Characterization of optical components of a laser amplifier via spectral interferometry

One of the largest branches of modern optics is laser technology, which offers a wide range of applications in industry, medicine and research. In particular, the generation of subpicosecond laser pulses has greatly expanded the possibilities for the application of laser systems. An important milestone in the development of pulsed laser systems was the chirped pulse amplification (CPA) principle developed by Strickland and Mourou in 1985 [1], which  earned them the Nobel Prize in 2018. The high peak powers that can be achieved by lasers employing the CPA schemes are used e.g. for experiments aiming at laser-driven proton acceleration. Such experiments are performed at the high intensity laser system POLARIS (Petawatt Optical Laser Amplifier for Radiation Intensive Experiments) which is operated at the Helmholtz Institute Jena. In order to characterize the plasma conditions during such experiments very precisely, an independent pump-probe setup is currently being developed. With this setup the precise characterization of the formation and temporal evolution of the plasma target becomes possible. For the plasma characterization setup, a CPA system consisting of a pulse stretcher, regenerative amplifier and pulse compressor is being developed.

In order to characterize the pre-plasma, knowledge of the temporal shape of the laser pulse is important. One of the main big issues is the measurement of ultra short laser pulses with durations of 50 fs, which is shorter than the response time of any electronic detector. In order to measure such kind of laser pulses, a reference pulse with a comparable duration can be used to sample the pulse to be measured by a suitable detector. Such a sample pulse can be the pulse itself or another well characterized femtosecond laser pulse [2]. It is important to measure both amplitude and phase of the spectrum, although it is usually not possible to measure the latter directly. One of the most effective methods of laser pulse characterization is called spectral interferometry (SI) which was pioneered by Froehly et al. in 1973 [3]. It offers the p ossibility to measure the spectral phase and has attracted the attention of researchers because of its large sensitivity and high spectral resolution.SI has many applications in spectroscopy [4], plasma probing [5], characterization of dispersion, studies of nonlinear processes [6], materials and characterization of crystals [7]. SI is also the basis for new techniques of spectral phase measurement, such as frequency resolved optical gating (FROG) [2] or spectral phase interferometry for direct electric field construction (SPIDER) [8].

In section 2 of this Bachelor thesis, the fundamentals of ultrafast optics based on Maxwell’s equations are presented, Gaussian beams, optical pulses and their propagation in dispersive media are introduced. The method of spectral interferometry (SI) is fundamentally introduced and explained in section 3, different possibilities for characterizing the spectral phase are presented. The experimental setup for the characterization and a referencing measurement to well characterized materials is done in section 4. It is also investigated in section 4 which experimental issues can occur, how large their influences on the measurement are and how they can be resolved. The derived methods of spectral phase characterization are used in section 5 to specify the optical components of an amplifier in a CPA laser system. The components of the laser amplifier are categorized and their effects on the spectral phase are compared and discussed. It is then summarized why dispersion measurements are important and how the method of SI can be utilized to select suitable components for a laser amplifier.