High-precision digital optical phase locking for 10^-12  W order weak light for a spaceborne gravitational wave...

Area of Science:

• Astrophysics and Space Science
• Optical Engineering
• Laser Physics

Background:

• Spaceborne gravitational wave interferometers (SGWIs) require highly stable laser phase locking for sensitive measurements.
• Existing methods face challenges in achieving phase locking with extremely weak light signals, below 100 picowatts (pW) as required by missions like LISA.
• The precise control of laser phase is critical for the signal-to-noise ratio in gravitational wave detection.

Purpose of the Study:

• To develop and validate a Digital Optical Phase-Locking Loop (DOPLL) system capable of phase locking weak light signals.
• To assess the stability, accuracy, and noise performance of the developed DOPLL for SGWI applications.
• To demonstrate the feasibility of using DOPLL for laser phase control in future spaceborne gravitational wave missions.

Main Methods:

• Implementation of a Digital Optical Phase-Locking Loop (DOPLL) system.
• Experimental phase locking of a slave laser to a master laser using picowatt-level optical power.
• Stability analysis using Bode diagrams and measurement of out-loop phase noise floor.
• Characterization of Allan standard deviation for the heterodyne signal.

Main Results:

• Successful phase locking of a slave laser to a master laser with optical power as low as several picowatts, significantly below the LISA requirement.
• The DOPLL system exhibits no steady-state errors, and stability is confirmed by Bode diagram analysis.
• The out-loop phase noise floor achieved is 2.3×10^-4 and 5.2×10^-4 rad/√Hz, approaching the shot noise limit.
• The Allan standard deviation of the heterodyne signal reached 3.1×10^-17 at 1000 seconds.

Conclusions:

• The developed DOPLL is highly effective for weak light phase locking in SGWIs.
• The system demonstrates superior stability and noise performance, meeting stringent requirements for gravitational wave detection.
• DOPLLs show significant promise for application in SGWI transducers and other sensitive optical systems.