Selected Papers

Multichannel Optical Coupling Techniqueand Its Application to a SIPAS Module
Akira Ohki†, Yasuo Shibata, Toshio Ito, and Yasuhiro Suzuki
Abstract
We propose a novel multi-channel optical coupling technique called MOCA (multichannel optical
coupling with an aspherical lens) to simplify the packaging of opto-electronic functional devices whose
optical I/O ports are arrayed optical waveguides. MOCA uses only an aspherical lens to couple an optical waveguide array to an optical fibre array. In spite of its simplicity, MOCA provides efficient optical
coupling for each optical I/O port and high performance for module reliability.
To evaluate the performance of MOCA, we applied it to an all-optical wavelength converter chip
known as a SIPAS (Sagnac interferometer integrated with parallel amplifier structure), which will be a
key device in near-future WDM networks. The fabricated module, which has a coupling loss of less than
3.5 dB per I/O port, acts as an all-optical wavelength converter for 10-Gbit/s RZ optical signals.

1. Introduction
The recent exponential growth of communication
traffic due to the deployment of multimedia and theInternet is creating a demand for higher network
throughput. Wavelength division multiplexing(WDM) is an effective way to raise network throughput and it is spreading widely. In the present WDM
network, the optical layer is mainly responsible forsignal transmission. Signal processing, such as pathrouting, multiplexing, and demultiplexing, is done
electrically in the network node system after opticalto-electrical (O/E) conversion. Further improvements
in network throughput require the development ofelectronic network nodes and management systems
whose capacities are much larger than present ones.
For lower power consumption, all-optical signal processing is far superior to large-capacity electronicsystems. Therefore, the development of all-opticalsignal processing modules will be important forlarge-capacity WDM optical networks. In spite of theneed for such modules, progress in their development
has been relatively slow. One of the major difficulties
in packaging these devices is the lack of a highly reli† NTT Photonics Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: ohki@aecl.ntt.co.jp

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able, cost-effective multichannel optical couplingtechnique.
We have developed a technique called MOCA,
which stands for multichannel optical coupling withan aspherical lens, and clarified its performance [1].
MOCA provides a simple, low-loss, and cost-effective multichannel optical coupling system. It has been
applied to the packaging of an all-optical wavelength
converter module consisting of a Sagnac interferometer monolithically integrated with a parallel amplifier structure (SIPAS) [2]. Successful wavelength conversion was achieved using a SIPAS module withMOCA.
2. Development of MOCA
2.1 Concept
To date, two optical coupling techniques havemainly been used in assembling multichannel opticalmodules:(a) Direct butting of the optical fibre array aligned on
V-grooves and optical device array [3], [4] and(b) Multichannel optical coupling using a micro-lensarray [5].
The optical coupling configurations for these techniques are shown in Figs. 1(a) and (b), respectively.
The former is cost effective, but not appropriate forthe above-mentioned devices because of its relatively
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large coupling loss and hermetic sealing problems.
The latter has sufficient coupling efficiency and reliability for all-optical signal processing modules, but
its assembly process is complex and the array
requires highly uniform lens properties.
MOCA was designed to solve these problems. A
typical optical coupling configuration of MOCA isschematically shown in Fig. 2. In this figure, the optical device has 4-channel waveguides as optical I/O
ports whose spot diameter and pitch are S µm and Lµm, respectively. These waveguides are coupled to a
4-ch single-mode fibre (SMF) array with a 250-µm
pitch by an aspherical lens whose magnification is M.
The values of M and L are simply determined by
S × M = 10 (spot diameter matching)(1)
L × M = 250 (array pitch matching).
(2)
The spot shape of a rectangular waveguide is usually elliptical. The effective spot diameter of the rectangular waveguide is estimated approximately by
Seff = (εl × εs)1/2,(3)

where εl and εs are the major and minor axes of theellipse, respectively. Seff is used instead of S in thedesign of MOCA for a rectangular waveguide array.
As one can see in Fig. 2, MOCA can cope with hermetic sealing as well as the insertion of an optical isolator, which are both essential for a highly reliablehigh-performance optical module. In addition, theassembly process with MOCA is almost the same asthat for ordinary laser transmitter modules. Theseadvantages make MOCA a simple, reliable, and costeffective multichannel optical coupling technique.
2.2 Optical coupling performance of MOCA
We measured the coupling efficiency of MOCA fora 4-ch waveguide array whose Seff was designed to be
2 µm. We also estimated the coupling efficiency by a
ray-tracing simulation. In the measurement and simulation, M and L were 5 and 50 µm, respectively; anaspherical lens whose cross sectional diameter andthickness were 1.8 and 1.3 mm, respectively, wasused; and the ratio of εl/εs was 2. The aspherical lens
and waveguide were located at the optimum positions.

