Field-programmable-gate-array-based digital frequency stabilization of low-phase-noise diode lasers.

Area of Science:

• Atomic, Molecular, and Optical Physics
• Laser Spectroscopy
• Control Systems Engineering

Background:

• Laser frequency stabilization is crucial for high-resolution spectroscopy and precision measurements.
• Traditional analog servo modules offer robust performance but lack flexibility.
• Field-Programmable Gate-Array (FPGA) technology presents an opportunity for advanced, adaptable control systems.

Purpose of the Study:

• To compare the performance of an FPGA-based digital servo module against a conventional analog servo module for laser frequency stabilization.
• To evaluate the effectiveness of proportional-integral-derivative (PID) control implemented on both digital and analog platforms.
• To assess the suitability of FPGA modules for low-phase-noise laser applications.

Main Methods:

• Characterized transfer functions of both digital (FPGA) and analog PID servo modules.
• Measured single-sideband power spectral density of phase noise for stabilized lasers using optical beat detection.
• Employed two low-phase-noise diode lasers at 1120 nm and 665 nm wavelengths.
• Utilized a high-finesse optical cavity for laser frequency locking.

Main Results:

• The FPGA-based digital servo module demonstrated performance comparable to the analog counterpart.
• Achieved integrated phase noise levels as low as 30 mrad and relative noise power of 10⁻³.
• Determined laser linewidths in the sub-kHz regime, limited primarily by the optical cavity.
• Validated the versatility of the STEMlab FPGA module with open-source modifications.

Conclusions:

• FPGA-based digital servo modules are viable alternatives to analog systems for laser frequency stabilization.
• These digital modules offer excellent performance for low-phase-noise applications.
• The flexibility of FPGA modules makes them suitable for remote-controlled experiments in atomic, molecular, and optical physics and laser spectroscopy.