Cladding mode coupling in highly localized fiber Bragg gratings: modal properties and transmission spectra
Published in Optics Express, Vol. 19 Issue 1, pp.325-341 (2011)
Spotlight summary: The writing of a Bragg grating inside an optical fiber is often used for making optical fiber sensors for measuring strain, bending, and temperature. The grating is typically manufactured by exposing the side of the fiber to an interference pattern of UV laser light. The UV photosensitivity of the fiber leads to a periodic refractive index change of the fiber core, the periodicity typically being on the order of the wavelength of light. This forms a grating that scatters some of the light guided in the core into reflected modes in the cladding surrounding the core. If the periodicity of the grating is significantly longer than the wavelength (a so-called Long Period Grating, LPG), the light is instead coupled into forward-propagating cladding modes with much higher losses than the core. Either way, the grating effectively works as a filter, hindering the transmission of light propagating at particular wavelengths determined by the pitch of the grating. Small changes in pitch, caused mechanically or by a change in temperature, modify the transmission and reflection spectra that can then be measured. The high sensitivity of cladding modes to bending and the surrounding environment is also exploited by measuring how the coupling between the forward-propagating core mode and the backward-propagating cladding modes affects the transmission spectrum.
The work by Thomas et al. addresses not only how to achieve a strong coupling to cladding modes, but also how to control precisely which cladding modes are excited. Strong coupling to higher-order cladding modes can lead to higher sensitivity of the fiber sensor to the surroundings. The researchers wrote gratings in a fiber using a femtosecond laser operating in the near-infrared. The nonlinear absorption of highly focused ultrashort pulses allowed point-by-point writing of highly localized refractive index changes, much smaller than the core to form the grating. The uniform transverse refractive index obtained with standard grating writing techniques limits both the strength of the coupling to cladding modes and the classes of cladding modes that can be excited. The presented work experimentally demonstrates a coupling to cladding modes with more than 99% efficiency, something that has been achieved previously only with gratings tilted at a large angle (80?) relative to the fiber length axis.
Furthermore, Thomas et al. make an in-depth theoretical analysis of how one can selectively excite particular high-order cladding modes by controlling the position of the highly localized refractive index defect in the core. For example, it is demonstrated that one can only achieve coupling into higher-order azimuthal modes by placing the defect off-center from the core.
The authors are already working on exploring further the efficient excitation of higher-order azimuthal modes. Future work could involve fiber sensors based on an asymmetric structure of the fiber core. Also intriguing is the potential of transferring the theoretical framework to design asymmetric grating structures to selectively excite particular higher-order core modes in large mode area multimode fibers for high-power fiber lasers.
Technical Division: Optoelectronics
ToC Category: Fiber Optics and Optical Communications
|OCIS Codes:||(060.0060) Fiber optics and optical communications : Fiber optics and optical communications|
|(060.2310) Fiber optics and optical communications : Fiber optics|
|(060.2340) Fiber optics and optical communications : Fiber optics components|
|(320.7130) Ultrafast optics : Ultrafast processes in condensed matter, including semiconductors|
|(060.3735) Fiber optics and optical communications : Fiber Bragg gratings|
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