We performed dual frequency comb spectroscopy for time-resolved methane detection in real time, which is of demanding interest for the natural gas industry, environmental science, atmospheric monitoring and geoscience research. Broadband MIR frequency combs provide high brightness and frequency precision for many spectroscopic applications. To increase the detection sensitivity, a multipass cell or a high finesse cavity with mid-infrared frequency comb produced by difference frequency generation is used. This setup increases also the interaction length between the detection target molecules and the MIR frequency combs. Both frequency comb sources are based on femtosecond Er-doped fiber laser oscillators with stabilized repetition rates at ∼250 MHz, which is stabilized and locked to sophisticated frequency references or GPS system in order to achieve comb-teeth resolved spectral measurements. We implemented the mid-infrared dual frequency comb spectroscopy in the spectral range between 2900 cm− 1 and 3150 cm− 1 with 0.07 cm− 1 resolution with a novel multipass cell of ∼580 m interaction length to detect the trace amount of methane in ambient air. We determined the methane concentration in the ambient air of the laboratory to be ∼ (1.5 ±  0.1) ppmv. The minimum detection limit for the current setup is ∼60 ppbv with an 80 ms data acquisition time.

We note that most of current existing frequency comb technologies have relatively narrow bandwidth and microwatt power levels limited by the applications for sensitive and real time multi-target gas detection. Another approach based on OPO (optical parametric oscillator) sources provided hundred milliwatt power and broad spectra, but the repetition rates or the carrier-envelope offset frequencies were not fully controlled, which led to distorted spectral measurements. We resolved this problem by applying dual frequency comb lasers DFCS to use the later as Vernier scale for correcting the distortion of the first laser comb system. A simplified diagram of the experimental setup is presented in Fig.1.

In Fig. 1, the experimental setup includes two MIR comb sources, mirrors and lenses allowing to couple the MIR comb2 into the multipass cell, one 50:50 beam splitter (BS) to obtain the reference and signal pulses, one 92:8 beam splitter to combine pulses from two comb sources, and an MCT photodetector with electronics for data acquisition. The spectra measured with a scanning monochromator are shown in inset (a) for MIR comb1 (cyan) and MIR comb2 (red). Interferometric autocorrelation traces are shown in inset (b) for MIR comb1 (cyan) and (c) for MIR comb 2 (red). The absorption features in the spectra are due to water vapor in the laboratory environment. Spot patterns on the mirrors of the multipass cell with the visible red laser are shown in inset (d) and are produced when the alignment red laser is introduced with the flip mirror off. The entrance/exit hole of 5 mm diameter can be seen in the top left part of the right side mirror.

Our system is referenced to a Rb frequency standard (Stanford Research, PSR10). One advantage of these DFG sources is the passive carrier-envelope offset (CEO) frequency stabilization. Since the pump and signal fields originate from the same source, the generated idler field is carrier-envelope phase slip free. The MIR comb1 has ∼120 mW output power, covering a spectral range from 2.8 to 3.6 μm (2700 cm− 1 to 3600 cm− 1). The pulse duration is ∼80 fs. The MIR comb2, employing a higher power Ytterbium doped fiber amplifier, generates an MIR comb of ∼300 mW with a similar spectrum and pulse duration.

We lock two femtosecond DFG MIR combs with slightly different repetition rates at fr1 = 249,998,633 Hz and fr2 = 250,000,122 Hz, thus the difference is δfr = 1489 Hz. In the time domain, when a pulse pair from two sources overlaps in time, the center burst of an interferogram is formed. Subsequent pulse pairs impinge on the detector with linearly increasing time delay. As a result, the detector records an interferogram formed by many pulse pairs of various delays. Because pulse pairs repeatedly move through each other, a new interferogram starts to form as soon as the previous is completed in 1/δfr∼0.672 ms. We record the interferogram train with an oscilloscope (Tektronix, MDO4104B-3) at a sampling rate of 250 MSPS with ∼10 bit resolution. The maximum record length is 20 Mega points, corresponding to 80 ms, or ∼118 complete interferograms. We first block the reference pulses, and record only signal interferogram train; which can be fast Fourier transformed to comb-teeth resolved spectrum with a simple software phase correction. The magnitude and phase radio frequency (RF) spectra are presented in Fig. 2.

In figure2, fourier transformed magnitude (black) and phase (green) RF spectra from an 80 ms signal interferogram train with a software phase correction: (a) broad range from 18 to 63 MHz, (b) zoomed-in narrow range from 40.25 MHz to 40.45 MHz. The phase is only perceptible and plotted at comb teeth positions since the RF signals between comb teeth are simply noise and therefore have a random phase between − π < /AσΣETHιγηλιγητ> to π. Because of the broad absorption features, the absorption dips can be observed in (a). The discrete comb lines with a spacing of δfr = 1489 Hz can be observed in (b).

Because of the passive CEO frequency stabilization, the DFG MIR frequency combs have a simple form of vm = mfr, where v is the optical frequency in MIR, m is an integer, and fr is the source repetition rate. For the measurements in the frequency domain, the individual comb lines from two sources beat between (N+2)fr1 and Nfr2, where N is an integer between ∼347,600 and ∼377,600, and down convert the optical frequency information at about 87∼95 THz to RF at about 18∼63 MHz with a simple formula Nfr2 − (N+2)fr1 = fRF. Thus, the optical up conversion follows fRFfr2/δfr+2fr1fr2/δfr. The measured complex spectrum of the signal is S(ν) = S0(v)exp[ − α(v)L/2 − iφ(v)], where S0(v) is the complex spectrum of the reference, α(v) is the molecular absorption coefficient, L is the path length of the multipass cell, φ(v) is the phase shift. We evaluate the DFCS quality factor, which is the product of the SNR and the number of resolved spectral elements normalized by the square root of the total acquisition time. With a SNR ∼100, in 80 ms acquisition time, we obtain the number of resolved spectral elements as 250 cm− 1/0.07 cm− 1 ∼ < /AσΣETHιγηλιγητ>3600, and the experimental quality factor is ∼1.3 × 106 Hz1/2.

This work was funded by the Robert A. Welch Foundation, Grant No. A1546 and the Qatar Foundation under Grant No. NPRP 6-465-1-091.


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