Photothermal Interferometry Research Articles

Fabry-Pérot photothermal interferometry in a hollow-core antiresonant fiber for gas detection in mid-infrared

Hollow-core antiresonant fiber can simultaneously act as a miniature gas cell and the light guidance with broad wavelength range, introducing an powerful tool to photothermal spectrosocpy and sensitive gas detection. Photothermal interferometry (PTI) is a spectroscopic tehcnique that employs an interferometer to finely measure the photothermal signal of trace gas. By splicing the HC-ARF between solid core fibers, a Fabry-Pérot (F-P) cavity is formed and enables PTI with a brief in-line design. However, the previous structure commonly adopted single-mode solid core fibers and might introduce large optical loss due to mode field mismatch, particularly for MIR pump laser. This could lead to significant decrease in sensitivity. Besides, the gas pressure effect on the HC-ARF-based PTI gas sensor had not been fully studied. In this work, we demonstrated the PTI for 13CO2 detection at 4.35 µm by employing an HC-ARF with a 100-µm core. An InF3 multimode fiber was utilized for the efficient delivery of MIR pump beam and successfully enhanced the pump power by a factor of 6.7. The limit of detection was determined to be 0.4 ppb at the integration time of 240 s. Finally, the relationship between gas pressure and photothermal signal was experimentally studied and compared with theoretical results.

Optimization of the optical path length amplitude for interferometric photothermal gas and aerosol sensing considering advection: A theoretical study

Photothermal spectroscopy, and more specifically photothermal interferometry (PTI), is a highly sensitive technique for measuring gas and aerosol concentrations. Numerous implementations of different PTI configurations have demonstrated the versatility of the technique. This theoretical study presents a comprehensive analysis and an optimization of the PTI optical path length (OPL) amplitude using characteristic times. We investigate how the OPL amplitude depends on the dimensions and orientations of the interferometer laser beam and the continuous-wave excitation laser beam. This analysis quantifies the impact of advection on the OPL amplitude based on the relative orientation of the two laser beams and the gas flow direction. It is analytically shown that the possibilities for photothermal OPL amplitude optimization are limited when thermal diffusion is dominant. Theoretically, advection has the potential to double or cancel the OPL amplitude, depending on the specific configurations. In summary, we provide an in-depth understanding of the design and parameter considerations required when tailoring and optimizing a PTI sensor for different fields of applications.

Pump–probe-alternating photothermal interferometry for two-component gas sensing

We demonstrate a high-sensitivity acetylene/methane gas sensor based on hollow-core fiber photothermal interferometry (PTI) with a pump–probe-alternating technique. This technique utilizes two distributed-feedback lasers as pump and probe beams alternatively for two gas components to facilitate photothermal phase modulation and detection through time-division multiplexing. With a 2.5-cm-long hollow-core conjoint-tube fiber, noise-equivalent concentrations of 370 ppb and 130 ppb are demonstrated for methane and acetylene, respectively. Noise characteristics of the PTI system are analyzed and experimentally tested. The proposed technique eliminates the need for an additional laser in the traditional PTI setup, enabling the construction of a sensitive yet more cost-effective multi-gas component detection system.

Low‐Coherence Photothermal Interferometry for Precision Spectroscopic Gas Sensing

AbstractLaser spectroscopy has shown great potential as a cost‐effective method for trace gas detection with high sensitivity and specificity. However, it still falls short in applications such as the measurement of stable isotope ratios, which require high precision and stability. Here, ultrasensitive gas detection with remarkable precision and stability as well as better immunity to isobaric interference is demonstrated by use of low‐coherence photothermal interferometry. With a 10 cm long hollow‐core fiber, acetylene detection is achieved with a noise‐equivalent concentration of 0.7 part‐per‐billion and measurement precision of 0.025%. The instability of the detection over a period of 3 h is ±0.038%, 10 times better than the state‐of‐the‐art photothermal spectroscopy. The measurement of the 13C/12C isotope ratio of acetylene is demonstrated with measurement precision of ≈0.01%. This study highlights the potential of low‐coherence photothermal interferometry as an alternative to mass spectrometers, offering reliable gas detection in a compact form.

