Tunable diode laser absorption spectroscopy (TDLAS) plays a key role in non-contact gas measurement, particularly under harsh environmental conditions, such as at high-temperature, and high-pressure situations. The emission of a tunable diode laser in a typical TDLAS system usually propagates in the free space and reaches the measurement zone. Such configuration inevitably suffers the natural diffraction of laser beams, which leads to a dramatic decrease in signal-to-noise ratio for remote measurement. In this case, the application of optical fibers provides a flexible way of delivering laser beams for TDLAS measurement, indicating excellent adaptability. However, under the mid-infrared length of no less than 2 μm, phonon absorption of fused silica will increase the material loss of silica glass optical fibers and reduces their long-distance transmission ability. Benefiting from the low material absorption, fluoride glass fibers based on ZBLAN and chalcogenide glass fibers based on As₂S₃ have become the main optical fibers operating at mid-infrared wavelengths. Unfortunately, such soft glass fibers have disadvantages including poor thermal stability, unstable chemical properties, and difficult preparation. Additionally, nearly all commercial fluoride and chalcogenide fibers on shelves are multimode fibers (MMF), which results in modal interference and poor laser beam quality, thereby leading to degraded TDLAS measurement performance. As a kind of hollow waveguide developed early for transmitting mid-infrared to far-infrared wavelengths, Capillary waveguides have been employed instead of soft glass fibers in high-temperature flow field detection due to their advantages of high-power threshold, low nonlinearity, and no-end reflection. However, they usually suffer high leakage losses and bending sensitivity. Anti-resonant hollow core fiber (AR-HCF) is a new type of microstructure hollow core fiber that features low loss, wide transmission bandwidth, and single mode transmission. AR-HCF is a novel transmission medium suitable for low-loss single mode transmission in the mid-infrared wavelength range, which has been successfully applied in high-power laser energy transmission, gas fiber laser technology, and other fields. Currently, the quartz-based AR-HCF exhibits lower transmission loss in the 2-5 μm range compared with commercial multi-mode fluoride fibers, demonstrating its enormous potential in mid-infrared region transmission. Moreover, due to the inherent advantages of quartz materials, mid-infrared quartz-based AR-HCF features excellent mechanical strength, physical and chemical stability, and good environmental adaptability. This paper constructs TDLAS systems based on AR-HCF and ZBLAN fibers respectively to carry out a combustion diagnostic by the high-temperature water vapor absorption at 2.5 μm. Unlike commercial fluoride and chalcogenide optical fibers, the AR-HCF is characterized by low loss and single-mode transmission in a broad spectral window from deep ultraviolet to mid-infrared. The TDLAS system is demonstrated to be capable of avoiding inter-modal interference that degrades measurement accuracy.

In this paper, the stack-and-draw method is employed to fabricate the AR-HCF operating at mid-infrared wavelengths. Firstly, some thin-wall capillaries are drawn from a silica glass tube. Then, the capillaries are stacked into a jacket tube to form a pre-designed structure. Next, the stack is drawn into preforms and then fiber, and a cut-back method is adopted to measure the transmission loss of the fiber. The transmission characteristics of fiber are also investigated numerically by COMSOL and the ability of low loss and single mode transmission in the AR-HCF is confirmed. Two TDLAS systems are built based on the homemade AR-HCFs and ZBLAN fibers respectively. The beam and spectrum of the system are collected through a pyroelectric array camera and photodetector. Analysis of the beam quality and signal-to-noise ratio for both systems exhibits the advantages of the AR-HCF-based TDLAS system. Additionally, the accuracy of the system is improved by evacuating the water vapor inside the AR-HCF.

The AR-HCF transmission band prepared in this paper is between 2.4-2.5 μm, and the loss at 2.5 μm is 0.06 dB/m (Fig. 3), which is lower than commercial fluoride glass fibers. Furthermore, COMSOL is adopted to build the AR-HCF mode. The simulation results show that the strong coupling between the second-order mode in the core and the modes in cladding holes results in energy leakage and high loss, thus enhancing the single-mode performance of the fiber (Fig. 4). With these advantages of AR-HCF, TDLAS system is preferred to be employed rather than free space. This paper leverages the homemade AR-HCF in the TDLAS system successfully to realize a signal-to-noise ratio of 31 dB (Fig. 9), which can output a collimated near-diffraction-limited beam with a measured diameter of 2.5 mm (Fig. 8 and Table 1) and divergence angle of 0.004 rad. The influence of residual water vapor in the hollow core of AR-HCF on the measurement of 4029.52 cm^-1 absorption line is studied, and the accuracy of the system is further improved by vacuuming the AR-HCF (Fig. 10).

This paper presents an AR-HCF-based TDLAS system and compares the performance of self-developed AR-HCF and commercial ZBLAN fiber in the high-temperature water vapor absorption measurement by TDLAS. Simulation and experimental results prove that the AR-HCF can achieve long-distance, low-loss, and single-mode transmission at 2.5 μm. The TDLAS system based on AR-HCF fundamentally eliminates multi-mode interference and has the advantages of the small beam diameter, small divergence angle, and high signal-to-noise ratio. The impact of residual trace water vapor in the AR-HCF on the measurement of the 4029.52 cm^-1 spectral line is also analyzed and the measurement accuracy of the system is improved by vacuuming it. This paper also designs and experimentally studies the mid-infrared TDLAS system based on AR-HCF. Finally, the system is confirmed to have the advantages of low transmission loss, long transmission distance, high laser beam quality, and high signal-to-noise ratio, which provides a new method for flow field detection in the mid-infrared band.