Gregor Angelow

Sub-two-cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser

Pulses shorter than two optical cycles with bandwidths in excess of 400 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser with a repetition rate of 90 MHz and an average power of 200 mW. Low-dispersion prisms and double-chirped mirrors provide broadband controlled dispersion and high reflectivity. These pulse durations are to our knowledge the shortest ever generated directly from a laser oscillator.

PHOTOREFRACTION IN THE ULTRAVIOLET : MATERIALS AND EFFECTS

Doped as well as nominally pure crystals of Lithium Niobate (LiNbO3), ι-Arginine Phosphate (LAP), Lithium Iodate (LiIO3), Potassium Dihydrogen Phosphate (KDP), Lithium Formate (LFM), Beta-Barium Borate (BBO), and lithium tetra borate were grown and investigated for photorefractive effects at ultraviolet wavelengths down to 333 nm. In nominally undoped LiNbO3 crystals strong beam coupling effects were observed. In contrast to the visible we revealed a diffusion-dominated charge transport mechanism based on holes, and a low photovoltaic field in the order of 550 V/cm. With such a crystal we investigated the modulation transfer function of a lensless image projection system based on a phase conjugation scheme. A spatial frequency response beyond 2800 line pairs per millimeter was observed. Photorefractive beam coupling was also obtained in LiIO3. Light-induced scattering was detected in iron-doped LiIO3 whereas as-grown LAP material did not exhibit any observable photorefractive effects. However, 100 kV X-ray irradiation seems to induce material defects which can lead to weak light-induced scattering at 351 nm. In all other above-mentioned materials, doped as well as undoped, light-induced scattering could not be observed. On the other hand, this is appreciated in all the applications where the crystals are used as nonlinear material for optical frequency conversion.

Quasisynchronous Pumping of Mode-locked Few-cycle Titanium Sapphire Lasers

We investigate the modelocking dynamics of quasi-synchronously pumped, dispersion managed Kerr-lens modelocked Titanium-Sapphire lasers. For the first time, self-starting few-cycle laser pulses with 6 fs pulse duration and ultrabroadband optical spectra are demonstrated without using any intracavity elements like saturable absorbers.

Designing resonators with large mode volume and high mode discrimination.

We present a new approach for the calculation of resonator mirror shapes based on a simulated annealing optimization algorithm. Compared with the possibilities of conventional resonators with spherical mirrors, the method presented permits a large mode volume of the stationary field distribution of the oscillating lowestloss mode and an increase in its loss discrimination against the remaining ensemble of other modes at the same time. We present results for circular symmetric graded-phase mirrors that yield a difference of 30% between the diffraction losses of the fundamental mode and of other evaluated higher modes.

Spatio-temporal coupling of laser fluctuations — Observations on a laser with internal frequency conversion

The amplitude of standing wave solid state lasers with frequency conversion inside the resonator can become unstable. We investigated the amplitude fluctuations of an end-pumped Nd:YAG-laser at low pump rates (below 160% threshold pump power) and also above. Detecting the individual mode instabilities separately and comparing them with a theoretical laser model, we can trace the origin of the mechanism leading to the observed fluctuations. At certain well defined discrete pump levels above 160% threshold the instabilities disappear. These stable emission states are associated with particular nonconventional mode patterns, some of them exhibiting a helical phase structure. Based on experimental observations and computer simulations of pump induced resonator aberrations we describe a mechanism for the emergence of the peculiar transversal modes and laser fluctuations.

Challenges and limitations on generating few cycle laser pulses directly from oscillators

Sub-two-cycle optical pulses with bandwidths in excess of 500 nm have been generated directly from a Ti:sapphire laser using double-chirped mirror pairs and exploiting dispersion-managed soliton formation in the cavity. For the first time sub-two-cycle pulses directly from the laser are generated with broad enough spectra to detect and control the evolution of the phase between carrier and envelope using interference between second and third harmonic of the fundamental spectrum.