Cascaded Tunable Optical Delay Line based on a Racetrack

Resonator with Tunable Coupling and Stable Wavelength

Solomon Getachew Hailu

and San-Liang Lee

Graduate Institute of Electro-Optical Engineering, National Taiwan University of Science and Technology,

No. 43, Sec. 4, Keelung Rd., Taipei 10607, Taiwan

Department of Electronic and Computer Engineering, National Taiwan University of Science and Technology,

No. 43, Sec. 4, Keelung Rd., Taipei 10607, Taiwan

Keywords: Group Delay, Racetrack Resonator, Tunable Optical Delay Line, Power Transmission, Thermal Tuning.

Abstract: We propose a novel integrated optical delay line based on a cascaded racetrack resonator with tunable coupler

by push-pull operation of each stage to stabilize the resonant wavelength. The thermal tuning effect and the

photonic characteristics of the whole integrated device is simulated to verify the characteristics of the tunable

ODLs. The tuning of hundreds of ps is achievable with a very compact device and very small power

consumption. The two-stage configuration can allow for reaching larger delay time or a wider bandwidth

depending on the operation condition.

1 INTRODUCTION

Integrated optical delay lines (ODLs) are key

components in photonic circuits for many

applications. The optical delay line can be realized

exploiting either a single stage or multiple cascaded

stages. The use of two cascaded stages allow to

increase either the delay tunability or the bandwidth

of device compared to a single-stage ODL (Melati &

Melloni, 2018). They can be found in numerous

applications, such as optical signal processing and

buffering in optical networks, beamforming and

filtering in microwave photonic systems, bio-medical

sensing, and 3D light scanning and ranging (Zhou, et

al., 2018) (Melati, et al., 2018) (Han, et al., 2013)

(Cardenas, et al., 2010).

Integrated on-chip ODLs exhibt multiple benefits

correlated to bulk optics or fiber-based ODLs, such as

the reduced cost, size, weight, and power

consumption (Zhuang, et al., 2013) (Melati, &

Melloni, 2018) (Melloni, et al., 2010) (Liu, et al.,

2018) (Balbas, et al., 2007). The integrated optical

delay lines have been illustrating for a variety of

utilization (Pegios, et al., 2018) (Park, et al., 2008)

(Capmany, & Novak, 2007) (Hyeon, et al., 2015).

Various integration platforms can be utilized to

construct ODLs. There are several trade-offs in the

delay line performances, like bandwidth and

maximum delay, integration density and waveguide

propagation loss needs to be acknowledging before

picking the proper platforms. Silicon photonics

platform based on the silicon-on-insulator (SOI) is

regarded as one of the most promising technologies

for large-scale high-density photonic integration

because of its large index contrast, small bending

radius and the use of compatible fabrication facilities

in semiconductor foundries (Melloni, et al., 2010)

(Xie, et al., 2007). There are several semiconductor

foundries and research institutes providing the multi-

project-wafer (MPW) process for SOI-based photonic

integration, which can reduce the research and

development cost for realizing tunable ODLs.

Therefore, our design is based on the SOI platforms.

There are two common used approaches to tune

the SOI-based ODLs: tuning with thermal heating or

carrier-induced effects. Both tuning mechanisms

change the refractive index of the waveguide and then

change the phase. Thermal tuning is usually the

choice for its simple device structure and low optical

loss as long as the tuning speed is not the major

concern. Tunable optical delay lines based on ring

resonators have been demonstrated by several groups

(Bogaerts, et al., 2012) (Poon, et al., 2004) (Katti, &

Prince, 2018) (Xie, et al., 2014). Most of the designs

are for optical signal processing or buffering. In these

applications, the critical requirements include large

enough true delay, wide bandwidth, and low higher-

Hailu, S. and Lee, S.

Cascaded Tunable Optical Delay Line based on a Racetrack Resonator with Tunable Coupling and Stable Wavelength.

DOI: 10.5220/0009164901570162

In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 157-162

ISBN: 978-989-758-401-5; ISSN: 2184-4364

157

order phase variations that will lead to dispersion and

signal distortion. Therefore, multiple stages of ring

resonators are employed to achieve the desired delay

with large bandwidth and low distortion (Cardenas, et

al., 2010).

In this work, we design an optical delay line based

on two-stage racetrack resonator equipped with

tuning heaters. The design aims at the applications for

optical and bio-medical sensing where the optical

signal usually narrow linewidth.

