1 Spontaneous m icrowave platicon frequency microcomb in dispersion -managed microresonators

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Spontaneous microwave platicon frequency microcomb in dispersion-managed

microresonators

Wenting Wang1,2,†,*, Jinkang Lim1†, Abhinav Kumar Vinod1, Mingbin Yu3, Dim-Lee Kwong3, and

Chee Wei Wong1,*

1 Fang Lu Mesoscopic Optics and Quantum Electronics Laboratory, University of California, Los

Angeles, CA 90095, United States of America

2 Communication and Integrated Photonics Laboratory, Xiongan Institute of Innovation,

Chinese Academy of Sciences, Xiong’an New Area, China

3 Institute of Microelectronics, A*STAR, Singapore 117865, Singapore

† These authors contributed equally to this work.

* Email: wenting.wang@xii.ac.cn; cheewei.wong@ucla.edu

Temporally stabilized optical pules, confined in microresonators driven by a continuous-wave

laser, have attracted tremendous attention due to their fascinating features with many applications.

Here we report the observations of mode-locked platicon frequency microcomb formation in

normal dispersion dispersion-managed microresonators operating at microwave K-band repetition

rate for the first time. Facilitated by the thermally controllable modulated background induced by

avoided mode-crossings, various platicon bound state patterns with regular and irregular temporal

separation are stably generated due to an additional balance between repulsive and attractive forces

resulting from non-trivial interpulse and background electromagnetic field interactions. The

number of mode-locked pulses can be switched by forward- and backward-cavity pump detuning

and, with increasing pump power, result in stationary bound-state complexes. These experimental

observations are in accordance with our nonlinear numerical simulations that includes avoided

mode-crossing, anomalous fourth-order dispersion and quality-factor spectral filtering. The

observed platicon mode-locked pulses have broad spectral profiles overlapping Kelly-sideband-

like parametric oscillation. The single-sideband phase noise of microcomb repetition rate is

characterized for the different mode-locked states, comparable with electronic microwave

oscillators. The ability to achieve mode-locking in dispersion-managed microresonators provides

a platform to reduce pulse timing jitter and enrich the exploration of ultrafast phenomena in

microresonators.

1. Introduction

Ultrafast nonlinear dynamics in continuous-wave-driven Kerr microresonators have recently

been investigated for disciplined ultrafast pulse pattern formation with repetition rates from

microwave to terahertz [1-5] and broadband coherent chip-scale frequency comb synthesis [6-8].

The demonstrated frequency microcomb has been applied for massively optical communication

[9-11], laser spectroscopy [12-14], precision distance metrology [15,16], coherent terahertz

generation [17] and astronomical spectroscopy [18,19]. The excitation of dissipative Kerr soliton

(DKS, bright and dark soliton determined by the cavity dispersion) [20,21] benefits from the

balance between anomalous dispersion and Kerr nonlinear phase shift, as well as internal

dissipation and externally coherent pump driving. The DKS can be spontaneously generated by

sweeping the pump laser wavelength across one of the microresonator resonances and anchoring

at an effective red pump-cavity detuning with appropriate pump power. Usually the bright DKS

undergoes rich cavity dynamics ranging from modulation instability-induced Turing rolls and

spatiotemporal chaos [15], to stable cavity solitons pumped at the anomalous dispersion regime.

During the soliton formation, the optical Čerenkov radiation [22-24] such as originating from high-

order dispersion and Raman-related phenomena in microresonators, can be observed as well [25].

Accessing the stable soliton state experimentally is often hindered due to strong thermal dynamics

resulting from thermo-optic effects in microresonators. To mitigate the underlying thermal

influences, techniques such as frequency ramping methods including forward- and backward-

pump laser scanning [5,26-28] and pump power kicking method [29], pump laser injection locking

[30] are proposed and experimentally demonstrated. Turn-key integrated soliton microcombs can

be obtained based on the photonic integrated circuits with heterogeneous integration using CMOS

foundry materials such as Si3N4 [31], AlGaAs [32], AlN [33] and CMOS-compatible fabrication

processes [34] or edge-coupling technologies combined with a compact laser diode [35,36].

In contrast to the soliton microcomb, the mode-locked microcomb can be formed in the normal

dispersion microresonators with different modalities such as dark soliton microcombs [5,21],

switching-waves [37] and platicon frequency microcombs [38-40] which exhibit an intrinsically

higher optical power conversion efficiency, and larger pulse duty cycle in the time domain. The

coherent normal dispersion microcomb can be generated with assistance of quality-factor-

dependent spectral filtering [5], mode-coupling such as spatial or polarization mode interaction

[41,42], pump laser self-injection locking [43], and external pump laser modulation [44].

Moreover, simultaneously normal-anomalous dispersion microresonators can be fabricated by

tuning the waveguide geometry for dispersion engineering to balance the material normal

dispersion [45, 46], while further expanding the stability zones.

Here we report the first observations of spontaneous mode-locked platicon frequency

microcomb formation with K-band repetition rate in normal dispersion dispersion-managed

dissipative microresonators. In analogy to dispersion-managed mode-locked fiber lasers [47], the

dispersion-managed dissipative microresonator enhances the mode-locking spectral range [45,

48,49]. The pulses are stabilized with the aid of avoided mode-crossings (AMX) and wavelength-

dependent quality-factor spectral filtering. By proper control of the pump laser power and pump-

cavity detuning, various mode-locked platicon pulse states are excited and switched. Mode-locking

dynamics and transitions related to the platicon pulse are further observed for the first time. The

tunability of the primary comb lines is realized by the normal group-velocity dispersion (GVD)

and temperature-dependent AMXs. By adjusting pump laser powers, bound states of double-,

three-, four-, six-, seven-, and eight-pulses are observed with varying relative temporal separation.

These experimental observations are supported by intensity noise characterization, single-sideband

phase noise, ultrafast intensity autocorrelation (AC) measurements, and frequency-resolved

optical gating (FROG) metrology, along with nonlinear numerical simulations. All measured

bound pulses are phase-locked in short- and long-range interactions, clearly manifested by optical

spectral interference fringes, single-sideband phase noise, and the intensity AC traces.

2. Results

Figure 1a shows the scanning electron micrograph of the dispersion-managed Si3N4

microresonator. The stoichiometric Si3N4 microresonator features a 7.7 mm circumference, a 1 m

× 0.76 m width-height cross-section over the bus and curved waveguides to ensure single-mode

operation, and seven tapered resonator-waveguide segments of 800 m lengths each. In each

tapered segment, the waveguide width adiabatically increases from 1 to 2.5 m and vice versa, to

finely tune the dispersion and enhance the single-mode mode-locking (detailed in Methods and

Supplementary Information Section I). The pulses can exist with the characteristic periodic

dispersion oscillation causing pulse stretching and compression behaviors. The numerically

evaluated GVD is varied from + 139.3 fs2/mm to -38.7 fs2/mm with the tapered waveguide width

from 1 to 2.5 m. The calculated path-averaged cavity GVD and TOD are +27.9 fs2/mm and -970.8

fs3/mm respectively for a 1590 nm pump wavelength. A commercial near-infrared continuous-

wave semiconductor laser pumps the microresonator for the mode-locked pulse generation where

a microwave signal is generated through a high-speed photodetector. The pump laser is directly

free-space edge-coupled to an engineered inverse taper waveguide on-chip, with coupling loss

around 3 dB per facet.

Figure 1. Mode-locked platicon frequency microcomb in dispersion-managed microresonators

with K-band repetition rate. (a) Schematic of the generation of platicon frequency microcomb in

the photonic-chip-based Si3N4 microresonator. The dispersion-managed microresonator

constitutes of seven tapered waveguides with varying widths from 1 to 2.5 μm to provide the

periodic oscillating group-velocity dispersion. Smaller panels show zoom-in views over the taper

waveguides and the curved single-mode spatial mode filter. Scale bar: 200 m. (b) Conceptual

illustration of the spontaneous platicon frequency microcomb formation facilitated by the tunable

intracavity modulated field background induced by the avoided mode-crossing (AMX). (c)

Measured integrated group velocity dispersion (GVD) of the Si3N4 microresonator with the swept-

wavelength interferometry. The measured normal GVD is β2 = 28.2 ± 6.4 fs2/mm, obtained from

