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  • 1.
    Axner, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ehlers, Patrick
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hausmaninger, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Noise-immune cavity-enhanced analytical atomic spectrometry — NICE-AAS: a technique for detection of elements down to zeptogram amounts2014In: Spectrochimica Acta Part B - Atomic Spectroscopy, ISSN 0584-8547, E-ISSN 1873-3565, Vol. 100, p. 211-235Article in journal (Refereed)
    Abstract [en]

    Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is a powerful technique for detection of molecular compounds in gas phase that is based on a combination of two important concepts: frequency modulation spectroscopy (FMS) for reduction of noise, and cavity enhancement, for prolongation of the interaction length between the light and the sample. Due to its unique properties, it has demonstrated unparalleled detection sensitivity when it comes to detection of molecular constituents in the gas phase. However, despite these, it has so far not been used for detection of atoms, i.e. for elemental analysis. The present work presents an assessment of the expected performance of Doppler-broadened (Db) NICE-OHMS for analytical atomic spectrometry, then referred to as noise-immune cavity-enhanced analytical atomic spectrometry (NICE-AAS). After a description of the basic principles of Db-NICE-OHMS, the modulation and detection conditions for optimum performance are identified. Based on a previous demonstrated detection sensitivity of Db-NICE-OHMS of 5×10−12 cm−1 Hz−1∕2 (corresponding to a single-pass absorbance of 7×10−11 over 10 s), the expected limits of detection (LODs) of Hg and Na by NICE-AAS are estimated. Hg is assumed to be detected in gas phase directly while Na is considered to be atomized in a graphite furnace (GF) prior to detection. It is shown that in the absence of spectral interferences, contaminated sample compartments, and optical saturation, it should be feasible to detect Hg down to 10 zg/cm3 (10 fg/m3 or 10-5 ng/m3), which corresponds to 25 atoms/cm3, and Na down to 0.5 zg (zg = zeptogram = 10-21 g), representing 50 zg/mL (parts-per-sextillion, pps, 1:1021) in liquid solution (assuming a sample of 10 µL) or solely 15 atoms injected into the GF, respectively. These LODs are several orders of magnitude lower (better) than any previous laser-based absorption technique previously has demonstrated under atmospheric pressure conditions. It is prophesied that NICE-AAS could provide such high detection sensitivity that the instrumentation should not, by itself, be the limiting factor of an assessment of elemental abundance; the accuracy of an assessment would then instead be limited by concomitant species, e.g. originating from the handling procedures of the sample or the environment.

  • 2.
    Axner, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
    Sub-Doppler dispersion and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy revised2008In: Journal of the Optical Society of America B, Vol. 25, no 7, p. 1166-1177Article in journal (Refereed)
    Abstract [en]

    An expression for the peak-to-peak sub-Doppler optical phase shift of two counter-propagating modes of light, to which the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) dispersion signal is proportional, valid for arbitrary degree of saturation, is derived.

    For low degrees of saturation it agrees with the expression for weakly saturating (ws) conditions, [(1+S)-1/2-(1+2S)-1/20/2, where S is the degree of saturation and α0 the unsaturated peak absorption.

    However, the new expression, which can be written as 0.45S(1+S)-1α0/2, does not predict a distinct maximum as the ws-expression does; instead it predicts an optical phase shift that increases monotonically with S and levels off to 0.45α0/2 for large S. This alters the optimum conditions for the sub-Doppler NICE-OHMS technique and improves its shot-noise-limited detectability.

    The new expression is based upon the same explicit assumptions as the ws-expression but not the Kramers-Kronig’s relations, which are not valid for nonlinear responses, and is supported by experimental results up to S = 100. The new expression is expected to be valid for all techniques measuring sub-Doppler dispersion signals

  • 3.
    Foltynowicz, Aleksandra
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Characterization of fiber-laser-based sub-Doppler NICE-OHMS for quantitative trace gas detection2008In: Optics Express, Vol. 16, no 19, p. 14689-14702Article in journal (Refereed)
    Abstract [en]

    The potential of fiber-laser-based sub-Doppler noise-immune cavity-enhanced optical heterodyne molecular spectrometry for trace gas detection is scrutinized. The non-linear dependence of the on-resonance sub-Doppler dispersion signal on the intracavity pressure and power is investigated and the optimum conditions with respect to these are determined. The linearity of the signal strength with concentration is demonstrated and the dynamic range of the technique is discussed. Measurements were performed on C2H2 at 1531 nm up to degrees of saturation of 100. The minimum detectable sub-Doppler optical phase shift was 5 x 10-11 cm-1 Hz-1/2, corresponding to a partial pressure of C2H2 of 1 x 10-12 atm for an intracavity pressure of 20 mTorr, and a concentration of 10 ppb at 400 mTorr.

