s GPON system

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Microelectronics Journal 39 (2008) 112–116 www.elsevier.com/locate/mejo

Burst-mode optical transmitter with DC-coupled burst-enable signal for 2.5-Gb/s GPON system Sang Hyun Parka,, Mikko Karppinenb, Quan Lec, Bin Young Yoonc a

Korea Institute of Energy Research, Future Fundamental Technology Research Department, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea b VTT Technical Research Center of Finland, MicroModule Centre, P.O. Box 1100, FI-90571 Oulu, Finland c Electronics and Telecommunications Research Institute, Optical Communications Research Center, 1110-6 Oryong-dong, Buk-gu, Gwangju 500-480, Republic of Korea Received 8 May 2007; accepted 9 October 2007 Available online 26 November 2007

Abstract In this paper, first burst-mode transmitter for 2.5-Gb/s gigabit-capable passive optical networks (GPON) system is evaluated. Small form factor (SFF) type optical network unit (ONU) generates 2.5-Gb/s optical eyes with commercial distributed feedback-laser diode (DFB-LD). For the fast burst signal access, burst enable (BEN) connection on the evaluation board utilizes DC-coupling configuration. Resultant optical output waveform shows 16.30 dB extinction ratio with wide eye opening for GPON mask test. No significant eye degradation is detected in high temperature operation and after 10 km distance transmission. Proposed GPON ONU successfully shows fast timing characteristics of 2.17 nsec for Tx enable stage and 1.17 ns for Tx disable stage. r 2007 Elsevier Ltd. All rights reserved. Keywords: Burst mode; Gigabit-capable passive optical networks (GPON); Optical transmitter; Distributed feedback-laser diode (DFB-LD)

1. Introduction With rapidly increasing bandwidth requirement, fiberto-the-home (FTTH) is emerged as a candidate access network system. Where broad-band passive optical network (BPON) and ethernet passive optical network (EPON) developments have shown their maturity, gigabitcapable passive optical network (GPON) is starting to be adopted by the industry for next generation system with its residential service capability and cost effectiveness [1]. Typical characteristics including high split ratio, line rate, and bandwidth efficiency increase usefulness of GPON system for the practical FTTH solution. The full-services access network (FSAN) study group firstly initiated GPON network standardization via recommendations for the GPON physical-media-dependent (PMD) layer. International Telecommunication Union-Telecommunication Corresponding author. Tel.: +82 62 970 6629; fax: +82 62 970 6989.

E-mail address: [email protected] (S.H. Park). 0026-2692/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2007.10.009

Standardization Sector (ITU-T) approved and ratified this as ITU-T Recommendation G.984.2 [2]. Based on this initial standardization work, intensive researches have been done to develop burst-mode transmitter, which requires several technological challenges like fast burst signal operation timing, large average output power for high split ratio, high extinction ratio (ER), and temperature stability. Previous works on vertical-cavity surface-emitting diode lasers [3,4] and planar avalanche photodiodes [5] enlarge the practical adaptation of fast optical data transmission like 2.5-Gb/s burst-mode signal. The recent works to develop GPON transmitter have reported mainly up to 1.25-Gb/s burst-mode data rate. The dependency in transmitter performance on data pattern is evaluated in [6]. Level monitoring with fast operating time technology is reported in [7]. Novel digital algorithm is proposed for automatic power control for burstmode transmitter in [8]. Li reported burst-mode transmitter which can be operated up to 2.5-Gb/s data rate but the relatively long Tx on/off time did not fulfill ITU-T G.984.2 recommendation [9].

ARTICLE IN PRESS S.H. Park et al. / Microelectronics Journal 39 (2008) 112–116

This paper presents a 2.5-Gb/s GPON transmitter using commercial laser driver IC. DC-coupling for burst-enable signal (BEN) is actively utilized to meet the tight timing specifications for GPON. High temperature stability and long distance transmission characteristic are also investigated with 2.5-Gb/s burst-data stream. To author’s best knowledge, this work is the first result which shows full compliances to the ITU-T G.984.2 of 2.5-Gb/s upstream burst-mode transmitter. 2. 2.5-Gb/s burst-mode upstream transmitter block The burst-mode transmitter test set for 2.5-Gb/s GPON system is presented in Fig. 1(a). Four generator modules in Par-BERT (Agilent 81250) are utilized to compose two input data sets and two BEN signal sets. Small form factor (SFF) type ETRI GPON optical network units (ONUs) generates 2.5-Gb/s optical eyes with commercial distributed feedback-laser diodes (DFB-LDs). To estimate the

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effect of emitted power during laser-off period, two ONUs are used in the measurement. With these two-ONU configuration, eye diagram which is the combination of the optical signal from both the transmitting ONU and the turned-off ONU can be investigated. Variable attenuators arrange power levels suitable for measurement with digital communication analyzer (Agilent DCA 86100). Single mode fiber pigtailed SFF ONU board is shown in Fig. 2. Upstream burst-mode overhead specified in ITU-T G.984.2 standard is depicted in Fig. 1(b). Total physical layer overhead time includes Tx enable, Tx disable, preamble and delimiter time. During the guard time which includes Tx enable and Tx disable, the ONU transmits no more power than the nominal zero level. During the delimiter time, ONU will transmit a special data pattern with optimal autocorrelation properties that enable the optical line termination (OLT) to find the beginning of the burst. For the fast burst signal access, BEN signal

Fig. 1. (a) Burst-mode transmitter test setup for 2.5-Gb/s GPON and (b) Test timing parameters following ITU-T G.984.2.

