Time and frequency controlled pulse formats from femtosecond regenerative amplifiers

Abstract

High energy femtosecond lasers offer great potential in science and industry. Such lasers are used in a number of applications ranging from material processing over strong field physics to precision spectroscopy. In order to produce high energy femtosecond laser pulses it is necessary, due to the spectral properties of widely used (ytterbium) gain media, to employ spectral shaping to avoid gain narrowing during amplification. A shaping scheme based on birefringent crystals without added phase distortion is presented. Another problem linked to the properties of gain media, the uorescence lifetime, is that regenerative amplifiers can operate in a bistable regime where the output energy jumps between two levels in consecutive amplification cycles. To stabilize an amplifier in this regime a method for inline control of the output energy was developed. As not for every wavelength, desired by e.g. spectroscopy or material processing applications, suitable gain media exist a method of nonlinear parametric frequency conversion by four-wave-mixing in hollow capillaries to extend the wavelength in the mid-infrared region is explored. As for a wide range of applications it is beneficial to have pulse bursts, packets of pulses with (much) smaller time separation that the repetition rate of the laser, the technique of generating pulse bursts by the Vernier technique is discussed to produce and stabilize pulse bursts with burst frequencies reaching from GHz to THz. The employed technique allows the control of the number of pulses, the pulse spacing and the amplitudes and phases of the pulses in the burst individually.