Fig. 1. Conventional multichannel optical coupling techniques.

Fig. 2. Concept of MOCA.

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The measured coupling losses for each channel ofthe SMF array are plotted in Fig. 3 along with thesimulated coupling loss curves. The horizontal axis
indicates the mis-alignment of the 4-ch SMF arrayfrom the optimum position. We think the deviation of
the simulation results from the measured values iscaused by Seff differing from the design value. That is
to say, Seff was actually larger than 2 µm and the projected spot diameter on the optical fibre cross-sectional surface was larger than 10 µm. This mismatchin spot size causes excess coupling loss around the
optimum position. On the other hand, the increase incoupling loss due to the misalignment was more
gradual than in the simulation because of the large
projected spot diameter. Therefore, we checked the
Seff value of each waveguide as follows. Each channel of the 4-ch waveguide array was coupled with an
SMF through the same aspherical lens as mentioned
above. The relationship between coupling losses and
M is shown in Fig. 4. The minimum coupling loss
was obtained at around M of 4.3. This indicates that
Seff of the 4-ch waveguide array is 2.3 µm.
In spite of the spot-size mismatch, each 4-ch SMF
can be coupled with the corresponding waveguidewith high efficiency (less than 2.5-dB coupling loss)
at the optimum position. The coupling loss deviation
among the four channels was suppressed to less than
0.7 dB. These results show that MOCA is a promising technique for multichannel optical coupling.
2.3 Expandability of MOCA
The optical coupling losses of MOCA for an 8-ch
waveguide array with 50-µm pitch were measured at
the optimum position (Fig. 5). The measurement was

Fig.3. Measured and calculated coupling losses of MOCA.

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done using the same configuration as mentioned
above. In this figure, the horizontal axis indicates the
distance between each waveguide channel and thecentral axis of the lens. The coupling losses increased
with distance because both the lens aberration andphoton incident angle increase with distance from thecentral axis. If the threshold coupling loss is set at 4dB, MOCA can be applied to a waveguide array withat most six channels. This means that a waveguidearray with at most 12 channels with 25-µm pitch canbe coupled with a 12-ch SMF array with 125-µm
pitch in the MOCA configuration. Further expansionof waveguide channels requires a larger lens diame-

Fig. 4. Relation between coupling loss and magnification M.

Fig. 5. Coupling losses of MOCA for an 8-ch
waveguide array.

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ter.
3. Application of MOCA to SIPAS packaging
The MOCA configuration was applied to an alloptical wavelength converter, or SIPAS, having 3-ch
optical waveguide I/O ports.
3.1 Structure of the SIPAS chip
Figure 6 is a photograph of the fabricated SIPAS
chip. SIPAS is a Sagnac interferometer with a parallel amplifier structure (PAS) consisting of a MachZehnder interferometer (MZI) having polarization
insensitive semiconductor optical amplifiers (SOAs)
in each arm. It was fabricated monolithically by buttcoupling the high mesa and buried waveguides [2].
Tensilely strained bulk InGaAsP was used for the
SOA active layer. The fabrication process is briefly as
follows: after 0.1%-tensile-strained SOA active layer
growth, 0.5-µm-thick InGaAsP core (λg = 1.05 µm)
and 1.0-µm-thick InP cladding layers were buttjoined. Next, the SOA stripe was dry etched, andembedded by a p-n blocking layer and p-doped InP
cladding layer. Then, the p-n blocking layer and pdoped InP cladding layer, which were grown over thepassive region, were removed to reduce the propagation loss of the passive waveguide. Finally, highmesa passive waveguides composing the Sagnac andMach-Zehnder interferometer were fabricated by
Br2-N2 reactive beam etching [6]. The propagation
loss of the high-mesa passive waveguide is about 5
dB/cm. The coupling loss between the SOA and passive regions is about 1 dB, including the active-topassive coupling loss. The SOA is 900 µm long andthe total chip size is 4.5 mm × 1.5 mm. The working
principle of our device is similar to that of the
SLALOM [7]. An input CW light is divided into

