Modal interferometer based on double cladding photonic crystal fiber for refractive index measurement by equalisation wavelength

We propose novel structure of double cladding Photonic crystal fiber (PCF) as a sensing element. First cladding allows one core mode propagation only. Second cladding consists of ring of holes with the average refractive index of the ring higher than the basic material of the fiber. Advantage of this arrangement against core - cladding interferometers is, that by suitable choice of refractive index of second cladding holes, the possibility to investigate the external refractive index in the region where phase constant of interfering modes shows an extreme (equalisation wavelength).