Quantum cascade laser: a promising candidate ...

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Email: [email protected] ... From our measured frequency noise PSD, we calculate a FWHM linewidth ... 1: Frequency noise PSD of a 4.6 µm DFB mid-IR.
Quantum cascade laser: a promising candidate toward an ultra-stable laser in the mid-IR L. Tombez, J. Di Francesco, S. Schilt, G. Di Domenico, D. Hofstetter, and P. Thomann Laboratoire Temps-Fréquence, Université de Neuchâtel, Bellevaux 51, 2000 Neuchâtel, Switzerland

Email: [email protected] An ultra-stable laser in the mid-IR range has been demonstrated in the past by Foreman et al.1 with a 3.39-µm HeNe laser stabilized to a methane transition. Such a reference laser has applications, for instance, in low phase noise microwave generation. Indeed, the relative stability of the ultra-stable optical reference can be transferred to the microwave range using a mid-IR optical frequency comb obtained by difference frequency generation (DFG) between two spectral regions of a near-IR comb. In this transfer scheme, and at the expense of the non-linear DFG process, the generated mid-IR comb has a null carrier-envelope offset (CEO) frequency. It implies that the original near-IR comb does not need to be self-referenced and the CEO frequency does not even need to be known, which relaxes the need of an octave spanning comb and f-to-2f interferometer. In this paper we present our preliminary results of frequency noise characterization of singlemode mid-IR quantum cascade lasers (QCLs) to evaluate their suitability as a potential frequency reference for low noise microwave generation. QCLs are semi-conductor lasers with a near-zero Henry’s linewidth enhancement factor, resulting in narrow intrinsic linewidth. We investigated the frequency noise properties of commercial free-running DFB QCLs emitting in the 4.6-µm wavelength range and operated in CW mode near room temperature. The measurement was performed using the side of a carbon monoxide absorption line as a frequency discriminator. The measured frequency noise power spectral density (PSD) (Fig. 1) revealed a 1/f-behavior from 3 Hz up to 10 kHz and 1/f3/2 at higher frequencies, with a level of Fig. 1: Frequency noise PSD of a 4.6 µm DFB mid-IR 108 Hz2/Hz at 100 Hz and 102 Hz2/Hz at QCL operated in CW mode near room temperature. 10 MHz. Even though one cannot certify whether the white frequency noise level was reached, an upper limit of 100 Hz2/Hz can be inferred from our measurement. This measured PSD is a factor of 100 lower than the one reported by Bartalini et al. 2, which was obtained with a similar laser (4.3 µm DFB) but operated at 77 K (instead of 277 K for ours). From our measured frequency noise PSD, we calculate a FWHM linewidth of 650 kHz at 15-ms observation time 3, which is in good agreement with the spectral width of the heterodyne beat signal between two identical QCLs observed in an additional experiment. With such a low frequency noise spectral density, linewidth narrowing of the QCL is possible using a moderate feedback loop bandwidth on the order of 100 kHz, leading to the possibility to use this laser as an ultra-stable reference for all-optical microwave generation. 1 2 3

S. M. Foreman et al.,Optics Letters, Vol. 30, No. 5, pp. 570-572, 2005 S. Bartalini et al., Phys. Rev. Lett., vol. 104, pp. 083904, 2010 G. Di Domenico et al., Applied Optics, Vol. 49, No. 25, pp. 4801-4807, 2010

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