Chip-scale ultrafast soliton laser

TL;DR

Chip-scale femtosecond lasers with high coherence and low power have been elusive. This work introduces photorefraction-assisted soliton (PAS) mode-locking in a hybrid InP RSOA and thin-film LiNbO3 (TFLN) external cavity to realize a self-starting, on-chip Kerr soliton laser within a Fabry–Pérot cavity. The device achieves sub-$90~\mathrm{fs}$ pulses at repetition rates up to $3~\mathrm{THz}$ with turn-key operation, and can be reconfigured via pump current while operating from a low-voltage supply of $1~\mathrm{V}$ and $75~\mathrm{mA}$ drive. It also delivers exceptionally high coherence, reporting an intrinsic comb linewidth of $53~\mathrm{Hz}$ (frequency-noise floor $17~\mathrm{Hz^2/Hz}$), orders of magnitude better than previous chip-scale MLLs, with potential for full integration enabled by LiNbO3 electro-optic functionality for on-chip signal processing and frequency conversion.

Abstract

Femtosecond laser, owing to their ultrafast time scales and broad frequency bandwidths, have substantially changed fundamental science over the past decades, from chemistry and bio-imaging to quantum physics. Critically, many emerging industrial-scale photonic technologies -- such as optical interconnects, AI accelerators, quantum computing, and LiDAR -- also stand to benefit from their massive frequency parallelism. However, achieving a femtosecond-scale laser on-chip, constrained by size and system power input, has remained a long-standing challenge. Here, we demonstrate the first on-chip femtosecond laser, enabled by a new mechanism -- photorefraction-assisted soliton (PAS) mode-locking. Operating from a simple, low-voltage electrical supply, the laser provides deterministic, turn-key generation of sub-90-fs solitons. Furthermore, it provides electronic reconfigurability of its pulse properties and features an exceptional optical coherence with a 53 Hz intrinsic comb linewidth. This demonstration removes a key barrier to the full integration of chip-scale photonic systems for next-generation sensing, communication, metrology, and computing.

Figures (4)

• Figure 1: Concept and implementation of on-chip soliton laser.(a, b) state-of-the-art approaches to chip-scale ultrashort pulse generation. (a) On-chip MLLs. Typically based on III-V platforms, these devices use a reverse-biased semiconductor saturable absorber to achieve self-starting pulse generation directly from an electrical current. However, their pulse duration is fundamentally limited to the sub-picosecond domain by the finite carrier response time of the semiconductor SA. (b) Externally-driven DKS. A high-Q microresonator is pumped by an external continuous-wave (CW) laser to have solitons through four-wave mixing (FWM). While this approach generates femtosecond pulses, the externally-driven parametric process is critically reliant on the external pump laser, introducing challenges such as difficult soliton initiation, low conversion efficiency, pedestal-present soliton, and pump-dependent comb linewidth. (c) Chip-scale femtosecond soliton laser enabled by PAS mode-locking. (c.I) Device architecture. A microring exhibiting a PR effect is embedded within a semiconductor-gain-based FP external cavity. (c.II) Frequency-domain picture of the soliton mode-locking mechanism. (c.III) Time-domain behavior of the mode-locking. (d) Schematic of the hybrid integrated laser architecture. (e,f) Photograph showing the hybrid integration and the device footprint in comparison with a one-cent coin. (g) Micrographs detailing the RSOA-TFLN coupling interface, embedded high-Q microresonator, and broadband Sagnac mirror.
• Figure 2: Performance of the soliton microcomb laser.(a) Optical (left) and RF (right) spectra of laser output at different RSOA driving currents. Dashed lines represent $sech^2$ fits. Each state was obtained by optimizing the relative position between the RSOA and TFLN chips to compensate for thermal-induced mechanical drift. (b) FROG traces for the 1.2-THz (top), 1.6-THz (middle), and 0.4-THz (bottom) soliton states at RSOA driving currents of 75, 120, and 151 mA, respectively. The center frequency ($\Delta \nu=0$) of the FROG traces is 386.83THz (corresponding wavelength of 775 nm). For a better comparison, the figures also show frequency and time scales normalized, respectively, by the 200-GHz FSR and 5-ps roundtrip time ($T_0$) of the mode-locking resonator. The relatively indistinct pulse temporal boundaries in the FROG traces result simply from limited gain bandwidth of the EDFA used to boost the soliton pulses before they are recorded by FROG (Fig. \ref{['Fig2']}d). This artifact is confirmed by performing the same FROG on similar comb states produced via conventional DKS excitation in a passive microresonator. Details are provided in SI. (c) Comparison with advanced on-chip MLLs on pulse duration and repetition rate. All data are listed in Table S1 of SI. The 132 mA state is 90 fs and 2 THz. The 80 fs state is given later in Fig. \ref{['Fig3']}e. (d) Schematic of experimental setup. LDC: laser diode controller; DCU: dispersion compensation unit; PM: power meter; FROG: frequency-resolved optical gating; OSA and ESA: optical and electrical spectrum analyzer.
• Figure 3: Dynamics and stability of turn-key operation.(a) Experimental setup to characterize the turn-key operation. (b) Time-dependent laser power (top) and beat-note spectrum (bottom) during five consecutive ON/OFF cycles (each with 20-s ON and 5-s OFF) of the RSOA current. Consistent recovery of both optical power and RF beatnote frequency confirms deterministic recurrence of the exact same soliton state. (c) Startup dynamics of the laser power. The dynamic process involves three phases: current ramp-up (grey), photorefraction-mediated transition (green), and steady-state soliton operation (purple). (d) Laser power (top), optical (middle) and RF (bottom) spectra as the RSOA current is ramped from 90 to 109 mA in 1-mA steps and $\sim$5-s duration per step. Representative spectra (right) show laser states before, during, and after the soliton regime. The lasing state becomes unstable when current $>$103 mA, so the optical and RF spectra shown in this region only repsent a snapshot of certain lasing states. See Fig. S1 for detailed discussion and detailed spectra at each current step. (e) Time-dependent laser power (top) and optical spectrum (bottom) over a long time duration, showing the long-term stability of the laser operation. Mechanical drift of the testing station perturbs the soliton operation after $\sim$18 minutes. A simple realignment of the mechanical stages readily restores the soliton state, indicating a robust mode-locking operation. The soliton state here exhibits a 3-dB optical bandwidth of 3.9 THz (corresponding to a transform-limited pulse duration of $\sim$80 fs) with a repetition rate of 1.2 THz. It was obtained with optimized gain at a RSOA current of 160 mA.
• Figure 4: Optical linewidth of the soliton microcomb laser.(a) Frequency noise spectrum of individual comb lines and the overall comb for the 1.6-THz soliton state. The overall comb has frequency noise floor lower than individual comb lines likely because of detector noises that limit the characterizaton of the latter due to their lower powers. (b) Spectral distribution of intrinsic optical linewidth for individual comb lines $\Delta \nu_m$, along with the soliton comb spectrum. (c) Comparison of intrinsic optical linewidth and optical efficiency across different on-chip ultrafast comb sources. Complete performance metrics and definitions are provided in Table S2 of SI.