Optical Transceiver Modules for Gigabit Ethernet PON FTTH System

Optical Transceiver Modules for Gigabit Ethernet PON
FTTH System
by Masayuki Iwase *, Yozo Ishikawa *, Takuya Komatsu *, Junichi Kasahara *,Nobuhiko Hattori *, Masayuki Miura *, Naoto Nakamura * and Kunio Odaka *The Gigabit Ethernet Passive Optical Network (GE-PON) Fiber To The Home
(FTTH) system is expected to be the standard optical communication accesssystem in the next generation. In Japan, full-scale introduction of FTTH services
will begin in 2005. We have developed bi-directional transceiver modules for an optical network
unit (ONU) and optical line terminal (OLT) that are compliant with the IEEE802.3ah standard for
GE-PON using optical sub-assembly (OSA) which has a unique passive alignment structure.

ABSTRACT1.

INTRODUCTION

FTTH systems are expected to be the key infrastructure
for broadband access systems in the future. Japan began
the introduction of FTTH systems in 2003, and according
to the Ministry of Internal Affairs and Communications,
there were over 2 million subscribers by July 2004. From
2005, the major carriers begin introduction of a new system based on the IEEE802.3ah 1000BASE-PX standard
for Gigabit Ethernet PON (GE-PON). We have developed
single-fiber bi-directional optical transceiver modules,
which are necessary both for optical network unit (ONU)
installed at the subscriber’s premises and for the opticalline terminal (OLT) at the central office. We have also
developed a new optical sub-assembly (OSA), which is
the key device for the transceivers. The compact highaccuracy plastic package encloses the optical fiber andthe active optical components in a passive-aligned configuration. In addition, a micro-miniature WDM filter
formed directly on the end facet of the optical fiber isused, assuring miniaturized, low-cost conversion. In thispaper we describe these optical transceivers.

2.

OVERVIEW OF GE-PON FTTH SYSTEM

This standard was adopted as the 802.3ah 1000BASE-PX
standard for Ethernet by the IEEE in June 2004. The
demand specification of the physical layer is specified as
PX10 for the 10-km version of transmission distance, andPX20 for the 20-km version.
The architecture of GE-PON is shown in Figure 1. The
difference from the current 100-Mbps FTTH system using
media converters which use one fiber for each subscriber
is that it uses 1310 nm and 1490 nm for the upstream anddownstream wavelengths, respectively, and provides a
maximum of 1.25-Gbps single fiber bidirectional commu* FITEL-Photonics Lab., R&D Div.

GE-PON FTTH System

ONU
PX10

Transmission speed 1.25Gbps
Transmission wavelength 1490nm SLM
Continuous mode, Extinction ratio 6dB
Transmission power Pf=+2~+7dBm

Transmission wavelength 1310nmMLM
Burst mode, Extinction ratio 6dB
Transmission power Pf=-1~+4dBm

Single mode
Optical fiber

Receiver wavelength1310nm
Burst mode,Bit error ratio=10 -12
Minimum received power=-27dBm
Over load=-6dBm

Receiver wavelength 1490nm
Continuous mode, Bit error ratio=10 -12
Minimum received power=-24dBm
Over load=-3dBm

1490nm

OLT
PX20

16 branch
1310nm
WDM coupler

Splitter

Subscriber A
WDM splitter

ONU
PX20

Video signal 1550nm

10 or 20km

Transmission
distance
Loss budget(Up/Down)

Transmission wavelength 1310nmSLM

Burst mode, Extinction ratio 6dB
E- ONUVideo signal Transmission power Pf= -1~+4dBm
1550nmCentral office
10km

20km

21dB/23dB

26dB/26dB

Figure 1

Receiver wavelength 1490nm
Continuous mode, Bit error ratio=10 -12
Minimum received power= -24dBm
Over load= -3dBm

Subscriber B

GE-PON FTTH system architecture.