We explain that with the proper control design, the

novel architecture doubles the group delay tunability

range compared to a single-stage racetrack resonator

ODL and achieves a limited delay-bandwidth product

(DBP). The ODL design demonstrated in this paper

can achieve a true delay ranging from tens of

picoseconds to hundreds of picoseconds at a stable

wavelength.

2 CASCADED INTEGRATED

OPTICAL DELAY LINE

Figure 1 (a) shows the schematic of the 2-stage ODL

where each stage is a tunable racetrack resonator with

a tunable coupler. The tunable coupler is a symmetric

Mach-Zehnder interferometer (MZI) with one of the

two arms equipped with a thermal heater based phase

shifter to tune the output ratio of the MZI. The peak

delay occurs at the resonance condition of the

racetrack resonator. The peak delay depends on the

coupling ratio of the MZI output. In general, as the

peak delay increases, the insertion loss of the ODL

rises and the wavelength bandwidth decreases. On the

same time, the peak wavelength where the peak delay

occurs moves with the coupling ratio. In order to

stabilize the peak wavelength in the application field

of optical sensing, the two phase shifters of each stage

is operated in push-pull mode. That is, as one phase

shifter is heating up, the other one will be cooling

down. The peak wavelength of the two stage can be

aligned to double the peak group delay or shifted

slightly to have a greater bandwidth.

To verify the operation characteristics of the

proposed 2-stage ODL, the simulation using the

Lumerical Solutions’ Photonic Design Tools

conducted here (Lumerical INTERCONNECT,

2019). The used to realize the two-stage integrated

ODL, which details as shown in Figure 1. The block-

diagrams component blocks used in the Lumerical

Interconnect is depicted in Figure 1(b). The device

architecture is constructed with six Lumerical MODE

waveguides, four thermal modulators, four 3-dB

couplers, four DC source components, and an optical

network analyzer. In each component block, we

import the exact file that comes from the MODE

simulation. We import the saved data from MODE

simulations into MODE waveguide and modulator

components. The change of effective index as a

function of input power of Heater 1, and Heater 3 can

be deposited into the "Optical Modulator 1", and

"Optical Modulator 3" respectively, and the change of

effective index as a function of input power of Heater

2, and Heater 4 can be loaded into "Optical Modulator

2", and "Optical Modulator 4" respectively. In each

component of MODE waveguide, we imported the

proper file of TE mode thermal waveguide profiles.

The response of the devices was measured through

the Optical Network Analyzer that permits

characterizing the power transmission spectrum and

the group delay spectrum.

(a)

(b)

Figure 1: (a) Schematic of 2-stage tunable delay lines, (b)

Lumerical INTERCONNECT blocks for the two cascaded

identical stages ODL based on racetrack resonator and MZI.

The box with blue shaded part includes the schematic of a

single stage delay line.

3 TWO STAGE INTEGRATED

OPTICAL DELAY LINE

SIMULATION RESULTS

Figure 2 shows the group delay, power transmission,

and wavelength at the resonant condition as a function

of the input power of Heater 4 when the input power

of Heater 1 and Heater 3 is fixed at 0 mW. It

corresponds to the case that only the coupling

PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology

158

coefficient of the ODL is tuned. By tuning the input

power of Heater 4, from 0 to 7mW, the group delay,

measured at the resonant peak varies from 35.62 ps to

201.43 ps, while the corresponding power

transmission efficiency at the transmission dip in the

spectrum, can be kept above 80% over the tuning

range. However, the peak wavelength shifts from

1550.74 nm to 1550.85 nm during tuning. That is, the

wavelength drifts at a rate of 0.0016 nm/mW

regarding the input power of Heater 4. This

wavelength drift will appear in a change in the group

delay for a fixed wavelength. Also, Figure 3 shows

the results for tuning the racetrack loop phase with

Heater 3 by fixing the input power of Heater 2 and

Heater 4 at 0 mW. During the tuning of Heater 3, the

peak group delay and power transmission are almost

constant, but the wavelength varies linearly with a

slope of 0.031 nm/mW. Hence, Heater 3 can be used

to correct the resonant wavelength without affecting

the group delay and power transmission.

Figure 2: Simulation of the group delay, power

transmission, and wavelength at the resonant peak versus

the input power of Heater 4 when the other heater is at 0

mW.

