A hyperfine-transition-referenced vector spectrum analyzer for visible-light integrated photonics

Article https://doi.org/10.1038/s41467-025-61970-0 A hyperfine-transition-referenced vector spectrum analyzer for visible-light integrated photonics Received: 24 August 2024 1234567890():,; 1234567890():,; Accepted: 7 July 2025 Baoqi Shi 1,2,10, Ming-Yang Zheng3,4,10, Yue Hu2,5, Yunkai Zhao2,5, Zhenyuan Shang2,5, Zeying Zhong2,5, Zhen Chen2,4, Yi-Han Luo2, Jinbao Long2, Wei Sun2, Wenbo Ma3, Xiu-Ping Xie 3,4, Lan Gao2, Chen Shen2,6, Anting Wang 1, Wei Liang7, Qiang Zhang 3,4,8,9 & Junqiu Liu 2,4 Check for updates Integrated photonics has been successfully established in the near-infrared (NIR) telecommunication bands. With the soaring demand in biosensing, quantum information and transportable atomic clocks, extensive endeavors have been stacked on translating integrated photonics into the visible spectrum. Demonstrations of visible-light lasers, frequency combs, and atom traps highlight the prospect of creating chip-based optical atomic clocks that can make timing and metrology ubiquitous. A pillar to the development of visiblelight integrated photonics is characterization techniques featuring high frequency resolution and wide spectral coverage, which however remain elusive. Here, we demonstrate a vector spectrum analyzer (VSA) for visible-light integrated photonics, offering spectral bandwidth of 766–795 and 518–541 nm. The VSA is rooted in widely chirping, high-power, narrow-linewidth, mode-hop-free lasers that are frequency-doubled from the near-infrared via efficient, broadband CPLN waveguides. The VSA is further referenced to hyperfine structures of alkaline atoms and iodine molecules, enabling megahertz frequency accuracy. We apply our VSA to showcase the characterization of loss, dispersion and phase response of passive integrated devices, as well as densely spaced spectra of mode-locked lasers. Leveraging individual operations at 518–541, 766–795, 1020–1098, and 1260–1640 nm bands, our VSA achieves an aggregate characterization bandwidth exceeding one octave. This capability establishes the VSA as an invaluable diagnostic tool for spectroscopy, nonlinear optical processing, imaging, and quantum interfaces with atomic systems. Integrated photonics1, which utilizes established semiconductor manufacturing technology for construction and mass production of chip-scale optical systems, has made explosive growth in the last decades. Enabling dense integration of lasers, modulators and detectors on monolithic substrates for optical information processing, integrated photonics has revolutionized telecommunications, sensing and computing. As such, it has been extensively developed and A full list of affiliations appears at the end of the paper. Nature Communications | (2025)16:7025 optimized in the near-infrared (NIR) wavelength, e.g., around 1550 nm where today’s telecommunications and datacenters operate2. Currently, there is surging interest in translating integrated photonics into the visible spectrum3–6. Fig. 1a presents diverse applications operated in the visible spectrum, such as biosensing7,8, augmented and virtual reality (AR/VR)9,10, AMO (atomic, molecular and optical) physics11,12, LiDAR (light detection and ranging)13,14, OCT (optical e-mail: 1 Article https://doi.org/10.1038/s41467-025-61970-0 a Frequency (THz) 550 500 450 400 350 300 Biosensing 250 200 LiDAR AR/VR OCT Quantum information Optical communication AMO physics Na I2 500 Li Ca+ 600 Sr Rb Cs K 700 800 900 Computation 1000 1100 1200 1300 1400 1500 1600 1700 Wavelength (nm) NIR chirping laser Trans. f Tunable laser Hyperfine reference CPLN EDFA Rb+K+I2 SH chirping laser SHG Frequency reference 2f Device under test DUT 2fNIR f Disp. fNIR Frequency ruler Fiber cavity Trans. b fNIR 2f 2f Fig. 1 | Principle and applications of vector spectrum analyzers. a Applications of integrated photonics in the NIR and visible spectra. LiDAR, light detection and ranging. AR/VR, augmented and virtual reality. OCT, optical coherence tomography. AMO, atomic, molecular and optical. b Principle and schematic of our vector spectrum analyzer in the visible spectrum. The near-infrared (NIR) chirping laser is a telecommunication-C-band, widely tunable, continuous-wave laser. After power amplification by an erbium-doped fiber amplifier (EDFA), a chirped periodically poled lithium niobate (CPLN) waveguide is used to frequency-double the NIR laser and generate the second-harmonic (SH) chirping laser. The relative frequency of both chirping lasers is calibrated by an FSR-calibrated fiber cavity that serves as a time-domain “frequency ruler”. The absolute frequencies of both chirping lasers are referenced to either Rb and K hyperfine structures in the 766–795 nm band, or to iodine hyperfine structures in the 518–541 nm band. The frequency-calibrated SH chirping laser is used to perform vector spectrum analysis of a device under test (DUT) in the visible spectrum. coherence tomography)15,16, and quantum information17,18. A notable application is chip-based atomic and optical clocks19–22. Despite that transportable clocks with precision reaching 10−18 have been demonstrated23,24, chip-based atomic and optical clocks bear further reduced size, weight and power consumption, and can allow frequency metrology ubiquitously deployed on mobile platforms and in space. Endeavors have created integrated components dedicated to clock systems, including low-noise lasers25–35, frequency combs36–40, surfaceelectrode ion traps41,42, and magneto-optic traps43. Recently, integrated low-noise lasers have been deployed to interrogate strontium (Sr) ion clocks44,45. Parallel to the progress in new architecture, fabrication, and application, characterization techniques of integrated photonics in the visible spectrum are equally pivotal, which however are inadequately developed. Wavemeter-calibrated tunable lasers and Mach-Zehnder interferometers25,46–48 have been employed to quantify optical loss or resonance linewidth of integrated microresonators. However, these methods require individual measurement for each resonance, and lack accuracy in determining resonance frequency. Cutback measurement can evaluate optical loss based on long waveguides with varying lengths26,49–51, however it suffers from mediocre precision and is invalid for cavity structures. None of these methods has sufficient measurement bandwidth to map the dispersion profiles, mainly due to the limitations of stable, widely tunable, narrow-linewidth, mode-hop-free lasers in the visible spectrum. Here, we demonstrate a wideband, high-resolution vector spectrum analyzer (VSA) for visible-light integrated photonics. In the visible spectrum, our VSA operates in two bands: the 766–795 nm band Nature Communications | (2025)16:7025 2 Article corresponding to the spectral region for rubidium clocks, and the 518–541 nm band relevant to iodine laser spectroscopy. The principle of our VSA is illustrated in Fig. 1b. While commerci (...truncated)