Broadband and fast frequency chirped FTIR spectroscopy with strongly modulated quantum cascade lasers

. While the opportunity to perform fast and broadband spectroscopy in the Mid-IR portion of the electromagnetic spectrum is very appealing, it requires the use of compatible light-sources. Here, we strongly modulate a Mid-Infrared Quantum Cascade Laser at a frequency in the RF domain, which is low compared to the natural repetition frequency of the device. In this way, we demonstrate that it is possible to obtain an emission bandwidth of up to 250 𝐜𝐦 −𝟏 . Finally, we employ it as a light source in an FTIR based on a rotational delay line, performing fast and broadband FTIR spectroscopy.


Introduction
Quantum cascade lasers frequency combs (QCL-FC) have been used more and more as bright and coherent light sources in the mid-infrared (Mid-IR) region to perform spectroscopy on a great number of molecules [1,2].Dualcomb spectroscopy, in particular, has established itself as a promising tool for fast and compact measurements.Nonetheless, the maximum bandwidth of the source laser at present can reach a maximum of about 100cm −1 [3], becoming a limiting factor for practical applications.Therefore, a solution allowing broadband operation of QCLs is highly desirable.Radio frequency (RF) modulation of the current injected in the devices has already been shown to improve the stability [4] and emission bandwidth [5] of the comb.For this reason, a study on the impact of high power (approximately 1W) RF injection at low frequencies (from 500 MHz up to 1 GHz) compared to the typical repetition frequency of the device has been performed.The device has then been used as a light source in an FTIR based on a recently conceived geometry, relying on a rotational delay line [6].In this way, the absorption spectrum of a plastic foil sample could be measured.

Results
When operated close to roll-over, the device shows an emission bandwidth of about 125cm −1 in free running operation.However, it lacks the features of a FC, as it is not possible to detect a natural beatnote at 15GHz, corresponding to the expected repetition frequency.As Fig. 1 shows, injecting an RF signal, a spectral broadening up to 250cm −1 is observed and the envelope shape improves in regularity.A linewidth study has been performed on the source with heterodyne spectroscopy.
To characterise the device, a high resolution FTIR in combination with an external-cavity QCL (EC-QCL) reference laser was used.The results indicate an increase of the linewidth of the laser in the injected case compared to the free running one, showing a compromise between the coherence of the emitted radiation and the envelope of the spectrum.The RF modulation makes it possible for the QCL to emit through the full gain bandwidth, while the mode competition in the free running case allows lasing only for modes experiencing higher gain.Due to this, it is impossible to drive the device in a frequency comb regime.This can be explained qualitatively as an effect due to the amplification of spontaneously emitted radiation.In fact, the device is rapidly switched on and off by the injection, implying that the light generated by stimulated emission does not have enough time to dominate over the one produced by spontaneous emission.Generally, it is possible to bring the FWHM of the emission down to a few hundred MHz, the linewidth remains reasonable, the bandwidth is large and the amplitude noise is not altered significantly.The possibility to access the full gain bandwidth of the system maintaining an acceptable degree of coherence makes it possible to use this laser in combination with fast spectrometers.Hence, it could possibly become a great tool for Mid-IR spectroscopy.To demonstrate this, an FTIR spectrometer based on a rotating octagram was used, which provides the delay required to obtain an interference pattern on a millisecond timescale.
Fig. 2 illustrates examples of the interferogram and spectrum of the RF-modulated device.By averaging 500 interferograms, resulting in a total integration time of 1 second, good quality measurements of the spectrum can be with a side-mode suppression ratio of almost 30dB.
In order to prove the capabilities of the system in the field of spectroscopy, the absorption spectrum of water vapor and of a germanium etalon were measured and are shown in Fig. 3 and Fig. 4 respectively.Each of these spectra was obtained merging 24 spectra measured at different currents.The absorption is computed as a ratio between a measurement with the sample in the beam path and a background one.Both measurements were taken with an integration time of 0.1s.
Notice that the absorption can be computed only in correspondence of the peaks of the source emission.Therefore, since increasing the DC bias current of the QCL shifts its emission frequencies, Fig. 3 and 4 were obtained by merging 24 spectra in order to provide a more dense spectral coverage.
To the authors' knowledge, this is the first time a spectrometer of this type is employed with a light source of this kind, combining high acquisition speed, broadband operation, brightness and a relatively high degree of coherence.

Summary
In conclusion, this work demonstrates the possibility to exploit the full gain bandwidth of a QCL by modulating its injected current in the RF domain.In this way, this device becomes a promising source for fast and broadband spectroscopy, as shown by the fast acquisition of the absorption spectrum of a plastic foil sample over a 200cm -1 bandwidth.

Fig. 1
Fig. 1 Emission spectrum of the QCL in free running operation (I = 410mA) and under strong RF injection (I = 360mA, f = 800MHz, RF-Power = 1W) acquired with a high resolution FTIR.

Fig. 2
Fig. 2 Interferogram and spectrum of the QCL under RF injection (I = 360mA, f = 800MHz, RF-Power = 1W), measured with a Doppler-based FTIR.The interferogram is obtained by co-adding 500 acquisitions, resulting in a total integration time of about 1s.

Fig. 2
Fig. 2 Absorption spectrum of water vapor.In red, the absorption spectrum from the HITRAN database is shown.The spectrum of the laser is also shown (light blue).

Fig. 2
Fig. 2 Absorption spectrum of a germanium etalon.The measurement is shown in a reduced wavenumber range for the sake of clarity.