780 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 52, NO. 11, NOVEMBER 2005

A CMOS Burst-Mode Optical Transmitter for1.25-Gb/s Ethernet PON Applications

Yong-Hun Oh, Sang-Gug Lee, Quan Le, Ho-Yong Kang, and Tae-Whan Yoo

Abstract—This brief presents a CMOS burst-mode opticaltransmitter suitable for use in 1.25-Gb/s Ethernet passive opticalnetwork applications. Based on feedback from the monitoringphotodiode, in order to control consecutive burst data the pro-posed transmitter in this brief uses a reset mechanism, whichallows fast responses from the beginning of a high-speed inputburst. The chip is fabricated in mixed-mode 0.18- m CMOS tech-nology and measurements are implemented in a chip-on-boardconfiguration using a pig-tailed type Fabry–Perot laser. Underburst-mode operation of 1.25-Gb/s pseudorandom binary se-quences, measurements show about 1-dBm averaged transmittedoptical power with an over 12-dB extinction ratio over a widetemperature range.

Index Terms—Burst mode, Ethernet passive optical network(E-PON), gigabit Ethernet, laser driver, optical transmitter.

I. INTRODUCTION

MOTIVATED by the huge data transmission capacity re-quired for multimedia communications, passive optical

network (PON) based on fiber-to-the-home (FTTH) is consid-ered an emerging access network technology to solve the lastmile problem of communications.

As illustrated in Fig. 1, a typical PON system is basically apoint-to-multipoint (PtMP) optical network with no active ele-ments in the signal path. Based on TDMA, this passive PtMPability shares a single optical fiber making it feasible to imple-ment a cost effective solution for the subscriber line that sup-ports broad-band voice, data, and even video services [1], [2].

A key component of such a PON system is the burst-modeup-stream transmitter located inside each subscriber optical net-work unit (ONU). For real burst-mode operation, together withthe stable transmitted optical power under wide temperaturevariation, laser turn-on/turn-off delay is one of the critical per-formance parameters of the up-stream transmitter.

This brief describes a 1.25-Gb/s optical transmitter for Eth-ernet up-stream PON applications using standard CMOS tech-nology. Though the ATM-PON was developed and first stan-dardized from full-service access network (FSAN) in the mid1990s, the Ethernet PON (E-PON) based on Internet Protocol

Manuscript received February 3, 2005. This paper was recommended by As-sociate Editor A. Apsel.

Y.-H. Oh is with the RF Microelectronics Laboratory, Electrical EngineeringDepartment, Information and Communications University (ICU), Daejeon,Korea, and also with the Electronics and Telecommunications ResearchInstitute, Daejeon 305-350, Korea(e-mail: yhoh0131@icu.ac.kr).

S.-G. Lee and Q. Le are with the School of Engineering, Information andCommunications University (ICU), Daejeon 305-732, Korea.

H.-Y. Kang and T.-W. Yoo are with the Electronics and TelecommunicationsResearch Institute, Daejeon 305-350, Korea.

Digital Object Identifier 10.1109/TCSII.2005.852928

Fig. 1. Architecture of typical PON system.

TABLE IPERFORMANCES SUMMARY OF PROPOSED WORK

(IP) technology tends to be thought of as a much more attrac-tive communication protocol because of its lower cost, greaterflexibility and higher speed [3]. Furthermore, in this brief, theincreasing demands for lower cost and higher integration canbe sufficiently satisfied with CMOS based implementation ofhigh-speed analog circuits. The general requirements [4] for theburst-mode transmitter to be used in E-PON are summarized inTable I.

For a more detailed description, Fig. 2 shows E-PON timingparameter definitions. The data packet in E-PON allows dy-namic and flexible burst length arrangement. As can be seen inFig. 2, the time gap between the start “ ” and the start ofa real packet is defined as a patterned idle signal. So in orderto maximize channel efficiency in one data frame, this gap timeshould be reduced so far as the system performance of the trans-mitter guarantees no errors in the optical-line-terminal (OTL)receiver. However, in the timing parameters given in Fig. 2,the only controllable feature of the transmitter is the laser turn-

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OH et al.: CMOS BURST-MODE OPTICAL TRANSMITTER FOR 1.25-Gb/s ETHERNET PON APPLICATIONS 781

Fig. 2. E-PON timing parameter definitions.

Fig. 3. Block diagram of the proposed transmitter with separated bias andmodulation currents control circuits.

on/off time. Receiver settling time or clock-and-data recovery(CDR) time is a parameter dependent on receiver performances.Therefore, it is important to make an effort to minimize the laserturn-on/off delay for a better up-stream transmitter.

II. TRANSMITTER ARCHITECTURE AND CIRCUITS

Most optical transmitters are required to stably maintainsystem performances- such as average transmitted opticalpower and extinction ratio- over a wide temperature range( C to 80 C). In order to obtain a reliable and constanttransmitted optical power over this wide temperature, thecharacteristics of laser diode should be confirmed as a functionof temperature.

