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actual coupler separation is larger than the designed value of
2 . 0 ~wing to the finite undercut which occurs during the RIE
process. In general, the discrepancy between BPM and measured
data is caused by the uncertainty in the actual coupler separation
and the limitation of BPM to accurately simulate wide angle
waveguide bends. E.g., our present BPM method is limited to S-
bend angles degrees to avoid spurious numerical loss. Our
devices have S-bends slightly >5 degrees. We suspect that with
improved BPM to handle larger S-bend angles, a better agreement
between design and experiment would be obtained.
References
1 SOREF, R.A., SCHMIDTCHEN,
J.,
and PETERMANN, K.: 'Large single-
mode rib waveguides in GaSi-Si and Si-on-SiO,', I EEE J . Quantum
Electron., 1986,QE-27 pp. 1971-1974
2 SCHMIDTCHEN, J., SPLETT, A., SCHUPPERT,
B ,
PETERMANN, K., and
BURBACH. G.:
'Low-loss singlemode optical waveguides with large
cross-section in silicon-on-insulator', Electron. Lett. , 1991,27, pp.
14861 87
3 R I C K MA N . A.G., and REED, G.T.: 'Silicon-on-insulator optical rib
waveguides: loss, mode characteristics, bends and y-junctions', IEE
Proc, Optoelectron.,
1994, 141,pp. 391-393
a
b 76 3
Fig.3
Measured near-field image and linescan output waveguides
of
3
dB directional coupler
h = 1.55
pm
a
Measured near-field image
b
Linescan
of
output waveguides
0 6
0 5
2 4
r
-
k 0 3
3
02
0 1
100
2 300 4 5
coupling 1eng th . p 117611
Fig. Power split ratio against coupling length
In conclusion, we have fabricated integrated optical directional
couplers using rib waveguides on
SO1
wafers. We have demon-
strated, for what we believe is the fr st time, a 3dB coupler on SO1
wafers. The device has an excess insertion loss as low as 1.9dB.
Such devices are useful for optical clock distribution in silicon
VLSI circuits and are also key building blocks of Mach-Zehnder
type wavelength
multiplcners/deniultiplexers.
This work
demon-
strates the potential of SO1 technology for low-cost monolithic
optoelectronic circuits.
Acknowledgment: This work has been sponsored by A R P N O N R
contract N 00014-95-1-0675.
EE 1995
Electronics Letters O nline No 19951453
P.D. Trinh, S. Yegnanarayanan and B. Jalali
(Department
of
Electrical
Engineering, University
of
California at
Los
Angeles,
Los
Angeles, CA
90095-1594, USA)
19 O ctober 1995
Long wavelength multimode waveguide
photodiodes suitable for hybrid optical
module integrated wi th planar lightwave
circuit
Y. Akatsu,
Y.
Muramoto, K. Kato,
M.
Ikeda, M. Ueki,
A.
Kozen, T. Kurosaki,
K.
Kawano and J. Yoshida
Indexing terms: Photodetectors, Optical waveguides, Integrated
optoelectronics
~ ~
The authors propose
a
multimode waveguide photodiode utilising
asymmetric waveguide structure which is suitable for optical
hybrid integration without coupling lenses, and has a high
responsivity of 0.95An;v
at a
wavelength of 1.31pm. Its 1dB
coupling tolerance to a planar lightwave circuit is S p m in the
vertical direction.
Introduction: A side-illuminated waveguide photodiode (WGPD)
is an attractive device for making compact and low-cost hybrid
optical modules because it can be coupled directly with fibres or
sihca-waveguides in planar lightwave circuits (PLCs). Moreover, it
can be attached more easily
on
the PLC than a surface-illuminated
photodiode, without additional mounting components such as
angled mirrors. The WGPD can also be integrated with a laser
diode on a PLC platform that has a silica-on-terraced-silicon
structure [l, 21, by using the same assembly method as the laser
diode.
