Design and Optimization of High-Channel Si

Based AWGs for

Medical Applications

D. Seyringer

, A. Maese-Novo

, P. Muellner

, R. Hainberger

, J. Kraft

, G. Koppitsch

G. Meinhardt

and M. Sagmeister

Research Centre for Microtechnology, Vorarlberg University of Applied Sciences,

Hochschulstrasse 1, 6850 Dornbirn, Austria

AIT Austrian Institute of Technology GmbH, Donau-City-Strasse 1, 1220 Vienna, Austria

ams AG, Tobelbader Strasse 30, 8141 Premstätten, Austria

Keywords: Arrayed Waveguide Gratings, AWG, AWG Design, Silicon Nitride Waveguides, Medicinal Applications,

Photonics, High-Channel AWG, Si

AWG, Optical Spectrometer, High-Index Contrast AWGs.

Abstract: We present the design and optimization of 80-channel, 50-GHz Si

based AWG. The AWG was designed

for TM-polarized light with a central wavelength of 850 nm. The simulations showed that, while the standard

channel count AWGs (up to 40) feature gut optical properties and are relatively easy to design, increasing the

channel counts (> 40 channels) leads to a rapid increase in the AWG size and this, in turn causes the

deterioration of optical performance like higher insertion loss and, in particular, higher channel crosstalk.

Optimizing the design we are able to design 80-channel, 50-GHz AWG with satisfying optical properties.

1 INTRODUCTION

Arrayed Waveguide Gratings (AWGs) are considered

an attractive Dense Wavelength Division

Multiplexing (DWDM) solution because they

represent a compact means of offering higher channel

count technology, have good performance

characteristics, and can be more cost-effective per

channel than other methods (Kaneko, 2002).

However, their performance characteristics depend

largely on the optical properties of the waveguide

materials used. In terms of material, they can all be

divided into two main groups, so-called low-index

and high-index contrast AWGs.

Low-index contrast AWGs (Silica-on-Silicon

(SoS) based waveguide devices) use SiO

-buried

rectangular waveguides, usually with a cross-section

of ~ (6x6) µm

and a low refractive index contrast

between the core (waveguide) and the cladding of



~ 0.75%. They feature many advantages such as low

fiber coupling losses in the order of 0.1 dB and low

propagation loss (Leijtens, 2006). However, the low

refractive index contrast means the bending radius of

the waveguides needs to be very large, which leads to

a rapid increase in the AWG size of several square

centimeters that limits the integration density of SiO

based photonic devices.

High-index contrast AWGs, such as Silicon-On-

Insulator (SOI) based waveguide devices, use a high

refractive index difference



n ~ 58%. This is

approximately one hundred times higher than that of

typical SoS waveguides. Due to the fact that a

waveguide size decreases proportionally to the

increase in refractive index contrast, the waveguide

size for this material composition shrinks into the

sub-micron scale. Such high-index contrast makes it

possible to guide light in waveguides with a far

smaller bending radius, which leads to a significant

reduction in the AWG size by more than two orders

of magnitude when compared to SoS based AWGs

(Pavesi, 2004). Such compact devices can easily be

implemented on-chip and have already found

applications in WDM systems as well as in emerging

applications such as optical sensors, devices for

biosensing and optical spectrometers for infrared

spectroscopy (Bradshaw, 2005). The main problem

arising from the reduced size of waveguides is the

coupling of light from the fiber into such small

waveguides, which causes much higher coupling

losses, in the order of 10 dB, than in silica AWGs.

The second drawback is the sensitivity of the mode

index to the dimensional fluctuations (e.g. roughness)

of the waveguide, which leads to a rapid increase in

random phase-errors in the fabricated array grating

Seyringer, D., Maese-Novo, A., Muellner, P., Hainberger, R., Kraft, J., Koppitsch, G., Meinhardt, G. and Sagmeister, M.

Design and Optimization of High-Channel Si

based AWGs for Medical Applications.

DOI: 10.5220/0006621002130220

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 213-220

ISBN: 978-989-758-286-8

213

arms (Lee, 2000). These technological imperfections

affect the AWG performance by causing a marked

increase in the channel crosstalk. In order to reduce

the roughness of the waveguide sidewalls the high

index contrast AWGs require very high-resolution

fabrication technology that still presents a

considerable challenge today.

An alternative to high-index contrast and low-

index contrast AWGs is the Si

material platform,

which has a moderate index contrast lying between

both main groups (Martens, 2015).