  • 4.
    Foltynowicz, Aleksandra
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry signals from optically saturated transitions under low pressure conditions2008In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 25, no 7, p. 1156-1165Article in journal (Refereed)
    Abstract [en]

    The influence of optical saturation on noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) signals from purely Doppler-broadened transitions is investigated experimentally. It is shown that the shape and the strength of the dispersion signal are virtually unaffected by optical saturation, whereas the strength of the absorption signal decreases as (1+G+-1)-1/2, where G+-1 is the degree of saturation induced by the sideband of the frequency modulated triplet, in agreement with theoretical predictions. This implies, first of all, that Doppler-broadened NICE-OHMS is affected less by optical saturation than other cavity enhanced techniques but also that it exhibits nonlinearities in the power and pressure dependence for all detection phases except pure dispersion. A methodology for assessments of the degree of saturation and the saturation power of a transition from Doppler-broadened NICE-OHMS signals is given. The implications of optical saturation for practical trace species detection by Doppler-broadened NICE-OHMS are discussed.

  • 5.
    Foltynowicz, Aleksandra
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmidt, Florian M
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wavelength modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signal line shapes in the Doppler limit2009In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 26, no 7, p. 1384-1394Article in journal (Refereed)
    Abstract [en]

    A thorough analysis of the shape and strength of Doppler-broadened wavelength modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy signals is presented and their dependence on modulation frequency, modulation amplitude and detection phase is investigated in detail. The conditions that maximize the on-resonance signal are identified. The analysis is based on the standard frequency modulation spectroscopy formalism and the Fourier description of wavelength modulation spectroscopy and verified by fits to experimental signals from C2H2 and CO2 measured at 1531 nm. In addition, the line strengths of two CO2 transitions in the v2→3v1+v2+v3 hot band [Pe(7) and Pe(9)] were found to differ by ~20% from those given in the HITRAN database.

  • 6.
    Foltynowicz, Aleksandra
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmidt, Florian M
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Noise-immune cavity-enhanced optical heterodyne molecular spectrometry: Current status and future potential2008In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 92, no 3, p. 313-326Article in journal (Refereed)
    Abstract [en]

    As a result of a combination of an external cavity and modulation techniques, noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is one of the most sensitive absorption techniques, capable of reaching close-to-shot-noise sensitivities, down to 5×10-13 fractional absorption at 1 s averaging. Due to its ability to provide sub-Doppler signals from weak molecular overtone transitions, the technique was first developed for frequency standard applications. It has since then also found use in fields of molecular spectroscopy of weak overtone transitions and trace gas detection. This paper describes the principles and the unique properties of NICE-OHMS. The historical background, the contributions of various groups, as well as the performance and present status of the technique are reviewed. Recent progress is highlighted and the future potential of the technique for trace species detection is discussed.

  • 7.
    Hausmaninger, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Model for molecular absorption spectroscopy in the 1-100 Torr range in the presence of vibrational depletion - Applied to CH4 in N2 and dry airManuscript (preprint) (Other academic)
    Abstract [en]

