Intracavity Absorption Research Articles

Precision Measurement of Vibrational Quanta in Tritium Hydride.

Saturated absorption measurements of transitions in the (2-0) band of radioactive tritium hydride are performed with the ultrasensitive noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy intracavity absorption technique in the range 1460-1510nm. The hyperfine structure of rovibrational transitions of tritium hydride, in contrast to that of hydrogen deuteride, exhibits a single isolated hyperfine component, allowing for the accurate determination of hyperfineless rovibrational transition frequencies, resulting in R(0)=203 396 426 692(22) kHz and R(1)=205 380 033 644(21) kHz. This corresponds to an accuracy 3 orders of magnitude better than previous measurements in tritiated hydrogen molecules. Observation of an isolated component in P(1) with reversed signal amplitude contradicts models for line shapes in hydrogen deuteride based on crossover resonances.

Hyperfine structure in a vibrational quadrupole transition of ortho-H2

ABSTRACT A Lamb dip spectroscopic measurement of the Q(1) transition in the (2-0) overtone band of ortho- H 2 is performed with the ultra-sensitive NICE-OHMS intracavity absorption technique at 1238 nm . For the first time individual hyperfine components of a vibrational transition in H 2 are resolved, displaying a pattern of 4 overlapping components with singly isolated components at the red and blue side. Weak components at a line strength of 4.4 × 10 − 28 cm/molecule could be observed as a Lamb dip. Analysis yields a hyperfineless purely rovibrational transition frequency for the Q(1) (2-0) line in H 2 at unprecedented accuracy for any rovibrational transition in a molecular hydrogen isotopologue. Its frequency is 242,091,630,140 ( 9 ) kHz under the assumption that only a blue recoil component is observed. A 1.7σ deviation from the most advanced quantum electrodynamics calculations is obtained, forming a challenge for theory.

Rapid and noninvasive cell assay by microfluidic-integrated intracavity evanescent field absorption in a fiber ring laser

BackgroundHighly sensitive and rapid detection of cell concentration and interfacial molecular events is of great value for biological, biomedical, and chemical research. Most traditional biosensors require large sample volumes and complicated functional modifications of the surface. It is of great significance to develop label-free biosensor platforms with minimal sample consumption for studying cell concentration changes and interfacial molecular events without labor-intensive procedures. ResultsHere, a fiber-optic biosensor based on intracavity evanescent field absorption sensing is designed for sensitive and label-free cell assays for the first time. The interaction between the cells and the evanescent field is enhanced by introducing microfluidic-integrated intracavity absorption in a fiber ring laser. This strategy extends the range of targeted analytes to include quantification of a large number of targets on a surface and improves the detection sensitivity of the fiber-optic biosensor. The level of sensing resolution could be improved from 10−4 RIU to 10−7 RIU using this strategy. The stem cells were studied over a wide concentration range (from 500 to 1.2 × 105 cells/ml) and were measured sequentially. By measuring the output power of the intracavity absorption sensing system, the cell concentration can be directly determined in a label-free manner. The results show that dozens of stem cells can be sensitively detected with a sample consumption of 72 μL. The response was fast (15 s) with a low temperature cross-sensitivity of 0.031 cells·ml−1/°C. SignificanceThe proposed method suggests its capacity for true label-free and noninvasive cell assays with a low limit of detection and small sample consumption. This has the potential to be used as a universal tool for quantitative and qualitative characterization of various cells and other biochemical analytes.

Self-calibrated NICE-OHMS based on an asymmetric signal: theoretical analysis and experimental validation.

