Optical frequency combs are innovative tools for broadband spectroscopy because a series of comb modes can serve as frequency markers that are traceable to a microwave frequency standard. However, a mode distribution that is too discrete limits the spectral sampling interval to the mode frequency spacing even though individual mode linewidth is sufficiently narrow. Here, using a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region, we achieved a spectral sampling interval equal to the mode linewidth rather than the mode spacing. The spectrally interleaved THz comb was realized by sweeping the laser repetition frequency and interleaving additional frequency marks. In low-pressure gas spectroscopy, we achieved an improved spectral sampling density of 2.5MHz and enhanced spectral accuracy of 8.39x10⁻⁷ in the THz region. The proposed method is a powerful tool for simultaneously achieving high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.

r0pt Figure We present a new scheme of mid-infrared dual-comb Fourier transform spectroscopy. It provides broad-spectral bandwidth absorption and dispersion spectra at a resolution limited by the Doppler width of the molecular profiles, with a negligible instrumental lineshape and self-calibration of the wavenumber scale within the accuracy of an atomic clock. The averaging times can be arbitrarily long and the spectra do not involve any phase corrections or other types of processing that could generate systematic effects or artifacts. Such results build on our concept of feed-forward dual-comb spectroscopyZ. Chen, M. Yan, T. W. Hänsch, and N. Picqué, A phase-stable dual-comb interferometer, preprint at arXiv:1705.04214 (2017). first demonstrated in the near-infrared 1.5 μm region. Here, we illustrate our latest results around 3000 cm⁻¹ measured at a signal-to-noise ratio that exceeds 1000 and a resolution of 3 10⁻³ cm⁻¹. A spectrum with resolved comb lines of acetylene in the region of the ν₃ band, measured within 35 minutes, is shown over its entire span in Fig.a. In an expanded view (Fig.b), the P(13) line of the ν₃ band of ¹²C₂H₂ is sampled by the individual comb lines of perfect cardinal sine instrumental line shape. Our technique opens up novel opportunities for broadband and precise investigation of molecular profiles.

---

Footnotes:

Z. Chen, M. Yan, T. W. Hänsch, and N. Picqué, A phase-stable dual-comb interferometer, preprint at arXiv:1705.04214 (2017).,

l0pt Figure We use a pair of highly-coherent subharmonic GaAs optical parametric oscillators with an instantaneous span 3.1-5.5 μm to demonstrate fast acquisition of 350,000 mode-resolved spectral data points and perform parallel detection in a mixture of 22 molecular species including N₂O, NO, CO, OCS, CH₄, C₂H₆, C₂H₄, C₂H₂, CO₂, H₂O, and their isotopologues containing ³³S, ³⁴S, ¹³C, ¹⁵N, ¹⁸O, ¹⁷O, and ²H (deuterium) isotopes. We demonstrate all the benefits of the mid-IR dual-comb spectroscopy including broadband coverage, fast acquisition of massive spectral data, ppb-level sensitivity, comb-tooth resolved spectra (with finesse 4000) and absolute optical frequency referencing to atomic clock. We sampled molecular spectra with the comb-tooth spacing (115 MHz), however, thanks to the narrow comb teeth (3-kHz absolute and 25-mHz relative linewidth between the two combs), much higher spectral resolution can be obtained in the scanning comb-tooth resolved mode. The Figure shows: (a) schematic of the dual-comb setup, (b) log-scale optical spectrum retrieved from a single coherently-averaged interferogram with an evacuated multipass gas cell, and (c) when the cell was filled with a mixture of gases. The two spectra are vertically offset for clarity.

