Free-running performance and full control of a passively phase-stable Er:fiber frequency comb

Abstract

Optical frequency combs based on erbium-doped fiber lasers are attractive tools in precision metrology due to their inherent compactness and stability. Here we study a femtosecond Er:fiber comb that passively eliminates the carrierenvelope phase slip by difference frequency generation. Quantum statistics inside the all-fiber soliton oscillator governs its free-running performance. Active stabilization of the repetition rate supports a subhertz optical linewidth and does not necessitate additional intracavity elements. Direct locking to an optical atomic frequency standard enables generation of a 100 MHz microwave signal with a stability of 3.4 mHz maintained over 15 min.

Deterministic Nonlinear Transformations of Phase Noise in Quantum-Limited Frequency Combs

“…Its broadband flat shape further supports the quantum origin of the intrinsic optical phase noise of the modelocked Er:fiber oscillator [33]. Interestingly, the noise spectrum of the DFG comb at 193 THz remains white but its level is significantly increased [28]. Therefore, nonlinear processes seem to crucially affect phase noise but in a way that it is still governed by its quantum origin.…”

“…Although the minimum DFG linewidth of the 100 MHz offset-free comb is below 100 kHz, we expect the fundamental comb to show significantly less optical phase noise since even non-quantum-limited sources have been reported to exhibit considerably lower phase jitter [37]. Indeed, a broadband study of the noise characteristics of comb lines at 141, 193, and 282 THz presented recently [28] confirms this assumption. The linewidths of the corresponding comb modes are depicted in magenta in Fig.…”

“…The setup of ultrafast laser systems used in our experiments is sketched in Fig. 1(a) [7,28]. A solitonic Er: fiber oscillator emits a train of Fourier-limited ultrashort pulses whose spectra exhibit a full width at half maximum (FWHM) of up to 18 nm.…”

“…Applying this methodology, we now compare two offset-free frequency comb systems with identical general design but operating at different repetition rates of 80 and 100 MHz, respectively. The optical linewidth of both DFG combs (δ DFG ) is determined at a frequency of 193 THz [see Figure 2 shows δ DFG of both sources as a function of optical bandwidth Δν of the fundamental soliton circulating in the oscillators which we control via the optical pump power [28]. For a quantumlimited phase jitter, δ DFG is expected to be proportional to the square of the pulse duration of the Fourier-limited intracavity soliton [34,35]: δ DFG ∝ ðΔνÞ −2 .…”

Frequency comb metrology with an optical parametric oscillator

“…An elegant approach to overcome this bandwidth requirement in fiber combs is based on passive CEO cancellation using a difference frequency generation (DFG) process between two distant spectral parts of the supercontinuum spectrum generated from a fibered femtosecond laser [14] As the resulting comb has only one remaining degree of freedom, full phase stabilization of the CEO-free comb can be achieved by stabilizing the repetition frequency, for instance to an optical frequency standard. This was recently demonstrated for the stabilization of an Yb-fiber comb to an ultra-stable laser at 1 μm [15], and for the stabilization of an Er-fiber comb to a sub-Doppler two-photon Rb transition [16] with a sub-hertz relative linewidth.…”