Three-Dimensional Printed Miniature Fiber-Coupled Multipass Cells with Dense Spot Patterns for ppb-Level Methane Detection Using a Near-IR Diode Laser

Tunable diode laser absorption spectroscopy (TDLAS) based on a multipass cell (MPC) is a powerful analytical tool and is widely applied to air quality monitoring, industrial process control, and medical diagnostics. However, the conventional MPC as a core component in TDLAS devices has a large size, low utilization efficiency of the mirror surfaces, and tight optical alignment tolerances. In this paper, we design and fabricate a mini-MPC with an optical absorption path length of 4.2 m and dimensions of 4 × 4 × 6 cm (open cavity), which, to our best knowledge, is the current smallest MPC in terms of the same optical path length. The mini-MPC generates a seven-nonintersecting-circle dense spot pattern on two 25.4 mm spherical mirror surfaces, providing a high fill factor of 21 cm−2. A fibercoupled collimator and an InGaAs photodetector are integrated into the miniMPC via a high-resolution three-dimensional printed frame, hence removing the requirement of active optical alignment. Using a 1.65 μm distributed-feedback laser, the performance of this mini-MPC for methane detection was evaluated in terms of linearity, flow response time, stability, minimum detectable limit, and measurement precision. Continuous measurements of methane near a sewer and in the atmosphere were performed to demonstrate the stability and robustness of the highly integrated mini-MPC-based gas sensor. Our analysis shows that a methane minimum detectable limit of 117 ppbv is achieved, paving the way toward a sensitive, low-cost, and miniature trace gas sensor inherently suitable for large-scale deployment of distributed sensor networks and for handheld mobile devices. O detection of trace gases has been demonstrated in a variety of sensing configurations. Compared with mass spectrometry or gas chromatography based on traditional methods, the optical method based on infrared (IR) absorption spectroscopy can quickly respond to the concentration of the measured gas without sample preparation. In particular, tunable diode laser absorption spectroscopy (TDLAS) based on the Lambert−Beer law has proven to be a tool unrivalled in both species selectivity and sensitivity by targeting ro-vibrational transitions of species in the near-to mid-IR (NIR to MIR) spectral regions. The extension of this technology to compact or handheld sensor systems for the identification and detection of trace gas concentrations is important for applications ranging from environmental monitoring to industrial process control and medical diagnostics. Particularly, an important application involves greenhouse gas sensing, which has recently undergone significant study and development on account of increasing emission control regulations. For example, the emissions of methane (CH4) contribute to the increasing greenhouse effect on earth’s surface. Some recent studies have shown significant CH4 production in sewers. 19 Water utilities are committed to reducing CH4 emissions from wastewater treatment plants and sewer networks. Furthermore, CH4 is the main component of natural gas used widely in industry and urban as an industrial fuel and domestic fuel, respectively. The boom in shale-gas production based on “fracking” technology raises concern about the environmental impact of shale-gas drilling. Therefore, reducing CH4 fugitive emissions is a key requirement for natural gas harvesting as a clean energy alternative. It is of great significance to study and monitor the change of CH4 concentrations in terms of climate change and industrial monitoring and CH4 production and emission from sewers. Ideally, a large-scale deployment of sensors in a network configuration will provide both spatial and temporal resolutions for CH4 emission localization and quantification, Received: May 5, 2020 Accepted: September 1, 2020 Published: September 1, 2020 Article pubs.acs. org/ac © 2020 American Chemical Society 13034 https://dx.doi. org/10.1021/acs.analchem. 0c01931 Anal. Chem. 2020, 92, 13034−13041 D ow nl oa de d vi a SH A N X I U N IV o n O ct ob er 6 , 2 02 0 at 0 7: 20 :2 5 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. respectively. Bench-top TDLAS sensor systems have been developed for on-line gas analysis, and higher sensitivities have been achieved using multipass cells (MPCs) with long interaction lengths. However, widespread use of these TDLAS sensor systems for distributed sensor networks (DSNs) or handheld mobile devices is limited by structure complexity, an extremely large size, and tight optical alignment tolerances of these systems. A desired TDLAS sensor system based on a simple structure with a small footprint, but high sensitivity, is preferred. To achieve a high detection sensitivity for TDLAS, the most effective method is to shift the target absorption line of the molecular compound from the NIR overtone spectral region to the MIR “fingerprint” spectral region and select the strongest MIR interference-free absorption line at the wavelength region that the employed MIR laser source covers. For example, an interband cascade laser operating at room temperature combined with a 54.6 m MPC, resulting in a CH4 sensor with the detection sensitivity of ∼10 ppb. However, MIR lasers and detectors are costly and nondurable compared to robust diode lasers operating in the NIR region. Furthermore, the MIR optical fiber coupling technique is not yet mature and not readily available for implementation of a cost-effective compact and robust MIR TDLAS sensor design. An alternative approach of obtaining high detection sensitivity for TDLAS is to increase the optical absorption path length by using a commercial or custom MPC, which is constructed with highly reflective mirrors. A variety of MPCbased TDLAS sensors were demonstrated for numerous spectroscopic applications. However, the traditional Herriott cell based on two spherical mirrors yields a standard circular/elliptical spot pattern with a low utilization efficiency of the mirror surfaces, which is difficult to realize in a small handheld sensor system. Many variants based on the Herriott cell, such as astigmatic mirror cells, can produce dense dot patterns to take advantage of the mirror surfaces, thus realizing a compact size with a long optical path length. However, the manufacturing of aspheric surfaces is more complex, and it is difficult to produce a mirror of sufficient surface accuracy to follow the designed curvature. Therefore, spherical mirrors are required in MPCs because of low requirements for mirror processing technology, more easily controlled surface quality, and more importantly lower cost. In this paper, we design and fabricate a mini-MPC with a 4.2 m optical absorption path length, which, to our best knowledge, is the current smallest MPC in terms of the same optical path length. The MPC employs two spherical mirrors and yields a seven-nonintersecting-circle spot pattern by means of its own spherical mirror aberration. A fibercoupled collimator and an uncooled InGaAs photodetector with a preamplifier circuit board are integrated into this MPC. The three-dimensional (3D) printed technique with a printing resolution of 100 μm is adopted to fabricate the MPC frame, which ensures a high integration of all components. As a result, the mini-MPC is lightweight, cost-effective, free from active optical alignment, and easy to fabricate and has a long optical path length. A NIR DFB laser emitting at 1.65 μm was used as a light source to detect methane achieving a ppb level detection sensitivity. A demonstration of continuous CH4 monitoring was carried out near sewers and in ambient air for 13 and 11 days, respectively. Our results indicate the potential for a highly integrated mini-MPC in a small handheld sensor system and in a large-scale deployment of DSN providing high detection sensitivity and selectivity for leak detection and monitoring atmospheric pollutants. ■ EXPERIMENTAL SECTION Conventional calculations of spherical mirror MPC, such as a Herriott cell is based on a standard ABCD matrix with paraxial approximation of a thin lens. However, with a more highly curved surface of the reflective mirrors and more pass counts in the MPCs, the conventional ray calculations based on a standard ABCD matrix, specifically marginal rays, generate deviations from the actual trajectories on account of spherical mirror aberrations. These deviations accumulate and are amplified, ultimately distorting the real spot pattern. In fact, these intrinsic aberrations can be effectively exploited to produce a set of intricate variants with respect to a Herriott circular/elliptical spot pattern, which can be completed by manipulating these parameters, such as the mirror spacing, the initial entry location, and the incident angle of the ray. The modified ABCD matrix describing the one pass count without the paraxial approximation of thin lens can be expressed as