Researchers at the California Institute of Technology, USA, have engineered a fully integrated photonic device that can produce coherent light at frequencies ranging from the visible range to the mid-infrared (Nat. Photon., doi: 10.1038/s41566-025-01753-7). The on-chip light source produces a continuous frequency comb, a series of discrete and evenly spaced spectral lines, with an efficiency orders of magnitude higher than existing schemes.

Potential spectral broadening

Frequency combs have become a vital tool for applications such as precision metrology, ultrafast physics and spectroscopy. However, generating these broadband spectra requires high-power laser pulses and bulky optical systems. Researchers have been working to create smaller and more efficient solutions, but these chip-scale devices cannot rival the spectral range of their tabletop counterparts.

The Caltech design is based on an optical parametric oscillator (OPO), a resonator that exploits a nonlinear material to convert laser light into two beams with widely separated frequencies. Recent studies have indicated the potential of chip-scale OPOs for spectral broadening, but dispersion and the need for high input energies has limited their use as broadband light sources.

Reducing dispersion

In this new work, the team specifically engineered the OPO to reduce the effects of dispersion. The resonator is designed to only couple light with frequencies around half that of the input laser, with higher harmonics remaining out of resonance. This strategy makes it possible to achieve near-zero dispersion for the resonant frequencies, yielding a large gain bandwidth and a record-low OPO operating threshold of just 18 femtojoules.

Researchers have been working to create smaller and more efficient solutions, but these chip-scale devices cannot rival the spectral range of their tabletop counterparts.

When pumped with a femtosecond laser, the researchers identified three distinct operating regimes. At low input energies, the device produces the classic OPO response, with coherent light generated over two narrow wavelength ranges. At higher pump powers, the output becomes unstable and incoherent, in common with other OPOs. But when the input energy is further increased to around 121 femtojoules, the device produces a continuous frequency comb extending from wavelengths of around 440 nm to more than 2670 nm.

A new regime

Further tests confirm the re-emergence of a coherent regime at these higher pump powers, running counter to expectations from current theoretical models. “We were surprised to find that the super-broad spectrum was actually coherent,” says team leader Alireza Marandi. “It took us maybe six months to discover that there is this new regime of OPO operation.”

The researchers used numerical simulations to understand this unexpected behavior, finding that coherent operation is re-established when the nonlinear phase shift is balanced by the detuning of the optical cavity. Their simulations also revealed a small adjustment to the design that could enable more efficient spectral broadening over a wider frequency range.