Roland Nagymihaly

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.

Drift and noise of the carrier–envelope phase in a Ti:sapphire amplifier

We report on the drift and noise measurement of the carrier–envelope phase (CEP) of ultrashort pulses in a three-pass Ti:sapphire-based amplifier. Spectrally and spatially resolved interferometry makes it possible to investigate the absolute CEP changes due exclusively to the amplifier, that is, entirely separated from the incidental phase fluctuations of the oscillator. We found that propagation through the amplifier crystal could result in an increase up to 30 mrad noise depending on the repetition rate, cooling, and pumping conditions. Most of this noise is related to mechanical vibrations and thermal instabilities. The absolute CEP drift of thermal origin can be as large as 11 mrad/°C for each mm of the amplifier crystal, originating from inefficient heat conduction during the absorption of pump pulses. The noise of the thermal CEP drift is inversely proportional to the repetition rate, as was shown experimentally and proven by simulations.

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.

Measurement of spectral phase noise in a cryogenically cooled Ti:Sa amplifier (Conference Presentation)

In most of cases the drift of the carrier envelope phase (CEP) of a chirped pulse amplifier (CPA) system is determined only [1], being the relevant parameter at laser-matter interactions. The need of coherent combination of multiple amplifier channels to further increase the peak power of pulses requires interferometric precision [2]. For this purpose, the stability of the group delay of the pulses may become equally important. Further development of amplifier systems requires the investigation of phase noise contributions of individual subsystems, like amplifier stages. Spectrally resolved interferometry (SRI), which is a completely linear optical method, makes the measurement of spectral phase noise possible of basically any part of a laser system [3]. By utilizing this method, the CEP stability of water-cooled Ti:Sa based amplifiers was investigated just recently, where the effects of seed and pump energy, repetition rate, and the cooling crystal mounts were thoroughly measured [4]. We present a systematic investigation on the noise of the spectral phase, including CEP, of laser pulses amplified in a cryogenically-cooled Ti:Sa amplifier of a CPA chain. The double-pass amplifier was built in the sample arm of a compact Michelson interferometer. The Ti:Sa crystal was cooled below 30 °K. The inherent phase noise was measured for different operation modes, as at various repetition rates, and pump depletion. Noise contributions of the vacuum pumps and the cryogenic refrigerator were found to be 43 and 47 mrad, respectively. We have also identified CEP noise having thermal as well as mechanical origin. Both showed a monotonically decreasing tendency towards higher repetition rates. We found that the widths of the noise distributions are getting broader towards lower repetition rates. Spectral phase noise with and without amplification was measured, and we found no significant difference in the phase noise distributions. The mechanical vibration was also measured in the setup by using an accelerometer synchronously with the optical measurements. The noise spectra of phase and vibration measurements were compared and the sources of individual noise components were identified. References [1] Sebastian Koke et al, Opt. Lett. 33, 2545-2547 (2008). [2] J. Limpert et al, IEEE J. of Sel. Top. in Quant. El. 20, 0901810 (2014). [3] A. Borzsonyi, A.P. Kovacs, K. Osvay, Appl. Sci. 3, 515-544 (2013). [4] A. Borzsonyi, R.S. Nagymihaly, K. Osvay, Las. Phys. Lett. 13, 015301 (2016).

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).

Spectral phase noise analysis of a cryogenically cooled Ti:Sapphire amplifier

The spectral phase noise of a cryogenically cooled Ti:Sapphire amplifier was analyzed by spectrally resolved interferometry. Since a relative phase difference measurement is performed, the effect of the amplifier stage can be determined with high precision. Contributions of the cooling system to the spectral phase noise were found to be below 50 mrad for both the vacuum pumps and the cryogenic system. The carrier-envelope phase noise of thermal and mechanical origin was also determined for different repetition rates of laser operation. Mechanical vibrational spectra were recorded by an accelerometer for different stages of operation and compared to the interferometric phase noise measurements.