Axel Ruehl

Direct frequency comb spectroscopy in the extreme ultraviolet

The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40u2009nanometres, by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82u2009nanometres and a neon transition at 63u2009nanometres, thus confirming the combs’ coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100u2009nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium, the development of next-generation ‘nuclear’ clocks and searches for variation of fundamental constants using the enhanced sensitivity of highly charged ions.

Widely-tunable mid-infrared frequency comb source based on difference frequency generation

We report on a mid-IR frequency comb source of unprecedented tunability covering the entire 3-10 μm molecular fingerprint region. The system is based on difference frequency generation in a GaSe crystal pumped by a 151 MHz Yb:fiber frequency comb. The process was seeded with Raman-shifted solitons generated in a highly nonlinear suspended-core fiber with the same source. Average powers up to 1.5 mW were achieved at the 4.7 μm wavelength.

Full phase stabilization of a Yb:fiber femtosecond frequency comb via high-bandwidth transducers

We present full phase stabilization of an amplified Yb:fiber femtosecond frequency comb using an intracavity electro-optic modulator and an acousto-optic modulator. These transducers provide high servo bandwidths of 580 kHz and 250 kHz for f(rep) and f(ceo), producing a robust and low phase noise fiber frequency comb. The comb was self-referenced with an f-2f interferometer and phase locked to an ultrastable optical reference used for the JILA Sr optical clock at 698 nm, exhibiting 0.21 rad and 0.47 rad of integrated phase errors (over 1 mHz-1 MHz), respectively. Alternatively, the comb was locked to two optical references at 698 nm and 1064 nm, obtaining 0.43 rad and 0.14 rad of integrated phase errors, respectively.

Power optimization of XUV frequency combs for spectroscopy applications [Invited]

We address technical impediments to the generation of high-photon flux XUV frequency combs through cavity-enhanced high harmonic generation. These difficulties arise from mirror damage, cavity nonlinearity, the intracavity plasma generated during the HHG process, and imperfect phase-matching. By eliminating or minimizing each of these effects we have developed a system capable of generating > 200 μW and delivering ~20 μW of average power for each spectrally separated harmonic (wavelengths ranging from 50 nm - 120 nm), to actual comb-based spectroscopy experiments.

Broadband phase noise suppression in a Yb-fiber frequency comb.

We report a simple technique to suppress high-frequency phase noise of a Yb-based fiber optical frequency comb using an active intensity noise servo. Out-of-loop measurements of the phase noise using an optical heterodyne beat with a cw laser show suppression of phase noise by ≥7u2009dB out to Fourier frequencies of 100u2009kHz with a unity-gain crossing of ∼700u2009kHz. These results are enabled by the strong correlation between the intensity and phase noise of the laser. Detailed measurements of intensity and phase noise spectra, as well as transfer functions, reveal that the dominant phase and intensity noise contribution above ∼100u2009kHz is due to amplified spontaneous emission or other quantum noise sources.

Fully Stabilized GHz Yb-Fiber Laser Frequency Comb

We demonstrate a fully stabilized GHz-spaced Yb-fiber laser frequency comb using a Yb-fiber femtosecond oscillator with 1.04 GHz fundamental repetition rate.

Fourier Transform Spectrometry Using a Single Cavity Length Modulated Mode-Locked Fiber Laser

We present a Fourier transform spectrometer based on a single repetition rate modulated mode locked Yb-fiber laser configured as a coherent scanning delay line using an imbalanced Mach-Zehnder interferometer. Effective mirror scan rate is 7.5 m/s.

Power Scaling of High-Repetition-Rate HHG

We report on cavity-enhanced HHG with a frequency comb delivering 120-fs pulses and 80-W average power at 154-MHz repetition rates. With 5-kW average intracavity powers, average HHG powers beyond the microwatt level have been achieved.

Coherent Tm-fiber Raman-soliton amplifier

The molecular fingerprint region from 2 – 10 °m is currently receiving a lot of interest due to its potential for enabling the identification of many important molecules in biological and medical applications. Particularly mid-IR comb laser systems are highly sought, since they could potentially revolutionize mid-IR and far IR spectroscopy. However, to date no robust comb technology operating in the 2 µm spectral region has been available. Tm fiber lasers have been known as versatile sub ps sources for the 2 µm spectral region for a long time, but their application to comb technology has been hampered by the long pulses generated by even the best Tm oscillators, i.e. to date the shortest pulses generated from Tm fiber oscillators were around 173 fs [1].

Mode-Locked Yb-Fiber Laser for Rapid Dual Pulse Scanning Applications

We demonstrate a mode-locked Yb fiber soliton oscillator for the generation of pulse pairs with rapidly scanning pulse separations at interferometric precision.