第一章 无源光网络的发展现状和趋势
1996年，13家大型网络运营商及设备供应商成立了全业务接入网联盟，其目标是建立PON的通用标准。ITU-T于1996年6月通过了第一个有关PON的国际建议G.982，后又在1998年提出了G.983建议，制定了基于ATM技术的PON的通用标准，选择ATM和PON分别作为网络协议和网络平台，将ATM作为在PON之上的第二层的帧封装技术，因为ATM被看作是能够提供各种类型通信的唯一协议，而PON是最经济的宽带光纤解决方案。APON可以通过利用ATM的集中和统计复用，再结合无源分光器对光纤和光线路终端的共享作用，使成本比传统的以电路交换为基础的PDH/SDH接入系统低20%~40%。APON能实现网上的设备共享，业务透明，可以支持良好的网络管理系统，实现多种业务的综合，这使它成为一种比较典型的光纤接入网。但由于APON存在协议复杂度高和数据传输效率低等问题，始终没有得到广泛的应用。EPON是IEEE于2000年11月提出的接入技术，并于2004年6月将IEEE802.3ah EPON协议方案正式批准为该组织的标准之一。EPON技术是以太网技术和PON技术的结合。在EPON系统中，数据采用IEEE 802.3以太网协议，最小程度地扩充以太网MAC协议，传送的是可变长度的以太网数据帧，最长是1518字节，最短是64字节。EPON技术是性价比较高的宽带接入技术，充分继承了以太网技术价格低、协议灵活、技术成熟的优势，具有广泛的市场和良好的兼容性。GPON是FSAN组织于2001年开始起草的传输速率超过1Gbps的PON标准。GPON的主要技术特点是采用全新的传输汇聚层协议“通用成帧协议”（GFP, Generic Framing Protocol），实现多种业务码流的通用成帧规程封装，另一方面又保持了G.983中与PON协议没有自接关系的许多功能特性，如OAM(运行、管理、维护)管理，DBA(动态带宽分配)等。GPON的帧结构不基于任何指定类型的格式，而是基于各种用户信号原有的格式进行封装。因此，它不但能提供高速的比特率，而且可以支持多种接入业务，特别是能够有效的支持原有的数据流量和TDM业务，所以它的出现引起了业界的广泛关注。随着宽带通信技术的发展，时分复用技术由于受光电器件工作频率的限制，速率难以进一步提高，不能充分利用光纤巨大的带宽资源，另一方面，新的数字业务(如高质量视音频、虚拟现实等高带宽业务)需要高带宽，因此，基于波分复用(WDM)的无源光网络技术成为PON的发展方向之一。
WDM技术使用多个波长信道增加系统容量，具有支持大量ONU的能力。令人感兴趣的问题是如何将WDM技术引入无源光接入网中，为住户和办公用户提供更高的带宽。传统的方法已经实现了将1310nm和1550nm通信波长窗口的光复用到同一根光纤，这只是粗波分复用。而只有引入密集波分复用技术的PON才称作WDM-PON。
一种典型的结构是用波长路由器代替传统PON中的无源光分路器，OLT端采用多波长光源，为每一个用户都分配一个信道波长。具有波长路由功能的器件可以是波导光栅路由器、阵列波导光栅复用器或光学相位阵列等。在上行方向，各ONU发出独立的波长，在OLT处通过解复用器后由接收机阵列收到各自波长的光信号。另外一种选择是采用WDM/TDMA的混合技术，下行方向采用WDM技术，如前例。而上行方向只有一个波长，采用传统PON的时分复用多路接入(TDMA)方式。
WDM技术能充分挖掘光纤的带宽潜力，同时考虑到接入网作为国家信息基础结构的重要组成部分，WDM技术具有长远的生命力，融合WDM技术的WDM-PON将是满足未来用户带宽需求增长的有效方案。WDM-PON的另外一个优点是下行方向比TDM方式有更好的保密性能。目前WDM-PON的主要实现困难是缺乏价格适中的多波长光源。而上行方向实现WDM的困难则要更大一些，包括上行光源、温漂、串扰等问题。可以预计，随着波分复用器件成本的下降以及高带宽网络应用业务需求的不断出现，PON技术将最终不可避免的演进到基于波分复用的无源光网络(WDM-PON)。
第二章 PON中几种常用Coupler的分析和比较
根据制作方法不同，光滤波技术主要分为五类：多腔介质膜光滤波器(TFF)、熔融拉锥型(FBT)全光纤器件、奇偶交错滤波器(Interleaver)、平面集成阵列波导光栅(AWG)、紫外光写入光纤布拉格光栅(FBG)。
2.1多腔介质膜光滤波器(TFF)
薄膜干涉滤光片属于窄带滤光片的一种。薄膜滤光片本身具有非常低的温度系数，能够保证长期的稳定性。镀膜一般镀低折射率的λ/4膜和高折射率的λ/2膜，光从λ/4膜入射到λ/2膜表面时反射率最低，透射率最高。这样，每两个λ/4膜之间可以形成一个F-P腔，可以作为WDM滤波器。
3.1 PON传输系统设计
为了解决人们对接入网下行带宽的巨大需求，在无源光网络下行方向采用密集波分复用（DWDM）技术，为一个或多个用户分配固定波长，能够大大提高带宽利用率。而在上行方向，由于流量较小，仍然采用传统的时分复用方式。由于基于时分多址技术的上行接入方式已经比较成熟，因此本文主要进行下行方向传输系统的研究。
实现密集波分复用技术的关键是性能稳定的复用/解复用器件，传统的复用/解复用方法是在发送端使用波分复用器，在PON的远端节点使用解复用器。这时，远端节点既要完成下行方向密集波长的解复用，又要完成上行方向功率的合路，功能实现较复杂，不易维护，且能够满足上述要求的器件成本偏高。同时由于远端节点位置和环境因地而异，使解复用器难以稳定的工作。
基于上述考虑，本文设计了基于光纤Bragg光栅解复用器的复合PON传输系统结构，如图，在OLT端，下行链路使用波长稳定、温度性好、输出功率大的多波长激光器作为光源，发射波长符合WDM标准信道波长，信道间隔0.8nm。上行链路仍然使用突发模式接收机。通过调制后的信号光经过复用器耦合进入光纤，传输距离为20km，能够覆盖目前绝大多数用户接入范围。在远端节点，使用光无源分路器进行功率分路。
