汪靜麗劉洋 鐘凱

1)(南京郵電大學(xué)光電工程學(xué)院,南京 210023)

2)(天津大學(xué),光電信息技術(shù)科學(xué)教育部重點(diǎn)實(shí)驗(yàn)室,天津 300072)

基于領(lǐng)結(jié)型多孔光纖的雙芯太赫茲偏振分束器?

汪靜麗1)?劉洋1)鐘凱2)

1)(南京郵電大學(xué)光電工程學(xué)院,南京 210023)

2)(天津大學(xué),光電信息技術(shù)科學(xué)教育部重點(diǎn)實(shí)驗(yàn)室,天津 300072)

(2016年8月1日收到;2016年10月18日收到修改稿)

領(lǐng)結(jié)型多孔光纖具有高雙折射的特性,本文基于此設(shè)計(jì)了一種新型的雙芯太赫茲(THz)偏振分束器,采用調(diào)整結(jié)構(gòu)法實(shí)現(xiàn)了折射率反轉(zhuǎn)匹配耦合,達(dá)到偏振分離.仿真結(jié)果表明：該偏振分束器在0.5-2.5 THz頻率范圍內(nèi)均可實(shí)現(xiàn)偏振分離,最小分離長(zhǎng)度僅為0.428 cm,且在整個(gè)頻率范圍內(nèi)分離長(zhǎng)度不超過(guò)2.5 cm.在2.3 THz,x,y兩偏振模的吸收損耗均小于0.35 dB;消光比高達(dá)22.9和19.2 dB.此外,與填充法實(shí)現(xiàn)折射率反轉(zhuǎn)匹配耦合的雙芯THz偏振分束器進(jìn)行比較,本文設(shè)計(jì)的偏振分束器實(shí)現(xiàn)簡(jiǎn)單,運(yùn)行的頻率范圍更寬,分離長(zhǎng)度更短,吸收損耗更低.

偏振分束器,太赫茲,調(diào)整結(jié)構(gòu),多孔光纖

1 引 言

偏振分束器是光學(xué)系統(tǒng)中一種重要器件,可將光信號(hào)分離成兩個(gè)相互正交的偏振光,并沿著不同路徑傳輸[1,2].近年來(lái),科研工作者對(duì)光學(xué)波段的偏振分束器已進(jìn)行了深入研究[3-8],而對(duì)THz波段偏振分束器的研究仍處于起步階段[9-12].THz波在電磁波譜中介于微波和紅外波之間,具有其他波段電磁波不具有的獨(dú)特優(yōu)異性能.設(shè)計(jì)性能優(yōu)良的THz偏振分束器對(duì)于THz器件的研究具有重要的意義[13,14].

目前,基于雙芯光纖設(shè)計(jì)THz偏振分束器是THz領(lǐng)域的研究熱點(diǎn)之一,通常有兩種實(shí)現(xiàn)方式：第一種利用雙折射效應(yīng),通過(guò)調(diào)整光纖結(jié)構(gòu)參數(shù)使x,y偏振模同時(shí)在雙芯間進(jìn)行耦合,傳輸一定距離后實(shí)現(xiàn)偏振分離[3-6];第二種利用諧振效應(yīng),令某一偏振模滿足諧振條件在雙芯間來(lái)回耦合,而另一偏振模不滿足該條件固定在某纖芯內(nèi)傳輸,當(dāng)滿足諧振條件的偏振模完全耦合至另一纖芯時(shí),實(shí)現(xiàn)偏振分離[7,8].

基于雙芯光纖的THz偏振分束器已有相關(guān)報(bào)道,2012年,白晉軍等[9]提出一種低損耗、寬頻段THz雙芯光子帶隙光纖定向耦合器用于偏振分離,可實(shí)現(xiàn)0.14 THz范圍內(nèi)的定向耦合且耦合長(zhǎng)度小于15 cm.2013年,姜子偉等[10]設(shè)計(jì)了一種低損耗THz雙芯光子帶隙光纖定向耦合器,這種光纖定向耦合器在1.55-1.80 THz范圍內(nèi)耦合長(zhǎng)度小于1.8 cm.2014年,Li等[11]提出了一種基于填充式多孔光纖的寬帶雙芯THz偏振分束器,在0.8-2.5 THz頻率范圍內(nèi)實(shí)現(xiàn)了偏振分離,分離長(zhǎng)度為0.4-33.56 cm.同年,祝遠(yuǎn)峰[12]提出兩種THz偏振分束器,一種基于懸浮芯光纖,另一種基于十字架型纖芯光纖,前者在1 THz的分離長(zhǎng)度為3.36 cm,后者在1 THz的分離長(zhǎng)度為11.4 cm.然而,上述設(shè)計(jì)的器件分離長(zhǎng)度隨頻率增長(zhǎng)較快,應(yīng)用的帶寬相對(duì)較窄.

