# High-Power, Narrow-Linewidth Distributed-Feedback Quantum-Cascade Laser for Molecular Spectroscopy

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## Abstract

**:**

## 1. Introduction

## 2. Methods

#### 2.1. Manufacturing

#### 2.2. Characterization

## 3. Discussion

## 4. Outlook

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

QCL | Quantum Cascade Laser |

FWHM | Full Width Halfg Maximum |

DFB | Distributed Feedback Bragg Reflector |

MOPA | Master Oscillator Power Amplifier |

FNSPD | Frequency Noise Power Spectral Density |

CW | Continuous Wave |

## References

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**Figure 1.**(

**a**) One period of the QCL active region together with the computed wavefunctions (grey). The wavefunctions of the upper and lower level of the lasing transitions occurring in this device are highlighted in orange and blue, respectively. (

**b**) Top-view photograph of the device. (

**c**) Schematic (not to scale) of the cross section of the device where the active region (AR), guiding layer, top-contact and InP are respectively colored in orange, blue, gold, and grey.

**Figure 2.**(

**a**) Light–current (L-I) curves of the present QCL for temperatures between −20 °C and 20 °C in increments of 10 °C. (

**b**) Spectral tunability map of the device as a function of current and temperature. The color map indicates the wavenumber difference to a reference spectroscopic transition in N${}_{2}$O at 2185.53 cm${}^{-1}$ represented by the dashed line. (

**c**) Spectrum recorded for a temperature of −20 °C and a current of 650 mA showing a side-mode-suppression ratio (SMSR) of ∼40 dB.

**Figure 3.**(

**a**) Schematic of the experimental setup for characterizing the laser linewidth. The QCL was controlled by a current driver (CD) which was modulated by a triangular waveform from a signal generator (SG). Radiation from the QCL passed through an optical isolator (OI) and a gas cell (GC) filled with N${}_{2}$O. The transmitted laser intensity was measured on an infrared-sensitive HgCdTe photodetector (PD) allowing the observation of absorption lines of N${}_{2}$O on an oscilloscope (OSC). A part of the laser light was split off and diverted to a wavemeter (WM) and a powermeter (PM). A computer (PC) controlled the experiment and was used to compute the FNPSD. (

**b**) Transmittance signal as a function of the frequency detuning from the laser set point of 2185.53 cm${}^{-1}$. The fit of a straight line to the linear region of the slope of the absorption line (black) yields a signal-to-frequency transfer factor of 1.13 mV/MHz.

**Figure 4.**The frequency noise power spectral density (FNPSD) of the laser (red trace) follows an 1/f trend up to 2 kHz. The contribution of the current driver (blue trace) to the frequency noise is negligible. The FNPSD of the laser is crossed by the beta separation line at 467 kHz resulting in a laser linewidth equal to 1.3 MHz for an observation time $\tau $${}_{o}$ = 10 ms. The operation current and temperature of the laser were set to 650 mA and −20 ºC during the FNPSD measurment.

**Figure 5.**Maximum output powers as a function of the ratio of the FWHM with the emitted frequency of free-running narrow linewidth QCLs reported since 2010.

**Table 1.**Maximum output powers, free-running linewidths (full width at half maximum, FWHM), observation times and central wavelengths (WL) of free-running narrow linewidth QCLs reported in the literature since 2010.

Current State of the Art | |||||
---|---|---|---|---|---|

Pow. [mW] | FWHM [kHz] | Obs. Time [ms] | WL [$\mathsf{\mu}$m] | Author | Ref. |

>300 | 1300 | 10 | 4.56 | Bertrand 2022 | this work |

20 | 400 | 10 | 4.36 | Bartalini 2011 | [33] |

6 | 550 | 5 | 4.6 | Tombez 2011 | [8] |

20 | 770 | 10 | 4.56 | Tombez 2012 | [34] |

10 | 500 | 1 | 4.3 | Cappelli 2012 | [35] |

20 | 2750 | 50 | 4.67 | Borri 2012 | [36] |

10 | 2000 | 10 | 4.55 | Tombez 2013 | [37] |

20 | 1700 | 10 | 7.9 | Sergachev 2014 | [38] |

50 | 3200 | 1 | 8.6 | Fasci 2014 | [39] |

40 | 300 | 1000 | 10.3 | Argence 2015 | [6] |

150 | 380 | 1 | 4.5 | Sergachev 2017 | [32] |

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**MDPI and ACS Style**

Bertrand, M.; Shlykov, A.; Shahmohamadi, M.; Beck, M.; Willitsch, S.; Faist, J.
High-Power, Narrow-Linewidth Distributed-Feedback Quantum-Cascade Laser for Molecular Spectroscopy. *Photonics* **2022**, *9*, 589.
https://doi.org/10.3390/photonics9080589

**AMA Style**

Bertrand M, Shlykov A, Shahmohamadi M, Beck M, Willitsch S, Faist J.
High-Power, Narrow-Linewidth Distributed-Feedback Quantum-Cascade Laser for Molecular Spectroscopy. *Photonics*. 2022; 9(8):589.
https://doi.org/10.3390/photonics9080589

**Chicago/Turabian Style**

Bertrand, Mathieu, Aleksandr Shlykov, Mehran Shahmohamadi, Mattias Beck, Stefan Willitsch, and Jérôme Faist.
2022. "High-Power, Narrow-Linewidth Distributed-Feedback Quantum-Cascade Laser for Molecular Spectroscopy" *Photonics* 9, no. 8: 589.
https://doi.org/10.3390/photonics9080589