Toward terawatt few-cycle pulses via optical parametric chirped-pulse amplification with oxide crystals Download: 586次
1 Introduction
Ultrafast light fields enable studies of ultrafast processes in physics, chemistry and biology at the femtosecond scale[1]. An intense ultrafast laser field can even ionize the electrons from the parent atom, which will be further accelerated in the light field and finally recombine with the atom. Such a strong-field process can emit high harmonics of the drive laser[2]. Due to the coherent nature of the process, high harmonic generation (HHG) can convert a femtosecond pulse to previously inaccessible attosecond pulses and thereby open a door to attosecond science[3]. In addition, an intense ultrafast light field can accelerate an electron or protons with an acceleration gradient that is three orders of magnitude higher than that of a conventional microwave accelerator and holds promise for constructing table-top laser accelerators[4].
One core parameter in strong-field physics is the electron quiver energy,
Owing to these interesting applications, the generation of intense mid-IR ultrafast lasers has attracted increasing interest from the optical community. Given the lack of traditional laser amplifiers beyond
Table 1. Performance characteristics of long-wavelength OPA/DFG systems.
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Table 2. Performance characteristics of long-wavelength OPCPA systems.
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The performance of ultrafast OPA and OPCPA significantly depends on nonlinear crystals. In the ultrafast systems listed in Tables
Fig. 1. Transparent regions and damage thresholds of commonly used nonlinear crystals. The full (color) bar marks the transparent regions at zero (half) transmittance. The black, red and blue bars correspond to conventional oxide crystals, new langasite oxide crystals and semiconductor crystals, respectively. The green circles mark the damage thresholds with 10 ns pulses at $2.05~\unicode[STIX]{x03BC}\text{m}$ for the ZGP crystal and 10 ns pulses at 1064 nm for other crystals. Most of the data come from the book, D. N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey (Springer, New York, 2006). Other data for langasite oxides come from Refs. [45–48] and [51].
In this paper, we put forward a
2 Source architecture
The phase-matching (PM) condition is the major factor that governs OPCPA. In the whole paper, we define the three interacting waves of the highest, moderate and lowest frequencies as ‘pump’, ‘signal’ and ‘idler’, respectively (i.e.,
Fig. 2. PM for noncollinear OPCPA pumped by 1030 nm laser. (a) Type-II ($e_{p}\rightarrow o_{s}+e_{i}$ ) PM in $XY$ plane of $\text{LiGaS}_{2}$ crystal; (b) Type-I ($o_{p}\rightarrow e_{s}+e_{i}$ ) PM in LGN crystal; (c) Type-II ($o_{p}\rightarrow e_{s}+o_{i}$ ) PM in LGN crystal. $\unicode[STIX]{x1D6FC}$ is the intersecting angle between pump and mid-IR beams inside the crystal; $\unicode[STIX]{x1D6FC}=0^{\circ }$ corresponds to the collinear configuration.
Fig. 3. Schematic setup of the proposed $5.2~\unicode[STIX]{x03BC}\text{m}$ TW-class OPCPA system based on oxide LGN crystals. All of the hardware devices in the gray background are commercially available. The reflection-induced losses in the LGN crystals and Si plate are neglected. Three OPCPA stages are pumped with the same intensity of $50~\text{GW}/\text{cm}^{2}$ .
Based on the unique PM characteristics of LGN, we design a
The mid-IR seeding pulse for
The high-power thin-disk Yb:YAG picosecond laser is selected as the pump source for our OPCPA system, which has many advantages over typical Nd:YAG or Nd:glass lasers. First, the laser can support a repetition rate above 1 kHz due to good heat dissipation of the thin-disk geometry. In contrast, a high-power Nd:YAG or Nd:glass rod-type laser usually operates at 10 Hz or less. Second, the picosecond thin-disk laser allows a higher pump intensity in the crystal because the damage threshold of nonlinear crystals increases as pump pulse duration decreases[53]. An increase in the pump intensity ensures a high gain in a shorter crystal and thereby favors a broader gain bandwidth. Third, a picosecond pumped OPCPA system naturally supports a high pulse contrast by confining the parametric fluorescence within the narrow window of the pump pulse[54]. The high-contrast mid-IR laser can avoid preplasma formation when interacting with solid targets[55]. Finally, the picosecond pump also reduces the required group delay dispersion (GDD) to chirp the mid-IR pulse and allows compression with bulk materials. The residual high-order dispersion after compression can be precompensated by a commercial acousto-optic programmable dispersion filter (AOPDF, e.g., Fastlite DAZZLER UWB-3500-7000).
