| Tunable solid state lasers are more practical because their wavelength can be continuously tuned within a certain range.Especially,visible and near-infrared lasers are important laser sources in industries,medicine,military and other research fields,which has broad application prospects in material processing,medical surgery,laser radar and photoelectric countermeasures,etc.Using laser diode(LD)as the pump source to realize the operation of solid-state laser has the advantages of small size,simple design,etc.,which is conducive to the development of laser towards all solid-state and miniaturization,thus attracting the attention of researchers in the optical field.Among many solid gain mediums,transition metal ion doped crystals generally have wide phonon vibration emission spectra.Ti:sapphire(Ti3+:Al2O3),alexandrite(Cr3+:BeAl2O4),Cr3+:LiSrAlF6 and Cr3+:LiCaAlF6 are broadband tunable laser gain mediums with excellent performance in visible and near-infrared bands.Among them,the alexandrite has the wavelength tuning range of 701-858 nm and has the advantages of high hardness,high damage threshold,long fluorescence life,LD-pumped and special temperature characteristics,which is an excellent broadband tunable laser crystal.Therefore,the LD pumped all solid-state alexandrite lasers have important application potential.The electron-phonon coupling effect refers to the interaction between the activated ions and the surrounding lattice to achieve multiphonon-assisted electron transition,so as to obtain broadband tunable phonon vibration laser emission.In 1950,Huang and Rhys established a theory of multiphonon-assisted electron transition.So far,this theoretical model has been widely used and studied in the optical field.Although this theory has achieved success in many fields,the underlying physical mechanism of electron-phonon-photon interaction in the process of multiphonon-assisted laser emission has not yet revealed.At present,almost all phonon vibrational lasers only judged in advance by experimental spectra,and there is no reliable guiding theory.Therefore,the study of the intrinsic relationship between the multiphonon-assisted transition process and laser emission is of great significance for the further exploration of many physical phenomena in the electron-phonon-photon coupling system.High repetition rate ultrafast laser plays an important role in many emerging scientific fields because of its high resolution and fast response.Transition metal ion doped crystals have strong electron-phonon coupling and generally have a wide phonon vibrational emission,so they are ideal gain materials for obtaining ultrafast lasers.Since the first laser operation of alexandrite crystal achieved,its research progress in the mode-locked has been slow until recent years.At present,the narrowest pulse width achieved in alexandrite crystal was 65 fs,and the repetition frequency was in the order of MHz.Therefore,based on the application prospect of high repetition rate ultrafast alexandrite laser in medicine,biology,etc.,it is necessary to find new methods to further improve the repetition rate and compression pulse width.This dissertation is based on the alexandrite crystal.First,the spectral characteristics were introduced and its electron-phonon coupling process was clarified.Then,based on the Huang-Rhys theory,the physical mechanism of the multiphonon-assisted photon emission was studied,and theoretical model of the multiphonon-assisted process in the strong electron-phonon coupling system was proposed.Wavelength tuning experiment of alexandrite was carried out to verify the theoretical model.Then,based on Kerr-lens mode-locking and F-P cavity,the performance of high repetition rate ultrafast laser in alexandrite was studied.In addition,based on the principle of loss modulation,the laser performance of ultrashort self-pulse of alexandrite was studied by spectral broadening technology.The main research contents of this dissertation are as follows:1.The spectral characteristics of alexandrite crystal were characterized.The absorption spectrum,fluorescence lifetime and polarization emission spectrum versus temperature,and Raman spectrum were measured.The electron-phonon coupling strength was calculated based on the emission spectrum.The valence bond assignment of the partial phonon frequency was simulated through the analysis of Raman spectrum,and the electron-phonon coupling process of alexandrite was clarified.2.Based on theory of Huang-Rhys and spectral lineshape function,the relationships between phonon-assisted electron transition and laser generation were established,and the internal mechanism of multiphonon-assisted photon emission was explored.The wavelength tuning experiment of alexandrite crystal was carried out to verify this theoretical prediction.In the experiments,728-815 nm phonon assisted tunable vibration laser emission was realized,which corresponded to 2-5 phonons participating.Meanwhile,the changes of laser output power and laser threshold with wavelength at different temperatures and laser threshold with temperature at different wavelengths in the experiment were in good agreement with the theoretical calculation,which proved the reliability of this theoretical hypothesis.This work theoretically explains the intrinsic relationship between the multiphonon-assisted transition and the laser performance in the strong electron-phonon-photon coupling system,and provides theoretical guidance for wavelength tuning and spectral broadening of phonon vibration lasers.3.Based on Kerr-lens mode-locking technology and F-P cavity effect,using a red LD as a pumping source and a simple linear cavity,stable first and second order high repetition rate self-mode-locking laser have been achieved in alexandrite.First,a mode-locked laser with pulse repetition rate of 3.6 GHz and pulse width of 237 fs was realized without F-P cavity.Then,the F-P cavity was formed between the residual reflection of the rear face of the crystal and the output-coupling mirror.The experiment realized the mode-locked pulses with the repetition rate of 7.5 GHz and the pulse width of 201 fs,respectively.This work is the first GHz pulse laser realized by Kerr-lens mode-locking in transition metal ion-doped crystals with LD as the pump source at present.The simple and compact cavity used in this work is easy to operate,which provids a reference for obtaining high repetition rate ultrafast laser.4.Based on the spectral broadening technology,a 0.5 mm thick quartz birefringent filter was used as spectral broadening element to further compress the pulse width while ensuring high repetition rate.Similarly,the alexandrite crystal was also pumped by the red LD,and stable ultrashort self-pulse laser was achieved in a compact flat concave cavity.The repetition rate was 2.48 GHz and the pulse width was as short as 36 fs,which was the narrowest pulse currently realized in the alexandrite crystal.The corresponding spectral width was approximately 20 nm,which was about 4-5 times wider than that without spectral modulation.Using only a quartz filter,high repetition rate ultrashort self-pulses were obtained in a compact cavity with LD as pump source.This method is simple in design,which makes it possible for its miniaturization application,and provides an effective means for further compressing the pulse width of ultrafast laser.In addition,the possibility of using the electron-phonon coupling effect to broaden the spectrum and further compress the pulse width was also discussed. |
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