為解決該問(wèn)題,本文提出一種基于領(lǐng)結(jié)型多孔光纖[15]的雙芯THz偏振分束器,通過(guò)調(diào)整雙芯中某一纖芯結(jié)構(gòu)實(shí)現(xiàn)折射率匹配耦合,達(dá)到偏振分離的目的.研究表明：該類(lèi)THz偏振分束器能夠在較寬的頻率范圍(0.5-2.5 THz)內(nèi)實(shí)現(xiàn)偏振分離,且分離長(zhǎng)度短,均不超過(guò)2.5 cm.此外,我們還將本文所設(shè)計(jì)的THz偏振分束器與目前較為流行的使用填充法實(shí)現(xiàn)折射率匹配耦合的偏振分束器進(jìn)行了比較,結(jié)果表明：我們?cè)O(shè)計(jì)的結(jié)構(gòu)制作方便,操作簡(jiǎn)單,且運(yùn)行的頻率范圍更寬,分離長(zhǎng)度更短,吸收損耗更低.

2 結(jié)構(gòu)設(shè)計(jì)和仿真

本文設(shè)計(jì)的基于領(lǐng)結(jié)型多孔光纖的雙芯THz偏振分束器,采用諧振效應(yīng)實(shí)現(xiàn)偏振分離.要實(shí)現(xiàn)諧振條件,可采用文獻(xiàn)[10]所提出的折射率反轉(zhuǎn)匹配耦合(ICMC)法,該方法的思路：首先需要光纖具有高雙折射特性,即實(shí)現(xiàn)x,y兩個(gè)偏振模的分裂;其次,為實(shí)現(xiàn)分離操作,兩根光纖纖芯結(jié)構(gòu)應(yīng)具有正交關(guān)系(一般可將另一根完全相同的光纖旋轉(zhuǎn)90°),再通過(guò)調(diào)整填充液的有效折射率實(shí)現(xiàn)x或者y偏振模式的匹配.然而,采用填充法需要給器件填充液體,操作較為復(fù)雜;且在THz波段中尋找到損耗低且折射率滿足要求的液體,非常困難.基于此,本文提出一種在雙芯THz偏振分束器中實(shí)現(xiàn)偏振分離的新方法：對(duì)某一纖芯進(jìn)行隔行調(diào)整結(jié)構(gòu),從而實(shí)現(xiàn)折射率匹配耦合.

本文對(duì)所設(shè)計(jì)的THz偏振分束器特性的分析與討論均是基于全矢量有限元法(finite element method,FEM)計(jì)算而來(lái),FEM是以變分原理和剖分插值為基礎(chǔ)的一種數(shù)值計(jì)算方法[16].提出的THz偏振分束器如圖1所示,它由兩根多孔光纖(纖芯為多孔結(jié)構(gòu),包層為空氣,導(dǎo)光機(jī)理為全內(nèi)反射)構(gòu)成,光纖A為高雙折射領(lǐng)結(jié)型多孔光纖;光纖B是依據(jù)ICMC法,由光纖A旋轉(zhuǎn)90°并隔行調(diào)整結(jié)構(gòu)為橢圓所成.多孔光纖的基底為聚合物材料TOPAS,它在THz波段具有相對(duì)恒定的折射率,且損耗較低.光纖A和B的纖芯直徑均為Dcore=490μm,光纖A中領(lǐng)結(jié)型結(jié)構(gòu)的兩個(gè)大空氣孔半徑為r2=13μm,小空氣孔半徑為r1=8μm,孔間距Λ=70μm,光纖B中橢圓長(zhǎng)軸r3=15μm,短軸r4=13μm;當(dāng)纖芯A與B相切(L=490μm)時(shí),耦合現(xiàn)象最明顯.

圖1 基于領(lǐng)結(jié)型多孔光纖的雙芯THz偏振分束器的截面圖Fig.1.Cross section of dual-core THz polarization splitter based on porous fibers with near-tie units.

