Optical frequency comb is one of the most powerful technologies available for modern molecular spectroscopy. Simultaneous measurements of multiple molecular transitions can be done with laser precision using the hundreds of thousands of highly coherent spectral lines produced by a single laser instrument, an optical frequency comb (OFC) generator. We develop methods and instrumentation for high-resolution OFC spectroscopy in the mid-infrared molecular fingerprint regions.
- Fully stabilized mid-infrared optical frequency comb. Opt. Express 25, 4190 (2017)
- Direct OFC spectroscopy: New method (photoacoustic OFC spectroscopy) and the first reported infrared spectrum of radiocarbon methane, 14CH4. Opt. Lett. 44, 1142 (2019)
- OFC-assisted spectroscopy: Novel methods for double-resonant molecular spectroscopy. Opt. Express 25, 4688 (2017), Opt. Express 26, 29086 (2018)
Near-infrared OFCs are conveniently produced by mode-locked femtosecond lasers, such as Er- and Yb-fiber lasers that emit light at 1 µm and 1.5 µm spectral regions, respectively. (Our mode-locked laser infrastructure is described here). Molecular spectroscopy experiments typically require light in the UV/VIS (electronic transitions) or mid-infrared (MIR, rotational-vibrational transitions). Nonlinear optics can be used to convert near-infrared light into these spectral regions, which are difficult to access directly with mode-locked lasers. In our lab, we develop and use MIR OFCs based on synchronously pumped femtosecond optical parametric oscillators, SP-OPOs. An example of a degenerate (subharmonic) SP-OPO is schematically shown below in Fig. 1a. This type of SP-OPO divides the frequency of input pump light by two, and leads to a broad spectrum whose repetition frequency and offset frequency are automatically locked to the pump comb. We have used this method to produce a fully stabilized OFC in the 3 to 4 µm atmospheric transmission window , which is one of the most important regions for molecular spectroscopy .
Fig. 1. Synchronously-pumped OPO schemes for mid-infrared comb generation. (a) Degenerate (subharmonic) SP-OPO, and (b) singly-resonant SP-OPO.
Another SP-OPO technology used in our lab is based on singly resonant optical parametric oscillation. In this case, the output light contains two clearly separate spectral features, the signal and idler combs (Fig. 1b). Of these, the center wavelength of the idler comb is typically tunable in the MIR region. An advantage of the singly-resonant SP-OPO configuration is high average output power, which can exceed 1 W. On the other hand, unlike with the subharmonic SP-OPO, the offset frequency of the comb is not automatically stabilized.
Both the subharmonic and singly resonant SP-OPO need to have the OPO cavity length locked to the repetition rate of incoming pump pulses, in order to maintain the synchronous pumping condition for extended periods of time. A common method is dither-and-lock method, which requires the OPO cavity length (or pump laser repetition frequency) to be slightly dithered. This method is robust and simple to implement, but leads to unwanted spectral jitter in the SP-OPO output. We have developed a dither-free method for SP-OPO cavity locking in order to have more stable output spectrum for spectroscopic applications .
Optical frequency comb spectroscopy can be divided into two main categories: In direct OFC spectroscopy, the frequency comb is used as a light source to probe molecules or atoms under investigation. In OFC-assisted spectroscopy, the comb is used as a stable optical frequency reference, in order to improve the precision and accuracy of molecular spectroscopy carried out by a tunable continuous-wave laser.
Direct OFC spectroscopy
An example of direct OFC spectroscopy in the 3 to 4 µm mid-infrared region is illustrated in Fig. 2, which shows an extract of the first infrared spectrum of radiocarbon methane (14CH4) ever recorded. In this work, we combined our high-power SP-OPO frequency comb with cantilever-enhanced photoacoustic spectroscopy (PAS; see the Photoacoustics page for more details about cantilever-enhanced PAS). The PAS cell is placed inside a scanning Michelson interferometer, which makes it possible to simultaneously record all molecular absorption lines that fall within the comb spectrum, analogous to conventional Fourier-Transform Infrared (FTIR) spectroscopy. The main difference compared to traditional FTIR spectroscopy is that a measurable PAS signal is only generated at frequencies where the gas sample absorbs, which makes the OFC-PAS method essentially background free. In addition, the use of an optical frequency comb instead of an incandescent light source enables a greatly improved spectroscopic selectivity and detection sensitivity. The experimental implementation of the OFC-PAS method is described in more detail in references [4-5].
Fig. 2. Photoacoustic spectrum of a mixture of 12CH4 and 14CH4 in nitrogen. The light source used in the measurement was a high-power infrared optical frequency comb. The absorption lines of 14CH4 are indicated by asterisks.
