1. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. Standard pump-probe saturation spectroscopy with electronic feedback to the laser. The QCL is housed in a continuous-flow cryostat, and its temperature stabilized around 80 K. After collimation, the beam is split in two parts for the pump-probe saturation spectroscopy. The SFG process in the 4-cm-long PPLN crystal produces near-IR radiation at approximately 850 nm, which is compared with that of a comb-referenced diode laser. The beat note is detected and its frequency is measured. The cell is 12 cm long and has AR-coated C a F 2 windows. P1 and P2 are wire-grid polarizers, T-λ/2 is a tunable half-wave plate, and BS is a beamsplitter. Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775

2. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. Saturation spectrum of the (01 1 1–01 1 0) P(30) CO 2 transition in direct-absorption (inset) and first-derivative detection, with a gas pressure in the cell of 20 mTorr (which leads to a relative absorption of about 10%). The pump intensity interacting with the gas sample is about a factor of 2 higher than the saturation level. The first-derivative signal was obtained by a lock-in amplifier with a 1- ms integration time constant. Its best fit is also shown. Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775

3. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. (a) Frequency-noise power spectral density of the laser radiation, recorded in both free-running and locked configuration. The comparison shows the locking bandwidth (≈500 Hz), as expected with a 1-ms lock-in time constant, (b) Comparison, in free-running and locked configurations, of the spreads of the beat-note frequency values as measured by the counter. The inset shows a 20-min- long acquisition of the beat-note frequency, with the laser locked. Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775

4. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. (a) Absolute frequency measurement of the 13 CO 2 (00 0 1–00 0 0) P(30) Doppler-broadened transition with a gas pressure P = 250 mTorr in the cell. The nonuniform distribution of the points is due to a slow temperature drift of the QCL during the acquisition. The red line represents the Voigt fit to data. (b) Measured beat-note frequencies acquired over a two-week period with the Lamb-dip- locked QCL. Each point, with its associated error bar (standard deviation of the mean), corresponds to the mean value of absolute frequency measurements taken over a long time interval [as that shown in the inset of Fig. ]. The mean value and standard deviation of these points are also shown (red line and gray area, respectively). (Color online only.) Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775

5. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. The (01 1 1–01 1 0) P(30) CO 2 transition is acquired with the double-balanced polarization technique (a) and with the sub-Doppler first- derivative lock-in technique (b). The experimental conditions are the same except for the acquisition times: 40 ms for the trace in (b), and 1.5 ms for the trace in (a). In the latter case the signal is sharper and its bandwidth larger, thanks to the absence of any dither. In (c) the two traces are overlapped, on the same vertical and horizontal scales, for a direct comparison. (Color online only.) Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775

6. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. Spectra of the laser frequency noise and jitter in locked conditions. The picture on the left shows the evolution, over 70 ms, of the beat note as detected by our real-time spectrum analyzer. The light blue profile on the right is the FFT spectra of 70-ms-long acquisitions of the beat note, while the dark red trace is the FFT of a 70-μs-long acquisition. (Color online only.) Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775

7. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. (a) The proportionality between the amplitude of the current modulation and the amplitude of the induced frequency modulation justifies the use of the “amplitude markers” for a direct comparison between driver current noise and QCL frequency noise. (b) The comparison between noise floors shows unequivocally that the current noise alone does not explain the whole observed laser frequency noise. The presence of some additional frequency noise source must be assumed. Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775

8. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. (a) Measurement of the frequency-noise PSD of a mid-IR QCL, in the 10-Hz to 100-MHz range, with two different current drivers. Orange line: the QCL is driven by a homemade ultralow-noise driver; gray line: the QCL is driven by a commercial driver. The red and black lines (smoothed) show the contributions to frequency noise arising from the current noise of the homemade and the commercial driver, respectively. (b) By applying Eq. to the frequency-noise PSD shown in (a), the QCL emission spectrum is recovered. A larger frequency noise leads to a broader spectrum, as the comparison between the two Gaussian-like profiles clearly shows. Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775

9. Date of download: 6/17/2016 Copyright © 2016 SPIE. All rights reserved. Comparison between the two methods used for the measurement of the QCL emission spectrum: direct observation of the laser emission by acquiring the beat-note spectrum (light blue), and integration, according to Eq., of the frequency-noise PSD (red line). Figure Legend: From: Quantum cascade lasers for high-resolution spectroscopy Opt. Eng. 2010;49(11):111122-111122-9. doi:10.1117/1.3498775