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Fig. 3. Optical output waveform of transmitter at room temperature.

Fig. 2. Single mode fiber pigtailed SFF ONU boards for 2.5-Gb/s GPON.

Table 1 Laser driver electrical characteristics Parameter

LDD

Units

Burst enable Burst disable Extinction ratio Deterministic jitter Supply voltage

o2.3 o2.0 10 o40 +3.3

ns ns dB psP–P V

connection on the evaluation board utilizes DC-coupling configuration. The laser driver chip (LDD) used in this transmitter is a commercial product burst-mode GPON application, with performances listed in Table 1. For the stable laser power control, minimum burst length is limited to the burst on time of 576 ns. The timing parameters in Table 1 suggest that the chip can support 2.5-Gb/s data rate with proper output matching circuit and careful PCB board design. 3. Burst-mode measurement results Fig. 3 shows the 1310 nm wavelength optical fiber output eye diagram of the transmitter in room operating temperature. The eye mask for the 2.5-Gb/s GPON is not specified in the standardization work at the time this paper is written. So, we followed previous 1.25-Gb/s mask rule in ITU-T G.984.2 which is rather strict to measure faster 2.5-Gb/s signal eye opening. The eye test shows wide opening and no mask violation more than 30 min pattern accumulation. The ER in this eye is measured 16.30 dB which gives enough margin to minimum requirement in 1.25-Gb/s standard of 10 dB. Rising time from 10% to 90% of eye opening is measured 215 ps with 60 ps deterministic jitter. Principally, burst-mode ONU transmitter should launch no optical power into the fiber in the time period that the

Fig. 4. Tx enable/disable characteristics of 2.5-Gb/s GPON transmitter (500 psec/div).

other ONU is assigned to. However, standardization allows an optical power less than or equal to the launched power without input data to the transmitter. The ONU should meet this requirement during the guard time period that are assigned to it with the exception of Tx enable bits which is used for laser pre-bias and Tx disable bits which the output falls to zero. The measurement results

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1.17 ns from the last eye pattern to bias disabled point and this also satisfies maximum Tx disable specification of 32 bits. Besides electrical characteristics of optical transmitter, thermal stability in operation temperature has another importance as reported in [10]. High temperature characteristics of burst-mode ONU is evaluated and shown in Fig. 5(a). At 70 1C of measured ONU board temperature, the optical output waveform still shows wide eye-opening. The comparison of peak-to-peak jitter shows 5% increase compared to room temperature operation in Fig. 3 and ER decreased down to 14.29 dB. Long distance transmission characteristic is also measured and presented in Fig. 5(b). After 10 km single fiber connection the peak-to-peak jitter increases 18%, from 60 to 71 ps due to fiber dispersion. Slight increase in rise time of 231 ps is measured but the output waveform still satisfies eye mask test shown in Fig. 3. The characteristics of our GPON ONU transmitter are summarized in Table 2. 4. Conclusion

Fig. 5. (a) Optical output waveforms at high temperature of 70 1C and (b) eye opening after 10 km single-mode fiber transmission.

Table 2 Burst-mode 2.5-Gb/s GPON transmitter performances

Nominal bit rate (Mbit/s) Mean launched power MAX (dBm) Extinction ratio (dB) Maximum Tx enable (ns) Maximum Tx disable (ns) Jitter (p–p) (ps)

ITU-T G.983.2

This work

2488.32 3a 410 dBa 12.8 12.8 n/a

2500 3.2 16.30 dB 2.17 1.17 60

This work evaluated first GPON ONU block for the upstream 2.5-Gb/s burst-mode signal transmission. The optical transmitter is implemented in SFF type module structure with a single 1310 nm fiber. DC-coupled BEN signal is utilized to meet the tight GPON timing requirement. The tested module showed wide enough eye-opening for GPON eye mask test with 16.30 dB ER. Early and last part of burst eye transmission satisfies minimum Tx enable and Tx disable specification in standardization. Stable eye characteristics are also measured at 70 1C high-temperature operation and even after 10 km distance transmission. The fast responses with stable eye characteristics indicate that proposed DC-coupled BEN ONU structure is practically suitable for ITU-T G.983.2 specification of 2.5-Gb/s GPON system recommendation. Acknowledgement This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-214-D00145).

a

Value for 2.5-Gb/s is still left for further study. Written in this table is the value for 1.25-Gb/s.

of Tx enable and Tx disable time are shown in detail in Fig. 4. In Tx enable stage when power-off laser starts to transmit next burst eye, no overshoot is detected. It measured 2.17 ns from first eye pattern to no overlapped clear eye opening. Considering maximum allowed Tx enable time of 32 bits (12.8 ns) in standard, this early stage eye-opening characteristics shows fast response performance of ONU. Tx disable time measurement shows

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