High-Performance and Low-Cost 40-Gb/s CWDM Optical Modules

644 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 32, NO. 3, AUGUST 2009

High-Performance and Low-Cost 40-Gb/s

CWDM Optical Modules

Min-Ching Lin, Tien-Tsorng Shih, Pei-Hao Tseng, Kuei-Ming Chu, Chieh Hu, and

Wood-Hi Cheng, Senior Member, IEEE

Abstract—High-performance and low-cost 40-Gb/s optical

modules using four different wavelength uncooled 10-Gb/s

dis-tributed-feedback (DFB) lasers are proposed and demonstrated.

The 40-Gb/s optical module was integrated with coarse wavelength

division multiplexing (CWDM) thin-film filters which enabled

four 10-Gb/s transmission channels output through a single fiber.

The 10-Gb/s DFB laser was packaged by commercialized low-cost

coaxial TO-Can technology. The results of the 40-Gb/s optical

module showed that the output optical power was above

1

dBm per channel and the system power budget was 12 dB. The

transmission distance with a single-mode fiber reached more than

30 km at a bit-error-rate of

10

. Compared with conventional

40-Gb/s optical modules, the module is easy to fabricate and is low

cost. This proposed high-performance 40-Gb/s CWDM optical

module demonstrates not only the feasibility of a 30 km

trans-mission, but also shows the low-cost possibility of ensuring the

application of WDM-passive optical network fiber-to-the-home

systems.

Index Terms—Coarse wavelength-division multiplex (CWDM),

fiber-to-the-home (FTTH), low-cost, OC-192, wavelength division

multiplexing-passive optical network (WDM-PON), 40-Gb/s.

I. I

T

HE high data rates requirements in telecom and datacom

are growing rapidly. The access network is the major

bot-tleneck in most modern telecom societies. Due to the

installa-tion cost reducinstalla-tion and the bandwidth demand of clients, these

access networks have been upgraded gradually from the

low-speed copper solution to the high-low-speed optical network. To

overcome access network bottleneck, a higher bit rate

transmis-sion is necessary.

Recently, the solutions of 40-Gb/s applications by using

ex-ternal modulated devices, such as electric-absorption

modula-tors or Mach–Zehnder modulamodula-tors have been developed [1]–[3].

However, the cost of these 40-Gb/s solutions is expensive for

the access network applications. In contrast, coarse wavelength

division multiplexing (CWDM) technology by using 10-Gb/s

Manuscript received December 18, 2007; revised May 03, 2008, March 18, 2009. Current version published August 05, 2009. This work was supported in part by MOE Program of the Aim for the Top University Plan, Taiwan. This work was recommended for publication by Associate Editor A. Shapiro upon evaluation of the reviewers comments.

M.-C. Lin, P.-H. Tseng, K.-M. Chu, and W.-H. Cheng are with the Insti-tute of Electro-Optical Engineering, National Sun Yat-Sen University, Kaoh-siung, Taiwan (e-mail: [email protected]; 1094319105@cc. kuas.edu.tw; [email protected]; [email protected]).

T.-T. Shih is with the Department of Electronic Engineering, National Kaoh-siung University of Applied Sciences, KaohKaoh-siung, Taiwan (e-mail: [email protected]. edu.tw).

C. Hu is with the Digital Imaging Department, Laser Application Technology Center, ITRIS/ITRI, Tainan, Taiwan (e-mail: [email protected]).

Digital Object Identifier 10.1109/TADVP.2009.2020691

components can leverage the advantage of moderate cost and

reliable technology effectively. Such technology is becoming

an attractive approach due to its low-cost requirements. Several

studies have developed parallel optical modules with an

aggre-gated transmission data rate up to 40-Gb/s [4]–[7]. These optical

modules required additional CWDM MUX/DEMUX couplers

to combine different wavelengths of signals into a single fiber.

This would not only increase the cost of the 40-Gb/s system, but

would also need more room to setup.

In this study, four different wavelength uncooled 10-Gb/s

distributed-feedback (DFB) lasers have been packaged into

a 40-Gb/s optical module. Due to cost consideration, these

10-Gb/s DFB lasers adopted the existing low-cost coaxial

TO-CAN package format [8]. In order to reduce the room

for additional CWDM MUX/DEMUX coupler, four coaxial

10-Gb/s DFB lasers were integrated with three band-pass

thin-film filters (TFFs) to form a zig-zag optical path inside

the module. The proposed optical module operating at 10-Gb/s

per channel could exceed over 30 km transmission at the

bit-error-rate (BER) of

. When the BER was set at

,

the module transmitted over 20 km. The average system power

penalty of the proposed optical module was about 12 dB. The

proposed high-performance 40-Gb/s CWDM module

demon-strates not only the feasibility of a 30 km single-mode fiber

(SMF) transmission, but also shows the low-cost possibility

that ensures the application of WDM-passive optical network

(WDM-PON) fiber-to-the-home (FTTH) systems.

