A high sensitivity and large dynamic input range O-E-O optical wavelength converter for hybrid PONs DENG Lei1,2, LIU De-ming1,2*, ZHANG Min-ming1,2, QIAN Yin-bo1,2 1. School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, 430074, Wuhan, China 2. Wuhan National Laboratory for Optoelectronics, 430074, Wuhan, China *
E-mail:
[email protected]
ABSTRACT Optical access networks are developing to be large bandwidth, high capacity and long distance. The Hybrid Passive Optical Networks (HPONs) using Time Division Multiplexing (TDM) technology and Wavelength Division Multiplexing (WDM) technology are becoming representative technologies that will be employed in the next generation access network. A novel scheme of HPON system architecture based on the wavelength conversion technology is presented. The advantages and disadvantages of a variety of wavelength conversion technologies are contrastively analyzed, and a novel 1.25Gbps burst-mode Optical-Electrical-Optical Wavelength Converter (BM-OEOWC) used in the uplink channel of this HPON system is realized and designed. The receiver sensitivity of this Optical Wavelength Converter reaches -30dBm, and the dynamic range of the input optical power reaches up to 20dB. Theoretical analysis is presented to explain why Bit Error Rate (BER) would obviously increase if more ONUs are attached to this converter. An improved scheme is proposed at last. Keywords: HPON, burst-mode, O-E-O optical wavelength conversion, dynamic range
Photonics and Optoelectronics Meetings (POEM) 2009: Fiber Optics Communication and Sensors, edited by Dieter Stefan Jäger, Hequan Wu, Shuisheng Jian, Desheng Jiang, Deming Liu, Proc. of SPIE Vol. 7514, 7514100 © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.843421 Proc. of SPIE Vol. 7514 751403-1
1.
INTRODUCTION
With the development of emerging applications such as HDTV, IPTV, Video conference etc, EPON or GPON which offer 1.25Gbps data rate for both upstream and downstream can not meet the growing demand for next-generation optical access network. PON technology is developing to large-capacity single wavelength as well as dense wavelength division multiplexing. The Hybrid Passive Optical Network (HPON) system proposed in this paper provides several Wavelength Division Multiplexing (WDM) channels in one single-mode fiber while each channel using Time Division Multiplexing (TDM) technology. This novel scheme which combines the low cost of TDM-PON and high bandwidth, easy scalability of WDM-PON becomes a representative technology [1]. As a key device for HPON, the wavelength converter which connects the uplink of each WDM channel and “colorless” ONUs should be easy transition, cost-effective and large dynamic range of the input optical power. The techniques of wavelength conversion which utilize active semiconductor optical devices such as SOAs suffer from extinction ratio degradation, narrow dynamic range of input signal and no regeneration [2]. In this paper, we have solved this problem by providing an Optical-Electrical-Optical Burst-mode Optical Wavelength Converter (BM-OEOWC) to convert upstream wavelengths. The receiver sensitivity of this converter reaches -30dBm, and the dynamic range of the input optical power reaches up to 20dB.
2.
SCHEME OF HPON SYSTEM
The proposed evolution architecture of HPON system is shown in Fig.1. At the central office (CO), 8 next-generation OLT nodes are divided into upstream and downstream by 8 WDM1.1310nm signal which is transmitted in the downstream direction of each channel is converted into L-Band (1540.94~1546.89nm) signal by Optical-Electrical-Optical continuous-mode optical wavelength converter. Then, the L-Band WDM multiplexes 8 L-Band signals into the fiber. In the upstream direction, the signals transmitted from the fiber are de-multiplexed by C-Band (1540.94~1546.89nm) WDM. At the remote node (RN), the BM-OEOWC is used to convert the 1530nm signal coupled by C1 to C-Band signal. The outputs of 8 converters are multiplexed into the fiber by C-Band WDM, and L-Band WDM is responsible for the de-multiplexing of downstream signal. Meanwhile, WDM2 is adopted to ensure backward compatibility with the existing EPON or GPON system. Optical network terminal (ONT) which is deployed as a tree topology by passive 1:32 optical splitters uses TDM protocol. Each ONU can offer 1.25Gbps data rate for both upstream and downstream, and the total transmission capacity of 8 WDM channels is up to 10Gbps. More users can be accessed in one single-mode fiber with the original
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bandwidth, thus it is able to take full advantage of the tremendous bandwidth of the fiber.
