Photoacoustic spectroscopy (PAS) is an ultrasensitive method for gas sensing, which usually using a capacitive microphone as a signal detector. However, the electric nature of the microphone limits their applications in the circumstances with high temperature, electromagnetic interference (EMI), as well as explosive environments. Due to the uncharged and high-sensitivity characteristics, optical acoustic sensor is an ideal substitute in the circumstances above. Thus, all-optical PAS can get better applications in a harsh environment. In this paper, a brief review of all-optical PAS used in gas sensing with different optical acoustic sensing technologies is presented, such as intensity-modulated technology, fiber Bragg grating (FBG)-based technology, and interferometer-based technology. Besides, using a fiber-optic photoacoustic (PA) sensor would allow the realization of remote detection, in-situ detection, real-time monitoring, and multiplexed multi-point detection within a fiber-optic network. Finally, the advantages and limitations of these different technologies are discussed.

A multiple reflections-enhanced fiber-optic photoacoustic (PA) gas sensor for gas micro-leakage is introduced. Multiple reflections of the excitation laser occur on the inner surface of a reflective ring to enhance the PA signal. The PA signal is obtained by measuring the deflection of the gold-coated poly (phenylene sulfide) (PPS) diaphragm with a Fabry-Perot interferometer (FPI). The second harmonic wavelength modulation spectrum (2f-WMS) technology can essentially eliminate the fundamental frequency noise generated by the wavelength-independent absorption of the reflective ring. Experimental results show that the PA signal can be effectively enhanced 11.7 times by the multiple reflections optical path compare with the double-pass optical path. The minimum detection limit of the system is achieved to be 23.6 ppb. The designed PA gas sensor is suited for remote detection of gas micro-leakage.

This paper presents the development of a highly sensitive gas detection system based on a resonant photoacoustic cell for detecting dissolved gases in transformer oil. A simulation model of the resonant photoacoustic cell was studied and optimized the buffer chamber volume while ensuring signal enhancement. The volume of the photoacoustic cell was reduced by about 80% compared to the classical model. A resonant photoacoustic cell was then fabricated based on the optimized simulation optimization. The dual-resonance photoacoustic system was constructed by combining the resonant PA cell with a handmade cantilever fiber acoustic sensor. The system’s sensitivity was further improved by using an erbium-doped fiber amplifier, wavelength modulation, and harmonic detection technology. The experimental results showed that the system achieved a detection limit of 6 ppb and an excellent linear range under 1000 ppm for C₂H₂ gas. The developed gas detection system has potential applications for monitoring the condition of power transformers in power grids.

A new method to strengthen nano porous broadband anti- reflectance silica films is described. Silica sols were made from TEOS with a base/acid two-step catalytic process. Properties of silica films deposited with a dip-coat method are characterized with AFM, SEM, spectrophotometer and abrasion test. Experimental results show that an average reflectance of a glass coated with the nano porous silica films is less than 1% in the visible region; and that the two-step catalysis obviously strengthens the films. Finally, the strengthening mechanism of the films is discussed.