We use offset-frequency-free difference frequency generation comb sources and a Fourier transform spectrometer with comb-mode-width limited resolution to measure and analyze spectra of molecular species of atmospheric relevance: CH3I and CH2Br2 around 3000 cm-1, and 14N216O around 1280 cm-1
Halogenated volatile organic compounds (HVOCs) play an important role in the photo-chemistry of the atmosphere, for example in ozone depletion [1]. They are produced naturally in the oceans but are also used in industrial and agricultural applications where they may pose a health-hazard due to their biological effects. Optical detection of these compounds would hence be of great value in, for example, atmospheric monitoring and leak detection in workplaces. Crucial for such detection schemes is access to accurate spectroscopic models, which in turn require high-precision laboratory measurements. Due to the combination of broad spectral coverage and high resolution, optical frequency comb Fourier transform spectroscopy is an excellent tool for providing the necessary spectroscopic data. We use a mid-infrared frequency comb and a Fourier transform spectrometer (FTS) to measure and assign high-resolution spectra of multiple absorption bands of two HVOCs: methyl iodide, CH 3 I [2] , and dibromomethane, CH 2 Br 2 , around 3.3m.
Continuous-filtering Vernier spectroscopy (CF-VS) is a laser-based detection technique that combines the broad spectral coverage of an optical frequency comb (OFC) with the enhanced interaction length provided by an optical cavity. The resonances of the cavity filter the OFC to a small group of comb modes that probe the transitions of the species present in the cavity. Controlling cavity resonances allows for a fast scanning of the selected comb modes across the full bandwidth of the comb. CF-VS delivers high detection sensitivity through its immunity to the frequency-to-amplitude-noise conversion. Previous works have shown the capability of CF-VS to perform sensitive and broadband measurements of multiple species in both the near-infrared (NIR) and the mid-infrared (MIR) regions. Those implementations required high-bandwidth stabilization via feedback to the comb sources, which resulted in bulky setups and complex operations. Moreover, they provided acquisition rates up to 20 Hz, limited by the mechanical design. Besides, the target species were measured under static conditions 一 CF-VS had not yet been employed to monitor any time-dependent processes.
The goal of the thesis was to address these issues. In the first project, we used CF-VS based on an Er:fiber comb to measure consecutive spectra of H2O and OH with 25 ms time resolution in a premixed flame whose fuel/air equivalence ratio was modulated with a square wave to simulate temporal perturbations. The concentrations of both species were retrieved with percent level precision, and their temporal profiles were repeatable in each modulation cycle. The steady-state concentrations were in good agreement with a static flame simulator. This work was the first demonstration of CF-VS and cavity-enhanced comb-based spectroscopy with ms time resolution.
In the second project, we implemented a new design of CF-VS that uses a compact Er:fiber comb and a custom-made moving aperture. This removes the requirement for high-bandwidth stabilization and allows acquisition rates up t0 100 Hz. To verify these capabilities, we measured CO2 and CH4 spectra in two spectral ranges. We developed a simple model to account for the influence of the high scanning speed above the adiabatic limit on the absorption signal.
The last project aimed to implement a robust and compact CF-VS spectrometer in the MIR region. For that, we improved an existing MIR source based on difference frequency generation (DFG) using a low-noise Yb:fiber pump, delay stabilization, and a novel polarization-maintaining silicon crystal fiber. The MIR comb uses a soliton generated in the fiber as the seed for DFG. We characterized the soliton using the pump laser. The wide tuning range of the soliton allows the idler to emit in the 2.7-4.2 μm range with high brightness. The MIR comb has a simple delay stabilization and a fixed zero-offset frequency and was successfully implemented to measure high-resolution and precision spectra of CH3I using a comb-resolved Fourier transform spectrometer. Finally, we used the source to perform CF-VS by measuring CH4 spectra at around 3.3 μm. We showed that a single-shot spectrum could be successfully retrieved under the robust operation in the fingerprint regime.
