Research Report on Hybrid Integrated Ultra-Narrow Linewidth Self-Injection Locking 780nm Laser

Background

In modern technology, narrow linewidth lasers play an essential role in various applications, including classical and quantum sensing, ion trap systems, positioning/navigation/timing systems, optical clocks, and microwave frequency synthesizers. In the visible and near-visible spectral ranges, low-noise lasers are particularly important, especially for laser trapping and cooling techniques used in quantum computing, sensing, and atomic clocks. This study demonstrates a hybrid integrated narrow linewidth laser operating at 780 nm with a self-differential linewidth of 105 Hz. The research not only demonstrates the technical feasibility of Hz-level narrow linewidth lasers but also lays the foundation for future exploration.

Paper Provenance

The main authors of this paper are Artem Prokoshin, Michael Gehl, Scott Madaras, Weng W. Chow, and Yating Wan, from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and Sandia National Laboratories in the United States. The paper was published in Volume 11, Issue 7 of Optica magazine in July 2024.

Original Research Workflow

Research Steps and Process

In this study, the authors used a self-injection locking (SIL) technique to achieve a self-differential linewidth of 105 Hz by hybrid integration of a distributed feedback (DFB) laser with a silicon nitride microring resonator (SiN MRR) with a high-quality factor (Q≈5×10^6). The research followed these steps and methods:

1. Device Integration:

   • The DFB laser was grown on a GaAs substrate, with the active region consisting of Al0.09Ga0.91As/Al0.30Ga0.70As quantum wells, and was butt-coupled to a high-Q SiN microring resonator.
   • The microring resonator was manufactured using a CMOS-compatible process with a propagation loss of 1.5 dB/m at a wavelength of 780nm.

2. Phase Adjustment:

   • An integrated thermo-optic phase shifter in the microring resonator allowed for tuning the resonant wavelength.
   • The laser was butt-coupled to the SiN photonic chip, and by setting the phase of the feedback signal, stable SIL operation was achieved.

3. Linewidth Measurement:

   • The linewidth of the laser was measured using a delayed self-heterodyne interferometric method, with a delay time shorter than the coherence time of the laser.

4. Numerical Analysis:

   • The gain spectrum and carrier-induced refractive index changes were calculated using many-body theory and traveling-wave laser dynamics models.
   • Using these parameters, a laser dynamics model was applied to predict the linewidth.

Research Subjects and Experimental Methods

The research subjects included Al0.09Ga0.91As/Al0.30Ga0.70As quantum well DFB lasers and high-Q SiN microring resonators. These components were butt-coupled, and fine adjustments were made through various phase shifters to ensure the SIL effect. Experimental methods covered the following aspects:

• Gain and Linewidth Enhancement Factor Calculations: Using many-body laser theory to calculate the gain spectrum and carrier-induced refractive index changes for Al0.09Ga0.91As/Al0.30Ga0.70As quantum wells.
• Laser Dynamics Model: Describing the electric field within the laser cavity using the traveling-wave approach, analyzing the dynamics changes after the laser is locked to the microring resonator, predicting the frequency noise spectrum, and linewidth.

Results

Main Findings and Supporting Data

With an injection current of 120mA, the SIL laser achieved a linewidth of 105 Hz, which is a significant improvement over the free-running laser with a 1.2 MHz linewidth. Numerical simulations, calculating the gain and linewidth enhancement factor in relation to wavelength, indicate that when the carrier density is above 3×10^12 cm^-2, αh ≈ 1.2. These parameters were used as input to the traveling-wave laser model to calculate the frequency noise spectrum and estimate the linewidth.

Further Research and Potential

Calculations show that narrower linewidths can be achieved with microring resonators with higher Q values. For example, when Q=50×10^6, the predicted background of the frequency noise spectrum is 1 Hz^2/Hz, corresponding to a linewidth of 3 Hz. Furthermore, coupling efficiency can be further improved through direct chip connection with quantum dot lasers.

Conclusion and Research Value

Scientific and Application Value

This study has successfully achieved a 780 nm wavelength laser with a 105 Hz linewidth through experimentation and has developed a comprehensive theoretical model to simulate the behavior of SIL lasers. This not only replicates experimental results but also provides guidance for the design of new SIL lasers, especially at different wavelengths and material platforms. By increasing the Q value of the resonator, linewidths below 10 Hz are predicted to be achievable, although this poses new challenges for device fabrication.

Research Highlights

This study used microring resonators manufactured with atomic-scale precision and commercially available laser components, demonstrating the feasibility of producing narrow linewidth lasers based on SIL technology on a larger scale. In particular, by utilizing many-body theory and traveling-wave laser models to study the impact of MRR parameters on laser performance, the paper provides theoretical support for further design improvements.

Conclusion

This study has successfully demonstrated a hybrid integrated ultra-narrow linewidth 780nm laser, providing a technical roadmap for the development of low-noise lasers in the near-visible spectrum. The results indicate that by further optimizing the resonator’s geometrical structure and increasing Q values, significant progress can be made in improving laser performance and reducing noise. In the future, as relevant technologies continue to develop, applications of Hz-level ultra-narrow linewidth lasers in quantum computing, atomic clocks, and