| A cylindrical microcavity laser is fabricated by inserting a bare optical fiber into a lasing dye solution of low refractive index (RI), optically pumped and gain coupled by evanescent-waves, Whispering-Gallery-Mode (WGM) lasing emission is found around the fiber. The characteristics of the WGM lasers, including the polarization, energy threshold, spectrum shifted and spectral line broaden have been investigated systematically in this doctoral dissertation. Based on the investigation, a novel multi-band WGM fiber laser buried into a PDMS substrate is successfully invented. The main research contents and results of the dissertation are consisted of six aspects as following:First of all, the optical gain of the WGM lasing in a cylindrical microcavity is analyzed and calculated on the base of evanescent wave pumped and coupled gain. The pump light is assumed to be a Gaussian beams and homogeneous distribution beams, and when the beams are tightly focused by a concave lens and coupled into an optical fiber, the distribution function of incident angle on the inner surface of the fiber is derived and calculated. The calculation results indicate that the distribution function curves shift to the small angle direction of the inner surface of the fiber, that is, the bigger incident angle in the end face of the fiber is beneficial to the evanescent field of the pump beams tunneling into the cladding dye solution and stimulates the lasing gain effectively.Secondly, the analytical approximate formulas for the Q values which corre-spond to the leakage losses in a cylindrical cavity are deduced, and various losses related to the microcavity are analyzed theoretically and calculated numerically. The results show that:various losses related to the cylindrical microcavity are mainly de-pendent on the absorption and leakage losses; for a large (or small) diameter fiber, the losses are mainly determined by absorption (or leakage) losses, while the absorp-tion and leakage losses are determined the losses together for a moderate diameter fiber.Thirdly, the polarization characteristics of the WGM lasing in a cylindrical microcavity have been systematically studied by using both axial and side pump schemes. For the laser gain is excited by side-pumping scheme, it is found that the polarization property of lasing emission is simply dependent on the polarized state of the pump beams. The polarization property of lasing emission depends on the prop-agating situation of the pump beams in an optical fiber if the laser gain is excited by evanescent-wave pumping scheme, that is, if the pump beams within the fiber are meridional beams, the lasing emission is a transverse electric (TE) wave that forms a special radial polarization emission. However, if the pump beams within the fiber are skew beams, both transverse magnetic (TM) and TE waves exist simultaneously in lasing emission that forms a special axially and radially mixed polarization emission. Pumped by skew beams, the wave-number differences between TE and TM waves are also investigated quantitatively, the results demonstrate that the wave-number difference decreases with the increase of the fiber diameter and the RI of cladding solution. The observed polarization characteristics have been well explained based on lasing radiation mechanism of WGM fiber laser of gain coupled by evanescent wave.Fourthly, pumped by evanescent-wave along the fiber axis, the energy thresh-old properties of the lasers, including the energy threshold varied with the RI of the dye solution for different fiber diameters, and the produced length of lasing emission along the fiber axis varied with the RI of the dye solution and the lasing dye solu-tion concentration, have been investigated. We find experimentally that the energy threshold is very sensitive to the RI of dye solution and the fiber diameter, with the in-crease of the RI of the mixed solution, the threshold energy increases monotonously for smaller diameter fibers, but decreases slowly for fibers of larger diameter; for a fiber of moderate diameter, the threshold energy decreases slowly at first, and then increases sharply, there is an optimum RI of the mixed solution which matches a minimum threshold energy. The produced length of lasing emission is dependent on the dye concentration and the RI of dye solution, for a given pumping energy, the lasing produced length decreases with the increase of the dye concentration and the RI of dye solution, and the lasing produced length logarithm increases with the pump energy.Fifthly, cavity-Q-driven lasing wavelength shift and line-width widening in an evanescent-wave pumped and gain coupled WGM fiber lasers have been investi-gated. We find that both lasing wavelengths and line-width of WGM peaks are sen-sitive to the RI of the dye solution and the fiber diameter, with the increase of the RI of dye solution or decrease of the fiber diameter, a blue shift of the lasing spectrum and a widening on the spectral line width are observed. The observed phenomena have been attributed to the change of quality factor of a circular cavity built in the fiber cross section, the achieved experimental data agree well with the calculated values of the quality factor.Finally, a novel multi-band WGM fiber laser buried into a PDMS substrate has been invented, that is based on evanescent-waves pumped and coupled gain. Three-wavelength-range WGM lasing radiation, and a dual wavelength-range with unidi-rectional WGM lasing emission from a PDMS chip have been successfully achieved. |
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