69 research outputs found
Line positions and intensities of the band of CHI using mid-infrared optical frequency comb Fourier transform spectroscopy
We present a new spectral analysis of the and
+- bands of CHI around 2971 cm
based on a high-resolution spectrum spanning from 2800 cm to 3160
cm, measured using an optical frequency comb Fourier transform
spectrometer. From this spectrum, we previously assigned the and
+- bands around 3060 cm using PGOPHER, and
the line list was incorporated in the HITRAN database. Here, we treat the two
fundamental bands, and , together with the perturbing
states, 2+ and +2, as a four-level
system connected via Coriolis and Fermi interactions. A similar four-level
system is assumed to connect the +- and
+- hot bands, which appear due to the population of
the low-lying state at room temperature, with the
2+2 and +- perturbing
states. This treatment provides a good global agreement of the simulated
spectra with experiment, and hence accurate line lists and band parameters of
the four connected vibrational states in each system. Overall, we assign 4665
transitions in the fundamental band system, with an average error of 0.00071
cm, a factor of two better than earlier work on the band using
conventional Fourier transform infrared spectroscopy. The band shows
hyperfine splitting, resolvable for transitions with J 2 x K. Finally,
the spectral intensities of 65 lines of the band and 7 lines of the
+- band are reported for the first time using the
Voigt line shape as a model in multispectral fitting
OPTICAL FREQUENCY COMB FOURIER TRANSFORM SPECTROSCOPY
Fourier transform spectroscopy (FTS) based on optical frequency combs offers a number of advantages over conventional Fourier transform infrared (FTIR) spectroscopy based on incoherent sources\footnote{J. Mandon, G. Guelachvili, and N. Picque, Nat. Photonics 3, 99 (2009).}. The high spectral brightness of the comb sources allows measuring spectra with high signal-to-noise ratios in acquisition times of the order of seconds. What is more, the resolution of comb-based FTS is given by the linewidth of the comb modes rather than the optical path difference (OPD) in the spectrometer, provided that the OPD is matched to the inverse of the comb mode spacing\footnote{P. Maslowski, et al., Phys. Rev. A 93, 021802 (2016); L. Rutkowski, et al., J. Quant. Spectrosc. Radiat. Transf. 204, 63 (2018).}. This implies that spectra with kHz resolution can be measured using OPD of the order of a few tens of cm\footnote{L. Rutkowski, et al., Opt. Express 25, 21711 (2017).}, which is impossible in conventional FTIR spectrometers. To increase the sensitivity of direct absorption measurements, frequency combs can be efficiently coupled into enhancement cavities that increase the interaction length with the sample\footnote{M. J. Thorpe, and J. Ye, Appl. Phys. B 91, 397 (2008); A. Foltynowicz, et al., Phys. Rev. Lett. 107, 233002 (2011).}. In another cavity-enhanced approach, the profiles of the cavity modes are measured directly, and complex refractive index spectra of entire molecular bands are determined from the broadening and shift of the cavity modes caused by the molecular sample\footnote{A. C. Johansson, et al., Opt. Express 26, 20633 (2018).}. Comb-based FTS can also be combined with other detection methods, such as Faraday rotation spectroscopy to detect broadband interference-free spectra of paramagnetic molecules\footnote{A. C. Johansson, J. Westberg, G. Wysocki, and A. Foltynowicz, Appl. Phys. B 124, 79 (2018).}, or photoacoustic spectroscopy that allows detection in a very small sample volume\footnote{I. Sadiek, et al., Phys. Chem. Chem. Phys. 20, 27849 (2018).}. I will present the various implementations of comb-based FTS and show examples of high-resolution measurements of entire absorption bands in the near- and mid-infrared wavelength range
Cavity-enhanced optical frequency comb spectroscopy in the mid-infrared - application to trace detection of H2O2
We demonstrate the first cavity-enhanced optical frequency comb spectroscopy
in the mid-infrared wavelength region and report the sensitive real-time trace
detection of hydrogen peroxide in the presence of a large amount of water. The
experimental apparatus is based on a mid-infrared optical parametric oscillator
synchronously pumped by a high power Yb:fiber laser, a high finesse broadband
cavity, and a fast-scanning Fourier transform spectrometer with autobalancing
detection. The comb spectrum with a bandwidth of 200 nm centered around 3.75
{\mu}m is simultaneously coupled to the cavity and both degrees of freedom of
the comb, i.e., the repetition rate and carrier envelope offset frequency, are
locked to the cavity to ensure stable transmission. The autobalancing detection
scheme reduces the intensity noise by a factor of 300, and a sensitivity of 5.4
{\times} 10^-9 cm^-1 Hz^-1/2 with a resolution of 800 MHz is achieved
(corresponding to 6.9 {\times} 10^-11 cm^-1 Hz^-1/2 per spectral element for
6000 resolved elements). This yields a noise equivalent detection limit for
hydrogen peroxide of 8 parts-per-billion (ppb); in the presence of 2.8% of
water the detection limit is 130 ppb. Spectra of acetylene, methane and nitrous
oxide at atmospheric pressure are also presented, and a line shape model is
developed to simulate the experimental data.Comment: submitted to special FLAIR 2011 issue of Appl. Phys.
