Single-branch Er:fiber frequency comb for precision optical metrology with 10^−18 fractional instability

Abstract

The comparison of optical atomic clocks with frequency instabilities reaching 1 part in 1016 at 1 s will enable more stringent tests of fundamental physics. These comparisons, mediated by optical frequency combs, require optical synthesis and measurement with a performance better than, or comparable to, the best optical clocks. Fiber-based mode-locked lasers have shown great potential for compact, robust, and efficient optical clockwork but typically require multiple amplifier and fiber optic paths that limit the achievable fractional frequency stability near 1 part in 1016 at 1 s. Here we describe an erbium-fiber laser frequency comb that overcomes these conventional challenges by ensuring that all critical fiber paths are common mode and within the servo-controlled feedback loop. Using this architecture, we demonstrate a fractional optical measurement uncertainty below 1×10−19 and fractional frequency instabilities less than 3×10−18 at 1 s and 1×10−19 at 1000 s.