2020-02-03, Liehl, Andreas, Sulzer, Philipp, Fehrenbacher, David, Eggert, Stefan, Ludwig, Markus, Ritzkowsky, Felix, Seletskiy, Denis V., Leitenstorfer, Alfred

Carrier-envelope and optical phase noise of a femtosecond frequency comb based on Er-doped fiber technology is investigated and minimized without exploiting active external references. Ultrabroadband, coherent, and tailorable supercontinua are generated in a highly nonlinear germanosilicate fiber assembly. Difference frequency mixing between comb modes in their spectral extrema passively eliminates the carrier-envelope phase slip. This step generates an inherently offset-free comb with a relative frequency stability better than 10⁻²¹. In contrast, the phase fluctuations at the carrier frequency of 193 THz are increased as compared to the fundamental comb. Their level matches the value found by parabolic extrapolation of the phase noise of the fundamental comb to zero frequency. The latter is unambiguously accessible by means of any beat note centered at the carrier-envelope offset frequency. All these findings rely on strong correlations between the comb modes that are quantitatively described by an elastic tape model, underlining the deterministic character of the processes involved. The superior optical phase noise of the fundamental comb is transferred to the difference-frequency comb while not compromising the inherent cancellation of the carrier-envelope offset frequency. In this way, the optical linewidth of the passively phase-locked comb is reduced from 100 kHz to a measured value of 5 kHz, which is limited by the cw laser reference used for out-of-loop characterization.

2017, Liehl, Andreas, Fehrenbacher, David, Sulzer, Philipp, Leitenstorfer, Alfred, Seletskiy, Denis V.

Recent demonstrations of passively phase-locked fiber-based combs motivate broadband characterization of the noise associated with the stabilized carrier-envelope offset frequency. In our study, we analyze the phase noise of a 100 MHz Er:fiber system in a wide range spanning from microhertz to the Nyquist frequency. An interferometric detection method enables analysis of the high-frequency output of an f-to-2f interferometer. The dominant contribution of a broadband white noise floor at high frequencies attests quantum-limited performance. An out-of-loop measurement of the carrier-envelope phase reveals its jitter to be as low as 250 mrad when integrated over 12 orders of magnitude of the radio-frequency spectrum.

2015, Seletskiy, Denis V., Fehrenbacher, David, Sulzer, Philipp, Leitenstorfer, Alfred

Modern precision metrology ever more strongly relies on the availability of optical frequency combs. We review our approach to passive stabilization of the carrier envelope phase (CEP) of a pulse train at full repetition rate via difference frequency generation. Using this approach, we demonstrate a inherently offset-free Er:fiber comb directly locked onto an optical reference ( 85 Rb) together with a possibility of an actively narrowed sub-500 mHz linewidth. This performance highlights an attractive potential of robust all-fiber comb systems toward applications in ultrahigh-precision metrology.

2019-05-24, Liehl, Andreas, Sulzer, Philipp, Fehrenbacher, David, Rybka, Tobias, Seletskiy, Denis V., Leitenstorfer, Alfred

Optical phase noise of femtosecond lasers is analyzed over various steps of broadband nonlinear frequency conversion. The intrinsic phase jitter of our system originates from quantum statistics in the mode-locked oscillator. Supercontinuum generation by four-wave-mixing processes preserves a noise minimum at the optical carrier frequency. From there, a quadratic increase of the comb linewidth results with mutually anticorrelated phase fluctuations of both spectral wings. Passive phase locking by difference frequency generation strongly enhances the optical phase noise to a level equaling the carrier-envelope phase jitter of the fundamental comb. The same value results from quadratic extrapolation of the optical phase noise to radio frequencies. Our findings are consistent with a fully deterministic transformation of phase noise according to the elastic tape model.

2017, Liehl, Andreas, Fehrenbacher, David, Sulzer, Philipp, Seletskiy, Denis V., Leitenstorfer, Alfred

Applications of optical frequency combs in high precision metrology [1] require low-noise stabilization of its carrier-envelope offset (CEO) frequency. This task is commonly achieved via active feedback. Fully passive elimination of the CEO frequency based on difference frequency generation (DFG) between two octave-separated comb sections followed by amplification of the DFG signal in the EDFA has been demonstrated recently in an all-fiber design [2]. In this work, we develop a novel broadband characterization method of carrier-envelope phase (CEP) noise and apply it to study the passively phase-locked 100 MHz Er:fiber comb [3].

2015, Fehrenbacher, David, Sulzer, Philipp, Seletskiy, Denis V., Leitenstorfer, Alfred

An offset-free frequency comb generated by difference frequency mixing is established and characterized. mHz-level direct locking of the repetition rate to ⁸⁵Rb and reference-limited linewidth narrowing via an extra-cavity electro-optic modulator are demonstrated.

2019, Liehl, Andreas, Sulzer, Philipp, Fehrenbacher, David, Seletskiy, Denis V., Leitenstorfer, Alfred

Phase noise in mode-locked Er:fiber systems shows strong spectral correlations emerging from broadband nonlinear-optical processes. Our fundamental insights are key to maximize the quality of passively phase-stable frequency combs and single-cycle pulse trains.

2017, Liehl, Andreas, Fehrenbacher, David, Sulzer, Philipp, Seletskiy, Denis V., Leitenstorfer, Alfred

We study the carrier-envelope phase noise of an Er:fiber frequency comb which is passively phase-locked at the full repetition rate of 100 MHz. A novel characterization method determines an out-of-loop phase jitter of only 250 mrad when integrated over 12 orders of magnitude: from 50 μHz up to the Nyquist frequency.

2015, Fehrenbacher, David, Sulzer, Philipp, Liehl, Andreas, Kälberer, Thomas, Riek, Claudius, Seletskiy, Denis V., Leitenstorfer, Alfred

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 carrier–envelope 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.