Characterization of Frequency-Doubled 1.5- m Lasers for High-Performance Rb Clocks.

Area of Science:

• Atomic Physics and Spectroscopy
• Laser Technology
• Metrology and Timekeeping

Background:

• Rubidium (Rb) vapor-cell atomic clocks are crucial for precise timekeeping.
• Current atomic clocks often utilize lasers emitting directly at 780 nm.
• Frequency-doubled diode lasers offer a potential alternative for improved clock performance.

Purpose of the Study:

• To characterize two fiber-coupled 1.5-µm diode lasers frequency-doubled to 780 nm.
• To evaluate their spectral properties and instabilities against a state-of-the-art 780-nm laser.
• To assess their suitability for Rubidium atomic clock applications.

Main Methods:

• Frequency stabilization of three lasers (two frequency-doubled, one reference) to Rubidium atomic resonances using Doppler-free spectroscopy.
• Measurement of long-term optical power fluctuations at 780 nm.
• Simultaneous measurement of frequency instability using beat notes between the lasers.

Main Results:

• One frequency-doubled laser system demonstrated excellent spectral properties with relative intensity noise one order of magnitude lower than the reference.
• Frequency noise was limited by the laser current source (<10^6 Hz^2/Hz).
• Optical frequency instability was limited by the reference laser, outperforming it on timescales up to one day.

Conclusions:

• Frequency-doubled diode lasers show promise as an alternative for Rubidium atomic clocks due to superior spectral properties and frequency stability.
• Optical power instabilities, primarily from the frequency-doubling stage, are identified as a key limitation for achieving high-performance atomic clocks.
• Further optimization of the frequency-doubling process is needed to fully realize the potential of these laser systems.