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A ultra-wideband highly-birefringent Bragg layered photonic bandgap fiber (BL-PBGF) with concave-index cladding is proposed and demonstrated, by incorporating PBG effect with Bragg multilayers for the first time to the best of our knowledge. The proposed BL-PBGF contains honeycomb capillary cladding and elliptical core with horizontal asymmetry for birefringence. By innovatively introducing three modified silica capillary layers with different refractive indices, the cladding with concave-convex refractive index distribution is combined, providing superior characteristics for optical polarization implementation, such as confinement loss, bending loss and birefringence. Results show that the confinement loss stays around 2 dB/km level within ultrawide 180 nm wavelength range, basically 3 times wider than conventional PBGF. The birefringence maintains at the order of 10 -3 across the entire bandwidth and the maximum value reaches 2.5×10 -3 . The bending loss at 1550 nm is significantly reduced to below one third of the conventional uniform index PBGF when the bending radius is less than 3.5 mm, and maintains below 1 dB/km level when the bending radius is beyond 10 mm. The proposed universal BL-PBGF has great potential in minimized freestanding fiber coil and small footprint fiber optical gyroscope applications.

With the benefits of low latency, wide transmission bandwidth, and large mode field area, hollow-core antiresonant fiber (HC-ARF) has been a research hotspot in the past decade. In this paper, a hollow core step-index antiresonant fiber (HC-SARF), with stepped refractive indices cladding, is proposed and numerically demonstrated with the benefits of loss reduction and bending improvement. Glass-based capillaries with both high (n = 1.45) and low (as low as n = 1.36) refractive indices layers are introduced and formatted in the cladding air holes. Using the finite element method to perform numerical analysis of the designed fiber, results show that at the laser wavelengths of 980 and 1064 nm, the confinement loss is favorably reduced by about 6 dB/km compared with the conventional uniform cladding HC-ARF. The bending loss, around 15 cm bending radius of this fiber, is also reduced by 2 dB/km. The cladding air hole radius in this fiber is further investigated to optimize the confinement loss and the mode field diameter with single-mode transmission behavior. This proposed HC-SARF has great potential in optical fiber transmission and high energy delivery.

Photoacoustic spectroscopy (PAS) has received extensive attention in optical gas sensing due to the advantages of high sensitivity, gas selectivity, and online detection. Here, a mid-infrared hollow-core fiber (HCF) based flexible longitudinal photoacoustic resonator for PAS-based gas sensing is proposed and theoretically demonstrated. A mid-infrared anti-resonant HCF is designed to innovatively replace the traditional metallic acoustic resonator and obtain a flexible photoacoustic cell in PAS. Optical transmission characteristics of the HCF are analyzed and discussed, achieving single mode operation with below 1 dB/m confinement loss between 3 and 8 μm and covering strong absorptions of some hydrocarbons and carbon oxides. With varied bending radii from 10 mm to 200 mm, the optical mode could be maintained in the hollow core. Based on the photoacoustic effect, generated acoustic mode distributions in the HCF-based flexible photoacoustic resonator are analyzed and compared. Results show that the PAS-based sensor has a stable and converged acoustic profile at the resonant frequency of around 16,787 Hz and a favorable linear response to light source power and gas concentration. The proposed novel photoacoustic resonator using HCF presents bring potential for advanced flexible PAS-based gas detection.