Mid-infrared optical frequency comb spectroscopy using an all-silica antiresonant hollow-core fiber.

We present the first mid-infrared optical frequency comb spectrometer employing an absorption cell based on self-fabricated, all-silica antiresonant hollow-core fiber (ARHCF). The spectrometer is capable of measuring sub-mL sample volumes with 26 m interaction length and noise equivalent absorption sensitivity of 8.3 × 10-8 cm-1 Hz-1/2 per spectral element in the range of 2900 cm-1 to 3100 cm-1. Compared to a commercially available multipass cell, the ARHCF offers a similar interaction length in a 1000 times lower gas sample volume and a 2.8 dB lower transmission loss, resulting in better absorption sensitivity. The broad transmission windows of ARHCFs, in combination with a tunable optical frequency comb, make them ideal for multispecies detection, while the prospect of measuring samples in small volumes makes them a competitive technique to photoacoustic spectroscopy along with the robustness and prospect of coiling the ARHCFs open doors for miniaturization and out-of-laboratory applications.

Procedure for automated low uncertainty assessment of empty cavity mode frequencies in Fabry-Pérot cavity based refractometry.

A procedure for automated low uncertainty assessment of empty cavity mode frequencies in Fabry-Pérot cavity based refractometry that does not require access to laser frequency measuring instrumentation is presented. It requires a previously well-characterized system regarding mirror phase shifts, Gouy phase, and mode number, and is based on the fact that the assessed refractivity should not change when mode jumps take place. It is demonstrated that the procedure is capable of assessing mode frequencies with an uncertainty of 30 MHz, which, when assessing pressure of nitrogen, corresponds to an uncertainty of 0.3 mPa.

Sub-Doppler optical-optical double-resonance spectroscopy using a cavity-enhanced frequency comb probe.

Accurate parameters of molecular hot-band transitions, i.e., those starting from vibrationally excited levels, are needed to accurately model high-temperature spectra in astrophysics and combustion, yet laboratory spectra measured at high temperatures are often unresolved and difficult to assign. Optical-optical double-resonance (OODR) spectroscopy allows the measurement and assignment of individual hot-band transitions from selectively pumped energy levels without the need to heat the sample. However, previous demonstrations lacked either sufficient resolution, spectral coverage, absorption sensitivity, or frequency accuracy. Here we demonstrate OODR spectroscopy using a cavity-enhanced frequency comb probe that combines all these advantages. We detect and assign sub-Doppler transitions in the spectral range of the 3ν3 ← ν3 resonance of methane with frequency precision and sensitivity more than an order of magnitude better than before. This technique will provide high-accuracy data about excited states of a wide range of molecules that is urgently needed for theoretical modeling of high-temperature data and cannot be obtained using other methods.

Optical frequency comb-based measurements and the revisited assignment of high-resolution spectra of CH2Br2 in the 2960 to 3120 cm-1 region.

Brominated organic compounds are toxic ocean-derived trace gases that affect the oxidation capacity of the atmosphere and contribute to its bromine burden. Quantitative spectroscopic detection of these gases is limited by the lack of accurate absorption cross-section data as well as rigorous spectroscopic models. This work presents measurements of high-resolution spectra of dibromomethane, CH2Br2, from 2960 cm-1 to 3120 cm-1 by two optical frequency comb-based methods, Fourier transform spectroscopy and a spatially dispersive method based on a virtually imaged phased array. The integrated absorption cross-sections measured using the two spectrometers are in excellent agreement with each other within 4%. A revisited rovibrational assignment of the measured spectra is introduced, in which the progressions of features are attributed to hot bands rather than different isotopologues as was previously done. Overall, twelve vibrational transitions, four for each of the three isotopologues CH281Br2, CH279Br81Br, and CH279Br2, are assigned. These four vibrational transitions are attributed to the fundamental ν6 band and the nearby nν4 + ν6 - nν4 hot bands (with n = 1-3) due to the population of the low-lying ν4 mode of the Br-C-Br bending vibration at room temperature. The new simulations show very good agreement in intensities with the experiment as predicted by the Boltzmann distribution factor. The spectra of the fundamental and the hot bands show progressions of strong QKa(J) rovibrational sub-clusters. The band heads of these sub-clusters are assigned and fitted to the measured spectra, providing accurate band origins and the rotational constants for the twelve states with an average error of 0.0084 cm-1. A detailed fit of the ν6 band of the CH279Br81Br isotopologue is commenced after assigning 1808 partially resolved rovibrational lines, with the band origin, rotational, and centrifugal constants as fit parameters, resulting in an average error of 0.0011 cm-1.

