Mid-Infrared Frequency Comb Generation and Spectroscopy with Few-Cycle Pulses and ${\chi }^{(2)}$ Nonlinear Optics

The mid-infrared atmospheric window of $3–5.5\mu \mathrm{m}$ holds valuable information regarding molecular composition and function for fundamental and applied spectroscopy. Using a robust, mode-locked fiber-laser source of $<11\mathrm{fs}$ pulses in the near infrared, we explore quadratic (${\chi }^{(2)}$) nonlinear optical processes leading to frequency comb generation across this entire mid-infrared atmospheric window. With experiments and modeling, we demonstrate intrapulse difference frequency generation that yields few-cycle mid-infrared pulses in a single pass through periodically poled lithium niobate. Harmonic and cascaded ${\chi }^{(2)}$ nonlinearities further provide direct access to the carrier-envelope offset frequency of the near infrared driving pulse train. The high frequency stability of the mid-infrared frequency comb is exploited for spectroscopy of acetone and carbonyl sulfide with simultaneous bandwidths exceeding 11 THz and with spectral resolution as high as $0.003{\mathrm{cm}}^{-1}$. The combination of low noise and broad spectral coverage enables detection of trace gases with concentrations in the part-per-billion range.

Figure 1

(a) Experimental setup of MIR generation and offset frequency ($f_0$) detection. (b) An optical spectrum of the ultrashort pulse. (c) Offset frequency beat notes measured at 600, 900, and 3500 nm (resolution bandwidth, $\mathrm{RBW}=1\mathrm{kHz}$). (d) Measured intrapulse DFG spectra with a commercial FTS ($4{\mathrm{cm}}^{-1}/120\mathrm{GHz}$ resolution). All spectra are normalized to the peak signal. The measured power for each spectrum is indicated by the square markers, referencing the axis on the right. Atmospheric ${\mathrm{CO}}_2$ absorption is visible in the spectrum centered around $4.3\mu \mathrm{m}$. (e) Predicted MIR spectra, also normalized. LPF: Longpass filter.

Figure 2

Broad bandwidth MIR generation. (a) Using a chirped PPLN crystal, we generate broader bandwidth MIR light. The blue and yellow curves correspond to different chirp profiles. ${\mathrm{CO}}_2$ absorption is also visible in the yellow spectrum at $4.3\mu \mathrm{m}$. Powers for the two spectra are 1.3 mW (blue) and $360\mu \mathrm{W}$ (yellow). (b) Modeling of the experimental spectra with chirp profiles shown in the inset. The red dashed line is a modeled profile which covers this full spectral region. The chirp rate for each spectrum is $0.8\mu \mathrm{m}/\mathrm{mm}$ (blue), $1.45\mu \mathrm{m}/\mathrm{mm}$ (yellow), and $7\mu \mathrm{m}/\mathrm{mm}$ (red dashed).

Figure 3

Simulation results in time and frequency domain for the modeled chirp profile. (a) Time domain propagation of the NIR ultrashort pulse. (b) Time domain propagation of the MIR pulse. (c) Frequency domain propagation. Most light is generated near the middle of the crystal, but some higher frequency components arise near the end of the PPLN. (d) Time domain traces of the MIR pulse at the output of the PPLN (blue) and after 0.5 mm of Ge (red). The transform-limited pulse duration is 20 fs. (e) Left axis: MIR spectrum at the output of the PPLN. Right axis: Spectral phase of the pulse at the output of the PPLN (black dashed) and after 0.5 mm of Ge (red dashed).

Figure 4

Dual-comb spectroscopy. (a) Two MIR combs with $\mathrm{\Delta }{f}_{\mathrm{rep}}=50\mathrm{Hz}$ (20 ms interferograms) combine on a $50/50$ ${\mathrm{CaF}}_2$ beamsplitter. One combined beam is directly detected on a MCT detector to provide a reference spectrum. The offset frequencies ($f_0$) are also detected from this detector. By tuning and locking the oscillator $f_0$ away from the DCS heterodyne that resides in an rf band between 27 and 50 MHz, we avoid interference between the two signals. The other beam is sent through a 15 cm-long gas cell and detected on a second MCT detector. The Fourier transform of the time-domain signal is shown in the inset. (b) Acetone dual comb spectroscopy. Experimental (blue) and predicted (red) spectra. (c), (d) OCS dual-comb spectroscopy. The inset in (c) shows data recorded with comb-tooth resolution. The zoomed-in portion of the OCS data in (d) highlights good agreement over absorbance ranging 0.02 to 0.5 in only 90 seconds of averaging. The small absorption features are evidence of the ${}^{16}\mathrm{O}{}^{12}\mathrm{C}{}^{34}\mathrm{S}$ isotopologue. CW: continuous wave.