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Fig.1. Evolution architecture of HPON system
3.
BURST-MODE O-E-O WAVELENGTH CONVERTER
The BM-OEOWC is employed to support the burst data on the upstream and achieve colorless ONUs. Recently, there are two solutions for wavelength conversion. The first method is to use Optical-Electronic-Optical (O-E-O) in lower data rate systems, where the optical signals are detected and electronically reshaped/retimed before being re-modulated onto a new wavelength optical source. As the second solution, all-optical techniques using cross-polarization modulation, cross-phase modulation and cross-gain modulation are demonstrated to achieve the wavelength conversion. At the emerging higher data rates of 40Gbps, the performance of O-E-O wavelength converter becomes worse. However, O-E-O solution is commercially viable at 1.25Gbps data rate due to the low-cost and regeneration [3]. ±L9U2W!W
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Fig.2. O-E-O burst-mode wavelength converter block diagram
The block diagram of the proposed BM-OEOWC is depicted in Fig.2.The wavelength converter is composed of optical receiver module and optical transmitter module. The optical receiver module consists of a PIN photodiode that converts input current from the photodiode to output voltage, a burst-mode limiting amplifier (LA) to amplify the signal
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to limited amplitude, and a burst-mode clock-and-data recovery chip to obtain the synchronous clock from the input data. The monitor circuit creates alarm signal when the module is abnormal. The optical transmitter module is constructed from a burst-mode laser driver chip that modulates the coaxial laser diode (LD) according to the input signal. Automatic power control (APC) circuit uses feedback from the laser to adjust the drive and keep the laser’s output constant. The experiment results at room temperature clearly show that the receiver sensitivity of this O-E-O converter reaches -30dBm. The performance has been measured using the packet loss rate (PLR) instead of the bit-error rate (BER). To measure the PLR of HPON, SmartBits 6000C is inserted between the ONU and the OLT. The experimental setup for measuring the PLR performance is shown in Fig.3 (a). The measured PLRs for the upstream of one channel with different numbers of ONUs accessing the system are shown in Fig.3 (b). Here, a PLR of 10-6 corresponds to a BER of 10-9, approximately [4]. As a result, the PLR of 10-8 for upstream can be achieved when only one ONU is online. The PLR performance is degraded when the input optical power is lower than -25dBm, the dynamic range of the input optical power reaches up to 20dB. However, it is concluded that a wide dynamic range of >20dB can be attained with the PLR lower than 7x10-6 when several ONUs access the HPON system. 2W9U
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Fig.3. (a) Schematic diagram of experimental setup (b) Measured upstream PLRs in HPON
4.
ANALYSIS OF PACKET LOSS RATE DETERIORATION
As shown in Fig.3, the PLR performance is degraded significantly for the upstream and dynamic input range drafts obviously when several ONUs access the HPON system. In accordance with the IEEE 802.3ah standard [5], time diagram depicting the ONU transmission is shown in Fig.4. Dynamic bandwidth allocation algorithm is used to assign time slots to each ONU by the OLT. Each ONU turns on the laser to transmit upstream data within the allocated timeslot. The draft IEEE802.3ah D1.414 specifies that the ONU laser on/off times shall not be greater than 512ns, automatic gain control (AGC) and clock-and-data (CDR) times shall not be less than 400ns. On the basis of the sum of these times which are called synchronization times, the burst-mode receiver module of OLT can achieve the required stability. Synchronous data are composed of IDLE bytes sent by the Physical Coding Sublayer (PCS) in the ONU, as shown in Fig.4.
