Significance As one of the most successful optical transmission media, optical fibers owe their widespread applications to their exceptional low transmission loss characteristics, which enable long-distance transmission. However, in practical applications, fibers are often exposed to environments with fluctuating temperatures, leading to variations in transmission phase and delay. This thermally-induced instability accumulates with increasing fiber length, potentially causing signal distortion, timing mismatches, and measurement errors in physical quantities. To mitigate the adverse effects of temperature sensitivity on system performance, it is common to either control the environmental temperature fluctuations surrounding the fiber or employ complex transmission schemes and data post-processing techniques to compensate for the optical signal, which often increase system complexity and cost. As a novel type of optical fiber, hollow core fibers have demonstrated unique light-guiding mechanisms and low-loss transmission characteristics over years of research, including reduced thermal sensitivity. With the rapid development and maturation of hollow core fibers over the past two decades, the feature of low thermal sensitivity has been further explored, holding promise for applications aimed at reducing environmental disturbances and enhancing temperature stability in optical transmission systems.
Progress First, the physical mechanisms underlying thermal sensitivity in optical fibers are introduced, which are summarized as the thermal expansion effect and the thermo-optic effect. The causes of the thermal expansion effect are essentially the same for both solid core fibers and hollow core fibers. On the other hand, for the thermo-optic effect, it primarily arises from the thermal properties of the silica material in solid core fibers, whereas in hollow core fibers, it originates from the combined contributions of the core gas and the microstructured cladding. Quantitative analysis reveals that the thermal sensitivity of solid core fibers is predominantly governed by the thermo-optic effect, while that of hollow core fibers is mainly determined by the thermal expansion effect. Depending on specific designs, the thermal coefficient of delay and thermal coefficient of phase in current hollow core fibers can be 3 to 20 times smaller than those of solid core fibers.
Next, we present recent advancements in reducing the thermal sensitivity of hollow core fibers. Broadly, the methods can be categorized into two types: direct approaches and compensation techniques. Direct approaches aim to reduce thermal sensitivity by directly minimizing the thermal expansion coefficient of the fiber. Specific methods include operating the fiber at extremely low temperatures, winding the hollow core fiber onto spools with near-zero thermal expansion coefficients, and reducing the thickness of the fiber coating. Compensation techniques, on the other hand, utilize a negative-thermal-sensitive thermo-optic effect to counterbalance the positive thermal sensitivity caused by thermal expansion effect. The thermo-optic effects used for compensation include the shift of transmission windows induced by heated microstructures (applicable only to photonic bandgap type of hollow core fibers) and refractive index changes caused by gas flow in fiber core.
Finally, this review highlights recent advancements in exploring practical applications of hollow core fibers with low thermal sensitivity. Newly identified application scenarios include highly stable optical interferometers, fiber optic gyroscopes, data center clock synchronization, microwave photonic devices, and time-frequency transfer systems. In these applications, the reduced thermal sensitivity of the fiber directly translates to decreased thermal sensitivity in key performance metrics, offering the potential to enhance the precision of device and reduce temperature control costs.
Conclusions and Prospects With the gradual maturation and transition of hollow core fiber technology toward practical applications, significant progress has been made in understanding and addressing its thermal sensitivity. The physical mechanisms underlying thermal sensitivity have been comprehensively elucidated. Research efforts are ongoing to further reduce thermal sensitivity, aiming to achieve complete thermal insensitivity, though current challenges lie in enhancing practicality. Applications leveraging the low thermal sensitivity of hollow core fibers are being actively explored, and their advantages are becoming increasingly evident. With breakthroughs in key metrics such as propagation loss in anti-resonant type of hollow core fibers over the past three years, it is anticipated that the low thermal sensitivity advantages of hollow core fibers will deliver substantial value in future applications.