PORTFOLIO

During this project, we developed a prototype of a microwave-optical frequency standard based on quantum cascade lasers and optical frequency combs. Thanks to the small physical dimensions and the excellent beam quality in terms of intrinsic linewidth, such lasers constitute an important building block for future optical frequency standards. For this purpose, we combined a QCL with an external Fabry- Perot cavity for the stabilization and linewidth reduction of the QCL. We are now about to stabilize (lock) the repetition frequency of the comb so that the difference between two distant teeth corresponds exactly to the QCL emission frequency. Due to the favorable carrier dynamics within the QC laser cavity - mainly based on the fact that only one type of carriers is used - we observed that its linewidth is very narrow and thus perfectly suited for frequency stabilization. Different measurements have actually confirmed this hypothesis.

Several potential end users have already indicated their interest in such devices, and the project has also initiated several important collaborations in the Time and Frequency domain within Switzerland, for instance with METAS, CSEM, and the HE-ARC. We have therefore paved the way for a prototype compact atomic clock based on a classical frequency comb equipped with a mid-infrared quantum cascade laser.

The frequency stabilized infrared lasers described in this project were useful for different real world products. Particularly, optical frequency standards based on such infrared lasers have various interesting applications, for instance in spectroscopy and high-precision time-keeping. Therefore, they can potentially result in a successful future commercialization of miniaturized atomic clocks. Since the described project felt somewhat in between basic research and the activities of a spin-off company, financing from Gebert Rüf Stiftung was ideal to bridge this funding gap.

We have fabricated and tested state-of-the-art singlemode quantum cascade lasers emitting at mid-infrared wavelengths around 4.6 m. They had threshold currents of 300 mA at 10 °C and delivered nearly 10 mW of continuous wave output power. Such lasers were perfectly suited for our experiments. Not only this wavelength was compatible with the absorption properties of the most important non-linear optical material, namely periodically poled lithium-niobate, but it also allowed one to perform stabilization onto a carbon-monoxide absorption line. By using this material for optical difference frequency generation, we are going to produce - in the very near future - an optical frequency comb at mid-infrared wavelengths. The latter will then be frequency-locked to the stabilized quantum cascade laser.

In a preliminary linewidth measurement, the emission of two slightly detuned quantum cascade lasers were precisely collimated using two high precision ZnSe lenses. The laser beams were then collinearly overlaid and detected in a fast Mercury- Cadmium-Telluride detector. The latter allowed us to perform a heterodyne mixing from the multi-THz optical frequencies down to the radio frequency range. Measurement of the laser spectrum was thus possible at frequencies on the order of 200 kHz. We observed a linewidth of 3 MHz integrated over 1 s and roughly 500 kHz integrated over 4 ms. These results corresponded exactly to our calculations based on a slightly modified Schawlow-Townes model.
In a more sophisticated line of experiments, we sent one of these laser beams through a high finesse Fabry-Perot resonator filled with carbon-monoxide and used the latter as a frequency discriminator. Such an experiment enabled us to convert the frequency noise of the laser into measurable intensity noise. The frequency noise power spectral density revealed a 1/f-like behavior, with a noise level of 2x108 Hz2/Hz at 100 Hz dropping down to a very low level below 100 Hz2/Hz at 10 MHz. Via Fourier transformation, the linewidth of the free-running laser could be determined to roughly 500 kHz. These experiments therefore confirmed the heterodyne measurements from above in an excellent way. Due to the nearly zero linewidth enhancement factor of quantum cascade lasers, this linewidth was roughly a factor of 100 smaller than in a standard diode laser. Finally, the measured noise behavior showed that a moderately fast feedback loop bandwidth on the order of 100 kHz is sufficient for a drastic reduction of the laser linewidth.

These findings are extremely important and promising for further applications in precise time-keeping. Currently, we are on the way to perform a frequency locking between two distant teeth of the frequency comb and the quantum cascade laser. As a final step, the laser will be frequency stabilized to an ultra-stable external Fabry-Perot resonator. Together with the unique know-how of the Laboratory for Time and Frequency at the University of Neuchatel, this project unified therefore several key ingredients for a highly innovative applied research project. It also led to several important collaborations in further applications.

M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, and H. Melchior, Continuous wave operation of a mid-infrared semiconductor laser at room temperature, Science, vol. 295, pp. 301 - 305, 2002;
D. Hofstetter, M. Graf, T. Aellen, J. Faist, L. Hvozdara, and S. Blaser, 23 GHz operation of a room temperature photovoltaic quantum cascade detector at 5.35 m, Appl. Phys. Lett., vol. 89, no. 6, pp. 061119, 2006;
P. Kroetz, D. Stupar, J. Krieg, G. Sonnabend, M. Sornig, F. Giorgetta, E. Baumann, M. Giovannini, N. Hoyler, D. Hofstetter, R. Schieder, Applications for quantum cascade lasers and detectors in mid-infrared high-resolution heterodyne astronomy, Appl. Phys. B., vol. 90, no. 2, pp. 187-190, 2008;
F.R. Giorgetta, E. Baumann, M. Graf, D. Hofstetter, Q. Yang. C. Manz, K. Köhler, H.E. Beere, D.A. Ritchie, E. Linfield, G. Davies, Y. Fedoryshyn, H. Jäckel, M. Fischer, J. Faist, and D. Hofstetter, Quantum Cascade Detectors, Journal of Quantum Electronics, vol. 45, no. 8, pp. 1029-1042, 2009;
L. Tombez, J. Di Francesco, S. Schilt, G. Di Domenico, J. Faist, P. Thomann, and D. Hofstetter, Frequency noise of free-running room temperature DFB quantum cascade lasers, Optics Letters, Vo. 36, No. 16, August 2011.

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