As an ultra-sensitive detection technique, the noise-immune cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) technique has great potential for assessment of the concentration of trace gases. To determine gas concentrations at the ppt or lower level with high accuracy, it is desirable that the technique exhibits self-calibration (or calibration-free) capabilities. Although being sensitive, NICE-OHMS has so far not demonstrated any such ability. To remedy this, this paper provides a self-calibrated realization of NICE-OHMS that is based on a switching of the feedback target of the DeVoe-Brewer (DVB) locking procedure from the modulation frequency of the frequency modulation spectroscopy (FMS) to the cavity length, which creates an asymmetrical signal whose form and size can be used to unambiguously assess the gas concentration. A comprehensive theoretical model for self-calibrated NICE-OHMS is established by analyzing the shift of cavity modes caused by intracavity absorption, demonstrating that gas absorption information can be encoded in both the laser frequency and the NICE-OHMS signal. To experimentally verify the methodology, we measure a series of dispersion signals under different levels of absorbance using a built experimental setup. An instrument factor and the partial pressure are obtained by fitting the measured signal through theoretical expressions. Our results demonstrate that fitted values are more accurate for higher partial pressures than for lower. To improve on the accuracy at low partial pressures, it is shown that the instrument factor obtained by fitting the signal at large partial pressures (in this case, above 7.8 µTorr) can be set to a fixed value for all fits. By this, the partial pressures can be assessed with a relative error below 0.65%. This technique has the potential to enable calibration-free ultra-sensitive gas detection.

Revisiting iron oxidation chemistry in synthesis flames: Insights from a shock-tube study with intracavity laser absorption spectroscopy tracking of FeO

The reaction between Fe atoms and O2 in a mixture of iron-pentacarbonyl (IPC, 2 ppm) and oxygen (100 ppm), diluted in argon, has been studied in a shock tube behind reflected shock waves over the temperature and pressure ranges of 1050–3400 K and 0.7–2.0 bar. Time-resolved measurements of Fe and FeO have been performed simultaneously using a combination of atomic resonance absorption spectroscopy (ARAS) and intracavity laser absorption spectroscopy (ICAS) with a custom-made broadband dye laser, respectively. For ICAS, absorption features in the spectral range from 16,316 to 16,353 cm−1 have been evaluated, and the oscillator strengths for all 41 assigned ro-vibronic transitions have been expressed. For most of the experimental cases, the measured Fe and FeO traces agreed well with the mechanism reported in this work. The quantitative and highly-sensitive measurements revealed the presence of FeO at temperatures below 1400 K, leading to a reconsideration of rate coefficients for different Fe oxidation channels.

Detection of C2H2 using a segmented HC-PCF gas cell

This article demonstrates a fiber laser system that can detect acetylene gas (C2H2) at parts per million (ppmv) levels. The authors investigated the use of off-resonance laser wavelength to develop a highly sensitive detection system. They have developed a gas cell using segmented Hollow-Core Photonic Crystal fiber (HC-PCF), which reduced the filling and evacuation time and thus greatly improved the response time. The gas cell was filled with the gas sample and placed inside the laser cavity, which formed an intracavity laser absorption spectroscopic system. The laser wavelength was selected using a fiber Bragg grating (FBG). The fiber Bragg grating wavelength was centered between the R (5) and R (4) rotational lines of the acetylene ν1 + ν3 combination band, at 1522.22 nm, and used to probe their absorption.

Rovibrational Polaritons in Gas-Phase Methane.

Polaritonic states arise when a bright optical transition of a molecular ensemble is resonantly matched to an optical cavity mode frequency. Here, we lay the groundwork to study the behavior of polaritons in clean, isolated systems by establishing a new platform for vibrational strong coupling in gas-phase molecules. We access the strong coupling regime in an intracavity cryogenic buffer gas cell optimized for the preparation of simultaneously cold and dense ensembles and report a proof-of-principle demonstration in gas-phase methane. We strongly cavity-couple individual rovibrational transitions and probe a range of coupling strengths and detunings. We reproduce our findings with classical cavity transmission simulations in the presence of strong intracavity absorbers. This infrastructure will provide a new testbed for benchmark studies of cavity-altered chemistry.

Multiplexed Photonic Crystal Fiber Gas-Sensing Network Based on Intracavity Absorption.

