Quantum fluctuations disrupt the cyclic motions of dissipative Kerr solitons
(DKSs) in nonlinear optical microresonators and consequently cause timing
jitter of the emitted pulse trains. This problem is translated to the performance
of several applications that employ DKSs as compact frequency comb sources.
Recently, device manufacturing and noise reduction technologies have
advanced to unveil the quantum properties of DKSs. Here we investigate the
quantum decoherence of DKSs existing in normal-dispersion microresonators
known as dark pulses. By virtue of the very large material nonlinearity, we
directly observe the quantum decoherence of dark pulses in an AlGaAs-on-insulator
microresonator, and the underlying dynamical processes are
resolved by injecting stochastic photons into the microresonators. Moreover,
phase correlation measurements show that the uniformity of comb spacing of
quantum-limited dark pulses is better than 1.2×10^−16 and 2.5 × 10^−13 when
normalized to the optical carrier frequencies and repetition frequencies,
respectively. Comparing DKSs generated in different material platforms
explicitly confirms the advantages of dark pulses over bright solitons in terms
of quantum-limited coherence. Our work establishes a critical performance
assessment of DKSs, providing guidelines for coherence engineering of chip scale
optical frequency combs.