Chai Yi Yoon, Zhang Jing, Pamidighantma V Ramana, Yap Guan Jie, John Lau Hon-Shing
"Development of a Low Cost 2.5-Gbps SFP Optical Transceiver using 0.18µm CMOS ICs ,”
Photonics Packaging, Integration, and Interconnects VIII, edited by Alexei L. Glebov, Ray T. Chen
Proc. of SPIE Vol. 6899, 68990U, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.764294
Copyright 2009 Society of Photo-Optical Instrumentation Engineers. One print or electronic
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http://dx.doi.org/doi: 10.1117/12.764294
Development of a Low Cost 2.5-Gbps SFP Optical Transceiver using
0.18µm CMOS ICs
Chai Yi Yoon, Zhang Jing, Pamidighantma V Ramana, Yap Guan Jie, John Lau Hon-Shing
Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 11 Science Park Road,
Singapore Science Park II, Singapore 117685
ABSTRACT
The high cost of optoelectronics components typically used for long-haul communication is prohibitive in the Fiber to
the Home (FTTH) and Passive Optical Networks (PONs). One method of cost reduction is through the reducing the cost
of the electronics in the transceiver and reducing the packaging cost. We report the development of low-cost 2.5-Gbps
optical transceiver for Gigabit Passive Optical Network (GPON) using CMOS driver ICs and chip-on-board assembly
method. We developed the Laser Diode Driver (LDD), Trans-impedance Amplifier (TIA), Limiting Amplifier (LA) and
the Clock and Data Recovery (CDR) using CMOS technology for short reach application and developed the burst mode
version of the ICs for PON applications. The ICs are designed in house and fabricated on a standard CMOS 8” wafer
with 0.18µm technology. The devices operate at 1.8V and are low power in nature, thus reducing the demand on power
dissipation. The transceiver consists of an un-cooled and direct modulated laser diode driven with a LDD, a high speed
PIN photo-diode with amplifier and CMOS ICs. The bare CMOS ICs are attached on a transceiver substrate that is
compliant with the small form-factor pluggable (SFP) package multisource agreement (MSA) and coupled to a 1310nm
FP laser TOSA and a PIN ROSA with LC connector. The integrated transceiver is characterized up to 2.5-Gbps. In this
publication, we present the detail of the module development, assembly methods and performance characterization at
1310nm.
Keywords: 2.5-Gbps SFP, optical transceiver, 0.18µm CMOS technology, FTTH, GPON
1. INTRODUCTION
With increased in demand for faster applications such as data transfer and video, the IP traffic grew rapidly. This
stimulated the development of low-cost and high performance optical access network. Passive Optical Network (PON) is
one of the emerging cost-effective and high-performance methodologies to be looked into for the last mile
communication. A typical PON consists of the Optical Line Terminal (OLT) located at the Central Office (CO) that is
responsible for broadcasting information downstream to the users through an Optical Network Unit (ONU). Information
is transmitted upstream from the users, superimposed at combiner and sent to the OLT at the CO. The data from the users
must be buffered and transmitted in short burst to avoid collision. The CO will decide which user to send a burst at
which point in time, thus upstream traffic flow, shown in Figure 1a, requires burst-mode transmission.
Photonics Packaging, Integration, and Interconnects VIII, edited by Alexei L. Glebov, Ray T. Chen
Proc. of SPIE Vol. 6899, 68990U, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.764294
Proc. of SPIE Vol. 6899 68990U-1
Transmitted in
ONU-specific
time slots
ONU #1
1
User 1
ONU #2
2
User 2
ONU 3
3
User 3
1
1
2
OLT
n
…
2
combiner
3
Fig. 1. (a) Upstream traffic flow in GPON
In the downstream traffic, Figure 1b, the OLT will tag the data with addresses and broadcast it to all the users
sequentially. Each user will merely select the data that has the appropriate address tag; therefore, conventional
continuous transmission can be exploited.
ONU #1
ONU-specific
packet
1
User 1
3
2
1
1
2
3
1
OLT
2
3
2
User 2
ONU #2
splitte
1
2
3
ONU #3
3
User 3
Fig. 1. (b) Downstream traffic flow in GPON
This paper looks into the development of a 2.5-Gbps optical transceiver for short reach application for G-PON (Gigabit
Passive Network) using 0.18µm CMOS ICs. To achieve low-cost in the GPON transceiver development, one method is
to reduce the cost of the electronics in the transceiver by going for CMOS ICs. On top of that, packaging cost of ICs can
be further reduced by using direct chip-on-board assembly method. The CMOS ICs are integrated onto the small form
factor transceiver. Over the years, there are quite a few research developments on ICs using 0.18µm CMOS technology
[1][2][3] to drive optoelectronics components. However, their experiment focuses on either the receiver or the
transmitter. The evaluation for CMOS IC transceivers has not been extensively carried out.
