Homebuilt Laser System for Frequency Domain Measurements of Gravity using Echo Atom Interferometry

We discuss the development of a homebuilt external cavity diode laser system (ECDL) based on Beica et al., RSI, 90, 086113 (2019) for measurements of gravitational acceleration using a frequency-domain echo atom interferometer (AI). Since the experimental setup utilizes a sample of laser cooled rubidium atoms confined in a magneto-optical trap (MOT), it is necessary to reduce atom number fluctuations that depend on both laser frequency and intensity. Additionally, it is necessary to derive slightly detuned optical frequencies for atom interferometry. To achieve these goals, we compare the performance of a commercial ECDL that is frequency stabilized using lock-in spectroscopy with a homebuilt ECDL frequency stabilized using a modulation-free technique, namely, Doppler Free Dichroic Atomic Vapour Laser Lock (DF-DAVLL). We find that the frequency stability of this homebuilt system, characterized by measurements of the Allan deviation (AD), has a floor of 7 × 10^-12 at ~40 s. We investigate the effect of frequency modulation on MOT temperature using both the commercial and homebuilt ECDLs. We have also intensity stabilized this laser system using a feedback loop with a time constant as small as ~500 us based on an acousto-optic modulator and an analog circuit comprised of a comparator, amplifier, integrator, and voltage-controlled radio frequency attenuator. We find that this feedback loop results in a three-fold reduction in intensity fluctuations. To derive detuned counter-propagating travelling wave pulses for atom interferometry, we have developed an inexpensive, dual output RF synthesizer with a tuning range of ~50 MHz and detuning step size of 1 µHz, which reproduces the stability of a master oscillator (10 MHz rubidium clock with a 1 s AD of 2 × 10^-11). This synthesizer has recently been used for realizing a cold atom velocimeter (Carlse et al., Submitted (2024)).