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Laser dynamics : laser linewidth broadening measurements

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Context : We propose an original method for the measurement of the Henry factor (αH) coupling the refraction index and the gain of the amplification medium of a semiconductor laser. This factor was introduced to account for the laser linewidth broadening (1+ αH2) with respect to the fundamental limit [1] given by the spontaneous emission rate of the gain medium which perturbs the laser field maintained by stimulated emission (Schawlow-Townes equation). Even though the dynamics of diode lasers has been investigated during a few decades, for quantum cascade lasers (QCLs) investigations of their linewidth are not many and results are sometimes contradictory. We propose an original method of measurement which should allow a better understanding of the dynamics of these lasers.

Method : The technique which will be applied consists in the observation of the laser response to optical feedback, in a regime of weak feedback levels [2]. Optical feedback induces a change of the emission frequency and power. One can show that a measurement of both these parameters allows to deduce the αH coefficient. The originality of this technique rests on the use of a resonant optical cavity in a V-shaped geometry (see picture below).

Objective : The goal is the determination of the Henry factor for different types of semiconductor lasers : Diode lasers in the near infrared ( 2μm) to obtain results to be compared to previously published values ; then in particular QCLs and ICLs (intra-band cascade lasers) in the mid infrared (around 5μm) which have become commercially available only these last few years.

Expected skills  : The candidate will have good knowledge of laser optics and an interest in instrumental developments and modeling.

Master 2 internship : position available from February 2019, for a duration of 2 to 6 months. PhD continuation possible (well suited).

Supervisor : Irène VENTRILLARD, LIphy, Grenoble, France

Collaboration : Jérôme MORVILLE, ILM, Lyon, France

Scheme of principle of a QCL laser

Figure 1 : Scheme of principle of a QCL laser subject to optical feedback generated by a V-shaped optical cavity formed by three high reflectivity mirrors (in blue). A mirror mounted on a piezoelectric translator (PZT) and an attenuator are used to adjust the phase and intensity of the optical feedback, respectively. The signal is measured at the cavity output by a photodiode (PD) and normalized by the laser power (measured by another PD). Phase control by acting on the PZT is performed by an electronic controller which extracts an error signal from the analysis of the shape of the signals transmitted by the cavity. We have a few of such setups which have been realized for application to high sensitivity laser absorption spectroscopy trace gas analysis [3].

[1] C. H. Henry, “Theory of the Linewidth of Semiconductor Lasers,” IEEE J. Quantum Electron., vol. 18, no. 2, pp. 259–264, 1982.
[2] C. Szwaj, E. Lacot, and O. Hugon, “Large linewidth-enhancement factor in a microchip laser,” Phys. Rev. A - At. Mol. Opt. Phys., vol. 70, no. 3, pp. 4–7, 2004.
[3] J. Morville, S. Kassi, M. Chenevier, and D. Romanini, “Fast, low-noise, mode-by- mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking,” Appl. Phys. B, vol. 80, no. 8, pp. 1027–1038, May 2005.