Coherent averaging quantum cascade laser based dual-comb spectrometer with near infrared light illumination control

. We present a dual-comb spectrometer based on quantum cascade lasers operating at 7.7 µ m with a stabilization scheme that enables coherent averaging. We show that by illuminating a low cost near-infrared light source of the front facet of the quantum cascade laser, we can tightly lock one comb line of the dual-comb spectrum, resulting in narrow linewidth with sub-radian integrated phase noise for all RF comb lines


Introduction
Frequency comb spectroscopy is a widely used technique and still in constant development as new opportunities of applications emerge [1].Dual-comb spectroscopy (DCS), namely the study of the multi-heterodyne beating between two mutually coherent combs in the radio-frequency domain (RF), also interests many researchers due to several advantages, such as the combined high acquisition rate and resolution, which surpasses more conventional techniques [2].In this context, performing DCS in the midinfrared (MIR) brings also one more advantage for applications since many relevant molecules show strong absorption features, meaning that short interaction lengths or low concentration gases can be targeted.MIR is difficult to reach, and common comb sources operating in this region often rely on a near-infrared (NIR) source that is nonlinearly frequency converted.However, a few sources can generate combs directly in the MIR, especially quantum cascade lasers (QCL) that can emit between 4 µm up to THz region depending on the chip used [3].DCS using QCLs has been demonstrated several times [4], and these lasers have the merit to be compact and with a low footprint, enabling commercially available MIR spectrometer [5].
Here, we present a stabilization scheme for DCS that allow us to perform coherent averaging.We show that illuminating the front facet of a QCL with a modulated NIR continuous wave (CW), and using a distributed feedback (DFB) QCL within the spectral range of the combs as an intermediate, a tight lock of one comb line of the multi-heterodyne RF comb can be achieved.This results in obtaining RF comb lines with narrow linewidths and sub-radian integrated phase noise.

Experimental setup
The dual-comb setup considered here is presented in This signal is divided and compared to a synthetizer, giving us an error signal that is fed to two servo controllers.One of them acts on the driving current of one QCL-comb whereas the other one acts, via an integrated electro-absorption modulator, on the intensity modulation of a NIR CW laser that is illuminating the front facet of the same QCL-comb.In the end, our QCL-combs operated in a master-follower configuration, enabling coherent averaging.

Characterization and coherent averaging
Once the setup is stabilized, we record a dual-comb spectrum and take an interest in the phase noise of individual lines.The results are presented in Figure 2. The dual-comb spectrum is composed of more than 50 lines with a signal-to-noise ratio higher than 50 dB for most comb lines in the center of the spectrum.The analysis of the integrated phase noise of individual lines show that it increases with the line number according to a quadratic law, as can be expected.Note that the main contribution to the phase noise comes from the bump around 3 kHz, which is due to one RF synthesizer we are using (see the synthesizer difference phase noise curve in Fig. 2 b)).Hence, further improvements are expected.
Next we take an interest in the demonstration of coherent averaging with our spectrometer.For this we record a 71 ms long interferogram and we compare one interferogram sample with an averaging of 355 555 samples.The results are displayed in Figure 3 and clearly demonstrate the possibility of coherent averaging.We verified that the dual-comb spectrum obtained either by coherent averaging of the interferograms or by averaging dual-comb spectra in the RF domain led to similar results, which validates the technique.

Conclusion
We presented a stabilization scheme for a MIR QCL-based dual-comb spectrometer that relies on NIR light illumination of the front facet of one QCL-comb and a DFB as a transfer mechanism.This master-follower configuration allowed us to tightly lock one comb line of the dual-comb spectrum and to reach sub-radians integrated phase noise for lines of the RF dual-comb spectrum.The results presented can be improved as we put into evidence the limitations of our current setup, but coherent averaging has been demonstrated and the results are nevertheless very promising towards further spectroscopic applications.

Figure 1 :
Figure 1: Diagram showing the dual-comb spectrometer and the stabilization part relying on the light illumination of a NIR CW light on the front facet of a QCL-comb. /doi.org/10.1051/epjconf/202328707022

Figure 2 :
Figure 2: a) Example of recorded dual-comb spectrum.b) Phase noise of different RF comb lines which positioning are pictured in a).

Figure 3 :
Figure 3: Graph showing one interferogram sample compared to an averaging over 355 555 samples, corresponding to a recording time of 71 ms.