    When molecules whose collision induced vibrational decay rates are small are probed by molecular absorption spectroscopic (MAS) techniques the absorption signal can, under certain conditions, be reduced and distorted. The reason has been attributed to the fact that a substantial fraction of the molecules in the interaction region will reside in excited vibrational states, which leads to a depletion of the vibrational ground state. One type of molecule in which this can take place is methane. A model for this phenomenon, based on CH4 in trace concentrations in either N2 or dry air in a cylindrical gas cell, detected by mid-infrared light in the 1 - 100 Torr pressure range, is presented. Due to a fast collisional coupling between various rotational states and velocity groups we suggest that depletion in MAS can be modeled adequately by a simple three-level system to which the transport of molecules in the system is coupled as diffusion according to Fick's law, applied to each level individually. The model is verified in a separate work [Hausmaninger T et al., J Quant Spectrosc Radiat Tr. 2017;205:59-70] with good agreement. It predicts that depletion has a strong pressure dependence in the 1 - 30 Torr range, that it is significantly more pronounced in N2 than in air, and that considerable degrees of depletion can be obtained for mW powers of light (> 10% for powers > 20 mW). The findings indicate that, unless precautions are taken, depletion can adversely affect quantitative assessments performed by MAS. Means of how to reduce depletion are given.

  • 8.
    Hausmaninger, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics. Shanxi University, Taiyuan 030006, China.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – II: experimental verification2016In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 168, p. 245-256Article in journal (Refereed)
    Abstract [en]

    Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) is normally described by an expression, here termed the conventional (CONV) description, that is restricted to the conventional cavity-limited weak absorption condition (CCLWA), i.e. when the single pass absorbance is significantly smaller than the empty cavity losses, i.e. when α0L<<π/F. To describe NICE-OHMS signals beyond this limit two simplified extended descriptions (termed the extended locking and extended transmission description, ELET, and the extended locking and full transmission description, ELFT), which are assumed to be valid under the relaxed cavity-limited weak absorption condition (RCLWA), i.e. when α0L<π/Fα0L<π/F, and a full description (denoted FULL), presumed to be valid also when the α0L<π/Fα0L<π/F condition does not hold, have recently been derived in an accompanying work (Ma W, et al. Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition - I. Theoretical Description. J Quant Spectrosc Radiat Transfer, 2015, http://dx.doi.org/10.1016/j.jqsrt.2015.09.007, this issue). The present work constitutes an experimental verification and assessment of the validity of these, performed in the Doppler limit for a set of Fα0L/πFα0L/π values (up to 3.5); it is shown under which conditions the various descriptions are valid. It is concluded that for samples with Fα0L/πFα0L/π up to 0.01, all descriptions replicate the data well. It is shown that the CONV description is adequate and provides accurate assessments of the signal strength (and thereby the analyte concentration) up to Fα0L/πFα0L/π of around 0.1, while the ELET is accurate for Fα0L/πFα0L/π up to around 0.3. The ELFT description mimics the Db NICE-OHMS signal well for Fα0L/πFα0L/π up to around unity, while the FULL description is adequate for all Fα0L/πFα0L/π values investigated. Access to these descriptions both increases considerably the dynamic range of the technique and facilitates calibration using certified reference gases, which thereby significantly broadens the applicability of the Db NICE-OHMS technique.

  • 9.
    Hausmaninger, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zhao, Gang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Depletion of the vibrational ground state of CH4 in absorption spectroscopy at 3.4 μm in N2 and air in the 1-100Torr range2018In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 205, p. 59-70Article in journal (Refereed)
    Abstract [en]

    A model presented in an accompanying work predicts that mid-IR absorption signals from methane in trace concentrations in various buffer gases detected at pressures in the 1-100Torr range can be reduced and distorted due to depletion of the vibrational ground state if the molecules are exposed to laser powers in the tens of mW range or above. This work provides experimental evidence of such depletion in a resonant cavity under a variety of conditions, e.g. for intracavity laser powers up to 2W and for buffer gases of N-2 or dry air, and verifies the applicability of the model. It was found that the degree of depletion is significantly larger in N-2 than dry air, and that it increases with pressure for pressures up to around 10Torr (attributed to a decreased diffusion rate) but decreases with pressure for pressures above 20Torr (caused by an increased collisional vibrational decay rate). The maximum degree of depletion (similar to 80%) was obtained for methane in N-2 at around 15Torr. This implies that absorption spectrometry of methane can experience significant non-linear dependencies on laser power, pressure, as well as buffer gas composition. It is shown that depletion takes place also in (CH4)-C-13, which verifies the applicability of the model also for this isotopologue, and that NICE-OHMS signals detected in absorption phase are less affected by depletion than in dispersion. It was concluded that the absorption mode of detection can provide concentration assessments that are virtually free of influence of depletion for intracavity powers below 0.8 W. 

