Analysis of surface plasmon resonance in bent single-mode waveguides with metal-coated cladding by eigenmode expansion method

Abstract

A numerical study is presented of surface plasmon waves excitation in a metal film applied to the cladding of a standard bent single-mode optical fiber. It was shown that by adjusting the bend radius and metal film thickness one can achieve effective coupling between the fiber fundamental mode and symmetric surface plasmon mode through the intermediary of whispering gallery modes supported by the cladding of the bent fiber. This effect is demonstrated to allow for refractometric measurement both in the wavelength and intensity-modulated regimes with a resolution of up to 10⁻⁸ RIU. Usage of standard noise reduction techniques for intensity-modulated optical signals promises further increase in accuracy.

Surface plasmon resonance in a metal-coated single-mode optical fiber with decreased normalized frequency

“…Figure 1 (curve 8) shows a characteristic profile of the electric field amplitude for one of the two modes calculated at a resonance wavelength of SM800 bent with R = 7 cm. Note that, in contrast to the case stud ied previously [8,9], where the energy of guided radi ation was transferred to a plasmon polariton wave via cladding modes, the field of these modes in the present case ( Fig. 1) is localized predominantly in the regions of core and metal film, while not reaching large values in the cladding.…”

“…2b) is close to linear and the spectral sensitivity amounts to ~70 μm/RIU, which significantly exceeds the analogous characteristics of most known optical fiber SPR refractometers [1,4,5,14]. In particular, the obtained sensitivity value is more than three times higher than the analogous estimation obtained previ ously [8,9] for a bent metal coated single mode waveguide structure in which the SPR was excited via whispering gallery cladding modes. Assuming the res olution of spectrum analyzer to be 0.02 nm, the reso lution of refractometric measurements using the pro posed system can be estimated at 3 × 10 -7 RIU.…”

“…Since the modes of bent seg ment 2 exhibit large ohmic losses, analysis of the tran sition phenomena at the boundaries between segments was performed as described in [8,9], using a noncon jugated formulation of the orthogonality criterion [11,13].…”

“…Recently, we have proposed [7][8][9] a new approach to the creation of optical fiber SPR sensors according to which a metal film is deposited immediately onto the optical cladding of a standard single mode fiber. The interaction of a fundamental mode (guided in the fiber core) with this film is then achieved due to bend ing of the optical fiber and is mediated by whispering gallery modes propagating in the bent fiber, which are strongly coupled with the fundamental mode at a cer tain radius of bending.…”

“…The interaction of a fundamental mode (guided in the fiber core) with this film is then achieved due to bend ing of the optical fiber and is mediated by whispering gallery modes propagating in the bent fiber, which are strongly coupled with the fundamental mode at a cer tain radius of bending. This configuration removes the need for violation of the structural integrity of the optical fiber and allows the resolution of measure ments to be significantly improved as compared to that provided by the existing optical fiber SPR refractome ters [8,9]. However, the interaction of a fundamental mode with the metal film deposited onto the optical cladding of a bent single mode fiber can be also achieved directly, provided that the fundamental mode field penetrates into the core to a sufficiently large extent.…”