## 1 Introduction  
Trace gas detection techniques play an important role in the fields of environment, industry, aerospace, and medical technology [1; 2; 3; 4; 5]. Various optical sensing methods for atmospheric monitoring have been reported by numerous researchers [6; 7; 8; 9; 10]. Compared with other optical spectroscopic techniques, photoacoustic spectroscopy (PAS) technology is one of the most effective approaches for trace gas sensing, in which a microphone is employed to detect acoustic signals generated by the modulated optical radiation in a weakly absorbing gas [11; 12; 13]. PAS technology has many advantages, such as wavelength independence and zero-background signal. However, the wideband-microphone based PAS system is sensitive to environmental noise and gas flow noise due to the low resonance frequency of photoacoustic cell. Quartz-enhanced photoacoustic spectroscopy (QEPAS), a variant of PAS, employs a low-cost and small-size quartz tuning fork (QTF) as a sharply resonant acoustic transducer to sense the weak light-induced sound wave. QEPAS has been widely used for detecting trace gas since its first demonstration in 2002 [14; 15; 16; 17; 18; 19; 20; 21]. It combines the main merits of conventional PAS with the advantages of a standard QTF which has a high resonance frequency and narrow resonance width, providing a robust, compact and reliable superchphone. To improve the photoacoustic signal, some architectures of QTF-based spectrophotbaves have been modified and successfully applied to QEPAS system, such as multi-quartz-enhanced photoacoustic spectroscopy sensors (M-QEPAS) [22], single-tube on-beam QEPAS sensors (SO-QEPAS) [23], and intracavity QEPAS sensors (I-QEPAS) [24]. In most QEPAS sensors, commercially available standard QTFs are usually used as acoustic transducer. The 32.7-kHz standard QTF with a quality factor about 10,000 has a spacing of 300 \(\upmu\)m between the QTF prongs and a prong length of 3.8 mm. Therefore, in order to pass through the QTF prong spacing without being blocked, the laser beam with a diameter less than 300 \(\upmu\)m is required at the center of the QTF gap. Considering that the photoacoustic signal from QEPAS system is proportional to the number of gas molecules which are excited to a high energy state by absorbing photons from an excitation source in unit time, the inner surface areas of the QTF prongs are underutilized with such a small circular laser spot. A circular laser beam creates a longitudinal (cylindrical) pressure source, whose geometry does not match well the geometry of the tuning fork prongs. In other words, the overlap integral between the excitation source and the inner surface of the prongs is very low. Hence some measures for optical beam shaping can be implemented to improve the area utilization ratio of the QTF prongs' inner surfaces and create a sound pressure source with a geometry matching more closely to the inner surface of the prongs. To date, three common methods which can transform a circular spot into a strip spot have been reported. The first method employs a Digital Micromirror Device (DMD) which consists of a highly reflective aluminum micromirror array [25; 26]. By operating with this method, the required light is reflected by the micromirror device and the unnecessary light can be absorbed by a laser absorber. In addition, the direction of its illumination is achieved by controlling the angle of the micromirror by means of electrostatic action. In the second approach, plano-convex cylindrical lenses were equipped to convert a circular beam spot into a strip spot, or change the height of the laser beam without changing the beam width [27]. However, the laser beam shaped by these two methods has a large central hotspot and faded edge distribution. Especially for laser beams with small size, they have a poor shaping ability. Therefore, more focusing lenses and shaping devices are needed to improve the beam quality. To address these issues, a third method has been proposed, in which a Powell lens is used for spatial beam shaping [28; 29].