The 3?5 μm mid-infrared spectrum covers the majority of radiation from high-temperature objects and the characteristic spectra of various molecular structures. Thus, mid-infrared spectral detection has crucial applications in fields such as medicine, national security, industry, and environmental monitoring. Mid-infrared coherent fiber bundle (CFB), as devices for delivering mid-infrared images, offer advantages such as light weight, flexibility, and resistance to electromagnetic interference, which makes them suitable for mid-infrared imaging in extreme environments. Currently, the performance testing of mid-infrared CFB remains underdeveloped due to limitations in mid-infrared light sources and testing conditions. This is especially evident in crosstalk rate testing, where controlling the size of the lens’s focal spot is challenging. Temperature resolution testing is influenced by numerous factors and requires strict control of experimental variables to ensure the reliability of the image data. Using As-S glass as the base material, we adjust the ratio of As to S to modify the refractive index of the core and cladding materials to meet the numerical aperture requirements of the imaging system. A stack-and-draw process is employed to fabricate the mid-infrared CFB. A mid-infrared imaging system based on the CFB is constructed, and experiments are conducted to evaluate the CFB’s breakage rate, spatial resolution, crosstalk rate, and temperature resolution.

The core diameter of the fiber preform is 11 mm, with an internal diameter of 11.6 mm and an external diameter of 14 mm for the cladding. A PEI tube, used as the coating material, has an internal diameter of 14.3 mm and an external diameter of 15.6 mm. Calculations show that for a core diameter of 30 μm, the internal cladding thickness is 3.3 μm, which meets the transmission requirements for the 3?5 μm wavelength range. High-purity S and As elements are utilized to prepare rods (As₄₀S₆₀) and tubes (As₃₈S₆₂). S distilled at 200 ℃ and As sublimed at 350 ℃ are encapsulated in the ampoule and then melted at 750 ℃ for 12 hours to obtain preform glasses. Mid-infrared fibers with a diameter of 280 μm are fabricated using the rod-in-tube method, and their optical loss in the mid-infrared range is tested using the cutoff method. The fibers are then drawn into CFBs via the stack-and-draw process. The theoretical filling factor and the measured filling factor of the CFB are determined using formulaic calculations and image processing techniques, respectively. The average loss of the CFB is also tested following the cutoff method. A 940 nm light source is used to observe the transparency of the CFB through a near-infrared microscope. Spatial resolution is evaluated by imaging a USAF1951 resolution target. The crosstalk rate is tested using a tungsten lamp as the radiation source and a 30 μm aperture created with femtosecond laser machining on aluminum foil, which ensures the light is coupled into the CFB’s core. The crosstalk rate is calculated based on the grayscale distribution captured by a mid-infrared camera. For temperature resolution testing, a heated iron nail is used as the target, and both the mid-infrared imaging system and thermal imager are employed to capture and record the grayscale and temperature distribution. The temperature resolution of the mid-infrared imaging system is calculated, and the effect of incorporating the CFB into the system is analyzed.

The optical loss baseline of the 280 μm fiber in the mid-infrared range is less than 0.2 dB/m, with a strong H—S absorption peak around 4 μm, resulting in absorption intensity of approximately 4.5 dB/m (Fig. 2). The optical microscope of the CFB reveals that the individual fiber diameters are approximately 40 μm (Fig. 3). The average optical loss of the CFB in the mid-infrared detection range is 0.99 dB/m. The increase in loss can be attributed to two factors: 1) The mid-infrared camera’s response range is between 3.8?4.7 μm, which overlaps with the H—S absorption peak; 2) Absorption caused by the PEI coating. Transparency tests show uniform light transmission across the fiber elements, with no black or dark fibers, indicating good consistency in light transmission (Fig. 4). The theoretical filling factor for the CFB is 51%, whereas the actual measured filling factor is 54%, which is marginally higher than the theoretical value. This discrepancy can be attributed to the fact that, during the stack-and-draw process, the gap is filled by the fiber cladding and coating layer, which slightly increases the overall filling factor. The theoretical spatial resolution of the CFB is calculated to be 14.43 lp/mm, and experimental results show that the resolution corresponds to the third group, the sixth element on the resolution target, but not the first element of the fourth group. Therefore, the measured spatial resolution is 14.25 lp/mm, which is close to the theoretical value (Fig. 5). The crosstalk rate of the CFB is found to be less than 1%. By adjusting the aperture position and coupling the light source to the fiber interface, it is observed that the fibers maintain a clear boundary, which indicates that the PEI layer suppresses crosstalk by absorbing the coupled light in the cladding (Fig. 6). However, a smaller cladding thickness is not always advantageous; an excessively thin cladding can result in increased light leakage into the coating layer, thereby raising the overall loss of the CFB. The temperature resolution of the imaging system is 0.25 K, and after incorporating the CFB, the resolution increases to 0.50 K. This degeneration is mainly due to two factors: 1) Fiber loss and f-number degradation result in a loss of energy reaching the detector; 2) The uncoated CFB exhibits significant Fresnel reflection loss due to the high refractive index of the material.

A mid-infrared CFB based on As-S glass has been fabricated and thoroughly characterized through various imaging system setups. The CFB exhibits a low breakage rate, and the individual fiber elements demonstrate consistent light transmission, which ensures no degradation of image quality due to defects. The measured spatial resolution is in close agreement with the theoretical calculation. The crosstalk rate of the CFB is found to be less than 1%, and a convenient method using femtosecond laser-machined apertures has been developed to measure the crosstalk rate reliably. The PEI coating on the CFB significantly suppresses crosstalk by absorbing mid-infrared stray light. Compared to the ribbon-stacking method, this configuration reduces cladding thickness and improves the filling factor. Temperature resolution testing reveals that the introduction of the CFB into the infrared imaging system reduces the temperature resolution from 0.25 K to 0.50 K. Further improvement in temperature resolution can be achieved by reducing fiber loss, coating anti-reflection films on the bundle end faces, and optimizing the subsequent coupling lens design.