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Theoretical And Experimental Studies On Passively Q-switched And Mode Locking Tm:YAP Lasers

Posted on:2011-03-05Degree:MasterType:Thesis
Country:ChinaCandidate:Y TianFull Text:PDF
GTID:2178330338480758Subject:Physical Electronics
Abstract/Summary:PDF Full Text Request
The laser diode (LD) pumped Tm:YAP laser's wavelength is 2μm which is in the atmospheric window and is applied in remote sensing, photoelectric countermeasures, and medical diagnoses widely. In this paper, we investigate and explore both theories and experiments in the field of the diode-end-pumped Tm:YAP continuous-wave (CW), passively Q-switched (PQS) and Q-switched mode-locking (PQML) lasers.Theoretically, our main work is concentrated on the following three aspects. Firstly, in order to investigate the mode radius in the laser cavity and design various resonators conveniently,"Resonator of Solid State Laser Computer Aid Design Software V1.0"is programmed according to ABCD matrix theory. Secondly, the CW, PQS and PQML models are built according to the rate equation theory, and"Solid State Laser Output Laser Characteristics Simulation Software V1.0"is programmed, which can be easily used to analyze the output laser power, energy, and pulse time characteristics under CW, PQS and PQML laser operation. Finally, in order to analyze the effect on the output laser power, energy, and stability of long cavity by thermal loading in the crystals, a time-dependent analytical thermal model of temperature and stresses is developed for the first time. What's more, the temperature contribution coefficient and stress contribution coefficient are firstly derived from the analytical model. Then, the software of"Laser Crystal Thermal Analysis Simulation V 1.0"is programmed according to the model. Under the condition of pump power of 30 W, dynamic thermal effects of Tm:YAP sample with a radius of 2 mm was analyzed. The results show that the thermal diffusion time constant of the crystal's end face was about 0.78 s, which is independent of the crystal radius. The temperature contribution coefficient obtained from analytical model is 1.29 K/W and the highest temperature in the crystal was 326 K. The maximum stress intensity at the edge of the crystal end face is 10.2 MPa. The thermal analysis also shows that the closer the distance to the center of the face, the faster it would reach steady state, and the corresponding stress intensity in the steady state would be smaller. For the stress contribution coefficient of 0.34 MPa/W, the permissible maximum pump power of 471 W is obtained.Experimentally, four experiments are mainly carried out as follows. The first one is thermal focus measurement experiment. When the pump power is between 16 W and 34 W, the focal lengths of the Tm:YAP is measured in the range of 40~90 mm, which is agreed with the simulating results very well. Simulating and experimental results show that changes in transient thermal and stress in the crystal can be well reflected by the model. The second one is CW operation of Tm:YAP laser experiment. For the Tm:YAP laser, the influences including the working temperature of LD, the cavity length and the different transmittances of output coupler on the output power are studied in details. Then, the convex lens and plano-concave mirror thermal lens compensation methods are proposed and applied to achieve high output power. In addition, to verify the CW operation laser model, an L shape cavity is built. The slope efficiencies of the theory and experiment are agreed well (34%). However, the threshold of the theory (5.28W) is smaller than the experimental result (6.35 W). And the reasons for this difference are analyzed. The third one is PQS Tm:YAP laser experiment. Dual-wavelength (1940 nm and 1986 nm) PQS Tm:YAP laser with InGaAs/GaAs as the saturable absorber is demonstrated firstly. The influences of the semiconductor saturable absorber's (SESA) position and thermal lens effect on the Q switch characteristics are investigated. At pump power of 35 W, the maximum pulse energy of 28.1μJ with a pulse width of 447 ns at the repetition rate of 43.7 kHz is obtained by optimizing the position of the SESA. The last one is PQML Tm:YAP laser experiment. The PQML Tm:YAP laser operating at the wavelength of 1.94μm with InGaAs/GaAs as the saturable absorber is demonstrated . Conversion efficiency of pulsed operation to free-running is estimated to be 46.6%. The experimental results show that the PQML laser has the 50% modulation depth and the maximum average output laser power is 480 mW. The experimental result certifies the the InGaAs/GaAs can be used as the saturable absorber in ultrashort-pulse solid-state lasers emitting in the range of 2μm.
Keywords/Search Tags:Solid State Laser, 2μm Laser, Tm:YAP, Passive Q-switch, Passive Mode-Lock
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