The 200 mJ energy from Yb:YAG laser is divided into three parts of 5, 45 and 150 mJ to pump three OPCPA stages with LGN crystal length of 15, 12 and 7 mm, respectively. Three beams of pump light are telescoped into beam sizes of 2.1, 6.1 and 11.2 mm (full width at half maximum (FWHM)), respectively, so that three OPCPA stages can be pumped by the same intensity of
3 Numerical model
All of the simulations in this paper are based on the refined Sellmeier equation of LGN given in Ref. [47],
4 Results and discussion
4.1 Intrapulse DFG for generating mid-IR seed pulses
The collinear intrapulse DFG between the short-wavelength (pump) and long-wavelength (signal) components of a single octave-spanning pulse can generate the mid-IR pulse (idler) for seeding subsequent OPCPA. Type-II PM (
In addition to broadband PM for the
Fig. 4. PM properties of Type-II collinear intrapulse DFG. (a) The attainable signal (blue) and idler (red) wavelengths under the condition of $\text{GVM}_{si}=0$ . In the calculation, the PM angle $\unicode[STIX]{x1D703}$ (green) is varied with the pump wavelength. (b) The phase-matched signal (blue) wavelength, idler (red) wavelength and the corresponding $\text{GVM}_{ps}$ (black) at $\unicode[STIX]{x1D703}=59.50^{\circ }$ .
Fig. 5. (a) Schematic of intrapulse DFG for passive CEP stability. (b) Calculated mid-IR idler intensity as a function of the polarization angle $\unicode[STIX]{x1D711}$ . The inset in (b) illustrates the definition of the angle $\unicode[STIX]{x1D711}$ . The calculation parameters are $\unicode[STIX]{x1D706}_{p}=770~\text{nm}$ , $\unicode[STIX]{x1D706}_{s}=904~\text{nm}$ , $\unicode[STIX]{x1D706}_{i}=5.2~\unicode[STIX]{x03BC}\text{m}$ , $\unicode[STIX]{x1D703}=59.50^{\circ }$ , $L=0.1~\text{mm}$ and $I_{0}=1~\text{TW}/\text{cm}^{2}$ .
Fig. 6. Simulation results for intrapulse DFG. (a) Input pump (black) and signal (red) spectral components. (b) Idler efficiency versus LGN crystal length. Inset shows the idler beam profile at $L=0.7~\text{mm}$ . (c) Output mid-IR idler spectrum (solid) and phase (dashed) at $L=0.7~\text{mm}$ . (d) Output idler pulse before (black) and after (red) dispersion compensation with GDD of $652~\text{fs}^{2}$ and TOD of $-5.84\times 10^{3}~\text{fs}^{3}$ . The blue curve shows the FTL pulse. The parameters used in the simulation are $\unicode[STIX]{x1D706}_{p}=770~\text{nm}$ , $\unicode[STIX]{x1D706}_{s}=904~\text{nm}$ , $\unicode[STIX]{x1D706}_{i}=5.2~\unicode[STIX]{x03BC}\text{m}$ , $\unicode[STIX]{x1D703}=59.50^{\circ }$ , $I_{p0}=0.75~\text{TW}/\text{cm}^{2}$ and $I_{s0}=0.25~\text{TW}/\text{cm}^{2}$ .