The goal of the silicon nitride waveguide based

AWG development reported in this paper is to take a

significant step towards the integration of spectral

domain optical coherence tomography (SD-OCT)

system operating in a wavelength range of 800 nm to

900 nm and having 0.1 nm resolution. OCT is a

contact-free imaging method, which has become

significantly important in ophthalmology to visualize

the retina. In the course of the project, key-

components of an SD-OCT system will be integrated

on a single optical waveguide chip employing CMOS

compatible processes.

2 AWG PRINCIPLE

Based on the substrate, an AWG consists of an array

of waveguides (also called phased array, PA) and two

star couplers (Fig. 1). One of the input waveguides

launches the light consisting of multiple wavelengths



into the input star coupler, which then

distributes the light amongst an array of waveguides.

The light subsequently propagates through the

waveguides to the output coupler. The length of these

waveguides is chosen such that the optical path length

difference between adjacent waveguides dL equals an

integer multiple of the central wavelength



of the

demultiplexer. For this wavelength, the fields in the

individual arrayed waveguides will arrive at the input

of the output coupler with equal phase, and the field

distribution at the output of the input coupler will be

reproduced at the input of the output coupler. In the

output star coupler the light beams interfere

constructively and converge at one single focal point

on the focal line in the image plane. In this way, for

the central wavelength



the input field at the object

plane of the input star coupler is transferred to the

center of the image plane of the output star coupler. If

the wavelength is shifted to





i.e.







..),

there will be a phase change in the individual PA

waveguides that increases linearly from the lower to

the upper channel. As a result, the phase front at the

input aperture of the output star coupler will be

slightly tilted, causing the beam to be focused on a

different position in the image plane. The positioning

of the output waveguides at the focal points in the

image plane allows the spatial separation of the

different wavelengths (Smit, 1996).

3 AWG DESIGN

AWG design begins with the calculation of its

dimensions, which are essential to create the AWG

layout. The dimensions are given by the geometrical

parameters, as shown in Fig. 1 (Seyringer

, 2016):

1. minimum waveguide separation between PA

waveguides (parameter dd),

2. minimum waveguide separation between

input/output waveguides (parameter dx),

3. length of the star coupler (parameter Lf), and

4. optical path length difference between adjacent

waveguides in the phased array (parameter dL).

Figure1: Principle of an AWG with its design parameters and used waveguide cross-section.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

214

Width of the coupler W is not a dominant

parameter and can be freely changed. In order to

minimize the loss of light capture in the arrayed

waveguides, the number of arrayed waveguides Na

should be sufficiently large. Num is a number of

output waveguides (transmitting channels) that the

AWG is designed for.

There are a couple of commercial photonics tools,

available on the market that can be used to design and

simulate AWGs. Particularly: WDM PHASAR from

Optiwave, APSS Suite from Apollo Photonics, RSoft

tool or Photon Design tool. For our AWG designs, we

used WDM PHASAR, APSS Suite and RSoft tool.

All AWGs were designed for the TM-like mode with

a central wavelength of



= 850 nm.

3.1 AWG-Parameters Tool

As the commercial photonic tools do not support (or

only partially support) the calculation of the AWG

geometrical parameters, we developed a new

software tool called “AWG-Parameters” (Seyringer,

2013). This tool significantly reduces the time needed

for AWG design and also facilitates an understanding

of the relationship between input design and

geometrical parameters. The calculations of this tool

are based on the model of Smit and van Dam (Smit,

1996). The tool was used in many low-index contrast

AWG designs and technologically well proven

(Seyringer

, 2016). Therefore, we used this tool to

design Si

based AWGs, too.

Figure 2 shows the user interface of this tool,

presenting the design of Si

based 8-channel, 100-

GHz AWG. The input parameters for the calculation

of AWG geometrical parameters are:

Technological parameters used to design waveguide

structure (“Material” window in Fig. 2):

▪ Waveguide structure: width w = 0.8 µm (see also

in Fig. 1 “waveguide cross-section”).

▪ Effective index of the TM-like mode, n

eff,TM

1.50912,

▪ n

out

is a refractive index of the cladding (n

) =

1.4575 (in Fig. 1 “waveguide cross-section”).

AWG type parameters (“Transmission Parameters

 AWG Parameters” window in Fig. 2):

▪ Number of transmitting channels (output

waveguides): Num = 8 (see also in Fig. 1).