  • 10.
    Ma, Weiguang
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Theoretical description of Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectroscopy under optically saturated conditions2008In: Journal of the Optical Society of America B, Vol. 25, no 7, p. 1144-1155Article in journal (Refereed)
    Abstract [en]

    A theoretical description of Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) under optically saturated conditions is presented. Expressions for the strength and shape of the Doppler-broadened NICE-OHMS signals are given for both absorption and dispersion phase, in the Voigt regime as well as in the Doppler limit. It is shown that Doppler-broadened NICE-OHMS is affected less by optical saturation than other cavity enhanced techniques; in the Doppler limit the absorption signal decreases by a factor of (1+G+-1)-1/2, where G+-1 is the degree of saturation for one of the frequency modulation sidebands, whereas the dispersion signal is virtually unaffected by optical saturation. In the Voigt regime both signals show additional dependence on optical saturation. The concept of saturation-insensitive detection is introduced and its conditions are identified.

  • 11.
    Ma, Weiguang
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi University, Taiyuan 030006, China.
    Silander, Isak
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hausmaninger, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – I. Theoretical Description2016In: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 168, p. 217-244Article in journal (Other academic)
    Abstract [en]

    Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) is conventionally described by an expression (here referred to as the CONV expression) that is restricted to the case when the single-pass absorbance, α0L, is much smaller than the empty cavity losses, π/F [here termed the conventional cavity-limited weak absorption (CCLWA) condition]. This limits the applicability of the technique, primarily its dynamic range and calibration capability. To remedy this, this work derives extended descriptions of Db NICEOHMS that are not restricted to the CCLWA condition. First, the general principles of Db NICEOHMS are scrutinized in some detail. Based solely upon a set of general assumptions, predominantly that it is appropriate to linearize the Beer–Lambert law, that the light is modulated to a triplet, and that the Pound–Drever–Hall sidebands are fully reflected, a general description of Db NICE-OHMS that is not limited to any specific restriction on α0L vs. π/F, here referred to as the FULL description, is derived. However, this description constitutes a set of equations to which no closed form solution has been found. Hence, it needs to be solved numerically (by iterations), which is inconvenient. To circumvent this, for the cases when α0Loπ/F but without the requirement that the stronger CCLWA condition needs to be fulfilled, a couple of simplified extended expressions that are expressible in closed analytical form, referred to as the extended locking and extended transmission description, ELET, and the extended locking and full transmission description, ELFT, have been derived. An analysis based on simulations validates the various descriptions and assesses to which extent they agree. It is shown that in the CCLWA limit, all extended descriptions revert to the CONV expression. The latter one deviates though from the extended ones for α0L around and above 0.1π/F. The two simplified extended descriptions agree with the FULL description for a larger range of α0L than the CONV expression, viz. for the ELET description for α0L up to 0.3π/F and for ELFT for α0L up to 0.6 or 1.0 π/F (depending on the mode of detection). It is then demonstrated that the conventional view of Db NICE-OHMS, which states that the out-of-phase and the in-phase signals can be referred to as a pure absorption and dispersion signal, respectively, breaks down when the CCLWA condition does not hold. In this case, the out-of-phase signal is additionally affected by the phase shifts of the laser components (i.e. dispersion) while the in-phase signal is also influenced by their attenuation. Access to new descriptions broadens considerably the dynamic range of Db NICE-OHMS and facilitates calibration using standard references samples, and thereby its applicability

  • 12.
    Schmidt, Florian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foltynowicz-Matyba, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lock, Tomas
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened fiber-laser-based NICE-OHMS: improved detectability2007In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 15, no 17, p. 10822-10831Article in journal (Refereed)
    Abstract [en]

    The performance of fiber-laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) has been improved by elimination of the technical constraints that limited its first demonstration. Doppler-broadened detection of C2H2 and CO2 at ~1531 nm is demonstrated using a cavity with a finesse of 4800. Frequency and wavelength modulated detection at absorption and dispersion phase are compared and the optimum mode of detection is discussed. A minimum detectable absorption of 8 × 10-11 cm-1, which corresponds to a detection limit of 4.5 ppt (2 ppt·m) for C2H2, was obtained for an acquisition time of 0.7 s by lineshape fitting. The linearity of the pressure dependence of the signal strengths is investigated for both C2H2 and CO2.