l0pt Figure Dual-comb spectroscopy (DCS) using quantum cascade laser (QCL) or interband cascade laser (ICL) frequency combs presents an opportunity for miniaturized and fully electronically controlled broadband spectrometers with no moving-parts and all-electrical control that can serve as alternatives to systems based on broadly tunable lasers or external cavity lasers. In contrast to systems based on conventional tunable laser sources, a DCS system gives instantaneous access to the optical information across the entire spectral bandwidth through a multi-parallel heterodyning process, which enables acquisition times in the μs-range. The main drawback of quantum and interband cascade laser frequency combs is the excess phase noise observed, which affects the averaging capabilities of the DCS systems. In principle, coherent averaging can be implemented using active feedback control, but this adds additional complexity to the systems, which negates the intrinsic advantage of the monolithic comb emitters. Here, we report on DCS using free-running lasers, where a coherent averaging algorithm is implemented to correct for phase noise via purely computational means. The correction algorithm leverages the temporal mode coherence masked by the noise and is generally applicable to all DCS systems affected by excessive phase noise. Spectroscopic detection of molecular species with broadband spectral signatures in the mid-infrared will be discussed. Further details on the spectroscopic systems, as well as a discussion of current limitations and future directions of this DCS technique will be given.

I will be discussing our recent work on the development and application of frequency-agile optical frequency combs generated with electro-optic modulators. Through the use of an arbitrary waveform generator we are able to digitally control the resulting frequency comb and then employ it in a self-heterodyne configuration in which we can exploit the common-mode nature of the probe and local oscillator beams for facile coherent averaging. Importantly, this approach allows for complete control over the comb tooth spacing, down to the 100’s of Hz level for ultrahigh resolution measurements. We have recently applied these approaches to atomic spectroscopy as well as for physical metrology using optical microcavities, areas in which the agility and resolution of these frequency combs are ideally suited.

r0pt Figure Single-cavity dual-comb fiber laser can generate two ultra-short pulse trains from the same cavity, which could have relatively low common-mode noises and good mutual coherence. They had been demonstrated to be applicable to various dual-comb metrology applications including optical spectroscopyX. Zhao, et al., Optics Express, vol. 24, 21833, 2016. absolute distance measurementB. Lin, et al., IEEE Photonics Journal, 99, 2017.nd so on. Here, broadband dual-comb spectroscopy measurement using a broadband mode-locked dual-comb fiber laser based on hybrid pulse formation mechanisms is demonstrated. Through the dispersion management in the cavity, the bandwidths of both pulses are greatly increased. Dual-comb pulse with broader spectra can be generated when the pump power is above the threshold power. After jointly being amplified by an Erbium-doped fiber amplifier, the 3-dB spectral bandwidths, at the center wavelength of 1550 nm and 1555 nm, have been broadened from 4.4 nm and 1.3 nmY. Liu, et al., Optics Express, vol. 24, 21392, 2016.o 58.8 nm and 8.5 nm, respectively. Also, the 10-dB width of the overlapped spectra is over 16 nm, covering 1546 nm to 1562 nm. The repetition rate difference of two pulse trains is 37 Hz, because of the small difference in their center wavelengths and the low cavity dispersion. Then, typical asynchronous experimental setup is used to measure transmission spectrum of an on-chip microring resonator by averaging over 30 interferograms. All the resonance spectral features are clearly resolved. This kind of dual-comb fiber lasers could offer extra options for low-complexity, dual-comb metrology applications.

---

Footnotes:

X. Zhao, et al., Optics Express, vol. 24, 21833, 2016., B. Lin, et al., IEEE Photonics Journal, 99, 2017.a Y. Liu, et al., Optics Express, vol. 24, 21392, 2016.t

