【作者】 郑史烈；

【导师】 陈抗生； 章献民；

【作者基本信息】 浙江大学 ， 电子科学与技术， 2003， 博士

【摘要】 塑料光纤由于制造简单、价格便宜、接续快捷等优点，已成为短距离宽带通信网的理想选择之一。随着塑料光纤应用领域的不断扩大，如何提高塑料光纤的带宽以及精确估算光纤的带宽对实际应用具有非常重要的意义。探究影响塑料光纤带宽的因素将有助于更好地选择制备塑料光纤的材料，改进生产工艺及合理采取措施以提高光纤带宽。塑料光纤的测量带宽远高于用石英光纤色散模型计算的理论带宽。对不同折射率分布的塑料光纤，其实测带宽高于理论带宽的原因迄今尚存在争议。此外，迄今还没有一个综合考虑所有影响塑料光纤带宽因素的色散模型。本文在大量实验基础上，系统地研究了不同折射率分布塑料光纤中影响带宽的众多因素，并建立了一个计算塑料光纤传输带宽的新的色散模型。 光纤的折射率分布决定光纤的传输性能，掌握光纤折射率分布是把握光纤特性的一个重要课题。本文根据聚焦法和近场折射法测量塑料光纤折射率分布的测量原理，设计并研制了两种测量装置。同时分析了用这两种方法测量塑料光纤的折射率分布的可行性及优缺点，并对实验结果做了误差分析。 利用光纤的远场辐射图，系统地研究了阶跃折射率分布型塑料光纤(SLPOF)和渐变折射率分布型塑料光纤(GI-POF)中的模式耦合。分析得到SI-POF和GI-POF的模式耦合长度。对SI-POF，入射光束的数值孔径(NA)越大，模式耦合越容易进行。入射光束NA=0.1时，光在光纤中传输5m之前没有发生模式耦合，在5～10m开始发生模式耦合；而入射光束NA=0.4时，光传输2.5m后就开始发生模式耦合。光在SI-POF中传输30m时耦合完成。对GI-POF，耦合在光传输1m后就开始发生，到光在光纤中传输10m左右完成。 比较SI-POF和GI-POF的模式耦合系数随模号增加的变化趋势，分析引起模式耦合的不同来源对不同折射率分布塑料光纤性能的影响。SI-POF的模式耦合系数随模号的增加先下降，到模号接近截止模的时候，模式耦合系数迅速上升；而GI-POF的模式耦合系数随模号的增加变化不大。实验结果还发现，SI-POF的模式耦合系数比GI-POF的模式耦合系数大3～4个数量级。根据塑料光纤的模式耦合理论，所有引起模式耦合的机理都可以归结为光纤折射率的随机扰动，只有当随机扰动的空间频率ω与光纤中某两个相邻模式之间的传播系数差相等时，才会引起模式耦合。基于这个理论，对折射率满足幂律分布的塑料光纤，幂指数α越大，引起模式耦合的随机扰动周期Λ的范围就越宽，模式耦合就越容易发生。因此，SI-POF比GI-POF更容易发生模式耦合。综合考虑光纤中模式耦合发生的几率和模式耦合发生的强弱发现，模式耦合对SI-POF性能的影响比对GI-POF的影响大得多。因此，分析SI-POF的色散模型时，必须考虑模式耦合的影响。而对GI-POF而言，由于模式耦合的作用相对较弱，可在一定程度上忽略模式耦合对带宽的作用。 用剪断法测量了SI-POF和GI-POF的微分模损耗曲线。结果表明，不同折射率分布的塑料光纤的微分模损耗随模号增加的变化趋势不同：对SI-POF而言，随着入射角度的增加，浙江大学博卜学位论文摘要激励起的模式的模号越大，其微分模损耗也越大;最高次模与最低次模的损耗差为80dB/km。对Gl一POF而言，m从了小于0.5的范围内.微分模损耗随模号变化不大;当模号尹刀为订大于0.5，光模的微分模损耗迅速增加，其最高次模与最低次模的损耗差可达350 dB/km。分析发现，如果光纤中激励的高次模损耗很大，它们传播一定距离后会变成辐射模消失。这样，在光纤输出端大大减小最高和最低次模间的相对延迟，光纤的带宽提高。因此，Gl一POF的微分模损耗特性是提高Gl一POF带宽的主要因素。而SI一POF中，微分模损耗对带宽的影响较小。 用频域相移法测量了SI一POF和GI一POF的微分模延迟。通过比较不同长度POF微分模延迟曲线的变化趋势，分析了模式祸合和微分模损耗对光纤模延迟的作用。结果表明:模式祸合和微分模损耗都减小塑料光纤的模延迟。对Sl一POF而言，模式祸合是降低光纤模间时延的主要因素;而对G卜 POF而言，微分模损耗在很大程度上降低了光纤中的模间时延。 采用频域法测量了不同数值孔径入射光束对SI一POF带宽的影响。实验结果发现，当光纤一长度小于祸合一民度时，入射光束的数值孔径越小，测量带宽值越高;当光纤长度超过祸合长度时，入射光束的数值孔径对测量带宽影响不大。与满入射下的理论带宽相比，由于模式祸合的存在，SI一POF带宽值均高于理论带宽。 全面分析了模式祸合对光纤带宽的影响，发现模式祸合对带宽有两个相反的作用:1)模式祸合的存在使得光在传播过程中激励新的模式，相当于增加了入射光束的数值孔径。这部分作用使带宽值下降;2)模式祸合的存在使得各模群间能量平均，传输速度平均，到光纤输出端时模群最大延迟减小。这部分作用增加SI一POF的带宽。不同注入条件下，模式祸合对带宽的影响是上述两个作用竞争的结果。当入射光束数值孔径小于被测光纤数值孔径时，模式祸合的第一个作用比第二个作用大，因此，实验带宽值比理论带宽值小;反之，当入对光束数值孔径接近或者超过被测光纤数值孔径时?更多还原