Vernier spectroscopy is a frequency comb-based technique employing optical cavities for filtering of the comb and for enhancement of the interaction length with the sample. Depending on the ratio of the cavity free spectral range and the comb repetition rate, the cavity transmits either widely spaced individual comb lines (comb-resolved Vernier spectroscopy) or groups of comb lines, called Vernier orders (continuous-filtering Vernier spectroscopy, CF-VS). The cavity filtering enables the use of low-resolution spectrometers to resolve the individual comb lines or Vernier orders. Vernier spectroscopy has been implemented using various near- and mid-infrared comb sources for applications ranging from trace gas detection to precision spectroscopy. Here, we present the principles of the technique and provide a review of previous demonstrations of comb-resolved and continuous-filtering Vernier spectroscopy. We also demonstrate two new implementations of CF-VS: one in the mid-infrared, based on a difference frequency generation comb source, with a new and more robust detection system design, and the other in the near-infrared, based on a Ti:sapphire laser, reaching high sensitivity and the fundamental resolution limit of the technique.
Sensitive in situ detection of multiple atmospheric species at fast acquisition rates is needed for environmental monitoring. For field applications, robust and compact design is also demanded. Continuous-filtering Vernier spectroscopy (CF-VS) [1] is a cavity-enhanced frequency-comb-based technique that provides broad spectral bandwidth and high absorption sensitivity in short acquisition times. In CF-VS, groups of comb lines (Vernier orders, VOs) are transmitted through the cavity when its free spectral range (FSR) is slightly detuned from the comb repetition rate ( f rep ) and continuously swept across the broadband laser spectrum (by scanning the FSR). In previous implementations [1] - [3] , a diffraction grating rotating on a galvo scanner was used to image one VO on the detector during the spectral scan, limiting the acquisition rates to 20 Hz. Moreover, high-bandwidth stabilization was needed to synchronize the scans of the galvo and the cavity FSR. Here we present an improved design of CF-VS based on a compact Er:fiber laser and a moving aperture that follows and selects one VO. This removes the requirement of tight active stabilization and enables faster acquisition rates.
We present a new design of a robust cavity-enhanced frequency comb-based spectrometer operating under the continuous-filtering Vernier principle. The spectrometer is based on a compact femtosecond Er-doped fiber laser, a medium finesse cavity, a diffraction grating, a custom-made moving aperture, and two photodetectors. The new design removes the requirement for high-bandwidth active stabilization present in the previous implementations of the technique, and allows scan rates up to 100 Hz. We demonstrate the spectrometer performance over a wide spectral range by detecting CO2 around 1575 nm (1.7 THz bandwidth and 6 GHz resolution) and CH4 around 1650 nm (2.7 THz bandwidth and 13 GHz resolution). We achieve absorption sensitivity of 5 × 10−9 cm-1 Hz-1/2 at 1575 nm, and 1 × 10−7 cm-1 Hz-1/2 cm-1 at 1650 nm. We discuss the influence of the scanning speed above the adiabatic limit on the amplitude of the absorption signal.
A continuous-filtering Vernier spectrometer based on an Er:fiber femtosecond laser was developed to acquire broadband H2O and OH spectra in a premixed CH4/air flame with 25 ms time resolution and percent precision on concentrations retrieval.
We use broadband near-infrared continuous-filtering Vernier spectroscopy (CF-VS) for time-resolved detection of H2O and OH radical in a premixed CH4/air flat flame. The CF-VS spectrometer is based on a femtosecond Er:fiber laser, an external cavity that contains the flame, and a detection system comprising a rotating diffraction grating and photodetectors. Spectra of H2O and OH radical around 1570 nm are continuously recorded with 6.6 GHz spectral resolution, 4.0 x 10-7 cm-1 absorption sensitivity, and 25 ms time resolution, while the fuel-air equivalence ratio is periodically modulated with a square wave. The concentrations of the two analytes are retrieved with percent level precision by a fit of a Vernier model to each spectrum spanning 13 nm. The temporal profiles of both concentrations in each modulation cycle are repeatable and the steady-state concentration levels are in good agreement with predictions based on one-dimensional simulations of a static flat flame. The robust CF-VS spectrometer opens up for quantitative monitoring of multiple products of time-varying combustion processes with relatively simple data acquisition procedures.
Broadband high-resolution spectra of two key atmospheric species, methyl iodide (CH3I) and dibromomethane (CH2Br2), are measured around 3 µm using a comb-based Fourier transform spectrometer and assigned with the help of the semi-automatic fitting in PGOPHER.
