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Dipartimento di Fisica - Politecnico di Milano

Mid-IR lAser for optiCaL spEctroscopy and metrology (MIRACLE)

The MIRACLE lab aims at the development and characterization of innovative laser sources with emission in the mid infrared spectral range, based both on chalcogenide crystals (ZnS, ZnSe, CdSe) doped with divalent chromium (Cr) and iron (Fe) ions and on special fibers doped with trivalent rare earth ions such as dysprosium (Dy), thulium (Tm), holmium (Ho) and praseodymium (Pr), for the direct synthesis of optical frequency combs in the mid-infrared spectral region (from 2 to 10 µm). Using the most sophisticated optoelectronic techniques for absolute frequency stabilization, these sources are used for high-precision and broadband molecular spectroscopy in the so called "molecular firgerprint region" from 2 to 15 µm. The laser sources are designed, manufactured and characterized with specific attention to the performance in terms of spectral purity and wide emission bandwidth and operate mostly in pulsed mode with a pulse duration of a few tens of femtoseconds (passive mode-locking regime). In addition, high-power optical amplifiers (in the chirped-pulse-amplification configuration) operating in the spectral region between 1.5 and 3.3 µm are developed for the realization of optical frequency combs with single-comb-mode power levels larger than 10 µW. This power content allows the implementation of novel direct-frequency-comb-spectroscopy (DFCS) methods able to detect molecular absoprtions in an extremely wide optical spectrum with very high sensitivity levels. These DFCS methods find interesting  applications in both fundamental physics (determination of fundamental constants, frequency metrology, and in general of atomic / molecular physics) and in applied physics (characterization of materials, gas analysis, remote atmospheric sensing and sensors in general). For example, with specific reference to fundamental physics, MIRACLE is involved in the development of comb-based spectroscopic methods for the determination of molecular spectral features with extreme characteristics: narrower than 10 Hz in the spectral region around 8.6 µm for the measurement of the constancy of the proton/electron mass ratio; broader than hundreds of MHz for the determination of the Boltzmann constant and primary thermometry in the spectral region from 1.5 to 3.3 µm.  

Coherent supercontinuum generation in the wavelength range from 1.5 to 4 µm using femtosecond Cr:ZnSe laser. 



The MIRACLE lab is equipped with: i) Fourier-Transform InfraRed (FTIR) spectrometer in the spectral range from from 30 cm-1 to 20000 cm-1 (from 500 nm to 300 µm) with a frequency resolution of  0.07 cm-1; ii) optical frequency comb syntheziser in the optical spectral range from 1 to 2 µm based on Er-fiber femtosecond laser systems; iii) synthesis of a tunable optical comb in the mid-IR spectral region from 7.5 to 14 µm; iv) prototype of a femtosecond Cr:ZnSe laser at 2.4 µm; v) novel Dy:ZBLAN fiber laser in the spectral range from 3 to 3.3 µm; vi) single-frequency quantum cascade lasers at  8.6 µm; vii) 10-W continuous-wave (CW) Er:ZBLAN fiber laser at 2.82 µm, 20-W CW Er:fiber laser at 1.56 µm, 20-W CW Tm:fiber laser at 1.94 µm; viii) mid-IR optical cavities and multipass cells; ix) general purpose opto-elettronics instrumentations (electrical spectrum analyzer, digital oscilloscopes, frequency counters, GPS-disciplined Rb clock, monochromator and wavemeter).

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