nication paths on the passive double star (PDS) transmission line with 16 branches. There is an economical advantage in sharing the optical fiber between OLT in a center
office and the branching point (splitter) by two or more
subscribers. Moreover, although not prescribed by theIEEE802.3ah standard, down-linking the video signal
which is based on the ITU-T983.3 standard is also taken
into consideration, whereby, using the 1550-nm wavelength band, video delivery through the fiber is carried out(3 wave multiplex).
In a GE-PON system, the OLT performs time sharing(TDM) communication between subscribers ONUs
through two or more optical fibers of different distances.
Therefore, in order for the OLT to receive the up-link signal without a collision with different power levels fromeach ONU, the receiver of the OLT must be able to follow
each power level instantaneously and to communicate inorder (burst mode communication). The down stream
direction is continuous mode, as in conventional communication systems.
The loss budget for GE-PON systems has the specification of a 10-km and a 20-km version according toIEEE802.3ah standard, in which the fiber loss differs

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Optical Transceiver Modules for Gigabit Ethernet PON FTTH System

accordingly. In an actual system, the practical course is to
use the 20-km specification for the OLT and to choose
either the 10-km or the 20-km version for the ONU according to the circumstances.

3.

DESIGN OF OPTICAL TRANSCEIVER
MODULES3.1 Transceiver Module for ONU
Figure 2 is a block diagram of the transceiver module
developed for the 10-km version ONU, and Figure 3 is aphotograph. Since the transceiver module for the ONU
was incorporated in the ONU apparatus installed in thesubscriber’s premises, it was designed in conformity withthe small form factor (SFF) standard. The SC type receptacle was adopted as the optical connector.
The transceiver module for the ONU consists of a case,an SC connector receptacle and a printed circuit board,
forming an optical sub-assembly (OSA) and driver circuit.
The OSA is the passive alignment assembly structure in aplastic package, and contains a band pass filter, photodetector (PD), an electric signal amplifier IC and a fiberwith WDM filter on the receiver side, and a Fabry-Perotlaser diode and monitor PD on the transmission side. The
OSA is described more fully in Section 4 below. Using acustom-designed IC which integrates trans-impedance
preamplifier and a limiting amplifier, the characteristic of
this device realizes 2R functions by the OSA alone, andachieves reduced component count and lower power
consumption.
SC receptacle

OSA(Optical Sub-Assembly)

Monitor PD
1310nm FP- LD
SWPF
Filter on fiber facet

Printed wiring board

LD burst
driver circuit

Vcc
in(-)
in(+)
Tx- GNDENABLE

3.2 Transceiver Module for OLT
Figure 4 is a block diagram of the transceiver module forthe 20-km version OLT and Figure 5 is a photograph.
Based on a need for installing high-density boards
inside the transmission equipment rack the height of thetransceiver module for the OLT had to be 8.5 mm or less,conforming to the transceiver size for the standard fullservice access network ATM-PON. The SC type receptacle was adopted as the optical connector.
In GE-PON, the optical output power of the OLT needs
a distributed feedback (DFB) laser having a 1490-nm single longitudinal mode wavelength of high output power.
For this reason, in the OSA a can package was adopted
for the transmitting laser and a high coupling efficiency
design was adopted using an aspheric lens. The receiverside had the same structure as the OSA for ONU, having
a fiber with WDM filter, receiving PD, and receiving amplifier with a passive alignment assembly in a plastic package. The receiving amplifier for the OLT is also a customdesigned IC corresponding to the 2R functional burst
receiver which integrates a trans-impedance preamplifier
and limiting amplifier.

4.

STRUCTURE AND CHARACTERISTICS
OF OSA

4.1 Structure of OSA for ONU
The performance and cost of a bi-directional transceivermodule depend on the OSA.
It is especially important for the OSA for the ONU that it
is small and low in power consumption. In our development of this transceiver module, a number of new techSC receptacle

OSA(Optical Sub-Assembly)
OpticalIsolator

Vcc

TIA

1490nmBPF Filter

1490nm DFB - LD

LIA

Vcc
in(-)
in(+)
Tx- GNDENABLELD driver circuit

LWPF
Filter on fiber facetout(-)Pin - PD

Printed wired board

Monitor PD

Vcc

out(+)
Signal Detect

Rx - GND

Rx- GND

TIA

out(-)

LIA

out(+)Pin - PD

Figure 2

Block diagram of transceiver module for ONU.

SWPF filter

Figure 4

Figure 3

Signal Detect
Rx- GND

Rx- GND

Block diagram of transceiver module for OLT.

Photo of SFF transceiver module for ONU.
Figure 5

Photo of transceiver module for OLT.

