Ultra-short Pulse And Multi-wavelength Fiber Lasers Based On Nonlinear Polarization Rotation

The advanced technique of low-loss optical fiber fabrication stimulatesthe rapid development of fiber laser technology. In1990s, by employing thenonlinear polarization rotation effect of the fiber, a mode locked fiber lasercalled nonlinear polarization rotation fiber laser was realized. D ue to man ynovel features, such as fiber waveguide, special mode-locking technique,simple structure and low cost, these fiber lasers have attracted intenseattention and gained a rapid development in the past20years. In some designs,these fiber lasers have achieved great results in aspects of pulse duration, peakpower, per-pulse energy and pulse repetition rate, which is comparable to theperformance of some bulk lasers. These results extend their applicationpotentials to the fields of fiber communication, optical sensor, biology,medical treatment, industry, and so forth. Therefore, it is meaningful toconduct study on these fiber lasers for the practical purpose. Moreover, studyon the lasers plays a significant role on developing nonlinear fiber optics aswell. Since the cavity feedback effect, the electric field inside the cavity canbe strongly enhanced, making the thresholds of many nonlinearity effectsreadily to achieve. To date, theoretical predictions of many different solitonoperations have been realized in these lasers, and also some interestingphenomena have been observed in these lasers, such as low-threshold super-continuum generation. As a result, the study on the mode-locked fiber lasers issignificant for both scientific research and practical applications.Due to the effects of fiber dispersion, nonlinearity and polarization, thelight propagation within the fiber shows complicated properties. In addition,the laser cavity further influences the light propagation, making the analysisof the laser more challengi ng. So far, no analytical solution has been achievedfor the light propagation in the cavity and the study on these lasers mainlyrelied on numerical simulations. Therefore, a proper analytical model is inurgent demand for deeply understanding the operati on principles of the laserand further improving the lasing performance. For this purpose, we developeda graphic analysis approach by analytically considering the cavity propagationwith reasonable approximations. The approach was proved to be feasible to reveal the physical mechanisms of various intrinsic properties of the laser.Using this approach and numerical simulation, we studied the tunability of thepulse width and wavelength dynamics of the laser, and designed several room-temperature dual-wavelength and multi-wavelength lasers. Details are shownin the following:Firstly, based on analytical results, we summarized a graphic approachfor analyzing the cavity transmission coefficient, which was proved to be easyand effective to reveal the p hysical mechanism of various properties of thelaser, such as pump hysteresis. Unlike traditional bulk lasers which use Kerrlensing effect or saturable absorber to realize mode locking, nonlinearpolarization rotating lasers rely on a new mode-locking technique, in whichthe light polarization is sensitive to the light power. Due to the complicity ofoperation principles, these lasers lack simple approach to reach the physicalmechanisms of laser properties. Therefore, a proper analytical model is highlydesired to better understand the principles of the laser and further enhance thelaser performance. Since no accurate analytical solution is available for thecavity transmission, we obtained the analytical results by making anapproximation of separately c onsidering the effects of dispersion andnonlinearity of the fiber inside the cavity. Firstly, cavity feedback effect andfiber propagation were considered by ignoring the dispersion, and atransmission coefficient was obtained. The dispersion and some oth er lasercomponent effects were then considered as related movements of the laserstate on the transmission curve. This approach is named as graphic approachsince it uses a graph to describe the physical origin of laser principles. Wefound this approach was able to easily and precisely analyze the mechanismsof various lasing features, since it take into account all the effects of fiberpropagation, cavity feedback, and the other components of the laser. Thisapproach was verified by analyzing classic f eatures of the laser and comparingthe results with others.Secondly, we studied the pulse-shape tunability of the laser and analyzedthe mechanism and effective factors. Tunability of a laser is acquired by manypractical applications. W e experimentally found that the pulse shape could bereadily adjusted by rotating the intra-cavity polarization controllers after theachievement of mode-locking. Simulation results also confirmed this result:when the linear delay bias was changed within a certain range, the peak power of the laser pulse changed accordingly. The mechanism can be understoodusing the graphic analysis approach. The transmission of th e cavity shows asinusoidal relationship versus light power within the cavity, which generates anonlinear cavity loss clamping the peak power to a fixed value. And the valueis determined by the initial position of the laser state at the curve. When thepolarization controller is adjusted, the initial position will changecorrespondingly, which leads to the pulse-shape variation.Thirdly, we summarized the wavelength dynamics during the tuningprocess of the laser, including wavelength collapse, dual-wavelength operation,and wavelength tunability. As a candidate for fiber communication and opticalsensing, lasing wavelength properties, especially the precise control of thelasing wavelength, are strongly concerned for these lasers. To date, only a fewworks have been reported on wavelength tunability of the nonlinearpolarization rotating lasers. However, in this laser, we obtained much morecomplicated wavelength dynamics, including wavelength evolution when thelaser operation changed from one state to another, and wavelength tuning forone certain operation state. Therefore, understanding the physics behind themwill not only improve the understanding of the principle of the laser, but alsoprovide guidance towards wavelength-controllable laser and dual-wavelength(or multi-wavelength) laser design s. We found the wavelength dynamicssprings from the dispersion of the fiber waveguide. For a certain light power,the transmission of the cavity varies from wavelength, and it also shows asinusoidal relationship versus the wavelength. As a result, the lasingwavelength is determined by both the gain and transmission spectra. When thecavity components is adjusted, the transmission spectrum will changecorrespondingly, resulting in the re-selection of the lasing wavelength andspectrum evolution dynamics. Taking the dispersion effect into account andusing graphic analytical approach, the physical origin of the evolution wassummarized and analyzed.Fourthly, we realized a combined dual-wavelength emission laser andanalyzed the mechanism of this dual-wavelength operation. It is always anattractive issue of a laser to oscillate in dual wavelengths simultaneously. Toachieve the dual-wavelength operation, filtering components were used to beemployed into a cavity to select the lasing wavelengths. However, we found dual-wavelength state can be realized in the polarization rotat ing laserswithout any such filters. As well as dual-wavelength continuous-waveoperation, the combined dual-wavelength states of continuous-wave and pulseoperation was obtained. It was also obtained that the states at the twowavelengths are switchable. Through the graphic analysis, we found the dual-wavelength operation is a unique feature of the cavity, which is due to theperiodic property of the cavity transmission dispersion. Numerical simulationsfurther reveal that the lasing wavelength is determined by both the gain andloss spectra, and the wavelength spacing is related with gain spect ral shapeand the birefringence strength of the cavity.Fifthly, several flexibly tuned room-temperature multi-wavelength fiberlasers were realized by nonlinear polarization rotation. The modern fibercommunication system, such as Dense Wavelength Division Multiplexing,acquires stable and tunable multi-wavelength lasers. Traditional realization ofsuch lasers is to insert a comb-like filter into the cavity of an inhomogeneousbroadened semiconductor laser. Compared with semiconductor gain media,erbium-doped fiber offers better gain performance. However, due to thehomogeneously broadened gain property, mode competition is strong so thatroom-temperature multi-wavelength operation is hard to be achieved by simplyusing comb-like filters inside the cavity. However, by analyzing the cavitytransmission, we found the peak-clamping effect caused by the nonlinear lossproperty could effectively balance the mode competition, and realized multi-wavelength oscillation of the laser in room temperature. Both the self-filteringeffect of the cavity and additional comb-filter could be used to realize themulti-wavelength lasing. Due to the loss dispersion, lasing wavelength couldbe tuned. In addition, the wavelength spacing could also be tuned by adjustingthe free spectral range of the comb-filter inside the cavity.