Huabao Cao

High peak and average power Ti: sapphire thin disk amplifier with extraction during pumping

The combination of the extraction during pumping (EDP) amplification scheme and the thin disk (TD) technology has been successfully applied to the Ti:sapphire (Ti:sa) laser medium for the first time, to the best of our knowledge. In a proof-of-principle experiment, we demonstrate high energy broadband amplification in a room temperature water cooled EDP-TD head of stretched femtosecond pulses at a 10 Hz repetition rate, instead of performing a cryogenically cooled traditional multi-pass scheme. Hence, the EDP-TD combination can overcome the limits associated with thermal effects and transverse amplified spontaneous emission, enabling Ti:sa laser systems to have a petawatt peak and hundreds of watts of average power.

Design of a thin disk amplifier with extraction during pumping for high peak and average power Ti:Sa systems (EDP-TD)

Combination of the scheme of extraction during pumping (EDP) and the Thin Disk (TD) technology is presented to overcome the limitations associated with thermal cooling of crystal and transverse amplified spontaneous emission in high average power laser systems based on Ti:Sa amplifiers. The optimized design of high repetition rate 1-10 PW Ti:Sapphire EDP-TD power amplifiers are discussed, including their thermal dynamic behavior.

Liquid-cooled Ti:Sapphire thin disk amplifiers for high average power 100-TW systems

In this work, numerical heat transfer simulations of direct water-cooled gain modules for thin disk (TD) Ti:Sapphire (Ti:Sa) power amplifiers are presented. By using the TD technique in combination with the extraction during pumping (EDP) method 100-TW class amplifiers operating around 300 W average power could be reached in the future. Single and double-sided cooling arrangements were investigated for several coolant flow velocities. Simulations which upscale the gain module for multiple kilowatts of average power were also performed for large aperture Ti:Sa disks and for multiple disks with several coolant channels.

Active spectral shaping with polarization encoding of chirped pulses in Ti:Sapphire amplifiers

Up to date, the majority of the existing petawatt class lasers are based on Ti: Sapphire (Ti:Sa) because of the wide emission spectrum, high thermal conductivity and loose requirement of the pump laser parameters [1]. However, petawatt systems with pulse duration considerably less than 25 fs are not yet available due to the gain narrowing effect which limits the achievable bandwidth [2]. The most widely used methods for overcoming gain narrowing are based on spectral shaping by inducing spectral losses (e.g. Dazzler), optical parametric amplification in the frontend and post compression. Unfortunately, critical all of them have imperfections, which limit their application. Polarization Encoded (PE) amplification in Ti: Sa was proposed to amplify pulses with spectral bandwidth supporting a few cycle duration even for final amplifiers of PW-level laser systems without energy losses, and the proof-of-principle experiments were recently reported [3]. A PE amplifier can preserve the bandwidth effectively during amplification by exploiting the difference between the π- and σ-emission cross-sections in a Ti: Sa medium, simultaneously the profile of the output spectrum can be conveniently shaped to either asymmetric red-, blue-shifted, or a symmetrically broad, only by rotating two λ/2 plates. These two features make the PE amplifier attractive for ultrashort high power laser systems.

Water-cooled thin disk Ti:Sapphire amplifiers for kW average power

Rapid development of ultrafast laser systems led to the generation of laser pulses with several PW peak power in table top arrangements [1]. Most of these systems rely on amplification using Ti:Sapphire (Ti:Sa), which has exceptional spectral and thermal properties [2]. Reaching ultrahigh peak power pulses at high repetition rates, which is favorable for many applications, is however still a great challange due to the thermal load in the gain medium. By using conventional cooling techniques non-uniform transverse temperature profile is created in the crystal, which leads to serious beam degradation during the amplification process. Thin disk (TD) technology may offer the possibility for Ti:Sa crystals to be used in with high average output power systems. Research and development has strongly focused on relatively narrow emission spectral band media (doped YAGs) and thus the pulse duration has been limited to the lower, sub-picosecond regime [3].

Active spectral pre-shaping with polarization encoded amplifiers (Conference Presentation)

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.

Thin Disk Ti:Sapphire amplifiers for Joule-class ultrashort pulses with high repetition rate (Conference Presentation)

High peak power CPA laser systems can deliver now few petawatt pulses [1]. Reaching the high energies with broad spectral bandwidth necessary for these pulses was possible by the use of large aperture Ti:Sa crystals as final amplifier media. Wide applications for these systems will be possible if the repetition rate could be increased. Therefore, thermal deposition in Ti:Sa amplifiers is a key issue, which has to be solved in case of high average power pumping. The thin disk (TD) laser technology, which is intensively developed nowadays by using new laser materials, is able to overcome thermal distortions and damages of laser crystals [2]. TD technique also has the potential to be used in systems with both high peak and average power. For this, the commonly used laser materials with low absorption and emission cross sections, also low heat conductivity, like Yb:YAG, need to be replaced by a gain medium that supports broad enough emission spectrum and high thermal conductivity to obtain few tens of fs pulses with high repetition rates. Parasitic effects during the amplification process however seriously limit the energy that can be extracted from the gain medium and also they distort the gain profile. Nevertheless, the application of the Extraction During Pumping (EDP) technique can mitigate the depopulation losses in the gain medium with high aspect ratio [3]. We proposed to use Ti:Sa in combination with TD and EDP techniques to reach high energies at high repetition rates, and we presented numerical simulations for different amplifier geometries and parameters of the amplification [4,5]. We present the results of the proof-of-principle experiment, where a EDP-TD Ti:Sa amplifier was tested for the first time. In our experiment, the final cryogenically cooled Ti:Sa amplifier in a 100 TW/10 Hz/28 fs laser system was replaced with the EDP-TD room temperature cooled arrangement. Amplified seed pulse energy of 2.6 J was reached only for 3 passes through TD with 0.5 J of input seed and 5 J of absorbed pump energy. We verified the excellent heat extraction capabilities of our amplifier module. Results of the scaling simulations on the base of this experiment for 100s of TW peak power laser systems operating at up to 100 Hz will be also presented. References 1. Y. Chu et al, Opt. Lett. 40, 5011-5014 (2015). 2. C. R. E. Baer et al, Opt. Exp. 20, 7054-7065 (2012). 3. V. Chvykov et al, Opt. Comm. 285, 2134-2136 (2012). 4. V. Chvykov, R. S. Nagymihaly, H. Cao, M. Kalashnikov, K. Osvay, Opt. Exp. 24, 3721 (2016). 5. V.Chvykov, R. S. Nagymihaly, H. Cao, M. Kalashnikov, K. Osvay, Opt. Lett. 41,13, 3017 (2016).