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Optical Transceiver Modules for Gigabit Ethernet PON FTTH System

nologies were developed for the purpose of simplifying
the process of OSA assembly, reducing the component
count, and improving integration density. These are:
1) Passive alignment assembly package equippedwith omega grooves for fiber fixation and molded
projections by high-precision plastic molding technology 1)~3).
2) Dielectric multilayer film WDM filter vapor deposition technology on a fiber facet polished at 30° 3).
3) Spot size conversion using quartz waveguide (SSCPLC) for passive alignment assemblies with Vgrooves 4).
Figure 6 shows a cross-sectional view of the OSA for
the ONU.
The OSA consists of a fiber with WDM filter, a leadframe package, and a plastic package containing aceramic ferrule. The lead-frame package contains a
receiver PD and electric amplifier mounted on a silicon
substrate on the receiver side, an FP-LD mounted on a silicon substrate on the transmission side, a monitor PD,
and pre-molded electric terminals.
Figure 7 is a photograph of the OSA for the ONU. It
measures 13.0 mm in length (without leads), 5.0 mm inheight and 6.2 mm in width. Details of the main parts aredescribed below.
4.2 Precision Molded Plastic Package
The high-precision plastic package was developed withthe molding technology for the MT connector, which is ahighly precise plastic ferrule, to perform relative positioning of the fiber and optical element by passive alignment.
Figure 8 is sectional views of the receiving (Rx) part of the
OSA, the SSC-PLC part, and the transmitting (Tx) part.
Plastic package with Ω shape groove
1550 nm
1490 nmThis plastic molding has three features. 1) A hole that
positions the fiber on the plastic package; 2) Projections
for aligning with V-grooves in the silicon substrate on
which the optical element is mounted; 3) Carry out incidence of the downstream light signal that is reflected by
the WDM filter on the facet of a polished fiber to the PD
through the side of the fiber, by having prepared an opening in the side of the fiber positioning hole in the package.
This fiber fixing groove, called an omega groove
because of its cross-section shaped like an Ω, has thefeature that the inserted fiber cannot come loose, unlikewith the conventional V-groove. We used a pin-PD with an
input window 80 µm in diameter to reduce coupling loss,
and a receiving sensitivity of more than 1.0 A/W was
obtained. Positioning of the transmission side light axis isdone automatically in such a way that the two V-grooves
provided on the silicon substrate (SiOB: Silicon opticalbench) on which the optical element was installed and on
the SSC-PLC mentioned below engage with the shaped
projections on the plastic molding. In order to realize low
loss coupling of a single mode fiber with the LD, the relative position accuracy of the plastic projections and thefiber hole is managed to ±0.5 µm, and the light axis of thesingle mode fiber, SSC-PLC and LD assembles are settled
in ±1 µm. The photographs, alignment state of the projection of the plastic package with the V-groove on the silicon substrate, and the cross-section of the omega groove
are shown in Figure 9.
Plastic packageΩ- shape groovePDRx part

Rx-SiOB

SSC-PLCSMF

PLC core

WDM filter on fiber facet

1310 nmPD

TIA/LIA

1310 nm
FP-LD

SSC part

SSC-PLC

Projection aligned in V-groove
SC feruleFigure 6

LD

Band pass filter
Transmission 1480-1500nm

Structure of OSA for ONU transceiver.

Tx partTx-SiOB

Figure 8

Plastic projection

Cross-sections of OSA.
Plastic package

Fitting

Epoxy resin for sample

Figure 9
Figure 7

Cross-sections of V-groove alignment on silicon substrate (left) and omega groove of plastic mold (right).

Photo of OSA for ONU transceiver.

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Optical Transceiver Modules for Gigabit Ethernet PON FTTH System

