1.5  μm Lasers with Sub-10 mHz Linewidth

Area of Science:

• Quantum optics
• Laser physics
• Metrology

Background:

• High-stability lasers are crucial for precision measurements.
• Thermal Brownian noise in optical cavities limits laser frequency stability.
• Understanding and mitigating noise sources is essential for advancing metrology.

Purpose of the Study:

• To report on two ultrastable lasers stabilized to independent silicon Fabry-Pérot cavities.
• To investigate the fundamental limits imposed by thermal Brownian noise on laser frequency instability.
• To develop methods for relating flicker frequency noise to observable linewidths and coherence times.

Main Methods:

• Stabilization of two independent lasers to silicon Fabry-Pérot cavities at 124 K.
• Characterization of fractional frequency instability and identification of noise sources.
• Analysis of phase noise spectra and beat notes between lasers.
• Derivation of methods to quantify noise-induced divergences.

Main Results:

• Achieved fractional frequency instability floor of 4×10⁻¹⁷, limited by thermal Brownian noise.
• Demonstrated laser linewidths as narrow as 5 mHz at 194 THz.
• Derived usable phase coherence times ranging from 11 to 55 s.

Conclusions:

• Thermal Brownian noise in mirror coatings is the dominant factor limiting laser instability.
• The developed methods provide practical links between noise phenomena and observable laser parameters.
• The achieved performance represents a significant advancement in ultrastable laser technology for metrology and fundamental science.