Dual-comb spectroscopy based on quantum-cascade-laser frequency combs

Remote sensing of trace gases in the atmosphere can be performed with numerous spectrometers relying on different sources of light. Incoherent sources such as those found with typical Fourier transform spectrometers provide broad spectral coverage, thus allowing to measure spectral signatures from multiple species simultaneously. However, this comes at the cost of limited sensitivity and spectral resolution. On the other hand, coherent sources such as lasers offer high spectral brightness and resolution, resulting in high sensitivity and selectivity at the cost of limited spectral coverage. Developed since the advent of the optical frequency comb (OFC) 25 years ago, state-of-the-art spectrometers operating with OFCs as probing light sources combine high sensitivity, high spectral resolution and broad spectral bandwidth. Among all comb-based spectroscopic techniques, dual-comb spectroscopy (DCS) does not require any dispersive or moving optical component to record a spectrum, allowing for relatively small footprints and mechanically robust instruments. This makes dual-comb spectrometers particularly suited for remote sensing [1] and field-deployed operation outside of the optical laboratory [2].Here, we present the recent technical developments of a near-infrared dual-comb spectrometer for open-path monitoring of greenhouse gases above the city of Heidelberg. The instrument is located at the top of the Institute of Environmental Physics in Heidelberg University campus. The light from two fibered OFCs, spanning 1.58-1.7 µm, is coupled into free space with a telescope, and propagates along a 1.5 km path to a retroreflector array. The reflected signal is picked up by the telescope and coupled back into fiber for detection and data acquisition. We discuss performance of the instrument and the results of our upcoming measurement campaign.[1] G. B. Rieker et al., "Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths," Optica 1, 290-298 (2014), DOI: 10.1364/OPTICA.1.000290.[2] S. Coburn et al., "Regional trace-gas source attribution using a field-deployed dual frequency comb spectrometer," Optica 5, 320-327 (2018), DOI: 10.1364/OPTICA.5.000320.

Mid infrared frequency combs allow for high resolution absorption spectroscopy of molecular species, which have strong signatures in this spectral region. Dual comb spectroscopy can provide broadband and high-resolution capability, but requires two fully stabilized frequency combs which adds complexity to the system. Previous work has demonstrated that frequency combs coupled with a high resolution spectrometer, consisting of a virtually imaged phased array (VIPA) along with a grating, can perform time-resolved, broadband and high- resolution absorption spectroscopy with a single frequency comb. The VIPA spectrometer disperses the spectrum in two dimensions and images it onto a focal plane detector array. If the comb teeth can be resolved, the VIPA is easily calibrated and provides comb-tooth resolved resolution and accuracy. However, in previous work, the repetition rate of the laser sources used was too low to be resolved directly, and additional passive filter cavities had to be employed to increase the effective repetition rate of the frequency comb. In this work we use a fully stabilized mid infrared frequency comb based on a 1.6 GHz repetition rate modelocked vertical external cavity surface emitting laser (VECSEL) and difference frequency generation to produce an off set free comb in the 3- 4 micron wavelength range. The source is directly coupled to the VIPA spectrometer to provide comb-tooth resolved absorption spectroscopy. We discuss the system's performance in gas absorption spectroscopy and its time resolving capabilities, which are limited only by the speed of the detector system.

In this Letter, we propose a high-resolution dual-comb spectroscopy (DCS) in the mid-infrared (MIR) region. A broadband electro-optic frequency comb (EOFC) with a line spacing of 13 GHz is generated in the near-infrared region. The injection locking technique is employed to lock the distributed feedback (DFB) laser to each comb line of the 34 comb lines as the seed laser for the subsequent electro-optic modulation. A dual radio frequency (RF) comb source with a 50 MHz line spacing and a 13 GHz bandwidth drives a single IQ Mach-Zehnder modulator (IQ-MZM), functioning as a single-sideband (SSB) generator and producing a DCS with high spectrum flatness and resolution flexibility. The generated DCS is converted to the MIR region via a nonlinear difference frequency generation (DFG) system. A DCS with a bandwidth of 442 GHz and a resolution of 50 MHz is achieved in the 3.3 µm region, and the figure of merit reaches 2.94×106 H z 12 in a 183.6 ms measurement time.

