Optics & Photonics News

supports linearly polarized (LP) modes. On the other hand, cladding modes are guided by a larger step discontinuity in refractive index, such that their vectorial nature cannot be ignored. In fiber-optic terms, the “LP mode” approximation no longer applies. Experimentally, we observe that high-order cladding modes—whose resonances occur at short wavelengths that are far from the Bragg resonance—do appear to be very strongly polarization-dependent. This is because higher-order cladding mode resonances actually come in pairs made up of nearly degenerate orthogonally polarized modes (the EH and HE vector modes of the cladding) and the TFBG automatically selects one or the other polarization when core mode light is

Polarization dependence of
high-order mode resonances [ ]
Transmission [dBm]

P polarization
S polarization
Polarization dependence of high-order mode resonances
in a TFBG transmission spectrum measured in air. S- and
P-polarization have their usual meaning relative to the tilted
grating planes.

HEx1, 44

HEy1, 44

S-polarized resonance Neff= 1.3319 S-polarized resonance Neff= 1.3319

EHx1, 44

EHy1, 44

P-polarized resonance Neff= 1.3320 P-polarized resonance Neff= 1.3320

S resonances have predominantly tangential electric fields around the fiber cladding boundary. In other words, the x component of the field is maximum at the top and bottom boundaries, and the y component is maximum at the left-right boundaries. P resonances have electric fields that are predominantly radial at the boundary. Therefore, P resonances can transfer energy to a plasmon wave on the metal surface (bright fringe around the fiber) but S resonances cannot.

linearly polarized either in the plane of the tilt (P-polarization) or perpendicular to it (S-polarization), respectively.

Apart from an obvious application as a narrowband, high-extinction-ratio polarizer, the polarization dependency of high-order modes in TFBGs has an important consequence for sensing. Polarization control allows the user to select only HE or EH modes across large portions of the spectrum. The electric field of high-order EH modes is oriented predominantly radially at the cladding boundary, while that of HE modes is mostly tangential. Therefore, any sensing modality that depends strongly on the polarization state of the light near the cladding boundary can be controlled very effectively with TFBGs.

Plasmonics on fibers

One of the most important consequences of this EH-HE discrimination is that TFBGs can be used to efficiently excite quasi-cylindrical surface plasmons on metal-coated optical fibers. Surface plasmons are highly confined electromagnetic waves that are propagating along a metal-dielectric interface with a well-defined propagation constant (completely determined by the permittivities of the metal and dielectric) and an electric field oriented perpendicular to the interface. Spectral or angular interrogation of such interfaces with light beams— using the so-called surface plasmon resonance, or SPR, instruments—is now done to detect minute changes in the properties of the metal or of the dielectric, and hence to sense chemical or biochemical reactions.

Researchers have pursued various fiber-based SPR techniques to miniaturize the sensing probe and to facilitate the input-output coupling of the light. The more successful fiber SPR techniques rely on measuring the transmission spectrum of etched cladding or tapered multimode fibers where the SPR shows up as a loss band that occurs only for modes whose propagation constant is equal to that of the plasmon wave for the external medium of the fiber. This requires accurate and stable control of the distribution of the light among all of the possible modes of the fiber, and polarization must either be scrambled or linearly polarized at the input of the fiber, thereby losing a great deal of the signal-to-noise ratio in either case because plasmons can only be excited by radially polarized light along the fiber boundary.

Both constraints also mean that the fiber must be kept relatively straight and cannot be moved during experiments without causing changes in the shape or position of the SPR signature.

On the other hand, with a TFBG in single-mode fiber, the light propagates as a single mode of the core everywhere but at the sensor itself. In other words, the fiber can be moved, and the light source and interrogation system can even be located many kilometers away from the sensor. In addition, cladding modes with the correct (radial) polarization and propagation constant to excite the plasmon can be automatically selected by the TFBG.