Polarization encoded (PE) Ti:sapphire amplifier can easily pre-shape the spectrum of amplified pulses. This property can be used to compensate for the spectral red-shifting and gain narrowing that are typically observed in Ti:Sapphire lasers. We demonstrate experimentally that active pre-shaping of the pulse spectrum in a PE amplifier combined with saturated amplification in the following conventional amplifier can conserve and even broaden the overall amplification bandwidth. A combined amplifier that includes PE- amplification (during the first passes) and a conventional one in the following saturation phase is also proposed and studied by computer modelling. This allows to achieve both the broad bandwidth and high efficiency in a single amplifier. A 5 passes combined PE amplifier was simulated. The seed was firstly amplified by 3 passes with the PE amplification scheme, then the seed was decoded and directed back to the crystal for 2 additional passes of a saturated conventional amplification. Because the seed was already decoded before the last saturation passes in the amplifier, the energy extraction efficiency reached 44% which is similar to that of a conventional Ti:sapphire amplifier. The amplified bandwidth of 125 nm was obtained with a Gaussian seed spectrum of 100nm. We show experimentally that the decoding efficiency of PE amplifier can be optimized by changing the thickness of the decoding quartz. At gain of ~30, the decoding efficiency of ~75% was achieved with the thickness of the decoding quartz of 35.1mm (thickness of the encoding quartz was 17.4mm), while the decoding efficiency of 80% was reached at gain of ~10. It shows that smaller gain guaranties better efficiency and also a smoother spectral profile. The compressibility of the PE amplified pulses close to the transform limit is verified experimentally.

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Huabao Cao, Mikhail P. Kalashnikov, Károly Osvay, Nikita Khodakovskiy, Roland Sándor Nagymihály, and Vladimir V. Chvykov, "Active spectral pre-shaping with polarization encoded amplifiers (Conference Presentation)," Proc. SPIE 10238, High-Power, High-Energy, and High-Intensity Laser Technology III, 1023809 (Presented at SPIE Optics + Optoelectronics: April 26, 2017; Published: 9 June 2017); https://doi.org/10.1117/12.2264702.5463398607001.