Cavity-enhanced Optical Heterodyne Molecular Spectroscopy Research Articles

Pressure control of tritiated hydrogen isotopologues in hermetically sealed vessels by non-evaporable getters for sub-Doppler-resolution spectroscopy

Small molecules serve as valuable benchmarks for testing quantum chemical theories. Ultra-high-resolution spectroscopy currently offers the capability of measuring molecular hydrogen energy levels with a relative accuracy on the order of 10-10. Expanding such investigations to encompass different isotopologues of molecular hydrogen has emerged as a promising avenue for examining mass-dependent contributions to the full description of molecular structure in the framework of relativistic quantum electrodynamics. However, the delicate nature of handling radioactive tritium in a high-resolution spectroscopy environment poses significant technological challenges, thereby limiting the extension of these studies to include T2, DT and HT isotopologues. In this work, we present the experimental setup of the very sensitive, Doppler-free technique of Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS), used to measure rovibrational transitions in the HT molecule. The NICE-OHMS measurements were performed in a closed cavity system without connection to an actively pumped vacuum system. For this reason, a new method for tritium storage and pressure regulation within 0.05–1Pa based on a temperature-controlled mini-getter system is employed using less activity than the tritium exemption limit (1GBq). Non-Evaporable Getters of the type SAES St171 were used for this purpose exploiting their property that hydrogen is released upon heating, while other gases remain being pumped. Prior to the NICE-OHMS experiment, the reversible sorption properties of the getter for hydrogen isotopes were investigated. We show measurements of the equilibrium (hydrogen) pressure for all three pure isotopes and for a H2:HT:T2 mixture (with H:T = 1:1) as a function of temperature for a system of defined volume and amount of gas. For the read-out of the temperature, a method using the resistance of the getter in combination with a pyrometric calibration has been implemented. We discuss the experiences from practical use of the tritium getter and show results obtained with the NICE-OHMS setup leading to frequency calibration of near-infrared transitions in HT at an accuracy, surpassing previous studies by nearly three orders of magnitude.

Optical feedback noise-immune cavity-enhanced optical heterodyne molecular spectrometry for sub-Doppler-broadened detection of C2H2: publisher's note.

This publisher's note contains a correction to Opt. Lett.49, 202 (2024)10.1364/OL.507004.

Optical feedback noise-immune cavity-enhanced optical heterodyne molecular spectrometry for sub-Doppler-broadened detection of C2H2.

A novel, to the best of our knowledge, noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) has been developed, utilizing optical feedback for laser-to-cavity locking with a common distributed-feedback diode laser. The system incorporates active control of the feedback phase and feedforward control of the laser current, allowing for consecutive laser frequency detuning by scanning a piezoelectric transducer (PZT) attached to the cavity. To enhance the fidelity of the spectroscopic signal, wavelength-modulated (wm) NICE-OHMS is implemented. Benefiting from the optical feedback, a modulation frequency of 15 kHz is achieved, surpassing the frequencies typically used in traditional NICE-OHMS setups. Then, the sub-Doppler-broadened wm-NICE-OHMS signal of acetylene at 1.53 µm is observed. A seven-fold improvement in signal to noise ratio has been demonstrated compared to NICE-OHMS alone and a limit of detection of 6.1 × 10-10cm-1 is achieved.

Hyperfine structure of ro-vibrational transition of HD in magnetic field

The precise measurement of the infrared transition of hydrogen-deuterium (HD) molecule is used to test quantum electrodynamics and determine the proton-to-electron mass ratio. The saturated absorption spectrum of the R(1) line in the first overtone (2–0) band of HD molecule has been measured by the comb locked cavity ring-down spectroscopy (CRDS) method in Hefei [Tao L G, et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevLett.120.153001"> 2018 <i>Phys. Rev. Lett.</i> <b>120</b> 153001</ext-link>], and also by the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) method in Amsterdam [Cozijn F M J, et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1103/PhysRevLett.120.153002">2018 <i>Phys. Rev. Lett.</i> <b>120</b> 153002 </ext-link>]. However, there is a significant difference between the line center positions obtained in these two studies. Later the discrepancy was found to be due to unexpected asymmetry in the line shape of the saturated absorption spectrum of the HD molecule. A possible reason is the superposition of multiple hyperfine splitting peaks in the saturated spectrum. However, this model strongly depends on the population transfer caused by intermolecular collisions, which is a lack of experimental and theoretical support. In this paper, the hyperfine structures of the ro-vibrational transition of HD are calculated in the coupled and uncoupled representations. The hyperfine structures of the R(0), P(1) and R(1) lines in the (2–0) band of HD molecule under different external magnetic fields are calculated. The corresponding spectral structures at a temperature of 10 K are simulated. The results show that the transition structure of HD molecule changes significantly with the externally applied magnetic field. The frequency shift of each hyperfine transition line also increases with the intensity of external magnetic field increasing. When the intensity of the external magnetic field is sufficiently high, the hyperfine lines are clearly divided into two branches, and they can be completely separated from each other. Because the dynamic effect of intermolecular collision and the energy level population transfer are very sensitive to the energy level structure, the comparison between experiment and theory will help us to analyze the mechanism of the observed special profiles. It will allow us to obtain accurate frequencies of these transitions, which can be used for testing the fundamental physics.