  • 13.
    Schmidt, Florian M.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Highly sensitive dispersion spectroscopy by probing the free spectral range of an optical cavity using dual-frequency modulation2010In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 101, no 3, p. 497-509Article in journal (Refereed)
    Abstract [en]

    Dual-frequency modulation (DFM) has been used to continuously track the frequency shifts of optical cavity modes in the vicinity of an optical transition of a gas inside the cavity for assessment of the gas concentration. A theoretical description of the size and lineshape of the DFM dispersion spectroscopy (DFM-DS) signal is given. Since the signal is measured in terms of a radio frequency the technique is insensitive to laser intensity fluctuations. The signal strength, which can accurately be obtained by curve fitting, only depends on fundamental parameters (including the line strength), thus enabling quantitative detection without calibration procedure. In a first demonstration, using a compact setup based on a narrowband fiber laser, the change in free spectral range around a value of 379.9 MHz due to an acetylene transition near 1531 nm was measured with a resolution of 6 Hz (i.e. with an accuracy of 1.5 parts in 10(8)) in 12.5 s acquisition time, which corresponds to a minimum detectable integrated absorption (SNR=3) of 3x10(-9) cm(-1)

  • 14.
    Silander, Isak
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hausmaninger, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ehlers, Patrick
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 4 x 10-13 cm-1 Hz-1/2: implementation of a 50,000 finesse cavity2015In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 9, p. 2004-2007Article in journal (Refereed)
    Abstract [en]

    We report on the realization of a Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) instrumentation based on a high-finesse (50,000) cavity with a detection sensitivity of 4 x 10(-13) cm(-1) Hz(-1/2). For the P-e(11) transition targeted at 1.5316 mu m, this corresponds to a C2H2 concentration of 240 ppq (parts-per-quadrillion) detected at 100 Torr. The setup was originally affected by recurrent dips in the cavity transmission, which were attributed to excitation of high-order transverse mode by scattering from the mirrors. The effect of these was reduced by insertion of a small pinhole in the cavity.

  • 15.
    Silander, Isak
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hausmaninger, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Harren, Frans J. M.
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Doppler-broadened mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometry based on an optical parametric oscillator for trace gas detection2015In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 40, no 4, p. 439-442Article in journal (Refereed)
    Abstract [en]

    An optical parametric oscillator based Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) system suitable for addressing fundamental vibrational transitions in the 3.2-3.9 mu m mid-infrared (MIR) region has been realized. An Allan-Werle analysis provides a detection sensitivity of methane of 1.5 x 10-9 cm(-1) with a 20 s integration time, which corresponds to 90 ppt of CH4 if detected at the strongest transition addressed at 40 Torr. This supersedes that of previous Db MIR NICE-OHMS demonstrations and suggests that the technique can be suitable for detection of both the environmentally important (CH4)-C-13 and CH3D isotopologues. It also opens up for detection of many other molecular species at ppt and sub-ppt concentration levels.

  • 16. Xue, Dong
    et al.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Magnetic Switching of a Stoner-Wohlfarth Particle Subjected to a Perpendicular Bias Field2019In: Electronics, ISSN 2079-9292, Vol. 8, no 3, article id 366Article in journal (Refereed)
    Abstract [en]

    Characterized by uniaxial magnetic anisotropy, the Stoner-Wohlfarth particle experiences a change in magnetization leading to a switch in behavior when tuned by an externally applied field, which relates to the perpendicular bias component (h(perp)) that remains substantially small in comparison with the constant switching field (h(0)). The dynamics of the magnetic moment that governs the magnetic switching is studied numerically by solving the Landau-Lifshitz-Gilbert (LLG) equation using the Mathematica code without any physical approximations; the results are compared with the switching time obtained from the analytic method that intricately treats the non-trivial bias field as a perturbation. A good agreement regarding the magnetic switching time (t(s)) between the numerical calculation and the analytic results is found over a wide initial angle range (0.01 < (0) < 0.3), as h(0) and h(perp) are 1.5 x K and 0.02 x K, where K represents the anisotropy constant. However, the quality of the analytic approximation starts to deteriorate slightly in contrast to the numerical approach when computing t(s) in terms of the field that satisfies h(perp) > 0.15 x K and h(0) = 1.5 x K. Additionally, existence of a comparably small perpendicular bias field (h(perp) << h(0)) causes t(s) to decrease in a roughly exponential manner when h(perp) increases.

