Simulation and Experimental Validation of a Silicon Photonics Ring

Assisted Mach-Zehnder Interferometer Filter

Ana Clara Marques

1,2

, Pedro Cabrita

, Maria João Carvalhais

, Catarina Novo

1,2

, Mário Lima

1,2

Francisco Rodrigues

and António Teixeira

1,2,3

Instituto de Telecomunicações, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal

University of Aveiro, Aveiro, Portugal

PICAdvanced AS, Ílhavo, Portugal

Keywords: Ring-Assisted Mach-Zehnder Interferometer, Photonic Integrated Circuits, Optical Filters.

Abstract: This paper presents a comprehensive study on the simulation and laboratorial validation of ring assisted Mach-

Zehnder interferometers (RAMZIs) for use as optical filters in demultiplexing systems in optical

communication. The study explores the integration of a Mach-Zehnder interferometer (MZI) and an optical

ring ressonator (ORR) to achieve a flat-top spectral response. The ORR and MZI are independently simulated

before combining them into a RAMZI structure. The experimental results, obtained from a fabricated PIC on

a SiOI platform, closely align with simulated spectrum, demonstrating the validity of the proposed model.

1 INTRODUCTION

Photonic integrated circuits (PICs) are considered a

significant advancement in the field of

telecommunications. Their capability of integrate

many optical components such as filters, modulators,

polarization elements, photodiodes or lasers into a

single chip, combined with system flexibility, low

cost and potential for large scale production, are some

of the key characteristics that have driven extensive

research into this technology over the past few years

(Dong et al., 2014).

Optical selective bandpass filters are a special

subgroup of components in optical transmission

systems, particularly in wavelength-division

multipplexing (WDM) systems, due to their ability to

perform channel selection, demultiplexing and

multichannel filtering (Kohli et al., 2021). These

filters enable precise isolation of specific

wavelengths or channels, ensuring efficient data

transmission and reception over multiple channels

simultaneously, thereby enhancing the overall

performance of optical networks (Song et al., 2008).

Over the years, numerous optical components

have been refined to perform as filters. Mach-

Zehnder interferometer (MZI) stand out as a key

example (Horst et al., 2013). It is a simple device

based on a Y-splitter structure that divides the

incoming light on the waveguide into two arms and

then recombine the two signals allowing them to

interfere and generate an output signal. The nature of

this interference, whether constructive or destructive,

depends on the optical path difference between the

two arms ().

Figure 1: MZI scheme.

Although MZIs are fundamental components in

Silicon Photonics (SiP), a single MZI is not capable

of delivering a flat-top spectral response. To obtain a

more rectangular shaped spectrum multiple stages,

MZIs can be designed, although this approach

increases the device footprint and complexity in

design and fabrication processes. As an alternative,

the design of ring assisted Mach-Zehnder

interferometers (RAMZIs) appeared. This

configuration offers a more box-like passband

response compared to cascaded optical ring

ressonators (ORRs) or MZIs with a simplified device

sctructure (Sabri et al., 2024).

Marques, A. C., Cabrita, P., Carvalhais, M. J., Novo, C., Lima, M., Rodrigues, F. and Teixeira, A.

Simulation and Experimental Validation of a Silicon Photonics Ring Assisted Mach-Zehnder Interferometer Filter.

DOI: 10.5220/0013397500003902

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 13th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2025), pages 147-151

ISBN: 978-989-758-736-8; ISSN: 2184-4364

147

Conventionally this component consists of a basic

MZI with a coupled ORR in its shorter arm.

A generic ring resonator consists of an optical

waveguide which is looped back on itself, such that a

resonance occurs when the optical path length of the

resonator is exactly a whole number of wavelengths .

In addition to that we have one straight waveguide

that allows the access to the loop through a coupling

mechanism. This specifically design is called all-pass

filter and a scheme of the described component is

shown in Figure 2 where the three main physical

parameters of the device are represented as 



for the

coupling length, that refers to the straight path that

separates the two halves of the ring, R for the radius

of the ring and the gap, g, between the ring and the

straight waveguide.

Figure 2: ORR scheme.

The addition of the ORR enables the RAMZI to

exploit both the interference and resonance

phenomena, making it suitable for applications

requiring enhanced wavelength filtering or precision

phase control, while retaining the basic functionality

of the MZI. This straightforward configuration is

known as a single-ring RAMZI, as showed in Figure

3. In recent years, various design modifications have

been explored, including two or more cascading rings

or parallel rings configurations (Kohli et al., 2021).

Figure 3: Single ring RAMZI scheme.

In this paper, for demonstration of the RAMZI

working principle, we will present a model of

simulation and experimental validation of the single

ring RAMZI.

2 SIMULATION MODEL

This simulation process involves two steps. First the

ORR and MZI are simulated independently to ensure

that each component adheres to the required

relationship between their Free Spectral Ranges

(FSR), with the MZI's FSR being twice that of the

ORR. Next, the two components are integrated to

form a RAMZI.

2.1 ORR Simulation

The simulation of the ORRs for the design of RAMZI

structures were carried out using the Variational

Finite-Difference Time-Domain (varFDTD) solver in

Ansys Lumerical. This solver provides an accurate

representation of light propagation in planar

integrated optical systems.

Simulating the rings is essential to determine the

parameters required to calculate the Q-factor defined

as the ratio between a resonant wavelength and its full

width at half maximum (FWHM), as described in

Equation 1. This Q-factor is then used to determine

the coupling coefficient, k, between the straight

waveguide and the ring, via Equation 2.













(1)





























(2)

Additionally, it is also essential to calculate the

total length of the ORR using Equation 3.