Dispersion turning point-enhanced photothermal interferometry gas sensor with an optical microfiber interferometer

Photothermal interferometry (PTI) spectroscopy is a background-free and ultrasensitive gas or chemical sensing method. To date, most works have focused on applying different techniques to improve the sensitivity of PTI gas sensors via enhancing the photothermal (PT) phase modulation. Herein, we develop an all-fiber dispersion turning point (DTP)-enhanced PTI gas sensor with a taper-based mode interferometer in which the PT phase detection sensitivity of the interferometer is enhanced in a millimeter-long single fiber PTI configuration, which has not been studied yet. The design is demonstrated by conducting the measurement of acetylene with a 3-mm-long, 2.29-μm-diamter microfiber interferometer. A lower detection limit of 965 parts per billion (ppb) acetylene is achieved when the probe wavelength is operating near DTP, showing 16 times enhancement. Besides, theoretical analysis shows that the sensitivity of phase detection would be enormously enhanced when the dispersion factor approaching zero. By optimal design of the taper-based mode interferometer, together with thinner fiber diameter and longer fiber length, gas detection may be further improved by double enhancement of both PT phase generation and phase detection. With advantages of high sensitivity, compact size and low cost, the proposed all-fiber DTP-enhanced PTI gas sensor is potentially promising for ultra-sensitive trace chemical or biochemical sensing.

Amplified Photothermal Phase Modulation for Carbon Dioxide Detection by Operating a Dual-Mode Interferometer at Destructive Interference.

Photothermal interferometry is a highly sensitive spectroscopic technique for trace gas detection. However, the performance of the state-of-the-art laser spectroscopic sensors is still insufficient for some high-precision applications. Here, we demonstrate optical phase-modulation amplification by operating a dual-mode optical fiber interferometer at destructive interference for ultrasensitive carbon dioxide detection. With a 50 cm long dual-mode hollow-core fiber, amplification of photothermal phase modulation by a factor of nearly 20 is achieved, which enables carbon dioxide detection down to 1 parts-per-billion with a dynamic range of over 7 orders of magnitude. This technique could be readily used to improve the sensitivity of phase modulation-based sensors with a compact and simple configuration.

Narrow-band multi-component gas analysis based on photothermal spectroscopy and partial least squares regression method

In spectroscopic multi-component gas sensing, the crosstalk in the overlapping region of gas absorption spectra will affect the measurement accuracy. This paper reports a new multi-component gas analyzing method based on partial least squares regression (PLSR), which can retrieve target gas concentration and the composition and concentration of interfering gas from an overlapping narrow-band spectrum. Two overlapping absorption spectra (acetylene and ammonia) around 1530 nm are selected to simulate the absorption crosstalk phenomenon. We built a fiber-optic photothermal interferometry system to verify the method on two-component gases in the overlapping region. The training of the PLSR model was based on 63 sets of photothermal second-harmonic signals, which acquired from the gases composed by 100–700 ppm ammonia and 100–900 ppm acetylene. The prediction error of PLSR model achieved 3.84 % when tested by extra data.

A dual-wavelength photothermal aerosol absorption monitor: design, calibration and performance