We have developed the OFC-PAS method particularly for measurements of small trace gas concentrations and with small sample volumes. When combined with a high-power laser source, cantilever-enhanced PAS provides extremely good detection sensitivity – down to the parts-per-trillion level or even below  – and requires just a few milliliters of sample gas. This makes the method particularly suitable for measurements of radioactive gases and other samples where the sample amount is limited due to safety regulations or other reasons. The first measurements of 14CH4 spectrum pave the way for real-time on-site detection of radiocarbon methane, for instance for applications in biogas biofraction detrmination, CH4 source apportion, and monitoring of radioactive CH4 emissions from light water nuclear reactors .
The spectral resolution of sub-Doppler molecular spectroscopy is often limited by excess linewidth and frequency jitter of the laser source(s) used for the experiment. Figure 3 illustrates an example of OFC-assisted sub-Doppler spectroscopy of acetylene. In this example, we studied symmetric vibrational states of acetylene, which are infrared inactive and thus cannot be accessed by one-photon transition from the vibrational ground state [7-9]. Instead, we use a ladder-type double-resonant excitation by two photons: The acetylene molecules are first pumped to an antisymmetric vibrational state using an in-house built continuous-wave optical parametric oscillator (CW OPO), which is widely tunable in the 3 to 4 µm wavelength region and has a linewidth of 1 MHz. The molecules are subsequently excited to the final symmetric vibrational state by a near-infrared external cavity diode laser (ECDL) at approximately 1.5 µm. This second step of the ladder-type transition is probed using cavity ring down spectroscopy (CRDS), while the first step to the intermediate energy state is carried out by passing the mid-infrared beam of the CW OPO through the acetylene gas sample once.
Fig. 3. (a) Layout of the 2-resonance laser spectrometer and (b) a sub-Doppler spectrum of acetylene measured using the setup.
Owing to double-resonant excitation and narrow linewidth of the CW-OPO, the absorption lines observed by CRDS are free from the Doppler broadening. In practice, the CW OPO frequency is tuned in resonance with the first transition, and CRDS spectrum of the second transition is recorded while scanning the ECDL frequency. In order to observe the narrow sub-Doppler absorption profiles without substantial instrumental broadening, the CW OPO frequency is locked to a fully stabilized mid-infrared OFC, which is based on the degenerate SP-OPO. For the same reason, the ECDL is tightly phase-locked to a fully-stabilized near-infrared OFC. Referencing of both the ECDL and CW OPO frequencies to an optical frequency comb improves not only the resolution but also the signal-to-noise ratio of the double-resonance absorption experiment . Additional improvement of the signal quality by background suppression can be obtained by using so-called step-modulated CRDS .
 M. Vainio and L. Halonen, “Mid-infrared optical parametric oscillators and frequency combs for molecular spectroscopy,” Phys. Chem. Chem. Phys. 18, 4266 (2016)
 M. Vainio and J. Karhu, "Fully stabilized mid-infrared frequency comb for high-precision molecular spectroscopy," Opt. Express 25, 4190 (2017)
 M. Vainio and L. Halonen, "Stabilization of femtosecond optical parametric oscillators for infrared frequency comb generation," Opt. Lett. 42, 2722 (2017)
 I. Sadiek, T. Mikkonen, M. Vainio, J. Toivonen, A Foltynowicz, "Optical frequency comb photoacoustic spectroscopy," Physical Chemistry Chemical Physics 20, 27849 (2018)
 J. Karhu et al., "Broadband photoacoustic spectroscopy of 14CH4 with a high-power mid-infrared optical frequency comb," Opt. Lett. 44, 1142 (2019)
 T. Tomberg, M. Vainio, T. Hieta, L. Halonen, "Sub-parts-per-trillion level sensitivity in trace gas detection by cantilever-enhanced photo-acoustic spectroscopy," Scientific reports 8, 1848 (2018)
 J. Karhu et al., “Double resonant absorption measurement of acetylene symmetric vibrational states probed with cavity ring down spectroscopy,” The Journal of Chemical Physics, 144, 244201 (2016)
 J. Karhu, M. Vainio, M. Metsälä, L. Halonen, “Frequency comb assisted two-photon vibrational spectroscopy,” Opt. Express, 25, 4688 (2017)
 J. Karhu, K. Lehmann, M. Vainio, M. Metsälä, L. Halonen, “Step-modulated decay cavity ring-down detection for double resonance spectroscopy,” Opt. Express, 26, 29086 (2018)