In situ determination of the penetration depth of mirrors in Fabry-Perot refractometers and its influence on assessment of refractivity and pressure.

A procedure is presented for in situ determination of the frequency penetration depth of coated mirrors in Fabry-Perot (FP) based refractometers and its influence on the assessment of refractivity and pressure. It is based on assessments of the absolute frequency of the laser and the free spectral range of the cavity. The procedure is demonstrated on an Invar-based FP cavity system with high-reflection mirrors working at 1.55 µm. The influence was assessed with such a low uncertainty that it does not significantly contribute to the uncertainties (k = 2) in the assessment of refractivity (<8 × 10-13) or pressure of nitrogen (<0.3 mPa).

Robust, fast and sensitive near-infrared continuous-filtering Vernier spectrometer.

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.

Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe.

We report the first measurement of sub-Doppler molecular response using a frequency comb by employing the comb as a probe in optical-optical double-resonance spectroscopy. We use a 3.3 µm continuous wave pump and a 1.67 µm comb probe to detect sub-Doppler transitions to the 2ν_{3} and 3ν_{3} bands of methane with ∼1.7 MHz center frequency accuracy. These measurements provide the first verification of the accuracy of theoretical predictions from highly vibrationally excited states, needed to model the high-temperature spectra of exoplanets. Transition frequencies to the 3ν_{3} band show good agreement with the TheoReTS line list.

Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution: erratum.

We correct the values of the group delay dispersion of the cavity mirrors and N2, as well as the concentration of CO2, obtained from the measurement of the center frequencies of cavity modes using a comb-based Fourier transform spectrometer. The corrected values of group delay dispersion are a factor of 3 higher, which implies that the precision and accuracy of the dispersion measurements is 0.3 fs2 and 3 fs2, respectively.

Stabilized all-fiber source for generation of tunable broadband fCEO-free mid-IR frequency comb in the 7 - 9 µm range.

A compact and robust all-fiber difference frequency generation-based source of broadband mid-infrared radiation is presented. The source emits tunable radiation in the range between 6.5 µm and 9 µm with an average output power up to 5 mW at 125 MHz repetition frequency. The all-in-fiber construction of the source along with active stabilization techniques results in long-term repetition rate stability of 3 Hz per 10 h and a standard deviation of the output power better than 0.8% per 1 h. The applicability of the presented source to laser spectroscopy is demonstrated by measuring the absorption spectrum of nitrous oxide (N2O) around 7.8 µm. The robustness and good long- and short-term stability of the source opens up for applications outside the laboratory.

Time-resolved continuous-filtering Vernier spectroscopy of H2O and OH radical in a flame.

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 × 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.

Optical frequency comb photoacoustic spectroscopy.

