Letters

A Compact Low-cost 4-channel × 10-Gbit/s Optical Module
for Coarse Wavelength Division Multiplexing Links
Takeshi Sakamoto†, Nobuo Sato, Shinji Koike,
Koichi Hadama, and Naoya Kukutsu
Abstract
We have developed a compact low-cost optical module that has four 10-Gbit/s transmission channelsof different wavelengths giving a total throughput of 40 Gbit/s. The target applications of the module areshort-reach links such as inter-rack connections in POP (point of presence) and WDM (wavelength division multiplexing) access networks. Although 40-Gbit/s laser modulators, photodiodes, and driver LSIs
(large-scale integrated circuits) are available, they are still expensive. However, a 10-Gbit/s optical link
can be made using cheap direct-modulation laser diodes and low-cost CMOS (complementary metaloxide semiconductor) driver LSIs. Thus, a four-channel CWDM (coarse wavelength division multiplexing) link is the most cost-effective way to achieve 40-Gbit/s transmission. Our module has a compact
package size of 33 × 30 × 6.3 mm3 and is assembled using a simple optical alignment process based ona novel silicon platform technique to reduce the assembly cost. We verified CWDM transmission over a
10-km fiber with a pair of commercial available CWDM multiplexer/demultiplexer filters.

1. Introduction
A CWDM (coarse wavelength division multiplexing) link has great potential to reduce network costs
because it can carry multiple applications on an optical fiber and is less expensive than a dense WDM(DWDM) link, which requires costly light sourcesand expensive optical filters with high wavelength
accuracy. These days, the application areas for
CWDM links are growing. One example of a new
application is the WDM PON (passive optical network) system (Fig. 1), which is a point-to-multipoint
subscriber access network that provides higher bandwidth than PONs based on time-division-multiplexing. Another application is VSR (very short reach)interconnection in POP (point of presence) (Fig. 2).
An IA (implementation agreement) for an OC-768
VSR link using four-wavelength CWDM has been
published by the Optical Internetworking Forum(OIF) [1]. An example configuration of the CWDM
† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: tsakamot@aecl.ntt.co.jp

Vol. 3 No. 2 Feb. 2005

ONU

WDM
filter
OLT
NTT

OLT: optical line terminal
ONU: optical network unit

Fig. 1. WDM PON system.

VSR module is shown in Fig. 3. This configuration
using multiple 10-Gbit/s units will enable a costeffective 40-Gbit/s module because 10-Gbit/s opticallinks can be made using cheap direct-modulation
laser diodes (LDs) and low-cost CMOS (complementary metal oxide semiconductor) driver LSIs (largescale integrated circuits).
Although CWDM has great potential to reduce thelink cost, there is still one major issue. A convention25

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WDM backbone

POP

ADM

One module with four
10-Gbit/s channels

Four 10-Gbit/s
modules

WDM

Fig. 4. A compact multi-channel optical module would
reduce the number of optical modules and makethe system compact.

VSR

VSR

IP network
ADM: add-drop multiplexer
1 cm

Fig. 2. Typical configuration of a POP (point of presence).
Fig. 5. Photograph of the 4-ch × 10-Gbit/s module.
40-Gbit/s
CWDM
VSR module

4-wavelength
CWDM

CWDM
filterTable 1. Module specifications.

4-ch × 10-Gbit/s
electrical signals

Four 10-Gbit/s
transmitter/receivermodulesFig. 3. Example configuration of CWDM VSR module.

