Polarization maintaining and spectral filtering properties of hollow-core optical fibers for near-infrared region

Abstract

Negative curvature hollow-core fibers (NCFs) offer attractive properties like ultralow losses, broad bandwidth, low latency, low nonlinearity, and a high damage threshold. Additionally, NCFs can overcome the inherent optical transparency limitations posed by solid-core fibers, which restrict their application to specific wavebands, thus enabling more flexible usage of NCF. These qualities make NCFs suitable for their utilization as polarization-maintaining (PM) fibers. In this thesis, the PM properties of NCFs are investigated through two different design approaches. In the first part of the thesis, a new design of polarization-maintaining and spectral filtering NCF tailored for the telecommunication bands in the near-infrared region is presented. The optical fiber, consisting of a six-tube silica structure, incorporates nested tubes anchored radially by pole structures in the vertical direction. Meanwhile, in the horizontal direction, nested tubes without pole anchors are employed, making the fiber structure asymmetric, which encounters birefringence. This unique fiber design not only preserves the polarization states of light but also exhibits frequency selective transmission exclusively in the vertical direction due to the pole structure. Through numerical modeling, transmission losses below 0.1 dB/km for spectrally filtered wavelengths are achieved, with birefringence on the order of 10−5 within the spectral range of 1.45 μm to 1.60 μm. These results demonstrate significant improvements in terms of birefringence, distinct loss separation between horizontally and vertically polarized states, and a reduced number of spectrally filtered wavelengths compared to previously reported findings. The proposed fiber design holds untapped potential for applications requiring selective transmission with specific polarization. The second part of the thesis focuses on the effect of ellipticity on the polarization maintaining performance of NCFs. The nested tube NCF is stretched vertically and compressed horizontally with the same ellipticity of 5%, 8%, and 10% to introduce ellipticity through the entire cross-section of the fiber. Through these modifications, the fiber successfully exhibits orthogonal polarization states compared to the original fiber with no ellipticity. Further investigations into the fiber with 8% ellipticity involve coupling it with bi-thickness (t1 and t2) and a manipulated nested tube diameter (dnx) along the horizontal axis of the fiber. The findings reveal that ellipticity has a limited effect on the improvement of the birefringence and polarization extinction ratio (PER) in NCFs, but it can be used as a complementary fine-tuning process. This suggests that ellipticity has the potential to be utilized as an additional mechanism for enhancing the overall polarization-maintaining characteristics of NCFs.