在用户端，下行链路使用光纤光栅作为解复用器。光纤光栅具有易耦合，附加损耗小、可微型化的特点，和环行器组合可以构成高边模抑制特性的窄带滤波器。通过调节光纤光栅的Bragg波长，使之与信道波长相对应，就可以完成对应的波长光信号解复用。采用1×N分路器+N个光纤光栅窄带滤波器就构成了1×N路解复用器。因此，在原有网络的基础，将远端1×N光分路器作为解复用器的一部分，而将光纤光栅滤波器布置到用户接收端。在上行方向仍然使用突发模式激光器作为光源。
这样，服务商可以在不改变网络结构的基础上，仅仅在OLT端和ONU端加入设备，就可以完成网络的升级。同时，光纤光栅可以集成在ONU内，避免室外环境的影响。对于传输网络，服务提供商可在带宽需要非常高的区域配置1×16分路，而在需求低的区域使用1×32或1×64分路器。只要PON的最大损耗预算没有超出，发射机和接收机不必为不同的分路比进行重新设计，集合比特率也将保持不变。
考虑到光环行器成本较高，本实验采用了光耦合器＋光纤Bragg光栅组成带通滤波器，其反射谱带宽决定了该信道宽度。实测光纤Bragg光栅传输损耗均小于0.5dB，光纤Bragg光栅滤波器插入损耗8.7dB。主要来自于光纤耦合器的插入损耗。采用光功率计直接测量输入和输出的光功率值，得到WDM解复用器的插入损耗为12.5dB。
以光纤Bragg光栅为基础的光通信滤波器，具有极低的传输损耗和较长的相互作用距离，并且易于与光纤系统耦合和接续，在与半导体激光器等光通信光源耦合时也可实现高效率，因此其应用领域广泛。尤其可用于密集波分复用光通信系统中作为去除杂散光频率成分的波长选择滤波器。光纤光栅具有制作简单、偏振不敏感、光谱响应特性(光谱形状、带宽、反射率、边模抑制比)的动态可调、附加损耗小、器件微型化、能与光纤很好的耦合、不受环境尘埃影响等一系列特殊优异性能，尤其具有优良的窄带、平顶的光谱响应特性，使用光纤光栅可制成光纤激光器、密集波分复用/解复用器、光分插复用器OADM，色散补偿器、滤波器、光放大器的增益平坦化、超窄光脉冲产生器、泵浦激光器的波长锁定等高性能光通信器件，目前正向实用化方向发展。光纤光栅将使全光纤器件的研制成为可能，因而所谓的全光纤一维光子集成(即将各种全光纤器件集成在一条光纤里，形成诸多集成型光纤信息系统)也将成为现实。
3.2 系统传输性能的仿真
在理论设计的基础上，通过计算机进行仿真分析，可以对系统的设计进行优化，对实验研究有一定的指导作用。本文使用的是Optiwave公司的OptiSystem光通信系统专业仿真软件，具有强大而新的环境仿真和组件及系统的真实等级定义。广泛应用于组件到物理层次中系统标准的光通信系统设计和计划；CATV or TDM/WDM网络设计；SONET/SDH环形网络设计；发射器、通道、放大器、接收器设计；色散分析等领域。通过对各仿真器件的性能参数的自定义来改变系统性能，最后通过系统的监视器可以生成光谱、眼图等可视化图样，以此观测系统性能。
Chapter passive optical network status and trends of development
In 1996, 13 major network operators and equipment suppliers set up a Full Service Access Network Alliance, whose goal is to establish common standards for PON. ITU-T in June 1996 adopted the first international recommendations on PON''s G.982, then in 1998 made G.983 recommendation to develop ATM-PON based on common criteria, select ATM, and PON, respectively, as network protocols and network platform that will be on top of ATM as a second layer of PON frame packaging technology, because ATM is seen as the only able to provide various types of communication protocols, while PON is the most economical broadband fiber optic solutions. APON can be concentrated through the use of ATM and statistical multiplexing, combined with passive splitter to the fiber and optical line terminal to share the role, so that cost than traditional, circuit-switched based on PDH / SDH access systems by 20% to 40% . APON to achieve line of equipment to share, business transparency, can support a good network management system to achieve a comprehensive