arXiv subjects

Aleksandra Foltynowicz

Publications and source records attributed to Aleksandra Foltynowicz.

35 records · Page 2Linked to original sources

arXiv2020

Compact mode-locked Er-doped fiber laser for broadband cavity-enhanced spectroscopy

We report the design and characteristics of a simple and compact mode-locked Er-doped fiber laser and its application to broadband cavity-enhanced spectroscopy. The graphene mode-locked polarization maintaining oscillator is thermally stabilized and equipped with three actuators that control the repetition rate: fast and slow fiber stretchers, and metal-coated fiber section. This allows wide tuning of the repetition rate and its stabilization to an external reference source. The applicability of the laser to molecular spectroscopy is demonstrated by detecting CO2 in air using continuous-filtering Vernier spectroscopy.

physics.optics↗

arXiv2020

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

We report the measurement of sub-Doppler double-resonance transitions in methane using a 3.3 $μ$m continuous wave optical parametric oscillator to pump transitions in the fundamental $ν_3$ band and a 1.67 $μ$m frequency comb to probe ladder-type transitions in the 3$ν_3$ band over 200 cm$^{-1}$ of bandwidth. We detected 36 ladder-type resonances for 9 pumped transitions with average center frequency accuracy of 1.7 MHz, limited by the pump frequency stability. The lines are assigned using the TheoReTS line list, and the intensity ratios for different relative pump/probe polarizations. This method provides accurate assignment of highly excited energy levels that cannot be done using high temperature spectra.

physics.chem-ph↗

arXiv2018

Optical frequency comb photoacoustic spectroscopy

We report the first photoacoustic detection scheme using an optical frequency comb - the 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 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 CH${_4}$, 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 higher than previously reported with broadband PAS is obtained, limited by the pressure broadening. A limit of detection of 0.8 ppm of methane in N${_2}$ is accomplished in a single interferogram measurement (200 s measurement time, 1000 MHz resolution, 1000 mbar total pressure) for an exciting power spectral density of 42 μW/cm${^{-1}}$. A normalized noise equivalent absorption of 8x10${^{-10}}$ 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 volume with high resolution and selectivity.

physics.optics↗

arXiv2018

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.

physics.optics↗

arXiv2018

Optical frequency comb Faraday rotation spectroscopy

We demonstrate optical frequency comb Faraday rotation spectroscopy (OFC-FRS) for broadband interference-free detection of paramagnetic species. The system is based on a femtosecond doubly resonant optical parametric oscillator and a fast-scanning Fourier transform spectrometer (FTS). The sample is placed in a DC magnetic field parallel to the light propagation. Efficient background suppression is implemented via switching the direction of the field on consecutive FTS scans and subtracting the consecutive spectra, which enables long term averaging. In this first demonstration, we measure the entire Q- and R-branches of the fundamental band of nitric oxide in the 5.2-5.4 μm range and achieve good agreement with a theoretical model.

physics.optics↗

arXiv2017

An experimental water line list at 1950 K in the 6250 -- 6670 \cm\ region

An absorption spectrum of H$_2$$^{16}$O at 1950 K is recorded in a premixed methane/air flat flame using a cavity-enhanced optical frequency comb-based Fourier transform spectrometer. 2417 absorption lines are identified in the 6250 -- 6670 cm region with an accuracy of about 0.01 cm. Absolute line intensities are retrieved using temperature and concentration values obtained by tunable diode laser absorption spectroscopy. Line assignments are made using a combination of empirically known energy levels and predictions from the new POKAZATEL variational line list. 2030 of the observed lines are assigned to 2937 transitions, once blends are taken into account. 126 new energy levels of H$_2$$^{16}$O are identified. The assigned transitions belong to 136 bands and span rotational states up to $J=27$.

physics.chem-ph↗

arXiv2017

Optical Frequency Comb Fourier Transform Spectroscopy with Sub-Nominal Resolution - Principles and Implementation