High-resolution trace gas detection by sub-Doppler noise-immune cavity-enhanced optical heterodyne molecular spectrometry: application to detection of acetylene in human breath.

A sensitive high-resolution sub-Doppler detecting spectrometer, based on noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS), for trace gas detection of species whose transitions have severe spectral overlap with abundant concomitant species is presented. It is designed around a NICE-OHMS instrumentation utilizing balanced detection that provides shot-noise limited Doppler-broadened (Db) detection. By synchronous dithering the positions of the two cavity mirrors, the effect of residual etalons between the cavity and other surfaces in the system could be reduced. An Allan deviation of the absorption coefficient of 2.2 × 10-13 cm-1 at 60 s, which, for the targeted transition in C2H2, corresponds to a 3σ detection sensitivity of 130 ppt, is demonstrated. It is shown that despite significant spectral interference from CO2 at the targeted transition, which precludes Db detection of C2H2, acetylene could be detected in exhaled breath of healthy smokers.

Laser frequency stabilization based on a universal sub-Doppler NICE-OHMS instrumentation for the potential application in atmospheric lidar

Abstract. Lidar is an effective tool for high-altitude atmospheric measurement in which a weak absorption line for the target gas is selected to ensure a large optical depth. The laser frequency stabilization to the line center is required, and a sub-Doppler (sD) spectroscopy of the target line is preferred as a frequency reference. In this paper, a novel universal sD noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) instrumentation based on a fiber-coupled optical single-sideband electro-optic modulator (f-SSM) for the potential application in atmospheric lidar for different target gases with different types of lasers is reported. The f-SSM can replace all frequency actuators in the system, so as to eliminate the individual design of feedback servos that often are tailored for each laser. The universality of the instrumentation was demonstrated by the alternative use of either an Er-doped fiber laser or a whispering-gallery-mode laser. Then the instruments based on both lasers were used to produce the sD signals of acetylene, which worked as a frequency reference to stabilize the laser. By performing the lockings, relative frequency stabilizations of 8.3×10-13 and 7.5×10-13 at an integration time of 240 s were demonstrated.

Sub-Doppler Frequency Metrology in HD for Tests of Fundamental Physics.

Weak transitions in the (2,0) overtone band of the hydrogen deuteride molecule at λ=1.38 μm were measured in saturated absorption using the technique of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Narrow Doppler-free lines were interrogated with a spectroscopy laser locked to a frequency comb laser referenced to an atomic clock to yield transition frequencies [R(1)=217105181895(20) kHz; R(2)=219042856621(28) kHz; R(3)=220704304951(28) kHz] at three orders of magnitude improved accuracy. These benchmark values provide a test of QED in the smallest neutral molecule, and they open up an avenue to resolve the proton radius puzzle, as well as constrain putative fifth forces and extra dimensions.

Shot-noise-limited Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry.

Shot-noise-limited Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) has been realized by implementation of balanced detection. A characterization of the system based on Allan-Werle plots of the absorption coefficient, retrieved by fitting a model function to data, shows that the system has a white noise equivalent absorption per unit length per square root of bandwidth of 2.3×10-13 cm-1 Hz-1/2, solely 44% above the shot noise limit, and a detection sensitivity of 2.2×10-14 cm-1 over 200 s, both being unprecedented for Db NICE-OHMS. The white noise response follows the expected inverse square root dependence on power that is representative of a shot-noise-limited response, which confirms that the system is shot-noise-limited.

Frequency locking of fiber laser to 1530.58 nm NH3 sub-Doppler saturation spectrum based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy technique

