Propagating OAM modes through a regular octagonal photonic crystal fiber with a flat dispersion

Abstract In this paper, a regular octagonal photonic crystal fiber (ROPCF) is proposed to support and improve the stable transmission of orbital angular momentum (OAM) modes. The proposed ROPCF is composed of a doped high refractive index ring and four layers of air hole rings, ensuring that the effective refractive index difference between OAM modes of the same order or OAM mode group is greater than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> , and can stably transmit 114 OAM modes in the wavelength range of 1.5 ∼ 1.6 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> . A finite element method (FEM) is used to analyze the designed ROPCF. The measured results show that the ROPCF has high mode quality (MQ) (all modes are greater than 95%), and the confinement losses (CL) of all vector modes are in the range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>14</mml:mn> </mml:mrow> </mml:msup> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>11</mml:mn> </mml:mrow> </mml:msup> </mml:math> dB/m. In addition, it also has a large effective mode area between 197.64 to 297.82 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:math> . Moreover, the nonlinear coefficients and chromatic dispersions (CD) are less than 0.6 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">W</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">km</mml:mi> </mml:math> and 276 ps/nm/km, respectively. And the dispersion change is relatively flat. The 10ps walk-off lengths of all vector modes supported by ROPCF at a wavelength of 1.55 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">μ</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:math> are also analyzed. The obtained data results show that the 10ps walk-off lengths of the most of supported vector modes are within the range of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mn>3</mml:mn> </mml:msup> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mn>5</mml:mn> </mml:msup> </mml:math> m, which is beneficial for long-distance transmission of OAM modes. These significant performances indicate that ROPCF has great application value in improving communication channel capacity (CC) and spectral efficiency (SE).