Fig. 7. Simulation results for the three-stage OPCPA. (a) Evolution of the mid-IR pulse energy in the first (green), second (red) and third (blue) OPCPA stages. (b) Evolution of chirped mid-IR pulse duration with amplification. The black curve represents the input chirped pulse. (c) Evolution of mid-IR spectrum with amplification. The black curve represents the input mid-IR spectrum. (d) FTL pulses after OPCPA-3 (blue) and seed mid-IR pulses before stretching (black). (e) Pump beam profile output from the first (left), second (middle) and third (right) OPCPA stages. (f) Mid-IR beam profile output from the first (left), second (middle) and third (right) OPCPA stages.
To accommodate Type-II DFG, the LGN crystal is rotated around the light path so that the linearly polarized input beam can have components along the
Based on the above parameters and analysis, we numerically solve Equations (
Figure
4.2 Three-stage OPCPA
Spectral width of the
The design parameters of the three-stage OPCPA are given in Figure
Three OPCPA stages, based on LGN crystals with lengths of 15, 12 and 7 mm, respectively, amplify the 90 nJ mid-IR seed to 0.1, 2.8 and 16.5 mJ successively, with the total pump-to-IR energy conversion efficiency of about 8% (Figure
Table 3. Optical parameters at $5.2~\unicode[STIX]{x03BC}\text{m}$ of six commonly used bulk materials.
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4.3 Dispersion management
The use of a picosecond pump pulse facilitates the compression of a
Table
The required length of Si is determined by the total negative dispersion experienced by the mid-IR chirped pulse. We calculate the GDD and TOD induced in the stretching and amplification processes. A 187.1 mm long Si plate is needed to fully compensate the negative GDD imposed by the AOPDF and three LGN crystals. As both the Si plate and LGN crystal have positive TOD, they can impose a total TOD of
Table 4. Dispersion management for the $5.2~\unicode[STIX]{x03BC}\text{m}$ OPCPA.
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The 16.5 mJ amplified mid-IR pulse before and after compression by the Si block has peak powers of 7.9 and 137.5 GW, respectively. The averaged peak power in the Si block is approximately 72.7 GW. To control the total
4.4 Performance scalability
We have numerically demonstrated the generation of
The seven-cycle mid-IR pulses from OPCPA can be further compressed down to sub-three-cycle or even sub-cycle pulses by nonlinear compression methods. Filamentation-assisted supercontinuum generation followed by anomalous dispersion compensation has been used to nonlinearly compress the mid-IR pulse[35, 60]. Recently, supercontinuum generation and self-compression in bulks have attracted much attention due to good shot-to-shot repeatability and the absence of complicated pulse splitting[61, 62]. In addition, nonlinear soliton compression via quadratic cascaded nonlinearity is another potential route to compress mid-IR pulses with good controllability[63, 64]. All these nonlinear compression methods usually have a high efficiency of
In addition to pulse shortening by nonlinear compression, the scaling of the peak power of mid-IR OPCPA ultimately relies on the available pump energy. The pulse energy of a commercial Yb:YAG thin-disk regenerative laser is currently a maximum of 200 mJ in the design, which may be further boosted up to Joule-level energy at the expense of cost[65]. With the development of a Joule-level, kHz, picosecond Yb:YAG thin-disk laser, OPCPA based on large-size langasite oxide crystals can support
Finally, we want to point out that the thermal effect has not been considered in our simulations due to the lack of temperature-dependent Sellmeier equations for LGN crystal. Because of the high transmittance of LGN to all the three interacting pulses, we expect that the thermal effect is not significant in our demonstrated OPCPA with a 16 W average power. With the increase of average power in the future, the thermal effect should be considered definitely[67]. A precise characterization of thermal parameters for LGN crystal, e.g., temperature-dependent Sellmeier equations, is the precondition to evaluate the influence of thermal effect on the power scalability of the mid-IR OPCPA.
5 Conclusion
In conclusion, we have proposed a design of a 0.13 TW, seven-cycle,
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Article Outline
Jinsheng Liu, Jingui Ma, Jing Wang, Peng Yuan, Guoqiang Xie, Liejia Qian. Toward terawatt few-cycle pulses via optical parametric chirped-pulse amplification with oxide crystals[J]. High Power Laser Science and Engineering, 2019, 7(4): 04000e61.