Slow-Light-Enhanced Polarization Division Multiplexing Infrared Absorption Spectroscopy for On-Chip Wideband Multigas Detection in a 1D Photonic Crystal Waveguide.

Slow-light photonic crystal waveguide (PCW) gas sensors based on infrared absorption spectroscopy play a pivotal role in enhancing the on-chip interaction between light and gas molecules, thereby significantly boosting sensor sensitivity. However, two-dimensional (2D) PCWs are limited by their narrow mode bandwidth and susceptibility to polarization, which restricts their ability for multigas measurement. Due to quasi-TE and quasi-TM mode guiding characteristics in one-dimensional (1D) PCW, a novel slow-light-enhanced polarization division multiplexing infrared absorption spectroscopy was proposed for on-chip wideband multigas detection. The optimized 1D PCW gas sensor experimentally shows an impressive slow-light mode bandwidth exceeding 100 nm (TM, 1500-1550 nm; TE, 1610-1660 nm) with a group index ranging from 4 to 25 for the two polarizations. The achieved bandwidth in the 1D PCW is 2-3 times that of the reported quasi-TE polarized 2D PCWs. By targeting the absorption lines of different gas species, multigas detection can be realized by modulating the lasers and demodulating the absorption signals at different frequencies. As an example, we performed dual-gas measurements with the 1D PCW sensor operating in TE mode at 1.65 µm for methane (CH4) detection and in TM mode at 1.53 µm for acetylene (C2H2) detection. The 1 mm long sensor achieved a remarkable limit of detection (LoD) of 0.055% for CH4 with an averaging time of 17.6 s, while for C2H2, the LoD was 0.18%. This polarization multiplexing sensor shows great potential for on-chip gas measurement because of the slow-light enhancement in the light-gas interaction effect as well as the large slow-light bandwidth for multigas detection.

Near-Infrared Off-Axis Cavity-Enhanced Optical Frequency Comb Spectroscopy for CO2/CO Dual-Gas Detection Assisted by Machine Learning.

Cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) is widely used as a highly sensitive gas sensing technology in various gas detection fields. For the on-axis coupling incidence scheme, the detection accuracy and stability are seriously affected by the cavity-mode noise, and therefore, stable operation inevitably requires external electronic mode-locking and sweeping devices, substantially increasing system complexity. To address this issue, we propose off-axis cavity-enhanced optical frequency comb spectroscopy from both theoretical and experimental aspects, which is applied to the detection of single- and dual-gas of carbon monoxide (CO) and carbon dioxide (CO2) in the near-infrared. An erbium-doped fiber frequency comb with a repetition frequency of ∼41.709 MHz is coupled into a resonant cavity with a length of ∼360 mm in an off-axis manner, exciting numerous high-order modes to effectively suppress cavity-mode noise. The performance of multiple machine learning models is compared for the inversion of a single/dual gas concentration. A few absorbance spectra are collected to build a sample data set, which is then utilized for model training and learning. The results demonstrate that the Particle Swarm Optimization Support Vector Machine (PSO-SVM) model achieves the highest predictive accuracy for gas concentration and is ultimately applied to the detection system. Based on Allan deviation, the detection limit for CO in single-gas detection can reach 8.247 parts per million by volume (ppmv) by averaging 87 spectra. Meanwhile, for simultaneous CO2/CO measurement with highly overlapping absorbance spectra, the LoD can be reduced to 13.196 and 4.658 ppmv, respectively. The proposed optical gas sensing technique indicates the potential for the development of a field-deployable and intelligent sensor system capable of simultaneous detection of multiple gases.

Mid-infrared auto-correction on-chip waveguide gas sensor based on 2f/1f wavelength modulation spectroscopy.

Compared to the most commonly used on-chip direct absorption spectroscopy (DAS) gas detection technique, the second harmonic (2f) based on-chip wavelength modulation spectroscopy (WMS) proposed by our group has the faculty to suppress noise and improve performance, but the accuracy of 2f WMS is easily affected by optical power variation. A mid-infrared auto-correction on-chip gas sensor based on 2f/1f WMS was proposed for decreasing the influence of the variation of optical power. The limit of detection of methane (CH4) obtained by a chalcogenide waveguide with a length of 10 mm is 0.031%. Compared with the 2f WMS, the maximum relative concentration error of the auto-correction on-chip gas sensor was decreased by ∼5.6 times. The measurement error is ≤2% in a temperature variation range of 30°C. This auto-correction sensor without a complicated manual calibration is helpful to the high accuracy measurement for on-chip integrated gas sensing.