Broadband precision spectroscopy is indispensable for providing high fidelity molecular parameters for spectroscopic databases. We have recently shown that mechanical Fourier transform spectrometers based on optical frequency combs can measure broadband high-resolution molecular spectra undistorted by the instrumental line shape (ILS) and with a highly precise frequency scale provided by the comb. The accurate measurement of the power of the comb modes interacting with the molecular sample was achieved by acquiring single-burst interferograms with nominal resolution precisely matched to the comb mode spacing. Here we give a full theoretical description of this sub-nominal resolution method and describe in detail the experimental and numerical steps needed to retrieve ILS-free molecular spectra, i.e. with ILS-induced distortion below the noise level. We investigate the accuracy of the transition line centers retrieved by fitting to the absorption lines measured using this method. We verify the performance by measuring an ILS-free cavity-enhanced low-pressure spectrum of the 3ν1+ν3 band of CO2. We observe and quantify collisional narrowing of absorption line shape, for the first time with a comb-based spectroscopic technique. Thus retrieval of line shape parameters with accuracy not limited by the Voigt profile is now possible for entire absorption bands acquired simultaneously.

physics.ins-det↗

arXiv2017

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${_2}$ with 0.1 fs${^2}$ precision and 1 fs${^2}$ accuracy, as well as the refractivity of the 3ν1+ν3 absorption band of CO${_2}$ with 5 x 10${^{-12}}$ precision. This opens up for broadband refractive index metrology and calibration-free spectroscopy of entire molecular bands.

physics.ins-det↗

arXiv2017

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 (PPLN) crystal. Mid-infrared 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.

physics.optics↗

arXiv2017

Mid-infrared continuous-filtering Vernier spectroscopy using a doubly resonant optical parametric oscillator

We present a continuous-filtering Vernier spectrometer operating in the 3.15-3.4 $μ$m range, based on a femtosecond doubly resonant optical parametric oscillator, a cavity with a finesse of 340, a grating mounted on a galvo scanner and two photodiodes. The spectrometer allows acquisition of one spectrum spanning 250 nm of bandwidth in 25 ms with 8 GHz resolution, sufficient for resolving molecular lines at atmospheric pressure. An active lock ensures good frequency and intensity stability of the consecutive spectra and enables continuous signal acquisition and efficient averaging. The relative frequency scale is calibrated using a Fabry-Perot etalon or, alternatively, the galvo scanner position signal. We measure spectra of pure CH${_4}$ as well as dry and laboratory air and extract CH${_4}$ and H${_2}$O concentrations by multiline fitting of model spectra. The figure of merit of the spectrometer is 1.7x10${^-}$${^9}$ cm${^-}$${^1}$ Hz${^-}$${^1}$${^/}$${^2}$ per spectral element and the minimum detectable concentration of CH${_4}$ is 360 ppt Hz${^-}$${^1}$${^/}$${^2}$, averaging down to 90 ppt after 16 s.

physics.ins-det↗

arXiv2016

Signal line shapes of Fourier transform cavity-enhanced frequency modulation spectroscopy with optical frequency combs

We present a thorough analysis of the signal line shapes of Fourier transform-based noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS). We discuss the signal dependence on the ratio of the modulation frequency, f${_m}$, to the molecular line width, Γ. We compare a full model of the signals and a simplified absorption-like analytical model that has high accuracy for low f${_m}$/Γ ratios and is much faster to compute. We verify the theory experimentally by measuring and fitting NICE-OFCS spectra of CO${_2}$ at 1575 nm using a system based on an Er:fiber femtosecond laser and a cavity with a finesse of ~11000.

physics.optics↗

arXiv2016

Optical frequency comb spectroscopy at 3-5.4 μm with a doubly resonant optical parametric oscillator