Section II gives details of the design configuration and circuitry of the transceiver module. Assembly and the transceiver
characterization methodology will be described in detail in Section III. Transceiver test results are presented in Section
IV. Last but not least, conclusion is covered in Section V.
Proc. of SPIE Vol. 6899 68990U-2
2. TRANSCEIVER DESIGN CONFIGURATION
ONU transceiver module
Optical block
Electrical block
Fiber
LD
LDD
DATA
IN
LA
DATA
OUT
1310nm
TOSA
Fiber
1310nm
PD
TIA
ROSA
Fig. 2. Transceiver module
The block diagram of the transceiver block is illustrated in Figure 2. It is sub-divided into optical and electrical block.
The optical block consists of a transmitter optical sub-assembly (TOSA) and a receiver optical subassembly (ROSA).
The commercial un-cooled Fabry Perot (FP) TOSA is a multi-rate 1310nm direct modulated source laser that is packaged
within an LC receptacled transmitter optical sub-assembly. The commercial receiver optical sub-assembly (ROSA)
integrates a 2.5-Gbps PIN Photodiode and a low noise trans-impedance amplifier (TIA) into a hermetic TO stem. The
PIN is composed of an InGaAs absorption layer, thus it is suitable for 1310-nm applications.
The electrical block is further divided into 2 subsections, namely the transmitter and the receiver sections. The
transmitter portion contains a laser diode driver (LDD) that is developed using 0.18µm CMOS technology. The LDD
requires a 1.8V and 3.3V supply voltage. It can modulate laser diodes up to 20mA and can supply bias current up to
30mA. The LDD is designed to operate up to 2.5-Gbps; with a rise and fall time of about 180ps. The achievable
extinction ratio with this driver is 8dB. The LDD can function in both continuous burst mode, simply by varying the
input level at burst enable control pins BEN+ and BEN-. By supplying a 1.8V to the BEN+ pin and connecting the BENpin to ground, the laser driver will function in continuous mode. Conversely, by applying a burst enable signal to the
BEN+ and BEN- pins and some additional circuitry, the LDD will then operate in burst mode. The receiver section is
composed of a limiting amplifier (LA), as illustrated in Figure 4. It is the stage following TIA, which further amplifies
voltage signal and generates constant output level. The amplifier has bandwidth of more than 2.5-Gbps and accepts input
range from 50mV to a maximum of 250mV. It provides constant current-level current-mode logic (CML) output
voltages of about 200mV.
In this work, we use the package type which is compliant with SFP package. This package has a very small physical
dimension and it is hot-pluggable, thus enables easy testing via host boards without having to turn off the DC bias [4].
The SFP transceiver is designed on a 4-layer FR4 material with board thickness of about 1mm. Split power DC power
planes is implemented for transmitter and receiver to prevent any crosstalk. The transceiver module is targeted to
operate up to 2.5-Gbps.
Proc. of SPIE Vol. 6899 68990U-3
Fig. 3. Laser diode driver (LDD) CMOS IC
Fig. 4. Limiting Amplifier CMOS IC
3. TRANSCEIVER ASSEMBLY AND CHARACTERIZATION
In order to eliminate the parasitic effects of the through-hole connections which will degrade the performance of the
transceiver, the TOSA and ROSA are both edge mounted onto the SFP substrate. The laser driver and the limiting
amplifier are assembled and connected to the transceiver substrate using chip-on-board (COB) method to further reduce
the parasitic effects introduced by leads of packages, in addition to the reduced cost due to the elimination of packaging
cost. The CMOS driver ICs are firstly wire-bonded onto the PCB board before the rest of the passive components are
reflow soldered onto the same board. A high purity liquid epoxy encapsulant used as glob top is applied onto the wirebonded driver ICs and cured in order to protect the bond wires during the high reflow temperature and handling during
characterization.