如果光纖B僅僅是將光纖A旋轉(zhuǎn)90°,那么由于nAx=nBy＞nAy=nBx(其中nAx和nAy分別為光纖A中x和y偏振模的有效折射率;nBx和nBy分別為光纖B中x和y偏振模的有效折射率),無(wú)法實(shí)現(xiàn)偏振模匹配.因此,嘗試對(duì)光纖B的結(jié)構(gòu)進(jìn)行改變,通過(guò)隔行調(diào)整領(lǐng)結(jié)型結(jié)構(gòu)為橢圓,以實(shí)現(xiàn)A,B光纖中x偏振模的匹配.仿真表明：當(dāng)橢圓參數(shù)設(shè)置為r3=15μm,r4=13μm 時(shí),A和B光纖中兩個(gè)偏振模式的有效折射率分別滿足nBy＞nBx(如圖2(a)所示)和nAx＞nAy(如圖2(b)所示),且此時(shí)nBx和nAx幾乎完全相等(如圖2(c)所示).因此,在0.5-2.5 THz頻率范圍內(nèi),x偏振模將會(huì)在兩個(gè)纖芯間強(qiáng)烈耦合,同時(shí)由于兩個(gè)纖芯中y偏振模的有效折射率相差較大,模式不匹配故不發(fā)生耦合.

圖3給出了工作頻率為0.5,1.0,1.5,2.0,2.5 THz時(shí),x,y偏振奇模和偶模的穩(wěn)態(tài)模場(chǎng)分布,下標(biāo)o,e分別表示奇模和偶模.如圖3所示,在0.5-2.5 THz的頻率范圍內(nèi),A和B光纖中的x偏振模因滿足模式匹配條件,在兩芯之間始終發(fā)生耦合,穩(wěn)態(tài)模場(chǎng)同時(shí)分布于雙芯中;而對(duì)于y偏振模而言,由于模式不匹配,始終不會(huì)發(fā)生耦合,穩(wěn)態(tài)模場(chǎng)只存在于某一纖芯中.

圖2 光纖中x和y偏振模的有效折射率隨頻率的變化 (a)光纖B;(b)光纖A;(c)光纖A與BFig.2.Effective refractive index of x and y polarization modes versus frequency：(a)Fiber B;(b)fiber A;(c)fiber A and B.

圖3 (網(wǎng)刊彩色)x,y偏振奇模和偶模在不同工作頻率時(shí)穩(wěn)態(tài)模場(chǎng)分布Fig.3.(color online)Modal distributions in steady state of even and odd modes for x and y polarization modes at different frequencies.

偏振分束器的分離長(zhǎng)度是衡量該類(lèi)器件性能的一個(gè)重要指標(biāo),定義如下：

式中Lc表示分離長(zhǎng)度,nxe和nxo分別是x偏振偶模和奇模的有效折射率,λ是入射波長(zhǎng).由圖4(b)可見(jiàn)：采用調(diào)整結(jié)構(gòu)法的THz偏振分束器,其分離長(zhǎng)度先隨著頻率的增加而增加,在f=1.5 THz的時(shí)候達(dá)到峰值(Lc=2.5 cm),之后隨著頻率的增加而減小,并且在整個(gè)頻率范圍內(nèi),分離長(zhǎng)度變化較為緩慢,控制在一個(gè)較小的范圍內(nèi)(0.428-2.5 cm).為了便于比較,我們還對(duì)基于領(lǐng)結(jié)型多孔光纖的雙芯THz偏振分束器進(jìn)行了填充(如圖4(a)所示,仿真計(jì)算表明對(duì)B芯進(jìn)行隔行填充折射率為1.28的液體時(shí),也可達(dá)到偏振分離),填充法是目前實(shí)現(xiàn)折射率匹配耦合的常用方法.如圖4(b)所示,采用填充法的THz偏振分束器,其分離長(zhǎng)度的變化趨勢(shì)和調(diào)整結(jié)構(gòu)法的THz偏振分束器相似,但在高頻處(f＞1.7 THz),本文所設(shè)計(jì)的偏振分束器的分離長(zhǎng)度明顯優(yōu)于用填充法設(shè)計(jì)的偏振分束器.

圖4 (a)采用填充法的THz偏振分束器結(jié)構(gòu);(b)填充法和調(diào)整結(jié)構(gòu)法所設(shè)計(jì)的THz偏振分束器的分離長(zhǎng)度隨頻率的變化Fig.4.(a)The structure of THz polarization splitter with filling methods;(b)splitting length of polarization splitter with two methods versus frequency.

損耗是衡量偏振分束器性能的另一重要指標(biāo).圖5(a)首先給出了器件的吸收損耗系數(shù)隨頻率的變化,該參數(shù)定義如下：

式中αeff是器件的損耗系數(shù);αm(r)是是光纖基底材料的體吸收系數(shù);Abackground和A∞分別是表示光纖橫截面區(qū)域和整個(gè)平面的面積;n(r)是基底材料的有效折射率;E和H分別是電場(chǎng)強(qiáng)度和磁場(chǎng)強(qiáng)度.由圖5(a)可見(jiàn)：不管填充法還是調(diào)整結(jié)構(gòu)法設(shè)計(jì)的THz偏振分束器,其吸收損耗系數(shù)均隨著頻率的增加而單調(diào)增加;在0.5 THz處,對(duì)于x,y偏振模而言,兩器件的吸收損耗系數(shù)都小于0.1 dB/cm.