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is a powerful tool for trace gas detection, which is based on the combination of frequency modulation spectroscopy (FMS) for reduction of 1/f noise, especially residual intensity noise, and cavity enhanced absorption spectroscopy (CEAS) for prolonging the interaction length between the laser and the targeted gas. Because of the locking of modulation frequency in FMS to the free spectral range (FSR) of the cavity, NICE-OHMS is immune to the frequency-to-amplitude noise, which is a main limitation to CEAS. Moreover, due to the building of high power inside the cavity, NICE-OHMS can easily saturate the molecular absorption thus obtain sub-Doppler spectroscopy, which possess a high resolution and odd symmetry, and thus can act as a frequency discriminator for the locking of the laser frequency to the transition center. In this paper, a fiber laser based NICE-OHMS system is established and the laser frequency is locked to the sub-Doppler absorption line of NH3 by sub-Doppler NICE-OHMS. To avoid the complex design of high-Q-factor bandpass filter at radio frequency, the frequency νpdh, used for Pound-Drever-Hall (PDH) locking, is generated by the beat frequencies νfsr and νdvb, which are used for NICE-OHMS signal and DeVoe-Brewer (DVB) locking, respectively. The performances of PDH and DVB locking are analysed by the frequency distribution deduced from the error signals, which result in frequency deviations of 4.3 kHz and 0.38 kHz, respectively. Then, the CEAS signal and NICE-OHMS signal in the dispersive phase for the measurement of NH3 at 1.53 μm under 70 mTorr are obtained, which show signal-to-noise ratios of 3.3 dB and 45.5 dB, respectively. Due to the high power built in the cavity, the sub-Doppler structure in the NICE-OHMS signal is obtained in the center of the absorption tansition with a satruation degree of 0.22, which is evaluated by the amplitude ratio between sub-Doppler and Doppler-broadened signals. The linewidth (full width at half maximum) of the sub-Doppler signal of 2.05 MHz is obtained, which is calibrated by the time interval between carrier and sideband. The free-running drift of the laser frequency is estimated by the NICE-OHMS signal and results in 50 MHz over 3 h. While, with locking, the relative deviation of the laser frequency is reduced to 16.3 kHz. In order to evaluate the long term stability of the system, the frequency deviation over 3 h is measured. The Allen deviation analysis shows that the white noise is the main noise of the system in the integration time shorter than 10 s. And the frequency stability can reach to 1.6×10-12 in an integration time of 136 s.

Differential noise-immune cavity-enhanced optical heterodyne molecular spectroscopy for improvement of the detection sensitivity by reduction of drifts from background signals

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 N ...

Whispering-gallery-mode laser-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry.

The whispering-gallery-mode (WGM) laser is a type of laser that has an exceptionally narrow linewidth. Noise-immune cavity-enhanced optical heterodyne molecular spectrometry, which is a detection technique with extraordinary properties that benefit from narrow linewidth lasers, has been realized with a WGM laser. By locking to a cavity with a finesse of 55 000, acetylene and carbon dioxide could be simultaneously detected down to an unprecedented noise equivalent absorption per unit length of 6.6×10-14 cm-1 over 150s, corresponding to 5ppt of C2H2.

Sub-Doppler resolution near-infrared spectroscopy at 1.28 μm with the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy method.

We report on the sub-Doppler saturation spectroscopy of the nitrous oxide (N2O) overtone transition at 1.28μm. This measurement is performed by the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy technique based on the quantum-dot (QD) laser. A high intra-cavity power, up to 10W, reaches the saturation limit of the overtone line using an optical cavity with a high finesse of 1.14(5)×105. At a pressure of several mTorr, the saturation dip is observed with a full width at half-maximum of about 2MHz and a signal-to-noise ratio of 71. To the best of our knowledge, this is the first saturation spectroscopy of molecular overtone transitions in the 1.3μm region. The QD laser is then locked to this dispersion signal with a stability of 15kHz at 1s integration time. We demonstrate the potential of the N2O as a marker because of its particularly rich spectrum at the vicinity of 1.28-1.30μm where there are several important forbidden transitions of atomic parity violation measurements and the 1.3μm O-band of optical communication.

Measuring finesse and gas absorption with Lorentzian recovery spectroscopy.

In this paper we present a method for obtaining accurate finesse by recovering the Lorentzian profile of cavity resonances with a laser continuously locked to the cavity and apply it to weak gas absorption measurements. The technique was implemented on our noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) experimental setup. The measurement is performed in the cavity-locked regime, leading to high repeatability and easy automation. The technique involves locking the carrier to a fundamental mode of the cavity and sweeping a second set of sidebands across adjacent cavity modes. The Lorentzian line shape can be reconstructed through a measurement of the transmitted optical power of the auxiliary sidebands. The cavity finesse and gas absorption can then be extracted from these power measurements. The accuracy of our measurements was verified by comparing our results to those obtained with the cavity ring down technique. We demonstrate the use of the technique in spectroscopy by measuring the absorption coefficient of the R(14) line of 12C16O that has been well characterized by others. The gas absorption results obtained were consistent with other experimental measurements and theoretical calculations.