TOSA (LD)
Laser diode driver (LDD)
ROSA
(PD+TIA)
Limiting Amplifier (LA)
Fig. 5. Assembled transceiver module with 1310nm TOSA and ROSA
Transmitter testing is carried out by injecting a pseudo-random bit stream (PRBS), non-return-to-zero (NRZ) 215-1
patterns into the input of the CMOS laser diode driver, through the host board. The optical output from the TOSA is
Proc. of SPIE Vol. 6899 68990U-4
directly connected, via single-mode fiber, to a digital sampling oscilloscope optical module where the eye diagram is
captured and measured.
Transceiver on test
board
supply
PBRS generator
Singlemode fiber
Optical eye
Fig. 6. Transmitter test set up
For the receiver testing as shown in Figure 7, a reference transmitter is used as the source to send PRBS signal to the
receiver. The reference transmitter optical signal, as shown in Figure 7, has an extinction ratio of about 10dB, optical
power of -5dBm and a rise/fall time of around 180/200ps. The receiver under test consists of a PIN photodiode and a
trans-impedance amplifier integrated in a commercial ROSA and a CMOS limiting amplifier. The PIN photodiode
converts the optical signal into electrical current while the trans-impedance is responsible for converting the photodiode
current into voltage with some amplification. The limiting amplifier further amplifies the electrical signal. The reference
optical signal is transmitted through a singlemode fiber and received at the receiver under test.
Test boards
Reference
transmitter
PRBS input
Receiver
tested
Electrical output
SMF
Fig. 7. Receiver test setup using a reference transmitter
Proc. of SPIE Vol. 6899 68990U-5
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F515.6 JJ
a v-isa — tII_ H* T
R/SApAqM©dSpM
ii
1111 HFIL
I cs
wfrD6 Cl
Iw1D6C3
I
-5.606927dB
1AOPdCS
1E<d6C3
7.73O369p
1RMSJCS
is2 .oooop
FM'
CS
292.oooop
jRC3
24 .930 W
'2500 .SRW
0'/475 .SRW
IF2J 192.09004/ j 1296.3004
MM[/
I
I 1100.000P'/
6:37 AM 11-004-07
Fig. 8. Reference transmitter eye diagram at 2.488-Gbps
4. RESULTS
Characterization of the SFP transceiver is performed up 2.488-Gbps. Optical power of -2dBm is attained and the
extinction ratio is 8dB. The extinction ratio can be fine-tuned by adjusting the biasing and modulating current, which are
controlled by the SMT trimmers assembled on the SFP transceiver. Some overshoot and undershoot are observed at the
optical eye. This phenomenon can be improved by selection of appropriate damping resistor on the SFP, connected at the
cathode of the laser diode. Alternatively, an RC shunt network consisting of a resistor and a capacitor connected to
between the laser diode and the laser driver can be fine tuned to further improve the ringing or overshooting. The
presence of lead and bond wires length that introduces parasitic inductance which may be the cause for the overshoot
observed in the eye diagram. More significant jitter can be observed when the bit rate is increased from 1.244-Gbps to
2.488-Gbps. The summary of the transmitter is shown in Table 1(a).
VA&!fl
IErJ iw HS±!J
Fig. 9. Optical eye diagram at 1.244-Gbps
Proc. of SPIE Vol. 6899 68990U-6
—
Fl. E
'It'. S
IELi 1100 .09W1
jjjj
U A. H* lTII
I M1 t I
1490
I I °•°°°P
-509.1[V
120 .235
I
Fig 10. Optical eye diagram at 2.488-Gbps
Parameters
Supply voltage
Center wavelength
Bit rate
Optical input power
(Reference transmitter)
Minimum Receiver
sensitivity at BER 10-11 at
1.244-Gbps
RMS jitter
Result
3.3
1310
2.488
-5
Unit
V
nm
Gbps
dBm
-22
dBm
10.9
ps
Parameters
Optical
power
at
2.488-Gbps
Supply voltage
Center wavelength
Bit rate
Extinction Ratio
Optical rise/fall time
RMS jitter at 2.488Gbps
Result
-2.5
Unit
dBm
3.3
1310
2.488
~6-8
180/200
11
V
nm
Gbps
dB
ps
ps
At BER = 1x10-11 using PBRS 27-1
(a)
(b)
Table 1 (a) Transmitter measurements using 1310nm Fabry Perot laser diode (b) Receiver measurements using 1310nm PIN photodiode, with the reference transmitter
Proc. of SPIE Vol. 6899 68990U-7
U Ai. H* Ti.d
Fl. E 'ltiq 9
IdgIEt[1Di[t
F515.6
ii rmI'
_2_ti o.oo[v/ jj [o.ov
I
I Foo.ooop
j20.1434
±EIicJ
Fig. 11. Recovered electrical eye diagram at 2.488-Gbps
It is observed that zero crossings of the recovered electrical eye diagram at 2.488-Gbps is shifted above the midpoint.