A highly sensitive hollow-core photonic crystal fiber (HC-PCF) gas-sensing network based on intracavity absorption is designed and experimentally verified. The capacity of the multichannel sensing network is expanded by time division multiplexing and wavelength division multiplexing technology. The voltage gradient method is employed in the wavelength scanning process of Fabry-Perot (F-P) filter to enhance the detection efficiency up to six times. The proposed sensing network has 16 sensing points. Experimental results show that the minimum detection limit (MDL) of this sensing system is 25.91 ppm and 26.85 ppm at the acetylene gas absorption peaks of 1530.371 nm and 1531.588 nm, respectively. As far as we know, it is the first time to obtain an intracavity sensing network via the application of an optical switch and DWDM at the same time. The sensing network can be used for high-capacity, low-concentration dangerous gas detection. It has great potential in environmental monitoring, industrial manufacturing, safety inspection and similar occasions.

Atomic-level-mixing contribution to the P,T -odd Faraday effect as an enhancement factor in the search for P,T -odd interactions in nature

Recently, a $\mathcal{P},\mathcal{T}$-odd Faraday effect (PTFE), optical rotation in an external electric field parallel to the light propagation direction, was discussed as a promising tool for observation of the electron electric dipole moment in the intracavity absorption spectroscopy with atoms or molecules. The main mechanism leading to this effect, the linear Stark splitting of atomic levels (PTFE, LS) is well known. This mechanism is similar to the Zeeman splitting mechanism in case of the ordinary Faraday effect, i.e., optical rotation in an external magnetic field. In this paper we analyze the other possible mechanisms of PTFE, in particular, the atomic-level-mixing mechanism (PTFE, LM). In our analysis we perform considering PTFE (as well as the ordinary FE) as a kind of electro- (magneto-) optical circular birefringence. For the ordinary FE the level-mixing mechanism (FE, LM) is known apart from the main Zeeman level splitting mechanism (FE, LS). However, (PTFE, LM) mechanism for the PTFE was never considered in literature. Usually, the (FE, LS) mechanism dominates and the same was expected for (PTFE, LS). We demonstrate, however, that with heavy diatomic molecules, in particular in PbF, the contribution of (PTFE, LM) may essentially exceed the (PTFE, LS). This promises an additional enhancement of $\mathcal{P},\mathcal{T}$-odd effects in experiments with PbF molecule and makes this molecule one of the best candidates for performing such experiments.

Intracavity absorption spectroscopy of HCl isotopes, H2O, CH4, C2H4, and C2H6 in the 3.1-3.4 µm spectral range using a Cr:CdSe laser.

We demonstrate the first application of a Cr:CdSe laser for highly-sensitive multicomponent intracavity absorption spectroscopy around λ = 3.1-3.4 µm. A detection scheme based on an integrated recording of multiple (∼70) individual Cr:CdSe laser pulses after a single pump-pulse excitation is reported. The sensitivity of our system corresponds to an effective absorption path length of Leff ≈ 850 m. Exemplary measurements of atmospheric H2O and CH4, and additionally introduced gas-phase HCl, C2H4, or C2H6 are presented. The achieved noise-equivalent detection limits are in the ppb range. Possibilities for further sensitivity enhancement by up to a factor of 104 are discussed.

Spectral Purity Improvement in Flickering Self-Injection Locked Lasers

We study improvements of the Self-Injection Locked (SIL) distributed feedback (DFB) semiconductor laser phase noise as well as integral linewidth on the amount of the SIL feedback and show that SIL reduces the laser noise nearly equally in a broad frequency range, spanning from 1 Hz to 10 MHz. In our experiment SIL operation is supported by a tunable resonant reflector consisting of a high quality factor (high-Q) monolithic whispering gallery mode resonator (WGMR) supplied with a controllable attenuator. Using the experimental setup we validated theoretical formula describing the SIL spectral purity and identified practical parameters of the system allowing reaching the phase noise limit resulting from the fundamental thermodynamic noise of the resonator. We show that the linewidth improvement due to SIL effect is drastically impacted by the frequency spectral dependence of the phase noise of the laser and that it drops significantly in a flickering laser. We also confirmed that the thermorefractive coefficient as well as mode volume of the WGMR imposes the major limitations on the SIL performance. Low intracavity absorption is needed to suppress the attenuation-related thermal effects in the cavity.

