If the refractive indexs of core layer and cladding layer of the optical waveguide satisfy nl≈n2,this optical waveguide can be seen as the weakly guiding optical waveguide.The amplitude distributions of diffractive fields of TEo and TM0 modes of weakly guiding optical waveguide are similar to each other.So, we take the TEo mode for example in the following discussions. As an example,the basic parameters of planar optical waveguide are nl=1.447 3,n2—1.442 0, =1.55 m and a=3 m.The normalized frequency of waveguide V is 1.507,which makes sure that there is only a single mode inside the planar optical waveguide,L,一0.916 8,w一1.196 0,R一0.1 m. 3.1 Slit Width Being Smaller Than Core Layer Thickness Based on the character of Dirac function,the Eq.(6)can be written as 5 一 + 矗 sin + .0一×\ ~J7l (8) If the half width of slit satisfies 《a,Eq.(8)can be expressed approximately as the spatial frequency spectrum of the single slit diffraction of plane wave as follows: s㈣一2 nc( ) ]●●●● , ●●J (9) As an example,d一1.5/zm.Through making 8 numerical calculation by combining Eq.(5)with Eq.(8),the amplitude distribution of diffractive field is displayed as the solid line in Fig.2.We can see that the central bright fringe extends quite 6 4 2 widely when d<a.Although there are side lobes in O two sides。its value is relatively smaller than the maximum amplitude. The amplitude distribution of diffractive field of 一2 l 一_0.5 O sin0 O.5 p1ane wave is disp1ayed as the dashed line in Fig.2 Fig.2・Ampntude dist u i。“。fdiffracti field wh “d<。 from Eqs.(5)and(9).Comparing the dashed line with the solid line,we can conclude that they are close to each other.The numerical calculation indicates that the solid line will approach the dashed line better and better,along with the decrease of the slit width.When d《a,the diffraction from the restricted output end surface of planar optical waveguide can be seen as the single slit diffraction of plane wave. From Eq.(8),we can also see that when the width of slit is smaller than or equal to the thickness of core layer,the spatial frequency spectrum of the restricted diffraction in the fundamental mode is the superimposition of frequency spectrum from the one—dimensional restricted diffraction of two plane waves. It reflects that the nature of the restricted diffractive field is a diffractive field of standing wave.In essence,it reflects that the mode field of planar optical waveguide is a standing wave field. 3.2 Slit Width Being Bigger Than Core Layer Thickness When 一6“m,from Eqs.(5)and(7),the amplitude distribution of the diffractive field is shown as the solid line in Fig.3. As shown in Fig.3。the maximum amplitude locates at the center.The minimum and the inferior maximum appear in turns in two sides and the intensity of side lobes in the solid line is far smaller than that 维普资讯 http://www.cqvip.com
86 Semiconductor Photonics and Technology Feb.2o07 of the central bright fringe. The spatial frequency spectrum of the diffraction from end facet of dielectric planar optical waveguide in fundamental mode can be expressed as[ ] s㈤= [ + (10) where F==kasin 0 is the spatial frequency and a is the half thickness of core layer. sin0 The amplitude distribution of diffractive field of Fig.3. Amplitude distribution of diffraction field when d>a end facet of planar optical waveguide is displayed as the dashed line in Fig.3.As compared with the solid line,the side lobes are relatively smal1.It is demonstrated by numerical calculation that along with the increase of the slit width,the solid line will be close to the dashed line more and more.If the slit width tends to infinity,i.e.d》a,the diffraction from the restricted output end surface of planar optical waveguide can be seen as the diffraction from end facet of planar optical waveguide. 4 Full Angle Width The size of the full angle width at half maximum 1.5 intensity of central bright fringe can reflect the l_2 degree of diffraction directly.Therefore,it is 0.9 significative to investigate the full angle width.Lets 司 take the diffraction from the restricted output end 0.6 surface of planar optical waveguide in fundamental 0_3 mode TE0 for an example,from Eqs.(5)~(7),the relationship between the full angle width and the 0 2 4 6 8 d,×l half width of slit is derived from numerical Fig.4. Relationship between the full angle width and the calculation and the result is shown as the solid line half width of slit in Fig.4. As shown in Fig.4,with the increase of the half width of slit,the full angle width decreases monotonously.When d<a,along with the increase of the half width of slit,the full angle width drops very quickly and when d>a,the full angle width changes relatively slow. For the convenience of analysis,fitting the solid line in Fig.4 by adopting the exponential function,we acquire the following equation A0—2.403exp(一102 60d。