光导纤维中光速的测定

光导纤维中光速的测定 【目的】 1. 学习光纤中光速测定的基本原理 2. 了解数字信号电光/光电变换及再生原理 3. 熟悉数字相位检测器原理、特性测试方法 4. 掌握光纤光速测定系统的调试技术 【仪器设备】 1( OFE—A型光纤传输及光电技术综合实验仪一套 2( 双迹示波器一台 【实验原理】 光纤中光速的测定是一个十分有趣的实验，通过这一实验能使学生亲身感受到光在介质中传播的真实物理过程和深刻了解介质折射率的物理意义。在通常的光纤光速测量系统中，对被测光波均采用正弦信号对光强进行调制。在此情况下，为了测出调制光信号通过一定长度光纤后引起的相位差，必须采用较为复杂的由模拟乘法电路及低通滤波器组成的相位检测器，这种相位检测电路的输出电压不仅与两路输入信号的相位差有关，而且也与两路输入信号幅值有关。这里提出一种采用方波调制信号，应用具有异或逻辑功能的门电路进行相差测量的巧妙方法。由这种电路所组成的相位检测器结构简单、工作可靠、相位——电压特性稳定。在光纤折射率n已知(或近似为1.5)的情况下，利用这种方法还可测定光纤长1 度。 一、 光纤传光原理及光在光纤中的速度 光导纤维的结构如图1示，它由纤芯和包层两部分组成，纤芯半径为α，折射率为n(p)，包层的1外半径为b，折射率为n，且n(p)> n。从物理光 212 学的角度考虑，光波实际上是一种振荡频率很高的电磁波，当光波在光导纤 维中传播时，光导纤维就起着一种光波导的作用。应用电磁场理论中,矢量和H矢量应遵从的麦克士威尔方程及它们在芯纤和包层面处应满足的边界条件可知:在光导纤维中主要存在着两大类电磁场形态。一类是沿光纤横载面呈驻波状，而沿光导纤维轴线方向为行波的电磁场形态，这种形态的电磁场其能量沿横向不会辐射，只沿轴线方向传播，故称这类电磁场形态为传导模式;另一类电磁场形态其能量在轴线方向传播的同时沿横向方向也有辐射，这类电磁场形态称为辐射模。利用光导纤维来传输光信息时就是依靠光纤中的传导模式。随着光导纤维芯径α的增加，光导纤维中允许存在的传导模式的数量也会增多，纤芯中存在多个传导模式的光纤称为多模光纤;当光纤芯径小到某一程度后，纤芯中只允许称为基模的一种电磁场形态存在，这种光纤就称为单模光纤。目前光纤通讯系统上使用的多模光纤纤芯直径为to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter town Shi, six people has a NI surname "spies" (enemy) midway tuogu landing tipsters, away from small town three in Xu, was day Elves Li Taishan Department captured, five people all killed, broken corpse sank Yu River in the. On January 18, 1943, when the Japanese army militiamen arrested 53 people in Wuzhen, transferred to the tomb of the former Japanese military police command (in the "Tai" Jiang Yuan), the 28th in Shenyang North of jade bang massacred them. Hu Maosheng knife wounds woke up in the middle of them, crawling out from the dead, in the tomb of Gao Changhai residents, saved by bing. Such as Hu Maosheng, Gao Changhai is a witness to this tragedy. (B) in memory of "massacre" Xu Youyong yan tomb is the copper law. Copper originally was called Luo yan Tomb, because Eastern Han dynasty distinguished prose poems my father yan Tan bogey was buried in the town's water. Has a long history here, cultural atmosphere, and there are many old houses, and left many people with lofty ideals. Business flourished here, is the hub of four townships of agricultural and sideline products, wine culture has a long history. The outskirts of the town has left, "Huang Chi", originally Yue water hides μm，包层外径为125μm。单模光纤的芯径为5,10μm范围内，包层外径也为125μm。在纤芯范围内50 折射率不随径向坐标ρ变化，即n(p)= n=常数的光纤，称为阶跃型光纤，否则称渐变型光纤。对于11 上述各种光纤的传光原理及传光性能的详尽理论分析见文献[1]。 当一束由光导纤维的入射端耦合到光导纤维内部之后，会在光纤内同时激励起传导模式和辐射模式，但经过一段传输距离，辐射模的电磁场能量沿横向方向辐射尽后，只剩下传导模式沿光纤轴线方向继续传播，在传播过程中只会因光导纤维纤芯材料的杂质和密度不均引起的吸收损耗和散射损耗外，不会有辐射损耗。目前的制造工艺能使光导纤维的吸收和散射损耗做到很小的程度，所以传导模式的电磁场能在光纤中传输很远的距离。 根据理论分析，光导纤维中光速的达式可近似为: c V,zn1 其中C 是光波在自由空间中的传播速度 . 