Recently, the Russell Advanced Lightwave Research Center at Hangzhou Optical Precision Machinery Institute (HPMI), together with Wuhan University of Technology (WUT) and Ningbo Aifab Optoelectronics Technology Co., Ltd. have made a breakthrough in ultra-high single-mode purity hollow-core anti-resonance microstructured optical fiber. The team designed and fabricated a hollow-core, anti-resonant microstructured fiber with a non-equal-pitch capillary distribution and a double-cladding structure, and demonstrated that the fiber's higher-order mode suppression near 1585 nm is about one to two orders of magnitude higher than that of reported hollow-core fibers.
The hollow-core anti-resonant microstructured fiber is a new type of microstructured fiber waveguide guided by low refractive index air holes, which has the advantages of broad spectral light guiding, low dispersion, low nonlinearities, large mode-field area, and high laser damage threshold, and has been attracting the attention of researchers because it provides an excellent transmission platform for research in the fields of laser transmission, fiber optic communication, fiber optic sensing, and nonlinear optics. On the one hand, good single-mode characteristics are important for hollow-core fibers in practical applications such as high-performance fiber sensing, fiber optic communications, and intelligent processing of laser energy transmission. However, due to the limitation of its light guiding principle, it is difficult to achieve effective suppression of higher-order modes in hollow-core fibers similar to that in solid-core fibers. The optimization of single-mode characteristics of optical fibers is often achieved through the design of the fiber structure size to achieve a higher-order modes in the core and the cladding modes to achieve phase matching conditions, increasing the loss of higher-order modes and filtering out a particular core mode. However, this type of solution does not effectively filter out other higher-order modes in the core, and the residual higher-order modes may still cause problems such as output power fluctuations due to intermodal interference or crosstalk of transmitted signals, especially in the case of applications using shorter optical fibers.
To address these issues, Russell's team explored a different approach to higher-order mode filtering, firstly, by introducing a larger hole spacing between the cladding capillaries of an empty-core, anti-resonant photonic crystal fiber with a uniform hole spacing to enhance the leakage of higher-order modes, and secondly, by introducing a suitable air layer outside the casing on the outside of the first cladding to construct a second anti-resonant cladding layer of the fiber, as shown in Fig. 1a below. Due to the differences in the effective transverse wavelengths of different modes, this structure significantly enhances the higher-order mode loss at specific wavelengths and keeps the loss of the fundamental mode relatively low. The team experimentally succeeded in preparing a double-cladding hollow-core anti-resonant fiber (Fig. 1b), which forms dense resonant peaks in the long wavelength interval above ~1 μm, providing a multilevel window for laser transmission with ultra-high single-mode purity, as shown in Fig. 1c. Further, as shown in Fig. 2, it is experimentally verified that the core LP11 higher-order mode rejection ratio of this double-clad hollow-core anti-resonant photonic crystal fiber near 1585 nm is as high as 60 dB/km, which is about one to two orders of magnitude higher than that of the reported fibers with optimized single-mode purity. In addition, this study verifies the flexible tunability of the high single-mode purity transmission interval of this hollow-core anti-resonant fiber with the variation of the filling gas pressure, which effectively expands the wavelength interval available for high-purity single-mode. The validation of ultrahigh single-mode purity in hollow-core microstructured fibers with double-cladding structure provides a new idea for applications requiring high purity of fiber modes, such as fiber laser energy transmission, fiber optic communication, or fiber optic sensing.
The related research results are published in the journal "Ultrahigh Transverse Mode Purity by Enhanced Modal Filtering in Double-Clad Single-Ring Hollow-Core Photonic Crystal Fiber "The research results were published in Laser & Photonics Reviews, a top journal of laser and photonics. Dr. Zhuozhao Luo, a joint PhD student of Wuhan University of Technology and Shanghai Institute of Optical Machinery (SIOM), was the first author, and Associate Researcher Jiapeng Huang, Researcher Xin Jiang and Researcher Mian Pang of Russell Center were the co-corresponding authors, along with Dr. Ruochen Yin, a PhD student of SIOM, and Dr. Yu Zheng of Ningbo Aifibo Optoelectronics Technology Co. This research work was supervised by Prof. Philip Russell, a foreign academician of the Chinese Academy of Sciences, and supported by the Shanghai Science and Technology Innovation Action Plan (21ZR1482700), the National Natural Science Foundation of China (62275254), the Zhangjiang Laboratory Construction and Operation Program (20DZ2210300), the National High-level Talent Youth Program, and Fuyang High-level Talent Program. High-level Talent Program, and other supports.
Fig. 1 (a) Theoretical design, (b) scanning electron microscopy, and (c) transmission spectrum test pattern of double-clad hollow-core anti-resonant photonic crystal fiber
Fig. 2 (a) LP01 and LP11 mode near-field maps obtained from mode-selective excitation, (b) LP01 and LP11 mode loss results, (c) higher-order mode suppression ratio FOM curves, (d) center wavelengths of the maximum FOM11 values (left axis) and high FOM11 value intervals (right axis) for the double-cladding hollow-core anti-resonant photonic crystal fibers with nitrogen filling from 1-25 bar, (e) center wavelengths at FOM curves measured at gas pressure values of 1 bar, 10 bar and 20 bar
