High-power optical frequency comb achieved through a new way of thinking: Keio University actively exploits crystal birefringence to achieve 28% conversion efficiency

A research group led by Doctoral Student Liu Yang (at the time of the research) in the Graduate School of Science and Technology and Undergraduate Student Keisuke Ogawa (fourth year) and Assistant Professor Shun Fujii from the Department of Physics, Faculty of Science and Technology at Keio University has succeeded in dramatically improving the output power and efficiency of microcombs through an international collaboration with Xi'an Jiaotong University in China. The group achieved this by exploiting a crystal optical property that had previously been considered unfavorable for optical frequency combs (microcombs) generated in optical microresonators. Their findings were published in Physical Review Letters.

An optical frequency comb (optical comb) is a light source that is sometimes called an "optical ruler," in which a large number of laser lines are evenly spaced. It holds great promise for applications across a wide range of fields, including high-speed optical communications, precision measurement, and low-noise microwave generation. In particular, microcombs (micro optical frequency combs) generated using optical microresonators are simple, compact, energy-efficient, high-repetition rate laser sources that offer major practical advantages, and research and development efforts are flourishing worldwide. Among microcombs, soliton combs with high coherence and low noise are well suited for many applications, and efforts toward their deployment in society have already begun.

However, soliton combs have a fundamental problem: their output power and the efficiency with which pump light is converted into comb lines are both low. In particular, in the case of soliton combs operating at high repetition frequencies of a few gigahertz to several hundred gigahertz, most of the continuous-wave (CW) pump laser light passes straight through the resonator without being coupled. As a result, the usable comb output is typically only a few percent or less of the pump power, and the output power generally remains below a few milliwatts. This has made external optical amplifiers necessary for high-performance applications, causing systems to grow larger and noisier.

The research group focused on ultrahigh-Q optical microresonators made from single-crystal magnesium fluoride (MgF₂). They achieved soliton combs with unprecedented high output power and high conversion efficiency by deliberately exploiting the crystal's strong birefringence and the optical properties it produces.

MgF₂ is an optically uniaxial crystal with birefringence, and the properties of light propagating inside the resonator vary greatly depending on the direction in which the crystal is cut relative to its optic axis. Conventionally, z-cut resonators—cut so that the optic axis aligns with the resonator's axis of symmetry—have been mainly used because they allow stable microcomb formation. However, in z-cut resonators, the refractive index for each polarization is constant and the group-velocity dispersion (GVD) is relatively mild, which limits the output power and conversion efficiency of soliton combs.

In contrast, the research group employed x-cut resonators, which are cut along the x-axis direction so that the optic axis is perpendicular to the resonator's axis of symmetry. In x-cut resonators, the birefringence of the crystal causes the refractive index of a specific polarization mode to vary periodically as light travels around the resonator. This effect generates strong interactions between polarization modes and induces large distortions in the resonator's dispersion profile.

Such strong mode interactions have generally been regarded as obstacles to soliton comb formation, but the research group discovered that this phenomenon produces very strong local anomalous dispersion within a specific wavelength band. By making use of this local dispersion region for microcomb generation, they were able to greatly enhance the energy of the soliton comb and enable high-output-power operation.

The experiments demonstrated successful generation of a microcomb in an x-cut MgF₂ resonator operating at a repetition frequency of 15.5 GHz, reaching a maximum average output power of approximately 38 mW and a maximum conversion efficiency of 28%. The resulting optical pulses had a pulse width of several picoseconds and showed stable, low-noise operation. This represents world-leading performance for a CW-pumped microcomb. Theoretical analysis clarified that strong anomalous dispersion dramatically boosts both output power and conversion efficiency, supporting the experimental results.

The high-power soliton microcomb demonstrated in this research enables efficient optical-to-electrical (OE) conversion without the use of optical amplifiers, offering significant advantages for next-generation communication applications such as low-noise microwave generation and reference clock signal distribution. Furthermore, this design approach is applicable to other wavelength bands and to integrated resonator devices, making it an important foundational technology for the development of practical, high-efficiency microcombs.

Journal Information
Publication: Physical Review Letters
Title: High-Power Picosecond Pulsed Kerr Soliton Microcombs
DOI: 10.1103/vdrs-2cvt