Design of ONU and transceiver module in E-pon

Absrtact: This paper introduces the technical requirements of E-PON (Ethernet passive optical network) system for ONU (Optical Network Unit) and the basic principle and design method of ONU transceiver module, and gives the experimental results.

Keywords: e-pon;onu; burst; transceiver module; BiDi

1. Foreword

With the rapid development of the information society, People's reliance on it becomes more and more high, and the demand for it is more and more. The popularization and development of Internet business provides a good platform for information acquisition and communication. In addition, with the increasing demand of high bandwidth information represented by HDTV, the new wideband transmission technology will replace the traditional narrowband transmission technology more and more. In these new technologies, the E-PON (Ethernet passive optical network) system, which combines mature Ethernet technology and passive optical network technology, becomes a good technology scheme for optical fiber broadband access because of the advantages of wide transmission bandwidth, transparent operation, easy maintenance and management, convenient upgrade and low cost.

The E-pon system is composed of Olt (optical circuit terminal-optical line termination), ONU (Optical Network Unit-optical Network) and fiber wiring unit, and the system uses Ethernet protocol. We call the downlink signal from Olt through 1:n optical passive splitter to ONU, from ONU to Olt for uplink signals. The data of the E-pon system is transmitted through an indefinite length of packet, and the packet length is 1518 bytes long according to the IEEE802.3 Ethernet protocol. Downlink information is broadcast from Olt to multiple ONU. Each packet carries a letterhead that uniquely identifies the specific ONU to which the data is to be reached, and some packets are sent to all ONU, called broadcast packets, and some packets are sent to a set of ONU, called multicast packets. The data flow through the optical splitter is divided into N-way independent signal, each signal contains a packet sent to all the specific ONU. When ONU receives the stream, only packets sent to them are extracted, and packets sent to other ONU are discarded. The uplink data from each ONU to the OLT is transmitted through the time Division multiple access (TDMA) mode, and Olt allocates a transmission slot for each ONU. These slots are synchronized, so when packets are coupled to a single fiber, there is no collision between the different ONU packets. Uplink technology is the core technology of EPON, and the main technologies of e-pon system with TDMA uplink technology include: timing and round-trip delay compensation, TDMA multiplexing, bandwidth dynamic allocation, initial ranging and plug and Play technology.

This paper describes the burst light emission for uplink signal transmission in e-pon system and the ONU Optical transceiver module for downlink continuous mode optical receiver.

Technical requirements of 2.e-pon for optoelectronic modules

It is known from the above introduction that the downlink of E-pon is a typical point to multiple point system, upstream is a number of ONU to Olt TDMA system. That is, the downlink can use the traditional continuous mode optical transmission technology, and the uplink must use the burst mode optical transmission technology to meet the requirements of the system. Therefore, the e-pon to the Photoelectric module technology has the following requirements:

(a) As a result of the use of a passive optical splitter to increase the loss of optical fiber line, the downlink signal emitting light power is large enough to receive a high sensitivity to meet the 20Km transmission distance.

b The Olt of the uplink line will bring together the light signals of each ONU, in order to ensure a very low ber and a certain transmission distance, it is required that there is a higher extinction ratio; ieee802.3ah requires each ONU to turn off the laser without transmitting the data to the Olt and make the output light power at this time less than -45dbm.

(c) Due to the e-pon rate of GB/s, in order to make full use of the launch time slot, the uplink ONU burst light emitting module is required to open/close less than 512ns.

D due to the different positions of various ONU, different distances and different optical lines, the amplitude of each ONU light signal reaching olt varies, and the receiving part of the Olt is abrupt and high dynamic range.

e) Due to the ONU placed in the client, it requires ONU photoelectric integrated module high reliability, low power consumption, low cost.

3.ONU Optical Transceiver Integrated Module design

A) Structural design

To meet the above requirements, the structure of ONU Optical transceiver module adopts 2*6 pin with tail fiber SFF (Small Form Factor, small external factor) structure, and optical interface is LC.

b) Bidi Component design

BiDi (bi-direction, single fiber bidirectional Ld/pin-tia component) is the core device in the ONU Optical transceiver module. There are the following requirements, one is to meet low-cost requirements, the choice of fp-ld, the central wavelength of 1310nm, spectral width is not more than 3nm, operating temperature range in -45℃~+85℃, the rate is greater than 1.25gb/s; the second is the response speed of the back probe is high, so that the LD device is suitable for burst mode work The rate of the three Pin-tia components is 1.25gb/s, the receiving sensitivity is higher than the -24DBM, the four are small size, low cost and high reliability. The samples we developed have passed the reliability assessment to fully meet the above requirements.

C Burst-type optical emission circuit design

Unlike the continuous mode LD drive circuit, in addition to the fast modulation of the drive current, the bias current of the LD requires a switch control, so the LD's drive unit has 4 driving ports. As shown in Figure 3. Td+, td-for the e-pon system ONU unit to the burst mode drive circuit differential input signal, en+, en-for the system control LD switch signal. That is, when the ONU is required to emit a light signal, the system control part gives the en+ and en-to turn on the signal so that the LD bias is above the threshold current, at this time, the ONU emits the normal modulation light signal to the OLT; when the system control part gives the en+, the en-sends to turn off the signal, at this time, The ONU no longer emits light signals to the olt. In addition, the design of fast APC should be considered in order to ensure the stability circuit design of output light signal. At present, IC vendors have provided integrated solutions for CMOS processes.

d) Receiving circuit design

Because this part is the traditional continuous mode of light receiving mode, the use of mature pin-tia devices and integrated amplifier IC technology scheme, this is no longer described.

e) test Results

All samples were tested using Agilent 86100A oscilloscope, MP1605 error meter and SDA6020 serial Data Analyzer.

4. Conclusion

At present, the module has passed the small batch reliability examination, and has been tested by the instrument and the actual use on the E-pon system, It is proved that the module index satisfies the Ieee802.3ah's suggestion request, the main technical index reaches the similar product level in the same period abroad, and the module is smaller, the structure accords with the SFF protocol, which brings convenience to the user.

Heng Bao Tong Optoelectronics Co., Ltd. Chai Guanghui, Chao Wentao, Chen Yiguo, Huang Feng

Department of Electronic Engineering, Tsinghua University, Chen Minghua, Xia, Sheshijong