al CWDM link system requires as many transmitterand receiver modules as there are wavelengths, as
shown in Fig. 3. This requires a large amount of board
space and results in a high system cost. For applications such as the OC-768 link and optical line terminal (OLT) in the WDM-PON system, a compact
multi-channel optical module (Fig. 4) would reduce
the number of optical modules for the CWDM links
and make the system compact and inexpensive.
We have developed compact low-cost 4-ch × 10Gbit/s optical transmitter and receiver modules forsuch CWDM applications. Each module has a total
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Data rate per channel

up to 10 Gbit/s

Number of channels

4Package size

33 mm × 30 mm × 6.3 mm

Output power per channel>+4 dBm

Receiver sensitivity–17 dBm

Transmission distance

10 km

Laser diodeDirect-modulation DFB-LD

PhotodiodePIN PD

Wavelengths1275.7, 1300.2, 1324.7, 1349.3 nm

throughput of 40 Gbit/s and its volume is only 6.2
cm3. It is assembled using a simple optical alignment
process based on a novel silicon platform technique,which we call the PALC (passive-alignment laser collimator) bench.
2. Module construction
A photograph of the module is shown in Fig. 5, andits specifications are shown in Table 1. The transmitter and receiver are separate modules and have thesame appearance. Each module has four transmission
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Groundcoplanar
lines

Lid

Opticalsubassembly
Baseplate

Optical bench
L-shaped metal
bracketMonitor PD
mounting bar
Fiber guide

10 mmDriver LSI

Fig. 7. Photograph of the ceramic-metal hybrid package.
Fiber guide
Bottom plate
Ceramic wiring board

Fig. 6. Exploded view of the transmitter module.

channels, each of which can carry data at a rate of up
to 10 Gbit/s. The lasers in the transmitter are directmodulation distributed-feedback laser diodes (DFBLDs), and the photodetectors in the receiver are PIN(positive-intrinsic-negative) PDs (photodiodes). The
transmitter has LD driver LSIs and monitor PDs. The
receiver has transimpedance amplifier (TIA) LSIs.
The transmission distance is up to 10 km, which isenough for inter-rack interconnections and mostaccess networks.
There are two different approaches to constructing
a multichannel optical module: using device arrays or
discrete devices. Optical device arrays and LSI arrays
can minimize the size of a multichannel module, but
optical device arrays are intrinsically expensive
because of their low product yield. On the other hand,single-channel LD drivers and TIAs have a broad
market and are getting cheaper. However, 10-Gbit/s
driver arrays and TIA arrays are not available yet, andwhen they are, they will be expensive because of theirlimited market. The use of single-channel devices has
another advantage. It suppresses inter-channel
crosstalk, which is a serious issue in arrayed devices,
especially for multi-gigabit-per-second transmission.
Thus, we chose an independent-channel structure,
which uses single-channel optical devices and LSIs.
An exploded view of the transmitter (Tx) module isVol. 3 No. 2 Feb. 2005

shown in Fig. 6. To reduce the cost, we designed it assimply as possible. A Tx optical subassembly (OSA)or receiver (Rx) OSA is assembled in advance andmounted in a package. Optical alignment is completed on the OSA, so the package does not require
mechanical accuracy for optical alignment. The driver LSIs or TIA LSIs are mounted on an LSI carrier
and connected to the OSA and high-speed signal lines
on the ceramic wiring board with gold wires. The LSI
carrier is separated from the OSA, so that heat fromthe LSIs is not conducted to the optical devices. The
Tx module has monitor PDs. A monitor PD mounting
bar, which contains four monitor PDs, is mounted on
the ceramic package to receive back illuminationfrom four LDs.
2.1 Package
A photograph of the ceramic-metal hybrid package
is shown in Fig. 7. A ceramic wiring board and a
metal fiber guide with fiber grooves are soldered to ametal base plate. The OSA and the LSI carrier areplaced on the baseplate. The wiring board contains
ground coplanar lines for 10-Gbit/s signals and feedthrough electrodes that allow hermetic sealing. The
package is designed to be easy to seal. The fiber feedthrough is hermetically sealed just by placing metalized fibers in the grooves of the fiber guide and soldering the lid on; the whole package is sealed at thesame time. The S-parameters of the package areshown in Fig. 8. The –3 dB bandwidth of S21 isgreater than 20 GHz, and S11 is better than –15 dB at
10 GHz, so the package can handle 10-Gbit/s signals.