Fig. 3 shows a simplified block diagram of the burst-mode op-tical transmitter, which is initially proposed in [5], and revisedin this work in order for the feedback loop to give better auto-matic power control (APC). The transmitter in [5] has a goodeye opening but much temperature variation in the averaged op-tical power due to the low gain of the feedback loop. As shown

Fig. 4. Simplified laser driver schematic.

in Fig. 3, for controlling the transmitted optical power of thelaser diode, this work uses an analog circuit technique based onconventional feedback, which is composed of a high-speed tran-simpedance amplifier (TIA) and top hold/bottom hold (TH/BH)peak detection circuits. This is in contrast to many previousworks [6]–[8] for the burst-mode transmitter that have been im-plemented using digital APC circuits.

As depicted in Fig. 3, in order to control the bias and modula-tion currents at the same time, feedback based on the monitoringphoto-diode (MPD) is separated by two independent paths. To-gether with the TH/BH peak detection circuits, the comparatorcircuits (ABC/AMC) control the bias and modulation currentsof the laser diode, respectively. For the given reference voltageswhich set the initial bias and modulation power level, as temper-ature increases the lower monitoring photodiode currentis fed into the high-speed TIA due to the decreased transmittedoptical power of the laser. This feedback current then generateshigher voltage at the output of peak detection circuits, whichresults in increasing bias and modulation currents of the laserdiode. Finally, the feedback loop adjusts the current driven tothe laser diode such that the output current from the MPD equalsa predefined reference.

Therefore, in this architecture, once the reference voltages ofthe AMC and ABC circuits are determined at the outside, the ini-tial bias and modulation currents for stable transmitted opticalpower are automatically installed. Here the initial bias currentis usually around the threshold in order to reduce the turn-ondelay of the laser. And, dc coupling between each functionalcircuit block is required for burst-mode operation.

Fig. 4 shows the simplified laser driver schematic depicted inFig. 3. In order to achieve proper dc bias and be compliant with50 input matching, the laser driver uses a resistive divider asa simple low-voltage positive emitter-coupled logic (LVPECL)interface. And in order to guarantee enough saturation at the dif-ferential stage, a typical dc feedback technique using two diodeconnected transistors is applied.

The peak detection circuits (TH and BH) are depicted inFig. 5. For burst-mode operation, the designed TH/BH peakdetection circuits are reset before each input burst. As canbe seen from Fig. 5, by using the rectifying diode and holdcapacitor the hold level is fed back into the negative input ofthe main amplifier through the source follower to make a unitygain feedback loop. The principle and concern on the detailedoperation of TH/BH peak detection circuits is fully discussed

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782 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 52, NO. 11, NOVEMBER 2005

Fig. 5. Peak detection circuits. (a) TH circuits. (b) BH circuits.

in [9], [10]. Together with the transmission gate logic switch,the reset created from the burst enable (BEN) signal provides afast response time for the laser on/off operation as well.

III. MEASUREMENT RESULTS

The proposed optical transmitter shown in Fig. 3 is realizedusing 0.18- m CMOS technology and tested in a chip-on-boardconfiguration using a pig-tailed type Fabry–Perot (FP) LD/MPDassembly. The chip microphotograph is shown in Fig. 6. Withsingle 3.3 V supply, the transmitter dissipates about 200 mWin an area of 1.0 0.75 mm . The modulation is loaded ontothe bias due to dc-coupled interface between the transmitter andthe laser diode. For the given optical power under temperaturevariation, the transmitter senses the MPD current variation of10 A in order to achieve constant averaged optical power andextinction ratio. The transmitter can drive most commercial gi-gabit laser diodes with a laser back-facet capacitance of about10 pF, and an MPD current ranging from 100 to 1500 uA.

In order to evaluate the performance of the proposed trans-mitter, we use the Agilent Parallel BER Tester, which can gen-erate up to a 3-Gb/s pseudorandom bit sequences (PRBS) burstsignal. Fig. 7 shows the measured transmitted optical power for1.25-Gb/s burst mode with PRBS data pattern. The choicefor this short pattern length is most likely due to the fact that thetransmitter will run with 8B10B line coding in E-PON and willtherefore have similar pattern characteristics. As can be seen

Fig. 6. Chip microphotograph.

Fig. 7. Measured output waveforms for 1.25-Gb/s burst-mode with 2 � 1

PRBS.

in Fig. 7, with the reset signal generated by the BEN, the pro-posed transmitter allows fast responses from the beginning of ahigh-speed input burst.

In order to meet the E-PON specifications summarized inTable I, the laser should be stably turned on within 512 ns withthe patterned preamble data. As shown in Fig. 3, the proposedfeedback, in this brief, directly controls the gate nodes of thecurrent source transistors’ driving the bias and modulationcurrents of the laser. Therefore, the critical delay comes fromthe threshold voltage of the current source transistors andthe threshold current of the laser diode. Moreover, these twothresholds vary according to temperature.