▪ AWG central wavelength (



): Lambda (µm) =

0.85.

▪ Channel spacing: df (GHz) = 100.

Transmission parameters (“Transmission Parameters

 AWG Parameters” window in Fig. 2):

▪ Adjacent channel crosstalk between output wave-

guides: Cr (dB) = -115.45.

▪ Adjacent channel crosstalk between arrayed

waveguides: CRaW (dB) = -20.7.

▪ Uniformity over all output channels (also called

non-uniformity): Lu

(dB) = 1.

When the “Calculate” button is pressed, the tool

calculates all necessary geometrical parameters given

in Fig. 1 and displays them in the “Transmission

Parameters  AWG Parameters” window (Fig. 2):

▪ Number of arrayed waveguides: Na = 51 (see also

in Fig. 1).

▪ Minimum waveguide separation between I/O

waveguides: dx (µm) = 4.000967.

▪ Minimum waveguide separation between PA

waveguides: dd (µm) = 1.200540.

▪ Coupler length: Lf (µm) = 181.927341.

▪ PA waveguide length difference: dL (µm) =

93.185026.

Figure 3 shows the spectral responses of this AWG

simulated with all three photonics tools together with

the measured characteristics of the fabricated AWG

(applying the special taper structures, the fibre

coupling efficiency for the used waveguide structure

is in the range of 1 dB - 1.5 dB).

Minimum Waveguide Separation Between PA

Waveguides (dd): One of the most important AWG

performance parameters is insertion loss. This loss

occurs due to reflection of the light at the facets of

interspaces between the individual PA waveguides.

Light penetrating the cladding material at these facets

is usually absorbed. This loss can be minimized by

maintaining only a small distance between the array

waveguides (parameter dd) or by adding linear tapers;

hence has to be considered already in the AWG

design. Therefore, in the first designs (8-channel,

100-GHz AWGs) we studied the influence of the dd

parameter on AWG performance, mainly on the

losses. We varied this parameter from 1 µm to 1.2

µm, 2 µm and 2.5 µm applying AWG-Parameters

tool. The design parameter dx was kept sufficiently

large, dx = 4 µm (Fig. 2 shows one of these designs).

Parameters dL and Lf were accordingly calculated.

From all simulations was evident that decreasing the

minimum waveguide separation between PA

waveguides led to a strong reduction of the insertion

loss, IL by about 4 dB. In comparison, the linear

tapers, applied in PA waveguides, reduced losses by

less than 1 dB (Seyringer

, 2016).

Based on this study and considering waveguide

width, w = 0.8 µm together with the fabrication

limitations we fixed this parameter to dd = 1.2 µm.

Design and Optimization of High-Channel Si

based AWGs for Medical Applications

215

Figure 2: User interface of the AWG-Parameters tool presenting design of 8-channel, 100-GHz AWG.

PHASAR

APSS

RSoft

MEASUREMENT

Figure 3: Simulated spectral responses of 8-channel, 100-GHz AWG from 3 photonics tools together with the measured

characteristics from the fabricated AWG.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

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Minimum Waveguide Separation Between

Input/Output Waveguides (dx): In the second step,

it was necessary to fix the design parameter dx, i.e.

minimum waveguide separation between input/

output waveguides. This parameter has an impact on

the crosstalk between adjacent output channels. To

this purpose, four 20-channel, 50-GHz AWGs with

different output waveguide separations were

designed with AWG-Parameters tool: dx = 2.5 µm, 3

µm, 3.5 µm and 4 µm and simulated applying all three

photonics tools (Seyringer, 2017). The simulations

showed that there is some minimum waveguide

separation dx necessary to keep the crosstalk between

output channels sufficiently low (in our AWG design,

dx = 3.5 µm). At this separation, the output

waveguides are positioned far enough from each

other, to prevent the focusing of the power from

selected channel into the neighbour output

waveguides and vice versa. This implies that by

increasing this value the performance of AWG did

not change much (case of dx = 4 µm). If the

waveguide separation was too small, the crosstalk

strongly increased. This is in particular the case of

waveguide separation dx = 2.5 µm and partially dx =

3 µm, where the AWG spectral response contained

side-lobes inducing high channel crosstalk. Based on

this study we fixed this parameter to dx = 3.5 µm.

We would like to point out that all above

described designs were technologically verified and

confirm the simulated results.

