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1.2 Ultrafast Laser Physics and Nonlinear Optics
Project coordinator(s): G. Steinmeyer, V. Petrov, U. Griebner
Ultrahigh intensity lasers
M. Kalashnikov, N. Khodakovskiy

    Polarization Encoded Chirped Pulse Amplification in Ti:sapphire (PE-CPA).

    Most of modern high peak power and short pulse laser systems of up to a Petawatt peak power rely upon Titanium Sapphire (Ti:Sa) as amplifier material due to its broad emission spectrum, high thermal conductivity and contrary tom OPCPA systems the relatively low quality requirements of the pump pulses. However, gain narrowing and saturation limits the achievable pulse duration to about 30-40 fs.
    At MBI we develop new technique called polarization encoded chirped pulse amplification (PE-CPA) [1] that allows to overcome restrictions given by gain narrowing. The major principle of this technique is demonstrated in Fig.1. Before amplification the optical rotary dispersion (ORD) is applied to the seed pulse to encode the polarization state of the amplified spectrum. This is being done by using an ORD quartz crystal (right rotating quartz in Fig. 1(b)) with a resulting spectral distribution between π- and σ- cross-sections (Fig. 1(a)). The components around the central and the most intense part of the spectrum are directed closer to the σ- axis, while the sides of the spectral band are aligned towards the π- axis. The emission cross-section of σ- polarized light is nearly 0.4 of the π- polarized light, using this difference results in the effective spectral gain being re-shaped. Hence the amplification bandwidth can be kept broad whilst maintaining the efficiency of amplification. As the effective spectral gain is tuned, unlike to spectral shaping of the beam, the overall amplification process can be kept lossless and efficient. This technique is especially advantageous for intermediate and power amplifiers of ultrashort pulse large scale systems where the energy extraction, rather than total gain, is of major importance. After amplification, the second quartz crystal (left rotating quartz in Fig.1(b)), with an opposite sign of ORD decodes the polarization state of spectral components. The two achromatic λ/2 plates are used to fit the whole polarization encoded spectral distribution between π- and σ-directions. Birefringence in Ti:Sa leading to a temporal walk-off between the π- and σ- components and can be compensated by the addition of an un-doped, but orthogonally oriented sapphire of the same thickness.



    Fig. 1. Distribution of polarization directions of spectral components by a 17.4 mm ORD quartz for shown in the figure wavelengths (a); Principle schematic of the polarization encoded amplifier (b)

     

     

    Fig.2 (a) - amplified spectra with (solid line) and without (dash line) the polarization encoding at gain ~200, (b)- spectra of the seed (solid) and amplified decoded π polarized (dash line) and σ -polarized (dot line) components for the case of low (~30) gain, (c) – computer modeling of a power PE-CPA amplifier, seed : 2J, Gaussian 180 nm FWHM, pump: 50 J

     

     

    Pfeil

    Fig.3 Pulse recompression of PE_CPA pulses

     

References

[1] Mikhail Kalashnikov, Huabao Cao, Károly Osvay, and Vladimir Chvykov, "Polarization-encoded chirped pulse amplification in Ti:sapphire: a way toward few-cycle petawatt lasers," Opt. Lett. 41, 25-28 (2016)

 

Degradation of picosecond temporal contrast of Ti:sapphire lasers with coherent pedestals

In addition to the main pulse, the recompressed temporal shape of a Ti:sapphire CPA typically contains relatively long pre- and post- pedestals and replicas of the main pulse arising from reflections on optical elements of the laser system. The long pedestals are observed in all high peak power laser systems. Depending on the system they typically start at intensity levels ~ 10-5-10-7 and only start to roll off at tens or even hundreds of picoseconds before the main pulse peak. Despite playing a key role in laser-matter interactions, these artifacts—especially the shape of the leading front of the recompressed pulses—are poorly investigated and understood.
Experiments performed with a millijoule Ti:sapphireCPA laser system at the Max Born Institute have shown that recompressed pulses of Ti:sapphire CPA systems are characterized by a postpedestal with ragged temporal structure. This pedestal is coherent to the main pulse and consists of numerous pulses with time separation in the picosecond range. In the presence of substantial selfphase modulation, the postpedestal may generate a symmetric chopped prepedestal after recompression. This process is similar to pre-pulses generated from post-pulses. This effect severely limits the achievable temporal contrast in the leading part of the pulse. It further appears that the duration of the resulting pre-pedestal and the accumulated energy density preceding the main pulse are proportional to the stretched pulse duration. Thus, in petawatt- class Ti:sapphire laser systems the stretched pulse duration needs to be carefully optimized to shorten the pre-pedestal. Moreover, it seems paramount to avoid any unnecessary increase of the B-integral.

