Sulzer, Philipp
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Deterministic Nonlinear Transformations of Phase Noise in Quantum-Limited Frequency Combs
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.
Passive Elimination of Spectrally Correlated Intensity Noise in Ultrabroadband Supercontinua from Highly Nonlinear Fibers
Amplitude fluctuations of pump pulses for frequency broadening by third-order processes are found to result in strong anti-correlations of spectral output components. We exploit this information to minimize intensity noise in subsequent nonlinear conversion steps.
Ultrabroadband out-of-loop characterization of the carrier-envelope phase noise of an offset-free Er:fiber frequency comb
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.
Analysis of the carrier-envelope phase noise of passively phase-locked Er:Fiber frequency combs up to the Nyquist frequency
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.
High power frequency comb delivered by a Tm-doped fiber laser
Frequency combs are an enabling technology for metrology and spectroscopic applications in fundamental and life sciences. While frequency combs in the 1 μm regime, produced from Yb-based systems have already exceeded the 100 W – level, high power coverage of the interesting mid-infrared wavelength range remains yet to be demonstrated. Tm- and Ho-doped laser systems have recently shown operation at high average power levels in the 2 μm wavelength regime. However, frequency combs in this wavelength range have not exceeded the 5 W-average power level. In this work, we present a high power frequency comb, delivered by a Tm-doped chirped-pulse amplifier with subsequent nonlinear pulse compression. With an integrated phase noise of <320 mrad, low relative intensity noise of <0.5% and an average power of 60 W at 100 MHz repetition rate (and <30 fs FWHM pulse duration), this system demonstrates high stability and broad spectral coverage at an unrivalled average power level in this wavelength regime. Therefore, this laser will enable metrology and spectroscopy with unprecedented sensitivity and acquisition time. It is our ongoing effort to extend the spectral coverage of this system through the utilization of parametric frequency conversion into the mid-IR, thus ultimately enabling high power fingerprint spectroscopy in the entire molecular fingerprint region (2 – 20 μm).
Spectral Correlations of Phase Noise in Ultrabroadband Femtosecond Lasers
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.
Characterization of carrier-envelope phase noise of passively phase-locked fiber-based frequency combs up to the nyquist frequency
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].
Broadband interferometric subtraction of optical fields
We present a Mach-Zehnder-like interferometer capable of simultaneous super-octave (950 – 2100 nm) destructive interference with an intensity extinction of 4 × 10−4. Achromatic nulling is achieved by unbalancing the number of Fresnel reflections off optically denser media in the two interferometer arms. With a methane gas sample in one interferometer arm, we isolate the coherent molecular vibrational emission from the broadband, impulsive excitation and quantitatively examine the potential improvement in detectable concentration, compared to direct transmission geometry. The novel concept will benefit sensing applications requiring high detection sensitivity and dynamic range, including time-domain and frequency-domain spectroscopy.
High-power frequency comb at 2  μm wavelength emitted by a Tm-doped fiber laser system
We report on the generation of a high-power frequency comb in the 2 μm wavelength regime featuring high amplitude and phase stability with unprecedented laser parameters, combining 60 W of average power with <30  fs pulse duration. The key components of the system are a mode-locked Er:fiber laser, a coherence-preserving nonlinear broadening stage, and a high-power Tm-doped fiber chirped-pulse amplifier with subsequent nonlinear self-compression of the pulses. Phase locking of the system resulted in a phase noise of less than 320 mrad measured within the 10 Hz–30 MHz band and 30 mrad in the band from 10 Hz to 1 MHz.