Broadband birefringence hollow-core anti-resonant optical fiber with elliptical air holes

As a fundamental attribute of light, polarization effects take an essential role in many optical applications, such as optical fiber gyroscopes (FOGs) [1], fiber sensors [2], and polarization-maintaining fiber lasers [3]. Generally, fiber structure will change unpredictably in the fabrication process, inevitably introducing a random birefringence and resulting in the limitations of its applications. To solve this problem, many kinds of polarization-maintaining fibers(PMFs) were demonstrated [4], [5]. But these fibers are solid core, having some fundamental limitations, including material absorption, low damage threshold, and high non-linearity [6], [7].

The above fundamental limitations can be addressed with hollow-core fibers (HCFs), since HCFs guide light in the hollow core. Over the past decades, scientists have studied two kinds of hollow-core fibers (HCFs). The first type is hollow-core photonic bandgap fibers (HC-PBGFs), which guide light in the air-core based on photonic bandgap effect. It was reported that a 19-cell HC-PBGF was fabricated with a minimum loss of 4.9 dB/km and a phase birefringence of 2.5 × 10−4, but with a narrow bandwidth of 14 nm [8]. A quasi-elliptical core HC-PBGF was designed with a birefringence >4.6 × 10−4 in the wavelength range of 1.46–1.59μm, and a low confinement loss of <0.02 dB/m [9]. However, PM-HC-PBGFs have some shortcomings, such as a narrow transmission band, poor mode purity, and a low damage threshold.

The second type of HCFs is hollow-core anti-resonant fibers (HC-ARFs). This kind of fiber limits beam in the air-core by inhibiting the coupling between the core mode and the cladding mode. Due to this light-guiding mechanism, it possesses greater optical properties than HC-PBGF, such as wider transmission bandwidth [10], [11], lower potential loss [10], [11], [12], [13], [14], [15], and lower anomalous dispersion [16]. Recently, they have attracted much attention from scientists for their excellent optical properties. Because ARF guides light in the air-core and has a low field overlap between the core and the tube cladding, it can achieve lower potential loss than HC-PBGF. But meanwhile, it is hard to achieve a high-birefringence (>10−4), which is comparatively easy for HC-PBGF fiber to realize. Therefore, there are not many theoretical works that are based on HC-ARF to introduce Hi-Bi. So far, most of the reported works obtained polarization-maintaining by coupling one polarized FM with cladding mode or silica mode. Mousavi et al. proposed a fiber design with a low loss of 40dB/km and high birefringence [17]. Yan et al. reported a single-polarization double-ring fiber that realizes polarization-maintaining by changing inner-ring radius and thickness to couple the fundamental mode and the silica mode. It can introduce a high polarization extinction ratio (PER) of 17662 [18]. Yan et al. demonstrated a new way to realize single-polarization via coating a different material with a specific thickness on one vertical cladding tube. They achieved a PER of 1732 at 1550nm [19]. Furthermore, researchers proposed an effective method to achieve polarization-maintaining by adding silicon whose refractive index is larger than silica. M. S. Habib et al. reported an HC-ARF design exhibiting a birefringence of 5 × 10−5 at 1064 nm by adding silicon layers [20]. Zhao et al. demonstrated a fiber structure with silicon tubes, silica tubes, and a cladding ring, which contributes to the birefringence of 3.07 × 10−4 at 1550 nm [21]. Recently, some experimental works have been reported. A PM-ARF was used in a polarization-maintaining laser with a minimum loss of 185 dB/km [22]. C. Goel et al. proposed a PM-ARF which was fusion spliced to a commercial ytterbium-doped fiber, achieving 25 W power output with a PER of >21 dB [3].

In traditional solid-core fibers, it is an effective way to obtain Hi-Bi by making the fiber core elliptical [23], but it seems to be an ineffective way in HC-ARF [24]. So in this paper, a quasi-elliptical hollow-core anti-resonant fiber with hybrid elliptical/circular tubes is proposed to investigate the effect of a quasi-elliptical core on fiber optical properties. It achieves a Hi-Bi of 1.33 × 10−4 at 1550 nm and a broad transmission band (1300–1760 nm) where the confinement loss is less than 1 dB/m. Besides, it has great bend resistance. How fiber parameters affect fiber optical properties is also investigated.

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