Peta watt (PW) level laser systems, with focused laser peak intensity of about or even higher, have been reported repeatedly[
High Power Laser Science and Engineering, Volume. 6, Issue 2, 02000e23(2018)
Linear angular dispersion compensation of cleaned self-diffraction light with a single prism
The linear angular dispersion of a self-diffraction (SD) pulse, from a femtosecond laser pulse cleaning device, is compensated for by the use of a single prism. More than $500~\unicode[STIX]{x03BC}\text{J}$ first-order SD pulse has a contrast of $10^{12}$, which is about five orders of magnitude improvement from the input fundamental pulse. The wings of the distribution away from the main pulse in $\pm 1$ ps are cleaned with a contrast improvement of about $10^{7}$, which verifies the pulse cleaning ability of the SD process.
1 Introduction
Peta watt (PW) level laser systems, with focused laser peak intensity of about or even higher, have been reported repeatedly[
To build high temporal contrast laser systems, optical parametric chirped-pulse amplification (OPCPA)[
In theory, the temporal contrast of the first-order SD signal is the cube of the temporal contrast of the incident pulse as , because the generated cleaned SD signals are spatially separated from the incident beams without use of any polarization discrimination devices. What is more, this technique can also achieve a high energy SD signal output using a cylinder mirror focusing on the incident beams, even with low energy-conversion efficiency[
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In this study, we report the highest pulse energy and highest temporal contrast enhancement pulses generation based on the SD effect with two cylindrical convex lenses so far. More than first-order SD signal with a temporal contrast of is generated with about five orders of magnitude improvement. The wings around the main pulse in ps are cleaned with a contrast improvement of about , which verifies the pulse cleaning ability of SD process. The cause of angular dispersion generation of the SD signals is also explored, and the linear angular dispersion is compensated with a single prism.
2 Principle and experimental setup
The SD process is a degenerated cascaded four-wave mixing (DCFWM) process[
Figure
3 Results and discussion
3.1 High energy and high temporal contrast SD pulse generation
To generate SD signals with high energy and temporal contrast, experiments were performed with a commercial Ti:sapphire CPA laser system (Legend Elite Cryo PA, Coherent Inc.). The laser system produces 1 kHz/50 fs/800 nm/10 mJ pulses with a diameter of about 15 mm. The pulse energies of the two incident beams before the Kerr medium P1 are both about 4.9 mJ. The beam size on the Kerr medium is about . About five diffracted orders of SD signals are generated beside each side of the two incident beams. The first-order SD signals and are about and , respectively. The energy-conversion efficiency from the two incident beams to is about 7.8%.
To characterize the temporal contrast of , the diameter of signal beam is reduced to about 3 mm by C1 (spherical concave reflective mirror, ) and C2 (spherical convex reflective mirror, mm) first. Then, a 2 mm thick VND filter is used to adjust the input energy to the correlator to about 200 mW. A 1 mm thick fused silica plate is inserted in the path of to introduce reference post-pulses.
The measured temporal contrast curves of the input pulse and are shown in Figure
3.2 Analyzation of angular dispersion generation
The generation of the angular dispersion of is caused by the different phase-matching condition of wavelength components of the two incident beams shown in Figure
For , its phase-matching condition is . In the direction, denotes the wave vector of the longest wavelength component and the wave vector of the shortest wavelength component. Similarly, in the direction, denotes the wave vector of the longest wavelength component and the wave vector of the shortest wavelength component. Then the generated longest wavelength and shortest wavelength in can be expressed as and , respectively. is the original longest and the shortest wavelength of the incident pulses. The phase-matching conditions for the longest and shortest wavelength SD signals are and , respectively. Then the dispersion angle of is the angle between and . It is related to the cross angle of the two incident beams, the shortest and the longest wavelength components of the two beams, and can be expressed as , , .
There is a little difference between the center wavelength of and that of the incident pulses, and can be calculated according to the law of cosines: , where is the new generated center wavelength of and the original center wavelength of the incident pulses. For nm, . The new generated center wavelength of nm. It can be concluded that for a small crossing angle of the two incident beams, the generated first-order SD signals almost keep the same center wavelength of the incident pulses, and the SD process can be looked on as a frequency-conserving process.
3.3 Angular dispersion compensation
For an incident beam with a spectrum as shown in Figure
In the experiment, after propagation of about 2000 mm in air, the width of beam is about 30 mm. We measured the spectra of at 30 different positions P0 to P29 in the horizontal direction, with every two positions separated by about 1 mm. Figure
The scheme of angular dispersion compensation is shown in Figure
For a prism with refractive index and apex angle , we can obtain the angular dispersion of the prism[
The relation between and can be calculated with Snell’s law , and rad. If the cross angle between the two output beams is about , the angle is the right output angle of the beam. The input angle of the beam can be calculated as about rad.
The angular dispersion compensated propagates about 1500 mm, and is expanded to about 20 mm in the horizontal direction. The spectra of at five different positions with 5 mm apart are shown in Figure
As can be seen in Figure
4 Conclusion
In conclusion, temporal contrast enhancement by the SD process in a bulk Kerr medium possesses a few advantages compared to many other pulse cleaning techniques. The SD signals are spatially separated from the incident beams without the use of any polarization discrimination devices. It can also achieve a high energy SD signal output with a cylindrical mirror focusing on the incident beams even with low energy-conversion efficiency. The temporal contrast of the signal is the cube of the temporal contrast of the incident pulse, in theory, which indicates a great potential of temporal contrast enhancement by the SD process.
In this study, a temporal contrast enhancement equipment based on SD effect with two cylindrical convex lenses is built. As high as first-order SD signal at 800 nm with contrast of is generated with about five orders of magnitude improvement. Wings around the main pulse in are cleaned with a contrast improvement of about , which verifies the pulse cleaning ability of SD process. The cause of angular dispersion generation of the SD signals is also explored, and the angular dispersion is compensated with a single prism. It is expected to extend the SD process as an effective pulse cleaning method for high power laser at 1053 nm with a narrow spectral bandwidth and hundreds of femtosecond pulse duration.
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Xiong Shen, Peng Wang, Jun Liu, Ruxin Li. Linear angular dispersion compensation of cleaned self-diffraction light with a single prism[J]. High Power Laser Science and Engineering, 2018, 6(2): 02000e23
Received: Nov. 2, 2017
Accepted: Feb. 9, 2018
Published Online: Jul. 4, 2018
The Author Email: Jun Liu (jliu@siom.ac.cn)