Fig. 4

Spectral dependencies of mode propagation constants for the tapered PCF with a polymer coating of thickness d = 5 μm surrounded with water; a -loss of the PCF mode, b -phase diagram. Curve 1 - quartz glass refractive index, 2 and 2′ – Reβ and Imβ of the PCF core mode, 3 and 3′ - Reβ and Imβ of TE mode of the coating, 4 and 4′ - Reβ and Imβ of TM mode of the coating. Dashed lines indicate wavelengths of phase synchronism of the PCF core and coating modes. They correspond to intersection points of curves 3 and 4 with curve 2. Dotted lines refer to minima of ׀β – β_TM (left line) and ׀β – β_TE׀ (right line)