Mid-Infrared fiber-based optical frequency synthesizer Laboratorio di ottica non lineare e spettroscopia di precisione – Sez. Napoli Extension of a near infrared Optical frequency synthesizer (OFS) to the mid infrared region via difference frequency generation Idler beam Signal beam Difference-frequency MIR comb Amplified NIR frequency comb Pre-spectral broadening output Span = 1540-1580 nm Ptot = 0.7 W Ptooth = 14 mW Comb “rainbow” recorded by a mid-IR camera Span = 3030-3390 nm Ptot= 5 mW Ptooth = 200 pW Offset frequency f0 = 0 Pump beam Amplified NIR laser Phase-locked to a NIR-OFS tooth N1 (l= 1055 nm Ptot = 0.8 W) PPLN crystal A second cw 1.5-mm, copropagating with the signal beam, is used to characterize the mid-infrared comb. In this configuration, a second DFG process with the pump radiation at 1055 nm occurs, that produces a mW-level cw idler beam around 3 mm. As a consequence, two rf beats at wcw-wn and wn+1-wcw are encoded in the idler radiation, respectively between the DFG cw at wcw and its two closest comb teeth at wn and wn+1. Detection of these beat notes while tuning the 1.5-mm cw wavelength allows to estimate the DFG comb span. Direct comb referencing of a mid-infrared cw-source for absolute frequency spectroscopy Any cw MIR source can now be comb-referenced by detecting the rf beat note fbeat with the closest MIR tooth and feeding back currections through a phase locking loop (PLL). In our case, by closing a PLL on the 1.5 cw laser the whole mid-infrared cw radiation is comb-referenced. The scheme is shown on the left. Details on experimental apparatus are on the right After phase-locking the 3-mm probe radiation to the MIR comb, its frequency can be tuned across a molecular resonance by changing the comb mode spacing fr by discrete steps (M is determined once for all). For each point, the absolute frequency (x-scale value) can be retrieved from the readings of fr , fbeat and f 1beat on frequency counters, while the DFG power transmitted through the cell is simultaneously recorded for the y-scale value. This acquisition procedure takes less than one minute. As an example, the recording of the (0000-0010) P(6) CH4 transition in pure gas at 50 mTorr is shown besides. A Doppler profile is also fitted to the experimental points to extract the line-center frequency. The result is consistent with the value provided by the HITRAN database