(3)

The calculated parameters, k and L, will serve as

essential inputs for simulating the RAMZI.

The resulting spectrum of the simulated ORR with a

radius of 5.5 µm, a coupling length of 1.5 µm and a

gap of 200 nm, using a waveguide with an effective

index, 



, of 2.5454, group index, 



, of 3.9030, a

width of 450 nm and a thickness of 120 nm is

presented at Figure 4 corresponding to the expected

response for this component compared to the

reviewed literature (Bogaerts et al., 2012).

2.2 MZI Simulation

For simulating the MZI Lumerical

INTERCONNECT was employed due to its

PHOTOPTICS 2025 - 13th International Conference on Photonics, Optics and Laser Technology

148

Figure 4: ORR spectrum.

capability to support block-based schematic designs.

This software facilitates the creation and visualization

of photonic circuits, enabling efficient simulation and

analysis of the MZI architecture. Lumerical

INTERCONNECT’s framework allows easy

parameter adjustments and comprehensive insights of

the configuration’s performance.

Figure 5 represents the MZI setup with the length

of each parameter represented in µm. The input

optical signal enters through the Y-branch splitter,

which divides it into two paths, forming the two arms

of the interferometer. In the upper arm, the signal

propagates through two waveguide segments, one of

them being 17.29µm, introducing a significant phase

delay. In the lower arm, the signal travels through

shorter waveguides. These two paths are then

recombined at the directional coupler where the

signals interfere. The output interference pattern is

analysed by the Optical Network Analyzer.

Figure 5: Block diagram of the simulated MZI.

The resultant spectra of this specific MZI is

showed in Figure 6, showcasing the two output

channels, and it also corresponds to the expected

response for the respective component as seen in

literature (Zheng et al., 2021).

Figure 6: MZI spectrum.

2.3 RAMZI Simulation

The RAMZI simulation is essentially identical to the

MZI, with the main difference being the addition of

the ORR in the smaller arm as depicted in Figure 7.

This ORR must follow the parameters established in

the varFDTD simulation. To ensure it matches the

original ring and maintains the same spectral

response, the length and coupling coefficient

calculated earlier are applied here.

Figure 7: Block diagram of the simulated RAMZI.

Finally, the spectrum of the final component is

depicted in Figure 8.

Figure 8: RAMZI spectrum.

L=10

L=17.29

L=5

L=37.5575

L=10

L=17.29

L=5

Simulation and Experimental Validation of a Silicon Photonics Ring Assisted Mach-Zehnder Interferometer Filter

149

As anticipated we can observe that the RAMZI exhibits

a more rectangular response compared to the individual

spectra of the ORR and MZI.

3 EXPERIMENTAL VALIDATION

To validate the simulation models, a laboratory test

was done to compare the real and simulated spectrum.

Both a RAMZI and an MZI from an existing PIC were

tested, and the results were analysed in comparison

with the simulated data.

The devices were designed using GDSfactory.

The PIC was fabricated on a multi-project wafer from

CORNERSTONE foundry and the stack is a SOI

platform with rib waveguides.

The optical responses of the components were

assessed by acquiring spectra at both ports of a MZI

and a RAMZI using the experimental setup a depicted

in Figure 9. In this setup, a C-L Band ASE Light

Source provides the input light in the system. An

Erbium Fiber amplifier(EDFA) is necessary to

amplify the signal before it enters the PIC. Finally the

output of the component is analysed with an Optical

Spectrum Analyser (OSA).

Figure 9: Experimental setup.

Figure 10 and Figure 11 show the experimental

spectrums of a MZI and RAMZI respectively

compared to its simulated spectrums.

Figure 10: MZI experimental vs simulation.

By examining the results, we can conclude that,

despite potential wavelength shift from the simulation

possibly due to lithography differences between the

simulated component and the fabricated one that will

be explored in future work, the overall shape of the

spectra aligns well. This consistency indicates that the

simulation models are valid and are reliable for future

analyses.

Figure 11: RAMZI experimental vs simulation.

4 CONCLUSIONS

In conclusion, this study successfully demonstrated

the simulation and experimental validation of a

RAMZI for optical filtering applications in photonic

integrated circuits. By individually simulating the

ORR and MZI and then integrating them into the

RAMZI structure, we achieved a detailed

understanding of their performance characteristics.

The inclusion of the ORR in the MZI configuration

enhanced the spectral response, offering a flatter

passband and improved wavelength selectivity

compared to a standard MZI. Experimental results

from fabricated photonic chip showed good

alignment with simulated spectra, validating the

reliability of the simulation models despite minor

deviations attributed to fabrication tolerances.

ACKNOWLEDGEMENTS

This work was supported by FCT – Fundação para a

Ciência e Tecnologia, I.P. by project reference

LA/P/0109/2020, and DOI identifier 10.54499/

LA/P/0109/2020, https://doi.org/10.54499/UIDB/

50008/2020

This work is supported by the European Regional

Development Fund (FEDER), through the

Competitiveness and Internationalization Operational

Programme (COMPETE 2020) of the Portugal 2020

framework [Project POWER with Nr. 070365 (POCI-

01-0247-FEDER-070365)] and Project SINO.

The University of Southampton and the UK

Engineering and Physical Sciences Research Council

(EPSRC) funded CORNERSTONE project

(EP/L021129/1), CORNERSTONE 2 project

PHOTOPTICS 2025 - 13th International Conference on Photonics, Optics and Laser Technology

150

(EP/T019697/1) or CORNERSTONE 2.5 project

(EP/W035995/1).

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