variety of services, which makes it a typical fiber-optic access network. However, there is an agreement APON complexity of high and low efficiency of data transmission problems, has not been widely used. EPON is the IEEE in November 2000 made access to technology, and in June 2004 will be IEEE802.3ah EPON protocol officially approved the program for the organization in the world. EPON Ethernet technology is a combination of technology and PON technology. In the EPON system, the data is IEEE 802.3 Ethernet protocol, the minimum degree of expansion of the Ethernet MAC protocol, transmission is the variable-length Ethernet frames, the longest is 1518 bytes, the shortest is 64 bytes. EPON technology is a high cost-effective broadband access technologies, fully inheriting the Ethernet technology, low price, the agreement flexible advantage of mature technology, with extensive market and good compatibility. GPON is the FSAN the organization in 2001 to begin drafting the transmission rate of more than 1Gbps of the PON standards. The main technical features GPON adoption of a new transport layer protocol convergence, "Generic Framing Protocol" (GFP, Generic Framing Protocol), to achieve a variety of business streams of the Generic Framing Procedure package, on the other hand maintained a G.983 with PON agreement does not take the relationship from a number of features, such as OAM (operation, management, maintenance) management, DBA (dynamic bandwidth allocation), etc.. GPON frame structure is not based on any of the specified type of format, but on the various user signals the format of the original package. Therefore, it not only provides high-speed bit rate, and can support multiple access services, in particular, can be effectively supported by existing data traffic and TDM services, so it caused the emergence of the industry''s attention. With the development of broadband communications technology, time-division multiplexing technology, optoelectronic devices operating frequency due to the restrictions, the rate is difficult to further improve and can not take full advantage of a huge fiber bandwidth resources, on the other hand, the new digital services (such as high-quality video and audio, virtual reality high-bandwidth services) that require high bandwidth, therefore, based on wavelength division multiplexing (WDM) passive optical network technology as the direction of development of one of the PON.