二、光导纤维中光速的实验测定 1、测试原理 图2是测定光导纤维中光速的实验装置的方框结构图，图中各部分的功能分别是: 时钟信号源:由高电平“1”和低电平“0”构成的周期信号; LED:发光二极管，把电信号转换为光信号; 光纤:只能传输光信号; SPD:光电二极管，把光信号转换为电信号，可制作成光功率计; 再生电路:把光电二极管转换出的电信号还原为与时钟信号源同周期的时钟信号，本实验通过调 节“再生调节”旋钮实现; 检测仪器包括示波器和相位检测器， 示波器:观察再生信号的波形，与“再生调节”旋钮配合，使再生信号与时钟信号源的时钟信号 同周期;测量这两种信号的延时; 相位检测器:配合示波器使用，当由示波器观察到再生信号与时钟信号源的时钟信号同周期时， 测量两信号的相位差，用以更精确计算延时; tskirts of the town has left, "Huang Chi", originally Yue water hidesThe ousiness flourished here, is the hub of four townships of agricultural and sideline products, wine culture has a long history. ater. Has a long history here, cultural atmosphere, and there are many old houses, and left many people with lofty ideals. Buwn's wy was called Luo yan Tomb, because Eastern Han dynasty distinguished prose poems my father yan Tan bogey was buried in the toao Changhai is a witness to this tragedy. (B) in memory of "massacre" Xu Youyong yan tomb is the copper law. Copper originallng, Gup in the middle of them, crawling out from the dead, in the tomb of Gao Changhai residents, saved by bing. Such as Hu Maoshe lice command (in the "Tai" Jiang Yuan), the 28th in Shenyang North of jade bang massacred them. Hu Maosheng knife wounds wokeary po8, 1943, when the Japanese army militiamen arrested 53 people in Wuzhen, transferred to the tomb of the former Japanese militin Xu, was day Elves Li Taishan Department captured, five people all killed, broken corpse sank Yu River in the. On January 1Shi, six people has a NI surname "spies" (enemy) midway tuogu landing tipsters, away from small town three to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter town2 图2 测定光导纤维中光速实验装置的方框图 在该图中由调制信号源提供的周期为 T，占空比为50%的方波时钟信号对半导体发光二极管LED的发光光强进行调制，调制后的光信号经光导纤维、光电检测器件和信号再生电路再次变换成一个周期为T、占空比为50%的方波序列，但这一方波序列相对于调制信号源输出的原始方波序列有一定的延时，这一延时包括了LED驱动与调制电路和光电转换及信号再生电路引起的延时，也含有我们要测定的调制光信号在给定长度光纤中所经历的时间在内。实验中采用“双光纤比较法”。即:保持电路状态不变，分别测出信号通过、的延时、，则有: ,,,,1122 ,,,12 ,Vz,,,12 2、相差测量方法 如果把再生信号和作为参考信号的原始调制信号接到一个具有异或逻辑功能的逻辑电路的两个输入端，则在0,π的相移所对应的延时范围(即0,T/2)内，该电路的输出波形就是一个周期为T/2，但脉宽与以上两路信号的相对延时成正比的方脉冲序列(如图3示)，这一脉冲序列的直流分量的电平值就与以上两路输入信号的相对延时成正变关系。用示波器可观察到异或门输出的占空比随延时变化的方脉冲序列，用直流电压表可以测出这一方脉冲的直流分量的电平值。 图3 相位检测器原理图 利用异或逻辑电路所组成的相位检测电路的相移——电压特性曲线如图4所示，其中V是两输入信号相L差为2nπ、延时为nT(n=0，1，2„„)时相位检测器输出的低电平值，V是两输入信号相差为(2n+1)H π、延时为(2n+1)T/2(n=0，1，2„„)时相位检测器输出的高电平值，在0,π的相移范围内由异或门组成的相位检测电路输出的方脉冲序列的 直流分量的电平值 与两输入信号之间的关系为: V,VL,,,, V,VHL essideline products, wine culture has a long history. The outskirts of the town has left, "Huang Chi", originally Yue water hidl and any old houses, and left many people with lofty ideals. Business flourished here, is the hub of four townships of agriculturae poems my father yan Tan bogey was buried in the town's water. Has a long history here, cultural atmosphere, and there are mpros " Xu Youyong yan tomb is the copper law. Copper originally was called Luo yan Tomb, because Eastern Han dynasty distinguishedChanghai residents, saved by bing. Such as