  • 17.
    Zhao, Gang
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi University, Taiyuan, China .
    Hausmaninger, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi University, Taiyuan, China .
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Differential noise-immune cavity-enhanced optical heterodyne molecular spectroscopy for improvement of the detection sensitivity by reduction of drifts from background signals2017In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 23, p. 29454-29471Article in journal (Refereed)
    Abstract [en]

    The detection sensitivity of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is often limited by background signals that bring in drifts. A novel realization of NICE-OHMS, termed differential NICE-OHMS, that both reduces such drifts and enlarges the molecular signal is presented. It is based on simultaneous detection of NICE-OHMS signals in reflection and transmission, followed by a subtraction of the former (properly weighted) from the latter. An Allan plot analysis shows that the instrumentation could demonstrate a noise equivalent absorption per unit length (NEAL) of 4.7 × 10−14 cm−1, obtained for an integration time of 170 s.

  • 18. Zhao, Gang
    et al.
    Tan, Wei
    Hou, Jiajia
    Qiu, Xiaodong
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi University ; Collaborative Innovation Center of Extreme Optics, Shanxi University.
    Li, Zhixin
    Dong, Lei
    Zhang, Lei
    Yin, Wangbao
    Xiao, Liantuan
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Suotang
    Calibration-free wavelength-modulation spectroscopy based on a swiftly determined wavelength-modulation frequency response function of a DFB laser2016In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 24, no 2, p. 1723-1733Article in journal (Refereed)
    Abstract [en]

    A methodology for calibration-free wavelength modulation spectroscopy (CF-WMS) that is based upon an extensive empirical description of the wavelength-modulation frequency response (WMFR) of DFB laser is presented. An assessment of the WMFR of a DFB laser by the use of an etalon confirms that it consists of two parts: a 1st harmonic component with an amplitude that is linear with the sweep and a nonlinear 2nd harmonic component with a constant amplitude. Simulations show that, among the various factors that affect the line shape of a background-subtracted peak-normalized 2f signal, such as concentration, phase shifts between intensity modulation and frequency modulation, and WMFR, only the last factor has a decisive impact. Based on this and to avoid the impractical use of an etalon, a novel method to pre-determine the parameters of the WMFR by fitting to a background-subtracted peak-normalized 2f signal has been developed. The accuracy of the new scheme to determine the WMFR is demonstrated and compared with that of conventional methods in CF-WMS by detection of trace acetylene. The results show that the new method provides a four times smaller fitting error than the conventional methods and retrieves concentration more accurately.

  • 19. Zhao, Gang
    et al.
    Tan, Wei
    Jia, Mengyuan
    Hou, Jiajuan
    Ma, Weiguang
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dong, Lei
    Zhang, Lei
    Feng, Xiaoxia
    Wu, Xuechun
    Yin, Wangbao
    Xiao, Liantuan
    Axner, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Suotang
    Intensity-Stabilized Fast-Scanned Direct Absorption Spectroscopy Instrumentation Based on a Distributed Feedback Laser with Detection Sensitivity down to 4 x 10(-6)2016In: Sensors, ISSN 1424-8220, E-ISSN 1424-8220, Vol. 16, no 9, article id 1544Article in journal (Refereed)
    Abstract [en]

    A novel, intensity-stabilized, fast-scanned, direct absorption spectroscopy (IS-FS-DAS) instrumentation, based on a distributed feedback (DFB) diode laser, is developed. A fiber-coupled polarization rotator and a fiber-coupled polarizer are used to stabilize the intensity of the laser, which significantly reduces its relative intensity noise (RIN). The influence of white noise is reduced by fast scanning over the spectral feature (at 1 kHz), followed by averaging. By combining these two noise-reducing techniques, it is demonstrated that direct absorption spectroscopy (DAS) can be swiftly performed down to a limit of detection (LOD) (1 sigma) of 4 x 10(-6), which opens up a number of new applications.

1 - 19 of 19
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