3.2 Design of 40-Channel, 50-GHz

AWG

Based on the previous study we designed 40-channel,

50-GHz AWG. The AWG structure was then

simulated applying WDM PHASAR tool and the

spectral response is shown in Fig. 4. As can be seen,

the transmitted optical signals are well separated from

each other which is confirmed by low background

crosstalk, BX = -42 dB (Fig. 4a). Adjacent channel

crosstalk reached its highest value at the side-lobes in

the middle of the spectrum, AX = 23.8 dB (Fig. 4b).

In addition, the losses are very low (Fig. 4c). There

are nearly no losses in the middle of the

characteristics (the highest peaks) and there is about

1.2 dB loss at the lowest peaks (insertion loss, IL).

This loss is mainly a result of the non-uniformity i.e.

difference between the highest- and the lowest peaks;

also called insertion loss uniformity, ILu = 1.18 dB

(in AWG-Parameters tool labelled as Lu parameter).

From this can be concluded that applying an

optimized design based on the previous study of the

design parameters in order to eliminate losses and

crosstalk, the insertion loss was suppressed nearly to

zero and the channel crosstalk is low, too.

4 DESIGN OF HIGH-CHANNEL

AWGS

Above described optimized design ensures rather

satisfying optical properties of AWGs up to 40

channels. However, the optical spectrometers for

medical applications require much higher AWG

channel counts. We will show that increasing the

number of output channels (parameter Num) brings

some additional design problems to be solved.

4.1 Design of 80-Channel, 50-GHz

AWG

To this purpose, we designed 80-channel, 50-GHz

AWG in which we used the same design parameters,

i.e. dd = 1.2 µm, dx = 3.5 µm; and the parameters Lf

and dL were calculated accordingly (we will call it

DESIGN1). The AWG structure was then simulated

applying WDM PHASAR tool keeping the same

calculation conditions. The simulated spectral

response is shown in Fig. 5. From the characteristics

is evident that the optical signals are much wider

compared to 40-channel, 50-GHz AWG (defined

through the bandwidth, B@5dB, and B@20dB, i.e. a

width of optical signal, measured at a −5 dB, and −20

dB drop from transmission peak), as can be seen in

Figs. 4b and 5b. It causes the increase of insertion loss

by nearly 1 dB (IL = 2.18 dB in Fig. 5c) and

particularly much higher adjacent channel crosstalk,

AX = 11.8 dB (Fig. 5b). Non-uniformity ILu is similar

in both cases, since both AWGs were designed to

have insertion loss uniformity about 1 dB.

Background crosstalk, BX increased from -42 dB (40-

channel AWG) to -38 dB (80-channel AWG, Fig. 5a).

4.2 Optimization of 80-Channel,

50-GHz AWG Design

Since the same design procedure as well as the same

simulation conditions were used in both designs the

question is what is the reason for such widening of the

optical signals causing deterioration of the optical

properties of designed 80-channel, 50-GHz AWG and

how can be this negative effect eliminated.

We have tested various AWG design parameters

and the deep study of the results showed that the

reason for widening of the optical signals is the

crosstalk caused by the coupling between PA

waveguides.

Design and Optimization of High-Channel Si

based AWGs for Medical Applications

217

Figure 4: a) Simulated spectral response of 40-ch, 50-GHz

AWG; b) detailed view of adjacent channel crosstalk,

AX in the middle of the spectrum, c) detailed view of

insertion loss, IL and insertion loss uniformity, ILu.

With increasing the number of output waveguides

(parameter Num) the number of waveguides in the

phased array (parameter Na) also increases and the

optical path length difference between waveguides

decreases. Therefore, the PA waveguides are placed

much closer to each other. From this follows that the

crosstalk caused by coupling in the PA can be avoided

by increasing the separation between arrayed

waveguides (Smit, 1996).

To show this influence we have designed the same

80-channel, 50-GHz AWG but with a different

parameters, dd = 1.75 µm (DESIGN2) and dd = 2.2

µm (DESIGN3) keeping the parameter dx = 3.5 µm.

Figure 5: DESIGN1 - a) Simulated spectral response of 80-

ch, 50-GHz Si

AWG; b) detailed view of adjacent

channel crosstalk, AX in the middle of the spectrum, c)

detailed view of insertion loss, IL and insertion loss

uniformity, ILu.

Figure 6 and Figure 7 show the simulated spectral

responses of both AWG designs.

