Passive harmonic mode-locked laser on lithium niobate integrated photonics

Abstract

Mode-locked lasers (MLLs) are essential for a wide range of photonic applications, such as frequency metrology, biological imaging, and high-bandwidth coherent communications. The growing demand for compact and scalable photonic systems is driving the development of MLLs on various integrated photonics material platforms. Along these lines, developing MLLs on the emerging thin-film lithium niobate (TFLN) platform holds the promise to greatly broaden the application space of MLLs by harnessing TFLN 's unique electro-optic (E-O) response and quadratic optical nonlinearity. Here, we demonstrate the first electrically pumped, self-starting passive MLL in lithium niobate integrated photonics based on its hybrid integration with a GaAs quantum-well gain medium and saturable absorber. Our demonstrated MLL generates 4.3-ps optical pulses centered around 1060 nm with on-chip peak power exceeding 44 mW. The pulse duration can be further compressed to 1.75 ps via linear dispersion compensation. Remarkably, passive mode-locking occurs exclusively at the second harmonic of the cavity free spectral range, exhibiting a high pulse repetition rate $\sim$20 GHz. We elucidate the temporal dynamics underlying this self-starting passive harmonic mode-locking behavior using a traveling-wave model. Our work offers new insights into the realization of compact, high-repetition-rate MLLs in the TFLN platform, with promising applications for monolithic ultrafast microwave waveform sampling and analog-to-digital conversion.

Figures (4)

• Figure 1: Hybrid GaAs–TFLN passive harmonic MLLa, Schematic of the passive harmonic MLL. The GaAs RSOA chip contains an SOA section and an SA section, and it is butt-coupled to a TFLN extended cavity to form a Fabry-Perot laser cavity. When passively mode-locking occurs, two ultrashort light pulses exist within one cavity round-trip. b, Optical microscope image of the TFLN nanophotonic waveguide forming a passive extended cavity. Scale bar: 100 µ m. c, Butt-coupling interface between the GaAs RSOA chip and the TFLN chip. d, Fundamental TE mode in the GaAs waveguide at 1060 nm. e Fundamental TE mode in the input TFLN waveguide at 1060 nm. f, SEM image of the CDC in the broadband Sagnac loop mirror. Scale bar: 2 µ m. The coupler has a total coupling length of 54 µ m and a gap of 600 nm between the waveguide top surfaces. g, Simulated (blue solid line) and measured (red symbols) reflection spectrum of the Sagnac loop mirror.
• Figure 2: Bias-dependent operating regimes of the passive harmonic MLLa, Schematic of the MLL measurement setup. The laser signal emitted from the output facet was collected using a single-mode lensed fiber and divided by beam splitters (BS) for simultaneous monitoring of the output power with a power meter (PM), the optical spectrum with an optical spectrum analyzer (OSA), the RF spectrum with an electrical signal analyzer (ESA) through a fast photodetector (FPD), and the pulse intensity autocorrelator. Additionally, we used a continuous tunable CW laser (CTL) for heterodyne beat note measurement. Before the autocorrelator, we employed a pulse shaper for dispersion compensation and a ytterbium-doped fiber amplifier (YDFA). b, Output optical power as a function of gain current ($I_\mathrm{g}$) and bias voltage ($V_\mathrm{b}$). The lasing threshold currents are 65, 85, and 95 mA for $V_\mathrm{b} = 0.2$, 1.6, and 2.8 V, respectively. c, SNR of RF signal around 20 GHz under various $I_\mathrm{g}$ and $V_\mathrm{b}$. d, Laser spectra under various $V_\mathrm{b}$ and $I_\mathrm{g}$. White dotted lines separate different operating regimes.
• Figure 3: Characterization of second harmonic passively MLLa, Laser output spectrum measured at $V_\mathrm{b} =$2.8 V and $I_\mathrm{g}$ = 290 mA. The 10 dB bandwidth is approximately 1.748 nm, containing 23 comb lines due to axial modes. The 3 dB bandwidth is around 0.691 nm by fitting with the Fourier transform of a Sech$^2$ function. b, Normalized RF signal (blue) and its Lorentzian fit (red) centered around 20.115 GHz, with a fitted 3 dB linewidth of 150 kHz and a SNR of 62 dB. c, Heterodyne RF beat note measurement showing three spectrally clean and sharp peaks at 8.28, 11.83, 20.11 GHz. Insets: Zoomed-in view of the heterodyne beat notes. Blue symbols are measured data, and red lines are Lorentzian fits. d, Autocorrelation traces together with Sech$^2$ autocorrelation fits of the pulse at $V_\mathrm{b} = 2.8$ V. The corresponding pulse durations are 1.75 ps with the waveshaper on (red) and 4.3 ps with the waveshaper off (blue), obtained by multiplying the fitted FWHM values of the autocorrelation traces by the deconvolution factor 0.6482. e, Minimum pulse widths after dispersion compensation at different $V_\mathrm{b}$ from 0.2 V to 2.8 V.
• Figure 4: Passive harmonic MLL dynamicsa, Comparison of experimental and simulated pulse trains. Top: measured pulse train using a fast photodetector and a high-speed oscilloscope; Bottom: simulated pulse train. Inset: Zoom-in of simulated pulse exhibiting a steep leading edge and a prolonged trailing tail at $V_\mathrm{b}$ = 2.8 V, $I_\mathrm{g}$ = 260 mA. b, Simulated laser optical spectrum showing comb line spacing of approximately 75 pm and a 10 dB bandwidth of 2.29 nm. c, Simulated pulse width as a function of $V_\mathrm{b}$ at $I_\mathrm{g}$ = 290 mA. d, Upper: Evolution of round-trip gain, loss, and net gain. Inset: Zoom-in view of the gain and loss dynamics responsible for the formation of the second net gain window. Lower: Round-trip pulse train formation. Inset: Zoom-in view of early round-trips showing a pulse and its satellite, compared with a uniformly spaced pulse train at steady state.