Active Cancellation of Servo-Induced Noise on Stabilized Lasers via Feedforward

Abstract

Many precision laser applications require active frequency stabilization. However, such stabilization loops operate by pushing noise to frequencies outside their bandwidth, leading to large ``servo bumps'' that can have deleterious effects for certain applications. The prevailing approach to filtering this noise is to pass the laser through a high-finesse optical cavity, which places constraints on the system design. Here, we propose and demonstrate a different approach where a frequency error signal is derived from a beat note between the laser and the light that passes through the reference cavity. The phase noise derived from this beat note is fed forward to an electro-optic modulator after the laser, carefully accounting for relative delay, for real-time frequency correction. With a hertz-line-width laser, we show $\ensuremath{\gtrsim}20\phantom{\rule{0.2em}{0ex}}\mathrm{dB}$ noise suppression at the peak of the servo bump (approximately $250\phantom{\rule{0.2em}{0ex}}\mathrm{kHz}$) and a noise-suppression bandwidth of approximately $5\phantom{\rule{0.2em}{0ex}}\mathrm{MHz}$---well beyond the servo bump. By simulating the Rabi dynamics of a two-level atom with our measured data, we demonstrate substantial improvements to the pulse fidelity over a wide range of Rabi frequencies. Our approach offers a simple and versatile method for obtaining a clean spectrum of a narrow-line-width laser, as required in many emerging applications of cold atoms, and is readily compatible with commercial systems that may even include wavelength conversion.