Estimating emissions of trace gases into the lower troposphere requires accurate concentration measurements of the species of interest. Most commonly, they are provided by networks of in-situ sensors or remote sensing instruments on satellites. In high-gradient environments (e.g. urban settings), in-situ instruments are only spatially representative for a small area. On the other hand, many satellites average on the kilometer scale on which also the aggregation of the data for inversion modelling takes place. But satellites can only provide data for sunny weather conditions, at best once a day in a specific region and typically lack sensitivity for local enhancements. Path averaged measurements of trace gases can potentially fill this observation gap. Between all the technological options for such measurements, dual comb spectroscopy (DCS) can provide high resolution spectra at high brightness with basically no instrument line function, all of which have already been demonstrated in the field [1]. But the high costs and the amount of experience required to set up and run such a system limit the application to metrology experts. With developments in recent years, like the commercial availability of turn-key frequency combs, DCS becomes a more realistic option for a wider scientific community and industry.Here, we present our DCS setup, which is intended for greenhouse gas quantification in the near infra-red. Where possible, we used readily available parts and solutions. We present our current setup and first results obtained, as well as lessons learned and experiences gained in the process.[1] Sean Coburn et al., "Regional trace-gas source attribution using a field-deployed dual frequency comb spectrometer," Optica 5, 320-327 (2018), DOI: 10.1364/OPTICA.5.000320.

Precision spectroscopy contributed significantly to the development of quantum mechanics in its early stages. In the twenty-first century, precision spectroscopy has played an important role in several fields, including fundamental physics, precision measurement, environmental monitoring, and medical diagnostics. An optical frequency comb is indispensable in determining the frequency axis in precision spectroscopy and it is useful as a light source for spectroscopy. Dual-comb spectroscopy uses two frequency combs with slightly different repetition rates and has the potential to surpass conventional Fourier-transform infrared spectrometers. The resolution of dual-comb spectroscopy is limited by the frequency spacing of the comb components, that is, the repetition rate of the comb. We demonstrate dual-comb spectroscopy in the visible-wavelength region using wavelength-converted frequency combs from Er-doped fiber combs. The repetition rates of the combs are relatively low at 19.8 MHz, resulting in relatively high resolution in the dual-comb spectroscopy. The observed spectral shape in dual-comb spectroscopy agrees well with the fitting result based on the hyperfine structure of molecular iodine. The realized dual-comb spectroscopy using wavelength-converted Er-doped fiber combs is reliable (maintenance free) and applicable in other experiments at visible wavelengths.

The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, that act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy, two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both cw laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Here I will touch on the application of this system for open-path measurements of atmospheric trace gases (CH4, CO2, CO, NH3, water, ethane, and N2O) and volatile organic compounds (acetone, isopropanol, propane) with field applications targeting industrial oil and gas monitoring and agriculture.

Optical multi-dimensional coherent spectroscopy is a powerful technique for studying the structure, properties and ultrafast dynamics of atoms, molecules, semiconductor materials and complex systems. Current implementations of multi-dimensional coherent spectroscopy have long acquisition times and/or limited spectral resolution. In addition, most of the techniques utilize complex geometries or phase cycling schemes to isolate non-linear signals. We demonstrate a novel approach of using frequency combs to perform rapid, high resolution and background free multi-dimensional coherent spectroscopy of semiconductor materials. Our approach is inspired by dual-comb spectroscopy, which has been proven to be a versatile tool for obtaining one dimensional absorption spectra with high resolution in a short acquisition time. We demonstrate the method using a GaAs multi-quantum well sample.