To ensure a high coupling efficiency between the WGPD and a
fibre or a silica-waveguide of a PLC, without using coupling
lenses, the photoabsorption layer of the photodiode must be thick.
However, a single thick photoabsorption layer is difficult to
deplete in a conventional InPLnGaAsRnP waveguide photodiode.
We proposed a multimode WGPD [3] that has a doped intermedi-
ate-bandgap layer between a core layer and a cladding layer to
enlarge optical field distribution. This multimode structure allows
the photoabsorption layer to be depleted. Conversely, from the
viewpoint of epitaxial growth and the fabrication process, it is
desirable to make epitaxial layers thin.
In this Let ter , we propose 1 . 3 ~ultimode waveguide photo-
diodes that are suitable for optical hybrid integration, and have a
high coupling efficiency and a large alignment tolerance. We also
use an asymmetric waveguide s tructure to reduce the total thick-
ness of the epitaxial layers without increasing coupling loss.
Design: To calculate the coupling efficiency, we considered two
WGP D models. One is a symmetric structure consisting of an InP
cladding layer, an InGaAsP intermediate-bandgap layer, an
InGaAsP core layer, an InGaAsP intermediate-bandgap ayer, and
an InP substrate. The other is an asymmetric structure that has
only an InGaAsP intermediate-bandgap layer on one side of the
core layer. The bandgap energy
of
the core layer is designed to
exhibit wavelength-dependent responsivity. The bandgap energy of
the intermediate-bandgap layers is set to be greater than that cor-
responding to a 1 . 3 ~avelength.
The coupling efficiency between a WGPD and a dispersion-
shifted fibre that produces a Gaussian beam with a 4 pm spot size,
was calculated by considering the overlap integral between the
optical field
of
the fibre and that of the WGPD [4]. This coupling
2098 ELECTRONlCS L E T R S 23rd November 7995
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4 5 6
7
total thickness, pm
Fig. 1 Calculated coupling efficiency against total thickness of core
layer and intermediate-bandgap layer
Thickness of core layer is a constant
of 3pm
when total thickness of
asymmetric structure slightly exceeds 4.5brn coupling efficiency
increases dramatically
to 92
efficiency is shown in Fig. 1against the total thickness of the core
layer and the intermediate-bandgap layer. When the total thick-
ness of the asymmetric structure slightly exceeds 4.5pn the cou-
pling efficiency increases dramatically to
92 .
This is because the
higher-order modes and odd-order modes contribute to coupling
efficiency. Conversely, for the symmetric structure, the coupling
efficiency increases gradually as the total thickness increases,
owing to contributions from only even-order modes, and reaches
92
at the total thickness of 6.0pm. Comparing these curves indi-
cates that it is possible to make the epitalxial layers thinner by
using the asymmetric structure.
Results: The epitaxial layers were grown on a semi-insulating InP
substrate by low-pressure metal-organic vapour phase epitaxy
(MOVPE). These layers comprised
a
1 O p hick unintentionally
doped InP cladding layer, a 3 . 0 ~hick unintentionally doped
InGaAsP (h, =
1 . 4 ~ )
ore layer, and
a
1 . 5 ~
hick n+-InGaAsP
(h, = 1 . 2 ~ )ntermediate-bandgap layer on the InP substrate. A
1
OW
thick p-type region that was 3 0 ~ide and 2 0 p ong was
formed in the InP cladding layer by the Zn-diffusion method.
After diffusion, the mesa was formed by the dry etching technique
and buried with polyimide.
10
m v I
-10 - 5
5
10
axial shift, m
Fig.
2
Measured coupling tolerance curves of WG PD to
D S F
and
PLC
with a refractive index difference
ofO.75 n
v,ertical directio n
Calculated tolerance is shown as
a
broken line
oupling tolerance curves of WGPD to DSF
coupling tolerance curves of WGPD to PLC
This asymmetric WGPD with an antireflection coating had a
high responsivity of 0.95AiW at a wavelength of
1 . 3 1 p .
