Toward chip-scale integration: microcomb-enabled ultra-broadband spectroscopy

The rapid progress of optics has created a pressing need for highly precise tools to measure the properties of advanced optical components. Applications such as dispersion engineering in integrated photonic devices and metasurface optics demand spectral analysis techniques that combine kilohertz-level resolution with measurement bandwidths spanning terahertz ranges. Conventional approaches rely on high-performance tunable lasers to sweep across wavelengths, but these systems are bulky, costly, and require elaborate frequency calibration. Traditional amplitude spectrometers also fall short, as they cannot access phase information—essential for next-generation devices.

To overcome these challenges, researchers form Nanjing University of Aeronautics and Astronautics have developed a compact optical vector spectrometer powered by a dissipative Kerr soliton microcomb. The system is based on a high-Q silicon nitride micro-resonator, producing a stable, low-noise single-soliton comb with a 103.9 GHz free spectral range and kilohertz-level linewidths. Remarkably, the design requires only a laser with an 8 GHz tuning range, yet achieves a measurement bandwidth of 5.2 THz, effectively covering the entire optical C-band—a 650-fold increase over the laser’s tuning range.

The setup employs an asymmetric dual-sideband microwave photonics architecture. An acousto-optic modulator generates an 80 MHz frequency-shifted reference, while a Mach-Zehnder modulator sweeps the measurement sideband. This design eliminates higher-order sideband interference, boosting dynamic range. A direct-through calibration algorithm and dual-channel balanced detection further enhance accuracy by compensating for photodetector response variations.

Experiments confirmed kilohertz-level resolution and sub-100 Hz frequency precision over 10 minutes. Tests on an H₁₃C₁₄N gas cavity matched HITRAN 2020 data, while measurements on a high-Q micro-resonator resolved 100 MHz resonance linewidths and extracted a precise second-order dispersion coefficient.

This work breaks the trade-off between tuning range and bandwidth in spectroscopy. By leveraging CMOS-compatible microcombs, it points toward chip-scale, fully integrated spectrum analyzers for next-generation photonics.