D2 = -c/nD12β2. The FSR (D1) of the device is 19.7 GHz in the microwave K-band. Inset: Multiple

measurements of the GVD. (d) Normalized cold-cavity transmission of the single-mode tapered

microresonator. The pump resonance at 1590 nm with a loaded Q of 1.79  106. (e) Measured

parametric primary comb lines can be continuously tuned – such as over 41 nm in this plot – by

（a）

（c）（d）

Tapered waveguide

（b）

5.3 THz

5.2 THz

7.5 GHz

5.2 THz

Pump

LD Wavelength

Pump

Sideband

resonance

Time

Pump resonance

Δω (T) = N ωFSR

Δt=

2π/Δω

Modulational

background

Roundtrip time

Pump

Primary

comb line

1600 1700

1500

Wavelength (nm)

Dispersion-managed

microresonator

Frequency (MHz)

Transmission (a.u.)

（e）

-60 -40 -20 020 40 60

Mode number,μ

-80

-60

-40

-20

(ωμ-ω0-D1μ)/(2π) (MHz)

Wavelength (nm)

1600 1590 1580

3.3

nm D2= 66.7 ± 6.5 kHz

-500 0500

0.2

0.4

0.6

0.8

Q =

1.79×106

λp= 1590 nm

1 2 3 4 5 6 7 8 9 10

100

D2(kHz)

β2(fs2/mm)

200 μm

Measurement

controlling the pump-cavity detuning, supported by the AMX or the equivalent anomalous fourth-

order dispersion.

Figure 1b shows schematic of the mode-locked platicon frequency microcomb generation in

the normal dispersion microresonator facilitated by the intracavity field modulated background,

triggered by the AMXs. Swept-wavelength interferometry is used to characterize the single-mode

cold-cavity dispersion at the fundamental transverse-electric mode (TE00) with the TE-polarized

pump laser as shown in Figure 1c (detailed in Supplementary Information Section II). The

microresonator dispersion is determined by analyzing the FSR wavelength dependence with the

relation   



 , where  is the angular

frequency of the resonances,  is the pump laser angular frequency, and D3 is the higher-order

dispersion parameter. The measured free spectral range D1/2π = 19.7 GHz with a normal GVD β2

of 28.2 ± 6.4 fs2/mm and the corresponding second-order dispersion parameter D2/2π = 66.7 ± 6.5

kHz, based on ten dispersion measurement sets as shown Figure 1c inset. The 1590 nm pump

resonance is fitted with a Lorentzian lineshape as shown in Figure 1d. The coupled resonance

linewidth κ/2π is  105 MHz, corresponding to the loaded quality factor Q of 1.79 × 106 (intrinsic

Q of 1.86 × 106 ). Figure 1e shows the widely tunable parametric modulation sidebands when the

pump-cavity detuning is continuously swept and controlled.

Mode-locked platicon frequency microcomb formation

To deterministically generate the mode-locked pulses, we implement a polarization-assisted

resonance pulling method (PARP), which mitigates thermal influences in the microresonator

benefiting from the short response time of the polarization modulator (shorter than thermal

relaxation time ≈ 1 µs). Instead of accessing the stable soliton step by sweeping pump wavelength,

in PARP the pump wavelength is firstly tuned into a close resonance and subsequently an abrupt

voltage is applied to the polarization modulator to change the pump laser polarization. This cools

down the cavity resonance such that the pump wavelength can access different mode-locking states.

To quantify the thermal dynamics in the microresonator, we scan the pump wavelength over the

cavity resonance with a 20 nm/s scanning rate to measure the hot cavity transmission as shown in

Figure 2a. It can be clearly observed that the cavity resonance is red-shifted by the thermo-optic

and Kerr nonlinearities with the characteristic discrete step plateau of a negative slope, indicating

the AMX after heating the microresonator. The corresponding discrete comb spectral structures

are shown in Figure 2b which are generated by controlling the pump laser power and pump-

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1Spontaneousmicrowaveplaticonfrequencymicrocombindispersion-managedmicroresonatorsWentingWang1,2,†,*,JinkangLim1†,AbhinavKumarVinod1,MingbinYu3,Dim-LeeKwong3,andCheeWeiWong1,*1FangLuMesoscopicOpticsandQuantumElectronicsLaboratory,UniversityofCalifornia,LosAngeles,CA90095,UnitedStatesofAmerica2Commun...

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1 Spontaneous m icrowave platicon frequency microcomb in dispersion -managed microresonators

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