圖5(b)給出了器件的吸收損耗隨頻率的變化,該參數(shù)定義如下：

其中Mloss是器件的吸收損耗,Lc器件的分離長(zhǎng)度.如圖5(b)所示：在整個(gè)頻率范圍內(nèi),兩種方法所設(shè)計(jì)的THz偏振分束器的吸收損耗變化趨勢(shì)一致,在低頻處均隨著頻率的增加而增加,到達(dá)峰值后逐漸減小.且正如圖4(b)和圖5(a)所示,由于低頻處兩種方法所設(shè)計(jì)的THz偏振分束器,其分離長(zhǎng)度和器件吸收損耗系數(shù)差別很小,所以器件的吸收損耗也相差不大.但是,隨著頻率的增加,兩者的器件長(zhǎng)度均縮短,從而吸收損耗也隨之減少.其中,當(dāng)頻率大于1.7 THz后,調(diào)整結(jié)構(gòu)法設(shè)計(jì)的THz偏振分束器的吸收損耗明顯小于填充法設(shè)計(jì)的偏振分束器,且在2.3 THz頻率處,x,y兩個(gè)偏振的吸收損耗均小于0.35 dB.

圖5 采用填充法和調(diào)整結(jié)構(gòu)法設(shè)計(jì)的THz偏振分束器的損耗特性 (a)器件的吸收損耗系數(shù)隨頻率的變化;(b)器件的吸收損耗隨頻率的變化Fig.5. Loss characteristics of THz polarization splitter with filling method and adjusting structure method：(a)Absorption loss coefficient versus frequency;(b)absorption loss of the device versus frequency.

圖6給出了兩種方法設(shè)計(jì)的THz偏振分束器的x,y偏振模的消光比隨頻率的變化曲線.偏振模的分離程度可用消光比來(lái)衡量,其公式為

式中ER表示消光比,px和py分別表示x,y偏振模的輸出功率.由圖6可知,采用填充法的THz偏振分束器,其x,y偏振模的消光比隨著頻率的增加而增加(即：隨頻率的增加,偏振模分離越徹底);當(dāng)f=2.3 THz,x,y偏振模的消光比最大,分別為25.15和24.92 dB.采用調(diào)整結(jié)構(gòu)法的THz偏振分束器,x偏振模的消光比隨著頻率的增加而增加,最大為22.94 dB(f=2.3 THz);而y偏振模的消光比先隨著頻率的增加而增加,在f=2.1 THz達(dá)到峰值20.51 dB,隨后變小.其中,在2.3 THz處,x,y偏振模的消光比均較好,分別達(dá)到22.94和19.2 dB.在整個(gè)頻率范圍內(nèi),填充法的THz偏振分束器的消光比要比調(diào)整結(jié)構(gòu)法的優(yōu)越.

圖6 (網(wǎng)刊彩色)基于填充法和調(diào)整結(jié)構(gòu)法的THz偏振分束器中,x,y偏振模的消光比隨頻率的變化Fig.6.(color online)THz polarization splitter withfilling method and adjusting structure method,extinction ratios for x,y polarization modes versus frequency.

3 結(jié) 論

本文基于領(lǐng)結(jié)型多孔光纖設(shè)計(jì)雙芯THz偏振分束器,采用調(diào)整結(jié)構(gòu)法實(shí)現(xiàn)折射率反轉(zhuǎn)匹配耦合.分別討論了分離長(zhǎng)度、損耗及消光比隨頻率的變化,研究表明：其分離長(zhǎng)度和損耗的變化趨勢(shì)一致,均先隨頻率的增加而增加,達(dá)到峰值后減小;并且在整個(gè)頻率范圍內(nèi),分離長(zhǎng)度變化較為緩慢,控制在一個(gè)較小的范圍內(nèi)(0.428-2.5 cm);x,y偏振模的吸收損耗較小(10-1數(shù)量級(jí)),最小值分別為0.0327和0.0334 dB(f=0.5 THz);x,y偏振模消光比大致是隨頻率的增加而增加,在2.3 THz處分別達(dá)到22.94和19.2 dB.此外,與采用填充法的THz偏振分束器進(jìn)行比較,除去操作方便,制作簡(jiǎn)單外,在高頻處的性能指標(biāo)(分離長(zhǎng)度和損耗)更具優(yōu)勢(shì).而且所設(shè)計(jì)的THz偏振分束器由兩根聚合物光纖構(gòu)成,目前制作聚合物多孔光纖的方法較多,常用的有：堆積法,擠壓法,打孔法[17]等,故而加工方便,較易實(shí)現(xiàn).