WDM technology uses multiple wavelength channels to increase system capacity, with the ability to support a large number of ONU. Interesting question is how to introduce WDM-PON technology in the household and office users with higher bandwidth. The traditional approach has been achieved to 1310nm and 1550nm communication wavelength window is used with a fiber liberation, this is only coarse wavelength division multiplexing. Only the introduction of dense wavelength division multiplexing PON technology, it is called WDM-PON.
A typical structure is to replace the traditional PON using wavelength router in the passive optical splitter, OLT-side multi-wavelength light source for each user is assigned a channel wavelength. With a wavelength routing device can be a waveguide grating router, arrayed waveguide grating multiplexer or optical phase arrays. In the upstream direction, each ONU to issue a separate wavelength, in the OLT through the demultiplexer at the receiver array after the receipt of the respective wavelengths of light signals. Another option is to use WDM / TDMA hybrid technology, downstream direction using WDM technologies, such as a precedent. The upstream direction there is only one wavelength, using traditional Time Division Multiplexing PON Multiple Access (TDMA) mode.
WDM technology can fully exploit the bandwidth potential of optical fiber, taking into account the access network as a national information infrastructure, an important component, WDM technology has a long-term vitality and integration of WDM technology, WDM-PON will be to meet future growth in demand for user bandwidth, effective programs . WDM-PON downstream Another advantage is that the direction of TDM methods are better than the performance of confidentiality. WDM-PON is currently the principal realization of the difficulties is the lack of affordable multi-wavelength light sources. The upstream direction will have to realize the difficulties of WDM larger, including the upstream source, drift, crosstalk and other issues. It can be expected, with the decline in the cost of WDM devices and high-bandwidth network applications of the emerging business needs, PON technology will ultimately inevitable evolution of WDM-based Passive Optical Network (WDM-PON).
Chapter II PON of several commonly used for analysis and comparison of Coupler
According to production methods, optical filtering techniques can be divided into five categories: Multi-cavity dielectric thin film optical filter (TFF), fused biconical taper (FBT) all-optical devices, interleaver (Interleaver), planar integrated arrayed waveguide grating ( AWG), UV-written fiber Bragg grating (FBG).
More than 2.1 cavity dielectric thin film optical filter (TFF)
Thin-film interference filters are a kind of narrow-band filter. Thin-film filter itself has a very low temperature coefficient to ensure long-term stability. Coating with low refractive index of the general-plated λ / 4 film and high refractive index of the λ / 2 film, the light from the λ / 4 film incident to the λ / 2 film surface reflectivity of the lowest transmission rate is highest. In this way, every two λ / 4 film can be formed between a FP cavity, can serve as a WDM filter.
3.1 PON Transmission System Design
In order to solve people''s butt huge demand for network bandwidth downstream, in the downstream direction of passive optical networks using dense wavelength division multiplexing (DWDM) technology, one or more users assigned a fixed wavelength, can greatly enhance the bandwidth utilization. In the upstream direction, due to a smaller flow rate is still using traditional time-division multiplexing method. As the technology based on TDMA uplink access methods are already quite mature, this paper mainly for downlink direction of transmission systems.
Dense Wavelength Division Multiplexing technology to achieve stable performance, the key to the reuse / de-multiplexer, the traditional multiplexing / demultiplexing method is the use of wavelength division multiplexer at the sending end, in the remote node in the use of PON demultiplexer Appliances. At this time, the remote node, it is necessary to complete the downstream direction of dense wavelength demultiplexing, but also the completion of upstream power combiners, functions to achieve more complex and difficult to maintain, and able to meet the above requirements of high cost devices. Same time, because the remote node location and the environment vary, so that demultiplexer is not stable work.
