Preparation of Nano Gain Materials of Infrared Lasers and the Investigation on Output Characteristics of a High Power Laser
|School||Zhejiang University of Technology|
|Course||Mechanical Manufacturing and Automation|
|Keywords||Quantum dot quantum dot preparation PbSe quantum dot doped glass quantum dot doped fiber laser CO2laser laser transverse mode|
Laser gain media generally include solids (such as Nd3+:YAG, Yd3+:YAG),semiconductors （such as GaAs, InP）, liquids (such as organic dye solution, Nd3+dopedinorganic solution), and gases （such as CO2, He-Ne）. Conventionally, these gain media arebulk materials. Recently, semiconductor nanocrystal materials, i.e., quantum dots （QDs）,which are used as laser gain media, have been attracting considerable attention in the world.Due to quantum confinement effects and quantum size effects, QDs exhibit unique optical andelectrical properties, for instance, discrete emission spectra, tunable emission wavelengths,and obvious Stokes shifts. Then, QD shows widely prospective applications in optoelectronicgain devices （optical amplifiers and lasers）, solar cells, biomedical fluorescence probes, andtracers.This dissertation is concentrated on the laser as a processing tool in the advancedmanufacturing technology. We begin with a preparation of the infrared （IR） lasing gain media,i.e., PbSe QD doped glasses and the QD fibers by using the high-temperature melting methoddue to usual energy source in laser processing is IR lasers. Following a QD doped fiber laser（QDFL） is proposed by analyzing its stimulated radiation mechanism, the lasing outputcharacteristics and temperature dependence in the fiber are simulated numerically. The resultsshow that QDFL has low saturated doping concentration, short saturated fiber length, andhigh pumping efficiency, however, temperature-rise effect can not be neglected. On the otherhand, in order to improve the beam quality of high power transverse-flow CO2laser, we makea detailed analysis of forming laser transverse mode by establishing and numerically solvingthe lasing level-population rate equations. Obtained results show that the main factorsinfluencing the laser transverse mode and power are the electrode configuration and the gasflowing rate in the CO2laser resonator.The study mainly includes the following aspects:1. Using ZnSe as the source of Se for PbSe QDs to replace Se, a silicate glass doped withhigh concentration PbSe QDs is successfully prepared by using the high-temperature meltingmethod. The X-ray diffraction （XRD）, transmission electron microscopy （TEM）, andfluorescence spectra （PL） show that QD concentration in the glass is higher than that ofelement Se for PbSe QDs in the same thermal treatment condition. Moreover, both the PLpeak intensity and the FWHM are also larger than that of element Se as the source of Se forPbSe QDs. Effect of thermal treatment conditions on the size of QDs, PL peak wavelength, peak intensity, and FWHM are also studied in chapter2.2. Some types of tellurite glasses doped with PbSe QDs are prepared in laboratory byusing the high-temperature melting method. However, the measured XRD shows that there isno crystal structure of PbSe QDs formed in such prepared glasses. The invalid causes of thismethod are analyzed in chapter3.3. The PbSe QD doped fiber is fabricated by using the melt-drawing method. Themeasurements of an optical microscope, XRD, and TEM show that the fiber has a fixeddiameter （about125μm, close to the ordinary fiber）, smooth surface, and circular cross section.The fiber contains a certain amount of PbSe QDs after thermal treatment. Its mechanicalstrength comes near usual SiO2fibers.4. A two-level model is established based on the observed single-peakphotoluminescence of PbSe QD used as the gain medium in the fiber. The optimal parameters（doping concentration, fiber length, and pumping power） of QDFL are determined bynumerically solving the rate equations, power-propagation equation, and heat conductionequation. The laser power and the temperature distribution under different pumpingconfigurations are analyzed. Comparing with the conventional Yd3+doped fiber laser, QDFLshows obvious the temperature rise effect of the fiber due to big absorption cross-section ofPbSe QD. Then, we propose an improvement in experiment to reduce the temperature riseeffect.5. The lasing transverse energy distribution of a high power tube-plate transverse-flowCO2laser is simulated numerically by solving the spacetime-resolving rate equations derivedfrom the discharging field determined by the Maxwell equation. It shows that the simulatedlasing distribution agrees with the experimental observation. Moreover, there is evidence toshow that the electrode configuration in the CO2laser resonator determines the peak value oflasing transverses modes, while the gas flowing rate determines the lasing gain peak and thepeak position along the gas flowing direction.The work in this dissertation can be helpful for the development of novel IR lasers bypreparing PbSe QD doped glasses, investigating the stimulated emission generated in the QDdoped glass, and the laser propagation in the QD doped fiber. Obtained results for thetransverse mode of high power CO2laser can be advisable for controlling lasing transversemodes, improving lasing stability, and enhancing the processing quality when the laser is usedas a light source.