Unified laser stabilization and isolation on a silicon chip

Abstract: Rapid progress in photonics has led to an explosion of integrated devices that promise to deliver the same performance as table-top technology at the nanoscale; heralding the next generation of optical communications, sensing and metrology, and quantum technologies. However, the challenge of co-integrating the multiple components of high-performance laser systems has left application of these nanoscale devices thwarted by bulky laser sources that are orders of magnitude larger than the devices themselves. Here we show that the two main ingredients for high-performance lasers -- noise reduction and isolation -- currently requiring serial combination of incompatible technologies, can be sourced simultaneously from a single, passive, CMOS-compatible nanophotonic device. To do this, we take advantage of both the long photon lifetime and the nonreciprocal Kerr nonlinearity of a high quality factor silicon nitride ring resonator to self-injection lock a semiconductor laser chip while also providing isolation. Additionally, we identify a previously unappreciated power regime limitation of current on-chip laser architectures which our system overcomes. Using our device, which we term a unified laser stabilizer, we demonstrate an on-chip integrated laser system with built-in isolation and noise reduction that operates with turnkey reliability. This approach departs from efforts to directly miniaturize and integrate traditional laser system components and serves to bridge the gap to fully integrated optical technologies.

• The paper introduces a unified laser stabilizer using high-Q silicon nitride ring resonators to achieve self-injection locking and significant noise reduction.
• It demonstrates 14 dB optical isolation and a 25-35 dB reduction in frequency noise in a compact, CMOS-compatible design.
• The work paves the way for scalable integrated photonic systems with applications in quantum computing, optical communications, and metrology.

Unified Laser Stabilization and Isolation on a Silicon Chip

Introduction

The integration of photonic devices has advanced significantly, yet the complexity of fully integrating high-performance laser systems remains a major challenge. This paper, titled "Unified Laser Stabilization and Isolation on a Silicon Chip" (2404.03093), proposes a novel solution to integrate key laser components on a silicon chip to achieve simultaneous noise reduction and isolation. This work leverages CMOS-compatible nanophotonic devices, specifically a silicon nitride ring resonator, to unlock new capabilities in photonics while maintaining the benefits of miniaturization.

Device Design and Theory

The proposed system introduces a unified laser stabilizer (ULS), which capitalizes on the long photon lifetime and the nonreciprocal Kerr nonlinearity of high-Q silicon nitride ring resonators. The ring resonator achieves self-injection locking of a semiconductor laser chip while providing optical isolation. The high-Q resonator serves as a feedback mechanism to stabilize the laser frequency, reducing noise through self-injection locking, and concurrently serves as an isolator using the nonreciprocal Kerr effect to mitigate back-reflections.

High-Q Stability and Noise Reduction

Traditionally, narrow-linewidth lasers require external feedback and isolation with bulky components that are incompatible with chip-scale integration. In this paper, the ULS design challenges the conventional approach by integrating feedback stabilization and isolation within the same resonator structure. Utilizing the nonreciprocal nature of the Kerr effect, the ULS avoids the feedback-induced destabilization typical in semiconductor lasers, maintaining a stable and narrow laser linewidth. It achieves a noise reduction factor (NRF) independent of the laser input power, a significant improvement over traditional designs.

Device Integration and Performance

The experimental results demonstrate that the ULS device achieves 14 dB of isolation and reduces the frequency noise of a distributed feedback (DFB) laser by 25-35 dB. The system operates at approximately 33 mW of on-chip power with an isolation ratio proving robust across different power levels. This capability is attributed to the deterministic nature of the feedback, which allows for passive, stable operation without the need for active thermal tuning.

Implications and Future Work

The integration of the ULS represents a significant step towards full-scale photonic integration, offering a pragmatic path forward in the miniaturization of laser systems. The demonstrated on-chip system opens avenues for more compact and efficient photonic devices, with potential applications in diverse fields, including quantum computing, optical communication, and metrology.

Further research could explore enhancing the performance of the ULS by increasing the device quality factor through commercial-scale fabrication. Such improvements are likely to reduce the power threshold for operation and further enhance the isolation and noise reduction capabilities of integrated photonic devices.

Conclusion

This study presents a pioneering approach to laser stabilization and isolation using a unified silicon chip, utilizing high-Q ring resonators to concurrently achieve linewidth stabilization and optical isolation. This innovation addresses critical limitations in current on-chip laser systems, and its adoption could lead to more reliable and scalable integrated photonic technologies. The successful implementation of the ULS paves the way for further advancements in integrated photonics, promising increased performance and reduced system complexity.