We report the first photoacoustic detection scheme using an optical frequency comb-optical frequency comb photoacoustic spectroscopy (OFC-PAS). OFC-PAS combines the broad spectral coverage and the high resolution of OFCs with the small sample volume of cantilever-enhanced PA detection. In OFC-PAS, a Fourier transform spectrometer (FTS) is used to modulate the intensity of the exciting comb source at a frequency determined by its scanning speed. One of the FTS outputs is directed to the PA cell and the other is measured simultaneously with a photodiode and used to normalize the PA signal. The cantilever-enhanced PA detector operates in a non-resonant mode, enabling detection of a broadband frequency response. The broadband and the high-resolution capabilities of OFC-PAS are demonstrated by measuring the rovibrational spectra of the fundamental C-H stretch band of CH4, with no instrumental line shape distortions, at total pressures of 1000 mbar, 650 mbar, and 400 mbar. In this first demonstration, a spectral resolution two orders of magnitude better than previously reported with broadband PAS is obtained, limited by the pressure broadening. A limit of detection of 0.8 ppm of methane in N2 is accomplished in a single interferogram measurement (200 s measurement time, 1000 MHz spectral resolution, 1000 mbar total pressure) for an exciting power spectral density of 42 µW/cm-1. A normalized noise equivalent absorption of 8 × 10-10 W cm-1 Hz-1/2 is obtained, which is only a factor of three higher than the best reported with PAS based on continuous wave lasers. A wide dynamic range of up to four orders of magnitude and a very good linearity (limited by the Beer-Lambert law) over two orders of magnitude are realized. OFC-PAS extends the capability of optical sensors for multispecies trace gas analysis in small sample volumes with high resolution and selectivity.

Broadband calibration-free cavity-enhanced complex refractive index spectroscopy using a frequency comb.

We present broadband cavity-enhanced complex refractive index spectroscopy (CE-CRIS), a technique for calibration-free determination of the complex refractive index of entire molecular bands via direct measurement of transmission modes of a Fabry-Perot cavity filled with the sample. The measurement of the cavity transmission spectrum is done using an optical frequency comb and a mechanical Fourier transform spectrometer with sub-nominal resolution. Molecular absorption and dispersion spectra (corresponding to the imaginary and real parts of the refractive index) are obtained from the cavity mode broadening and shift retrieved from fits of Lorentzian profiles to the individual cavity modes. This method is calibration-free because the mode broadening and shift are independent of the cavity parameters such as the length and mirror reflectivity. In this first demonstration of broadband CE-CRIS we measure simultaneously the absorption and dispersion spectra of three combination bands of CO2 in the range between 1525 nm and 1620 nm and achieve good agreement with theoretical models. This opens up for precision spectroscopy of the complex refractive index of several molecular bands simultaneously.

Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution.

Optical cavities provide high sensitivity to dispersion since their resonance frequencies depend on the index of refraction. We present a direct, broadband, and accurate measurement of the modes of a high finesse cavity using an optical frequency comb and a mechanical Fourier transform spectrometer with a kHz-level resolution. We characterize 16000 longitudinal cavity modes spanning 16 THz of bandwidth in terms of center frequency, linewidth, and amplitude. Using the center frequencies we retrieve the group delay dispersion of the cavity mirror coatings and pure N2 with 0.1 fs2 precision and 1 fs2 accuracy, as well as the refractivity of the 3ν1 + ν3 absorption band of CO2 with 5 × 10-12 precision. This opens up for broadband refractive index metrology and calibration-free spectroscopy of entire molecular bands.

High-power frequency comb source tunable from 2.7 to 4.2 µm based on difference frequency generation pumped by an Yb-doped fiber laser.

We demonstrate a broadband mid-infrared (MIR) frequency comb source based on difference frequency generation (DFG) in periodically poled lithium niobate crystal. MIR radiation is obtained via mixing of the output of a 125 MHz repetition rate Yb-doped fiber laser with Raman-shifted solitons generated from the same source in a highly nonlinear fiber. The resulting idler is tunable in the range of 2.7-4.2 µm, with average output power reaching 237 mW and pulses as short as 115 fs. The coherence of the MIR comb is confirmed by spectral interferometry and heterodyne beat measurements. Applicability of the developed DFG source for laser spectroscopy is demonstrated by measuring absorption spectrum of acetylene at 3.0-3.1 µm.

Fourier transform and Vernier spectroscopy using an optical frequency comb at 3-5.4 µm.