・ 。 。)+0.128 1 (11) From Eq.(1 1)。we can conclude that along with the increase of the half width of slit,the full angle width attenuates exponentially.The numerical calculation indicates that so long as the half width of slit is bigger than 0.2 um and does not tend to infinity,the dotted line as shown in Fig.4 which is derived from Eq.(11)is close to the solid line. Two kinds of situations d《a and d》口are discussed below. 4.1 Slit Width Being Far Smaller than Thickness of Core Layer When 《口。the diffraction of the restricted output end surface of single mode dielectric planar optical waveguide is equivalent to the single slit diffraction of plane wave.We can deduce from Eq.(9)that the full angle width must satisfy the following formula A0—2arcsin( ) (12) 维普资讯 http://www.cqvip.com
Vo1.13 No.1 LI L H,et a1.Chara.Analysis of Diff.from the Res.Output End Surface of Single Mode Planar Waveguide 87 From Eq.(12),the numerical calculation indicates that,when 1.35 btm≤d≤2.75 btm,the full angle width is consistent with the full width in Fig.4.As to the single slit diffraction of plane wave,neglecting the influence of inclination factor under the paraxial approximation,when <1.35 um,the full width is bigger than the full width in Fig.4 and when d>2.75 ttm,the full width is smaller than the full width in Fig.4. 4.2 Slit Width Being Far Bigger Than Thickness of Core Layer Under the paraxial approximation,the spatial frequency at half maximum intensity of the central bright fringe of the end facet diffraction should satisfy the following equation[ ] FU + ’ F+U 一 【Ⅳw … where F—kasin 0 is the spatial frequency.From the Eq.(13),we can acquire that the full width of the central bright fringe A0 is 0.1 1 1 8.Neglecting the influence of inclination factor,if the slit width tends to infinity,that is to say,the half width of slit satisfies 》n,the numerical calculation indicates that the full width tends to 0.111 6.As compared with these two kinds of results,they are quite consistent. Therefore,we can conclude that when the slit width is far bigger than the thickness of core layer,the diffraction from the restricted output end surface of planar optical waveguide can be seen as the diffraction from the end facet of planar optical waveguide. 5 Conclusion The diffractive field distribution from the restricted output end surface of single mode dielectric planar oDtica1 waveguide is the convolution between the Dirac function and the Sinc and Lorenz functions. When the slit width tends to zero,it is equivalent to the single slit diffraction of plane wave.When the slit width tends to infinity,it can be seen as the diffraction from end facet of planar optical waveguide.The numerical calculation indicates that along with the increase of the slit width,the full angle width attenuates exponentially.These conclusions are helpful to deepen the recognition of the propagation characteristic of the light beam,and have some instructive significance for developing and designing the integrated optics devj ces. References: E l i Pennings E,Khoe G D,Smit M K,et a1.Integrated—optic versus microoptic devices for fiber-optic telecommunication systems.A comparison[J].IEEE Journal of Selected Topics in Quantum Electronics,1996,2(2):151—164- [2]Huang D W,Chin T H,Lai Y.Arrayed waveguide grating DWDM interleaver[J].Proceeding of Optical Fiber Communication Conference and Exhibit(OFC’20014),2001,3:WDD80—1一WDD80—3. t M K.Van Dam C.PHASAR—based WDM—devices:Principles,design and applications[J].IEEE Journal of [3] SmiSelected Topics in Quantum Electronics,1996,2(2):236—250. —yan.LEI Hong-bing.YANG Qin-qing,et a1.Simple method for designing 1×8 arrayed—waveguide grating [4] OU Haimuhi/demultiplexers[J].Chinese Journal of Semiconductors,2000,21(8):798—802(in Chinese). a Ke-miao,Li Bai-yang,et a1.Silica—based arrayed waveguide grating with flattened spectral response [5] Tang Yan-zhe,Jiusing a muhimode interference coupler[J].Chinese Physics Letters,2004,21(6):1 064—1 066. Greer K A.Diffraction from concave gratings in planar waveguides[J].IEEE Photonics Technology Letters,1995,7 [6] Mc(3):324-326. Yoshimura H,Takai N,Asakura T.Far-field analysis of fluctuating supermodes radiated from phase-locked array lasers [7] [J].IEEE Journal of Quantum Electronics,1994,30(2):222—229. Fu-yuan,LIN Bin,CH EN Yu-qing.Diffraction characteristic of planar waveguide in fundamental mode[J].Laser [83 GUO &Infrared.2002.32(5):323—326(in Chinese). 9] TENG Shu—yun,LIU Li—ren,LUAN Zhu,et a1.Axial distribution of Gaussian beam limited by a hard—edged aperture 维普资讯 http://www.cqvip.com
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