Optical frequency combs emit a spectrum of frequencies that are perfectly spaced like the teeth on a comb. Two such frequency combs with slightly different pulse-repetition rates are sometimes combined to create a dual-comb setup emitting a stream of short pulses. Dual-comb lasers emit a train of pulses particularly useful for extremely sensitive and fast spectroscopy measurements and precisely measuring distances via laser ranging. Yet, the need for two stabilized combs plus complex synchronization electronics has restricted such measurements to laboratory equipment.
Now, researchers have developed a free-running approach that uses a single laser cavity to create two high-power optical frequency combs emitting high-power femtosecond pulses that offers equal or improved timing precision without any stabilization electronics. According to Benjamin Willenberg from ETH Zurich in Switzerland, who presented the development at the 2020 OSA Laser Congress, the approach allows generation of a pair of frequency combs with a small and passively stable offset in their repetition rate.
“This resolves the long-standing problem of the high complexity of dual-comb systems without compromising on laser performance,” Willenberg explains, “enabling potential sensing applications in time-domain spectroscopy for nondestructive testing, trace-gas detection for industrial and environmental monitoring, and laser ranging for machine-vision applications.”
By replacing complex electronic stabilization systems with a simpler, passively stable optical approach, the researchers have achieved dual-comb operation using a single laser cavity multiplexed with birefringent calcite crystals that allow for lasing in the two polarization states. By combining this multiplexing technique with a diode-pumped solid-state laser crystal, the researchers have come up with a dual comb with a continuously tunable repetition rate difference of 40 Hz to 3.35 kHz without impacting laser output.
Because the new design creates two frequency comb lasers using a single optical cavity, it could enable the development of more-compact dual combs that offer flexibility in power, wavelength, bandwidth, and pulse repetition rates. With the new setup, the researchers achieved pulses with a 175 fs duration and 440 mW of power in two 1050 nm beams with a repetition rate difference of 1 kHz. Next steps for this technology include developing prototype systems in a robust and portable package, demonstrating scientific and industrial applications, scaling to higher powers and higher repetition rates for faster measurements, and setting up channels to offer the laser commercially. Reference: B. Willenberg et al., Opt. Express, 28, 30275–30288 (2020); https://doi.org/10.1364/oe.403072.