Mid-infrared concentration-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy of a continuous supersonic expansion discharge source

Concentration-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is implemented for the first time on a continuous gas-flow pinhole supersonic expansion discharge source for the study of cooled molecular ions. The instrument utilizes a continuous-wave optical parametric oscillator easily tunable from 2.5 to 3.9 μm and demonstrates a noise equivalent absorption of ∼1 × 10(-9) cm(-1). The effectiveness of concentration-modulated NICE-OHMS is tested through the acquisition of transitions in the ν1 fundamental band of HN2 (+) centered near 3234 cm(-1), with a signal-to-noise of ∼40 obtained for the strongest transitions. The technique is used to characterize the cooling abilities of the supersonic expansion discharge source itself, and a Boltzmann analysis determines a rotational temperature of ∼29 K for low rotational states of HN2 (+). Further improvements are discussed that will enable concentration-modulated NICE-OHMS to reach its full potential for the detection of molecular ions formed in supersonic expansion discharges.

Molecular frequency reference at 1.56 μm using a 12C16O overtone transition with the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy method.

We report on a molecular clock based on the interrogation of the 3ν rotational-vibrational combination band at 1563 nm of carbon monoxide C1612O. The laser stabilization scheme is based on the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) technique in frequency modulation (FM) saturation spectroscopy. We use a high-finesse ultra-low expansion (ULE) glass optical cavity with CO as the molecular reference for long-term stabilization of the cavity resonance. We report an Allan deviation of 1.8×10-12 at 1 s that improves to ∼3.5×10-14 with 1000 s of averaging.

Dual-laser absorption spectroscopy ofC2H2at1.4μm

Spectroscopic parameters (line intensity factor, pressure self-broadening, and shifting coefficients) of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ at $1.4\ensuremath{\mu}\mathrm{m}$ were accurately measured using a dual-laser approach, based upon the technique of optical phase locking. This generated an absolute frequency scale underneath the absorption spectra. A pair of extended-cavity diode lasers was used. One of them, the probe laser, is forced to maintain a precise frequency offset from a reference laser, which is an optical frequency standard based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Laser-gas interaction takes place inside an isothermal multipass cell that is stabilized at the temperature of the triple point of water. The fidelity in the observation of the shape associated to the ${P}_{e}$(14) line of the $2{\ensuremath{\nu}}_{3}+{\ensuremath{\nu}}_{5}$ band allowed us to measure the spectroscopic parameters, with a global uncertainty for the line strength of 0.22%.

Narrowing of the linewidth of an optical parametric oscillator by an acousto-optic modulator for the realization of mid-IR noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 10⁻¹⁰ cm⁻¹ Hz⁻¹/².

The linewidth of a singly resonant optical parametric oscillator (OPO) has been narrowed with respect to an external cavity by the use of an acousto-optic modulator (AOM). This made possible an improvement of the sensitivity of a previously realized OPO-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry instrument for the 3.2 - 3.9 µm mid-infrared region by one order of magnitude. The resulting system shows a detection sensitivity for methane of 2.4 × 10(-10) cm(-1) Hz(-1∕2) and 1.3 × 10(-10) cm(-1) at 20 s, which allows for detection of both the environmentally important (13)CH(4) and CH(3)D isotopologues in atmospheric samples.

Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – II: Experimental verification

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/F, and a full description (denoted FULL), presumed to be valid also when the α0L<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). The present work constitutes an experimental verification and assessment of the validity of these, performed in the Doppler limit for a set of Fα0L/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/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/π of around 0.1, while the ELET is accurate for Fα0L/π up to around 0.3. The ELFT description mimics the Db NICE-OHMS signal well for Fα0L/π up to around unity, while the FULL description is adequate for all Fα0L/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.

Doppler-broadened NICE-OHMS beyond the cavity-limited weak absorption condition – I. Theoretical description

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 NICE-OHMS that are not restricted to the CCLWA condition. First, the general principles of Db NICE-OHMS 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 α0L<π/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.

Noise-immune cavity-enhanced optical heterodyne molecular spectrometry on N₂O 1.283 μm transition based on a quantum-dot external-cavity diode laser.

To access the wavelength within the 1.1-1.3 μm region, we have developed a quantum-dot (QD) laser with an external-cavity configuration and a linewidth of kilohertz at a 1 ms integration time. The residual electroluminescence, due to the inhomogeneous broadening of the QD gain medium, was observed and filtered out using a grating. While a fiber-coupled electro-optical modulator was employed, this laser system was locked to a high-finesse (F∼18,500) optical cavity, and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy was used to observe weak transitions. The Doppler-broadened spectra of a weak N(2)O transition at 1.283 μm are obtained with a signal-to-noise ratio of 30 for a gas pressure of 54 mTorr. The minimum noise-equivalent absorption coefficient is 5.3×10(-10) cm(-1) Hz(-1/2). This system can be a powerful and stable light source for atomic parity nonconservation measurements using thallium, ytterbium, lead, and iodine.