Frequency comb assisted two-photon vibrational spectroscopy

Abstract

We report a setup for high-resolution two-photon spectroscopy using a mid-infrared continuous wave optical parametric oscillator (CW-OPO) and a near-infrared diode laser as the excitation sources, both of which are locked to fully stabilized optical frequency combs. The diode laser is directly locked to a commercial near-infrared optical frequency comb using an optical phaselocked loop. The near-infrared frequency comb is also used to synchronously pump a degenerate femtosecond optical parametric oscillator to produce a fully stabilized mid-infrared frequency comb. The beat frequency between the mid-infrared comb and the CW-OPO is then stabilized through frequency locking. We used the setup to measure a double resonant twophoton transition to a symmetric vibrational state of acetylene with a sub-Doppler resolution and high signal-to-noise ratio.© 2017 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited. Bartalini, and P. De Natale, "Subkilohertz linewidth room-temperature mid-infrared quantum cascade laser using a molecular sub-Doppler reference," Opt. Lett. 37(23), 4811-4813 (2012). 9.

Step-modulated decay cavity ring-down detection for double resonance spectroscopy

“…Weak absorption by a gas sample inside the cavity can then be calculated from the difference in the ring-down rate between the cavity with the gas sample and that of the empty cavity. The CRDS method has also been used to measure weak multiphoton transitions, for example in Raman [2] and double resonance spectroscopy [3]. Two-photon transitions can be used to measure spectral features with resolution beyond the Doppler limit [4], without the need of cooling the sample, and they allow accessing energy states that are inaccessible with single photon transitions due to spectroscopic selection rules [5].…”

“…The fast scan speed can be especially advantageous when measuring narrow sub-Doppler transitions, since instabilities in the pump laser wavelength and power may degrade the line shape during the scan over the transition line profile. This degradation can be alleviated for example by using stabilized lasers to excite the transitions, but this generally requires significantly more complex measurement setups [3].…”

High-precision methanol spectroscopy with a widely tunable SI-traceable frequency-comb-based mid-infrared QCL

“…This allows the reference laser ultra-low frequency instability to be transferred to the QCL and our measurement absolute frequency scale to be controlled with an uncertainty at the hertz level, thanks to the traceability to the primary frequency standards developed at LNE-SYRTE [41]. This is a factor of 250 better than GPS-disciplined radiofrequency reference based measurements [15,[17][18][19][20]67,68] and an improvement of more than six orders of magnitude compared to using traditional frequency synthesizers. These two methods in addition are not SI-traceable.…”

Continuous-wave optical parametric oscillators for mid-infrared spectroscopy

“…If a better stability or accurate knowledge of the absolute frequency is needed, the CW-SRO can be locked to a fully stabilized optical frequency comb, OFC. [23][24][25][26] True phase locking of a CW-SRO is technically quite challenging, because a pump laser with fast frequency modulation capability would be required. However, simple frequency locking or tracking can be realized by counting the beat frequency of the CW-SRO idler and the OFC.…”

“…24 In addition, we have developed a two-photon method, which combines two OFC-stabilized sources -a CW-SRO and a near-infrared diode laser -for Doppler-free molecular spectroscopy. 26,30 This new method gives access to vibrational energy states that are inaccessible directly by single-photon absorption. As an example, we have accurately determined energy levels of symmetric vibrational states of acetylene, C2H2.…”