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 ...

The dynamical behavior of intramolecular electronic excitation transfer in 9,9′-bifluorene and 2,2′-binaphthyl has been investigated in various solvents using a heterodyned and background-subtracted femtosecond polarization technique. It has been found that the excitation transfer between fluorenyl moieties occurs on a time scale of around 300 fs in hexane, but evidently slows down to around 970 fs in CCl4, and there appears little observable variation in transfer dynamics when the solvent is changed through the hexane–decane–hexadecane series. It has also been found that excitation transfer between naphthyl moieties in CCl4 undergoes damped oscillations which have an apparent period of 1.2±0.1 ps and a damping time constant of 180±20 fs. While there is no clear sign of any oscillation in hexane, the transfer dynamics decays only slightly faster. All the phenomena observed can be neatly categorized, within the simple phenomenological Bloch theory, as either over- or underdamped motion relevant to the ratio of the exciton splitting to the pure dephasing rate. However, it is the thorough analysis of all the secular elements of the Redfield relaxation matrix that pinpoints the role of correlated fluctuations in the excitation transfer, and provides a quantitative relation of the pure dephasing between the excited local states to that between the excited and ground states. The equivalency of the pure dephasing rate between the two local states to the population transfer rate between the two delocalized states also prompts us to propose that a local libration of solvent CCl4 could play a key role in underdamping the excitation transfer coherence.

We utilize mid-infrared dual frequency comb spectroscopy for the detection of methane in ambient air. Two mid-infrared frequency comb sources based on femtosecond Er:fiber oscillators are produced through difference frequency generation with periodically poled MgO-doped lithium niobate crystals and stabilized at slightly different repetition rates at about 250 MHz. We performed dual frequency comb spectroscopy in the spectral range between 2900 cm −1 and 3150 cm −1 with 0.07 cm −1 resolution using a multipass cell of ~580 m path length, and achieved the sensitivity about 7.6 × 10 −7 cm −1 with 80 ms data acquisition time. We determined the methane concentration as ~1.5 ppmv in the ambient air of the laboratory, and the detection limit as ~60 ppbv for the current setup.

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We use two femtosecond Erbium-doped fiber lasers with slightly different repetition rates to perform a modern type of Fourier transform spectroscopy without moving parts. The measurements are done in real time, and it takes less than 50 µs. We work with somewhat different spectral outputs from two Erbium-doped fiber lasers and employ spectral filtering based on a 2f-2f grating setup to select the common spectral region of interest, thereby increasing the signal-to-noise ratio. The interferogram is taken with a 20 cm long gas cell, containing a mixture of acetylene and air at atmospheric pressure, and is fast-Fourier-transformed to obtain the broadband spectral fingerprint of the gas.Ever since the femtosecond frequency comb was invented in the late 1990s [1], there have been ongoing revolutions in the field of spectroscopy. Many methods have been and are being developed to utilize the regular comb structure of millions of laser modes in spectroscopy ranging from the XUV to the mid IR [2][3][4][5]. Among these, dual frequency comb spectroscopy (DFCS) emerges as a promising, highly sensitive, superior fast spectroscopy with high resolution to complement the traditional Fourier Transform Spectroscopy (FTS) [2,[6][7][8][9][10][11][12]. Especially in the near and mid IR, a plethora of greenhouse and other gases have molecular fingerprint spectra that can be studied with DFCS based mainly on Er-or Yb-doped fiber lasers and their wavelength ranges extended by optical parametric oscillation process [13], supercontinuum, or difference frequency generation [14].DFCS uses two femtosecond frequency combs with slightly different repetition rates. In the time domain, pulse pairs arrive at a photo detector (PD) with a linear increasing time delay. Each time a pulse pair overlaps in time, like the zero optical path difference in FTS, the central burst of an interferogram is formed. Subsequent pairs of pulses impinge on the PD with varying delay, analogous to the delay introduced by FTS, except no moving mirror parts are needed. As a result, the PD records an interferogram formed by many pulse pairs of various delay. Because pulse pairs repeatedly move through each other, a new interferogram starts to form as soon as the previous is completed. In the frequency domain, comb lines of one source beat with the same order comb lines of the other source, and the optical frequency information is down converted to the radio frequency range, which is the Fourier transform of the interferogram. After frequency up conversion and calibration, the optical spectrum is recovered.Compared to traditional FTS, DFCS is extremely fast, and an interferogram can be recorded in less than 25 µs [9]. A fast oscilloscope can display an interferogram and the broadband spectrum by Fast Fourier Tranform (FFT) in real time. If the repetition rates and carrier envelope offset (CEO) frequencies of both combs are well stabilized, several seconds of the interference signal can be transformed to a spectrum with high resolution at high signal-to-noise ratio (SNR) ...