Automatic digital optical heterodyne phase locking loop in the milliradian domain for spaceborne laser interferometry

Area of Science:

• Physics
• Astronomy
• Engineering

Background:

• Spaceborne laser interferometers are essential for detecting gravitational waves by measuring minute displacements over vast distances.
• Picometer-level accuracy is required for pathlength variation measurements, necessitating highly precise optical phase locking.

Purpose of the Study:

• To develop and demonstrate a digital optical phase locking loop (OPLL) with enhanced precision, control bandwidth, and an automatic laser locking strategy.
• To achieve performance suitable for future spaceborne gravitational wave detection missions.

Main Methods:

• Development of a digital OPLL implemented on a field-programmable gate array (FPGA).
• Experimental demonstration of automatic phase locking between two independent free-running Nd:YAG lasers.
• Characterization of residual phase error and control bandwidth.

Main Results:

• Achieved residual phase error below 1 mrad/Hz above 0.01 Hz, the best performance for digital servos to date.
• Demonstrated a high control bandwidth of up to 2.8 MHz.
• Successfully implemented an automatic laser locking strategy.

Conclusions:

• The developed digital OPLL offers high precision and bandwidth, suitable for gravitational wave detection.
• The automatic locking strategy and FPGA implementation pave the way for future spaceborne interferometers.
• This work highlights the distinction between optical phase tracking and optical phase locking for phasemeter design.