Subhertz linewidth laser by locking to a fiber delay line.

Area of Science:

• Optics and Photonics
• Laser Physics
• Metrology

Background:

• High-stability lasers are crucial for precision measurements.
• Fiber lasers offer advantages in compactness and robustness.
• Achieving subhertz linewidths remains a significant challenge.

Purpose of the Study:

• To demonstrate an ultralow-noise, subhertz 1.55 μm erbium-doped fiber laser.
• To achieve high frequency stability using an all-fiber Michelson interferometer.
• To characterize the laser's frequency noise and instability.

Main Methods:

• Locking an erbium-doped fiber laser to an all-fiber Michelson interferometer.
• Utilizing 500 m of SMF-28 optical fiber and an acousto-optic modulator for heterodyne detection.
• Comparing beat-note signals from two identical laser systems.

Main Results:

• Achieved a subhertz (0.67 ± 0.21 Hz) linewidth beat-note signal.
• Obtained a fractional frequency instability of 7×10⁻¹⁵ at short timescales (0.1–1 s).
• Frequency noise power spectral density below -1 dB·Hz²/Hz at 1 Hz, reaching -18 dB·Hz²/Hz from 200 Hz to 1 kHz.

Conclusions:

• The presented fiber laser system offers ultralow frequency noise.
• This laser demonstrates high potential for precision spectroscopy and metrology.
• The all-fiber approach provides a robust platform for stable laser generation.