From the simulations is evident that the shape of

optical signals is similar to the signal shape presented

in Fig. 4 (40-channel, 50-GHz AWG). It ensures

sufficient separation of the signals leading to the

improvement of AWG optical properties.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

218

Figure 6: DESIGN2 - a) Simulated spectral response of 80-

ch, 50-GHz Si

AWG; b) detailed view of the adjacent

channel crosstalk, AX in the middle of the spectrum, c)

detailed view of the insertion loss, IL and insertion loss

uniformity, ILu.

5 DISCUSSION

Table 1 summarizes the most important performance

parameters calculated from all four optical responses:

▪ Insertion Loss (IL): As described in section 3,

decreasing the minimum waveguide separation

between PA waveguides (parameter dd) led to a

strong reduction of the insertion loss, IL. This is

in particular the case of 40-channel, 50-GHz

AWG (dd = 1.2 µm) that reached the loss IL = -

1.2 dB. Opposite to this, increasing the distance

between PA waveguides, the losses increased

accordingly, as can be seen in DESIGN2 (dd =

1.75 µm, IL = -1.45 dB) and in DESIGN3 (dd =

2.2 µm, IL = -3 dB). In comparison, the 80-

channel, 50-GHz AWG (DESIGN1), even having

dd = 1.2 µm, features higher losses caused by

widening of the optical signals (IL = -2.18 dB).

▪ Insertion Loss uniformity (ILu): Due to the fact,

that the transmitted wavelengths follow the

envelope described by the far-field of the array

waveguides, there will always be non-uniformity

ILu in the intensity of focal sum-fields (Smit,

1996). Therefore this performance parameter is

also used as an input parameter in AWG design

(see sub-section 3.1), which was set to Lu = 1 dB

(Fig. 2). The non-uniformity calculated from the

simulated spectral responses is similar to this

value in all designs. Here, the small deviations are

the result of the calculation accuracy.

Figure 7: DESIGN3 - a) Simulated spectral response of

optimized 80-ch, 50-GHz Si

AWG, b) detailed view of

the adjacent channel crosstalk, AX in the middle of the

spectrum, c) detailed view of the insertion loss, IL and

insertion loss uniformity, ILu.

Design and Optimization of High-Channel Si

based AWGs for Medical Applications

219

▪ Adjacent Channel Crosstalk (AX): the highest

channel crosstalk between the neighbour output

waveguides (AX = 11.8 dB) was reached in

DESIGN1 with the widest shape of optical

signals. Since the other designs (40-channel, 50-

GHz AWG, DESIGN2 and DESIGN3) feature

similar spectral responses also the calculated

channel crosstalks are similar to each other (AX =

22-23 dB) and they all are much lower compared

to DESIGN1. Lower channel crosstalk means

higher channel isolation.

▪ Background Crosstalk (BX): this parameter

follows the adjacent channel crosstalk parameter,

AX, i.e. the worst value was reached for the

DESIGN1 (BX = -38 dB). All other designs

reached similar values ensuring much higher

background isolation.

Table 1: Performance parameters calculated from all optical

responses.

IL (dB)

ILu (dB)

AX(dB)

BX(dB)

40-ch AWG

-1.2

1.18

23.8

-42

DESIGN1

-2.18

0.9

11.8

-38

DESIGN2

-1.45

0.83

-45

DESIGN3

-3

1.8

-45

6 CONCLUSIONS

We showed that the design of standard channel count

AWGs (up to 40) is relatively easy and ensures good

AWG performance, too. Increasing the channel

counts (> 40 channels) leads to the deterioration of

optical properties like higher insertion loss and, in

particular, higher channel crosstalk. This appears due

to widening of the optical signals caused by light

coupling between PA waveguides. To avoid this

effect, the minimum separation between arrayed

waveguides has to be increased, i.e. the PA

waveguides have to be placed further apart. However,

it is important to point out that the greater distance

between PA waveguides means the higher losses.

Therefore, the DESIGN2 is the most suitable since

this AWG has optical properties the closest to

optimized 40-channel, 50-GHz AWG.

Finally, based on this study we designed 160-

channel, 50-GHz AWG having the optical properties

very similar to 80-channel, 50-GHz AWG. All high-

channel AWG designs are going to be technologically

verified. Based on the measured data, the next step is

the design of 320-channel, 42-GHz AWG.

ACKNOWLEDGEMENTS

This work was carried out in the framework of the

project COHESION, no. 848588, funded by the

Austrian Research Promotion Agency (FFG).

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