A robust, low-cost mode-locked laser oscillator for deployed frequency comb sensor applications
Authors: Timmers Henry , Tooley Dylan , Sodergren Bennett , Robinson Ryan , Vogel Kurt , Knabe Kevin 
Published: September 2020

1. Introduction Frequency combs have been investigated in the laboratory over the course of the last 25 years in a wide range of implementations and applications including but not limited to precision metrology, optical atomic clocks, LIDAR, low-phase-noise RF generation, and precision spectroscopy [1]. While the Nobel prize winning technology of frequency combs have shown their usefulness in a variety of applications, there have been few demonstrations of this technology in real-world applications [2, 3]. Here we present a mode-locked oscillator that has been designed to be environmentally robust and low cost, while maintaining suitability for use in frequency comb applications. Vescent Photonics is designing and manufacturing fiber mode-locked oscillators and frequency combs for government customers whose application deployments include satellites and terrestrial moving platforms. These designs allow for repetition rate matching at the time of manufacture, which is an important consideration for integration of this technology into optical atomic clocks, dual comb spectroscopy, and low-phase-noise radio frequency generation. Vescent currently is manufacturing oscillators at 100 and 200 MHz repetition rates, and these designs could be modified to give repetition rates between 80 and 250 MHz. While performance and environmental robustness are necessary metrics for real-world applications of frequency combs, the cost of these devices can also be a barrier to wide-spread deployment. Vescent Photonics is diligently working on optimizing all three parameters (performance, environmental robustness, and cost) to bring a deployable mode-locked laser and frequency comb solution to market. 2. Mode-locked oscillator performance The compact mode-locked oscillators presented here have been designed to be low-cost, low-volume, highperformance, and environmentally robust for field-deployed applications. Figure 1a shows the mechanical enclosure for the fiber oscillator measuring 0.12 L (10.75 cm x 6.35 cm x 1.72 cm) in volume; it should be noted that pump diodes have not been included in this packaging due to thermal considerations. An enclosure for a frequency comb system (optic only) has also been designed with a volume of 0.62 L (19.05 cm x 11.30 cm x 2.86 cm). Two length actuators are included in this design; a fast PZT with >200 kHz bandwidth that allows 150 Hz of tuning on a 100 MHz repetition rate, and slow temperature control of the fiber with a ~10 s thermal time constant and >40 kHz of tuning on a 100 MHz repetition rate. The oscillator optical spectrum is included in Figure 1b. Average optical powers out of the oscillators are on the order of a few milliwatts, and pulses are close to transform limited (~250 fs). Erbium-doped fiber amplifiers and pulse compression stages are routinely added to the oscillator outputs to achieve average powers >200 mW and pulse durations <100 fs. While these parameters are useful for a wide range of mode-locked laser applications, it also allows the detection of the carrier envelope offset frequency ( , a fundamental comb parameter) in an f-2f interferometer with greater than 40 dB SNR in 300 kHz resolution bandwidth (Figure 1c). To show suitability of these fiber mode-locked oscillators in frequency comb applications, fractional Allan Deviations of the in-loop stabilized signals have been included in Figure 1d where the absolute frequency deviations have been divided by the distance between the frequency comb lock points. Typical optical clock performance has also been plotted, where the performance of the locks is more than suitable for this application. 3. Summary A compact, low-cost, environmentally robust mode-locked oscillator has been shown to be suitable for frequency comb experiments. Detection and stabilization of the fundamental comb parameters is suitable for optical atomic clocks, as well as a host of real-world frequency comb applications including dual comb spectroscopy, low phase noise RF generation, and optical metrology.