Study On Multiwavelength Fiber Laser Based On Long-period Fiber Gratings&Nonlinear Effects

Multiwavelength fiber laser (MWFL) outputs dozens of wavelengths in spectral range oftens of nanometers. It is hopeful to be applied in potential dense wavelength-divisionmultiplexing (DWDM) optical communication system, optical fiber sensing, high-precisionspectroscopy and optical instrument testing, etc. The thesis presents a novel switchable andtunable MWFL based on cascaded mismatching long-period fiber gratings (LPFG) and somesimple-structure but high-effect MWFLs utilizing nonlinear effects of fiber. Meanwhile, wealso research LPFG-fabrication technologies and find some novel characteristic of LPFGs.The main and original achievements include:Firstly, we design and establish an automatic LPFG-fabrication setup by use of line-focusedand long-pulse CO2laser, which is one of the most low-cost and simplified LPFG fabricationtechnologies. We explain the dependence of mode coupling between fiber core mode andasymmetric cladding mode on irradiation-spot size of CO2laser, during the LPFG fabrication.Simulation based on finite element method (FEM) and cladding-etching experiment bothagree with our explanation.Secondly, we research cascaded LPFGs with differences, named “cascaded mismatchingLPFGs”, for the spectral characteristics in theory and experiment. We derive the analyticalexpressions of the upper and lower envelopes around the transmission spectrum of cascadedmismatching LPFGs. We prove the upper envelope of the comb spectrum is intrinsicallycurved, but less than the lower envelope, which is sensitive to the cladding mode loss. Inexperiment, through bending the grating pair of cascaded mismatching LPFGs properly,continuously tunable and high-contrast comb spectrum is realized in wavelength range of 1530nm~1560nm.Thirdly, we produced cascaded mismatching LPFGs with polarization-dependent loss(PDL) property. The upper envelope of such cascaded mismatching LPFGs is fluctuated withincident state of polarization (SOP), i.e., the spectral peaks will raise or fall with incident SOPinhomogeneously, and then a channel-transmissivity-individually-variable comb filter isformed. When such a pair of LPFGs is inserted into MWFL, it can act as a wavelengthselector and a switching filter simultaneously. With fine cooperation of a high-birefringenceSagnac loop mirror (HiBi-SLM) to tune the gain region，our individually switchable andwidely tunable MW-EDFL can output tens of types of lasing-wavelength combinations from1540nm to1570nm, which is difficult to achieve by other means.Fourthly, a length of dispersion-shifted fiber (DSF) is used in MWFL as a nonlinear mediumto induce both nonlinear-polarization-rotation (NPR) and four-wave-mixing (FWM) effects.The former induces intensity-dependent loss (IDL) to suppress the mode competition resultedfrom homogenous linewidth broadening of erbium-doped fiber (EDF); the latter equalizespower distribution on wavelengths, and both of them perform in favor of multiwavelengthgeneration. Comparative experiment demonstrates DSF has higher efficiencies thanconventional single-mode fiber (SMF) with the same length in term of wavelength count.From a simple ring-cavity MWFL, up to38wavelengths with channel spacing of0.4nm arerealized.Fifthly, a MWFL based on a nonlinear amplifying loop mirror (NALM) is demonstrated.Our theoretical and experimental researches both prove NALM has a unique advantage thatmodifying the transmission characteristic of NALM through controlling the cavity loss of thelaser, can optimizes the MWFL performance conveniently. In the “8”-figure-cavity MWFLassisted by a section of un-pumped EDF to adjust the cavity loss, as many as62wavelengthswith0.45nm channel spacing are achieved, with only70mW pump power. Meanwhile, wenumerically calculate the atomic-rate equation and cavity-rate equation of laser with software of MATLAB. We carry out dynamic simulation of MWFL based on a NALM, which ismeaningful for deep study on MWFL principles in theory.At last, we research a novel two-state MWFL based on a NALM, in which several hundredsof meters of SMFs are used as the nonlinear medium. In passively mode-locked state, thisMWFL can output nanosecond-scale square pulses at the fundamental cavity-frequency, whenas many as seventy-eight stable wavelengths are observed on the optical spectrum. Throughadjusting the polarization controllers, the laser turns to continous-wave state, generatingtunable four wavelengths with high signal-noise-ratio and narrow linewidth.