Low-loss 1310nm/1550nm Integrated Silicon Duplexer based on a

Directional Coupler

Daivid Fowler

, Bertrand Szelag

, Vincent Hugues

, Christian Sillans

Stéphane Bernabé

and Christophe Kopp

Univ. Grenoble Alpes, CEA, LETI, MINATEC campus, F38054 Grenoble, France

IFOTEC, ZAC de Champfeuillet, Route des Bois, 38500 Voiron, France

Keywords: Silicon Photonics, Photonic Integrated Circuits, Passive Components, Multiplexer, Duplexer.

Abstract: We describe the design, fabrication and measurement of an integrated silicon photonics O-band/C-band

duplexer. The duplexer is based on a directional coupler and is designed to be polarisation insensitive at

1550nm in order to be used in conjunction with a broadband integrated edge coupler. The measured insertion

losses are <1.3dB with a -1dB bandwidth of ~80nm.

1 INTRODUCTION

The increasing maturity of integrated silicon

photonics technology means that it is becoming

increasingly attractive to substitute traditional

discrete optical componentry with a low cost

integrated solution. Here, we report the design,

fabrication and measurement of a 1310/1550nm

duplexer for use in bi-directional high-bitrate single-

fiber systems, which is relevant for 1000BASE-BX10

and FTTH.

The use of integrated silicon photonics in this type

of application could lead to significant cost reductions

by eliminating many of the precise mechanical

operations involved in the assembly of discrete

optical components, leaving only the attachment of

the fiber to the integrated source-detector-duplexer.

This potentially decreases the module costs by a

significant margin.

Figure 1: Typical circuit schematic for a 1000BASE-BX10

type gigabit Ethernet module.

Figure 1 shows a schematic of the circuit

considered here. A modulated source laser operating

at 1310nm passes through a duplexer before leaving

downstream on a single mode fibre via a broadband

fibre coupler. Incoming, upstream data arrives in the

same fibre, using a modulated 1550nm signal. The

state of polarisation of the incoming light from the

network is intrinsically unknown and unstable. This

light will then be transferred into a combination of TE

and TM modes in the on-chip waveguide via the

broadband edge-type fibre coupler (Vivien et al,

2009). While integrated germanium photodiodes

(Cardenas et al, 2014) are largely insensitive to the

polarisation state in the incoming waveguide,

photonic circuit waveguides tend to be designed in

order to avoid unwanted crosstalk between

polarisation states, and therefore have unequal

refractive index values for TM and TE modes.

Therefore, care must be taken that the duplexer not

only splits/combines 1310/1550nm light, but also

exhibits low polarisation dependent loss for the

1550nm light.

2 DESIGN

Silicon photonics devices for multiplexing have been

widely developed to address single-band operation

(e.g. DWDM or CWDM) (Okamoto, 2014), however,

these solutions are not necessarily appropriate for

inter-band applications. Such duplexing devices have

been demonstrated on a silicon platform using

diffractive gratings (Roelkens, 2007), the multimode

interference effect (Hong and Lee 2007) and planar

reflective gratings (S. Bidnyk et al, 2007). Here we

Fowler D., Szelag B., Hugues V., Sillans C., Bernabe S. and Kopp C.

Low-loss 1310nm/1550nm Integrated Silicon Duplexer based on a Directional Coupler.

DOI: 10.5220/0006158000950098

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 95-98

ISBN: 978-989-758-223-3

describe the use of a directional coupler to obtain very

low (<1dB) insertion losses.

Directional couplers are based on the transfer of

an optical field between two identical adjacent

waveguides via the coupling of evanescent modes.

They are often used in photonic circuits as a low-loss

component to separate the optical field in a single

waveguide into two waveguides with a pre-selected

intensity ratio. If a first waveguide carrying an optical

mode with a power, P

, runs parallel to a second

waveguide, the amount of transmitted power, P

, is

given by



















⁄

, (1)

where L is the length of the parallel waveguides,

lambda is the operating wavelength and 



is the

difference between the symmetric and antisymmetric

coupled modes, which is a function of wavelength,

waveguide geometry and the gap between the two

waveguides. Thus, the length of parallel waveguide

required in order to fully transfer the optical field

from one waveguide to another and back again, L

tot,

given by





 



⁄

(2)

With the use of a 2D mode solver (in this case RSOFT

FEMSIM), it is therefore possible to determine the

theoretically required coupling length in order to split

the 1310/1550nm wavelengths while implementing

polarisation insensitivity at 1550nm.

Figure 2a: L

tot

versus waveguide rib width. Inset: waveguide

geometry in coupling region. Figure 2b: Electrical field

intensity plots along the coupling region illustrating the

multiplexing mechanism.

Figure 2 shows L

tot,

as a function of waveguide rib

width for a waveguide spacing of 200nm for a

1.31µm TE mode (blue circles), a 1.55µm TE mode

(red circles) and 1.55µm TM mode (black circles).

