Kerr frequency comb (microcomb) generation based on parametric four-wave mixing (FWM) in monolithic microresonators with high quality (
Photonics Research, Volume. 9, Issue 7, 1351(2021)
Directly accessing octave-spanning dissipative Kerr soliton frequency combs in an AlN microresonator
Self-referenced dissipative Kerr solitons (DKSs) based on optical microresonators offer prominent characteristics allowing for various applications from precision measurement to astronomical spectrometer calibration. To date, direct octave-spanning DKS generation has been achieved only in ultrahigh-Q silicon nitride microresonators under optimized laser tuning speed or bi-directional tuning. Here we propose a simple method to easily access the octave-spanning DKS in an aluminum nitride (AlN) microresonator. In the design, two modes that belong to different families but with the same polarization are nearly degenerate and act as a pump and an auxiliary resonance, respectively. The presence of the auxiliary resonance can balance the thermal dragging effect, crucially simplifying the DKS generation with a single pump and leading to an enhanced soliton access window. We experimentally demonstrate the long-lived DKS operation with a record single-soliton step (10.4 GHz or 83 pm) and an octave-spanning bandwidth (1100–2300 nm) through adiabatic pump tuning. Our scheme also allows for direct creation of the DKS state with high probability and without elaborate wavelength or power schemes being required to stabilize the soliton behavior.
1. INTRODUCTION
Kerr frequency comb (microcomb) generation based on parametric four-wave mixing (FWM) in monolithic microresonators with high quality (
However, accessing soliton states in microresonators is still challenging since the DKS generation requires keeping the pump in an effective red-detuned regime, where the thermo-optic instability in the microresonator causes complex behavior [6]. Solitons have been demonstrated in
An alternative method to manipulate the power coupled into the resonance is using an auxiliary laser to pump another cavity mode, which can suppress thermal dragging dynamics during soliton formation and improve the soliton stability and access window [27–31]. However, generally for
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AlN, with a similar optical refractive index to
In this work, we reveal that an adjacent mode near (e.g., 35 pm longer than) the pump mode can help to mitigate the cavity thermo-optical effects, thereby allowing stable access to DKSs in an AlN microresonator via slow laser tuning. By pumping the fundamental TE (
2. DEVICE CHARACTERIZATION AND PRINCIPLE
Figure 1 illustrates the device characterization and the approach taken in this work. Using standard photolithography and inductively coupled plasma etching processes [39], we fabricated the devices in a quarter of a 2 inch wafer, as shown in Fig. 1(b), that consists of a 1.2-μm-thick epitaxial single-crystal AlN film and a sapphire substrate [40]. We employed a microring resonator with a radius of 60 μm and a cross section of
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Figure 1.(a) Scanning electron micrograph of the AlN microresonator after dry etching. (b) Photograph of a quarter of a wafer fabricated with standard photolithography. (c) Microscope image of one fabricated device. (d) Transmission spectrum of the microresonator used for octave-spanning soliton generation. (e) Zoomed-in region of the two close resonances near 1550.6 nm, with fits to determine the
Besides the high
3. EXPERIMENTAL AND SIMULATION RESULTS
A. Accessing Octave-DKS via Pump Sweeping
As the experimental setup depicted in Fig. 2, the light source (Santec TSL-710) is amplified by an erbium-doped fiber amplifier (EDFA) and injected into the waveguide through a lensed fiber. The output including transmitted pump light and generated comb lines is divided into two branches, one of which is connected with two optical spectrum analyzers (OSAs) for recording the spectra. The other branch is injected into a fiber Bragg grating (FBG) filter (bandwidth 0.4 nm) to differentiate the pump transmission (bandpass, BP) and comb lines (band rejection, BR) separately. The BR part is used for comb coherence characterization with the electrical spectrum analyzer (ESA) and autocorrelator (AC) after a photodiode (PD) and a fiber polarization controller (FPC), respectively. The BP part is sent to an oscilloscope or a powermeter for the pump transmission measurement. The powermeter with two synchronized channels enables the transmission measurement of all comb lines and the filtered pump at the same time.
Figure 2.Experimental setup used for single DKS generation and characterization. FPC, fiber polarization controller; EDFA, erbium-doped fiber amplifier; OSA, optical spectrum analyzer; OSA1, 600–1700 nm; OSA2, 1200–2400 nm; AC, autocorrelator; PD, photodiode; OSC, oscilloscope; ESA, electrical spectrum analyzer; FBG, fiber Bragg grating.