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 cavity modes spanning 16 THz of bandwidth in
terms of center frequency, linewidth, and amplitude. We retrieve the group
delay dispersion of the cavity mirror coatings and pure N with 0.1
fs precision and 1 fs accuracy, as well as the refractivity of the
3{\nu}1+{\nu}3 absorption band of CO with 5 x 10 precision.
This opens up for broadband refractive index metrology and calibration-free
spectroscopy of entire molecular bands
Quantum-noise-limited optical frequency comb spectroscopy
We achieve a quantum-noise-limited absorption sensitivity of
1.7/times10 cm 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
ultra-low-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 lineshapes are in excellent agreement with theoretical
predictions.Comment: 18 pages, 4 figures; http://prl.aps.org/pdf/PRL/v107/i23/e23300
Line Positions and Intensities of the {\nu} Band of Methyl Iodide Using Mid-Infrared Optical Frequency Comb Fourier Transform Spectroscopy
We use optical frequency comb Fourier transform spectroscopy to measure
high-resolution spectra of iodomethane, CHI in the C-H stretch region from
2800 to 3160 cm. The fast-scanning Fourier transform spectrometer with
auto-balanced detection is based on a difference frequency generation comb with
repetition rate, f, of 125 MHz. A series of spectra with sample point
spacing equal to f are measured at different f 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, the symmetric stretch band centered at 2971
cm, and the asymmetric stretch band centered at 3060
cm. The spectra of the band and the nearby
+- 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 band with
standard deviation (observed - calculated) of 0.00034 cm, and 831
transitions to the +- hot band with standard
deviation of 0.00084 cm. The hyperfine splittings due to the I
nuclear quadrupole moment are observed for transitions with J2xK.
Finally, intensities of 157 isolated transitions in the band are
reported for the first time using the Voigt line shape as a model in
multispectral fitting
Optical Frequency Comb Fourier Transform Spectroscopy of NO at 7.8 {\mu}m
We use a Fourier transform spectrometer based on a compact mid-infrared
difference frequency generation comb source to perform broadband
high-resolution measurements of nitrous oxide, NO, and
retrieve line center frequencies of the fundamental band and the
+ - hot band. The spectrum spans 90 cm
around 1285 cm with a sample point spacing of 3 10
cm (9 MHz). We report line positions of 67 lines in the
fundamental band between P(37) and R(39), and 78 lines in the +
- hot band (split into two components with e/f rotationless
parity) between P(34) and R(33), with uncertainties in the range of 90-600 kHz.
We derive upper state constants of both bands from a fit of the effective
ro-vibrational Hamiltonian to the line center positions. For the fundamental
band, we observe excellent agreement in the retrieved line positions and upper
state constants with those reported in a recent study by AlSaif et al. using a
comb-referenced quantum cascade laser [J Quant Spectrosc Radiat Transf,
2018;211:172-178]. We determine the origin of the hot band with precision one
order of magnitude better than previous work based on FTIR measurements by Toth
[http://mark4sun.jpl.nasa.gov/n2o.html], which is the source of the HITRAN2016
data for these bands
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