WMS-based near-infrared on-chip acetylene sensor using polymeric SU8 Archimedean spiral waveguide with Euler S-bend.

SU8 is a cost-effective polymer material that is highly suitable for large-scale fabrication of waveguides. However, it has not been employed for on-chip gas measurement utilizing infrared absorption spectroscopy. In this study, we propose a near-infrared on-chip acetylene (C2H2) sensor using SU8 polymer spiral waveguides for the first time to our knowledge. The performance of the sensor based on wavelength modulation spectroscopy (WMS) was experimentally validated. By incorporating the proposed Euler-S bend and Archimedean spiral SU8 waveguide, we achieved a reduction in the sensor's size by over fifty percent. Leveraging the WMS technique, we evaluated the C2H2 sensing performance at 1532.83 nm for SU8 waveguides of lengths 7.4 cm and 13 cm. The limit of detection (LoD) values were 2197.1 ppm (parts per million) and 425.5 ppm, respectively, with an averaging time of 0.2 s. Furthermore, the experimentally obtained optical power confinement factor (PCF) closely approximated the simulated value, with a value of 0.0172 compared to the simulated value of 0.016. The waveguide loss is measured to be 3 dB/cm. The rise time and fall time were approximately 2.05 s and 3.27 s, respectively. This study concludes that the SU8 waveguide exhibits significant potential for high-performance on-chip gas sensing in the near-infrared wavelength range.

Ultra-Wideband Mid-Infrared Chalcogenide Suspended Nanorib Waveguide Gas Sensors with Exceptionally High External Confinement Factor beyond Free-Space.

On-chip waveguide sensors are potential candidates for deep-space exploration because of their high integration and low power consumption. Since the fundamental absorption of most gas molecules exists in the mid-infrared (e.g., 3-12 µm), it is of great significance to fabricate wideband mid-infrared sensors with high external confinement factor (ECF). To overcome the limited transparency window and strong waveguide dispersion, a chalcogenide suspended nanorib waveguide sensor was proposed for ultra-wideband mid-infrared gas sensing, and three waveguide sensors (WG1-WG3) with optimized dimensions exhibit a wide waveband of 3.2-5.6 µm, 5.4-8.2 µm, and 8.1-11.5 µm with exceptionally high ECFs of 107-116%, 107-116%, and 116-128%, respectively. The waveguide sensors were fabricated by a two-step lift-off method without dry etching to reduce the process complexity. Experimental ECFs of 112%, 110%, and 110% were obtained at 3.291 µm, 4.319 µm, and 7.625 µm, respectively, through methane (CH4) and carbon dioxide (CO2) measurements. A limit of detection of 5.9 ppm was achieved for an averaging time of 64.2 s through the Allan deviation analysis of CH4 at 3.291 µm, leading to a comparable noise equivalent absorption sensitivity of 2.3 × 10-5 cm-1 Hz-1/2 as compared to the hollow-core fiber and on-chip gas sensors.

On-chip mid-infrared silicon-on-insulator waveguide methane sensor using two measurement schemes at 3.291 µm.

Portable or even on-chip detection of methane (CH4) is significant for environmental protection and production safety. However, optical sensing systems are usually based on discrete optical elements, which makes them unsuitable for the occasions with high portability requirement. In this work, we report on-chip silicon-on-insulator (SOI) waveguide CH4 sensors at 3.291 µm based on two measurement schemes including direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). In order to suppress noise, Kalman filter was adopted in signal processing. By optimizing the waveguide cross-section structure, an etch depth of 220 nm was selected with an experimentally high power confinement factor (PCF) of 23% and a low loss of only 0.71 dB/cm. A limit of detection (LoD) of 155 parts-per-million (ppm) by DAS and 78 ppm by WMS at an averaging time of 0.2 s were obtained for a 2 cm-long waveguide sensor. Compared to the chalcogenide (ChG) waveguide CH4 sensors at the same wavelength, the reported sensor reveals the minimum waveguide loss and the lowest LoD. Therefore the SOI waveguide sensor has the potential of on-chip gas sensing in the mid-infrared (MIR) waveband.