Fig. 8 shows the measured laser turn-on/off delay at high tem-perature (worst case). We can see that about 250 ns is requiredfor stable transmitted optical power. But when considering onlythe bias, just about 100 ns is enough to turn-on and set a properzero bias of the laser. At room temperature turn-on delay is lessthan 40 ns. On the other hand, as can be seen in Fig. 8, when theBEN is off, the laser is instantly turned off. In addition, Fig. 8clearly shows that there is an overshoot of optical power of thelaser. Even so, this undesired overshoot can be minimized withproper filtering and use of the shunt R–C network on the evalu-ation board.

To provide more insights, Fig. 9 provides a more detailed de-scription of the eye diagram of one of the two bursts shown in

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OH et al.: CMOS BURST-MODE OPTICAL TRANSMITTER FOR 1.25-Gb/s ETHERNET PON APPLICATIONS 783

Fig. 8. Laser turn-on/turn-off characteristics at high temperature.

Fig. 9. Eye diagram of one of the two bursts shown in Fig. 7 at roomtemperature.

Fig. 7. All the measurements are evaluated at the same offsetcondition and the optical power of the center of the verticalscale is 1 mW (0 dBm) in the measured eye diagrams. The eyeshown in Fig. 9 is clearly opened with the patterned preambleand PRBS payload data at room temperature (less than 160-psrising/falling times). It also shows an over 12-dB extinction ratiowith about 1-dBm averaged transmitted power.

Finally, Fig. 10 compares the eye diagrams under two extremetemperatures. These are measured at temperatures both lowerthan C and higher than 80 C, respectively. To test the tem-perature performances of the proposed transmitter, a constanttemperature chamber is used. Unlike the measurement results in[5] which are tested using a hot blow heater, we confirm that thefeedback proposed in this work is working properly by showingthat it can provide constant averaged transmitted optical powerwith a less than 10% variation.

IV. CONCLUSION

A burst-mode optical transmitter for 1.25-Gb/s E-PON appli-cations has been integrated in a mixed-signal 0.18- m CMOStechnology. The overall performance of this work is summarizedin Table I. Thanks to the analog APC technique, the proposedtransmitter does not require any other adjustments except settingthe initial laser power and extinction ratio. Based on our mea-surements, the proposed feedback mechanism can be used forgigabit burst-mode applications, and this work complies with

Fig. 10. Eye diagrams of the proposed transmitter under two extremetemperatures.

the E-PON IEEE P802.ah standard. In terms of system inte-gration, small form factor (SFF) package for the implementedchip is currently in development. As well, an improved ver-sion will focus on ESD protection circuits and faster laser turn-on/turn-off control.

ACKNOWLEDGMENT

The author would like to thank the Korea Ministry of Infor-mation and Communications, and researchers in ETRI (Elec-tronics and Telecommunications Research Institute) for theirencouragement and fruitful discussions.

REFERENCES

[1] G. Kramer and G. Pesavento, “Ethernet passive optical network(EPON): Building a next-generation optical access network,” IEEECommun. Mag., no. 2, pp. 66–73, Feb. 2002.

[2] H. Frazier and G. Pesavento, “Ethernet takes on the first mile,” IT Pro,pp. 17–21, Jul. 2001.

[3] Broadband Optical Access Systems Based on Passive Optical Networks(PON), Recomm. ITU-T G.983.

[4] Ethernet in the First Mile Task Force, IEEE P802.3ah , Apr. 2004. .[5] Y.-H. Oh et al., “Burst-mode transmitter for 1.25-Gb/s Ethernet PON

applications,” in Proc. 30th Eur. Solid-State Circuits Conf., Sep. 2004,pp. 283–286.

[6] J. Bauwelinck et al., “DC-coupled burst-mode transmitter for 1.25-Gb/supstream PON,” Electron. Lett., vol. 40, no. 8, Apr. 2004.

[7] E. Sackinger et al., “A 15-mW, 155-Mb/s CMOS burst-mode laser driverwith automatic power control and end-of-life detection,” IEEE J. Solid-State Circuits, vol. 34, no. 12, pp. 1944–1950, Dec. 1999.

[8] K. Nishimura et al., “A 1.25-Gb/s CMOS burst-mode optical transceiverfor Ethernet PON system,” in VLSI Circuits Symp. Tech. Dig., 2004, pp.414–417.

[9] M. Nakamura, N. Ishihara, and Y. Akazawa, “156-Mbit/s CMOS opticalreceiver for burst-mode transmission,” IEEE J. Solid-State Circuits, vol.33, no. 8, pp. 1179–1187, Aug. 1998.

[10] Q. Le, S.-G. Lee, Y.-H. Oh, H.-Y. Kang, and T.-H. Yoo, “A burst-modereceiver for 1.25-Gb/s Ethernet PON with AGC and internally cre-ated reset signal,” IEEE J. Solid-State Circuits, vol. 39, no. 12, pp.2379–2388, Dec. 2004.

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