Standard Fourier Transform Infrared Spectrometers (FTIR) rely on a Michelson Interferometer scheme which uses a linear delay line to retrieve an interference pattern. Here, we demonstrate a fast FTIR based on a rotational delay line which allows us to achieve kHz acquisition rates. We perform spectrometry measurements using it in combination either with a Mid-IR Quantum Cascade Laser (QCL) frequency comb or a strongly, low-frequency, RF-modulated QCL. Regarding the latter, the modulation enables to broaden the laser emission up to 250cm^-1 (from 6.5µm to 7.5µm) and to reduce its amplitude noise compared to the free-running case. The combination of a strongly modulated QCL with a rotational FTIR opens the possibility to fast and broadband spectroscopy in the Mid-IR region, with possible applications spanning from gas detection to process control
In recent years, quantum cascade lasers have matured to become compact, powerful sources of coherent midinfrared light. Yet, the ultrafast carrier dynamics in these sources has so far restricted the formation of highintensity ultrashort pulses. In this work, we demonstrate the formation of ∼ 630 fs QCL pulses with a peak power of ∼ 4.3 W. We break the picosecond barrier in an approach similar to chirped pulse amplification, where we externally recompress the maximally chirped output of a quantum cascade laser frequency comb. Ultrashort pulse formation is confirmed with a novel asynchronous optical sampling technique. These results emphasise the potential of quantum cascade lasers also as sources for non-linear experiments in the mid-infrared.
In this paper, we present a thorough comparison of mid-infrared techniques, focusing on the two dominant solutions: QCL and FTIRs. Consequently, we will cover the technical challenges the DCS technique has to overcome to be superior to the FTIR technique. Pushing the DCS technique, we manage to get µs time resolution for up to 131 ms acquisition time as well as < 1 ms time-resolution for reactions which take > 10s. Furthermore, we have improved the spectral coverage of QCL DCS covering more than 100 cm-1 . Overall, the combination of high-speed, spectral bandwidth and high-brightness of this highly coherent source puts DCS at an advantage compared to FTIRs for a plethora of applications, such as liquid analysis (e.g. protein analysis, dioxin measurement, stopped flow), fiber applications and high-resolution spectroscopy. As such, we will give a comprehensive review of applications which are targeted today using QCL DCS. This covers bio-, environmental/gas, combustion as well as water analysis.
It has long been established that the combs emitted by quantum cascade lasers (QCLs) cannot be described as a train of short pulses, separated by the cavity roundtrip time. Instead, simulations made for typical device parameters suggest that, in steady state, these four-wave mixing driven combs have a constant temporal envelope, and undergo periodic rapid and complicated swings in frequency.
Recent work in characterising the modal phases has revealed a state which, somewhat unexpectedly, has a simple parabolic phase profile, corresponding to a linearly chirped output field. Moreover, this phase relationship was shown to be stable over time, and to be recoverable after the laser’s power had been cycled; from the perspective of a fixed external pulse compression scheme, these last two properties are critical.
In this work, we use a pair of gratings and lenses in a 4-f Martinez-type scheme to modify the phase of a high-power (~1 W) QCL comb emitted at 8.2 um with more than 100 cm-1 spectral bandwidth. By changing the position of the second grating, a parabolic phase can be added to or subtracted from the field. Employing this scheme, we demonstrate a compression of the QCL output from a 133 ps continuous wave waveform, to a train of pulses of width < 20 ps, and a peak power more than 10x that of the original. With this proof-of-principle work, we highlight the potential of the QCL system to deliver short, powerful pulses, with applications in nonlinear spectroscopy, for example.
Time-resolved vibrational spectroscopy is an important tool for understanding biological processes and chemical reaction pathways [1]. Today, all available methods to our knowledge require many repetitions of an experiment to acquire a microsecond time-res. mid-IR spectrum.
We present the IRspectrometer, a quantum cascade laser dual frequency comb spectrometer [2-3]. It allows for parallel acquisition of hundreds of mid-infrared wavelengths with microsecond time resolution. The formation of the light-activated L, M and N-states in bacteriorhodopsin – which only have µs to ms lifetimes – has been recorded that show the infrared response of bacteriorhodopsin to 10 ns visible light pulses with microsecond time-resolution. The different wavelengths were all measured in parallel thanks to the dual-comb approach. The spectra as well as the kinetics show good agreement with those from step-scan FT-IR measurements. As a benchmark, the spectral signature of several intermediate states of the bacteriorhodopsin photocycle has been recorded in a single shot measurement. This approach greatly reduces the complexity of time-resolved bio-spectroscopy measurements in the mid-infrared which currently require many repetitions.
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