Dual comb spectroscopy holds the promise of bringing the bandwidth, resolution and sensitivity advantages of direct frequency comb spectroscopy to the mainstream. Replacing complex Fourier transform interferometer (FTIR) or virtually imaged phased array (VIPA) detectors with a simple photo diode potentially leads to compact, robust, field-usable measurement setups. In exchange, some of that complexity is then shifted to the laser source, usually leading to the requirement of two identical mode-locked lasers, actively stabilized to each other. However, such a dual comb source can be significantly simplified by generating two pulse trains using a single laser cavity, with the advantage of passive mutual coherence due to common-mode noise cancellation in the down converted radio frequency (RF) comb. We introduce a new, particularly flexible, method to generate a single-cavity dual comb, exploiting independent mode-locking within two isolated spectral regions of the gain profile, created by intra-cavity spectral filtering. By setting a non-zero cavity dispersion we generate stable pulse trains with different repetition rates. These are made to overlap spectrally via successive spectral broadening in a non-linear fiber, yielding a compact, fully useable dual comb source. The underlying concept is generally applicable to nearly all kinds of passively mode-locked lasers, including state-of-the-art all-fiber types. We demonstrate its feasibility in a nonlinear polarization evolution (NPE) mode-locked Yb:fiber laser. Spectral filtering is introduced in the free-space section of the grating compressor by mechanically blocking a central frequency band using a needle-shaped object. Fine control over its position and thickness allows for easy tuneability of the spectral separation and dual-laser operation. The laser features two independently mode-locked pulse trains centered around 1015 nm and 1040 nm, respectively, with a spectral width of about 10 nm each. Their pump-power limited repetition rates are around 20 MHz with a difference tuneable from 10 kHz down to 750 Hz. In a proof-of-principle experiment the laser output was spectrally broadened and further amplified, producing a spectral overlap around 1030 nm with an average power of 40 mW over a bandwidth of 10 nm. Once properly stabilized and brought to higher repetition rates we expect to produce a stable downconverted RF comb. Even more robust implementations might for example involve the integration of spectral filtering in form of fiber Bragg gratings or dielectric filters into and all-fiber figure-9 laser. Nonlinear conversion schemes will further extend the spetral range into the Mid-IR or XUV regions.

Coherent Raman microscopy can intrinsically enable label-free imaging by measuring vibrational spectra of biomolecules. Nevertheless, trade-off between high chemical-specificity and high imaging-speed currently exists in transition from spectroscopy to spectroscopic imaging when capturing dynamics in complex living systems. Here, we present a dual-comb scheme to substantially beat this trade-off and facilitate high-resolution broadband coherent anti-Stokes Raman spectroscopic imaging by combining dual-comb asynchronous optical scanning and phase-controlled spectral focusing excitation. A rapid measurement of vibrational micro-spectroscopy on sub-microsecond scale over a spectral span  700 cm⁻¹with solid signal-to-noise ratio provides access to well-resolved molecular signatures within the fingerprint region. We demonstrate this high-performance spectroscopic imaging for spatially inhomogeneous distributions of chemical substances. This technique offers an unprecedented route for broadband CARS imaging with hundreds of kHz spectroscopic refresh rate using available 1-GHz oscillators.

Since its inception, dual comb spectroscopy (DCS) has enabled a variety of new spectroscopic applicationsIan Coddington, Nathan Newbury, and William Swann, "Dual-comb spectroscopy," Optica 3, 414-426 (2016) Here, we report on the extension of a broad bandwidth, high coherence DCS system to mid-infrared (MIR) spectroscopy of liquid-phase samples in the C-H stretch region ( ∼ 2800 to 3100 cm⁻¹). Each comb originates from a self-referenced Er:fiber oscillator with a repetition rate of 200 MHz. Using a two-branch difference frequency generation (DFG) configuration, each comb is downconverted to the MIR, which yields output tunable from 2.6 to 5.2 μm with up to 100 mW in a single instantaneous bandwidth. Using attenuated total reflectance (ATR) coupling techniques, spectra of the pure isopropanol C-H stretch are recorded in 1.5 second integration periods consisting of  ∼ 100 phase-corrected, averaged interferograms and show excellent agreement with reference spectra. A fitted average of 50 integration periods is shown in the abstract image. Initial estimates show that baseline fluctuations on the order of 2% will dominate sensitivity models. Further, we demonstrate progress towards the use of our DCS system as an analytical tool for organic reaction monitoring. We use an aldol condensation reaction as a test case where we detect C_sp³-H stretches in the reactant (acetone) and C_sp²-H stretches in the product (mesityl oxide). Methods for quantitative analysis of the aldol condensation will be discussed. Finally, this talk will explore the compatibility of DCS and newly-developing micro-reactor waveguide technology.

---

Footnotes:

Ian Coddington, Nathan Newbury, and William Swann, "Dual-comb spectroscopy," Optica 3, 414-426 (2016).