We report on a high-power mid-infrared frequency comb source based on a femtosecond Er:fiber oscillator with a stabilized repetition rate at 250 MHz. The mid-infrared frequency comb is produced through difference frequency generation in a periodically poled MgO-doped lithium niobate crystal. The output power is about 120 mW with a pulse duration of about 80 fs, and spectrum coverage from 2.9 to 3.6 µm. The coherence properties of the produced high-power broadband mid-infrared frequency comb are maintained, which was verified by heterodyne measurements. As the first application, the spectrum of a ~200 ppm methane-air mixture in a short 20 cm glass cell at ambient atmospheric pressure and temperature was measured.Currently there is a large demand for gas detection systems in the mid-infrared (MIR) in many areas of science and technology. For these applications high repetition rate femtosecond lasers and frequency combs are being developed actively due to fast data acquisition rates, high sensitivity, and multi-target detection properties inherent of broadband frequency comb spectroscopy [1,2]. The 3 to 4 µm MIR range is of particular interest since it contains strong absorption features of the C-H stretching vibrational mode of methane (v3 band) and many other more complex hydrocarbons. For a multitude of measuring tasks in the booming natural gas industry, agriculture, atmospheric and geosciences researches, methane detection with real time monitoring, and quantifying different hydrocarbon isotopes are important [3,4]. In addition there is a growing interest to detect volatile organic compounds such as benzene, toluene and ethyl benzene, which are precursors of atmospheric nanoaerosols and can contribute to poor indoor air quality.Different versions of frequency comb spectroscopies have been developed since the invention of frequency comb [2,[5][6][7][8][9][10][11]. Broadband MIR frequency combs provide useful light sources for many spectroscopic applications. In particular, several MIR sources using single pass difference frequency generation (DFG) have been developed and are attractive because of their relative simplicity and the benefit of passive carrier-envelope offset frequency stabilization [12][13][14]. If the pump and signal fields are phase coherent and originated from the same source, the generated idler field is carrier envelope phase slip free and requires only stabilization of the comb spacing, which is relatively easy to implement by stabilizing the source repetition rate. Hence it was shown to provide a frequency synthesizer in the MIR and can be used in frequency standard applications [15]. Several MIR sources based on DFG have been reported [12][13][14]; however in some cases involving Raman shifting the coherence was reduced and even lost [13]. In addition, the available power levels have been moderate with about 1.5 mW at 4.7 µm having been reached [14].In this letter, we demonstrate a high-power MIR frequency comb source based on a femtosecond Er:fiber oscillator with a stabilized repetition ...
We present a sub-85 fs self-starting stretched-pulse passively mode-locked Erbium-fiber oscillator in a sigma setup with tunable repetition rate. The sigma cavity included a movable mirror enabling a tunable pulse repetition rate variation of +/- 1 % from 55.3 MHz to 56.4 MHz with continuous, uninterrupted mode-locked operation and an output power around 14 mW. Based on the wide tuning range of the repetition rate the presented fiber oscillator is a suitable candidate for applications in femtosecond spectroscopy or precision metrology around 1.56 microm.
We report on the measurement of an Erbium-fiber oscillator's carrier-envelope-offset frequency using an extruded SF6 photonic crystal fiber for the generation of a more than two octave-spanning supercontinuum from 400 nm to beyond 1750 nm. A modified type of f-2f-interferometer was employed, beating the frequency doubled input signal of the fiber oscillator with the supercontinuum to generate the carrier-envelope-offset beat. Controlling the fiber oscillator's pump power with an electronic feedback loop, we phase-locked the carrier-envelope-offset frequency to an external reference source. The resulting residual phase excursions correspond to fractional frequency instabilities of the oscillator's frequency comb of the order of 10(-16) for averaging times longer than 10 s.
We demonstrate the generation of broad spectra with a flat intensity distribution from originally highly structured supercontinua, obtained with femtosecond pulses in a photonic crystal fiber. This is accomplished by truncating the spectra at a constant level using a liquid crystal based spatial light modulator. The technique is useful for astronomical spectrograph calibration using frequency combs, where it allows to equalize the optical power of the calibration lines. This enables an improved calibration accuracy by maximizing each line's signal-to-noise ratio.
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