Absolute frequency readout derived from ULE cavity for next generation geodesy missions

Area of Science:

• Geodesy
• Astrophysics
• Optical Physics

Background:

• Future Gravity Recovery and Climate Experiment (GRACE)-like missions depend on inter-spacecraft laser interferometry.
• Laser frequency stability is critical for measuring local gravity and long-term mass changes (water/ice) from space.

Purpose of the Study:

• To demonstrate a simple phase modulation scheme for measuring long-term laser frequency variations.
• To assess laser frequency stability for next-generation satellite geodesy missions.

Main Methods:

• Comparing an on-board Ultra-Stable Oscillator (USO) frequency reference to the Free Spectral Range (FSR) of an optical cavity.
• Utilizing a phase modulation scheme to directly measure laser frequency change.
• Calculating a scale correction factor for a laser locked to a specific longitudinal mode.

Main Results:

• Achieved fractional absolute laser frequency stability at the 10 ppb (10^-8) level.
• Demonstrated stability over time scales greater than 10,000 seconds.
• The results indicate suitability for future mission requirements.

Conclusions:

• The developed phase modulation scheme effectively measures long-term laser frequency drift.
• The achieved laser frequency stability meets the stringent requirements for next-generation satellite geodesy missions.