Mid-infrared ChG-on-MgF2 waveguide gas sensor based on wavelength modulation spectroscopy: publisher's note.

This publisher's note contains corrections to Opt. Lett.46, 4797 (2021)OPLEDP0146-959210.1364/OL.440361.

Mid-infrared ChG-on-MgF2 waveguide gas sensor based on wavelength modulation spectroscopy.

A novel, to the best of our knowledge, mid-infrared chalcogenide (ChG) on magnesium fluoride (MgF2) waveguide gas sensor was fabricated by using the lift-off method. MgF2 was used as a lower cladding layer to increase the external confinement factor for enhancing light-gas interaction. Wavelength modulation spectroscopy (WMS) was used in carbon dioxide (CO2) detection at the wavelength of 4319 nm (2315.2cm-1). The limit of detection for the 1-cm-long sensing waveguide based on WMS is ∼0.3%, which is >8 times lower than the same sensor using direct absorption spectroscopy (DAS). The combination of WMS with the waveguide gas sensor provides a new measurement scheme for the performance improvement of on-chip gas detection.

Surface-Enhanced Infrared Absorption Spectroscopic Chalcogenide Waveguide Sensor Using a Silver Island Film.

A surface-enhanced infrared absorption spectroscopic chalcogenide waveguide sensor based on the silver island film was proposed for the first time to enhance the sensing performance in both liquid and gas phases. The chalcogenide waveguide sensor was fabricated by the lift-off and oblique angle deposition methods. The surface morphology of the silver island film with different thicknesses was characterized. The absorption of ethanol (liquid) at a wavelength of 1654 nm and that of methane (gas) at 3291 nm were measured using the fabricated chalcogenide waveguide sensor. The chalcogenide waveguide sensor integrated with the 1.8 nm-thick silver island film revealed the best sensing performance. With an acceptable increased waveguide loss resulting from the fabrication of the film, the absorbance enhancement factors for ethanol and methane were experimentally obtained to be >1.5 and >2.3, respectively. The 1σ limit of detection of methane for the sensor integrated with the 1.8 nm-thick silver island film was ∼4.11% for an averaging time of 0.2 s. The mathematic relation between the absorbance enhancement factor and the waveguide loss was derived for sensing performance improvement. Also, the proposed rectangular waveguide sensor provides an idea for the design of a sensor-on-a-chip instead of other waveguide sensors with a high requirement of fabrication accuracy, for example, a slot waveguide or a photonic crystal waveguide.

Theoretical study of microcavity-enhanced absorption spectroscopy for mid-infrared methane detection using a chalcogenide/silica-on-fluoride horizontal slot-waveguide racetrack resonator.

The reported chalcogenide (ChG) rectangular waveguide sensors with a small evanescent field need a large waveguide length to obtain an enhanced light-gas interaction effect. To make such sensors compact and improve the light-gas interaction effect, a microcavity-enhanced absorption spectroscopy technique for methane (CH4) detection was proposed using a mid-infrared chalcogenide/silica-on-fluoride horizontal slot-waveguide racetrack resonator. For the horizontal slot waveguide, an equivalent sensor model (ESM) and related formulations were proposed to simplify the analysis of the racetrack resonator sensor model (RRSM), and the ESM was verified through a comparison between the theoretical result of ESM and the simulation result of RRSM based on the finite element method (FEM). Due to the use of a chalcogenide/silica-on-fluoride horizontal slot-waveguide structure, the waveguide parameters were optimized to obtain a high power confinement factor of 44.63% at the wavelength of 3291 nm, which is at least 5 times higher than other ChG rectangular waveguides. The waveguide length is reduced at least 30 times due to the use of the optimized chalcogenide/silica-on-fluoride horizontal slot-waveguide and racetrack resonator. The limit of detection (LoD) is 3.87 ppm with an intrinsic waveguide loss of 3 dB/cm and an amplitude coupling ratio of 0.1 for the resonator. The response time is less than 5 µs due to the small light-gas interaction area. The influences of environmental pressure and waveguide intrinsic loss on the sensing characteristics were discussed. The compact racetrack resonator sensor structure and equivalent analytical model can also be adopted in the design of an on-chip waveguide sensor for the detection of other gas species.