Research on uncertainty of optical feedback linear cavity ringdown spectroscopy

Cavity ring-down spectroscopy (CRDS) is a highly sensitive molecular absorption spectroscopic technology, which has been widely used in mirror reflectance measurement, atmospheric trace gas detection, molecular precision spectroscopy and other fields. It deduces the intracavity absorption by measuring the rapid variation of the ringdown signal. As a result, detector with high linearity, broad bandwidth and low electrical noise is indispensable. Additionally, due to the large noise in laser frequency, low laser to cavity coupling efficiency is resulted. Consequently, the cavity transmission is faint, which deteriorates the detection sensitivity. Optical feedback could address this problem by locking the laser to the cavity longitudinal mode. Then, the laser frequency noise is suppressed and hence better detection sensitivity is expected. Optical feedback CRDS has been widely studied with V-shape cavity. Compared to Fabry-Perot cavity, this cavity geometry is more sensitive to mechanical vibration and possesses lower finesse with an additional mirror. In this paper, optical feedback linear cavity ring-down spectroscopy based on a Fabry-Perot cavity with a finesse of 7800 is presented. The principle of the combination of optical feedback and linear cavity is explained from the perspective of the light phase, which shows the reflection would not generate efficient optical feedback if the feedback phase is properly controlled and laser to cavity locking could be therefore realized. And then, the factors influencing the stability of ring-down signal is analyzed, including the feedback ratio, the trigger voltage for the ringdown event and the distance between the light spot and the detector center. The experimental results show a superior fractional uncertainty of the empty ringdown time of 0.026% could be attained with a low feedback rate (3% FSR), a high ringdown signal trigger threshold (90% cavity mode amplitude) and superposition of the light spot with the detector center. With Allan variance analysis, the white noise response of 1.56×10<sup>-9</sup> cm<sup>-1</sup>/ HZ<sup>-1/2</sup> and the detection sensitivity of .29×10<sup>-10</sup> cm<sup>-1</sup> for trace gas detection could be achieved at the integration time of 180 s, corresponding to the lowest CH<sub>4</sub>concentration detection of 0.35 ppb at 6046.9cm<sup>-1</sup>. This robust spectroscopic technique paves the way for the construction of high sensitive and stable cavity based instrument for trace gas detection.

Two-Mode Biomedical Sensor Build-up: Characterization of Optical Amplifier

Intracavity absorption spectroscopy is a strikingly sensitive technique that has been integrated with a two-wavelength setup to develop a sensor for human breath. Various factors are considered in such a scenario, out of which Relative Intensity Noise (RIN) has been exploited as an important parameter to characterize and calibrate the said setup. During the performance of an electrical based assessment arrangement which has been developed in the laboratory as an alternative to the expensive Agilent setup, the optical amplifier plays a pivotal role in its development and operation, along with other components and their significance. Therefore, the investigation and technical analysis of the amplifier in the system has been explored in detail. The algorithm developed for the automatic measurements of the system has been effectively deployed in terms of the laser's performance. With this in perspective, a frequency dependent calibration has been pursued in depth with this scheme which enhances the sensor's efficiency in terms of its sensitivity. In this way, our investigation helps us in a better understanding and implementation perspective of the proposed system, as the outcomes of our analysis adds to the precision and accuracy of the entire system.

