Under project ВУФ-02/2005 titled "Modern Laboratory of femtosecond photonics" with project coordinator Prof. Ivan Christov, the first in Bulgaria and on the east Balcans, Laboratory of femtosecond photonics was created. The project was financed by the Ministry of Education and Science for three year.

For the completion of the project, the procurement of a high-quality pumping source, was required, without which, mode-locked regime of the Ti:Sapphire femtosecond laser is impossible. As the most suitable, the Verdi v5 (Coherent), was chosen.

Purchased and installed continuous-wave single-frequency pumping laser Verdi V5.

Since, at that time in Bulgaria, there was almost no experience working with solid-state femtosecond lasers, and the team needed preliminary testing, the group of Associate Professor Dr. Ivan Buchvarov, with their own components, launched the first femtosecond oscillator and investigated the regimes and settings.

The purchased elements, needed for the Ti:Sapphire oscillator, laser crystal, chirped mirrors, stands and tables for mounting and tuning the optical components in the laser resonator. The laser oscillator was mounted, tuned and optimized to produce ultrashort pulses. on the figures, the chosen scheme for the femtosecond oscillator is shown.

For controlling the in-resonator dispersion, from the active medium, a system of mirrors (CM1-n), with a group delay on each of them is chosen. The output mirror is on a thin surface, designed ina way, that it has minimal dispersions. In the free-generation regime the oscillator's output power is ~800mW. On the figures below, the construction of the laser oscillator is shown. With the aid of the specifically built in our laboratory autocorellator, with second harmonic generations, autocorellator pulse length of 38fs was measured, corresponding to a real pulse duration of 28 fs with central wavelength of 790 nm and energy of 4 nJ in a single pulse with 83 MHz repetition rate.

Afterwards, a GRENOUILLE femtosecond measuring system was purchased, whick works based on the FROG (frequency-resolved optical gating) technique, together with the necessary software for data analysis. Additionally power and energy meters and an ultrafast oscilloscope were also purchased.

FROG data, aquired during optimal laser regime and small de-tuning.

Differentregimes of the built femtosecond oscillator were investigated, by using an in-reseonator compression with chirped mirrors. In the figure below the FROG-data, aquired, are shown when the laser pulse is optimized, and with a small de-tuning. With optimised pulse, the duration is 67 fs, and in the other case - more than 80fs.

Having this experience, under different project, another femtosecond oscillator was purchased from Quantonix (Ti:Light), as well as an amplifier. The output energy of a single femtosecond pulse was up to 2.5 mJ with 1kHz repetition rate. The laser is pumped via SHG of CW Nd:YVO4 laser ( λ=532nm ) with pumping power of around 3W, the output is around 200mW with the central wavelength of 800nm.

The ability of controlling pulse duration between 110 fs and 35 fs, by changing the depth of penetration of the beam in the in-resonator prism is shown in the figure below.

Dependancy of the pulse length duration on depth of prism penetration, lead by the total dispersion of the resonator. Black curve (circles) - experimental data; red curve (squared) - numerical data. Images (right) - measured FROG-data at three characteristic positions.

Additionally built cascade of chirped mirrors, onto which, the beam has multiple reflections (see figure below),with which the Ti:Light laser steadily delivers sub-25fs pulses.

Under another project, XUUS (KMLabs–USA) equipment for coherent X-ray light in the vacuum UV and soft X-ray range was delivered. It is the main module in a system for coherent laser UV radiation. Special CCD-cameras were purchased, needed for registering the radiation, sensitive in the spectral regions needed, as well as the needed vacuum equipment.

X-ray CCD-camera, registering the output beam of the nonlinear transformer.

The next figure shows an overview of the system for coherent short wavelength generation.

X-ray monochromator, combined with sensors and pump.

Overview of the laser system for generating coherent short wavelengths, using HHG (high harmonics generation) in hollow waveguide

Next figure represents the first successful generation of 25-th, 27-th и 29-th harmonic of the laser in Ar gas and the tunability, depending on the gas pressure.

Relative intensities of 25-th, 27-th и 29-th harmonic of Ti:Sapphire laser in Ar.