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2.2 Optical subassemblies
A simple optical coupling structure is the mosteffective way to reduce the cost of the module
because the dominant cost is for optical alignment.
We have developed new Tx and Rx OSAs using a silicon technique that greatly simplifies optical alignment. Details are given below.
2.2.1 Tx OSA
We chose a two-lens optical configuration for theTx OSA to obtain high coupling efficiency because
the output power of a CWDM Tx module must be
higher than that of a non-CWDM module to compensate for optical loss of CWDM filters. A high coupling efficiency also enables us to reduce the laser
current and the power to the laser drivers. This resultsin lower power consumption, which helps with thermal management. However, the alignment process
for two-lens optics is much more complex than thatfor single-lens optics. To simplify the alignment, weinvented a new optical platform for the Tx OSA,
which we call the PALC bench (Fig. 9) [2]. It consistsof a silicon optical platform, four LDs, and four 1st
lenses. The LDs and 1st lenses are mounted on theplatform by a passive alignment process. The silicon
optical platform has four lens pockets for the 1st lenses and four electrode patterns to mount LDs. The LDs
are mounted on the electrode pattern by flip-chipbonding and a visual alignment technique having
positional accuracy within 1 µm [3]. The lens pockets are made by cutting the silicon bench with a dicing saw. The 1st lenses, which have a cubic shape areput in the lens pockets and pushed against the pocket

walls, so they are aligned mechanically by the edgesof the pockets. These lens pockets enable us to mountthe 1st lenses without a complex active alignment
process. A schematic of the Tx OSA is shown in Fig.
10. The PALC bench and four fiber-with-lens assemblies are mounted on the L-shaped metal bracket.
Four fiber-with-lens modules are mounted using anactive alignment technique, which does not require
submicrometer accuracy because the ±1-dB toleranceof the fiber-with-lens module is nearly ±10 µm. Thiswide tolerance comes from the large beam size emitted from the PALC bench, which is over 500 µm, anddrastically reduces the time required for active alignment. A photograph of the complete Tx OSA in the
package is shown in Fig. 11. The Tx OSA is soldered
onto the package baseplate and connected electrically to the driver LSIs with gold wires.
2.2.2 Rx OSA
A schematic of the Rx OSA, which has four Rx
optical benches, is shown in Fig. 12. Each bench consists of a silicon platform with a V-groove, a fiberwith an angled facet, and a PD. The PD is mounted on
the silicon bench using a visual alignment process.

1st lens

Electrode

Silicon platform

14 mm
Lens pocket

0

0

–10

–1

Fig. 9. Passive alignment laser collimator (PALC) bench.

–20

–2

–30

–3
S21 (dB)–40

S21 (dB)

S11 (dB)

1st lens

L-shaped metal bracket

Laser
diode

–4
S11 (dB)
PALC bench

–50

0

5

10

15

Frequency (GHz)

Fig. 8. S-parameters of the package.

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–5
20

Fiber-with-lens assembly

Fig. 10. Transmitter optical subassembly (Tx OSA).

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Letters

Tx OSARx OSA

Fig. 11. Tx OSA mounted in the package.

Fig. 13. Rx OSA mounted on the package.

Rx benchwith 1 channel operating
45-degreecut fiberwith 4 channels operating

PD
Siliconplatform

ch 1

ch 2

V-groove

ch 3

ch 4
Metal
block

Fig. 12. Rx OSA.

Fig. 14 . Eye diagrams for the Tx module.