Uncertainty of optical feedback linear cavity ringdown spectroscopy

Cavity ring-down spectroscopy (CRDS) is a highly sensitive molecular absorption spectroscopic technology, which has been widely used in mirror reflectance measurement, atmospheric trace gas detection, molecular precision spectroscopy and other fields. It deduces the intracavity absorption by measuring the rapid variation of the ringdown signal. As a result, detector with high linearity, broad bandwidth and low electrical noise is indispensable. Additionally, owing to the large noise in laser frequency, low laser-to-cavity coupling efficiency is obtained. Consequently, the cavity transmission is faint, which deteriorates the detection sensitivity. Optical feedback can address this problem by locking the laser to the cavity longitudinal mode. Then, the laser frequency noise is suppressed and hence better detection sensitivity is expected. Optical feedback CRDS with V-shape cavity has been widely studied. Compared with Fabry-Perot cavity, this cavity geometry is very sensitive to mechanical vibration and possesses low degree of fineness due to an additional mirror. In this paper, optical feedback linear cavity ring-down spectroscopy based on a Fabry-Perot cavity with a degree of fineness of 7800 is presented. The principle of the combination of optical feedback and linear cavity is explained from the perspective of the light phase, which shows that the reflection will not generate efficient optical feedback if the feedback phase is appropriately controlled and laser to cavity locking can be therefore realized. And then, the factors influencing the stability of ring-down signal are analyzed, including the feedback ratio, the trigger voltage for the ringdown event, and the distance between the light spot and the detector center. The experimental results show that a superior fractional uncertainty of the empty ringdown time of 0.026% can be obtained with a low feedback rate (3% FSR), a high ringdown signal trigger threshold (90% cavity mode amplitude) and superposition of the light spot with the detector center. With Allan variance analysis, the white noise response of 1.6 × 10<sup>–9</sup> cm<sup>–1</sup>·Hz<sup>–1/2</sup> and the detection sensitivity of 1.3 × 10<sup>–10</sup> cm<sup>–1</sup> for trace gas detection can be achieved in an integration time of 180 s, corresponding to the lowest CH<sub>4</sub> concentration detection of 0.35 × 10<sup>–9</sup> at 6046.9 cm<sup>–1</sup>. This robust spectroscopic technique paves the way for constructing high-sensitive and stable-cavity based instrument for trace gas detection.

A Review: Application and Implementation of Optic Fibre Sensors for Gas Detection.

At the present time, there are major concerns regarding global warming and the possible catastrophic influence of greenhouse gases on climate change has spurred the research community to investigate and develop new gas-sensing methods and devices for remote and continuous sensing. Furthermore, there are a myriad of workplaces, such as petrochemical and pharmacological industries, where reliable remote gas tests are needed so that operatives have a safe working environment. The authors have concentrated their efforts on optical fibre sensing of gases, as we became aware of their increasing range of applications. Optical fibre gas sensors are capable of remote sensing, working in various environments, and have the potential to outperform conventional metal oxide semiconductor (MOS) gas sensors. Researchers are studying a number of configurations and mechanisms to detect specific gases and ways to enhance their performances. Evidence is growing that optical fibre gas sensors are superior in a number of ways, and are likely to replace MOS gas sensors in some application areas. All sensors use a transducer to produce chemical selectivity by means of an overlay coating material that yields a binding reaction. A number of different structural designs have been, and are, under investigation. Examples include tilted Bragg gratings and long period gratings embedded in optical fibres, as well as surface plasmon resonance and intra-cavity absorption. The authors believe that a review of optical fibre gas sensing is now timely and appropriate, as it will assist current researchers and encourage research into new photonic methods and techniques.

Polarization impedance measurement cavity enhanced laser absorption spectroscopy.

We present a theoretical overview and experimental demonstration of a continuous-wave, cavity-enhanced optical absorption spectrometry method to detect molecular gas. This technique utilizes the two non-degenerate polarization modes of a birefringent cavity to obtain a zero background readout of the intra-cavity absorption. We use a double-pass equilateral triangle optical cavity design with additional feed-forward frequency noise correction to measure the R14e absorption line in the 30012←00001 band of CO2 at 1572.655 nm. We demonstrate a shot noise equivalent absorption of 3 × 10-13 cm-1 Hz-1/2.