4.4 SSC-PLC
As compared with earlier optical fiber communications
systems, GE-PON system requires a higher power signal
light because of PDS system of 16 light branches by the
splitter. The optical output demand specification for the
ONU is from -1 to +4 dBm. On the other hand, in thepassive alignment assembly structure used to meet theneed for lower cost, although it is joined together by coupling a fiber and LD directly without lens, the couplingloss due to the difference of the mode field diameter ofthe fiber and the LD was about 10 dB, and it is difficult toobtain high coupling efficiency beyond that level.
Furthermore, by the single fiber bi-directional module,
since the insertion loss of the WDM filter is also added, forobtaining a desired optical output, it is indispensable toreduce the coupling loss between the LD and the fiber.
In order to meet the conflicting demands of low costand high output power, we have achieved a coupling loss
improvement as well as passive alignment within a highprecision plastic package, by inserting between the LD
and the fiber an SSC-PLC that effects beam spot size
conversion. The optical coupling structure using an SSCPLC is shown in Figure 12. The SSC-PLC is a minute optical waveguide with a length of 500 µm, which has a taper
part with a length of 300 µm, and connection loss is
reduced by changing the spot size of LD light into that of
the fiber. Figure 13 shows the results of calculations ofcoupling loss vs. waveguide width for each coupling of
the SSC-PLC with the LD and with the single-mode fiber.
The refractive-index difference of the waveguide core andclad is 0.8 %. It is expected as a result of calculation thatby making the LD side and the fiber side of the waveguide
4.5 µm and 10 µm in width, respectively, it would be possible to set the coupling loss between the LD and a single-mode fiber to a minimum of 4 dB.

Total loss = 4.0 dB

SSC
100 µm300 µm
100 µmFigure 10 WDM filter vapor-deposited on the facet of angle-polished fiber.

SMF coupling loss (dB)

Transmittance (dB)

WLD

SiOB

12

1.2
20 dB

-20
-30
-40
-50
-60
1200

LD
MPD

WfiberFigure 12 Structure of SSP-PLC.

0-101360 nm
1250

1300

1350

1400

1480 nm
1450

1500

1550

1600

Wavelength (nm)Figure 11 Characteristic of the short wavelength pass filter(SWPF) on a 30 °polished fiber facet.

Fiber

SOLUTION
4.5 µm - 10.0 µm

1.0

0.80%

10
8

0.8
0.6 dB

0.6

6
3.4 dB

0.4
SMF
LD

0.2
0.0

4

0

2

2
4

6

8

10

12

14

16

LD coupling loss (dB)

4.3 WDM Filter on the Fiber Facet
In order to separate the upstream and downstream signals in a GE-PON system, wavelength separation with a
WDM filter is required. The conventional WDM filter isdeposited onto a glass substrate etc., cut into chips several millimeters square, and mounted in the OSA. We
have developed a new technology whereby a WDM filter
is deposited on the facet of a polished fiber with 30° angle
directly in order to minimize filter size. Insertion fixation isthen carried out, the fiber with its filter is seated into theomega groove of the above-mentioned plastic package,and the signal light is coupled with the PD by filter reflection. Thus the WDM filter circuit, which accounts for the
major portion of the OSA cost, was realized simply by thefiber assembly process within a package. Fiber couplingwith the LD has been done within the omega groove inthe plastic molding by butt coupling a fiber polished at
30°. Figure 10 is a photograph of the filter formed of thefiber facet. Figure 11 shows the transmission spectrum
wavelength characteristic of the WDM filter. It gets 20 dB
as the transmission loss difference between the maximum
wavelength of 1360 nm in the upstream band and theminimum wavelength of 1480 nm in the downstream
band. Furthermore, in order to set both the crosstalk within the module of the wavelength of 1310 nm of LD fortransmission and the isolation between the transmission
light and the downstream video signal of 1550 nm to 40
dB or more, a band pass filter with a pass band from 1480
nm to 1500 nm is inserted in front of the PD in addition to
the fiber end filter. In order to fully acquire the characteristic of a band pass filter, the PD was mounted with anangle of 30°, and it is so devised that the reflected light
impinges almost at right angles to the PD.

0

Waveguide width of SSC-PLC (µm)

Figure 13 Calculated result of coupling loss between fiber and
LD and SSC-PLC.

