| Abstract [eng] |
Since invention of a first solid state laser scientists have tried to achieve highest possible peak power in order to fulfil the need for industrial and fundamental research applications. The two main mechanisms for increasing peak power of a pulsed laser are by increasing pulse energy or by decreasing its temporal width. Soon however the limits of the amplifying medium were reached as the short became powerful enough to damage the amplifying medium. Things started to move forward again when G. Mourou introduced the method of chirped pulse amplification (CPA). The idea behind this method is that short low power pulse is stret- ched temporarily to decrease its peak power, amplified to a certain level, while avoiding to reach the damage threshold for amplifying medium, and compressed again to the initial duration. Two of the main parts in such system are pulse stretcher and compressor. There are many ways of stretching and compressing the pulse but they all have their limitations. Diffraction gratings are sensitive to alignment precission, prisms require huge spatial separation for better stretching or compression, fiber require a lot of material, chirped fiber bragg grating have low damage threshold and chirped mirror require multiple pass to stretch pulse to the proper level. As a possible alternative to these methods we investigate a chirped volume Bragg grating with a temperature controlled dispersion, which allows to achieve high peak power due to its large volume and compensates large amount of dispersion in very short length. In this experiment two chirped volume Bragg gratings were used. Stretcher had thermal control over its two ends to create temperature gradient along the crystal and to compensate dispersion of compressor as well as dispersion of components between them. Experiment was conducted using narrow and broad spectrum pulses. Narrow spectrum pulses were amplified and compressed to 670 fs (transform limited 570 fs) but had a smooth shape therefore we think its possible to reach transform limited pulse length by optimizing our setup. Broadband pulse had higher deviation from transform limited pulse and changed its shape from Gaussian to sinc function. There were also some attempts made to compensate single mode fiber dispersion. By adding 40 m fiber between compressor and stretcher, we were able to compensate fiber dispersion by creating a temperature gradient of dT=29 °C between the two ends of CVBG stretcher. Ability to control pulse length was also demonstrated. By under compensation of the compressor dispersion its is able to compress the pulse to higher pulse length if there is a need for that. In our case we were able to extend pulse from 670 fs to 19,4 ps by using largest possible temperature gradient in our system. |
