Mid-infrared continua via spectral broadening and difference frequency generation in a nanophotonic lithium niobate waveguide

Abstract

Periodically poled thin film lithium niobate waveguides provide simultaneous access to efficient second and third order nonlinear processes, enabling broadband generation of coherent laser light. Here, we demonstrate the generation of a broadband mid-infrared continuum in a nanophotonic lithium niobate waveguide pumped by a telecom-wavelength femtosecond source. Specifically, our dual-stage design includes both third-order nonlinear spectral broadening followed by a dedicated periodically poled waveguide section performing efficient broadband intrapulse difference frequency generation. Driven by sub-100 fs pulses with approximately 200 pJ pulse energy, the generated mid-infrared light covers wavelengths from 3200 to 4800 nm. Cascaded harmonic generation also extends the spectrum into the visible and ultraviolet domains, resulting in an overall spectral bandwidth ranging from 350 to 4800 nm.

Figures (4)

• Figure 1: Concept of MIR generation in a dual stage lithium niobate waveguide. A femtosecond pulse from a telecom-wavelength fiber laser experiences spectral broadening via supercontinuum generation (SC) due to the third-order nonlinearity in the first stage. In the second stage, mid-infrared light is generated via difference frequency generation (DFG), facilitated by a quasi-phase matched second-order nonlinearity; in addition second harmonic generation (SHG) and higher harmonics generation may occur.
• Figure 2: Supercontinuum generation in an unpoled LiNbO$_\mathbf{3}$ waveguide.a) Experimental setup. A telecom-wavelength femtosecond oscillator (OSC) seeds an Er:fiber amplifier (EDFA). The pulses propagate through 1 m of standard polarization maintaining (PM) fiber. A prism compressor (PC) isolates the resulting soliton pulses. Parabolic mirrors couple light into and out of the LiNbO$_{3}$ waveguides. Optical spectrum analyzers (OSA) are used to measure the output spectra. The photograph shows a LiNbO$_{3}$ waveguide in operation. b) Experimental FROG trace (left) and reconstructed trace (right) indicate a bandwidth limited pulse of 83 fs at a central wavelength of 1680 nm. c) Supercontinuum generation in LiNbO$_{3}$ waveguides without poling at an on-chip pulse energy of 300 pJ. Generated supercontinua as a function of waveguide (top) width. A bandwidth exceeding 100 THz at the 10% intensity level can be achieved.
• Figure 3: Waveguide designa) False color scanning electron micrograph of a waveguide cross section. b) Calculated group velocity dispersion as a function of wavelength for different waveguide (top) widths. c) Calculated group index as a function of wavelength for different waveguide widths. d) Layout of the two-stage waveguide. In a narrow section with length L$_1=4$ mm a supercontinuum is generated. Subsequent IDFG is performed in a wider, periodically poled section of length L$_2=1$ mm for minimal temporal walk-off and improved long-wavelength mode confinement. e) False color scanning electron micrograph indicating the poled material before etching. Shown are the electrodes spaced by a poling period $2\Lambda$ and the resulting domain inversion after poling. The horizontal light gray bar indicates the location of the later etched waveguide. f) Calculated quasi-phase matching for a waveguide width of 3000 nm. Top: Color-coded domain width (half poling period) as a function of two input wavelengths (1&2) driving IDFG. Bottom: Resulting output IDFG wavelengths as a function of the two input wavelengths. The nearly constant color code inside the dashed triangle (top graph) indicates a nearly uniform requirement for the poling period for IDFG output wavelengths between $\mathrm{3-6\,\mu m}$ (bottom graph).
• Figure 4: MIR comb generation via IDFGa) Output spectra of an unpoled waveguide as a function of estimated on-chip input pulse energy. b) Output spectra of an identical waveguide geometry as in a) but with periodic poling in the IDFG stage. Spectral slices at 236 pJ indicated by the red dotted lines are shown in panel c). In contrast to the unpoled waveguide, where no light generation beyond 2.5 $\mu$m wavelength is observed, the poled waveguide shows significant MIR light generation, 20dB above the noise floor. The dashed lines indicate waveguide losses calculated for the two involved waveguide widths.