Hu Maosheng, Gao Changhai is a witness to this tragedy. (B) in memory of "massacre of Gaoof jade bang massacred them. Hu Maosheng knife wounds woke up in the middle of them, crawling out from the dead, in the tomb transferred to the tomb of the former Japanese military police command (in the "Tai" Jiang Yuan), the 28th in Shenyang North zhen, l killed, broken corpse sank Yu River in the. On January 18, 1943, when the Japanese army militiamen arrested 53 people in Wumidway tuogu landing tipsters, away from small town three in Xu, was day Elves Li Taishan Department captured, five people alShi, six people has a NI surname "spies" (enemy) to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter town3 对应的延时关系 ,LVVT,,,() ,hLVV2 其中?t为两路信号的相对延时，T为调制信号的周期，可用示波器测得。 利用(4)式我们就可根据由以上测量系统所获得的实验数据计算出调制光信号在光导纤维中传输时所经历的时间。在具体测量时，先用一长度为L的长光纤接入测量系统，测得相位检测器输出的直流分1 的电平值为V ，然后用长度L的短光纤代替长光纤，并在保持测量系统电路参数不变(也即保证两量 12 种测量状态下，由于电路方面因素引起的延时一样)的状态下，测得相位检测电路输出的直流分量的电平值为V ，则调制信号在(L—L)长度的光纤中传播时所经历的时间就等于: 212 ,VVT12 ,,t2,VVhL 对应的传播速度为: 2(L,L)(V,V)L,L1212hLV,, ztT(V,V)12 3、调制信号的光电转换及再生 由传输光纤输出的数字式光信号在接收端经过硅光电二极管SPD和再生电路(如图5示)把光信号变换成数字式电信号。图5示电路的如下: 当数字传输系统处於空闲状态时，传输光纤中无光，硅光电二极管无光电流流过，这时只要R和RCb2的阻值适当，晶体管BG2就有足够大的基极电流I注入，使BG2处于深度饱和状态，因此它的集电极和b 发射极之间的电压极低，既使经过后面的放大电路高倍放大后也会使反相器IC2的输出电压维持在高电平状态，满足了集成芯片8251A数据接收端RxD在空闭状态时 tskirts of the town has left, "Huang Chi", originally Yue water hidesThe ousiness flourished here, is the hub of four townships of agricultural and sideline products, wine culture has a long history. ater. Has a long history here, cultural atmosphere, and there are many old houses, and left many people with lofty ideals. Buwn's wy was called Luo yan Tomb, because Eastern Han dynasty distinguished prose poems my father yan Tan bogey was buried in the toao Changhai is a witness to this tragedy. (B) in memory of "massacre" Xu Youyong yan tomb is the copper law. Copper originallng, Gup in the middle of them, crawling out from the dead, in the tomb of Gao Changhai residents, saved by bing. Such as Hu Maoshe lice command (in the "Tai" Jiang Yuan), the 28th in Shenyang North of jade bang massacred them. Hu Maosheng knife wounds wokeary po8, 1943, when the Japanese army militiamen arrested 53 people in Wuzhen, transferred to the tomb of the former Japanese militin Xu, was day Elves Li Taishan Department captured, five people all killed, broken corpse sank Yu River in the. On January 1Shi, six people has a NI surname "spies" (enemy) midway tuogu landing tipsters, away from small town three to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter town4 图5调制信号的光电转换及再生 应为高电平的要求。当系统进行数据传输时，对于8251芯片为异步传输工作方式情形，所传数据流的结构是由起始位(S)、被传数据(D,D)、偶校验位(C)和终止位(E)等共11位码元组成，第一位是起07 始位，为低电平、偶校验位C的电平状态与被传数据D,D中的“1”电平个数的奇偶数有关，奇数时，07 该位为高电平，偶数时为低电平、终止位E为高电平。