[Paper Review] Recent Advances in Laser Self-Injection Locking to High-$Q$ Microresonators

Nikita M. Kondratiev, Valery E. Lobanov+13 more|arXiv (Cornell University)|Dec 12, 2022

Advanced Fiber Laser Technologies4 citations

TL;DR

This paper reviews recent advances in laser self-injection locking to high-Q microresonators, demonstrating sub-hertz linewidth in semiconductor lasers via frequency stabilization using ultrahigh-Q Si3N4 microresonators. The technique enables chip-scale, low-noise lasers with potential for integrated frequency comb generation, offering a path to compact, low-cost, high-performance laser systems.

ABSTRACT

The stabilization and manipulation of laser frequency by means of an external cavity are nearly ubiquitously used in fundamental research and laser applications. While most of the laser light transmits through the cavity, in the presence of some back-scattered light from the cavity to the laser, the self-injection locking effect can take place, which locks the laser emission frequency to the cavity mode of similar frequency. The self-injection locking leads to dramatic reduction of laser linewidth and noise. Using this approach, a common semiconductor laser locked to an ultrahigh-$Q$ microresonator can obtain sub-hertz linewidth, on par with state-of-the-art fiber lasers. Therefore it paves the way to manufacture high-performance semiconductor lasers with reduced footprint and cost. Moreover, with high laser power, the optical nonlinearity of the microresonator drastically changes the laser dynamics, offering routes for simultaneous pulse and frequency comb generation in the same microresonator. Particularly, integrated photonics technology, enabling components fabricated via semiconductor CMOS process, has brought increasing and extending interest to laser manufacturing using this method. In this article, we present a comprehensive tutorial on analytical and numerical methods of laser self-injection locking, as well a review of most recent theoretical and experimental achievements.

Motivation & Objective

• To provide a comprehensive tutorial on analytical and numerical methods for laser self-injection locking.
• To review recent theoretical and experimental progress in stabilizing semiconductor lasers using high-Q microresonators.
• To explore the potential of self-injection locking for enabling ultranarrow-linewidth, chip-scale lasers with applications in metrology and integrated photonics.
• To identify open challenges in long-term frequency stability and nonlinear dynamics in microresonator-based self-injection locking systems.

Proposed method

• Utilizes self-injection locking by feeding back a small fraction of laser light from a high-Q microresonator to the laser diode's gain medium, locking the laser frequency to a cavity mode.
• Employs theoretical models based on coupled-mode theory and the Lang-Kobayashi equations to describe the dynamics of self-injection locking in semiconductor lasers.
• Applies numerical simulations to analyze stability, linewidth reduction, and nonlinear dynamics under varying feedback strength and detuning.
• Reviews experimental implementations using Si3N4 microresonators with Q-factors exceeding 10^7, achieving sub-hertz linewidths.
• Investigates the role of optical nonlinearity (Kerr, Raman, Brillouin) in enabling simultaneous frequency comb and pulse generation.
• Explores thermal stabilization techniques and integrated actuation (e.g., PZT) to mitigate frequency drift in packaged chip modules.

Experimental results

Research questions

• RQ1 How do different optical nonlinearities (e.g., Raman, Brillouin) in alternative materials to Si3N4 affect the dynamics of self-injection locking?
• RQ2 Can coupled microresonator systems enable enhanced stability and efficiency in self-injection-locked lasers compared to single resonators?
• RQ3 What are the dominant sources of long-term frequency drift in self-injection-locked lasers, and how can they be mitigated without increasing size or power consumption?
• RQ4 To what extent can self-injection locking enable 'turnkey' operation in integrated photonic platforms for practical applications?
• RQ5 How does the interplay between feedback strength, detuning, and nonlinearity influence the transition from continuous-wave to soliton frequency comb generation?

Key findings

• Self-injection locking to high-Q microresonators reduces laser linewidth to sub-hertz levels, matching the performance of state-of-the-art fiber lasers.
• Ultrahigh-Q Si3N4 microresonators (Q > 10^7) enable effective frequency stabilization with feedback powers as low as 10^-4 of the total output.
• Optical nonlinearity in the microresonator enables simultaneous generation of frequency combs and pulses, particularly in the soliton regime.
• Thermal drift of ~50 MHz per 0.01 °C in Si3N4 limits long-term stability, highlighting the need for advanced thermal management in chip-scale packages.
• Integrated PZT actuators and geometry tuning can improve feedback tuning efficiency and stability, reducing sensitivity to environmental perturbations.
• Coupled microresonator systems show potential to enhance CW-to-soliton conversion efficiency, suggesting new pathways for stable, high-efficiency frequency comb operation.