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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