| Mid-infrared laser has attracted large amount of attentions because of their wide applications in the fields of national defense,biomedicine,atmospheric environment monitoring and so on.Compared with the traditional solid and gas laser,as a new technology developed in recent years,mid-infrared fiber lasers have many advantages such as good beam quality,good heat dispersion,high conversion efficiency,simple structure and so on.On the other hand,Q-switched pulse fiber laser,compared with CW fiber laser,can achieve higher peak power and pulse energy with same average power.Therefore,mid-infrared Q-switched pulse fiber lasers have more important application value in the fields of industry processing,medical treatment and optical-electrical countermeasure.Up to now,there have been a lot of experimental reports of mid-infrared Q-switched pulse fiber laser,but there are few reports about theoretical research and optimization of this laser.The main work of this thesis is as follows:1.The numerical model of actively Q-switched Er3+-doped ZBLAN fiber laser and actively Q-switched cascade Ho3+-doped ZBLAN fiber laser were established.The output characteristics of Q-switched pulse were investigated for different fiber core diameters,pump power,repetition rate,fiber length,fiber loss and acousto-optic efficiency.The results show that Er3+-doped fiber under forward pump,Ho3+-doped fiber under bidirectional pump will cause the increase of pulse energy,peak power,stored energy and cause the decreasing of pulse width.Through improving the pump power or reducing the repetition frequency,higher output energy,more narrow pulse width,higher peak power pulse and higher fiber stored energy can be achieved.Using the gain fiber with large core diameter will cause the increase of the pulse energy,pulse width and energy storage,and the decrease of the peak power.Overlength gain fiber will result in the decrease of pulse energy,pulse width and the increase of the pluse buildup time.The loss(AOM loss,fiber loss,output coupling loss)will lead to the decrease of output energy and peak power,but has no effect on the pulse width.2.Based on the theoretical optimization results,an actively Q-switched cascade Ho3+-doped ZBLAN fiber laser was bulit,high energy and high power stable output pluse operated at 3?m and 2?m waveband were realised.Pulse at 3?m waveband with maximun pulse energy of 29.8?J and minimun pulse width of 380 ns at repetition rate of 25 kHz was produced.Correspondingly,pulse at 2?m waveband with maximun pulse energy 9.2?J and maximun peak power 49 W at repetition rate of 17 kHz was realised.Then,we set up a Q-switched induced gain switching cascade Ho3+-doped ZBLAN fiber laser.Through using a plane diffraction grating to tune wavelength,stable pulse trains from the 5I6?5I7 transition were observed from 2.95?m to 3.031?m and gain-switched pulse at 2?m waveband induced by the 5I6 ?5I7 Q-switched pulses were also produced over the entire tuning range.At maximum pump power and at an acousto-optic modulator repetition rate of 40 kHz,for pluse at 3?m waveband,maximun pulse energy of 17?J,maximum peak power of 43 W and pluse width of 410 ns was produced,corresponding to the 2?m waveband,maximum pulse energy of 3.7?J,pulse width of 550 ns at repetition of 40 kHz was realised,and the output wavelength is basically unchanged. |
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