Formula omitted]-odd Faraday rotation in intracavity absorption spectroscopy with particle beam as a possible way to improve the sensitivity of the search for the time reflection noninvariant effects in nature

Possible experiments on the observation of the P,T - odd Faraday effect (rotation of polarization plane for the light, propagating through a medium in the presence of an external electric field, P - is the space inversion, T - is the time reflection) caused by the electron electric dipole moment in atoms and molecules, are discussed theoretically. An observation is assumed to be performed on the basis of modern intra-cavity absorption spectroscopy (ICAS) combined with atomic (molecular) beam techniques. The P,T - odd optical rotation is due to the linear Stark splitting of atomic or molecular levels. This leads to the difference between the refractive indexes for the right and left circularly polarized photons.

Room-temperature Fe:ZnSe laser tunable in the spectral range of 3.7-5.3 µm applied for intracavity absorption spectroscopy of CO2 isotopes, CO and N2O.

We demonstrate an intracavity absorption spectroscopy system based on a broadband single-crystal pulsed Fe:ZnSe laser. The laser operates at room-temperature and is continuously tunable in the spectral range of 3.76-5.29 µm. The long-wavelength emission up to 5.29 µm is a record achievement for Fe:ZnSe lasers, to the best of our knowledge. The developed laser system is applied for measurements of gaseous absorption inside the laser resonator. We demonstrate sensitive detection of (i) CO2 isotopes in the atmosphere and in human breath, (ii) CO in breath (after cigarette smoking) and in the smoke of a smoldering paper, and (iii) N2O in a gas flow. The achieved detection limits are: 0.1 ppm for 12CO2 and 13CO2, 3 ppm for CO, and 1 ppm for N2O. The sensitivity of the current system is primarily limited by the short pump-pulse duration of 40 ns. Possibilities for sensitivity enhancement by up to a factor of 107 are discussed.

Photon-spin-dependent contribution to the -odd Faraday rotation effect for atoms

The photon-spin-dependent (PS) contribution to the ordinary Faraday effect—optical rotation in an external magnetic field is well known. This contribution is proportional to factor where is the photon spin. Recently a -odd Faraday effect—optical rotation in an external electric field —was discussed as a promising tool for observation of the electron electric dipole moment in the intra-cavity absorption spectroscopy. In the present paper, we introduce the PS contribution to the -odd Faraday rotation effect. This contribution is proportional to and has to be taken into account in the calculations of the -odd Faraday effect.

Widely-Tunable Quantum Cascade-Based Sources for the Development of Optical Gas Sensors.

Spectroscopic techniques based on Distributed FeedBack (DFB) Quantum Cascade Lasers (QCL) provide good results for gas detection in the mid-infrared region in terms of sensibility and selectivity. The main limitation is the QCL relatively low tuning range (~10 cm−1) that prevents from monitoring complex species with broad absorption spectra in the infrared region or performing multi-gas sensing. To obtain a wider tuning range, the first solution presented in this paper consists of the use of a DFB QCL array. Tuning ranges from 1335 to 1387 cm−1 and from 2190 to 2220 cm−1 have been demonstrated. A more common technique that will be presented in a second part is to implement a Fabry–Perot QCL chip in an external-cavity (EC) system so that the laser could be tuned on its whole gain curve. The use of an EC system also allows to perform Intra-Cavity Laser Absorption Spectroscopy, where the gas sample is placed within the laser resonator. Moreover, a technique only using the QCL compliance voltage technique can be used to retrieve the spectrum of the gas inside the cavity, thus no detector outside the cavity is needed. Finally, a specific scheme using an EC coherent QCL array can be developed. All these widely-tunable Quantum Cascade-based sources can be used to demonstrate the development of optical gas sensors.