Weak-light phase tracking with a low cycle slip rate.

Area of Science:

• Space physics
• Optical engineering
• Signal processing

Background:

• The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission relies on precise optical signal tracking between spacecraft.
• Beam diffraction in space significantly weakens optical signals, complicating phase-locked loop (PLL) tracking and increasing susceptibility to cycle slips.
• Previous weak-light phase locking was limited to 40 fW with a cycle slip rate of 1 cycle per second.

Purpose of the Study:

• To enhance the capability of tracking weak optical signals in space.
• To improve the cycle slip rate in phase-locked loop systems under low signal power conditions.
• To enable new space-based interferometry missions by improving signal tracking performance.

Main Methods:

• Implemented a phase-locked loop (PLL) to track optical signal phase changes between two spacecraft over long distances.
• Selected a specific PLL bandwidth to minimize signal variance caused by shot noise and laser phase fluctuations.
• Demonstrated tracking of optical signals at power levels significantly below previously reported thresholds.

Main Results:

• Successfully tracked an optical signal at 30 fW, a 25% reduction in power compared to previous records.
• Achieved a cycle slip rate of less than 0.01 cycles per second, representing a 100-fold improvement.
• Demonstrated robust weak-light phase locking crucial for inter-spacecraft optical links.

Conclusions:

• Optimized PLL bandwidth selection is critical for improving weak-light signal tracking performance in space.
• The enhanced tracking capability significantly reduces cycle slip rates, making inter-spacecraft optical communication more reliable.
• This advancement opens possibilities for a new generation of space-based interferometry missions.