Figure 4 shows the contour plot of the wavelength

of the proposed ODL as a function of the input power

of the two heaters. The contour plot will be used to

illustrate how to tune the group delay and keep the

peak wavelength intact. Figure 4 shows the tuning of

the peak wavelength versus the power to heater 3 and

4, as indicated by the red arrows in Figure 4. The

triangular background pattern shows in Figure 4 is the

wavelength due to the push pull operation of the

heaters tuning. This figure shows particularly one of

the peaks between the several peaks caused by the

ring resonance.

Figure 3: Simulation of the group delay, power

transmission, and wavelength at the resonant peak versus

the input power of Heater 3 when the other heater is at 0

mW.

Figure 4: Contour plot of wavelength as a function of the

input power of heaters.

Figure 5 shows that the peak group delay can be

continuously tuned by simply changing the input

power to Heater 4 while it remains fixed in the tuning

of Heater 3. From Figures 2 & 3, the wavelength drift

rate is different for tuning Heater 4 and Heater 3,

respectively. Moreover, the tuning of Heater 3 and

Heater 4, the shift of the wavelength is in the same

direction (longer wavelength). To retain the

wavelength in tuning the group delay, the two heaters

require to be operated at the push-pull mode. That is,

the heater input power to heater 3, must be decreasing

as the input power to heater 4, increases.

As designated by the yellow arrows in Figure 4

and 5 as an example, the input power of Heater 4 is

increased to increase the group delay, then the input

power to Heater 3 needs to be decreased to keep the

same peak wavelength.

Cascaded Tunable Optical Delay Line based on a Racetrack Resonator with Tunable Coupling and Stable Wavelength

159

Figure 5: Contour plot of peak group delay as a function of

the input power of heaters.

Figure 6: Group delay as a function of the wavelength for

[(H4, H3) = (0,5), (1,4.5), (2,4), (3,3.5), (4,3), (5,2.5), (6,2),

(7,1.5), (8,1), (9,0.5)] mW, where H3 and H4 indicate the

power to Heater 3 and 4, respectively.

Figure 7: Peak group delay, power transmission, and

wavelength as function of the input power of heater 4.

Figure 8: Group delay as a function of the wavelength for

[(H4, H3) = (0, 0-28)] mW.

Figure 9: Group delay as a function of the wavelength for

[(H4, H3) = (8, 0-28)] mW.

Figure 6 depicts the tuning spectrum for the push-

pull operation. Since the wavelength drift rate for

tuning Heater 3 is about double of that for tuning

Heater 4, the input power for Heater 4 and Heater 3 is

of a step of 1 mW and 0.5 mW, respectively, for the

spectra shown in Figure 7. During group delay tuning,

the resonant wavelength is almost fixed. The

summary of the group delay, power transmission, and

wavelength by tuning the two heaters together at the

push-pull mode is as shown in Figure 7. The

wavelength keeps constant while the group delay is

tuned by more than one order of magnitude.

Figures 8 and 9 show that the peak group delay can

be tuned over one free spectral range (FSR) by using

less than 30 mW of the Heater 3 power when the input

power of Heater 4 is 0 and 8 mW for different delay

time. Due to the periodic delay-time characteristics,

Figures 8 and 9 indicates that the proposed two-stage

tunable delay lines can be tuned to track the

wavelength of the input light sources over a wide

wavelength range and provide the needed group delay.

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4 CONCLUSIONS

We present here a novel way of tuning the group

delay of a two-stage optical delay line by operating

the thermal heater at push-pull mode. The peak group

delay and resonant peak wavelength can be tuned

almost independently. The group delay is tuned by the

heater in one arm of the balanced MZI coupler to vary

the effective coupling coefficient to the resonator. On

the other hand, the thermal tuning on the racetrack

loop can correct the wavelength drift without

affecting the group delay. The simulation using the

practical device structure of a semiconductor foundry

is pursued to demonstrate the group-delay tuning

characteristics of such a racetrack-resonator.

The targeted applications for such tunable optical

delay lines are for optical sensing and/or bio-medical

sensing, where the light source usually has very

narrow linewidth. We demonstrate the tuning of

group delay to nearly 40 ps while fixing the resonant

wavelength by adjusting the heating power of Heater

3. The maximum group delay can be achieved by

tuning the coupling coefficient to have an even

sharper resonant peak with a tradeoff on the

transmission power. The tuning of hundreds of

picosecond is achievable with a very compact device

and very small power consumption. The use of two

cascaded identical stages with the described control

scheme allows to double both the tunability range of

the group delay compared to a single-stage ODL.

ACKNOWLEDGEMENTS

This research was supported by the Ministry of Science and

Technology, Taiwan, under Grant MOST 108-2622-E-011-

CC1.

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