Based on the above considerations, this paper is designed based on fiber Bragg grating demultiplexer composite PON transmission system structure, shown in Figure, in the OLT side, the downlink using the wavelength stability, the temperature is good, the output power of a large multi-wavelength laser as the light source, meet the standards of emission wavelength WDM channel wavelength, channel spacing 0.8nm. Uplink still using burst mode receiver. Through the modulated signal through the multiplexer coupled into optical fiber, transmission distance 20km, to cover the present range of the vast majority of user access. In the remote node, the use of optical passive splitter to power splitter.
The user side, the downlink using the fiber Bragg grating as a demultiplexer. Fiber Bragg Grating coupled with the easy, the additional loss is small, can be miniaturized characteristics, and the circulator combination can constitute a high side-mode suppression characteristics of narrow-band filters. By adjusting the fiber Bragg grating wavelength, so that the wavelength that corresponds with the channel, they can complete the corresponding wavelength demultiplexing optical signals. Using 1 × N splitter + N a fiber Bragg grating narrow-band filter constituted a 1 × N demultiplexer way. Therefore, the basis of the original network, the remote 1 × N optical splitter as a demultiplexer a part of the layout of the fiber grating filter to the user receiver. In the upstream direction are still using burst-mode laser as light sources.
In this way, service providers can be achieved without changing the network structure, based on the only side in the OLT and ONU-side by adding equipment, you can complete the network upgrade. Meanwhile, the Fiber Bragg Grating can be integrated in the ONU and avoid outdoor environment. For the transmission network, service providers may require very high bandwidth, regional configurations 1 × 16 splitter, while in regions with low demand for the use of 1 × 32 or 1 × 64 splitter. PON as long as the budget did not exceed the maximum loss, the transmitter and receiver do not have the splitter for different than the re-design, a collection of bit-rate will remain unchanged.
Taking into account the high cost of optical circulator, the experiment using fiber Bragg grating coupler + composition of band-pass filter, the reflection spectrum bandwidth determines the width of the channel. Measured fiber Bragg grating transmission loss less than 0.5dB, fiber Bragg grating filter insertion loss of 8.7dB. Mainly from the optical fiber coupler insertion loss. Direct measurement using optical power meter input and output optical power values obtained WDM demultiplexer insertion loss of 12.5dB.
With fiber Bragg grating-based optical filter, with extremely low transmission loss and a longer interaction distance, and easy coupling with optical fiber systems and the continuation, with the semiconductor lasers and other optical sources may also be coupled to achieve high efficiency , so its widespread applications. In particular, can be used for dense wavelength division multiplexing optical communication system as to remove stray light frequency components of the wavelength selection filter. Fiber grating has produced a simple, polarization is not sensitive to spectral response characteristics (spectral shape, bandwidth, reflectivity, side-mode suppression ratio) of the dynamic adjustable, additional loss of small, device miniaturization, coupled with the fiber well, without Environmental impact of a series of special dust, excellent performance, especially with excellent narrowband, flat-topped spectral response characteristics, the use of fiber gratings can be made into fiber lasers, dense wavelength division multiplexing / de-multiplexer, optical add-drop multiplexer OADM , dispersion compensation, filters, optical amplifier gain flattening, ultra-narrow optical pulse generator, pump laser wavelength locking and other high-performance optics, are currently being practical direction. Fiber Bragg Grating will develop all-fiber devices possible, and thus the so-called all-optical one-dimensional photonic integrated circuit (about a variety of all-optical devices integrated in an optical fiber, the formation of a number of integrated optical fiber information system) will also become a reality.
3.2 Simulation of the system transmission performance
In the theoretical basis for the design, through computer simulation, it can optimize the design of the system, the experimental study of a certain guide. This article uses the company''s Optiwave professional OptiSystem optical communication system simulation software, has powerful new environment simulation and component and system level definition of the real. Is widely used in the system components to the physical level, the standard optical communication system design and planning; CATV or TDM / WDM network design; SONET / SDH ring network design; transmitter, channel, amplifier, receiver design; dispersion analysis and other fields. Through the simulation of device performance parameters of a custom to change the system performance, the final adoption of the system monitor can generate spectrum, eye diagram and other visual patterns, thus observing system performance.