The fiber is aligned along the V-groove. Input signal
light is reflected at the angled facet of the fiber andthereby coupled to the PD. This simple configuration
makes the Rx OSA module inexpensive. The complete Rx OSA mounted on the package is shown in
Fig. 13. The Rx OSA is also connected to the TIA
LSIs with gold wire after being soldered onto thebaseplate.

shows eye diagrams when only one channel was
operating, while the right one shows eye diagrams
when all the channels were operating. The diagrams
all show good eye openings, and the waveforms arealmost the same regardless of the number of channels
operating. These results show that crosstalk is quite
low, even at 10 Gbit/s.
Figure 15 shows eye diagrams for the receiver for
input power of –20 dBm without signals on the adjacent channels (a), when the input power to the adjacent channel was also –20 dBm (b), and when it was–10 dBm (c). Even when the power to a neighboring
channel was 10 dB higher than that to the observed
channel, very little jitter due to crosstalk was

3. Evaluation results
Filtered eye diagrams for the Tx module at 10
Gbit/s with a pseudorandom binary sequence (PRBS)
223–1 pattern are shown in Fig. 14. The left column
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observed. These results demonstrate that the crosstalkin the module is quite low.
A block diagram of the CWDM transmission evaluation system is shown in Fig. 16. Optical outputs
from the four-wavelength Tx module were wavelength-division multiplexed by a commercially available CWDM multiplexer (MUX) module and transmitted on a single-mode fiber. The transmitted WDM
signal was demultiplexed by a CWDM demultiplexer(DEMUX) module and received by the Rx module.
The spectrum of the Tx output after the CWDM MUX
module is shown in Fig. 17. There are four peaks,
which correspond to the channel output wavelengths.
The bit-error-rate (BER) characteristics at 10 Gbit/s
are shown in Fig. 18. At a BER of 10–12, the received
power was –17 dBm. This indicates that the sensitivity of the receiver module is high enough. In addition,
all the lines nearly overlap regardless of the transmission distance and number of operating channels.
From this result, we can conclude that the module issuitable for transmission over a distance of 10 km andthat the crosstalk is very low.

(a)

(b)

(c)

Fig. 15. Eye diagrams of Rx output in 10-Gbit/s operation.
Received power of the observed channel was –20
dBm. (a) No input on the disturbance channel. (b)–20 dBm on the disturbance channel. (c) –10
dBm on the disturbance channel.

10 Gbit/s
×4

λ1
λ2
λ3
λ4

Tx

4. Conclusion
We have developed a compact low-cost opticalmodule for CWDM applications. It has four 10Gbit/s transmission channels of different wave-

CWDM
MUX

CWDM
DEMUX
λ1 to λ4

λ1
λ2
λ3
λ4

Rx

Fig. 16. Block diagram of the CWDM evaluation system.

–8.6 REF
dBm
5.0dBm
/div

–33.6
dBm

–58.6
dBm
1260.0 nm

10 nm/div

1310.0 nm

in Vac1360.0 nm

Fig. 17. Tx output spectrum after the CWDM MUX module.

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10–3

10–4

Bit error rate

lengths giving a total throughput of 40 Gbit/s. We
designed the new transmitter and receiver OSAs to
reduce the assembly time and cost. We verified thatthe module is suitable for 40-Gbit/s 10-km CWDM
transmission.

PRBS 223–1
with CWDM filter
1 ch operating(back to back)
4 chs operating(back-to-back)
1 ch operating(10 km)4 chs operating(10 km)

10–5
10–6

References
[1]

10–7
10–8
10–9

[2]

10–10
10–11
10–12–22

–20

–18

–16

–14

–12

–10

Received power (dBm)

Fig. 18. Bit-error-rate characteristics for CWDM
transmission.