Figure 3(a) shows the measured all and pump transmissions at 350 mW on-chip power and a pump tuning speed of 1 nm/s, as well as the calculated difference between them. A pronounced 63-pm-wide step-like structure characteristic of soliton formation is formed. The low
Figure 3.(a) Resonance transmissions at an on-chip power of 350 mW and a pump tuning speed of 1 nm/s. (b) Frequency comb evolution map. (c) Optical spectral snapshots of different comb states, corresponding to the dash lines in (b). The dashed lines show the fitted
The top stack of Fig. 3(c) is the generated MI comb spectrum (i), which ranges from 140 to 260 THz with a dispersive wave (DW) bump around 255 THz. Typical spectra of solitons 1 [region (ii) of Fig. 3(c)] and 2 [region (iii) of Fig. 3(c)] are presented in Fig. 3(c) subsequently, which cover an octave-spanning range from 130 to 273 THz (1100–2300 nm). The measured soliton spectra are significantly extended toward shorter wavelengths due to the emission of the DW via soliton-induced Cherenkov radiation at
We also observed the shiny green light emission during the soliton formation [see insets in Fig. 3(c)], corresponding to the third-harmonic generation (THG) at
The transition from the MI comb to the soliton comb is verified by the drastic reduction of low-frequency intensity noise [see Fig. 3(d)]. To further confirm the single DKS states, the temporal characteristics were also carried out through the second-harmonic generation (SHG)-based autocorrelation measurement. Figure 3(e) shows the measured soliton autocorrelation trace, which has a poor signal-to-noise ratio (SNR) since the soliton comb power is at the limit that can be detected by the AC. However, the pulses are clearly separated by
To explore the dependence of soliton existence range on the pump power, we characterized the all and pump transmissions, under various powers, and we plot their difference in Fig. 4. A striking soliton step with 10.4 GHz (83 pm) width is observed at 335 mW, which is the widest soliton existence window as far as we know. When further increasing the power, the soliton step declines nearly linearly (
Figure 4.Power difference between all and pump transmissions measured at on-chip powers with a laser tuning speed of 1 nm/s. The green polygon indicates the soliton existence regime.
B. Accessing Octave-DKS by Tuning the Pump Wavelength with One Step
One highlight of our system is the direct creation of the DKS state by simply red tuning the pump light with one step, thereby eliminating the requirement for complex pump tuning or power kicking techniques. As the schematic depicted in Fig. 5(a), the laser wavelength was switched 50 times between an off- (
Figure 5.(a) Schematic of the laser wavelength tuning with step mode. (b) Soliton accessing possibility among 50 attempts, versus the
The possibility of accessible soliton versus the
4. DISCUSSION
As investigated in Refs. [18,20], with an auxiliary mode, the intracavity power is modified such that a part of the soliton regime becomes thermally stable. Therefore, the soliton behavior is affected by the mode separation and a thermally induced cavity redshift, which is related to the pump power, the mode
To increase the overall yield for ensuring the pump and auxiliary modes are close, we will design and fabricate microresonators with minor changes around the target parameters by taking the fabrication variation into account. A patterned metal contact can be deposited near the devices for attempting to fine-tune the mode separation and coupling. This provides another dimension, together with optimized pump tuning speed and power, to control the thermal effect for accessing soliton states more easily. We can estimate that the pump power required for soliton generation can be decreased to tens of mW assuming the intrinsic
5. CONCLUSION
A simple route is proposed and demonstrated to stably achieve the octave-spanning DKS in an AlN microresonator, in which a nearby auxiliary mode is slightly red-detuned from the pump mode. The auxiliary resonance can compensate for the intracavity power change and balance the thermal effects in the resonator, thus producing and broadening the soliton step significantly. A comparison of different nonlinear material platforms for single soliton generation is shown in Table 1. In this work, an octave-spanning soliton microcomb ranging from 130 to 273 THz, with a repetition rate of
Comparison of Single Kerr Solitons Generated with Various Chip-Integrated Microresonators
Material | FSR (GHz) | On-Chip Power (mW) | Spectral Range (nm) | Tuning Method | Soliton Step | Reference | |
---|---|---|---|---|---|---|---|
200 | – | 71 | 1470–1620 | Thermally tuned resonance | – | [ | |
230 | 200 | 1400–1700 | Forward and backward tuning | – | [ | ||
1000 | 1100–2320 | Forward sweeping, 0.8 nm/s | [ | ||||
1000 | 455 | 1100–2300 | Forward and backward tuning | – | [ | ||
99 | 6.2 | 1540–1620 | Piezo laser tuning | [ | |||
40 | 30 | 1520–1600 | Turn-key soliton | [ | |||
199.7 | 33 | 1470–1650 | Forward or backward sweeping | [ | |||
335 | 240 | 1190–2140 | Backward sweeping, | [ | |||
AlN | 225 | 1400–1700 | Forward sweeping, 20 nm/s | 0.2 pm | [ | ||
AlN | 433 | 1050–2400 | Single-sideband modulation, forward sweeping 60 pm/μs and backward tuning | – | [ | ||
AlN | 374 | 1100–2300 | Forward sweeping, 1 nm/s, or manually tuning, or step mode | This work |
Represents the loaded
Represents the octave-spanning spectral range.
Acknowledgment
Acknowledgment. The authors would like to thank Prof. Liam Barry for the technical assistance.
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Haizhong Weng, Jia Liu, Adnan Ali Afridi, Jing Li, Jiangnan Dai, Xiang Ma, Yi Zhang, Qiaoyin Lu, John F. Donegan, Weihua Guo, "Directly accessing octave-spanning dissipative Kerr soliton frequency combs in an AlN microresonator," Photonics Res. 9, 1351 (2021)
Category: Nonlinear Optics
Received: Apr. 12, 2021
Accepted: May. 13, 2021
Published Online: Jul. 5, 2021
The Author Email: John F. Donegan (jdonegan@tcd.ie), Weihua Guo (guow@mail.hust.edu.cn)