clockwise (CLW) and counterclockwise (CCW) traveling lights. Since the PAS is asymmetrically placedin the loop, these lights reach the SOAs at different
times, which leads to different phase modulation(DPM) [8] between them when the signal light is
input into the SOAs. After traveling around the loop,
the CLW and CCW lights are superimposed andtransmitted to the output port by means of DPM. We
placed the PAS asymmetrically by 0.5 mm so that theswitching window due to DPM was about 10 ps,
which enables high-speed operation at over 10 Gbit/s.
As the PAS is set in the cross state, the signal lightcannot enter the loop, resulting in filter-free wavelength conversion.
3.2 Performance of the SIPAS module
A photograph and schema of the module are shownin Fig. 7. The module is compact and has a volume of
1.6 cm3 (8.2 mm long, 16 mm wide, and 12 mmhigh). Among the 3-ch waveguide I/O ports, conversion output and pumping input ports are located on
the same edge of the SIPAS chip. The effective spot
diameters of these two ports were designed to be 2.5µm. MOCA was used for their coupling. In order toachieve efficient optical coupling, the magnificationof the aspherical lens was determined so as to makethe spot size of the waveguide around 10 µm in diameter; as a result, it was 4. Then the pitches of the
waveguide and SMF arrays were designed to be 62.5
and 250 µm, respectively. After assembling the
SIPAS module, we evaluated the actual coupling
losses and obtained 3.2 and 3.4 dB. The excess coupling losses are mainly caused by a post-welding shiftof the SMF array and the lens after the YAG laser
welding process and by spot size mismatch.
To evaluate the performance of the SIPAS module,
we performed a wavelength conversion experimentFig. 6. Photograph of a SIPAS chip.

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Fig. 7. Photograph and schema of the SIPAS module with MOCA.

Fig. 8. Eye diagrams. (a) Input signal (1552. 3 nm),(b) Conversion output (1548.0 nm).

using a 1552.3-nm 10-Gbit/s RZ-PRBS input signal
and a 1548.5-nm continuous-wave pumping input.
The results are shown in Fig. 8. The wavelength ofthe output optical signal was successfully converted
to 1548.5 nm.
4. Conclusion
MOCA (multichannel optical coupling with anaspherical lens) is a simple, low-loss, cost-effective
multichannel optical coupling technique. It has been
applied to the assembly of an all-optical wavelength
converter (or SIPAS) module. The assembled module
successfully provides wavelength conversion. This
indicates that MOCA is a promising packaging technique for optical functional modules that have multiple optical I/O ports.
References[1] A. Ohki, Y. Shibata, N. Kikuchi, T. Ito, H.
50

Okamoto, Y. Suzuki, and N. Ishihara, “Multichannel Optical Coupling with an AsphericalLens and Its Application to an All-Optical
Monolithic Wavelength Converter Module,”
52nd ECTC, San Diego, U.S.A., pp. 29-33, May
2002.
[2] Y. Shibata, N. Kikuchi, S. Oku, T. Ito, H.
Okamoto, Y. Kawaguchi, Y. Kondo, and Y.
Suzuki, “Filter-free Wavelength Conversion
using a Sagnac Interferometer Integrated with
Parallel Amplifier Structure,” OECC/IOOC
2001, Sydney, Australia, pp. 212-213, Jul.
2001.
[3] J. Eichenberger, N. Takezawa, C. Keller, and S.
Toyoda, “1.25-3.2 Gbit/s/ch Multi-channel
Optical Interconnection Modules and their
Mass-production,” 13th LEOS, Rio Grande,
Puerto Rico, Vol. II, pp. 675-676, Nov. 2000.
[4] M. S. Cohen, M. F. Cina, E. Bassous, M.M.
Opyrsko, J. L. Speidell, F. J. Canora, and M. J.
DeFranza, “Packaging of High-Density
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[5]

[6]

[7]

[8]