The
responsivity at a wavelength
of
1 . 5 5 ~a:j 27dB lower than tha t
at 1.31pm owing to wavelength-dependent responsivity.
The
3dB
bandwidth of the PD was >3GHz.
The measured average coupling
loss
between WGPDs and
cleaved dispersion-shifted fibres was 0.44dB without coupling
lenses, assuming no reflection from the
surface
of the WGPD.
This result indicates that the proposed asymmetric structure has a
high coupling efficiency and is suitable for optical hybrid integra-
tion. Fig. 2shows the measured coupling tolerance curves of the
WGP D with the dispersion-shifted fibre and with the PLC that
produces
a
4pm spot-size Gaussian beam and has a 0.75% refrac-
tive index difference between the core and the cladding layer, in
the vertical direction. The measured tolerance is consistent with
the calculated curve (a broken line). The 1dB tolerance is k2pn in
the vertical direction. We also obtained a 1dB tolerance of fl7pm
in { he horizontal direction and S 7 p m in the light-incident direc-
tion. These results reveal that this WGPD has a large alignment
tolerance, which is sufficient for coupling with PLC and fibre by
a
passive alignment technique.
4 2 r 1 I I
0
-45
-40 - 35
average received power dBm
Fig. 3Bit error rate performance of28 .8M bit/s NRZ lightwave signal
i t error rate of WGPD module
conventional PD
h
=
1 . 3 1 ~
Sensitivity is 41.7dBm
Fig. 3 shows the bit error rate performance of the WGPD meas-
ured for a pseudorandom (P3 ), 28.8MbiUs [5]NRZ lightwave
signal using a CMOS receiver IC [6].The open squares show the
bit error rate of the WGPD module and the sensitivity at a bit
error rate of
1
x 1W is 41.7dBm, which is the same as that
of
the
conventional surface-illuminated photodiode (solid circles).
Co,sclusion: We have designed and fabricated an improved multi-
mode waveguide photodiode using an asymmetric structure to
reduce the thickness of the epitaxial layers. This proposed photo-
diode had a high responsivity of 0.95AiW at
a
wavelength of
1.3
1
pm. The average coupling loss between photodiodes and
cleaved dispersion-shifted fibres was 0.44dB without coupling
lenses. The
1
dB coupling tolerance to fibre and PLC was
2 p
n
the vertical direction. These results indicate that the proposed
asymmetric waveguide photodiode is a promising device for opti-
cal hybrid integration on PLCs, by the use of the passive align-
ment technique.
EE 1995
Electronics Letters Online No: 19951444
Y .Akatsu, Y. Muramoto, K . Kato,
M.
Ikeda,
M.
Ueki, A. Kozen, T.
Kurosaki, K. Kawano and J. Yoshida
NTT
Opto-electronics
Laboratories, 3-1 Morinosato W akam iya, Atsugi, Kanagawa, 243-01
Japun)
21 September 1995
ELECTRONICS
LETERS
23rd November 1995 Vol. 31
No
24 2099
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3/3
References
YAMADA, Y.,
TAKAGI,
A.,
OGAWA,
I. KAWACHI,
M.,
and
KOBAYASHI,
M.: 'Silica-based optical waveguide on terraced silicon
substrate as hybrid integration platform', Electron. Lett., 1,993,
29
pp. 444-446
YAMADA, Y., SUZUKI, S MORIWAKI, K. , TOHMORI, Y., AKATSU, Y.,
NAKASUGA, Y . ,
HASHIMOTO,
T ,
TERUI,
H., YANAGISAWA, M.,
INOUE,
Y., AKAHORI, Y . ,
and NAGASE, R.: 'A hybrid integrated optical
WDM transmitter/receiver module for optical subscriber systems
utilizing a planar lightwave circuit platform'. Tech. Dig. OFC'95,
1995, (San Diego), PD-12
KATO,
K ,
HATA,
S.,
KOZEN,
A., and KAWANO, K.: 'High-efficiency
waveguide InGaAs pin photodiode with bandwidth
of
over
40GHz', IEEE Photonics Technol. Lett.,
1991,
3 pp. 473474
KATO,
K., HATA,
s., KAWANO, K.,
YOSHIDA, J and KOZEN, A.:
A
high-
efficiency 50 GHz InGaAs multimode waveguide photodetector',
I E E E
J.
