Research ArticleLASER PHYSICS

Spectral purity and tunability of terahertz quantum cascade laser sources based on intracavity difference-frequency generation

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Science Advances  01 Sep 2017:
Vol. 3, no. 9, e1603317
DOI: 10.1126/sciadv.1603317

Figures

  • Fig. 1 Emission spectra and power output of the DFG-QCL used in our study.

    (A and B) FTIR emission spectra of the two mid-IR pumps and the resulting THz DFG, collected at heat-sink temperatures (TH) of 65 K (A) and 85 K (B) in rapid-scan mode with a 0.125 cm−1 (3.75 GHz) spectral resolution, plotted as a function of the driving current. Identical colors in the mid-IR and THz spectra correspond to identical driving current conditions. (C) Current-voltage and light-current characteristics of the mid-IR pumps and THz DFG for the device operated in CW mode at 85 K.

  • Fig. 2 Frequency tuning.

    Dependence of mid-IR pump frequencies (A) and THz difference frequency (B) of the THz DFG-QCL on the TH. The device was operated in pulsed mode, as described in the main text. The solid line in (B) represents the frequency difference of the two mid-IR peaks as extracted from (A), and the round symbols with error bars indicate the experimentally measured THz peak positions.

  • Fig. 3 Experimental setup.

    (A) Schematic of the experimental setup: The THz frequency comb is generated in a MgO-doped lithium niobate waveguide by optical rectification with Cherenkov phase matching of a femtosecond mode-locked fiber laser, working at 1.5-μm wavelength. The optical rectification process produces a zero-offset free-space THz comb with a repetition rate of approximately 250 MHz of the pump femtosecond laser. The THz frequency comb and the emission from the DFG-QCL were overlapped on the sensor element of the HEB with a 250-MHz electrical bandwidth. The beat note signals of the THz DFG-QCL emission with the nearby frequency comb lines were analyzed by an FFT spectrum analyzer, having a 40-MHz real-time bandwidth. (B) Typical beat note spectrum observed on a spectrum analyzer for 2-ms integration time. The value of fb obtained by fitting is used in Eq. 2 for the determination of the QCL emission frequency fQCL.

  • Fig. 4 LW measurements.

    (A) Width of the beat note at different time scales measured at two different operating temperatures of the device. The use of an FFT spectrum analyzer allows retrieval of the beat note spectra over different integration times and therefore evaluation of the THz DFG-QCL emission LW at different time scales (ranging from 20 μs to approximately 20 ms). Solid lines are fits with a logarithmic function. The logarithmic dependence of the LW on time confirms the 1/f nature of the QCL frequency noise (see Methods for details). (B) Reconstruction of the FNPSD of the THz DFG-QCL emission. The plots have been obtained by inverting the approach developed by Di Domenico et al. 36, as discussed in Methods. This procedure allows retrieving the FNPSD for frequencies where it is larger than the β-line [8/π2ln(2)f, dotted in the lower part of the graph. Given the smallest time scale of 20 μs of our setup, our measurements do not include frequencies higher than 50 kHz. The dashed red and green lines refer to the measurements performed by Bartalini et al. (34) and Vitiello et al. (7) for the mid-IR and THz QCLs, respectively, whereas the solid lines refer to the measurements presented in this work.

  • Fig. 5 Beat note frequency measurements.

    (A) Frequency of the beat note fb, plotted as a function of the THz frequency comb repetition rate, while keeping the QCL driving current at 540 mA and the TH fixed at 75 K. This measurement is used for the determination of N. (B) Frequency tuning of the 2.58-THz emission line of the DFG-QCL as a function of the QCL TH, measured while keeping the QCL driving current fixed at 540 mA. The 2.49-THz emission line shows a similar dependence. (C) QCL frequency tuning of the 2.58-THz emission line as a function of the driving current at different TH. The 2.49-THz emission line shows a similar dependence.

  • Fig. 6 Order N determination.

    Schematic representation of the process used to retrieve the dependence of fb on frep by measuring the beat notes in the 125- to 250-MHz range.

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