Digital Doppler-Cancellation Servo for Ultrastable Optical Frequency Dissemination Over Fiber

Area of Science:

• Optical physics and metrology
• Telecommunications engineering
• Signal processing

Background:

• Advancements in optical frequency standards necessitate stable optical fiber networks.
• Existing methods for transporting optical signals face limitations in precision over distance.

Purpose of the Study:

• To present a novel Doppler-cancellation setup for ultrastable optical signal dissemination.
• To achieve high-fidelity optical signal transport over fiber optic links.

Main Methods:

• Implementation of a digital phase-locked loop (PLL) using software-defined radio (SDR) on a field-programmable gate array (FPGA) board.
• Utilizing a heterodyne Michelson interferometer for optical phase stabilization.
• Employing an undersampling method within the PLL configuration.

Main Results:

• Demonstrated a 20-m fiber link with residual fractional frequency instability as low as 10⁻¹⁸ at 1000 s.
• Achieved optical phase noise of -70 dBc/Hz at 1 Hz for a telecom frequency carrier.
• Validated the effectiveness of the digital PLL Doppler-cancellation technique.

Conclusions:

• The developed digital PLL setup enables ultrastable optical signal dissemination over fiber.
• This method significantly reduces frequency instability and optical phase noise.
• The system is suitable for advancing precise optical timing and frequency transfer applications.