Abstract. There exists a lack of aerosol absorption measurement techniques with low uncertainties and without artefacts. We have developed the two-wavelength Photothermal Aerosol Absorption Monitor (PTAAM-2λ), which measures the aerosol absorption coefficient at 532 and 1064 nm. Here we describe its design, calibration and mode of operation and evaluate its applicability, limits and uncertainties. The 532 nm channel was calibrated with ∼ 1 µmol mol−1 NO2, whereas the 1064 nm channel was calibrated using measured size distribution spectra of nigrosin particles and a Mie calculation. Since the aerosolized nigrosin used for calibration was dry, we determined the imaginary part of the refractive index of nigrosin from the absorbance measurements on solid thin film samples. The obtained refractive index differed considerably from the one determined using aqueous nigrosin solution. PTAAM-2λ has no scattering artefact and features very low uncertainties: 4 % and 6 % for the absorption coefficient at 532 and 1064 nm, respectively, and 9 % for the absorption Ångström exponent. The artefact-free nature of the measurement method allowed us to investigate the artefacts of filter photometers. Both the Aethalometer AE33 and CLAP suffer from cross-sensitivity to scattering – this scattering artefact is most pronounced for particles smaller than 70 nm. We observed a strong dependence of the filter multiple scattering parameter on the particle size in the 100–500 nm range. The results from the winter ambient campaign in Ljubljana showed similar multiple scattering parameter values for ambient aerosols and laboratory experiments. The spectral dependence of this parameter resulted in AE33 reporting the absorption Ångström exponent for different soot samples with values biased 0.23–0.35 higher than the PTAAM-2λ measurement. Photothermal interferometry is a promising method for reference aerosol absorption measurements.

Mid-infrared photothermal gas sensor enabled by core-cladding mode interference in a hollow-core fiber

Photothermal spectroscopy provides an attractive way of sensitive gas detection with a large dynamic range. Previous photothermal gas sensors normally use an optical interferometer for measuring the slight phase change caused by gas absorption. However, an active opto-mechanical stabilization system is required in the photothermal interferometry, making the sensor complex, bulky and hard for field applications. Here, we report a new mid-infrared photothermal gas sensor based on core-cladding mode interference in a tellurite hollow-core antiresonant fiber (HC-ARF). This method relies on the co-propagation of the HE₁₁ mode of a mid-infrared pump laser and the core-cladding modes of a probe laser in the tellurite HC-ARF. As a proof-of-principle demonstration, we detect nitric oxide in the 35 cm long fiber by using a quantum cascade laser (5.2 µm, 21.7 mW) as the pump laser. We achieve a noise equivalent concentration of 50 ppbv, corresponding to a normalized noise equivalent absorption coefficient of 8.6 × 10^-9 cm^-1WHz^-1/2. Our technique has long-term stability without using any active stabilization devices and holds promise for fast, sensitive and background-free gas detection with a simplified configuration.

Phase-Modulation-Amplifying Hollow-Core Fiber Photothermal Interferometry for Ultrasensitive Gas Sensing

Photothermal interferometry (PTI) with hollow core fibers (HCFs) have enabled highly sensitive spectroscopic gas sensors in an all-fiber format. Here we report remarkable improvement in the limit of detection of HCF-PTI, in terms of noise equivalent concentration (NEC), by exploiting the optical-phase-modulation amplifying (OPMA) effect of an HCF resonating cavity. By locking the wavelength of a 1550 nm probe laser to the resonance of a 10-cm-long HCF Fabry-Pérot cavity with a finesse of ∼700, OPMA of more than two orders of magnitude is achieved, which enables ultra-sensitive gas detection with large dynamic range. With 1654 nm, 1532 nm, and 761 nm pump lasers, we demonstrate detection of methane, acetylene, and oxygen with noise-equivalent-concentration of 15 parts-per-trillion (ppt), 2.7 ppt, and 0.56 parts-per-million (ppm), respectively. Further improvement in NEC is possible by use of a higher finesse cavity with a longer length of HCF. Extension of the technique to other gases, other types of phase or dispersion modulation-based sensors, and other optical resonating cavities is straightforward.

Multicomponent trace gas detection with hollow-core fiber photothermal interferometry and time-division multiplexing

We report a multicomponent photothermal gas sensor with a conjoined-tube hollow-core optical fiber gas cell. With a common Fabry-Perot probe interferometer and a common gas cell, simultaneous detection of methane, acetylene and ammonia can be achieved by time-division multiplexing. Experiments with a 15-cm-long hollow-core fiber demonstrated noise-equivalent concentration of 24.2 parts-per-billion (ppb) for methane, 11.6 ppb for acetylene, and 46.1 ppb for ammonia. The dynamic range is measured to be around 5 orders of magnitude. The crosstalk issue is addressed by spectrum fitting. Assisted with an air pump and a compact gas chamber, the response time of less than 10 s is achieved.

