  We demonstrate the use of a flexible digital servo system for the optical
stabilization of both the repetition rate and carrier-envelope offset frequency
of a laser frequency comb. The servo system is based entirely on a low-cost
field programmable gate array, simple electronic components, and existing
open-source software. Utilizing both slow and fast feedback actuators of a
commercial mode-locked laser frequency comb, we maintain cycle-slip free
locking of optically-derived beatnotes over a 30 hour period, and measure
residual phase noise at or below ~0.1 rad, corresponding to <100 attosecond
timing jitter on the optical phase locks. This stability is sufficient for
high-precision frequency comb applications, and indicates comparable
performance to existing frequency control systems. The modularity of this
system allows for it to be easily adapted to suit the servo actuators of a wide
variety of laser frequency combs and continuous-wave lasers, reducing cost and
complexity barriers, and enabling digital phase control in a wide range of
settings.
