Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator

Summary

Here, we present a protocol to generate soliton crystals in a butterfly-packaged micro-ring resonator using a thermal tuned method. Further, the repetition rate fluctuations of a soliton crystal with a single vacancy are measured using a delayed self-heterodyne method.

Abstract

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the nonlinearity in a propagation Kerr medium. The development of dissipative Kerr solitons (DKSs) in high-Q microcavities drives a novel, compact, chip-scale soliton source. When DKSs serve as femtosecond pulses, the repetition rate fluctuation can be applied to ultrahigh precision metrology, high-speed optical sampling, and optical clocks, etc. In this paper, the rapid repetition rate fluctuation of soliton crystals (SCs), a special state of DKSs where particle-like solitons are tightly packed and fully occupy a resonator, is measured based on the well-known delayed self-heterodyne method. The SCs are generated using a thermal-controlled method. The pump is a frequency fixed laser with a linewidth of 100 Hz. The integral time in frequency fluctuation measurements is controlled by the length of the delay fiber. For a SC with a single vacancy, the repetition rate fluctuations are ~53.24 Hz within 10 µs and ~509.32 Hz within 125 µs, respectively.

Introduction

The steady DKSs in microresonators, where the cavity dispersion is balanced by Kerr nonlinearity, as well as the Kerr gain and cavity dissipation¹, have attracted great interest in the scientific research community for their ultra-high repetition rate, compact size, and low cost². In the time domain, DKSs are stable pulse trains that have been used for high-speed ranging measurement³ and molecular spectroscopy⁴. In the frequency domain, DKSs have a series of frequency lines with equal frequency spacing that are suitable for wavelength-division-multiplex (WDM) communications....

Protocol

1. Optical coupling

1. Polish the end-face of the MRR on a grinding plate using 1.5 μm abrasive powders (aluminum oxide) mixed with water for 5 min.
2. Fix the MRR with a chip fixture and place an eight-channel FA on a six-axis coupling stage, which includes three linear stages with a resolution of 50 nm and three angle stages with a resolution of 0.003°. The patches of the MRR and FA are 250 μm.
3. Use a 1,550 nm laser as an optical source for real-time monitoring of the coupling efficiency. Carefully adjust the position of the FA until the inset loss reaches the minimum value, typically less than 6 dB, corresponding to a couplin....

Results

Figure 3 shows the transmission power trace while a resonance thermal was tuned across the pump. There was an obvious power step that indicated the generation of SCs. The step had similar power compared with its precursor, the modulational instability comb. Therefore, the generation of SCs was not tuning speed dependent. The SCs exhibited a great variety of states, including vacancies (Schottky defects), Frenkel defects, and superstructure^12,

Discussion

On-chip DKSs provide novel compact coherent optical sources and exhibit excellent application prospects in optical metrology, molecular spectroscopy, and other functions. For commercial applications, compact packaged micro-comb sources are essential. This protocol provides a practical approach to make a packaged micro-comb that benefits from the reliable, low coupling loss connection between the MRR and FA, as well as a robust thermal-controlled DKS generation method. Therefore, our experiments are no longer coupling sta.......

Materials

Name	Company	Catalog Number	Comments
6-axis coupling stage	Suruga Seiki	KXC620G KGW060	Contains 3 linear motorized translation states and 3 angular motorized rotational stages. Linear state: Minimum stepping: 0.05 μm; Travel: 20mm; Max.speed: 25mm/s; Repeatability: +/-0.3 μm; Rotational stage:Travel: ±8°; Resolution/pulse: 0.003 degree; Repeatability:±0.005°
Abrasive powder	Shenyang Kejing Auto-Instrument Co., LTD	2980002	Silicon carbide, granularity: 1.5 μm
Glue 3410	Electronic Materials Incorporated	Optocast 3410	Optocast 3410 is an ultra violet light and heat curable epoxy suitable for opto-electronic assembly. It cures rapidly when exposed to U.V. light in the 320-380 nm.
High-index doped silica glass	Home-made	-	The MRR is fabricated by a high index doped silica glass platform. The waveguide section is 2×3 μm and radius is 592.1 μm, corresponding to FSR of 49 GHz.
Pump laser	NKT Photonics	E15	It is a continuous wave fiber laser with linewidth of 100 Hz.
Ultrastable Laser	Menlosystems	ORS	State-of-the-art linewidth (<1Hz) and stability (<2 x 10-15 Hz)