We present a versatile mid-infrared frequency comb spectroscopy system based on a doubly resonant optical parametric oscillator tunable in the 3-5.4 µm range and two detection methods: a Fourier transform spectrometer (FTS) and a continuous-filtering Vernier spectrometer (CF-VS). Using the FTS with a multipass cell, we measure high precision broadband absorption spectra of CH4 at 3.3 µm and NO at 5.25 µm, the latter for the first time with comb spectroscopy, and we detect atmospheric species (CH4, CO, CO2, and H2O) in air in the signal and idler ranges. Multiline fitting yields minimum detectable concentrations of 10-20 ppb Hz-1/2 for CH4, NO, and CO. For the first time in the mid-infrared, we perform CF-VS using an enhancement cavity, a grating, and a single detector, and we measure the absorption spectrum of CH4 and H2O in ambient air at ∼3.3 µm, reaching a 40 ppb concentration detection limit for CH4 in 2 ms.

Noise-immune cavity-enhanced optical frequency comb spectroscopy.

We present a new method of optical frequency comb spectroscopy that combines cavity enhancement with frequency modulation to obtain immunity to laser frequency-to-amplitude noise conversion by the cavity modes and, thus, high absorption sensitivity over a broad spectral range. A frequency comb is locked to a cavity with a free spectral range (FSR) equal to 4/3 times the repetition rate of the laser, and phase-modulated at a frequency equal to the cavity FSR. The transmitted light is analyzed by a Fourier transform spectrometer with a high bandwidth detector. Phase-sensitive detection of the interferogram yields a noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS) signal. In the first demonstration, we record NICE-OFCS signals from the overtone CO2 band at 1575 nm with absorption sensitivity of 4.3×10(-10) cm(-1) Hz(-1/2) per spectral element, close to the shot noise limit.

Cavity-enhanced optical frequency comb spectroscopy of high-temperature H2O in a flame.

We demonstrate near-infrared cavity-enhanced optical frequency comb spectroscopy of water in a premixed methane/air flat flame. The detection system is based on an Er:fiber femtosecond laser, a high finesse optical cavity containing the flame, and a fast-scanning Fourier transform spectrometer (FTS). High absorption sensitivity is obtained by the combination of a high-bandwidth two-point comb-cavity lock and auto-balanced detection in the FTS. The system allows recording high-temperature water absorption spectra with a resolution of 1 GHz and a bandwidth of 50 nm in an acquisition time of 0.4 s, with absorption sensitivity of 4.2 × 10(-9) cm(-1) Hz(-1/2) per spectral element.

Fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry incorporating an optical circulator.

To reduce the complexity of fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry, a system incorporating a fiber-coupled optical circulator to deflect the cavity-reflected light for laser stabilization has been realized. Detection near the shot-noise limit has been demonstrated for both Doppler-broadened and sub-Doppler signals, yielding a lowest detectable absorption and optical phase shift of 2.2×10(-12) cm(-1) and 4.0×10(-12) cm(-1), respectively, both for a 10 s integration time, where the former corresponds to a detection limit of C2H2 of 5 ppt.

Quantum-noise-limited optical frequency comb spectroscopy.

We achieve a quantum-noise-limited absorption sensitivity of 1.7×10(-12) cm(-1) per spectral element at 400 s of acquisition time with cavity-enhanced frequency comb spectroscopy, the highest demonstrated for a comb-based technique. The system comprises a frequency comb locked to a high-finesse cavity and a fast-scanning Fourier transform spectrometer with an ultralow-noise autobalancing detector. Spectra with a signal-to-noise ratio above 1000 and a resolution of 380 MHz are acquired within a few seconds. The measured absorption line shapes are in excellent agreement with theoretical predictions.

Distributed-feedback-laser-based NICE-OHMS in the pressure-broadened regime.

A compact noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) system based on a narrow linewidth distributed-feedback laser and fiber-coupled acousto-optic and electro-optic modulators has been developed. Measurements of absorption and dispersion signals have been performed at pressures up to 1/3 atmosphere on weak acetylene transitions at 1551 nm. Multiline fitting routines were implemented to obtain transition parameters, i.e., center frequencies, linestrengths, and pressure broadening coefficients. The signal strength was shown to be linear with pressure and concentration, and independent of detection phase. The minimum detectable on-resonance absorption with a cavity with a finesse of 460 was 2 × 10(-10) cm(-1) for 1 minute of integration time.