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PACS:42.81.Gs,87.50.U-,42.81.Qb DOI:10.7498/aps.66.024209

Dual-core terahertz polarization splitter based on porous fibers with near-tie units?

Wang Jing-Li1)?Liu Yang1)Zhong Kai2)
1)(Department of Opto-Electronic Engineering,Nanjing University of Posts and Telecommunications,Nanjing 210023,China)
2)(Key Laboratory of Optoelectronic Information Science and Technology(Ministry of Education),Tianjin University,Tianjin 300072,China)

1 August 2016;revised manuscript

18 October 2016)

Terahertz(THz)radiation,which is defined as the electromagnetic wave with a frequency ranging from 0.1 THz to 10 THz,has attracted widespread attention in recent years because of its unique possibilities in many fields.Highperformance THz polarization splitter,a key device in THz manipulation,is of great significance for studying the THz devices.In the present paper,a novel dual-core THz polarization splitter is proposed,which is based on porous fiber with near-tie units.The introduction of near-tie units into the fiber core can enhance asymmetry to realize high mode birefringence.And the results show that the porous THz fiber exhibits high birefringence at a level of 10-2over a wide frequency range.An index converse matching coupling(ICMC)method,which exhibits several advantages(such as short splitting length,high extinction ratio,low loss,and broad operation bandwidth),is used to allow for the coupling of one polarization mode within a broad operation band,while the coupling of the other polarization component is effectively inhibited.The splitting length is equal to one coupling length of x-or y-polarization component for which inter-core coupling occurs,and short splitting length means low transmission loss.Unlike the reported filling method,an adjusting structure method is proposed in the paper to satisfy the condition of index converse matching coupling.The full vectorfinite element method(FEM),which is based on the variational principle and the subdivision interpolation,is used to analyze the guiding properties of the proposed THz polarization splitter.The FEM is a widely used numerical method in physical modeling and simulation.Simulation results show that the THz polarization splitter operates within a wide frequency range of 0.5-2.5 THz.The splitting length does not exceed 2.5 cm in the whole frequency range and the minimum is only 0.428 cm.At 2.3 THz,the material absorption losses of x-and y-polarization are both less than 0.35 dB,and the extinction ratios for x-and y-polarization are 2.9 and 19.2 dB,respectively.Moreover,by comparing with a THz polarization splitter with filling method,the proposed THz polarization with adjusting structure method is easier to realize,the operating frequency range is wider,the splitting length is shorter,and the material absorption loss is lower.Finally,we note that the fabrication of such THz porous fiber designs could be realized by several methods,such as a capillary stacking technique,a polymer casting technique,a hole drilling technique,etc.

polarization splitter,terahertz,adjusting structure,porous fiber

:42.81.Gs,87.50.U-,42.81.Qb

10.7498/aps.66.024209

?光電信息技術(shù)教育重點(diǎn)實(shí)驗(yàn)室(天津大學(xué))開(kāi)放基金(批準(zhǔn)號(hào)：2014KFKT003)、國(guó)家自然科學(xué)基金(批準(zhǔn)號(hào)：61571237)、國(guó)家自然科學(xué)基金青年科學(xué)基金(批準(zhǔn)號(hào)：61405096)、區(qū)域光纖通信網(wǎng)與新型光通信系統(tǒng)國(guó)家重點(diǎn)實(shí)驗(yàn)室開(kāi)放基金資助項(xiàng)目(批準(zhǔn)號(hào)：2015GZKF03006)和江蘇省光通信工程技術(shù)研究中心資助項(xiàng)目(批準(zhǔn)號(hào)：ZSF0201)資助的課題.

?通信作者.E-mail：jlwang@njupt.edu.cn

*Project supported by the Key laboratory of Opto-electronic Information Technology,Ministry of Education(Tianjin University),China(Grant No.2014KFKT003),the National Natural Science Foundation of China(Grant No.61571237),the Young Scientists Fund of the National Natural Science Foundation of China(Grant No.61405096),the Open Fund of State Key Laboratory of Advanced Optical Communication Systems and Networks,Shanghai Jiao Tong University,China(Grant No.2015GZKF03006),and the Research Center of Optical Communications Engineering&Technology,Jiangsu Province,China(Grant No.ZSF0201).

?Corresponding author.E-mail：jlwang@njupt.edu.cn