Antiresonant hollow core fiber-assisted photothermal spectroscopy of nitric oxide at 5.26 μm with parts-per-billion sensitivity

Laser-based gas sensors are undergoing constant and rapid development due to their usability in numerous applications focused on e.g. monitoring of greenhouse gases concentration, safety in harsh environments or medicine. Amongst many types of such sensors, gas detectors employing the photoacoustic or photothermal effect are characterized by an exceptional capability of delivering high selectivity combined with superb signal-to-noise ratio (SNR). In this work, we demonstrate a novel, compact and sensitive nitric oxide (NO) sensor targeting its strong transition near 5.26 μm by combining the photothermal interferometry (PTI) technique and a gas absorption cell based on a self-fabricated antiresonant hollow-core fiber (ARHCF). Having only a 25 cm long interaction length, the sensor reached a minimum detection limit (MDL) of 11 ppbv (parts per billion by volume) for an integration time of 144 s. Proper design of the ARHCF utilized as the gas cell enabled combining a mid-infrared (mid-IR) pump laser to induce the refractive index modulation and a Fabry-Perot interferometer-based readout of the photothermal effect with a near-infrared probe laser. The presented results document the first experimental attempt to detect gas molecules having transitions beyond 4.5 μm in an ARHCF-aided photothermal detection scheme.

Sensitivity enhancement in photothermal interferometry by balanced detection of the complex response to moving excitation.

The sensitivity of photothermal detection relies on both the magnitude of the response of a sample to excitation and the way the response is sensed. We propose a highly sensitive photothermal interferometry by addressing the above two issues. One is the use of moving excitation to enable a different manner in sample heating and cooling, which results in a strong thermoelastic response of the sample. The other is the use of a balanced Mach-Zehnder interferometer with a defocused probe beam to sense the complex response induced by the phase delays taking place at the sample surface and in the surrounding air. The method was verified experimentally with a Nd-doped glass to have 68-fold sensitivity enhancement over the classical photothermal common-path interferometry.

Hollow-core fiber photothermal methane sensor with temperature compensation.

We demonstrate a high sensitivity all-fiber spectroscopic methane sensor based on photothermal interferometry. With a 2.4-m-long anti-resonant hollow-core fiber, a 1654 nm distributed feedback laser, and a Raman fiber amplifier, a noise-equivalent concentration of ${\sim}{4.3}\;{\rm ppb}$ methane is achieved at the room temperature and pressure of ${\sim}{1}\;{\rm bar}$. The effects of temperature on the photothermal phase modulation as well as the stability of the interferometer are studied. By introducing a temperature-dependent compensation factor and stabilizing the interferometer at quadrature, signal instability of ${\sim}{2.1}\%$ is demonstrated for temperature variation from 296 to 373 K.

3D characterization of low optical absorption structures in large crystalline sapphire substrates for gravitational wave detectors

Very high-quality sapphire substrates are key elements of the cryogenic Japanese gravitational interferometer KAGRA, in which they are used to build the main mirrors, working as the test masses to sense the gravitational waves. To meet the extreme requirements of this system, the sapphire test masses must possess an extremely low optical absorption, which makes their study challenging using standard methods. In this paper, we illustrate the results obtained on two typical samples using a specialized absorption setup based on the technique of Photo-thermal Common-path Interferometry (PCI). Our system combines a very high sensitivity to small absorption features with the possibility to perform a full three-dimensional mapping of the sample volume. Our results elucidate how the ultra-low absorption variations inside the sample possess a structure that is probably inherited from the growth history of the sample. Some conclusions on the role of structural defects as preferential sites for the inclusion of absorbing centers are drawn.

