The spectral characteristics of a fiber Bragg grating (FBG) with a transversely inhomogeneous refractive index profile, differs considerably from that of a transversely uniform one. Transmission spectra of inhomogeneous and asymmetric FBGs that have been inscribed with focused ultrashort pulses with the so-called point-by-point technique are investigated. The cladding mode resonances of such FBGs can span a full octave in the spectrum and are very pronounced (deeper than 20dB). Using a coupled-mode approach, we compute the strength of resonant coupling and find that coupling into cladding modes of higher azimuthal order is very sensitive to the position of the modification in the core. Exploiting these properties allows precise control of such reflections and may lead to many new sensing applications.

The use of ultrashort laser pulses for fiber grating inscription has many advantages in comparison to continuous wave and long pulse lasers. The most important one is that it allows inscription in nonphotosensitive fiber materials. In this paper the principal inscription techniques and the physical properties of femtosecond (fs) pulse written in-fiber gratings are reviewed. The role of focusing and order walk-off on the inscribed structures is emphasized. A fs pulse written fiber Bragg grating (FBG) also has a unique coupling behavior, due to a refractive index change that is independent from the fiber geometry. Selected applications of such gratings for sensing and fiber lasers are discussed.

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We present the realisation of volume Bragg gratings (VBGs) in different transparent non-photosensitive glasses by using femtosecond laser pulses to enable the monolithic integration of VBGs in various optical setups

Femtosecond laser-written micro void FBG's offer control of core-cladding coupling. Careful placement of localized defects within the core cross section permits tailoring of the reflection into modes of higher azimuthal order.Many sensing applications harness core-cladding coupling of FBG. In particular, fiber based spectrographs rely on slanted FBGs for coupling into cladding modes. However, strong coupling to higher azimuthal modes is limited by the extreme (over 80 degrees) tilt angles required [1]. In contrast to UV single photon refractive index changes, which are determined by the photosensitive regions of the fiber, fs pulse induced modifications can be controlled by the focussing conditions of the writing laser. Modifications can be strong (up to ∆n = 0.44), and highly localized. This enables the design of FBGs, that are not constrained by the fiber geometry. Here, we exploit this new degree of freedom for tailored core-cladding coupling. 1535 1540 1545 1550 1555 −20 −10 0 wavelength λ [nm] Transmission [dB] 1535 1540 1545 1550 1555 −20 −10 0 wavelength λ [nm] Transmission [dB] x y w h a) 20µm b) d) a) c) Fig. 1. Micrograph (a) and transmission spectrum (b) of a micro void FBG. Below, the schematic of the model cross section (c) is shown, that was used to compute the transmission spectrum (d).The FBGs for our investigation consist of a periodic chains of fs pulse induced micro voids (Fig. 1a)) in the core of an uncoated SMF-28e fiber. They were inscribed by focussing the light of an ultra short pulse laser (Spectra Physics Hurricane, pulse duration < 110 fs at 800 nm) with a 20× oil immersion objective (NA= 0.8) into the fiber core. Second order FBGs were written with pulse energies between 200 and 275 nJ [2]. Fig. 1b) depicts a typical transmission spectrum of such a grating. At wavelength below 1554 nm, the coupling to even and odd cladding modes is clearly visible. A setup similar to that of [3] was used to image the reflected cladding modes. Based on the observed a779_1.pdf a779_1.pdf OSA / BGPP 2010 BThC6.pdf BThC6.pdf

The advantages of femtosecond laser written fiber gratings are reviewed: Directly inscribed into highly doped fibers, this technique provides an outstanding flexibility to tailor the optical properties of FBGs for monolithic fiber laser setups

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