In this paper we report a mid-infrared sensor based on an anti-resonant hollow core fiber. A quantum cascade laser operating around 4.53 µm is used to target one of the strongest transition of ...nitrous oxide near 2203.7 cm
. The system provides 1-second minimum detection limit at single parts-per-billion level using 3.2-m-long fiber with the response time of less than 30 seconds. Presented sensing approach shows a good perspective for compact and sensitive mid-infrared fiber-based spectrometers.
For over a decade hollow-core fibers have been used in optical gas sensors in the role of gas cells. However, very few examples of actual real-life applications of those sensors have been ...demonstrated so far. In this paper, we present a highly-sensitive hollow-core fiber based methane sensor. Mid-infrared distributed feedback interband cascade laser operating near 3.27 µm is used to detect gas inside anti-resonant hollow-core fiber. R(3) line near 3057.71 cm
located in ν
band of methane is targeted. Compact, lens-free optical setup with an all-silica negative curvature hollow-core fiber as the gas cell is demonstrated. Using wavelength modulation spectroscopy and 7.5-m-long fiber the detection limit as low as 1.54 ppbv (at 20 s) is obtained. The demonstrated system is applied for a week-long continuous monitoring of ambient methane and water vapor in atmospheric air at ground level. Diurnal cycles in methane concentrations are observed, what proves the sensor's usability in environmental monitoring.
Multimodal interference is frequently pointed out as one of the main limiting factors in gas sensing setups which utilize hollow-core optical fibers as gas cells. In this paper we demonstrate ...suppression of optical fringes that originate from propagation of higher order modes in an anti-resonant hollow-core fiber by controlled fiber bending. Near-infrared broadband absorption spectroscopy is used to characterize the fringe patterns observed when higher order modes are excited and suppressed. A tunable laser diode emitting near 1687 nm is used to present positive impact of bending the hollow-core fiber on signals recorded with both direct absorption spectroscopy and wavelength modulation spectroscopy of methane. Bending radius of 30 mm was found optimal for the setup presented in this work, leading to sub-ppm-level detection limit at 1687 nm with 7.35-m-long fiber.
Hollow-core optical fibers have been used for laser gas sensing for almost two decades. However, to date, the vast majority of sensing systems have used hollow-core fiber only as a replacement for ...bulky gas cells (single- or multi-pass). Here, we investigate a reflective, dual-pass configuration in which light enters and exits hollow-core fiber from the same side. The advantage of this approach is the fact that the hollow-core fiber can be used not only as a cell but also as an endoscope-like probe, which does not need any additional tubing for sample delivery.
In this paper, we identify unwanted reflections from various fiber-to-fiber connections in the setup as the main challenge limiting the performance of the sensor in reflective configuration. We propose and experimentally demonstrate that the problem of reflections can be addressed using heterodyne-based detection, which allows for identifying and extracting useful signals in the radio frequency domain. Absorption and dispersion spectroscopy of methane are performed as a proof-of-concept with a detection limit of methane concentration below 10 ppm. Discussion insights into further development are provided, including strategies for miniaturization and detection limit improvement.
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.
In this paper, we present a laser-based sensing inside anti-resonant hollow core fiber. A distributed feedback laser diode operating near 2004 nm and a 1.35-m-long silica-based fiber are used to ...demonstrate carbon dioxide detection with sensitivity down to ~5 ppmv. Gas exchange time as low as 5 seconds is obtained. This performance was achieved in a very simple optical configuration, without any mirrors or lenses in the setup.
•A new design of fast-response multi-segment hollow-core fiber gas cell for laser spectroscopy is demonstrated.•The setup utilizes 1.35-m-long hollow-core fiber segments arranged in series which are ...simultaneously filled with gas.•Direct absorption spectroscopy of methane at 1687 nm is used for setup characterization. Its pros and cons are discussed.•The pressure buildup and drawdown times are identified as a critical factor limiting the total response time the sensor.
Typically, hollow-core fiber (HCF)-based laser gas sensing systems use a single monolithic fiber as a gas cell. This results in a tradeoff between sensitivity which requires long optical fiber and sensor’s response time, that grows with the fiber length. Here we present a simple approach to solve this issue with new all-fiber modular gas cell design. The setup uses modified fiber mating sleeves to connect multiple HCFs in series, with optical loss of typically from 2 to 3 dB per connection. By injecting the sample gas at every second HCF-to-HCF connection, all HCF segments are filled simultaneously. We demonstrated the setups with two and four HCF segments, each with a length of 1.35 m. Laser absorption spectroscopy of methane near 1687 nm is used for setup characterization. Using the inlet pressure of 2 bar, the gas filling time is reduced over 13 times, from 77.9 s for a monolithic fiber, to only 5.9 s for the four-segment design (total HCF length of 5.4 m). With the pressure increased to 3 bar, the response time is further decreased to 3.6 s. Pressure buildup and drawdown times in the HCF were also examined.
In this paper we present a quartz-enhanced photoacoustic spectroscopy (QEPAS) of methane near 1651 nm. QEPAS is a high-sensitivity gas sensing method that relies on detecting acoustic waves generated ...by gas molecules. The sensor setup consists of a bismuth-doped fiber amplifier (BDFA) operating at 1651 nm that is used to enhance the amplitude of the QEPAS signal and increase the detection sensitivity. With the BDFA delivering ∼250 mW of optical power to the sample, the minimum detection limit of ∼11 ppb was achieved for the integration time of 150 s.
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