Operation of a Single-Frequency Bismuth-Doped Fiber Power Amplifier near 1.65 µm

The spectral range between 1650 and 1700 nm is an interesting region due to its potential applications in optical telecommunication and optical-based methane sensing. Unfortunately, the availability of compact and simple optical amplifiers with output powers exceeding tens of milliwatts in this spectral region is still limited. In this paper, a single-frequency continuous-wave bismuth-doped fiber amplifier (BDFA) operating at 1651 and 1687 nm is presented. With the improved signal/pump coupling and modified pump source design, the output powers of 163 mW (at 1651 nm) and 197 mW (at 1687 nm) were obtained. Application of the BDFA to the optical spectroscopy of methane near 1651 nm is also described. We demonstrate that the BDFA can be effectively used for signal amplitude enhancement in photothermal interferometry.

Influence of edge effects on laser-induced surface displacement of opaque materials by photothermal interferometry

We demonstrate the influence of edge effects on the photothermal-induced phase shift measured by a homodyne quadrature laser interferometer and compare the experiments with rigorous theoretical descriptions of thermoelastic surface displacement of metals. The finite geometry of the samples is crucial in determining how the temperature is distributed across the material and how this affects the interferometer phase shift measurements. The optical path change due to the surface thermoelastic deformation and thermal lens in the surrounding air is decoded from the interferometric signal using analytical and numerical tools. The boundary/edge effects are found to be relevant to properly describe the interferometric signals. The tools developed in this study provide a framework for the study of finite size effects in heat transport in opaque materials and are applicable to describe not only the phase shift sensed by the interferometer but also to contribute to the photothermal-based technologies employing similar detection mechanisms.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy.

An optical cavity can be utilized as an excellent transducer for highly sensitive gas detection with the application of photothermal spectroscopy, featuring the beneficial property of an ultra-low absorption volume within a rugged sensing element. We report the novel implementation of balanced detection in Fabry-Perot photothermal interferometry via two identical 1 mm-spaced cavities. That way, excess noise limiting the sensitivity of previous cavity-based photothermal sensors was effectively rejected close to the fundamental limit of shot noise. A quantum cascade laser served as mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced variations. The metrological qualities of the sensor were investigated by SO2 detection. For the targeted absorption band centered at 1380.93 cm-1, a 5 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 7.5 × 10-9 cm-1 W Hz-1/2. Additionally, the sensor showed excellent long-term stability, enabling integration times of a few thousand seconds.

Localized Chemical Detection in Quasi-Distributed Multi-Node Fiber-Ring Network

Laser-based molecular spectroscopy plays an important role in chemical analysis and gas detection. Here we present how optical spectroscopy can be combined with quasi-distributed fiber-based passive sensing networks. Simultaneous gas detection and localization is demonstrated using photothermal interferometry at 1.55 μm. With heterodyne-based signal retrieval sensor node position can be determined through measurement of phase/time delay of photothermal signal. This is performed using simple and passive network configuration. Proof-of-concept experiments are presented in which distance resolving power of approximately 5 meters was achieved with 1 second integration time. System performance, limitations, and capabilities are analyzed and discussed.

Photothermal Interferometry Gas Sensor Based on the First Harmonic Signal

Photothermal interferometry (PTI) incorporated with wavelength modulation spectroscopy is a typical ultrasensitive spectroscopic technique for trace gas detection. In this paper, to our best knowledge, we demonstrate that the first harmonic signal is the best choice for a PTI spectroscopy gas sensing system for the first time. Both the theoretical and experimental results confirm that the first harmonics signal has the largest peak-to-peak amplitude in all harmonic signals. The noise measurement indicates that the all harmonic signals nearly have the same noise level. The detection limit in terms of noise equivalent concentration is improved by 30%, from 114 to 78 ppb ( ${\text{9}}\,\times \,{\text{10}}^{-8}\,{\text{cm}}^{-1}$ in absorption coefficient) by use of the first harmonic signal instead of the second harmonic signal.