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 Vernier spectrometer. Using the FTS with a multipass cell we measure high-precision broadband absorption spectra of CH$_4$ and NO at ~3.3 μm and ~5.2 μm, respectively, and of atmospheric species (CH$_4$, CO, CO$_2$ and H$_2$O) in air in the signal and idler wavelength range. The figure of merit of the system is on the order of 10$^{-8}$ cm$^{-1}$ Hz$^{-1/2}$ per spectral element, and multiline fitting yields minimum detectable concentrations of 10-20 ppb Hz$^{-1/2}$ for CH$_4$, NO and CO. For the first time in the mid-infrared, we perform continuous-filtering Vernier spectroscopy using a low finesse enhancement cavity, a grating and a single detector, and measure the absorption spectrum of CH$_4$ and H$_2$O in ambient air at ~3.3 μm.

physics.optics↗

arXiv2015

Surpassing the Path-Limited Resolution of a Fourier Transform Spectrometer with Frequency Combs

Fourier transform spectroscopy based on incoherent light sources is a well-established tool in research fields from molecular spectroscopy and atmospheric monitoring to material science and biophysics. It provides broadband molecular spectra and information about the molecular structure and composition of absorptive media. However, the spectral resolution is fundamentally limited by the maximum delay range (Δ$_{max}$) of the interferometer, so acquisition of high-resolution spectra implies long measurement times and large instrument size. We overcome this limit by combining the Fourier transform spectrometer with an optical frequency comb and measuring the intensities of individual comb lines by precisely matching the Δ$_{max}$ to the comb line spacing. This allows measurements of absorption lines narrower than the nominal (optical path-limited) resolution without ringing effects from the instrumental lineshape and reduces the acquisition time and interferometer length by orders of magnitude.

physics.optics↗

arXiv2014

Noise-immune cavity-enhanced optical frequency comb spectroscopy: A sensitive technique for high-resolution broadband molecular detection

Noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS) is a recently developed technique that utilizes phase modulation to obtain immunity to frequency-to-amplitude noise conversion by the cavity modes and yields high absorption sensitivity over a broad spectral range. We describe the principles of the technique and discuss possible comb-cavity matching solutions. We present a theoretical description of NICE-OFCS signals detected with a Fourier transform spectrometer (FTS), and validate the model by comparing it to experimental CO2 spectra around 1575 nm. Our system is based on an Er:fiber femtosecond laser locked to a cavity and phase-modulated at a frequency equal to a multiple of the cavity free spectral range (FSR). The NICE-OFCS signal is detected by a fast-scanning FTS equipped with a high-bandwidth commercial detector. We demonstrate a simple method of passive locking of the modulation frequency to the cavity FSR that significantly improves the long term stability of the system, allowing averaging times on the order of minutes. Using a cavity with a finesse of ~9000 we obtain absorption sensitivity of 6.4 x 10^{-11} cm^{-1} Hz^{-1/2} per spectral element, and concentration detection limit for CO2 of 450 ppb Hz^{-1/2}, determined by multiline fitting.

physics.optics↗

arXiv2012

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 μm 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.

physics.optics↗

arXiv2011

Quantum-noise-limited optical frequency comb spectroscopy

We achieve a quantum-noise-limited absorption sensitivity of 1.7/times10$^{-12}$ 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 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.

physics.optics↗

arXiv2010

Mid-Infrared Frequency Comb Fourier Transform Spectrometer

Optical frequency-comb-based-high-resolution spectrometers offer enormous potential for spectroscopic applications. Although various implementations have been demonstrated, the lack of suitable mid-infrared comb sources has impeded explorations of molecular fingerprinting. Here we present for the first time a frequency-comb Fourier transform spectrometer operating in the 2100-to-3700-cm-1 spectral region that allows fast and simultaneous acquisitions of broadband absorption spectra with up to 0.0056 cm-1 resolution. We demonstrate part-per-billion detection limits in 30 seconds of integration time for various important molecules including methane, ethane, isoprene, and nitrous oxide. Our system enables precise concentration measurements even in gas mixtures that exhibit continuous absorption bands, and it allows detection of molecules at levels below the noise floor via simultaneous analysis of multiple spectral features. This system represents a near real-time, high-resolution, high-bandwidth mid-infrared spectrometer which is ready to replace traditional Fourier transform spectrometers for many applications in trace gas detection, atmospheric science, and medical diagnostics.

physics.optics↗