We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH3I, in the C-H stretch region from 2800 to 3160 cm(-1). The fast-scanning Fourier transform spectrometer with auto-balanced detection is based on a difference frequency generation comb with repetition rate, f(rep), of 125 MHz. A series of spectra with sample point spacing equal to f rep are measured at different f rep settings and interleaved to yield sampling point spacing of 11 MHz. Iodomethane is introduced into a 76 m long multipass absorption cell by its vapor pressure at room temperature. The measured spectrum contains three main ro-vibrational features: the parallel vibrational overtone and combination bands centered around 2850 cm(-1), the symmetric stretch nu(1) band centered at 2971 cm(-1), and the asymmetric stretch nu(4) band centered at 3060 cm(-1). The spectra of the nu(4) band and the nearby nu(3)+nu(4)-nu(3) hot band are simulated using PGOPHER and a new assignment of these bands is presented. The resolved ro-vibrational structures are used in a least square fit together with the microwave data to provide the upper state parameters. We assign 2603 transitions to the nu(4) band with a standard deviation (observed - calculated) of 0.00034 cm(-1), and 831 transitions to the nu(3)+nu(4)-nu(3) hot band with a standard deviation of 0.00084 cm(-1). For comparison, in the earlier work using standard FT-IR with 162 MHz resolution [Anttila, et al., J. Mol. Spectrosc. 1986; 119:190-200] 1830 transition were assigned to the nu(4) band, and 380 transitions to the nu(3)+nu(4)-nu(3) hot band, with standard deviations of 0.00083 cm(-1) and 0.0013 cm(-1), respectively. The hyperfine splittings due to the 127 I nuclear quadrupole moment are observed for transitions with J <= 2 x K. Finally, intensities of 157 isolated transitions in the nu(4) band are reported for the first time using the Voigt line shape as a model in multispectral fitting.
We report the design of a microstructured silica-based fiber for widely tunable soliton self-frequency shift, suitable for pumping with two most common fiber laser wavelengths: 1.04 μm and 1.55 μm. Depending on the pump source, the output spectrum can be continuously tuned up to 1.67 μm (pump at 1.04 μm) or 1.95 μm (pump at 1.55 μm) in the same 1.5 m-long fiber sample, with pump-to-soliton conversion efficiency higher than 20%. The fiber is highly birefringent, which results in an excellent polarization extinction ratio of the soliton, reaching 26 dB. The shifted solitons have a high degree of coherence confirmed by pulse-to-pulse interference measurement. The available soliton tuning range covers the wavelengths inaccessible for fiber lasers, e.g., 1.3 μm and 1.7 μm, highly important for multi-photon microscopy and imaging. Our work shows that it is possible to design and fabricate one universal optical fiber that supports soliton shift when pumped at two different wavelengths separated by over 500 nm.
The need for fast and simultaneous detection of multiple combustion-related chemical species in various industrial environments has motivated the use of comb-based spectroscopic techniques in these applications. Dual comb spectroscopy has been employed to simultaneously measure CO2 and H2O concentrations in a gas turbine exhaust with 1% precision [1], and cavity-enhanced optical frequency comb Fourier transform spectroscopy has been used to detect H2O and OH radical in a flame [2,3]. However, the time resolution of these measurements has so far been of the order of seconds. Here, we present a cavity-enhanced continuous-filtering Vernier spectrometer (CF-VS) that allows detection of H2O and OH in a flame with a time resolution of 25 ms.
We present a new robust approach to cavity-enhanced comb spectroscopy based on Vernier filtering, a fixed diffraction grating, custom-made chopper wheel, and a low bandwidth comb-cavity stabilization scheme. We measure a CO2 spectrum with a few GHz resolution and 5 x 10-8 cm-1 sensitivity in 9.4 ms.
We present a robust cavity-enhanced comb-based spectrometer with a 6.6 GHz resolution and 60 Hz acquisition rate, based on the continuous-filtering Vernier principle, a fixed diffraction grating, a custom-made chopper wheel, and a low-bandwidth comb-cavity stabilization scheme. We measure a CO2 spectrum with 5 x 10-8 cm-1 sensitivity in 17 ms.