Mid-infrared (MIR) spectrometers are invaluable tools for molecular fingerprinting and hyper-spectral imaging. Among the available spectroscopic approaches, GHz MIR dual-comb absorption spectrometers have the potential to simultaneously combine the high-speed, high spectral resolution, and broad optical bandwidth needed to accurately study complex, transient events in chemistry, combustion, and microscopy. However, such a spectrometer has not yet been demonstrated due to the lack of GHz MIR frequency combs with broad and full spectral coverage. Here, we introduce the first broadband MIR frequency comb laser platform at 1 GHz repetition rate that achieves spectral coverage from 3 to 13 µm. This frequency comb is based on a commercially available 1.56 µm mode-locked laser, robust all-fiber Er amplifiers and intra-pulse difference frequency generation (IP-DFG) of few-cycle pulses in χ(2) nonlinear crystals. When used in a dual comb spectroscopy (DCS) configuration, this source will simultaneously enable measurements with μs time resolution, 1 GHz (0.03 cm−1) spectral point spacing and a full bandwidth of >5 THz (>166 cm−1) anywhere within the MIR atmospheric windows. This represents a unique spectroscopic resource for characterizing fast and non-repetitive events that are currently inaccessible with other sources.

Two frequency combs can be used to measure the full complex response of a sample in a configuration which can be alternatively viewed as the equivalent of a dispersive Fourier transform spectrometer, infrared time domain spectrometer, or a multiheterodyne laser spectrometer. This dual comb spectrometer retains the frequency accuracy and resolution inherent to the comb sources. We discuss, in detail, the specific design of our coherent dual-comb spectrometer and demonstrate the potential of this technique by measuring the first overtone vibration of hydrogen cyanide, centered at 194 THz (1545 nm). We measure the fully normalized, complex response of the gas over a 9 THz bandwidth at 220 MHz frequency resolution yielding 41,000 resolution elements. The average spectral signal-to-noise ratio (SNR) is 2,500 for both the fractional absorption and the phase, with a peak SNR of 4,000 corresponding to a fractional absorption sensitivity of 0.025% and phase sensitivity of 250 microradians. As the spectral coverage of combs expands, this dual-comb spectroscopy could provide high frequency accuracy and resolution measurements of a complex sample response across a range of spectral regions.

Dual-comb spectroscopy can provide broad spectral bandwidth and high spectral resolution in a short acquisition time, enabling time-resolved measurements. Specifically, spectroscopy in the mid-infrared wavelength range is of particular interest, since most of the molecules have their strongest rotational-vibrational transitions in this “fingerprint” region. Here we report time-resolved mid-infrared dual-comb spectroscopy, covering ~300 nm bandwidth around 3.3 μm with 6 GHz spectral resolution and 20 μs temporal resolution. As a demonstration, we study a CH4/He gas mixture in an electric discharge, while the discharge is modulated between dark and glow regimes. We simultaneously monitor the production of C2H6 and the vibrational excitation of CH4 molecules, observing the dynamics of both processes. This approach to broadband, high-resolution, and time-resolved mid-infrared spectroscopy provides a new tool for monitoring the kinetics of fast chemical reactions, with potential applications in various fields such as physical chemistry and plasma/combustion analysis.

Temperature and concentration measurements in a high-pressure gasifier enabled by cepstral analysis of dual frequency comb spectroscopy

Dual-comb spectroscopy (DCS) is a modern method using two frequency combs with slightly different repetition rates [1,2]. The DCS down-converts a material response in the optical frequency domain to a rf signal that is easy to handle. It provides significant advantages over conventional spectroscopy such as high spectral resolution, broad spectral range, frequency precision, short measurement time, and so on. The DCS has mainly been applied to precise molecular gas spectroscopy [3]. There are several attempts to expand its application to solid state physics. One of the applications is the determination of complex refractive index of materials [4]. As changes in both the amplitude and phase inside the material can be measured by DCS, one can determine both the real and imaginary part of the refractive index without utilizing the Kramers-Kronig relation. In addition, we consider physically interesting materials such as low-dimensional nanostructures and polymers with polarization-dependent complex refractive index (optical anisotropy). For investigating their physical property, it is important to add polarization sensitiveness to the DCS. So far, we have developed a polarization-sensitive (PS) DCS using a rotating compensator polarimetry [5].