The inset in the figure shows the waveguide geometry

used. We opted for a ‘rib’ type waveguide with a

continuous 65nm slab region for consistency with our

standard process, although other waveguide

geometries could offer valid starting points. For a

fixed gap between the waveguides of 200nm, the

values of L

tot

can be obtained as the rib width is

varied. In order to satisfy the function defined in

figure 1, we require

tot

(1.55

) = L

tot

(1.55

) = L

tot

(1.31

)/2. (3)

When this condition is satisfied, the duplexing

functionality of the directional coupler is as illustrated

in the electric field intensity plots in figure 2b. This is

well approximated for a rib width of 315/320nm,

where L

tot

(1.55

) = 13.0/13.5µm, L

tot

(1.55

) =

13.0/13.3µm and L

tot

(1.31

)/2 = 14.0/14.7µm.

While the above analysis provides a good starting

point for the directional coupler design, in reality, the

parallel waveguides must approach one other and

separate gradually, so as not to incur unwanted

bending losses. The actual design is shown in figure

3a. The waveguides are brought together using a sin-

function bend whose minimum bending radius is

11µm. Although the coupling strength between the

two waveguides decreases exponentially with the gap

between them, the power transfer which occurs in the

input and output bent waveguide sections is not

negligible and is most simply simulated using 3D-

FDTD (RSOFT FULLWAVE).

Figure 3a: Implemented directional coupler geometry

including input/output S-bends. 3D-FDTD simulated

insertion losses (data points in circles, the solid lines are a

guide for the eye).

Figure 3 shows the simulated insertion losses for the

1.31

TE,

1.55

and the

1.55

waveguide modes as a

function of the length of the straight coupling section.

In order to obtain less than 1dB insertion losses per

channel, the FDTD simulations, which included the

initial S-bends, revealed that a straight coupling

length of between 7 and 9 µm is required. This is

rather less than the value attained in the simple model

6.5 7 7.5 8 8.5 9

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

Insertion losses (dB)

320nm

200nm

7µm

7,5µm

0,5µm

Straight coupling section length (µm)

1310nm

cross

1550nm

bar

1550nm

bar

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

described above, which is due to the significant

coupling which takes place in the input/output bent

waveguide sections.

3 FABRICATION AND TEST

Test structures were fabricated at CEA-LETI on

200mm wafers with a SOI/BOX thickness of

310/800nm. Waveguides were defined using 193nm-

DUV lithography and a dry etching process (B.

Szelag, 2016).

Figure 4: Measured insertion losses of best performing

fabricated design variant. Solid lines are fit lines to the data

(points), dashed lines are 3D-FDTD simulations of the

nominal structure.

Test structures with a range of nominal waveguide

widths and coupling section lengths were fabricated.

Each variant was triplicated and connected to vertical

fibre grating couplers designed to couple

1.31µmTE/1.55µmTE/1.55µmTM light into the

waveguides. In order to evaluate the insertion losses

for each channel, a reference waveguide was

measured with each type of fibre coupler. The ‘cross’

transmission (corresponding to light crossing from

one guide to another) and the ‘bar’ transmission

(corresponding to light propagation along the same

waveguide) of each variant was then measured for

each channel. The directional coupler transmission

spectra were then obtained by subtracting the

corresponding fibre coupler spectra from the

directional coupler transmission data.

The transmission spectra of the best performing

variant (rib width = 320nm, gap = 200nm, coupling

section length = 7µm) are shown in figure 4. The data

points are shown as well as polynomial fits (solid

lines). The high noise level of the data can be

attributed to the summing of the noise in the

transmission spectra and the reference spectra. The

measured insertion losses for the

1.31µmTE/1.55µmTE/1.55µmTM channels were -

0.6/-0.55/-1.2dB. The measured -1dB bandwidth is in

the region of 80nm. The use narrowband fibre grating

couplers prevented crosstalk measurements, although

FDTD simulations show values in the -10 to -15dB

range.

For comparison, the 3D-FDTD simulated

transmission spectra for this nominal structure are

also shown as dashed lines on figure 4. The observed

discrepancy as well as the fact that the best

performing variant (rib width = 320nm, L = 7µm) was

not exactly that identified via FDTD simulation may

be firstly attributed to the unsimulated optical

coupling in the waveguide routing required to access

the test structure and secondly, to departures from the

nominal waveguide geometry in the processed

devices.

4 CONCLUSIONS AND

PERSPECTIVES

We have designed, fabricated and measured an

integrated silicon low loss O-band/C-band duplexer

with polarisation insensitivity at 1550nm. The device

shows low measured losses (<1.3dB) at each design

wavelength. Further design refinements will likely

reduce insertion losses significantly below 1dB,

which compares favourably with conventional,

discrete optical componentry. Based on this positive

result, we are now fabricating a complete circuit,

including integrated laser, photodiode and broadband

edge coupler to demonstrate the potential of

integrated silicon photonics for this application.

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PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology