Abstract
The use of near infrared, high intensity femtosecond laser pulses for the inscription of long period fiber gratings in photonic crystal fiber is reported. The formation of grating structures in photonic crystal fiber is complicated by the fiber structure that allows wave-guidance but that impairs and scatters the femtosecond inscription beam. The effects of symmetric and asymmetric femtosecond laser inscriptions are compared and the polarization characteristics of long period gratings and their responses to external perturbations are reported.
Highlights
Long period gratings (LPG) result from periodic refractive index variations inscribed axially along the core of an optical fiber and guided light is coupled out of core region, where it interacts with a potentially infinite number of cladding modes
The index modulation change within the core of a single mode optical fiber can vary depending upon the inscription method used, tending to be in the range of 10–4 to 0.1, whereas the period for LPGs is typically between 100 μm and 600 μm
The inscription can be induced by using ultra-violet laser irradiation [1]; through fusion arc or laser heating inscription or more recently by femtosecond lasers [9,10,11], where in the near infrared the dominant inscription mechanism appears to be a combination of effects that include void creation, material compaction, and the photo-elastic effect induced by thermal strain
Summary
Long period gratings (LPG) result from periodic refractive index variations inscribed axially along the core of an optical fiber and guided light is coupled out of core region, where it interacts with a potentially infinite number of cladding modes. The consistent manufacture of long period gratings in photonic crystal fiber is the subject of considerable research interest due to their potential applications as filters and sensing devices, responsive to strain, temperature, bending and refractive index. The design of fiber and the choice of grating period can have a considerable influence on the sensitivity of various parameters, for example allowing the creation of a bend sensor with minimal temperature cross-sensitivity. This is not readily possible with FBG sensors. We compare the effects of symmetric and asymmetric femtosecond laser inscriptions on LPG wavelength spectra and report on the characterization of the gratings to external perturbations
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