Modeling of er3+ doped fiber lasers

Resumen

Today the erbium-doped silica fibers are widely used in fiber laser industry as an active element in fiber amplifiers, continuous wave or pulsed lasers operating in the S, C and L communication bands (from 1480 to 1625 nm). Moreover, erbium doped fiber lasers constitute a suitable alternative to laser diodes. They are potentially powerful sources of electromagnetic radiation and have some attractive advantages as small size, low weight, and relatively simple designing concept. The Q-switched erbium doped fiber laser is capable of producing pulses with hundreds of Watts of peak power, but as observed in many types of actively and passively Q-switch fiber lasers, the output has a multi-peak shape, with a time interval between the neighboring peaks being approximately equal to the laser cavity round-trip period. The most common explanation of the origin of this multi-peak pulse structure is a partial ¿self-mode-locking¿ effect. The lack of a proper understanding of the ¿self mode-locking¿ effect leads to the wrong conclusion that Q-switched erbium doped fiber laser is in particular and Q-switched fiber laser is in general are unsuitable as a master oscillator due to its incapacity to produce a smooth clean and short pulse. Along this thesis, a model for an actively QS-EDFL is developed; the model takes into account the distributed nature of the laser, the real function that describes an opening process of the Q-modulator and all the distributed and local losses that exist in the fiber cavity including the excited state absorption losses inherent in erbium doped fibers. In addition, the master system of differential equations that describes the EDFL behavior is solved in the steady state case that corresponds to the continuum wave laser regime. Furthermore, the model of the erbium doped fiber laser is used to study of the influence of the exited state absorption on the laser performance for continuum wave, distributed feedback and Q-switched erbium doped lasers. The comparison of theoretical values and the experimental data shows that the erbium doped fiber parameters used in the modeling match with the electromagnetic field distribution across the fiber. Moreover, our simulations satisfy energy and photon balances. The numerical solution is corroborated by the experimental data and shows that the ¿self mode-lock¿ hypothesis is inconsistent and the source of the multi-peak structure of Q-switched erbium doped fiber laser pulses is the spatial-temporal oscillations of the electromagnetic field inside the cavity. In this case, the Q-modulator properties determine the shape of sub-peaks fronts, while the laser level depopulation dynamics defines the ¿exponential¿ shape of their rear-edges. Furthermore, the depopulation mechanism of the laser level is similar to the mechanism accepted for the description of pulse distortions in erbium doped fiber amplifiers. In conclusion: in this thesis, the realistic distributed models of the erbium doped fiber lasers including continuum waves Fabry-Pérot fiber lasers, distributed feedback fiber lasers, and Q-switch Fabry-Pérot fiber lasers is presented. Therefore, it is shown that the same system of differential equations can be used for simulation of the continuum wave and Q-switch laser. It is demonstrated that the shape of output pulses from Q-switched erbium doped fiber lasers with the long cavity essentially originates from its distributed nature, rather than from a ¿self-mode-locking¿ effect. At least, this is the case when the rise-time of the acousto optic modulator is less than the cavity round-trip period or has the same order of magnitude. The losses originated from the exited state absorption, always observed in erbium-doped fibers, have a strong effect on the optimal parameters of the distributed feedback erbium doped fiber lasers and lasers working in continuum wave regime. Thus, the exited state absorption losses are the main limiting factor of the erbium doped fiber laser efficiency. Moreover, it is demonstrated that one-side efficiency of erbium doped distributed feedback fiber laser can be increased by approximately 60% by shifting the phase defect from the middle of the fiber Bragg grating towards the laser output, increasing at the same time the fiber Bragg grating strength. Finally, the design of a Q-switch fiber laser with new properties is carried out. The designed laser is able to generate ¿on-demand¿ single Gaussian-like pulses with the duration of 17.5 ns and 76 W of peak power.