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Optical Transceiver Modules for Gigabit Ethernet PON FTTH System

The optical output-current characteristic of the OSA for
the ONU without a fiber filter obtained experimentally toverify the effect of SSC-PLC is shown in Figure 14. The
coupling loss with the single-mode fiber and the 1310-nm
FP-LD through the SSC-PLC was estimated as 4.4 dB
showing good agreement with the design. As for the fiber
output Pf at 85°C, more than +3 dBm was obtained.
4.5 Characteristics of OSA for ONU
Figure 15 shows the optical output-current characteristicof the OSA for the ONU using SSC-PLC, and a histogramof the total coupling loss. As a result of adding WDM filter
loss, mounting variation, and drop in emitting efficiency of
LD caused by the generation of heat due to the built-in
amplifier IC etc., the total loss was approximately 8 dB±1
dB. The optical coupling efficiency within the sameassembly lot shows normal distribution, so that reproducibility of assembly is obtained.
On the other hand, as for the receiving sensitivity of the
OSA for the ONU for the 1490-nm wavelength, more than
1.0 A/W was obtained. As for the overload, +3 dBm was
obtained.
4.6 Structure and Characteristics of OSA for OLT
For the OSA for the OLT of a GE-PON system it is indispensable for reasons of the specification to use a DFB-LD
having a 1490-nm wavelength. Particularly in the 20-km
version an optical output of from ±2 to ±7 dBm is called
for. Figure 16 shows the OSA for the OLT developed here.
It is of hybrid construction with a metal can packaged
DFB laser having an aspheric surface lens and a plastic
package OSA containing a PIN-TIA/LIA with a built-in
WDM filter (LWPF) that is the same as for the receiver side

Ω shape groove plastic package
Metal ferrule

SMF

1.49um DFB-LD CAN

WDM filter on fiber facet

1310 nm
1490 nmPD

TIA/LIA

Optical isolator
Aspheric lensSC ferrule

Figure 16 Schematic diagram of OSA for OLT transceiver.

of the OSA for the ONU. In order to perform alignment
and laser welding fixation, the metal ferrule was insertionmolded to a high-precision plastic mold package. To
reduce the influence of the reflected return light, an optical isolator was mounted on the facet of the metal ferrule.
Figure 17 is a photograph of the OSA for the OLT. Its sizeis 24 mm in length (without leads), a maximum of 5.6 mm
in height, and 6.2 mm in width.
Figure 18 shows the temperature dependence of the
optical output vs. current characteristic of the OSA for the
OLT. The optical output was more than 5 mW (+7 dBm),
which satisfies the upper limit of the target specification
for 85°C. The total insertion loss of the DFB-LD assuming
a maximum filter transmission loss of 1 dB was 3 dB. As
for the receiving sensitivity at 1310-nm wavelength, 1.0 A/W
or more was obtained.

5.

TRANSCEIVER CHARACTERISTICS

5.1 Transceiver for ONU
The result of evaluating the burst mode transmitter characteristic required for an ONU transceiver is shown in
Figure 19. Burst transmission was carried out in the wave-

6
-40°C
5

0°C
25°C

Pf (mW)

4
3

70°C
85°C

2
1
0

0

10

20

30

40

Figure 17 OSA for OLT transceiver.

50

If (mA)

16

Figure 14 L-I characteristics of ONU OSA without WDM filter.

14

25°C

8

70°C
85°C

0

20

40
If (mA)

60

80

n=20

6

Pf (mW)

10
Frequency

-15°C

10

6
4

2

2
-5

-6 -7 -8 -9 -10 Next
Coupling Loss(dB)Figure 15 L-I characteristics and histogram of coupling loss of
ONU OSA.

70° C

8

4

0

25° C

12

12

Pf (mW)

10
9
8
7
6
5
4
3
2
1
0

-10° C
0° C

0

85° C

0

20

40
If (mA)

60

80

Figure 18 Temperature dependence of L-I characteristics of
OSA for OLT transceiver.

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Optical Transceiver Modules for Gigabit Ethernet PON FTTH System

Trace <up>
Burst enable signal(Reverse)

Trace <low>
Burst optical signal(a)200ns/div(c)(b)

Figure 21 Eye diagram of OLT transceiver.
200ps/div
After 30nsec of Ton

200ps/div
Before 2bit time of Toff(d)

200ps/div
Eye diagram under continuous mode operation

Figure 19 Eye diagram of ONU transceiver in burst mode operation.

form shown in (a) in the figure, and measurement of therise time and fall time and the transmitting eye at the timeof burst transmission was obtained.
Both Ton and Toff times were several nanoseconds. Theeye patterns under the burst transmission mode shown in(b) and (c) in the figure are equivalent to the eye patternshown in (d) measured in continuous light emitting mode,
and in all cases sufficient margin was obtained for theIEEE802.3ah standard.
The receiving characteristic of the transceiver for the
ONU is shown in Figure 20. The minimum received powerof -29.0 dBm was obtained at a bit error rate of 10-12 insimultaneous transmission/reception operation. A 5-dB
margin was obtained with respect to the target minimum
1e-03
1e-04

