| Author: | Li, Taige |
| Title: | Miniature optical fiber photoacoustic gas sensors based on 3D micro-printed optomechanical microresonators |
| Advisors: | Zhang, A. Ping (EEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Electrical and Electronic Engineering |
| Pages: | xxiv, 117 pages : color illustrations |
| Language: | English |
| Abstract: | Gas sensors have become an important tool for various industrial, agricultural, and environmental applications, such as hazardous gas leakage detection, food production/storage monitoring, and air quality real-time assessment. Compared with electrical gas sensors, which typically possess inherent limitations, such as poor selectivity, slow response, susceptibility to electromagnetic interference, optical gas sensors have not only the advantages of high selectivity and fast response but also the ability for remote monitoring in harsh environments. Most of the optical gas sensors are based on laser absorption spectroscopy (LAS), which renders the advantage for very high selectivity because of the use of narrow-linewidth excitation lasers matching well with specific gas absorption lines. One of the most sensitive and robust LAS gas sensing technologies is photoacoustic spectroscopy (PAS), wherein absorbed modulated light induces acoustic waves proportional to gas concentration, enabling direct transduction of molecular absorption into quantifiable acoustic signals. To detect the induced acoustic signal, existing optical PAS gas sensors typically require the incorporation of acoustic wave transducers, such as tuning forks, diaphragms, and cantilevers. However, these detection approaches suffer from inherent trade-offs between miniaturization, sensitivity, and operational simplicity. To address these limitations, this thesis is devoted to developing miniature optical fiber PAS gas sensors with directly 3D micro-printed spirally-suspended optomechanical microresonators (OMRs). A special design of planar-spiral spring OMR (PSS-OMR) is employed to tackle the trade-off between small size and high sensitivity for PAS gas sensing. The use of planar-spiral springs can not only reduce the size of the entire structure for sensor minimization but also tailor vibration resonance for sensitivity enhancement. A thin-film reflector is introduced in the OMR to form Fabry-Pérot (F-P) micro-interferometer together with the end-face of optical fiber for optical interferometric readout of acoustic signals. Moreover, to precisely fabricate such exquisite and complex PSS-OMRs, an optical 3D micro-printing technology has been developed to enable rapid in-situ printing of micrometer-scale OMRs using SU-8 epoxy photoresist, which has advantages of high optical transparency, high mechanical strength, and superb chemical and temperature resistance, for development of small-size PAS sensors. Firstly, we designed and fabricated a miniature optical fiber PAS gas sensor based on a 3D micro-printed ferrule-top PSS-OMR. The end-face of a fiber-optic ferrule is a platform directly exposed with light without need of additional coupling, offering opportunities for micro-engineering 2D/3D microstructures for development of novel miniature optical fiber sensors. In the experiments, we directly printed a PSS-OMR with a diameter of 400 μm at a 1.25-mm wide fiber-optic ferrule to make a miniature optical fiber PAS sensor. The testing results showed that this sensor can very sensitively detect acoustic and ultrasonic signals from 100 Hz to 60 kHz. Compared with non-resonance response, the sensor’s acoustic response can be greatly enhanced by 102 times when it operates at the fundamental resonance frequency. After integrating the sensor with a 1.4μL non-resonant photoacoustic cell, the optical fiber PAS sensors can be used to detect acetylene (C₂H₂) gas at the concentration level of 45 ppb, and the sensor’s response time can be as short as 0.2 seconds. To develop ultrasmall PAS for applications in highly space-constrained environments, we further developed an ultraminiature optical fiber-tip PAS gas sensor based on a 3D micro-printed fiber-top PSS-OMR. Compared with the abovementioned optical fiber PAS sensor, it is a monolithic design with a much smaller sensor head and without the need of extra PA cell. Its sensor head has the same size as a standard single-mode optical fiber, i.e., with a diameter of 125 μm. Experimental results demonstrate that mechanical vibrational resonance of PSS-OMR can enhance the sensor's acoustic sensitivity by about two orders of magnitude. Gas sensor testing experiments reveal that, comparing with off-resonance operation, the optical fiber-tip PAS sensor at on-resonance operational mode can achieve a remarkably stronger PA signal whose signal-to-noise ratio (SNR) is improved by up to 550 %. The sensor can detect acetylene (C₂H₂) at a very low concentration level of 55 ppb, and its response time is less than 0.2 seconds. These small-size high-sensitivity optical fiber PAS gas sensors provide new trace-gas detection technology for various applications such as environmental monitoring and industrial process controls in many space-constrained applications, such as in-reactor or battery monitoring and medical diagnosis. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/14094