The other sections of this paper are organized as follows.

Sec-tion II describes the device structure and fabricaSec-tion process.

The measurement results of the 40-Gb/s CWDM optical module

and discussion are presented in Section III. A brief conclusion

is given in Section IV.

II. M

S

F

A. Fabrication of the DFB Laser With Collimator Output

For cost-efficiency consideration, an uncooled 10-Gb/s DFB

laser diode was bonded on a conventional transistor-outline

(TO)-56 header. A ball lens cap was welded on the top of the

header to focus the laser light. The resonant effect induced

by the TO-Can package degraded the radio-frequency (RF)

performance. In order to reduce the impedance mismatch and

the parasitic effects, a built-in matching resistor was used inside

a low-cost TO laser module [8]. The impedance of the DFB

laser was designed at 25

. This low-cost coaxial type package

is suitable for 10-Gb/s applications.

In order to extend the propagation distance of the light source

in free-space, an optical collimator was integrated with the

LIN et al.: HIGH-PERFORMANCE AND LOW-COST 40-GB/S CWDM OPTICAL MODULES 645

Fig. 1. Photograph of the DFB laser module with a collimated light output.

Fig. 2. Coupling efficiency of the optical collimator as a function of position.

cost coaxial type DFB laser by adopting the laser welding

tech-nique [8]. Fig. 1 shows the photograph of the DFB laser with a

collimator output. Because the conventional optical collimator

with gradient index (GRIN) lens finds it difficult to obtain a long

working distance in free-space transmission, an optical

colli-mator with C-lens was used to increase the working distance and

the coupling efficiency, without increasing the cost. The

cou-pling efficiency between two optical collimators at different

po-sitions is shown in Fig. 2. The working distance of each optical

collimator was chosen at 85, 95, 115, and 125 mm in order to

propagate different optical paths. The coupling efficiency

main-tained above 70% within a propagation distance of 60–150 mm.

This coupling efficiency could be converted into 0.72 dB for the

insertion loss. The beam diameters of these optical collimators

were about 0.5 mm. This C-lens optical collimator was less

sen-sitive in working distances than the conventional collimator with

GRIN lens. The dimension of the DFB laser with a collimator

output was 3.0

0.5

0.5 cm . To ease the RF response

mea-surement, the DFB laser was attached to a FR4 printed-circuit

board (PCB) with an SMA connector. The performance of the

DFB laser will be described in Section III.

B. Structure and Fabrication of the Proposed 40-Gb/s CWDM

Optical Module

Fig. 3 shows the prototype of the 4-cahnnel CWDM optical

module with each channel modulated at 10-Gb/s giving an

ag-gregated data rate of 40-Gb/s. This CWDM module consisted of

four uncooled 10-Gb/s DFB lasers as the input port 1–4 and one

single-mode fiber as the output port. The emission wavelengths

Fig. 3. Photograph of the 40-Gb/s CWDM module.

of port 1–4 were 1275, 1350, 1325, and 1300 nm, respectively.

The DFB lasers were attached at the side of the module. Since

the propagation distances of these four DFB lasers with

inte-grated collimators were different, the working distance of the

devices were different. Because the 1300 nm DFB laser was

arranged at the longest propagation channel, it should be

inte-grated with the longest working distance collimator.

The zig-zag architecture was adopted as the optical path by

using three pieces of band-pass thin-film filters to transmit or

reflect the collimated laser beams. The dimensions of the TFFs

were 1.4

1.4

1.0 mm . The band-pass wavelengths of the

TFF1, TFF2, and TFF3 were 1275, 1350, and 1325 nm,

respec-tively. The transmission bandwidth of the TFFs was about 16

nm. The transmission-band and stop-band losses of the TFFs

were 0.3 and 30 dB, respectively. Because a larger incident

angle would shift the bandwidth range of band-pass TFFs to

short wavelengths, the incident angle of every TFF was limited

by 12 . Therefore, the maximum full angle of every light trace

in the optical module was designed at 22 . The coupling

toler-ance of optical collimator alignment was very sensitive at the

tilted angle (smaller than 0.1 ). The coupling tolerance at the

-direction was less sensitive by adopting the C-lens. The

di-mension of the zig-zag architecture can be reduced by adopting

wider acceptance-angle TTFs.

The fabrication process of the zig-zag architecture TFFs

CWDM coupler is described as follows. First, the ouput port

collimator with an SMF was attached to the metal base. Then

the TFFs were placed at the mark position on the metal base.

The rotation angles of the TFFs can influence the light coupling

severely. The optimized angle of TFF1 was determined by

the maximum coupling power of 1350-nm DFB laser to the

output port fiber. The TFF was fixed by using UV epoxy after

the alignment process. After the coupling measurements of

the 1325 and 1300 nm DFB lasers, the angles of TFF2 and

TFF3 were determined. The dimension of the metal base was

63

46

5 mm . The insertion loss of the zig-zag architecture

TFFs CWDM coupler was less than 2.5 dB for every channel.