Possible causes of the above phenomenon could be due to differences in the current paths during the rise and fall time.
This happens because each path may present distinct charging/discharging characteristics. [5] Possible solutions could be
to select a proper damping resistor that is connected between the laser diode cathode and the LDD. Nevertheless, the eye
diagram is wide and clean from jitters, even at 2.488-Gbps.
Figure 13 shows the BER characteristics of optical the transceiver. A variable optical attenuator (VOA) is connected
between the reference transmitter source with output optical power of -5dBm and the receiver under test. The VOA is
adjusted to vary the optical signal received at the receiver and the recovered electrical eye and BER is measured at the
sampling oscilloscope and Anritsu BERT scope respectively. It is found that the BER at -23dBm and -22dBm of
received optical power are 10-11 and 10-9 respectively at 1.244-Gbps. Therefore, we conclude that the receiver sensitivity
at 1.244-Gbps is -23dBm. A summary of the characteristics of the receiver is shown in Table 1(b).
Edit Vi2 S &2p U tiIiti HIp IT tiggtd
I
•I*1s1A131C1c IMv1IA.qMX.IUrcS zJTISDt zJP
FIAPItt2d ii niI
1°.°°v
I_255 •o V
1.—Iu
Mit1 1aL!i 1200 .OOOP
121 .SOOo
Fig. 12. Recovered electrical eye at 1.244-Gbps with receiver sensitivity of -23dBm, at BER = 1.2x10-9
Proc. of SPIE Vol. 6899 68990U-8
Fig. 13. Receiver sensitivity at 1.244-Gbps
5. CONCLUSIONS
This work demonstrates the integration of 0.18um chip on board CMOS ICs on standard SFP transceiver to attain a low
cost optical transceiver that is targeted to operate up to 2.5-Gbps. The transceiver exploited the due to its small size and
hot-pluggable feature. The transmitter is able to output around -2dBm of optical power and achieving up to 8dB
extinction ratio at room temperature. The receiver has proven to achieve a sensitivity of -22dBm with BER 10-11. Further
work is being carried out for future developments on the burst-mode features of the CMOS ICs.
6. ACKNOWLEDGEMENT
This work is carried out as a part of Optical Network Focus Interest Group- II (ONFIG - II) Programme sponsored by
Science Engineering and Research Council (SERC), Agency for Science, Technology and Research (A*STAR),
Singapore.
REFERENCES
[1] Carolien Hermans, Student Member, IEEE, and Michiel S. J. Steyaert, Fellow, IEEE, “A High-Speed 850-nm Optical Receiver
Front-End in 0.18µm CMOS,” IEEE Journal of Solid-State Circuits, vol. 41, no. 7, July 2006.
[2] Yong-Hun Oh*, Quan Le*, Sang-Gug Lee*, Nguyen Duy Bien Yen*, Ho-Yong Kang**, and Tae-Whan Yoo**, “Burst-mode
Transmitter for 1.25-Gbps Ethernet PON Applications,” 2004 IEEE.
[3] Quan Le, Student Member, IEEE, Sang-Gng Lee, Yong-Hun Oh, Ho-Yong Kang, and Tae-Hwan Yoo, “A Burst-Mode Receiver
for 1.25-Gb/s Ethernet PON With AGC and Internally Created Reset Signal”. IEEE Journal of Solid-State Circuits, vol. 39, no.
12, December 2004.
[4] Jin-Wook Kwon, Joong-Hee Lee, Member, IEEE, Jae- Myung Baek, Joo-Chul Cho, Ja-Won Seom Sung-Soo Park,
Jung-Kee Lee, Yun-Kyung Oh, and Dong-Hoon Jang, “AC- Coupled Burst-Mode OLT SFP Transceiver for Gigabit Ethernet
PON Systems,” IEEE Photonics Technology Letters, vol. 17, no.7, July 2005.
[5] Interfacing Maxim Laser Drivers and Laser diodes Application Note: HFAN-2.0
Proc. of SPIE Vol. 6899 68990U-9