Quantum Electvon., 1992,
QE-28
pp. 2128-2735
OKADA, K., and
MIKI,
N.: 'Fiber-optics subscriber systems for point-
to-multipoint transmission architecture'. ECOC'92, 1992, (Berlin),
We
A11.2
N A K A MU R A , M., ISHIHARA, N., A K A Z A WA , Y., and KIMURA, H.: Proc.
1994Custom Integrated Circuits Conf., 1994, pp. 629-632
i optical waveguide 1.31/1.55pm
WDM
wi th 50dB crosstalk over
100nm
bandwidth
Y.P.
Li,
C.H. Henry, E.J. Laskowski, H.H. Yaffe and
R.L.
Sweatt
Indexing t er m : Comm unity antenna television, Wavelength division
multiplexing, Optical waveguide components
The authors have designed and fabricated monolithc optical
waveguide 1.31/1.55pWDMs wth -50dB crosstalk over
l o o m
bandwidth and fibre-to-fibre insertion loss of
IdB.
They have
used these WDMs to multiplex and demultiplex 60 analogue
CATV channels at 1 . 3 1 ~nd 85 digital video channels at
1 . 5 5 ~n a single optical fibre.
Many telecommunications applications seek
a
broadband wave-
length division
multiplexer/demultiplexer
(WDM) with rectangular
amplitude response and low crosstalk to combindseparate the 1.31
and 1 . 5 5 ~ommunication bands. Various devices have been
proposed or used to fiil these demanding requirements, but none
were fully satisfactory. Mach-Zehnder (MZ) interferometers [11
have been widely employed, but they have a sinusoidal response,
giving rise to strongly wavelength-dependent transmission and
a
narrow rejection band. The resonant coupler
[2]
has an inherently
narrow stopband. Lattice and transversal fiters [3, 41 have been
used only in narrowband applications. In some commercial
devices, th in f h ilters are employed to reduce the crosstalk of
a
simple coupler or MZ WDMS, but these hybrid devices are more
expensive to fabricate.
We have designed and fabricated monolithic optical waveguide
1.31/1.SSpn WDMs with high performance that were previously
only achievable with hybrid thin
film
filters. Moreover, our
WDMS are made by mass production integrated circuit tech-
niques, and can be integrated with other components to perform
complex circuit functionalities.
Our waveguide WDM comprises
a
chain of optical couplers
linked by differential delays. Its design is based on the following
principle of the sum of all optical paths [5]. The transfer function
from any input port to any output port of a chain of N couplers
and N-l differential delays consists of the unweighted
sum
of con-
tributions of all
(2N-1)
istinct optical paths. The contribution of
each path is a product of 2N-1 factors: traversing a coupler with-
out crossing gives cos , and
i
sin with crossing; traversing the
longer arm
of
a
differential delay gives ea@ nd the shorter armele
Here = 7c1/2L,where is the length of the coupler, L is the cou-
pling length, 8 = mii/h, s and n are the length difference and
effective refractive index of the delay waveguides, and
h
is the
wavelength.
phase responses
[SI.
I
Fig. 1 Layout
o
a 1.3/1.55
we have used these
ultiplex
60
analogue
CATV
chan-
channels
at 1 . 5 5 ~nto
a
sin-
(49dB) is degraded by only
fabricated monolithic 1.3
1/
tential applications in optical
EE
1995
I 1
September 1995
C.H. Henry and
E.J.
Lasko
600, Murray H ill, NJ 07974,
T T Bell Laboratories, PO B ox
~
2100
ELECTRONICS LETERS
23rd
Noverhber
995 Vol 3
No
24
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