[3]

Optical Internetworking Forum (OIF) Implementation Agreement,“OIF-VSR5-01.0: Very Short Reach Interface Level 5 (VSR-5):
SONET/SDH OC-768 Interface for Very Short Reach (VSR) Applications,” http://www.oiforum.com/public/documents/OIF-VSR501.0.pdf.
T. Sakamoto, N. Sato, S. Koike, K. Hadama, N. Kukutsu, T.
Hashimoto, and T. Ohyama, “4 channel × 10 Gbit/s Parallel Optical
Module for Short Reach Optical Links,” 2003 IEEE LEOS Annual
meeting, Vol. 2, pp. 567-568, Oct. 2003.
T. Hashimoto, A. Kanda, R. Kasahara, I. Ogawa, Y. Shuto, M. Yanagisawa, A. Ohki, S. Mino, M. Ishii, Y. Suzuki, R. Nagase, and T. Kitagawa, “A Bidirectional Single Fiber 1.25 Gb/s Optical Transceiver
Module with SFP Package using PLC,” Proc. of 53rd ECTC, pp. 279283, 2003.

Takeshi Sakamoto

Shinji Koike

Research Engineer, Smart Devices Laboratory,
NTT Microsystem Integration Laboratories.
He received the B.E. and M.E. degrees in electronic engineering from Kyoto University, Kyoto
in 1994 and 1996, respectively. In 1996, hejoined NTT Opto-electronics Laboratories,
where he studied coding systems for parallel
optical interconnection. His current researchinterest is high-capacity parallel optical link
modules. He received the IEEE CPMT YoungAward in 2002. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE) of Japan and IEEE.

Senior Research Engineer, Photonic Processing Devices Research Group, Photonics Integration Laboratory, NTT Photonics Laboratories.
He received the B.E. and M.E. degrees in precision engineering and Ph.D. degree in material
and life science from Osaka University, Suita,
Osaka in 1986, 1988, and 1997, respectively. In
1988, he joined NTT Applied Electronics Laboratories, where he engaged in R&D related tomicrowave packaging technology. He engaged
in research on optical coupling devices for optoelectronics multichipmodules (OE-MCMs) in
NTT Interdisciplinary Research Laboratories
from 1989 to 1995. He engaged in R&D concerning packaging systems for high-throughput
switching at NTT Network Service Systems Laboratories from 1995 to 1997. He was engaged indeveloping photonic packaging systems (PhaiPAS) from 1997 to 2000 in NTT Opto-electronics Laboratories and NTT Telecommunications
Energy Laboratories. He was engaged in developing optical transceiver modules from 2001 to
2003 in NTT Telecommunications Energy Laboratories and NTT Microsystem Integration Laboratories. He is a member of IEEE, Japan Societyof Applied Physics, IEICE, and Japan Institute ofElectronics Packaging.

Nobuo Sato
Senior Research Engineer, Smart Devices Laboratory, NTT Microsystem Integration Laboratories.
He received the B.S. degree in physics from
Tokyo University of Science, Tokyo in 1977. In
1970 he joined NTT Electrical Communications
Laboratories, Tokyo, Japan, where he has been
engaged in research on packaging technology for
optical interconnection module. He is a memberof IEICE.

Naoya Kukutsu

Koichi Hadama
Smart Devices Laboratory, NTT Microsystem
Integration Laboratories.
He received the B.E. and M.E. degrees inapplied physics from the University of Tokyo,
Tokyo in 1999 and 2001, respectively. In 2001,
he joined NTT Telecommunication Energy Laboratories. He is currently with NTT Microsystem
Integration Laboratories, engaged in developing
microoptics modules for fiber communication
networks. He is a member of IEICE and the Optical Society of Japan.

Vol. 3 No. 2 Feb. 2005

Senior Manager, R&D Vision Group, Department III (R&D Strategy).
He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from Hokkaido University, Sapporo, Hokkaido in 1986, 1988, and1991, respectively. In 1991, he joined NTT
Applied Electronics Laboratories, where heengaged in R&D of packaging technology forhigh-speed and high-frequency electronic
devices. In 1996, he moved to Multimedia Networks Laboratories and engaged in R&D of IP
network systems. From 1999, he researched optical interconnection technologies for VSR. In
2004, he moved to Department III. He is a member of IEICE and IEEE.

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