Hertz-linewidth semiconductor lasers using CMOS-ready ultra-high-$Q$ microresonators (2009.07390v1)

Abstract: Driven by narrow-linewidth bench-top lasers, coherent optical systems spanning optical communications, metrology and sensing provide unrivalled performance. To transfer these capabilities from the laboratory to the real world, a key missing ingredient is a mass-produced integrated laser with superior coherence. Here, we bridge conventional semiconductor lasers and coherent optical systems using CMOS-foundry-fabricated microresonators with record high $Q$ factor over 260 million and finesse over 42,000. Five orders-of-magnitude noise reduction in the pump laser is demonstrated, and for the first time, fundamental noise below 1 Hz$^2$ Hz$^{-1}$ is achieved in an electrically-pumped integrated laser. Moreover, the same configuration is shown to relieve dispersion requirements for microcomb generation that have handicapped certain nonlinear platforms. The simultaneous realization of record-high $Q$ factor, highly coherent lasers and frequency combs using foundry-based technologies paves the way for volume manufacturing of a wide range of coherent optical systems.

• The paper achieves hertz-level linewidth reduction by coupling semiconductor lasers with CMOS-compatible ultra-high-Q microresonators through self-injection locking.
• It demonstrates a noise reduction of five orders of magnitude with frequency noise below 1 Hz²/Hz, enabling robust performance for precision applications.
• The work paves the way for scalable, chip-based integration of high-coherence laser systems, significantly impacting telecommunications and sensing technologies.

Overview of "Hertz-linewidth semiconductor lasers using CMOS-ready ultra-high-Q microresonators"

The paper under discussion advances the field of coherent optical systems by addressing the need for semiconductor lasers with extremely narrow linewidths suitable for mass production. This work employs CMOS-compatible ultra-high-Q silicon nitride (Si$_3$N$_4$) microresonators to achieve significant linewidth narrowing, essential for applications such as atomic clocks, laser gyroscopes, LIDAR, and coherent communications. The presented laser system demonstrates a fundamental noise reduction of five orders of magnitude and realizes a frequency noise below 1~Hz$^2$~Hz$^{-1}$. This achievement enables further integration of high coherence laser systems with existing semiconductor technology.

High-Q Microresonators

The crux of the paper is the integration of ultra-high-Q microresonators, fabricated using a CMOS-compatible foundry process. These microresonators achieve a Q factor exceeding 260 million and finesse over 42,000, marking them among the highest for integrated platforms. The Si$_3$N$_4$ waveguides are pivotal in maintaining low loss propagation, attributed to precision manufacturing advancements such as increased core thickness to 100 nm, aiding compact integration with sub-millimeter bending radii.

Semiconductor Laser Integration

Semiconductor lasers typically exhibit linewidths on the order of 100 kHz to several MHz, inadequate for applications requiring high coherence. This work bridges this gap by coupling semiconductor distributed feedback (DFB) lasers with ultra-high-Q microresonators, utilizing self-injection locking. Intrinsic frequency noise reduction is achieved without relying on conventional intricate laser designs, making it feasible to transition high-performance lasers from benchtop to scalable chip-based implementations.

Experimental Results

Experimentation revealed unprecedented noise suppression across significant frequency offsets. The paper showcases successful integration with noise floors below 1~Hz$^2$~Hz$^{-1}$ by implementing these resonators with different free spectral ranges (FSR), demonstrating superior performance due to reduced thermorefractive noise. Furthermore, the paper unlocks Kerr frequency combs in normally dispersive regimes without avoided mode crossings or in previously challenging systems, thus expanding the applications and capabilities of integrated photonics.

Implications and Future Directions

The implications of this research are profound, offering a transformative approach to achieving high coherence in mass-produced laser systems utilizing CMOS technology. The demonstrated high-Q, high-finesse resonators offer potential for significant impact in telecommunications, sensing, and beyond. Moreover, the advancements make it possible to utilize high coherence sources in compact and portable formats at lower costs.

Future work might focus on enhancing integration densities and further reducing propagation losses in these Si$_3$N$_4$ platforms. Additionally, exploring heterogeneous integration of active components onto a single chip could lead to fully integrated photonic circuits possessing both laser and microresonator functionalities, thereby revolutionizing the fabrication of highly coherent laser systems. These advancements set the stage for innovative applications in coherent optical systems, leveraging the mature ecosystem of semiconductor manufacturing to widespread practical deployment.