Fibre/Laser Modules Using Passive Alignment
Techniques,” IEEE Trans. On Components,
Hybrid, and Manufacturing Tech., Vol. 15, No.
6, pp. 944-955, 1992.
A. Takai, H. Furuichi, A. Miura, K. Tonemura,
K. Kawamoto, S. Ueno, Y. Fukashiro, T. Haga,
T. Toyonaka, K. Yamada, T. Suejima, and K.
Saitoh, “800 Mbit/s/ch × 12 ch, True DC-Coupled Parallel Optical Interconnects Using Single-Mode Fibre and 1310 nm LD Array,” 50th
ECTC, Las Vegas, U.S.A., pp. 491-496, May
2000.
S. Oku, Y. Shibata, and K. Ochiai, “Controlled
Beam Dry Etching of InP by Using Br2-N 2
Gas,” Journal of Electronic Materials, Vol. 25,
No. 4, pp. 585-591, 1996.
M . E i s e l t , W. P i p e r, a n d H . G . We b e r,
“SLALOM: Semiconductor Laser Amplifier ina Loop Mirror,” IEEE Journal of Lightwave
Tech., Vol. 13, pp. 2099-2112, 1995.
Y. Ueno, S. Nakamura, H. Hatakeyama, T.
Tamanuki, T. Sasaki, and K. Tajima, “168-Gb/s
OTDM Wavelength Conversion Using an SMZType All-Optical Switch,” ECOC 2000,
Munich, Germany, pp. 13-14, 2000.

Akira Ohki
Senior Research Engineer, Photonics Integration Project, NTT Photonics Laboratories.
He received B.E. and M. E. degrees from
Nagoya University in 1984 and 1986, respectively, and a Ph. D. in electrical engineering from
Nagoya University in 1994. In 1986, he joined
NTT Ibaraki Electrical Communication Laboratories, Ibaraki, where he engaged in research on
MOVPE growth of compound semiconductormaterials. In 1993, he moved to NTT Opto-Electronics Laboratories, where he engaged inresearch on laser diodes based on ZnMgSSe quaternary alloy crystals. Since 1997, he has been
engaged in the design and development of optoelectronics modules for fibre optic communication network. Dr. Ohki is a member of the Institute of Electronics, Information and Communication Engineers of Japan and the Japan Societyof Applied Physics.

Yasuo Shibata
Senior Research Engineer, Photonic Device
Laboratory, NTT Photonics Laboratories.
He received the B.S. and M.E. degrees in electrical engineering from Keio University, Kanagawa, Japan in 1985 and 1987, respectively. In
1987, he joined NTT Optoelectronics Laboratories, where he has been engaged in research on
optical switches and optical filters. He is currently with NTT Photonics Laboratories, engaged inresearch on SOA and wavelength-conversion
devices. Mr. Shibata is a member of the Japan
Society of Applied Physics, and the Institute ofElectronics, Information and Communication
Engineers (IEICE).

Toshio Ito
Senior Research Engineer, Photonics Integration Project, NTT Photonics Laboratories.
He received the B.S. and M.E. degrees from
Keio University, Kanagawa, Japan, in 1986 and1988, respectively. In 1988, he joined theMusashino Electrical Communications Laboratories, where he engaged in research on parallel
processing systems and communication switching systems. Since 1993, he has been engaged inresearch on the InGaAlAs/InAlAs multi-quantum well optical switches, InGaAsP semiconductor optical amplifiers, and wavelength converters
at NTT Photonics Laboratories. Dr. Ito is a member of the IEICE, JSAP, and OSA.

Yasuhiro Suzuki
Senior Research Engineer, Supervisor, Photonics
Integration Project, NTT Photonics Laboratories.
He received the B.S. and M.S. degrees in applied
physics from the University of Tokyo, Tokyo,
Japan, in 1980 and 1982, respectively. In 1982, hejoined the Musashino Electrical Communications
Laboratories, Nippon Telegraph and Telephone
public Corporation (now NTT), Tokyo, Japan.
After that, he was engaged in research and development on semiconductor optical devices. In
1987, he moved to concentrate on supperlatticedisordering, polarization control devices, opticalamplifiers, and optical integrated circuits. He iscurrently engaged in the development of opticalsignal processing devices including wavelength
conversion devices and optical module assembly
technologies for optical metropolitan/access networks. He is currently engaged in the development of optical signal processing devices including wavelength conversion devices and opticalmodule assembly technologies for optical metropolitan/access networks. Mr. Suzuki is a memberof the Institute of Electronics, Information andCommunication Engineers, the Japan Society ofApplied Physics and the Institute of Electrical andElectronics Engineers. He has received the IEICE
Inose Award.

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