Dual-comb spectroscopy (DCS) is a powerful tool for gas spectroscopy due to high resolution, high accuracy, broadband spectral coverage, and rapid data acquisition, based on optical frequency comb (OFC) traceable to a frequency standard. In DCS, after a temporal waveform of interferogram is acquired in time domain, the corresponding mode-resolved OFC spectrum is obtained by fast Fourier transform (FFT) calculation of the acquired interferogram. However, FFT calculation of huge-sized temporal data spends significantly longer time than the acquisition time of interferogram, making it difficult to response the transient signal change. In this article, we demonstrate frequency-domain DCS by a combination of DCS with lock-in detection (LID), namely LID-DCS. LID-DCS directly extracts an arbitrary OFC mode from a vast number of OFC modes without the need for FFT calculation by the synchronous detection at a LID reference frequency while maintaining high resolution and high accuracy. Usefulness of LID-DCS is demonstrated in rapid monitoring of transient signal change and spectroscopy of hydrogen cyanide gas by comparing with usual DCS.

Optical frequency comb consists of a series of equally spaced and highly stable frequency lines. Due to the advantages of the ultra-high frequency stability and ultra-low phase noise, the optical frequency combs have important applications in high precision spectroscopy, imaging, communications, etc. In the terahertz frequency range, semiconductor-based electrically pumped terahertz quantum cascade lasers have the characteristics of high output power and wide frequency coverage, and are the ideal candidates for generating terahertz optical frequency combs. In this article, we first briefly introduce the research progress of the optical frequency comb in the communication and the mid-infrared bands. Then we mainly review the research progress of the optical frequency combs based on the terahertz semiconductor quantum cascade laser (QCL) operating in free-running, active frequency stabilization and passive frequency stabilization modes. In free running mode, the terahertz QCL frequency comb is mainly limited by the large group velocity dispersion which results in a small comb bandwidth. Therefore, the dispersion compensation is one of the important methods to stabilize the optical frequency comb and broaden the spectral bandwidth. At present, the active frequency stabilization mode is a relatively matured method to realize the optical frequency combs in terahertz QCLs. In this article, we also detail the methods and applications of terahertz QCL dual-comb operations, including on-chip dual-comb and dual-comb spectroscopy. Compared with the Fourier transform infrared spectroscopy and time domain spectroscopy, the terahertz dual-comb spectroscopy has advantages in fast data acquisition (real-time) and high spectral resolution. The emergence of the dual-comb technique not only verifies the concept of optical frequency combs, but also further promotes the applications of frequency combs.

Dual-comb spectroscopy has emerged as an indispensable analytical technique in applications that require high resolution and broadband coverage within short acquisition times. Its experimental realization, however, remains hampered by intricate experimental setups with large power consumption. Here, we demonstrate an ultrasimple free-running dual-comb spectrometer realized in a single all-fiber cavity suitable for the most demanding Doppler-limited measurements. Our dual-comb laser utilizes just a few basic fiber components, allows us to tailor the repetition rate difference, and requires only 350 mW of electrical power for sustained operation over a dozen of hours. As a demonstration, we measure low-pressure hydrogen cyanide within 1.7 THz bandwidth and obtain better than 1% transmittance precision over a terahertz in 200 ms enabled by an all-computational phase retrieval and correction algorithm. The combination of the setup simplicity, comb tooth resolution, and high spectroscopic precision paves the way for proliferation of frequency comb spectroscopy on a larger scale.