Abstract
We model the generation threshold and conversion efficiency of microcombs by scaling the cavity coupling. With the Lugiato–Lefever equation (LLE), quantitative analysis of threshold is established in the parameter space of pump power and coupling. Considering the large detuning and Kerr-induced phase shift, the threshold power is numerically solved with the minimum at over-coupling, in agreement with that from the traveling wave theory. Furthermore, the coupling dependence on microcomb generation is discussed, providing the accessibility of high-efficient, stable combs (≥ 40%) around the threshold. This work offers universal guidelines for the design of microcombs with low-power and high-efficient operation.
Highlights
To characterize the cold-cavity threshold, we vary the input power at the bus waveguide slowly from zero and keep the pump detuning the same. This can be achieved by changing the current of the integrated semiconductor optical amplifier (SOA) and stabilizing the frequency with an integrated sensor[23]
The minimal position of the threshold power exhibits a strong dependence on the cavity coupling
With a large phase shift, the coupling is optimized to be strongly over-coupled; while with zero phase shift, the optimized point moves to the under-coupled regime
Summary
We start our approach by numerically solving the generalized mean-field LLE, written a s12: TR. The wavelength tunability and linewidth reduction have been well-established by the control of external cavities and microheaters[22,23], developments of fixed wavelength on-chip lasers with ultra-narrow linewidth[24] are still advantageous for high-purity comb operation. This idea has been demonstrated by mode-locking solitons in microresonators with the aid of thermal tuning[25]. With the LLE, we would be able to model the comb threshold in relation to the pump power and cavity coupling while a locked, low-noise fixed-frequency laser is used as the pump
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