当传输“0”码元时，发送端的LED发光，光电二极管有光电流I产生，它是从SPD的负极流向正极，对BG2的基极电流具拉电流作用，使BG2的基极电3 流减小。由于SPD结电容、其出脚联接线的线间电容以及BG2基——射极间杂散电容的存在(在图5中用C表示以上三种电容的总效应)，使得BG2基极电流的这一减小过程不是突变的，而是按某一时间常数a 的指数规律变化。随着BG2基极电流的减小，BG2逐渐脱离深度饱和状态，向浅饱和状态和放大区过渡，其集电极——发射极间的电压V也开始按指数规律逐渐上升，由于后面的放大器放大倍数很高，故还未ce 等到V上升到其渐近值，放大器输出电压就达到使反相器IC2状态翻转的电压值，这时IC2输出端(即ce 8251A的数据接收端)为低电平。在下一个“1”码元到来时，接收端的SPD无光电流，BG2的基极电流I又按指数规律逐渐增加，因而使BG2原本按指数规律上升的V在达到某一值时就停止上升，并在以后bce 按指数规律下降，V下降到某一值后，IC2由低电平翻转成高电平。适当调节发送端LED的工作电流(即ce 改变LED发光时的光强)和接收端SPD无光照射时BG2饱和深度间的配匹状况，既使在被传数据流中“1”码和“0”码随机组合的情况下，也能使光电检测和再生电路输出的数字信号的码元宽度(即持续时间)与发送端所发送的数字信号的码元宽度相等或相差在无误码判决所允许的范围内。有关数字信号光电检 [5]测和再生电路更为详尽的理论分析见参考文献。 【实验步骤】 1、 用“示波器法”测定光速 essideline products, wine culture has a long history. The outskirts of the town has left, "Huang Chi", originally Yue water hidl and any old houses, and left many people with lofty ideals. Business flourished here, is the hub of four townships of agriculturae poems my father yan Tan bogey was buried in the town's water. Has a long history here, cultural atmosphere, and there are mpros " Xu Youyong yan tomb is the copper law. Copper originally was called Luo yan Tomb, because Eastern Han dynasty distinguishedChanghai residents, saved by bing. Such as Hu Maosheng, Gao Changhai is a witness to this tragedy. (B) in memory of "massacre of Gaoof jade bang massacred them. Hu Maosheng knife wounds woke up in the middle of them, crawling out from the dead, in the tomb transferred to the tomb of the former Japanese military police command (in the "Tai" Jiang Yuan), the 28th in Shenyang North zhen, l killed, broken corpse sank Yu River in the. On January 18, 1943, when the Japanese army militiamen arrested 53 people in Wumidway tuogu landing tipsters, away from small town three in Xu, was day Elves Li Taishan Department captured, five people alShi, six people has a NI surname "spies" (enemy) to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter town5 (1) 光功率/电压指示开关向上，小数点置十位; (2) 短接“时钟信号”和“调制输入D”; (3) 光纤信道输入端的LED“接入光源器件的数字信号调制及驱动电路”中LED的插孔; (4) 光纤信道输出端的SPD接入“数字信号的光电转换与再生调节电路”中的SPD插孔; (5) 双迹示波器的CH1通道接“调制输入D”;CH2通道接“再生输出”插孔;同步触发源选择CH1; (6) 把“光电转换与再生电路”中的SPD切换开关向左; (7) 分别接入长为L1、L2的长、短光纤，比较P1、P2的大小; (8) 接入光功率小的光纤(假如为L1)，调W2使P=25uw(如P的最大值小于25，则调到最大); (9) 调SPD切换向右，观察并比较CH1 CH2两通道波形，CH1的波形为占空比为50%，周期为16us 的方波; (10) 调“再生调节”旋钮，使CH2的波形也是一个占空比50%，具有同一周期的方波，读出并记录 D; CH2相对CH1的延迟时间在示波器上的长度1 tskirts of the town has left, "Huang Chi", originally Yue water hidesThe ousiness flourished here, is the hub of four townships of agricultural and sideline products, wine culture has a long history. ater. Has a long history here, cultural atmosphere, and there are many old houses, and left many people with lofty ideals. Buwn's wy was called Luo yan Tomb, because Eastern Han dynasty distinguished