Fig.4. Time diagram of the transmitter
Part of synchronization times are consumed by the burst-mode optical receiver module of this BM-OEOWC when the burst upstream data arrive. Owing to the reduction of the synchronization bytes, some bytes of the data frame are required to achieve the burst optical receiver synchronization in OLT. Therefore, the PLR performance is deteriorated. By comparing the input and the output signal waveforms of this BM-OEOWC in Fig.5 (a) and (b), it can be seen that
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the time interval between burst data being transmitted by two ONUs is significantly prolonged from 120ns to 160ns. In addition to the periodical discovery time window performed to offer the opportunity to those unregistered ONUs to be active, the OLT allocates all the rest of time slots to the ONU when there is only one ONU online. Thus, dynamic bandwidth allocation algorithm switches the time slot infrequently and the optical receiver in the OLT performs less number of synchronization. As a result, HPON system is consistent with a good PLR performance. In conclusion, The PLR performance deterioration is attributed to the frequently time slot switching of ONUs and synchronization of the OLT receiver when there are several ONUs online.
Fig.5. (a) BM-OEOWC input waveform (b) BM-OEOWC output waveform
Based on the analysis above, the main reason why the PLR performance of HPON deteriorates is that the synchronization bytes consumed by the burst-mode receiver module are not regenerated by the continuous transmitter module of the BM-OEOWC. To solve this problem, an advanced scheme of O-E-O burst-mode wavelength converter is presented as shown in Fig.6. The sending of the synchronization bytes is controlled by PCS layer according to IEEE802.3ah standard [6]. Therefore, the PCS module should be added to regenerate the synchronization data. Fig.7 shows the flow chart of regeneration process. Data detector in the PCS layer of transmitter module will check whether the incoming data is IDLE byte. IDLE code will be sent to the Physical Medium Attachment (PMA) layer when there is no data available. Thus, the OLT will receive the synchronization bytes in the transmission interval between two burst packets. We believe that the use of this advanced BM-OEOWC can significantly improve the PLR performance.
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Fig.6. Advanced O-E-O wavelength conversion
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Fig.7. Flow chart of transmitter module
5.
CONCLUSION
We have demonstrated the operation of the Hybrid Passive Optical Networks using Time Division Multiplexing (TDM) technology and Wavelength Division Multiplexing (WDM) technology. The high sensitivity and large dynamic input range burst-mode O-E-O wavelength converter for HPONs is also realized to achieve cost saving enormously. In addition, analysis and experiments show that the problem of PLR performance deterioration can be solved by adding PCS module in the BM-OEOWC. In conclusion, HPONs provide the opportunity to upgrade the existing EPON or GPON in a low-cost way.
REFERENCES [1] Kazovsky, L., G., Shaw, W., T., Gutierrez, D., Cheng, N. and Wong, S., W., “Next-Generation Optical Access Networks” J. Lightw. Technol., 25 (11), 3428-3440 (2007). [2] Yoo, S., J., B., “Wavelength conversion technologies for WDM network applications”, J. Lightw. Technol., 14 (6), 955-966 (1996). [3] Davey, R., P., Healey, P., Hope, I., Watkinson, P., Payne, D., B., Marmur, O., Ruhmann, J. and Zuiderveld, Y., “DWDM Reach Extension of a GPON to 135 km”, J. Lightw. Technol., 24(1), 29-30 (2006). [4] Choi, K., M., Lee, S., M., Kim, M., H. and Lee, C., H., “An Efficient Evolution Method From TDM-PON to Next-Generation PON”, IEEE Photon. Technol. Lett., 19 (9), 647-649 (2007). [5] IEEE Std 802.3ahTM, Part 3：Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, 299-315 (2004). [6] Kramer, G., “Ethernet Passive Optical Networks”, McGraw-Hill Professional, New York, 78-79(2005).
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