The fabrication flow chart of the 40-Gb/s CWDM module

is shown as Fig. 4. Compared with the conventional CWDM

MUX/DeMUX coupler which dimension was 110

80

20

mm

[9], the dimension of this proposed module was

110

60

20 mm . The designed module is compact, which

can save a lot of installation space.

LIN et al.: HIGH-PERFORMANCE AND LOW-COST 40-GB/S CWDM OPTICAL MODULES 649

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Min-Ching Lin was born in Taipei, Taiwan, in 1979.

He received the B.S. degree in electrical engineering from Yuan-Ze University, Taoyuan, Taiwan, in 2001, the M.S. and Ph.D. degrees in electro-optical engi-neering from National Sun Yat-Sen University, Kaoh-siung, Taiwan, in 2003 and 2009, respectively.

His main research interests are high-speed opto-electronic packaging and radio-frequency circuit de-sign for telecommunication applications.

Tien-Tsorng Shih was born in Taiwan, in 1965. He

received the B.S. and Ph.D. degrees from the National Chiao Tung University, Taiwan, in 1986 and 1991, respectively.

In 1991, he joined Telecommunication Laborato-ries, Taiwan, as a Research Associate. From 1996 to 2000, he was a Project Manager at Chunghwa Telecommunication Laboratories, Taiwan. In 2000, he founded Infomax Optical Technology Corpo-ration and was the CEO during 2000–2003. He is now an Assistant Professor at the Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Taiwan. His main research interests include the theoretical study of optical waveguides and III-V optoelectronic devices, fabrication of laser diodes, photo-diodes, and planar lightwave circuits, packaging technology for optoelectronic devices, transceiver modules, and transmission technologies for the fiber optics communication applications.

Pei-Hao Tseng was born in Kaohsiung, Taiwan, on

October 30, 1981. He received the B.S. degree in electrical engineering from Chung Hua University, Hsinchu, Taiwan, in 2004 and the M.S. degree in Institute of Photonics and Communications engi-neering at the National Kaohsiung University of Applied Sciences. He is currently working Ph.D. degree in electro-optical engineering, National Sun Yat-sen University, Kaohsiung, Taiwan.

His research interests include radio-frequency cir-cuit design and measurement for telecommunication applications.

Kuei-Ming Chu was born in Kaohsiung, Taiwan.

He received the B.S. degree in electrical engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 2008. He is currently working toward the M.S. degree in electro-optical engineering, National Sun Yat-sen University, Kaohsiung, Taiwan.

His main research is high-speed optoelectronics packaging.

Chieh Hu was born in Taichung, Taiwan, in 1967, He

received the M.S. and Ph.D. degrees in mechanical engineering from the University of National Central, Taoyuan, Taiwan, in 1991 and 1996, respectively.

In 1996, he joined the Industrial Technology Research Institute (ITRI), Hsin Chu, Taiwan. Since 2000, he was the Head of the Department of Laser Lightwave Technology. His main research interests include scientific and technical aspects associated with the device of optical fiber communication and fiber laser.

Wood-Hi Cheng (M’95–SM’00) was born in

Changhua, Taiwan, in 1944. He received the Ph.D. degree in physics from Oklahoma State University, Stillwater, in 1978.

From 1978 to 1980, he was a Research Asso-ciate at Telecommunication Laboratories, Taiwan. From 1980 to 1984, he was a Research Engineer at General Optronics, Edison, NJ. From 1984 to 1991, he was a Principal Design Engineer at Rockwell International, Newbury Park, CA. From 1991 to 1994, he was an Optoelectronic Packaging Manager at Tacan Corporation. Carlsbad, CA. He is now a Professor at the Institute of Electro-Optical Engineering and Director of Southern Taiwan Opto-Electronic Center of Excellence, National Sun Yat-Sen University, Kaoshiung, Taiwan. His research and development activities have been focused on the design and fabrication of high-speed semiconductor lasers for lightwave communications, highly efficient light coupling from lasers into fibers, fiber couplers, character-ization of III-V semiconductors materials, and optoelectronic packaging. His current research interests are the design, fabrication, and finite-element-method analysis for laser module packaging, high-speed laser module packaging for digital lightwave systems, fabrication of high density WDM components, and novel materials for electromagnetic shielding. He served as a consultant for Chunghwa Telecom Laboratories, Opto-Electronics and System Laboratories, and Chung-Shan Institute of Science and Technology, all from Taiwan.

Dr. Cheng is a member of the Optical Society of America (OSA) and the Photonics Society of Chinese-Americans. He served as a Chair for the IEEE Lasers and Electro-Optics Society (LEOS), Taipei Chapter during 1999-2000, and served as a Chair for the OSA, Taiwan Chapter, during 2005-2006.