 

Fig. 2 (a) Pulses of a Ti:sapphire oscillator/amplifier system (black), pulse of a Ti:sapphire master oscillator only (red), and white light seed amplified in a hybrid OPCPA/Ti:sapphire amplifier (blue). (b) The leading pedestal generated from the trailing pedestal by SPM.

References

[1] Nikita Khodakovskiy, Mikhail Kalashnikov, Emilien Gontier, Franck Falcoz, and Pierre-Mary Paul, "Degradation of picosecond temporal contrast of Ti:sapphire lasers with coherent pedestals." Opt. Lett.41, 4441-4444(2016)

High peak and average power Ti:sapphire Thin Disk amplifier with extraction during pumping (EDP-TD)

Thin Disk technology may offer the potential to be used in systems with both high peak and average output power too. So far, research and development have been mainly devoted to a relatively narrow emission spectral band media (doped YAGs), and hence, the pulse duration is limited to a picosecond (ps) range. The application of this technology to Ti:sa laser materials may allow the combination of broadband pulse amplification at a kilowatt (kW) average power. In this configuration, on one hand, the suppression of the transverse amplified spontaneous emission (TASE) and the transverse parasitic generation (TPG) are the primary challenges. On the other hand, the low gain and low pump absorption of the TD amplifiers complicate its scheme and leads to multi-passing for both the pump and seed. A medium with a large emission cross section and a high concentration of active ions, such as Ti:sa, can help to overcome this problem too.

Recently in a collaboration of MBI and ELI-ALPS [ELI-ALPS (GINOP 2.3.6-15-2015-00001); LASERLAB-EUROPE (284464, EC-FP7)] 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 The obtained results demonstrate the capacity to build a room temperature cooled final amplifier, providing few Joules of energy of the seed laser pulses with in a 100 s TW/10 s Hz CPA laser system. The EDP-TD amplifier was brought to saturation, reaching the close to theoretical maximum efficiency of ∼50% at only three amplification passes. This is a considerable simplification of the amplifier scheme compared to the conventional ten passes TD-amplifiers. Simulations also supported the findings that the new amplifier design overcomes the limitations associated with thermal wave front distortion of the amplified pulse, and losses due to TASE and TPG in high peak and average power laser systems, enabling the scaling up of the system to the repetition rate of hundreds of Hz, or to a PW-level output peak power for tens of Hz repetition rates.

Fig. 1. Crystal mount for Thin Disk Ti:sapphire.

Fig. 2. Schematic diagram of the experimental setup. The green beams are the pump; the red beams are seed passes. Mirrors P1-1and P1-2 were used for pump laser 1, mirrors P2-1 and P2-2 were used for pump 2, mirrors P3-1, P3-2, and P3-3 were used for pump 3. The layout of the seed amplification consists of mirrors S1–S5. The incidence angles are exaggerated for clarity.

Fig. 3. Temperature distribution through the amplifier crystal after temperature stabilization during pumping by the 4 J per pulse at a10 Hz repetition rate. The curve shows the temperature distribution of the horizontal mean section of the pumped area (red, solid line) and temperature distribution in the amplifier head (inset).

 

References

[1] Vladimir Chvykov, Roland S. Nagymihaly, Huabao Cao, Mikhail Kalashnikov, and Karoly Osvay, "Design of a thin disk amplifier with extraction during pumping for high peak and average power Ti:Sa systems (EDP-TD)," Opt. Express 24, 3721-3733 (2016)
[2] Vladimir Chvykov, Huabao Cao, Roland Nagymihaly, Mikhail P. Kalashnikov, Nikita Khodakovskiy, Richard Glassock, Lutz Ehrentraut, Matthias Schnuerer, and Károly Osvay, "High peak and average power Ti:sapphire thin disk amplifier with extraction during pumping," Opt. Lett. 41, 3017-3020 (2016)

Collaborations and Funding.

LaserLab Europe
ELI-ALPS
Marie Skłodowska-Curie Actions