Visible dual-comb spectroscopy across more than 100 THz with lithium niobate nanophotonic waveguides

TL;DR

This work addresses broadband, high-resolution UV-VIS spectroscopy using visibly generated dual-comb spectra from thin-film lithium niobate nanophotonic waveguides pumped by low-power Er:fiber combs. It demonstrates generation of UV-VIS-NIR frequency combs spanning >$100~$THz with ~100 MHz resolution, enabling direct, high-precision spectroscopy of iodine, NO2, and atomic rubidium and sodium, all within a compact, low-SWaP platform. The approach leverages cascaded $\chi^{(2)}$ processes and dispersion-engineered TFLN waveguides with real-time GPU-based phase correction and averaging to realize fast, coherent, broadband measurements. These results establish a path toward portable, lab-grade broadband spectroscopy across 500 THz from the UV to the near-infrared, with applications in fundamental spectroscopy, atmospheric sensing, and astronomical calibration.

Abstract

Broadband and high-resolution spectroscopy in the visible and ultraviolet is central to advances in multiple fields, including fundamental quantum physics, biology, atmospheric science and astronomy. Traditionally, these measurements are performed with grating or Fourier-transform spectrometers using incoherent light sources. Leveraging coherent light enables powerful frequency-comb-based techniques, but is limited by the technical complexity of efficiently generating broad spectral bandwidths from relatively narrowband and spectrally distant laser sources. Current visible dual-comb spectrometers require implicit compromises between optical bandwidth, experimental simplicity, and acquisition speed. In this work, we introduce a simple and efficient dual-comb spectrometer that converts robust Er:fiber frequency combs from the near-infrared to the ultraviolet and visible with thin-film lithium niobate (TFLN) nanophotonic waveguides. Using real-time signal processing, we retrieve coherently averaged dual-comb spectra over nearly 120 THz of simultaneous bandwidth in the visible with 100 MHz spectral resolution. With these capabilities, we measure the broadband absorption spectrum of molecular iodine (I2), demonstrating the broadest visible spectral coverage of a dual-comb spectrometer to date. Additional measurements of NO2, atomic rubidium, and atomic sodium further illustrate the achievable combination of spectroscopic bandwidth, resolution, and intrinsic frequency accuracy. Our results demonstrate the powerful integration of low-power frequency combs, nonlinear nanophotonics, and digital signal processing to enable a compact, efficient and versatile approach to high-resolution mapping of complex absorption spectra across 500 THz in the UV-visible and near-infrared spectral regions for multiple applications beyond the research lab

Figures (8)

• Figure 1: Experimental setup for dual-comb spectroscopy (a) Two frequency combs independently generate broadband UV-VIS spectra in thin-film lithium niobate nanophotonic waveguides for dual-comb spectroscopy. (b) Schematic of the thin-film lithium niobate waveguide with chirped poling. (c) Generated UV-visible-NIR spectrum and the isolated region utilized for dual-comb measurements of iodine (yellow region). Summary of abbreviations in panel (a): Osc: Er:fiber oscillator; Amp: Er:fiber amplifier; HWP: half-wave plate; L: lens; TFLN: thin-film lithium niobate; MO: microscope objective; BS: beamsplitter; OF: optical filters; PD: photodetector.
• Figure 2: Dual-comb interferogram and spectrum from iodine spectroscopy. (a) Time-averaged interferogram. (b) Corresponding dual-comb spectrum with $f_0$ tone present. The black curve shows the entire spectrum, while the colored regions are expanded in (c)-(f). The inset in (d) shows a single absorption feature and the 100 MHz point spacing.
• Figure 3: Processed iodine dual-comb spectrum and comparison to iodine atlas. (a) Measured dual comb spectrum (green) and inferred background obtained through cepstral analysis (blue). Inset shows an expanded region for comparison. The spectral artifact near 507 THz is an $f_0$ tone. (b) Baseline-corrected transmission spectrum. (c) Comparison of baseline-corrected transmission spectrum to iodine atlasSALAMIROSS. The iodine atlas has been inverted, scaled, and vertically shifted to aid comparison. (d) Expanded region of (c), showing agreement in positions of absorption features between our measurement and the iodine atlas.
• Figure 4: Comparison of optical bandwidths of recent dual-comb works in the ultraviolet and visible spectral ranges
• Figure 5: Dual comb spectroscopy of atomic rubidium and sodium. Broadband dual-comb spectrum with coverage of (a) atomic rubidium and (b) atomic sodium. The insets shows a zoom-in of the measured transitions. (c)-(d) Measured rubidium D1 and D2 transitions after cepstral analysis (orange) compared to the model (yellow). Transition labels denote the respective ground-hyperfine state and isotope. Residuals (Measurement-Model) are shown in the bottom panel. For the D1 transitions, $\nu_{CM}$ = 377,107.409 GHz, and for the D2 transitions $\nu_{CM}$ = 384,230.436 GHz. The D1 and D2 transitions were measured at two different temperatures, with details found in the Supplement. (e)-(f) Measured sodium D1 and D2 transitions after cepstral analysis (green) compared to the model (yellow). Residuals (Model-Measurment) are shown in the bottom panel. For D1 $\nu_{CM}$ = 508,333.152 GHz, for D2 $\nu_{CM}$ = 508,848.718 GHz.