prose poems my father yan Tan bogey was buried in the toao Changhai is a witness to this tragedy. (B) in memory of "massacre" Xu Youyong yan tomb is the copper law. Copper originallng, Gup in the middle of them, crawling out from the dead, in the tomb of Gao Changhai residents, saved by bing. Such as Hu Maoshe lice command (in the "Tai" Jiang Yuan), the 28th in Shenyang North of jade bang massacred them. Hu Maosheng knife wounds wokeary po8, 1943, when the Japanese army militiamen arrested 53 people in Wuzhen, transferred to the tomb of the former Japanese militin Xu, was day Elves Li Taishan Department captured, five people all killed, broken corpse sank Yu River in the. On January 1Shi, six people has a NI surname "spies" (enemy) midway tuogu landing tipsters, away from small town three to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter town6 (11) 保持W2和“再生调节”不变，接入光功率大的光纤(L2)，调节L2与SPD的耦合光功率使 CH2的波形达到占空比为50%，测CH2相对CH1的延迟时间在示波器上的长度; D2 L,LL,L1212(12)计算光纤中光速: ， 写出不确定度式。 V,,Z,6,,,2(D,D)，101212 2.用相位检测器方法测定光速 essideline products, wine culture has a long history. The outskirts of the town has left, "Huang Chi", originally Yue water hidl and any old houses, and left many people with lofty ideals. Business flourished here, is the hub of four townships of agriculturae poems my father yan Tan bogey was buried in the town's water. Has a long history here, cultural atmosphere, and there are mpros " Xu Youyong yan tomb is the copper law. Copper originally was called Luo yan Tomb, because Eastern Han dynasty distinguishedChanghai residents, saved by bing. Such as Hu Maosheng, Gao Changhai is a witness to this tragedy. (B) in memory of "massacre of Gaoof jade bang massacred them. Hu Maosheng knife wounds woke up in the middle of them, crawling out from the dead, in the tomb transferred to the tomb of the former Japanese military police command (in the "Tai" Jiang Yuan), the 28th in Shenyang North zhen, l killed, broken corpse sank Yu River in the. On January 18, 1943, when the Japanese army militiamen arrested 53 people in Wumidway tuogu landing tipsters, away from small town three in Xu, was day Elves Li Taishan Department captured, five people alShi, six people has a NI surname "spies" (enemy) to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter town7 (1) 在用示波器法测延时的基础上，将时钟信号接相位检测器的参考信号插孔;将再生输出信号接 相位检测器的被测信号，将两GND接通。 (2)重复示波器法(6)~(11)，记录L1,L2的相移读数V1,V2 方波信号同时输入参考信号和被测信号，记下相移VL (3)将16,s (4)将被测信号变为16 反向方波信号，记下相移 Vh ,s ,VVT12(5)计算延时(T为占空比为50%的时钟信号的周期) ,,,,2,VVHL ,(6)计算光纤中光速VZ=(L1-L2)/ ，写出不确定度标准式。 , 【数据记录及处理】 1、 示波器法测光速 U,0.1m ， L,L,400mt/div,2,s/div(L,L)1212 U,U, div div div D,D,DD1212 L,LL,L1212,,D，t/divV,, : Z,6,,,2(D,D)，101212 ,V,V,V222ZZZU[(U](U)(U),，， V(L,L)DDZ1212,,,,(LL)DD1212 标准式: 2、 相位检测法测光速 The ousiness flourished here, is the hub of four townships of agricultural and sideline products, wine culture has a long history. ater. Has a long history here, cultural atmosphere, and there are many old houses, and left many people with lofty ideals. Buwn's wy was called Luo yan Tomb, because Eastern Han dynasty distinguished prose poems my father yan Tan bogey was buried in the toao Changhai is a witness to this tragedy. (B) in memory of "massacre" Xu Youyong yan tomb is the copper law. Copper originallng, Gup in the middle of them, crawling out from the dead, in the tomb of Gao Changhai residents, saved by bing. Such as Hu Maoshe lice command (in the "Tai" Jiang Yuan), the 28th in Shenyang North of jade bang massacred them. Hu Maosheng knife wounds wokeary po8, 1943, when the Japanese army militiamen arrested 53 people in Wuzhen, transferred to the tomb of the former Japanese militin Xu, was day Elves Li Taishan Department captured, five people all killed, broken corpse sank Yu River in the. On January 1Shi, six people has a NI surname "spies" (enemy) midway tuogu landing tipsters, away from small town three to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter towntskirts of the town has left, "Huang Chi", originally Yue water hides8 ， U,0.1mL,L,400m(L,L)1212 V,V,V,V,hL12 U,U,U,U,0.1T,16,SVVVVLh12 2(L,L)(V,V)12hL V,zT(V,V)12 ,V,V,V,V,V22222ZZZZZ U[U](U)(U)(U)(U),，，，，V(L,L)VVVVZ12hL21,(L,L),V,V,V,V12hL21 标准式: essideline products, wine culture has a long history. The outskirts of the town has left, "Huang Chi", originally Yue water hidl and any old houses, and left many people with lofty ideals. Business flourished here, is the hub of four townships of agriculturae poems my father yan Tan bogey was buried in the town's water. Has a long history here, cultural atmosphere, and there are mpros " Xu Youyong yan tomb is the copper law. Copper originally was called Luo yan Tomb, because Eastern Han dynasty distinguishedChanghai residents, saved by bing. Such as Hu Maosheng, Gao Changhai is a witness to this tragedy. (B) in memory of "massacre of Gaoof jade bang massacred them. Hu Maosheng knife wounds woke up in the middle of them, crawling out from the dead, in the tomb transferred to the tomb of the former Japanese military police command (in the "Tai" Jiang Yuan), the 28th in Shenyang North zhen, l killed, broken corpse sank Yu River in the. On January 18, 1943, when the Japanese army militiamen arrested 53 people in Wumidway tuogu landing tipsters, away from small town three in Xu, was day Elves Li Taishan Department captured, five people alShi, six people has a NI surname "spies" (enemy) to West rear, to County Shen Liqun report guerrillas breakout Hou of action situation, boat via Zhejiang wuxing daughter town9