Optimization of pump absorption in MOF lasers via multi-long-period gratings: design strategies

Abstract

Different strategies for designing optical couplers, optimized to enhance the pump absorption in the rare-earth-doped core of microstructured fiber lasers, are illustrated. Three kinds/configurations of optical couplers have been designed and compared as examples of the different design strategies which can be followed. Their effectiveness to enhance the performance of an ytterbium-doped, double cladding, microstructured optical fiber laser has been accurately simulated. They consist of a suitable cascade of multiple long-period gratings (MLPGs) inscribed in the fiber core region. The characteristics of the MLPG couplers have been simulated via a homemade computer code based on both rate equations and an extended coupled mode theory. The proposed MLPG couplers seem particularly useful in the case of low rare-earth concentration but, even for a middle-high ytterbium concentration, as N(Yb)=5×10(25) ions/m(3), the slope efficiency S can be increased up to 20%, depending on the fiber length.

Advances on Optical Fiber Sensors

“…Laser sensors, include active behavior (e.g., due to the rare earth doped materials) and grating transmission spectra (e.g., FBG or LPG), which can be highly sensitive to the variation of some measurands. In [82,83] the design criterion to choose the suitable spatial distribution of multiple long period gratings MLPG couplers with respect to a MOF laser cavity was identified. The reported model extensively illustrates the behavior of the LPG in active medium, via the coupled mode equation and the rate equation of the rare earth ion population.…”

“…In fact, it describes an Yb 3+ -doped MOF laser cavity by taking into account the presence of different MLPGs inscribed within the core. The MLPGs reported in [83] enable the interaction among the two-fold degenerate fundamental mode guided in the fiber core (HE 11 ) and the inner cladding modes, at the pump wavelength. The model includes the following kinds of couplers: (a) MLPGs-P, Multiple Long Period Gratings in Passive fibers, i.e., grating cascades inscribed in the undoped region, just outside the laser cavity; (b) MLPGs-A, Multiple Long Period Gratings in Active fibers, i.e., grating cascades inscribed within the laser cavity (see Figure 13); (c) DS-MLPGs, Double Stage Multiple Long Period Gratings, i.e., a combination of passive grating cascades, inscribed outside the laser cavity, and active ones, inscribed within the laser cavity, are employed).…”

“…The vertical lines represent the grating cascade along the fiber axis. Even if MLPGs-P, MLPGs-A and DS-MLPGs were designed in [83] to increase the pump power transfer from the inner cladding modes at the pump power towards the fundamental core mode of the MOF laser, the model can be employed for designing active MLPG sensors since both (i) the LPG transmission spectrum; and (ii) its efficiency as power coupler can be affected by different measurands (e.g., bending or strain).…”

“…Power P clad (λ p ), P core (λ p ), P + core (λ s ), P − core (λ s ) versus the position along the fiber longitudinal axis z, DS-MLPGs[83].…”

Bend measurement using an etched fiber incorporating a fiber Bragg grating

“…The sensor has short measurement range of ±10°. Also, as the LPG couples forward propagating guided mode to forward propagating cladding modes [8,9], the sensor can only be used in transmission configuration and may not be preferable for single-ended sensor applications. A compact fiber optic vector inclinometer based on taper and tilted fiber Bragg grating (TFBG) concatenation is presented by Shao and Albert [10].…”