Doppler-Based FTIR Spectroscopy

Real-time monitoring of non-repetitive events is currently a rapidly developing area in the field of instrumentation [1]. Rapid optical analysis in applications such as chemical reaction monitoring [2], protein dynamics [3], micromachining [4] or phase transitions in novel material systems[5] is of uttermost importance to understand the underlying complex dynamics. A general purpose instrument for spectroscopic characterisation is a Fourier-Transform (FT) spectrometer. However, the scan rate of the traditional FT spectrometer is often too slow when compared to the timescale of the physical processes of interest [6]. This is due to the fact that the information is acquired via the linear displacement of a mirror, which results in a trade-off between the temporal and spectral resolutions given that the system is not limited by the signal-to-noise ratio. Through a FT spectrometer based on a rotational delay line and a single frequency comb light source, we show it is possible to achieve ms acquisition times allowing real-time monitoring of gas absorption lines in the Mid-IR spectral region. Such a high acquisition speed results in a high signal-to-noise-ratio (SNR), due to the minimization of the 1/f noise within the signal bandwidth; such dependence has been observed to reduce the noise by two orders of magnitude moving from Hz to kHz for a free running mid-IR QCL (comb) [7,8].

The schematic of the rotational delay line (eight-point star) is shown in Fig1 together with the instrumental setup. The laser light is first divided into two arms, one is sent to a fixed delay line while the other one passes through the rotational delay line. During the star rotation, the light experiences multiple reflections and is subject to a Doppler shift due to the high speed of the rotating mirrors. Once it is recombined with the first light beam is sent to two detectors, one is used for reference while the other one is used after a sample compartment. The same beam path is experienced by a near-IR laser used as a reference to convert the interferogram scales, which are usually acquired as a function of time, to total delay. An example of interferogram is provided in Fig1(d).

Enlarged view: doppler_fig_1 — Fig1: On the left, the schematic of the setup. On the right, a schematic of the rotational delay line. The light is reflected multiple times on the star faces through 2 back reflectors(a). The insets show the way the light can be reflects at different star positions (b-c). An example of a typical QCL interferogram is also provided (d).

The combination of our rotational FT spectrometer with the frequency comb as a light source does not limit our system in spectral resolution given by the total optical path length. As a proof-of-principle, by exploiting a quantum cascade laser frequency comb as a light source, we demonstrate high spectral resolution measurements leveraging on the interleaving technique already employed in dual-comb spectrometers[9,10]. We can resolve the transmission resonances of a silicon etalon and the absorption lines of a Doppler-broadened low-pressure methane gas sample (CH4) with a resolution < 250 MHz as shown in Fig2. More details about the system are available in [11].

Enlarged view: doppler_fig_2 — Fig2. (a) Measured interleaving spectrum (blue dots, error bars smaller than the dots) of ≈ 500 μm thick silicon etalon within 7 s with a binned frequency resolution down to 5 GHz. Theoretical computed etalon transmission spectrum (red line). (b) Zoom in into (a). (c) Doppler broaden methane (CH4) spectrum recorded via interleaving (blue dots) at a pressure of 200 mbar within 25 s with frequency binned resolution down to 250 MHz and the HITRAN data base reference (red line). (d) Zoom in into (c) with sub-GHz resolved methane absorption lines.

References:

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