  Ultra-low frequency noise lasers have been widely used in laser-based
experiments. Most narrow-linewidth lasers are implemented by actively
suppressing their frequency noise through a frequency noise servo loop (FNSL).
The loop bandwidths (LBW) of FNSLs are currently below megahertz, which is
gradually tricky to meet application requirements, especially for wideband
quantum sensing experiments. This article has experimentally implemented an
FNSL with loop-delay-limited 3.5 MHz LBW, which is an order higher than the
usual FNSLs. Using this FNSL, we achieved 70 dB laser frequency noise
suppression over 100 kHz Fourier frequency range. This technology has broad
applications in vast fields where wideband laser frequency noise suppression is
inevitable.

Optics;Instrumentation and Detectors;Quantum Physics