Bit error rate

1e-05
1e-06

Ambient temperature 25°C
Supply voltage 3.3 V
1.25 Gb/s, PRBS 27-1

receiving sensitivity specification of -24 dBm. In addition,
sensitivity degradation by crosstalk in simultaneous transmission/reception operation (Tx on) was 0.5 dB.
5.2 Transceiver for OLT
The eye pattern of the transceiver for the OLT under continuous mode operation is shown in Figure 21. A goodeye pattern was obtained at 1.25 Gbps.
The burst receiving characteristic of the transceiver for
the OLT was measured connecting two ONUs to an OLT
transceiver under the input conditions shown in Figure 22.
It was measured under the severest conditions for burst
received signals, i.e., inputting an optical signal overload
as a dummy signal, inputting an optical signal of the minimum receiving sensitivity into the measurement signal,and setting the burst interval to 0 ns. The settling time of
the receiver was about 300 ns, and it was confirmed that
there was sufficient margin with respect to the 400 ns of
the IEEE802.3ah standard.

Measurement condition: Burst guard time = 0 nsec
ONU#1
(Dummy signal)
ONU#2(Measured signal)

Input opticalsignal to OLT

Transmitter extinction ratio:
9.0 dB:Transmitter OFF:Transmitter ON

Output electricalsignal

1e-07
1e-08
1e-09
1e-10
1e-11
1e-12
1e-13
1e-14

Expand of C-C'
1µsec after receive signal

-35
-30
-25
-20
Average received power (dBm)

Figure 20 Bit error rate of ONU transceiver.

Figure 22 Burst mode receiver characteristics of OLT transceiver.

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Optical Transceiver Modules for Gigabit Ethernet PON FTTH System
1e-03

Ambient temperature 25°C
Supply voltage 3.3 V
1.25 Gb/s, PRBS 27-1

1e-04

Transmitter extinction ratio:
9.0 dBBit error rate

1e-05

:Transmitter OFF:Transmitter ON

1e-06
1e-07
1e-08
1e-09
1e-10
1e-11
1e-12
1e-13
1e-14

6.

CONCLUSIONS

We have developed bi-directional transceiver modules for
GE-PON FTTH systems.
The OSA developed here was able to achieve miniaturization of the transceiver module, and simultaneously
reduce the cost by means of a passive alignment structure. The completed transceiver module fully satisfies theIEEE802.3ah standard, and application to ONU and OLT
apparatus can be expected.

-35

-30
-25
-20
Average received power (dBm)

Figure 23 Bit error rate of OLT transceiver.

Figure 23 is the bit error rate of the transceiver for the
OLT. The minimum receiving sensitivity at a bit error rateof 10-12 in simultaneous transmission/reception operation
was -28.5 dBm, and a 1.5-dB margin was obtained withrespect to the target minimum receiving sensitivity specification of -27 dBm.
Sensitivity degradation by crosstalk in simultaneoustransmitting/receiving operation (Tx on) was 0.5 dB.

REFERENCES
1) M. Iwase, “Passive Alignment Optical Modules using High
Precision Plastic Package and Silicon Optical Bench Technologies”,
OECC2002 10C1-1, pp134-135.
2) M. Iwase, T. Nomura, A. Izawa, H. Mori, S. Tamura, T. Shirai, T.
Kamiya, “Single Mode Fiber MT-RJ SFF Transceiver Module Using
Optical Subassembly With a New Shielded Silicon Optical Bench”
IEEE Trans. Advanced Packaging, Vol. 24, No.4 (2001) pp419-428.
3) M. Iwase, Y. Ishikawa, A. Izawa, K. Mizuno, H. Abe, H. Kawashima,
K. Nara, Proceedings of the 2004 IEICE General Conference, C-362, p235. (in Japanese)
4) H. Kawashima, K. Nara, Y. Ishikawa, M. Iwase, Proceedings of the2004 IEICE General Conference, C-3-112, p286. (in Japanese)

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