【Abstract】 Plastic optical fibers (POFs) are being considered for high-performance fiber links at very short distances because of their low-cost, robustness and ease of preparation and connection. With the extending application of POFs, to increase the bandwidth of POFs and predict it exactly are of great importance in the practice. To clarify the parameters that affect the bandwidth of POFs helps to choose the raw materials of POFs, to improve the preparation technology and to choose reasonable methods to increase the bandwidth of POFs. The theoretical bandwidth properties of the POFs with different refractive index profiles disagree to some extent with those measured. However, the reason of the disagreement still remains a puzzle. In addition, the precise predicting tool for bandwidth of POFs is still absence at present time. The simulation models developed for glass optical fibers (GOFs) systems some decades ago appear to be invalid in consideration of POFs. In this paper, the factors affecting the bandwidth of POFs with different refractive index profiles were systematically analyzed. A new dispersion model was developed to predict precisely the bandwidth of POFs.Determination of the refractive index profiles of POFs is of great importance in the evaluation of performance, and design, manufacture of the fibers. Two sets of the equipments for measuring the refractive index of POFs were designed and manufactured on the basis of the measuring principle of both the focusing method and the refracted near-field method. The applicability and the advantages and disadvantages of these two methods were analyzed theoretically and verified by experiments.With the help of the far-field pattern of the POFs, the mode coupling effects in both the SI-POF and the GI-POF were systematically analyzed. The coupling length of both the SI-POF and the GI-POF was obtained. As for the SI-POF, the larger the numerical aperture (NA) of the incident rays is, the easier the mode coupling happens. When the NA of the incident rays is 0.1, mode coupling happens after transmitting for 5-10 in; whilst the NA of the incident rays is 0.4, mode coupling happens only after transmitting for 2.5 m. After transmitting for about 30 m, mode coupling finishes and the equilibrium mode distribution is established. As for the GI-POF, mode coupling happens after transmitting for 1 m and finishes after transmitting for 10 in.The effect of different sources to cause mode coupling for the POFs with different refractive index profiles was analyzed by comparison of mode coupling coefficient of both the SI-POF and the GI-POF. It is shown that the mode couplingcoefficient of the SI-POF decreases first with the increasing mode number and then increases quickly when the mode number approaches the cutoff value. As for the GI-POF, the mode coupling coefficient does not change significantly with the increasing mode number. Furthermore, the mode coupling coefficient of SI-POF is 3-4 orders larger than that of the GI-POF. According to the mode coupling theory of POFs, all the mechanisms to cause mode coupling can be represented by a general refractive index perturbation. Coupling between neighboring modes occurs only if the perturbations with a spatial frequency component a> equals to the difference in the propagation constants between these two modes. Assuming that the refractive index profile of POFs meets the power-law, the larger the power index a is, the wider the perturbation period A spectrum of perturbations contributing to the mode coupling, that is, the easier the mode coupling happens. Considering both the probability and the level of the mode coupling, its effect on the properties of the SI-POF is found to be greater than on the GI-POF. Thus, when considering the dispersion property of the SI-POF, the effect of mode coupling cannot be neglected; while for the GI-POF, it is ignorable.The differential mode attenuation (DMA) profile of both the SI-POF and the GI-POF was obtained by the cut-back method. With the increasing mode number, the DMA properties of更多还原